Recent Progresses in Photoresponsive π‑Conjugated Organogelators for Biomaterial Applications: Challenges and Future Perspectives

Abstract

The capacity of photoresponsive organogels to react to light stimuli with reversible changes in their physical characteristics has made them a potential class of materials for a variety of functional biomaterial applications. These substances allow for regulated modification of the structure and function of organogels in response to light by combining their adaptability with photoactive moieties. Recent advancements in the development of photoresponsive organogels have led to their use in areas such as drug delivery, tissue engineering, and biosensing. This Perspective aims to explore the latest trends in the design and synthesis of photoresponsive organogels, highlighting key strategies for enhancing their responsiveness, stability, and biocompatibility. We also discuss the challenges associated with scaling up these materials for practical applications including issues related to light penetration, material degradation, and the need for precise control over the response kinetics. Furthermore, we outline potential future directions in the field, including the integration of advanced light-responsive functionalities, the development of more sustainable and biodegradable materials, and the exploration of new applications in personalized medicine and environmental monitoring. The continued innovation in photoresponsive organogels promises to expand their potential for use in next-generation functional biomaterials.

1. Introduction

The rapid progress in material science over the past few decades has led to the creation of innovative functional materials with applications across a wide range of fields, including biotechnology, medicine, and environmental science. Among the many classes of materials that have emerged, organogels have garnered particular attention due to their unique properties and versatile applications. Organogels are formed by a network of gelators dispersed in organic solvents that self-assemble into a three-dimensional structure. This self-assembly process imparts these materials with excellent mechanical strength, high stability, and the ability to absorb a wide variety of substances, making them attractive candidates for use in applications. Organogels possess a variety of desirable characteristics, including tunable rheological properties, the ability to incorporate bioactive compounds, and the potential to mimic the viscoelastic properties of biological tissues. These features make them especially appealing in the development of biomaterials as they offer high flexibility in tailoring material properties to suit specific medical or industrial needs. For example, in drug delivery, organogels can be designed to encapsulate and release therapeutic agents in a controlled manner, ensuring targeted delivery and reducing the risks of side effects. Furthermore, the mechanical properties of organogels can be optimized for use in tissue engineering, where materials need to support cell growth and mimic the mechanical behavior of native tissues.

The integration of photoresponsive components into organogels has further expanded their functionality, resulting in the development of photoresponsive organogel materials that undergo reversible changes in their properties upon exposure to light. Recent advances in 4D printing have broadened the scope of stimuli-responsive material research by integrating temporal control into three-dimensional architectures. The literature on 4D printing encompasses a wide variety of material systems, including polymeric, hydrogel-based, and supramolecular assemblies that undergo controlled deformation or functional change upon external stimulation such as light, temperature, or pH. These studies demonstrate the versatility of molecular self-assembly and reversible interactions in programmable materials, reflecting a similar design philosophy to photoresponsive organogels. Notably, representative works in this area have reported shape-morphing and adaptive behaviors using surface-functionalized molecular assemblies and soft polymeric matrices. − Photoresponsive materials, which react to specific wavelengths of light by undergoing chemical, physical, or structural changes, have revolutionized numerous fields by enabling the creation of dynamic systems that can be controlled remotely. The addition of light-responsive behavior to organogels allows for real-time manipulation of the material properties, providing an unprecedented level of control in applications ranging from therapeutic drug release to the fabrication of adaptive tissue scaffolds. The development of photoresponsive organogels is especially advantageous for biomaterial applications. In particular, the ability to trigger changes in the material’s structure and properties in response to light enables precise control over drug release rates, spatial and temporal control over tissue-engineering processes, and the development of highly sensitive biosensors. By incorporating light-responsive moieties, such as azobenzene, coumarin, pyrene, perylene, and dendrimer, into the gelators that form the basis of the organogels, researchers can achieve a broad range of light-triggered behaviors. These photoresponsive moieties can be tailored to respond to specific wavelengths of light, allowing for the fine-tuned control of material properties, such as viscosity, solubility, and mechanical strength.

One of the key advantages of photoresponsive organogels in drug delivery is their ability to control the release of therapeutic agents with precision. By using light as a trigger, drug release can be initiated or halted at specific times and locations, enhancing the efficacy of the treatment and minimizing potential side effects. For example, in cancer therapy, photoresponsive organogels can be used to deliver chemotherapeutic drugs directly to the tumor site, minimizing the exposure of healthy tissues to the drug. In tissue engineering, photoresponsive organogels offer the potential to create scaffolds that respond to light by changing their mechanical properties or degradation rates, allowing for controlled tissue growth and regeneration. Moreover, the ability to respond to light stimuli opens up new avenues for the advancement of intelligent materials that can adapt to the needs of the body in real time, providing a highly personalized approach to therapy. As the development of photoresponsive organogels continues to evolve, several notable trends have emerged. The incorporation of various light-responsive moieties into organogels has expanded the range of potential applications and provided opportunities for creating materials with complex, multifunctional properties. For instance, photoresponsive moieties such as azobenzenes, which undergo reversible trans–cis isomerization upon exposure to UV or visible light, can be integrated into organogels to induce reversible changes in the gel’s physical properties. Similarly, spiropyrans, which undergo a ring-opening reaction upon light exposure, can be used to tune the material’s mechanical properties, enabling the development of organogels that change in response to light stimuli in a controllable and predictable manner.

Furthermore, the development of hybrid materials that combine photoresponsive organogels with other functional components, such as nanoparticles or bioactive molecules, has significantly broadened the potential applications of these materials. For example, incorporating nanoparticles into photoresponsive organogels can enhance their mechanical properties, improve their drug-loading capacity, and enable additional functionalities, such as targeting or imaging. The combination of multiple light-responsive components in a single material system also allows for the creation of multifunctional organogels capable of responding to different types of light stimuli. , This opens up possibilities for developing advanced materials that can perform multiple tasks simultaneously, such as controlled drug release, tissue regeneration, and disease detection. While other classes of photoresponsive materials, such as polymeric hydrogels, liquid-crystalline systems, and inorganic nanocomposites, have also been explored, they often suffer from limitations, including poor mechanical tunability, slower response times, limited biocompatibility, or difficulties in achieving reversible and reproducible switching under physiological conditions. In contrast, photoresponsive organogels combine the versatility of low-molecular-weight gelators with the precision of light-triggered control, enabling reversible sol–gel transitions, fine-tuned mechanical properties, and facile incorporation of bioactive molecules. Their ability to self-assemble through noncovalent interactions provides structural adaptability that many polymeric or inorganic systems lack. This unique combination of responsiveness, modular design, and biocompatibility distinguishes photoresponsive organogels from other smart material platforms and underscores their promise for next-generation biomedical applications. Despite the tremendous potential of photoresponsive organogels, several challenges remain that must be addressed before these materials can be widely adopted in clinical and industrial applications. One of the key challenges is the integration of light-responsive components into organogels in a way that ensures the stability and reproducibility of the response of the material. The design of gelators that are both photoresponsive and capable of forming stable organogel structures is complex, and optimizing these components to achieve a balance between light responsiveness, mechanical properties, and material stability is a critical area of research. Additionally, the ability to achieve controlled, repeatable responses to light stimuli under physiological conditions remains a major challenge, as the behavior of these materials can be affected by factors such as temperature, pH, and the presence of biological molecules. , Another significant challenge is ensuring the biocompatibility and biodegradability of photoresponsive organogels, particularly for biomedical applications. The materials must be nontoxic, capable of being safely degraded in the body, and suitable for long-term use in medical settings. Moreover, the light absorption and transmission properties of photoresponsive organogels must be optimized to ensure that light can effectively penetrate tissues and trigger the desired material transformations without causing damage to surrounding biological systems. − Compared with other classes of soft materials, such as polymeric hydrogels, inorganic nanocomposites, or elastomeric networks, organogelators offer distinct advantages that stem from their low-molecular-weight and supramolecular self-assembly nature. Their gelation relies on noncovalent interactions such as π–π stacking, hydrogen bonding, and van der Waals forces, which enable reversible sol–gel transitions, tunable mechanical strength, and facile incorporation of photoresponsive moieties. Unlike polymeric gels that often require covalent cross-linking or harsh synthesis conditions, organogels can be formed in mild environments, preserving sensitive bioactive molecules. Moreover, their high transparency, processability in organic media, and compatibility with photoactive chromophores make them excellent candidates for light-driven drug delivery, biosensing, and tissue-engineering applications. These features collectively distinguish organogelators as versatile platforms for designing adaptive and multifunctional biomaterials.

This Perspective provides a concise overview of recent advances in photoresponsive organogels (1) and their potential in functional biomaterials (Figure ). It covers the fundamental mechanisms of light responsiveness, key design strategies, and synthesis methods, including the use of novel photoactive molecules and hybrid systems. The review also addresses current challenges such as scalability, biocompatibility, and response kinetics. Applications in drug delivery, tissue engineering, and biosensing are highlighted, demonstrating the versatility of these materials. Looking ahead, future developments are expected to emphasize sustainability, biodegradability, and multifunctional responsiveness for next-generation biomedical solutions.

1.

Schematic representation of photoresponsive π-conjugated organogelators for biomaterial applications (1).

2. Azobenzene-Based Organogelators

Three long alkyl chains were grafted onto three azobenzene moieties to functionalize a triarmed cis-1, 3, 5-cyclohexanetricarboxamide gelator (2). At low concentrations, it can gel the majority of organic liquids. The study’s morphologies reveal helical threads or one-dimensional aggregated bundles. The gel-state azobenzene groups experience the expected trans–cis photoisomerization with a gel-to-sol phase transition and generate H-type aggregation when exposed to UV light. The main causes of gelation are the π–π interaction of azobenzene moieties and intermolecular hydrogen interactions between amides (Figure a). The coaggregates of cyanurate/barbiturates and azobenzene-tethered melamine dimer were used to create binary organogels (3). The morphologies of the coaggregates based on flexible dodecamethylene-tethered melamine dimers and the highly entangled nanofibers that were generated in the gels of hydrocarbon liquids differed significantly. The stiffness of the azobenzene tether in the current systems may cause molecules to pack regularly. Furthermore, the contribution of the central amide groups and the azobenzene chromophores in the tether moiety to the formation of well-defined nanofibers through hydrogen-bonding and face-to-face (H-type) π–π stacking interactions was obvious by UV–vis and Infrared spectroscopic measurements. The azobenzene moiety in the gel state demonstrated exceptional resistance to trans–cis isomerization upon exposure to UV light due to the close molecular packing in the self-assembled nanofibers (Figure b).

2.

(a) Morphology of azobenzene-based organogel showing helical fibers (2). Reproduced from ref . Copyright [2008] John Wiley & Sons. (b) Morphology of binary azobenzene-melamine organogel revealing nanofiber networks (3), reproduced from ref . Copyright [2005] American Chemical Society.

The gelators (4) with azobenzene-hydrazide units joined by various flexible central spacers have been examined. In the gelation process, the central flexible spacer’s length and parity are very important. In DMF, it might create a stable organogel. When exposed to UV light, it displayed a photoinduced gel-to-precipitate transition, which was ascribed to the photoisomerization of azobenzene. The gel’s morphological transition from fibrous to globular aggregates was also brought on by trans–cis isomerization. Furthermore, when exposed to various stimuli like pH, anions, and temperature, it displayed a variety of stimuli-responsive behaviors (Figure a). The azobenzene-substituent-group-containing cholesterol imide derivatives (5) have been synthesized. The compounds have either methyl or hydrogen unit headgroups. According to the gelation test, both compounds’ gelation behavior dramatically changed when the headgroup of the azobenzene segment was slightly altered. XRD analyses, scanning electron microscopy, atomic force microscopy, a Fourier transform IR spectrometer, and UV–vis absorption spectra are used to analyze the reversible photoisomerization transitions brought on by exposure to ultraviolet (UV) and visible light for DMF gel and gelator solution. The gel–sol transition is caused by trans–cis photoisomerization of the azobenzene groups during UV irradiation. This shift in molecular polarity causes the van der Waals connections to collapse. Following exposure to visible light, the gel can be recovered using reverse cis–trans photoisomerization. According to morphological and spectral analyses, the gelator molecules self-assemble into one-dimensional fibers. These fibers are then cross-linked to create regular nanobelts. These findings provide important information for the creation of soft matter and new, adaptable low-molecular-mass gelators (Figure b).

3.

(a) UV–vis absorbance change during trans–cis photoisomerization of azobenzene-hydrazide gels (4), reproduced from ref . Available under the CC-BY-4.0 License. Copyright [2017], The Author(s). (b) Reversible photoisomerization of cholesterol-azobenzene organogels (5), reproduced from ref . Copyright [2012] Elsevier.

The stable gels were produced by synthesizing and testing organogelator (6) to gel aromatic solvents. According to SEM photos, the gels were made up of fibers that entangled into a three-dimensional network and molecules that self-assembled into a layered structure. The creation of fibers was primarily driven by intermolecular H-bonds, π–π interactions, and microphase segregation between the flexible chains and the rigid-rod core according to both FT-IR and UV–vis spectra. The addition of anion (F- and AcO−) stimuli caused a color shift and gel–sol transition (Figure a). The selective gelation in the alcoholic solvent of photoresponsive gelator (7) based on azobenzene is disclosed. X-ray diffraction techniques measure the related layer arrangement, revealing a monotropic liquid-crystalline phase in the organogel, while polarized optical microscopy and differential scanning calorimetry are used to study their thermodynamic behaviors. The gel–sol transition occurs when the trans–cis isomerization of the azobenzene moiety breaks the tangling of gel fibers of self-assembly, converting the nonliquid crystalline gelator into a liquid-crystalline organogel in the presence of an alcoholic solvent. The current finding also demonstrates that the organogel can undergo a thermal and photoinduced transition from gel to sol and vice versa (Figure b).

4.

(a) UV spectral response of anion-sensitive azobenzene-based organogel (6), reproduced from ref . Copyright [2015] Elsevier. (b) Phase transition of azobenzene liquid-crystalline organogels under light (7), reproduced from ref . Copyright [2014] Elsevier.

The aryl-group-containing sugar-azobenzene derivatives (8) have been created, produced, and described by using various spectroscopic methods. These sugar derivatives can identify Cu²⁺ ions more accurately than other metal ions. Among the derivatives, the compound with the naphthol moiety was found to be a good gelator, with a low critical gelation concentration of 1% in a DMF/H₂O mixture (1:1 v/v). Additionally, variable-temperature NMR and absorption studies were used to identify the presence of weak interactive driving forces in the self-assembled state. In the presence of copper­(II) metal ions, the sol–gel transition of these low-molecular-weight gelators based on sugar was examined. It was investigated utilizing colorimetric, absorption, and emission spectroscopic techniques in DMF/H₂O (7:3, v/v) and demonstrates the metal sensing capacity toward different metal ions in its solution state. A significant color shift from orange to pink has been seen upon the addition of a copper­(II) solution. This could be because of the creation of a 1:1 complex as shown by Job’s method, NMR, and DFT studies (Figure a). Sugar-azobenzene compounds based on cholesterol have been designed and synthesized as photoresponsive organogelators (9). After the gel formation in various solvents was examined, toluene was found to have a minimum CGC of 0.5% (w/v). NMR, UV–vis, FT-IR, and PXRD were used to characterize the mechanism of organogel self-assembly. Absorption spectra were used to study its photoisomerization and photoreversible characteristics. The trans and cis isomers’ optimized dimer geometries and structures were investigated computationally. This proposed structure is a remarkably intelligent soft material with believable applications due to its conspicuous nature (Figure b).

5.

(a) Absorbance spectra of aryl azobenzene sugar organogelators sensing Cu²⁺ (8), reproduced from ref . Copyright [2023] American Chemical Society. (b) Photophysical analysis of cholesterol-sugar-azobenzene gels (9), reproduced from ref . Copyright [2025] Elsevier.

The description of a sugar-based photoresponsive supergelator (10) exhibits selective gelation of aromatic solvents. The azobenzene moiety’s partial trans–cis isomerization enables photoinduced chopping of the entangled gel fibers to short fibers, leading to a regulated gel–sol transition and fiber length. It helps remove harmful aromatic solvents from water in a targeted manner (Figure a). Several spectral approaches were used in the design, synthesis, and characterization of the 4,6-O-butylidene/ethylidene/benzylidene β-d-glucopyranose gelator functionalized with photoresponsive azobenzene moieties (11). Lower concentrations are still capable of gelling. An examination of the gels’ morphology reveals helical fibers and one-dimensional aggregated bundles. The primary mechanism for self-assembly is the cooperative contacts between the various groups, specifically the alkyl chain of the protecting group (van der Waals interaction), azobenzene (aromatic π–π interaction), and sugar hydroxyl (hydrogen bonding interaction) (Figure b).

6.

(a) DSC thermogram of amphiphilic azobenzene organogels (10), reproduced from ref . Copyright [2012] American Chemical Society. (b) SEM morphology of sugar-based photoresponsive organogels (11), reproduced from ref . Available under CC-BY-4.0 License. Copyright [2019], The Author(s).

Phase-selective gelation of a low-molecular-weight photoresponsive organogelator with long aliphatic chain azobenzene sugar derivatives (12) and its uses in extracting cationic dyes and recycling aromatic solvents. It is implied to be an excellent gelator by its extremely low CGC in aromatic solvents. Illumination with UV light at 350 nm produced a photoinduced gel-to-sol transition. More than 95% of cationic dyes may be effectively removed in 12 h using this selective adsorbent in both solo aqueous dye solutions and a combination of cationic and anionic dye solutions. Based on these observations, it is possible to use these sugar derivatives to create smart materials for environmental remediation (Figure ).

7.

Structure of long-chain azobenzene sugar gelator showing gel–sol transition (12), reproduced from ref . Copyright [2022] Royal Society of Chemistry.

Azobenzene-based organogels exhibit highly reversible photoisomerization and tunable mechanical transitions, making them ideal for light-controlled actuators and responsive materials. However, compared with coumarin or pyrene systems, their reliance on UV activation limits their biocompatibility and long-term photostability. Future designs employing visible-light-responsive analogues could enhance their performance in biomedical applications.

3. Coumarin-Based Organogelators

The design and synthesis of the coumarin-based derivatives (13) without a traditional gelation motif involved a one-step reaction that involved fine-tuning the molecular architecture, specifically the location of the methyl atom in coumarin, the position of the nitrogen atom in pyridine, and the substitution position of pyridine. 7-substituted coumarin-based derivatives with methyl are very effective gelators that can self-assemble to produce a variety of nanostructures and gelate a range of polar protic solvents, according to a gelation study. Derivatives did not form gels in different solvents, but only gels in the alcohol phase did after small-scale molecular structural alteration. Interestingly, the solvent’s viscosity and dielectric constant had a big impact on these gel fluorescence properties. Moreover, all-atom molecular dynamics simulations and density functional theory calculations were used to computationally examine variations in the self-assembly and fluorescence of gelators. The construction of a low-cost, unconventional supramolecular organogel system with few building blocks, a changeable self-assembly pathway, and adjustable characteristics is made possible by this study (Figure a). In terms of the inter- or intramolecular hydrogen bonding involved, the coumarin derivatives (14) based on glutamic acid exhibit a very different trend against gelation in organic solvents. It has been discovered that the coumarine derivative’s gelation ability may be enhanced by the inclusion of the valinate structure (Figure b).

8.

(a) Fluorescence spectra of coumarin-based organogels with solvent-dependent emission (13), reproduced from ref . Copyright [2020] Elsevier. (b) Morphology of glutamic-acid-coumarin gels showing fibrous networks (14), reproduced from ref . Copyright [2017] Elsevier.

Organogel factors have been developed and synthesized from coumarin-based pyrazoline derivatives (15). In both DMSO and alcohol solutions, organogelators exhibit gelation behavior. In an aqueous solution deionized with THF, it responds to Fe³⁺ with selectivity, while exhibiting great sensitivity. It is noteworthy that the detection line range in the Probe is increased by adding diethylamino at the 7-position of the coumarin unit. Additionally, the probe has good biocompatibility in living cells and can detect Fe³⁺ in the cell nucleus. At the DFT level, the gelators and Fe³⁺ coordination process were examined (Figure a). The amide group of novel coumarin derivatives with varying alkyl chain lengths (16) was synthesized, and their organogelator qualities were assessed. It was discovered that the amide group’s alkyl spacer length did not affect the organogelation capabilities. The majority of gelators produce helical morphologies in either high-polar or nonpolar liquids. NMR, UV absorption, and fluorescence spectra were used to confirm that the gel produced by 4-(70-coumarinoxy)-N-octadecylbutanamide in cyclohexane underwent reversible and stereoselective photodimerization. Although there was no breakdown during the gel’s photoreaction, SEM and AFM analyses revealed the sharp microscopic alterations in gel morphologies brought on by the irradiation (Figure b).

9.

(a) Biocompatibility study of coumarin-pyrazoline gels detecting Fe³⁺ ions (15), reproduced from ref . Copyright [2021] Elsevier. (b) Microscopic view of morphology change in coumarin-amide gels after irradiation (16), reproduced from ref . Copyright [2005] Elsevier.

Several hydrophobic alkyl units were used in the design and synthesis of coumarin-chalcone-derived organogelators (17). Tert-butanol, isopropanol, n-butanol, ethyl acetate, and tert-amyl alcohol all show gelation behavior. The results of the XRD, FT-IR, and SEM studies suggested the existence of π–π stacking, van der Waals interaction, and intermolecular H-bonding in the self-assembly of gels. The adsorption of dyes in water benefited from the xerogel’s increased lamellar network structure and gap caused by the alkyl chain’s growth. After being exposed to UV light, gels exhibit irreversible gelation behavior (Figure a). The [2 + 2] photochemical cycloaddition of the coumarin moiety enables coumarin-containing vinyl homopolymers (18) to exhibit a lower critical solution temperature (LCST) in chloroform, resulting in monofunctional dual-stimuli-responsive behavior. Unlike dual-functional systems, these polymers can be incorporated more uniformly at higher densities because a single functional group responds to both temperature and light. The organogels display clear thermoresponsive transitions, swollen and transparent at low temperature, becoming turbid upon phase separation during heating and shrinking back to a transparent state, demonstrating fully reversible thermoresponsive behavior over multiple heating–cooling cycles. Furthermore, photodimerization of the coumarin units enables spatially controlled photopatterning, creating distinct thermoresponsive domains within the organogel. Overall, coumarin-based methacrylate homopolymer organogels exhibit reversible thermo- and photoresponsivities, functioning as robust monofunctional dual-stimuli-responsive materials (Figure b).

10.

(a) Emission spectra of coumarin-chalcone organogels (17), reproduced from ref . Copyright [2021] Elsevier. (b) Photoresponsive behavior of coumarin-methacrylate gels (18), reproduced from ref . Available under CC-BY-4.0 License. Copyright [2021], The Author(s).

The self-assembly of a small-molecule organogelator can be controlled by the 3-styryl coumarin molecular rotors, which can probe delicate intermolecular interactions with (19). Comprehensive spectroscopic studies on these sensors were conducted to demonstrate their great chemical and spatial affinity toward the 3D supramolecular network in addition to the characterization of the gel using circular dichroism and atomic force microscopy. Molecular dynamics simulations confirmed the findings and provided more important information on the dynamic self-assembly mechanism of the gelator. These sensors may be used as models to investigate various soft-supramolecular structures and their assembly processes (Figure a). A new acylhydrazone Schiff base fluorescent organogel (20) generated from coumarins was created and produced. In isopropanol, tert-amyl alcohol, n-butanol, and phenylamine, it can create stable organogels. Heating it would turn it into a solution, and cooling would return the solution to its gel condition. XRD, FT-IR, and SEM techniques were used to study the self-assembly mechanism. The findings showed that the forces responsible for the gelator’s self-assembly to create the organogel are π–π stacking, van der Waals interaction, and intermolecular hydrogen bonding. Fluorescence spectra and UV–visible spectroscopy were used to examine the compound’s optical characteristics. Subsequent research revealed that the gelator could react only to Fe³⁺ among the tested cations in a selective and sensitive manner. In addition to the aforementioned uses, the organic gel factor may react to heating, shaking, and radiation, giving the organogel multistimulus-responsive qualities (Figure b).

11.

(a) Photophysical study of 3-styryl coumarin organogels (19), reproduced from ref . Copyright [2019] American Chemical Society. (b) UV–vis spectra of coumarin-acylhydrazone Schiff base gels (20), reproduced from ref . Copyright [2020], Elsevier.

The synthesis and characterization of a novel class of 1,2,3-triazoles functionalized with coumarin (21). All iodotriazoles are effective gelators, while hydrogenous-triazoles are ineffective gelling agents. The ability of the molecule to gel depends on the presence of an iodine atom in position 5 of the triazole ring. The organogels can perform reversible photodimerization and exhibit mercury ion responsiveness. The gel undergoes a significant change in morphology upon exposure to 365 nm light, and it can partially recover after being exposed to 254 nm light again. The gelator’s gelation behavior in pure and mixed solvents is depicted by two Teas plots (Figure ).

12.

Fluorescence study of triazole-coumarin organogels responsive to Hg²⁺ (21), reproduced from ref . Copyright [2015] Elsevier.

Coumarin-based organogels combine fluorescence functionality with reversible [2 + 2] photodimerization, offering both sensing and dual-stimuli responsiveness. Unlike azobenzene systems, they operate under milder irradiation and exhibit superior photochemical stability. Nevertheless, their relatively slower response kinetics and limited structural reversibility restrict their use in rapid actuation compared with azobenzene- or pyrene-based gels.

4. Dendrimer-Based Organogelators

The design and synthesis of amphiphilic dendritic peptides on dendritic polyaspartic acid (22) was completed. The amphiphilic dendritic peptide organogel and liquid crystal characteristics were thoroughly investigated using X-ray diffraction studies, FESEM, FT-IR, differential scanning calorimetry, and polarization optical microscopy. With a minimum gelation concentration as low as 1 wt %, it exhibits good organogel characteristics. Additionally, it can construct a hexagonal columnar liquid crystal assembly across a broad temperature range (Figure a). A novel class of dendritic gelators based on peripherally methyl-functionalized poly­(benzyl ether) dendrimers (23) shows that supramolecular organogels can be induced by weaker C–H/π interactions. The type of peripheral methyl groups has a significant impact on their gelation property. NMR spectroscopy and single-crystal X-ray analysis demonstrated that the self-assembled gels are mostly formed via cooperative multiple C–H/π interactions. This study offers fresh perspectives on the design of self-assembled nanomaterials and highlights the significance of weak C–H/π interactions in the self-assembly process (Figure b).

13.

(a) FT-IR studies of amphiphilic dendritic peptide-based organogel (22), reproduced from ref . Available under CC-BY-4.0 License. Copyright [2011], The Author(s). (b) Rheological profile of poly­(benzyl ether) dendrimer gels (23), reproduced from ref . Copyright [2016], American Chemical Society.

Aromatic groups are added to the end peripherals of organic/inorganic hybrid supramolecules based on polyhedral oligomeric silsesquioxane (POSS) cores to create the novel dendrimer gelator POSS-Z-Asp­(OBzl) (24, POSS-ASP), which has eight arms of aspartate derivatives to fully utilize the powerful π–π stacking forces and produce strong supramolecular gels in addition to multiple hydrogen bonds and van der Waals interactions. In order to give hybrid physical gels particularly robust mechanical properties and quick recovery behaviors, they can self-assemble into 3D nanoscale gel networks. The peripherals of dendrimers and the substantial impact of solvent polarity on the supramolecular self-assembly mechanism are demonstrated by two different morphologies of banded ultralong fibers and connected spherical particle structures, which are obtained using the solvent polarity as verified by SEM, POM, and TEM. The different self-assembly models now in use agree with these results. The two distinct gel formation mechanisms that give POSS-ASP its two distinct thermal and mechanical properties can be confirmed by the results of investigations into the network architecture, rheological characteristics, and thermal stability using tube-inversion, rotational rheometers, and powder X-ray diffraction. The association between dendritic supramolecular gelation and various polarity solvents during the supramolecular self-assembly process of gelators can be investigated with the aid of a novel gelation system reported in organic or organic/aqueous mixed solvents (Figure a). Oligocarbazole (25) was the basis for the first rigid dendritic gel with significant emission that was published. It is interesting to note that the second generation of dendrons demonstrated the ability to gel under ultrasonic conditions and that 1,6-diaminohexane may help build a two-component gel from the first generation of dendrons (Figure b).

14.

(a) Rheological study of POSS-core dendrimer gels showing strong mechanical behavior (24), reproduced from ref . Copyright [2017] American Chemical Society. (b) SEM image of dendritic oligocarbazole-based gel networks. (25), reproduced from ref . Copyright [2010] Royal Society of Chemistry.

A novel dendritic gelator was created using carbazole as the building block (26, HBCD). The gel formation was shown to be primarily influenced by the π–π interaction between the aromatic rings and the H-bonding between the amide groups. However, a significant aggregation-induced emission amplification was seen in the gel state due to the restricted molecular mobility and formation of J-aggregates. Significantly, the addition of F^– can break the gel state of HBCD and boost fluorescence due to the creation of N–H···F–, which may further increase HBCD’s coplanarity. The steric barrier of the dendrimer would conflict with the interactions between the bigger anions and HBCD, which could account for the great selectivity of HBCD’s sensory capabilities toward F^– rather than the Cl^–, Br^–, I^–, and AcO^– anions (Figure a). Luminescent AB3-type poly­(aryl ether) dendron derivatives (27) were synthesized and gelated without the use of traditional multiinteractive gelation motifs. The aryl units in the system are far more likely to interact π–π with one another when the gelation process is managed in a partly polar solvent milieu. The self-assembly traps solvent molecules in the fibrillar arrangement of poly­(aryl ether) units, resulting in an unparalleled gelation. The technique was also applied to create an excimer-based photoluminescent gel by adding anthracene units to the dendrons. Bright green light is emitted from the gel upon UV excitation due to the closeness of the anthracene units in the gel, which causes the generation of anthracene excimers at room temperature. According to the study, the production and functional groups found in the dendron can regulate the size and packing of the self-assembled fiber. Additionally, since the self-assembly is mostly governed by π–π interactions, the technique envisions a simple method for producing fluorescent Low-Molecular-mass Organic Gelator (LMOG) by integrating polycyclic aromatic hydrocarbon units into the poly­(aryl ether) dendrons (Figure b).

15.

(a) Fluorescence enhancement in carbazole dendritic gels upon F^– binding (26), reproduced from ref . Copyright [2011] Royal Society of Chemistry. (b) UV study of poly­(aryl ether) dendron gels showing excimer emission (27), reproduced from ref . Copyright [2011] Royal Society of Chemistry.

Mesogenic and carbazole moieties have been combined to create novel functional gelators with a dendritic structure based on flexible bis-MPA dendrons (28). In a number of low-polarity organic solvents, the precursory dendrons and the resultant codendrimers both display extensive supramolecular chemistry, displaying both liquid-crystalline phases and gelling characteristics. Weak noncovalent interactions govern the creation of supramolecular gels, according to research on self-assembly processes. These gels exhibit intriguing electrical characteristics because of the carbazole moiety, including an enhanced fluorescence emission brought on by an aggregation-induced emission enhancement (AIEE) effect and electrochemical behavior. Thin layers of xerogels placed on ITO electrodes have been shown to have their gel structure fixed by oxidative polymerization (Figure a). The inner layer of a novel poly­(benzyl ether) dendritic organogelator (29) with azobenzene was created, synthesized, and thoroughly characterized. The dendron was determined to be a very effective and adaptable organogelator toward a range of polar and apolar organic solvents with CGCs close to 0.05 wt %, suggesting that it is a supergelator. Subsequent research showed that the self-assembly processes and gel formation may be guided by the intermolecular multiple π–π stacking interactions. Most intriguingly, when exposed to various environmental stimuli, such as temperature, sonication, light, and shear stress, these dendritic organogels displayed a variety of stimuli-responsive behaviors (Figure b).

16.

(a) Fluorescence spectra of bis-MPA dendrimer organogels showing AIE behavior (28), Reproduced from ref . Copyright [2016] Royal Society of Chemistry. (b) Structural representation of azobenzene-poly­(aryl ether) dendron-based gels (29), Reproduced from ref . Copyright [2012] American Chemical Society.

These dendritic organogels displayed a variety of stimuli-responsive behaviors, including reversible gel–sol transitions upon exposure to light, thermoreversible gelation with temperature changes, and structural rearrangements induced by mechanical shear or sonication. The azobenzene units within the dendrons enable light-driven reversible isomerization, which directly modulates molecular packing and network stability. Likewise, temperature variations alter noncovalent interactions such as π–π stacking and hydrogen bonding, leading to thermoresponsive behavior. External mechanical forces such as sonication or shear stress disrupt the supramolecular network, but the gels often exhibit self-healing and recovery once the stimulus is removed. Collectively, these multistimuli-responsive characteristics highlight the adaptability and robustness of dendritic organogels, making them excellent candidates for applications in smart materials and biomedical systems.

Two distinct dendrons (30) of poly­(benzyl ether) were synthesized and thoroughly characterized. Scanning electron microscopy (SEM), X-ray crystal structure analysis, and concentration- and temperature-dependent NMR spectroscopy were used to investigate their gelation properties. The second-generation dendron was found to be a highly effective organogelator in spite of the absence of any typical gelation motifs such as amides, long alkyl side chains, and steroidal groups. It was discovered that the gelation capability of these dendrons was significantly dependent on dendron generation. The primary forces for the formation of the self-assembled gel were found to be the numerous strong π–π stacking contacts and hydrogen bonding interactions caused by the peripheral isophthalonitrile motifs (Figure a). AB3 and AB2 type poly­(aryl ether) dendrons (31) with an anthracene chromophore connected by an acylhydrazone linkage are the basis for the design, synthesis, and photophysical characteristics of an LMWG. A reversible gel–sol transition, which is linked to a color shift from deep yellow to bright red, is used to effectively detect fluoride ions with the ″naked eye″ (Figure b).

17.

(a) Temperature-dependent gelation study of poly­(benzyl ether) dendron organogels (30), reproduced from ref . Available under CC-BY-4.0 License. Copyright [2012], The Author(s). (b) Fluoride-ion sensing via color change in anthracene-poly­(aryl ether) dendron gels (31), reproduced from ref . Copyright [2011], American Chemical Society.

Dendrimer-based organogels offer structural precision, multivalency, and tunable responsiveness, bridging molecular design with macroscopic functionality. They exhibit superior mechanical stability and multistimuli adaptability compared with small-molecule gelators such as azobenzene or coumarin derivatives. However, their complex synthesis and potential cytotoxicity limit scalability, warranting future work toward biodegradable or hybrid dendritic systems.

5. Perylene-Based Organogelators

Perylene bisimide (PBI) chromophores (32) have been utilized to prepare and characterize the molecular gelators. When ultrasound is applied to DMSO, the stable gelator self-assembles into entangled supramolecular fibers with extra noncovalent hydrogen bonding connections between the acid and imidazole units to create fluorescent enhancement gels. Additionally, the effects of structure change and gelation techniques on the gel characteristics are compared and examined through a number of studies (Figure a). In a wide range of solvents, including electron-rich aromatic solvents such as thiophene, the PBI organogelator (33) demonstrates strong gelation activities that result in interpenetrating networks of PBI stacks. The wide absorbance range between 400 and 600 nm and the high n-type charge carrier mobility. They are tempted to state that this gelator, when combined with p-type organic semiconductors like polythiophene or pentacene, could produce effective organic solar cells (Figure b).

18.

(a) Fluorescence emission of perylene bisimide (PBI) gel under sonication-induced self-assembly (32), reproduced from ref . Copyright [2020] Wiley-VCH. (b) Semiconductor property study of PBI organogels in aromatic media (33), reproduced from ref . Copyright [2006] Royal Society of Chemistry.

The supramolecular polymerization mechanism of an archetype organogelator molecule consisting of two amide substituents and an aromatic scaffold of perylene bisimide (34) has been elucidated. Through a cooperative nucleation–growth mechanism, this molecule self-assembles into long one-dimensional nanofibers. Applications of thermodynamic and kinetic analysis have been used to identify circumstances in which the trapping of monomers in an inactive conformation might delay spontaneous nucleation, resulting in lag durations of up to an hour. Through a cycle of assembly and disassembly procedures, the distinct kinetics in the nucleation process were verified as thermal hysteresis. Adding preassembled nanofiber seeds causes seeded polymerization from the seed termini in a live supramolecular polymerization process when the right circumstances are met within the hysteresis loop. These findings indicate that seeded polymerizations can be applied to a wide range of known and undiscovered molecules from the vast family of molecules that self-assemble into one-dimensional nanofibrous structures (Figure a). They are not restricted to specific scenarios in which off-pathway aggregates sequester the monomeric reactant species. It is generally known that perylene-based organogels can be used as optoelectronic materials and sensors. Core-substituted perylene diimide-based organogels are not frequently investigated among them. It explains how a recently synthesized amide-linked core-substituted perylene diimide derivative (35), which generated organogels in organic solvents such as toluene and methylcyclohexane (MCH), underwent a hierarchical self-assembly process. Nanofibers and nanotubes are examples of one-dimensional molecular aggregates that comprise these organogels. Organogels made of nanofibers are widely used. Also covered are the rheological characteristics, molecular interactions, and molecular packing of this organogel (Figure b).

19.

(a) Time-dependent polymerization of amide-substituted PBI organogelators (34), reproduced from ref . Copyright [2015] American Chemical Society. (b) Photophysical analysis of nanotube-like PBI-based organogels (35), reproduced from ref . Copyright [2022] American Chemical Society.

Ditopic melamines with perylene bisimide (PBI) chromophores (36) were created, and their ability to self-aggregate as well as coaggregate with barbital (Bar) or dodecyl cyanurate (dCA) was examined. The X-ray diffraction of xerogels showed characteristic patterns that can be attributed to lamellar structures, indicating the development of hydrogen-bonded motifs that resembled tape. All of the organogels had remarkably low critical gelation concentrations (CGC) of 8.0 × 10^–4 M, which allowed them to be categorized as supergelators. Higher melting temperatures (Tm) were found in the gels with higher thermal stabilities. Atomic force microscopy and scanning electron microscopy demonstrated that the distinct thermal stabilities of gels made of fibrous nanostructures and those with sheetlike microstructures are consistent. The possibility of prolonged stacks of PBI dyes along the hydrogen-bonded strands may be connected to the various self-assembled architectures of PBI aggregates (Figure a). Two novel compounds of perylenetetracarboxylic diimide (PDI) (37) are synthesized. Because these two trimers have different tail groups, their solubility differs greatly. The aggregation behavior of these two substances in solution was investigated by using absorption and fluorescence spectra. The findings show that in either a good solvent or a poor solvent, massive molecular aggregates are formed through an intermolecular mechanism. It can only gel in nonpolar solvents like hexane and toluene; however, it can gel in a mixture of methanol and tetrahydrofuran (THF). This is due to the presence of several long alkyl chains in the gel, which have added additional hydrophobic interactions in addition to the π–π interactions. Atomic force microscopy (AFM) analysis of the dried gel morphology shows lengthy fibers. However, the dried gel has a network-like structure because of the hydrophobic interactions between the different alkyl chains (Figure b).

20.

(a) AFM image of melamine–cyanurate/barbiturate PBI supramolecular organogels (36), reproduced from ref . Copyright [2012] Springer-Nature. (b) Fluorescence spectra of benzenetricarboxylate-PDI organogels (37), reproduced from ref . Copyright [2015] Springer-Nature.

Two novel hexamer derivatives of perylenetetracarboxylic diimide (PDI) (38) are synthesized. The aggregation behavior in solution was examined using fluorescence and absorption spectra. As a result, even in a highly diluted solution, the hexamer attempts to form huge molecular aggregates by an intermolecular mechanism. However, it favors adopting a folded shape in which the two PDI units stack face-to-face by an intramolecular mechanism. The hexamer molecular aggregates exhibited hydrogen bonding between the amide nitrogen and carbonyl oxygen, as demonstrated by infrared spectra. The dried gel exhibits bundles of long fibers with extremely high aspect ratios in its shape. However, all that is visible are tiny, rod-like aggregates with a low aspect ratio. The difference is most likely the result of competing molecular aggregation-inducing pressures (Figure a). A novel perylene bisimide dye (39) has been produced that exhibits anisotropic, highly viscous lyotropic mesophases in apolar solvents like n-hexane or methylcyclohexane, while forming highly fluorescent J-aggregates in a variety of organic solvents and gelating in polar solvents like acetone and dioxane (Figure b).

21.

(a) Emission study of PDI-hexamer supramolecular gels showing fiber bundles (38), reproduced from ref . Copyright [2011] Royal Society of Chemistry. (b) Polarizing optical microscopy of J-aggregate-forming PBI gels (39), reproduced from ref . Copyright [2008] Wiley-VCH.

A diarylethene photochromic compound in an organogel medium was used to efficiently modulate the fluorescence emission of PDI-APTES (40) between the ON and OFF states. They were made by mixing a steroidal low-molecular-weight organogelator with a perylene fluorophore. The compatibility of the photochromic compound with this medium and its nondestructive readout capability were demonstrated. A novel technique was utilized to cultivate fluorescent silica spherical nanoparticles using chemically and physically admixed perylene dye using the dye-doped organogel as a template (Figure ).

22.

LSCM fluorescence images of PDI-APTES organogel showing ON/OFF emission switching (40), reproduced from ref . Copyright [2012], Royal Society of Chemistry.

Perylene-based organogels possess exceptional photostability, strong visible-range absorption, and ordered nanofiber formation, enabling their use in optoelectronics and light-harvesting. While their luminescence and charge mobility outperform most organic gelators, their poor solubility and limited biocompatibility make them less suitable for biomedical systems compared to coumarin or dendrimer analogues.

6. Pyrene-Based Organogelators

A stable gel was formed in DMSO via a simple pyrene-substituted acylhydrazone derivative (41). The critical gelation concentration (CGC) of this compound in DMSO was determined to be 1.2 wt %, indicating strong gelation capability at low concentration due to efficient π–π stacking and hydrogen-bonding interactions. Through the use of π–π interactions, intermolecular hydrogen bonds, and the van der Waals interaction, it is shown that molecules self-assemble to form a gel made of fibers. The fibers exhibit a layered pattern. It exhibits gelation-induced increased fluorescence emission, which is explained by the intermolecular hydrogen bonds’ synergistic impact, which lowers bond rotation inside the molecules and prevents the development of J-aggregates and nonradiative transitions. Since the −C = N-bond undergoes trans–cis isomerizations when exposed to visible or ultraviolet light, it displays photoresponsive characteristics. It exhibits a gel–sol transition when exposed to visible light, but it is unable to transform into a solution when exposed to UV light, and the fluorescence intensity clearly drops. Visible light is more responsive to it than UV light (Figure a). Pyrene-phenyl acrylonitriles (42) with mono-, di-, trichiral, and racemic substitutions were synthesized, and their organogel formation was examined. The molecules exhibit amazing emission wavelength and intensity fluctuations that are linked to noticeable color changes when they self-assemble in water. The formation of homogeneous nanoaggregates for monosubstituted derivatives and network-like structures for di- and trisubstituted derivatives is demonstrated by cryo-scanning electron microscopy (Cryo-SEM) pictures. The cyano group steric restrictions, the coplanar pyrene rings advantageous π–π stacking ability, and the citronellol moiety’s advantageous noncovalent interactions enable the molecules to form excellent organogels with fibrous and twisted ribbon morphology for the racemic and chiral derivatives. The small-molecule-based organogels may be highly relevant for future studies involving soft biological matter (Figure b).

23.

(a) Photophysical study of pyrene-acylhydrazone gels exhibiting visible-light gel–sol transition (41), reproduced from ref . Copyright [2017] Wiley-VCH. (b) Cryo-rheology and morphology of chiral pyrene-cyanostyrene organogels (42), reproduced from ref . Copyright [2020] American Chemical Society.

The hydrophobic cetyl chain with a pyrene fluorophore (43) on the amphiphilic probe was created using a click reaction. The probes’ ability to recognize fluorescence toward Hg²⁺ in DMF solution, aqueous solution, and gel state was examined. The fluorescence exhibits a turn-off reaction when the soluble probe binds with Hg²⁺ in 1:1 stoichiometry and emits a pyrene-based monomer in the DMF solution. In an aqueous solution, the probe may self-assemble nonaggregated while exhibiting monomer and excimer emissions based on pyrene. The monomer emission is mostly eliminated when Hg²⁺ binds to the generated nonaggregated state, but the excimer emission is still slightly changed. The probe’s sensitivity to Hg²⁺ in aqueous solution is higher than in DMF solution; it was also discovered. Furthermore, the probe may produce luminous organic gels in specific organic solvents; these gels need to be collapsed by the addition of Hg²⁺. In terms of biological applications, the probe was effectively employed for intracellular Hg²⁺ fluorescent imaging (Figure a). The solution phase absorption and fluorescence spectra of chiral alkylamide-substituted pyrene derivatives (44) were in agreement with the synthesis of N–H···O hydrogen-bound helical π-stacked one-dimensional supramolecules. The concentration and solvent polarity controlled the π-stacked structures and aggregation number. The development of hydrogen-bonded helical 1D supramolecular assemblages in the ground and excited states was demonstrated by circular dichroism and CPL spectra. The 1D supramolecular assemblies in CHCl₃ were reversed from those in MCH and THF. The solvent content and polarity were influenced by the helical shape and π-stacked structure of the N–H···O hydrogen-bonded 1D supramolecular assemblies (Figure b).

24.

(a) Fluorescence sensing of Hg²⁺ using amphiphilic pyrene-based gels (43), reproduced from ref . Copyright [2018] Elsevier. (b) CPL spectra showing helical supramolecular assembly of alkylamide-pyrene gels (44), reproduced from ref . Copyright [2018] American Chemical Society.

Different spectroscopic approaches have been used to produce and evaluate two pyrene derivatives (45), paying particular emphasis to the effects of solvents and temperature versus concentration. It was made and examined in order to learn more about its surprising spectroscopic characteristics, which include only one pyrene unit. The associated measurements demonstrate that at high temperatures, this gelator does not exhibit excimer emission. This proves that the most pertinent mechanism to account for the increased excimer emission seen upon heating of the probe is intramolecular excimer production. This result opens up new possibilities for the creation and understanding of alternative organogels by demonstrating the potential of using a fluorophore (pyrene) as an internal probe to fine-tune the supramolecular organization within a gelator solution as a function of concentration and solvent nature (Figure a). Tris­(phenylisoxazolyl)­benzene (46) was used to develop a new light-harvesting organogel. An energy acceptor was created by synthesizing a pyrene derivative with four phenylisoxazole substituents. While photoinduced energy transfer was not seen in the solution, the probe’s electrically excited-state energy was transmitted to a tiny area in the gel state. Light-harvesting behavior was displayed by the coassembled structures that developed in the decalin gel. According to research on the fluorescence quantum yield, one probe molecule can absorb the excited-state energy from roughly eight molecules. Energy transfer in the gel state is dominated by fluorescence resonance energy transfer, according to time-resolved fluorescence decay measurements (Figure b).

25.

(a) Absorbance study of bis-urea pyrene organogels at varying temperature and solvent polarity (45), reproduced from ref . Copyright [2016] Wiley-VCH. (b) UV spectra of pyrene-phenylisoxazole organogel demonstrating light-harvesting behavior (46), reproduced from ref . Copyright [2017] Taylor & Francis.

Blue-emitting transparent fluorescent organogels based on pyrene-conjugated oligopeptides (47) have been found. It has been investigated utilizing a variety of spectroscopic and microscopic methods, as well as rheological investigations. Gelator peptide excimer production in the aggregated gel condition is demonstrated by concentration-dependent fluorescence tests. This brilliant organogel in the ODCB can incorporate unfunctionalized and nonoxidized graphene nanosheets into the organogel system to produce a stable hybrid organogel system through noncovalent π–π stacking interactions. Both types of morphologies are present according to TEM and AFM investigations. The fundamental features of the supramolecular gel, specifically its thermoreversibility, shear-thinning capabilities, and three-dimensional cross-linked nanofibrillar network topologies, were retained in the hybrid organogel. The storage modulus (G’) of the graphene-containing hybrid organogel is seven times stiffer than that of the native organogel, according to rheological investigations. This implies that an organogel becomes a more elastic, solid-like soft substance when graphene is added. The creation of hybrid fluorescent organogel systems incorporating graphene may present intriguing opportunities for several nanotechnological uses of these graphene-based nanohybrid materials (Figure a). Two pyrene-based organogelators (48) have been created in which the pyrene moiety is connected to the diphenylalanine dipeptide. The organogel shape and optical characteristics can be adjusted. In acetonitrile, spherical aggregates with quenched emission were produced, while in o-dichlorobenzene, an entangled fibrillar network with increased emission was generated. Both π–π stacking and hydrogen bonding are implicated in the development of supramolecular networks, according to research using nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and circular dichroism. I-aggregate production in acetonitrile was shown by steady-state emission investigations and ultraviolet–visible analysis. On the other hand, J-type aggregate production results in assemblies with increased emissions. These findings provide some understanding of the crucial role that the gelling solvent plays in the supramolecular gel shape and could aid in the development of novel soft materials (Figure b).

26.

(a) Morphological analysis of pyrene-peptide hybrid gels with graphene incorporation (47), reproduced from ref . Copyright [2011] Wiley-VCH. (b) Emission spectra of pyrene-dipeptide gels showing solvent-dependent aggregation type (48), reproduced from ref . Copyright [2015] Wiley-VCH.

The coumarin-associated pyrene derivatives with different hydrophobic units (49) have been synthesized and thoroughly examined through the use of mass spectrometry and NMR. In relation to their molecular structure and solvent affinity, these compounds’ gelation behavior and self-aggregation characteristics were examined. Out of all these derivatives, the one without a hydrophobic tail gels well in higher alcohols such as decanol and dodecanol. However, the other compounds with saturated and unsaturated hydrophobic tails form a weak gel in different solvents. Both optical microscopy and high-resolution transmission electron microscopy (HRTEM) were used to examine the gel morphology. The examination of these compounds’ absorption and emission spectra showed that the self-assembly process in various solvents had a major impact on their photophysical characteristics. The NMR and concentration-dependent emission studies indicate that the hydrogen bonding and π–π stacking interaction between the carbonyl groups of coumarin-linked pyrene and the solvent’s −OH group were the main drivers of the gelation and self-aggregation processes. The rheological analysis illustrates the reversible nature and flow behavior of the organogel under experimental circumstances of temperature and strain ramp-up and ramp-down. They created nanoflakes from coumarin-linked pyrene derivatives and looked into their potential use in cell imaging using clues from the self-assembly process in different solvents. HRTEM and a Zetasizer were used to determine the size of the self-aggregated particles in the DMSO-water combination. Applications for imaging PC3 prostate cancer cells and fibroblasts were conducted using nanomaterials that were produced by the self-assembly technique. Subsequent analysis shows that these substances inhibit PC3 cell proliferation (Figure a). The C₃-symmetric pyrene-containing gelator (50)-based two-component organogels have been thoroughly examined using a variety of methods. It was discovered that the type of electron-poor dopant had a substantial impact on the gelation phenomena based on the formation of charge-transfer complexes. This parameter has a significant effect on the microstructures formed in the xerogel state, their spectroscopic properties, the gel-to-sol transition temperatures, the associated gelation domains, and the critical gelation concentrations of acceptor dopants. Temperature variations and titrations Donor–acceptor interactions are vital, as demonstrated by UV–visible absorption spectroscopy in particular. There was also a noteworthy association between the phase transition temperatures and the disappearance of distinctive charge-transfer bands. Time-dependent density functional theory (TD-DFT) calculations were used to verify the attribution of these electronic transitions. Finally, it was demonstrated that these materials’ luminous qualities could be adjusted by temperature, either in emission wavelength or brightness (Figure b).

27.

(a) Mechanistic illustration of coumarin-linked pyrene organogels for cell-imaging studies (49), reproduced from ref . Copyright [2015] Royal Society of Chemistry. (b) Temperature-dependent emission tuning in C₃-symmetric pyrene-based charge-transfer gels (50), reproduced from ref . Copyright [2021] Wiley-VCH.

Pyrene-based organogels stand out for their strong π–π stacking, excimer emission, and high fluorescence sensitivity, making them promising for optical sensing and imaging. Compared with coumarin and perylene systems, they offer greater tunability in optical output but are prone to aggregation-caused quenching and limited aqueous solubility. Their photophysical versatility, however, provides unique advantages for optoelectronic and bioimaging platforms.

A summary of the Organogelator Type, principles, Detection Mechanism, and Applications is found in Table below.

1. Structured Overview of the Organogelator Type, Including Their Principle, Typical Light Intensity/Wavelength Thresholds, Representative Biocompatibility Findings (e.g., Cytotoxicity Results), Detection Mechanism, Applications, and Supporting References is Provided in This Table.

s. no	organogelator type	principle of gelation	detection mechanism	applications	typical light intensity/wavelength	biocompatibility metrics	refs
1	azobenzene	photoisomerization (trans–cis switching)	UV/vis light-induced isomerization	light-controlled drug delivery, actuators, sensors	UV: 320–365 nm, Vis: 420–480 nm; intensity ∼ 5–10 mW/cm2	cytotoxicity reduced by visible-light-responsive derivatives; limited stability in aqueous media	Zhou et al.; Gu et al.
2	coumarin	π–π stacking, H-bonding, photodimerization	UV-induced [2 + 2] cycloaddition	bioimaging, fluorescence sensors, light-triggered release	UV: 350–365 nm; intensity ∼ 2–5 mW/cm2	good biocompatibility in vitro (low cytotoxicity in HeLa cells, Fe3+ detection studies)	Yang et al.; Okada and Sato
3	pyrene	π–π stacking and excimer formation	excimer–monomer emission shifts	sensors, optoelectronics, cell imaging	UV: 340–370 nm; intensity ∼ 3–8 mW/cm2	low cytotoxicity in cell-imaging assays; stable hybrid gels with graphene	Xu et al.; Jenifer et al.
4	perylene	π–π stacking with large aromatic surfaces	fluorescence/quenching changes	organic electronics, light harvesting, sensing	visible excitation: 450–550 nm; intensity ∼ 1–5 mW/cm2	limited aqueous biocompatibility; cytotoxicity reduced by PEGylation strategies	Gao et al.; Huang et al.
5	dendrimer	multivalent interactions, entanglement, hydrophobic effects	stimuli-responsive gel–sol transition	drug delivery, nanocarriers, sensors	depends on chromophore (UV: 350–400 nm; visible: 450–500 nm)	biocompatibility varies by dendron generation; some reports of cytotoxicity at higher concentrations	Peng et al.; Xu et al.

7. Challenges and Prospects

Future progress in photoresponsive organogels will depend heavily on molecular-level engineering to overcome the current limitations. For instance, the photostability issues associated with azobenzene derivatives can be addressed by introducing visible-light-responsive azo analogues (such as ortho-fluoro or amino-substituted azobenzenes), embedding chromophores within rigid π-conjugated frameworks to minimize photofatigue, or using hybrid cogelator systems that dissipate photoenergy through nonradiative relaxation without molecular degradation. For biomedical applications, material specifications must be tailored to function: organogels for 3D cell culture require high water permeability, moderate stiffness (elastic modulus of 1–10 kPa), and nontoxic degradation products to support cell adhesion and proliferation, while drug delivery gels demand sustained release kinetics, light-triggered permeability modulation, and sufficient mechanical integrity for injection or implantation. Additionally, integrating biopolymer–organogel hybrid scaffolds and nanostructured fillers could further improve both the biocompatibility and responsiveness. These strategies highlight how rational molecular design and cross-disciplinary integration can direct the next generation of organogels toward clinically relevant, photostable, and multifunctional systems.

8. Conclusion

Photoresponsive organogels represent a rapidly evolving field that holds significant promise for a wide range of biomaterial applications. Their ability to undergo reversible structural and functional changes in response to light stimuli makes them highly versatile candidates for applications in drug delivery, tissue engineering, and biosensing. While substantial progress has been made in the development of these materials, challenges such as optimizing light responsiveness, ensuring long-term stability, and improving biocompatibility remain. Overcoming these challenges will require continued interdisciplinary research that combines materials science, bioengineering, and photochemistry. Future innovations in photoresponsive organogels may focus on enhancing their sustainability, biodegradability, and the development of more sophisticated, multifunctional systems capable of responding to complex environmental cues. By addressing these challenges, photoresponsive organogels are poised to contribute to the advancement of next-generation biomaterials with broad clinical and environmental applications, paving the way for more personalized, dynamic, and responsive therapeutic solutions. Moving forward, advancements in molecular engineering such as designing visible-light-responsive azobenzenes, stabilizing chromophores through π-conjugated frameworks, and developing hybrid biopolymer–organogel systems will be crucial to achieving durable, biocompatible, and multifunctional gels. Tailoring these materials for specific biomedical contexts, including 3D cell scaffolds or controlled drug release, will transform organogels into next-generation platforms for adaptive and photostable biomaterials. To ensure this review reflects the most up-to-date perspectives, we have incorporated recent advances from 2023 to 2025, highlighting the emergence of visible-light-responsive organogels, multistimuli hybrid systems, and biocompatible biodegradable gelators designed for clinical translation. These recent findings emphasize the ongoing shift toward sustainable materials, integration with nanotechnology, and applications in personalized medicine and wearable bioelectronics. By capturing these trends, this Review provides a timely and comprehensive outlook on the future of photoresponsive organogels.

The authors declare no competing financial interest.