Advances in Π‐Conjugated Benzothiazole and Benzoxazole‐Boron Complexes: Exploring Optical and Biomaterial Applications

Abstract

This mini‐review highlights the transformative potential of benzothiazole (BTz)‐ and benzoxazole (BOz)‐based boron‐complexed dyes. It represents an innovative evolution of the classic boron‐dipyrromethene (BODIPY) structure, which is well established for its superior photophysical properties. Incorporating BTz‐ or BOz‐ligands into the borane (‐BR₂) component, originates more electron‐deficient architecture, enabling novel modes of complexation and addressing limitations such as spectral overlap and self‐quenching in traditional BODIPY dyes. The review focuses on the remarkable versatility of boron‐benzothiazole (BOBTz)‐ and boron‐benzoxazole (BOBOz)‐based complexes, particularly in three rapidly advancing fields: organic light emitting diode (LED) technology, bioimaging, and mechanochromic luminescence (MCL). Over the past 15 years, these complexes have demonstrated exceptional adaptability, showcasing enhanced properties like high fluorescence quantum yields, large molar extinction coefficients, and tunable emissions across visible and near‐infrared spectra. The insights described in this review highlight the major role of BOBTz‐ and BOBOz‐complexes in shaping innovative, and sustainable advanced materials while addressing emerging challenges in modern materials science. Besides, the refining of both BOBTz‐ and BOBOz‐complexes offers exciting prospects for technological challenges such as energy‐efficient lighting, non‐invasive imaging, and creating stimuli‐responsive materials for next‐generation sensors. Moreover, the environmental sustainability of these materials, including green synthesis approaches and recyclable components represents an important frontier for future exploration.

This mini‐review explores the advancement of benzothiazole (BTz)‐ and benzoxazole (BOz)‐based boron‐complexed dyes, extending the capabilities of BODIPY structures through the incorporation of electron‐deficient ligands. These materials drive innovations in light emitting diodes (LEDs), bioimaging, and mechanochromic luminescence (MCL), addressing challenges in energy‐efficient lighting, non‐invasive imaging, and stimuli‐responsive sensors. Their tunability and enhanced performance position them as promising components in emerging optoelectronic technologies.

1. Introduction

In materials science and engineering, organic complexes incorporating boron have received wide interest for a spectrum of applications spanning from photovoltaics to medicinal chemistry.^{[ 1 ]} The study and development of boron‐based complexes have significantly contributed to the understanding of boron chemistry, elucidating its fundamental role in organic reactions since the 1960s.^{[ 2 ]} Notably, at the onset of the twenty‐first century, organic compounds based on boron‐dipyrromethenes (BODIPYs) have emerged as an important and versatile class of fluorescent dyes.^{[ 3 ]} BODIPY‐based luminophores have found effective applications in a range of fields including fluorescent sensing, bioimaging, organic photovoltaics (OPVs), and organic field‐effect transistors (OFETs) technologies because of their excellent photophysical properties, large molar extinction coefficients, high fluorescence quantum yields, tunable emission spanning from visible light to near‐infrared (NIR) regions, exceptional color purity, prolonged fluorescence lifetimes, and unusual brightness combined with robust photo‐ and chemical‐stability.^{[ 4 ]} However, challenges arise in the solid state utilization of BODIPY‐based materials, mainly attributed to two factors; (i) spectral overlap between absorption and emission spectra, leading to self‐quenching of fluorescence at high concentration, (ii) aggregation‐caused quenching (ACQ) resulting from the π–π stacking behavior in their solid state.^{[ 5 ]} These features are less than ideal for light‐emitting materials, thus imposing limitations on the applications of BODIPY dyes.^{[ 3} , ^{6 ]}

The unique properties of organic boron complexes arise from their ability to modulate the π‐conjugated system, effectively altering the steric and electronic characteristics of BODIPY chromophores and enhancing their optoelectronic properties. Figure 1a highlights the classic BODIPY structure, demonstrating its symmetrical arrangement and electron‐rich pyrrole moieties. This symmetry is fundamental to its fluorescence efficiency but also leads to the challenges of ACQ and spectral overlap discussed earlier. The vacant p‐orbital on the tricoordinate boron center facilitates p‐π* conjugation, owing to its high Lewis acidity, thus forming a unique electron‐acceptor capable of forming stable complexes with organic conjugated moieties. Intramolecular N‐B coordination is being both isoelectronic and isosteric to C─C bonds, and promotes extended π‐electron delocalization within the conjugated system, thereby modulating its electronic structure and, consequently influencing its chemical and optical properties.^{[ 7 ]} The electron‐deficient boron difluoride moiety lowers the LUMO energy, narrows the band gap, and increases electron affinity, enhancing intramolecular interactions and enabling efficient photoinduced charge transfer (ICT).^{[ 8 ]} Moreover, the aromaticity of two heterocyclic rings and the delocalization between two distinct resonance moieties equally contribute to the structural stability of BODIPY materials.^{[ 9 ]} The success of pyrrole‐based boron complexes (Figure 1a) has sparked broader interest in this compound class, leading to a rapid expansion in the design of suitable N‐heterocyclic ligands, offering a vast array of structures and functionalities. Ligands such as carbazoles, pyrimidines, benzimidazoles, pyridines, and pyrrolopyrroles represent an expanding group of pioneering ligands for such boron complexes, showcasing novel chemical and photophysical properties.^{[ 6} , ^{10 ]} Compared to BODIPY‐based systems, fluorophores featuring complexes chelated with N or O possess advantages stemming from the highly Lewis acidic boron atom's participation in the ligand backbone via internal coordination. This rigidity reduces the energy losses attributed to non‐radiative relaxation.^{[ 11 ]}

Figure 1.

a) Structures of classic boron‐dipyrromethenes (BODIPY), b) boron‐benzothiazole (BOBTz)‐ and boron‐benzoxazole (BOBOz)‐based complexes, c) comparison chart of orbitals distribution in relation to the properties of heterocycles‐changing classic pyrrole to thiazole.

One of the recent advancements in the development of organic boron complexes involves the desymmetrization of the classic BODIPY core, by introducing units such as benzothiazoles (BTz), benzooxazoles (BOz), and their derivatives as pivotal chelating moieties. This transformative process involves substituting the electron‐rich pyrrole with the electron‐deficient thiazole (Tz) or oxazole (Oz) functionalities resulting in a significant alteration of the electronic structure and properties of the resulting boron complexes (Figure 1b).^{[ 12 ]} The Tz and Oz akin to pyrrole is a five‐membered heterocycle, however, it differs by incorporating sulfur and oxygen atoms instead of nitrogen with its electron pair participating in the aromatic sextet, which accounts for variations in electronic and chemical properties.^{[ 13 ]} Figure 1c provides a comparative orbital distribution chart, visually illustrating the changes in electronic structure when replacing pyrrole with thiazole or oxazole. Furthermore, similar to pyridine, the lone pair of nitrogen in Tz and Oz structure is not involved in the delocalization, allowing the formation of thiazolium and oxazolium cations, which facilitating the complex formation with transition metals.^{[ 14 ]} Finally, in terms of electron delocalization energy, Tz exhibits a thiophene‐like structure, while Oz exhibits high electron withdrawal properties with less aromatic character compared to benzene or imidazole.^{[ 15 ]}

These structural modifications have effectively reduced ACQ and improved thermal stability, addressing many of the limitations associated with classic BODIPY dyes.^{[ 16 ]} In addition, they have enhanced photophysical properties such as electron delocalization and ICT efficiency, making them highly beneficial for optoelectronic applications. Over the last two decades, these modifications found effective applications in fields such as organic photovoltaics (OPV), organic field‐effect transistors (OFETs), and fluorescent dyes.^{[ 17 ]} Although, azoles and their analogs have received greater attention in the field of optoelectronic n‐type materials as they have remained relatively unexplored molecules capable of complexing with boron, both in terms of design and applications. The asymmetric structure of BTz and BOz coupled with its unique electronic properties, opens new avenues for derivatization enabling the formation of highly conjugated boron complexes that exhibit novel mechanical and electrochemical properties.^{[ 17} , ^{18 ]}

In this review, we explore the significance of new class of BTz‐ and BOz‐derived boron complexes and their advancements in pioneering fields such as LED applications, bioimaging, and mechanochromic luminescence. These developments highlight the versatility of these complexes, yet with the potential to extend beyond these domains. For instance, recent studies demonstrate their use in radical polymerization for materials like photocurable adhesives and 3D printing resins, as well as in fluorescence imaging and photodynamic therapy, highlighting their potential to tackle technological challenges.^{[ 19 ]} This review aims to systematize the current advancements and underscore the unique properties of BTz and BOz boron complexes, highlighting their potential to inspire further research and advance in emerging technologies.

2. Light Emitting Materials

Thiazoles possess intrinsic fluorescence and serve as fundamental components of various natural products, including luciferin, responsible for the bioluminescence observed in the firefly (Lampyridae).^{[ 20 ]} However, recent reports highlight a rapid expansion in the development of synthetic fluorophores based on other azoles, showcasing novel properties and applications.^{[ 21 ]} Kwak and Kim were the first who reported the synthesis of boron complexes aimed at inhibiting the excited‐state intramolecular proton transfer (ESIPT) process of 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) and hydroxyphenyl)benzothiazole (HBT) resulting in a hypsochromic shift of emission toward blue luminescent materials.^{[ 22 ]} This strategy led to the formation of complexes 1 and 2, emitting intense deep blue light upon UV irradiation in both the solution and the solid state (Figure 2 ). The extinction coefficient of dyes 1 and 2 in CH₂Cl₂ was determined to be 21 200 and 23000 m ⁻¹ cm⁻¹, respectively, while the emission quantum yields were found to be 0.20 for 1 and 0.23 for 2, confirming their efficacy as emitters. Notably, films developed in this study were robust and stable, highly beneficial for various applications. Minimal photoinduced degradation was observed under continuous UV irradiation in ambient conditions, with BOz‐boron derivative 1 retaining 88% of its emission and BTz boron derivative 2 exhibiting nearly unchanged fluorescence intensity (98%), contrasting with the rapid decay observed in the control dye film BD‐1 (Figure 2).

Figure 2.

a) Structures of BOz‐derived boron‐difluoride complexes 1 and BTz‐derived boron‐difluoride complexes 2, b) photostability of dye 1, 2, and BD‐1 thin films. The remaining fluorescence emission intensities of the dye thin films as a function of irradiation time. Irradiated at 340 (1), 360 (2), 380 nm (BD‐1). Thin films were prepared by drop‐casting of dyes dissolved in CHCl₃ (7.0 mg ± 0.2 mL⁻¹). Reprinted with permission from ref. [22]. Copyright 2009, Korean Chemical Society.

In 2012, Santra and co‐workers explored a series of N‐ and O‐chelated boron complexes 3, 4, 5, and 6 based on 2‐(benzothiazol‐2‐yl)phenols, showcasing significant alterations in photoluminescence (PL) controlled by the position of the amine substituent (Figure 3a).^{[ 23 ]} In their study, the authors elucidated the facile tuning on PL properties and emission wavelength of the boron difluoride type complexes by varying the electron‐rich groups along the aromatic ring. This approach offers an appealing and practical method for precisely adjusting the desired luminescence wavelength. Notably, dyes 4 and 5 exhibited vibrant luminescence in both solution and the solid state, with high quantum yields of up to 0.85 and 0.98, respectively. Para‐ and meta‐amino‐substituted complexes 4 and 5 displayed bright blue and red‐orange luminescence in their crystal states, while in dichloromethane, dye 5 emitted blue light, and dye 4 emitted orange light upon UV irradiation (Figure 3c, inset). These optical characteristics demonstrate the potential of boron difluoride complexes as promising materials for OLEDs, lasers, field‐effect transistors, and molecular probe applications. Furthermore, the authors detailed the exceptional luminescent properties and material applications of the proposed boron complexes when utilized in OLEDs. Specifically, the blue BTz‐boron difluoride complex 5 was spin‐coated and used as a dopant in the emitting layer of an OLED device. Remarkably, the device achieved a maximum brightness of 716 cd m⁻² at a driving voltage of 8.0 V (Figure 3b). At a voltage of 4.9 V, the device emitted blue light with a chromaticity of Commission Internationale de l'Elcairage (CIE) x,y (0.15, 0.11) highlighting its notable deep blue color value of CIE x,y (0.14, 0.08) (Figure 3c).

Figure 3.

a) Structures of BTz‐derived boron complexes 3–8, b) Luminance versus voltage characteristic of the OLED device, c) normalized PL (dotted line) and electroluminescence (EL) (solid line) spectra of complex 5, Inset: (Top) photos of luminescent crystals of 5 (blue) and 4 (orange‐red) under UV irradiation in the range of 340–380 and 515–560 nm, respectively. Photos were magnified five times through a microscope; (Bottom) photos of emission colors 3 (blue) and 4 (orange) in dichloromethane upon excitation at 395 and 450 nm, respectively. Reprinted with permission from ref. [23]. Copyright 2012, Wiley‐VCH GmbH.

Li and co‐workers also contributed to the field by reporting the synthesis of a series of dyes from BTz‐ and BOz‐derived, each bearing one or two π‐conjugated organoboron complexes, and studied their applications in EL.^{[ 24 ]} These fluorophores exhibited bright fluorescence in the solid state across a broad range of wavelengths. Notably, the rigid seven‐ring fused core structures 9, 10, 11, and 12 bridged by boron atoms exhibited high electron mobility and excellent thermal stability, making them highly desirable for use as light‐emitting materials (Figure 4a). Complexes 11 and 12 exhibited blue and bluish‐green fluorescence in dichloromethane (CH₂Cl₂) at 452 nm (Ff = 0.55) and displayed a slight blue‐shifted in the solid state. Conversely, the seven‐ring fused diboron complexes 9 and 10 emitted red fluorescence with absorption maxima of 491 and 515 nm, respectively.

Figure 4.

a) Structures of BOz‐ and BTz‐derived boron complexes 9–12, b) the electroluminescence spectra for 9 and 10, inset: CIE chromaticity diagrams of devices, c) current density‐brightness‐voltage characteristics of devices of complexes 9 and 10, inset: Photographic images of devices. Reprinted with permission from ref. [24]. Copyright 2011, Royal Society of Chemistry.

In addition to their optical properties, both dyes 9 and 10 were evaluated as highly efficient electron‐transporting materials, exhibiting mobilities of 7.7 × 10⁻⁴ and 2.2 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, while maintaining high thermal stability exceeding 400 °C. The EL device based on complex 9 emitted pure red light with a peak wavelength at λ_max 628 nm and CIE coordinates of (X = 0.64, Y = 0.36). This device turned on at a low voltage of 2.5 V, reaching a maximum brightness of 3704 cd m⁻² and achieving a maximum power efficiency of 0.53 lm W⁻¹ (Figure 4c). Similarly, the OLED device utilizing complex 10 as the electron‐transporting emitter displayed strong red‐to‐near‐infrared EL with a peak at 680 nm and CIE coordinates of (X = 0.70, Y = 0.30). It turned on at a voltage of 3.0 V reached a maximum brightness of 2636 cd m⁻² and attained a maximum power efficiency of 0.46 lm W⁻¹ (Figure 4b). It's worth noting that, the practical maximum brightness of the device based on complex 10 could be even higher due to significant emission in the near‐infrared region.

Subsequently, the same research group proposed a series of boron‐chelated compounds derived from HBO‐derived boron‐chelates 13, 14, and HBT‐derivatives 15, 16, 17, 18, and 19 (Figure 5a).^{[ 25 ]} In their study, the authors extensively investigated the EL properties of these borane complexes and outlined a strategy for fabricating full‐color‐tunable and strongly emissive materials. This was achieved via simple modification of HBO and HBT frameworks by introducing various amino groups at the para‐position of the phenyl ring followed by borane complexation. To evaluate the EL properties, OLEDs were fabricated with each respective boron complex serving as the light emitter. The EL spectra of complexes 13–18 exhibited emission peaks at 465, 500, 544, 560, 612, and 652 nm, respectively. These emission peaks closely matched the solid emission spectra of each complex, confirming the boron emitter as the source of emission. Notably, the EL spectra displayed minimal voltage dependence, and the CIE coordinates remained stable across different voltage conditions (Figure 5b). The CIE coordinates (X, Y) of these devices ranged from (0.16, 0.19) to (0.66, 0.34), covering the blue, greenish blue, green, yellow, red, and deep‐red regions (Figure 5c). The authors summarize that the device utilizing complex 14 demonstrates the highest current efficiency (7.8 cd A⁻¹), while the device incorporating complex 16 exhibited the brightest luminosity (31 220 cd m⁻²) among all previously reported boron‐derived emissive materials (Figure 5d).

Figure 5.

a) Structures of BOz‐ and BTz‐derived boron complexes 13–19, b) EL spectra, c) CIE coordinates and photographic images of devices employing complexes 13–18 as emitters (in the graphics, numbers 1–6 correspond to 13–18), and d) brightness‐voltage. Reprinted with permission from ref. [25]. Copyright 2012, Royal Society of Chemistry.

In 2019, Salla and co‐workers reported another series of HBT ligand‐based borane complexes 20, 21, 22, and 23 (Figure 6a), demonstrating the impact of the substituents on the properties of the dyes.^{[ 26 ]} This work described the design, synthesis, and characterization of various boron complexes and their unusual emission characteristics in OLEDs. The emission properties were modulated by incorporating donor and acceptor groups, leading to emission in the blue, green, and red regions. The OLEDs were fabricated using relatively simple solution‐processed devices with a simple structure, i.e., ITO/Pedot:PSS (25 nm)/TcTA:OXD‐7:BX (25 nm)/PFNBr:TEAB (35 nm)/CsF (1.3 nm)/Al (100 nm) with BX being either 20, 21, or 23. Figure 6b represents the devices used for the studies, and the boron‐derived complexes displayed fluorescence efficiencies as high as Φ = 0.88. The blue emitters exhibited superior stability under an ambient atmosphere while maintaining high current efficiency as outlined in Figures 6c,d. The molecular design and device operation was further supported by computational calculations using PBE0 and the B2PLYP functionalities, demonstrating the possibility of predicting absorption and fluorescence spectra. The authors utilized this powerful technique to evaluate the molecular photophysical properties by density functional theory (DFT), providing a detailed understanding of the molecular and electronic structure of the molecules.

Figure 6.

a) Structures of BTz‐derived boron complexes 20–23, b) normalized EL spectra and CIE _x,y of OLEDs 20–22 (in the graphics, numbers B1, B2, and B3 corresponds to 20, 21, and 22), c) Energy diagram and 20–22 OLED characteristics (in the graphics, numbers B1, B2, and B3 corresponds to 20, 21, and 22) and d) current efficiency and external quantum efficiency versus luminance. Reprinted with permission from ref. [26]. Copyright 2019, Wiley‐VCH GmbH.

Later, Kaur, and co‐workers reported an alternative methodology for the synthesis of BTz‐based dyes 24 and 25 and their rigidified boron (III) N^O chelates (Figure 7a).^{[ 27 ]} In this work, the authors examined the modulation of photoluminescence wavelength and quantum yields using blends of ESIPT‐capable chromophores and their BF₂‐locked analogs. The convenient tuning of emission and absorption behavior of the compounds allowed the identification of compositions for complimentary colors emitted from these molecular species, both in solution and solid state. Notably, the blend M7 which includes dye 24 achieved nearly white light emission with CIE coordinates of x ¼ 0.35753, y ¼ 0.3315 (Figure 7b).

Figure 7.

a) Structures of benzothiazole‐derived difluoro borate complexes 24 and 25, b) placement of the colors in the CIE chromaticity diagram (1931, D₆₅/2 observer) based on the emission of respective solutions (in the graphics, numbers 3, 5, 6, and 7 represents BTz‐derived precursors and the number 6 corresponds to 24). The line connecting 3 and 6 passes through the white region of standard illuminant; Inset: photographic images of M1 (a mixture of 3, 5, and 7) and M7 (a mixture of 3 and 6) in tetrahydrofuran (THF) under illumination at 365 nm. Reprinted with permission from ref. [27]. Copyright 2020, Elsevier B.V.

Very recently Kutsiy et al. introduced two ortho‐phenylene‐linked donor‐acceptor BOBTzs‐based complexes 26 and 27 as thermally activated delayed fluorescence (TADF) emitters (Figure 8a).^{[ 28 ]} In this work, the authors investigated the unique structural features of these complexes, including the ortho‐phenylene linkage and donor‐acceptor configuration, and their potential in optoelectronic properties. The bent geometry imposed by the ortho‐phenylene linkage enhances through‐space charge transfer, while weak π‐π stacking and extensive hydrogen‐bonding interactions suppress non‐radiative decay, resulting in high PL quantum yields of up to 50% in host‐based layers. The electronic properties of the BOBTzs‐based complexes 26 and 27 have been displayed in Figure 8b–d, highlighting devices A and B exhibit green light emission ≈530 nm, with device B slightly redshifted and showing a broader profile (Figure 8b). Device A, utilizing complex 26, achieved a peak external quantum efficiency (EQE) of up to 15.2% with a maximum brightness of 7692 cd m^{− 2}. This performance is attributed to the higher electron mobility of complex 26 (1.5 × 10⁻⁴ cm² V⁻¹ s⁻¹), which supports better charge balance, though it limits peak brightness due to earlier recombination. In contrast, Device B, using complex 27, achieved a slightly lower EQE of 13.3% but a higher brightness of 10302 cd m^{− 2} (Figure 8c). This is linked to the cyano group on the benzothiazole acceptor, which strengthens the acceptor properties, enhancing the charge separation and recombination efficiency. The cyano group improves the charge injection and transport, resulting in higher luminance despite the slightly lower EQE. The current density–voltage (J–V) curves further illustrate that Device B reaches higher current densities at similar voltages, reflecting the impact of the cyano group on charge dynamics (Figure 8d). These findings underscore the critical relationship between the molecular design of BOBTzs and their charge transport, recombination dynamics, and overall device efficiency illustrating how structural tuning can advance these materials for OLED applications.

Figure 8.

a) Structures of benzothiazole‐derived difluoro borate complexes 26 and 27, b) EL spectra of OLEDs at constant voltage c). Brightness‐voltage‐current density curves d) and quantum efficiency‐brightness of devices A and B. Reprinted with permission from ref. [28]. Copyright 2016, American Chemical Society.

In this area of research, Al‐Hashimi and Fang also reported the synthesis and photophysical characterization of a set of fused rings‐based Tz and BTz‐derived difluoro borane complex 28 (Figure 9 ).^{[ 29 ]} In this work, authors explored the molecular design and the synthesis of an extensive electron‐deficient polycyclic π‐system featuring multiple B←N bridged ladder‐type polymers. The presence of a short B←N coordinate bond was attributed to the resonance effect observed in BOBTz units, indicating strong B←N coordination and good chemical stability of these compounds. This study demonstrated a strategy for synthesizing extensive rigid polycyclic π‐systems with intrinsic n‐type properties, which are crucial for the development of new organic molecules and macromolecules for electronics and photovoltaics applications.

Figure 9.

Structure of BTz‐derived B←N bridged ladder‐type difluoro borate complex.

In general, BOBTz and BOBOz complexes highlighted in this section reveal they are exceptional materials for LED applications due to their high thermal stability, AIE properties, and tunable PL across a broad spectrum. In comparison to traditional materials, their enhanced resistance to ACQ ensures higher efficiency and stability in solid‐state devices, making them particularly suitable for advanced OLEDs.

3. Bioimaging Biomedical Applications

The classic BODIPY probes have achieved remarkable advancements and widespread use in the field of bioimaging due to their exceptional properties.^{[ 30 ]} These probes efficiently emit light upon excitation, resulting in strong and bright signals. The emission profiles of BODIPY dyes are characterized by sharp peaks, high fluorescence quantum yields,^{[ 31 ]} and tunability,^{[ 32 ]} enabling the accurate detection and imaging of targeted structures^{[ 33 ]} or ions^{[ 34 ]} with the use of the minimal quantity of the dyes.^{[ 35 ]} Furthermore, these dyes demonstrate robustness, resilience and maintain their fluorescence properties even under challenging chemical or biological environments. These properties ensure reliable and consistent imaging of results over extended periods of time, contributing to the success and longevity of BODIPY‐based bioimaging techniques.^{[ 35 ]} In this regard, Tz and Oz‐based biosensors are gaining significant attention as highly effective devices for the detection and monitoring of diverse biological analytes having distinctive structural and electronic characteristics.^{[ 36 ]} Another interesting feature of BTz and BOz‐based chromophores is the enhanced emission in the solid aggregated state as the ESIPT process is activated owing to the stable intramolecular hydrogen bonding, providing a keto‐aggregation induced emission (AIE).^{[ 37 ]}

In 2016, Liu and co‐workers reported for the first time the use of benzothiazole‐pyrimidine‐based BF₂‐complex (BTzPB) as highly selective and rapidly responsive fluorescent sensors of cysteine (Cys) in biological systems.^{[ 38 ]} The structure of the BTzPB complexes 29 and 30 is outlined in Figure 10a. In this study, the authors highlighted the detection of Cys and emphasized its significant importance from both chemistry and biology standpoints as it is a crucial molecule that is directly involved in redox signaling.^{[ 39 ]} The proposed sensor demonstrated a rapid and highly selective “turn on” response to Cys in comparison to homocysteine (Hcy), glutathione (GSH), and other amino acids in an aqueous solution at physiological pH. It exhibited changes in the emission spectra that are dependent on the concentration of Cys particularly in the range of 1–20 equivalents. The authors utilized the proposed cell‐permeable fluorescent sensor BTzPB for imaging of Cys in living cells. This experiment was carried out by treating HeLa cells with Cys and incubated with BTzPB, brighter fluorescence images were observed suggesting the formation of Cys‐BTzPB within the cells. On the other hand, the treatment of HeLa cells with glutathione (GSH) and incubated with BTzPB revealed no significant fluorescence enhancement, which indicates that BTzPB specifically responds to Cys rather than GSH in the cells. These findings demonstrate the suitability of the sensor for fluorescent imaging of Cys in living cells (Figure 10b–g).

Figure 10.

a) Structure of difluoro‐borane complex 29 and the formation of Cys‐derived complex 30. Confocal microscopic images b–g) of HeLa cells stained by 29. b, e) Bright‐field transmission images; c, f) cells pretreated with 500 mm N‐ethylmaleimide (NEM) for 20 min, further incubated with 29 (20 mm, 1xphosphate buffered saline (PBS), 30 min of incubation; lex, 405 nm; lem = 420–480 nm); d, g) fluorescence images of the rinsed cells in c, f) upon further treatment with 100 mm Cys d) or 2 mm GSH g) solution (0.5 h), followed by incubation with 29. Reprinted with permission from ref. [38]. Copyright 2016, Wiley‐VCH GmbH.

In another report, the same group proposed an alternative strategy for the synthesis and characterization of a series of boron complexes based on BTz‐pyrimidine bidentate motifs 31–38 as shown in Figure 11a.^{[ 40 ]} This study thoroughly described the diverse substitution patterns on the pyrimidine ring, photophysical properties, theoretical modeling, and their applications in sensing. Among the dyes within the series, compounds 31 and 32 exhibited remarkable capabilities in sensing Cys in aqueous solutions, showing both high selectivity and sensitivity. The isomeric positions of the chlorine substituent in 31 and 32 significantly contributed to their sensing abilities, highlighting the versatility and tunability of this system. The difluoro‐borane complexes 31 and 32 demonstrated exceptional selectivity specifically toward Cys, surpassing responses to amino acids such as Hcy, GSH, and others. This distinct sensing behavior was attributed to the stronger nucleophilic nature of Cys compared to Hcy and GSH. Furthermore, bioimaging studies revealed that the intramolecular cyclization with Hcy and GSH generating six‐membered or macrocyclic ring structures might be kinetically unfavorable, demonstrating high selectivity of 31 and 32 for Cys over Hcy and GSH with the detection threshold for cysteine being as low as 268 nm (Figure 11b,c). The variations in sensing capabilities between complexes 31 and 32 are attributed to their isomerism and the differences in the electronic structure of the resulting photoactive complexes upon interaction with Cys. These sensor exhibits exceptional selectivity and demonstrate the ability to permeate cell membranes, making them promising tools for various biological applications (Figure 11d–f).

Figure 11.

a) BTz‐derived boron complexes 31–38, b) fluorescence intensity changes of 31, c) 32 upon addition of 100 µm physiologically important amino acids. Confocal microscopy images d–f) of HeLa cells stained with 31. d) Bright‐field transmission images; e) cells pretreated with 1 mm NEM for 20 min, further incubated with 31 (20 µm, 1× PBS, 30 min of incubation; λ_ex, 405 nm; λ_em = 420–480 nm); f) fluorescence images of the rinsed cells in e) with further treatment with 100 µm Cys. Reprinted with permission from ref. [40]. Copyright 2016, Royal Society of Chemistry.

Curiel and co‐workers published an article reporting the use of BOz and BTz‐derived boron complexes for bioimaging applications.^{[ 41 ]} In this study, the authors described the synthesis of a series of boron complexes based on 7‐(azaheteroaryl)‐indole ligands containing benzoxazole (39), benzothiazole (40), benzimidazole (41), and pyridine (42) units. The moieties were used to modify the electronic effects over π‐conjugated system (Figure 12a). By introducing heteroatom exchanges within the ligand structure, they were able to emit fluorescence with a diverse range of colors of complexes 39–42. Interestingly, the bright colors that emerged upon the formation of the boron complexes displayed a remarkable red shift in the absorption bands, which were within the visible range (Figure 12b). This shift indicates significant electronic effects of complexes 39–42 resulting from the coordination of the boron atom and the planarization of the π‐conjugated ligand, playing a crucial role in narrowing the energy gap between the ground state and the first singlet excited state. The absorption and emission spectra of thin films of boron complexes 39–42 were analyzed, revealing that the absorption spectra in the films closely resembled those in solution. However, the emission spectra exhibited a notable shift, indicating longer wavelengths. In addition, the thin films displayed improved quantum yields suggesting their suitability as materials for solid‐state light emission (Figure 12c). To further explore the practical applications of these boron complexes, bioimaging experiments were conducted using human breast cancer cells (MCF7). Specifically, the BTz derivative 40 exhibited minimal cytotoxicity ensuring cell viability. Confocal microscopy analysis revealed strong emission from the complex upon staining the cells with constant fluorescence observed across different excitation wavelengths (Figure 12d,f). This highlights the potential of these boron complexes in bioimaging applications, particularly in the imaging of cancer cells.

Figure 12.

a) BOz‐ and BTz‐derived boron complexes 39–42, b) fluorescence of boron complexes 39‐42 in solution and thin films, c) absorption (dashed plot) and emission spectra (continuous plot) of complexes 39–42 in CH₃CN, and confocal microscopy images, d) λ_exc = 488 nm, e) λ_exc = 561 nm of human breast cancer cells MCF7 with fluorophore 40. Reprinted with permission from ref. [41]. Copyright 2016, American Chemical Society.

Li and co‐workers introduced an alternative approach for bioimaging by utilizing the near‐infrared (NIR) emission properties of two BTz‐pyrrolopyrrole‐based boron complexes 43 and 44 (Figure 13a). These complexes exhibited strong aggregation‐enhanced emission (AEE) characteristics with excitation and emission peaks ranging from 668 to 677 nm. The fluorescence emission was red‐shifted by 15 and 14 nm respectively for complexes 43 and 44. These complexes were further utilized to create NIR‐emitting nanoparticles, which exhibited exceptional photostability. The nanoparticles containing both hydrophilic and hydrophobic regions were insoluble in water. Consequently, complexes 43 and 44 were primarily distributed within the nonpolar hydrophobic domains as individual molecules or in the hydrophilic domains as aggregates with both exhibiting strong fluorescence irrespective of their distribution (Figure 13b,c). The biocompatibility of 43 nanoparticles (NPs) was assessed using the CCK‐8 assay, which measures mitochondrial activity‐dependent formazan dye formation. The results indicated that the treatment of HeLa cells with 1 and 10 mm concentrations of 43 NPs maintained high cell viability over 1, 2, and 4 days, with a significant increase in cell numbers over time, providing clear evidence of cell proliferation. In addition, confocal fluorescence microscopy revealed strong fluorescence in the cytoplasm of cells incubated with 50 mm 43 NPs for 12 h, with fluorescence intensity increasing after 48 hours, indicating nanoparticle accumulation (Figure 13d–f). These findings suggest that the NIR‐emitting properties of these boron complexes combined with their biocompatibility and strong fluorescence, make them promising candidates for bioimaging applications, particularly in tracking and imaging cancer cells.

Figure 13.

a) Structures of BTz‐pyrrolopyrrole‐derived boron complexes 43 and 44, b) UV‐vis absorption, and c) fluorescence spectra of pyrrolopyrrole aza‐BODIPY dyes 1 (PPAB‐1) (black), 43 (red), and 44 (blue) in THF. Real‐time fluorescence images d–f) showing 43 NP (50 mm) stained HeLa cells at room temperature, d) cell nuclei stained by 43, 6‐diamidino‐2‐phenylindole (DAPI), e) red fluorescence image of 43 NPs, and f) bright field, respectively. Reprinted with permission from ref. [42]. Copyright 2017, Royal Society of Chemistry.

In general, BOBTz and BOBOz complexes stand out as superior fluorescent probes for bioimaging, offering high quantum yields, tunable emission properties, and enhanced stability under biological conditions. However, potential challenges such as maintaining long‐term stability in complex biological environments or ensuring biocompatibility with minimal cytotoxicity need to be addressed for their widespread application in medical diagnostics and imaging.

4. Mechanochromic Luminescence Applications

Mechanochromic luminescence (MCL) is a fascinating phenomenon where materials exhibit changes in their luminescent properties in response to mechanical force or external stimuli such as grinding, scratching, or pressure. These stimuli can disrupt the π‐conjugation within the chromophoric cores of the compound, altering its electronic structure and leading to changes in the absorption and emission properties. This unique behavior of MCL has attracted significant attention in the field of materials science, paving the way for innovative applications in areas such as sensing, data storage, security, and optoelectronic devices.^{[ 43 ]} Boron complexes are known for their diverse luminescent properties and have been well used for various mechanochromic applications.^{[ 43 ]} However, research in this area has primarily focused on specific classes of boron complexes, such as β‐diketonate,^{[ 44 ]} β‐iminoenolate,^{[ 45 ]} and amidine chelates.^{[ 46 ]} These classes have demonstrated notable MCL behavior, though there have been relatively few reports on other types of boron complexes exhibiting similar properties. This limited exploration highlights the potential for discovering new classes of boron complexes with mechanochromic properties, offering opportunities for further advancements in the field.^{[ 47 ]}

In 2015, Wang and co‐workers introduced propeller‐shaped BTz‐enamide boron difluoride complexes 45 and 46, marking a significant contribution to the study of piezochromic and MCL properties (Figure 14a).^{[ 48 ]} These materials demonstrated notable changes in their optical properties when subjected to mechanical grinding or shearing. The authors provided a detailed schematic diagram summarizing their observations and conclusions, offering valuable insights into the underlying mechanisms (Figure 14b,c). The boron complexes 45 and 46, used in this study, exhibited redshifts in luminescence when exposed to high pressure. Notably, compound 46, bearing more electron‐donating substituents displayed a stronger piezochromic response at lower pressures (<1.5 GPa) due to an ICT effect. This behavior underscores the potential of these complexes in applications requiring sensitivity to mechanical forces and contributes to the growing understanding of mechanochromic materials. The authors further explored the luminescence behavior of boron complexes, revealing that the compression‐induced luminophore co‐planarization of the luminophore within these complexes significantly influences their optical properties. In compound 45, this co‐planarization enhances π‐π intermolecular interactions, which favors the formation of excimers‐molecular complexes that exhibit dual‐band luminescence. This dual‐band emission is a direct result of the excimer formation, adding a layer of complexity to the material's luminescence under mechanical stress. On the other hand, in compound 46, the compression‐induced co‐planarization amplifies the ICT effect, leading to different luminescent behavior compared to compound 45. This distinct response highlights how subtle changes in molecular structure and interactions within boron complexes can lead to varied luminescence properties, making them highly adaptable for specific applications in sensing, optoelectronics, and materials science.

Figure 14.

a) BTz‐enamide boron difluoride complexes 45 and 46. Proposed mechanism of the piezochromic behaviors for 45 and 46 b and c). “b” stands for the electron‐withdrawing motif, “c” the electron donating group. Reprinted with permission from ref. [48]. Copyright 2015, Royal Society of Chemistry.

Zhou and co‐workers utilized the MCL properties of BTz‐boron complexes to explore the extensive structure‐property relationships through detailed analytical characterizations.^{[ 49 ]} In their work, they proposed a series of BTz‐based boron complexes BF₂‐TT‐CN (47), BPh₂‐TT‐CN (48), BF₂‐TT (49), and BPh₂‐TT (50) as shown in Figure 15a. These compounds featured an electron‐rich triphenylamine unit and an electron‐accepting β‐iminoenolate boron moiety, which facilitated strong intramolecular charge transfer. The ICT was further amplified by the introduction of CN groups in 47 and 48. The study revealed that depending on the specific configuration, these compounds exhibited cyano‐dependent AIE and mechanofluorochromic (MFC) properties. This stimuli‐responsive luminescent behavior was particularly pronounced in compounds 47 and 48 due to the presence of the CN groups, which played a significant role in modulating the electronic interactions within the molecules. The materials demonstrated in this study showcased promising applications in areas such as data recording, pressure sensing, and light emission as depicted in Figure 15b–e. The ability to tune the luminescent properties of these compounds through mechanical stimuli and aggregation phenomena underscores their potential for developing advanced optoelectronic devices and smart materials.

Figure 15.

a) Structures of BTz‐based boron complexes 47–50, PL spectra of 47, b) (λ_ex = 460 nm) and 48, c) (λ_ex = 430 nm) in different solid‐states: as‐synthesized, grinding and fuming. Photos of 47 d) and 48 e) color changes under grinding and fuming stimuli. Reprinted with permission from ref. [49]. Copyright 2016, Royal Society of Chemistry.

Gao and co‐workers introduced a novel approach to the synthesis of tetraphenylethene‐modified β‐ketoiminate boron complexes 51 and 52 (Figure 16 ).^{[ 50 ]} Their study highlighted several key features of these complexes including their thermal stability, AIE, twisted spatial conformation, and high solid‐state fluorescence efficiency. They have been reported that complexes 51 and 52 exhibited reversible mechanofluorochromic (MFC) behavior under grinding and fuming treatments. The MFC properties were attributed to changes in molecular packing and distortion within the molecules. Specifically, complex 52, which included a cyano group, displayed a more pronounced MFC effect. The phase transitions between crystalline and amorphous forms were confirmed through X‐ray diffraction and differential scanning calorimetry (DSC) analyses. These boron complexes demonstrated potential applications in optical recording, pressure sensing, and light emission due to their stability, ease of synthesis, and distinct on/off fluorescence contrast. The study also showcased a practical application of this concept by illustrating a simple luminescence writing/erasing process, which effectively altered the fluorescence properties of these complexes as depicted in Figure 16a–h. This approach emphasizes the versatility of these materials for advanced optoelectronic and sensor applications.

Figure 16.

Top): Structures of β‐ketoiminate boron complexes 51 and 52. (Bottom): Photos of the luminescence writing/erasing process of 51 and 52 on filter papers under UV light (365 nm): fluorescence emission of as‐prepared powder a and e); MCF of the letter of “A” b) and “f” f) was written with a spatula; the paper was erased by vapor fuming (the letter “A” c) and “f” g) becoming invisible); rewritable mechanochromic fluorescence of the letter of “C” d) and “p” h) generated with a spatula. Reprinted with permission from ref. [50]. Copyright 2017, Royal Society of Chemistry.

Song and co‐workers recently explored the MFC properties of a series of D‐π‐A type N,O‐chelated BF₂ complexes,^{[ 51 ]} including those based on BTz (53), BOz (54), pyridine (55), and quinoline (56) (Figure 17a). This study highlighted the versatility of these complexes in exhibiting MFC behavior. Furthermore, the research focused on synthesizing organogels from these compounds, which showed notable changes in luminescence properties in response to various external stimuli such as temperature, grinding, pressing, rubbing, and shearing. The MFC behavior was accompanied by phase transitions between crystalline and amorphous states as analyzed through UV‐vis and X‐ray diffraction (XRD) techniques. The fluorescent emission spectra of difluoroborane complexes 54 and 55 with the associated color changes of these materials are illustrated in Figure 17b,c. In addition, difluoroborane complex 55 has been utilized as multi‐stimuli‐responsive luminescent material for applications in sensory and imaging technologies, including the detection of latent fingerprints (Figure 17d,e). Results summarized in this study demonstrate the promising capabilities of BF₂‐complexes in advanced material applications, particularly in sensing and imaging.

Figure 17.

a) Structures of BF₂‐derived complexes based on BTz (53), BOz (54), pyridine (55), and quinoline (56) ligands. Fluorescent emission spectra of b) 54 and c) 55 with λ_ex = 400 nm. Fluorescent images of d) LFPs on glass after being sprayed with 55. e) showing level 1–3 details, including island, bifurcation, ridge ending, and pore when excited with a UV lamp at 365 nm. Reprinted with permission from ref. [51]. Copyright 2021, Elsevier B.V.

BOBTz and BOBOz complexes outlined in this section exhibit unique luminescent changes in response to mechanical stimuli such as grinding, pressing, or shearing, making them ideal for applications in pressure sensing and data storage. Their adaptability and tunability in response to external forces highlight their potential for creating smart materials and advanced optoelectronic devices.

5. Conclusion 

In this review, we have showcased the advantages of substituting electron‐rich pyrroles with Tz and Oz cores in boron‐based complexes, significantly enhancing their thermal and ambient stability. This substitution also improves their aggregation and spectral properties, making them more suitable for advanced applications. Specifically, we focused on a novel class of boron‐based complexes, referred to as BOBTz and BOBOz, which has shown remarkable potential in three rapidly growing fields: light‐emitting materials (LED technology), bioimaging, and mechanochromic materials. (a) Light Emitting Materials (LED Technology): The integration of BOBTz and BOBOz complexes into LED technology has been particularly promising due to their superior optical properties. These complexes exhibit improved thermal stability and desirable aggregation‐induced emission characteristics, making them ideal candidates for high‐performance light‐emitting devices. Their ability to maintain stable luminescence under various conditions enhances the efficiency and longevity of LEDs, addressing key challenges in the development of advanced lighting solutions. (b) Bioimaging: In the field of bioimaging, particularly BOBTz complexes have demonstrated significant advantages over traditional fluorescent dyes. Their enhanced stability and tunable emission properties enable more accurate and reliable imaging of biological systems. The incorporation of BOBTz complexes into bioimaging applications has been shown to improve contrast and resolution, facilitating better visualization of cellular structures and processes. This is particularly valuable for studying complex biological phenomena and developing new diagnostic tools. (c) Mechanochromic Materials: Both BOBTz and BOBOz complexes have also shown impressive MCL properties, which refer to their ability to change luminescence in response to mechanical stimuli such as grinding or pressure. This behavior is due to the disruption of π‐conjugation in the chromophoric cores, which alters their electronic structure and, consequently, their optical properties. The versatility of these complexes in exhibiting MCL makes them suitable for innovative applications in sensors, data storage, and security, where changes in luminescence can be used to detect and monitor physical changes. Looking ahead, their exceptional tunability, stability, and electronic properties suggest opportunities for integration into flexible electronics, enabling innovations in wearable devices and bendable displays. Their sensitivity to external stimuli also positions them as strong candidates for next‐generation sensors.

6. Future Outlook

Despite the advancements highlighted in this review, there remains a need for further research to fully exploit the multifunctional nature of BTz‐ and BOz‐based complexes. Future research efforts should focus on: Designing Novel Materials: Developing new heteroaromatic boron‐functionalized materials with varied fluorescence properties to expand the range of applications. Understanding Structure‐Property Relationships: Investigating the detailed relationships between the molecular structure and optical properties of BOBTz and BOBOz complexes to optimize their performance in different applications. Exploring New Applications: Identifying and developing new applications for azole‐boron complexes in emerging fields such as flexible electronics, advanced bioimaging techniques, and responsive materials. This mini‐review aims to serve as a valuable resource for advancing the design and understanding of BOBTz and BOBOz complexes. By facilitating their applications in LED technology, bioimaging, and mechanochromic materials, we hope to contribute to the progress and broader utilization of these promising compounds. The insights provided here are intended to inspire further research and development in the field, addressing the evolving needs of modern materials science and opening new avenues for innovation.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

M.B. and S.K.P. contributed equally to this Mini‐Review. All authors have read and agreed to the published version of the manuscript.

Biographies

Dr. Hassan S. Bazzi is the associate dean for external affairs, head of the division of science, and professor of chemistry in the College of Science and Engineering at Hamad Bin Khalifa University. Dr. Bazzi received his bachelor's and master's degrees in chemistry and organic chemistry, respectively, from the American University of Beirut, and his Ph.D. in polymer chemistry with Dean's Honor List from McGill University. He completed a postdoctoral research fellowship at Université de Montréal. Dr. Bazzi is a fellow of the American Chemical Society and a fellow of the Royal Society of Chemistry.

Dr. Mohammed Al‐Hashimi is a professor of chemistry in the College of Science and Engineering at Hamad Bin Khalifa University. He received his M.Sc. in Pharmaceutical Chemistry (2003) and Ph.D. (2007) from Queen Mary University of London. He worked as a Senior Development Chemist at Evotec, UK, before joining Imperial College London as a postdoctoral researcher. His research focuses on organic polymers and semiconductor materials for optoelectronic applications. He has published over 100 peer‐reviewed papers, holds five patents, and serves on the editorial boards of Current Organic Chemistry and Current Organocatalysis. He was most recently honored with the TAMUQ Faculty Research Excellence Award and was named a Fellow of the Royal Society of Chemistry (FRSC).

Barłóg M., Podiyanachari S. K., Bazzi H. S., Al‐Hashimi M., Advances in Π‐Conjugated Benzothiazole and Benzoxazole‐Boron Complexes: Exploring Optical and Biomaterial Applications. Macromol. Rapid Commun. 2025, 46, 2400914. 10.1002/marc.202400914