Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Jul 24;146(31):22056–22063. doi: 10.1021/jacs.4c07730

The Delayed Box: Biphenyl Bisimide Cyclophane, a Supramolecular Nano-environment for the Efficient Generation of Delayed Fluorescence

Swadhin Garain †, Kazutaka Shoyama †,‡, Lea-Marleen Ginder †, Menyhárt Sárosi †,‡, Frank Würthner †,‡,*
PMCID: PMC11311229  PMID: 39047068

Abstract

graphic file with name ja4c07730_0006.jpg

Activating delayed fluorescence emission in a dilute solution via a non-covalent approach is a formidable challenge. In this report, we propose a strategy for efficient delayed fluorescence generation in dilute solution using a non-covalent approach via supramolecularly engineered cyclophane-based nanoenvironments that provide sufficient binding strength to π-conjugated guests and that can stabilize triplet excitons by reducing vibrational dissipation and lowering the singlet–triplet energy gap for efficient delayed fluorescence emission. Toward this goal, a novel biphenyl bisimide-derived cyclophane is introduced as an electron-deficient and efficient triplet-generating host. Upon encapsulation of various carbazole-derived guests inside the nanocavity of this cyclophane, emissive charge transfer (CT) states close to the triplet energy level of the biphenyl bisimide are generated. The experimental results of host–guest studies manifest high association constants up to 104 M–1 as the prerequisite for inclusion complex formation, the generation of emissive CT states, and triplet-state stabilization in a diluted solution state. By means of different carbazole guest molecules, we could realize tunable delayed fluorescence emission in this carbazole-encapsulated biphenyl bisimide cyclophane in methylcyclohexane/carbon tetrachloride solutions with a quantum yield (QY) of up to 15.6%. Crystal structure analyses and solid-state photophysical studies validate the conclusions from our solution studies and provide insights into the delayed fluorescence emission mechanism.

Introduction

Recently, metal-free, purely organic triplet harnessers have evolved as an important research field in the optoelectronic realm, especially in organic light-emitting diodes (OLEDs) due to their theoretically allowed near-unity exciton–photon conversion efficiency.1−3 Organic triplet harnessers are also very promising in optical sensing, bioimaging, and photocatalysis due to their high oxygen sensitivity and prolonged lifetime.1−3 Until now, triplet exciton harnessing was achieved in three ways: phosphorescence, triplet–triplet annihilation (TTA), and thermally activated delayed fluorescence (TADF).1−3 However, a purely organic phosphor always requires special measures to accomplish significant spin–orbit coupling and to improve the phosphorescence quantum efficiency.4 Significant advancements were made by Adachi and co-workers by designing spatially separated donor–acceptor systems for TADF.5,6 Hitherto, this design principle has mainly been exploited in covalent molecular systems that often suffer from arduous syntheses with meager reaction yields and aggregation-caused quenching. A promising alternative approach is the non-covalent supramolecular strategy, where specifically tailored subunits provided by the host and the guest afford desirable photofunctional properties in their host–guest complexes.7−13 A first very successful example for this approach was recently reported by Wong and Chou where the stabilization of triplet excitons and exploration of photofunctional properties from purely organic triplet generators in solution have been accomplished with the help of a guest embedded in a host cage, leading to solid-state materials with excellent OLED performance.14 However, in this system, the binding strength between host and guest in solution remained low compared to cyclophane-based host–guest systems,15,16 and TADF properties could only be studied in a reliable way in the solid state, like for other donor–acceptor TADF systems relying on the non-covalent approach.17−27 Further, activating the triplet harnessing pathways via a non-covalent strategy using organic chromophores remains a formidable challenge because, in addition to a sufficient binding strength, such a heavy atom-free, purely organic solution-state triplet generator demands an efficient triplet state population, a low singlet–triplet energy gap (ΔES–T), and its stabilization from vibrational dissipation and oxygen quenching. Here, an advanced molecular design strategy, including the creation of a tailored supramolecular nano-environment, is necessary to unveil the desired scenario. In this regard, cyclophanes are a most promising choice as they are constituted by two larger-sized parallel aromatic subunits having the ability to create sequestering supramolecular nano-environments and acting as strongly binding hosts for planar aromatic hydrocarbons via strong π–π and CH−π interactions.

Here, we show a supramolecular strategy based on complexes formed between a cyclophane host and aromatic guests for the efficient generation of delayed fluorescence in the solution state. This work utilizes a through-space intermolecular CT approach17−27 upon encapsulation of carbazole-derived guests inside an arylene bisimide-derived cyclophane. We hypothesized that the arylene bisimide family is prone to generate excellent triplet yields due to multiple carbonyl groups inherently in the nascent molecular design (i.e., El Sayed’s rule).28−33 Further, carbazole derivatives are well-established triplet harnessers reported in the literature,34 whose electron-rich character should be suitable for the CT approach in combination with an electron-poor arylene bisimide cyclophane. To alleviate our hypothesis, we synthesized a novel triplet-generating host biphenyl bisimide (BPCy) and carbazole derivatives (EtCz, PhCz, and ExCz) as guests, which can form emissive CT states close to the locally excited triplet state (3LE) of BPCy and decrease the ΔES–T. Further, upon encapsulation of the relevant guest, the nanocavity of the cyclophane supports reduced vibrational dissipation and oxygen penetration for triplet exciton stabilization, even in diluted conditions in the solution state, as desired for delayed fluorescence emission. Substantiating our hypothesis, the experimental observations revealed delayed fluorescence emission even in dilute solutions with a quantum efficiency of up to 15.6%. Further delayed fluorescence emission proved to be tunable from cyan to yellow by varying the guest. The host–guest titration studies showed association constants up to 104 M–1 for the largest π-conjugated guest. We further experimentally proved our hypothesis through single-crystal X-ray diffraction analysis and solid-state photophysical studies. We elucidated that ISC (kISC = ∼107 s–1) and reverse intersystem (RISC) (kRISC = ∼104 s–1) happened efficiently to the 1CT state by forming the close-lying triplet CT state (3CT) to accomplish delayed fluorescence emission in solution.

Results and Discussion

Synthesis and Characterization

For the synthesis of BPCy cyclophane, one of the starting materials, i.e., (2,5-dihexyl-1,4-phenylene)dimethanamine linker 3, was synthesized by following a literature procedure for a similar compound, as shown in Scheme S1. The other starting material, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride 4, was commercially available and used as purchased. With these two precursor molecules, we synthesized BPCy cyclophane in a one-pot imidization reaction of dianhydride 4 and 1 equiv of diamine 3 in an acetic acid:toluene mixture (1:4 ratio) under diluted conditions (Scheme 1). Initial purification of the BPCy cyclophane was accomplished by silica-gel column chromatography using 1% methanol in dichloromethane as an eluent to remove undesirable oligomeric and polymeric byproducts. In a second purification step, the cyclic dimers were separated from the larger macrocyclic byproduct by recycling gel-permeation chromatography (GPC) in chloroform. BPCy cyclophane was characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analysis. All carbazole-derived guests were synthesized in our own laboratory to avoid isomeric impurity-induced long-lived emission (Scheme S2).

Scheme 1. Synthesis of Biphenyl Bisimide Cyclophane Host (BPCy).

Scheme 1

Open in a new tab

Chemical structures of the investigated carbazole-derived guests (EtCz, PhCz, and ExCz) and reference molecule (BP).

Binding Studies in Solution

First, we performed temperature-dependent NMR studies of BPCy to ensure the absence of isomeric species of BPCy in solution (Figure S1) at room temperature (295 K). According to these studies, we conclude that sufficiently fast rotations of all subunits are possible. Next, we tried UV–vis titration studies to investigate the host–guest complexation in a carbon tetrachloride (CCl4) solution. However, due to the significant overlap between the absorption bands of the host and the guests, we are unable to monitor the absorption change properly upon guest addition. However, we could evaluate the binding constants using 1H NMR titration studies for all guests PhCz, EtCz, and ExCz in a deuterated methylcyclohexane (MCH)/CCl4 solvent mixture at a 1:3 ratio by nonlinear curve fitting (Figures S2–S4, Table 1). The complexation studies reveal that the binding affinity of host BPCy for ExCz is higher than those for the smaller guests EtCz and PhCz, which we attribute to the larger possible π–π contact surface as well as a preference for the nanocavity of BPCy for nonplanar guests. The decreased binding affinity for PhCz could be due to steric hindrances caused by the phenyl group.

Table 1. Binding Constants K (M–1) and Gibbs Free Energies ΔG0 (kJ mol–1) of 1:1 Host–Guest Complexes.

host–guest complex K (M–1)a ΔG0295 (kJ mol–1)b
PhCz@BPCy (7.7 ± 0.5) × 101 –8.2
EtCz@ BPCy (9.3 ± 0.2) × 102 –16.8
ExCz@BPCy (5.5 ± 1.5) × 104 –26.8
Open in a new tab
a

Association constants K determined by fitting of 1H NMR titration data with the program bindfit35 for a 1:1 binding model in a deuterated MCH/CCl4 solvent mixture at a 1:3 ratio at 295 K.

b

Gibbs free energies ΔG0 (295 K) calculated from K according to ΔG0 (295 K) = −RT ln K. All binding constants reported are within 0.5 kJ mol–1 of error in Gibbs free energy. Experimental details, errors, and mole fraction plots are given in the Supporting Information.

Single-Crystal XRD Analysis

To gain structural insight into the guest encapsulation by the host cavity, we grew single crystals of the host and the host–guest complexes by slow vapor diffusion of methanol into the chloroform solution. The pure host molecule (BPCy) crystallizes in the C2/m space group with two cyclophanes and eight asymmetric units per unit cell (Figure S5, Table S1). Single-crystal analysis affirmed that the host forms a box-like nanocavity with a centroid-to-centroid distance of 8.8 Å between the two biphenyl bisimide units and a distance between two linker units of 13.1 Å, making the host ideal for guest encapsulation via π–π and CH−π interactions (Figure S5). The host–guest complex EtCz@BPCy crystallizes in the P21/c space group with two host–guest complexes and four asymmetric units in the unit cell (Figure S6, Table S2). Crystallographic analysis of the host–guest complex EtCz@BPCy uncovered that the guest is stabilized inside the host cavity through two π–π interactions and four CH−π interactions, the latter with distances of 3.1, 3.2, 2.7, and 4.1 Å. We hypothesize that such a multipoint embedment is beneficial for the reduction of vibrational dissipation in the photoexcited state (Figure 1a–c). Importantly, for the creation of CT states, there is a π–π contact (centroid-to-centroid) between the electron-deficient host (BPCy) and the electron-rich guest (EtCz) at distances of 3.7 and 4.05 Å, respectively (Figure 1a). A similar crystal structure was observed for the PhCz guest (Figure S7 and Table S3). Here, the extra phenyl ring caused a looser complexation between the host and guest, which is also reflected in our binding study (Figure S7). Notably, the relatively weak CT interaction inside the nanocavity is aiding in the spatial separation of the occupied and unoccupied frontier molecular orbitals between the donor and acceptor, which is a prerequisite for fast RISC by reducing the ΔES–T. Theoretical calculations corroborate this hypothesis and show spatially separated highest-occupied (HOMO) and lowest-unoccupied (LUMO) molecular orbitals in the optimized structures. Thus, the HOMO is located on the guest, and the degenerate LUMO and LUMO + 1 are located on the host cyclophane for all of the host–guest complexes (Figures 1d and S8). The computed vertical singlet excited states for EtCz@BPCy confirm the donor–acceptor CT in the host–guest complex (Figure S8).

Figure 1.

Figure 1

Open in a new tab

Single-crystal X-ray analysis of the EtCz@BPCy complex: the front view of the host–guest complex showing stabilization of the EtCz guest inside the BPCy cyclophane via (a) π–π and (b) CH−π interactions. The top view (c) of EtCz@BPCy uncovers π–π interactions as an important prerequisite for CT. (d) Optimized geometry and frontier molecular orbitals of EtCz@BPCy. HOMO: localized on EtCz (yellow/blue). Degenerate LUMO and LUMO+1: localized on BPCy (orange/green). Hydrogen atoms are removed for clarity.

Spectroscopic Studies of Individual Molecules in Solution

First, we recorded UV–vis absorption spectra of the cyclophane- and carbazole-derived guests in CCl4 solutions. BPCy cyclophane shows a broad π–π* absorption band with a maximum of 312 nm, and the carbazole-derived guests EtCz and PhCz show absorption bands with S0–S2 transition in the range up to 300 nm with maxima at 295 and 297 nm, respectively, and S0–S1 transitions from 300 to 360 nm (with maxima at 342 and 347 nm for EtCz and PhCz, respectively), along with vibrational progression (Figure 2a). The absorption spectra of ExCz are more bathochromically shifted compared to EtCz due to the increased conjugation length, showing a maximum for the S0–S1 absorption band at 372 nm (Figure 2a). Next, we examined the underlying locally excited singlet (1LE) and triplet (3LE) states of BPCy and carbazole-derived guests in CCl4 solution ([c] = 5 × 10–5 M) at 295 and 77 K (Figures 2b and S9). At 295 K, we observed only one emission band for BPCy cyclophane, spanning from 350 to 700 nm with a maximum at 400 nm (λex = 320 nm) (Figure 2b). Interestingly, at 77 K, a new low energy emission band is seen at λmax = 534 nm, along with the higher energy emission band at λmax = 400 nm (λex = 320 nm) (Figure 2b). In the delayed emission spectrum (λex = 320 nm, delay time = 0.1 ms), only the low energy emission band was observed, which suggests the short-lived nature of the former high energy emission (λmax = 400 nm) and the long-lived nature of the latter low energy emission band (Figure 2b). The delayed emission spectra and lifetime decay profile helped us to identify that the emission with a maximum of 400 nm originates from the singlet state, while the emission band with a maximum of 534 nm can be ascribed to the 3LE triplet state (Figure 2b, Table S4). Similarly, we experimentally determined the triplet levels (3LE) of the guest derivatives and found that 3LE for EtCz and PhCz are almost the same, with maxima at 410 and 430 nm, respectively, along with strong vibrational progression (λex = 320 nm) (Figure S9). The 3LE for ExCz is further red-shifted (λmax = 525 nm) compared to EtCz, due to the extended conjugation length of the chromophore (Figure S9). We anticipate that the underlying 3LE of the cyclophane host and the guest are prerequisites for achieving delayed fluorescence emission from the singlet CT state for efficient RISC by decreasing the ΔES–T.

Figure 2.

Figure 2

Open in a new tab

Spectroscopic studies in solution: (a) UV–vis absorption spectra of BPCy (host molecule), PhCz, EtCz, and ExCz (guest molecules) in CCl4 at 295 K. (b) Steady-state emission of BPCy cyclophane at 295 and 77 K and delayed emission spectra at 77 K showing 1LE emission at 295 K and dual 1LE and 3LE emission at 77 K of the individual host and guest (λex = 320 nm, delay time = 0.1 ms) in CCl4 (c[BPCy] = 5 × 10–5 M). (c) Zoomed absorption spectra of BPCy cyclophane (black line) with increasing equivalent (0 to 140 equiv) of EtCz guest showing the formation of a CT band upon host–guest complexation in CCl4 at 295 K (c[BPCy] = 5 × 10–5 M). (d) Emission spectra of the EtCz@BPCy complex upon changing the solvent polarity, showing the bathochromic shift in the emission spectra (λex = 405 nm) in CCl4 and CHCl3 at 295 K (c[BPCy] = 5 × 10–5 M). (e) Emission and (f) excitation spectra of the PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy host–guest complexes showing emissive 1CT state formation and its origin upon host–guest complexation (λex = 405 nm, λobs = 500, 520, and 540 nm for PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy host–guest complex) in an MCH/CCl4 (1:3) solvent mixture at 295 K (c[BPCy] = 5 × 10–5 M, c[PhCz] = 5 × 10–2 M, c[EtCz] = 9 × 10–3 M, and c[ExCz] = 7 × 10–4 M).

Spectroscopic Studies of Host–Guest Complexes in Solution

Further, we performed the photophysical studies of the host–guest complexes in the solution state. For these studies, it was very helpful that new broad CT bands arise at wavelengths above the lowest energy absorption bands of the individual molecules, e.g., between 350 and 450 nm, upon addition of EtCz to BPCy (Figures 2c and S10). Accordingly, selective excitation of the complexes becomes possible even in the presence of a larger excess of guest molecules (as desired for high degrees of host complexation) in the solutions. The corresponding emission spectra also showed a new broad emission band in the 450–700 nm region (λex = 405 nm) (Figure S10). These results indicate the involvement of CT states in the formation of host–guest complexes (Figures 2c and S10). Further, excitation spectra monitored at 520 nm reveal a clear CT band in the 350 to 470 nm range, confirming the origin of the CT emission (Figure S10). The bathochromic shift of the broad emission band upon changing the solvent polarity from nonpolar CCl4 to polar CHCl3 provides further evidence for the CT nature of the emission (λex = 405 nm) (Figure 2d). Next, we studied the photoluminescence of the host–guest complex in an MCH/CCl4 (1:3) solvent mixture upon selective excitation of the CT band at 405 nm where the individual host and guest have no absorption (Figure 2a,c). Upon complexation with BPCy, all the guests (PhCz, EtCz, and ExCz) formed emissive CT states with maxima of 490, 510, and 540 nm, respectively (Figure 2e). Corresponding broad excitation spectra (bands ranging from 350 to 500 nm) (λobs = 500, 520, and 560 nm for PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy complexes, respectively) explained the origin of the CT emission (Figure 2f). The experimental results proved our hypothesis and showcased that host–guest complexation can establish emissive 1CT states, which motivated us toward further investigations of the triplet-induced functional properties.

Delayed Fluorescence Properties of Host–Guest Complexes in Solution

In order to evaluate the triplet contribution to the emissive 1CT state, we performed a series of photophysical studies under aerated and inert atmospheric conditions (Figures 3, S11, and S12). Intriguingly, we noticed that both emission intensity and time-resolved emission half-life increase significantly upon argon purging (λex = 405 nm) (Figures 3, S11, and S12). The emission intensity of the 1CT state increases 2.6, 6.5, and 2.4 times for the PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy complexes, respectively, indicating the involvement of the triplet state in the 1CT emission (Figures 3a, S11, and S12). Further, the similarity of the steady-state emission and the delayed emission spectra (delay time = 50 μs) under an inert atmosphere corroborated the triplet contribution to the 1CT emissions (λex = 405 nm) (Figures 3b, S11, and S12). This experimental finding revealed that the 1CT state has two lifetime components: prompt fluorescence with a nanosecond time scale and longer-lived delayed fluorescence with a microsecond time scale. Therefore, we obtained prompt fluorescence with a lifetime of 60 to 80 ns and long-lived delayed fluorescence with 50 to 140 μs, attested to the delayed fluorescence emission from the 1CT state (λex = 405 nm) (Figures 3c,d, S11, and S12). The delayed fluorescence lifetimes are 141, 122, and 58 μs for PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy, respectively, in an argon atmosphere (λex = 405 nm, λobs = 500, 520, and 540 nm for PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy host–guest complexes, respectively) (Figures 3d, S11, and S12, and Tables S5 and S6). However, we have not detected any delayed fluorescence contribution in the delayed lifetime decay profile under aerated conditions (Figures 3d, S11, and S12). The experimentally observed photoluminescence quantum yields of the PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy are 5.5, 10.3, and 15.6% (upon selective excitation of the CT band), where the quantum yield of the individual host and guests are <1% (Table 2). Under an argon atmosphere, we experimentally determined efficient ISC (kISC) and RISC (kRISC) rates of ∼107 and ∼104 s–1, respectively, responsible for efficient delayed fluorescence in solution (Table 2).36 Notably, we have not observed any 1CT emission upon mixing the reference molecule (BP) and the carbazole-derived guests even at very high concentrations, revealing the importance of the guest encapsulation in the BPCy supramolecular nanocavity for delayed fluorescence emission (Figure S13). The experimental evidence proved that our hypothesis toward the molecular design of the biphenyl bisimide host and carbazole-derived guest is adequate, and we envisage that the underlying triplet state of the host and guest, close to the emissive CT state of the host-guest complex, was essential to achieve delayed fluorescence emission from the emissive CT state.

Figure 3.

Figure 3

Open in a new tab

Delayed fluorescence studies of EtCz@BPCy host–guest complexes in MCH/CCl4 (1:3) solvent mixture at 295 K: (a) steady-state emission under air and argon atmospheres, showing a significant increase of emission intensity upon argon purging, suggesting the triplet contribution of the 1CT emission. (b) Normalized steady-state and delayed emission spectra under air and argon conditions showing similarity of the delayed emission to the steady-state emission (delay time = 50 μs), indicating the delayed nature of 1CT emission. (c) Prompt and (d) delayed fluorescence lifetime decay profiles under air and Ar atmospheres, explaining the prompt and delayed fluorescence contribution in the total 1CT emission. (For all cases, λex = 405 nm, λobs = 520 nm, c[BPCy] = 3.2 × 10–5 M, c[EtCz] = 5.8 × 10–3 M.)

Table 2. Photoluminescence Decay Times and Rates for Intersystem Crossing (kISC) and Reverse Intersystem Crossing (kRISC) of the 1:1 Host–Guest Complexes under an Argon Atmosphere.

host–guest complex τPF (ns) τDF (μs) ΦPF (%) ΦDF (%) ΦISC (%)a kISC (s–1)a kRISC (s–1)a
PhCz@BPCy 57.1 141 2.2 3.4 61 1.1 × 107 1.8 × 104
EtCz@BPCy 73.7 122 1.6 8.7 85 1.2 × 107 5.3 × 104
ExCz@BPCy 61.7 51.1 6.5 9.1 59 1.0 × 107 4.7 × 104
Open in a new tab
a

ΦISC, kISC, and kRISC were calculated from prompt and delayed fluorescence quantum yield and lifetime for the 1:1 host–guest complex according to the following equations36 ΦISC = ΦDF/(ΦDF + ΦPF), kISC (s–1) = ΦISC/τPF, and kRISC (s–1) = (ΦDF/ΦPF)/(ΦISC × τDF).

It is also interesting to mention that the lifetime of the prompt fluorescence emission of these host–guest complexes is surprisingly high, with a significant increase under inert conditions (Figures 3d, S11, and S12, and Table S5). This is explained by the pronounced separation of the HOMO of the guest and the LUMO of the host, leading to the meagre oscillator strength of the CT transition, which affords a small radiative rate. In such a situation, fluorescence becomes only observable if the nonradiative decay by internal conversion is slow, which we attribute to the rigid nanoenvironment created by the host–guest complexation, leading to restricted molecular motion. For such long-lived prompt fluorescence, quenching by molecular oxygen and concomitantly increased fluorescence decay times in argon-purged solutions are indeed quite common,37,38 thereby rationalizing the observed increase in fluorescence decay time seen in Figure 3c.

Thermally Activated Delayed Fluorescence Properties of the Host–Guest Complex in the Solid State

To understand the nature of the delayed fluorescence emission and the underlying mechanism, we also studied the host–guest complexes in the solid state of amorphous films. Further, we performed a temperature-dependent study and observed that the emission intensity and lifetime decrease upon decreasing the temperature from 295 to 77 K for both EtCz@BPCy and PhCz@BPCy complexes, which indicates that the delayed fluorescence mechanism is a thermally activated process (Figures 4a,b, S14, and S15) (λex = 405 nm, λobs = 500 and 520 nm for PhCz@BPCy and EtCz@BPCy host–guest complexes, respectively). We also noticed that the nanosecond component increases upon decreasing the temperature, confirming the impact of rigidification on the prompt (Figures S14 and S15, and Tables S7 and S8) fluorescence from the 1CT state (λex = 405 nm, λobs = 500 and 520 nm for PhCz@BPCy and EtCz@BPCy host–guest complexes, respectively). The absolute photoluminescence quantum efficiencies of the PhCz@BPCy, EtCz@BPCy, and ExCz@BPCy complexes in the solid state are 17, 22, and 16% (λex = 405 nm), respectively, at room temperature (295 K).

Figure 4.

Figure 4

Open in a new tab

Temperature-dependent (a) steady-state emission and (b) delayed fluorescence lifetime decay profiles of EtCz@BPCy in the solid state, showing a decrease in emission intensity and lifetime upon decreasing the temperature as expected for the TADF process. (For all cases, λex = 405 nm, λobs = 520 nm.) (c) Simplified Jablonski diagram showing the plausible excited-state formation upon host–guest complexation and the mechanism of delayed fluorescence emission.

Interestingly, when recording delayed emission spectra upon excitation at 405 nm for the delay time of 1 ms, a new long-lived emission band with maxima at 550 and 560 nm, respectively, is observed for both EtCz@BpCy and PhCz@BpCy complexes at 77 K (Figures S14 and S15). The lifetime of these emission bands is 325 and 322 ms, respectively, for PhCz@BPCy and EtCz@BPCy (Figures S14 and S15, and Table S9) (λex = 405 nm, λobs = 560 nm). We hypothesize that this broad and long-lived emission band is the phosphorescence band, which is 3CT in nature. The 3CT phosphorescence band of the host–guest complex should be vibronically coupled with the 3LE of the BPCy, which results in efficient RISC (Figure 4c).39

Conclusions

In conclusion, we synthesized a biphenyl bisimide-derived cyclophane and different carbazole-derived aromatic guests for the demonstration of efficient delayed fluorescence in solution for a supramolecular host–guest system. First, our detailed spectroscopic studies revealed the formation of host–guest inclusion complexes in the MCH/CCl4 solvent mixture under dilute conditions with moderate-to-high association constants. As expected from our design of the HOMO and LUMO levels of host and guest species, the formation of emissive CT states was observed upon guest encapsulation inside the cyclophane nanocavity. Moreover, the host’s nanocavity upon encapsulation of the suitable guest helped to produce color-tunable, efficient delayed fluorescence emission (QY = 15.6%) in solution via reducing vibrational dissipation as a prerequisite for triplet exciton stabilization. The detailed spectroscopic characterization supported by structural insights from single-crystal X-ray analysis enabled us to understand the non-covalent host–guest complexation, guest stabilization, and CT-state formation via CH−π and π–π interactions. Further, theoretical investigations demonstrated the role of spatially separated molecular orbitals of the donor and acceptor in non-covalent arrangements for TADF emission. Finally, detailed temperature-dependent spectroscopic studies in the solid state attested to the TADF emission and provided an understanding of the decay mechanism for the photoexcited states.

The current report shows that tailored supramolecular nanoenvironments can exert control of emission (quantum yield, lifetime) beyond simple fluorescence, as reported in our previous research for perylene bisimide cyclophanes16 or for natural green fluorescent protein40 and a variety of synthetic analogues.41 Here, we showed that efficient triplet exciton harnessing via TADF toward emissive 1CT state formation is also possible in solution by embedding suitable guests in structurally and electronically tailored supramolecular nano-environments. We envision that our herein proposed concept can be further elaborated with new arylene bisimide-derived cyclophanes toward exciting triplet-induced photofunctions, including circularly polarized delayed luminescence.

Acknowledgments

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 787937; recipient F.W.). We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III, and we would like to thank Dr. Johanna Hakanpää for assistance in using P11. Beamtime was allocated for proposal I-20230262.

Data Availability Statement

All the additional spectroscopic, NMR titration, and all additional data underlying this study are openly available in Zenodo at 10.5281/zenodo.11652166.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c07730.

  • Experimental methods, synthetic procedures and characterization of new compounds, spectroscopic studies, binding constant determination, and single-crystal X-ray analysis (PDF)

Author Contributions

All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ja4c07730_si_001.pdf (5.6MB, pdf)

References

  1. Hirata S. Molecular Physics of Persistent Room Temperature Phosphorescence and Long-Lived Triplet Excitons. Appl. Phys. Rev. 2022, 9, 011304. 10.1063/5.0066613. [DOI] [Google Scholar]
  2. Xu Y.; Xu P.; Hu D.; Ma Y. Recent progress in hot exciton materials for organic light-emitting diodes. Chem. Soc. Rev. 2021, 50, 1030–1069. 10.1039/D0CS00391C. [DOI] [PubMed] [Google Scholar]
  3. Wong M. Y.; Zysman-Colman E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1605444. 10.1002/adma.201605444. [DOI] [PubMed] [Google Scholar]
  4. Sasikumar D.; John A. T.; Sunny J.; Hariharan M. Access to the triplet excited states of organic chromophores. Chem. Soc. Rev. 2020, 49, 6122–6140. 10.1039/d0cs00484g. [DOI] [PubMed] [Google Scholar]
  5. Uoyama H.; Goushi K.; Shizu K.; Nomura H.; Adachi C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234–238. 10.1038/nature11687. [DOI] [PubMed] [Google Scholar]
  6. Zhang Q.; Kuwabara H.; Potscavage W. J.; Huang S.; Hatae Y.; Shibata T.; Adachi C. Anthraquinone-Based Intramolecular Charge-Transfer Compounds: Computational Molecular Design, Thermally Activated Delayed Fluorescence, and Highly Efficient Red Electroluminescence. J. Am. Chem. Soc. 2014, 136, 18070–18081. 10.1021/ja510144h. [DOI] [PubMed] [Google Scholar]
  7. Zhou H.-Y.; Zhang D.-W.; Li M.; Chen C.-F. A Calix [3]-acridan-Based Host–Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence. Angew. Chem., Int. Ed. 2022, 61, e202117872 10.1002/anie.202117872. [DOI] [PubMed] [Google Scholar]
  8. Xue N.; Zhou H. Y.; Han Y.; Li M.; Lu H. Y.; Chen C.-F. A general supramolecular strategy for fabricating full-color-tunable thermally activated delayed fluorescence materials. Nat. Commun. 2024, 15, 1425. 10.1038/s41467-024-45717-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Roy D.; Paul S.; Dasgupta J. Photocatalytic Terminal C-C Coupling Reaction Inside Water Soluble Nanocages. Angew. Chem., Int. Ed. 2023, 62, e202312500 10.1002/anie.202312500. [DOI] [PubMed] [Google Scholar]
  10. Li D.; Lu F.; Wang J.; Hu W.; Cao X.; Ma X.; Tian H. Amorphous Metal-Free Room-Temperature Phosphorescent Small Molecules with Multicolor Photoluminescence via a Host-Guest and Dual-Emission Strategy. J. Am. Chem. Soc. 2018, 140, 1916–1923. 10.1021/jacs.7b12800. [DOI] [PubMed] [Google Scholar]
  11. Garci A.; Abid S.; David A. H. G.; Jones L. O.; Azad C. S.; Ovalle M.; Brown P. J.; Stern C. L.; Zhao X.; Malaisrie L.; Schatz G. C.; Young R. M.; Wasielewski M. R.; Stoddart J. F. Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes. J. Am. Chem. Soc. 2023, 145, 18391–18401. 10.1021/jacs.3c04213. [DOI] [PubMed] [Google Scholar]
  12. Voll C.-C. A.; Markopoulos G.; Wu T. B.; Welborn M.; Engelhart J. U.; Rochat S.; Han G. G. D.; Sazama G. T.; Lin T.-A.; Van Voorhis T.; Baldo M. A.; Swager T. M. Lock-and-Key Exciplexes for Thermally Activated Delayed Fluorescence. Org. Mater. 2020, 02, 001–010. 10.1055/s-0039-3402059. [DOI] [Google Scholar]
  13. Chen H.; Roy I.; Myong M. S.; Seale J. S. W.; Cai K.; Jiao Y.; Liu W.; Song B.; Zhang L.; Zhao X.; Feng Y.; Liu F.; Young R. M.; Wasielewski M. R.; Stoddart J. F. Triplet–Triplet Annihilation Upconversion in a Porphyrinic Molecular Container. J. Am. Chem. Soc. 2023, 145, 10061–10070. 10.1021/jacs.2c13846. [DOI] [PubMed] [Google Scholar]
  14. Lin C.-Y.; Hsu C.-H.; Hung C.-M.; Wu C.-C.; Liu Y.-H.; Shi E. H.-C.; Lin T.-H.; Hu Y. C.; Hung W.-Y.; Wong K.-T.; Chou P.-T. Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient OLEDs. Nat. Chem. 2024, 16, 98–106. 10.1038/s41557-023-01357-0. [DOI] [PubMed] [Google Scholar]
  15. Barnes J. C.; Juricek M.; Strutt N. L.; Frasconi M.; Sampath S.; Giesener M. A.; McGrier P. L.; Bruns C. J.; Stern C. L.; Sarjeant A. A.; Stoddart J. F. ExBox: A Polycyclic Aromatic Hydrocarbon Scavenger. J. Am. Chem. Soc. 2013, 135, 183–192. 10.1021/ja307360n. [DOI] [PubMed] [Google Scholar]
  16. Spenst P.; Würthner F. A Perylene Bisimide Cyclophane as a “Turn-On” and “Turn-Off” Fluorescence Probe. Angew. Chem., Int. Ed. 2015, 54, 10165–10168. 10.1002/anie.201503542. [DOI] [PubMed] [Google Scholar]
  17. Chen L.; Li C.; Fu E.; Li M.; Kuboi Y.; Li Z.-Y.; Chen Z.; Chen J.; Liu X.; Tang X.; Frédéric L.; Maurel F.; Adachi C.; Mathevet F.; Zhang S. A donor-acceptor cage for thermally activated delayed fluorescence: toward a new kind of TADF exciplex emitters. ACS Mater. Lett. 2023, 5, 1450–1455. 10.1021/acsmaterialslett.3c00310. [DOI] [Google Scholar]
  18. Tang X.; Cui L.-S.; Li H.-C.; Gillett A. J.; Auras F.; Qu Y.-K.; Zhong C.; Jones S. T. E.; Jiang Z.-Q.; Friend R. H.; Liao L.-S. Highly efficient luminescence from space-confined charge-transfer emitters. Nat. Mater. 2020, 19, 1332–1338. 10.1038/s41563-020-0710-z. [DOI] [PubMed] [Google Scholar]
  19. Jankus V.; Chiang C.-J.; Dias F.; Monkman A. P. Deep Blue Exciplex Organic Light-Emitting Diodes with Enhanced Efficiency; Ptype or E-type Triplet Conversion to Singlet Excitons?. Adv. Mater. 2013, 25, 1455–1459. 10.1002/adma.201203615. [DOI] [PubMed] [Google Scholar]
  20. Kongasseri A. A.; Garain S.; Ansari S. N.; Garain B. C.; Wagalgave S. M.; Singh U.; Pati S. K.; George S. J. Unleashing Ambient Triplet Harvesting Pathways in Arylene Diimides via Modular, Noncovalent Charge Transfer Interactions. Chem. Mater. 2023, 35, 7781–7788. 10.1021/acs.chemmater.3c01667. [DOI] [Google Scholar]
  21. Barman D.; Annadhasan M.; Bidkar A. P.; Rajamalli P.; Barman D.; Ghosh S. S.; Chandrasekar R.; Iyer P. K. Highly efficient color-tunable organic co-crystals unveiling polymorphism, isomerism, delayed fluorescence for optical waveguides and cell imaging. Nat. Commun. 2023, 14, 6648. 10.1038/s41467-023-42017-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Feng S.; Wang L.; Milián-Medina B.; Meixner A. J.; Kwon M. S.; Park S. Y.; Wannemacher R.; Gierschner J. Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design. Adv. Mater. 2023, 35, 2306678. 10.1002/adma.202306678. [DOI] [PubMed] [Google Scholar]
  23. Park S. K.; Kim J. H.; Ohto T.; Yamada R.; Jones A. O. F.; Whang D. R.; Cho I.; Oh S.; Hong S. H.; Kwon J. E.; Kim J. H.; Olivier Y.; Fischer R.; Resel R.; Gierschner J.; Tada H.; Park S. Y. Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials. Adv. Mater. 2017, 29, 1701346. 10.1002/adma.201701346. [DOI] [PubMed] [Google Scholar]
  24. Zhu W.; Zheng R.; Zhen Y.; Yu Z.; Dong H.; Fu H.; Shi Q.; Hu W. Rational Design of Charge-Transfer Interactions in Halogen-Bonded Co-Crystals toward Versatile Solid-State Optoelectronics. J. Am. Chem. Soc. 2015, 137, 11038–11046. 10.1021/jacs.5b05586. [DOI] [PubMed] [Google Scholar]
  25. Han J.; Yang D.; Jin X.; Jiang Y.; Liu M.; Duan P. Enhanced Circularly Polarized Luminescence in Emissive Charge-Transfer Complexes. Angew. Chem., Int. Ed. 2019, 58, 7013–7019. 10.1002/anie.201902090. [DOI] [PubMed] [Google Scholar]
  26. Tsujimoto H.; Ha D.-G.; Markopoulos G.; Chae H. S.; Baldo M. A.; Swager T. M. Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through Space Charge Transfer. J. Am. Chem. Soc. 2017, 139, 4894–4900. 10.1021/jacs.7b00873. [DOI] [PubMed] [Google Scholar]
  27. Sun L.; Hua W.; Liu Y.; Tian G.; Chen M.; Chen M.; Yang F.; Wang S.; Zhang X.; Luo Y.; Hu W. Thermally Activated Delayed Fluorescence in an Organic Cocrystal: Narrowing the Singlet–Triplet Energy Gap via Charge Transfer. Angew. Chem., Int. Ed. 2019, 58, 11311–11316. 10.1002/anie.201904427. [DOI] [PubMed] [Google Scholar]
  28. Feng W.; Chen D.; Zhao Y.; Mu B.; Yan H.; Barboiu M. Modulation of Deep-Red to Near-Infrared Room-Temperature Charge-Transfer Phosphorescence of Crystalline “Pyrene Box” Cages by Coupled Ion/Guest Structural Self-Assembly. J. Am. Chem. Soc. 2024, 146, 2484–2493. 10.1021/jacs.3c10206. [DOI] [PubMed] [Google Scholar]
  29. Goudappagouda; Manthanath A.; Wakchaure V. C.; Ranjeesh K. C.; Das T.; Vanka K.; Nakanishi T.; Babu S. S. Paintable Room Temperature Phosphorescent Liquid Formulations of Alkylated Bromonaphthalimide. Angew. Chem., Int. Ed. 2019, 58, 2284–2288. 10.1002/anie.201811834. [DOI] [PubMed] [Google Scholar]
  30. Niyas M. A.; Ramakrishnan R.; Vijay V.; Sebastian E.; Hariharan M. Anomalous Halogen-Halogen Interaction Assists Radial Chromophoric Assembly. J. Am. Chem. Soc. 2019, 141, 4536–4540. 10.1021/jacs.8b13754. [DOI] [PubMed] [Google Scholar]
  31. Garain S.; Ansari S. N.; Kongasseri A. A.; Garain B. C.; Pati S. K.; George S. J. Room Temperature Charge-Transfer Phosphorescence from Organic Donor-Acceptor Co-Crystals. Chem. Sci. 2022, 13, 10011–10019. 10.1039/d2sc03343g. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Garain S.; Kuila S.; Garain B. C.; Kataria M.; Borah A.; Pati S. K.; George S. J. Arylene Diimide Phosphors: Aggregation Modulated Twin Room Temperature Phosphorescence from Pyromellitic Diimides. Angew. Chem., Int. Ed. 2021, 60, 12323–12327. 10.1002/anie.202101538. [DOI] [PubMed] [Google Scholar]
  33. Sun M.-J.; Anhalt O.; Sárosi M. B.; Stolte M.; Würthner F. Activating Organic Phosphorescence via Heavy Metal−π Interaction Induced Intersystem Crossing. Adv. Mater. 2022, 34, 2207331. 10.1002/adma.202207331. [DOI] [PubMed] [Google Scholar]
  34. Chen C.; Chi Z.; Chong K. C.; Batsanov A. S.; Yang Z.; Mao Z.; Yang Z.; Liu B. Carbazole isomers induce ultralong organic phosphorescence. Nat. Mater. 2021, 20, 175–180. 10.1038/s41563-020-0797-2. [DOI] [PubMed] [Google Scholar]
  35. Bindfit (Supramolecular). 2020, http://supramolecular.org.
  36. Dias F. B.; Penfold T. J.; Monkman A. P. Photophysics of thermally activated delayed fluorescence molecules. Methods Appl. Fluoresc. 2017, 5, 012001. 10.1088/2050-6120/aa537e. [DOI] [PubMed] [Google Scholar]
  37. Pigulski B.; Shoyama K.; Sun M.-J.; Würthner F. Fluorescence Enhancement by Supramolecular Sequestration of a C54-Nanographene Trisimide by Hexabenzocoronene. J. Am. Chem. Soc. 2022, 144, 5718–5722. 10.1021/jacs.2c00142. [DOI] [PubMed] [Google Scholar]
  38. Urban M.; Marek-Urban P. H.; Durka K.; Luliński S.; Pander P.; Monkman A. P. TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone Triarylboron Acceptor. Angew. Chem., Int. Ed. 2023, 62, e202217530 10.1002/anie.202217530. [DOI] [PubMed] [Google Scholar]
  39. Gibson J.; Monkman A. P.; Penfold T. J. The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules. ChemPhysChem 2016, 17, 2956–2961. 10.1002/cphc.201600662. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Zimmer M. Green Fluorescent Protein (GFP): Applications, Structure, and Related Photophysical Behavior. Chem. Rev. 2002, 102, 759–782. 10.1021/cr010142r. [DOI] [PubMed] [Google Scholar]
  41. Tsai M.-S.; Ou C.-L.; Tsai C.-J.; Huang Y.-C.; Cheng Y.-C.; Sun S. S.; Yang J.-S. Fluorescence Enhancement of Unconstrained GFP Chromophore Analogues Based on the Push–Pull Substituent Effect. J. Org. Chem. 2017, 82, 8031–8039. 10.1021/acs.joc.7b01260. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ja4c07730_si_001.pdf (5.6MB, pdf)

Data Availability Statement

All the additional spectroscopic, NMR titration, and all additional data underlying this study are openly available in Zenodo at 10.5281/zenodo.11652166.

Articles from Journal of the American Chemical Society are provided here courtesy of American Chemical Society

RESOURCES