Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence Congdi Shang†, Gang Wang†, Yu-Chen Wei†, Qingwei Jiang, Ke Liu, Meiling Zhang, Yi-Yun Chen, Xingmao Chang, Fengyi Liu, Shiwei Yin, Pi-Tai Chou and Yu Fang Congdi Shang† College of Food Science and Engineering, Northwest A&F University, Shaanxi, Yangling 712100 Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 †C. Shang, G. Wang, and Y.-C. Wei contributed equally to this work.Google Scholar More articles by this author , Gang Wang† Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 †C. Shang, G. Wang, and Y.-C. Wei contributed equally to this work.Google Scholar More articles by this author , Yu-Chen Wei† Department of Chemistry, Taiwan University, Taipei 10617 †C. Shang, G. Wang, and Y.-C. Wei contributed equally to this work.Google Scholar More articles by this author , Qingwei Jiang Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Ke Liu Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Meiling Zhang Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Yi-Yun Chen Department of Chemistry, Taiwan University, Taipei 10617 Google Scholar More articles by this author , Xingmao Chang Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Fengyi Liu Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Shiwei Yin Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author , Pi-Tai Chou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Taiwan University, Taipei 10617 Google Scholar More articles by this author and Yu Fang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi’an 710062 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100871 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail We have fabricated a new perylene bisimide (PBI) folda-dimer ( BPBI-CB-1) by tethering two PBI moieties to the ortho-carbon positions of a carborane unit. The synthesized compound adopted distinct configurations in different solvents with dual emissions as its characteristic. The two PBI moieties in the molecule appeared either in a weakly interacted, monomer-like state or brought into close π–π contact with each other, forming an interacted stacking state. The equilibrium between these two states was governed by the nature of solvents and testing temperature. Spectroscopic and theoretical studies concluded that dual emission bands originated from intramolecular monomer-like and stacking states, respectively. Remarkably, in a solvent like 1,2-dichloroethane (DCE), both emission intensities increased with rising temperatures. The positive temperature response of the monomer emission was ascribed to an increased amount of monomer-like population, owing to its endothermic energy state, while the excimer emission was rationalized by increased population of the bright exciton state, resulting in an increased emission yield that compensated the diminished population of the stacking state. To our knowledge, this is the first report on positive temperature-responsive dual emissions associated with the synergism of intramolecular intersubunit aggregation/dissociation and excimer transformation. Download figure Download PowerPoint Introduction Exploration of the structure–functionality relationship is an eternal topic of chemical science. One of the challenging issues is the correlation among various specific configurations of a compound endowed with relatively diversified chemical/physical properties such that multifunctionality could be accessed in a single molecular unit. Driven by these issues, research endeavors on tuning photophysical properties of organic conjugated structures via configurational change have received intense attention.1–4 One popular approach is the exploitation of steric hindrance effect via anchoring bulky substituents or rigid units into molecules to increase the hindrance, stabilizing an otherwise unaffordable configuration.5–8 Meanwhile, the “rotational isomerism” is another strategy to fine-tune the configuration,9,10 in which two functional moieties are connected by a designated bridge, which could be C–C single, double, triple bonds, or even extended structures.11–15 Accordingly, mutual arrangement of the two moieties can be altered via rotation,16,17 bending,18 and structural reorganization19–21 of the bridge such that various thermally stable configurations may exist, which purportedly exhibit different properties in terms of physical or photophysical behaviors.22–25 While each of the above-mentioned approaches has been widely studied, the integration of multiple strategies into a single molecular composite, to our knowledge, has not been fully explored. In a dual moiety featured molecular dyad, the chemical nature of both functional moieties and linkers plays a crucial role in determining the configurational diversity, and hence, the associated physical and photophysical properties. In this regard, perylene bisimide (PBI) has been widely used as a functional unit to create molecular dyads. This mainly stems from its outstanding properties, such as high fluorescence quantum yield (QY), high photochemical stability, and multiple modification sites.26–29 However, PBI-based dyes are subject to dimerization and/or aggregation, which on the one hand, could extend the optoelectronic properties such as the emission Stokes shift. And on the other hand, it becomes a shortcoming when used in a monomeric state for applications such as cytometry, microscopy, sensor, photon concentrators,30–32 and so on. In fact, various covalently linked PBI dyads were reported to have face-to-face and side-by-side interactions;33–40 among these systems are the PBI molecular dimers that demonstrate rather complicated photophysical properties due to their subtle intersubunit electronic interactions in the excited state. The associated fundamental challenge has been well described by the excitonic coupling theory developed by Kasha et al.41 and extended by others.42–44 According to the theory, the interaction between the transition dipole moment vectors associated with the two fluorescent moieties causes a two-fold splitting.45–48 The relative intensities of the two absorptions depend on the mutual alignment of the respective chromophores.43 For the two transition-dipole alignments in line, one or the other of the two absorption transitions is forbidden due to accidental cancellation of the relevant transition dipole moment vectors, resulting in a non-emissive state (dark state) and a highly emissive one (bright state).49,50 The two packing alignments correspond to the well-known H- and J-type aggregates of the fluorescent dyad.51–53 Thus, ingeniously regulating the transformation of the two aggregation forms is of paramount importance in probing the structure–functionality relationship of PBIs.54–57 Herein, we created two PBI moieties, connected chemically to a carborane unit with phenylene ethynylene as linkers, which resided mutually at the ortho-carbon positions ( BPBI-CB-1; Figure 1). The designed structure is believed to separate the two PBI units at a suitable distance owing to the steric configuration of the o-carborane, endowing them with partial rotational freedom.58 The bay position modified PBI derivative was selected as the chromophore not only because of its aforementioned superior fluorescence properties but also because of its acceptable solubility in common organic solvents, avoiding the strong tendency of forming intermolecular aggregates. This design enabled us to focus mainly on the intramolecular excitonic interactions of two PBI moieties and their interplay between two different configurations (vide infra). For controls, two other carborane derivatives of PBI, namely, BPBI-CB-2 and PBI-CB-ref (Figure 1), were synthesized, which in theory, have no possibility of forming PBI aggregate intramolecularly. As a result, BPBI-CB-1 showed dual fluorescence in suitable solvents where intermolecular aggregates did not exist. Remarkably, both emission bands exhibited a positive temperature effect, which was an ultra-rare event. Comprehensive spectroscopy, dynamics, and theoretical approaches unveiled the nature of dual emissions and the origins of anomalous photophysical properties. Detailed results and relevant discussions are elaborated in the following sections. Figure 1 | The chemical structures of BPBI-CB-1, BPBI-CB-2, and PBI-CB-ref. Download figure Download PowerPoint Experimental Methods 3,4,9,10-Perylene-tetracarboxylic acid diimide was purchased from Adamas Reagent Co. Ltd. (98%; Shanghai, China). Imidazole (99%) and copper(I) iodide (98%) were purchased from Shanghai Titan Scientific Co. Ltd. (Shanghai, China). Bromine (≥97%), trimethylsilylacetylene (98%), and bis(triphenyl-phosphine)palladium(II) dichloride (98%) were purchased from TCI Shanghai (Shanghai, China). Toluene (TOL), tetrahydrofuran (THF), and diisopropylamine (DIPA) were freshly distilled from sodium benzophenone ketyl under a nitrogen atmosphere before use. Other chemicals used were of the highest grade commercially available and did not require further purification. Water used in this work was obtained from a Milli-Q reference system (Massachusetts, USA). 1H NMR, 11B NMR, and 13C NMR spectra were obtained on a Bruker AV 600 NMR spectrometer (Bruker, Karlsruhe, Germany). The high-resolution mass spectra (HRMS) were acquired in atmospheric pressure chemical ionization (APCI) sources using a Bruker maxis Ultra High Resolution Time of Flight (UHR-TOF) mass spectrometer (Bruker, Karlsruhe, Germany). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) data were collected on a MALDI-TOF Bruker maxis mass spectrometer in electrospray ionization (ESI) positive mode (Bruker, Karlsruhe, Germany). Steady-state fluorescence excitation and emission spectra were obtained using a time-correlated single-photon counting (TCSPC) fluorescence spectrometer (FLS920; Edinburgh Instruments Ltd., Livingston, UK) with a xenon lamp as the light source at room temperature. Lifetimes were measured on the same system using an EPL-485 picosecond pulsed diode laser (Edinburgh Instruments Ltd., Livingston, UK) as the light source. The absolute fluorescence QYs were measured on the Hamamatsu C9920 (Hamamatsu Photonics K.K., Hamamatsu, Japan) Quantum Efficiency Instrument. UV–vis absorption spectra were performed on a JASCO 770V spectrometer (JASCO, Tokyo, Japan) with a spectral bandwidth of 1 nm and a scan rate of 400 nm min−1 using quartz cell cuvettes of 10 cm. The ultrafast spectroscopic studies were conducted on another TCSPC system (OB-900 L lifetime spectrometer, Edinburgh Instruments Ltd., Livingston, UK). The excitation light source at 800 nm from the Ti-Sapphire oscillator (82 MHz, Spectra-Physics, Milpitas, USA) was pulse-selected to reduce its repetition rate to typically 0.8–8 MHz and then used to generate second-harmonic (400 nm). The fluorescence was collected at a right angle with respect to the pump beam path and passed through a polarizer, which set the polarization at a magic angle (54.7°) to eliminate anisotropy. Similar data analysis and fitting procedures were compared with the previous TCSPC system were applied. The temporal resolution, after partial removal of the instrumental time broadening, was ∼20 ps. Results and Discussion As shown in Figure 1, all the title PBI derivatives contain a common component of o-carborane that was intentionally selected to direct the mutual orientations of the relevant PBI moieties, as well as to ensure solubility of the molecular system. Detailed synthesis and characterization of BPBI-CB-1 and the two references, BPBI-CB-2 and BPBI-CB-ref, are provided in Supporting Information Scheme S1 and Supporting Information Characterization Data section. To gain the basic photophysical properties of BPBI-CB-1, steady-state UV/vis absorption and fluorescence emission measurements were conducted in a wide variety of solvents ranging from less polar TOL to polar solvents like 2-methoxyethyl ether (MOE). The results depicted in Figure 2a show dual emission, specified as F1 (short wavelength, λmax ∼ 560 nm) and F2 (long wavelength, λmax ∼ 640 nm) bands with different ratiometers in different solvents, even though the spectra were recorded at a concentration as low as 2 × 10−7 mol·L−1 at room temperature. The spectra of the controls, BPBI-CB-2 and BPBI-CB-ref, in the same solvents were also recorded; the acquired spectra are shown in Supporting Information Figures S1 and S2, respectively. Based on the comprehensive solvent-dependent studies (for details, see the Supporting Information), we found that in 1,1,2,2-tetrachloroethane (TCE) and 1,2-dichloroethane (DCE), the target compound BPBI-CB-1 demonstrated unique solvent-dependent dual emission behaviors; therefore, these two solvents were chosen for conducting further research to explore the structure-functionality relationship. To ascertain the intramolecular effect of the origin of the observed dual emission of BPBI-CB-1 in the examined solvents, concentration-dependent fluorescence spectra of the ensemble in TCE and DCE were recorded at room temperature, as shown in Figures 2b and 2c, respectively. We explored the boundary between intramolecular intersubunit aggregation and intermolecular aggregation by plotting the intensity ratio for F2 versus F1 bands against the PBI derivatives concentration. The gradients of the two plots depicted in the insets of Figures 2b and 2c results from a drastic change in concentrations from 1 × 10−6 to 4 × 10−7 mol·L−1, respectively. The ratio of the dual emission remains constant at lower concentrations (2 × 10−7 mol·L−1), revealing that the dual emissions were authentic in the dilute solution where the intermolecular BPBI-CB-1 interaction, and hence, the associated emission had been eliminated. These results indicated the existence of intramolecular interaction, intrinsically, between two PBI units in BPBI-CB-1, giving the anomalous dual emission. Figure 2 | The absorption and fluorescence emission spectra (λex = 470 nm) of BPBI-CB-1 in different solvents, recorded at a concentration of 2 × 10−7 mol·L−1 at room temperature (a). The fluorescence emission spectra (λex = 470 nm) of BPBI-CB-1, recorded at different concentrations in TCE (b) and DCE (c) at room temperature; the insets are the plots of ratios of emission bands with changes in concentration. Note: TCM, trichloromethane, DMF, dimethyl-formamide. Download figure Download PowerPoint To gain further insights into the photophysical behavior, the absorption/excitation and emission spectra of BPBI-CB-1, BPBI-CB-2, and BPBI-CB-ref in TCE and/or DCE were recorded at a concentration of ∼2 × 10−7 mol·L−1; the results are depicted in Figures 3a–3d. As expected, PBI-CB-ref, which represented the prototypical PBI monomer property, was characterized by a structural emission band (the F1 band) consisting of two prominent vibronic progressive peaks at ∼540 and 590 nm and a shoulder in the longer wavelength side (Figure 3a). This spectral feature was ascribed to a typical monomer-relevant emission of the PBI derivatives. In comparison, the emission spectrum of BPBI-CB-2 (Figure 3b) resembled that of PBI-CB-ref, possessing the vibronic peaks at 560 and 610 nm, which unambiguously resulted from the derivatized PBI monomer emission, without any sign of the F2 emission band originating from the intra-PBI aggregate. Thus, the monomeric emission nature of BPBI-CB-2 in the solution ruled out the possibility of both intermolecular and intramolecular aggregation. Figure 3 | The absorption and fluorescence excitation and emission spectra of PBI-CB-ref (a) and BPBI-CB-2 (b) in TCE, recorded at a concentration of 5 × 10−7 mol·L−1 and at room temperature (λex = 470 nm). The absorption and fluorescence excitation and emission spectra of BPBI-CB-1 in TCE (c) and DCE (d), recorded at a concentration of 2 × 10−7 mol·L−1 at room temperature (λex = 470 nm). Download figure Download PowerPoint Detailed monomer behavior for PBI-CB-ref and BPBI-CB-2 was also provided by their associated absorption spectra ( Supporting Information Figures S1 and S2). The absorption bands of PBI-CB-ref at ∼550 and ∼500 nm (Figure 3a) corresponded to the 0–0 and 0–1 absorption transitions from the ground singlet state (S0) to the first excited singlet state (S1), respectively, attributed to a vibronic coupling between the electronic transition and C–C-stretching modes of the perylene core of PBI moiety. The large ratio (∼1.50) of the two vibronic absorption transitions (A0–0/A0–1) for BPBI-CB-ref revealed the monomer nature of PBI.22 Similar elucidation is also applicable for the absorption of PBI-CB-2 where the ratio for 0–0 (∼530 nm) versus 0–1 (∼490 nm), that is, A0–0/A0–1, was calculated to be 1.52, which was within the category of the monomer behavior. The excitation spectra profiles of both PBI-CB-ref and PBI-CB-2 ( Figures 3a and 3b), independent of the monitored emission wavelength, were identical and the same as the absorption profile, further confirming the monomolecular feature of both compounds. This result is taken for granted for PBI-CB-ref because it possesses only one PBI unit. As for BPBI-CB-2, the result is not difficult to apprehend by considering that the dual PBI units were far away from each other, according to the structure depicted in Figure 1. Therefore, in sufficiently diluted solution, both intra- and inter-PBI interactions could be eliminated. The monomeric behavior led to high fluorescence QY (>80%) for both BPBI-CB-2 and PBI-CB-ref in solution. In stark contrast, as shown in Figure 3c, PBI-CB-1 dissolved in TCE revealed a significantly decreased absorbance A0–0/A0–1 ratio of ∼1.18 (cf. 1.52 in BPBI-CB-2). Since the concentration was well below the threshold for forming intermolecular aggregation (vide supra), the results manifested intramolecular interaction of the two PBI units in PBI-CB-1. The corresponding emission (Figure 3c) of PBI-CB-1 showed a more pronounced effect regarding the intramolecular interaction, which, except for the two prominent vibronic peaks (∼570 and ∼615 nm), associated with the monomer PBI unit (the F1 band); the longer wavelength F2 band was non-negligible, broad, structureless, and most plausibly, ascribed to the presence of an intramolecular excimer induced by intra-PBI aggregation. Such intramolecular PBI–PBI interaction became more enhanced when PBI-CB-1 was dissolved in DCE (Figure 3d), supported by the decreased intensity ratio (A0–0/A0–1) for the absorption bands of PBI-CB-1 from 1.18 in TCE to 0.78 in DCE, pointing out to stronger intrasubunit interaction of the two PBI units. This is further indisputably verified by the corresponding emission spectra in DCE, consisting of a strong, dominant, and long-wavelength F2 emission maximized at 650 nm, while the F1 emission at ∼560 nm became minimized. Thus, the F1 and F2 emission bands were reasonably ascribed to the PBI monomer and PBI-associated excimer emission, respectively. Undoubtedly, the behavior of the dual PBI units of BPBI-CB-1 was distinct from the dual PBI units of BPBI-CB-2. While the two PBI-subunits have negligible interaction in BPBI-CB-2, BPBI-CB-1 tended to allow intramolecular excimer formation via the interaction of the two PBI-subunits. Moreover, closer inspection of the excitation spectra monitoring at F1 (e.g., 570 nm) and F2 (e.g., 650 nm) bands for BPBI-CB-1 (Figures 3c and 3d) produced a slightly different spectral feature in which the former revealed a profile similar to regular monomer absorption, with A0–0/A0–1 ratio of ∼1.52. The latter showed an A0–0/A0–1 ratio of ∼1.20 that supported the intrasubunit PBI interaction. Thus, unambiguously, there existed two types of ground-state species for BPBI-CB-1 in DCE (and TCE), consisting of one configuration where two PBI subunits were in an orientation/distance that has negligible interaction, giving PBI a monomer-like emission and another configuration where two PBI subunits were in an orientation/distance that was slightly coupled, giving excimer emission upon excitation. For the clarity of discussion, these two conformers were denoted as intra-PBI monomer and intra-PBI aggregate, which are in equilibrium in the ground state. Notably, the results also demonstrated that the PBI moieties of BPBI-CB-1 showed much more pronounced excimer emission in DCE than in TCE. The difference in ratiometric emission between two solvents reflected different degrees of equilibrium between intra-PBI monomer and intra-PBI aggregate in these two solvents, though the corresponding structures have not yet been disclosed (vide infra). The changes in equilibrium could be fine-tuned by mixing TCE and DCE solvents; the results showed that the monomer dominated the absorption and emission recorded in pure TCE, which gradually changed to dimer or excimer dominated absorption and emission in pure DCE with an increasing volume ratio of the latter (see Supporting Information Figures S3 and S4). For further exploration, we carried out temperature-dependent fluorescence measurements of the titled compounds in two solutions (DCE and TCE). To our surprise, both F1 and F2 emission intensity of BPBI-CB-1 in DCE increased significantly upon raising the temperature from 253 to 343 K (Figure 4a). In other words, both intensities of F1 and F2 showed positive temperature responsiveness, suggesting that at high temperatures, the population of the intramolecular PBI aggregates rose from the less-emissive state to the bright state. This statement was further supported by the temperature independence of population decay lifetimes for both F1 and F2 emissions ( Supporting Information Figure S5). BPBI-CB-1 in TCE revealed a similar effect in the same temperature range, but the changes were relatively small and became irregular at certain temperatures (Figure 4b). For comparison, the fluorescence spectra of the two reference compounds, BPBI-CB-2 and BPBI-CB-ref, in the two solvents were also recorded at different temperatures. Note that the experiment was performed under a diluted concentration of 5 × 10−7 mol·L−1 to eliminate the aggregation effect. The results presented in Supporting Information Figures S6 and S7 for BPBI-CB-2 and BPBI-CB-ref, respectively, showed no noticeable temperature-dependent emission behavior in the two solvents. Figure 4 | Fluorescence emission spectra of BPBI-CB-1 in DCE (a) and TCE (b), recorded at different temperatures at a concentration of 2 × 10−7 mol·L−1 (λex = 470 nm). Download figure Download PowerPoint Repetitive tests have been performed in the BPBI-CB-1/DCE system. The results shown in Supporting Information Figure S8 revealed that the positive temperature responsiveness of the emission intensity is fully reversible, affirming temperature-dependent changes in equilibrium rather than any irreversible chemical process. Further support is given by the temperature-dependent absorption measurements of BPBI-CB-1 in DCE ( Supporting Information Figure S9), where the spectra showed isosbestic points at ∼530 nm and ∼550 nm, concluding the existence of an equilibrium between two species, viz, the proposed intra-PBI monomer and intra-PBI aggregate. Moreover, upon increasing temperature from 298 K to 343 K, the intensity ratio between A0-0 and A0-1 rose from 0.75 to 0.80, indicating an increase in intra-PBI monomer species, and hence, an enhanced dissociation of the intra-PBI aggregate at a higher temperature. This could well be explained by the thermodynamic relationship, as follows59: intra − PBI monomer ↔ intra − PBI aggregate (1) ln K e q = − Δ H ! R T + Δ S ! R (2)where Δ H ! and, respectively, Δ S ! are the standard enthalpy and entropy changes of the concerned process, and Keq denotes the equilibrium constant between intra-PBI monomer and intra-PBI aggregate. Thermodynamically, upon increasing temperature, the exothermic reaction (negative Δ H ! ) from intra-PBI monomer to intra-PBI aggregate led to an increase of the intra-PBI monomer population (eq. 2), rationalizing the positive temperature responsiveness of the monomer emission. However, the decrease of the PBI aggregate was accompanied by an increase in PBI excimer emission, contrary to the conventional expectation. We intend to rationalize this anomalous observation later, after performing both time-resolved spectroscopic and computational approaches, elaborated as follows: The time-resolved fluorescence decays of BPBI-CB-1 in DCE were monitored at 530 and 690 nm, mainly dominated by typical monomer and excimer emissions of the PBI units, respectively (Figures 2 and 3). Upon 400 nm excitation and monitoring at 530 nm for the monomer emission (Figure 5), there appeared a fast decay component of 250 ps and a much slower population decay component of 6.6 ns (Table 1). Contrarily, the excimer emission monitored at 690 nm consisted of a fast rise component of 168 ps, accompanied by a long population decay time of 17 ns. Significantly, the fast 250 ps decay of the monomer emission, within the experimental uncertainty, correlated well with the rise lifetime (168 ps) of the excimer emission, supporting a precursor–successor type of kinetic relationship. In other words, it took ∼168–250 ps time constant for the excimer formation. Comparing with the results monitored at 530 nm, the relatively faster rise lifetime and the smaller pre-exponential factor at 690 nm were engendered by the interference of the decay signals from the F1 emission band. However, the differences in the slow population decay lifetimes of the monomer (6.6 ns) and excimer (17 ns) revealed a nonequilibrium characteristic in the transition between these two excited states. This result is consistent with the conclusion made from the steady-state observation, showing the existence of two types of BPBI-CB-1 configuration in DCE, the intra-PBI monomer and intra-PBI aggregate, denoted as BPBI-CB-1-M (M) and PBI-CB-1-M ( D), respectively. The absorption spectra of M and D were somewhat similar, except for the ratiometric difference in vibronic absorption peaks due to the PBI-PBI interaction in D. For example, the excitation of the M configuration at 400 nm gives a monomer-like emission, maximized at 560 nm with a population decay time of 6.6 ns. Similar excitation (400 nm) for D rendered a fast decay (168–250 ps) of the monomer-like species, which mainly underwent excimer formation, giving rise to an excimer emission with a rise and decay time of 220–250 ps and 17 ns, respectively. Note that such an excimer formation kinetics is relatively slow compared to other singlet excited-state electronic transition such as photointroduced intramo