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

Abstract

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 to monomer with A0–0/A0–1 ratio of The showed an A0–0/A0–1 ratio of that supported the intrasubunit PBI Thus, two of for BPBI-CB-1 in DCE consisting of one configuration where two PBI were in an that has negligible interaction, giving PBI a monomer-like emission and another configuration where two PBI were in an that was slightly giving excimer emission For the of these two were as intra-PBI monomer and intra-PBI which are in equilibrium in the ground state. the results also demonstrated that the PBI moieties of BPBI-CB-1 showed more pronounced excimer emission in DCE in The in emission between two solvents different of equilibrium between intra-PBI monomer and intra-PBI aggregate in these two solvents, though the corresponding structures have not been (vide infra). The changes in equilibrium could be by TCE and DCE the results showed that the monomer the absorption and emission recorded in TCE, which to or excimer absorption and emission in DCE with an ratio of the Supporting Information Figures and For further we out fluorescence measurements of the in two and To our both F1 and F2 emission intensity of BPBI-CB-1 in DCE increased significantly the temperature from to (Figure In other both intensities of F1 and F2 showed positive temperature that at high the population of the intramolecular PBI aggregates from the state to the bright state. This was further supported by the temperature of population for both F1 and F2 emissions ( Supporting Information Figure BPBI-CB-1 in TCE revealed a effect in the same temperature but the changes were relatively and became at (Figure For comparison, the fluorescence spectra of the two reference BPBI-CB-2 and BPBI-CB-ref, in the two solvents were also recorded at different temperatures. that the was performed under a diluted concentration of 5 × 10−7 mol·L−1 to eliminate the aggregation The results in Supporting Information Figures and for BPBI-CB-2 and BPBI-CB-ref, respectively, showed no emission behavior in the two Figure 4 | Fluorescence emission spectra of BPBI-CB-1 in DCE (a) and TCE recorded at different at a concentration of 2 × 10−7 mol·L−1 (λex = 470 nm). Download figure Download PowerPoint have been performed in the system. The results shown in Supporting Information Figure revealed that the positive temperature of the emission intensity is fully changes in equilibrium rather any chemical is by the absorption measurements of BPBI-CB-1 in DCE ( Supporting Information Figure where the spectra showed at nm and ∼550 nm, the existence of an equilibrium between two the intra-PBI monomer and intra-PBI aggregate. Moreover, temperature from to the intensity ratio between and from to an increase in intra-PBI monomer and hence, an enhanced of the intra-PBI aggregate at a temperature. This could well be by the as PBI monomer PBI aggregate = respectively, are the and changes of the and the equilibrium constant between intra-PBI monomer and intra-PBI aggregate. temperature, the from intra-PBI monomer to intra-PBI aggregate led to an increase of the intra-PBI monomer population the positive temperature of the monomer emission. However, the of the PBI aggregate was by an increase in PBI excimer emission, to the We to this anomalous after both spectroscopic and elaborated as The fluorescence of BPBI-CB-1 in DCE were monitored at and nm, mainly by typical monomer and excimer emissions of the PBI (Figures 2 and 400 nm excitation and monitoring at nm for the monomer emission (Figure appeared a component of and a population component of 1). the excimer emission monitored at nm of a component of by a population time of the of the monomer emission, within the well with the lifetime of the excimer emission, a of relationship. In other it time constant for the excimer with the results monitored at nm, the relatively lifetime and the at nm were by the of the from the F1 emission However, the in the population of the monomer and excimer revealed a in the transition between these two excited This result is with the from the steady-state the existence of two of BPBI-CB-1 configuration in DCE, the intra-PBI monomer and intra-PBI as and ( respectively. The absorption spectra of and were except for the in vibronic absorption peaks due to the interaction in For the excitation of the configuration at 400 nm a monomer-like emission, maximized at 560 nm with a population time of Similar excitation (400 nm) for a of the monomer-like which mainly excimer giving to an excimer emission with a and time of and respectively. that such an excimer formation is relatively compared to other singlet electronic transition such as

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