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Spectroscopy and Photochemistry; General Theory

Solvent-Modulated Charge-Transfer Resonance Enhancement in the Excimer State of a Bay-Substituted Perylene Bisimide Cyclophane Woojae Kim, Agnieszka Nowak-Król, Yongseok Hong, Felix Schlosser, Frank Würthner, and Dongho Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00357 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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The Journal of Physical Chemistry Letters

Solvent-Modulated Charge-Transfer Resonance Enhancement in the Excimer State of a BaySubstituted Perylene Bisimide Cyclophane Woojae Kim,† Agnieszka Nowak-Król,‡ Yongseok Hong,† Felix Schlosser,‡ Frank Würthner,*,‡ and Dongho Kim*,† †Department of Chemistry and Spectroscopy Laboratory for Functional π-Electronic Systems, Yonsei University, Seoul 03722, Korea. ‡Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.

AUTHOR INFORMATION Corresponding Author [email protected], [email protected]

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Excimer, a configurational mixing between Frenkel exciton and charge transfer resonance states, is typically regarded as a trap state that hinders desired energy or charge transfer processes in artificial molecular assemblies. However, in recent days, the excimer has received much attention as a functional intermediate in the excited-state dynamics such as singlet fission or charge separation processes. In this work, we show that the relative contribution to charge transfer resonance of the excimer state in a bay-substituted perylene bisimide dimer cyclophane can be modulated by dielectric properties of the solvents employed. Solvent-dependent time-resolved fluorescence and absorption measurements reveal that an enhancement of charge transfer resonance in the excimer state is reflected by incomplete symmetry breaking charge separation processes from the structurally relaxed excimer state by means of dipolar solvation processes in the high dielectric environment.

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When solar energy is absorbed by light-harvesting complexes (LHCs) in natural photosynthetic systems, the captured energy is consecutively funneled into a reaction center (RC) which is the last place of the light reaction.1 Here, the primary charge separation (CS) reaction occurs in a πstacked cofacial homodimer of bacteriochlorophyll (BChl) pigment called the special pair.2 Despite of quasi-C2-symmetry about the axis from the special pair to a non-heme iron atom of the RC, a symmetry-breaking (SB) CS reaction in the special pair, owing to its significant charge transfer (CT) character and the slightly asymmetric environment, generates the CS state with more than 90% quantum efficiency and unidirectionally transfers an electron to the A branch.3,4 In order to mimic this type of functional features of the natural light-harvesting systems, a number of cofacial dimeric and oligomeric assemblies based on organic dyes including boron dipyrromethene (BODIPY), perylene, naphthalene bisimide (NBI, also abbreviated as NDI), perylene bisimide (PBI, also abbreviated as PDI), etc., have been synthesized and characterized.5-10 However, excitons in cofacially-arranged π-stacks are generally trapped in the low-energy excimer state which disrupts desired energy and/or charge transport processes in the landscape of applications for light harvesting.11-13 However, new perspectives on the excimer that behaves as a functional intermediate in the excited-state dynamics have recently been reported. For instance, it has been suggested that the singlet fission (SF), a spin-conserving process that one photoexcited singlet state decays to the triplet pair, can occur via an excimer intermediate state.14-20 Although the actual mechanism for SF is a subject still under debate, in highly concentrated solutions of TIPS-pentacene14 and TIPStetracene,15 the fluorescence lifetime of the excimer state is in good agreement with the rise time of the triplet pair formation, implying that the excimer is possible to mediate the SF process. In this case, since the ground state monomers are electronically decoupled and the excimer is formed


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solely through diffusion, the SF rate can be controlled by the sample concentration.14 Moreover, the SF from the excimer intermediate state has been discussed in the solution phase of a cofacial alkynyl-tetracene dimer16 and polycrystalline thin films of diketopyrrolopyrrole (DPP) derivatives,17 terrylene bisimide (TBI, also abbreviated as TDI),18,19 perfluoro-pentacene (PFP),20 etc. in detail. As another example, our group has reported that SB-CS in a cofacially-stacked dimer of a non-core substituted PBI tethered by a cyclophane can be mediated by the excimer state,21 which is evident from the unexpectedly short lifetime (~ 33-36 ps) of the excimer fluorescence. This was the first example of disclosing the functional role of the excimer in the electron transfer dynamics. [Chart 1] Here we present another facet of the excimer in a cofacially-stacked dimer of 1,6,7,12-tetra(4tert-butyl-phenoxy)perylene bisimide in a cyclophane, rPBI-CP (Chart 1; rPBI = red PBI, CP = cyclophane), which can undergo conformational changes between closed and open cavities depending on its redox state, according to our previous study.22 Through solvent-polaritydependent experiments based on time-resolved fluorescence and transient absorption spectroscopy, we could capture the spectroscopic signatures of solvent-modulated CT resonance enhancement in the excimer state. The time-resolved fluorescence and absorption data are in excellent agreement with each other and suggest that highly polar solvents can increase the contribution of CT resonance in the structurally relaxed excimer state of rPBI-CP. The enhancement of CT resonance in the excimer state is evident from incomplete SB-CS occurring on the time scale of solvation dynamics. [Figure 1]


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The intensity-normalized steady-state absorption and fluorescence spectra of rPBI-CP in THF (εr = 7.58), DCM (εr = 8.93), and BCN (εr = 26.0) are shown in Figure 1. In the absorption spectra, contrary to the reference monomer, rPBI (Figures 1 and S1), rPBI-CP shows the more intense 01 vibronic band as compared to the 0-0 band, indicative of prominent H-type excitonic coupling in closely face-to-face stacked PBI dimer regardless of dielectric properties of the solvents.21-23 This spectroscopic feature is insensitive to the sample concentration so that we can rule out the presence of aggregate species beyond pure cyclophane-embedded “stacked-dimer” in further analyses.22 This is in line with the fact that 1,6,7,12-bay-substituted PBI, especially tetra(4-tertbutyl-phenoxy)-substituted ones, have a low propensity towards aggregation due to a significant distortion (dihedral angle of around 27°) of the PBI core and sterically demanding phenoxy substituents.13,22. Meanwhile, broad and featureless fluorescence spectra with a large Stokes shift were observed in the investigated solvents, which can be attributed to the fluorescence from the excimer state.23,24 But interestingly, rPBI-CP shows a remarkably low fluorescence quantum yield (0.6%) in highly polar BCN as compared to those in moderately polar THF and DCM (7 and 5%, respectively). This solvent-dependent fluorescence strongly implies the existence of an additional non-radiative deactivation channel that greatly quenches the excimer fluorescence only in the most polar solvent BCN. In this respect, the steady-state measurements (solvent-insensitive absorption and solvent-sensitive fluorescence) illustrate the pivotal role of a rigid ethynylene-phenylenebutadinylene-based cyclophane backbone in rPBI-CP. Geometry optimization of rPBI-CP based on density functional theory (DFT) method (B97D3 functional and def2SVP basis set)52,53 also supports the cofacially-stacked structure with the aid of the cyclophane backbone (Figure 1). A rotational displacement and an intermolecular distance between PBI units were calculated to be 19.62° and 3.484 Å , respectively. The cyclophane backbone confines not only the spatial


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arrangement but also the coupling between the constituent PBIs, so we can purely investigate the impact of surrounding environment, especially in terms of the dielectric properties of the solvents, on the excited-state dynamics of the well-defined rather rigid stacked PBI dimer.21 In the following, we analyzed and compared the spectroscopic results in DCM and BCN, since the basic photophysical properties in THF is nearly the same as in DCM. [Figure 2] In order to investigate the solvent-sensitive fluorescence dynamics of rPBI-CP from the excimer state, we first have utilized femtosecond broadband fluorescence upconversion spectroscopy (FLUPS).25,26 The pump pulse was tuned to be peaked at around 520 nm corresponding to the upper Frenkel excitonic state (Figure S2). The obtained time-resolved fluorescence spectra (TRFS) for rPBI-CP in DCM and BCN are shown in Figures 2a and 2b, respectively.27 In DCM, rPBICP shows spectral relaxation with a fast decay up to ca. 30 ps. Afterwards, the shape and intensity of TRFS remain unchanged. From the total fluorescence intensity (signal integration over 600 – 780 nm region) kinetic traces, we could find only a single exponential decay with a time constant of 2 ps (Figure 2c, top), which can be assigned to the structural relaxation processes from the unrelaxed excimer.21,28,29 The shorter time constant for the structural relaxation of rPBI-CP as compared to the case of self-assembled helically-stacked PBI dimers and oligomers without linkers can be attributed to the structural rigidity imparted by the cyclophane geometry.13,21,29,30 After this relaxation, a long-lived component corresponding to the excimer-state lifetime was observed. But interestingly, the transient fluorescence in BCN from the excimer state is continuously quenched up to a few hundreds of picoseconds, implying that in highly polar BCN rPBI-CP portrays quite different excited-state dynamics. The total fluorescence intensity decay profiles reveal that there are two consecutive relaxation channels before the excited rPBI-CP returns to the ground state


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(Figure 2c, bottom). The fastest decay component of 2 ps, which is the same value observed in DCM can be assigned to the structural relaxation of the unrelaxed excimer. After that, an additional fluorescence quenching process takes place with the time constant of 23 ps. In particular, the quenching process and its time constant are very similar to the SB-CS dynamics from the excimer intermediate state of the non-core substituted PBI dimer having the same cyclophane geometry.21 This process occurred exceptionally in chlorinated solvents with the formation of solvation shell around the individual PBI unit. But the most important feature here is that the excimer fluorescence of rPBI-CP was not totally quenched, signifying that the time constant of 23 ps does not correspond to the lifetime of the excimer state. In this regard, to precisely determine the excimerstate lifetime of rPBI-CP, we also have performed time-correlated single photon counting (TCSPC) measurements (Figure 2d). We found that the excimer-state lifetime of rPBI-CP in BCN to be about 2.8 ns, which is 3.4 times as short as that in DCM (9.6 ns) and even nearly twice as short as that of the reference monomer (rPBI) in BCN (5.3 ns). Given that the fluorescence lifetime of the excimer state is typically much longer than that of monomer, this result seems to be quite unusual. Brown et al. exceptionally reported a shorter excimer-state lifetime (~ 2.5 ns) of a cofacial unsubstituted PBI dimer linked by the aromatic spacers as compared to that of a PBI monomer (~ 4 ns).29 But in their case, it should be noted that the distinctive solvent dependence was not revealed. Furthermore, the short excimer-state lifetime was tentatively explained by two possible reasons: 1) increased vibrational interactions in the tightly packed PBI dimer or 2) an enhanced internal conversion rate due to a decreased energy gap by the ground-state destabilization. We also performed a global target analysis of the TRFS by using Glotaran program31 to extract evolution associated spectra (EAS) assuming a sequential scheme with increasing time constants and analyze their temporal and spectral evolutions.32 The slowest decay components were fixed as


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the fluorescence lifetimes measured by TCSPC method due to the limited time window of FLUPS setup. We found that two (A → B → Ground state (GS)) and three (A → B → C → GS) steps are prerequisite to fully reproduce the excited-state dynamics in DCM and BCN, respectively (Figure S4). The time constants obtained from the global target analysis were pretty well matched with the constants derived from the total fluorescence intensity decay profiles. In DCM, EASA (the EAS of A) transforms into EASB (the EAS of B) with a time constant of 2 ps. This spectral evolution accompanies a ~ 15 nm red-shift with a significant intensity drop, which again justifies the conviction that EASA and EASB corresponds to the unrelaxed and relaxed excimer states, respectively. In BCN, nearly the same spectral evolution (EASA, unrelaxed excimer → EASB, relaxed excimer, τ = 2 ps) was also observed. However, the second EASB further evolves into the third EASC (the EAS of C) with a time constant of 24.5 ps, which additionally entails a ~ 25 nm red-shift with a drastic intensity drop. These results clearly indicate that in BCN the structurally relaxed excimer can be further stabilized into not only energetically more relaxed but weakly fluorescent state. However, time-resolved fluorescence results were not enough to fully explain the details of the additionally relaxed state. Therefore, we needed a more conclusive evidence to figure out the origin of additional fluorescence quenching process and the exceptional emissive nature of rPBI-CP in highly polar BCN. [Figure 3] In this sense, we have carried out visible and near-infrared (VIS-NIR) femtosecond transient absorption measurements (fs-TA) (Figure 3 and Figures S5). To scrutinize the temporal and spectral evolutions of fs-TA spectra, we also extracted evolution-associated difference spectra (EADS) on the basis of a global target analysis. First, in line with time-resolved fluorescence results, we found that two sequential exponential steps (A → B → GS) are suitable for


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characterizing the excited-state dynamics in DCM (Figure 3a). Both EADSA (the EADS of A) and EADSB (the EADS of B) show the ground-state bleaching (GSB) up to 610 nm and the excitedstate absorption (ESA) bands in the range from 610 to 1350 nm. As opposed to rPBI (Figure S7), fairly broad and featureless ESA bands are shown, unambiguously representing the complicated electronic structure of the excimer state.13,29,30 The EADS evolve from A into B with the time constant of 2 ps, which coincides with the 2 ps decay component observed in FLUPS results. Furthermore, considering that there is no remarkable spectral change, we can safely assign the EADSA and EADSB to the unrelaxed and the relaxed excimer, respectively. On the other hand, in accordance with the FLUPS results, three sequential exponential steps, A → B → C → GS, were necessary to reproduce the fs-TA data in BCN. EADSA in BCN shows almost identical spectral features to those in DCM and also has the same lifetime of 2 ps, indicative of the unrelaxed excimer. Therefore, EADSB can be interpreted as the relaxed excimer. In addition, the spectrum of the relaxed excimer changes further to EADSC (the EADS of C), which is completely invisible in DCM, with the time constant of 25 ps. Importantly, EADSC apparently reveals new positive bands at around 970 and 1075 nm, which are the spectral characteristics of a radical anion of tetraphenoxy-substituted PBI (the core of rPBI).32,33 The most intense band for the PBI radical anion at 780 nm is rather unclear due to the spectral artifact in our experimental condition. However, through the comparison of kinetic traces at 780 nm in the two solvents (Figure S6), we observed the 25 ps rise component only in BCN. These results are the striking evidence that 25 ps is the CS time constant, thus it can be said that EADSC represents the CS state. We suggest that the radical anion species is made by SB-CS between the two rPBIs rather than the electron transfer process from the phenylene-ethynylene backbone to rPBI monomer. The reasons are as follows: 1) in the case of rPBI, electron transfer from the backbone to the PBI core does not occur. Even


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in BCN, the fluorescence quantum yield of the reference monomer is quite high (85%) and the structural relaxation, corresponding to flattening processes of the twisted aromatic core induced by bay-substituted bulky tetra-phenoxy groups, was solely observed in the fs-TA spectra (Figure S7). 2) in cyclic voltammetry, the first one-electron oxidation and first one-electron reduction processes are both attributable to the PBI core with almost the same oxidation and reduction potentials in the monomer (E1/2,ox = 0.92 V and E1/2,red = -1.16 V) and the dimer (E1/2,ox = 0.92 V and E1/2,red = -1.10 V).22 On the one hand, TA bands at NIR region of a radical cationic rPBI are relatively not distinctive. This is presumably due to their quite low extinction coefficient with broad spectral shape.33,34 Therefore, EADSC, the SB-CS state, finally decays to the ground state, and thus this process can be assigned to the charge recombination (CR) between adjacent radical cationic and anionic PBIs with a time constant of 2.5 ns. Combining the time-resolved fluorescence and absorption results, important conclusions can be draw regarding the solvent-sensitive excited-state dynamics of rPBI-CP. In highly polar BCN, the additional fluorescence quenching and energetic relaxation processes from the relaxed excimer measured by FLUPS is consistent with the SB-CS process observed by fs-TA (Figure S8). This coincidence is intuitively reasonable since the electron/hole transfer generally diminishes the oscillator strength of the fluorescent state.35 But more importantly, although the SB-CS occurs, fluorescence from the relaxed state (corresponding to EASC in FLUPS and EADSC in fs-TA) of rPBI-CP is not fully quenched. This means that the final state has a smaller but finite radiative property as compared to the structurally relaxed excimer state. Then, what is the nature of the final state of rPBI-CP in BCN? In order to answer this question, it is necessary to recall the theoretical expression of the excimer, which is now widely accepted as a configurational mixing between


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Frenkel exciton (FE, |𝑀𝑀⟩∗𝐹𝐸 = |𝑀1∗ 𝑀2 ⟩ + |𝑀1 𝑀2∗ ⟩ ) and charge transfer (CT, |𝑀𝑀⟩∗𝐶𝑇 = |𝑀1+ 𝑀2− ⟩ + |𝑀1− 𝑀2+ ⟩) resonance states.36-39 |𝑀𝑀⟩∗𝐸𝑥 = 𝛼(|𝑀𝑀⟩∗𝐹𝐸 ) + 𝛽(|𝑀𝑀⟩∗𝐶𝑇 )

(1)

This equation indicates that the characteristics of the excimer state are determined by the relative contribution ( and ) of each eigenstate.  and  terms rely on not only the distance and relative orientation between the chromophores but also the dielectric properties of the solvents.29,38-41 However, in the general case of aromatic excimers,  is relatively much larger than  so that distinct spectroscopic signatures induced by varying the  term have been rarely revealed. An example is the negligible solvatochromism of excimers consisting of polyaromatic hydrocarbons (PAHs), such as naphthalene, pyrene, and perylene, etc.6,29,42 A recent work on cofacial perylene dimers by Cook et al. has highlighted that the excimer can be formed by the CR processes from a small portion of the compartmental SB-CS state in highly polar acetonitrile (εr = 37.5).6 However, the excimer-state lifetimes of the perylene dimers were not affected by the solvent polarity, reflecting a very weak CT character of the excimer state. Several previous works also reported the role of highly polar solvents which stabilize the CT resonance.41 In these cases, the highly polar solvents lower the activation energy barrier of the structural rearrangement which helps the formation of the excimer state. On the contrary, in the case of rPBI-CP, excimer formation and structural relaxation processes are preceded regardless of dielectric properties of the solvents,21,42,43 and then the structurally relaxed excimer state is subsequently affected only by the high dielectric environment. We propose that this phenomenon is CT resonance enhancement (an increased contribution of the  term in eq 1) in the excimer state with the aid of the high dielectric environment.


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In particular, we found that some spectroscopic signatures of the CT resonance enhanced excimer for rPBI-CP are fairly similar to those of a tight exciplex which is typically regarded as an incomplete photoinduced CS product during the electron transfer processes between the electron donor and acceptor.45-47 Compared with the fully separated ion pair state, the tight exciplex has a few distinctive spectroscopic characteristics as follows: 1) In TA spectra, the peak positions of radical (cation or anion) bands are analogous to each other but the bandshape of the tight exciplex is broader, 2) Of the two, the exciplex can exclusively emit fluorescence with a very small but finite radiative rate constant due to its partially charged nature. On the other hand, the ion pair state is not fluorescent, because the charges are fully separated. The reason why these kinds of differences arise is that the partially charged exciplex is formed only when the CS driving force (ΔGCS) is small and a spatial overlap of molecular orbitals exists between donor and acceptor at short distance.46,47 [Figure 4] The CT resonance enhanced excimer of rPBI-CP shows both characteristics in our measurements. First, as shown in Figure 4a and 4b, the peak positions of radical bands in EADSC are similar as compared with those of simulated transient absorption spectra for the fully separated ion pair, whereas the bandshape of EADSC is much broader.34 This implies that through incomplete SB-CS the structurally relaxed excimer gets partial positive and negative charges. The charges cannot be fully separated, which is due to the enforced π-π stacking owing to the rigid cyclophane geometry. Second, as mentioned above, the final state, which is now able to be called as the CT resonance enhanced excimer, has a finite but smaller radiative property as compared with the structurally relaxed excimer. As a result of partial positive and negative charges, the fluorescence from the CT resonance enhanced excimer is not fully quenched.


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We could get additional evidence on incomplete SB-CS in the excimer state by monitoring timedependent emission transition dipole moment (μem) based on the FLUPS data (see Supporting Information for details). This method was recently suggested by Beckwith et al., which can be a specific real-time observation of the excited SB processes in symmetric molecular systems based on the FLUPS setup.48 As can be seen in Figure 4c, the initial μem values are nearly the same as 2.2 and 2.1 D for BCN and DCM, respectively, reflecting their similar oscillator strengths at early times. In DCM, μem value drops to 1.7 D (a ~ 19% reduction) with the time constant of 2 ps, which can be assigned as the structural relaxation toward more relaxed excimer, as we discussed above.21 The smaller μem for structurally relaxed excimer as compared to that for unrelaxed excimer is reasonable because the more perfect cofacial geometry the excimer has, the weaker fluorescence it emits.29 In BCN, μem value also declines to around 1.7 D (a ~ 22% reduction) with the time constant of 2 ps, however it additionally drops to 1.0 D (a ~ 32% reduction, a total of ~ 54% reduction) with the time constant of 23 ps. The former decline of μem occurs due to the structural relaxation as in the case in DCM, and therefore the latter decline of μem is responsible for the SBCS processes. μem remains after SB-CS, clearly reflecting that the positive and negative charges are partially separated between the PBI dimer in BCN. The role of solvent must be considered significant as well. Generally, for the CS process energetically to be favorable, solvation (dipolar solvent relaxation) of the CS state is indispensable. Thus, CS is typically not faster than the dipolar solvation.49 The time constant of SB-CS for rPBICP, τ = 23-25 ps, is similar to the slowest component of the solvation, corresponding to its diffusive part, of BCN (τ = 25 ps).50 Compared to the average time constant considering both inertial and diffusive solvation processes (τ = 5.1 ps),50 the SB-CS time constant is quite slow. This is attributed to a large molecular size of rPBI-CP, which can make the solvation process slower.51


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From a thermodynamical point of view, although the conventional Weller analysis cannot be purely applied in a π-π stacked system because it does not follow the hard sphere model,29 the feasibility of incomplete SB-CS from the relaxed excimer state for rPBI-CP implies that it may have a small driving force in BCN as in the case of a tight exciplex. [Figure 5] In conclusion, we have experimentally observed solvent-modulated CT resonance enhancement in the excimer state of a cofacially-stacked 1,6,7,12-tetra(4-tert-butyl-phenoxy)-substituted PBI dimer in a rigid cyclophane by using time-resolved fluorescence and absorption methods. Our results reveal that the CT resonance enhanced excimer can be formed through incomplete SB-CS from the relaxed excimer state with the assistance of dipolar solvation of the high dielectric environment (Figure 5). As briefly discussed above, it is widely accepted that the excimer acts as a trap state that impedes efficient energy and charge transport in molecular self-assemblies. Thus, strenuous efforts to develop various molecular architectures have been made to obstruct the unwanted exciton trapping processes. However, as the positive role of the excimer state has been spotlighted again, especially in the field of singlet fission or electron transfer, we believe that our experimental findings and suggestions can pave a new way for controlling the characteristics of the excimer state in the field of artificial photosynthetic systems to more perfectly mimic the role of the special pair in natural light harvesting systems. ASSOCIATED CONTENT Supporting Information.


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Experimental details, photophysical parameters, time-resolved fluorescence and absorption data for rPBI-CP and rPBI, Simulation of TA spectra for the fully separated ion pair state, Determination of time-dependent μem for rPBI-CP. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ORCID Agnieszka Nowak-Król: 0000-0003-0454-2491 Frank Würthner: 0000-0001-7245-0471 Dongho Kim: 0000-0001-8668-2644 Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The work at Yonsei University was supported the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2016R1E1A1A01943379). The quantum mechanical calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with supercomputing resources including technical support (KSC-2018-CRE-0051). The research at the University of Würzburg has been supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the research unit FOR 1809. REFERENCES


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(1) Mirkovic,T.; Ostroumov, E. E.; Anna, J. M.; van Grondelle, R.; Govindjee; Scholes, G. D. Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms. Chem. Rev. 2017, 117, 249−293. (2) Mamedov, M.; Govindjee; Nadtochenko, V.; Semenov, A. Primary Electron Transfer Processes in Photosynthetic Reaction Centers from Oxygenic Organisms. Photosynth. Res. 2015, 125, 51−63. (3) Wraight, C. A.; Clayton, R. K. The Absolute Quantum Efficiency of Bacteriochlorophyll Photooxidation in Reaction Centers of Rhodopseudomonas Spheroides. Biochim. Biophys. Acta, Bioenerg. 1974, 333, 246−260. (4) Levanon, H.; Möbius. K. Advanced EPR Spectroscopy on Electron Transfer Processes in Photosynthesis and Biomimetic Model Systems. Annu. Rev. Biophys. Biomol. Struct. 1997, 26, 495−540. (5) Saki, N.; Dinc, T.; Akkaya, E. U. Excimer Emission and Energy Transfer in Cofacial Boradiazaindacene (BODIPY) Dimers Built on a Xanthene Scaffold. Tetrahedron 2006, 62, 2721−2725. (6) Cook, R. E.; Phelan, B. T.; Kamire, R. J.; Majewski, M. B.; Young, R. M.; Wasielewski, M. R. Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers. J. Phys. Chem. A 2017, 121, 1607−1615. (7) Wu, Y.; Frasconi, M.; Gardner, D. M.; McGonigal, P. R.; Schneebeli, S. T.; Wasielewski, M. R.; Stoddart, J. F. Electron Delocalization in a Rigid Cofacial Naphthalene-1,8:4,5bis(dicarboximide) Dimer. Angew. Chem. Int. Ed. 2014, 53, 9476−9481. (8) Sakai, N.; Lista, M.; Kel, O.; Sakurai, S.; Emery, D.; Mareda, J.; Vauthey, N.; Matile, S. Self-Organizing Surface-Initiated Polymerization: Facile Access to Complex Functional Systems. J. Am. Chem. Soc. 2011, 133, 15224−15227. (9) Würthner, F.; Saha-Möller, C. R.; Fimmel, B.; Ogi, S.; Leowanawat, P.; Schmidt, D. Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials. Chem. Rev. 2016, 116, 962−1052.


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Wang, D.; Ivanov, M. V.; Kokkin, D.; Loman, J.; Cai, J.-Z.; Reid, S. A.; Rathore, R. The

Role of Torsional Dynamics on Hole and Exciton Stabilization in π-Stacked Assemblies: Design of Rigid Torsionomers of a Cofacial Bifluorene. Angew. Chem. Int. Ed. 2018, 130, 8321−8325. (11)

Fink, R. F.; Seibt, J.; Engel, V.; Renz, M.; Kaupp, M.; Lochbrunner, S.; Zhao, H.-M.;

Pfister, J.; Würthner, F.; Engels, B. Exciton Trapping in π-Conjugated Materials: A Quantum-Chemistry-Based Protocol Applied to Perylene Bisimide Dye Aggregates. J. Am. Chem. Soc. 2008, 130, 12858−12859. (12)

Lindquist, R. J.; Lefler, K. M.; Brown, K. E.; Dyar, S. M.; Margulies, E. A.; Young, R.

M.; Wasielewski, M. R. Energy Flow Dynamics within Cofacial and Slip-Stacked Perylene3,4- dicarboximide Dimer Models of π‑Aggregates. J. Am. Chem. Soc. 2014, 136, 14912−14923. (13)

Lim, J. M.; Kim, P.; Yoon, M.−C.; Sung, J.; Dehm, V.; Chem, Z.; Würthner, F.; Kim, D.

Exciton Delocalization and Dynamics in Helical π-Stacks of Self-Assembled Perylene Bisimides. Chem. Sci. 2013, 4, 388−397. (14)

Walker, B. J.; Musser, A. J.; Beljonne, D.; Friend, R. H. Singlet Exciton Fission in

Solution. Nat. Chem. 2013, 5, 1019−1024. (15)

Stern, H. L.; Musser, A. J.; Gelinas, S.; Parkinson, P.; Herz, L. M.; Bruzek, M. J.;

Anthony J.; Friend, R. H.; Walker, B. J. Identification of a Triplet Pair Intermediate in Singlet Exciton Fission in Solution. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 7656−7661. (16)

Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth,

S. E.; Thompson, M. E. Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer. J. Am. Chem. Soc. 2016, 138, 617−627. (17)

Mauck, C. M.; Hartnett, P. E.; Margulies, E. A.; Ma, L.; Miller, C. E.; Schatz, G. C.;

Marks, T. J.; Wasielewski, M. R. Singlet Fission via an Excimer-Like Intermediate in 3,6Bis(thiophen-2-yl)diketopyrrolopyrrole Derivatives. J. Am. Chem. Soc. 2016, 136, 11749−11761.


17


(18)

Page 18 of 28

Margulies, E. A.; Miller, C. E.; Wu, Y.; Schatz, G. C.; Young, R. M.; Wasielewski, M. R.

Enabling Singlet Fission by Controlling Intramolecular Charge Transfer in π-Stacked Covalent Terrylenediimide Dimers. Nat. Chem. 2016, 8, 1120−1125. (19)

Margulies, E. A.; Logsdon, J. L.; Miller, C. E.; Ma, L.; Simonoff, E.; Young, R. M.;

Schatz, G. C.; Wasielewski, M. R. Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films. J. Am. Chem. Soc. 2017, 139, 663−671. (20)

Kolata, K.; Breuer, T.; Witte, G.; Chatterjee, S. Molecular Packing Determines Singlet

Exciton Fission in Organic Semiconductors. ACS Nano 2014, 8, 7377−7383. (21)

Sung, J.; Nowak-Król, A.; Schlosser, F.; Fimmel, B.; Kim, W.; Kim, D.; Würthner, F.

Direct Observation of Excimer-Mediated Intramolecular Electron Transfer in a CofaciallyStacked Perylene Bisimide Pair. J. Am. Chem. Soc. 2016, 138, 9029−9032. (22)

Schlosser, F.; Moos, M.; Lambert, C.; Würthner, F. Redox-Switchable Intramolecular π-

π-Stacking of Perylene Bisimide Dyes in a Cyclophane. Adv. Mater. 2013, 25, 410−414. (23)

Chen, Z.; Stepanenko, V.; Dehm, V.; Prins, P.; Siebbeles, L. D. A.; Seibt, J.; Marquetand,

P.; Engel, V.; Würthner, F. Photoluminescence and Conductivity of Self-Assembled π- πStacks of Perylene Bisimide Dyes. Chem. Eur. J. 2007, 13, 436−449. (24)

Pensack, R. D.; Ashmore, R. J.; Paoletta, A. L.; Scholes, G. D. The Nature of Excimer

Formation in Crystalline Pyrene Nanoparticles. J. Phys. Chem. C 2018, 122, 21004–21017. (25)

Zhang, X.−X.; Würth, C.; Zhao, L.; Resch-Genger, U.; Ernsting, N. P.; Sajadi, M.

Femtosecond Broadband Fluorescence Upconversion Spectroscopy: Improved Setup and Photometric Correction. Rev. Sci. Instrum. 2011, 82, 062108. (26)

Gerecke, M.; Bierhance, G.; Gutmann, M.; Ernsting, N. P.; Rosspeintner, A.

Femtosecond Broadband Fluorescence Upconversion Spectroscopy: Spectral Coverage Versus Efficiency. Rev. Sci. Instrum. 2016, 87, 053115.


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Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27)

For a detailed analysis, we used TRFS from 500 fs due to the strong background

scattering (sum-frequency generation signal between pump and gate pulses) under very weak fluorescence of samples, which distorts bandshapes as well as kinetic profiles of pure transient fluorescence at early times. Thus, the initial transient fluorescence from the lower Frenkel excitonic state could not be resolved. However, the early time dynamics is beyond the scope of this work, so we instead focused on the excited-state dynamics after the excimer formation. (28)

Mauck, C. M.; Young, R. M.; Wasielewski, M. R. Characterization of Excimer

Relaxation via Femtosecond Shortwave- and Mid-Infrared Spectroscopy. J. Phys. Chem. A 2017, 121, 784−792. (29)

Brown, K. E.; Salamant, W. A.; Shoer, L. E.; Young, R. M.; Wasielewski, M. R. Direct

Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers Using Near-Infrared Transient Absorption Spectroscopy. J. Phys. Chem. Lett. 2014, 5, 2588−2593. (30)

Son, M.; Park, K. H.; Shao, C.; Würthner, F.; Kim, D. Spectroscopic Demonstration of

Exciton Dynamics and Excimer Formation in a Sterically Controlled Perylene Bisimide Dimer Aggregate. J. Phys. Chem. Lett. 2014, 5, 3601−3607. (31)

Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M.

Glotaran: a Java-Based Graphical User Interface for the R Package TIMP. J. Stat. Softw. 2012, 49, 1−22. (32)

If the sequential scheme with increasing time constants represents the correct

physicochemical picture, evolution associated spectra correspond to true species associated spectra indicating the intermediate states. See the reference: van Stokkum, I. H. M.; Larsen, D. S.; van Grondelle, R. Global and Target Analysis of Time-Resolved Spectra. Biochim. Biophys. Acta, Bioenerg. 2004, 1657, 82-104. (33)

Würthner, F.; Sautter, A.; Schmid, D.; Weber, P. J. A. Fluorescent and Electroactive

Cyclic Assemblies from Perylene Tetracarboxylic Acid Bisimide Ligands and Metal Phosphane Triflates. Chem. Eur. J. 2001, 7, 894−902.


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(34)

Page 20 of 28

Wu. Y.; Young, R. M.; Frasconi, M.; Schneebeli, S. T.; Spenst, P.; Gardner, D. M.;

Brown, K. E.; Würthner, F.; Stoddart, J. F.; Wasielewski, M. R. Ultrafast Photoinduced Symmetry-Breaking Charge Separation and Electron Sharing in Perylenediimide Molecular Triangles. J. Am. Chem. Soc. 2015, 137, 13236−13239. (35)

Scarongella, M.; Laktionov, A.; Rothlisberger, U.; Banerji, N. Charge Transfer

Relaxation in Donor-Acceptor Type Conjugated Materials. J. Mater. Chem. C 2013, 1, 2308−2319. (36)

Schubert, A.; Settels, V.; Liu, W.; Würthner, F.; Meier, C.; Fink, R. F.; Schindlbeck, S.;

Lochbrunner, S.; Engels, B.; Engel, V. Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates. J. Phys. Chem. Lett. 2013, 4, 792−796. (37)

Birks, J. B. Excimers, Rep. Prog. Phys. 1975, 38, 903−974.

(38)

Kalinowski, J. Excimers and Exciplexes in Organic Electroluminescence. Mater. Sci.-

Poland 2009, 27, 735−756. (39)

Azumi, T.; Armstrong, A. T.; McGlynn, S. P. Energy of Excimer Luminescence. II.

Configuration Interaction between Molecular Exciton States and Charge Resonance States. J. Chem. Phys. 1964, 41, 3839–3852. (40)

Wu, Y.; Zhou, J.; Phelan, B. T.; Mauck, C. M.; Stoddart, J. F.; Young, R. M.;

Wasielewski, M. R. Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy. J. Am. Chem. Soc. 2017, 139, 14265–14276. (41)

Tamai, Y.; Ohkita, H.; Shimada, J.; Benten, H.; Ito, S.; Yamanaka, S.; Hisada, K.; Tani,

K.; Kubono, K.; Shinmyozu, T. Dynamical Excimer Formation in Rigid Carbazolophane via Charge Transfer State. J. Phys. Chem. A 2013, 117, 7776–7785. (42)

Castanheira, E. M. S.; Martinho, J. M. G. Solvatochromic Shifts of Naphthalene and

Pyrene Excimers. J. Photochem. Photobiol. A 1994, 80, 151–156.


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Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43)

Sung, J.; Kim, P.; Fimmel, B.; Würthner, F.; Kim, D. Direct Observation of Ultrafast

Coherent Exciton Dynamics in Helical π-Stacks of Self-Assembled Perylene Bisimides. Nat. Commun. 2015, 6, 8646. (44)

Kaufmann, C.; Kim, W.; Nowak-Król, A.; Hong, Y.; Kim, D.; Würthner, F. Ultrafast

Exciton Delocalization, Localization, and Excimer Formation Dynamics in a Highly Defined Perylene Bisimide Quadruple π-Stack. J. Am. Chem. Soc. 2018, 140, 4253–4258. (45)

Gould, I. R.; Young, R. H.; Mueller, L. J.; Albrecht, A. C.; Farid, S. Electronic Structures

of Exciplexes and Excited Charge-Transfer Complexes. J. Am. Chem. Soc. 1994, 116, 8188– 8199. (46)

Koch, M.; Licari, G.; Vauthey, E. Bimodal Exciplex Formation in Bimolecular

Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption. J. Phys. Chem. B 2015, 119, 11846–11857. (47)

Rosspeintner, A.; Vauthey, E. Bimolecular Photoinduced Electron Transfer Reactions in

Liquids under the Gaze of Ultrafast Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 25741–25754. (48)

Beckwith, J. S.; Rosspeintner, A.; Licari, G.; Lunzer, M.; Holzer, B.; Fröhlich, J.;

Vauthey, E. Specific Monitoring of Excited-State Symmetry Breaking by Femtosecond Broadband Fluorescence Upconversion Spectroscopy. J. Phys. Chem. Lett. 2017, 8, 5878– 5883. (49)

Vauthey, E. Photoinduced Symmetry-Breaking Charge Separation. ChemPhysChem

2012, 13, 2001–2011. (50)

Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. Subpicosecond

Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited. J. Phys. Chem. 1995, 99, 17311–17337. (51)

Gerecke, M.; Richter, C.; Quick, M.; Ioffe, I. N.; Mahrwald, R.; Kovalenko, S. A.;

Ernsting, N. P. Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy. J. Phys. Chem. B 2017, 121, 9631–9638.


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(52)

Page 22 of 28

Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion

Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465. (53)

Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and

Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305.


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Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ar H13C6 H13C6

Ar OO

O

C6H13

O

N

N

O

O

C6H 13

OO Ar Ar OO

O H13C6 H13C6

Ar Ar O

N

N

O

O

C6H 13 C6H13

OO Ar

Ar Ar =

Chart 1. Molecular structure of rPBI-CP.


23


(a)

THF

BCN

rPBI monomer in BCN

Absorbance (Norm.)

DCM

Fl. intensity (Norm.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

350

400

450

500

550

600

650

700

750

Wavelength (nm)

(b)

Top view

19.62°

Side view

3.484 Å

Figure 1. (a) Normalized steady-state absorption (solid lines) and fluorescence (dashed lines) spectra of rPBI-CP in various solvents. (THF, tetrahydrofuran; DCM, dichloromethane; BCN, benzonitrile). The spectra of rPBI monomer in BCN are also shown for comparison. (b) Geometry optimized structure of rPBI-CP based on B97D3 functional and def2SVP basis set. 4-tert-Butyl groups in phenoxy substituents and n-hexyl groups in cyclophane backbones were replaced by hydrogens and methyl groups in DFT calculations, respectively, in order to reduce computational costs.


24

Page 25 of 28

(a)

(b) -0.13

0.55

1.2

1.9

-0.18

2.5

1.8

3.8

5.8

7.0

100

Time (ps)

Time (ps)

100 10 2 0

10 2 0

Time (ps)

2

0.5 0.75 1.3 2.6 5 10 20 40 100 200 300 Steady -state

Fl. Intensity (a.u.)

1

4

2

0

0 600

625

650

675

700

725

750

6 4

725

2

A (2 ps) B (24.5 ps) C (2.8 ns, fixed)

8

690

685

Intensity (Norm.)

10

A (2 ps) B (9.6 ns, fixed)

700

Intensity (Norm.)

0

700

Fl. Intensity (a.u.)

6

2 0 600

775

625

650

Wavelength (nm)

675

700

725

750

775

Wavelength (nm)

(c)

(d) 600

DCM

τfl (ns)

rPBI-CP (DCM) rPBI-CP (BCN) rPBI (DCM) rPBI (BCN)

2 ps

400 200 0 0

50

100

150

200

250

800

300

BCN

600

2 ps

400

Fl. Intensity (counts)

Total fl. Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

9.6 2.8 5.7 5.3

100

23 ps

200

10

0 0

50

100

150

200

250

300

0

10

20

30

40

50

60

70

80

Time (ns)

Time (ps)

Figure 2. Time-resolved fluorescence contour map (top), spectra (middle) and evolution associated spectra (bottom) of rPBI-CP in (a) DCM and (b) BCN. In both solvents, the sample was photoexcited with 900 μW (90 nJ/pulse) (c) Total fluorescence intensity (integrated area from 600 to 780 nm) kinetic traces of rPBI-CP in DCM (top) and BCN (bottom) from 0.5 to 300 ps. (d) Fluorescence decay profiles of rPBI-CP and rPBI in DCM and BCN measured by TCSPC method under photoexcitation at 450 nm. The probe wavelength was selected at each fluorescence maximum of corresponding steady-state fluorescence spectra.


25


OD -0.045 -0.030 -0.015

0.0

OD -0.045 -0.030 -0.015

(b)

0.015 0.030

1000

100

100

Time (ps)

1000

50 40 30 20 10 0

-0.03 0.03 0.00 A (2 ps) B (> 8000 ps)

-0.03

500

600

700

800 900 1000 1100 1200 1300

0 0.03 ΔOD 580 nm 655 nm 780 nm 950 nm 1110 nm

0.0

0.015 0.030

50 40 30 20 10 0

0

Intensity (a.u.)

Time (ps)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 28

0.03 0.00 A (2 ps) B (25 ps) C (2500 ps)

-0.03 500

600

700

0.02 ΔOD 480 nm 640 nm 780 nm 970 nm 1080 nm 1145 nm

800 900 1000 1100 1200 1300

Wavelength (nm)

Wavelength (nm)

Figure 3. Transient absorption contour map (top left), decay profiles at different probe wavelengths (top right), and evolution-associated difference spectra (bottom left) obtained from a global target analysis of rPBI-CP in (a) DCM and (b) BCN, respectively. In both solvents, the sample was pumped with 500 μW (500 nJ/pulse) at 540 nm.


26

(a)

(b)

(c) 3 rPBI+ rPBI− rPBI++rPBI− EADSC

2

800

0

0 rPBI+ rPBI− rPBI++rPBI− EADSC

-2

-2

2

(rPBI-CP in BCN)

970 1075

1 900

(rPBI-CP in BCN)

-4 500

600

700

Wavelength (nm)

800

2

1

972

2.2

2.2 Intensity (x 10-2, a.u.)

780

ΔOD (x 10-3)

2 ΔOD (x 10-3)

3

4 Intensity (x 10-2, a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


2.0 1.8

2.0 μem (Debye)

Page 27 of 28

2 ps

1.8

0 900

1000

1100

1200

1300

Wavelength (nm)

2

4

6

8

2 ps & 23 ps

1.4 1.2

DCM BCN

1260

0

0

1.6

1.0

1080

1.6

0.8 0

50 100 150 200 250 300 Time (ps)

Figure 4. Comparison between the third evolution associated difference spectra (EADSC) of rPBI-CP in BCN corresponding to the CT resonance enhanced excimer and simulated transient absorption spectra of fully separated rPBI+/rPBI– ion pairs in (a) VIS and (b) NIR region. Transient absorption spectra of rPBI+ and rPBI– were measured by bimolecular charge separation experiments. See Figure S9 for details. (c) Time-dependent μem profiles of rPBI-CP in DCM and BCN.


27


Page 28 of 28

(1) Excimer formation

(1PBI*-PBI)

(τ =

Frenkel Exciton

200 fs)

δ+ 1(PBI-PBI)*

δ-

Excimer

hν

1(PBIδ+-PBIδ-)*

(2) Structural relaxation (τ =

2 ps)

(3’) Incomplete SB-CS with solvation (τ =

23-25 ps)

(4) CT resonance enhanced excimer lifetime

(3) Excimer lifetime

(τ =

(τ = 9.6 ns)

(PBI-PBI)

2.5-2.8 ns)

DCM : (1) → (2) → (3) BCN : (1) → (2) → (3’) → (4)

Figure 5. Jablonski diagram summarizing the solvent-modulated excited-state dynamics of rPBICP measured by time-resolved fluorescence and absorption techniques. Cyclophane backbones and 4-tert-butyl-phenoxy substituents of rPBI-CP are omitted for clarity.


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Solvent-Modulated Charge-Transfer Resonance ... - ACS Publications

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