DOI: 10.1002/cphc.201500156

Articles

Folding-Induced Modulation of Excited-State Dynamics in an Oligophenylene–Ethynylene-Tethered Spiral Perylene Bisimide Aggregate Minjung Son,[a] Benjamin Fimmel,[b] Volker Dehm,[b] Frank Wìrthner,*[b] and Dongho Kim*[a] The excited-state photophysical behavior of a spiral perylene bisimide (PBI) folda-octamer (F8) tethered to an oligophenylene–ethynylene scaffold is comprehensively investigated. Solvent-dependent UV/Vis and fluorescence studies reveal that the degree of folding in this foldamer is extremely sensitive to the solvent, thus giving rise to an extended conformation in CHCl3 and a folded helical aggregate in methylcyclohexane (MCH). The exciton-deactivation dynamics are largely governed

by the supramolecular structure of F8. Femtosecond transient absorption (TA) in the near-infrared region indicates a photoinduced electron-transfer process from the backbone to the PBI core in the extended conformation, whereas excitation powerand polarization-dependent TA measurements combined with computational modeling showed that excitation energy transfer between the unit PBI chromophores is the major deactivation pathway in the folded counterpart.

1. Introduction Self-assembly of perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI) derivatives has attracted increasing attention for over a decade with respect to both the synthesis routes and potential applications in organic nanodevices.[1] In addition to the outstanding fluorescence properties and chemical robustness of the PBI monomer,[2] the planar structure of bay-unsubstituted PBIs enables them to undergo facile p–p stacking interactions with adjacent molecules to form supramolecular self-assembled architectures in many solvents, in particular in aliphatic solvents such as methylcyclohexane (MCH).[3] Taking advantage of the strong intermolecular interactions between PBIs, PBI-bearing foldamers, in which the individual chromophores are covalently appended to external scaffolds and are capable of p–p stacking at the same time, have recently emerged as a new class of PBI-based supramolecular architectures. Such foldamers are potential candidates as advanced functional materials in the following two aspects. First, whereas most self-assemblies with a purely noncovalent nature have a major drawback that no unambiguous information can be given about their accurate spatial arrangement or size, the preorganization of chromophores with the aid of a rigid backbone renders it feasible to investigate the struc[a] M. Son, Prof. Dr. D. Kim Spectroscopy Laboratory for Functional p-Electronic Systems and Department of Chemistry Yonsei University, Seoul 120-749 (Korea) E-mail: [email protected] [b] B. Fimmel, Dr. V. Dehm, Prof. Dr. F. Wìrthner Institut fìr Organische Chemie & Center for Nanosystems Chemistry Universit•t Wìrzburg Am Hubland, 97074 Wìrzburg (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500156.

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ture–property relationship and material properties of three-dimensional dye assemblies.[4] Second, with p-conjugated backbones it is possible to achieve intramolecular supramolecular p–n heterojunctions[5] by modulating the donor–acceptor relationship established between the scaffold and electron-deficient PBI units. Recent examples of PBI-based intramolecular supramolecular p–n heterojunctions, albeit small in size, include xanthene-linked PBI dimer aggregates investigated by the groups of Wasielewski and Janssen,[6] and calix[4]arenelinked PBI dimer aggregates and PBI–pyrene hybrids investigated by the groups of Williams and Wìrthner.[7] In recent work, we have developed the first class of p–n heterojunction-type PBI foldamers Fn (n = 1, 2, 3, and 8) by introducing a rigid ortho–meta alternating oligophenylene–ethynylene (OPE) backbone,[8, 9] and demonstrated that their degree of folding can be easily controlled by changing the solvent. In other words, when dissolved in poor solvents, the individual PBI dyes covalently appended to the OPE platform are capable of folding into an H-aggregate-type intramolecular spiral assembly with a closer p–p distance of 3.4 æ even at low concentrations,[8, 9] as evidenced by the dramatic spectral changes in the steady-state absorption spectra. From our previous spectroscopic analysis of smaller foldamer systems (F1, F2, and F3)[8] we noticed a remarkable feature, that is, that the phenylene–ethynylene moiety serves as an electron donor, leading to ultrafast photoinduced electron transfer (PET) (1.5–4.5 ps) from OPE to the electron-accepting PBI cores; this is in line with the previous findings on the aforementioned xanthene- and calix[4]arene-tethered dimeric PBIs.[6, 7] Notably, studies on the photophysics of PBI-based foldamer systems remain at a rudimentary stage. Profound investigations are only available on small model compounds derived from PBI-dimer aggregates, whereas the excited-state photophysical characteristics of larger oligomer and/or polymer ana-

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Articles 2. Results 2.1. Steady-State Absorption, Fluorescence, and Fluorescence Excitation Anisotropy The ground-state absorption and fluorescence spectra of the monomer (F1) and the foldamers F2 and F8 were measured in CHCl3, tetrahydrofuran (THF), and MCH (Figure 2). The solvents

Figure 1. a) Molecular structures of helical PBI foldamer F8 and reference molecules F1 and F2 studied in this work.[11] b) Schematic drawings of AMBER force-field geometry-optimized structures of F8 in its unfolded (left) and folded form (right) (a: azimuth angle between the in-plane polarized transition dipole moments of two adjacent PBI units assumed to be stacked perpendicular to the helix axis).

logues are of pivotal importance for fabricating functional molecular wires and examining device performance, for example, energy- and charge-transport efficiencies.[10] In this context, herein, we focus on the folda-octamer (F8) of the Fn series (Figure 1) and present one of the first in-depth photophysical investigations on an oligomeric PBI foldamer system along with direct comparisons to its monomer (F1) and dimer (F2) model compounds. In a similar fashion to the dimer (F2), the transformation from the extended and conformationally inhomogeneous structure of F8 into a compact folded form with much enhanced regularity, exhibiting identical p–p distances of 3.4 æ, can be realized by choosing the appropriate solvent. A particular feature of the folded structure of F8 is that the increased number of constituent dye units leads to a nearly regular rotational displacement (a) of approximately 488.[9] Taking it a step further than our latest work on the folding behaviors of the foldamers and their influence on the PET process from the electron-donating OPE backbones to the PBI cores,[8] here we focus on the interplay between the excitation energy transfer (EET) that occurs among the constituent PBI units and PET pathways by using a combination of various time-resolved absorption and fluorescence techniques with varying excitation laser fluence and polarization, as well as computational modeling. Interestingly, these two competing processes (PET and EET) were found to occur in a switchable manner that is governed by the three-dimensional supramolecular conformation of the foldamer. The extended form revealed direct spectral signatures of a moderately efficient PET process, whereas the folded helix, reminiscent of its natural counterpart, DNA, showed almost completely diminished PET capability and instead an EET deactivation pathway between the constituent PBIs. ChemPhysChem 2015, 16, 1757 – 1767

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Figure 2. Steady-state absorption (black) and fluorescence emission spectra (gray; excitation wavelength (lexc) = 480 nm) of F1, F2, and F8 in CHCl3 (solid lines), THF (dashed lines), and MCH (dotted lines). The absorption spectra of F2 and F8 are normalized to the 0–1 absorption bands at approximately 490 nm.

were carefully selected, so as not to induce formation of unwanted intermolecular aggregates by self-assembly. To exclusively achieve the desired intramolecular aggregate by folding, THF had to be used for dimer F2, as self-assembly into larger aggregate structures was observed in MCH (Figure S2 in the Supporting Information); a similar case to this has also been observed for a bis(merocyanine) dye.[8, 12] The monomeric reference compound F1 was almost insoluble in MCH, so it had to be dissolved in THF as well for comparison with the larger oligomer analogues. The absorption of F1 exhibited the characteristic features of a PBI monomer consisting of well-resolved S0–S1 vibronic structures at 425–550 nm without appreciable solvent dependence,[13] whereas clear differences were observed in the absorption spectra of F2 and F8 depending on the number of building blocks and the solvent. When the solvent was changed from CHCl3 to THF or MCH, broadening of the vibronic progression, a bathochromic shift of the 0–0 absorption band, and, in particular, a reversal in the intensities of the 0–0 and 0–1 absorption bands were observed; these phenomena are all highly indicative of helically p–p stacked PBI units with predominant H-aggregate-type excitonic coupling.[8, 9, 14, 15] The ratio of absorbance, A0¢0/A0¢1, can be used as a useful index of the degree of aggregation, or folding, of the PBI dyes, where the value for a monomer is 1.65.[16] F1 displayed a value close to 1.65 in both CHCl3 and THF, whereas those for F2 and

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Articles wavelength irrespective of the solvent, whereas those of F1 were not. When the emission wavelength was changed from the monomer-like fluorescence band (540 nm) to the excimer fluorescence band (625 nm), F2 and F8 exhibited a decrease in the absolute anisotropy values from 0.03–0.04 to nearly zero. This is because the transition dipoles oriented along the long axis of each unit PBI chromophore are displaced in a nonparallel, helical arrangement (a = 35–488) in the aggregated oligomer species, leading to the canceling out of the dipole vectors.

Table 1. Photophysical properties of F1, F2 and F8 in different solvents at room temperature.[8–9] Cmpd.

Solvent

Ffl[a]

A0¢0/A0¢1[b]

r540nm[c]

r625nm[c]

F1

CHCl3 THF CHCl3 THF CHCl3 MCH

0.21 0.36 0.28 0.16 0.015 0.008

1.64 1.63 1.27 0.78 1.12 0.88

0.05 0.04 0.03 0.04 0.04 0.03

0.05 0.04 0.01 0.01 0.007 0.007

F2 F8

[a] Fluorescence quantum yield (reference: Rhodamine 6G (Ffl = 0.95) in ethanol). [b] Ratio of the absorbance at 0–0 and 0–1 absorption bands. [c] Steady-state fluorescence excitation anisotropy values measured at emission wavelengths of 540 and 625 nm.

2.2. Time-Resolved Fluorescence and Fluorescence Anisotropy

F8 were moderately lower in CHCl3 and decreased to less than 1 in THF for F2 (0.78) and in MCH for F8 (0.88; see Table 1). Comparing the values with those of extended columnar selfassembled PBI stacks with a = 308 (0.54),[14a] we observe a nice match between these experimentally observed values and the prediction by exciton coupling theory[15, 17] that the relative intensity of 0–0 absorption bands increases as a increases; this implies that the theoretically forbidden 0–0 transition in H-aggregates becomes partially allowed by the rotational offsets between dye units. Yet, considering that A0¢0/A0¢1 band ratios of the F2 and F8 were all smaller than 1.65 in CHCl3, even the extended conformations of the foldamers are thought to be not strictly devoid of dye–dye interactions. The fluorescence spectra also exhibit distinctive features that depend on the conformation of the molecules. F1 again showed clear vibronic progression in the fluorescence spectrum, which is almost the mirror image of the absorption spectrum, whereas a broad structureless emission band peaking at approximately 625 nm appeared for F2 and F8; this band is ascribable to the excimer emission coming from stacking between PBI units.[14, 18] A fair amount of monomer-like fluorescence remained for F2 in both solvents, thus implying that the interaction between the two PBI units in F2 is not as strong as that in the folded F8. The most striking solvent dependence in the fluorescence spectra was noticed for F8; thus, F8 is the best system thereof to study the structure-dependent photophysical properties of these foldamers. The fluorescence quantum yields of all three compounds were strongly reduced in comparison to a typical PBI monomer; this is mainly owing to nonradiative deactivation through the PET process occurring from the electron-donating backbone to the PBI units.[8, 19] By examining the steady-state fluorescence excitation anisotropy spectra, we were able to elucidate the relative orientations of emissive transition dipole moments with respect to those of excitation, where several depolarization pathways such as rotational diffusion and EET can affect the anisotropy values.[20] We separately measured the anisotropy values by employing two emission wavelengths of 540 and 625 nm, which correspond to the monomer-type and excimer-type fluorescence, respectively (see Figure S3 in the Supporting Information and Table 1). Notably, the anisotropy values of F2 and F8 proved to be strongly dependent on the monitoring ChemPhysChem 2015, 16, 1757 – 1767

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To shed light on the fluorescence behaviors of the foldamer systems, their time-resolved fluorescence and fluorescence anisotropy decay profiles (Figure S4 and Figure 3) were obtained by a time-correlated single photon counting (TCSPC) technique

Figure 3. Time-resolved fluorescence anisotropy decay profiles (lexc = 450 nm, lem = 625 nm) of F1, F2, and F8 in a) CHCl3 and b) THF (F1, F2) and MCH (F8).

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Articles Table 2. Fluorescence lifetimes and fluorescence anisotropy decay profiles of F1, F2 and F8 in different solvents at room temperature. Cmpd.

Solvent

t1[a,b] [ns]

t2[a] [ns]

t3[a] [ns]

r0[c]

tr[d] [ns]

F1

CHCl3 THF CHCl3 THF CHCl3 MCH

< 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05

2.7 2.6 2.7 2.0 3.0 3.0

– – 20.0 20.0 20.1 19.5

0.35 0.34 0.15 0.15 0.08 0.02

0.78 0.66 1.1 0.89 1.5 1.7

F2 F8

[a] Components from the fitted fluorescence lifetimes. [b] Accurate timescales of the short components could not be determined due to the instrumental response (~ 50 ps) of our setup. [c] Initial anisotropy values at t = 0. [d] Fitted rotational diffusion times.

(the fitted parameters are shown in Table 2 and Table S1 in the Supporting Information). The monitoring wavelengths (lem) were gradually moved from the monomer-like fluorescence band (530–540 nm) to the red edge of the fluorescence spectra (675–730 nm), where the predominant population is expected to be the excimeric species. The fluorescence decay profiles of F2 and F8 decomposed primarily into two time components with a gradually increasing proportion of excimer fluorescence, wherein the shorter time components (2.0– 3.0 ns) are ascribable to the fluorescence lifetimes of the noninteracting dyes with monomer-type spectral features and the longer ones (19.5–20.1 ns) to the contribution from the excimer fluorescence from strongly coupled aggregated PBIs. When the 0–0 emission bands were probed, fast sub-nanosecond components were detected for all three compounds, the exact time constants of which could not be deduced owing to the instrumental response ( … 50 ps) of our TCSPC setup and had to be further examined by transient absorption (see below). These short components likely represent rapid fluorescence quenching, which results from the PET process from the backbone to the PBI cores; this is also in line with the significantly reduced fluorescence quantum yields observed in the steady-state fluorescence spectra of the foldamers. It is this fast relaxation pathway that makes the fluorescence lifetime of the monomer (F1; 2.3–2.7 ns) perceptibly shorter than those reported for PBI monomers.[8] Moreover, when the fluorescence decay profiles of F1 and the backbone-free monomer M were compared, M revealed a single exponential decay of 3.9–4.0 ns without any fast component (Figure S5 in the Supporting Information); this finding further substantiates the strong quenching effect from the electron-rich OPE moiety. The relative amplitudes of these fast components in F1 and F2 were higher in CHCl3 than in THF; this led us to expect that the contribution of the PET process would be greater in the extended form of aggregate. The rotational diffusion times acquired from the fluorescence anisotropy decays of F1, F2, and F8 provided us with information on the approximate size (hydrodynamic volume) of the molecules, with the diffusion times gradually increasing as the number of building blocks in the oligomers was increased, starting from 780 ps (CHCl3) and 660 ps (THF) for F1 and increasing to 1.5 and 1.7 ns for F8. The slight variations in the ChemPhysChem 2015, 16, 1757 – 1767

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time constants upon changing the solvent can be attributed to differences in the viscosity of the solvents (CHCl3 : 5.7 Õ 10¢4 Pa Õ s; THF: 5.5 Õ 10¢4 Pa Õ s; MCH: 6.7 Õ 10¢4 Pa Õ s at 25 8C), which affects the rotational diffusion constant, thereby the rotational reorientation time of the molecule.[21] From the progressively decreased initial anisotropy values (r0) observed in the fluorescence anisotropy decay profiles, we can conjecture that not only the nonparallel spatial distribution of the transition dipole moments, but also an ultrafast depolarization channel such as excitation energy migration, affects the anisotropy of the foldamers.[14b, 22] F1 exhibited the initial anisotropy values of 0.35 (CHCl3) and 0.38 (THF), which are only slightly smaller than 0.4, the theoretically expected maximum anisotropy value we can get when no depolarization takes place at all, whereas those of F2 and F8 were significantly lower than this value, thus indicating the intervention of an excitation energy migration process occurring between the unit PBI chromophores. In particular, the initial anisotropy values of F8 were remarkably lower than those of F1 and F2, likely owing to additional exciton–exciton interactions with surrounding PBI units originating from the increased number of chromophores.[22] Furthermore, judging from the clearly solvent-dependent, that is, conformation-dependent initial anisotropy values of F8 in CHCl3 (0.08) and MCH (0.02), which is in contrast to those of F1 and F2, we can surmise that the efficiency, or the timescale, of this energy-transfer process may also be dependent to a significant degree on whether the foldamers adopt an extended or a folded structure. 2.3. Femtosecond Transient Absorption and Transient Absorption Anisotropy Our aforementioned presumption of the conformation dependence on the EET process was further examined by femtosecond transient absorption (TA) spectroscopy with varying excitation power density. The possibility of an additional energytransfer process between the backbone and the PBI unit can be neglected, as there is no significant spectral overlap between the steady-state fluorescence spectrum of the OPE backbone B (Figure S6 in the Supporting Information), which mainly emits at 335–435 nm, and the S0-S1 transition (425– 550 nm) of PBI. An excitation with high photon density could lead to multiple excitations of the PBI dye arrays, which can result in fast deactivation by a singlet-singlet annihilation (SSA) process,[23] therefore a careful analysis of the pump-power dependence of the TA kinetic traces can offer us clues on the dynamics of the energy-hopping process between PBI dyes. As the most common outcome of SSA is one of the two excitons being promoted to an upper excited singlet state and the other quickly relaxing back to the lowest excited singlet state, the net effect is the loss of one exciton, and this depopulation of S1 state can be observed as a fast decay component in the TA kinetic traces.[10b, 24] To minimize any possible complications in our analysis owing to the contribution from the overlap between the stimulated emission (SE) and the ground-state bleaching (GSB)

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Articles signals, we have chosen the excited-state absorption (ESA) signals at approximately 710 (F1) and 825 nm (F2 and F8) as the probe region when analyzing the pump-power dependent TA decay profiles. The monomer (F1) revealed no power dependence, whereas the decay profiles of F2 and F8 exhibited short decay components that turned out to be dependent on the excitation photon density during the initial 15 ps (Figure 4). The decay profiles of F2 were fitted to a biexponential function, in which the long residuals were fixed as the singlet excited-state lifetimes (2700 and 2000 ps), as revealed in the fluorescence

Figure 4. Pump-power dependent transient absorption decay profiles of F1, F2, and F8 in a) CHCl3 and b) THF (F1, F2) and MCH (F8) by employing the pump wavelength of 530 nm and the probe wavelengths of 710 nm (F1) and 825 nm (F2 and F8). The pump beam intensities used are 0.8 W cm¢2 (4 mW), 0.4 W cm¢2 (2 mW), 0.2 W cm¢2 (1 mW), and 0.1 W cm¢2 (0.5 mW) for F1 and F2, and 0.4 W cm¢2 (2 mW), 0.2 W cm¢2 (1 mW), 0.1 W cm¢2 (0.5 mW), and 0.05 W cm¢2 (0.25 mW) for F8.

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Table 3. Fitted transient absorption and transient absorption anisotropy decay parameters for F2 and F8 in different solvents. Cmpd.

Solvent

Power-dependent TA t1 [ps] t2 [ps]

TAA t1 [ps]s

t2 [ps]

F2

CHCl3 THF CHCl3 THF

2.0 1.0 8.9 4.9

0.88 œ 0.03 0.47 œ 0.03 0.90 œ 0.07 0.45 œ 0.08

1100 890 1500 1700

F8[a]

2700 2000 3000 3000

[a] For F8, the fastest decay components (< 180 fs) were neglected, as the slower of the power-dependent decay profiles is regarded as the SSA signal.

decay profiles (Table 3). In contrast, in the case of F8, the TA decay traces were best fitted into a triexponential function, due to the fast (< 180 fs) components, the time constants of which cannot be resolved in the time resolution of our instrument ( … 180 fs), which might be associated with high-order annihilation processes between more than two chromophores.[25] Notably, F8 exhibited an SSA time of 8.9 ps in its extended form, that is, in CHCl3, but this was prominently decreased by approximately 45 % upon folding (in MCH; 4.9 ps). This result is in excellent agreement with our previous findings that in MCH the constituent PBI units are structurally restricted to maintain a short interchromophoric distance of approximately 3.4 æ, driven by the extremely strong tendency for folding in nonpolar solvents through pronounced p–p stacking interactions.[3a, 14a, 26] A nearly similar trend was observed for the SSA times of the model compound F2, in which the SSA time in CHCl3 (2 ps) was two-fold higher than 1 ps in THF. In the case of F2, however, the amplitudes of the annihilation components are much smaller, as the probability that both of the two PBI units are simultaneously photoexcited is significantly lower than in the larger F8 oligomer. Polarization-dependent TA measurements, which employ different polarizations of the pump and probe pulses, provide valuable insights into the previously mentioned ultrafast depolarization channel;[20b, 24, 27] this channel is too fast to be elucidated by the TCSPC technique with the time resolution of approximately 50 ps. As opposed to PBI J-aggregate systems, in which the transition dipole moments are placed parallel to each other, the helically organized nature of our aggregate system allows for a fast decay component in the time-dependent transient absorption anisotropy (TAA) profiles, which is indicative of an EET process between the constituent units. As previously mentioned, the TAA decay profiles were probed at the ESA signals ( … 710 nm) and the fitted decay profiles are shown in Figure 5 and Table 3. In the TAA decay profiles of the monomer (F1), no fast depolarization components were found, whereas those of F2 and F8 were fitted with two time components, the longer of which represent the rotational diffusion motion of the molecules. The fitted depolarization times of F2 (F8) became 46 % (50 %) shorter upon folding in THF (MCH), which is in line with the trend observed in the annihilation times probed by pumppower dependent TA. 1761

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Articles

Figure 5. Transient absorption anisotropy decay profiles of F1 (top), F2 (middle), and F8 (bottom) in CHCl3 (circles), THF (diamonds), and MCH (triangles). The pump and probe wavelengths employed were 530 and 710 nm, respectively. All the experiments were carried out under low pump-power conditions (< 0.5 mW) to avoid multiple-exciton related processes.

By shifting the probe wavelength to the near-infrared (NIR) region, we can acquire an in-depth understanding on the process responsible for the rigorously quenched fluorescence of F8, that is, the PET process. As the formation of PBI radical anions is easily recognized by an decrease of neutral PBI absorption in the 425–550 nm region and a concomitant increase of new absorption bands at 713, 800, and 960 nm,[5b, 7b, 28] we focused our attention on the ESA signals, which should contain the spectral signature of absorption from the PBI radical anion (PBI·¢) formed during the photoinduced charge separation (CS) process. Comparative analysis of the TA spectra of F8 measured in CHCl3 and MCH in both the visible and NIR probe regions allowed us to assess the charge-transfer process between the OPE backbone and PBI cores (Figure 6). The two spectra revealed clear differences in the structure of the ESA signals: the extended form of the aggregate exhibited relatively sharp spectral features at 710 and 960 nm, which correspond to the absorption features of a PBI radical anion, whereas the stacked counterpart only showed rather broad bands in the same region without perceptible band structures, thus suggesting that the PET process is highly suppressed in the folded form. As it is impossible to completely exclude the contribution from the SE of the excimer species to the ESA signals at 710 nm, the NIR band at 960 nm is a better candidate for probing the PET dynamics. The temporal evolution of the NIR TA spectra in CHCl3 was globally analyzed (Figure S7 in the Supporting Information), because the PBI radical anion peak at 960 nm, although sufficiently sharp, was largely immersed in the broad ESA signal of a neutral PBI (1*PBI) decaying throughout the entire range of our observation, thus making it impossible to exclusively probe the emerging radical anion absorpChemPhysChem 2015, 16, 1757 – 1767

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Figure 6. Transient absorption spectra of F8 measured in a) CHCl3 and b) MCH by using the pump wavelength of 530 nm. The arrows indicate the decaying band profiles with time. Insets show the enlarged NIR TA spectra (900–1100 nm) up to a time delay of 100 ps (arrows indicate the PBI radical anion bands at approximately 960 nm).

tion. Whereas the transient species with the shorter time constant (1.6 ps) exhibited broad and intense signals, which much resemble the broad ESA signals of 1*PBI, the species with the longer time constant (100 ps) showed a broad diminished band and partially resolved radical anion peak at 960 nm. This feature indicates that the CS process occurs within several picoseconds to form the radical cation of the innately electronrich OPE backbone and the anionic counterpart of the electron-deficient PBI moiety, which subsequently undergo charge recombination (CR) in the next few hundred picoseconds. On the contrary, no distinct spectroscopic signatures of the electron-transfer process were observed in the same region for the folded structure; this suggests that PET might take place, but is likely to be extremely inefficient compared to the extended form.

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Articles 3. Discussion The EET times of F8 and its model compound F2 can be experimentally deduced from the fast decay components of the TAA decay profiles. For the dimer (F2), estimating the EET time is a simple procedure, as the energy-migration process is considered reversible between the two constituent PBI units following a simple equilibrium as indicated below [Eq. (1)]: PBI1* PBI2 Ð PBI1PBI2*

ð1Þ

in which PBI and PBI* represent the ground and excited states of the PBI units, respectively. If we denote the forward and backward reaction rate constants kEET and k¢EET, respectively, the EET time (tEET = kEET¢1) can be derived from [Eq. (2)]: tdep ¼ kdep ¢1 ¼ 1=ðkEET þ k¢EET Þ

ð2Þ

in which tdep is the depolarization time, which is experimentally obtained from the TAA decay profiles.[29] As the two PBI units in F2 are identical, it follows that the EET time is twice that of depolarization (Table 3), thus giving rise to EET times of 1.76 œ 0.06 ps (CHCl3) and 0.95 œ 0.06 ps (THF), respectively (Table 4).

r0

a [8]

d

kEET [s¢1]

tEET[a] [ps]

tEET(F2)[b] [ps]

CHCl3 MCH

0.35 0.34

117.2[c] 47.9[c]

¢0.19 0.17

5.46 Õ 1011 1.04 Õ 1012

1.83 0.96

1.76 0.95[d]

[a] Calculated EET times by fitting with the DNA helix model (see Equations (3)–(4)). [b] Estimated EET times for F2 by using Equation (2). [c] Averaged values determined from the AMBER force-field optimized structure of each conformation (Table S2, in the Supporting Information). [d] Measured in THF.

In contrast, the depolarization dynamics of the spiral octamer F8 are rather complicated and so a robust model to express the EET process in a helical molecular array had to be introduced. By assuming that the eight PBI units are located with a regular rotational displacement and interplanar distance (the separation between one PBI plane and the adjacent PBI plane), it is possible to derive the rate constant of energy migration (kEET) to the adjacent PBI units by simulating the experimentally acquired anisotropy decay profiles into the DNA helix model [Eq. (3)],[30] which describes one of the most commonly encountered natural counterparts with helical geometry: rðtÞ=r 0 ¼ 0:5½ð1¢dÞexpð¢2kEET tÞ þ 1 þ d¤

ð3Þ

in which r(t) is the time-dependent anisotropy, r0 is the anisotropy without energy transfer, and d is the depolarization factor [Eq. (4)]: ChemPhysChem 2015, 16, 1757 – 1767

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d ¼ ð3cos2 a¢1Þ=2

ð4Þ

in which a represents the azimuth angle between the in-plane polarized transition dipole moments of each donor–acceptor pair assumed to be stacked perpendicular to the helix axis (Figure 7; see Table 4 for the individual parameters used for calculation). To evaluate the a values, we computed the optimized structure of each conformation by using DFT-D calculations at the B97D/STO-3G[31] level (F2) and by the cost-effective AMBER force-field method (F8), and the averaged values of the seven individual a values were employed to calculate EET times (Table S2; see the Supporting Information for details on the computational methods).

Table 4. Fitting parameters for the calculation of EET times in F8 in its extended (CHCl3) and folded forms (MCH) by using the DNA model and the fitted results. Solvent

Figure 7. Ratio of the time-dependent anisotropy r(t) to r0 for F8 in CHCl3 (top) and MCH (bottom) plotted against time. Solid lines represent the respective fit curves. Insets: selected parts of force-field optimized structures of F8 in its unfolded (top) and folded form (bottom). The double-headed arrows represent the orientations of the transition dipole moments, and the azimuth angles (a) between them are also indicated.

For the stacked structure of F8 in MCH, d = 0.17 (a = 47.98) and kEET = 1.04 Õ 1012 s¢1, which resulted in a fitted EET time (tEET = kEET¢1) of 0.96 œ 0.09 ps. For the extended form in CHCl3, the averaged a value was determined to be 117.28, which led to a d value of ¢0.19 and kEET = 5.46 Õ 1011 s¢1 (tEET = 1.83 œ 0.13 ps). The DFT-D calculation results for the dimer F2 in its unfolded form gave an a value of 165.58, suggesting that the two PBI units are located as far as possible from each other with minimal PBI–PBI interactions,[8] whereas F8 maintains relatively low a values, owing to the higher number of unit chromophores, and thus, additional interactions with adjacent PBI units. Strikingly, the derived time constants exhibit little deviation from those calculated for F2 by using Equation (2) and are almost identical to the SSA times (tannih) of F2. Considering that exciton–exciton annihilation between the two constituents in a dimer system can be directly related to the mutual excitation energy migration process between the two units, tannih = tEET in a dimer,[32] and the EET times of F2 obtained from two different experimental techniques interestingly indicate almost the same energy transfer times of … 2 ps (extended) and 1 ps (folded form), respectively.

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Articles The migration-limited character[23a, 33] of the SSA process also enables us to extract the timescale of the EET process from the fast decay components observed in pump-power dependent TA profiles. The EET times of F8 can be estimated by adopting a modified version of the well-known relationship between SSA and EET times for the cyclic bacterial light-harvesting complex of LH1; this can be applied to linear molecular array systems [Eqs. (5)–(6)][34]: 2 tring annih ¼ tEET =8 sin ðp=2NÞ

ð5Þ

ring tchain annih ¼ 2tannih

ð6Þ

in which tring annih is the SSA time in a ring structure, tEET is the EET time, N is the number of hopping sites in the ring and tchain annih is the SSA time in a chain system). This linear model still shares its roots with the ring model; SSA is regarded as a random-walk process of an excitation with respect to another fixed excitation,[34] with only simple symmetry arguments added to account for a linear-chain system. In our regime, where we exclusively consider the homotransfer occurring between one of the PBI units to another, N = 8, and tchain annih values were determined to be 8.9 ps (CHCl3) and 4.9 ps (MCH), which led to calculated EET times of 1.35 and 0.75 ps, respectively; these values are fairly consistent with those acquired from the analysis of the anisotropy decays. The subtle differences between the EET times obtained by the two separate experimental methods might originate from the more complicated nature and possible heterogeneity of the spatial arrangement of F8 than in smaller systems, despite the solvent-induced ordering into relatively well-defined supramolecular architectures. Moreover, despite carefully choosing the probe wavelength ( … 710 nm), we cannot completely exclude the influence from charge transfer, which is one of the primary deactivation pathways that compete with EET process. Nevertheless, what we stress here is that a clear modification in the energy-transfer times is observed depending on the conformation. The extended structure of F8, which maintains a sufficiently large distance between the PBI units (16.3 æ), can be treated with the point-dipole approximation, and the EET rates can also be mechanistically estimated by the Fçrster-type incoherent energy hopping model[21, 35] [Eq. (7)]: Z kEET ðrÞ ¼ ðQD k2 =tD r 6 Þð9000ðln 10Þ=128p5 Nn4 Þ

0

1

FD ðlÞeA ðlÞl4 dl ð7Þ

in which QD is the quantum yield of the donor, k2 is the orientation factor, tD is the fluorescence lifetime of the donor, r is the donor–acceptor distance, eA is the extinction coefficient of the acceptor at l in M¢1 cm¢1, N is Avogadro’s number, n is the refractive index of the medium and, FD(l) is the fluorescence intensity. Assuming only homotransfer between identical PBI units, we acquired the calculated EET rate of 1.04 Õ 1012 s¢1, and hence, ChemPhysChem 2015, 16, 1757 – 1767

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the EET time of 0.96 ps (see the Supporting Information for details of the Fçrster energy-transfer calculations). A similar analysis is not applicable for the stacked foldamer F8, owing to the breakdown of the point-dipole approximation resulting from the significantly decreased PBI–PBI separations.[36] Although efficient exciton self-trapping to the excimer state is expected to almost prevent appreciable exciton migration or EET processes in an H-type PBI aggregate,[37] the structural constraints imposed by the rigid covalent network of the OPE spacers might modulate the properties of excitonic states and allow the identification of unprecedented SSA dynamics, as the energy levels and transitions between states in H aggregates are known to be highly sensitive to their geometry.[17a, 38] Nevertheless, our results, which exhibit only a 1.8-fold enhancement in EET rate despite a non-negligible 4.1-fold decrease in the averaged PBI– PBI distance (r, 16.3!4.0 æ) upon folding, also point to the fact that self-trapping to the excimer state should greatly reduce the EET efficiency. It is worth noting how the supramolecular arrangement of our dye aggregate system affects the kinetics of the two competing deactivation pathways. Interestingly, while the solventinduced folding of our PBI oligomers rendered the EET process to be almost two-fold accelerated, strong solvophobic effect exerted by MCH was found to simultaneously suppress the electron-transfer process between the OPE linker and the PBI units, as manifested in the clearly distinctive spectral features of the TA spectra in different solvents. In other words, the strong tendency of PBI p surfaces to elude contact with such “bad” solvent forces the constituent dye units to be helically wired, thus maintaining as close a distance to nearby dyes as possible, and this characteristic congregated structure eventually leads to the predominance of EET and exciton self-trapping over PET. In contrast, the structurally less-demanding configuration of F8 in CHCl3, a “good” solvent, makes it more probable for the backbone and the PBI moiety to interact, resulting in a remarkable enhancement of the PET efficiency.

4. Conclusions We designed a helical PBI foldamer aggregate, the constituent dyes of which are covalently appended to rigid oligophenylene–ethynylene linkers, and investigated the folding-induced modification of its excited-state dynamics. This unique system features characteristic folding into a spiral staircase structure driven by the p–p stacking interactions of the PBI units when placed in solvents of low polarity. Femtosecond transient absorption measurements by varying experimental parameters such as the excitation laser power, polarization of the pump and probe beams, and probe region revealed the coexistence of ultrafast excitation energy-transfer and photoinduced electron-transfer dynamics within approximately 2 ps. Moreover, the efficiency of these two representative excited-state dynamics could be tuned by simply controlling the degree of folding. The extended structure affords only an efficient charge separation and not energy migration, whereas a nearly contrasting behavior is observed for the stacked structure. Owing to their well-defined conformational arrangement and suitable size for

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Articles studying fundamental photophysical properties, we envisage that foldable oligomer aggregates such as F8 provide ideal intermediate states between isolated molecules and solid-state materials for the elucidation of fundamental processes originating at interfaces such as p–n heterojunctions[39] or in the bulk such as exciton transport[40] in organic solar cells.

Experimental Section Steady-state spectroscopy: Steady-state absorption spectra of M, F1, F2, and F8 were recorded on a commercial spectrometer (Cary5000, Varian). Fluorescence spectra were measured by a spectrophotometer (FL2500, Hitachi) and the spectral sensitivity was corrected by comparison with well-known reference chromophores such as rhodamine and coumarin dyes.[21] For measuring the steady-state fluorescence excitation anisotropy, Glan laser and sheet polarizers were added into the excitation and monitoring paths, respectively. The anisotropy (r) at a specific monitoring wavelength (lem) as a function of excitation wavelength (lexc) was the given by [Eq. (8)]: rðlexc Þ ¼ ½IVV ðlexc Þ¢GIVH ðlexc Þ¤=½IVV ðlexc Þ þ 2GIVH ðlexc Þ¤

ð8Þ

where IVV(lexc) [or IVH(lexc)] is the fluorescence intensity with the photoexcitation at lexc when the excitation light is vertically polarized and only the vertically (or horizontally) polarized portion of the fluorescence is detected, and the first and second subscripts indicate the excitation and detection polarizations, respectively. The factor G is defined by [IHV(lem)/IHH(lem)], which is equal to the ratio of the sensitivity of the detection system for vertically and horizontally polarized light at a given emission wavelength lem. All steady-state measurements were carried out by using a quartz cuvette with a pathlength of 1 cm at ambient temperatures. Fluorescence decay and fluorescence anisotropy decay: Time-resolved fluorescence decays were obtained by using a TCSPC technique. A mode-locked Ti:sapphire oscillator (MaiTai-BB, SpectraPhysics) was used as the excitation light source, which provides a fwhm (full width at half maximum) of 80 fs with a high repetition rate of 80 MHz. To minimize artifacts such as thermal lensing and accumulation effect, the repetition rate was reduced to 800 kHz by using a homemade acousto-optic pulse selector. The chosen fundamental pulses were frequency doubled by a b-barium borate (BBO) nonlinear crystal (Eksma) of 1 mm thickness. The fluorescence was collected by a microchannel plate photomultiplier (MCP-PMT, R3809U-51, Hamamatsu) with a thermoelectric cooler (C4878, Hamamatsu). Time-resolved fluorescence signals were calculated by a TCSPC board (SPC-130, Becker & Hickel GmbH). The overall instrumental response function (IRF) was determined to be less than 30 ps (fwhm) in all spectral regions. The polarization of the photoexcitation pulses was set to be vertical to the laboratory frame by both a half-wave retarder and a Glan laser polarizer, and sheet polarizers were used in the fluorescence collection path at the magic angle (54.78) to obtain polarization-independent population decays when measuring the fluorescence decay profiles. Timeresolved fluorescence anisotropy was obtained by changing the detection polarization on the fluorescence path to parallel or perpendicular to the polarization of the excitation pulses. The calculation of anisotropy decay at a specific monitoring wavelength is as follows [Eq. (9)]: rðtÞ ¼ ½IVV ðtÞ¢GIVH ðtÞ¤=½IVV ðtÞ þ 2GIVH ðtÞ¤ ChemPhysChem 2015, 16, 1757 – 1767

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Femtosecond transient absorption and transient absorption anisotropy: The femtosecond time-resolved transient absorption (fs-TA) spectrometer consists of an optical parametric amplifier (OPA; Palitra, Quantronix) pumped by a Ti:sapphire regenerative amplifier system (Integra-C, Quantronix), operating at 1 kHz repetition rate, and an optical detection system. The generated OPA pulses have a pulse width of approximately 100 fs and an average power of 1 mW in the range of 280–2700 nm, which are used as pump pulses. White light continuum (WLC) probe pulses were generated using a sapphire window (3 mm thick) by focusing a small portion of the fundamental 800 nm pulses, which was picked off by a quartz plate before entering the OPA. The time delay between the pump and probe beams was carefully controlled by making the pump beam travel along a variable optical delay (ILS250, Newport). Intensities of the spectrally dispersed WLC probe pulses were monitored by a high speed spectrometer (Ultrafast Systems) for both visible and near-infrared measurements. To obtain the time-resolved transient absorption difference signal (DA) at a specific time, the pump pulses were cut at 500 Hz and absorption spectra intensities were saved alternately with or without a pump pulse. Typically, 4000 pulses were used to excite the samples to obtain a fs-TA spectra at each delay time. The polarization angle between pump and probe beam was set at the magic angle (54.78) by using a Glan laser polarizer with a half-wave retarder to prevent polarization-dependent signals. Cross-correlation fwhm in pump–probe experiments was less than 200 fs and the chirp of WLC probe pulses was measured to be 800 fs in the 400–800 nm region. To minimize chirp, all reflection optics in the probe beam path and a quartz cell with a 2 mm pathlength were used. After fsTA experiments, the absorption spectra of all compounds were carefully examined to detect if there were artifacts due to degradation and photooxidation of the samples. The three-dimensional data sets of DA versus time and wavelength were subjected to single value decomposition and global fitting to obtain the kinetic time constants and their associated spectra by using Surface Xplorer software (Ultrafast Systems). For TAA measurement, both I//(t) and I ? (t) signals were collected simultaneously by a combination of polarizing beam-splitter cube and dual lock-in amplifiers as [Eq. (10)]: rðtÞ ¼ ½I== ðtÞ¢I? ðtÞ¤=½I== ðtÞ þ 2I? ðtÞ¤

ð10Þ

where I//(t) and I ? (t) represent TA signals with the polarization of the pump and probe pulses being mutually parallel and perpendicular, respectively. The pump pulses were set to vertical polarization and the probe pulse was set to 458 with respect to the pump pulse by using Glan laser polarizers and half-wave plates. After the probe pulse passed through the sample cell, it was split by apolarizing beam-splitter cube, and then detected by using two separate photodiodes. Two gated integrators and two lock-in amplifiers recorded the signal simultaneously within a single scan. A standard anisotropy measurement with rhodamine 6G dye in methanol showed a clean single exponential decay with reorientational relaxation times of 122.1 œ 0.3 ps and an initial anisotropy r0 value of 0.39 œ 0.02, which are in agreement with literature.[41] For all TAA measurements, a thin absorption cell with a pathlength of 1 mm was used to eliminate additional chirping. More experimental/computational details and additional data are available in the Supporting Information.

ð9Þ

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Articles Acknowledgements The work at Yonsei was financially supported by the Mid-career Researcher Program (2005–0093839) administered through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). The researchers from Wìrzburg are grateful to the Deutsche Forschungsgemeinschaft for financial support of their work within the research graduate school (GRK 1221) “Control of electronic properties of aggregates of p-conjugated molecules” and the research group (FOR 1809) “Light-induced dynamics in molecular aggregates”. We would also like to thank Prof. Bernd Engels for helpful discussions on the evaluation of structural aspects for our reference compound F2. Keywords: excited state · foldamers · self-assembly · perylene bisimide · time-resolved spectroscopy [1] a) F. Wìrthner, Chem. Commun. 2004, 1564 – 1579; b) M. R. Wasielewski, J. Org. Chem. 2006, 71, 5051 – 5066; c) J. A. A. W. Elemans, R. van Hameren, R. J. M. Nolte, A. E. Rowan, Adv. Mater. 2006, 18, 1251 – 1266; d) S. Huang, S. Barlow, S. R. Marder, J. Org. Chem. 2011, 76, 2386 – 2407; e) D. Gçrl, X. Zhang, F. Wìrthner, Angew. Chem. Int. Ed. 2012, 51, 6328 – 6348; Angew. Chem. 2012, 124, 6434 – 6455; f) W. Jiang, Y. Li, Z. Wang, Acc. Chem. Res. 2014, 47, 3135 – 3147. [2] H. Langhals, Heterocycles 1995, 40, 477 – 500. [3] a) Z. Chen, B. Fimmel, F. Wìrthner, Org. Biomol. Chem. 2012, 10, 5845 – 5855; b) Z. Chen, A. Lohr, C. R. Saha-Mçller, F. Wìrthner, Chem. Soc. Rev. 2009, 38, 564 – 584. [4] E. Schwartz, S. Le Gac, J. J. L. M. Cornelissen, R. J. M. Nolte, A. E. Rowan, Chem. Soc. Rev. 2010, 39, 1576 – 1599. [5] a) F. Wìrthner, Z. Chen, F. J. M. Hoeben, P. Osswald, C.-C. You, P. Jonkheijm, J. van Herrikhuyzen, A. P. H. J. Schenning, P. P. A. M. van der Schoot, E. W. Meijer, E. H. A. Beckers, S. C. J. Meskers, R. A. J. Janssen, J. Am. Chem. Soc. 2004, 126, 10611 – 10618; b) M. Wolffs, N. Delsuc, D. Veldman, N. V–n Anh, R. M. Williams, S. C. J. Meskers, R. A. J. Janssen, I. Huc, A. P. H. J. Schenning, J. Am. Chem. Soc. 2009, 131, 4819 – 4829; c) R. Bhosale, J. Misek, N. Sakai, S. Matile, Chem. Soc. Rev. 2010, 39, 138 – 149. [6] a) J. M. Giaimo, A. V. Gusev, M. R. Wasielewski, J. Am. Chem. Soc. 2002, 124, 8530 – 8531; b) D. Veldman, S. M. A. Chopin, S. C. J. Meskers, M. M. Groeneveld, R. M. Williams, R. A. J. Janssen, J. Phys. Chem. A 2008, 112, 5846 – 5857. [7] a) C. Hippius, I. H. M. van Stokkum, E. Zangrando, R. M. Williams, M. Wykes, D. Beljonne, F. Wìrthner, J. Phys. Chem. C 2008, 112, 14626 – 14638; b) N. V–n Anh, F. Schlosser, M. M. Groeneveld, I. H. M. van Stokkum, F. Wìrthner, R. M. Williams, J. Phys. Chem. C 2009, 113, 18358 – 18368. [8] B. Fimmel, M. Son, Y. M. Sung, M. Grìne, B. Engels, D. Kim, F. Wìrthner, Chem. Eur. J. 2015, 21, 615 – 630. [9] V. Dehm, M. Bìchner, J. Seibt, V. Engel, F. Wìrthner, Chem. Sci. 2011, 2, 2094 – 2100. [10] a) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910 – 1921; b) M. J. Ahrens, L. E. Sinks, B. Rybtchinski, W. Liu, B. A. Jones, J. M. Giaimo, A. V. Gusev, A. J. Goshe, D. M. Tiede, M. R. Wasielewski, J. Am. Chem. Soc. 2004, 126, 8284 – 8294; c) C. Li, H. Wonneberger, Adv. Mater. 2012, 24, 613 – 635. [11] We would like to note that the number of repeating units in F8 is not exactly defined since this molecule has been synthesized by a co- polymerization reaction. Purification by recycling gel permeation chromatography, however, provided a product with a small polydispersity index (D = 1.1; see ref. [9]). [12] A. Zitzler-Kunkel, M. R. Lenze, K. Meerholz, F. Wìrthner, Chem. Sci. 2013, 4, 2071 – 2075. [13] M. Sadrai, L. Hadel, R. R. Sauers, S. Husain, K. Krogh-Jespersen, J. D. Westbrook, G. R. Bird, J. Phys. Chem. 1992, 96, 7988 – 7996.

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Received: February 25, 2015 Published online on March 31, 2015

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Folding-induced modulation of excited-state dynamics in an oligophenylene-ethynylene-tethered spiral perylene bisimide aggregate.

The excited-state photophysical behavior of a spiral perylene bisimide (PBI) folda-octamer (F8) tethered to an oligophenylene-ethynylene scaffold is c...
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