Direct Observation of Ultrafast Excimer Formation ... - ACS Publications

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Direct Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers Using Near-Infrared Transient Absorption Spectroscopy Kristen E. Brown, Walter A. Salamant, Leah E. Shoer, Ryan M. Young,* and Michael R. Wasielewski* Department of Chemistry and Argonne−Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: Energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is often limited by formation of a low-energy excimer state. Formation dynamics of excimer states are often characterized by line shape changes and peak shift dynamics in femtosecond visible transient absorption spectra. Femtosecond near-infrared transient absorption experiments reveal a unique low-energy transition that can be used to identify and characterize this state without overlapping excited singlet-state absorption. Three covalently bound PDI dimers with differing PDI−PDI distances were studied to probe the influence of interchromophore electronic coupling on the PDI excimer transient spectra and dynamics.

SECTION: Spectroscopy, Photochemistry, and Excited States

U

absorption band in the near-infrared (NIR) region.20 This region provides a clearer picture of excimer behavior and kinetics because the excimer transition is not masked by overlapping S1 → Sn excited-state absorptions. Here, we present a NIR fsTA study of the energetics and dynamics of PDI excimer state formation using a series of covalently bound PDIs that provide control over chromophore distance and orientation (2−4, Scheme 1). In particular, the geometry of 4 closely mimics interchromophore π-stacking in PDI films due to its rigid cyclophane structure.21 The syntheses of compounds 1−3 have been previously reported,16 and the synthesis of cyclophane 4 is described in detail in the Supporting Information (SI). The substituents on the aromatic bridge of 4 were chosen such that no electron transfer occurs from the aromatic spacer to PDI, as has been previously observed in covalently linked J-aggregate mimics.17 Average interplanar distances, R, for 2−4 were calculated using the semiempirical PM3 method and are shown in Table 1.16 In particular, the calculated average PDI interplanar distance for 4 is 3.8 Å, which is similar to that observed between PDI monomers in solution aggregates and films.10 The steady-state absorption and emission spectra of 1−4 in toluene provide insights into the electronic interaction between the PDI monomers (Figure 1). Consistent with the Kasha exciton model,22 4 shows the largest degree of electronic

nderstanding the formation of low-energy states in molecular aggregates is of great interest for organic electronics and photovoltaics.1,2 In particular, the excimer state can act as an exciton trap site, decreasing the quantum yield of desired products or device performance when the excimer formation rate is faster than targeted processes such as exciton diffusion,3−5 charge transfer (CT) and transport,6,7 and singlet fission.8−10 In contrast, excimer state formation was recently found to precede singlet fission in a concentrated solution of 6,13-Bis(triisopropylsilylethynyl) (TIPS)−pentacene9 and thus can also gate desirable processes. Of particular importance is the nature of excimers in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates, commonly used in organic electronics because of their thermal and photochemical stability.2,11 The role of the excimer in PDI energy transfer oligomers has prompted several recent theoretical12−15 and experimental7,16,17 studies on the formation of these energetically lower-lying states. Femtosecond transient absorption (fsTA) spectroscopy has proven to be a powerful tool in studying excimer formation in both covalently bound16−18 and self-assembled PDI systems.6,11,19 For instance, a slower rate of excimer formation has been observed in covalent PDI dimer 2 relative to that in 3 as a result of steric interference between the two branched alkyl groups in 2.16 In these studies, excimer state formation was monitored by observing line shape changes in the visible excited-state absorption and emission spectra. A more detailed study of PDI excimer formation was prompted by the expected observation of an excimer © 2014 American Chemical Society

Received: June 9, 2014 Accepted: July 14, 2014 Published: July 14, 2014 2588

dx.doi.org/10.1021/jz5011797 | J. Phys. Chem. Lett. 2014, 5, 2588−2593

The Journal of Physical Chemistry Letters

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Scheme 1. Structures of Molecules Used in This Study

Figure 2. Schematic detailing the relative energy level ordering of molecules 1−4.

coupled in the excited state. The very low excimer fluorescence quantum yield of 4 (ΦFL = 0.001) is consistent with strong electronic coupling between the two PDIs enforced by the cyclophane structure, which enhances nonradiative decay. The vibronic structure on the blue edge of the emission spectrum of 4 is attributed to a minor monomeric impurity having a high ΦFL (SI). In keeping with the steady-state absorption spectrum, molecule 2 shows the least interaction between the PDIs (ΦFL = 0.19). The two shoulders on the blue edge of the excimer emission spectrum of 2 are approximately mirror images of the respective absorption features of 2 and are attributed to transitions from both initially formed exciton levels, which are both allowed as a result of the two PDIs being forced out of parallel alignment by the 12-tricosanyl tails. Figure 2 shows the relative energy level ordering of 2−4 determined from their absorption and emission data. As stated previously, stronger electronic coupling results in greater exciton splitting, increasing the energy gap between the ground state and the upper exciton transition. In addition, the excimer emission energy, Exmr, reflects the energy gap between the vibrationally relaxed excimer state and ground state. The observed trend in Exmr for 2−4 results from an interplay between conformational relaxation and electronic coupling produced by the close proximity of the PDIs. Computational studies at the SCS-CC2 and TD-DFT levels of theory have provided insights into the geometric dependence of PDI dimer ground- and exciton-state energies.12−14 In the relaxed ground state, the torsional angle, φ, between the N−N axes of cofacial

Table 1. Steady-State Spectral Data in Toluene at 298 K

1 2 3 4 a

λmax,abs, nm

λmax,em, nm

R, Å

0−1/0−0 transition ratio

ΦFL

526 490 491 493

533 650 733 790

n/a 4.5 4.4 3.8

0.65 1.4 1.5 2.3

0.98a 0.19a 0.02a 0.001b

See ref 16. bDetails on ΦFL measurements are given in the SI.

coupling. It is well established that when the monomers are bound in a face-to-face geometry, the first excited state splits into higher and lower Frenkel exciton states. In an H-aggregate, the higher-energy transition is symmetry-allowed (Figure 2). Indeed, the UV−vis spectra of 2−4 are consistent with an extension of the exciton model, which shows that the ratio of the vibronic bands is modulated by electronic coupling.23−25 The ratio of the 0−1 to 0−0 band intensity, signifying Haggregation, is greatest for molecule 4 at 2.3 (Table 1), while the corresponding ratio for molecule 2, 1.4, reveals the smallest degree of association due to the mutual steric interference between the two 12-tricosanyl tails. Interchromophore coupling is also observed in the steadystate emission data (Figure 1). Monomeric PDI is characterized by a vibronically resolved emission spectrum, mirroring the steady-state absorption spectrum. The broad, red-shifted emission observed for 2−4 is characteristic of excimer state formation, in which the PDI molecules are electronically

Figure 1. Normalized steady-state absorption (left) and fluorescence data (right) in toluene of 1 (black), 2 (red). 3 (green), and 4 (blue). Emission data are generated with 490 nm excitation. Dashed lines indicate emission peak maxima (λmax). 2589



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PDIs is 30°. In noncovalent solution-based aggregates, PDI monomers having no sterically encumbering groups are generally able to adopt this minimum-energy structure. Following vertical excitation of the ground-state aggregates, the nascent excimer state requires a structural change to achieve the most stable conformation at φ = 0°. Structural constraints in 3 and 4 set the ground-state conformation at φ ≅ 0°, while steric interactions between the two 12-tricosanyl groups in 2 make φ somewhat larger than 0°. Thus, all three covalent PDI dimers cannot adopt the fully stabilized ground-state geometry (φ ≅ 30°) predicted by theory.16 The computations indicate that the electronic repulsion between two PDIs constrained to φ = 0° destabilizes the ground state ∼0.6−0.7 eV.12 While the observed energy difference between the emission maxima of monomer 1 and dimer 4 is comparable to this value, if the red shift of the excimer emission band relative to that of the monomeric emission were due purely to ground-state destabilization, then the PDI dimer absorption band would also be expected to red shift to ∼580 nm, which is not observed. Theoretical investigations of PDI dimers by Gao et al. suggest that mixing between Frenkel and CT excitons yields a low-intensity, red-shifted, and broadened emission spectrum with respect to monomeric PDI.24 For 2 and 3, the red shift most likely results from both Frenkel−CT exciton mixing and geometric rearrangement following photoexcitation. However, the corresponding red shift in 4, which is bound by the phenyl moieties and unable to undergo large rearrangements, may be due largely to Frenkel−CT exciton mixing. The higher-energy emission maxima observed for 2 and 3 relative to 4 support the distance dependence analysis by Gao et al., which shows that more stabilized excimers in systems with smaller interchromophore spacings result from greater Frenkel−CT exciton mixing. Additional information regarding the excimer state was obtained using fsTA spectroscopy. FsTA measurements on 1− 4 were conducted with an instrument that has been previously described in detail.26 The visible femtosecond probe pulse (VIS, 380−800 nm) was generated using a 5 mm cell with a 50/50 mix of H2O/D2O, while the NIR probe pulse (800− 1600 nm) was generated and detected using a customized Helios system (Ultrafast Systems, LLC). Femtosecond absorption data were acquired after 490 or 525 nm, 150 fs, 500 nJ/pulse excitation using the appropriate VIS or NIR detector. All samples were prepared with an optical density between 0.5 and 1 at the excitation wavelength. The VIS fsTA data of 1−3 in toluene have been reported previously16 but were reacquired using the new experimental setup. Both the VIS and NIR fsTA spectra of PDI monomer 1 following excitation at 525 nm are shown in Figure 3. The ground-state bleach extends from 420 to 530 nm and is accompanied by the stimulated emission feature from 530 to 640 nm. The excited-state absorption band peaks at 690 nm and extends broadly into the NIR, with a small absorption tail extending to 1600 nm. The VIS fsTA spectra of 2−4 are shown in Figure 4. The excited-state absorption of 4 resembles that of molecules 2 and 3, displaying no stimulated emission and indicating excimer formation. The excimer band in the VIS spectra is well-known to overlap the S1 absorption band.16 The NIR fsTA data of 2−4 (Figure 5), on the other hand, reveal a background-free absorption band on the red edge of the probed spectral window near 1600 nm. The band is absent in the NIR spectrum of 1 and decays in all cases with a time constant matching that

Figure 3. VIS and NIR fsTA spectra of 1 in toluene. The sample was excited with 525 nm, 500 nJ excitation. Time delays are reported in ps.

Figure 4. Comparison of the VIS fsTA data of 2−4. Spectra were acquired in toluene following 490 nm, 500 nJ excitation. Time delays are reported in ps.

acquired with VIS fsTA (discussed below). This indicates that the band apparent in the NIR region reflects the transition from S1 to a higher lower state than the upper transition probed in the VIS region. Katoh et al. observed a similar NIR feature in a series of polycyclic aromatic excimers,20,27 which was assigned to the transition from the lowest excimer Frenkel state 1*(MM) to a higher-energy CT state (M+M−). Engels et al. recently computed the adiabatic potential surfaces of the two localized Frenkel and two localized CT excimer states for a noncovalent PDI dimer.13,28 The computed energy gap between 1*(PDI− PDI) and 1(PDI+−PDI−), ∼0.5−0.6 eV, closely matches the NIR vertical transition energy, ENIR, observed for 3 and 4 (Table 2). ENIR for 3 and 4 is approximated by fitting a Gaussian function to the NIR band; however, this transition cannot be fit for 2 because the band maximum occurs well beyond the wavelength range of the detector. ENIR clearly increases with increasing electronic coupling between the two PDIs, due in part to the geometric dependence of Exmr outlined 2590



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polar solvents precludes determining the corresponding solvent dependence for these molecules.17 The absence of a solvent dependence for 4 is consistent with the expected very short lifetime of the 1(PDI+−PDI−) state (1CT) based on energy gap law considerations, given that ENIR is only ∼0.75 eV for 1 (PDI+−PDI−) → 1*(PDI−PDI). If the 1(PDI+−PDI−) lifetime is short relative to the reorientation time of the solvent dipoles, the CT state derives little or no stabilization from the fully relaxed solvent as captured by the low-frequency (static) solvent dielectric constant (εs). Only the residual polarization of the medium, given by its high-frequency dielectric constant εop ≅ n2, where n is the refractive index, contributes to overall stabilization of 1(PDI+−PDI−). The energy of a photogenerated ion pair in polar media can be estimated by the sum of the one-electron oxidation, Eox, and reduction, Ered, potentials of the donor and acceptor, respectively. The Coulomb stabilization energy of the ion pair in polar media is greatly reduced by the charge screening effect of the high dielectric medium when the ions are fully solvated. In contrast, under conditions in which the medium has a low dielectric constant or the ion pair lifetime is short relative to solvent reorientation times, the impact of the Coulomb attraction of the ions and the absence of solvation stabilization of the ions become significant. Various treatments based on dielectric continuum models of solvation have been widely used to estimate the ion pair energies under these conditions.29 For example, the ion pair energy, Ecalc IP , can be estimated using eq 2 Figure 5. Comparison of the NIR fsTA data of 2−4. Spectra were acquired in toluene following 490 nm, 500 nJ excitation. Time delays are reported in ps.

calc = Eox − Ered + E IP

2 3 4 4 4 a

ENIR, eV

EXMR, eV

Eexp IP , eV

toluene toluene toluene CH2Cl2 benzonitrile

a 0.70 0.78 0.72 0.80

1.91 1.69 1.57 1.57 1.57

2.39 2.35 2.29 2.37

A Gaussian fit for 2 could not be obtained.

by Engel et al.13 For instance, the face-on alignment imposed by the cyclophane structure in 4 stabilizes 1*(PDI−PDI), likely resulting in a higher-energy transition from 1*(PDI−PDI) to 1 (PDI+−PDI−) compared to 2 and 3. The same argument can be extended to a comparison between 2 and 3, considering the steric effect of the 12-tricosanyl tails in 2. The role of 1(PDI+−PDI−) in the trend observed in ENIR can be further investigated using an analysis similar to that of Katoh et al.20,27 Again, we attribute the observed NIR transition of the excimer to 1*(PDI−PDI) → 1(PDI+−PDI−), the ion pair state. The ion pair state energy, Eexp IP , can be estimated as the sum of ENIR and Exmr exp E IP = Exmr + E NIR

(2)

where e is the electronic charge, ε is the dielectric constant of the medium, RDA is donor−acceptor distance, and RD and RA are the radii of the ions, which are assumed to be hard spheres. However, there are several problems with using this treatment to estimate Ecalc IP for large π-stacked donor−acceptor systems. First, the ionic radii of the donor and acceptor are frequently larger than the distance between them; for example, RD = RA = 4.5 Å for PDI (see the SI), while RDA = 3.7 Å for PDI−PDI in 4, which violates the assumptions of a hard-sphere model surrounded by a continuous dielectric that is intrinsic to eq 2. Second, one entire face of each PDI molecule is shielded from the surrounding medium by the presence of the other PDI; thus, each PDI experiences a significant contribution from the effective dielectric constant of the neighboring PDI, which in general differs from that of the bulk solvent. Comparing Eexp IP = + − 2.4 to Ecalc IP = Eox − Ered = 2.2 eV for PDI −PDI (Eox = 1.64 V and Ered = −0.53 V versus SCE),11 the overall effect of Coulombic stabilization and solvent destabilization on the formation of 1(PDI+−PDI−) is to destabilize the CT state by calc only ∼0.2 eV relative to Ecalc IP , whereas eq 2 yields EIP = 3.8 eV for 4 using the parameters given above and RD = RA = 1.9 Å, which is RDA/2. This example highlights the importance of finding direct methods of estimating ion pair energies in media that have low polarity or are unable to reorganize to stabilize the ion pair. Molecules in solid solutions, especially at low temperatures exemplify the latter effect.30 Kinetic analysis of the data for 2−4 reveals additional information about the nature of the excimer state. The excimer band of 4 decays to the ground state in 2.5 ns, significantly faster compared to 2 and 3, which decay in 29 and 9 ns, respectively,16 and 1, which decays from its S1 state in 3.5 ns. A

Table 2. Energetic Parameters solvent

e2 ⎛ 1 1 1 ⎞ + − ⎜ ⎟ ε ⎝ 2RD 2RA RDA ⎠

(1)

Eexp IP

The experimental values of for 3 and 4 are nearly identical (∼2.4 eV) regardless of the differences in Exmr. Moreover, Eexp IP for 4 is essentially solvent-independent, as indicated by the data obtained in toluene, CH2Cl2, and benzonitrile, which cluster at ∼2.4 eV (Table 2). Photoinduced electron transfer from the xanthene spacer to PDI in 2 and 3 in 2591



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Table 3. fsTA Kinetics for Compounds 1−4 λ, nm rise

600−750

decay rise

0.69 ± 0.60

decay rise decay

4230 ± 50 2.0 ± 0.2 4110 ± 10

rise

b

decay

b

950

1450−1550

a

1, ps 2 ± 0.1 (25%) 3920 ± 40 (75%)

490−540a

2, ps

3, ps

4, ps

44.7 ± 5.6

7.1 ± 3.8

0.38 ± 0.05

≫7000 4.6 ± 0.4 83 ± 7 ≫7000

≫7000 8.5 ± 0.2

2350 ± 12 3.2 ± 0.4

≫7000

2340 ± 16

1.8 ± 0.1 (25%) 53.3 ± 2.2 (30%) ≫7000 (55%) 17.4 ± 3.0 (20%) 192 ± 25 (80%) ≫7000

0.8 ± 0.2 (10%) 29.9 ± 4.7 (25%) ≫7000 (65%) 1.8 ± 0.7 (50%) 14.1 ± 3.5 (50%) ≫7000

3.9 ± 0.5 (20%) 2854 ± 83 (80%) 6.6 ± 0.7 2720 ± 91

Fit along the ground-state bleach. bSignal-to-noise prohibits accurate kinetics from being acquired.

observed following excitation at 525 nm (not shown), which directly populates the lower exciton state. In contrast to 2 and 3, the NIR band in 4 rises monoexponentially. Because the line shape change and peak shift of the VIS absorption peak of 4 are small, the VIS and NIR kinetics are more consistent with the NIR data than those of 2 and 3. The lack of peak shift in the VIS spectrum and the strong coupling indicated by the steady-state data suggest that population of the excimer state occurs within the instrument response time in 4 and that the short ∼3−6 ps component is due to a small degree of conformational relaxation to form the preferred cofacial excimer geometry from the slightly slipped ground-state geometry. The rigidity of the phenyl linkers that lock the PDIs in a nearly ideal geometry for excimer formation also prohibit large degrees of reorientation following photoexcitation. While visible fsTA spectroscopy has been relied on previously to determine the kinetics of the excimer state, we have demonstrated a method not only to directly access this information but also to provide information about the nature of the higher-lying CT state accessible from the excimer state. The three excimer-forming PDI dimers reveal a trend in formation kinetics, demonstrating that as the degree of electronic coupling between monomers increases and structural mobility decreases, the excimer formation times decrease. Although this study was performed on covalent aggregates with relaxation constraints, we anticipate similar relations in thin films and solution-based aggregates where structural relaxation is possible. This analysis can provide important information necessary to gauge energy and charge-transport efficiencies and mechanisms in those systems.

similarly short 0.9 ns excimer decay time has been observed in PDI thin films in which the PDI−PDI π-stacking distance is 3.5 Å.10 The increased vibrational interactions between closely bound PDI monomers could account for the decreased lifetime in 4. Previous work by Kelley et al.31 demonstrates that the zero-order exciton model breaks down in merocyanines with intermolecular distances less than 4 Å. At this point, the electronic interaction between monomers increases such that the dimer should be treated as a new molecule. To accurately model the effects of aggregation, the point-dipole approximation is replaced with an extended dipole model considering the full spatial expansion of the transition density matrix. One would expect that the relaxation mechanism commonly understood for monomeric PDI would not hold in such a supramolecular structure. Alternatively, destabilization of the ground state may lead to enhanced internal conversion, which would decrease the excimer lifetime in 4. Previous discussions of aromatic excimers have identified a two-step formation mechanism where the formation of the relaxed excimer state is preceded by an unrelaxed, hot excimer state.32,33 While earlier studies have observed this mechanism in PDI dimers, the excimer formation dynamics are often elucidated by analyzing peak shifts and line shape changes using VIS fsTA data. The VIS fsTA of 4 reveals only a slight narrowing of the excited-state absorption (τ ≈ 3 ps), contrary to molecules 2 and 3, whose peak maxima were reported to blue shift in τ = 69 and 1.2 ps,16 respectively. The excimer formation kinetics are also complicated by overlapping absorption features, which result in multiple kinetic components. For instance, single-wavelength analyses of 2 and 3 result in a wavelength-dependent, multiexponential rise preceding the nanosecond decay (Table 3). The time dependence of the NIR band appearance provides a more direct measure of the excimer formation kinetics. The NIR features of 2 and 3 rise biexponentially; 2 rises with τ1 = 17 ps and τ2 = 200 ps and 3 with τ1 = 2 ps and τ2 = 14 ps. Similar time scales were obtained for 3 by monitoring the ∼15 nm peak shift in the VIS data (SI). The shorter rise component is assigned to the initial population of the excimer state from the lower exciton state, and the second component is assigned to subsequent geometric rearrangement in the excimer state.16 Although excitation at 490 nm populates the upper Frenkel state, internal conversion to the lower state in ∼200 fs13 is not observable by our apparatus and will not contribute to the observed kinetics. Furthermore, identical transient behavior is

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of 4, additional transient absorption spectra and associated kinetics, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.R.W.). *E-mail: [email protected] (R.M.Y.). Notes

The authors declare no competing financial interest. 2592



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(16) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.; Wasielewski, M. R. Excited Singlet States of Covalently Bound, Cofacial Dimers and Trimers of Perylene-3,4:9,10-bis(dicarboximide)s. J. Phys. Chem. A 2008, 112, 2322−2330. (17) Lefler, K. M.; Brown, K. E.; Salamant, W. A.; Dyar, S. M.; Knowles, K. E.; Wasielewski, M. R. Triplet State Formation in Photoexcited Slip-Stacked Perylene-3,4:9,10-bis(dicarboximide) Dimers on a Xanthene Scaffold. J. Phys. Chem. A 2013, 117, 10333− 10345. (18) Veldman, D.; Chopin, S. p. M. A.; Meskers, S. C. J.; Groeneveld, M. M.; Williams, R. M.; Janssen, R. A. J. Triplet Formation Involving a Polar Transition State in a Well-Defined Intramolecular Perylenediimide Dimeric Aggregate. J. Phys. Chem. A 2008, 112, 5846−5857. (19) Würthner, F.; Chen, Z.; Dehm, V.; Stepanenko, V. OneDimensional Luminescent Nanoaggregates of Perylene Bisimides. Chem. Commun. 2006, 1188−1190. (20) Katoh, R.; Katoh, E.; Nakashima, N.; Yuuki, M.; Kotani, M. Near-IR Absorption Spectrum of Aromatic Excimers. J. Phys. Chem. A 1997, 101, 7725−7728. (21) Shirai, S.; Iwata, S.; Maegawa, Y.; Tani, T.; Inagaki, S. Ab Initio Molecular Orbital Study on the Excited States of [2.2]-, [3.3]-, and Siloxane-Bridged Paracyclophanes. J. Phys. Chem. A 2012, 116, 10194−10202. (22) Kasha, M. Energy Transfer Mechanisms and the Molecular Exciton Model for Molecular Aggregates. Radiat. Res. 1963, 20, 55−70. (23) Spano, F. C. The Spectral Signatures of Frenkel Polarons in Hand J-Aggregates. Acc. Chem. Res. 2009, 43, 429−439. (24) Gao, F.; Zhao, Y.; Liang, W. Vibronic Spectra of Perylene Bisimide Oligomers: Effects of Intermolecular Charge-Transfer Excitation and Conformational Flexibility. J. Phys. Chem. B 2011, 115, 2699−2708. (25) Kelley, A. M. A Multimode Vibronic Treatment of Absorption, Resonance Raman, and Hyper-Rayleigh Scattering of Excitonically Coupled Molecular Dimers. J. Chem. Phys. 2003, 119, 3320−3331. (26) Young, R. M.; Dyar, S. M.; Barnes, J. C.; Juríček, M.; Stoddart, J. F.; Co, D. T.; Wasielewski, M. R. Ultrafast Conformational Dynamics of Electron Transfer in ExBox4+⊂Perylene. J. Phys. Chem. A 2013, 117, 12438−12448. (27) Katoh, R.; Sinha, S.; Murata, S.; Tachiya, M. Origin of the Stabilization Energy of Perylene Excimer as Studied by Fluorescence and Near-IR Transient Absorption Spectroscopy. J. Photochem. Photobiol., A 2001, 145, 23−34. (28) Schubert, A.; Falge, M.; Kess, M.; Settels, V.; Lochbrunner, S.; Strunz, W. T.; Würthner, F.; Engels, B.; Engel, V. Theoretical Analysis of the Relaxation Dynamics in Perylene Bisimide Dimers Excited by Femtosecond Laser Pulses. J. Phys. Chem. A 2014, 118, 1403−1412. (29) Weller, A. Z. Phys. Chem. 1982, 130, 129. (30) Gaines, G. L.; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.; Wasielewski, M. R. Photoinduced Electron Transfer in the Solid State: Rate vs. Free Energy Dependence in Fixed-Distance Porphyrin− Acceptor Molecules. J. Am. Chem. Soc. 1991, 113, 719−721. (31) Leng, W.; Würthner, F.; Kelley, A. M. Resonance Raman Intensity Analysis of Merocyanine Dimers in Solution. J. Phys. Chem. B 2004, 108, 10284−10294. (32) Walker, B.; Port, H.; Wolf, H. C. The Two-Step Excimer Formation in Perylene Crystals. Chem. Phys. 1985, 92, 177−185. (33) Ma, L.; Tan, K. J.; Jiang, H.; Kloc, C.; Michel-Beyerle, M.-E.; Gurzadyan, G. G. Excited-State Dynamics in an α-Perylene Single Crystal: Two-Photon- and Consecutive Two-Quantum-Induced Singlet Fission. J. Phys. Chem. A 2014, 118, 838−843.

ACKNOWLEDGMENTS The authors acknowledge Yilei Wu for assistance in steady-state fluorescence measurements and thank Eric Margulies and Patrick Hartnett for useful discussions. This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, DOE under Grant No. DE-FG02-99ER14999. R.M.Y. thanks the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry for support.

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