π-Stacking-Dependent Vibronic Couplings Drive Excited-State Dynamics in Perylenediimide Assemblies

Abstract

Vibronic coupling, the interplay of electronic and nuclear vibrational motion, is considered a critical mechanism in photoinduced reactions such as energy transfer, charge transfer, and singlet fission. However, our understanding of how particular vibronic couplings impact excited-state dynamics is lacking due to the limited number of experimental studies of model molecular systems. Herein, we use two-dimensional electronic spectroscopy (2DES) to launch and interrogate a range of vibronic coherences in two distinct types of perylenediimide slip stacksalong the short and long molecular axes, which form either an excimer or a mixed state between the Frenkel exciton (FE) and charge transfer states. We explore the functionality of these vibronic coherences using quantum beatmaps, which display the Fourier amplitude signal oscillations as a function of pump and probe frequencies, along with knowledge of the characteristic signatures of the FE, ionic, and excimer species. We find that a low-frequency vibrational mode of the short-axis slip stack appears concomitantly with the formation of the excimer state, survives 2-fold longer than in the FE state in the reference monomer, and shows a phase shift compared to other modes. For the long-axis slip stacks, a pair of low-frequency modes coupled to a high-frequency coordinate of the FE state were found to play a critical role in mixed-state generation. Our findings thus experimentally reveal the complex and varying roles of vibronic couplings in tightly packed multimers undergoing a range of photoinduced processes.

Coherent Vibronic Wavepackets Show Structure-Directed Charge Flow in Host–Guest Donor–Acceptor Complexes

“…The spectrograms of EP and MP in the low-frequency region highlight the fact that these collective oop vibrations dephase with time constants of less than 400 fs (Figure S23). The dephasing time constants are shorter than the localization kinetics in the BBTA results (Figure ) as these modes broaden , the wavepackets rapidly with the various solute–solvent interactions along the charge localization coordinate. The spectrograms of the high-frequency region provide detailed information of the CT character.…”

“…22−24 Using these tools, we found recently that vibronic coupling effectively drives symmetry-breaking charge separation in a slip-stacked perylenediimide trimer through vibronically mixed Frenkel exciton and CT states. 25,26 Given that this trimer model is symmetric, charge separation can occur in two directions with equal probability. Directional CT may impact overall power conversion efficiencies in OPVs in comparison to random CT, when donor−acceptor orientations in OPVs are favorable, especially in ternary blends.…”

Rescuing the solid-state fluorescence of perylene diimide dyes by host–guest isolation

“…Perylene diimide (PDI) dyes are an important class of dyes. By varying the substituents on the nitrogen at the diimide or perylene bay positions, PDI dyes can be endowed with various attractive features 1–5 and applications, 1,6–14 wherein many applications and desired features are related to the fluorescence property and aggregation behaviour of PDI dyes in solid state. However, due to the large and flat π surface of the perylene core, PDI dyes usually suffer from severe face-to-face (H-type) self-aggregation.…”

Nondestructive All-Optical Readout through Photoswitching of Intramolecular Excimer Emission

“…38 Interestingly, the interchromophoric distance of 3.5 Å found between the two pyrenes in A(o) leads to ground-state interactions, whereas similar distances calculated in PDI dimers exhibit a monomer-like absorption feature likely because of higher rigidity and slip stacks. 39 One can notice that in A(o), the two differently branched pyrenes allow an optimized overlap with very short slip stacks of 1.4 and 0.73 Å along the short and long pyrene axes, respectively.…”