Intermolecular vibrations mediate ultrafast singlet fission

Abstract

Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. We study the primary steps of singlet fission in a pentacene film by using a combination of TG and 2D electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. We show that the coherent vibrational dynamics induces the ultrafast transition from the singlet excited electronic state to the triplet-pair state via a degeneracy of potential energy surfaces, i.e., a multidimensional conical intersection. Significant vibronic coupling of the electronic wave packet to a few key intermolecular rocking modes in the low-frequency region connect the excited singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, they open a new channel for the ultrafast exciton transfer through the resulting conical intersection.

Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission

“…Evolution along this coordinate results in an avoided crossing at resonance between S 1 and TT, enabling ultrafast population transfer despite extremely small electronic coupling between the states. Thus, the 1435 cm −1 vibration plays the essential role of a tuning mode, similar to its role in coherent SF in the pentacene films above [45][46][47].…”

“…Two dominant modes were observed, at 315 and 760 cm −1 , and from their intensity distribution with probe wavelength they could be assigned uniquely to S 1 and 1 TT, respectively. The authors analysed the temporal evolution of the VC frequency contents using a sliding-window Fourier transform with 1 ps window, which can provide similar information to the wavelet analysis used by Duan et al [45]. In TIPS-tetracene, this analysis showed that on early times (0-1 ps) the 315 cm −1 S 1 VC is prominently observed.…”

“…In 2020, Duan et al employed a simpler conceptual scheme to qualitatively describe comparable CI-mediated SF dynamics in polycrystalline pentacene films [45]. This work utilized a combination of 2DES and heterodyne-detected TG spectroscopy in one-colour experiments using ~18 fs pulses spanning 650-750 nm.…”

“…The resulting timescales could often be described by multiple competing theories, and thus the principal tools for tracking the SF reaction provided little insight into the core questions about the nature of the relevant couplings. The chief exception, the breakthrough report of instantaneous coherent 1 TT formation using photoemission spectroscopy, which implied very strong electronic coupling [42,43], subsequently proved controversial [44,45]. A broad study of structure-dependent SF rates in polyacene materials formulated an adiabatic mechanism of continuously evolving wavefunction character from S 1 to 1 TT along an arbitrary reaction coordinate, but no explicit link was made to nuclear dynamics [44].…”

Vibrational Excitation Initiates Biomimetic Charge-Coupled Motions in the Electronic Ground State

“…For more complex molecular systems, however, there exists no generic prescription for the a priori identification of an experimentally accessible handlea strongly infrared-absorbing vibrational mode that also couples strongly to the reaction coordinateas a precursor to photo-induced ground-state chemistry. This is in stark contrast with excited-state chemistry, where barrier crossing is readily facilitated by photoactivation, and the reactive (as well as the spectator) modes can be easily discerned by various time-resolved spectroscopic and diffractive-imaging techniques [10][11][12] . A recipe to determine the key ground-state reactive modesa formidable challenge, especially for complex molecules with ~3N degrees of freedom administers the parameter space along which a judicious preparation of a ground-state coherent vibrational wave-packet may usher in exciting new chemistry.…”