Effect of high-frequency modes on singlet fission dynamics

Abstract

Singlet fission is a spin-allowed energy conversion process whereby a singlet excitation splits into two spin-correlated triplet excitations residing on adjacent molecules and has a potential to dramatically increase the efficiency of organic photovoltaics. Recent time-resolved nonlinear spectra of pentacene derivatives have shown the importance of high frequency vibrational modes in efficient fission. In this work, we explore impacts of vibration-induced fluctuations on fission dynamics through quantum dynamics calculations with parameters from fitting measured linear and nonlinear spectra. We demonstrate that fission dynamics strongly depends on the frequency of the intramolecular vibrational mode. Furthermore, we examine the effect of two vibrational modes on fission dynamics. Inclusion of a second vibrational mode creates an additional fission channel even when its Huang-Rhys factor is relatively small. Addition of more vibrational modes may not enhance the fission per se, but can dramatically affect the interplay between fission dynamics and the dominant vibrational mode.

Effect of Off-Diagonal Exciton–Phonon Coupling on Intramolecular Singlet Fission

“…Following the Dirac-Frenkel time-dependent variational principle, we provide an accurate description for iSF using the multi-D 2 Ansatz, a superposition of the usual Davydov D 2 states. To our knowledge, diagonal coupling has been reported to aid efficient SF if excitonic coupling is weak, [19,46] and it has also been shown in this work to facilitate efficient fission if excitonic coupling is strong. Furthermore, we have demonstrated for the first time that off-diagonal coupling plays a crucial role in the fission process only if excitonic coupling is weak.…”

“…×δ (ǫ S1,g − λ S1,g − ǫ T T,g + λ T T,g − ν ω diag ) (C1) where χ S1 0 |χ T T 0 2 is the Frank-Condon factor associated with the vibronic transition from the 0-th vibrational level on the S 1 to the νth vibrational level on T T . [46] Thus for the case shown in Fig. 7, the transition from S 1 to T T is driven by the phonon mode at around ω diag = 40 meV satisfying the condition in which the energy of S 1 including the vibrational energy matches its counterpart in the T T state.…”

Vibronic exciton theory of singlet fission. III. How vibronic coupling and thermodynamics promote rapid triplet generation in pentacene crystals

“…We note that this procedure has in essence been followed by studies on singlet fission using few-state models as proxies for a extended crystals. 8,11,13,14,51,52 Lastly, a truncation of the Redfield tensor (see Sec. II B) is performed such that adiabatic states having energies higher than 1.92 eV (15 500 cm −1 ) are excluded.…”

“…Both Holstein and Peierls coupling have been theoretically demonstrated to benefit the intramolecular fission rate in chemically linked pentacenes. 16 Nevertheless, non-perturbative treatments of these mechanisms in a crystalline setting have remained limited to phenomenological models based on few-state systems, 8,11,[13][14][15] in which the potentially extended character of the involved excited states are not accounted for. As such, the significance of such modes for capturing the realistic, microscopic behavior representative of extended crystals remains to be verified.…”

Quantum Master Equation Approach to Singlet Fission Dynamics in Pentacene Linear Aggregate Models: Size Dependences of Excitonic Coupling Effects

“…Namely, in addition to the energy level matching conditions in step (i), design of intermolecular configuration (crystal packing) is found to be indispensable for describing the SF rate, which is proportional to transition probability from FE to TT state . Another essential factor for determining the SF efficiency is involved in the exciton dynamics with vibronic couplings, which is the main topic in step (iii). In step (iii), various results obtained in steps (i) and (ii) are combined and are mutually correlated with the exciton dynamics caused by vibronic couplings.…”