Microscopic theory of singlet exciton fission. III. Crystalline pentacene

Abstract

We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.

Singlet fission in linear chains of molecules

“…If ′ is reduced from 0.15 to 0.10 eV, the computed rate decreases by approximately one order of magnitude reconciling our results with other theoretical works (100-500 fs). 34,42,47 A similar reduction of one order of magnitude in the rate is observed by keeping the absolute value of ′ equal to 0.15 eV and changing sign of the Fock matrix elements in such a way that the coupling between HOMO orbitals has opposite sign than the coupling between LUMO orbitals (a result of negative interference between coupling paths). These observations collectively suggest that the accuracy of the Fock matrix elements is crucial if one is interested in quantitative evaluation of the rate for realistic system Regardless of the specific choice of parameters, our model implies that the interpretation of the experimental results may be more complicated than previously thought.…”

“…34,42 Our computed rates for the ME←S1 non radiative decay for a pentacene-like scenario at low Δ are higher than the experimental 22 and theoretical 34,42,47 SF rates for pentacene crystals and dimer models, respectively. In particular, we obtain transition times of the order of tens fs, which are approximately one order of magnitude faster than SF times, obtained by Zhu and coworkers via time-resolved two-photon photoemission spectroscopy (>100 fs).…”

“…47 Feng et al criticize the diabatization approach of Reichman's group in ref. 33,34 and they propose the use of adiabatic wave-function and the calculation of non-adiabatic couplings.…”

“…[32][33][34] Very recently, Reichman's group extended the application of this theoretical framework to the study of crystalline pentacene. 47 This was achieved with a combination of ab initio calculations and semi-empirical modeling. The results were compared to the one obtained in their previous works on dimers; 33,34 while they found the kinetics of SF being consistent with the one observed for the dimer case, a sensible qualitative difference between the two cases was noted.…”

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

“…Such arXiv:1703.01173v2 [cond-mat.mtrl-sci] 11 May 2017 has been proven successful in several studies on fission materials, [24][25][26][27][28][29][30] however, applications did not address the fission dynamics, 24,26,30 or were limited to a perturbative treatment of electronic couplings 25 or the vibrational degrees of freedom. [27][28][29] The present article forms the first entry in a series of studies in which we present a microscopic vibronic exciton model aimed at unraveling the mechanistic principles underlying singlet fission, with a particular emphasis placed on the role of vibronic coupling. We provide a detailed description of the model in which both the electronic degrees of freedom and a selected intramolecular vibrational mode are treated non-perturbatively.…”