Vibronic fingerprint of singlet fission in hexacene

Abstract

Note: This paper is part of the JCP special collection on Singlet Fission.

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…In hexacene, electron-phonon coupling proceeds through a few dominant vibrational modes, with energies of $180-200 meV. 36,41 This can be clearly seen in the vibronic overtones in the linear optical absorption spectrum (Fig. 2), with an energy spacing approximately matching this energy.…”

“…35,39 Experimental signatures of vibrational coherences have been detected using ultrafast vibrational and 2D electronic spectroscopy, where both inter-and intramolecular vibrational modes have been found to be important. [40][41][42][43][44] For incoherent triplet pair formation, the signature of vibrational mediation has been the dependence of the singlet ssion rate constant on the energetic driving force DE S-TT . This driving force increases with n, the number of rings in the oligoacene chromophore, such that tetracene (Tc, n ¼ 4) < pentacene (Pc, n ¼ 5) < hexacene (Hc, n ¼ 6).…”

“…Unlike analogous molecular crystals, we do not observe instantaneous triplet pair formation via a coherent process. 41 Still, the dependence of the singlet fission rate on the exoergicity implies that vibrations play a large role in the determining the dynamics of the incoherent generation process. In hexacene, electron-phonon coupling proceeds through a few dominant vibrational modes, with energies of ∼180–200 meV.…”

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

“…Finally, we briefly point out the contributions to vibronic studies of SF made with frequency-domain vibrational methods, such as time-resolved infrared and picosecond/femtosecond stimulated Raman spectroscopy [86,88,89,159,160]. Beyond their use to track the fingerprints and populations of electronic states [86,88], these measurements are more advantageous for tracking time-dependent changes in vibrational frequencies than those exploiting VCs.…”

“…Beyond their use to track the fingerprints and populations of electronic states [86,88], these measurements are more advantageous for tracking time-dependent changes in vibrational frequencies than those exploiting VCs. For instance, Deng et al report the progressive blueshift of a high-frequency mode in crystalline hexacene over precisely the same timescale as SF occurs [159]. Using functional mode analysis, the authors assign this mode as the key reaction coordinate driving ultrafast exothermic SF.…”