Polariton-assisted ultrafast energy delocalization in a donor-acceptor organic microcavity probed by two-dimensional electronic spectroscopy

Singlet fission, in which two triplet excitons are formed from one photon, has the potential to increase the power conversion efficiency of solar cells. While singlet fission has been studied extensively over the past decade, the role of vibrational coupling remain unclear. Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that investigates vibrational structures of molecules providing both high temporal resolution (∼100 fs) and reasonable spectral resolution (10cm−1). In this three−pulse technique, excited and ground state vibrational spectra are obtained to gain structural information about singlet and triplet kinetics in a fast singlet fission process. We have investigated the excited-state dynamics of 6,13-bis triiso-propylsilylethynyl (TIPS) Pentacene in concentrated solution to determine the role of excimer and aggregate formation in singlet fission solutions. Using transient absorption, we find that the mechanism of singlet fission remains dominated by diffusive encounters in highly-concentration solutions (Dvořák et al. [2]). Our research extends to track vibrations during the singlet fission process using FSRS. Here we present the time-resolved vibrational spectrum of highly-concentrated TIPS-Pentacene solutions. The molecular structural evolution is probed by monitoring vibrational changes in Raman transitions after a variable time delay. The data is compared with DFT computational methods to identify the excited state vibrational species. These results will be used to gain further insights into the singlet fission process.