Difference in intramolecular singlet fission trajectory for protonated and deuterated TIPS-pentacene dimers: plausible effect of H-bonding and vibrational phase matching*

Exciton multiplication property makes sensitizers that undergo singlet fission ideal to increase the efficiency of photovoltaic cells beyond the Shockley-Queisser limit. Understanding of the trajectory of singlet fission in atomistic detail is imperative to control the efficiency of singlet fission by targeted molecular engineering thus improving its applications. Femtosecond stimulated Raman spectroscopy (FSRS) allows us to extract the time-resolved structural evolution of photoexcited molecules. In this contribution, we used FSRS under double pulse excitation to coherently amplify a particular vibration to examine its effect on the intramolecular singlet fission dynamics in the synthesized TIPS-pentacene dimers. We found that the C-H wagging mode and the pentacene core out-of-plane bending mode affect the intramolecular singlet fission dynamics differently. Most interestingly, the same vibrational modes for the TIPS-pentacene dimer where all the pentacene core hydrogens are substituted by deuterium showed a completely opposite trend in affecting the dynamics. The opposite effect for C-H wagging and C-D wagging on the intramolecular singlet fission point toward plausible effects of the H-bonding and vibrational phase matching on the excited state dynamics. Moreover, our findings showed that the double pulse excitation technique to amplify a particular vibrational mode coherently can be utilized to control the photophysical outcomes which is crucial for molecular engineering.