Theory of Transient Excited State Absorptions in Solid Pentacene with Implications for Singlet Fission

We report the first theoretical calculations of excited state absorptions (ESAs) from the singlet and triplet excitons, as well as the key intermediate in the singlet fission (SF) process, the spin singlet multiexciton triplet-triplet state, for solid pentacene with herringbone crystal structure. Our goal is to compare theoretical results against ultrafast transient photoinduced absorption (PA) measurements and their interpretations, which have remained controversial. We show that the elusive triplet-triplet state absorbs both in the visible and near infrared (NIR), at or close to the PA energies assigned to the free triplet exciton. In contrast, the triplet PA has nearly vanishing oscillator strength in the NIR within the rigid herringbone structure. Observable oscillator strength for NIR triplet PA requires photoinduced enhancement of coupling between a pair of neighboring pentacene molecules that confers significant charge-transfer (CT) character to the triplet exciton. We discuss the implication of our results for efficient SF in pentacene and related materials.