  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.
