Theory of Transient Excited State Absorptions in Pentacene and Derivatives: The Triplet-Triplet Biexciton Versus Free Triplets^a

Recent experiments in several singlet-fission materials have found that the triplet-triplet
biexciton is either the primary product of photoexcitation, or has much longer lifetime than
believed until now. It thus becomes essential to determine the difference in the spectro-
scopic signatures of the bound triplet-triplet and free triplets to distinguish between them
optically. We report calculations of excited state absorptions (ESAs) from the singlet and
triplet excitons and from the triplet-triplet biexciton for pentacene crystal with the herring-
bone structure and for nanocrystals of bis(triisopropylsilylethynyl) (TIPS)-pentacene. The
triplet-triplet biexciton absorbs both in the visible and the near infrared (NIR), while the
monomer free triplet absorbs only in the visible. The intensity of the NIR absorption depends
on the extent of intermolecular coupling, in agreement with observations in TIPS-pentacene
nanocrystals. We predict additional weak ESA from the triplet-triplet, but not from the
triplet, at still lower energy.