Theory of Transient Excited State Absorptions in Pentacene and Derivatives: Triplet–Triplet Biexciton versus Free Triplets

Abstract

Recent experiments in several singlet-fission materials have found that the triplet-triplet biexciton either is the primary product of photoexcitation or has a much longer lifetime than believed until now. It thus becomes essential to determine the difference in the spectroscopic 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 a pentacene crystal with the herringbone structure and for nanocrystals of bis(triisopropylsilylethynyl) (TIPS)-pentacene. The triplet-triplet biexciton absorbs in both 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.

Striking the right balance of intermolecular coupling for high-efficiency singlet fission

“…We note that a recent theory study sheds additional insight on how molecular packing and intermolecular coupling influences these transient absorption signals. 108 …”

Multiexcitonic Triplet Pair Generation in Oligoacene Dendrimers as Amorphous Solid‐State Miniatures

“…ESA bands for free triplet and multiexcitonic triplet pair have been estimated by Khan et al. based on Pariser‐Parr‐Pople (PPP) π‐electron model [74] . According to their results, the ESA band in the visible region (500–600 nm) red‐shifts for free triplet compared to that of multiexcitonic triplet pair and the ESA band spanning from 700 to 800 nm disappears for free triplet species.…”

Singlet Fission in Carbene‐Derived Diradicaloids

“…In this particular case, the corresponding lifetimes would be expected to be in the same range. Overall, it is impossible to dissect the spectral shape of the triplet pair state spectrum and to directly relate it to that of an isolated, fully independent triplet state . Still, the similarities between sensitized and observed TA spectra serve as strong evidence for the presence of a triplet excited state (For 2 , see Figure S8 of the Supporting Informatino).…”