Assignment of the Triplet-to-Triplet Absorption Spectrum of Pyrene

The triplet-to-triplet absorption spectrum of pyrene underpins technological applications and has been measured many times, with most studies focusing on two intense transitions spanning the green to the UV regions. Nevertheless, there has been little attention paid to its very weak IR absorption and weak red absorption, even though the IR band provides critical information for many practical applications. Holistically, a critical work is the 1970 extensive spectral measurement and assignment of Langelaar et al. Using calculations of very different types (the CAM-B3LYP density-functional, coupled-cluster, and complete active space self-consistent field (CASSCF) approaches), we vindicate their assignment of four Franck-Condon spectral progressions. Three new regions exhibiting unaccounted absorption are also identified, one of which is identified as an additional dark state. Across the whole spectrum, strong vibronic coupling is predicted to lead to Herzberg-Teller effects that account for the unassigned absorptions and drive nonadiabatic couplings that would provide for very rapid energy relaxation, within the triplet manifold, to the lowest-energy triplet excited state T2 of symmetry 3B2g. Evidence is also obtained indicating the presence of low-lying conical intersections involving the next two triplet states, (1)3B2u and (1)3Ag.