Retrieving the Rate of Reverse Intersystem Crossing from Ultrafast Spectroscopy

Among various nonradiative photophysical processes related to energy migration in singlet-triplet coupled molecular systems, unlike such processes as internal conversion, intersystem crossing (ISC), and intramolecular vibrational relaxation that have been extensively addressed, the reverse ISC (rISC) is a unique one that apparently lacks sufficient interrogation probably owing to its intrinsically elusive nature. In particular, it still remains a nontrivial task to quantitatively describe the rISC pathway. Here we introduce a new, simple route to this end, just through explicit modeling and simulations on routinely available, experimental data from ultrafast transient absorption spectroscopy. We demonstrate on a proof-of-concept, rare-earth chelate molecular system that our approach, featuring spectral profile analysis together with wavelength-dependent global kinetics fitting, enables facile retrieval of the rISC rate from experimental data.