Mechanistic insights into the photosensitizing activity of Eosin Y and Rose Bengal: a comparative computational and experimental study.

Photosensitizing activity, including Type I radical pathways and Type II singlet oxygen (1O2) generation, is central to photodynamic therapy, redox biology, and biosensing technologies, yet the mechanistic factors controlling photosensitizer performance are often inferred from bulk reactive oxygen species (ROS) measurements. In this study, we introduce a state-resolved computational framework to describe the excited state evolution of two xanthene-based dyes and widely used photosensitizers, Eosin Y (EY) and Rose Bengal (RB). The approach explicitly analyzes intersystem crossing (ISC) pathways, spin-orbit coupling matrix elements (SOCME), state-to-state transition rates, and resulting triplet-state populations as important molecular determinants of photosensitizing activity, including both Type I and Type II pathways. Experimental evaluation of photosensitizing activity was performed using 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) photodegradation assays, electron paramagnetic resonance (EPR) spectroscopy, and chronoamperometry, providing complementary evidence that both dyes generate photoinduced oxidative species under wavelength specific visible excitation. Computational results show that both dyes possess accessible singlet-triplet pathways, but Rose Bengal exhibits stronger spin-orbit coupling, more favorable energetic alignment between excited S1 and Tn states, and larger steady-state triplet population than EY. The evolution of a closed-system unimolecular model indicates that the triplet population of RB is approximately 86-fold larger than that of EY, with population entering the triplet manifold primarily through the T2 state. These trends are consistent with the stronger experimental response observed for RB under the irradiation conditions used and also indicate that triplet-state population, absorbed photon flux, oxygen quenching, and competing relaxation pathways must be considered together when comparing photosensitizer performance.