Resolving the S0/S1 Crossing Topology for the Excited-State Origin of Watasenia scintillans Bioluminescence.

Although the firefly squid Watasenia scintillans produces blue bioluminescence, the electronic origin of its chemiexcitation has remained elusive. Previous time-dependent density functional theory (TDDFT) calculations suggested two distinct crossing regions between S0 and S1, but the corresponding S0-S1 energy gaps are too large to enable an efficient nonadiabatic S0 → S1 transition. In this work, we employ state-of-the-art multireference wave function theory (WFT), the density matrix renormalization group (DMRG) method, employing large active spaces comprising up to 36 orbitals, to elucidate the excited-state generation pathway. By providing a more accurate description of strong electron correlation in the biradical regime, our DMRG-based potential energy profile resolves the crossing topology into a single S0/S1 near-degenerate region, with the energy gap reduced by nearly an order of magnitude. This mechanistic picture is further supported by good agreement between computed and experimental emission wavelengths, as well as a Landau-Zener model analysis that predicts a high nonadiabatic transition probability.