  Nonadiabatic couplings between several electronic excited states are
ubiquitous in many organic chromophores and can significantly influence optical
properties. A recent experimental study demonstrated that the proflavine
molecule exhibits surprising dual fluorescence in the gas phase, that is
suppressed in polar solvent environments. Here, we uncover the origin of this
phenomenon by parameterizing a linear-vibronic coupling (LVC) Hamiltonian from
spectral densities of system-bath coupling constructed along molecular dynamics
trajectories, fully accounting for interactions with the condensed-phase
environment. The finite-temperature absorption, steady-stat emission, and
time-resolved emission spectra are then computed using powerful, numerically
exact tensor network approaches. We find that the dual fluorescence in vacuum
is driven by a single well-defined coupling mode, but is quenched in solution
due to dynamic solvent-driven symmetry-breaking that mixes the two low-lying
electronic states. We expect the computational framework developed here to be
widely applicable to the study of non-Condon effects in complex condensed-phase
environments.
