Deciphering the Luminescence Spectral Shape of an Oxyluciferin Analogue through a Mixed Quantum-Classical Approach

In this contribution, we present a computational study on the absorption and emission spectra of the cproxy^- anion in water, an analogue of the firefly oxyluciferin phenolate keto form. This compound displays a broad absorption spectrum and a large Stokes shift, two features that remain elusive to computational approaches, preventing a complete understanding of the photophysics behind this molecule. Here we attempt a fully first-principles computation of both absorption and emission spectral shapes and positions, explicitly including the effect of soft molecular flexible modes and of the stiff vibrational motions as well as those of the solvent. Namely, we adopt a recently developed mixed-quantum classical approach, the so-called Adiabatic Molecular Dynamics-generalized vertical Hessian (Ad-MD|gVH) method, which has been revealed to be well suited to reproduce band shapes in condensed phases. We also explore the performance of DFT functionals to build the potential energy surfaces and investigate the possible role of interstate couplings. By this means, we are able to obtain a first-principles simulation of the emission band shape close to the experimental one, and we correctly reproduce the two-peak shape of the absorption spectrum, both in terms of their spacing and relative intensity. However, the low-energy band of the computed absorption spectrum is too narrow, and the Stokes shift is remarkably underestimated. Through a careful analysis of different computational settings, we are able to identify some key aspects that partly explain these discrepancies, including the limitations of TD-DFT to properly describe the electronic energy along the flexible torsional degree of freedom in the lowest-excited state and the key role of mutual polarization of the solvent and the dye.