Theoretical spectroscopy of BASHY dyes

This works constitutes an ab initio investigation of a recently disclosed class of fluorophores, BASHY (Santos et al. in Chem Eur J 22:1631–1637, 2016). We use state-of-the-art approaches including an electronic structure method partially accounting for the contributions of double excitations and a solvent model dependent of the change of dipole moment between the states, to provide accurate estimates of all key optical signatures, including 0–0 energies and, for a prototype structure, the topology of the absorption and emission bands. Besides comparing the pros and cons of several theoretical models and showing that theory adequately reproduces the experimental trends, our calculations demonstrate that despite significant displacement of the electron density from the ground- to the excited-state, typical of charge-transfer states, a cyanine-like character partly pertains in the transition of BASHY. We also rationalize why the most planar structure has one of the smallest quantum yields of emission.