Fluorescence of all-trans retinal as a crystal and in a dense solution phase

The fluorescence spectra of all-trans-retinal as a polycrystalline solid and in highly concentrated n-hexane solutions have been measured at 77 K as a function of the exciting wavelength between 420 and 500 nm, i.e., below the energy of the lowest excited state of all-trans-retinal. The fluorescence spectra of concentrated solutions have been deconvolved in terms of four Gaussian bands, independent of the excitation wavelength. Emissions from complexes with water and from retinal dimers have been assigned in the solution spectrum, in agreement with early reports. Stable 1:1 and 1:2 complexes with protic solvents (H2O, CH3OH, C2H5OH, phenol) have been calculated by ab initio density functional (DF) methods (B3-LYP/6-31G*), enforcing the experimental assignment. Due to extensive drying operations on the solid, the fluorescence from water complexes is negligible in the crystal spectrum of all-trans-retinal. In this case the fluorescence has been attributed to dimers and higher order clusters. DF calculations on the isolated dimer and tetramer as well as on the same species of a retinal homologue indicate that these systems are stable. Dimers and tetramers of all-trans-retinal may be identified in the crystal structure, justifying the fluorescence from these species in the crystal spectrum.