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 an d 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 th e energy of the lowest excited state of all-trans-retinal. The fluorescence spectra of concentrated solutions have been deconvolved in terms of four G aussian bands, independent of the excitation wavelength. Emissions from com plexes with water and from retinal dimers have been assigned in the solutio n spectrum, in agreement with early reports. Stable 1:1 and 1:2 complexes w ith protic solvents (H2O, CH3OH, C2H5OH, phenol) have been calculated by ab initio density functional (DF) methods (B3-LYP/6-31G*), enforcing the expe rimental assignment. Due to extensive drying operations on the solid, the f luorescence from water complexes is negligible in the crystal spectrum of a ll-trans-retinal. In this case the fluorescence has been attributed to dime rs and higher order clusters. DF calculations on the isolated dimer and tet ramer as well as on the same species of a retinal homologue indicate that t hese systems are stable. Dimers and tetramers of all-trans-retinal may be i dentified in the crystal structure, justifying the fluorescence from these species in the crystal spectrum.