Electronic spectroscopy of lumiflavin in superfluid helium nanodroplets

We present the fluorescence excitation and dispersed emission spectra of lumiflavin doped into superfluid He nanodroplets. Both spectra show well resolved vibrational structure. The electronic origin transition at 21511 cm−1 is the strongest line in both spectra. Quantum chemical calculations with DFT and CASSCF methods support the assignment of S1 to a ππ∗ excited state. We obtain vibrational frequencies in the ground and lowest excited singlet state that can serve to test the validity of quantum chemical calculations. Multidimensional Franck-Condon factors are in good agreement with the intensities within the vibrational structure for S0 and S1. The strongest progression forming mode has a frequency of 164 cm−1 in both states and is assigned to an in-plane bending mode of the whole flavin chromophore with a large amplitude on the two methyl groups at ring I.

Highlights► We present the first electronic spectra of a flavin compound in a molecular beam. ► The low temperature of He-droplets (370 mK) permits high vibrational resolution. ► Vibrational frequencies of S0 and S1 agree well with quantum chemical calculations. ► These data can serve as reference for simulations on flavin containing enzymes.