Vibronic absorption, fluorescence, and phosphorescence spectra of psoralen: a quantum chemical investigation

Abstract

Excited state potential energy hypersurfaces of 7H-furo[3,2-g][1]benzopyran-7-one (psoralen) have been explored employing (time-dependent) Kohn–Sham density functional theory. At selected points, we have determined electronic excitation energies and electric dipole (transition) moments utilizing a combined density functional/multireference configuration interaction method. Spin–orbit coupling has been taken into account employing an efficient, non-empirical spin–orbit mean-field Hamiltonian. Franck–Condon factors have been computed for vibrational modes with large displacements in the respective Dushinsky transformations. The simulated band spectra closely resemble experimental band shapes and thus validate the theoretically determined nuclear structures at the S₀, S₁, and T₁ minima. In the S₁ (π_HOMO → π^*_LUMO) state, the lactone bond of the pyrone ring is significantly elongated. From excited vibrational levels of the S₁ state a conical intersection between a (π → σ*) excited state and the electronic ground state may be energetically accessible. Fast non-radiative decay via this relaxation pathway could explain the low fluorescence quantum yield of psoralen. The T₁ (π_HOMO−1 → π^*_LUMO) exhibits a diradicaloid electronic structure with a broken C₅–C₆ double bond in the pyrone ring. A variational multireference spin–orbit configuration interaction procedure yields a phosphorescence lifetime of 3 s, in excellent agreement with experimental estimates.