Electronic excitation spectra and singlet–triplet coupling in psoralen and its sulfur and selenium analogs

We have calculated the vertical singlet and triplet excitation spectra and spin–orbit coupling matrix elements for psoralen and its derivatives resulting from the replacement of intracyclic oxygen by sulfur or selenium. Molecular ground state equilibrium geometries have been determined employing Kohn–Sham density functional theory. Electronic excitation energies and oscillatory strengths have been obtained utilizing a combined density functional/multi-reference configuration interaction method. Spin–orbit coupling matrix elements for correlated wavefunctions have been computed applying the efficient, purely non-empirical spin–orbit mean-field approximation. eoretical data allows for a detailed assignment of experimental absorption bands [J. Photochem. Photobiol. B: Biol. 35 (1996) 221]. The computed excitation energy of the first π→π∗ singlet transition varies from 3.81 (7H-furo [3,2-g] [1] benzopyran-7-one) to 3.12 eV (7H-selenolo [3,2-g] [1] benzoselenopyran-7-one). The energy of the lowest triplet π→π∗ state T1 is remarkably constant in all cases (2.95–2.73 eV). The energies of the dark n→π∗ states are found to be lowered considerably (up to ≈0.80 eV) upon replacing intracyclic oxygen at the pyrone side by sulfur or selenium, but much less upon hetero-atom substitution solely at the furan side. For all the heteropsoralens, additional low-lying π→σ∗ states have been found that are important for photochemical ring opening reactions. in–orbit coupling between the T1 state and the ground state S0 amounts to less than 2 cm−1 for all cases. Between n→π∗ and π→π∗ states appreciable spin–orbit coupling matrix elements are observed which indicate a probable channel for singlet–triplet radiationless transitions. Their size varies from several ten wavenumbers for those psoralens which have oxygen or sulfur in the pyrone ring to several hundred wavenumbers in the case when selenium is present in the pyrone ring.