Intersystem crossing driven by vibronic spin-orbit coupling: a case study on psoralen

For 7H-furo[3,2- g][1]benzopyran-7-one (psoralen), intersystem crossing (ISC) rate constants have been computed. Employing the Fermi golden rule, the harmonic approximation, and a pure-spin Born-Oppenheimer basis, both direct and vibronic spin-orbit (SO) coupling has been taken into account. Necessary data on electronic excitation energies and potential energy hypersurfaces originate from correlated all-electron calculations applying (time-dependent) density functional theory and the density functional theory/multireference configuration interaction approach. SO coupling has been treated by means of the one-center mean-field approximation. Vibronic SO couplings have been evaluated via numerical differentiation of SO matrix elements. Accounting only for direct SO coupling, rate constants of the order of k(ISC) approximate to 10(10) s(-1) result for S-2(n -> pi*) -> T-1( pi -> pi*) ISC, whereas the rates of the channels S-1( pi -> pi*) -> T-{1,T-2,T-3}(pi -> pi*) do not exceed k(ISC) approximate to 10(5) s(-1). Including vibronic SO coupling, rate constants of k(ISC) approximate to 3 x 10(8) s(-1) are obtained for the S-1(pi -> pi*) -> T-1(pi -> pi*) ISC. The radiationless transition from the S-1(pi -> pi*) state to the nearly degenerate T-3(pi -> pi*) state has been estimated to be slightly less efficient (k(ISC) approximate to 10(7) s(-1)). Based on our computed rates of ISC and excited state solvent shifts, we conclude that the experimentally observed appreciable triplet quantum yields of psoralen in polar protic media are primarily due to S-1(pi -> pi*) -> T(pi -> pi*) channels. For heteroaromatic systems, (pi -> pi*)/(pi -> pi*) ISC driven by vibronic SO coupling is expected to be a common triplet state population mechanism.