Light-Induced Spin Crossover in an Intermediate-Spin Penta-Coordinated Iron(III) Complex.

(PMe3)2FeCl3 is an Fe(III) complex that exists in the intermediate-spin ground state in a distorted trigonal bipyramidal geometry. An electronic state with high-spin configuration lies in close vicinity to the ground state, making it a potential spin crossover candidate. A mechanistic account of the spin crossover from the lowest quartet state (Q0) to the lowest sextet state (S1) of this complex is provided by exploring both thermal and light-induced pathways. The presence of a large barrier between the two spin states suggests a possible thermal spin crossover at a rather high temperature. The light-induced spin crossover is investigated by employing complete active space self-consistent field calculations together with dynamic correlation and spin-orbit coupling for the lowest seven quartet and lowest five sextet states. The system in the Q0 state upon light absorption is excited to the optically bright Q4 LMCT state. By following minimum energy pathways along the electronic states, two light-induced pathways for spin crossover are identified. From the Q4 state, the system can photo-regenerate the ground intermediate-spin state (Q0) through an internal conversion of Q4/Q3 followed by Q3/S1 and S1/Q0 intersystem crossings. In an alternate route, through Q4/S2 intersystem crossing followed by S2/S1 internal conversion, the system can complete the spin crossover from the Q0 to S1 state.