Light-induced electron spin polarization in vanadyl octaethylporphyrin: I. Characterization of the excited quartet state.

Laser flash induced spin-polarized transient electron paramagnetic resonance (TREPR) spectra for vanadyl octaethylporphyrin in isotropic and partially ordered frozen solutions are presented and compared with corresponding luminescence data. The TREPR spectra show well-resolved hyperfine couplings to the vanadium nucleus and a multiplet polarization pattern with features typical of zero-field splitting (ZFS). The principal values of the vanadium hyperfine coupling tensor evaluated from the spectra are 1/3 of the corresponding values found from steady-state EPR spectra of the ground state. On the basis of these characteristics and numerical simulations, the polarization patterns are assigned to the excited quartet state. The values of the ZFS parameters of the trip-quartet obtained from simulation of the spectra (D = 17.5 mT and E = 1.5 mT) are comparable to those of the triplet state of the zinc and free base octaethyl porphyrin. The lifetime of the spin polarization is found to be temperature dependent and is essentially the same as that of the optical emission. The temperature dependence is rationalized using a model in which the decay to the ground state occurs from both the trip-quartet and trip-doublet, which are in thermal equilibrium even at 15 K. A fit of the model to the observed spin polarization lifetimes yields an energy gap of 47 cm(-1) between the trip-quartet and trip-doublet. It is shown that the spin polarization evolves from a multiplet pattern at early times to a net absorptive pattern at late times following the laser flash. It is proposed that the establishment of thermal equilibrium leads to the evolution of the spin from multiplet to net polarization.