Light-induced electron spin polarization in vanadyl octaethylporphyrin: II. Dynamics of the excited states.

The dynamics of the low-lying excited states of vanadyl octaethylporphyrin (OEPVO) in frozen solution is investigated by transient electron paramagnetic resonance (TREPR). The observation of spin-polarized TREPR spectra from the lowest excited trip-quartet state of OEPVO, reported in the preceding paper, opens a new avenue for investigation of the excited states of such molecules. Here, a model based on the back-and-forth transitions between the trip-quartet and trip-doublet states is developed and used to explain the time dependence of the low-temperature laser flash-induced electron spin polarization of OEPVO. At early times, the TREPR spectra show predominantly multiplet polarization, whereas strong net polarization develops at later times. An analysis of the time dependence reveals two well-separated processes: (i) fast evolution of the polarization from the multiplet pattern to the net absorptive pattern and (ii) very slow decay of the net polarization. Both processes are temperature dependent and are faster at higher temperature. All of these observed features can be reproduced, and the experimental data can be simulated within the framework of the model. For simplicity, only the two nearly degenerate orbital states resulting from the a(1) --> e triplet excitation of the porphyrin are considered. Each of these is split into a trip-doublet and trip-quartet giving a total of four low-lying excited states. Transitions between the trip-doublet and trip-quartet states are assumed to be governed by spin-orbit coupling, which mixes the four low-lying states. It is known that following light excitation, the molecule initially decays to the lowest trip-doublet state and then to the trip-quartet state. In agreement with the observed TREPR spectra, the model predicts that this decay results in predominantly multiplet polarization of the trip-quartet. However, a small amount of net polarization is also predicted due to the spin selectivity associated with the Zeeman interaction. Because the energy gap between the trip-doublet and trip-quartet states is small, back-and-forth electronic transitions between the trip-doublet and trip-quartet are expected to occur as thermal equilibrium is established. The model predicts that it is these transitions that lead to the observed evolution of the initial multiplet polarization to net absorptive polarization.