Light-Induced Electron Spin Polarization of a Weakly Coupled Triplet−Doublet Spin Pair in a Covalently Linked Porphyrin Dimer

Abstract

The spin-polarization patterns of a weakly coupled triplet−doublet spin pair are studied in a covalently linked copper (II)−free base porphyrin dimer partially oriented in a liquid crystalline solvent. The triplet−doublet pair is generated in the dimer by either spin−orbit intersystem crossing (ISC) within the free-base moiety or via energy transfer (EnT) from the copper porphyrin. The two pathways result in two different spin-polarization patterns, which can be observed separately by selectively exciting either the free base porphyrin at 640 nm or the copper porphyrin at 540 nm. The time development and orientation dependence of the two spin-polarization patterns is investigated at two microwave frequencies. An analysis of the EPR transients reveals that the polarization associated with the EnT pathway rises monoexponentially, while the ISC polarization rises rapidly and decays biexponentially. The rise of the EnT polarization pattern and the fast decay component of the polarization from ISC are governed by the same characteristic lifetime of 2 μs independent of the molecular orientation. However, the ratio of the two decay components of the ISC polarization is strongly orientation dependent. Separately, a general treatment of the spin-polarized transient EPR spectra of coupled triplet−doublet spin pairs is presented and two possible mechanisms for the kinetic behavior involving spin-selective depopulation or relaxation between the spin sublevels of the triplet−doublet pair are investigated. It is shown that all the observed features of the polarization patterns including orientation and excitation wavelength dependence as well as the kinetics can be explained in terms of the model and are consistent with either of the two mechanisms. In addition to the weakly coupled triplet−doublet pair, narrow signal components are observed near the center of the spectrum. The microwave frequency dependence of the spectra confirms the previous assignment that these components are the quartet and doublet states of a strongly coupled triplet−doublet pair generated in conformations of the dimer in which the two porphyrins are in close proximity.