Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10,15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(Pyr)(2)(tpfc) and Al(pyr)(2)(tpfc-Br-8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal ET Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)(2)(tpfc-Br-8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)(2)(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)(2)(tpfc-Br-8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameter D < 0 for the Sn(Cl)(tpfc) and Al(pyr)(2)(tpfc-Br-9), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e., A(X), A(Y) >> A(Z). This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole pi-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)(2)(tpfc-Br-8), however, the out-of-plane triplet sublevel is predominantly populated, i.e., A(Z) > A(X), A(Y), which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.