The electronic structure of the photoexcited triplet state of free-base (tetraphenyl)porphyrin by time-resolved electron-nuclear double resonance and density functional theory.

The photoexcited triplet states of free-base porphyrin (H(2)P) and free-base tetraphenylporphyrin (H(2)TPP) have been investigated by time-resolved electron paramagnetic resonance and electron-nuclear double resonance in a toluene glass at 80 K. Both the zero-field splitting parameters, D and E, and the proton A(zz) hyperfine coupling tensor components could be determined. D is about 13% larger in H(2)P than in H(2)TPP. In contrast, however, the A(zz) hyperfine coupling tensor components showed differences of less than 2%. To aid the understanding of these results, the electronic structures of H(2)P and H(2)TPP have been modeled using density functional theory. The geometrical structures of both molecules in their lowest triplet states were calculated using the Becke3 Lee-Yang-Parr composite exchange correlation functional and the 6-31G* basis set. Hyperfine couplings for these structures were calculated using the same functional but with the extended EPR-II basis set. These allow unambiguous assignment of the experimentally determined couplings. The theoretical values for H(2)P and H(2)TPP agree with the experimental values in that the presence of the phenyl groups has only a small effect on the unpaired electron spin-density distribution. The difference in sensitivity of the zero-field splitting parameters and the hyperfine couplings to mesophenyl substitution is discussed in terms of the wave functions of the four frontier orbitals of porphyrins introduced by Gouterman.