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| - The Electronic Structure of the Photoexcited Triplet State ofFree-Base (Tetraphenyl)porphyrin by Time-ResolvedElectron−Nuclear Double Resonance and Density FunctionalTheory
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| - The photoexcited triplet states of free-base porphyrin (H2P) and free-base tetraphenylporphyrin(H2TPP) have been investigated by time-resolved electron paramagnetic resonance and electron−nucleardouble resonance in a toluene glass at 80 K. Both the zero-field splitting parameters, D and E, and theproton Azz hyperfine coupling tensor components could be determined. D is about 13% larger in H2P thanin H2TPP. In contrast, however, the Azz hyperfine coupling tensor components showed differences of lessthan 2%. To aid the understanding of these results, the electronic structures of H2P and H2TPP have beenmodeled using density functional theory. The geometrical structures of both molecules in their lowest tripletstates were calculated using the Becke3 Lee−Yang−Parr composite exchange correlation functional andthe 6-31G* basis set. Hyperfine couplings for these structures were calculated using the same functionalbut with the extended EPR-II basis set. These allow unambiguous assignment of the experimentallydetermined couplings. The theoretical values for H2P and H2TPP agree with the experimental values inthat the presence of the phenyl groups has only a small effect on the unpaired electron spin-densitydistribution. The difference in sensitivity of the zero-field splitting parameters and the hyperfine couplingsto mesophenyl substitution is discussed in terms of the wave functions of the four frontier orbitals ofporphyrins introduced by Gouterman.
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