Time‐resolved electron paramagnetic resonance spectra of photoexcited triplet states of electron–donor–acceptor complexes in frozen solution: Methylated benzenes and chlorinated phthalic anhydrides

The excited triplet states of phthalic anhydride and its chlorinated derivatives in methyl substituted benzene derivatives were studied at 77 K by a time‐resolved EPR technique. The increase of the number of the methyl group on the benzene decreased the zero‐field splitting parameters of their triplet states. This was explained by the contribution of the electron–donor–acceptor (EDA) complex. Especially, in the case of tetrachlorophthalic anhydride in hexamethylbenzene, the apparent inversion of the spin‐polarization was observed. These data evidenced the negative sign of the D value in highly charge‐transferred (CT) complex. This experiment reveals that the employment of D value instead of D* provides much better estimation of the CT character of the EDA complex.