Spectral manifestations of the splitting of the lowest triplet levels in Pd complexes of nonsymmetric porphyrins

Based on the study of the phosphorescence and phosphorescence excitation spectra of Pd(II) tetramethylporphyrin (PdTMP) and Pd(II) tetramethyldiethylporphyrin (PdTMDEP) in solutions in 2-methyltetrahydrofuran and dodecane in the temperature range 77–283 K, the occurrence of the splitting of the lowest degenerate singlet (S1) and triplet (T1, T2) levels of porphyrin molecules is established. In the absorption of molecules of the compounds studied, two components, S1 and S2, are revealed in the range of allowed long-wavelength Q(0,0) transitions (530–550 nm) and four components, T1–T4, are observed in the range of spin-forbidden intersystem crossing transitions S0 → Tn (560–670 nm), with all the triplet levels T1-T4 being located below the S1 level on the energy scale. It is shown that an increase in the degree of deformation of the porphyrin macrocycle caused by steric interactions between β alkyls and hydrogen meso atoms is accompanied by an increase in the splitting in the system of singlet levels δE(S2–S1) from 120 cm−1 for PdTMDEP to 215 cm−1 for PdTMP, as well as by an increase in the splitting in the system of triplet levels δE(T2–T1) from 190 cm−1 for PdTMDEP to 250 cm−1 for PdTMP.