Luminescence and Geometric and Electronic Structures of Porphyrazines with Annelated 1,4-Diazepine Rings

Absorption, fluorescence, and phosphorescence spectra of metal complexes of 4-tert-butylphenyl-substituted tetra(1,4-diazepino)porphyrazine MPADz₄Ph₈′ (M = Mg, Zn; PA = porphyrazine; Dz = diazepine; Ph = phenyl; Ph′ = 4-tert-butylphenyl) were recorded in DMF at 293 and 77 K. The Q(0–0) absorption bands of MgPADz₄Ph₈′ and ZnPADz₄Ph₈′ did not show signs of splitting, i.e., corresponded to the monomeric form. Singlet–triplet gaps of 4700 cm^–1 for the Mg complex and 4800 cm^–1 for the Zn complex were determined from fluorescence and phosphorescence spectra. Fluorescence quantum yields and lifetimes were measured. Rate constants of S_1,2 → S₀ radiative transitions and total rate constants of nonradiative deactivation of the S_1,2 states were determined. Internal conversion rate constants were estimated. Conformer geometries of MgPADz₄Ph₈ (in the 6H tautomeric form) were calculated by density functional theory in the PBE/TZVP version. The conformer of symmetry S_4v (D_2d ) had the lowest energy. Nonplanar diazepine rings induced small but perceptible out-of-plane distortions in the central porphyrazine ring of MgPADz₄Ph₈. This feature correlated with the observed Stokes shifts of 400 and 350 cm^–1 (at 293 K) for MgPADz₄Ph₈′ and ZnPADz₄Ph₈′ whereas the metal phthalocyanines had shifts of ~50 cm^–1. Excited electronic states of MgPADz₄Ph₈ were calculated using INDO/Sm. The Q-state energy of 15,200 cm^–1 agreed well with the observed value of 14,800 cm^–1. Strong featureless absorption in the range 330–450 nm (Soret band analog) with maxima at ~25,000 and ~29,000 cm^–1 was due mainly to two strong electronic transitions with calculated energies of 24,100 and 31,500 cm^–1; a shoulder on the long-wavelength side of the Soret band at ~23,000 cm^–1, to a transition of calculated energy 23,800 cm^–1.