Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

Properties of the triplet states of octaethylporphyrins with the steric hin drance (free bases and Pd complexes) are studied by the methods of stationa ry and kinetic spectroscopy in the temperature range from 77 to 293 K. The mono-mesophenyl substitution results in a decrease in the quantum yield and shortening of the phosphorescence lifetime of Pd complexes by 250-3500 tim es in degassed toluene at 293 K. The phosphorescence quenching is caused by nonplanar dynamic conformations of the porphyrin macrocycle in the T-1 sta te, which also lead to the appearance of new bands at lambda similar to 100 0 nm in the T-T absorption spectra. As the number of meso-phenyls (Pd-octae tyltetraphenylporphyrin) increases, the quantum yield of phosphorescence fu rther decreases (<10(-5)) at 293 K, the lifetime of the T-1 state shortens (<50 ns), and the efficiency of the singlet oxygen generation abruptly decr eases (<0.01). The intense bathochromic emission of this compound at 705 nm with a lifetime of I ms at 77 K is assigned to the phosphorescence of a no nplanar conformation. Upon meso-ortho-nitrophenyl substitution, the quenchi ng of phosphorescence of Pd complexes (by more than 10(4) times at 293 K) i s caused by direct nonadiabatic photoinduced electron transfer from the T1 state to the nearest charge-transfer state with the probability k(et)(T) = (1.5-4.0) x 10(6) s(-1). The induced absorption of ortho-nitro derivatives in the region between 110 and 1400 nm is caused by mixing of pure pi pi* st ates with charge-transfer states. (C) 2001 MAIK "Nauka/lnterperiodica".