FLUORESCENT PROPERTIES OF PHENYL - CONTAINING ISOMERS OF PALLADIUM COMPLEXES OF OCTAETHYLPORPHIN IN ACETONITRILE
Abstract
The fluorimetric measurements of the acetonitrile solutions of palladium complexes of the octaethylporphyrin, 5,10- biphenyl octaethylporphyrin and 5,15-biphenyl octaethylporphyrin at 293 K were performed. Tetraphenylporfin was used in toluene as standards. It was determined that the studied compounds differ in the architecture of peripheral substitution, and as a result in molecular conformation and symmetry, which determine the rates of intramolecular processes of deactivation of electronic excitation energy. It was established that the introduction of two phenyl fragments into the meso-positions of the tetrapyrrole macrocycle of the octaethylporphyrin molecule markedly changes the spectral-luminescent properties of their palladium complexes. The compound with the adjacent arrangement of phenyl fragments of 5,10-diphenylctaethylporphyrin is characterized by concentrated steric effects in the region of one quadrant of the macrocycle containing two phenyl groups and the pyrrole fragment located between them. This arrangement of phenyl fragments leads to the formation of an asymmetrically distorted saddle-shaped conformer, as evidenced by the bathochromic shift of the bands in the electronic absorption spectrum by 495 cm–1. The quantum yield of S1→S0 fluorescence decreases slightly to 4.6·10-3, which is explained by the formation of a new competing non-radiative deactivation channel — the internal S1→S0 conversion, which is characteristic of distorted saddle-shaped macrocycle. When phenyl groups at opposite meso-positions of the 5.15-diphenyl octaethylporphyrin macrocycle flanked by alkyl groups in the Cb positions of the pyrrole rings, a conformer is formed with a macrocycle elongated along the Cm-Cm axis. This is probably due to the stricter molecular structure of this conformer compared to palladium octaethylporphyrin and the absence of conformational dynamics in the S1 excited state. As a result, the bathochromic shift of the bands in the electronic absorption spectrum of 5.15 – diphenyl octaethylporphyrin is less than 100 cm-1 and the quantum yield of S1→S0 fluorescence is almost equal to the quantum yield of the palladium complex of octaethylporphyrin - 5.4·10-3, which is probably due to the low non-radiative internal S1→S0 conversion for the 5.15-diphenyl octaethylporphyrin compared to the 5.10-diphenyl octaethylporphyrin.
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