Photophysical properties of 5-(4-acetamidophenyl)-10,15,20-tris (N-methylpyridinium-3-yl)porphyrin trichloride and its Zn(II) complex

The spectral and photophysical properties of two porphyrins, 5-(4-acetamidophenyl)-10,15,20-tris(N-methyl-pyridinium-3-yl)porphyrin trichloride (1) and its Zn(II) complex 2 were investigated in different solvents by absorption, steady-state and time-resolved fluorescence spectroscopy, and laser flash photolysis. The findings were corroborated by computations at the TD-B3LYP/6-31++G(d,p) level of theory. The absorption spectra of 1 and 2 are the typical for porphyrins and their Zn complexes with the Soret bands at approximate to 420 nm and 430 nm, respectively, and the Q-bands at 515-650 nm, and 560-605 nm. Smaller differences in the absorption spectra compared to the symmetric porphyrins were assigned to more pronounced HOMO and HOMO-1 energy levels splitting in the asymmetric molecules. The temperature dependence of the absorption spectra with hypsochromic shifts suggests the formation of H-aggregates. The emission spectra are situated at 600-700 nm, and the vibronic resolution of the spectra for 1 depends on the solvent and the excitation wavelength, which was assigned due to the tautomerism by H-transfer within the porphyrin core. Quantum yields of fluorescence for 1 and 2 in methanolic solutions are phi(f )= 0.072 and 0.051, respectively. Although, the steady-state fluorescence spectra do not indicate that 1 and 2 form aggregates in methanolic and aqueous solutions (at <10(-4) M), singlet decays measured by time-correlated single photon counting for both compounds were fit to multiexponential functions, which was explained by formation of aggregates in more concentrated solutions. Furthermore, at very high concentrations of 1 in CH3OH (>10(-4) M), the dynamic formation of aggregates was evidenced by new bathochromically shifted band in the emission spectrum and the growth kinetics in the time-resolved fluorescence measurement. Triplet excited states of 1 and 2 were probed by laser flash photolysis. The triplets are populated with quantum yields of intersystem crossing, phi(ISC )= 0.34 and 0.41, respectively and lifetimes of the triplet excited states are in millisecond time-scale. Both triplets are quenched by O-2 with the rate constants approaching diffusion limits. Relative quantum yields for the singlet oxygen formation are 0.86 and 0.92 respectively, rendering porphyrins 1 and 2 applicable for photodynamic therapy.