On the nature of the fluorescent state in beta-nitrotetraarylporphyrins

2-NO2-5,10,15,20-tetraphenylporphyrin (H2TPP-NO2) is shown to exist in solu tion as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluores cence spectra of the tautomers contain two well-resolved narrow bands that are slightly (similar to 300 cm(-1)) Stokes-shifted with respect to the cor responding Q(xoo) absorption bands, with the spectrum of the more stable ta utomer being similar to 500 cm(-1) red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shift ed relative to the longest wavelength absorption band. The Stokes shift inc reases with an increase of solvent polarity, being as large as similar to 2 000 cm(-1) in N,N-dimethylformamide. Fluorescence lifetimes of the tautomer s are found to be markedly (about a factor of 2) different, with both decre asing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical qua ntum chemical calculations. Charge-transfer(CT) states are found to be loca ted between the porphyrinic Q and B states for both tautomers, and a differ ent energy of the CT slates gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q(x) stat e energy and an increase of its dipole moment mu with a decrease of the ang le theta between the plane of the NO2 group and the porphyrin plane (relati ve to the ground-state equilibrium geometry with theta = 80 degrees). We su ggest that, in solution, such an increase in the mu value should result in an increase of the stabilization interaction between the polar porphyrin an d solvent molecules and, consequently, in flattening of the Qx state energy curve as a function of theta and a shift of the minimum of this curve to a values less than 80 degrees. Both quantum chemical calculations and picose cond transient absorption measurements of the Sl --> S-n absorption show th at the fluorescent Q(x) state of H2TPP-NO2 has mainly (1)(pi,pi*) character with a relatively small charge-transfer admixture.