2-NO2-5,10,15,20-tetraphenylporphyrin (H2TPP-NO2) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (similar to 300 cm(-1)) Stokes-shifted with respect to the corresponding Q(xoo) absorption bands, with the spectrum of the more stable tautomer 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-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as similar to 2000 cm(-1) in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer(CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT slates gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q(x) state energy and an increase of its dipole moment mu with a decrease of the angle theta between the plane of the NO2 group and the porphyrin plane (relative to the ground-state equilibrium geometry with theta = 80 degrees). We suggest that, in solution, such an increase in the mu value should result in an increase of the stabilization interaction between the polar porphyrin and 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 picosecond transient absorption measurements of the Sl --> S-n absorption show that the fluorescent Q(x) state of H2TPP-NO2 has mainly (1)(pi,pi*) character with a relatively small charge-transfer admixture.