Photophysical and structural properties of saddle-shaped free base porphyrins: Evidence for an "orthogonal" dipole moment

Ground- and excited-state absorption and fluorescence properties of three free base porphyrins with graded degrees of a macrocycle distortion have been studied. The different degrees of nonplanarity were introduced by successive addition of ethyl groups at the beta -pyrrole positions of free base 5,10,15,20-tetraphenylporphyrin (H2TPP): from four ethyl groups in cis-tetraethyl-TPP (H(2)cTETPP) to six ethyl groups in hexaethyl-TPP (H2HETPP) and eight ethyl groups in octaethyl-TPP (H2OETPP). The static and dynamic optical properties of the compounds change systematically with an increase of the porphyrin macrocycle nonplanarity. These perturbations include significant broadening of the absorption and fluorescence bands, an increased spacing between the long-wavelength absorption and short-wavelength emission maxima, and reduced excited-state lifetimes. In nonpolar solvents, these perturbations directly reflect the steric/electronic consequences of the distortion of the porphyrin 7-system. In polar media, all the photophysical consequences of nonplanar distortion are markedly enhanced as a function of solvent polarity. These effects derive from electronic interactions between the polar solvent molecules and the polar S-1(pi,pi*) excited state of the nonplanar free base porphyrin. The origin of the polar nature of these nonplanar chromophores is indicated by semiempirical calculations, which show that free-base porphyrins with saddle-type macrocycle distortions have a permanent dipole moment (1-2 D) with a significant projection orthogonal to the nitrogen mean plane. Among the contributions to this macroscopic dipole moment are structural/electronic asymmetries derived from the pyrrole rings, N-H bonds and nitrogen lone pairs. The specific factors and the macroscopic dipole moment provide foci for solvent interactions that are amplified in nonplanar porphyrins relative to their planar counterparts. The studies demonstrate the strong interrelated effects of the conformation(s) involving the porphyrin macrocycle and its peripheral substituents, electronic structure, and solvent interactions (including macrocycle-nonplanarity-induced dipole moments) in dictating the photophysical properties of distorted porphyrins. The findings have implications for the function of tetrapyrrole cofactors in the biological proteins and for the use of nonplanar porphyrins in molecular optoelectronics.