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 f ree base porphyrins with graded degrees of a macrocycle distortion have bee n studied. The different degrees of nonplanarity were introduced by success ive addition of ethyl groups at the beta -pyrrole positions of free base 5, 10,15,20-tetraphenylporphyrin (H2TPP): from four ethyl groups in cis-tetrae thyl-TPP (H(2)cTETPP) to six ethyl groups in hexaethyl-TPP (H2HETPP) and ei ght 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 b etween the long-wavelength absorption and short-wavelength emission maxima, and reduced excited-state lifetimes. In nonpolar solvents, these perturbat ions directly reflect the steric/electronic consequences of the distortion of the porphyrin 7-system. In polar media, all the photophysical consequenc es of nonplanar distortion are markedly enhanced as a function of solvent p olarity. These effects derive from electronic interactions between the pola r solvent molecules and the polar S-1(pi,pi*) excited state of the nonplana r free base porphyrin. The origin of the polar nature of these nonplanar ch romophores is indicated by semiempirical calculations, which show that free -base porphyrins with saddle-type macrocycle distortions have a permanent d ipole moment (1-2 D) with a significant projection orthogonal to the nitrog en mean plane. Among the contributions to this macroscopic dipole moment ar e structural/electronic asymmetries derived from the pyrrole rings, N-H bon ds and nitrogen lone pairs. The specific factors and the macroscopic dipole moment provide foci for solvent interactions that are amplified in nonplan ar porphyrins relative to their planar counterparts. The studies demonstrat e the strong interrelated effects of the conformation(s) involving the porp hyrin macrocycle and its peripheral substituents, electronic structure, and solvent interactions (including macrocycle-nonplanarity-induced dipole mom ents) in dictating the photophysical properties of distorted porphyrins. Th e findings have implications for the function of tetrapyrrole cofactors in the biological proteins and for the use of nonplanar porphyrins in molecula r optoelectronics.