Photophysical and Structural Properties of Saddle-Shaped Free Base Porphyrins:  Evidence for an “Orthogonal” Dipole Moment

Abstract

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 β-pyrrole positions of free base 5,10,15,20-tetraphenylporphyrin (H2TPP): from four ethyl groups in cis-tetraethyl-TPP (H2cTETPP) 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 π-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 S1(π,π*) 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.

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

“…2). These spectroscopic features are characteristic of a nonplanar porphyrin (16)(17)(18)(19)(20)(21)(22). Steric repulsion between the β-cyano groups and the meso-phenyl substituents leads to deformation of the tetrapyrrolic ring from its original planar geometry.…”

Influence of Macrocycle Protonation on the Photophysical Properties of Porphyrins

“…[37] This observation is in disagreement on a matter of principle with the concept proposed earlier. [17,18,24,70] According to that the DP species have unconditionally high efficiency of the internal S 1 →S 0 conversion. In the case described above, the quantum yield of the internal conversion Φ ISC is 0 (within experimental error).…”

Correlation of photophysical parameters with macrocycle distortion in porphyrins with graded degree of saddle distortion

“…In ref. 20 the authors report about a comparative study on basic electronic parameters of free base cTEPP, HETPP and OETPP in three solvents of different polarity: acetonitrile, n-hexane and toluene. They report a two exponential decay for each of the compounds independent on the solvent.…”

“…This fact could explain the two-exponential decay described in ref. 20. Our studies were performed in DMF because of the much higher photostability of the compounds and may give a rationale for the observed strongly mono-exponential fluorescence decay.…”