SOLVENT EFFECTS ON THE VISIBLE ABSORPTION MAXIMA OF TETRAPYRROLIC PIGMENTS

Visible absorption spectra of several porphyrins, phthalocyanine, their metal complexes, chlorin, and bacteriochlorophyll a are recorded in nonpolar and aprotic polar solvents at room temperature. Excellent linear dependence of the absorption band maxima on the Lorentz-Lorenz function phi(n2) = (n2-1)/(n2+2) (n is the refractive index) is observed for most of the compounds in n-alkanes. Spectral bands are red-shifted on the increase of phi(n2) due to the dispersive stabilization of the excited state. Solvent shifts were rationalized in terms of the Liptay and the Bakhshiev equations by using transition energies and dipole moments as well as the changes in polarizability (DELTAalpha) as molecular parameters, respectively. The DELTAalpha and transition energy for the free molecule is estimated from the dependence of the band maximum on phi(n2). The difference of the matrix-induced dipole moment in the ground and the excited state of centrosymmetric molecules (DELTAmu(ind)BAR)calculated from the linear Stark broadening of spectral holes, is proportional to the DELTAalpha divided by the relative molecular weight of the pigment. Dielectric properties of the environment affect the S1-S2 level splitting of free-base macrocycles by changing the degree of inequivalence of the perpendicular N ... N and N-H ... H-N axes. The positively charged inner hydrogen atoms move closer to each other in polar solvents, which produces a hypsochromic shift of the S1 band (like in metalloporphyrins). Another unusual solute-solvent interaction mechanism was suggested for meso-tetraphenyltetrabenzoporphine, consisting in the modulation of coplanarity of this sterically crowded molecule.