SOLVENT EFFECTS ON THE VISIBLE ABSORPTION MAXIMA OF TETRAPYRROLIC PIGMENTS

Visible absorption spectra of several porphyrins, phthalocyanine, thei r metal complexes, chlorin, and bacteriochlorophyll a are recorded in nonpolar and aprotic polar solvents at room temperature. Excellent lin ear dependence of the absorption band maxima on the Lorentz-Lorenz fun ction 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 Lipta y and the Bakhshiev equations by using transition energies and dipole moments as well as the changes in polarizability (DELTAalpha) as molec ular parameters, respectively. The DELTAalpha and transition energy fo r the free molecule is estimated from the dependence of the band maxim um on phi(n2). The difference of the matrix-induced dipole moment in t he 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 w eight of the pigment. Dielectric properties of the environment affect the S1-S2 level splitting of free-base macrocycles by changing the deg ree of inequivalence of the perpendicular N ... N and N-H ... H-N axes . The positively charged inner hydrogen atoms move closer to each othe r in polar solvents, which produces a hypsochromic shift of the S1 ban d (like in metalloporphyrins). Another unusual solute-solvent interact ion mechanism was suggested for meso-tetraphenyltetrabenzoporphine, co nsisting in the modulation of coplanarity of this sterically crowded m olecule.