Effect of solvent on the electronic absorption spectral properties of mixed β-octasubstituted free base tetraphenylporphyrins

A near planar macrocycle containing 2,3,12,13-tetraphenylethynyl-5,10,15,20-tetrakis-(4′-n-butylphenyl)porphyrin, H2T(4′-n-Bu Ph)P(PE)4 and a series of antipodally mixed substituted nonplanar porphyrins, 2,3,5,10,12,13,15,20-octaphenyl-7,8,17,18-tetra(2′-thienyl/phenylethynyl, PE)porphyrin, H2OPP(2′-Th/PE)4 and 2,3,12,13-tetramethyl-7,8,17,18-tetra(2′-thienyl/PE)-5,10,15,20-tetraphenylporphyrin, H2TPP(CH3)4(2′-Th/PE)4were examined by electronic absorption spectroscopy in twenty different solvents. The presence of push-pull substituents at the antipodal β-pyrrole positions of the nonplanar macrocycle induces varying degrees of orthogonal dipole moments to the porphyrin ring. The influence of different solvents on the degree of nonplanarity and electronic nature of the macrocycle on their electronic absorption spectral properties were examined. Generally, free base porphyrins showed dramatic solvent dependent absorption spectral band shifts and follow the order: H2OPP(PE)4 > H2OPP(2′-Th)4 ≥ H2TPP(CH3)4(2′-Th)4 >H2TPP(CH3)4(PE)4 > H2T(4′-nBu Ph)P(PE)4. Absorption spectral data in different solvents was analyzed using selected solvatochromic parameters, (η2-1)/(2η2 + 1), ET (30),β, and π*. The enhanced red-shift of the absorption bands of the mixed substituted porphyrins in polar solvents was influenced by solvent-core (porphyrin) interaction and is reflected from the 1H NMR chemical shift of the core imino-hydrogens in polar solvents relative that observed in less polar solvents. The magnitude of the difference in chemical shift (Δδ, ppm) of imino-hydrogens in DMSO-d6 relative to that in CDCl3 follow the order: H2TPP(CH3)4(2′-Th)4 (1.37 ppm) > H2OPP(PE)4 (1.17 ppm) > H2TPP(CH3)4(PE)4 (0.57 ppm). The large red-shift in B and Q bands in polar solvents relative to apolar (or less polar) solvents has been possible due to the combined effect of the electronic nature of the macrocycle, its nonplanarity, and solvent-porphyrin core interactions.