ABSTRACT
In addition to the central atom, substituents in the porphyrin ring also affect the absorption spectrum. The number of bromine atoms in the pyrrole position in CoTPP was correlated with the red shift of the Soret band [9], while an effect of distortion of the porphyrin ring should be considered [10] (Fig. 5). As antici pated from the molecular orbital coefficient (see Fig. 3) the aJu orbital would be more dependent on the substitution on the pyrrole position than the a2u orbital. A strong electron-donating substituent was reported to induce an inversion from a2u > a]u to a2u < álu, i.e., the a]u of 2-aminoporphyrin was reported to be more destabilized by 0.71 eV than that of 2-nitroporphyrin, while a2u suffers destabili-
zation of only 0.14 eV [12]. The relative positioning between aju and a2u should largely affect the chemical reactivity of the excited porphyrins in which the differ ence would be more evident in the lowest excited state [325]. Axial ligands induce a pretty large shift in many cases [11]. Electron-donating axial ligands can inter act with the a2u π orbital having a large contribution of the nitrogen orbital of porphyrin and stabilize the a2ueg excited state [11].
