The photophysical and metal coordination properties of the N-CH3 substituted porphyrins: H(N-CH3) TPP vs H(N-CH3)OEP

Abstract

The effect of N -methyl substitution on photophysical and metal coordination properties of the respective derivatives of octaethylporphyrin ( H 2 OEP ) and tetraphenylporphyrin ( H 2 TPP ) was studied by means of steady-state and time-resolved optical spectroscopies combined with semi-empirical quantum-chemical calculations and coordination chemistry methods. In case of H 2 TPP , the insertion of the methyl substituent into the center of the porphyrin macrocycle leads to noticeable nonplanar distortions of the molecule and is accompanied by changes of its photophysical and physicochemical properties towards those manifested by “classical” nonplanar porphyrins. Contrasting to that, N -methyl substituted H 2 OEP does not undergo significant nonplanar distortions and possesses photophysical characteristics mainly similar to unsubstituted H 2 OEP , except for the long-wavelength shift of the absorption and emission bands. The Zn coordination/ Zn complex dissociation and macrocycle thermal stability parameters were also determined for both N -methyl substituted and parent unsubstituted macrocycles, which correlate well with a higher degree of nonplanarity of the N -methyl substituted H 2 TPP as compared to H 2 OEP . Basing on the results of this study the conclusion postulated is that N -methyl substitution has a different effect on the photophysical and coordination properties of H 2 TPP vs. H 2 OEP .