Acid-base and photophysical properties of asymmetric dendrimers with porphyrin core

Abstract

The spectral and photophysical properties of mixed 5,10,15,20-arylporphyrins with А 2В2 - cis and А 2В2 - trans peripheral substitution and the porphyrin core dendrimers with the same spacer architecture between tetrapyrrolic macrocycle and indolocarbazole dendrons as well as those of their mono- and doubly protonated forms have been studied. Sequential formation of both mono- and doubly protonated forms of all the studied compounds was found during the acid-base titration. However, the concentration of monoprotonated form is small compared to those of the free base and doubly protonated one due to close values of protonation constants р K[a1](Н[2]P ↔Н[3]P{+}) and рK[а2](Н[3]P{+} ↔ Н[4]P{2+}) in both ground singlet S[0] state and first excited singlet S[1] state. Different position of peripheral substituents/spacers of А (phenyls) and В (mesityls) type, which is known to have different sterical hindrances with tetrapyrrolic macrocycle, does not result in any noticeable differences in the acid-base properties. The free bases of all the compounds have similar photophysical properties. Doubly protonated forms of dendrimers and parent porphyrins were found to have different probabilities of lowest singlet S[1] state deactivation. Compared to the free base molecules the fluorescen ce quantum yield value for dendrimers reduces for 50 −1,000 times, whereas the parent porphyrins demonstrate about three fold increase.