Novel cycloketo tetraphenylporphyrins: spectroscopic study of structure-properties relationships

Abstract

The developments in porphyrin chemistry over the last decades give great advantages for the practical use of porphyrin-based compounds. The properties of these compounds can be systematically tuned by rational utilization of substituents on meso-and/or β-positions as well as by using different metal atoms in the center of the tetrapyrrole macrocycle.Recently we prepared novel mono-and bis-functionalized cycloketo-porphyrins (CKPors). In this work the results of detailed spectroscopic investigations of these compounds are presented. It was found that a seven-membered ketone exocycle remarkably influences the photophysical properties of the CKPor systems. For mono-functionalized CKPors it results in strongly enhanced probability of intersystem crossing S 1 → T 1 with an ISC quantum yield up to 90 %. Moreover, the absorption of all CKPors undergoes a bathochromic shift and the Q-bands extinction is above two times higher compared to that of H 2 TPP, what makes these compounds promising candidates for use as photosensitizers in photodynamic therapy of tumors.For the first time two NH-tautomers of nonsymmetrical CKPors were experimentally resolved at room temperature using optical spectroscopic methods. It was found that the concentration of tautomer A with a lower frequency of the S 0,0 → S 1,0 transition is higher than that one of tautomer B at room temperature, and becomes dominant with cooling down. In contrast -and as it is expected -only one optical active species was observed for nonsymmetrical CKPor with a central Zn(II) atom as well as for symmetrical bis-CKPor.