The NH-tautomerism is a fundamental property of all metal-free porphyrins. In the present study it was investigated for three novel nonsymmetrical cycloketo-tetraphenylporphyrins in isotropic solutions. The combination of steady-state absorption and fluorescence techniques with decay-associated fluorescence spectroscopy was demonstrated to be a powerful tool to bring into light the photophysical properties of individual NH-tautomers even at room temperature. For all investigated free-base porphyrins, the equilibrium concentration of one tautomer, named T1, which has a lower frequency of the S(0,0)-->S(1,0) transition, was found to be higher than the concentration of the other tautomer, named T2, even at room temperature. Lowering the temperature enriches the population of tautomer T1 and it becomes dominant in absorption and fluorescence. The fluorescence decay time of tautomer T1 was found to be approximately three times shorter compared to that of tautomer T2, whereas both tautomers have similar fluorescence quantum yields. In contrast only one conformation exists for zinc cycloketo-porphyrin, which was used as reference compound.
New tetraphenylporphyrins 2a and 2b have been synthesized and characterized. Both compounds possess an ethanone bridge that connects the porphyrin ring with one of the phenyl groups at the periphery. Tetraphenylporphyrin 2a also has a methyl group on the same phenyl ring, whereas tetraphenylporphyrin 2b lacks this substituent. This small difference results in 2a and 2b having markedly different photophysical properties. In particular, 2a proved to be a highly efficient sensitizer for singlet oxygen generation with a quantum yield of 0.85. A reference compound 2c has also been synthesized in which the porphyrin ring is connected to one of the peripheral phenyl groups through just a keto function. Thus, the effect of the size of the exocyclic ring on conjugation and photophysical properties has been studied. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
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.
The cover picture shows three tetraphenylporphyrins that possess an exocyclic keto‐group‐bearing ring connecting the central chromophore with one of the phenyl rings. Depending on the size of the exocyclic ring and on the additional steric strain induced by a methyl group on the substituted phenyl ring, the dihedral angle between the central chromophore and the phenyl ring varies between 15 and 55 °. These topological changes have a dramatic influence on, for example, the singlet‐oxygen quantum yield. As the generation of 1O2 is induced by red light, the setting sun (taken at San Diego beach in 2003 by Michael Köhl) seemed to be an appropriate background for the porphyrins. Details of the syntheses and characterization are reported in the article by N. Jux, B. Röder et al. on p. 1075 ff.
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