Photophysical Behavior of Zinc(II) Tetraphenylporphyrin Covalently Incorporated in a Cholesterol-Bearing Polymethacrylate

Abstract

Photophysical behavior of a zinc(II) tetraphenylporphyrin (ZnTPP) moiety in a copolymer of 0.1 mol % zinc(II) 5-[4-(6-methacryloyloxyhexanoyloxy)phenyl]-10,15,20-triphenylporphynate (ZnTPP-C5-MA) and 99.9 mol % cholesteryl 6-methacryloyloxyhexanate (Chol-C5-MA) was investigated in comparison with that of monomeric ZnTPP-C5-MA. Pendant cholesterol (Chol) groups in the copolymer form stacks in n-hexane (a poor solvent for the copolymer), while no Chol stacks are formed in benzene (a good solvent). The absorption maxima of the Soret and Q bands of monomeric ZnTPP-C5-MA were solvent dependent, i.e. the maxima in n-hexane were 7.6 nm shorter than those in benzene. In contrast, no such solvent dependence was observed for the polymer-bound ZnTPP moiety. Spectral profiles for the fluorescence bands for the polymer-bound ZnTPP moiety and monomeric ZnTPP-C5-MA were very different in n-hexane, i.e. the relative intensity of the 0–0 to 0–1 fluorescence bands for the former was 0.52 while that for the latter was 1.10. The triplet excited lifetime for the polymer-bound Zn-TPP moiety at room temperature was much longer in n-hexane (22 ms) than in benzene (3.7 ms), while the fluorescence lifetime at room temperature was slightly longer in n-hexane (2.52 ns) than in benzene (2.09 ns). The polymer-bound ZnTPP moiety emitted phosphorescence and E-type delayed fluorescence in n-hexane at room temperature, arising from the long triplet lifetime. Fluorescence quenching for the polymer-bound ZnTPP moiety by vitamin-K3 was suppressed in n-hexane, as compared to the quenching of the monomer. All these observations indicate that the ZnTPP chromophores in the copolymer are “protected” in the Chol stacks in n-hexane, leading to an isolation of ZnTPP from the bulk solution phase.