Photophysical investigation of aminoalkylsiloxy silicon phthalocyanines: Intramolecular exciplex formation

The fundamental understanding of the rules governing the quantitative structure-activity relationships of silicon phthalocyanines is crucial to the goal of rationally designing improved phthalocyanine photosensitizers. Four sets of aminoalkylsiloxy silicon phthalocyanines have been prepared in Dr. Kenney's laboratory in order to examine structure-activity relationships. A comprehensive photophysical investigation on the compounds has been performed in this study. Time-correlated single-photon counting spectrometry is used to evaluate the fluorescence lifetimes of the compounds. The results demonstrate that every set has member compounds that shown unexpected bi-exponential decay. Specifically, Pcs116, 34, 121, 25, 122, 123, 83, 12, 41, 81, and 82 have bi-exponential fluorescence lifetimes. These same compounds also show lower fluorescence quantum yields, lower triplet quantum yields, and lower singlet oxygen quantum yields. Studies of the temperature dependence of the fluorescence lifetimes indicate that the singlet states of these 11 phthalocyanines form intramolecular exciplexes as a result of charge transfer interactions between the electronically excited Pc π-system and the terminal amine residues on the flexible axial ligands. The temperature dependence experiments also yield thermodynamic parameters. This intramolecular exciplex formation adds another rate parameter to the excited state decay algorithm which shortens the singlet state lifetime, thereby causing the loss of fluorescence intensity and the lowering of the efficiency of intersystem crossing from singlet state to triplet state, hence affecting the singlet oxygen quantum yield. Conformational factors are clearly important since bi-exponential behavior of the fluorescence is observed only when the alkyl group in the aminoalkylsiloxy ligand is C₂ or C₃ group.; Upon addition of toluic acid to the solutions in toluene as solvent, the fluorescence decay profiles are changed from bi-exponential to mono-exponential. This can be understood if in the presence of the acid the electron donating amine groups become protonated or participate in H bond formation with the acid such that they are unable to now form the intramolecular exciplex.; In DPPC liposome, the structural dependency of the intramolecular exciplex formation might be completely or partially removed, since the conformational change of the ligand is restricted and the precursor of the intramolecular exciplex can not be formed efficiently. The photophysical properties of the compounds in liposome have been observed to be very close with each other. As such, similar PDT efficacy is expected for all the four set compounds.