Synthesis and photophysical properties of nonaggregated phthalocyanines bearing dendritic substituents

A series of phthalocyanines containing four dendritic substituents with terminal ester or carboxylate functionalities have been synthesized. These compounds having bulky substituents are essentially nonaggregated in organic solvents (for the ester series) or water (for the [G(1)] and [G(2)] carboxylate series) as demonstrated by the absorption and fluorescence spectroscopy. Photoinduced electron transfer in these compounds has been investigated by a fluorescence quenching method using a series of neutral, anionic, and cationic quenchers. The rates of quenching depend largely on the size of the dendrons. For the carboxylate series with cationic (TMePyP)(4+) as the electron acceptor, quenching is very efficient, which can be attributed to a static quenching mechanism. Upon excitation of the polyaryl ether dendritic fragments, the ester series show an intramolecular singlet-singlet energy transfer from the dendrons to the phthalocyanine core. The energy transfer quantum yields as determined by steady-state fluorescence spectroscopy are moderate (10.3-13.3%). The singlet oxygen quantum yields of the ester series are comparable with that of unsubstituted ZnPc in DMF, while those of the carboxylate series measured in DMSO are considerably lower.