Synthesis and photophysical studies of new porphyrin-phthalocyanine dyads with hindered rotation

A series of novel porphyrin-phthalocyanine (Por-Pc) dyads 1-3 have been synthesized by using standard methodologies for unsymmetrically substituted phthalocyanine preparation. These two chromophoric units have been directly linked for the first time, that is, without any spacer, through the P-pyrrolic position of a meso-tetraphenylporphyrin, thus allowing a close proximity of the two units in a rigid arrangement. For this, a novel porphyrin-phthalonitrile precursor 4 had to be prepared. The UV/Vis spectra indicate that the basic electronic characteristics of both individual units (i.e., porphyrin and phthalocyanine) are retained in the hybrid Por-Pc molecules. The short Por-Pc separation in dyads 1-3 leads to strong excitonic coupling and ultrafast energy transfer (ca. 10(12)s(-1)), as determined by feratosecond transient absorption measurements, from the highly energetic (1.) (ZnPor) unit to the (1.)(ZnPc), which evolves to populate the long-lived (3.)(ZnPc) by intersystem crossing. Interestingly, the energy transfer seems to occur more efficiently in dyads 2 and 3, which have smaller HOMO-LUMO energy gaps. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).