Synthesis and Ultrafast Time Resolved Spectroscopy of Peripherally Functionalized Zinc Phthalocyanine Bearing Oligothienylene-ethynylene Subunits

Two new soluble tri-tert-butyl zinc(II) phthalocyanines, 1 and 2, bearing dendritic oligothienylene-ethynylene (DOT) groups as one of the peripheral substituents, have been prepared. The conjugated DOT moieties were introduced to cover the spectral window between 380 and 550 nm, where the ZnPc does not exhibit a strong absorption, in order to improve light harvesting. For their preparation, a convergent approach has been used starting from the corresponding iodoPc as precursor. Further transformation of the iodo groups by a Pd-catalyzed Sonogashira reaction with the appropriate DOT-functionalized terminal alkyne allowed the easy preparation of extended pi-conjugated compounds 1 and 2. The compounds have been characterized by standard spectroscopic methods, and their photophysical behaviors have been established by using ultrafast time-resolved techniques. Femtosecond upconversion measurements showed an ultrafast energy transfer from the DOT to zinc phthalocyanine in a time scale of 300 fs. As the number of thiophene groups increases in the dyads, the extent of ultrafast energy transfer was found to increase. Compounds 1 and 2 have been tested as donor components in bulk heterojunction (BHJ) solar cells. Their efficiencies are compared with RuPc analogues previously reported by us.