"Two-Point" Self-Assembly and Photoinduced Electron Transfer in meso-Donor-Carrying Bis(styryl crown ether)-BODIPY-Bis(alkylammonium)fullerene Donor-Acceptor Conjugates.

BF2 -chelated dipyrromethene, BODIPY, was functionalized to carry two styryl crown ether tails and a secondary electron donor at the meso position. By using a "two-point" self-assembly strategy, a bis-alkylammonium-functionalized fullerene (C60 ) was allowed to self-assemble the crown ether voids of BODIPY to obtain multimodular donor-acceptor conjugates. As a consequence of the two-point binding, the 1:1 stoichiometric complexes formed yielded complexes of higher stability in which fluorescence of BODIPY was found to be quenched; this suggested the occurrence of excited-state processes. The geometry and electronic structure of the self-assembled complexes were derived from B3LYP/3-21G(*) methods in which no steric constraints between the entities was observed. An energy-level diagram was established by using spectral, electrochemical, and computational results to help understand the mechanistic details of excited-state processes originating from 1 bis-styryl-BODIPY*. Femtosecond transient absorbance studies were indicative of the formation of an exciplex state prior to the charge-separation process to yield a bis-styryl-BODIPY.+ -C60.- radical ion pair. The time constants for charge separation were generally lower than charge-recombination processes. The present studies bring out the importance of multimode binding strategies to obtain stable self-assembled donor-acceptor conjugates capable of undergoing photoinduced charge separation needed in artificial photosynthetic applications.