Multi-Step Energy and Electron Transfer in a 'v-Configured' Supramolecular Bodipy-Azabodipy-Fullerene Triad

Supramolecular multi-modular donor-acceptor systems are useful for solar-fuel and solar-electricity generation, and also for building optoelectronic devices.  In the present contribution, a novel photosynthetic antenna-reaction center model compound composed of covalently linked BF₂-chelated dipyrromethene (BODIPY), BF₂-chelated azadipyrromethene (azaBODIPY) and fullerene (C₆₀), in a ‘V-Configuration’, has been newly synthesized and characterized using a multi-step synthetic procedure.¹  Optical absorbance and steady-state fluorescence, computational, and electrochemical studies were systematically performed in nonpolar, toluene and polar, benzonitrile solvents to establish the molecular integrity of the triad and establish an energy level diagram revealing different photochemical events.  The geometry obtained by B3LYP/6-31G* calculations revealed the anticipated V-configuration of the BODIPY-azaBODIPY-C₆₀ triad.  The location of the frontier orbitals in the triad tracked the site of electron transfer determined from electrochemical studies.  The different photochemical events originated from ¹BODIPY* were realized from the energy level diagram.  Accordingly, ¹BODIPY* resulted in competitive ultrafast energy transfer to produce BODIPY-¹azaBODIPY*-C₆₀ and electron transfer to produce BODIPY^·+-azaBODIPY-C₆₀^·- as major photochemical events.  The charge separated state persisted for few nanoseconds prior populating ³C₆₀^* which in turn revealed an unusual triplet-triplet energy transfer to produce ³azaBODIPY^* prior returning to the ground state.  These findings delineate the importance of multi-modular systems in energy harvesting, and more importantly, their utility in building multi-function performing optoelectronic devices.  

1. V. Bandi, F. P. D’Souza, H. Gobeze, F. D’Souza, Chem. Eur. J. 2015, in press.