Strongly Coupled Bisdonor-BF2 Chelated Azadipyrromethene-Fullerene Tetrads: Synthesis and Charge Separation Studies

The efficiency and mechanism of electron- and energy transfer events occurring in both in natural and synthetic donor-acceptor systems depend on their distance, relative orientation, and the nature of the surrounding media. Fundamental knowledge gained from model studies is key in building efficient energy harvesting devices. Faster charge separation and slower charge recombination in donor-acceptor systems is often sought out. In our continued effort to build donor-acceptor systems using near-IR sensitizers, in the present study, we report ultrafast charge separation in newly synthesized, strongly coupled, tetrads featuring bisdonor (donor = phenothiazine and ferrocene), BF2-chealted azadipyrromethane (azaBODIPY) and fullerene (C60) entities. The tetrads synthesized using multi-step synthetic procedure revealed strong charge transfer interactions in the ground state involving the donor and azaBODIPY entities. The near-IR emitting azaBODIPY acted as a photosensitizing electron acceptor along with fullerene while the phenothiazine and ferrocene entities acted as electron donors. The control triads (bisdonor-azaBODIPY, 1d and 2d) and tetrads (1 and 2) revealed ultrafast photoinduced charge separation leading to charge separated states as probed by femtosecond transient absorption spectroscopy in both polar and nonpolar solvent media. Figure 1