Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY–aluminum(iii) porphyrin–fullerene supramolecular triads

Abstract

Three, broad band capturing, vertically aligned supramolecular triads, R₂-BDP-AlPorF₃←Im-C₆₀ [R = H, styryl (C₂H₂-Ph), C₂H₂-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph₂ or BDP-TPA₂), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(III) porphyrin (AlPorF₃) and fullerene (C₆₀) entities. The C₆₀ and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C₆₀ through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF₃ dyad, selective excitation of BDP resulted in singlet–singlet energy transfer to AlPorF₃ (k_EnT = 1.0 × 10¹⁰ s⁻¹). On the other hand, excitation of the AlPorF₃ entity in the BDP-AlPorF₃←Im-C₆₀ triad revealed charge separation leading to the BDP-(AlPorF₃)˙⁺-(C₆₀)˙⁻ charge separated state (k_CS = 2.43 × 10⁹ s⁻¹). In the case of the Ph₂-BDP-AlPorF₃ dyad, energy transfer from ¹AlPorF₃* to ¹(Ph₂-BDP)* was witnessed (k_EnT = 1.0 × 10¹⁰ s⁻¹); however, upon assembling the supramolecular triad, (Ph₂-BDP)-AlPorF₃←Im-C₆₀, electron transfer from ¹AlPorF₃* to C₆₀ (k_CS = 3.35 × 10⁹ s⁻¹), followed by hole shift (k_HS = 1.00 × 10⁹ s⁻¹) to Ph₂-BDP, was witnessed. Finally, in the case of the TPA₂-BDP-AlPorF₃←Im-C₆₀ triad, only electron transfer leading to the (TPA₂-BDP)˙⁺-AlPorF₃←Im-(C₆₀)˙⁻ charge separated state, and no energy transfer, was observed. The facile oxidation of Ph₂-BDP and TPA₂-BDP compared to AlPorF₃ in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.