Bis(subphthalocyanine)–azaBODIPY triad for ultrafast photochemical processes

Abstract

Multi-modular supramolecular systems capable of undergoing photoinduced energy and electron transfer are of paramount importance to design light-to-energy and light-to-fuel converting devices. Often, this has been achieved by linking two or more photo-active or redox-active entities with complementary spectral and photochemical properties. In the present study, we report a new triad made out of two entities of subphthalocyanine covalently linked to BF₂-chelated azadipyrromethene ((SubPc)₂–azaBODIPY). The triad was fully characterized by spectral, computational, electrochemical and photochemical techniques. The B3LYP/6-31G* calculations revealed a structure wherein the donor, SubPc, and the acceptor, azaBODIPY, were well separated with no steric crowding. The different redox states were established from the differential pulse voltammetry studies and the data were used to estimate free-energy change associated with charge separation. Such calculations revealed the charge separation from either the ¹SubPc* or ¹azaBODIPY* to be thermodynamically feasible for yielding the (SubPc)SubPc˙⁺–azaBODIPY˙⁻ radical ion-pair. Steady-state fluorescence studies revealed quantitative quenching of ¹SubPc* in the triad and solvent dependent quenching of ¹azaBODIPY* indicating participation of both fluorophores in promoting photochemical events. In nonpolar toluene, singlet–singlet energy transfer from the ¹SubPc* to azaBODIPY was observed, while in polar benzonitrile, evidence of energy transfer was feeble. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of ultrafast photoinduced electron transfer by providing spectral proof for the formation of the (SubPc)SubPc˙⁺–azaBODIPY˙⁻ charge separated state. The charge recombination followed populating the ³azaBODIPY* prior to returning to the ground state.