Multichromophoric Perylenediimide-Silicon Phthalocyanine-C₆₀ System as an Artificial Photosynthetic Analogue

Sequential photoinduced energy transfer followed by electron transfer and the formation of charge-separated states, which are primary events of natural photosynthesis, have been demonstrated in a newly synthesized multichromophoric covalently linked triad, PDI-SiPc-C₆₀ . The triad comprises a perylenediimide (PDI), which primarily fulfils antenna and electron-acceptor functionalities, silicon phthalocyanine (SiPc) as an electron donor, and fulleropyrrolidine (C₆₀ ) as a second electron acceptor. The multi-step convergent synthetic procedure developed here produced good yields of the triad and control dyads, PDI-SiPc and SiPc-C₆₀ . The structures and geometries of the newly synthesized donor-acceptor systems have been established from spectral, computational, and electrochemical studies with reference to appropriate control compounds. Ultrafast energy transfer from ¹ PDI* to SiPc in the case of PDI-SiPc and PDI-SiPc-C₆₀ was witnessed. An energy-level diagram established from spectral and electrochemical data suggested the formation of two types of charge-separated states, that is, PDI-SiPc^.+ -C₆₀^.- and PDI^.- -SiPc^.+ -C₆₀ from the ¹ SiPc* in the triad, with generation of the latter being energetically more favorable. However, photochemical studies involving femtosecond transient spectroscopy revealed the formation of PDI-SiPc^.+ -C₆₀^.- as a major charge-separated product. This observation may be rationalized in terms of the closer spatial proximity to SiPc of C₆₀ compared to PDI in the triad. The charge-separated state persisted for a few nanoseconds prior to populating the ³ SiPc* state during charge recombination.