Distance Matters: Effect of the Spacer Length on the Photophysical Properties of Multimodular Perylenediimide-Silicon Phthalocyanine-Fullerene Triads.

A multimodular donor-acceptor conjugate featuring silicon phthalocyanine (SiPc) as the electron donor, and two electron acceptors, namely tetrachloroperylenediimide (PDI) and C60 , placed at the opposite ends of the SiPc axial positions, was newly designed and synthesized, and the results were compared to the earlier reported PDI-SiPc-C60 triad. Minimal intramolecular interactions between the entities was observed. Absorption, fluorescence, computational and electrochemical studies were performed to evaluate the excitation energy, geometry and electronic structure, and energy levels of different photoevents. Steady-state absorption, fluorescence and excitation spectral studies revealed efficient singlet-singlet energy transfer from 1 PDI* to SiPc in the PDI-SiPc dyad and the PDI-SiPc-C60 triad. The measured rates for these photochemical events were found to be much higher than those reported earlier for the triad, due to closer proximity between the PDI and SiPc entities. The distance also affected the charge separation path in which involvement of PDI, and not C60 , in charge separation in the present triad was witnessed. The present investigation brings out the importance of donor-acceptor distances in channeling photochemical events in a multimodular system.