Excited charge separation in silicon(IV) phthalocyanine with axially bound C60 via azobenzene spacers

Abstract

A novel silicon(IV) phthalocyanine, SiPc-derived donor-acceptor, has been synthesized,with two C60 electron-accepting groups attached to the axial positions of the SiPc via azobenzene spacers.The trans-to-cis isomerization, which adjusts the donor-acceptor distance, was achievable uponphotoirradiation; however, this process was accompanied by considerable photodecomposition of the triad.Electrochemical and spectroelectrochemical studies were performed to determine the redox potentials andhelp identify the electron transfer products. Compared to the SiPc control, the trans C60-azo-SiPc-azo-C60showed strong fluorescence quenching, suggesting the potential for excited-state nonradiative processesupon photoexcitation. The constructed energy diagram indicated that electron transfer occurs from 1SiPc*to C60 in the triad, with the azobenzene spacer serving as a facilitator. The electron transfer product thenrelaxes to the 3SiPc* state before returning to the ground state. This electron transfer mechanism was alsosupported by the performed DFT and TD-DFT calculations, which confirmed electron transfer between theelectron-rich SiPc core and the electron-poor C60 moiety from different excited states. Femtosecondtransient absorption studies verified electron transfer from 1SiPc* to C60 in the triad, with the products ofelectron transfer, SiPc•+ and C60•-, being spectrally identifiable. Furthermore, the transient data wereanalyzed using both global and target analysis. In the donor-acceptor triad, the 1SiPc* had lifetimes of86.81 and 65.91 ps in toluene and benzonitrile, respectively, which were significantly shorter than those ofthe SiPc-control. Most of the charge-separated state relaxed within 3 ns; however, a small fraction persistedbeyond this timescale. The final lifetimes of the radical ion pair SiPc•+-C60•- in the triad, based onnanosecond transient absorption studies, were determined to be 589.5 and 561.4 ns for SiPc•+ and C60•-signals, respectively, in toluene, and 433.8 and 488.0 ns for SiPc•+ and C60•- signals, respectively, inbenzonitrile. The lifetime of the 3SiPc* formed during charge recombination was found to be 6.04 and 5.95µs in toluene and benzonitrile, respectively, close to that of the SiPc-control in the respective solvents. Thelong-lived charge-separated states allow this triad to open the possibility of utilizing it for further photocatalytic and opptoelectronic aplications.