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

This is the authors' abstract. We don't add key points for this paper.

A novel silicon(IV) phthalocyanine, SiPc-derived donor-acceptor, has been synthesized, with two C[Formula: see text] 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 upon photoirradiation; however, this process was accompanied by considerable photodecomposition of the triad. Electrochemical and spectroelectrochemical studies were performed to determine the redox potentials and help identify the electron transfer products. Compared to the SiPc control, the trans C[Formula: see text]-azo-SiPc-azo-C[Formula: see text] showed strong fluorescence quenching, suggesting the potential for excited-state nonradiative processes upon photoexcitation. The constructed energy diagram indicated that electron transfer occurs from 1 SiPc* to C[Formula: see text] in the triad, with the azobenzene spacer serving as a facilitator. The electron transfer product then relaxes to the 3 SiPc* state before returning to the ground state. This electron transfer mechanism was also supported by the performed DFT and TD-DFT calculations, which confirmed electron transfer between the electron-rich SiPc core and the electron-poor C[Formula: see text] moiety from different excited states. Femtosecond transient absorption studies verified electron transfer from 1 SiPc* to C[Formula: see text] in the triad, with the products of electron transfer, SiPc[Formula: see text] and C[Formula: see text], being spectrally identifiable. Furthermore, the transient data were analyzed using both global and target analysis. In the donor-acceptor triad, the 1 SiPc* had lifetimes of 86.81 and 65.91 ps in toluene and benzonitrile, respectively, which were significantly shorter than those of the SiPc-control. Most of the charge-separated state relaxed within 3 ns; however, a small fraction persisted beyond this timescale. The final lifetimes of the radical ion pair SiPc[Formula: see text]-C[Formula: see text] in the triad, based on nanosecond transient absorption studies, were determined to be 589.5 and 561.4 ns for SiPc[Formula: see text] and C[Formula: see text] signals, respectively, in toluene, and 433.8 and 488.0 ns for SiPc[Formula: see text] and C[Formula: see text] signals, respectively, in benzonitrile. The lifetime of the 3 SiPc* formed during charge recombination was found to be 6.04 and 5.95 [Formula: see text] s in toluene and benzonitrile, respectively, close to that of the SiPc-control in the respective solvents. The long-lived charge-separated states allow this triad to open the possibility of utilizing it for further photocatalytic and optoelectronic applications.