Synthesis and Excited State Charge Separation Involving Coordinated C60 of π-Extended Pyrazinepyrene Fused Zinc Phthalocyanines

Abstract

Polycyclic aromatic hydrocarbons (PAHs) play an important role in organic electronics.[1] To increase the stability and the conjugation, pyrene units could be further attached through heteroatoms to improve the photo/chemical stability to oxygen or light.[2] In terms of the synthesis of these types of large acenes through pyrene, many routes have been studied and one of the best protocols is to synthesize the quinoxaline derivative from the dione and the corresponding diamino subunit.[3] Herein, we will present a series of pyrazinepyrene fused zinc phthalocyanines (ZnPc-Pyrn) as a new class of π-extended phthalocyanine systems. Bathochromically shifted absorption as a function of the number of pyrazinepyrene entities due to extended π-conjugation, and quenched fluorescence due to the presence of fused pyrazinepyrene were witnessed. The electronic structures of these phthalocyanines were probed by systematic computational and electrochemical studies while the excited state properties were examined by pump-probe spectroscopies operating at the femto- and nanosecond time scales. Like the excited singlet lifetimes, the excited triplet states also revealed diminished lifetimes with an increased number of pyrazinepyrene entities. Further, the coordinatively unsaturated zinc in these molecules was coordinated with phenyl imidazole functionalized fullerene, ImC60 to form a new series of donor-acceptor conjugates. Upon full characterization of these conjugates, the occurrence of excited-state charge separation was established by transient pump-probe spectroscopy covering wide temporal and spatial regions. With an increase in pyrazinepyrene entities, an increased rate of forward charge separation was witnessed while rate constants for charge recombination were slower resulting in the final lifetime of charge-separated states of about 40-50 ns, better than that reported earlier for a similar pristine zinc phthalocyanine derived dyad. [4]AcknowledgmentsAuthors acknowledge the support from the Spanish Ministerio de Ciencia e Innovación (MICINN/FEDER, PID2020-117855RB-I00 to ASS) and Generalitat Valenciana (CIPROM/2021/059 and MFA/2022/028 to ASS) and US-National Science Foundation (2000988 to FD). The computational work was completed at the Holland Computing Center of the University of Nebraska, which receives support from the Nebraska Research Initiative.