Donor—Acceptor-Substituted Tetrakis(phenylethynyl)benzenes as Emissive Molecules during Pulse Radiolysis in Benzene

摘要

Emission from charge recombination between radical cations and anions of various tetrakis(phenylethynyl)-benzenes (TPEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TPEB in the singlet excited state (~1TPEB~*) can be attributed to the charge recombination between TPEB~(·+) and TPEB~(·-), which are initially generated from the radiolytic reaction in Bz. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (—ΔH°) for the charge recombination of TPEB~(·+) and TPEB~(·-) and excitation energy of ~1TPEB~*. It was found that the charge recombination between TPEB~(·+) and TPEB~(·-) occurred to give ~1TPEB~* as the emissive species, but not the excimers because of the large repulsion between substituents caused by the rotation around C—C single bonds of TPEBs. Since donor—acceptor-substituted TPEBs possess three types of charge-transfer pathways (linear-conjugated, cross-conjugated, and "bent" conjugated pathways between the donor and acceptor substituents through the ethynyl linkage), the emission spectra of ~1TPEBs~* with intramolecular charge transfer (ICT) character depend on the substitution pattern and the various kinds of donor and acceptor groups during pulse radiolysis in Bz.