Fluorescent Charge-Transfer Excited States in Acceptor Derivatized Thiophene Oligomers

J Phys Chem A. 2020 Sep 3;124(35):7001-7013. doi: 10.1021/acs.jpca.0c05561. Epub 2020 Aug 19.

Abstract

A family of thiophene oligomers with lengths of 3, 4, 5, 6, and 8 units were synthesized and end-capped with a strongly coupled naphthalimide acceptor (TnNIF) which produces an emissive intramolecular charge-transfer state. A thorough photophysical study was performed on the oligomers including UV-vis absorption, fluorescence, and picosecond transient absorption spectroscopy to investigate the effect of thiophene oligomer length/donor strength and solvent polarity on the intramolecular charge-transfer properties. In hexane, the TnNIF compounds behave in a manner similar to that of oligothiophenes as fluorescence from a local singlet excited state and intersystem crossing to the triplet state dominates the excited-state dynamics. Interestingly, the excited-state dynamics become much more complicated with increasing solvent polarity, from ether to acetone, where emission from a charge-transfer state (δ+TnNIF) and quenching from a charge-separated state (•+TnNIF-•) become competitive. A mechanism is proposed that consists of a four-state diagram including a locally excited singlet state (1TnNIF), a triplet state (3TnNIF), an emissive charge-transfer state, and a nonemissive charge-separated state. The population of each of these states is highly dependent on both the thiophene oligomer length and solvent polarity which results in a mixture of excited states.