Excited State Characterization of Oligoand Polythiophene Derivatives: Tetraphenylethene-Substituted Thiophenes, Aryl-Bithiophenes and Alternating Donor-Acceptor Indigo- Cyclopentadithiophene Copolymers

Abstract

A comprehensive investigation of the excited state properties of polythiophenes and aryl-bithiophenes has been undertaken in solution at different temperatures. The investigated polythiophenes include the tetraphenylethene-substituted -thiophene (HomoPT) and -bithiophene building blocks (coPT). In addition, and for comparison purposes, the parent poly(3-hexylthiophene), P3HT, together with the monomeric percursor tetraphenylethene-thiophene and the bromide-substituted tetraphenylethene chromophore (Br-TPE) have also been investigated. The arylbithiophenes consist in a common core of a benzene-bithiophene unit substituted with five different side groups; again and for comparison purposes the results of the corresponding oligothiophene terthiophene (α3) was used as a model compound. This includes the determination of fluorescence quantum yields, fluorescence lifetimes both in solution and in thin films at room temperature and singlet oxygen yields yields. From the obtained data, the overall set of rate constants for all the deactivation processes fluorescence, intersystem crossing, and internal conversion, were obtained; from the data, it was found that both intersystem crossing and internal conversion are important in the decay of the lowest singlet excited state of the polythiophenes and arylthiophenes in solution. An aggregation induced emission (AIE) study was also performed for both the polythiophenes and aryl-bithiophenes; in the case of the these did not display AIE in constrast with the aryl-bithiophenes where AIE is found to be present. A complementary comprehensive investigation of the excited state characteristics of an alternating donor-acceptor Indigo-[dodecyl-cyclopentadithiophene] copolymer together with the oligomeric counterpart has been undertaken aiming to rationalize its photophysical properties. The energy transfer processes between the cyclopentadithiophene (donor) moiety and indigo (acceptor) unit and/or the occurrence of fast conformational relaxation processes in the excited state of these systems was investigated. From the overall photophysical and spectroscopic data it was seen for the oligomeric model compound and indigo-[dodecyl-cyclopentaditiophene] a high degree of electronic interaction/delocalization between the indigo and dodecylcyclopentaditiophene units both in the singlet and triplet excited states.