Structure‐Dependent Electronic Nature of Star‐Shaped Oligothiophenes, Probed by Ensemble and Single‐Molecule Spectroscopy. Issue 29 (4th June 2013)

• Record Type: Journal Article
• Authors: Chung, Heejae; Narita, Tomoyuki; Yang, Jaesung; Kim, Pyosang; Takase, Masayoshi; Iyoda, Masahiko; Kim, Dongho
• Is Part Of: Chemistry. Volume 19:Issue 29(2013)
• Page Start: 9699
• Page End: 9709
• Publication Date: 2013-06-04
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201300313
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>We have investigated the photophysical properties of star‐shaped oligothiophenes with three terthiophene arms (<italic>meta</italic> to each other, <bold>S3</bold>) or six terthiophene arms (<italic>ortho</italic>‐, <italic>meta</italic>‐, and <italic>para</italic>‐arranged, <bold>S6</bold>) connected to an ethynylbenzene core to elucidate the relationship between their molecular structure and electronic properties by using a combination of ensemble and single‐molecule spectroscopic techniques. We postulate two different conformations for molecules <bold>S3</bold> and <bold>S6</bold> on the basis of the X‐ray structure of hexakis(5‐hexyl‐2‐thienlyethynyl)benzene and suggest the coexistence of these conformers by using spectroscopic methods. From the steady‐state spectroscopic data of compound <bold>S6</bold>, we show that the exciton is delocalized over the core structure, but that the <italic>meta</italic>‐linkage in compound <bold>S3</bold> prevents the electronic communication between the arms. However, in single‐molecule spectroscopic measurements, we observed that some molecules of compound <bold>S3</bold> showed long fluorescence lifetimes (about 1.4 ns) in the fluorescence‐intensity trajectories, which indicated that π electrons were delocalized along the <italic>meta</italic> linker. Based on these observations, we suggest that the delocalized exciton is intensely sensitive towards the dihedral angle between the core and the adjacent thiophene ring, as well as to the substituted position of the terthiophene arms. Our results highlight that the fluorescence lifetimes of compounds <bold>S3</bold> and <bold>S6</bold> are strongly correlated with the spatial location of their excitons, which is mainly affected by their conformation, that is, whether the innermost thiophene rings are facing each other or not. More interestingly, we observed that the difference between the degrees of ring‐torsional flexibility of compounds <bold>S3</bold> and <bold>S6</bold> results in their sharply contrasting fluorescence properties, such as a change in fluorescence intensity as a function of temperature.</p> </abstract>