title: Defining Cyclic-Acyclic Exciton Transition at the Single-Molecule Level: Size-Dependent Conformational Heterogeneity and Exciton Delocalization in Ethynylene-Bridged Cyclic Oligothiophenes.
abstract: Conformational disorder in π-conjugated cyclic systems plays a crucial role in controlling the extent of exciton delocalization in much the same way as that in linear counterparts. However, to date, there have been no detailed spectroscopic investigations on the nature of excitons in π-conjugated cyclic systems at the single-molecule level. Herein, we studied the effect of conformational disorder on the excitonic behaviors of cyclic oligothiophenes composed of 6, 8, 10, and 12 subunits (C-6T, C-8T, C-10T, and C-12T, respectively) by employing single-molecule fluorescence spectroscopy. We found that, due to the cyclic symmetry constraint which suppresses S1-S0 transition, small and rigid C-6T and C-8T exhibit extremely long fluorescence lifetimes, while short lifetimes typical of linear systems are dominant in large, flexible C-10T and C-12T. Two-dimensional correlation maps between fluorescence lifetimes and spectral positions show that, by torsional defects created through continued photoexcitation, fully delocalized cyclic excitons shrink to form acyclic excitons in the case of C-10T, while localized acyclic excitons from initial states are maintained in the case of C-12T. The distribution of linear dichroism values from C-6T to C-10T gradually broadens but narrows in C-12T, suggesting a cyclic-to-acyclic transition in excitonic nature between C-10T and C-12T.
authors.affiliation: Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University , Seoul 03722, Korea.
authors.forename: Kyu Hyung
authors.initials: KH
authors.lastname: Park
authors.affiliation: Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University , Seoul 03722, Korea.
authors.forename: Jae-Won
authors.initials: JW
authors.lastname: Cho
authors.affiliation: Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University , Seoul 03722, Korea.
authors.forename: Tae-Woo
authors.initials: TW
authors.lastname: Kim
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University , Hachioji, Tokyo 192-0397, Japan.
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Shimizu
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University , Hachioji, Tokyo 192-0397, Japan.
authors.forename: Kazumi
authors.initials: K
authors.lastname: Nakao
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University , Hachioji, Tokyo 192-0397, Japan.
authors.forename: Masahiko
authors.initials: M
authors.lastname: Iyoda
authors.affiliation: Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University , Seoul 03722, Korea.
authors.forename: Dongho
authors.initials: D
authors.lastname: Kim
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D003500:Cyclization; D006572:Heterocyclic Compounds, Bridged-Ring / Q000737:chemistry*; D008968:Molecular Conformation; D013050:Spectrometry, Fluorescence; D013876:Thiophenes / Q000737:chemistry*
chemical_list: D000480:Alkynes; D006572:Heterocyclic Compounds, Bridged-Ring; D013876:Thiophenes