Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units

Keishiro Tahara; Haruya Koyama; Mamoru Fujitsuka; Ken Tokunaga; Xu Lei; Tetsuro Majima; Jun-Ichi Kikuchi; Yoshiki Ozawa; Masaaki Abe

doi:10.1021/acs.joc.9b00836

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units

J Org Chem. 2019 Jul 19;84(14):8910-8920. doi: 10.1021/acs.joc.9b00836. Epub 2019 May 22.

Authors

Keishiro Tahara¹, Haruya Koyama², Mamoru Fujitsuka³, Ken Tokunaga⁴, Xu Lei³, Tetsuro Majima³, Jun-Ichi Kikuchi², Yoshiki Ozawa¹, Masaaki Abe¹

Affiliations

¹ Department of Material Science and Research Center for New Functional Materials, Graduate School of Material Science , University of Hyogo , 3-2-1, Kouto , Kamigori, Ako , Hyogo 678-1297 , Japan.
² Graduate School of Materials Science , Nara Institute of Science and Technology , 8916-5, Takayama , Ikoma, Nara 630-0192 , Japan.
³ Institute of Scientific and Industrial Research (SANKEN) , Osaka University , 8-1, Mihogaoka , Ibaraki , Osaka 567-0047 , Japan.
⁴ Division of Liberal Arts, Centre for Promotion of Higher Education , Kogakuin University , 2665-1, Nakano , Hachioji, Tokyo 192-0015 , Japan.

PMID: 31072099
DOI: 10.1021/acs.joc.9b00836

Abstract

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr₃) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr₃)^•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr₃ to NAr₂ sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.