A new series of π-conjugated oligomers based on the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene repeating unit has been prepared and characterized by X-ray, electrochemical, spectroscopic (UV-Vis absorption, emission and Raman) and density functional theory methods. The oligomers in their neutral, oxidized and reduced forms have been investigated. The neutral compounds show a longer mean conjugation length than oligothiophenes and oligothiophene-vinylenes and display very rich redox chemistry with the stabilization of polycationic states of which the radical cations and dications are strong NIR absorbers, the latter displaying singlet diradicaloid character. An interesting complementarity between the sequence of aromatic-quinoidal structural segments in the radical cations and dications has been described and interpreted. Two derivatives with the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene unit, disubstituted either with electron donor, bis(triaryl amino) groups, or acceptors bis(dicyano-methylene) caps enforcing a quinoidal structure in the dithiophene-vinylene bridge, have been also synthesized and characterized. The radical cation of the triarylamine compound and the radical anion of the tetracyano compound similarly display hole and electron charge localization, or confinement, in the nitrogen and dicyano surrounding parts, or class II mixed valence systems, while their dication and dianion species, conversely, are open-shell diradical (i.e., polaron pair) and closed-shell (i.e., bipolaron), respectively. The preparation of these new π-conjugated oligomers gives way to the realization of compounds with new electronic properties and unique structures potentially exploitable in organic electronics.
Bibliographical noteFunding Information:
We thank the nancial support from MINECO, Spain (CTQ2013-48252-P, CTQ2016-79189-R and CTQ CTQ2015-69391-P) and Junta de Comunidades de Castilla-La Mancha, Spain (PEII-2014-014-P). P. M. B. acknowledges nancial support from the Juan de la Cierva-Formación 2015 programme supported by MINECO. RD thanks the Ministerio de Educación, Cultura y Deporte of Spain for an FPU grant. We thank the Research Central Services (SCAI) of the University of Málaga. T.M.P. acknowledges the University of Minnesota, Morris (UMM) Faculty Research Enhancement Funds supported by the University of Minnesota Office of the Vice President for Research and the UMM Division of Science and Mathematics for nancial assistance and the Supercomputing Institute of the University of Minnesota. T.M.P. and D.E.J. acknowledge the National Science Foundation Major Research Instrumentation Award #1125975 for funding the acquisition of the X-ray diffractometer used for data collection in this investigation.
© 2017 The Royal Society of Chemistry.