Quantum Chemical Study of the Ground and Excited State Electronic Structures of Carbazole Oligomers with and without Triarylborane Substitutes

• Published in: Journal of physical chemistry. C Vol. 116; no. 23; pp. 12434 - 12442
• Main Authors: Zhang, Shushu, Qu, Zexing, Tao, Peng, Brooks, Bernard, Shao, Yihan, Chen, Xiaoyuan, Liu, Chungen
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 14.06.2012
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron states; Exact sciences and technology; Methods of electronic structure calculations; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Physics; Molecular orbital; Electronic properties; Quantum chemistry; Ground states; Frontier orbital; Macromolecules; Theoretical study; Polymerization; Excited states; Monomers; Oligomers; Time dependence; Energy gap; Carbazoles; Electronic structure; Molecular orbital method; Optical properties; Electron state; Density functional method; Charge transfer
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp3027447

Recent experimental investigation (Reitzenstein and Lambert, Macromolecules 2009, 42, 773) indicated that the quite different optical properties of 2,7- and 3,6-linkage triarylboryl carbazole oligomers may arise from the different nature of their low-lying excited states: a low-lying delocalized within-backbone excitation in longer 2,7-linked oligomers vs a backbone-to-side chain charge-transfer (CT) excitation independent of the polymerization length in 3,6-linked oligomers. In this paper, two long-range corrected functionals, CAM-B3LYP and ωB97X, are applied together with the traditional B3LYP functional in time-dependent density functional theory (TDDFT) calculations to systematically investigate the low-lying electronic excitations in both oligomers. Our calculations indicate that an extensive conjugation exists between monomer molecular orbitals in 2,7-linked oligomers, which is absent in those of 3,6-linked structures, resulting in a considerable narrowing of the HOMO–LUMO gap of their backbone moiety, while having little effect on the side chains. CAM-B3LYP and ωB97x calculations confirm that the lowest-energy absorption is a within-backbone excitation in longer 2,7-linked oligomers as opposed to a backbone to side-chain charge transfer excitation in 2,7-linked oligmers of shorter length and 3,6-linked oligomers of any length. All these findings are consistent with the experimental findings and the qualitative energy diagram proposed by Reitzenstein and Lambert.