Optical, Redox, and NLO Properties of Tricyanovinyl Oligothiophenes: Comparisons between Symmetric and Asymmetric Substitution Patterns

Abstract

A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits pi-stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs).

Quinoidal Oligothiophenes: Towards Biradical Ground‐State Species

“…[16] This effect must be ascribed to partial aromatization of the quinoid backbone on oxidation (see Figure S3 in the Supporting Information). [2] The quinoid structure of compounds Q2-Q6 therefore has a remarkable effect on the redox potentials, more pronounced than that expected by only introducing electron-acceptor groups stronger than the CN group, [17] since the anodic potentials are greatly shifted to more positive values without significant alteration of the cathodic potentials.…”

Building and testing correlations for the estimation of one‐electron reduction potentials of a diverse set of organic molecules

“…As an alternative to the Born–Haber cycle methods, linear relationships between molecular orbital energies and the ability of a molecule to accept or donate an electron are among the earliest relationships that were considered in the literature and have been used extensively for specific families of molecules. The use of a linear correlation of calculated frontier orbital energies (highest occupied molecular orbital (HOMO) energy for oxidation, lowest unoccupied molecular orbital (LUMO) energy for reduction) of the singlet ground state molecules with their experimentally measured redox potentials is an even simpler way to estimate the redox potentials of unknown molecules.…”

Steric and Electronic Influence on Photochromic Switching of N,C‐Chelate Four‐Coordinate Organoboron Compounds

“…We have thus designed a dicyanomethylene‐substituted cyclopenta[ b ]thiophene (BCN) unit. Although a series of dicyanovinylene‐substituted conjugated oligomers,21, 22 tricyanovinylene‐substituted oligomers,23, 24 dicyanomethylene‐substituted quinoidal oligomers,25–31 dicyanomethylene‐bridged oligomers,32, 33 dicyanomethylene‐bridged polymers,8, 34, 35 cyano‐substituted phenylene‐vinylene oligomers,36, 37 cyano‐substituted phenylene‐vinylene polymers,38 dicyano‐substituted phenylene‐vinylene polymers,39 and cyano‐capped oligothiophenes40 have been reported, only a few π‐conjugated systems are known to fulfill both requirements of air‐stability and solution‐processability in n‐type OFET devices 8. 30, 31, 35 In this study, we have synthesized BCN‐terminated oligomers with hexyl‐substituted bithiophene (BCN‐HH‐BCN), dihexyl‐bridged bithiophene (BCN‐2Tb‐BCN), and dioctyl‐substituted fluorene (BCN‐2Pb‐BCN) inner conjugated units (shown here).…”