Theoretical Analysis of Effects of π-Conjugating Substituents on Building Blocks for Conducting Polymers

Abstract

Geometries of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′] dithiophene 1 and its CdO, CdS, CdCH 2 , CdCF 2 , and CdC(SR) 2 analogues were optimized using density functional theory. Three of the above groups, CdC(CN) 2 , CdO, and CdS, were also examined on dipyrrole, difuran, dicyclopentadiene, and diborole. Electronic structures were analyzed with respect to their suitability as building blocks for conducting polymers with the natural bond orbital (NBO) method. All bridging groups investigated decrease HOMO-LUMO gaps compared to the unsubstituted parent dimers. Substitution affects HOMO and LUMO energies. Energy gap reduction is caused by a stronger decrease of LUMO energies compared to HOMO energies. The CdS group leads to even smaller energy gaps than the dicyanomethylene group since the HOMO is lowered less in energy with CdS. Compared to unsubstituted dimers, the strongest substituent effects are found with pyrroles and furans. Boroles and thiophenes are least affected. The smallest HOMO-LUMO gaps are obtained for electron-poor systems such as boroles followed by cyclopentadienes. This is analogous to the trend for the unsubstituted parent systems. All of the bridging groups are potential π-acceptors due to their low-lying π*-orbitals, and the corresponding polymers are predicted to be n-dopable. In aromatic structures, the LUMO is localized around the bridging substituent and the coefficients at the R-carbon atoms that reflect electron density are small. This might contribute to the poor conductivity of the n-doped form of poly-1. Electron-poor monomers and polymers tend to switch to quinoid structures. In quinoid repeat units, the HOMO is localized but not as strongly as the LUMO in the aromatic repeat units. The LUMO in quinoid repeat units is delocalized with large coefficients at the R-carbon atoms. Quinoid polymers could therefore be good conductors in the n-doped state.

Electronic Structure Analysis of a New Quinoid Conjugated Polymer

“…Theoretical investigations of poly-1 [14,22] and related systems as well as calculations on the monomeric building blocks [23] support Huang and Pickup's findings. The conduction band widths, which can be very roughly correlated with conductivity, [8] are small for these systems [24] and orbital coefficients at the α-carbon atoms of the building blocks are small.…”

“…The conduction band widths, which can be very roughly correlated with conductivity, [8] are small for these systems [24] and orbital coefficients at the α-carbon atoms of the building blocks are small. [23] Thus, the dicyanomethylene group seems to increase the electron affinity of polythiophene by introducing low lying but localized π*-orbitals. This means that in the conduction "band", the orbitals do not overlap strongly.…”

Time-Dependent Density Functional Theory Study on Cyclopentadithiophene-Benzothiadiazole-Based Push-Pull-Type Copolymers for New Design of Donor Materials in Bulk Heterojunction Organic Solar Cells

“…First, in Section 3.1, we consider the derivatives 4-9 ( Figure 1) because their electron-rich components 4'-9', that is, dialkylmethylene (4'), sulfide (5'), carbonothioyl (6'), carbonyl (7'), dicyanomethylene (8'), and dicyano (9') derivatives of CPDT 1', have been proposed as promising candidates for optoelectronic applications owing to their narrow band gaps and low LUMO levels, 23,24 although the band gaps were significantly overestimated in these DFT (not TDDFT) studies at the level of B3LYP/6-31(d).…”

Trends in molecular design strategies for ambient stable n-channel organic field effect transistors

“…Although CN is considered to be strongly electron withdrawing, it actually does not attract electrons in the neutral ground state. 211,212 The high electron affinity of CN substituted systems is the result of p-accepting rather than inductive effects. CN has a low lying unoccupied p*-orbital which conjugates with the p-system of OSCs, polarizing the LUMO toward itself.…”

“…As a result of the localization effect of CN described above, many cyanosubstituted polymers are stable in the reduced form but have very flat conduction bands. [212][213][214] Theory predicts therefore limited intramolecular electron transport in these materials. 213,214 Because the valence band is delocalized, intramolecular hole transport is comparable to that of unsubstituted systems.…”