Comparative Study of the Semiconducting Properties of Benzothiadiazole and Benzobis(thiadiazole) Derivatives Using Computational Techniques

Abstract

Recent literature reports indicate that derivatives of benzothiadiazole (BT) and benzobis(thiadiazole) (BBT), which differs from BT by an extra thiadiazole ring, exhibit good semiconducting properties, such as high electron mobility and low-lying lowest unoccupied molecular-orbital (LUMO) levels. In this study herein, computational techniques like density functional theory (DFT), spin-flip DFT and valence-bond methods are used to analyze the semiconducting properties of these molecules. Calculations at the B3LYP/cc-pVTZ level reveal that all the BBT molecules, including the bare BBT ring, have lower lying LUMO energies (3.70-4.11 eV) compared to the BT derivatives (2.56-3.41 eV) with similar substitution. The reorganization energies (λ(+)/λ(-)) obtained at this level of theory of the BT derivatives are around (225-333)/(246-315) meV, while BBT derivatives have much smaller reorganization energies and these are in the range of (129-259)/(150-230) meV. We observe that the different behavior of BBT is due to the inherited biradicaloid character from the parent molecule tetramethylenebenzene (TMB), a disjoint non-Kekule biradical having non-bonding molecular orbitals (NBMOs) as the highest occupied molecular orbital (HOMO) and LUMO. Additionally, the perturbation of the orbitals of the biradical TMB to obtain BBT is the major cause for the BBT derivatives to have a larger electron affinity (EA) and a smaller HOMO-LUMO gap (HLG) compared to BT derivatives.

Importance of unpaired electrons in organic electronics

“…While the aromatic character of the benzene ring in B excludes the formation of an open‐shell structure, it is less certain for BT. However, a comparative DFT study between BT and BBT performed by Thomas et al confirms this, showing that no open‐shell contribution exists for BT, while biradicaloid character was determined for BBT . Figure (b) shows the ESR spectra for the BBT family.…”

“…The existence of a biradicaloid state in BBT‐based molecules has recently been noted by Thomas et al through DFT calculations . Biradicaloids have been investigated for quite some time, but recently have resurged, stimulated by the predicted biradical character of extended acenes and subsequent development of stable materials with biradicaloid character .…”

“…The existence of a biradicaloid state in BBT-based molecules has recently been noted by Thomas et al through DFT calculations. [8][9][10] Biradicaloids have been investigated for quite some time, [11][12][13] but recently have resurged, stimulated by the predicted biradical character of extended acenes 14 and subsequent development of stable materials with biradicaloid character. [15][16][17][18][19][20] The biradicaloid state differs from the biradical state in that the singlet-triplet degeneracy is perturbed and weakly separated, resulting in a singlet biradical ground state (S 0 ) with a thermally accessible triplet biradical state (T 1 ).…”

“…Figure showing that no open-shell contribution exists for BT, while biradicaloid character was determined for BBT. 10 Figure 4(b) shows the ESR spectra for the BBT family. Both BBT and BBTPD show ESR signals of similar strength, indicating spin concentrations of similar order.…”

Quinoidal conjugated polymers with open-shell character

“…32,33,44,47,[64][65][66][67] In this context, only several recent representative copolymers with open-shell character are shown here. Benzo[1,2-d:4,5-d′] bistriazole 68,69 and benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole [70][71][72][73][74][75] are proquinoidal while electron-accepting building blocks. Conjugated polymers including these two moieties, P1-4 78 and P5 and P6, 79 were reported by Tam and Xu.…”