Symmetrically Disubstituted Bithiophene Derivatives of 1,3,4-Oxadiazole, 1,3,4-Thiadiazole, and 1,2,4-Triazole – Spectroscopic, Electrochemical, and Spectroelectrochemical Properties

Abstract

Electrochemical and spectroelectrochemical properties of a series of new penta-ring donor -acceptor compounds, comprising 1,3,4-oxadiazole, 1,3,4-thiadiazole and 1,2,4-triazole central ring, symmetrically connected to substituted bithiophenes, were investigated. Aromaticity and electrophilic -nucleophilic traits of the aza-heterocyclic units, fostering inductive and resonance effects that translate to conjugation enhancement and electron (de)localization, were found a major factor determining the key electron properties of ionization potential (IP) and electron affinity (EA) of these molecules. Replacing the alkyl thiophene substituent for an alkoxy one, afforded certain control over the two parameters as well. All studied compounds were found to undergo electrochemical polymerization giving p and n-dopable products, featuring good electrochemical reversibility of their oxidative doping process, as demonstrated by cyclic voltammetry and UV-Vis-NIR, EPR spectroelectrochemistry. While electropolymerisation of entities differing in the heterodiazole unit was found to conserve the EA value, the IP parameter of polymerisation products was found to decrease by 0.6 to 0.7 eV affording an asymmetric narrowing of the frontier energy levels gap. Aided by quantum chemical computations, the effects of structure tailoring of the investigated systems are rationalized, pointing to conscious ways of shaping the electronic properties of thiophene class polymers using synthetically convenient heterodiazole π-conjugated units.

Star‐Shaped Conjugated Molecules with Oxa‐ or Thiadiazole Bithiophene Side Arms

“…Vibrational structure, although not very highly pronounced, is observed in the emission spectrao fm olecules with two alkyl substituents (S*TBT2 and S*OBT2),w hich may be interpreted in terms of some rigidification of the molecular structure in the excited state, possibly throughb enzoid-quinoid type of transformationo ccurringu pon excitation. [41] Relatively large Stokes shifts are measured for S*TBT1 and S*OBT1, 90 and 104 nm, respectively.T hey are larger than the shifts reported for the corresponding linear compounds containing oxadiazoleo rt hiadiazole units [37][38][39][40] and additionally corroborate significant,e xcitation-induced changesint heir molecular structure. Smaller Stokes shifts are found for S*TBT2 and S*OBT2 (59 and 71 nm) comparable to those noted for the linear compounds.…”

“…Although trisubstituted benzene as the central unit of the studied star-shaped molecules should assure their good planarity and conjugation, the planarity-sensitive p-p*b and in the spectra of these molecules is hypsochromically shiftedb y 13 to 16 nm, as compared to the case of the corresponding linear oxa-or thiadiazoles disubstituted with either alkylbithiophene or dialkylbithiophene. [37,39,40] The p-p*b and in the thin film spectra of molecules containing one alkyl solubilizing group in each arm (S*TBT1 and S*OBT1) is bathochromically shifted with respect to the position of this band in the solution spectra (see Table 1). This means that lower planarity of these molecules in solution has its origin in the interaction with the solvent molecules and in the solid state this planarity is improved.…”

“…[38,43] The reduction process of all four molecules is irreversible, similarly as linear oxa-and thiadiazoles disubstitutedw ith alkyl-o r dialkylbithiophenes ( Figure 2). [37,39,40] Molecules containing thiadiazole electron withdrawing units are easier to reduce and easier to oxidize as compared to their oxadiazole analogues (see Figure2and data presented in Ta ble 2) yielding lower ionization potential (IP) and highere lectron affinity (EA) values. As ar esult, the electrochemical gaps (E g (electro)) of S*TBT1 and S*TBT2 are smaller than those of S*OBT1 and S*OBT2.T he trend is similara si nt he case of linear donor-acceptor compounds consisting of the same electron-withdrawing and -donating groups, however,i nt he star-shaped molecules the electrochemical band gaps are smaller by 120 to 170 meV.…”

Spectroelectrochemistry of alternating ambipolar copolymers of 4,4′- and 2,2′-bipyridine isomers and quaterthiophene

“…While p2's linear structure and extended conjugation help it to accommodate charge carriers characteristic for heavily doped polythiophene [66][67][68], the conjugation break afforded by the meta substitution pattern across the pirydyl unit makes p1 behave like an ensemble of shorter oligomeric segments with better defined electronic structure. In such setting, 4,4'-bipirydyl tunes the electron energy levels of this thiophene copolymer, an approach effectively demonstrated with other heterocyclic units whose doping behaviour resembles that observed for p1 [69,70]. Compared to other electron deficient heterocyclic units, bipyridine's basic character offers auxiliary means to modify its electrophilic character by way of quaternisation of the nitrogen atom's lone electron pair.…”

Synthesis, spectral characteristics and electrochemistry of symmetrically substituted hybrids derived from 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole under Suzuki cross-coupling reaction

“…Electrochemical redox potentials provide a good estimate of the ionization potential (IP) and electron affinity (EA) parameters. 49 Minor shifts in E red have been observed, indicating that the electrochemically generated negative charge is located primarily on the electronaccepting 1,3,4-oxadiazole moiety, 50 with some contribution of the electron-donating 4-arylphenyl substituents. Oxidation peaks of most compounds are not observed for compounds 5bg in the potential window of the utilized supporting electrolyte, concurrent with the estimated energies of the electron transitions, believed to be HOMO-LUMO transitions, observed via UV-Vis spectroscopy.…”