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Photo- and electroluminescence of five bithiophene disubstituted 1,3,4-thiadiazoles, constituting a new class of solution processable materials for organic opto-electronics, were studied. It was found that the introduction of alkyl solubilizing substituents bathochromically shifted the photo- and electroluminescence bands. The most pronounced effect was observed for the substitution at the Cα position which changed the emitting light color from bluish to green. All five derivatives were tested in host/guest type organic light emitting diodes (OLEDs) with either poly(N-vinylcarbazole) (PVK) or poly(N-vinylcarbazole) + 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK + PBD) matrices. The latter matrix turned out especially well suited for these guest molecules yielding devices of varying color coordinates. The best luminance (750 cd/m2) was measured for 2,5-bis(5'-octyl-2,2'-bithiophene-5-yl)-1,3,4-thiadiazole with the luminous efficiency exceeding 0.4 cd/A. © 2014 Elsevier B.V. All rights reserved.

A series of copolymers consisting of alternating thiadiazole and unsubstituted or alkyl disubstituted quaterthiophene units, were synthesized by electropolymerization of 2,5-bis(2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P1 ), 2,5-bis(4′-octyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P2 ), 2,5-bis(3′-octyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P3 ), 2,5-bis(3-decyl-2,2′-bithiophene-5-yl)-1,3,4-thiadiazole (polymer P4 ). For comparative reasons P3 was also obtained via Suzuki coupling of 2,5-bis(5-bromo-2-thienyl)-1,3,4-thiadiazole and neopentyl ester of 4,4′-dioctyl-2,2′-dithienyl-5,5′-diboronic acid. As evidenced by cyclic voltammetry studies the presence of electron accepting thiadiazole unit in the main polymer chain results in an increase of the reductive doping potential of the studied compounds as compared to polythiophene or poly(alkylthiophene). Electrochemically determined electron affinities values were found in the range from −3.10 eV to −3.14 eV, showing a negligible effect of the alkyl substituent on this parameter. To the contrary, the oxidation potential of the studied copolymers strongly depended on the presence and the position of the alkyl group. For P3 the electron donating properties of the substituents were particularly pronounced leading to a decrease of its oxidative doping potential by 210 mV, as compared to the case of the unsubstituted polymer ( P1 ), and the corresponding drop of the ionization potentials from +5.75 eV to +5.54 eV. The presence of a characteristic capacitive plateau's following the reductive and oxidative dopings suggests that both redox reactions are true doping reactions and the synthesized polymers can be transformed either in n-type or p-type conductors. For P3 and P4 these findings are additionally corroborated by UV–vis-NIR spectroelectrochemical data which unequivocally show the formation of polaronic/bipolaronic bands upon reductive and oxidative dopings. The analysis of the Raman spectroelectrochemical data obtained for P3 , supported by theoretical calculations of the vibrational model, leads to the conclusion that the mechanism of the electrochemical doping in this polymers is the same as in poly(alkylthiophene) homopolymers and involves the transformation of the benzoid-like structure into quinoid one. As judged from the redox properties of the synthesized copolymers, P3 seems to be the most promising candidate for application in such organic electronic devices such as p-channel field effect transistors (FETs), photodiodes (PD) or photovoltaic cells (PC), however its use in air operating n-channel and ambipolar FETs seems to be excluded due to relatively high absolute value of electron affinity.

Unsubstituted 2,5-bis(2,2′-bithiophene-5-yl)-1,3,4-thiadiazole and four of its derivatives containing solubilizing octyl groups in different positions of the terminal thiophene ring were synthesized (3 new compounds and 2 already reported). Their UV–vis absorption and emission spectra turned out to be strongly dependent on the position of the substituent and showed significant bathochromic shifts of the dominant transition for compounds with the substituents attached to Cα of the terminal ring (λmax > 425 nm). A good correlation was found for the experimentally determined and theoretically calculated excitation energies employing the time-dependent Coulomb-attenuated hybrid exchange-correlation functional using the 6-31G* basis set (TD CAM-B3LYP/6-31G*). The calculations showed, in addition, that the alkyl substituents improved the planarity of the molecule and its aromaticity, and that they raised the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels (the lat...

Two series of new soluble conjugated compounds containing tetrazine central ring have been synthesized. The three-ring compounds have been synthesized by the reaction of aryl cyanide (where aryl = thienyl, alkylthienyl, phenyl or pyridyl) with hydrazine followed by oxidation of the intermediate product with diethyl azodicarboxylate. The five-ring compounds have been prepared using two pathways: (i) reaction of 5-cyano-2,2'-bithiophene (or its alkyl derivative) with hydrazine; (ii) via Suzuki or Stille coupling of 3,6-bis(5-bromo-2-thienyl)-1,2,4,5-tetrazine with a stannyl or boronate derivative of alkylthiophene. UV-vis spectroscopic properties of the synthesized compounds are strongly dependent on the nature of the aryl group, the position of the solubilizing substituent and the length of the molecule, showing the highest bathochromic shift (λ(max)440 nm) for five-ring compounds with alkyl groups attached to C(α) carbon in the terminal thienyl ring. An excellent linear correlation has been found for spectroscopically determined and theoretically calculated (TD-B3LYP/6-31G*) excitation energies. With the exception of dipyridyl derivative, the calculated lowest unoccupied molecular orbital (LUMO) level of the investigated molecules changes within a narrow range (from -2.63 to -2.41 eV), in line with the electrochemical data, which show a reversible reduction process with the redox potential varying from -1.23 V to -1.33 V (vs. Fc/Fc(+)). The electrochemically determined positions of the LUMO levels are consistently lower by 0.9 to 1.2 eV with respect to the calculated ones. All molecules readily crystallize. Single crystal studies of 3,6-bis(2,2'-bithien-5-yl)-1,2,4,5-tetrazine show that it crystallizes in a P2(1)/c space group whose structural arrangement is not very favorable to the charge carriers flow within the crystal. Powder diffraction studies of other derivatives have shown that their structural organization is sensitive to the position of the solubilizing substituent. In particular, the presence of alkyl groups attached to C(α) carbon in the terminal thienyl ring promotes the formation of a lamellar-type supramolecular organization.

Scanning tunneling microscopy was used to study the effect of the electron-accepting unit and the alkyl substituent's position on the type and extent of 2D supramolecular organization of penta-ring donor-acceptor-donor (DAD) semiconductors, consisting of either tetrazine or thiadiazole central acceptor ring symmetrically attached to two bithienyl groups. Microscopic observations of monomolecular layers on HOPG of four alkyl derivatives of the studied adsorbates indicate significant differences in their 2D organizations. Ordered monolayers of thiadiazole derivatives are relatively loose and, independent of the position of alkyl substituents, characterized by large intermolecular separation of acceptor units in the adjacent molecules located in the face-to-face configuration. The 2D supramolecular architecture in both derivatives of thiadiazole is very sensitive to the alkyl substituent's position. Significantly different behavior is observed for derivatives of tetrazine (which is a stronger electron acceptor). Stronger intermolecular DA interactions in these adsorbates generate an intermolecular shift in the monolayer, which is a dominant factor determining the 2D structural organization. As a consequence of this molecular arrangement, tetrazine groups (A segments) face thiophene rings (D segments) of the neighboring molecules. Monolayers of tetrazine derivatives are therefore much more densely packed and characterized by similar π-stacking of molecules independently of the position of alkyl substituents. Moreover, a comparative study of 3D supramolecular organization, deduced from the X-ray diffraction patterns, is also presented clearly confirming the polymorphism of the studied adsorbates.