Anjan Bedi

Cyclopenta[c]selenophene based cooligomers and their polymers: comparative study with thiophene analogues.

Selenophene and thiophene capped cyclopenta[c]selenophenes were synthesized and characterized. Crystal structure determination of some representative compounds revealed that the substitution at 3,4-position in the form of cyclopentane ring of selenophene or thiophene does not make any significant twist in the trimer backbone, making the cooligomer nearly planar. All the cooligomers were electrochemically polymerized and compared with thiophene capped cyclopenta[c]thiophene polymer. DFT calculations predict that the cyclopentane substitution on the third repeating unit (and in general) of one dimensional polymer neither disturb the planarity nor causes any significant twist on the polymeric backbone unlike the 3,4-dialkyl substitution. The electrochemically prepared selenophene based polymers showed low band gap compared to that of thiophene analogues. Cyclopentane substitution on selenophene as well as thiophene makes the resulting polymer oxidatively more stable when compared to more familiar poly-ethylenedioxythiophene (PEDOT) or poly-ethylenedioxyselenophene (PEDOS) systems. Alternate polymers of cyclopenta[c]selenophenes (CPS)/cyclopenta[c]thiophene (CPT) and thiophene/selenophene possess the energy of HOMO and LUMO significantly lower than that of homopolymers of CPS and CPT, however, possess higher band gap than PCPS.

Cyclopenta[c]thiophene oligomers based solution processable D–A copolymers and their application as FET materials

Two new solution processable, low band gap donor–acceptor (D–A) copolymers (P1 and P2) comprising a cyclopenta[c]thiophene (CPT) based oligomers as donors and benzo[c][1,2,5]selenadiazole (BDS) and 2-dodecyl[1,2,3]-benzotriazole (BTAz) as acceptors were synthesized and characterized and their field effect transistor properties were studied. The internal charge transfer interaction between the electron-donating CPT based oligothiophene and the electron-accepting BDS or BTAz unit effectively reduces the band gap in polymers to 1.3 and 1.66 eV with low lying highest occupied molecular orbital (HOMO). The absorption spectrum of P1 was found to be more red shifted than that of P2 because of incorporation of the more electron-withdrawing BDS unit. The color of neutral P1 was found to be green in both solution and film states with two major bands in the absorption spectra; however, neutral P2 revealed one dominant absorption exhibiting red color in both solution and film state which could be attributed to the less electron-withdrawing effect of the BTAz unit. The polymers were further characterized by GPC, TGA, DSC and cyclic voltammetry. P1 and P2 exhibited charge carrier mobilities as high as 9 × 10−3 cm2 V−1 s−1 and 2.56 × 10−3 cm2 V−1 s−1, respectively with the current on/off ratio (Ion/Ioff) in the order of 102.

Thienopentathiepine: a sulfur containing fused heterocycle for conjugated systems and their electrochemical polymerization

A series of conjugated building blocks based on the thieno[3,4-f][1,2,3,4,5]pentathiepine (C4S6) core has been synthesized by a new synthetic approach. The structural and optoelectronic properties of C4S6-derivatives are tuned by a judicious choice of end-capping. Chalcogenophene-capped derivatives (2d, 2e, 2f) were successfully electrochemically polymerized. The optical band gaps (Eoptg) of P1, P2 and P3 were found to be in the range of 1.7–1.8 eV. A spectroelectrochemical study of the polymers showed reversibility in the formation of singly (polaron) and doubly charged (bipolaron) species.

Thienopyrrole and selenophenopyrrole donor fused with benzotriazole acceptor: microwave assisted synthesis and electrochemical polymerization

Thieno-/selenophenopyrrole fused substituted benzotriazoles were synthesized by microwave assisted cyclization which dramatically reduced the reaction time. Incorporation of benzotriazole as an acceptor moiety in a fused system was studied for the first time. Single crystal X-ray crystallographic studies were performed on thienopyrrole and selenophenopyrrole fused benzotriazoles 4, 5, 6 and 7. Compound 4 and 5 crystalize via hydrogen bonding with an ethyl acetate molecule from the solvent. N-Methyl substituted compounds 6 and 7 were electrochemically polymerized and characterized by spectroelectrochemistry. The effects of changing the heteroatom of the terminally fused heterocycles on their photophysical and electrochemical properties were also explored in this report. Emission spectra of compound 6 and 7 showed solvatochromic effects with a large Stokes shift.

Phenyl-capped cyclopenta[ c ]chalcogenophenes: synthesis, crystal structures, electrochemistry and theoretical insights

Two new phenyl capped cyclopenta[c]chalcogenophenes (PhCPTPh and PhCPSPh) were synthesized by zirconocene mediated coupling and characterized by single crystal X-ray diffraction (SCXRD). Replacement of sulfur by selenium destabilized the HOMO level and increased the structural rigidity. The crystal structures of both the compounds were found to be nearly planar. PhCPSPh, though isostructural with PhCPTPh, packed in a very-ordered and specific pattern by Se⋯π interactions between two neighboring molecules. Hirshfeld surface analysis supported the crystal structure study. Theoretical calculations exhibited very close reorganization energies of 0.23 and 0.22 eV for PhCPTPh and PhCPSPh, respectively.

Selenium-Containing Fused Bicyclic Heterocycle Diselenolodiselenole: Field Effect Transistor Study and Structure–Property Relationship

The first application of the diselenolodiselenole (C4Se4) heterocycle as an active organic field effect transistor materials is demonstrated here. C4Se4 derivatives (2a-2d) were obtained by using a newly developed straightforward diselenocyclization protocol, which includes the reaction of diynes with selenium powder at elevated temperature. C4Se4 derivatives exhibit strong donor characteristics and planar structure (except 2d). The atomic force microscopic analysis and thin-film X-ray diffraction pattern of compounds 2a-2d indicated the formation of distinct crystalline films that contain large domains. A scanning electron microscopy study of compound 2b showed development of symmetrical grains with an average diameter of 150 nm. Interestingly, 2b exhibited superior hole mobility, approaching 0.027 cm(2) V(-1) s(-1) with a transconductance of 9.2 μS. This study correlate the effect of π-stacking, Se···Se intermolecular interaction, and planarity with the charge transport properties and performance in the field effect transistor devices. We have shown that the planarity in C4Se4 derivatives was achieved by varying the end groups attached to the C4Se4 core. In turn, optoelectronic properties can also be tuned for all these derivatives by end-group variation.