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A series of donor-acceptor low band gap polymers (DINIBT-C8, DINIBT-C10 and DINIBT-EH) containing 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donating unit and benzothiadiazole (BT) as an electron accepting unit with various kinds of alkyl chains (octyl; C8, decyl; C10 and ethyl hexyl; EH) on the donor unit have been designed, synthesized, and characterized for bulk heterojunction polymer solar cell applications. Due to the presence of long alkyl chains on the donor unit, all the polymers showing good solubility in common organic solvents. The frontier orbital energy levels of polymers were calculated using cyclic voltametry analysis. The influence of alkyl chains on the photo-physical properties of polymers were carried out. Among the three polymers, the DINIBT-C8 shows higher absorption coverage and good energy level matching with acceptor PC71BM than that of the other two polymers. Thus, the bulk-heterojunction polymer solar cells (BHJ PSCs) were fabricated using DINIBT-C8:PC71BM as an active...

Two fullerene-terthiophene dyads without hexyl chains (3T-C₆₀) and with hexyl chains (3TH-C₆₀) on the terthiophene substituent are synthesized by 1,3-dipolar cycloaddition of corresponding azomethine ylides to C₆₀. The cyclic voltammetry studies indicate no apparent electronic communication between the terthiophene pendent group and the fulleropyrrolidine core in the ground state. However, a significant florescence quenching is observed for 3T-C₆₀ and 3TH-C₆₀, compared to their fluorescent terthiophene (3T) and 3TH precursors, respectively, suggesting the occurrence of strong intramolecular electron/energy transfers in the photoexcited state. Furthermore, these new fulleropyrrolidine derivatives are applied as electron acceptors to fabricate poly(3-hexylthiophene) (P3HT) based bulk heterojunction solar cells. The incident photon-to-current efficiency (IPCE) value of the P3HT/3T-C₆₀ device is significantly higher than that of the P3HT/PCBM cell in wavelengths of 350-420 nm. This finding provides direct evidence for the contribution of 3T excitons to the photocurrent. Replacing 3T-C₆₀ with 3TH-C₆₀ effectively improves the morphology of the photoactive layer and widens the window of optimal D/A ratios, raising the power conversion efficiency (PCE) from 2.14% to 2.54%. Importantly, these devices exhibit superior stability of PCE against high-temperature aging.

Two series of click chemistry assisted alkoxymethyl-1H-[1,2,3]-triazol-1-yl containing side-chain liquid-crystalline polymethacrylates were synthesized by free radical polymerization technique. Mesogen was linked to backbone through various spacer units. Monomers and polymers were characterized by FT-IR, 1H and 13C-NMR spectral techniques. Thermal stability of polymers was confirmed by thermogravimetric analysis. Mesomorphic property and phase transition temperature of polymers were analysed by differential scanning calorimetry and polarized optical microscopy. Phase transition temperature and mesomorphic property of polymers with respect to insertion of polar alkoxy group on terminal triazole ring and spacer length between backbone and mesogen were investigated. Polymers exhibited grainy like textures under polarized optical microscopy. Spacer length between mesogen and backbone alters phase transition temperature of the polymers.

Visibly observed photocrosslinkable pendant liquid crystalline polymers containing indolyl based chalcone were synthesized by free-radical polymerization and characterized spectroscopically. The differential scanning calorimetry and polarized optical microscopy were used to examine liquid crystalline property. The photocrosslinking and luminescence properties were monitored by UV-Vis spectrophotometer and spectrofluorimeter, respectively. It demonstrates the chalcone unit did not manifest cis ↔ trans-isomerization reaction along with 2π+2π photodimerization upon irradiation with UV light like other chalcones hitherto reported. The photocrosslinking was visibly monitored in solution through change of fluorescent color to colorless. The spacer lengths play a key role in the reaction. The fluorescence maximum was blue shifted around 70 nm in chloroform solution upon irradiation with UV light confirms the 2π+2π photodimerization of chalcone unit.

Two series of heterocyclic liquid crystalline compounds with [1,2,3]-triazole ring at the terminal position were synthesized and analyzed for mesomorphic properties and its characteristics. Heterocyclic triazole core was introduced in to the target compound through click reaction between aromatic phenolic azide and alkyl propargyl ethers. The structure of the target compounds and intermediates was conformed by the 1 H NMR, 13 C NMR and IR spectral techniques. All the compounds in both the series were exhibited thermotropic liquid crystalline mesophase. These compounds show enantiotropic Smectic A phase on heating and cooling experiments. This was further confirmed using differential scanning calorimetric investigations. Interestingly, earlier reports on non-polar alkyl group attached to [1,2,3]-triazole heterocyclic ring was not showing any liquid crystalline properties but in the present investigation introduction of polar alkoxy group very near to [1,2,3]-triazole heterocyclic ring displayed liquid crystalline properties.

Azobenzene-based monomers and their polymers containing donor or acceptor groups at terminal positions have been synthesized. The liquid crystalline (LC) mesophase stability of one type of polymer and mixtures of donor or acceptor substituted polymers have been investigated. Mesophase formation and stability of the mixtures were studied using differential scanning calorimetry (DSC) and a hot-stage optical polarizing microscope (HOPM). The mesophase stability of the mixtures was better than those of the individual components. Binary mixture studies of acceptor monomers (a) and donor monomers (d) show that the pure low molecular weight compounds did not form any molecular aggregates and failed to show LC properties, whereas the mixture of donor substituted azobenzene and acceptor substituted azobenzene exhibits LC property. In the case of polymers, the individual acceptors (A) or donors (D) exhibit LC property, and their mixture stabilizes the crystallinity of the mesophase. The results of correlation studies of various proportions of mixtures of a/d and A/D with regard to LC properties and their mesophase stabilities are presented

Three series of side chain liquid crystalline polymers containing terminally substituted biphenyl cinnamoyl esters were synthesized and characterized. The para position in the cinnamoyl group was connected with polymer backbone through various even numbers of methylene spacers. The terminal electron acceptor and donor substituted biphenyl groups were linked to cinnamoyl group through ester linkages. Polarizing microscopic and DSC studies confirm the formation of thermotropic mesophase up on heating. All the polymers exhibited nematic/grainy mesophases. The TGA and DSC studies showed that the nature of the terminal substituents have significant effect on mesophase temperatures as well as in thermal stability of these liquid crystalline polymers. UV light promoted photocrosslinking studies reveal that the clear involvement on photocrosslinking efficiency, although they are linked to the olefinic bond of cinnamoyl group through their ester linkage. It is observed that electron-donating group accelerate the photocrosslinking rate, while acceptor group retard the phase of the reaction on the other hand, the unsubstituted polymers show an intermediate rate.