Malapaka Chandrasekharam

Novel Carbazole-Phenothiazine Dyads for Dye-Sensitized Solar Cells: A Combined Experimental and Theoretical Study

We report a joint experimental and computational work on new organic donor-acceptor dye sensitizers in which a carbazole (CZ) and a phenothiazine (PTZ) units are linked together by an alkyl C6H13, while two different anchoring groups are employed: the cyanoacrylic acid (CS1A, CSORG1) and the rhodanine-3-acetic acid (CS4A, CSORG4). The CZ moiety has multiple roles of (i) acting as an extra-electron donor portion, providing more electron density on the PTZ; (ii) suppressing the back-electron transfer from TiO2 to the electrolyte by forming a compact insulating dye layer; (iii) modulating dye aggregation on the semiconductor surface; and (iv) acting as an antenna, collecting photons and, through long-range energy transfer, redirecting the captured energy to the dye sensitizer. We show that the introduction of the CZ donor remarkably enhances the photovoltaic performances of the rhodanine-based dye, compared to the corresponding simple PTZ dye, with more than a two-fold increase in the overall efficiencies, while it does not bring beneficial effects in the case of the cyanoacrylic-based sensitizer. Based on quantum mechanical calculations and experimental measurements, we show that, in addition to a favored long-range energy transfer, which increases the light absorption in the blue region of the spectrum, the presence of the CZ unit in the CSORG4 dye effectively induces a beneficial aggregation pattern on the semiconductor surface, yielding a broadened and red-shifted light absorption, accounting for the two-fold increase in the generated photocurrent.

High molar extinction coefficient amphiphilic ruthenium sensitizers for efficient and stable mesoscopic dye-sensitized solar cells

Two efficient heteroleptic ruthenium (II) complexes, cis-di(thiocyanato)(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(2-(3,5-di tert-butylphenyl)ethenyl)-2,2′-bipyridine) ruthenium (II) (HRD-1) and cis-di(thiocyanato)(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(2-(2,3,5-trimethylphenyl)ethenyl)-2,2′-bipyridine) ruthenium (II) (HRD-2) were synthesized and characterized, which when anchored onto nanocrystalline TiO2 shows efficiency of 5.77 and 4.87% respectively, using durable redox electrolytes.

Indole and triisopropyl phenyl as capping units for a diketopyrrolopyrrole (DPP) acceptor central unit: an efficient D–A–D type small molecule for organic solar cells

Two simple D–A–D structured small molecules based on diketopyrrolopyrrole as central acceptor unit and different terminal units (indole and 2,4,6-triisopropylphenyl for CSDPP1 and CSDPP3 respectively) with broad absorption band and suitable energy levels were synthesized. The new DPP molecules are fully characterized and their photovoltaic properties were investigated using them as donor components along with PC70BM as acceptor in the bulk heterojunction (BHJ) active layer. The organic BHJ solar cells based on CSDPP1:PC70BM and CSDPP3:PC70BM (1:1 D–A ratio) cast from chloroform solvent showed moderate power conversion efficiency of about 3.26% and 2.42%, respectively. The higher power conversion efficiency (PCE) of organic solar cells based on CSDPP1 as donor as compared to CSDPP3 has been attributed to the higher hole mobility and absorption coefficient and low optical band gap of CSDPP1. The PCE of the thermally annealed device based on CSDPP1:PC70BM has been improved up to 4.96% attributed to the better film morphology and increased absorption of the BHJ active layer. We attribute the increase in absorption intensity upon thermal annealing to the stronger electrical polarization in the BHJ active layer resulting from morphological changes i.e. crystallinity and molecular aggregation. Finally, the capacitance–voltage measurements indicates that the increasing local electrical polarizations upon thermal annealing can lead to an enhancement of the charge generation from the excitons at donor–acceptor interfaces and their transportation to the respective electrodes in the BHJ device, leading to better short circuit photocurrents and fill factors.

One bipyridine and triple advantages: tailoring ancillary ligands in ruthenium complexes for efficient sensitization in dye solar cells†

Four new ruthenium bipyridyl complexes denoted as MC103–MC106, with novel unsymmetrical bipyridines as ancillary ligands have been successfully synthesized as efficient sensitizers for application in dye-sensitized solar cells (DSC). The spectral, electrochemical and photovoltaic properties of the new sensitizers have been investigated. Alkyl thiophene/alkyl bithiophene and trialkyl phenyl as substituents on ancillary ligands improved the spectral properties and hence better efficiencies of DSCs are achieved. The higher efficiencies of the new sensitizers obtained are 9.56%, 9.58%, 8.34% and 8.32%, respectively, compared to 7.2% for the standard N719 sensitizer fabricated and evaluated under similar conditions.

Synthesis of new praziquantel analogues: Potential candidates for the treatment of schistosomiasis

An efficient synthesis of antischistosomal drug praziquantel and analogues was achieved and the synthetic route designed was to afford structurally diverse analogues for better structure-activity relationship understanding. Total of nineteen PZQ analogues with structural variations at amide, piperazine and aromatic moieties have been synthesized and fully characterized. Among all the new analogues tested for antischistosomal activity, one dimethoxy tetrahydroisoquinoline analogue and two tetrahydro-β-carboline analogues exhibited moderate activity against adult Schistosoma mansoni. Tetrahydro-β-carboline analogues showed moderate activity whereas the presence of p-trifluoromethylbenzoyl and p-toluenesulphonyl moieties resulted in complete suppression of antischistosomal activity.

2,6-Bis(1-methylbenzimidazol-2-yl)pyridine: A New Ancillary Ligand for Efficient Thiocyanate-Free Ruthenium Sensitizer in Dye- Sensitized Solar Cell Applications

We have designed and synthesized a new thiocyante-free ruthenium complex containing 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, coded as SPS-G3, and it has been used as an efficient photosensitizer for dye-sensitized solar cells (DSSCs). Upon sensitization of SPS-G3 on nanocrystalline TiO2 film, the DSSC test cell yielded a large short-circuit photocurrent (16.15 mA cm(-2)), an open circuit voltage of 0.52 V, and a fill factor (FF) of 0.72, resulting in an overall power conversion efficiency (PCE) of 6.04% under simulated AM 1.5 solar irradiation (100 mW cm(-2)). DSSCs were prepared by adding various concentrations of multiwall carbon nanotubes (MWCNTs) (up to 0.5 wt %) into the TiO2 nanoparticles. Optimization of MWCNT concentration (0.3 wt %) lead to PCE values as high as 7.76%, while the test cells employing pure TiO2 photoanode obtained an efficiency of 6.04%. The results indicate that the PCE of MWCNTs/TiO2 composite DSSCs are dependent on the quantity of MWCNTs loading on the photoanodes. A small amount (0.3 wt %) clearly enhances the PCE of DSSC, while the excessive MWCNT loading lowers the photovoltaic performance of the DSSC. The increase in the PCE has been attributed to the decrease in charge-transport resistance, charge-transport time, and electron lifetime, which are estimated from electrochemical impedance spectra.

Copper-catalyzed regio and diastereoselective three component C–N, C–C and C–O bond forming reaction: oxidative sp3 C–H functionalization

A highly regio and diastereoselective copper catalyzed three component reaction of tetrahydroisoquinolines (THIQs), aldehydes, and naphthols to furnish naphthoxazine derivatives under mild conditions was developed. The initial generation of iminium ions followed by the nucleophilic attack of naphthol presumed to deliver 15-phenyl-7a,12,13,15-tetrahydronaphtho[1′,2′:5,6][1,3]oxazino[2,3-a]isoquinoline derivatives. The reaction ensured the formation of C–C, C–N and C–O bonds in a one-pot reaction to produce ring fused N,O-acetals in good yields. The three component reaction, the inexpensive copper catalyst in the absence of a co-oxidant/additive and the direct regioselective oxidative sp3 C–H functionalization are the attractive features of this approach. The preliminary photophysical properties obtained for selected compounds were presented.

A novel carbazole–phenothiazine dyad small molecule as a non-fullerene electron acceptor for polymer bulk heterojunction solar cells

A new organic small molecule, 2-((10-(6-(3,6-di-tert-butyl-9H-carbazole-9-yl)hexyl)-10H-phenothiazine-7-yl)methylene) malononitrile named CSORG5 has been synthesized and successfully applied for organic bulk heterojunction (BHJ) solar cells as a novel electron acceptor along with P3HT as a donor. The CSORG5 molecule possess facile synthesis, solution processability, visible light absorption, a properly matched lowest unoccupied molecular orbital (LUMO) energy level with conjugated polymers and moderate electron mobility, making the molecule an ideal acceptor for P3HT and other low band gap conjugated polymers. The BHJ organic solar cell constructed from a P3HT:CSORG5 (1:1) active layer processed from a DIO/THF solvent, exhibits a high open circuit voltage of up to 0.98 V resulting in an overall power conversion efficiency of 2.80%. Furthermore, the PCE is enhanced up to 4.16% when a thin layer of TiO2 is inserted between the active layer and the Al electrode.

Microwave-Assisted, Rapid, Solvent-Free Aza-Michael Reaction by Perchloric Acid Impregnated on Silica Gel

Abstract Highly efficient solvent-free aza-Michael additions of a variety of amines to α,β-unsaturated carbonyl compounds under microwave-irradiation conditions catalyzed by perchloric acid impregnated on silica gel (HClO4/SiO2) is reported. The reactions are completed within 2–7 min in a microwave oven to produce the corresponding adducts in excellent yields, and the catalyst can be recovered and reused.

SYNTHESIS OF β-IONONE#

Preparation of β-ionone by a reaction sequence starting from acetone is described.