[Show abstract][Hide abstract] ABSTRACT: Three pyrene-oxadiazole derivatives were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and electron transporters in three different device configurations, involving hole-injection/hole-blocking materials that showed good current and power efficiencies. To understand the underlying mechanisms involved in the application of these molecules as emitters and transporters a detailed photophysical characterization of molecules 4-6 was carried out. The absorption, steady state fluorescence, phosphorescence, fluorescence lifetime and phosphorescence lifetime measurements were carried out. The high quantum yield and efficient reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a good emitter and the charge delocalization properties leading to excimer formation makes them efficient electron transporters. Iso-energetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in absence of thermal activation.
Full-text · Article · Dec 2015 · The Journal of Organic Chemistry
[Show abstract][Hide abstract] ABSTRACT: Four new solution-processable donor-acceptor-donor (D-A-D) structured organic small molecules (CSDPP9, CSDPP10, CSDPP11 and CSDPP12) with diketopyrrolopyrrole (DPP) as the central core unit and different electron donating groups as the terminal units, were designed and synthesized. These molecules employed as donors along with PC71BM as an electron acceptor in solution processed bulk heterojunction (BHJ) solar cells, exhibited a strong visible (450-650 nm) absorption band with suitable molecular orbital energy levels. OSCs based on these small molecules achieved moderate power conversion efficiency (PCE) in the range of 2.86-3.55% along with high open circuit voltage (Voc) and the difference is correlated with the different electron donating units used in the DPP. The observed high open circuit voltage (Voc) of the device is directly related to the HOMO energy level of the DPP. The BHJ devices, based on CSDPP11:PC71BM and CSDPP12:PC71BM subjected to solvent and thermal annealing showed improved PCEs of 5.47% and 4.88% respectively. The enhancement in the PCE has been attributed to the improved charge carrier mobility, crystallinity and light harvesting ability of the active layer.
[Show abstract][Hide abstract] ABSTRACT: For the first time we report the design and syntheses of phosphonite coordinated ruthenium (II) sensitizers bearing ĈN̂N ligand and/or terpyridine derivatives carboxylate anchor (GS11, 12 and GS13) and its application in sensitized carboxylate anchor for hydrogen production over Pt-TiO2 system. These heteroleptic complexes exhibit broad metal-to-ligand charge transfer transition band over the whole visible regime extending up to 900 nm. DFT calculations of these complexes show that the HOMO is distributed over the Ru and Cl atom whereas; LUMO is localized on the polypyridile ligand, which are anchored on TiO2 surface. Among the sensitizers tested for photocatalytic hydrogen evolution, GS12 exhibited a maximum turnover number (TON) 8605 (for 8h) which is very high compared to the reference sensitizer (N719) with TON 163 under similar evaluation condition. The dependence of the hydrogen evolution rate at different pH using GS11, GS12, GS13 and DX-1 sensitized Pt-TiO2 has been studied and the maximum H2 production yield was obtained at pH 7 for all sensitizers.
No preview · Article · Aug 2015 · ACS Applied Materials & Interfaces
[Show abstract][Hide abstract] ABSTRACT: Two novel heteroleptic Ru[3+2+1] sensitizers, 1 and 2, with unsymmetrical bipyridine as ancillary ligand and electron donating 4-methylstyryl group in the anchoring π-extended terpyridyl ligand were synthesized and characterized. DFT studies reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine. The two new sensitizers showed an improvement in the molar extinction coefficient compared to reference standard N749 dye. Among the two new sensitizers, 1 exhibited maximum solar to electric conversion efficiency (η) of 5.19% with short circuit current density of 14.032 mA cm−2, open circuit voltage of 0.520 V and fill factor of 0.712, under Air Mass (AM) 1.5 sunlight, while the reference N749 sensitized solar cell exhibited η-value of 6.44%.
No preview · Article · Aug 2015 · Inorganica Chimica Acta
[Show abstract][Hide abstract] ABSTRACT: Tuning the parameters to enhance the efficiencies of a novel set of metal free sensitizers for dye sensitized solar cells (DSSC) is carried out by varying the π-conjugated spacers that link the donor benzocarbazole to the acceptor cyanoacrylic acid. The molecules are synthesized by different combinations of spacers, namely fluorene-thiophene (BFT), fluorene-furan (BFF), fluorene-phenyl (BFB), and thiophene-phenyl (BTB). The molar extinction coefficients of all the dyes are high which is attributed to benzocarbazole, but it is higher in the dyes in which fluorene is one of the spacers. But interestingly, in the photovoltaic device when the non-fluorene dye BTB is the sensitizer, red-shifted and broader incident photon-to-current efficiency (IPCE) curves are obtained leading to larger short circuit current density, Jsc, almost double when compared to BFB-based cell. The efficiency of the device with this dye as the sensitizer is also the highest in this series. The reasons behind these observations are investigated using computational techniques.
Full-text · Article · Jul 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: With a general aim to analyze molecules containing 1,3,4-oxadiazole moiety for charge transport in OLED devices, in this study we use DFT methods to compute reorganization energies, transfer integrals and hole and electron mobilities of a very large number of symmetrically substituted small and large oxadiazole derivatives retrieved from Cambridge Structural Database (CSD). The study reveals that for the majority of small planar molecules, the reorganization energies are large and the transfer integral plays a crucial role in the charge transport. In the case of large planar molecules, small reorganization energies leading to large drift mobility for both hole and electron transport is observed. The detailed analyses of the geometry and molecular orbitals of the molecules along with the pathways in the solid state have been carried out which should be helpful for improving the design of oxadiazole derivatives for multifunctional bipolar properties.
No preview · Article · May 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We report here a simple one-pot synthesis, characterization and cation recognition properties of two benzothiazole based ferrocenyl pyrazoline derivatives (4 and 5). These compounds behave as highly selective multichannel sensors towards Hg2+ and Cu2+ ions in acetonitrile solution. An anodic shift of the ferrocene/ferrocenium couple, with a progressive perturbation in the lower energy band, was observed upon interaction of the cations with these receptors. A colour change, from yellow to brown and red, for Hg2+ and Cu2+ ions respectively enabled facile “naked eye” detection using the ‘test strips’ prepared from these receptors. The competitive metal-ion binding experiments indicated high selectivity towards Hg2+ as compared to Cu2+ ions. A 45–59 fold chelation enhancement of fluorescence (CHEF) towards Hg2+ ion was observed in the emission spectrum of 4 and 5. 1H NMR studies and DFT calculations were used to determine the binding sites of receptor 4 involved in the complexation process.
No preview · Article · Mar 2015 · Journal of Organometallic Chemistry
[Show abstract][Hide abstract] ABSTRACT: We designed and synthesized a new ruthenium complex using terpyridine as an anchoring ligand and BOC (tertbutyloxycarbonyl) protected bidentate benzimidazole derivative as an ancillary ligand, coded as GS7. The complex was characterized using H-1 NMR, FTIR, elemental analysis, UV-vis spectrophotometer, and cyclic voltammetry. We also tested photovoltaic performance of this complex for dye-sensitized solar cell (DSSCs). GS7 when used as a sensitizer for DSSCs with iodine triiodide electrolyte, showed a J(sc) of 15.25 mA cm(-2), a V-oc of 0.576 V, a FF of 0.691 and overall power conversion efficiency of (eta) 6.07%. (C) 2014 Published by Elsevier B.V.
No preview · Article · Jan 2015 · Inorganic Chemistry Communications
[Show abstract][Hide abstract] ABSTRACT: Keeping in view to suggest one more class of molecules in order to make a choice and assessment of exchange-correlation (XC) functionals, symmetrical squarylium dye (SQ) derivatives have been considered and Le Bahers's diagnostic indexes have been applied to study the electronic transition character of these molecular systems through TD-DFT and SAC-CI methods. Unlike calculated absorption using SAC-CI, the TD-DFT results are not matching with experimental absorption data. However, the diagnostic indexes obtained with TD-DFT and SAC-CI are apparently similar for all methods. This indicates that care should be taken while choosing XC functionals and assessing the nature of electronic transitions of a specific class of molecules. The centroids of charges associated with the density increase and depletion regions are localized on central C4 ring and carbonyl groups and with a small extension up to side aromatic substitution of the SQ dye derivative. Hence the electronic transition occurring in this class of molecules is confined mainly within the central part of molecule. This is in contrast to Donor-Acceptor-Donor type structure in which charge depletion region is expected at side aromatic substitution of the molecule. The small values of calculated transferred charge (qCT) upon excitation lend support to the theory that the electronic transition in this class of molecules is not CT excitation. This is in agreement with our earlier finding that biradicaloid character and orbital interactions are playing key role in their NIR absorption. Hence highly correlated, single reference and multi-determinant SAC-CI method is able to explain the nature of electronic excitations in these molecules rather than TD-DFT with various type of XC functionals.
[Show abstract][Hide abstract] ABSTRACT: Two new Ru(II) complexes bearing with substituted dipyrrinato ligands (GS3 and GS10) have been successfully synthesized and characterized. These sensitizers exhibit panchromatic light harvesting properties, rendering them promising for DSSC applications. GS3 has shown high overall power conversion efficiency (η = 2.79%) compared to GS10 which is the highest so far in the case of Ru (II) dipyrrinato dyes. Density functional theory (DFT) and time-dependent DFT calculations were measured for GS3 and GS10 to understand their structural, electronic and photophysical properties.
No preview · Article · Jan 2015 · Electrochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Palmierite Ba3V2O8 and related Cu- and N-doped congeners were prepared by sol-gel, ion-exchange, and solid-state reactions, respectively, with the aim of tailoring their optical, electronic, and photocatalytic properties. The catalysts were characterized by XRD analyis, UV/Vis diffuse reflectance spectroscopy, SEM with energy-dispersive X-ray spectroscopy, photoluminence spectroscopy, BET measurements, thermogravimetric analysis, X-ray photoelectron spectroscopy, and TEM analysis. The electronic structures of Cu- and N-doped optimal compositions were calculated theoretically on the basis of density functional theory. A series of photocatalytic degradation experiments of methylene blue under visible light irradiation disclosed the optimal dopant concentrations of Cu and N into Ba3V2O8, and the photocatalytic ability of the optimal compositions was also tested by the degradation of aniline. The differences in the degradation performance of Cu- and N-doped samples are discussed on the basis of inextricably linked parameters such as photoabsorption, electronic structure, and mobility of the photoinduced charge carriers.
No preview · Article · Nov 2014 · Berichte der deutschen chemischen Gesellschaft
[Show abstract][Hide abstract] ABSTRACT: Molecular first hyperpolarizabilities (nonlinear optical responses) of selected s-triazine based heteroaromatic molecules are determined using experimental methods. A large enhancement in nonlinear optical response, in spite of a relatively weak donor–acceptor system, is observed. We have carried out a detailed analysis using computational chemistry techniques to account for this behavior.
No preview · Article · Oct 2014 · Journal of Molecular Structure
[Show abstract][Hide abstract] ABSTRACT: Four new organic dyes, coded as CSORG6, CSORG7, CSORG8 and CSORG9, comprising electron rich thiophene derivatives as antennas and cyanoacrylic acid as acceptor, bridged by phenothiazine or phenoxazine were designed and synthesized for dye-sensitized solar cells applications. They were fully characterized with their photophysical, electrochemical properties, density functional theory (DFT), time dependent density DFT (TDDFT) and the light-harvesting properties of the new sensitizers were evaluated with nanocrystalline TiO2-based devices. Among the four diheteroanthracene based dyes, phenoxazine (CSORG7 and CSORG9) based devices afforded the best photovoltaic performance (eta) of 6% under standard AM 1.5G solar irradiation, whereas, N719 showed (eta) 6.4% under the similar fabrication and evolution conditions.
[Show abstract][Hide abstract] ABSTRACT: In an attempt to develop small organic molecules with potential applications as donors in organic photovoltaic (OPV) devices, we have synthesized and characterized four novel benzothiadiazole (A) core structured D-π-A-π-D dyes featuring carbazole and benzocarbazole as donors (D) and fluorene and thiophene as spacers (π). The effects of the π-spacer units and variations in donor strength on the photophysical, electrochemical and thermal properties of the molecules have been investigated in detail. The replacement of fluorene by thiophene as a π-spacer promotes planarity, resulting in a larger bathochromic absorption shift, enhanced emission profiles and an enhanced intramolecular charge transfer (ICT) transition. The introduction of the benzocarbazole unit creates a low-lying HOMO level, as inferred from cyclic voltammetry studies. All the dyes exhibit remarkable thermal robustness. Theoretical calculations have been carried out to understand the structure–property relationships of the synthesized materials. The results obtained from the characterization methods reveal that the dyes with thiophene π-spacers show better optoelectronic properties compared to their fluorene counterparts. Solution-processable bulk-heterojunction devices with a structure of ITO/PEDOT:PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm) were fabricated using the materials investigated in this study as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC61BM) as an acceptor. A power conversion efficiency of 1.62% for the molecule with thiophene as a spacer and carbazole as donor/PC61BM was achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm−2).
[Show abstract][Hide abstract] ABSTRACT: We report the improvement of the light-harvesting property in the bulk heterojunction organic polymer solar cell based on poly(3-hexylthiophene) (P3HT) and PC70BM, with the incorporation of a nearinfrared absorbing squaraine (SQ) dye, bis[4-(2,6-di-tert-butyl)vinylpyrylium] squaraine (TBU-SQ). With the incorporation of TBU-SQ dye (2.5% by wt) in a P3HT:PC70BM (1:1 wt ratio) blend, the power conversion efficiency (PCE) has been enhanced up to 4.55% as compared to 3.47% for the device based on the P3HT:PC70BM binary blend. The
improvement in the photovoltaic performance with the incorporation of TBU-SQ attributed to the improvement in the light-harvesting efficiency in the near-infrared region of solar spectrum and increased exciton dissociation into free charge carriers in the ternary blended film. The PCE has been further enhanced to 5.15% when the thermally annealed P3HT:TBUSQ:PC70BM blend was used as the photoactive layer. It was observed that the absorption profile of the active layer was broadened upon thermal treatment as a result of the red shift as well as widening of the P3HT absorption band and the slight red shift of the TBU-SQ absorption peak in the blended film. The improved light-harvesting property of the thermally annealed film
and balanced charge transport in the device were attributed to the improvement in the PCE. These results show that TBU-SQ is a promising molecular sensitizer for increasing the PCE of P3HT:PC70BM-based polymer solar cells.
[Show abstract][Hide abstract] ABSTRACT: A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H-2[Porph] (ZnP-H2P), where Zn[Porph] and H-2[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycinemoiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO2 electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H2P/DC) and D (DC/ZnP-H2P). The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H2P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J(sc) = 11.72 mA/cm(2)), open circuit voltage (V-oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J-V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J(sc) = 13.45 mA/cm(2), V-oc = 0.76V, and FF = 0.75), when a formic acid treated TiO2 ZnP-H2P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J(sc) value (= 13.45 mA/cm(2)), which resulted from an increased dye loading, and an enhanced V-oc value (= 0.76 V), attributed to an upward shift and increased of electron density in the TiO2 CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO2 photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (tau(d)), and longer electron lifetime (tau(e)).
[Show abstract][Hide abstract] ABSTRACT: In this work, we present a detailed analysis on electron transport studies of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene (OXD-PH, OXD-PTOL and OXD-OTOL). The effect of methyl substitution at ortho (OXD-OTOL) and para position (OXD-PTOL) on the phenyl ring on the electron transport properties was studied and the results were compared with the anthracene derivative without any substitution at the phenyl ring. Electron transport was found to be highly dependent on the methyl substitution and electron mobilities in OXD-PTOL and OXD-OTOL were found to be lower than in OXD-PH. Mobilities were also found to be different for OXD-PTOL and OXD-OTOL, which indicates that the substitution at different places did not have a similar effect on charge transport properties. Thickness dependent trap states were observed for all three molecules with thickness dependent electron mobilities. Electron mobility was found to increase in all three molecules with the decrease in thickness, which favors their use for organic electronic devices and all three molecules had a better electron transport in comparison to Alq3. These results were explained by the DFT calculation which showed a dihedral structure. The dihedral angle was found to reduce in the anionic form of these molecules. Therefore, these molecules are likely to favor a proper stacking in the solid state form.
[Show abstract][Hide abstract] ABSTRACT: We have designed and synthesized four new metal free D–A–π–A type dyes (9–12) with variations in their acceptor/anchor groups. The four dyes carry tert-butyl substituted triphenylamine as donor, thiadiazole as acceptor and bithiophene as π-spacer. Cyanoacetic acid, rhodanine-3-acetic acid, 2-(4-methoxyphenyl)acetic acid and 2-phenylacetic acid are used as acceptor/anchor groups, respectively in the dyes 9–12. The acceptor/anchor effect on their photophysical, electrochemical and photovoltaic properties was investigated. The dyes exhibited good power conversion efficiency ranging from 1.95–4.12%. Among the four dyes, 9 showed the best photovoltaic performance: short-circuit current density (Jsc) of 8.50 mA cm−2, open-circuit voltage (Voc) of 645 mV and fill factor (FF) of 0.75, corresponding to an overall conversion efficiency of 4.12% under standard global AM 1.5 solar light conditions.