K. Bhanuprakash

Photophysical, electrochemical and photovoltaic properties of dye sensitized solar cells using a series of pyridyl functionalized porphyrin dyes

Three porphyrin dyes, P1, P2 and P3, bearing one, two and four pyridyl groups, respectively, in the meso positions, acting as electron acceptor anchoring groups, were synthesized, characterized and investigated as sensitizers for the fabrication of dye sensitized solar cells (DSSCs). The overall power conversion efficiencies (PCEs) of DSSCs based on these dyes lay in the range 2.46–3.9% using a 12 μm thick TiO2 photoanode. Porphyrin P2 achieved the maximum performance, which can be rationalized by the high dye loading, efficient electron injection, dye regeneration process and longer electron lifetime, as demonstrated by the electrochemical impedance spectroscopy (EIS) measurements. The PCE of the DSSC based on the P2 sensitizer when the photoanode was treated with formic acid, showed an enhanced efficiency of 5.23%. This improvement, attributed to multifunctional properties such as higher dye uptake, reduced recombination process and enhanced charge collection efficiency. Deoxycholic acid (DCA) was also used as a coadsorbent in order to prevent dye aggregation and it was found that the PCE improved up to 6.12% for sensitizer P2 and the modified TiO2 photoanode, which can be attributed to further improvement in the electron injection efficiency and charge collection efficiency.

Phosphine oxide functionalized pyrenes as efficient blue light emitting multifunctional materials for organic light emitting diodes

In a search for blue light emitting multifunctional materials, the electron transport enhancing diphenyl phosphine–oxide (Ph2PO) group has been appended to blue light emitting pyrene derivatives. This design, we observe, leads to highly efficient electron transporting blue-emitters for non-doped organic light emitting devices (OLEDs) with good film formation characteristics. The superior performance is attributed to enhanced charge transport and formation of pyrene excimers assisted by thermally activated delayed fluorescence (TADF) in the device. We report the synthesis and characterization using experimental and computational methods of six such pyrene derivatives. Although three of these derivatives show quenching of luminescence in solvents at higher concentrations, in the thin film invariably all six of them exhibit typical pyrene excimer emission. X-ray crystal analysis reveals π–π stacking and the C–H⋯O interactions in the solid due to the PO group. The measured electron mobilities for all the compounds are higher in comparison to the standard electron transport material, Alq3. Non-doped OLEDs with the pyrene derivatives as emitters (multi-layer configuration) as well as electron transport cum emitters (bilayer configuration) exhibit excellent efficiencies. The derivatives as emitters display a performance with current efficiencies (ηc) in the range 21.1–30.1 cd A−1, power efficiencies (ηp) 11.0–15.76 lm W−1, external quantum efficiencies (EQE) 7.2–9.1% and brightness 28 500–42 750 cd m−2. In addition, the derivatives as electron transporting emitters demonstrate very good external quantum efficiencies in the range of 3.0–4.0%. These results demonstrate a successful strategy to obtain blue light emitting multifunctional materials for OLED applications.

Synthesis and characterization of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene: Efficient n-type emitter for organic light-emitting diodes

With a general aim to make anthracene derivatives multifunctional (n-type emitter) and also study their suitability as electron transport layers for organic light emitting diodes (OLED), and with a more specific interest to understand the charge transport and packing pattern in the solid state due to the rotating side rings, we report the synthesis and characterization of six novel molecules (5–10) in which the 9 and 10 positions of anthracene have been directly substituted by phenyloxadiazole groups. We have carried out detailed studies of these molecules including photophysical, electrochemical, electroluminescent studies and solid state structure determination through crystallographic techniques. The electron affinity is very high, around 3.1–3.2 eV, and the ionization potential is around 5.9–6.0 eV, comparable to the more commonly used electron transport electroluminescent layer Alq3. The studies reveal that the new molecules being reported by us, in addition to the high thermal stability, are quite efficient in a two layer unoptimized device with the device structure ITO/α-NPD/5–10/LiF/Al and have an emission in pure green. They also show very high efficiency as electron transport layer in device structure ITO(120nm)/α-NPD(30nm)/Ir(ppy)3 doped CBP(35nm)/BCP(6nm)/5(28nm)/Al(150nm). From these studies we conclude that the anthracene derivatives also have considerable potential as multifunctional layers and as electron transport layers in OLED.

Pyrene–Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials

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 make them efficient electron transporters. Isoenergetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in the absence of thermal activation.

β-Sugar aminoxy peptides as rigid secondary structural scaffolds

Short homo-oligomers of a new building block, cis-beta(2,3)-furanoid sugar aminoxy acid, are designed, characterized, and found to exhibit rigid ribbon-like secondary structures composed of 5/7 bifurcated intramolecular hydrogen bonds.

Cyclometalated Iridium(III) Complexes Containing Hydroxide/Chloride Ligands: Isolation of Heterobridged Dinuclear Iridium(III) Compounds Containing μ-OH and μ-Pyrazole Ligands

The reaction of the cyclometalated chloro-bridged iridium(III) dimers [(ppy)(2) Ir(μ-Cl)](2) (ppyH = 2-phenyl pyridine) and [(tpy)(2)Ir(μ-Cl)](2) (tpyH = 2-p-tolylpyridine) with 3,5-diphenylpyrazole (Ph(2)PzH) in the presence of sodium methoxide resulted in the formation of heterobridged dimers [(ppy)(2)Ir(μ-OH)(μ-Ph(2)Pz)Ir(ppy)(2)] (1) and [(tpy)(2)Ir(μ-OH)(μ-Ph(2)Pz)Ir(tpy)(2)] (2). Interestingly, the reaction of [(ppy)(2)Ir(μ-Cl)](2) with 3(5)-methyl-5(3)-phenylpyrazole (PhMePzH) afforded both a heterobridged dimer, [(ppy)(2)Ir(μ-OH)(μ-PhMePz)Ir(ppy)(2)] (3), and the monomer [(ppy)(2)Ir(PhMePz)Cl] (4). The compound [(ppy)(2)Ir(PhMePz)OH] (5) containing a terminal OH was obtained in a hydrolysis reaction involving 4, sodium methoxide, and PhMePzH. Complexes 1-5 were characterized by X-ray crystallography and electrospray ionization high-resolution mass spectrometry. All of the complexes are luminescent at room temperature in their dichloromethane solutions. The luminescence of the dinuclear complexes is characterized by a single structureless band centered at λ(max) = 550 nm (1 and 3) and 546 nm (2). The emission spectra of the mononuclear complexes 4 and 5 display vibronic structures with their λ(max) values at 497 nm (4) and 513 nm (5). In each case, the main emission bands are accompanied by shoulder bands at 526 (4) and 534 nm (5). The quantum yields, calculated with reference to [Ir(ppy)(2)(bpy)]PF(6) (Φ(CH(3)CN) = 0.0622), range from 0.11 to 0.17 for the dinuclear complexes and 0.045 to 0.048 for the mononuclear complexes. The lifetimes of the emission are in the microsecond region, suggesting the phosphorescent nature of the emission. Density functional theory (DFT) and time-dependent DFT calculations were performed on complexes 1 and 4 in the ground state to gain insight into the structural, electronic, and photophysical properties. Electrochemical studies on complexes 1-3 showed the presence of two consecutive one-electron-oxidation processes, assigned as the stepwise oxidation of the two Ir(III) centers, i.e., Ir(III)-Ir(III)/Ir(III)-Ir(IV) and Ir(III)-Ir(IV)/Ir(IV)-Ir(IV) couples, respectively. The monomers displayed single-oxidation peaks. No reduction process was observed within the solvent cathodic potential limit.

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.

NLO Activity in some non-conjugated 3D triazine derivatives: a non-centrosymmetric crystal through conformational flexibility

Four derivatives of 2,4,6-tris(benzyloxy)-1,3,5-triazine are synthesized and detailed computational and non-linear optical investigations are carried out. Computations indicate four conformations with different energies in all the systems in the gas phase; and the individual dipole moments are also of different magnitude. HRS measurements of these molecules in solution reveal moderately large β values; structure–property relations are analyzed through computations. These molecules have one added advantage of nearly 100% optical transmission through the visible range, due to the non-conjugated structure. Molecule 1 [2,4,6-tris(benzyloxy)-1,3,5-triazine] crystallizes in a non-centrosymmetric space group by adopting the conformation with the lowest dipole moment but not the lowest energy. It also shows SHG activity in the solid state.

Theoretical studies on the non-linear optical properties of some organic molecules: effect of π–σ–π through-bond coupling on the first hyperpolarisability

Abstract First hyperpolarisabilities (β) are calculated by means of semi-empirical molecular orbital methods for a series of organic molecules containing a π-electron donor and a π-electron acceptor separated by a saturated C–C σ bond. The π orbitals of the donor and the acceptor interact through the σ skeleton (π–σ–π through-bond coupling), giving rise to intramolecular charger transfer in the excited states. In the case of zwitterions (D−–π–C–C–π–A+) very large hyperpolarisabilities and dipole moments are obtained. These studies suggest that this structural motif may have potential applications in the development of non-linear optical materials.

First hyperpolarizability of some nonconjugated donor–acceptor 3D molecules: noncentrosymmetric crystal through conformational flexibility

Four derivatives of 2,4,6-tris(benzylamino)-1,3,5-triazine are synthesized and detailed computational and nonlinear optical investigations are carried out. Computations indicate near degenerate conformations (C3 and C1 symmetry) in all systems in the gas phase; the dipole moments are also of similar magnitude in all of them except molecule 4 [2,4,6-tris(4-fluorobenzylamino)-1,3,5-triazine]. HRS measurements of these molecules in solution reveal moderately large β values; structure–property relations are analyzed through computations. These molecules have one added advantage of nearly 100% optical transmission through the visible range, due to the nonconjugated structure. Molecule 4 crystallizes in a noncentrosymmetric structure by adopting the conformation with the lower dipole moment; it also shows optical SHG in the solid state.