The synthesis of novel,visible-wavelength,oxidizable polymerization sensitizers based on the 8-halogeno-5,12-dihydroquinoxalino[2,3-b]quinoxaline skeleton

Novel dyes, based on the 8-halogeno-5,12-dihydroquinoxalino[2,3-b]quinoxaline skeleton, were synthesized and characterized using ¹H NMR spectroscopy and chemical ionization mass spectroscopy. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence and quantum yield of singlet oxygen generation, were also measured. These dyes were used as oxidizable sensitizers for diphenyliodonium and N-alkoxypyridinium salts. Photoredox pairs, consisting of dyes and pyridinium or iodonium salts, were found to be effective visible-wavelength initiators of free radical or cationic polymerization, respectively. The ability of each dye to act as a photoinitiator strongly depended upon its chemical structure. The heavy atoms present in the chemical structure could lead to excited triplet states within the dye, thereby facilitating electron transfer from these states.     

Fluorene-based organic dyes containing acetylene linkage for dye-sensitized solar cells

New organic dyes based on diphenylaminofluorene donors, cyanoacrylic acid acceptors and either ethynylbenzene or ethynylthiophene π-spacers have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The dye with thiophene in the conjugation pathway exhibited longer wavelength absorption due to the significant lowering of the LUMO level when compared to the phenyl analog. However, the dye with the phenylacetylene linker displayed promising DSSC characteristics such as short circuit current, open circuit voltage and fill factor indicative of efficient charge generation and injection. The solvatochromic behavior of the dyes were examined in solvents of different polarity and found to exhibit negative solvatochromism of the fluorescence emission suggestive of a nonpolar solvent stabilized excited state with a significant structural reorganization. The TDDFT computations were used to explain the optical properties of the dyes.     

Bandgap and electronic structure of CaSiN2: Experiment and theory

We interrogate the electronic properties of the nitridosilicate CaSiN₂ as determined using soft X-ray absorption and emission spectroscopy, which probe directly the partial electronic density of states. We compare our measurements to full potential, all electron density functional theory (DFT) calculations. Good agreement between experiment and theory is obtained, and the electronic bandgap of CaSiN₂ is determined to be 3.95 ± 0.3 eV, in contrast with our predicted value of 4.78 eV that was calculated using the modified Becke–Johnson exchange-correlation functional with the DFT framework.     

Synthesis of novel oxidizable polymerization sensitizers based on the dithiinoquinoxaline skeleton

Novel dyes, based on the dithiinoquinoxaline skeleton, were synthesized and characterized using ¹H NMR spectroscopy and chemical ionization mass spectroscopy. Their spectral properties, such as absorption, emission spectra and quantum yield of fluorescence, were also measured. Electron donating properties of the title compounds were estimated on the basis of DFT calculations. The studied dyes were used as oxidizable sensitizers for 2,4,6-tris(trichloromethyl)-1,3,5-triazine (Tz). The dye/Tz photoredox pairs were found to be effective visible-wavelength initiators of free radical polymerization. The ability of these systems to act as photoinitiators strongly depended upon the free energy change of the photoinduced electron transfer from the excited dyes to Tz. It has been shown that the intermolecular electron transfer is the limiting step in the photopolymerization initiated by these studied initiator systems.     

DFT Study of the Molecular and Electronic Structure of Metal-Free Tetrabenzoporphyrin and Its Metal Complexes with Zn,Cd, Al,Ga, In

The electronic and molecular structures of metal-free tetrabenzoporphyrin (H₂TBP) and its complexes with zinc, cadmium, aluminum, gallium and indium were investigated by density functional theory (DFT) calculations with a def2-TZVP basis set. A geometrical structure of ZnTBP and CdTBP was found to possess D_4h symmetry; AlClTBP, GaClTBP and InClTBP were non-planar complexes with C_4v symmetry. The molecular structure of H₂TBP belonged to the point symmetry group of D_2h. According to the results of the natural bond orbital (NBO) analysis, the M-N bonds had a substantial ionic character in the cases of the Zn(II) and Cd(II) complexes, with a noticeably increased covalent contribution for Al(III), Ga(III) and In(III) complexes with an axial –Cl ligand. The lowest excited states were computed with the use of time-dependent density functional theory (TDDFT) calculations. The model electronic absorption spectra indicated a weak influence of the nature of the metal on the Q-band position.     

Electronic structure and spectral properties of the triarylamine-dithienosilole dyes for efficient organic solar cells

The recently synthesized high-performance triarylamine dyes with the dithienosilole π-conjugated spacer for efficient organic solar cells are calculated at the density functional theory (DFT) level with the Bader approach for the quantum theory of atoms in molecule (QTAIM) analysis. The presence of stabilizing intramolecular hydrogen bonds and Van der Waals interactions in the dye molecules is predicted and the energies of these interactions are estimated. The electronic bands nature in absorption spectra of the dyes is determined by the time-dependent DFT calculations with a linear response methodology using B3LYP and BMK hybrid functionals. Relations between incident light absorption intensity in the first long-wavelength band of the dye, its polarization, HOMO-LUMO orbital nature and the driving force of electron injection to the semiconductor are discussed.     

Passivating ligand and solvent contributions to the electronic properties of semiconductor nanocrystals

We examine in detail the impact of passivating ligands (i.e., amines, phosphines, phosphine oxides and pyridines) on the electronic and optical spectra of Cd(33)Se(33) quantum dots (QDs) using density functional theory (DFT) and time-dependent DFT (TDDFT) quantum-chemical methodologies. Most ligand orbitals are found deep inside in the valence and conduction bands of the QD, with pyridine being an exception by introducing new states close to the conduction band edge. Importantly, all ligands contribute states which are highly delocalized over both the QD surface and ligands, forming hybridized orbitals rather than ligand-localized trap states. In contrast, the states close to the band gap are delocalized over the QD atoms only and define the lower energy absorption spectra. The random detachment of one of ligands from the QD surface results in the appearance of a highly localized unoccupied state inside the energy gap of the QD. Such changes in the electronic structure are correlated with the respective QD-ligand binding energy and steric ligand-ligand interactions. Polar solvent significantly reduces both effects leading to delocalization and stabilization of the surface states. Thus, trap and surface states are substantially eliminated by the solvent. Polar solvent also blue-shifts (e.g., 0.3-0.4 eV in acetonitrile) the calculated absorption spectra. This shift increases with an increase of the dielectric constant of the solvent. We also found that the approximate single-particle Kohn-Sham (KS) approach is adequate for calculating the absorption spectra of the ligated QDs. Besides a systematic blue-shift, the KS spectra are in very good agreement with their respective counterparts calculated with the more accurate TDDFT method.     

Photophysical properties of arylcarbonitrile derivatives: Synthesis, absorption and emission spectra, and quantum chemical studies

A new series of aromatic cyanovinyl compounds were synthesized via one-pot reactions of tri- or tetracyanoethylenes with nucleophilic reagents. The ground-state geometries and UV-vis absorption spectra of the compounds were computationally analyzed by means of density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, respectively. None of the compounds were fluorescent in solution, but some showed intense emission in the solid state. The first excited singlet states (S₁) potential energy surfaces (PESs) were explored using complete active space SCF (CASSCF) calculations for the compounds in order to elucidate nonradiative decay mechanism that takes into account the involvement of conical intersections (CI).     

Pore size dependence of self-assembled type photonic crystal on dye-sensitized solar cells efficiency utilising Chlorine e6

There are few reports on photoelectric conversion efficiency using naturally-occurring dyes for dye-sensitized solar cells (DSSC). This is because the matching with an excited electronic level of naturally-occurring dye to the conduction band of semiconductor is problematic; the excited electrons are easily relaxed to the steady state with fluorescence or heat emission. We examined the fluorescence inhibition effect of a self-assembled photonic crystal using Chlorine e6 dye. Chlorine e6 is derived from chlorophyll and has a long excited electron lifetime. We prepared TiO₂ inverse opals with various particle sizes by liquid phase deposition and described their effect on DSSCs with regard to structural, optical and electrochemical properties. In addition, we explored the implications of fluorescence lifetime measurements relative to the photonic band diagram and the amount of adsorbed dye. Although the main factor affecting the external photoelectric conversion efficiency was the diffusion resistance of the electrolyte and the contact resistance between TiO₂ interfaces, the possibility that the dye fluorescence lifetime, i.e. the photonic band structure, can affect the internal quantum efficiency per one dye molecule was also investigated.     

Enhancement of the photoelectric performance of dye-sensitized solar cells using Ag-doped TiO<Subscript>2</Subscript> nanofibers in a TiO<Subscript>2</Subscript> film as electrode

For high solar conversion efficiency of dye-sensitized solar cells [DSSCs], TiO₂ nanofiber [TN] and Ag-doped TiO₂ nanofiber [ATN] have been extended to be included in TiO₂ films to increase the amount of dye loading for a higher short-circuit current. The ATN was used on affected DSSCs to increase the open circuit voltage. This process had enhanced the exit in dye molecules which were rapidly split into electrons, and the DSSCs with ATN stop the recombination of the electronic process. The conversion efficiency of TiO₂ photoelectrode-based DSSCs was 4.74%; it was increased to 6.13% after adding 5 wt.% ATN into TiO₂ films. The electron lifetime of DSSCs with ATN increased from 0.29 to 0.34 s and that electron recombination was reduced.     

Theoretical investigation on the ground- and excited-state properties of novel octupolar oligothiophene-functionalized truxenes and dipolar analogs

The density functional theory (DFT) is used to compute the ground-state geometries, ionization potentials (IPs), electron affinities (EAs) and HOMO-LUMO gaps (ΔE_H-L) of novel octupolar oligothiophene-functionalized truxenes and the corresponding dipolar analogs. The lowest singlet excited-state geometries of some oligomers have been investigated by the singles configuration interaction (CIS) method. The absorption and emission spectra are calculated by time-dependent DFT (TDDFT) on the basis of the ground- and excited-state geometries, respectively. Our calculations are in good agreement with the available experimental results. The calculated results show that all oligomers are nonplanar in their ground electronic states. The structural relaxation upon excitation results in the planarity of dipolar oligomers and one branch of octupolar oligomers. Because fluorene and truxene segments in two series are not the repeated units of oligomers, the chain dependence of IPs, EAs, ΔE_H-L and excitation energies do not follow the usual linear 1/n rule. We have used three plotted points closest to polymer to plot lines; and extrapolated the resultant linear relationship to infinite chain length; and finally obtained the relative properties of polymers.     

A theoretical investigation on the properties of the new poly(N‐vinylcarbazole)‐3‐methylthiophene (PVK‐3MeT) synthesized graft copolymer

In this article, we present quantum chemical calculations, based on density functional theory (DFT), performed to investigate the geometries and the opto‐electronic properties of a new synthesized graft copolymer based on poly(N‐vinylcarbazole) (PVK) and poly(3‐methylthiophene) (PMeT) named PVK‐3MeT. First, we have theoretically computed and compared the structural, optical, and vibrational parameters of both neutral and doped states. In addition, the excited state was theoretically obtained by the ab initio RCIS/STO‐3G method. To assign the absorption and emission peaks observed experimentally, we computed the energies of the lowest singlet excited state with the time‐dependent density functional theory (TD‐DFT) method. Electronic parameters such as the HOMO‐LUMO band gap, the ionization potential (IP), and electron affinity (EA) are extracted. Calculations show that the PVK‐3MeT copolymer is nonplanar in its ground neutral state. Meanwhile, upon doping or photoexcitation, an enhancement of the planarity is observed, resulting on a decrease of the inter‐ring torsion angle between 3‐methylthiophene units. Such modifications in the geometric parameters induce a dramatic change on the HOMO and LUMO orbitals in the doped or excited states. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Phenyltrialkylborates as co-initiators with cyanine dyes in visible light polymerization of acrylates

Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic monomer. The influence of the sensitizers and electron donor structure on the photopolymerization kinetics of multiacrylate monomer was investigated by photo-DSC. It was found that the polymerization rate and the final conversion degree were dependent on both dye and borate structure. The kinetic studies of the free radical polymerization revealed an increase in the polymerization rate with a decrease of the borate oxidation potentials which was additionally reflected by the linear relationship between the Hammett constant and rate of polymerization. The efficiency of these initiators was discussed on the basis of the free energy change for electron transfer from an excited cyanine dye cation to a borate anion. The ΔG_el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one selenocarbocyanine dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoreduction of cyanine phenyltrialkylborate complex was studied using nanosecond laser flash photolysis. The dye triplet was found to be quenched by the electron donors via an electron transfer process. Rate constants (k_q) for the quenching of the excited states were high and approached diffusion-controlled limits and were found to depend on the borate structure.     

The synthesis, spectroscopic and electrochemical properties, and application of new dyeing photoinitiator systems for acrylate monomers polymerization

New symmetrical bicationic polymethine dyes were synthesized and their spectroscopic and electrochemical properties were described. The bichromophoric dyes (benzothiazole, benzoxazole, indolinium derivatives) were investigated as sensitizers in the free radical photopolymerization initiated by their borate salts. The obtained kinetic results shown that bicationic polymethine dyes as the organoborate salts are much efficient photoinitiating systems of acrylate monomers polymerization than monocationic parent dyes. The rate of polymerization depends on ΔG_ET of electron transfer from borate anion to the excited singlet state of bicationic polymethine dye. The relationship between the rate of polymerization and the free energy of electron transfer process shows the dependence predicted by the classical theory of electron transfer.     

Dopant-induced 2D-3D transition in small Au-containing clusters: DFT-global optimisation of 8-atom Au-Ag nanoalloys

A genetic algorithm (GA) coupled with density functional theory (DFT) calculations is used to perform global optimisations for all compositions of 8-atom Au-Ag bimetallic clusters. The performance of this novel GA-DFT approach for bimetallic nanoparticles is tested for structures reported in the literature. New global minimum structures for various compositions are predicted and the 2D-3D transition is located. Results are explained with the aid of an analysis of the electronic density of states. The chemical ordering of the predicted lowest energy isomers are explained via a detailed analysis of the charge separation and mixing energies of the bimetallic clusters. Finally, dielectric properties are computed and the composition and dimensionality dependence of the electronic polarizability and dipole moment is discussed, enabling predictions to be made for future electric beam deflection experiments.     

Synthesis and photovoltaic properties of main chain polymeric metal complexes containing 8‐hydroxyquinoline metal complexes conjugating alkyl fluorene or alkoxy benzene by CN bridge for dye‐sensitized solar cells

Four main chain polymeric metal complexes ( P1–P4 ) based on 8‐hydroxyquinoline metal complexes have been synthesized by the Heck coupling and characterized by gel‐permeation chromatography, Fourier transform infrared spectroscopy, proton‐nuclear magnetic resonance, thermogravimetric analysis, ultraviolet–visible absorption, photoluminescence spectroscopy, elemental analysis, and cyclic voltammetry. The applications of these polymers as dye sensitizers in dye‐sensitized solar cells (DSSCs) are also studied. The study results show that the solar cells have good device performance with power conversion efficiency of P1–P4 up to 2.17, 2.04, 2.45, and 2.18%, respectively. The highest power conversion efficiency of DSSCs is up to 2.45%, for P3 consisting of alkoxy benzene unit and Cd–metal complex under the illumination of air mass 1.5 G, 100 mW cm⁻². POLYM. COMPOS., 34:1629–1639, 2013. © 2013 Society of Plastics Engineers     

pH dependent spectral properties and electronic structure of benzothiazol containing cyanine dyes

The absorption and emission properties of some cyanine dyes over a range of 8 pH units are presented. Theoretical investigations of their electronic structure in the ground and excited states have been undertaken and the nature of the electronic transitions in the protonated and non-protonated forms is rationalised.     

Photocatalytic Activity and Electronic Structure Analysis of N‐doped Anatase TiO2: A Combined Experimental and Theoretical Study

N‐Doped TiO₂ photocatalysts were prepared by a hydrothermal method with tetra‐n‐butyl titanate (TTNB) and triethanolamine as precursors. The obtained samples were characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and UV‐visible diffuse reflectance spectra (DRS), respectively. Photocatalytic activities of the anatase products were investigated on the degradation of methyl orange (MO). The incorporation of nitrogen impurity in anatase TiO₂ was studied by the first‐principles calculations based on the density functional theory (DFT). The calculated electronic band structures for substitutional and interstitial N‐doped TiO₂ indicated the formation of localized states in the band gap, which lied above the valence band. Excitation from the impurity states of N 2p to the conduction band could account for the optical absorption edge shift toward the lower energies. It was consistent with the experimentally observed absorption of N‐doped samples in the visible region.     

Synthesis of ruthenium complex and its application in dye-sensitized solar cells

An amphiphilic bipyridyl ligand, 4,4′-dicarboxy-octyl-2,2′-bipyridine, and its ruthenium(□) complex (termed as S8) were synthesized and characterized by UV/Vis, IR and NMR spectroscopy. The performance of this S8 complex as charge transfer photo-sensitizer in TiO₂-based dye-sensitized solar cells was studied under standard AM 1.5 sunlight and by using an electrolyte consisting of 0.70 M 1,2-dimethyl-3-propyl-imidazolium iodide, 0.10 M LiI, 40 mM iodine and 0.125 M 4-tert-butylpyridine in acetonitrile. Aliphatic chains linking to carboxylate groups of S8 act as an effective electron donor and carboxylate groups act as an effective electron withdrawing between the TiO₂ layer and the carboxylate linking TiO₂ layer leading to increasing of electron density at this interface, which is attributed to increasing efficiency of electron injection to the TiO₂ conduction band from the excited state of dye. The complex, S8, gave a photocurrent density of 13.02 mA/cm², 0.60 V open circuit voltage and 0.69 fill factor yielding 5.36% efficiency. The S8 dye with aliphatic chain improved conversion efficiency of the resulting DSSCs compared with a cell fabricated using the N3 dye.