Solution-based synthesis of crystalline silicon from liquid silane through laser and chemical annealing.

We report a solution process for the synthesis of crystalline silicon from the liquid silane precursor cyclohexasilane (Si(6)H(12)). Polysilane films were crystallized through thermal and laser annealing, with plasma hydrogenation at atmospheric pressure generating further structural changes in the films. The evolution from amorphous to microcrystalline is characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and impedance spectroscopy. A four-decade enhancement in the electrical conductivity is attributed to a disorder-order transition in a bonded Si network. Our results demonstrate a potentially attractive approach that employs a solution process coupled with ambient postprocessing to produce crystalline silicon thin films.

Nano-engineering lattice dimensionality for a soft matter organic functional material.

A high performing electro-optic (EO) chromophore with covalently attached coumarin-based pendant groups exhibits intermolecular correlation of coumarin units through molecular dynamics (MD) simulations. Unique, orthogonal molecular orientations of the chromophore and coumarin units are also evident when investigated optically. Such molecular orientation translates to reduced lattice dimensionality of the bulk C1 soft matter material system, leading to increased acentric order and EO activity. Results are corroborated by nanorheological experimental methods.

Rational enhancement of second-order nonlinearity: bis-(4-methoxyphenyl)hetero-aryl-amino donor-based chromophores: design, synthesis, and electrooptic activity.

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborator's, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

Structure-property relationships in conjugated donor-acceptor molecules based on cyanoanthracene: computational and experimental studies.

[reaction: see text] Two series of pi-conjugated bipolar compounds, namely, 9-phenyl-10-anthronitriles (PAN series) and 9-phenylethynyl-10-anthronitriles (PEAN series), having inherent redox centers have been synthesized and their electronic absorption, fluorescence emission, and electrochemical behavior have been studied. Electrochemiluminescence of these molecules bearing weak, strong, and spin-polarized donors is also studied. The observed electronic properties are explained with the help of results obtained from density functional theory (DFT- B3LYP/6-31G) calculations. The structure-property relationships of all the molecules are discussed.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage.

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.

Synthesis, electronic properties, and electrochemiluminescence of donor-substituted phenylethynylanthronitriles.

A new series of pi-conjugated donor-acceptor compounds (1-6) with inherent redox centers have been prepared and studied with respect to their electronic properties. The photophysical characteristics of these compounds have been studied in relation to their structures. Cyclic voltammetry and UV-vis spectroelectrochemistry were used to probe the ground-state electronic properties of the neutral and charged species. The observed electronic absorption properties of the neutral and charged molecules are explained with the help of frontier orbital structures and electrostatic potential maps obtained from density functional theory (DFT, B3LYP/6-31G) calculations. Electrochemiluminescence (ECL) of this series of donor-substituted phenylethynylanthronitriles with different donors was also studied. The structure-property relationship of all of the compounds is discussed.

Donor-substituted phenyl-pi-chromones: electrochemiluminescence and intriguing electronic properties.

Phenylethynylchromones bearing different donor groups at the phenyl moiety have been prepared and their photophysical and electrogenerated chemiluminescence (ECL) properties have been studied with respect to their structural features. Intriguingly, the presence and variation of donor groups do not much influence the absorption spectra, which can be compared with the spectrum of unsubstituted chromone, whereas the photoluminescence (PL) spectra show pronounced changes. Density functional theory (DFT) calculations indicate enhancement of HOMO energy levels upon increasing the donor strength. The photophysical properties have also been studied in various solvents, and the PL spectra in particular show the anticipated trend. The introduction of pi-extension imparts ECL to the new molecules and the electronic coupling between the donor and the acceptor moieties through C-C triple bond influences ECL emission maxima. Weaker donors impart excimer ECL while stronger donors impart monomeric intramolecular charge transfer (ICT) ECL.

Highly fluorescent N,N-dimethylaminophenylethynylarenes: synthesis, photophysical properties, and electrochemiluminescence.

A new family of aryl-pi-donor-aryl molecules has been synthesized and studied with respect to their photophysical properties and electrogenerated chemiluminscence (ECL) for the first time. Anthracene, phenanthrene, naphthalene, biphenyl, and fluorene were coupled with N,N-dimethylanilino moiety via a C-C triple bond (1-7). Introduction of such a strong electron-donating moiety as N,N-dimethylanilino group through a triple bond imparts new properties to the resultant molecules that are not commonly observed for the parent arenes. All molecules show absorption in the near-visible region and emission totally in the visible region with high fluorescence quantum yields. Bright solid-state photoluminescence has also been noticed for all the compounds in the visible region. 9-Anthryl- and 1-naphthyl- derivatives exhibited blue-shifted electrochemiluminescence (ECL) relative to their photoluminescence because of aggregation. 9-Phenanthryl- and 2-naphthyl- derivatives did not show ECL. 2-Biphenyl derivative showed monomeric ECL while 4-biphenyl counterpart exhibited excimer ECL. No ECL was observed for 2-fluorenyl derivative. The observed electronic properties are discussed with regard to the structure of the molecules. The characteristics of the molecules chosen in the present study open up new prospects and promises for novel tunable organic materials, on the basis of simple extension of conjugation to promote intramolecular communication, for ECL, OLED, and other optoelectronic applications.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent substituted quinolines with donor-acceptor character.

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes and singlet excitation energies of PnQ. The formation of the intramolecular charge transfer state is assumed for PnQ in the singlet excited state with a strong electron donating substituent. The emission from PnQ is suggested to originate from PnQ in the singlet excited state formed from the charge recombination between the PnQ radical cation and the PnQ radical anion during the pulse radiolysis. This is strong evidence for the efficient electrogenerated chemiluminescence of PnQ.

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes.

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylethynylcoumarins.

Two series of donor-bearing phenylethynylcoumarins have been synthesized, and their photophysical properties have been evaluated. Chemiluminescence was observed through the annihilation of their electrogenerated radical ions and was found to be only slightly affected by the presence of various donor groups on the phenyl moiety linked through the C-C triple bond. The overall properties of the two series of compounds are discussed with respect to their structures. The observed electronic absorption properties are explained with the help of computational studies.

Arylethynylacridines: electrochemiluminescence and photophysical properties.

Electrogenerated chemiluminescence (ECL) of six new ethyne-based acridine derivatives (1-6) has been studied. The new acridine derivatives were synthesized by cross-coupling of 9-chloroacridine and corresponding donor-substituted phenylethynes under modified Sonogashira conditions. The donor groups were varied in the order of increasing steric hindrance and donor strength at the donor site. The solution phase photophysical properties and ECL of these compounds were studied comparatively in acetonitrile solvent. The UV-Visible spectra of compounds 1-5 exhibit closely the same maxima. Density functional theory (DFT) has been invoked to analyze and understand the unexpected UV-Visible absorption behavior. Compounds with weak electron donors produce excimer ECL irrespective of steric hindrance at the donor site, while the compound with a stronger donor gives rise to ECL that is blue-shifted with respect to its photoluminescence spectrum. All except one of these compounds also exhibit solid state fluorescence which may be useful for solid state devices such as organic light emitting diodes (OLEDs) and as laser dyes. The observed properties are discussed with reference to the structure of the compounds synthesized.

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylquinolinylethynes and phenylisoquinolinylethynes: effect of positional isomerism.

In furtherance of our research on the design, synthesis and study of electrogenerated chemiluminescence (ECL) of new donor substituted phenylquinolinylethynes, we report here more new series with the aim of studying the effect of positional isomerism on their overall photophysical properties with a special focus on ECL. For this study we have chosen 2-, 3-, and 4-(p-substituted phenyl)ethynylquinolines, and 1- and 4-(p-substituted phenyl)ethynylisoquinolines. These ethynes were synthesized in good yields by modified Sonogashira coupling of the corresponding terminal alkyne with the respective haloquinolines. The photophysical properties and ECL were studied in acetonitrile solvent and the various results are discussed.

First synthesis and electrogenerated chemiluminescence of novel p-substituted phenyl-2-quinolinylethynes.

A series of p-substituted phenyl-2-quinolinylethynes as blue-green emitters were synthesized using a modified Sonogashira coupling reaction and their electrogenerated chemiluminescence properties are reported.

Sonogashira coupling reaction with diminished homocoupling.

[reaction: see text] The side product from homocoupling reaction of two terminal acetylenes in the Sonogashira reaction can be reduced to about 2% using an atmosphere of hydrogen gas diluted with nitrogen or argon. Terminal arylethynes, diarylethynes, and a few new arylpyridylethynes with donor substituents have been synthesized in very good yields. Comparative control experiments suggest that the homocoupling yield is determined by concentration of both catalyst and oxygen.