Structures, electronic states, photoluminescence, and carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles

The excellent electroluminescent (EL) properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), and 1,1,2,3,4,5-hexaphenylsilole (HPS) have been found. Despite some studies devoted to these materials, very little is known about the real origin of their unique EL properties. Therefore, we investigated the structures, photoluminescence (PL), and charge carrier transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles as well as the effect of substituents on these characteristics. The single crystals of the three siloles involving 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), MPPS, and HPS were grown and their crystal structures were determined by X-ray diffraction. Three siloles have nonplanar molecular structures. The substituents at 1,1-positions enhance the steric hindrance and have predominant influence on the twisted degree of phenyl groups at ring carbons. This nonplanar structure reduces the intermolecular interaction and the likelihood of excimer formation, and increases PL efficiency in the solid state. The silole films show high fluorescence quantum yields (75-85%), whereas their dilute solutions exhibit a faint emission. The electronic structures of the three siloles were investigated using quantum chemical calculations. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the silole ring and two phenyl groups at 2,5-positions in all cases, while the LUMOs have a significant orbital density at two exocyclic Si-C bonds. The extremely theoretical studies of luminescent properties were carried out. We calculated the nonradiative decay rate of the first excited state as well as the radiative one. It is found that the faint emission of DMTPS in solutions mainly results from the huge nonradiative decay rate. In solid states, molecular packing can remarkably restrict the intramolecular rotation of the peripheral side phenyl ring, which has a large contribution to the nonradiative transition process. This explains why the 1,1-disubstituted 2,3,4,5-tetraphenylsiloles in the thin films exhibit high fluorescence quantum yields. The charge carrier mobilities of the MPPS and HPS films were measured using a transient EL technique. We obtained a mobility of 2.1 x 10(-)(6) cm(2)/V.s in the MPPS film at an electric field of 1.2 x 10(6) V/cm. This mobility is comparable to that of Alq(3), which is one of the most extensively used electron transport materials in organic light-emitting diodes (LEDs), at the same electric field. The electron mobility of the HPS film is about approximately 1.5 times higher than that of the MPPS film. To the best of our knowledge, this kind of material is one of the most excellent emissive materials that possess both high charge carrier mobility and high PL efficiency in the solid states simultaneously. The excellent EL performances of MPPS and HPS are presumably ascribed to these characteristics.     

Excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles: A quantum chemical characterization

Structure, electronic states, photoluminescence, and carries transport properties of 1,1-disubstituted 2,3,4,5-tetraphenylsiloles for light-emitting diodes were investigated experimentally [G. Yu, et al, J. Am. Chem. Soc. 2005, 127, 6335], and the excellent electroluminescent (EL) properties of them have been found. In this paper, excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles are studied with quantum chemistry method as well as the 3D real space analysis methods. The transition densities of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show that the orientations and strengths of dipole moments the neutral, anionic and cationic ones are significantly different, where the neutral 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show the Frenkel character; while the anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles show the plasmon character. The charge difference densities of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles reveal the important diversity of the orientations and the results of intramolecular charge transfer (ICT). The theoretical results also reveal the contribution of the substituents at the 1,1-position to the neutral and charged excited state properties of 2,3,4,5-tetraphenylsiloles. The calculated results are consistent with the experimental results, and further provide the insight of understanding to the excited state properties of neutral, anionic and cationic 1,1-disubstituted 2,3,4,5-tetraphenylsiloles.     

Thermodynamics of apoplastocyanin folding: comparison between experimental and theoretical results

It has been experimentally shown that the folding of apoplastocyanin (apoPC) accompanies a very large enthalpic loss [N. Baden et al., J. Chem. Phys. 127, 175103 (2007)]. This implies that an even larger entropic gain occurs in stabilizing the folded structure to overcome the enthalpic loss. Here, we calculate the water-entropy gain upon the folding of apoPC using the angle-dependent integral equation theory combined with the multipolar water model and the recently developed morphometric approach. It is demonstrated that the calculated value is in quantitatively good accord with the value estimated from the experimental data by accounting for the conformational-entropy loss. According to a prevailing view, the water adjacent to a hydrophobic group is unstable especially in terms of the rotational entropy and the folding is driven primarily by the release of such unfavorable water to the bulk through the burial of nonpolar side chains. We show, however, that the resultant entropic gain is too small to elucidate the experimental result. The great entropic gain observed is ascribed to the reduction in the restriction for the translational motion of water molecules in the whole system.     

Principle and mechanism of direct porphyrin metalation: joint experimental and theoretical investigation

The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).     

Electron affinities of small uracil-water complexes: a comparison of benchmark CCSD(T) calculations with DFT

We present benchmark CCSD(T) calculations of the adiabatic electron affinities (AEA) and the vertical detachment energies (VDE) of the uracil molecule interacting with one to three water molecules. Calculations with rather large aug-cc-pVTZ basis set were only tractable when the space of virtual orbitals was reduced to about 60% of the full space employing the OVOS (Optimized Virtual Orbital Space) technique. Because of the microhydration, the valence-bound uracil anion is stabilized leading to gradually more positive values of both AEA and VDE with increasing number of participating water molecules. This agrees with experimental findings. Upon hydration by three water molecules, the electron affinity of uracil increased in comparison with AEA of the isolated molecule by about 250 up to 570 meV, depending on the geometry of the complex. CCSD(T) results confirm trends determined by DFT calculations of the microhydrated uracil and its anion, even if electron affinities of the free and hydrated uracil molecule are overestimated by DFT by up to 300 meV.     

Problems in the comparison of theoretical and experimental hyperpolarizabilities revisited

The relationship between nonlinear susceptibilities and hyperpolarizabilities defined using different conventions is reexamined. In previous work [Willetts et al., J. Chem. Phys. 97, 7590 (1992)], relations between different conventions for microscopic hyperpolarizabilities have been derived, but the application of the corresponding conversion factors led to several inconsistencies. It is shown that different conventions for macroscopic susceptibilities have to be taken into account, too, in order to arrive at consistently comparable values. The complete set of conversion factors between several conventions are given for second harmonic generation, electric field induced second harmonic (EFISH) generation, and third harmonic generation. As an illustration, experimental EFISH and hyper-Rayleigh scattering results of p-nitroaniline are compared with each other and with recent results of ab initio computations including solvation effects. Several problems in the comparison of computational and experimental values are also discussed.     

Open-shell coupled-cluster method: Electron affinities of Li and Na

The open-shell coupled-cluster method and the diagrams needed for its implementation are described. The method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations. The two schemes give essentially the same results, in very good agreement (<0.02 eV) with experiment. Three-body effects are negligible.     

Crystal,molecular and electronic structure of 1,1-diaryl-2-halogenoethylenes

On the basis of semiempirical calculations, the conformation of the isolated molecule of 2-bromo-1,1-diphenylprop-1-ene corresponding to the minimum of total energy was determined. Assuming some information on the localization of the molecules in the crystal, also the conformation in a theoretical crystal was studied. The results have been compared with the crystal structure of 2-bromo-1,1-diphenylprop-1-ene, as obtained by X-ray diffraction. From the comparison a slight displacement of the C2 atom was suggested.

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Zusammenfassung Die dem Minimum der Gesamtenergie entsprechende Konformation des isolierten Moleküls von 2-Brom-1,1-diphenylprop-1-en wurde durch halb-empirische Methoden bestimmt. Gewisse Ergebnisse über die Anordnung der Moleküle im Kristall machten es möglich, die Konformation in einem theoretischen Kristall zu untersuchen. Nach Vergleich der Resultate mit der durch Röntgen-Analyse bestimmten Kristallstruktur des 2-Brom-1,1-diphenylprop-1-en wurde eine kleine Verschiebung des C2-Atoms vorgeschlagen.

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Résumé Par le moyen de calculs semiempiriques on a déterminé la conformation de la molécule isolée de 2-bromo-1, 1-diphénilprop-1-ène, correspondant au minimum d'énergie totale. On a aussi étudié la conformation dans le cristal théorique en se basant sur des informations concernant la localisation des molécules dans le cristal. On a comparé ces résultats avec ceux que nous avons obtenus par la détermination de la structure cristalline du 2-bromo-1, 1-diphénilprop-1-ène avec la diffraction des rayons X: cette comparaison nous a porté à déplacer un petit peu l'atome C 2.

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This work was supported by a grant from the Italian Consiglio Nazionale delle Ricerche.     

Experimental validation of theoretical potassium and sodium cation affinities of amides by mass spectrometric kinetic method measurements

In this study the theoretical Gaussian-2 K(+)/Na(+) binding affinities (enthalpies) at 0 K (in kJ mol(-1)) for six amides in the order: formamide (109.2/138.5) < N-methylformamide (117.7/148.6) < acetamide (118.7/149.5) < N,N-dimethylformamide (123.9/156.4) < N-methylacetamide (125.6/157.7) < N,N-dimethylacetamide (129.2/162.6), reported previously (Siu et al., J. Chem. Phys. 2001; 114: 7045-7051), were validated experimentally by mass spectrometric kinetic method measurements. By monitoring the collision-induced dissociation (CID) of K(+)/Na(+)-bound heterodimers of the amides, the relative affinities were shown to be accurate to within +/-2 kJ mol(-1). With these six theoretical K(+)/Na(+) binding affinities as reference values, the absolute K(+)/Na(+) affinities of imidazole, 1-methylimidazole, pyridazine and 1,2-dimethoxyethane were determined by the extended kinetic method, and found to be consistent (to within +/-9 kJ mol(-1)) with literature experimental values obtained by threshold-CID, equilibrium high-pressure mass spectrometry, and Fourier transform ion cyclotron resonance/ligand-exchange equilibrium methods. A self-consistent resolution is proposed for the inconsistencies in the relative order of K(+)/Na(+) affinities of amides reported in the literature. These two sets of validated K(+) and Na(+) affinity values are useful as reference values in kinetic method measurements of K(+)/Na(+) affinity of model biological ligands, such as the K(+) affinities of aliphatic amino acids.     

A comparison of experimental measurements and theoretical predictions regarding the behavior of a turbulent argon plasma jet discharging into air

Computed results are presented describing the temperature and concentration fields obtained when an argon plasma jet is being discharged into ambient air. A previously published mathematical model for turbulent plasma plumes is used for the calculations. These predictions are compared with recent), published experimental measurements by Brossa and Pfender, performed with an enthalpy probe. The theoretical predictions appear to agree reasonably well with the measurements of both the temperature and concentration profiles, with a maximum deviation in the 10–20% range.Notation A _max maximum temperature or velocity in the torch exit profile - C ₁ C ₂ C _D constants inK- model - h enthalpy - I torch current - K turbulent kinetic energy per unit mass - m mass concentration of plasma p pressure - Q How rate of argon through the torch - r radial coordinate - r _n nozzle radius (inside) - S source term for dependent variable      

Solvent tuning of a conical intersection: direct experimental verification of a theoretical prediction

We report an ultrafast study of a merocyanine molecule, whose fluorescence lifetime was tuned by changing the solvent's polarity. A recent theoretical prediction that the fluorescence lifetime is considerably shortened upon lowering the polarity of the solvent, due to tuning of the conical intersection properties, is fully confirmed (Xu et al. J. Phys. Chem. A 2009, 113, 9779-9791). This constitutes a direct measurement of a previously predicted tunable property of a conical intersection.     

Electron attachment and detachment: electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced M?ller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.     

Electron affinities of uracil: microsolvation effects and polarizable continuum model

We present adiabatic electron affinities (AEAs) and the vertical detachment energies (VDEs) of the uracil molecule interacting with one to five water molecules. Credibility of MP2 and DFT/B3LYP calculations is supported by comparison with available benchmark CCSD(T) data. AEAs and VDEs obtained by MP2 and DFT/B3LYP methods copy trends of benchmark CCSD(T) results for the free uracil and uracil-water complexes in the gas phase being by 0.20 - 0.28 eV higher than CCSD(T) values depending on the particular structure of the complex. AEAs and VDEs from MP2 are underestimated by 0.09-0.15 eV. For the free uracil and uracil-(H(2)O)(n) (n = 1,2,3,5) complexes, we also consider the polarizable continuum model (PCM) and discuss the importance of the microsolvation when combined with PCM. AEAs and VDEs of uracil and uracil-water complexes enhance rapidly with increasing relative dielectric constant (ε) of the solvent. Highest AEAs and VDEs of the U(H(2)O)(5) complexes from B3LYP with ε = 78.4 are 2.03 and 2.81 eV, respectively, utilizing the correction from CCSD(T). Specific structural features of the microsolvated uracil-(H(2)O)(n) complexes and their anions are preserved also upon considering PCM in calculations of AEAs and VDEs.     

Diorganostannylenes : III. Formation of dimethylstannylene from 1,1-dimethyl-2,3,4,5-tetraphenyl-1-stannacyclopentadiene and acetylenes

Dimethylstannylene, (CH₃)₂Sn:, was formed transiently in the reaction of 1,1-dimethyl-2,3,4,5-tetraphenyl-1-stannacyclopentadiene (stannole) with dimethyl acetylenedicarboxylate or phenylacetylene. When this reaction was carried out in the presence of n-butyl bromide, dimethylstannylene inserted into the CBr bond of n-butyl bromide to give n-butyldimethyltin bromide in 12% yield, but most of the dimethylstannylene was captured by dimethyl acetylenedicarboxylate to give a complex. The complex was also produced in the photolytic reaction of the dimethyltin polymer, [(CH₃)₂Sn]_m, with dimethyl acetylenedicarboxylate. With phenylacetylene, dimethylstannylene did not give a similar complex, but polymerized to give the dimethyltin polymer.     

Double-resonance versus pulsed Fourier transform two-dimensional infrared spectroscopy: an experimental and theoretical comparison

In this study we focus on the differences and analogies of two experimental implementations of two-dimensional infrared (2D-IR) spectroscopy: double-resonance or dynamic hole burning 2D-IR spectroscopy and pulsed Fourier transform or heterodyne detected photon echo spectroscopy. A comparison is done theoretically as well as experimentally by contrasting data obtained from both methods. As an example we have studied the strongly coupled asymmetric and symmetric carbonyl stretching vibrations of dicarbonylacetylacetonato rhodium dissolved in hexane. Both methods yield the same peaks in a 2D-IR spectrum. Within certain approximations we derive an analytic expression which shows that the 2D-IR spectra are broadened in one frequency dimension in the double-resonance experiment by convolution with the pump pulse spectral width, while the spectral resolution in the other frequency direction is the same in both cases.     

A comparison of239+240Pu post-mortem measurements with estimates based on current ICRP models

Measurements of^239+240Pu in human tissues, from nuclear weapons testing, provide an invaluable source of data for verifying the uptake and distribution of radionuclides in the body. Measured concentrations of^239+240Pu in lung, tracheobronchial lymph nodes, liver and skeleton have been compared with concentrations calculated using estimated plutonium intakes, the ICRP Publication 66 Respiratory Tract Model and the ICRP Publication 67 biokinetic model for plutonium. Measurement data tend to fall between the concentrations estimated on the basis of Type M and Type S absorption parameters. This indicates that the models represent the movement of plutonium through the body reasonably well.     

Electron affinities of the DNA and RNA bases.

Adiabatic electron affinities (AEAs) for the DNA and RNA bases are predicted by using a range of density functionals with a double-zeta plus polarization plus diffuse (DZP++) basis set in an effort to bracket the true EAs. Although the AEAs exhibit moderate fluctuations with respect to the choice of functional, systematic trends show that the covalent uracil (U) and thymine (T) anions are bound by 0.05-0.25 eV while the adenine (A) anion is clearly unbound. The computed AEAs for cytosine (C) and guanine (G) oscillate between small positive and negative values for the three most reliable functional combinations (BP86, B3LYP, and BLYP), and it remains unclear if either covalent anion is bound. AEAs with B3LYP/TZ2P++ single points are 0.19 (U), 0.16 (T), 0.07 (G), -0.02 (C), and -0.17 eV (A). Favorable comparisons are made to experimental estimates extrapolated from photoelectron spectra data for the complexes of the nucleobases with water. However, experimental values scaled from liquid-phase reduction potentials are shown to overestimate the AEAs by as much as 1.5 eV. Because the uracil and thymine covalent EAs are in energy ranges near those of their dipole-bound counterparts, preparation and precise experimental measurement of the thermodynamically stable covalent anions may prove challenging.