The Influence of Temperature on Coumarin 153 Fluorescence Kinetics

The influence of temperature varied in the range 183 K–323 K on the fluorescence quantum yield, fluorescence lifetime, absorption and emission transition moments and non-radiative deactivation rate was determined for the well known and largely used dye Coumarin 153, dissolved in 1-chloropropane. The Kennard-Stepanov relation connecting the absorption and emission spectra was used to check for the presence of more than one absorbing/emitting species and to investigate whether intramolecular vibrational redistribution completes in the C153 excited S ₁ state before the emission takes place. The emission spectrum corresponding to S ₁→S ₀ transition, was fitted at each temperature to the model function including the information on the dye vibrational modes coupling. In this way the displacement in equilibrium distance for the most active vibrational mode was determined for C153 in S ₁ and in S ₀. Using the temperature dependence of the fluorescence decay time and quantum yield, the non-radiative deactivation rate was determined. Its temperature dependence was compared to that calculated using the theoretical model with the most active vibrational mode displacement values taken from steady-state spectra analysis. The somewhat surprising dependence of the fluorescence decay time and quantum yield on temperature was related to non-trivial coupling between low-frequency vibrational modes of C153 in the excited and ground states.     

Effect of substitution of oxygen by sulfur in the nonleaving group of a carbonate: kinetics of the phenolysis and benzenethiolysis of S‐methyl aryl thiocarbonates

The phenolysis and benzenethiolysis of S‐methyl 4‐nitrophenyl thiocarbonate ( 1 ) and S‐methyl 2,4‐dinitrophenyl thiocarbonate ( 2 ) in water are studied kinetically. The Brønsted plots (log k _N versus nucleophile basicity) are linear for all reactions. The Brønsted slopes for 1 and 2 are, 0.51 and 0.66 (phenolysis) and 0.55 and 0.70 (benzenethiolysis), respectively. These values suggest a concerted mechanism for these reactions, as found in the corresponding carbonates. Namely, substitution of OMe by SMe in the nonleaving group does not change the mechanism. Copyright © 2007 John Wiley & Sons, Ltd.     

Theoretical study on the isomerization mechanisms of phenylazopyridine on S0 and S1 states

In this work, some important structures tied closely to the isomerization of 2‐(phenylazo)pyridine (2‐PAPy) and 4‐(phenylazo)pyridine (4‐PAPy) on the S₀ and S₁ states were characterized in detail by using the complete active space SCF (CASSCF) theory. The isomerization mechanism was discussed on the basis of the mapped potential energy surfaces (PESs) and conical intersections (CIs). A comparison of PAPy with azobenzene was carried out to stress the effect of molecular structure on the photoisomerization details. The results indicate that the thermal isomerization for both 2‐PAPy and 4‐PAPy are mainly attributed to the inversion of CNN angle on the side of the pyridine ring. In view of the energy, an optimized CI_rot with a twisting structure supports the rotation mechanism in the photoisomerization of PAPy on the S₁ state. However, it was found that another conical intersection (CI_inv) with a planar structure is higher in energy than the corresponding trans‐FC structure, via which only the PAPy excited on S₂ state or vibrationally hot S₁ state can relax their excitation energy. Minimum energy paths (MEPs) showed that the relaxation process of cis‐PAPy being excited on the S₁ state is characterized by a smoothly falling curve, which is very similar to that of azobenzene. Furthermore, a S₁ minimum and a transition state (TS₁) were found to exist on the MEP starting from the trans‐FC point to the CI_rot for 4‐PAPy, but these two typical structures were not found on the MEP of 2‐PAPy. Compared with azobenzene, 4‐PAPy exhibits a very similar photoisomerization PES, but a subtly different one can be predicted in the case of 2‐PAPy. The present results are expected to provide useful information for the design of photoresponsive materials based on the PAPy units. Copyright © 2009 John Wiley & Sons, Ltd.     

Luminescence from the Pr 4f5d and S0 states in LiLaP4O12

The photoluminescence and excitation spectra of Pr³⁺ activated LiLaP₄O₁₂ has been investigated in the 10-300 K temperature region. At all temperatures, the luminescence consists of optical transitions emanating from both the Pr³⁺ 4f¹5d¹ and the ¹S₀ states. However, at low temperatures the emission spectrum is dominated by the intraconfiguration emission transitions emanating from the Pr³⁺¹S₀ state. With increasing temperature, there is an exchange of intensity between the two emitting states; emission transitions from the ¹S₀ state exhibit strong intensity quenching while the 4f¹5d¹→4f² emission transitions reveal intensity gain. These results are explained on the basis of thermal population of the 4f¹5d¹ state by the ¹S₀ state. The energy barrier of 0.05 eV (403 cm⁻¹) for the nonradiative process is determined from the temperature dependence of the ¹S₀ lifetime.     

Theoretical design study on multifunctional triphenyl amino‐based derivatives for OLEDs

The use of triphenyl amino‐based derivatives in organic light‐emitting diodes (OLEDs) can significantly improve their efficiency and stability and especially their electroluminescence characteristics – most of the new hole‐transport materials have this feature. In this study, a series of triphenyl amino‐based compounds were computed, including two newly designed molecules. They can function as charge transport materials and emitters with high efficiency and stability. To reveal the relationship between the properties and structures of these bifunctional and multifunctional electroluminescent materials, the ground and excited state geometries were optimized at the B3LYP/6‐31G(d), HF/6‐31G(d), TD‐B3LYP/6‐31G(d), and CIS/6‐31G(d) levels, respectively. The ionization potentials (IPs) and electron affinities (EAs) were computed. The lowest excitation energies, the maximum absorption, and emission wavelengths of these compounds were calculated by employing the time‐dependent density functional theory (TD‐DFT) method. Also, the mobilities of holes and electrons were studied computationally based on the Marcus electron transfer theory. The CH₂Cl₂ solvent effect on the absorption spectra of N,N′‐di‐1‐naphthyl‐N,N′‐diphenylbenzidine ( NPB ) was considered by polarizable continuum model (PCM). The results obtained for these compounds are in good agreement with the experimental values. These data show that the proposed compounds 1 and 2 (N,B‐di‐1‐naphthyl‐N,B‐diphenylbenzidine and Mes₂N[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)]), are multifunctional and bifunctional materials similar to Mes₂B[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)] ( BNPB ) and NPB , respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

An ultrafast time‐resolved infrared and UV–vis spectroscopic and computational study of the photochemistry of acyl azides

The photochemistry of pivaloyl, benzoyl, 4‐phenylbenzoyl, and 2‐anthroyl azides has been studied using femtosecond (fs) time‐resolved infrared (TRIR) and UV–vis spectroscopy and interpreted with the aid of computational chemistry. Density functional theory calculations revealed a significant difference in the nature of the lowest singlet excited state for these carbonyl azides. The lowest singlet excited states (S₁) of p‐phenylbenzoyl and 2‐anthroyl azides are (π,π*) in nature, while the pivaloyl and benzoyl azides S₁ states involve (n,π*) excitations. Nevertheless, for all acyl azides studied here, a similar, and intense, IR band at about 2100 cm^?1 has been detected in the ultrafast TRIR experiments following 270 nm excitation. These bands were shifted to lower energy by about 100 cm^?1 relative to the N₃ stretching mode for the ground states of these azides. These 2100 cm^?1 vibrational bands were assigned to the S₁ states of acyl azides in agreement with density functional theory calculations. The decay of the acyl azide S₁ states was described by bi‐exponential functions. The fast component was attributed to the decay of the hot S₁ state and the longer component to the decay of the thermally relaxed S₁ state. A strong and broad transient absorption in the 350–650 nm spectral range was observed in the fs UV–vis experiments for p‐phenylbenzoyl and 2‐anthroyl azides. The carrier of this absorption also decayed bi‐exponentially, and the time constants were in excellent agreement with those found in the fs TRIR experiments. The slow component of the S₁ state decay was found to be dependent on the solvent polarity. When the lifetime of the acyl azide S₁ state is substantially longer than the time constant for vibrational cooling of nascent (hot) isocyanate, the correlation between the S₁ decay and isocyanate formation was clear. The 270 nm excitation populates the S_n (n ≥ 2) states of these acyl azides. It was established that a hot nitrene is produced more efficiently from both the S_n and hot S₁ states than from the relaxed S₁ state of these acyl azides. Thus, time‐resolved study provides direct experimental evidence that the S₁ state is the precursor of nitrene only when the S₁ state is pumped directly and when the S₁ state lifetime is longer than the time constant of vibrational cooling of the newborn nitrene. All of these results are consistent with the data obtained recently for 2‐napththoyl azide. Copyright © 2012 John Wiley & Sons, Ltd.     

The SN3–SN2 spectrum. Rate constants and product selectivities for solvolyses of benzenesulfonyl chlorides in aqueous alcohols

Rate constants for a wide range of binary aqueous mixtures and product selectivities (S) in ethanol–water (EW) and methanol–water (MW) mixtures, are reported at 25 °C for solvolyses of benzenesulfonyl chloride and the 4‐chloro‐derivative. S is defined as follows using molar concentrations: S = ([ester product]/[acid product]) × ([water solvent]/[alcohol solvent]). Additional selectivity data are reported for solvolyses of 4‐Z‐substituted sulfonyl chlorides (Z = OMe, Me, H, Cl and NO₂) in 2,2,2‐trifluoroethanol–water. To explain these results and previously published data on kinetic solvent isotope effects (KSIEs) and on other solvolyses of 4‐nitro and 4‐methoxybenzenesulfonyl chloride, a mechanistic spectrum involving a change from third order to second order is proposed. The molecularity of these reactions is discussed, along with new term ‘S_N3–S_N2 spectrum’ and its connection with the better established term ‘S_N2–S_N1 spectrum’. Copyright © 2009 John Wiley & Sons, Ltd.     

Solvent network at the transition state in the solvolysis of hindered sulfonyl compounds

Alcoholysis rates of unhindered benzenesulfonyl chlorides (X‐ArSO₂Cl, X = H‐; 4‐Br‐; 4‐Me‐) are similar in methanol; the same behavior is also observed in ethanol, whereas the reactivity order in iso‐propanol is 4 Me‐ < H‐ < 4‐Br‐. On the other hand, alcoholysis of sterically hindered arenesulfonyl chlorides (X‐ArSO₂Cl) (X = 2,4,6‐Me₃‐3‐NO₂‐; 2,6‐Me₂‐4‐tBu‐; 2,4,6‐Me₃‐; 2,3,5,6‐Me₄‐; 2,4,6‐iPr₃‐; 2,4‐Me₂‐; 2,4,6‐(OMe)₃‐) in all studied alcohols show a significant increase in reactivity, the so‐called positive steric effect. Most of the substrates showed a reaction order b ~ 2 with respect to the nucleophile in methanol and ethanol, and b ~ 3 in iso‐propanol. The correlation between reactivity and the Kirkwood function (1/ξ) gives negative sensitivity (U) for all systems. All substrates showed high sensitivity to media nucleophilicity that depends on Σσ_X. Obtained results suggest the alcoholysis of benzenesulfonyl chlorides proceeds through S_N2 mechanism where the transition state (TS) involves the participation of 2–3 alcohol molecules; such a TS can be cyclic, in the case of unbranched alcohols, or linear, for alcohols with bulkier hydrocarbon groups like iso‐propanol. To include the number of alcohol molecules playing such a role in the TS, the following terminology is proposed: cS_N2s_n for S_N2 reactions involving n solvent molecules in a cyclic (c) TS, where “s” stands for the solvent and “n” is either the closest integer or half‐integer to the reaction order relative to the solvent or, in computational studies, the proposed number of solvent molecules taking part in the TS, whereas S_N2s_n is proposed when the TS is not cyclic. Copyright © 2016 John Wiley & Sons, Ltd.     

Observation of 1D−1S forbidden optical emission of atomic oxygen in atmospheric-pressure N2/O2 plasma jet

We observed green optical emission from an atmospheric-pressure N₂/O₂ plasma jet. The green optical emission was composed of a line emission at λ = 557.71 ± 0.03 nm and a broadband component at 530 ≤ λ ≤ 560 nm . The line emission was assigned to the ¹D−¹S forbidden transition of atomic oxygen, whereas the broadband emission was due to the formation of O(¹S)N₂ excimer. We measured the absolute densities of O(¹S) and O(¹S)N₂ using a spectrograph with the absolute sensitivity calibration, and we discussed the kinetics in the green plasma jet on the basis of the absolute O(¹S) and O(¹S)N₂ densities. According to the rate coefficients and the transition probabilities reported in literature, the present experimental results are explained if the densities of and O(³P) are 9 × 10¹³ and 3 × 10¹³cm⁻³ , respectively.     

Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Multiple fluorescence and the electronic relaxation processes of coronene vapor: The fluorescence from the S1, S2, and S3 states

Fluorescence and excitation spectra of coronene vapor have been measured under different conditions. Weak emission which can be regarded as the fluorescence from the third excited singlet state, S₃(¹E_1u), was observed in addition to the S₁(¹B_2u) and S₂(¹B_1u) fluorescence. The observed S₂ and S₃ fluorescence are substantially different from those reported previously for coronene vapor. Addition of oxygen resulted in significant decrease of the S₁ fluorescence intensity, but did not affect the S₂ fluorescence intensity, indicating the faster decay rate of the S₂ state than that of S₁. Excitation energy dependence of the S₁, S₂ and S₃ fluorescence quantum yields (Φ_F(S₁), Φ_F(S₂) and Φ_F(S₃), respectively) revealed that Φ_F(S₁) decreases with increasing excitation energy, while Φ_F(S₂) and Φ_F(S₃) increase significantly. The quantum yield ratios, Φ_F(S₂)/Φ_F(S₁) and Φ_F(S₃)/Φ_F(S₂), obtained as a function of excitation energy are correlated with the ratios of the relative internal conversion rates.     

Laser induced collisional energy transfer processes in a rubidium-potassium mixture

Two laser induced collisional energy transfer processes in alkali vapours are investigated. For process K(4²S_1/2)+ Rb(5²P_3/2)+?ω_T→K(6²S_1/2)+Rb(5²S_1/2), a population inversion between the final state K(6²S_1/2) and lower states, leading to a superradiant emission has been observed. Comparison between experiment and theory shows a qualitative agreement in the limit of low atomic densities and laser power. However the transfer profile width is about two times larger than the calculated one for both processes.     

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes: synthesis,structure, and conformational analysis

4‐Alkyl‐2,2,6,6‐tetramethyl‐1,4,2,6‐oxaazadisilinanes RN[CH₂Si(Me)₂]₂O [R = Me ( 1 ), i‐Pr ( 2 )] were synthesized by two methods which provided good yields up to 84%. Low temperature NMR study of compounds ( 1 ) and ( 2 ) revealed a frozen ring inversion with the energy barriers of 8.5 and 7.7 kcal/mol at 163 and 143 K, respectively, which is substantially lower than that for their carbon analog, N‐methylmorpholine. DFT calculations performed on the example of molecule ( 1 ) showed that N? Me_ax conformer to exist in the sofa conformation with the coplanar fragment C? Si? O? Si? C, and its N? Me_eq conformer in a flattened chair conformation. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state

The short‐time structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S₂(ππ*) state were studied by using resonance Raman spectroscopy and quantum mechanical calculations. The vibrational spectra and ultraviolet absorption spectra of 4‐formaldehyde imidazole were assigned. The resonance Raman spectra of imidazole and 4‐formaldehyde imidazole were obtained in methanol and acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the short‐time structural dynamics. complete active space self‐consistent field calculations were carried out to determine the minimal singlet excitation energies and structures of S₁(nπ*), S₂(ππ*), and conical intersection point S₁(nπ*)/S₂(ππ*). The results show that the A‐band structural dynamics of imidazole is predominantly along the N₁H/C₄H/C₅H/C₂H in‐plane bending reaction coordinate, which suggests that excited state proton or hydrogen transfer reaction takes place somewhere nearby the Franck–Condon region. The significant difference in the short‐time structural dynamics between 4‐formaldehyde imidazole and imidazole is observed, and the underlying mechanism is interpreted in term of excited state charge redistribution. Copyright © 2015 John Wiley & Sons, Ltd.     

The synthesis and photolysis mechanisms of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid

Photolabile protecting groups have been extensively studied and applied for protection of small biological molecules, which make it convenient to detect the biological processes of the caged compounds. In this study, a series of 8‐nitroquinoline‐based photolabile caging groups for carboxylic acid were synthesized with improved photolysis efficiency. Among them, 6‐bromo‐8‐nitro‐1, 2‐dihydroquinolinyl chromophore was proven the best derivative on account of its longest absorption wavelength (345 nm), highest caging ability, and quantum yield (Φ = 0.003). Moreover, density functional theory calculations were performed in order to study the photolysis mechanisms. Theoretical calculations revealed that the reaction was kinetically inert under general mild condition with the high barrier height of 34.3 kcal/mol at carbonyl migration step, while under the photolysis condition, because of the large energy gap (64.5 kcal/mol) between S₀ and S₁ states, the reaction should be accessible in the triplet ground state (T₁) through successive excitation of S₀ → S₁ states, subsequent intersystem crossing of S₁ → T₁ states, and finally returned to the stable S₀ state for product via potential energy surface crossing between T₁ and S₀ states. Copyright © 2014 John Wiley & Sons, Ltd.     

Steady‐State and Time‐Resolved Emission Studies of 6‐Methoxy Quinoline

Abstract

Steady‐state absorption, fluorescence excitation, and emission spectra of 6‐methoxy quinoline (6‐MQ) were measured at room temperature in cyclohexane, dioxane, ethanol, acetonitrile, water, and water–dioxane solvents. Absorption spectra of cyclohexane, n‐hexane, and isopentane solutions show resolved vibronic structure at room temperature. However, the excitation spectrum of cyclohexane solution is structureless and is found to be emission wavelength dependent, indicating the formation of at least two distinct species in the ground state. Similar behavior was observed in dioxane and water–dioxane solutions. For all other solutions, the fluorescence excitation spectrum of 6‐MQ was found to be the same for different emissions. Emission of 6‐MQ in all solvents consisted of two bands with their maxima around 355 nm (I) and 430 nm (II), the actual positions and the relative intensities being dependent on the solvent used. The bands I and II were respectively attributed to normal and protonated/H‐bonded species of either ¹L_a or ¹L_b states or mixed (¹L_a/¹L_b) state of ππ* character. Fluorescence decay of this dye in all solvents monitored over each emission maximum showed biexponential behavior, and the analysis yielded two different lifetime components for each emission band. The short and long fluorescence decay components were respectively in the range of 0.30–3.00 ns and 18–20 ns. The observed emission characteristics coupled with the nature of the fluorescence polarization spectra and two different decay components for each emission suggest the existence of two different conformers having two different excited electronic states.     

Vibronic coupling and excited‐state reaction dynamics of pyrazine in 1 1B2u (1ππ*) state by resonance Raman spectroscopy and CASSCF calculation

The photophysics and photochemistry of pyrazine (C₄H₄N₂, D_2h) after excitation to the S₂ (1 ¹B_2u, ¹ππ*) electronic state were studied by using the resonance Raman spectroscopy and complete active space self‐consistent field method calculations. The B‐band resonance Raman spectra in cyclohexane solvent were obtained at 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of pyrazine in the S₂ (1 ¹B_2u, ¹ππ*) state. Three electronic states 1 ¹B_3u, 1 ¹B_1g, and 1 ¹B_2g were found to couple with the S₂ (1 ¹B_2u, ¹ππ*) state. Two conical intersection (CI) points CI[S₂(B_2u)/S₁(B_3u)] and CI[S₁/S₀] and one transition state of the isomerization between pyrazine and pyrimidine were predicted to play important roles in the photochemistry of pyrazine. On the basis of the calculations, the mechanism of the photoisomerization reaction between pyrazine and pyrimidine has been proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Interference Effects from Easily Ionizable Elements in Flame AES and ICP‐OES: A Proposed Simplified Rate Model Based on Collisional Charge Transfer Between Analyte and Interferent Species

The effects of excess Na and K on K and Mg atom line emission in the air‐acetylene flame and of excess Li and K on Ca, Mg, and Sr atom and ion lines in inductively coupled plasma spectroscopy were studied using emission signal ratios, I′/I as probes, where I′ and I are the emission readings in the presence and absence of the interferent respectively. The I′/I plots as a function of analyte concentration in the test solution for the ICP experiments were similar to those obtained for the flame experiments in the analytical range 0–10 mg/L. A simplified rate model based on analyte excitation via charge transfer between analyte ions and activated interferent atoms is proposed to account for the emission signal enhancement observed at low analyte concentrations (<1 mg/L) for both ICP‐AES and flame atomic emission spectroscopy (AES). Data are presented showing good agreement between experimental E′ calibration curves and theoretical E′ calibration curves computed using the simplified rate model.     

Radiative decay of the metastable quartet state of copperporphin

In CuP the low temperature luminescence originates from eight transitions between a quartet (M = ±3/2, ±1/2) and a doublet (M′ = ±1/2) with M and M′ denoting the (approximate) eigenvalues of the spin angular momentum S_z along the fourfold axis. Here we report (1) the selection rules governing the polarizations of the transitions; (2) the zero-field splitting ξ between the ±3/2 and ±1/2 components of the quartet for CuP in an n-octane crystal (ξ = 1·1 ± 0·2 cm^-1); (3) a photo-selection experiment on CuP in an isopentane glass from which it is concluded that the ratio of in-plane to out-of-plane polarization in the 0-0 band at 2·1 K amounts to an intensity ratio I _‖/I _⊥ ≈ 2. The implications of these results for the different SOC pathways are analysed.     

Hydrostatic pressure effect on Tc of new BiS2‐based Bi4O4S3 and NdO0.5F0.5BiS2 layered superconductors

We investigate the external hydrostatic pressure effect on the superconducting transition temperature (T_c) of new layered superconductors Bi₄O₄S₃ and NdO_0.5F_0.5BiS₂. Though the T_c is found to have a moderate decrease from 4.8 K to 4.3 K (dT_c^onset/dP = –0.28 K/GPa) for Bi₄O₄S₃ superconductor, the same increases from 4.6 K to 5 K (dT_c^onset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO_0.5F_0.5BiS₂ superconductor. The variation of T_c in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)