Heterogeneity in binary mixtures of (water + tertiary butanol): temperature dependence across mixture composition

The temperature dependence of solution heterogeneity in binary mixtures of water and tertiary butanol (TBA) and its effects on a chemical reaction have been investigated by using steady-state and time-resolved spectroscopic experiments within the temperature range of 278 ≤ T/K ≤ 373. Eleven different mole fractions of TBA, covering extremely low TBA mole fractions to pure TBA, have been considered. An organic chromophore that undergoes a photoexcited intramolecular charge-transfer reaction is employed to reveal the signature of the solution heterogeneity. Upon increasing the solution temperature, the absorption spectrum of the dissolved chromophore exhibits a red shift at very low TBA concentrations but shifts toward higher energy (blue shift) at higher alcohol concentrations. This is a reflection of temperature-assisted aggregation of TBA molecules in very dilute aqueous solutions. The magnitude of the temperature-induced red shift is the largest at around 0.04 mol fraction of TBA, and a larger variation of the spectral line width across the temperature suggests enhanced solution heterogeneity. Reaction time constants measured at various mixture compositions are found to follow an Arrhenius-type temperature dependence. The average activation energy, when plotted as a function of mixture composition, steeply rises with TBA concentration in the limit of the very low TBA mole fraction and then suddenly levels off to a plateau upon further addition of TBA. The alcohol concentration-dependent activation energy abruptly changes its slope at a TBA mole fraction ～0.1, at which a transition from the three-dimensional water-type network to the zigzag alcohol chain structure is known to occur. The plateau value of the activation energy is ～6k(B)T and agrees well with the earlier estimate for the same chromophore from the pure solvent data at room temperature. The observed increase in the spectral red shift with temperature at low TBA mole fractions is in general agreement with the existing experimental results which support the view that temperature assists the aggregation of TBA molecules in dilute aqueous solutions of TBA. However, unlike in the small-angle neutron scattering study [ Bowron, D. T.; Finney, J. L. J. Phys. Chem. B 2007, 111, 9838], which finds clustering of TBA molecules reaching a maximum at ～353 K, the present data do not indicate any such temperature maximum within the temperature range of 278 ≤ T/K ≤ 373.     

Structural transition in alcohol-water binary mixtures: A spectroscopic study

The strengthening of the hydrogen bonding (H-bond) network as well as transition from the tetrahedral-like water network to the zigzag chain structure of alcohol upon increasing the alcohol concentration in ethanol-water and tertiary butanol (TBA) — water mixtures have been studied by using both steady state and time resolved spectroscopy. Absorption and emission characteristics of coumarin 153 (C153), a widely used non-reactive solvation probe, have been monitored to investigate the structural transition in these binary mixtures. The effects of the hydrogen bond (H-bond) network with alcohol concentration are revealed by a minimum in the peak frequency of the absorption spectrum of C153 which occur at alcohol mole fraction ∼0·10 for water-ethanol and at ∼0·04 for water-TBA mixtures. These are the mole fractions around which several thermodynamic properties of these mixtures show anomalous change due to the enhancement of H-bonding network. While the strengthening of H-bond network is revealed by the absorption spectra, the emission characteristics show the typical non-ideal alcohol mole fraction dependence at all concentrations. The time resolved anisotropy decay of C153 has been found to be bi-exponential at all alcohol mole fractions. The sharp change in slopes of average rotational correlation time with alcohol mole fraction indicates the structural transition in the environment around the rotating solute. The changes in slopes occur at mole fraction ∼0·10 for TBA-water and at ∼0·2 for ethanol-water mixtures, which are believed to reflect alcohol mole fraction induced structural changes in these alcohol-water binary mixtures.     

Concentration fluctuations and collective properties in mixed liquid systems: Rayleigh-Brillouin spectra of tert-butyl alcohol/2,2'-dimethylbutane liquid mixture

Rayleigh-Brillouin spectra have been measured in a range of temperatures and compositions of t-butyl alcohol/2,2(')-dimethylbutane liquid mixture. The mixture mole fraction has been varied from pure alkane (x(TBA)=0) to pure alcohol (x(TBA)=1) at temperatures between 283 and 323 K. In the same composition and temperature ranges the authors also executed measurements of mass density, shear viscosity, and refractive index. From light scattering spectra the authors have extracted the hypersound velocities and adiabatic compressibilities and evaluated their excess values. Moreover, the authors attempted to evaluate the isothermal (40 degrees C) Landau-Placzek ratios at various mole fractions, but these values proved to be subject to significant errors due to great uncertainty in the central component intensity measurements. Thus, in discussing the results, this latter quantity was considered only from a qualitative point of view. These results highlight a nonideal behavior of the studied liquid mixture with a probable azeotropic composition around x(TBA)=0.7 due to formation of small clusters of hydrogen-bonded alcohol tetramers that are completely surrounded by solvent molecules and analogous or smaller clusters. These clusters, shaped as inverse micelles, offer their hydrophobic moiety towards the molecules that constitute the solvation shell, resulting in a low polarity solution structure that minimizes the solute-solvent interactions. Differences in thermal and compositional behavior of excess molar volumes and adiabatic compressibilities have been interpreted by attributing different weights to the solute-solvent interaction forces and to the hydrogen bond connectivity effects.     

Femtosecond dynamics of isolated phenylcarbenes

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A' state, corresponding to excitation from a pi-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with tau1 approximately 40 fs, a second time constant of tau2 approximatley 350 fs, and an additional time constant of tau3 approximately 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck-Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A' state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9-5.0 eV down to the A 1A' state.     

Influence of localized excited states on the transition moment directions of charge transfer complex absorptions

The influence of localized excited (LE) states on the spectroscopy of charge transfer (CT) complexes has been examined for a series of complexes formed between methyl-substituted benzene donors and 1,2,4,5-tetracyanobenzene as acceptor in 1,2-dichloroethane and octanenitrile solvents. A molecular orbital model was used to describe the appearance of multiple CT absorption bands that occur in the spectra of these complexes. The influence of LE states in these CT absorptions was explored using time-resolved linear dichroism spectroscopy where the direction of the CT transition moment vector (TMV) was used to probe the magnitude of intensity borrowing. The TMV directions for each of the observed CT transitions within the absorption spectra were determined for several complexes. In some cases, the observed CT transitions were interpreted as being pure CT transitions; in others the observed transitions are influenced significantly by a LE transition. The correlation between the TMV directions and the transition energy suggests that the magnitude of intensity borrowing is influenced not only by the energy difference between the CT and LE transitions but also by the specific character of the transitions under consideration.     

Global and target analysis of time-resolved fluorescence spectra of Di-9H-fluoren-9-yldimethylsilane: dynamics and energetics for intramolecular excimer formation

Dynamics and energetics for intramolecular excimer formation of a diarylsilane, di-9H-fluoren-9-yldimethylsilane (DFYDMS) have been investigated by means of ps time-resolved fluorescence spectroscopy and ab initio calculation. Multiple fluorescence decay curves were globally deconvolved to generate time-resolved fluorescence spectra and decay-associated spectra (DAS), from which species-associated spectra (SAS) were obtained. It is shown in the global analysis that there are at least three excited states: Two states are the locally excited (LE) states (lambda(max) approximately 320 nm) having lifetimes of 0.70 +/- 0.04 and 1.75 +/- 0.02 ns, and another is the excimer state (lambda(max) approximately 400 nm) having a lifetime of 7.34 +/- 0.02 ns. The species which decays with 0.70 ns evolves into a species with a red-shifted spectrum, which in turn decays in 7.34 ns. The experimental and ab initio results indicate that the rise time of 0.70 ns corresponds to the conversion of the initial S(1) LE state having a near sandwich geometry to the S(1) excimer state adopting a true sandwich geometry.     

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

The photophysical properties of the newly synthesized unsymmetrically substituted aromatic acetylene derivative 9-(2-(4-(N,N-dimethylamino)phenyl)ethynyl)anthracene-10-carbonitrile (CNAacDMA) were investigated with the steady-state and time-resolved fluorometry. In saturated hydrocarbon solvents, only fluorescence from a locally-excited state (LE) is recorded. In more polar solvents however, excitation of this dye leads to a charge transfer state (CT). In moderate polar solvents (ϵ=4–8) dual emission is observed as a result of competition between structural change and intramolecular charge transfer in the excited state. In polar solvents only one emission band, at shorter wavelength than CT emission, is observed, indicating a bidirectional solvatofluorochromism.     

Electron transfer dynamics from organic adsorbate to a semiconductor surface: zinc phthalocyanine on TiO2(110)

The femtosecond time evolutions of excited states in zinc phthalocyanine (ZnPC) films and at the interface with TiO2(110) have been studied by using time-resolved two-photon photoelectron spectroscopy (TR-2PPE). The excited states are prepared in the first singlet excited state (S1) with excess vibrational energy. Two different films are examined: ultrathin (monolayer) and thick films of approximately 30 A in thickness. The decay behavior depends on the thickness of the film. In the case of the thick film, TR-2PPE spectra are dominated by the signals from ZnPC in the film. The excited states decay with tau = 118 fs mainly by intramolecular vibrational relaxation. After the excited states cascaded down to near the bottom of the S1 manifold, they decay slowly (tau = 56 ps) although the states are located at above the conduction band minimum of the bulk TiO2. The exciton migration in the thick film is the rate-determining step for the electron transfer from the film to the bulk TiO2. In the case of the ultrathin film, the contribution of electron transfer is more evident. The excited states decay faster than those in the thick film, because the electron transfer competes with the intramolecular relaxation processes. The electronic coupling with empty bands in the conduction band of TiO2 plays an important role in the electron transfer. The lower limit of the electron-transfer rate was estimated to be 1/296 fs(-1). After the excited states relax to the states whose energy is below the conduction band minimum of TiO2, they decay much more slowly because the electron-transfer channel is not available for these states.     

Dynamics of ultrafast intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile

The intramolecular charge transfer (ICT) reaction of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile (MeCN) is investigated by picosecond fluorescence experiments as a function of temperature and by femtosecond transient absorption measurements at room temperature. NTC6 in n-hexane is dual fluorescent from a locally excited (LE) and an ICT state, with a quantum yield ratio Phi'(ICT)/Phi(LE) of 0.35 at +25 degrees C and 0.67 at -95 degrees C, whereas in MeCN mainly an ICT emission is observed. From the temperature dependence of Phi'(ICT)/Phi(LE) for NTC6 in n-hexane, an LE/ICT enthalpy difference DeltaH of -2.4 kJ/mol is determined. For comparison, 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6) is also investigated. This molecule does not undergo an ICT reaction, because of its larger energy gap DeltaE(S1,S2). From the molar absorption coefficient epsilonmax of NTC6 as compared with other aminobenzonitriles, a ground-state amino twist angle theta of approximately 22 degrees is deduced. The increase of epsilonmax between n-hexane and MeCN indicates that theta decreases when the solvent polarity becomes larger. Whereas single-exponential LE fluorescence decays are obtained for NIC6 in n-hexane and MeCN, the LE and ICT decays of NTC6 in these solvents are double exponential. For NTC6 in n-hexane at -95 degrees C, with a shortest decay time of 20 ps, the forward (ka=2.5x10(10) s(-1)) and backward (kd=2.7x10(10) s(-1)) rate constants for the LE<-->ICT reaction are determined from the time-resolved LE and ICT fluorescence spectra. For NTC6 in n-hexane and MeCN, the excited-state absorption (ESA) spectrum at 200 fs after excitation is similar to the LE(ESA) spectra of NIC6 and 4-(dimethylamino)benzonitrile (DMABN), showing that LE is the initially excited state for NTC6. These results indicate that the LE states of NTC6, NIC6, and DMABN have a comparable molecular structure. The ICT(ESA) spectrum of NTC6 in n-hexane and MeCN resembles that of DMABN in MeCN, likewise indicating a similar ICT structure for NTC6 and DMABN. From the decay of the LE absorption and the corresponding growing-in for the ICT state of NTC6, it is concluded that the ICT state originates from the LE precursor and is not formed by direct excitation from S0, nor via an S2/ICT conical intersection. The same conclusion was made from the time-resolved (picosecond) fluorescence spectra, where there is no ICT emission at time zero. The decay of the LE(ESA) band of NTC6 in n-hexane occurs with a shortest time tau2 of 2.2 ps. The ICT reaction is much faster (tau2 = 0.82 ps) in the strongly polar MeCN. The absence of excitation wavelength dependence (290 and 266 nm) for the ESA spectra in MeCN also shows that LE is the ICT precursor. With NIC6 in n-hexane and MeCN, a decay or growing-in of the femtosecond ESA spectra is not observed, in line with the absence of an ICT reaction involving an S2/ICT conical intersection.     

A femtosecond time-resolved investigation of dual fluorescence from N6,N6-dimethyladenine

The excited electronic state dynamics of N(6),N(6)-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at lambda= 258 nm were measured at a series of wavelengths in the range 320 nm or= 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (tau(3)) dominated, the second component (tau(2)) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a approximately 0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence-time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state.     

Charge-transfer-type fluorescence of 4-(1H-pyrrol-1-yl)benzonitrile (PBN) and N-phenylpyrrole (PP) in cryogenic matrixes: evidence for direct excitation of the CT band

The fluorescence spectrum of PBN in a neat argon matrix is excitation-wavelength-dependent: at short excitation wavelengths, it consists of dual emission assigned to a charge-transfer (CT) state and a much weaker band assigned to the locally excited (LE) state. The CT emission is broad and almost completely devoid of vibrational structure, whereas the LE band is characterized by vibrationally resolved emission. At long excitation wavelengths, only CT emission is observed, indicating that the CT state is populated directly by light absorption and not via the LE state. Comparison with jet-cooled spectra of the bare molecule allows the unambiguous assignment of the LE spectrum and the location of the 0,0 band. The matrix LE emission spectrum is blue-shifted with respect to that of the gas phase, showing that the dipole moment of the LE state is smaller than that of the ground state. The fluorescence spectrum of PBN in an argon matrix does not change appreciably when acetonitrile (AN) is added to the matrix, in contrast to the case of N-phenylpyrrol (PP) (Schweke, D.; Haas, Y. J. Phys. Chem. A 2003, 107, 9554), for which addition of AN results in the appearance of two well separated emission bands. The different photophysical behaviors of PP and PBN in an argon matrix (and in supersonic jets) are analyzed by a simple model that considers the restriction of large-amplitude motions in the matrix. The implications of these low-temperature studies for understanding the properties of these systems in liquid solution are discussed.     

Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers

The strong solvatochromism observed for two fluorene-dibenzothiophene-S,S-dioxide oligomers in polar solvents has been investigated using steady-state and time-resolved fluorescence techniques. A low-energy absorption band, attributed to a charge-transfer (CT) state, is identified by its red shift with increasing solvent polarity. In nonpolar solvents, the emission of these conjugated luminescent oligomers shows narrow and well-resolved features, suggesting that the emission comes from a local excited state (LE), by analogy to their conjugated fluorene-based polymer counterparts. However, in polar solvents, only a featureless broad emission is observed at longer wavelengths (CT emission). A linear correlation between the energy maximum of the fluorescence emission and the solvent orientation polarizability factor Deltaf (Lippert-Mataga equation) is observed through a large range of solvents. In ethanol, below 230 K, the emission spectra of both oligomers show dual fluorescence (LE-like and CT) with the observation of a red-edge excitation effect. The stabilization of the CT emissive state by solvent polarity is accompanied/followed by structural changes to adapt the molecular structure to the new electronic density distribution. In ethanol, above 220 K, the solvent reorganization occurs on a faster time scale (less than 10 ps at 290 K), and the structural relaxation of the molecule (CT(unrelaxed) --> CT(Relaxed)) can be followed independently. The magnitude of the forward rate constant, k(1)(20 degrees C) approximately 20 x 10(9) s(-1), and the reaction energy barrier, E(a) approximately 3.9 kcal mol(-1), close to the energy barrier for viscous flow in ethanol (3.54 kcal mol(-1)), show that large-amplitude molecular motions are present in the stabilization of the CT state.     

Photoproduct formation with 4-aminobenzonitriles in acetonitrile and its effect on photophysical measurements

Upon photoexcitation of 4-(dimethylamino)benzonitrile (DMABN) in the polar solvent acetonitrile (MeCN), a methyl group is subtracted from the dimethylamino substituent, producing 4-(methylamino)benzonitrile (MABN). The fluorescence of this photoproduct MABN occurs in the same spectral range as that of the locally excited (LE) state of DMABN. As DMABN undergoes efficient fluorescence quenching in MeCN, leading to a decrease of the LE fluorescence yield by a factor of 290 at 25 degrees C, whereas MABN is not quenched at all, even small amounts of this photoproduct strongly increase the apparent contribution of the LE emission to the total dual fluorescence spectrum of DMABN. As a further consequence of the photoproduct formation, the nanosecond decay time, tau1, in the double-exponential LE fluorescence decay of DMABN in MeCN increases in relative intensity as compared to its picosecond counterpart, tau2, as the fluorescence lifetime of MABN is similar to the tau1 decay time of DMABN. The presence of the photoproduct MABN therefore can lead to a misinterpretation of the kinetic data derived from photostationary and time-resolved fluorescence experiments with DMABN in polar solvents. Photoproducts are also observed with 4-(N-pyrrolidinyl)aminobenzonitrile (P5C) and 4-(N-piperidinyl)aminobenzonitrile (P6C) in MeCN. In the case of P5C, 4-cyano-N-phenylpyrrole (PP4C) is the main product, whereas photolysis of P6C produces 4-aminobenzonitrile (ABN), among other photoproducts. This photodegradation, leading to the appearance of multiexponential decays, likewise has a negative influence on the ICT and LE fluorescence spectra and fluorescence decays of P6C and P5C, again impairing the validity of the kinetic analysis of these data. The isosbestic (absorption) and isoemission (fluorescence) points encountered in the spectra of DMABN and P6C during photoirradiation indicate that at least one photoproduct is formed.     

A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.     

Cryogenic studies, site selectivity and discrete fluorescence in salicylic acid dimer

Cryogenic effects (10–293 K) on the photophysics of salicylic acid (SA) dimer have been using steady state and time-resolved spectroscopic techniques. SA dimer shows two emissions at approximately 390 nm (dimer, D) and approximately 430 nm (tautomer, T), formed by cyclic double proton transfer plus fast excited state intramolecular proton transfer (ESIPT), at low temperatures; a vibrational structure also develops which is due to C = O and OH stretches. On red edge excitation (REE), only the dimer-type (UV) emission is observed, which shifts with excitation energy resembling emission due to site selectivity. Due to the asymmetry of the double potential energy curves of D and T, all dimers can be trapped in the D minimum. The UV emission of the dimer is accompanied by the Stokes' shifted tautomer emission on excitation at 1050 cm⁻¹ higher than the (0,0) band of D, which is interpreted as the barrier height of the double potential energy curves of D and T. Time-resolved studies at various temperatures have helped to clarify the photophysics of crystalline SA.     

Photophysics of phenylpyrrole derivatives and their acetonitrile clusters in the gas phase and in argon matrixes: simulations of structure and reactivity

Recent experiments on the dual fluorescence of phenylpyrrole (PP) and pyrrolobenzonitrile (PBN) in supersonic jets and in cryogenic matrixes are analyzed. The structures of the 1:1 clusters are calculated using ab initio, density functional theory (DFT) and molecular mechanics (MM) methods. In these calculations, the structures of PP and PBN in the ground state and in two possible minima on the charge-transfer excited state are taken from a recent theoretical analysis. The structures of PP and PBN clusters with a larger number of acetonitrile molecules are also calculated using the molecular mechanics method. It is shown that the fact that small PP:AN and PBN:AN clusters do not exhibit any charge-transfer (CT) type emission, whereas for PBN:AN(n) clusters (n > or = 4) CT emission is observed, can be understood on the basis of the calculated structures. The trapping of PP and of PBN in an argon matrix (neat and doped with acetonitrile) is simulated by a molecular dynamics procedure. The observation of locally excited (LE) fluorescence only from PP in neat argon, whereas from argon-trapped PBN both CT and LE emission bands are observed, is readily understood on the basis of these simulations. Moreover, the appearance of CT emission from PP-doped argon matrixes when acetonitrile is added is also explained, as well as the relatively small spectral shift observed upon addition of acetonitrile to PBN-doped argon matrixes.     

甲烷在水-叔丁醇混合溶剂中的溶解度

在303.15 K、313.15 K、323.15 K、333.15 K温度下，0－6 MPa压力范围内测定了甲烷在水－叔丁醇混合溶剂中的溶解度．溶剂中叔丁醇的摩尔分数(x_(TBA))从0到1．结果表明，在温度和溶剂组成一定条件下，甲烷的溶解度随其分区的增加而增大，随x_(TBA)的增加，在富水区内，甲烷的溶解度变化较缓慢，当x_(TBA)超过某值时，甲烷的溶解度随x_(TBA)的增加而增大，并且幅度较大；在x_(TBA)约为0－0.045范围内，甲烷的溶解度随温度增加而减小，x_(TBA)约在0.045－0.15范围内，溶解度随温度增加而增加，x_(TBA)约在0.15～1.0范围内，溶解度随温度增加而减小．根据溶解度与温度和溶剂组成的关系可以推测，在303.15－333.15 K、0－6 MPa范围内，水－叙丁醇混合溶剂中仍存在笼合物结构．根据溶解度与温度、压力的关系讨论了甲烷在此混合溶剂中的亨利常数、偏摩尔体积、标准溶解自由能、标准溶解焓和标准溶解熵．     

Vibrational spectroscopy and dynamics of azide ion in ionic liquid and dimethyl sulfoxide water mixtures

Steady-state and time-resolved infrared spectroscopy of the azide (N(3)-) anion has been used to characterize aqueous mixtures both with the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF(4)]) and with dimethyl sulfoxide (DMSO). In the DMSO-water mixtures, two anion vibrational bands are observed for low water mole fractions (0 > X(w) > 0.25), which indicates a heterogeneous ion solvation environment. The band at 2000 cm(-1) observed for neat DMSO does not shift but decreases in amplitude as the amount of water is increased. Another band appears at slightly higher frequency at low X(w) (=0.05). As the amount of water is increased, this band shifts to higher frequency and becomes stronger and is attributed to azide with an increasing degree of hydration. At intermediate and high X(w), a single band is observed that shifts almost linearly with water mole fraction toward the bulk water value. The heterogeneity is evident from the infrared pump-probe studies in which the decay times depend on probe frequency at low mole fraction. For the azide spectra in IL-water mixtures, a single azide band is observed for each mole fraction mixture. The azide band shifts almost linearly with mole fraction, indicating nearly ideal mixing behavior. As with the DMSO-water mixtures, the time-resolved IR decay times are probe-frequency-dependent at low mole fraction, again indicating heterogeneous solvation. In both the DMSO and IL mixtures with water, the relaxation times are slower than would be expected from ideal mixing, suggesting that vibrational relaxation of azide is more sensitive than its vibrational frequency to the solvent structure. The results are discussed in terms of preferential solvation and the degree to which the azide shift and vibrational relaxation depend on the degree of water association in the mixtures.     

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.     

New insights on the behavior of PRODAN in homogeneous media and in large unilamellar vesicles

The behavior of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) was studied in homogeneous media and in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization, and time-resolved spectroscopies. In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (pi) and the hydrogen bond donor ability (alpha) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (beta) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands gives a linear correlation with the well-known polarity parameter E(T30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes, 2.27 and 0.65 ns, while the aggregate has a lifetime of 14.6 ns. Using steady-state anisotropy measurements, the calculated volumes of the aggregate and the monomer are 5590 and 222 mL mol(-1), respectively. In DOPC LUVs, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant (K(p)) value is large enough that only at [DOPC] below 0.15 mg/mL PRODAN in water can be detected. PRODAN dissolved in LUVs at [DOPC] > 1 mg/mL exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipid tails. These environments were characterized by their fluorescence lifetimes (tau), showing that PRODAN in the polar microenvironment has a tau value of approximately 4 ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yields values similar to that of methanol and tetrahydrofuran.