Ethanol Glass Research Articles

Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer.

Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety (1-3) were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that 1-3 exist as both enol and keto tautomers, with the enol form being predominant. Irradiation of 1 at 310 nm in ethanol glass (77 K) yielded an absorption band with a λmax at ∼405 nm, which was assigned to the trans-keto tautomer (trans-1K). In contrast, laser flash photolysis of 1-3 in methanol or acetonitrile resulted in a transient absorption with λmax at 440-460 nm. This transient, which decayed on the microsecond timescale and was significantly shorter lived in methanol than in acetonitrile, was assigned to the triplet excited state (T1) of the cis-keto tautomer (cis-1K-3K) and residual absorption of trans-1K-3K by comparison with TD-DFT calculations. The assignment of the T1 of cis-1K was further supported by quenching studies with anthracene and 2,5-dimethyl-2,4-hexadiene. Laser flash photolysis of 1 in the temperature range of 173-293 K gave an activation barrier of 6.7 kcal/mol for the decay of the T1 of cis-1K. In contrast, the calculated activation barrier for cis-1K to undergo a 1,5-H atom shift to reform 1 was smaller, indicating that intersystem crossing of the T1 of cis-1K is the rate-determining step in the regeneration of 1.

Electron spin echo spectra and characterization of electronic ground states for lanthanide ions in acidic water:ethanol glasses

X-band EPR absorption spectra were obtained by field-swept echo-detection for 10 mM Ce3+, Nd3+, Sm3+, Tb3+, Er3+, Tm3+, and Yb3+ in glassy acidic 1:1 water:ethanol at 4.2 to 6 K. The slowly-varying signals extend over thousands of gauss and are more readily detected by spin echo than by continuous wave EPR. For lanthanide ions spin–orbit coupling is strong and J is a good quantum number. The microwave power required to achieve a π/2 pulse depends on quantum numbers J and mJ and the gJ for the levels involved in the EPR transition, so it can be used to characterize the electronic ground state. For Nd3+, Sm3+, Er3+, and Tm3+ the power required for a π/2 pulse is consistent with values calculated for the expected value of gJ and transitions involving mJ = ±1/2 or mJ = ±1,0. However for Ce3+, Gd3+, Tb3+, and Yb3+ the microwave power required for a π/2 pulse is larger than predicted. For Ce3+, Tb3+, and Yb3+ it is proposed that mixing of levels results in contributions to the transitions from higher values of mJ. For Gd3+ the discrepancy is attributed to overlapping transitions with varying values of mJ.

Stark Spectroscopy of Lumichrome: A Possible Candidate for Stand-Off Detection of Bacterial Quorum Sensing.

Lumichrome (7,8-dimethylalloxazine, LC) is a natural photodegradation product and catabolite of flavin coenzymes. Although not a coenzyme itself, LC is used for biosignaling in plants and single-celled organisms, including quorum sensing in the formation of biofilms. The noninvasive detection of in vivo lumichrome would be useful for monitoring this signaling event. For molecules that undergo significant charge redistribution upon light excitation (e.g., intramolecular charge transfer), there are optical detection methods (e.g., second-harmonic generation) that would be well suited to this task. Here, we have used Stark spectroscopy to measure the extent and direction of charge redistribution in photoexcited LC. Stark and low-temperature absorption spectra were obtained at 77 K on LC in ethanol glasses and analyzed using the Liptay analysis to obtain the difference dipole moments and polarizabilities. These data were complemented by a computational analysis of the excited states using density functional theory (DFT) at the TD-B3LYP/6-311+G(2d,p) level of theory.

Photophysics of a mono-nuclear tetrahedral silver(I)N4 core and its copper(I) analog

A neutral ligand L is prepared by condensation of benzil dihydrazone and acetone in 1:2 mol ratio and used for the syntheses of [CuL2]ClO4, [CuL2]PF6, [AgL2]ClO4 and [AgL2]PF6. The X-ray crystal structures of L and the two perchlorate salts have been determined. The metal complexes are found to be mononuclear containing tetrahedral N4 coordination spheres for Cu(I) and Ag(I). Cyclic voltammetrically, the Cu(II/I) and Ag(II/I) potentials are 1.23 and 0.76 V vs NHE respectively in CH2Cl2 at a Pt electrode. The reason for the such a low Ag(II/I) potential is that the silver(I) complex is adsorbed on the electrode surface with a free energy of adsorption of ∼ −14.99 kcal mol−1. DFT calculations at the BP86/LanL2DZ level show that the HOMO’s in [CuL2]ClO4 and [AgL2]ClO4 are both metal based and the LUMO’s have no contribution from the metals. Both complexes show weak emissions from the MLCT states upon excitation at 270 nm in ethanol solution at room temperature. Changing the counterion to PF6− leads to higher quantum yields for these processes. It is consistent with the general observation that ClO4− being more coordinating than PF6−, it binds the metals in the MLCT state to form exciplexes leading to a lower ϕ or total quenching. At 77 K in ethanol glass, L, [CuL2]+ and [AgL2]+ show ligand centered (LC) emissions. Thus the MLCT and LC states are thermally equilibrated in the silver(I) and copper(I) complexes. In keeping with this, a mixture of the two processes, MLCT and LC emissions, is observed at room temperature for [AgL2]ClO4 and [CuL2]ClO4 when they are incorporated in a rigid polymethyl methacrylate matrix.

Effect of nanoparticles on the hydrolytic degradation of PLA-nanocomposites by water-ethanol solutions

Hydrolytic degradation of poly(lactic acid) (PLA-C) and PLA-nanocomposite produced with organomodified montmorillonite at 5 wt% (PLA-OMMT) were investigated in pure water, 50% and 95% ethanol solutions at 40 °C. The change in molecular weight, crystallinity, release of lactic acid (LA), and release of the organomodifier (QAC) were monitored. Ethanol sorption and glass transition temperature (Tg) vs ethanol fraction content were determined. Release of Al as a nanoclay marker was studied when PLA-OMMT was exposed to 50% ethanol solution. PLA-C and PLA-OMMT films had faster hydrolysis in 50% ethanol than in 95% ethanol and water. The incorporation of nanoclay did not have an effect on the hydrolytic degradation. The nanoclay doubled the sorption of ethanol into the film due to the sorption of ethanol into the nanoclay galleries. The nanoclay also acted as an anchor, restricting the movement of the polymer chains as evidenced by the ΔTg and the crystallization behavior. Hydrolysis of PLA-C and PLA-OMMT was related with the release of LA during exposure. The release of QAC from PLA-OMMT films was higher in 95% ethanol at the early stages, but then it exponentially increased in 50% ethanol due to the faster hydrolytic degradation. The clay released from PLA-OMMT films during hydrolysis in 50% ethanol was 0.58% wt. at 90 d of the initial amount of nanoclay in the PLA film.

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion.

Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR(3)), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S1 over 800 fs are followed by bond heterolysis in ∼1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ∼50 ps followed by intersystem crossing in ∼300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR(3) experiments, permitting a more clear assignment and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ∼4 K and observing a triplet species by cryogenic EPR spectroscopy.

Frozen-in disorder probed by electron spin relaxation

Here we demonstrate that two extreme cases of molecular packing, i.e. the crystalline and glassy state of solid ethanol, which define the complete experimental span of attainable host matrix frustration, can be efficiently probed by measuring the electron spin phase memory relaxation time, TM, of an incorporated TEMPO nitroxyl radical at X-band EPR frequency. The sensitivity of TM is based on the spectral and spin diffusion effects which reflect the spatial distribution of involved spins as a function of molecular frozen-in disorder. By keeping the excited region of the EPR spectrum fixed one can monitor the changes in the density of spin packets, upon the state transformation of the host. EPR experiments performed at 94GHz show a clear difference in local distribution of paramagnetic centers in ethanol glass as compared to crystalline ethanol. The results support the idea that, by measuring TM and properties of density of spin packets, one can explore the extent of molecular disorder.

Fluorescence line-narrowing difference spectra: Dependence of Huang–Rhys factor on excitation wavelength

It is well known that fluorescence line narrowed (FLN) difference spectra (ΔFLN spectra) reduce to single-site fluorescence line shape function in the low-fluence limit. This finding is confirmed by the observation of different line shapes of FLN and ΔFLN spectra obtained for pyrene (Py) in ethanol glass at various excitation wavelengths using time-resolved fluorescence measurements. A peculiar dependence of the Huang–Rhys factor (S) on excitation wavelength was discovered, and discussed in terms of the solute–solvent, intermolecular Lennard–Jones potential. A V-shaped frequency dependence of the S-factor suggests that the repulsive–dispersive potential is not sufficient to account for a complex ‘color effect’.

Fluorescence and phosphorescence of rutin

Rutin is one of the most promising flavonoid from a pharmacological and biochemical point of view. Here we have explored its spectroscopic and photophysical properties at room temperature and 77K using steady-state absorption-luminescence methods and pulse spectroscopy equipment. By excitation into the absorption band 1 of rutin in methanol at room temperature the normal Stokes׳ shifted fluorescence with a maximum at 415nm and quantum yield of 2×10−4 was revealed. However, by excitation into the bands 2 and 3 any emission wasn’t observed. At 77K in ethanol glass we have observed fluorescence at 410nm and phosphorescence at 540nm for the first time. As a result the adequate energetic scheme including the lowest electronic excited singlet at 26000cm−1 and triplet at 19600cm−1 states was proposed.

Synthesis, spectroscopic, electrochemical and computational studies of rhenium(i) dicarbonyl complexes based on meridionally-coordinated 2,2′:6′,2′′-terpyridine

A homologous series of meridionally-coordinated tridentate 2,2':6',2''-terpyridine Re(I) dicarbonyl complexes have been prepared and investigated. The complexes mer,cis-[Re(tpy-κ(3)N)(CO)2(L)](n) (L = Cl(-) (1), OSO2CF3(-) (2), NCCH3 (3), CN(-) (4), NC5H5 (5), PMe3 (6), PEt3 (7), PPh3 (8), P(OMe)3 (9) and P(OEt)3 (10); n = 0 or +1) have been synthesized and characterized by elemental analysis, (1)H NMR and infrared spectroscopy. The electrochemistry of these compounds has been studied and compared to that of other known rhenium compounds using an electrochemical parameterization model. Cyclic voltammetry measurements have shown that the first oxidation of the complexes varies systematically in potential as the ligand L is altered. Many of these one-electron oxidations occur at lower potentials compared to more familiar bidentate diimine Re(I) tricarbonyl complexes. A correlation exists between CO, a strong π-acidic ligand, and deviations of the observed reduction potentials from the calculated values. Many of the complexes absorb light throughout a significant portion of the visible spectrum. Two of these complexes, mer,cis-Re(tpy-κ(3)N)(CO)2Cl (1) and mer,cis-Re(tpy-κ(3)N)(CO)2CN (4), absorb light throughout the entire visible spectrum. Low temperature emission spectra were obtained for the compounds mer,cis-Re(tpy-κ(3)N)(CO)2Cl (1) and mer,cis-[Re(tpy-κ(3)N)(CO)2(P(OEt)3)](+) (10) at 77 K in a 4 : 1 methanol-ethanol glass matrix and give metal-to-ligand charge transfer ((3)MLCT) luminescence. Density functional theory (DFT) calculations of the electronic structure are in good agreement with the experimental data.

Photoinduced Coupled Charge and Proton Transfers in Gradually Twisted Phenol−Pyridinium Biaryl Series

A detailed photophysical analysis of a phenol-pyridinium biphenyl series with gradual twisted geometry is presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of intensity with a progressive blue shift by increasing the twist angle of the central bond (Theta(AD)). These effects are well described and quantified within the framework of the Mulliken-Murrel approach, which allows us to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond, excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet-Taft parameters (pi*, alpha, and beta). The correlation first confirms the pi* dependence of the CSh band shift but also demonstrates a clear beta dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing with Theta(AD). Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected. The relative increase of its fluorescence band intensity with Theta(AD) confirms the gradual enhancement of the excited states acidity.

Change in Electronic Structure upon Optical Excitation of 8-Vinyladenosine: An Experimental and Theoretical Study

8-Vinyladenosine (8VA) is an adenosine analog, like 2-aminopurine (2AP), that has a red-shifted absorption and high fluorescence quantum yield. When introduced into double-stranded DNA (dsDNA), its base-pairing and base-stacking properties are similar to those of adenine. Of particular interest, the fluorescence quantum yield of 8VA is sensitive to base stacking, making it a very useful real-time probe of DNA structure. The fundamental photophysics underlying this fluorescence quenching by base stacking is not well understood, and thus exploring the excited state electronic structure of the analog is warranted. In this study, we report on changes in the electronic structure of 8VA upon optical excitation. Stark spectroscopy was performed on 8VA monomer in frozen ethanol glass at 77 K to obtain the direction and degree of charge redistribution in the form of the difference dipole moment, Deltamu(01) = 4.7 +/- 0.3 D, and difference static polarizability, tr(Delta(alpha)01) = 21 +/- 11 A(3), for the S(0)-->S(1) transition. In addition, solvatochromism experiments were performed on 8VA in various solvents and analyzed using Bakhshiev's model. High level ab initio methods were employed to calculate transition energies, oscillator strengths, and dipole moments of the ground and excited states of 8VA. The direction of Deltamu(01) was assigned in the molecular frame for the lowest optically accessible state. Our study shows that the angle between ground and excited state dipole moment plays a critical role in understanding the change in electronic structure upon optical excitation. Compared to 2AP, 8VA has a larger difference dipole moment which, with twice the extinction coefficient, suggests that 8VA is superior as a two-photon probe for microscopy studies. To this end, we have measured the ratio of the two-photon fluorescence yields of the two analogs by excitation at the respective monomer absorption maxima. We show that 8VA is indeed a significantly brighter two-photon fluorophore, based on our experimental and computational results.

Electron spin-lattice relaxation in solid ethanol: Effect of nitroxyl radical hydrogen bonding and matrix disorder

The electron spin-lattice relaxation of TEMPO and TEMPONE was measured at temperatures between 5 and 80 K in crystalline and glassy ethanol using X-band electron paramagnetic resonance spectroscopy. The experimental data at the lowest temperatures studied were explained in terms of electron-nuclear dipolar interaction between the paramagnetic center and the localized excitations, whereas at higher temperatures low-frequency vibrational modes from the host matrix and Raman processes should be considered. The strong impact of hydrogen bonding between the dopant molecule and ethanol host on the spin relaxation was observed in ethanol glass whereas in crystalline ethanol both paramagnetic guest molecules behaved similarly.

EPR study of crystalline and glassy ethanol

Abstract Pulsed X-band electron paramagnetic resonance (EPR) spectroscopy was applied in studying molecular dynamics in two different solid ethanol matrices. Nitroxyl radicals as paramagnetic reporter groups were embedded in crystalline and glassy ethanol and the phase memory time, T m , was investigated at 5–80 K. Temperature variation revealed a maximum in 1/ T m centered around 50 K and a small linear decrease with temperature, below ca. 25 K. Faster phase memory time relaxation in crystalline ethanol than in ethanol glass was observed throughout the temperature range studied. This can be attributed to differences in spectral diffusion due to distinct molecular packing densities.

Low-temperature electron-spin relaxation in the crystalline and glassy states of solid ethanol

X-band electron paramagnetic resonance (EPR) spectroscopy was used to study the spectral properties of a nitroxide spin probe in ethanol glass and crystalline ethanol, at 5 - 11.5 K. The different anisotropy of molecular packing in the two host matrices was evidenced by different rigid limit values for maximal hyperfine splitting in the signal of the spin probe. The significantly shorter phase memory time, , for the spin probe dissolved in crystalline ethanol, as compared to ethanol glass, was discussed in terms of contribution from spectral diffusion. The effect of low-frequency dynamics was manifested in the temperature dependence of and in the difference between the data measured at different spectral positions. This phenomenon was addressed within the framework of the slow-motional isotropic diffusion model [S. Lee, and S. Z. Tang, Phys. Rev. B 31, 1308 (1985)] predicting the spin probe dynamics within the millisecond range, at very low temperatures. The shorter spin-lattice relaxation time of the spin probe in ethanol glass was interpreted in terms of enhanced energy exchange between the spin system and the lattice in the glass matrix due to boson peak excitations.

Photophysics and photochemistry of tetrahydrocurcuminoids

The photochemical properties of 10 tetrahydrocurcuminoids (THCs) have been studied for the first time. They display very low fluorescence in ethanol solution at room temperature with quantum yields between 0.9 and 13 × 10 −3 whereas their phosphorescence quantum yields in ethanol glass at 77 K are between 0.025 and 0.5. Their disappearance quantum yield measured in ethanol solution, between 5 × 10 −4 and 10 −2, follows the same trend as their antiradical power efficiency (ARP) previously measured. The reactivity of THCs is enhanced if they include a phenol group in meta or para of the linking chain and a phenol or methoxy group as neighbor. UV absorption difference spectra of the irradiated solutions, HPLC and GC–MS analyses performed on 4-propylguaiacol (4PG), which displays similar behavior as tetrahydrocurcumin (THC2) indicates the formation of quinone methides and ortho-quinone. The formation of a phenoxy radical by photooxidation appears to be the first reaction step. This demonstrates that the general photodegradation mechanism of phenols found for guaiacyl lignins applies also to simplest molecules of natural origin such as THCs. The irradiation of the THCs in carboxymethylcellulose films follows the same trends as the solution studies, except that the quinone methides and ortho-quinones are not detected, probably because they react with the cellulosic matrix. The knowledge obtained in this study probably will help to promote the use of tetrahydrocurcuminoids in various applications where it is necessary to develop antioxidant properties in presence of solar light.

Direct evidence for the glass-crystalline transformation in solid ethanol by means of a nitroxide spin probe

The phase transition phenomenon in solid ethanol doped with nitroxyl radicals was studied by continuous and pulsed electron paramagnetic resonance (EPR) spectroscopy. The appearance of the crystalline state from the ethanol glass was for the first time directly monitored in the course of an EPR experiment. The switching of dynamical constraints in the nitroxide local environment was thermally controlled within the same host matrix. The achieved distinct molecular order was further characterized by measuring the effective spin lattice relaxation time ( T 1) of the entrapped nitroxides from 80 to 20 K and simulating data assuming a two-phonon Raman process.

Charge Shift and Triplet State Formation in the 9-Mesityl-10-methylacridinium Cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

Restricted orientational motion of nitroxides in molecular glasses: Direct estimation of the motional time scale basing on the comparative study of primary and stimulated electron spin echo decays

A comparative study of anisotropic relaxation in two-pulse primary and three-pulse stimulated electron spin echo decays provides a direct way to distinguish fast (correlation time tau(c)<10(-6) s) and slow (tau(c)>10(-6) s) motions. Anisotropic relaxation is detected as a difference of the decay rates for different resonance field positions in anisotropic electron paramagnetic resonance spectra. For fast motion anisotropic relaxation influences the primary echo decay and does not influence the stimulated echo decay. For slow motion it is seen in both two-pulse echo and three-pulse stimulated echo decays. For nitroxide spin probes dissolved in glassy glycerol only fast motion was found below 200 K. Increase of temperature above 200 K results in the appearance of slow motion. Its amplitude increases rapidly with temperature increase. While in glycerol glass slow motion appears above glass transition temperature T(g), in ethanol glass it is observable below T(g). The scenario of motional dynamics in glasses is proposed which involves the broadening of the correlation time distribution with increasing temperature.

Is o-carborane photoluminescent?

At 77 K in the solid state and in ethanol glasses, o-carborane (1,2-C 2B 10H 12) shows a relatively intense ( ϕ ∼ 10 −3 at λ exc = 260 nm) structured photoluminescence ( λ max = 441 nm).In agreement with the rather slow decay of this emission ( τ ∼ 4 s) it is suggested to be a phosphorescence. While it appears to be a genuine property of o-carborane an unknown impurity as origin of this luminescence is not completely excluded.