Expression of Concern: Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water.

Expression of concern for 'Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water' by Andrzej Dobek et al., Phys. Chem. Chem. Phys., 2017, 19, 26749-26757, https://doi.org/10.1039/c7cp04061j.

Optical aberrations and modulation transfer function of a thermal lens for use in imaging.

The optical aberrations of the thermal lens formed in a liquid are reported for the first time as expressed in terms of the Zernike polynomials coefficients. Convection of the liquid in which the thermal lens is formed is shown to strongly modify the lens imaging properties, even for the case of vertical heating laser illumination, as studied herein. The time evolution of the optical properties of the thermal lens is investigated. The direction from which the heating laser beam illuminates the thermo-optical liquid, in which the thermal lens is formed, has been found to be crucial for the lens optical power, type, and level of aberrations observed and the way these lens properties change in time.

Thermal lens with passively tuned focal length formed in a photochromic material.

A new, to the best of our knowledge, way of tuning the focal length of a thermal lens is presented. The thermal lens is formed in a photochromic thermo-optical material, through illumination of the material by a heating laser beam of constant power. The tunability of the lens is achieved by changing the absorption coefficient of the thermo-optical material at the heating laser beam wavelength. This change is obtained by modifying the intensity of the additional, flat in profile, light of a wavelength different than that of the heating laser beam. The change in intensity of this light may follow from the effects of external factors, as, e.g., decrease in daylight intensity. Therefore, the lens may be considered as a passive auto-tuning lens, with focal length auto-adjusting to the external illumination.

Multi-planar imaging using the thermal lensing effect.

The application of a thermal lens, formed by a laser beam, in the imaging of objects lying in three different planes is reported for the first time, to the best of our knowledge. The objects' planes are separated by a distance surpassing the depth-of-field of the imaging setup. It is the local formation of two thermal lenses near the intermediate image plane in a thermo-optical material that allows for the observation of two additional objects.

Multi-planar imaging using the thermal lensing effect.

The application of a thermal lens, formed by a laser beam, in the imaging of objects lying in three different planes is reported for the first time, to the best of our knowledge. The objects' planes are separated by a distance surpassing the depth-of-field of the imaging setup. It is the local formation of two thermal lenses near the intermediate image plane in a thermo-optical material that allows for the observation of two additional objects.

Motionless microscopy with tunable thermal lens.

The ability to modify the lens focal length has an advantage of giving sharp images without moving the lens. This work presents for the first time the use of the thermal lens as a tunable imaging element of the microscope. It shows also that the thermal lens can modify the image of a selected area of the sample, leaving the rest part of the image unaffected. Thus, appropriate tuning of thermal lens can lead to a simultaneously sharp image of two objects lying in different object planes separated by a distance exceeding the depth of field of the imaging optics.

Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water.

Nonlinear THz pump-optical probe (THz-OKE) measurements in deionized, distilled and buffered (PBS) water are reported. A laser system that produces pulses at 800 nm, 30 fs FWHM, at a repetition rate of 1 kHz and an energy of 6 µJ per pulse, was a source of the probe optical beam and of the pump beam of THz 2 ps pulses. The electric field strength inducing birefringence in water samples was ETHz = 1.35 × 106 V m-1. These samples were chosen in order to study the effect of ionic concentration on water behavior in the ultrafast time scale. Differentiation between ultrafast effects resulting from internal H2O properties and those resulting from H2O-ion interactions was analyzed. These two effects may be connected to a difference in the fluctuations of the network of intermolecular hydrogen bonds of water molecules in the presence and absence of ions in solution. The results indicate that such fluctuations significantly alter water birefringence amplitude and its dynamics.

Deactivation of 6-aminocoumarin intramolecular charge transfer excited state through hydrogen bonding.

This paper presents results of the spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in various aprotic hydrogen-bond forming solvents. It was established that solvent polarity as well as hydrogen-bonding ability influence solute properties. The hydrogen-bonding interactions between S1-electronic excited solute and solvent molecules were found to facilitate the nonradiative deactivation processes. The energy-gap dependence on radiationless deactivation in aprotic solvents was found to be similar to that in protic solvents.

An intermolecular hydrogen-bonding effect on spectral and photophysical properties of 6-aminocoumarin in protic solvents.

The paper presents results of the spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in strong protic solvents with different hydrogen-bond acidity. The measurements of the fluorescence lifetime and quantum yield clearly show that in protic solvents intermolecular hydrogen-bonding interactions between solute and solvent molecules facilitate the nonradiative processes. It is suggested that in these solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. A large isotope effect is found for 6AC in deuterated water.

Three modes of proton transfer in one chromophore: photoinduced tautomerization in 2-(1H-pyrazol-5-yl)pyridines, their dimers and alcohol complexes.

Studies of 2-(1H-pyrazol-5-yl)pyridine (PPP) and its derivatives 2-(4-methyl-1H-pyrazol-5-yl)pyridine (MPP) and 2-(3-bromo-1H-pyrazol-5-yl)pyridine (BPP) by stationary and time-resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited-state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited-state intermolecular double-proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent-assisted double-proton transfer in hydrogen-bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited-state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground-state syn-anti equilibria are detected and discussed. The fraction of the higher-energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen-bond donor or acceptor abilities.

The influence of temperature on C153 steady-state absorption and fluorescence kinetics in hydrogen bonding solvents.

In a recent paper (J Fluoresc (2011) 21:1547-1557) a temperature induced modulation of Coumarin 153 (C153) fluorescence lifetime and quantum yield for the probe dissolved in the polar, nonspecifically interacting 1-chloropropane was reported. This modulation was also observed in temperature dependencies of the radiative and nonradiative rates. Here, we show that the modulation is also observed in another 1-chloroalkane-1-chlorohexane, as well as in hydrogen bonding propionitrile, ethanol and trifluoroethanol. Change in the equilibrium distance between S (0) an S (1) potential energies surfaces was identified as the source of this modulation. This change is driven by temperature changes. It leads to a modulation of the fluorescence transition dipole moment and it is the primary source of the experimental effects observed. Additionally, we have found that proticity of the solvent induces a rise in the fluorescence transition dipole moment, which leads to a shortening of the fluorescence lifetime. Hydrogen bonds are formed by C153 also with hydrogen accepting solvents like propionitrile. We show that while such bonds do not affect the transition probability, they do change the S(0) an S(1) energy gap which in turn implies a change in non-radiative transition rate in a similar way as in protic solvents, as well as in the fluorescence spectrum position. Finally, the influence of temperature on the energies of hydrogen bonds formed by C153 when acting as hydrogen donor or acceptor is reported.

Radiationless deactivation of 6-aminocoumarin from the S1-ICT state in nonspecifically interacting solvents.

This paper presents the results of a spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in the solvents with which this molecule interacts only nonspecifically (n-alkanes, tetrachloromethane and 1-chloro-n-alkanes) and in nitriles. The strong effects of the solvents on the emission spectra, fluorescence quantum yield and lifetime of 6AC were observed. The results of the steady-state and time-resolved photophysical study suggest the presence of very fast nonradiative deactivation processes. It is concluded that besides fluorescence, the efficient S(1)-ICT → S(0) internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying S(1)-ICT(π,π*) and S(2)(n,π*) states. Moreover, unexpectedly efficient triplet state formation occurs. In nitriles the intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative decay process involving the S(1)-exciplex.

Temperature influence on 4-aminophthalimide emission in 1-chloroalkanes plus water mixtures.

Thermochromic emission shifts of 4-aminophthalimide (4-AP) dissolved in three 1-chloroalkanes, namely, 1-chloropropane, 1-chlorobutane, and 1-chlorohexane, together with fluorescence decays measured at different wavelengths, all solvents containing different small amounts of water (10(-4)-10(-2) M), are reported in a broad temperature range covering the bulk water melting point. Our studies have shown that 4-AP is an effective indicator of water presence even at concentrations given by solvent suppliers for so-called "dry" solvents. Additionally, indications of water ice-clusters formation in 1-chloroalkanes, starting at different temperatures depending on water concentration, have been found. Finally, 4-AP in 1-chloroalkane + water mixture is shown to present at different temperatures three basic types of sources of time-resolved emission spectra (TRES) time/spectral evolution: emission of two kinetically coupled species, solvent relaxation, and simultaneous emission of two independent species.

Photon coincidences in spontaneous parametric down-converted radiation excited by a blue LED in bulk LiIO3 crystal.

We report on experimental and numerical investigation of two-photon coincidence properties of the parametric spontaneous down-converted field excited by a high brightness blue LED in bulk lithium iodate crystal. Ratio of up to 11.5% of coincidence, which cannot be attributed to classical coincidences, to single photon counts was recorded at the outputs of multimode fibers, demonstrating well-preserved biphoton property. This result, combined with practically useful power of the source, suggests its possible application for a class of quantum experiments.

Influence of hydrogen bonds and nonspecific interactions on the spectral and photophysical properties of the excited singlet states of 4-aminophthalimide in amine solution.

The hydrogen-bond and nonspecific interaction energies for 4-aminophthalimide (4-AP), often used as a probe, in the ground electronic and excited singlet states are determined using ab initio computational methods. It is shown that the 4-AP molecule can form three relatively strong hydrogen bonds with trimethylamine (TMA) and triethylamine (TEA), which leads to the formation of S(0)-complexes between the solute and solvent molecules. Only two of the hydrogen bonds with the amine group of 4-AP change significantly their energies upon excitation and deactivation. The theoretical results are necessary to explain the spectral and unusual photophysical properties of 4-AP in amine solutions.

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.

Temperature influence on the energy of nonspecific and specific interactions taking place between 4-aminophthalimide (4-AP) and homogeneous solvents.

Temperature induced spectral shifts of the 4-aminophthalimide (4-AP) emission spectra have been measured and compared to the predictions of the McRae solvent induced shift theory (J. Phys. Chem., 1957, 61, 562-572). Three moderately polar chloroalkanes selected as nonspecifically interacting media, and six hydrogen accepting or/and electron pair donating solvents have been used as the media in which the temperature influence on 4-AP-solvent interactions has been studied in the range of 180-320 K. Using the ab initio determined 4-AP ground state dipole moment and fitting appropriate expression originating from the mentioned theory to the shifts found in the chloroalkanes it has been possible to estimate the 4-AP excited state dipole moment, the probe excited state Onsager radius and its gas phase emission spectrum position. Using these values the thermochromic shifts of 4-AP emission spectra in hydrogen bond forming solvents have been predicted and compared to the experimental one. Temperature has been found to have different impact on the changes, upon excitation of the probe, in the mean values of the energies of different hydrogen bonds formed by 4-AP with solvents molecules.

Different sources of 4-aminophthalimide solvation dynamics retardation inside micellar systems.

The solvation dynamics of 4-aminophthalimide (4-AP) in two micellar systems (cetyltrimethylammonium bromide (M-CTAB) and Triton X-100 (M-TX-100)) has been studied. The results presented are in agreement with our earlier findings in sodium dodecyl sulfate (M-SDS) micelle (J. Phys. Chem. B 107 (2003) 13,986, J. Phys. Chem. B 109 (2005) 9422). They have confirmed that the main reason for the observed shape and position of the time changes in 4-AP time-resolved emission spectra (TRES) is the process of establishing a new equilibrium between two emissive species present in micellar systems, the excited 4-AP in the intramolecular charge transfer state (S(1)-ICT) and the exciplex formed between 4-AP in the S1-ICT state and water molecules dissolved inside micelles. In M-TX-100 and in M-CTAB this process has been found to be slower than in the earlier studied M-SDS. The presence of two emitting species has been concluded on the basis of observation of the isoemissive point in the time-resolved area-normalized emission spectra (TRANES) of 4-AP in micellar systems studied. It has been shown that the distance between the 4-AP molecule and the water molecules present inside the micelles can be one of the parameters responsible for the long-time duration of the exciplex formation and solvation process in the micelles.

Ground- and excited-state double proton transfer in lumichrome/acetic acid system: theoretical and experimental approach.

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.

Shape and position of 4-aminophthalimide (4-AP) time-resolved emission spectra (TRES) versus sodium dodecyl sulfate sds concentration in micellar solutions: the partitioning of 4-AP in the micellar phase and in water surrounding the micelles.

Time-resolved emission spectra (TRES) of 4-aminophthalimide (4-AP) dissolved in water solutions of sodium dodecyl sulfate (SDS) for three surfactant concentrations (0.05, 0.15, and 0.45 M) have been determined. The fraction of 4-AP dissolved in the water phase surrounding the micelles has been shown to increase with decreasing concentration of the surfactant. To obtain TRES of 4-AP present exclusively in micelles, a method of subtraction of the contribution of the emission originating from 4-AP present in the water phase surrounding the micelles from the total emission of the probe dissolved in SDS solution has been proposed. The consequences of failing to take into account the partitioning of 4-AP between the water and micellar phases are illustrated by some exemplary TRES results, taken before and after the subtraction of the emission originating from 4-AP present in the water phase. Together with the time of appearance and presence of isoemissive points in the time-resolved area-normalized emission spectra (TRANES), these results have shown a clear dependence of the rate and character of the 4-AP TRES changes on the SDS concentration. In connection with our earlier results and literature data, it has been concluded that the concentration of the water solubilized in micelles is the main factor determining the rate and character of these changes.