Proton Phototransfer Research Articles

Derivatives of 1,3,5 triaryl-2-pyrazoline with additional heterocyclic moieties in position 1 as potential fluorescent chemosensing compounds for detection of polyvalent metals cations

A series of 1,3,5-triaryl-2-pyrazoline derivatives with pyridine or 8'-hydroxyquinoline fragments in position 1 have been synthesized, some of which also contain a hydroxy group in the ortho-position of benzene ring-3, which makes possible the their OH-group proton phototransfer reaction to the nitrogen atom of pyrazoline cycle. The nitrogen atoms in molecules of the investigated compounds form chelate cavities, binding of the ions of polyvalent metals to which should lead to changes in the spectral characteristics. The spectral-luminescent properties of the synthesized compounds in solvents of different polarity and their interaction with salts of several polyvalent metals in acetonitrile were investigated. The proton phototransfer reaction was shown to result in significant fluorescence quenching, the most prominent for the derivative with the hydroxyquinoline fragment in position 1 of pyrazoline cycle. The proton transfer reaction rate constants vary within the interval of 2-4∙109 s-1 in aprotic solvents of different polarity. The prospects for application of the synthesized derivatives as fluorescent chemosensor compounds for the analysis of polyvalent metals were examined. In most of the studied cases, their interaction with heavy metal ions led to decrease of fluorescence intensity, however for compounds with intramolecular proton phototransfer reaction, no significant quenching by heavy metal ions was observed. The exception was the mercury ions, in the interaction with which compounds with proton phototransfer reaction demonstrated intensity redistribution of their two emission bands on the background of their general fluorescence quenching. The latter circumstance indicates the possibility of application of the modern methods of ratiometric fluorescence detection for the analysis of Hg2+ ions with their help.

Photoprocesses in 7-hydroxy coumarins molecules in solutions and SiO2 sol–gel films – Materials for optical chemical sensorics

The paper investigates the spectral properties of protolytic forms of three substituted 7-hydroxy coumarin molecules in aqueous ethanol solutions and SiO2 films. It explains spectral-and-luminescent properties of coumarins in the produced films. It is found that in aqueous ethanol solutions and sol–gel films, upon excitation in the S1 state, there is a reaction of proton phototransfer from the О–Н group of neutral coumarins molecules to the water molecule with development of anionic forms solvated by hydroxonium ions. We studied sensory properties of compounds in the films to the ammonia gas in terms of changes in their fluorescent properties and revealed the reasons causing such changes. We demonstrated the prospects of using films with one of the investigated coumarins as the ammonia gas sensitive material of an optical chemical sensor.

Mathematical Modelling of Dynamic Quenching of Dual-Band Fluorescence Pulses of Molecules with Intramolecular Hydrogen Bonds

Dynamic quenching of the intensity of dual-band fluorescence pulses is modelled using numerical solutions of the equations for the population matrix of a model molecule with five states. The state with the highest energy is assumed to be resonantly excited by light and two other excited states that are populated by subsequent relaxation processes are assumed to be the initial states for molecular transitions with emission of dual-band fluorescence photons. The model parameters for the molecule are chosen to match the typical parameters of molecules with intramolecular proton phototransfer. Quenching is treated as a consequence of nonradiative decay of the excited states of the fluorescing molecule owing to collisions with quenchant molecules. Examples of different dual-band fl uorescence quenching mechanisms for the model molecule are given, depending on the decay of particular excited states of the molecule involving nonradiative intramolecular transitions or transitions into states of the quenchant molecules.

Resonant light-scattering spectra of molecules with double-well vibrational potentials

Introduction. Oscillators, the motion of which is determined by a potential with a single minimum for a molecule in both the ground and excited electronic states, are used as a model for the vibrational subsystem in the overwhelming majority of theoretical studies of Raman scattering (RS) of light by molecules. The success of the theory is due mainly to the fact that the overlap integrals for wave functions of oscillators for the ground and excited electronic states of the molecule that are required to derive expressions for the RS line intensity are used in the analytical form. However, many vibrational states of various molecules should be examined using models of oscillators characterized by a potential with two minima (double-well potential). For example, this refers to models describing the Jahn–Teller and Renner–Teller effects, the rotation dynamics of a fragment of an organic molecule around a single or double bond, and the states of a proton in an intra- or intermolecular hydrogen bond (H-bond). Exact expressions for the steady-state wave functions of a particle in a double-well potential have not been obtained in the analytical form. Therefore, approximate wave functions that were found by numerical solution of the corresponding Schrodinger equation with the sought wave functions represented as a linear combination of the eigenfunctions of a harmonic oscillator were used as the vibrational wave functions in one of the first theoretical studies [1] of RS of light by a molecule with a vibrational subsystem characterized by a double-well potential in the lower of two interacting electronic states taking into account Jahn–Teller and Renner–Teller effects. Superpositions of two harmonic oscillator functions, each of which belonged to one of the potential wells, were taken as the approximate vibrational wave functions in theoretical studies [2–4] of RS of light by a molecule with a vibrational subsystem, the motion of which was determined by a double-well potential in the ground electronic state also. Obviously such an examination has a very limited scope of application. Only qualitative conclusions about certain features of the RS process in molecules with double-well vibrational potentials were drawn. Moreover, knowledge of the rules for RS of light by molecules with a double-well potential for the examined vibrational mode is of purely theoretical and great practical interest because RS spectroscopy, especially for irradiation of molecules in the frequency range of their self-absorption [resonance RS spectroscopy (RRS)], is an effective method for establishing, e.g., the mechanism of the position change of a proton in an intramolecular H-bond in the excited electronic state of the molecule during light transformation by it (proton phototransfer). Much attention has re

Molecular Stark effect spectroscopy of diflavonol and inhomogeneous broadening of its electronic spectra in erythrocyte membranes

Using spectroscopy of the molecular Stark effect and fluorescence spectroscopy, we study the characteristics of diflavonol 3,7-dihydroxy-2,8-di(4-dimethylaminophenyl)-4H,6H-pyrano[3,2-g]chromene-4,6-dione (DFME), which demonstrates intramolecular charge and proton phototransfer. In the ground state, this dye has only one form and, in the excited state, it has two forms, i.e., normal and phototautomeric. We found that, for the normal form of DFME, the transition dipole moment that is responsible for the absorption (m a ), the dipole moment in the equilibrium ground state (μ g ), and the change of the dipole moment upon transition of the molecule in the excited Franck-Condon state (Δ a μ) are parallel. In the ground equilibrium state, the dipole moments in 1,4-dioxane and cyclohexane are equal to μg = 12.2 × 10−30 C m and μ g = 11.0 × 10−30 C m, respectively. Upon excitation, they increase by Δ a μ = 61 × 10−30 C m and Δ a μ = 50.2 × 10−30 C m in these solvents. We study the spectral characteristics of DFME in organic solvents and erythrocyte membranes. A spectral inhomogeneity of DFME in erythrocyte ghosts is found. The inhomogeneous broadening of fluorescence spectra is manifested as a long-wavelength shift of the band of the normal form of DFME by 1640 cm−1 upon excitation at the red edge of the absorption spectrum.

Mathematical modeling of photoisomerization with intramolecular proton transfer

A system of equations describing time changes in the matrix elements of the density operator of a seven-level model of a molecule interacting with a light pulse taking into account spontaneous (including collective) decays of molecule excited states is suggested. Model parameters were selected to allow us to perform modeling of the photoisomerization of a molecule with two isomeric states with different stable proton positions on an intramolecular H-bond by numerically solving the suggested system of equations for density operator matrix elements. An analysis of the characteristic time dependences of the population of states of the model under consideration showed that proton phototransfer in the collective decay of various isomeric states of a molecule in an excited electronic state can be one of effective mechanisms of the photoisomerization of molecules whose structure is described by the model.

A mathematical simulation of intramolecular proton phototransfer

A system of equations for the matrix elements of the density operator of a seven-level model molecule interacting with a light pulse was solved numerically to determine the time dependences of the populations of molecule states at various radiation pulse parameters and parameters characterizing radiative and nonradiative spontaneous molecule transitions and reversible transitions between some of its states. The results were used to characterize the photoisomerization of molecules between states with different positions of the proton of the intramolecular H-bond (the keto and enol forms). Examples of oscillating molecular state population modulation in isomer-isomer tunnel proton shifts are given. Changes in the development of photoionization in time as molecular parameters and radiation pulse width and intensity changed were considered. An analysis of the results obtained is an example of the use of mathematical simulation of intramolecular dynamics for increasing the effectiveness of using spectral-time data in the determination of the mechanism of proton phototransfer in molecules with intramolecular H-bonds

On the Doubly hydrogen bonded dimer of 7-azaindole (0.1 M) as a model for DNA base pairs in acetonitrile solutions at rt

AbstractMultiple H-bonded base-pairing as a fundamental element of DNA structure was first described by Watson and Crick using stable keto tautomer forms. In their analysis, they considered the possibility of mutations via tautomeric proton transfer shifts. Among other phenomena, such shifts can be caused by electronic excitation; for example, anomalous adenine-cytosine pairing may be a result of two-proton phototautomerism. One suitable model base pair for two-proton translocating tautomerization is the C2h dimer of 7-azaindole (7AI) proposed by Taylor et al. in 1969. The most salient contribu&x00AC;tion of their work was that the double proton transfer in such a dimer occurs in a concerted manner. After the strong controversy raised in 1995 by the proposal of Douhal et al. of a stepwise mechanism for the process was overcome, its concerted nature has been strongly supported by available evidence (see references and references therein). Very recently, however, Kwon and Zewail claimed to have obtained new supportive evidence that the proton phototransfer in 7AI dimer in polar solvents is in fact a stepwise rather than concerted process. In this communication, we conducted a systematic spectroscopic study of the dimerization of 7AI at a 0.1 M concentration in acetonitrile that may allow us to demonstrate whether the 0.1 M 7AI solutions in acetonitrile at room temperature used by Kwon and Zewail contained enough 7AI doubly hydrogen bonded dimer to enable its photoselection and hence its photophysical characterization with femtosecond / fluorescence spectroscopy.

Dipolar switch of the proton phototransfer in the doubly hydrogen-bonded 7-azaindole dimer with C2hsymmetry

The potential loss of the C2h symmetry in 7-azaindole dimer caused by the interaction with a polar molecule or environment was examined in a 10−4 M solution of this compound in 2-methylbutane containing 4 × 10−5 M 1-chlorobutane and in pure 1-chlorobutane at a low temperature, respectively, in the light of the proton phototransfer processes potentially undergone by the dimers. A scheme for the proton phototransfer in the dimer is proposed based on which the transfer involves two protons if the dimer possesses C2h symmetry and a single one if such a symmetry is lost through complexing with a polar molecule or under the action of a polar environment. Based on the results, the proton transfers in 7-azaindole dimer take place via different mechanisms and the single-proton process is not an intermediate step in the formation of the tautomer dimer that results from the two-proton process.

Radiationless deactivation of the excited phototautomer form and molecular structure of ESIPT-compounds.

Correlation between the character of electronic density redistribution in the excited normal and phototautomer forms, which are involved in the excited state intramolecular proton transfer (ESIPT) reaction, and the rates of primary photoprocesses--proton phototransfer and radiationless deactivation of the phototautomer form--was analysed on the qualitative level. It was revealed that noticeable improvement of fluorescence properties of ESIPT molecules could be achieved by directed modification of their chemical structure, most importantly by the introduction of electron withdrawing substituents into their proton donor moieties. Several facts are presented in favour of the last statement.

Excited state intramolecular proton transfer reaction and luminescent properties of theortho-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole

Luminescent properties of several ortho-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole were studied in solvents of different polarity and protolytic ability and compared with the analogous derivatives of 1,3-oxazole, studied by us earlier. Evaluations of rate constants for the excited state intramolecular proton transfer (ESIPT) reaction and radiationless degradation of the energy of electronic excitation in the ESIPT reaction product, phototautomer form, were made on the basis of fluorescence quantum yields and fluorescence kinetic measurements. The proton phototransfer reaction is the most efficient primary photoprocess in the examined series of compounds. With rate constants of 1011–1012 s−1 it exceeds the rates of other primary photophysical processes in these molecules by up to 2–3 orders of magnitude. A connection between the rate of the ESIPT reaction and the electron density redistribution on electronic excitation was revealed for the molecules under study. The electron donor substituents in 5-phenyl decrease the proton phototransfer rate constant up to complete inhibition of the ESIPT process in the case of the dimethylamino derivative of the oxazole series, whereas the electron acceptor substituents must accelerate this reaction. The determining role of the excited state increase of the proton donor group acidity was assumed on the basis of experimental data and quantum chemical calculations. The temperature investigations showed that the ESIPT reaction in the series of ortho-hydroxy derivatives of 2,5-diphenyloxazole and 2,5-diphenyl-1,3,4-oxadiazole has a very low activation barrier. However, the radiationless processes in the phototautomer form are characterized by the presence of a certain barrier in both examined series. A dependence of the radiationless dissipation rate in the phototautomer form on the excited state redistribution of the electron density in it was also found. The more efficient is the charge transfer from the ortho-phenolate fragment to the protonated heterocycle and the rest of the molecule, the more efficient are the radiationless losses. It was revealed that structural changes, which prevent the mentioned movement of electron density, result in an increase in the fluorescence efficiency of the phototautomer form and, hence, also to an increase in the total fluorescence quantum yield of the ESIPT molecule. Copyright © 2000 John Wiley & Sons, Ltd.

Photoinitiated processes in 1-p-tolylsulfonylazo-2,4,6,8-tetrakis(tert-butyl)phenoxazine

New photochromic compound 1-p-tolylsulfonylazo-2,4,6,8-tetrakis(tert-butyl)phenoxazine containing the intramolecular hydrogen bond NH...N and the corresponding model structures 2,4,6,8-tetrakis(tert-butyl)-1-(veratroylazo)phenoxazine and 2,4,6,8-tetrakis(tert-butyl)-N-acetyl-1-(p-tolylsulfonylazo)phenoxazine were synthesized and studied. The data obtained suggested the mechanism of the photoreaction resulting in the accumulation of betaine 1-hydroxy-2,4,6,8-tetrakis(tert-butyl)-10-tolylsulfonylphenoxazin-9-one. The photochromic transformations in the molecule under study are due to intramolecular proton phototransfer followed by E–Z-isomerization about the N–N bond and the formation of betaine 1-hydroxy-2,4,6,8-tetrakis(tert-butyl)-10-tolylsulfonylphenoxazin-9-one. The molecular and crystal structure of the photoproduct was studied by X-ray analysis.

Proton phototransfer in a series of ortho-hydroxy derivatives of 2,5-diaryl-1,3-oxazole and 2,5-diaryl-1,3,4-oxadiazole in polystyrene films

Proton transfer in the lower excited singlet state has been examined for ortho-hydroxy derivatives of 2,5-diaryl-1,3-oxazole and 2,5-diaryl-1,3,4-oxadiazole in polystyrene films, as this largely determines the spectral characteristics. There is found to be a substantial reduction in the radiationless deactivation in the product from that reaction, namely the phototautomeric form, which is the basic reason for the low quantum yield in fluorescence in liquid solutions. These changes are explained from the viewpoint of a reduction in the probabilities of high-amplitude intramolecular motions in the excited phototautomeric forms of these ortho-hydroxy derivatives in a polymer.

Proton phototransfer kinetics in a solution of 3-metoxybenzanthrone in a polar aprotic solvent with weak donor additives

Proton phototransfer kinetics in a solution of 3-metoxybenzanthrone in a polar aprotic solvent with weak donor additives

Static and dynamic fluorescence quenching of carbazole by tropanic alkaloids

Steady state fluorescence quenching of carbazole by tropanic derivatives in cyclohexane and acetonitrile was performed. The results obtained using cyclohexane as solvent support the existence of two species in equilibrium: free and hydrogen-bonded complexed carbazole in both the ground and excited states. The parameters involved in the static and dynamic quenching were obtained separately and the thermodynamic parameters corresponding to these associations were calculated. In acetonitrile, the equilibrium in the excited state is established between carbazole and its proton phototransfer product.

Broadening of the tuning range of dye laser radiation by utilization of excited-state relaxation processes

A review is given of lasers utilizing multicomponent dye solutions and photochemical lasers relying on fast proton phototransfer reactions in organic compounds. An analysis is made of the influence of fast reversible photochemical reactions and electron energy transfer processes on broadening of the spectral tuning range of dye laser radiation. Criteria guiding the selection of acid-base systems for the active media of photochemical lasers and of donor-acceptor systems for the active media of energy-transfer lasers are formulated. Spectral characteristics of the active media of photochemical lasers and also of binary dye mixtures of energy-transfer lasers are presented.

Picosecond dynamics of proton phototransfer reactions in polar solutions of organic dyes

Picosecond dynamics of proton phototransfer reactions in polar solutions of organic dyes

Aspects of the structure of the nonphotosensitive crystal hydrate of p-bromo-[N?-(p-nitrobenzylidene)]benzhydrazide

1. An x-ray diffraction structural analysis has shown the p-bromo-[N′-(p-nitrobenzyli-dene)]benzyhydrazide forms a 1∶1 crystal hydrate. 2. Replacement of an N′-furfurylidene substituent by the N′-benzylidene substituent leads to a significant change in the structure of the crystal hydrate. The water molecules in N′-furfurylidenebenzhydrazide form unidimensional linear hydrogen bond chains between stacks of ketohydrazone molecules. The water molecules in N′-benzylidenebenzhydrazide form a three-dimensional hydrogen bond network, which prevents stacking of the ketohydrazone molecules. In the former case, the hydrogen bond system persumably is favorable for an intermolecular N → O proton phototransfer, while such a transformation is not favored in the latter case. Thus, the crystals of p-bromo-[N′-(p-nitrobenzylidene)]benzhydrazide are non-photosensitive.

Effect of a methyl substituent on the molecular and crystal structure of (2-acetyl-5-nitrofuran)-N-benzoylhydrazone

An x-ray diffraction study of (2-acetyl-5-nitrofuran)-N-benzoylhydrazone has shown that the molecules of this compound have a nonplanar structure with syn arrangement of the oxygen and nitrogen atoms of the keto and hydrazone groups and an anti arrangement of the oxygen atom of the furan fragment. With such a conformation, the molecules do not form crystal hydrates; in the crystal, they are connected with one another into chains by intermolecular hydrogen bonds N-H...O 2.26 A. However, intermolecular N → O proton phototransfer does not occur between them.

Effect of the proton phototransfer reaction on the laser radiation of heteroaryloxazoles

the SA by high-dimensional noise for values of r comparable with the variance of this noise, "breaks" should be observed on the curves in Figs. 4a and b. For r less than the coordinates of these breaks, the slope of in C(r)(m) equals the dimension of the imbedded phase space, i.e., m. Of course, if the intensity of the technical noise is so high that the fractal structure of the SA is "smeared" completely, then the curves of in C(r)(m). versus In do r not have a section with equal slope, and therefore such noise dominates in the laser radiation.