Effects of long-wavelength fluorescence excitation in N, N′-dimethylaminobenzonitrile liquid solutions

We have studied the properties of the emission, absorption, and excitation of dual fluorescence of N,N??-dimethylaminobenzonitrile in a set of solvents of different polarity under selective irradiation of solutions by light with different energies of quanta in the range of the long-wavelength absorption band. In all cases, dual fluorescence is observed, which is caused by emission from locally excited Franck-Condon and charge-transfer states. A change in the energy of excitation quanta has no effect on the position of the fluorescence bands; however, the intensity ratio between the bands noticeably changes in favor of the intensity of the long-wavelength band, which belongs to the charge-transfer state. To explain the observed effects, we involve data of quantum-mechanical calculations, which show that there is a considerable probability of occurrence in solutions of these systems of rotational isomers that differ in the orientation of the dimethylamino group with respect to benzonitrile. In the excited state, these rotamers have different charge-transfer reaction rates, which leads to a change in the intensity ratio of the observed fluorescence bands upon using the selective excitation.     

Modulation of charge transfer rate in (N,N′-dimethylamino)benzonitrile liquid solutions

We studied the properties of the emission, absorption and excitation of dual fluorescence of (N,N′?dimethylamino)benzonitrile in a polar aprotic solvent acetonitrile under selective irradiation of solutions by light with different energies of quanta to elucidate mechanisms of dual fluorescence arising in this solvent at different temperatures in the range 274–313 K. In all cases, dual fluorescence of the solute in acetonitrile was observed, which is caused by emission from locally excited Franck-Condon and charge-transfer states. A change in the energy of excitation quanta has a weak effect on the position of the fluorescence bands; however, the intensity ratio between the bands noticeably changes in favour of the intensity of the long-wavelength band at excitation in the range of the long-wavelength absorption band. An interesting and unusual fact is that solution heating is accompanied by essential growth of quantum yield of dual fluorescence at all wavelengths of the excitation. To explain the observed effects, the same dependences were measured and analysed for DMABN in neutral solvent n-hexane in the same conditions. We involve also the data of quantum-mechanical calculations, which show that there is a considerable probability of occurrence in solutions of DMABN rotational isomers with differing orientation of the dimethylamino group with respect to the benzonitrile. In the excited state, these have different charge-transfer rates, resulting in a modulation in the intensity ratio of the observed fluorescence bands with change excitation energy quanta on the red wing of the absorption band, doi: 10.1134/S0030400X12050219.     

The influence of temperature on red-edge excitation effects in liquid solutions of N,N′-dimethylaminobenzonitrile

A systematic study of new photophysical and photochemical processes in solutions is continued by the example of a recently found phenomenon of redistribution of the intensities of two fluorescence bands of N,N′-Dimethylaminobenzonitrile (DMABN) in polar solutions at room temperatures under selective irradiation by light with different photon energies in the region of the long-wavelength absorption band. The effects observed are explained using data of quantum-mechanical calculations, which reveal that solutions of these systems are very likely to contain rotational isomers with different orientations of the dimethylamino group with respect to the plane of the benzonitrile residue. The excited-state charge transfer reactions in these rotamers occur in different ways and, hence, with different rates, because of which the intensity ratio of recorded fluorescence bands is different for different wavelengths of selective excitation. In this study, the influence of the temperature on the red-edge excitation effect observed in the fluorescence of DMABN solutions in acetonitrile is studied in the temperature range of 274–313 K using the previously used selective excitation method. It is found that these effects manifest themselves at any temperature within this range, but are especially strong at 313 K. The parameters of the dual fluorescence that are most sensitive for recording of the considered effects are determined, and the obtained temperature dependences are interpreted.     

Fluorescent probe based on proton transfer in 4′-(diethylamino)-3-hydroxyflavone: Spectral and luminescent characteristics upon selective excitation

The spectral characteristics of acetonitrile solutions of 4′-(diethylamino)-3-hydroxyflavone dye with dual fluorescence are studied under selective excitation. This dye is a structural analog of 3-hydroxyflavone and exhibits excited-state proton transfer, which, in contrast to 3-hydroxyflavone, has a thermodynamic rather than a kinetic character. The fluorescence spectra at different excitation photon energies and the excitation spectra of different fluorescence bands are studied. It is found that the intensity ratio of the normal and tautomeric fluorescence bands lying near 507 and 570 nm, respectively, depends on the excitation wavelength, namely, this ratio is 1.45 and almost does not change in the region of the main absorption band (370–420 nm), while, in the region of the second singlet band (near 280 nm), it decreases to 1.15. This can be explained by an increase in the probability of proton transfer with formation of a tautomeric form in the case of excitation into the second band. Another interesting feature is the existence of a latent third emission band peaked at 535 nm, which was found and reliably recorded upon excitation at wavelengths of 470–500 nm. Addition of water quenches this emission, which indicates that it belongs to the anionic form of the dye.     

Deactivation of an excited charge-transfer complex II. Influence of rigidity of medium on radiative and radiationless transitions

Fluorescence spectra, quantum yields and decay times for charge-transfer complexes of tetracyanoethylene with benzene, toluene, ortho-xylene and para-xylene in low-temperature glassy solutions of different viscosities have been investigated. It was established that the influence of viscosity or rigidity of medium on charge-transfer fluorescence consists in the decrease of efficiency of radiationless processes when viscosity or rigidity increases. Fluorescence spectra of complexes of tetracyanoethylene with toluene, o-xylene and p-xylene have a composite double-band character at high viscosities of medium. The intensity of both subbands changes in a different way with change of viscosity. The excitation spectra of these complexes are dependent on the wavelength of observation. These facts are discussed in terms of an existence of different orientational isomers of a charge-transfer complex of low symmetry.     

Effect of temperature on the dynamic quenching of the dual fluorescence of molecules

The properties of the dual fluorescence of 3-hydroxyflavone in acetonitrile are studied under conditions of dynamic quenching by the spin quencher TEMPO in the temperature range from 20 to 80°C. 3-Hydroxyflavone is characterized by the intramolecular excited state proton transfer and its spectrum consists of two well-spaced fluorescence bands belonging to the normal and tautomeric forms. The fluorescence was selectively excited at wavelengths of 290, 304, and 340 nm, belonging to different absorption bands of the luminophore. The character and degree of the temperature quenching of the fluorescence depend on the excitation wavelength and are considerably different for normal and tautomeric fluorescence bands. The Stern-Volmer constants for both forms, calculated from the experimental data on the assumption of the diffusion mechanism of quenching of the excited states, increase with temperature. Both in the pure solutions and in the solutions with the quencher, the intensity ratio of the fluorescence of the initial form and the product increases with heating in the entire temperature range from 20 to 80°C for all the above excitation wavelengths. Original Russian Text ? V.I. Tomin, 2008, published in Optika i Spektroskopiya, 2008, Vol. 104, No. 6, pp. 926–933.     

Excitation Wavelength Dependence of Dual Fluorescence of DMABN in Polar Solvents

Steady-state absorption, fluorescence excitation and emission spectra of 4-(N,N-dimethylamino)benzonitrile (DMABN) have been measured at room temperature in cyclohexane, 1,4-dioxane, dichloromethane, and acetonitrile solutions. The fluorescence spectra of DMABN are found to exhibit dual emission in 1,4-dioxane, dichloromethane, and acetonitrile solutions and single emission in cyclohexane solution. The effect of solvent polarity and excitation wavelength on the emission spectra has also been studied. The fluorescence excitation spectra of DMABN monitored at the emission bands are different. The presence of two different conformations 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 compound. The experimental studies were supported by ab initio density functional theory (DFT) calculations performed at the B3LYP/6-31Gd level of theory. On the basis of the experimental results and our theoretical calculations, we suggest that there are two conformers of DMABN, which are stable in the ground state, equilibrated in solution at room temperature that give rise dual fluorescence upon excitation.     

Spectral properties of molecular charge-transfer probe QMOM

The spectral characteristics of solutions of a dye with dual fluorescence, 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone, in acetonitrile are studied upon selective excitation. This dye is a structural analogue of 3-hydroxyflavone and also exhibits excited-state proton transfer, which, as well as in the case of 3-hydroxyflavone, has a kinetic nature. The fluorescence spectra are studied upon excitation by photons of various energies, and the excitation spectra are recorded at wavelengths of different fluorescence bands. It is found that the intensity ratio of the emission of the normal and tautomeric forms (at wavelength of 415 and 518 nm, respectively) is almost the same (0.23–0.25) for excitation in the regions of the main and the second absorption bands. At the same time, in the case of excitation between these bands, this ratio decreases to 0.19. The second interesting feature is the existence of a third latent emission band peaked at about 480 nm, which is reliably detected upon excitation at wavelengths in the region of 400–450 nm. This study shows that this emission belongs to the anionic form of the dye. This form is also responsible for a decrease in the intensity ratio of the emission of the two main forms in the case of excitation between the first and second absorption bands.     

Intramolecular proton transfer from the highest singlet states in 3-hydroxyflavone

It is found that the excitation spectra of the dual fluorescence of 3-hydroxyflavone are different for different recording wavelengths and that the intensity ratio of the emission of the normal and tautomeric (with intramolecular proton transfer) forms upon selective UV excitation in the regions of the S ₁, S ₂, and S ₃ singlet absorption bands strongly depends on the excitation wavelength. The results obtained directly point to the existence of an additional channel of population of the excited state of the tautomeric form and are explained by the intramolecular proton transfer through the S ₂ and S ₃ excited singlet states of fluorophore molecules. The constants of this transfer are estimated using analytical relations for the steady-state fluorescence excitation.     

Solvent Polarity and Excitation Wavelength Dependence of the Dual Fluorescence in N,N-Diethyl-4-Nitrosoaniline

Dual fluorescence in N,N-Diethyl-4-nitrosoaniline (DENA) has been studied employing absorption, excitation and emission spectroscopic techniques and computational methods. The absorption and fluorescence spectra of DENA were measured in solvents of various polarities at room temperature. The emission spectra of DENA were found to exhibit a single emission band in non polar solvent (cyclohexane) and in a highly polar solvent (acetonitrile). In the contrary, two emission bands were observed in medium polar solvents (tetrahydrofuran, 1,2-dichloroethane and dichloromethane) whereby the short (local excited; LE) and long (charge transfer; CT) emission maxima correspond to the emission maxima of the compound observed in cyclohexane and acetonitrile solutions, respectively. Moreover, the two emission bands have shown strong excitation wavelength dependence, and area normalization resulted in an iso-emissive point. The two emission maxima were in addition found to correspond to two excitation maxima in 3D fluorescence spectra. Further, two minima were obtained in potential energy surface calculation of DENA. From the experimental and computational results it was concluded that the dual fluorescence may be attributed to the presence of two different ground state structural conformers of DENA in equilibrium that are stabilized through solute-solvent interaction.     

On estimating probability of excited-state intramolecular proton transfer

Higher singlet states can play an important role in various intramolecular processes. Recent investigations of the time-resolved (with a picosecond resolution) spectra of the dual fluorescence of 3-hydroxyflavone molecules excited in the region of the S ₁ and S ₂ absorption bands by pulses with a duration of ∼44 ps have directly shown the occurrence of the proton transfer from the carboxyl to the carbonyl group of the molecule upon excitation into the second singlet absorption band. The reaction times estimated from the emission characteristics are comparable with the electronic level lifetime (several picoseconds), as a result of which the direct measurements are rather difficult. The proton transfer through the S ₂ state is also recorded in the steady-state fluorescence excitation spectra. In this study, it is shown how the reaction rate can be estimated from these data.     

Molecular probe FET: Long-wavelength fluorescence excitation effect and proton transfer reaction

We studied the steady-state fluorescence spectra of solutions of FET (4′-(diethylamino)-3-hydroxyflavone) in acetonitrile that were excited at different temperatures by quanta with different energies located in the range of the main absorption band and in its long-wavelength wing. We found that, at room temperature, the emission intensity ratio of the bands of the normal and tautomeric forms, which are located at 505 and 570 nm, respectively, depends on the excitation wavelength. In the range of the main absorption band 300–360 nm, this ratio remains nearly the same, i.e., 1.45, while, upon excitation in the range of the long-wavelength wing 360–380 nm of the main band, it decreases to 1.33 at a wavelength of 460 nm. In this same range, a long-wavelength excitation effect that is unusual for liquid inviscid solvents at room temperature, i.e., a bathochromic shift of the entire short-wavelength emission band by 11 nm, manifests itself. We propose to explain these dependences using energy diagrams, which take into account the dependence of free energy on the orientational polarization of the polar solvent. The observed effect of the long-wavelength shift of the fluorescence spectrum with increasing excitation wavelength is explained in terms of the inhomogeneous broadening of electronic spectra of polar solutions, and it should be described using the scheme of energy states that takes into account sublevels of orientational broadening due to orientational dipole-dipole interactions of the fluorophore with nearest molecules of the polar solvent, as well as the relation between the fluorophore lifetime in the excited state and the dielectric relaxation time of solvent molecules in the field of the fluorophore dipole.     

Extreme narrowing of bands in the fluorescence excitation spectra of organic molecules in solid solutions

Extreme narrowing of bands in the fluorescence excitation spectra of some organic molecules is found. This narrowing is observed when the excitation spectrum of a narrow spectral interval of the fluorescence band is recorded. The effect is observed in a wide variety of matrices and permits one to determine exactly the vibronic structure of the excited electronic state of the dissolved molecule.     

Luminescence of solid cyclic polynitramines

The solid form of the cyclic polynitramines known as RDX and HMX have a weak absorption band in the near ultraviolet which is not observed in spectra of the solvated compounds. Fluorescence measurements on solid samples of these materials show a weak emission, the excitation spectrum of which corresponds to this absorption band. Phosphorescence measurements on the solids at 77 K reveal a long-lived emission, the excitation spectrum again corresponding to the weak absorption band. The absorption and the luminescence bands are attributed to charge-transfer self-complex formation in the solid state, and estimates of the energies of the emissive singlet and triplet excited states are given.     

Studying reactions that proceed from highest excited states of molecules using fluorescence quenching

The steady-state monochromatic excitation of a luminophore that has fluorescing products is considered. The effect of dynamic quenching of highest excited states on the fluorescence of singlet states under its excitation via singlet S ₁ and S _n (n ≥ 2) states is discussed. It is shown that the use of the method of fluorescence dynamic quenching by foreign impurities opens new possibilities for studying photoreactions that proceed via S _n singlet states. A large number of primary photoprocesses are considered which include the electron density redistribution (the internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bonding, and formation of excimers and exciplexes. It is shown that, upon dynamic quenching, the bimolecular quenching constant of an excited level depends on the amount of thermal energy released in the luminophore before the occurrence of the light emission event. Based on the experimental measurements of the fluorescence spectra at different quencher contents, the calculation of the Stern-Volmer constant for reaction products is considered in detail. It is shown that this constant can be most reliably determined from the dependence of the fluorescence intensity ratio of the initial reagents and the quencher product rather than from the dependence of the fluorescence intensity of the products on the concentration of the quencher. The relations determined are used in analysis of the experimental fluorescence spectra of solutions of 3-hydroxyflavone excited by radiation with different wavelengths lying in the range of the S ₁ and S ₂ absorption bands. The temperature behavior of the Stern-Volmer constant for different fluorescence bands of 3-hydroxyflavone is considered. It is shown that, if these constants for the normal and tautomeric forms are correctly determined, their temperature dependences are similar.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Separation of a weak fluorescence signal of 3-hydroxyflavone using dynamic quenching

The spectral and time characteristics of the dual fluorescence of a 3-hydroxyflavone probe in a solution with a fluorescence quencher are studied in the temperature range of 20–80°C. At room temperature, the fluorescence consists of two bands that belong to the normal and tautomeric forms of the luminophore, while heating of the solution results in the appearance of an additional fluorescence band belonging to the anionic form of the luminophore. The additional band is strongly overlapped with the band of the normal form, and its intensity rapidly increases with temperature to exceed the intensity of the normal band. The introduction of the TEMPO spin quencher of excited states into the solution completely quenches the fluorescence of the anionic form, which allows one to record the pure dual fluorescence of 3-hydroxyflavone in the entire temperature range studied. The detection of probe signals in the pure form is important for applications of proton transfer molecular probes using the intensity ratio of the fluorescence bands as the main sensitive parameter.     

Temperature dependences of dual fluorescence as criterion for determining character of photoreactions

We performed comparative studies of the temperature quenching of dual fluorescence of acetonitrile solutions of several molecular probes with proton transfer reaction in an excited singlet state of 4′-(dieth-ylamino)-3-hydroxyflavone (FET), 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone (QMOM), and 3-hydroxyflavone (3HF) parent molecule at different energies of excitation quanta. In accordance with expressions obtained from balance equations for photoreactions of the kinetic and thermodynamic character, the intensity ratios of fluorescence bands as functions of the degree of quenching behave differently. Namely, the quenching increases the relative intensity of bands normal form/tautomer for reactions of the kinetic type, retaining this ratio unchanged for reactions of the thermodynamic character. Our experimental studies showed that, for fluorescent probes with the kinetic reaction (3HF and QMOM), the intensity ratio fluorescence bands increases almost linearly with the degree of quenching, whereas, in the thermodynamic case (FET), this ratio is independent of this parameter. Conclusions about the character of reactions that we obtained in this work agree well with data of independent investigations of these molecules by laser spectroscopy with high time resolution, and the obtained relations allow us also to judge the mechanism of temperature quenching in the case of the reaction of the kinetic type. The method can be used for comparatively simple express selection of molecular probes, candidate for new applications.     

High resolution laser induced fluorescence excitation spectroscopy of jet-cooled terrylene

High resolution laser induced fluorescence excitation spectra upon absorption in the A¹B_3u ← X¹A_g band of jet-cooled terrylene have been recorded. Precise energies of three vibronic transitions are deduced. Low lying vibrations are found in both electronic states. Rotational constants in ground and excited state are determined by band contour analysis. Terrylene is a medium-size polycyclic aromatic hydrocarbon (PAH) and a possible carrier of diffuse interstellar bands (DIB). The results of the jet experiments are discussed regarding the PAH-DIB hypothesis.     

Dynamic quenching of the dual fluorescence of molecules

Mathematical relations describing the properties of spontaneous steady-state dual fluorescence under conditions of dynamic quenching of excited states by foreign impurities are derived. It is shown that, in the case of a kinetic character of the reaction, the initial form of the dye and its photoproduct are quenched, the intensity ratio of the fluorescence bands of the initial form and the product linearly increasing with the quencher concentration. Analysis performed is applicable to a wide range of photoreactions accompanied by the dual fluorescence (charge transfer, proton transfer, complexation, etc.). The properties of the fluorescence, absorption, and dual fluorescence excitation for 3-hydroxyflavone in acetonitrile under conditions of dynamic quenching by the TEMPO spin quencher with a concentration below 1.25 × 10^?2 M are studied. 3-Hydroxyflavone is characterized by the excited-state intramolecular proton transfer and by the fluorescence spectrum consisting of two well-spaced bands. The observed dependences of the intensity of both fluorescence bands on the quencher concentration correspond to the theoretical conclusions. The Stern-Volmer constants calculated from the experimental data on the assumption of diffusion quenching of the excited states are 858 and 1141 M^?1 for the normal and tautomeric fluorescence bands, respectively. The experimental results reveal the kinetic character of the excited-state proton transfer in 3-hydroxyflavone in acetonitrile.