ISSN 0030400X, Optics and Spectroscopy, 2013, Vol. 114, No. 5, pp. 729–736. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.I. Tomin, R. Jaworski, 2013, published in Optika i Spektroskopiya, 2013, Vol. 114, No. 5, pp. 794–802.
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Modulation of the Proton Transfer Rate by Excitation Photons V. I. Tomin and R. Jaworski Institute of Physics, Pomeranian Pedagogical University, S l upsk, 76200 Poland email:
[email protected] Received November 22, 2012
Abstract—The effect of the exciting photon energy on the excited state proton transfer in a dye with dual flu orescence—FET (4'diethylamino)3hydroxyflavone)—is studied. The steadystate fluorescence spectra are studied upon selective excitation by photons with different energies in the region of the main absorption band, as well as at its longwavelength wing, in the temperature range of 2–30°C. It is found that, at all tem peratures, the ratio of the integral emission of the normal and tautomeric forms, which are observed at 480 and 570 nm, respectively, depends on the excitation wavelength; namely, this ratio noticeably decreases with increasing excitation wavelength in the region of the main absorption band and its longwavelength wing at 390–440 nm, and the rate of this decrease depends on temperature. In the same region, the longwave length excitation effect, which is atypical for inviscid solvents at room temperature, is observed; i.e., a short wavelength emission band is bathochromically shifted by 6–15 nm depending on temperature. This spectral shift is directly related to the inhomogeneous broadening of the electronic spectra of the normal FET form, which is very large due to a considerable (>10 D) difference in the dye dipole moments. Most probably, the excitation creates the possibility of emission from nonrelaxed nonequilibrium orientational sublevels because their lifetime becomes shorter due to the proton transfer reaction, the rate of which in acetonitrile is compa rable with the rate of intermolecular orientational relaxation. It is proposed to explain these dependences using energy diagram taking into account the dependence of the free energy on the orientational polarization of the solvent. DOI: 10.1134/S0030400X13050196
INTRODUCTION In recent decades, molecular fluorescent probes, the operation mechanism of which is based on their sensitivity to local polarity, dipole orientational relax ation of the nearest environment, and hydrogen bonds, have found wide application [1–4]. Efficient probes usually exhibit a pronounced redistribution of electron density in the excited state (charge transfer) during the electronic transition, and, in some probes, the charge transfer also continues in the excited state, if the geometry changes before the emission event. The study of these fluorescent probes allowed one to observe clearly pronounced and very interesting spec troscopic features—namely, the timedependent Stokes shift of instantaneous emission spectra [2, 5– 9], which was observed for the first time in [10]; static and dynamic inhomogeneous broadening of elec tronic spectra [6–9]; such rededge effects [2, 3] as the directed Förster homotransfer of electronic excitation energy [7–9], which occurs in systems of chemically identical molecules; and the Brownian rotational depolarization of fluorescence, which depends on the stored intramolecular orientational energy. These effects have been successfully used in recent years to study the structural and molecular dynamics in differ ent physicochemical and biological systems [7–9]. Further progress in this field is related to the appear ance and efficient use of probes with dual fluores
cence, the spectral properties of which additionally depend on other parameters of the microenvironment and which are also called multiparametric probes [4, 8, 11–13]. As is known, both the initial excited mole cules and the products of their photoreactions very often exhibit efficient luminescence with two spectral bands, and the luminescent methods with the use of such systems are indispensable for investigations in various fields of physics, chemistry, and biology. The most wellknown photoreactions, the products of which exhibit intense fluorescence are the charge transfer (CT) and proton transfer (PT) reactions [1–4, 8, 11–15], and the wellknown prototypes of such sys tems are, respectively, DMABN (N,N'dimethylami nobenzonitrile) and 3hydroxyflavone molecules, as well as some of their derivatives. An important property of systems with dual fluo rescence is that intermolecular interactions cause con sistent changes in the intensity of the two fluorescence bands; this property allows calibration of the response of such systems to the action of the environment, which is widely used, for example, for investigating various physicochemical objects by molecular probes with excitedstate intramolecular proton transfer (IPT) [4, 8, 15]. It is of interest for practice to know the mechanisms of the influence of various physico chemical factors on the properties of the dual fluores cence and on the proton transfer rates, which deter
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S1N
k+ S1P
knRP
νN kRP
kRN
knRN
νe
k−
νP
S0N
P
S0P
N
Fig. 1. Energylevel diagram describing the reaction of for mation of the fluorescent photoproduct (Р) upon excita tion into the absorption band of the normal (N) form of the fluorophore.
mine the relative intensity on the emission bands of the luminophore and the reaction product. In this connection, the study of the fundamental properties of these molecular probes remains important. It should be noted that chemists recently managed to synthesize a series of molecules in which the IPT reactions are accompanied by CT reactions, which may occur both before and after IPT [4, 12, 13, 15–18]. Such systems provide additional possibilities for probing due to the heterogeneity of the solvates of such dyes, which leads to inhomogeneous broadening of their electronic spectra and, as a result, to the rededge excitation effects. The dyes with the dual fluorescence make it possible not only to perform highly sensitive recording of fluorescent responses using comparatively simple equipment, but also to avoid many problems related to obtaining quantitative data. The ratio of the signals at different wavelengths can be easily calibrated, is insen sitive to instrumental factors, and does not depend on the dye concentration, which allows one to obtain reproducible signals during longterm operation. To clarify the IPT specific features, we studied the spectral characteristics of a dye with dual fluores cence, namely, 4'(diethylamino)3hydroxyflavone (FET), in dichloromethane upon selective excitation. This dye is a structural analog of 3hydroxyflavone and also demonstrates excitedstate IPT. In addition, this dye in the excited state of its initial normal form has a considerably higher polarity than in the ground state; i.e., it is a system with a strong CT preceding the IPT. It is found that the relative yields of the integral lumi nescence of the normal and tautomeric forms at room temperatures depend on the excitation wavelength and temperature. In the region of the main absorption band and its longwavelength wing at 390–440 nm, we observed a longwavelength excitation effect, namely, a bathochromic shift of the entire shortwavelength emission band by 10–25 nm depending on tempera
ture, which is untypical for liquid inviscid solvents at room temperature. This spectral shift is directly related to the inhomogeneous broadening of the FET electronic spectra, which is very strong for the solution under study since the difference in the dye dipole moments exceeds 10 D. Most probably, the excitation creates the possibility of emission from unrelaxed nonequilibrium orientational sublevels due to short ening of the lifetime of emitting centers as a result of PT, the rate of which is comparable with the rate of ori entational relaxation of solution molecules. It is pro posed to explain these dependences using energy dia grams taking into account the dependence of free energy on the orientational polarization of the polar solvent. This study continues the detailed investiga tions of the rededge shift of the fluorescence spec trum, which was previously observed for FET in a strongly polar solvent, acetonitrile [19]. SCHEME OF THE DUAL FLUORESCENCE Figure 1 presents the scheme that describes an arbi trary photoreaction in the excited state. After absorp tion of light with frequency νe lying within the main absorption band, the molecule passes from the ground state S 0N to the excited level S1N , which is responsible for the shortwavelength fluorescence band (the S1N → S 0N transition with frequency νN and probability k RN ). From the S1N state, nonradiative transitions to the N ground state with probability k nR also occur, as well as the reaction of formation of the excited form of the product in the singlet S1p state with rate k+. In most cases, the formation of a photoproduct is a more ener getically favorable process; hence, its singlet level S1p usually lies slightly lower than the S1N level. The S1p state spontaneously decays within the lifetime τР through transitions with probability kRP (fluorescence), P nonradiative transitions with probability k nR , and N reverse transitions to the S1 level with probability k–. Thus, the second band of the spontaneous emission relates to the transitions from the excited S1P state of the photoproduct to the ground S 0P state. The study of this dual fluorescence is an efficient method of investigating both the N and P systems themselves and the effect exerted on them by the fields of the environment or host matrix. However, the scheme shown in Fig. 1 is very general and only roughly explains the formation of dual fluorescence in the PT process. At the same time, some dependences of this fluorescence on the physical conditions of the experiment (temperature, excitation photon energy, solvent or matrix characteristics) cannot be under OPTICS AND SPECTROSCOPY
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stood using such simplified diagrams. It should also be noted that the PT reaction is one of the most funda mental reactions in chemistry and biology and is very important for practice.
IF, arb. units 300
400 O
420 380
We used solutions of FET in dichloromethane (Aldrich, pure grade, for UV–VIS spectroscopy) with a concentration of 10–5 M. The fluorescence spectra were measured using a Hitachi F2500 spectrofluo rimeter, and the absorption spectra were recorded on a Hitachi U2810 spectrophotometer. The fluorescence was recorded using a scheme at an angle of 90° to the excitation beam. RESULTS AND DISCUSSION The structural formula of FET is shown in Fig. 2. Two active chemical groups, 3OH (proton donor) and 4carbonyl (proton acceptor) are bound to the basic molecular skeleton and are continuously con nected by hydrogen bonds, which predetermines the fastest PT way along this bond. Being an isomer with respect to the main form, the tautomeric form exhibits efficient fluorescence with a wide band shifted to the red. Thus, dual fluorescence appears, the properties of which can provide rich information on the PT and its dependence on various physical factors. This mecha nism of dual fluorescence is typical for some deriva tives of 3hydroxyflavone. The dual fluorescence related to the tautomer formation, as well as its mech anism typical for a large number of chemical com pounds, was described for the first time in [20, 21] for 3hydroxyflavone molecules. Figure 2 also presents the fluorescence spectra of FET in dichloromethane at a temperature of 10°С measured upon excitation at different wavelengths λех in the range from 380 to 460 nm. As is seen, the irradiation of the solution in the region of the main absorption maximum leads to the appearance of two absorption bands peaking at ~485 (shortwavelength band) and ~570nm band (longwavelength band). The first band is more effi cient than the second one and, according to the scheme in Fig. 1, is identified as the fluorescence of the normal form. The second fluorescence band is assigned to the S1P state of the tautomeric form, which is a photoproduct of the normal form appearing upon photoexcitation to the singlet S1N state. The positions of the two bands correspond to the previously observed spectra of this compound [22, 23]. The shift of the tau tomeric fluorescence with respect to the normal band is ~85 nm. The tautomer has the same chemical for mula as the initial parent molecule, with the only dif ference being that a proton of the hydroxyl group is transferred to the oxygen atom of the carbonyl group of the molecule. In contrast to 3hydroxyflavone (classical compound with IPT [13, 21]), the intensity Vol. 114
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N
C2H5
C2H5
430 440
100 450
0 400
500
600
λ, nm
Fig. 2. Fluorescence spectra of FET for different excitation wavelengths in the range of 380–460 nm (figures near cor responding curves). The inset shows the structural formula of FET. T = 10°С.
ratio I N I P of the two bands in FET is higher than unity, which testifies that reverse reaction rate k– is high or comparable with the decay rate of the N* level. The tautomer fluorescence band position does not depend on the excitation wavelength in the studied range of 390–460 nm, while the emission band of the normal form shifts to the red with increasing excita tion wavelength. The shift of the short wavelength as a whole is continuous and rather strong, and, hence, the spacing between the two bands decreases. Similar changes were observed at all temperatures used, and the shifts of the maxima of the normal form at differ ent temperatures vary from 7 to 15 nm (Fig. 3). In addition to the shift of the N band maximum, one can see a clear modulation of the intensity ratio of these bands. Figure 4 shows the dependence of the intensity ratio I N I P of the initial and tautomeric fluorescence bands on the excitation wavelength. As is seen, this ratio at first decreases in the region of 390–420 nm for all temperatures and then slightly increases. For better understanding of these dependences, the intensities in this case must be estimated taking into account the rather noticeable overlap of the two broad fluores cence bands of the dye, because of which the measured intensities in the maxima are not true values. Keeping in mind that the measured fluorescence intensities only approximately reflect the total fluorescence yield (more precise measurements must take into account emission from all sublevels of the given electronic state), we tried to process the obtained data more accurately and estimated the relative fluorescence yields by the areas under the fitting curves plotted so that their sum represented the measured spectrum.
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492 λmax, N, nm
3 488 2 484 1
480 400
370
460 λex, nm
(b)
100 Δλ0.5, N, nm
430
60
400
430
460 λex, nm
Fig. 3. Dependences of the (a) peak position and (b) half width of the fluorescence band of the FET normal form on the excitation wavelength for temperatures T = (䊊) 2, (+) 10, and (䉭) 20°C.
The fluorescence profile was approximated using asymmetric sigmoid functions I F of the form IF =
300
350
400
450 λex, nm
Fig. 4. Intensity ratio I N I T (solid curve, ×20) of the flu orescence of the normal and tautomeric forms as a func tion of the excitation wavelength and absorption spectra at T = (䊊) 2, (+) 10, and (䉭) 20°C.
80
40 370
0
A 1 + exp[− (λ − λ 0 + w1 2) w2]
⎛ ⎞ 1 × ⎜1 − ⎟, ⎝ 1 + exp[− (λ − λ 0 − w1 2) w3]⎠
(1)
where А, λ0, w1, w2, and w3 are the constants depend ing on the shape of the approximated spectrum. The results of this approximation are shown in Fig. 5 for two spectra excited at wavelengths of (a) 390 and (b) 450 nm. It is seen that the fitting curves for individual bands describe very well the total experimental spec trum of two bands. As a result of this simulation, we find a more detailed shape of the spectra—in particu lar, determine their halfwidths—and, hence, can determine the fluorescence yields of the bands more accurately. The halfwidths of the fluorescence bands of the normal form given in Fig. 3 for different excitation wavelengths are found using the described fitting. As is seen from Fig. 3, at all temperatures, the spectrum becomes noticeably shorter with increasing excitation wavelength. This fact may directly point to selective
excitation of some groups of the spectral centers of the inhomogeneously broadened ensemble. As was noted above, the estimation of areas SN and SР of the corre sponding bands using the fitting curves of the experi mental spectrum must more correctly reflect the total number of photons emitted from all sublevels of the given vibronic state than the approximate estimation of intensity by the band peaks. Therefore, the SN/SР ratio must yield a more correct dependence of the integral fluorescence on the excitation wavelength. The dependences obtained in this way are shown in Fig. 6 for all temperatures. It is clearly seen that the excitation wavelength strongly affects the SN/SР ratio; namely, this ratio decreases with wavelength increas ing from 380 to ~420 nm at all temperatures and, at longer wavelengths, increases weakly for 2 and 10°С and more strongly for 20°С. It is interesting that heat ing from 2 to 20°С also leads to an increase in SN/SР at all excitation energies. Thus, it is seen that, in the range of 380–420 nm, the stored vibrational energy affects the fluorescence spectrum identically for both methods of the transfer of this energy to the molecule; this conclusion corresponds to the classical concepts of the luminescence of complex molecules. Note also that the SN/SР ratio is the most convenient and sensi tive parameter for studying the effect of physicochem ical parameters on the emission of fluorophore mole cules This parameter is called ratiometric and does not depend on the dye concentration, variations in the absorption at different wavelengths (as in our case, since the excitation occurs at different wavelengths at the steep slope of the absorption curve), quenching factors, and photochemical reactions in samples. Thus, the experiments described show that a decrease in the excitation photon energy leads to interesting dependences in the solution of FET in dichlo romethane in the temperature range studied. The main features are related to (1) a change in the relative OPTICS AND SPECTROSCOPY
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SN/SP
483
4 566
IF, arb. units
100
0 400
2
500
0 380
600 (b)
40
400
420
440
460 λex, nm
Fig. 6. Ratio SN/SP of the integral emission of the normal and tautomeric forms vs. excitation wavelength at temper atures T = (䊊) 2, (+) 10, and (䉭) 20°C.
492
566
20
0 400
500
600
λ, nm
Fig. 5. Experimental fluorescence spectra and their fitting by function (1) upon excitation at (a) 390 and (b) 450 nm at a temperature of 10°C.
emission of the two forms (the relative contribution of the N band almost always decreases with increasing wavelength at temperatures of 2 and 10°С, while at room temperature it decreases only in the range of 380–420 nm and then increases in the range of 420– 460 nm) and (2) a longwavelength shift of the lumi nescence band of the normal form and simultaneous shortening of its halfwidth (without considerable vari ations in the band shape). The first feature is explained very simply: the yield of the photoproduct (in our case, tautomer) is proportional to the rate of the corre sponding photoreaction, and an increase in the contri bution of the longwavelength emission band in this case testifies to an increase in this rate [11, 12]. The behavior observed at room temperature needs addi tional consideration. We may suppose that the temper ature quenching of the fluorescence in this case changes the SN/SР ratio because, as is known, the longwavelength emission of the reaction product is quenched stronger and, hence, the SN/SР ratio increases. To interpret the second specific feature, let us look at Fig. 3, which clearly shows the bathochro mic shift of the entire shortwavelength band with decreasing excitation photon energy. These effects are well studied for polar solutions of molecules without excited state CT and IPT. For these molecules, the effects related to changes of the main characteristics upon longwavelength excitation are called rededge or longwavelength excitation effects [2–4, 7–9]. All the found dependences, namely, the decrease in the SN/SР ratio and the longwavelength shift of the emis OPTICS AND SPECTROSCOPY
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sion band of the main form, cannot be explained using the simple scheme of the PT reaction shown in Fig. 1. It is obvious that the occurrence of this reaction must strongly depend on the solvation processes, which may considerably change the relative position of singlet levels participating in the reaction and the reaction barriers. The solvation processes are directly caused by intermolecular van der Waals interactions and are especially strong for polar fluorophores in polar sol vents. The FET molecules have rather large dipole moments, which, according to electrooptic measure ments, are 6.8 and 16 D for the ground and excited states of the normal form, respectively; the angle between these dipoles is 25° [24]. Quantummechan ical calculations of the dipoles of the ground and excited states of the tautomeric form yield 5.5 and 5.2 D, respectively [23]. Thus, the excitation of the dye causes a very strong CT or electron density redis tribution with a change of the dipole moment by 10.2 D, which is followed by the PT reaction. The change in the dipole moment of the normal form determines the sensitivity of the corresponding luminescence band to the solvent polarity—this band undergoes a large bathochromic shift with increasing polarity of the medium. At the same time, the tautomer is almost insensitive to polarity since the dipole moments of its combining levels are almost the same. As is known, thermal and structural fluctuations cause inhomogeneous broadening of electronic spec tra of dipole fluorophores in polar solvents, which leads to interesting features in the fluorescence char acteristics with changing excitation photon energy, i.e., to rededge excitation effects [2, 3]. To describe these effects and take into account the inhomoge neous broadening, it is convenient to use the field dia gram of the polar solution. This diagram represents the dependence of the free energy of the solvate, which contains a dye molecule and neighboring solvent mol ecules in each electronic state, on the coordinate char acterizing the degree of orientation of molecules in the
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halfwidth of this distribution is readily estimated by the formula [7–9]
ρN*, ν1
N*
Δν 0.5 = Δμ ( 2 f κT ) , 12
k+
ρN*, ν2
T*
νN'
νN' ν1
νT
ν2
T N ρN0 RI
RII
R
Fig. 7. Energy diagram of free energy U of the singlet levels of the normal and tautomeric forms in a polar solvent as a function of reactive field strength R.
solvate shell or their polarization by the dye dipole field. As such a coordinate, it was proposed [7, 25, 26] to use reaction field R created by the solvate shell as a result of its polarization by dye dipole μ,
R = f µ,
(2)
where f is the reaction rate or the dielectric permittiv ity of solvent molecules in the solvate shell of the dye molecule, which is easily calculated using a particular solvate model, for example, the Onsager model. The minimum and the steepness of the curve are deter mined by the electric dipole moment, and the poten tial curves of the main four states calculated taking into account the known FET dipole moments μN = 6.8 D, μN* = 16 D, μT* = 5.5 D, and μT = 5.2 D [23, 24] have the form shown in Fig. 7. This scheme allows us to estimate the change in the fluorophore energy occurring in the process of molecular relaxation and as a result of the PT reaction. The minima of the curves for the ground states correspond to the equilibrium configuration of all dipoles of solvate molecules, at which the vectors of the dye dipole and the reaction field are oriented so that the solvate has the minimum free energy. In the ground state, the equilibrium field strength is RI and all the solvates of the solution under the thermodynamic equilibrium condition are distrib uted over broadening states (points along the R axis) corresponding to the Gaussian distribution ρN for dif ferent field strengths R with the maximum at R1. The
(3)
where Δμ = μe – μg is the difference between the dipole moments of the fluorophore in the ground (μg) and excited (μe) states. As is seen, halfwidth Δν 0.5 depends on difference Δμ, solvent reaction rate f, and the absolute temperature of the solution. From expression (3), using the Onsager model, which allows one to calculate the f function (see, for example, [5– 8]), it follows that halfwidth Δν 0.5 for FET molecules in dichloromethane (CH2Cl2) at room temperature and ε = 9.1 is significant, ~1200 cm–1. The radius of the Onsager sphere used in calculations is 0.46 nm. It was calculated from the volume of this molecule, which was determined using an ACDLABS 11/0 ChemSketch software. It is obvious that the inhomo geneous broadening halfwidth will further increase with increasing temperature. The potential curve of the N* excited state is shifted with respect to the curve of the N state to the right along the electric field axis and its minimum corre sponds to field strength RII exceeding field strength RI because the dipole moment of the excited state is con siderably larger than the dipole moment of the ground state (μN* > μN). The N* and T* curves for the main and tautomeric forms may be separated by an energy barrier, which characterizes activation energy Еа of the PT reaction (not shown in the figure). The reaction can occur only if the fluorophore is excited by an energy no lower than Ea. Photons with different ener gies can excite solvates with different orientational energies. In particular, the excitation in the absorption maximum corresponds to the absorption of equilib rium and neighboring centers, the stored orientational intermolecular energy of which exceeds the reaction barrier. Then, the relaxation of excited solvates may occur via three processes: the intermolecular orienta tional relaxation (wavy arrows along the potential curves), the IPT reaction with the rate k+, and the internal radiative (kr) and nonradiative (knr) conver sion to the ground state. All three processes occur in parallel and, if the IPT reaction successfully competes with intermolecular relaxation, the fluorescence occurs from the states with intermolecular energy stored in the excitation process and with the frequen cies of 0–0 transitions corresponding to the vertical (in accordance with the Franck–Condon principle) tran sitions from these states. The tautomeric forms in this case also appear from unrelaxed states (vertical arrows k+). The experimentally measured kinetics of both flu orescence bands of FET in acetonitrile can be found in the literature. The decay kinetics of the excited singlets of the normal form have the fast components τ1 < 0.1 ps and τ2 ~ 0.44 ps with relative contributions of OPTICS AND SPECTROSCOPY
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0.42 and 0.51, respectively, and a long component (0.43 ns) [22]. The fast components have positive amplitudes for the normal form and approximately the same negative amplitudes for the tautomeric form and, hence, should be assigned to the characteristic times of the PT reaction. The orientational relaxation times of dichloromethane in our temperature range vary from 2.2 to 1.6 ps, decreasing with heating [27]. We may assume that the PT rates for FET molecules in dichlo romethane will not be strongly different from the data of [22], since dichloromethane, similar to acetonitrile, is also an aprotic solvent and the PT reaction will not be hindered by intermolecular hydrogen bonds. Therefore, the direct PT in our solution occurs before, during, and after establishment of the equilibrium configuration in the process of orientational relax ation leading to an equilibrium distribution of excited molecules over orientational degrees of freedom. The excitation in the absorption maximum of sol vates with 0–0 transition frequency ν1 transfers to the fluorescent level of the stable solvates without orienta tional energy in the ground state (corresponding to reaction field RI). These solvates in the excited state will have the maximal store of orientational excitation and create some nonequilibrium distribution of sol vates over orientational degrees of freedom ρ N*, ν1 , which then must relax to a stable equilibrium distribu tion with a maximum corresponding to the minimum of the N* curve, if this process is not terminated by a faster transfer of molecules to the tautomeric form. Instantaneous distribution ρ N*, ν1 created by fre quency ν1 determines the fluorescence spectrum of the normal form and, under the conditions of a fast PT reaction, the tautomer is formed directly from the states with the maximum orientational energy, because the relaxation has no time to occur com pletely due to a fast depopulation of these states caused by the PT reaction. As a hypothesis, we may assume that the excess of the orientational energy hinders the PT process by disturbing, for example, the intramo lecular hydrogen bond between the hydrogen of the hydroxyl group and the oxygen of the carbonyl group, which predetermines the natural occurrence of this reaction. A decrease in the exciting radiation energy (the 0–0 transition frequency ν2 in Fig. 7) creates dis tribution ρ N*, ν2 of excited centers with lower orienta tional energy and 0–0 transition frequencies. They lie closer to the minimum of the potential curve N* and are characterized by fluorescence at longer wave lengths. This distribution is closer to equilibrium and weaker changes with time, because of which emission occurs from a narrower group of spectral centers with respect to orientational energies, which is testified by the smaller halfwidth of the spectra (Fig. 3b). In addi tion, the IPT reaction barrier for “redder” centers should be lower, although they also may partially relax to a more stable state with the minimum potential OPTICS AND SPECTROSCOPY
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energy. The lower reaction barrier directly manifests itself in a higher PT reaction rate, which is demon strated by the experimental data given in Fig. 6. Obvi ously, stable or close to stable configurations of solvate molecules cannot strongly reduce the development of the reaction along the intramolecular hydrogen bond because these states have no excess orientational energy. Thus, the observed effect of the longwavelength shift of the fluorescence spectrum with increasing excitation wavelength can be explained based on inho mogeneous broadening of the electronic spectra of polar solutions and can be well described using the energy level diagram taking into account the orienta tional broadening sublevels formed due to dipole– dipole interactions of the fluorophore with the nearest molecules of the polar solvent and the thermal fluctu ations of the structure of solvates. In this scheme, it is also necessary to take into account the relation between the lifetime of fluorophore in the excited state and the relaxation time of solvent molecules in the electric field of the fluorophore dipole. The excited state lifetime is determined not only by the radiative and nonradiative processes in its own systems of levels, but also by fast PT processes. Note that the observed nonequilibrium of the excited state directly points to one more important fact, namely, to a high PT rate, which correlates with the femtosecond measurements performed in [22]. Owing to this nonequilibrium, it is possible to estimate these rates from known dielectric relaxation times without using complicated femtosec ond laser spectroscopy. CONCLUSIONS The fluorescence spectra of FET are studied upon excitation by photons with different energies in the region of the main absorption band and at its long wavelength wing at different temperatures. It is found that the ratio of the integral emission of the normal and tautometric forms depends on the excitation wavelength, namely, it noticeably decreases with the excitation wavelength in the region of the main absorption band and at its longwavelength wing (380–440 nm) and the decrease rate depends on tem perature. In the same region, a longwavelength exci tation effect is observed, i.e., a bathochromic shift of the entire shortwavelength emission band by 7– 15 nm depending on temperature, which is unusual for liquid inviscid solvents at room temperature. This spectral shift is directly related to the inhomogeneous broadening of the FET electronic spectra, which is very strong (1200 cm–1) for the solution studied due to a considerable difference in the fluorophore dipole moments. Most probably, emission from excited unre laxed nonequilibrium orientational sublevels becomes possible due to shortening of the lifetime of emitting centers as a result of PT, the rate of which in acetoni trile is higher than the rate of orientational relaxation.
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TOMIN, JAWORSKI
These dependences are explained using energy dia grams taking into account the dependence of the free energy on the orientational polarization of the polar solvent and its thermal fluctuations. ACKNOWLEDGMENTS We are grateful to D.A. Yushchenko for providing FET molecules. This work was supported by the Pomeranian Uni versity, project no. BW 12/1/11. REFERENCES 1. Molecular Interactions, Ed. by H. Ratajczak and W. J. OrtwilleThomas (Willey, New York, 1983), Vol. 2. 2. J. Lakowicz, Principles of Fluorescent Spectroscopy (Plenum, New York, 1984). 3. B. Valeur, Molecular Fluorescence. Principles and Appli cations, 4th ed. (WileyVCH, New York, 2007). 4. A. P. Demchenko, Introduction to Fluorescence Sensing, (SpringerVerlag, Berlin, 2009). 5. N.G. Bakhshiev, Spectroscopy of Intermolecular Inter actions (Nauka, Leningrad, 1972) [in Russian]. 6. N. G. Bakhshiev, Photophysics of Dipole–Dipole Inter actions: Solvation and Complexation Processes (St. Petersburg Gos. Univ., St. Petersburg, 2005) [in Russian]. 7. A. Nemkovich, A. N. Rubinov, and V. I. Tomin, in Top ics in Fluorescence Spectroscopy: Principles, Ed. by J. R. Lakowicz (Plenum, New York, 1991), Vol. 2, p. 367. 8. V. I. Tomin, in Springer Series on Fluorescence, Methods and Applications. Advanced Fluorescence Reporters in Chemistry and Biology I. Fundamentals and Molecular Design., Ed. by A. Demchenko (Springer, New York, 2010), Vol. 8, p. 189. 9. A. P. Demchenko, J. Lumin. 17, 19 (2002). 10. W. R. Ware, Chem. Phys. Lett. 2, 356 (1968).
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Translated by M. Basieva
OPTICS AND SPECTROSCOPY
Vol. 114
No. 5
2013
