- Email: [email protected]
Tuning the mechanism of proton-transfer in a hydroxyflavone derivative
Chemical Physics Letters 379 (2003) 53–59 www.elsevier.com/locate/cplett
Tuning the mechanism of proton-transfer in a hydroxyflavone derivative A.D. Roshal, J.A. Organero, A. Douhal
*
Departamento de Quımica Fısica, Secci on de Quımicas, Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain Received 25 July 2003; in final form 11 August 2003 Published online: 3 September 2003
Abstract In this Letter, we report on steady-state and time-resolved emission studies of 40 -dimethylaminoflavonol (DAMF) in two families of solvents where we observed a reversible or an irreversible proton motion in the ultrafast formed chargetransfer (CT) state of DMAF. The slower (picosecond) proton-transfer time constant of DMAF compared to the femtosecond one observed in non-substituted 3-hydroxyflavones is due the involvement of an intramolecular CT. For the reversible situation, increasing the polarity of the medium increases the rate of proton motion. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Charge-, electron- and proton-transfer processes are among the most important elementary key events of reactions occurring in nature, and their dynamics is very fast [1–6]. 3-Hydroxyflavones (flavonols) are prototype molecules for investigating excited-state intramolecular protontransfer (ESIPT) reactions in which the proton motion is very fast giving birth to a new phototautomer, which emits at around 560 nm [7–13]. Beside the Ôsimplest conceptualÕ proton motion between two oxygen atoms positioned on the same ring and separated by a relatively short distance,
*
Corresponding author. Fax: +34-925-268840. E-mail address: [email protected] (A. Douhal).
the interest to flavonols and their derivatives spectroscopy resides on a possible use of these molecules as fluorescent probes in biological media [14,15], and as starting materials for designing other molecular probes [16]. Depending on the nature of the used solvent, two types of mechanisms have been proposed: a proton-tunnelling process in non-polar solvents [7,9,12], and a mixed tunnelling-Arrhenius type mechanism in polar media [10]. Furthermore, dialkylamino substituted flavonols (DAAF), showed spectral properties different from those of flavonols and its other derivatives. In aprotic non-polar solvents, DAAF shows a dual fluorescence [17]. The long-wavelength emission band (560 nm) is similar to that of flavonols, and it is due to the emission of phototautomers (zwitterionic-type, Z), produced by an ESIPT process in the excited enol form
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.08.008
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A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
Scheme 1. Molecular structures of 40 -dimethylaminoflavonol (DMAF) in its enol (E), zwitterionic-type (Z) and solventbonded species.
(Scheme 1). The normal emission band (440–480 nm), very week in non-polar solvents, comes from enol (E) structure. Therefore, the presence of a dual fluorescence of 40 -diethylaminoflavonol in aprotic non-polar solvents has suggested the existence of a reversible excited state proton transfer between E* and Z* [17]. A similar conclusion was reached for 40 -diethylaminoflavonol and 40 -dimethylaminoflavonol (DMAF) in mixtures of benzene-acetonitrile solutions [18]. In this Letter, we report on the steady-state and time-resolved picosecond emission studies of DMAF in two families of solvents (Scheme 1). For each family, we observed a different mechanism of the transfer: a reversible or an irreversible reaction, but also involving a CT state. Measurements of temperature effect on the emission decays give the energetics of both direct and reverse processes at the excited state, and on the fluorescence deactivation of Z tautomers.
2. Experimental DMAF was synthesized using Algar–Flynn– Oyamada reaction [19] and purified by means of a repeated recrystallisation from methanol solu-
tions. All the used solvents (spectroscopy grade, Sigma–Aldrich) were purified by standard procedures, before use. Steady-state absorption and emission spectra were recorded on a Varian (Cary E1) and Perkin–Elmer (LS-50B) spectrophotometers, respectively. Time-resolved emission measurements were done using a time-correlated single-photon counting picosecond spectrophotometer (FluoTime 200) described earlier [20]. Magic-angle fluorescence decays were convoluted with the instrumental response function (65 ps) and fitted to a multi-exponential function using the Fluofit package. The quality of the fit was characterized in terms of the reduced v2 value and the distribution of residuals. Emission quantum yields measurements were done using quinine sulphate in 0.1 N H2 SO4 (/ ¼ 0:54) as a reference. All experiments were done at 293 1 K except for temperature effects on the emission decays.
3. Results and discussion 3.1. Steady-state observation Experiments using DMAF and 40 -dimethylaminoflavones suggested that the excited enol form suffers an intramolecular charge-transfer (CT) reaction from the 40 -dimethylaminophenyl part to the chromone one [17,21], and thus the emission might originate from an intramolecular CT state. However, fluorescence quantum yields of CT and Z structures do not show any dependency on the solvent nature [12]. Therefore, to understand this abnormal behaviour, we studied the emission spectra of DMAF in different solvents where we observed two kinds of behaviour (Fig. 1). We divided the used solvents into two families. In the first family, the intensity of the short-wavelength band is weak, and almost independent on solvent polarity. For example, for a non-polar solvent (dioxane), and for a polar one (tetrahydrofurane, THF), having reaction dielectric field parameter, F ðn; eÞ ¼ 0:03 and 0.44, respectively, the intensity is almost the same in both media (Fig. 1A). For the second family (chloroform, dichloromethane and acetonitrile), the emission of the short-wavelength band is more intense than that observed in the first
Fluorescence intensity (a.u.)
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
b
55
d a
450
c 500
(A)
550
600
650
600
650
Wavelength (nm)
efg Fluorescence intensity (a.u.)
g
e
450
f
500
5 50
(B) Fig. 1. Normalized (at the maximum intensity of Z band) fluorescence spectra of DMAF in the used solvents. (A) cyclohexane (a), dioxane (b), diethylic ether (c) and tetrahydrofurane (d). (B) chloroform (e), dichloromethane (f) and acetonitrile (g). Excitation wavelength, 400 nm and T , 293 K.
one (Fig. 1B). Furthermore, in chloroform ðF ðn; eÞ ¼ 0:29) the emission is three times more intense than that observed in THF, a more polar solvent ðF ðn; eÞ ¼ 0:44). The intensity of this emission relative to that of Z-type structure increases with the value of F ðn; eÞ. Clearly, this behaviour is related to the nature (mechanism) of proton transfer and the energy barrier for nonradiative processes of each structure. 3.2. Picosecond time-resolved emission measurements To get information on the mechanism of proton motion in excited DMAF in both solvent families, we measured emission decays at different wavelengths of observation. Fig. 2 shows two typical examples of decays at the blue (CT) and the red (Z-type) sides demonstrating different dynamical behaviours. Table 1 contains the obtained data.
Fig. 2. Emission decays of DMAF in (A) tetrahydrofurane and (B) dichloromethane at 460 nm (a) and 680 nm (b), respectively. The instrumental response function (IRF, 65 ps) and the distribution of the residuals are shown. The lifetimes values of the multi-exponential fits ðv2 < 1:2Þ are posted in Table 1. Excitation at 393 nm, detection at magic angle and T ¼ 293 K.
For the first family of solvents (dioxane, diethylic ether and tetrahydrofurane), the decays at the blue band were fitted by a multi-exponential function having a fast component (sB1 10–16 ps,
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A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
Table 1 Values of the wavelength of the maximum of UV–visible absorption (kabs ), blue (kBfl ) and red (kR fl ) emission bands of DMAF in the used solvents Solvent
F ðe; nÞ
kabs (nm)
kBfl (nm)
kR fl (nm)
sB1 ; sB2 (ps)
R sR 1 ; s2 (ps)
Cyclohexane Dioxane Diethyl ether Tetrahydrofurane Chloroform Dichloromethane Acetonitrile
0.00 0.03 0.30 0.44 0.29 0.47 0.71
385 392 390 396 393 403 396
440 462 466 485 477 488 518
556 570 567 574 560 568 570
<10, 1400 15, 700 11, 1450 16, 1190 79, 773 43, 672 52, 362
(-)10, (-)15, (-)11, (-)16, (-)79, (-)43, (-)52,
1450 400 360 308 773 672 362
BðRÞ
ii is the time constant of the emission decays at the blue (B) or red (R) band at 293 K. The sign (-) before the values of sR 1 means a rising component. F ðe; nÞ is the reaction dielectric field parameter, where e is the dielectric constant and n is the refractive index, respectively, of the solvent.
99%) and a slow one (sB2 ¼ 0:7–2:0 ns, 1%) (Fig. 2A and Table 1). The ns component is due to the low population of DMAF having established an intermolecular H-bond with the etheric solvents (Scheme 1). The small contribution of this component increases when using H-bonding solvents. In fact, addition of small amounts of methanol to a cyclohexane solution leads to a growth of the pre-exponential coefficient of this component, but does not change significantly the value of the time constant (1.4 ns). At the red region of the spectrum, the decays were fitted by a bi-exponential function as well (Fig. 2A). A rising ps component, B sR 1 , similar to the fast one at the blue region (s1 ), and thus suggesting a common channel for these components, and a slow time, sR 2 300–400 ps, assigned to the fluorescence decay Z-type structure (Table 1). This time constant is different from the one observed at the blue side of the spectrum (sB2 ). In cyclohexane, the observation is similar to that described above. A fast decay component (<10 ps) and a very weak (about 0.3%) 1.4 ns contribution at the blue band and fast rising component with a time constant less than 10 ps and a 1.45 ns decay at the red region assigned to the lifetime of the produced Z phototautomers. The ns component at the blue side in cyclohexane might be due to the presence of a very small amount of water or a contribution from Z. Therefore, in this family of solvents, the observed behaviour of DMAF is similar to that of non-substituted flavonol [12], and reveals that in
these solvents the time constant of ESIPT reaction is short (10–16 ps) and that the process is irreversible. In fact, femtosecond experiments on DMAF show that the intramolecular CT and the ESIPT reactions occur in less than 2 and 10 ps, respectively, in agreement with the above results [22]. For the second family of solvents (Figs. 1B and 2B), the blue emission shows a bi-exponential decay, while that of the Z-type shows a rise and a decay components. The time constants for both decays (Table 1) are similar indicating the existence of an equilibrium between both fluorophores at the excited state. To get the corresponding time (rate) constants of the equilibrium and those of relaxation processes to the ground state, we applied the Birks model for equilibrated fluorophores [23,24]. It is worth to note that the ratio of the preexponential factors of the decay and rise components, when gating the red emission, is almost )1, indicating that Z-type structure is not reached by a direct excitation, but rather coming from the excited enol form upon a fast CT reaction followed by a proton motion in the formed CT state. Thus, based on Birks kinetics model ESIPT time (rate) constants were calculated using the data of the bi-exponential fits at the blue and red regions of DMAF emission. The blue (IB ) and red (IR ) emission intensities can be written as: s1
s2
s
t2
IB ¼ A1B e t þ A2B e t IR ¼ A1R e
t1
þ A2R e
and
s
;
ð1Þ
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
s1 (equal to sB1 and sR 1 in Table 1) and s2 (equal to sB2 and sR 2 ) are the times corresponding to fast and slow decays (rise) at the blue and red regions, respectively. A1B , A2B , A1R , A2R are the pre-exponential factors. The analysis was carried out at a short-wavelength region of the blue band (460 nm) and at a long-wavelength region of the red one (680 nm), where the spectral overlap between both bands is very weak. Resolution of (1) gives the following equations for calculating the time (rate) constant of direct and reverse proton transfer: 1 s1 1 A s2 ; where 1þA A1R A2R A¼ at condition that ¼ 1; A1B A2B
DMAF and formation of intermolecular H-bond with methanol solvent lead to the appearance of a new decay component. Its time constant, sHB was used for the determination of sDPT and sRPT , and Table 2 contains the obtained values. To check if the value of sHB has some influence on the obtained values, we have also used the lifetime of 3-methoxy-40 -dimethylaminoflavone, a molecule where ESIPT reaction is not possible [18]. Using the reported value of sNI ¼ 2:71 ns in acetonitrile [18], a difference in the values of rate constants of less than 3% was obtained with respect to the previous calculations. This good result may be explained by the fact that sHB (1–2 ns), sNI and therefore the sN real value of DMAF are very large (by more than a factor of 10) when compared to the time constant of direct ESIPT reaction (50–90 ps). We note also that the fluorescence lifetime (sZ ) of Z type is not very different from the time constant of reverse proton transfer, suggesting that the supposition in [18] about ESIPT reversibility based on the condition sZ sRPT does not stand. The data listed in Tables 1 and 2 show that the time constant (ps regime) of direct ESIPT reaction in DMAF is longer than those (fs regime) found in non-substituted flavonols [12]. This may be explained by the fact that the proton transfer in DMAF is governed by a CT process induced by the presence of the electron-donating dimethylamino group. Upon excitation the CT from this group precludes the normal increase in acidity of the OH group and thus decreases the rate constant of proton motion in DMAF. The relationship between the CT and PT and related equilibrium constant (K) at the excited state can be seen in Table 2. K and the rate constant of reverse proton motion increases with the value of F ðe; nÞ. Note
X ¼
ð2Þ
1 Y ¼ s1 1 þ s2 X ;
ð3Þ
1 and sDPT sRPT ¼ X Y s1 1 s2 ;
s1 N
s1 DPT ,
s1 Z
s1 RPT ,
57
ð4Þ s1 N
þ Y ¼ þ and s1 where X ¼ Z correspond to the rate constants of radiative and non-radiative (excluding ESIPT reactions) deactivation of charge transfer and Z-type forms, respectively. sDPT and sRPT are the time constants for direct and reverse proton-transfer reactions, respectively. To determine sDPT and sRPT it is necessary to know an experimental value of sN or of sZ . Since these values cannot be directly obtained, we used lifetimes of a model molecule, where ESIPT is not possible. Therefore, the complexes of DMAF with methanol (Scheme 1) were used. The complexes were obtained by adding little quantities of methanol (<0.01% v/v) into each sample solutions. Breaking the intramolecular hydrogen bonds in
Table 2 Experimental and calculated time constants for reversible ESIPT processes Solvent
F ðe; nÞ
sHB (ns)
sZ (ps)
sDPT (ps)
sRPT (ps)
K(sRPT =sDPT )
Chloroform Dichloromethane Acetonitrile
0.29 0.47 0.71
1.63 1.93 0.71
780 576 358
86 54 70
980 362 140
0.09 0.15 0.50
sHB is the emission lifetime of methanol-bonded DMFA in the used solvent and emitting at the blue side. sZ is the emission lifetime of Z phototautomer. sDPT and sRPT are the time constants for direct and reverse proton-transfer reactions, respectively. K is the value of equilibrium constant between excited CT and Z structures at 293 K. F ðe; nÞ is the reaction dielectric field parameter.
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A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
that changing the polarity of the medium will affect not only the energy gap between the CT and Z-type structures, but also the energy barrier (the transition-state (TS) energy) between the involved states. An increase of the polarity will certainly stabilizes the polar TS. Thus, the solvent coordinate in this intramolecular proton transfer plays a key role in the dynamics. 3.3. Energetics of the irreversible and reversible ESIPT reactions To get information on the energetics involved in the ESIPT reaction, we have measured the temperature effect on the time constants deduced from the fluorescence decays of DMAF in THF (irreversible process) and in dichloromethane (reversible process). Analysis of the fluorescent decays in THF solutions has shown that the change of the ps component (12–16 ps) is beyond the time resolution of the used apparatus, and thus we suggest that the energy barrier if it exists should be very small (<1 kcal/mol). However, the slow decay component sR 2 corresponding to the lifetime of Ztype structure decreases from 365 to 163 ps. Taking into account that quantum yield of Z in THF at 293 K is 0.057, and that the fluorescence lifetime is 308 ps, the time constant for radiative deactivation is 5.7 ns. With this value (which will not depend on the temperature) and the obtained sZnr (time constant of non-radiative processes at temperature T), we obtained the values of non-radiative rate constant at each temperature. Fig. 3 shows an Arrhenius plot (ln sZnr vs. 1=T ), which gives the activation energy value (Ea ¼ 4:0 0:4 kcal/mol) for non-radiative relaxation of Z-type structure. In dichloromethane solutions of DMAF, where ESIPT is a reversible process, the increase of the temperature decreases the value of the time constant of both decay components. When the temperature changes from 284 to 308 K, the fastest one changes from 60 to 37 ps, while the slowest one decreases from 893 to 492 ps, respectively. The values of sZnr were obtained by the same way used for tetrahydrofurane solutions. The fluorescence quantum yield of Z-type in dichloromethane at 293 K is 0.063. Therefore, we calculated the value
Fig. 3. Arrhenius plots of (a) ln sDPT , (b) ln sRPT and (c) ln snr (Z) vs. 1/T. Solvent: dichloromethane (see text).
of the time (rate) constants for forward and reverse proton-transfer reactions (Fig. 3). From the Arrhenius plots, the calculated activation energy, Ea (0.3 kcal/mol), for direct and reverse ESIPT processes are 4.6 and 5.7 kcal/mol, respectively. This suggests about 1 kcal/mol as an energy stabilization of Z relatively to the CT structure. The barrier energy of non-radiative deactivation of Z phototautomer is 5.0 0.3 kcal/mol, not different from that observed for the irreversible ESIPT process in THF. Note that both solvents have similar reaction dielectric field parameters (Table 1), and thus a similar change in the energy gap between the p; p* and n,p* states, which may be important for the efficiency of non-radiative channels of Z structure.
4. Conclusion The results reported here indicate that the ESIPT reaction in DMAF is governed by a fast intramolecular charge transfer. For solvents where the process is reversible, the equilibrium constant value depends on the polarity of the medium, indicating the importance of solvation in the dynamics of this molecular system. The ps time constant in DMAF when compared to the fs-time one observed in flavonols is due the in-
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
volvement of a CT process induced by the dimethylamino group leading to an energy barrier for the ESIPT reaction. We believe that these results are relevant for a better understanding of flavonol derivatives when used as fluorescent molecular probes [25]. Encapsulation by a biological medium might provoke a variation in the mechanism of proton transfer and thus the exploration of the fluorescence intensity change is not so evident for this kind of molecules.
Acknowledgements This work was supported by the MCYT (Spain) and the JCCM through projects MAT-2002-00301 and PAI-02-004. A.D.R. thanks NATO for supporting his sabbatical stay.
References [1] A.M. Kusnezov, J. Ulstrup, Electron Transfer in Chemistry and Biology, Wiley, Chichester, 1998. [2] V. May, O. K€ uhn, Charge and Energy Transfer Dynamics in Molecular Systems, Wiley-VCH, Berlin, 2000. [3] A. Douhal, S.K. Kim, A.H. Zewail, Nature 378 (1995) 260. [4] A. Douhal, F. Lahmani, A.H. Zewail, Chem. Phys. 207 (1996) 447. [5] T. Elsaesser, H.J. Bakker (Eds.), Ultrafast H-bonding Dynamics and Proton transfer Processes in the Condensed Phase, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.
59
[6] A. Douhal, J. Santamaria (Eds.), Femtochemistry and Femtobiology, Ultrafast Dynamics in Molecular Science, World Scientific, Singapore, 2002. [7] D. McMorrow, M. Kasha, J. Am. Chem. Soc. 105 (1983) 5133. [8] A.J.G. Strandjord, P.F. Barbara, J. Phys. Chem. 89 (1985) 2355. [9] N.P. Ernsting, B. Dick, Chem. Phys. 136 (1989) 181. [10] G.A. Brucker, T.C. Swinney, D.F. Kelley, J. Phys. Chem. 95 (1991) 3190. [11] B.J. Shwartz, L.A. Peteanu, C.B. Harris, J. Phys. Chem. 96 (1992) 3591. [12] S.J. Formosinho, L.J. Arnaut, J. Photochem. Photobiol. A 75 (1993) 21. [13] S. Ameer-Beg, S.M. Ormson, R.G. Brown, P. Matousek, M. Towrie, E.T.J. Nibbering, P. Foggi, V.R. Neuwahl, J. Phys. Chem. A 105 (2001) 3709. [14] A. Sytnik, D. Gormin, M. Kasha, Proc. Natl. Acad. Sci. USA 91 (1994) 11968. [15] S.M. Dennison, J. Guharay, P.K. Sengupta, Spectrochim. Acta A 55 (1999) 903. [16] A.S. Klymchenko, A.P. Demchenko, J. Am. Chem. Soc. 124 (2002) 12372. [17] P.T. Chou, M.L. Martinez, J.H. Clements, J. Phys. Chem. 97 (1993) 2618. [18] T.C. Swinney, D.F. Kelley, J. Chem. Phys. 99 (1993) 211. [19] M.A. Smith, R.M. Neumann, R.A. Webb, J. Heterocycl. Chem. 5 (1968) 425. [20] J.A. Organero, L. Tormo, A. Douhal, Chem. Phys. Lett. 363 (2002) 409. [21] P. Wang, S. Wu, J. Lumin. 62 (1994) 33. [22] A. Douhal et al. (to be published). [23] J.B. Birks, Photophysics of Aromatic Molecules, Wiley, London, 1970. [24] A. Demeter, T. Berces, K.A. Zachariasse, J. Phys. Chem. A 105 (2001) 4611. [25] A.S. Klymchenko, V.G. Pivovarenko, A.P. Demchenko, J. Phys. Chem. A 107 (2003) 4211.
Tuning the mechanism of proton-transfer in a hydroxyflavone derivative A.D. Roshal, J.A. Organero, A. Douhal
*
Departamento de Quımica Fısica, Secci on de Quımicas, Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain Received 25 July 2003; in final form 11 August 2003 Published online: 3 September 2003
Abstract In this Letter, we report on steady-state and time-resolved emission studies of 40 -dimethylaminoflavonol (DAMF) in two families of solvents where we observed a reversible or an irreversible proton motion in the ultrafast formed chargetransfer (CT) state of DMAF. The slower (picosecond) proton-transfer time constant of DMAF compared to the femtosecond one observed in non-substituted 3-hydroxyflavones is due the involvement of an intramolecular CT. For the reversible situation, increasing the polarity of the medium increases the rate of proton motion. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Charge-, electron- and proton-transfer processes are among the most important elementary key events of reactions occurring in nature, and their dynamics is very fast [1–6]. 3-Hydroxyflavones (flavonols) are prototype molecules for investigating excited-state intramolecular protontransfer (ESIPT) reactions in which the proton motion is very fast giving birth to a new phototautomer, which emits at around 560 nm [7–13]. Beside the Ôsimplest conceptualÕ proton motion between two oxygen atoms positioned on the same ring and separated by a relatively short distance,
*
Corresponding author. Fax: +34-925-268840. E-mail address: [email protected] (A. Douhal).
the interest to flavonols and their derivatives spectroscopy resides on a possible use of these molecules as fluorescent probes in biological media [14,15], and as starting materials for designing other molecular probes [16]. Depending on the nature of the used solvent, two types of mechanisms have been proposed: a proton-tunnelling process in non-polar solvents [7,9,12], and a mixed tunnelling-Arrhenius type mechanism in polar media [10]. Furthermore, dialkylamino substituted flavonols (DAAF), showed spectral properties different from those of flavonols and its other derivatives. In aprotic non-polar solvents, DAAF shows a dual fluorescence [17]. The long-wavelength emission band (560 nm) is similar to that of flavonols, and it is due to the emission of phototautomers (zwitterionic-type, Z), produced by an ESIPT process in the excited enol form
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.08.008
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A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
Scheme 1. Molecular structures of 40 -dimethylaminoflavonol (DMAF) in its enol (E), zwitterionic-type (Z) and solventbonded species.
(Scheme 1). The normal emission band (440–480 nm), very week in non-polar solvents, comes from enol (E) structure. Therefore, the presence of a dual fluorescence of 40 -diethylaminoflavonol in aprotic non-polar solvents has suggested the existence of a reversible excited state proton transfer between E* and Z* [17]. A similar conclusion was reached for 40 -diethylaminoflavonol and 40 -dimethylaminoflavonol (DMAF) in mixtures of benzene-acetonitrile solutions [18]. In this Letter, we report on the steady-state and time-resolved picosecond emission studies of DMAF in two families of solvents (Scheme 1). For each family, we observed a different mechanism of the transfer: a reversible or an irreversible reaction, but also involving a CT state. Measurements of temperature effect on the emission decays give the energetics of both direct and reverse processes at the excited state, and on the fluorescence deactivation of Z tautomers.
2. Experimental DMAF was synthesized using Algar–Flynn– Oyamada reaction [19] and purified by means of a repeated recrystallisation from methanol solu-
tions. All the used solvents (spectroscopy grade, Sigma–Aldrich) were purified by standard procedures, before use. Steady-state absorption and emission spectra were recorded on a Varian (Cary E1) and Perkin–Elmer (LS-50B) spectrophotometers, respectively. Time-resolved emission measurements were done using a time-correlated single-photon counting picosecond spectrophotometer (FluoTime 200) described earlier [20]. Magic-angle fluorescence decays were convoluted with the instrumental response function (65 ps) and fitted to a multi-exponential function using the Fluofit package. The quality of the fit was characterized in terms of the reduced v2 value and the distribution of residuals. Emission quantum yields measurements were done using quinine sulphate in 0.1 N H2 SO4 (/ ¼ 0:54) as a reference. All experiments were done at 293 1 K except for temperature effects on the emission decays.
3. Results and discussion 3.1. Steady-state observation Experiments using DMAF and 40 -dimethylaminoflavones suggested that the excited enol form suffers an intramolecular charge-transfer (CT) reaction from the 40 -dimethylaminophenyl part to the chromone one [17,21], and thus the emission might originate from an intramolecular CT state. However, fluorescence quantum yields of CT and Z structures do not show any dependency on the solvent nature [12]. Therefore, to understand this abnormal behaviour, we studied the emission spectra of DMAF in different solvents where we observed two kinds of behaviour (Fig. 1). We divided the used solvents into two families. In the first family, the intensity of the short-wavelength band is weak, and almost independent on solvent polarity. For example, for a non-polar solvent (dioxane), and for a polar one (tetrahydrofurane, THF), having reaction dielectric field parameter, F ðn; eÞ ¼ 0:03 and 0.44, respectively, the intensity is almost the same in both media (Fig. 1A). For the second family (chloroform, dichloromethane and acetonitrile), the emission of the short-wavelength band is more intense than that observed in the first
Fluorescence intensity (a.u.)
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
b
55
d a
450
c 500
(A)
550
600
650
600
650
Wavelength (nm)
efg Fluorescence intensity (a.u.)
g
e
450
f
500
5 50
(B) Fig. 1. Normalized (at the maximum intensity of Z band) fluorescence spectra of DMAF in the used solvents. (A) cyclohexane (a), dioxane (b), diethylic ether (c) and tetrahydrofurane (d). (B) chloroform (e), dichloromethane (f) and acetonitrile (g). Excitation wavelength, 400 nm and T , 293 K.
one (Fig. 1B). Furthermore, in chloroform ðF ðn; eÞ ¼ 0:29) the emission is three times more intense than that observed in THF, a more polar solvent ðF ðn; eÞ ¼ 0:44). The intensity of this emission relative to that of Z-type structure increases with the value of F ðn; eÞ. Clearly, this behaviour is related to the nature (mechanism) of proton transfer and the energy barrier for nonradiative processes of each structure. 3.2. Picosecond time-resolved emission measurements To get information on the mechanism of proton motion in excited DMAF in both solvent families, we measured emission decays at different wavelengths of observation. Fig. 2 shows two typical examples of decays at the blue (CT) and the red (Z-type) sides demonstrating different dynamical behaviours. Table 1 contains the obtained data.
Fig. 2. Emission decays of DMAF in (A) tetrahydrofurane and (B) dichloromethane at 460 nm (a) and 680 nm (b), respectively. The instrumental response function (IRF, 65 ps) and the distribution of the residuals are shown. The lifetimes values of the multi-exponential fits ðv2 < 1:2Þ are posted in Table 1. Excitation at 393 nm, detection at magic angle and T ¼ 293 K.
For the first family of solvents (dioxane, diethylic ether and tetrahydrofurane), the decays at the blue band were fitted by a multi-exponential function having a fast component (sB1 10–16 ps,
56
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
Table 1 Values of the wavelength of the maximum of UV–visible absorption (kabs ), blue (kBfl ) and red (kR fl ) emission bands of DMAF in the used solvents Solvent
F ðe; nÞ
kabs (nm)
kBfl (nm)
kR fl (nm)
sB1 ; sB2 (ps)
R sR 1 ; s2 (ps)
Cyclohexane Dioxane Diethyl ether Tetrahydrofurane Chloroform Dichloromethane Acetonitrile
0.00 0.03 0.30 0.44 0.29 0.47 0.71
385 392 390 396 393 403 396
440 462 466 485 477 488 518
556 570 567 574 560 568 570
<10, 1400 15, 700 11, 1450 16, 1190 79, 773 43, 672 52, 362
(-)10, (-)15, (-)11, (-)16, (-)79, (-)43, (-)52,
1450 400 360 308 773 672 362
BðRÞ
ii is the time constant of the emission decays at the blue (B) or red (R) band at 293 K. The sign (-) before the values of sR 1 means a rising component. F ðe; nÞ is the reaction dielectric field parameter, where e is the dielectric constant and n is the refractive index, respectively, of the solvent.
99%) and a slow one (sB2 ¼ 0:7–2:0 ns, 1%) (Fig. 2A and Table 1). The ns component is due to the low population of DMAF having established an intermolecular H-bond with the etheric solvents (Scheme 1). The small contribution of this component increases when using H-bonding solvents. In fact, addition of small amounts of methanol to a cyclohexane solution leads to a growth of the pre-exponential coefficient of this component, but does not change significantly the value of the time constant (1.4 ns). At the red region of the spectrum, the decays were fitted by a bi-exponential function as well (Fig. 2A). A rising ps component, B sR 1 , similar to the fast one at the blue region (s1 ), and thus suggesting a common channel for these components, and a slow time, sR 2 300–400 ps, assigned to the fluorescence decay Z-type structure (Table 1). This time constant is different from the one observed at the blue side of the spectrum (sB2 ). In cyclohexane, the observation is similar to that described above. A fast decay component (<10 ps) and a very weak (about 0.3%) 1.4 ns contribution at the blue band and fast rising component with a time constant less than 10 ps and a 1.45 ns decay at the red region assigned to the lifetime of the produced Z phototautomers. The ns component at the blue side in cyclohexane might be due to the presence of a very small amount of water or a contribution from Z. Therefore, in this family of solvents, the observed behaviour of DMAF is similar to that of non-substituted flavonol [12], and reveals that in
these solvents the time constant of ESIPT reaction is short (10–16 ps) and that the process is irreversible. In fact, femtosecond experiments on DMAF show that the intramolecular CT and the ESIPT reactions occur in less than 2 and 10 ps, respectively, in agreement with the above results [22]. For the second family of solvents (Figs. 1B and 2B), the blue emission shows a bi-exponential decay, while that of the Z-type shows a rise and a decay components. The time constants for both decays (Table 1) are similar indicating the existence of an equilibrium between both fluorophores at the excited state. To get the corresponding time (rate) constants of the equilibrium and those of relaxation processes to the ground state, we applied the Birks model for equilibrated fluorophores [23,24]. It is worth to note that the ratio of the preexponential factors of the decay and rise components, when gating the red emission, is almost )1, indicating that Z-type structure is not reached by a direct excitation, but rather coming from the excited enol form upon a fast CT reaction followed by a proton motion in the formed CT state. Thus, based on Birks kinetics model ESIPT time (rate) constants were calculated using the data of the bi-exponential fits at the blue and red regions of DMAF emission. The blue (IB ) and red (IR ) emission intensities can be written as: s1
s2
s
t2
IB ¼ A1B e t þ A2B e t IR ¼ A1R e
t1
þ A2R e
and
s
;
ð1Þ
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
s1 (equal to sB1 and sR 1 in Table 1) and s2 (equal to sB2 and sR 2 ) are the times corresponding to fast and slow decays (rise) at the blue and red regions, respectively. A1B , A2B , A1R , A2R are the pre-exponential factors. The analysis was carried out at a short-wavelength region of the blue band (460 nm) and at a long-wavelength region of the red one (680 nm), where the spectral overlap between both bands is very weak. Resolution of (1) gives the following equations for calculating the time (rate) constant of direct and reverse proton transfer: 1 s1 1 A s2 ; where 1þA A1R A2R A¼ at condition that ¼ 1; A1B A2B
DMAF and formation of intermolecular H-bond with methanol solvent lead to the appearance of a new decay component. Its time constant, sHB was used for the determination of sDPT and sRPT , and Table 2 contains the obtained values. To check if the value of sHB has some influence on the obtained values, we have also used the lifetime of 3-methoxy-40 -dimethylaminoflavone, a molecule where ESIPT reaction is not possible [18]. Using the reported value of sNI ¼ 2:71 ns in acetonitrile [18], a difference in the values of rate constants of less than 3% was obtained with respect to the previous calculations. This good result may be explained by the fact that sHB (1–2 ns), sNI and therefore the sN real value of DMAF are very large (by more than a factor of 10) when compared to the time constant of direct ESIPT reaction (50–90 ps). We note also that the fluorescence lifetime (sZ ) of Z type is not very different from the time constant of reverse proton transfer, suggesting that the supposition in [18] about ESIPT reversibility based on the condition sZ sRPT does not stand. The data listed in Tables 1 and 2 show that the time constant (ps regime) of direct ESIPT reaction in DMAF is longer than those (fs regime) found in non-substituted flavonols [12]. This may be explained by the fact that the proton transfer in DMAF is governed by a CT process induced by the presence of the electron-donating dimethylamino group. Upon excitation the CT from this group precludes the normal increase in acidity of the OH group and thus decreases the rate constant of proton motion in DMAF. The relationship between the CT and PT and related equilibrium constant (K) at the excited state can be seen in Table 2. K and the rate constant of reverse proton motion increases with the value of F ðe; nÞ. Note
X ¼
ð2Þ
1 Y ¼ s1 1 þ s2 X ;
ð3Þ
1 and sDPT sRPT ¼ X Y s1 1 s2 ;
s1 N
s1 DPT ,
s1 Z
s1 RPT ,
57
ð4Þ s1 N
þ Y ¼ þ and s1 where X ¼ Z correspond to the rate constants of radiative and non-radiative (excluding ESIPT reactions) deactivation of charge transfer and Z-type forms, respectively. sDPT and sRPT are the time constants for direct and reverse proton-transfer reactions, respectively. To determine sDPT and sRPT it is necessary to know an experimental value of sN or of sZ . Since these values cannot be directly obtained, we used lifetimes of a model molecule, where ESIPT is not possible. Therefore, the complexes of DMAF with methanol (Scheme 1) were used. The complexes were obtained by adding little quantities of methanol (<0.01% v/v) into each sample solutions. Breaking the intramolecular hydrogen bonds in
Table 2 Experimental and calculated time constants for reversible ESIPT processes Solvent
F ðe; nÞ
sHB (ns)
sZ (ps)
sDPT (ps)
sRPT (ps)
K(sRPT =sDPT )
Chloroform Dichloromethane Acetonitrile
0.29 0.47 0.71
1.63 1.93 0.71
780 576 358
86 54 70
980 362 140
0.09 0.15 0.50
sHB is the emission lifetime of methanol-bonded DMFA in the used solvent and emitting at the blue side. sZ is the emission lifetime of Z phototautomer. sDPT and sRPT are the time constants for direct and reverse proton-transfer reactions, respectively. K is the value of equilibrium constant between excited CT and Z structures at 293 K. F ðe; nÞ is the reaction dielectric field parameter.
58
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
that changing the polarity of the medium will affect not only the energy gap between the CT and Z-type structures, but also the energy barrier (the transition-state (TS) energy) between the involved states. An increase of the polarity will certainly stabilizes the polar TS. Thus, the solvent coordinate in this intramolecular proton transfer plays a key role in the dynamics. 3.3. Energetics of the irreversible and reversible ESIPT reactions To get information on the energetics involved in the ESIPT reaction, we have measured the temperature effect on the time constants deduced from the fluorescence decays of DMAF in THF (irreversible process) and in dichloromethane (reversible process). Analysis of the fluorescent decays in THF solutions has shown that the change of the ps component (12–16 ps) is beyond the time resolution of the used apparatus, and thus we suggest that the energy barrier if it exists should be very small (<1 kcal/mol). However, the slow decay component sR 2 corresponding to the lifetime of Ztype structure decreases from 365 to 163 ps. Taking into account that quantum yield of Z in THF at 293 K is 0.057, and that the fluorescence lifetime is 308 ps, the time constant for radiative deactivation is 5.7 ns. With this value (which will not depend on the temperature) and the obtained sZnr (time constant of non-radiative processes at temperature T), we obtained the values of non-radiative rate constant at each temperature. Fig. 3 shows an Arrhenius plot (ln sZnr vs. 1=T ), which gives the activation energy value (Ea ¼ 4:0 0:4 kcal/mol) for non-radiative relaxation of Z-type structure. In dichloromethane solutions of DMAF, where ESIPT is a reversible process, the increase of the temperature decreases the value of the time constant of both decay components. When the temperature changes from 284 to 308 K, the fastest one changes from 60 to 37 ps, while the slowest one decreases from 893 to 492 ps, respectively. The values of sZnr were obtained by the same way used for tetrahydrofurane solutions. The fluorescence quantum yield of Z-type in dichloromethane at 293 K is 0.063. Therefore, we calculated the value
Fig. 3. Arrhenius plots of (a) ln sDPT , (b) ln sRPT and (c) ln snr (Z) vs. 1/T. Solvent: dichloromethane (see text).
of the time (rate) constants for forward and reverse proton-transfer reactions (Fig. 3). From the Arrhenius plots, the calculated activation energy, Ea (0.3 kcal/mol), for direct and reverse ESIPT processes are 4.6 and 5.7 kcal/mol, respectively. This suggests about 1 kcal/mol as an energy stabilization of Z relatively to the CT structure. The barrier energy of non-radiative deactivation of Z phototautomer is 5.0 0.3 kcal/mol, not different from that observed for the irreversible ESIPT process in THF. Note that both solvents have similar reaction dielectric field parameters (Table 1), and thus a similar change in the energy gap between the p; p* and n,p* states, which may be important for the efficiency of non-radiative channels of Z structure.
4. Conclusion The results reported here indicate that the ESIPT reaction in DMAF is governed by a fast intramolecular charge transfer. For solvents where the process is reversible, the equilibrium constant value depends on the polarity of the medium, indicating the importance of solvation in the dynamics of this molecular system. The ps time constant in DMAF when compared to the fs-time one observed in flavonols is due the in-
A.D. Roshal et al. / Chemical Physics Letters 379 (2003) 53–59
volvement of a CT process induced by the dimethylamino group leading to an energy barrier for the ESIPT reaction. We believe that these results are relevant for a better understanding of flavonol derivatives when used as fluorescent molecular probes [25]. Encapsulation by a biological medium might provoke a variation in the mechanism of proton transfer and thus the exploration of the fluorescence intensity change is not so evident for this kind of molecules.
Acknowledgements This work was supported by the MCYT (Spain) and the JCCM through projects MAT-2002-00301 and PAI-02-004. A.D.R. thanks NATO for supporting his sabbatical stay.
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