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Temperature and solvent effects on the photodynamics of 1′-hydroxy-2′-acetonaphthone
Chemical Physics Letters 381 (2003) 759–765 www.elsevier.com/locate/cplett
Temperature and solvent effects on the photodynamics of 10 -hydroxy-20-acetonaphthone J.A. Organero, A. Douhal
*
Departamento de Quımica Fısica, Secci on de Quımicas, Facultad de Ciencias del Medio Ambiente, Campus Tecnol ogico de Toledo, Avenida Carlos III, S.N., Universidad de Castilla-La Mancha, 45071 Toledo, Spain Received 4 September 2003; in final form 10 October 2003 Published online: 4 November 2003
Abstract We present studies of temperature viscosity and polarity effects on the photodynamics of 10 -hydroxy-20 -acetonaphthone. We observed a striking temperature effect on the non-radiative decays and which explains the low emission quantum yield of the formed keto-type phototautomers. The thermal activation energy is lower for the keto structure (0.96 kcal/mol) than for the keto rotamer one (3.4 kcal/mol). For the alcohols family of solvents, the keto-type tautomer is more sensitive to solvent response than the keto rotamer. However for non-hydroxylic media the low influence of solvent polarity is similar for both keto type structures. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction In previous reports, we have studied the photophysics of 10 -hydroxy-20 -acetonaphthone (HAN) using laser fluorescence spectroscopy under jet-cooled molecular beam conditions, quantum chemistry at both S0 and S1 states, and in solution nanocavities [1–7]. The results show that the most stable conformer of HAN (enol structure, E) is that having an intramolecular H-bond between the OH and COCH3 group, and electronic excitation of E induces through or over a small energy barrier an excited-state intramolecular
*
Corresponding author. Fax: +34-925-268840. E-mail address: [email protected] (A. Douhal).
proton transfer (ESIPT) reaction leading to a ketotype tautomer (K*) (Scheme 1). This structure subsequently may lead to a rotamer (KR*) though a twisting motion of C–C bond linking the now protonated acetyl group and the naphthalene frame [4]. Femtosecond experiment in gas phase at room temperature performed by Cheng group found that the time constant for shifting the proton within E* to rapidly equilibrate with the generated keto type K* structure is in the picosecond time scale (60–85 ps), in agreement with the gas phase jet-cooled molecular beam conclusion [8]. Earlier, nanosecond–microsecond time-resolved experiments in solution carried out by Tobita et al. [9] showed a long lived ground keto tautomer after relaxation of K*. To explain the result, the authors
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.10.039
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J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
O H
O C
O CH3
Proton
H O C
O CH3
Transfer
HO C
Twisting
CH3
Motion
Scheme 1. Molecular structures of enol (E*), keto (K*) and keto rotamer (KR*) of 10 -hydroxy-20 -acetonaphthone (HAN). The arrows indicate the involved motion at the S1 state.
2. Experimental HAN (from Aldrich) were used as received. All the solvents (spectrograde quality) were used as received and these purity was checked before used. Steady-state emission spectra were recorded on Perkin–Elmer (LS 50B) spectrophotometer. Timeresolved emission measurements were done using a picosecond time-correlated single-photon counting spectrophotometer described earlier [6]. For measurements of emission decays at different temper-
atures we used a Cryostat (Oxford OptistatDN) with a digital controller ITC 601. Decays and anisotropy data were analysed using the FluoFit software package. Exponential decay functions were convoluted with the experimental response function (65 ps) and fitted to the experimental decay. The quality of the fits were characterized in terms of the reduced v2 value and the distribution of residuals.
3. Results and discussion 3.1. Temperature effect The emission spectrum of HAN in methyl cyclohexane (MC, an inert solvent) at 298 K (Fig. 1) is broad with the maximum at 480 nm and a shoulder at 450 nm. Upon lowering the temper-
Normalized intensity (a.u.)
suggested the involvement of a structural change of K* to produce a metastable keto rotamer KR*. Stolow and co-workers found a 30 ps component in the femtosecond transient of excited (excess energy 2500 cm1 ) HAN in molecular beam, which it was assigned to an internal conversion processes enhanced by a close lying n,p* state [10]. Lochbrunner et al. [11] using a femtosecond pump–probe technique observed that the ESIPT of HAN occurs in 30 fs. However, reports by Catalan et al. [12,13] did not recognize the occurrence of ESIPT reaction in HAN [14]. Recently, we have reported on experiments using a-, b- and c-cyclodextrins (CDÕs) as caging media of HAN and concluded that K* may suffer a twisting motion in few tens of ps along the naphthyl-COCH3 bond yielding to a nanosecond-living tautomer which emits at the red side of the K* spectrum. This photoisomerization is largely affected by the size of the nanocavity and stochiometry of the complex [6,7]. In continuation to the previous efforts for a better understanding of HAN fast dynamics, we report here studies of temperature and solvent effects on its photophysics.
0.5
0.0 400
440
480
520
560
600
Fig. 1. Steady-state emission spectra of lM HAN in methyl cyclohexane (MC) at 298 (dotted line) and 77 K (solid line) when excited at 380 nm.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
ature to 77 K, the emission exhibits a structured band with maxima at 440 and 480 nm. Besides these peaks, other blue (420 nm) and green (500 nm) shoulders appear at this temperature. The structural difference between both spectra when lowering the temperature is interpreted in terms of the involvement of a twisting motion in the formed phototautomer at S1 , in agreement with previous findings [4–10]. A study of 30 ,40 benzo-20 -hydroxychalcone, a comparable system, at different temperatures has reached a similar conclusion [15]. To obtain more information about the effect of the temperature on the photodynamics of HAN in MC when exciting at 393 nm (almost without excess energy at E*), we gated the emission at different wavelengths and at different temperatures (80–298 K). Fig. 2 shows the emission decays at 480 nm. To begin with, the decay at 298 K could not be fitted to a single exponential function and the fitting needed a biexponential with time constants of 39 ps (22%) and 101 ps (78%). The shortest time, assigned to K*, might be considered as a limit value for the twisting motion of this structure to produce KR* which has the longest time constant. Femtosecond experiments showed that the time constant of the proton transfer in E* is 30 fs [11]. Under the assumption of equilibrium between K* and KR*, and using 5 ns as a fluorescence lifetime value for KR* at room temperature (similar to that observed in caging nanocavities [6,7]), we obtained 660 ps for the conversion of K* to KR* using the Birks kinetics model and proceeding as in a previous contribution [16]. This time is clearly longer than 90 ps, the fluorescence decay time of K* deduced from the same supposed equilibrium scheme. Therefore, in this solvent at room temperature we suggest that K* and KR* structures are not equilibrated. Lowering the temperature to 80 K, the global emission decays show a remarkable change. At 480 nm for example, the lifetime values change from 39 ps to 2.16 ns (4%) and from 101 ps to 8.38 ns (96%) assigned for K* and KR*, respectively (Table 1). The presence of efficient non-radiative processes at higher temperatures like those due to conformational motions of the phototautomers is the reason of short lifetimes at room temperature [5–7]. We
761
Fig. 2. (a) 480 nm and magic-angle emission decays of HAN in MC at 298, 260, 240, 220, 210, 200, 190, 180, 170, 160, 150, 130, 100, 80 K when excited at 393 nm. (b) Arrhenius plot (solid line) of the non-radiative rate constants (knr1;2 ) of HAN in MC. Ea1 and Ea2 are the activation energies for the non-radiative processes of K* and KR*, respectively. The dotted line indicates the deviation from the Arrhenius behaviour.
identify these motions as twisting motion along the naphtyl-COCH3 bond, torsion and out-of-plane (dragging) motion of the naphthalene molecular frame in K* and in KR*, respectively. Theoretical results suggested a loss of aromaticity of the naphthalene ring and a lower conjugation of the twisted and protonated acetyl group in KR* [4]. Similar emission deactivation processes with a dependency on the viscosity and polarity of the medium have been observed in methyl salicylate, a molecule similar to HAN and which suffers a proton transfer at the S1 state [17].
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J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
Table 1 Fluorescence lifetimes (s1;2 ), pre-exponential factors (between brackets) and non-radiative rate constants (knr1;2 ) of HAN in MC at different temperatures Temperature (K)
s1 (ps) (%)
s2 (ps) (%)
(knr1 ) (1010 s1 )
(knr2 ) (108 s1 )
298 260 240 220 210 200 190 180 170 150 130 100 80
39 (22) 65 (23) 55 (29) 77 (21) 90 (19) 90 (20) 105 (19) 119 (18) 160 (15) 156 (12) 873 (8) 893 (7) 2156 (4)
101 (78) 230 (77) 431 (71) 788 (79) 1114 (81) 1608 (80) 2294 (81) 3103 (82) 4078 (85) 5860 (88) 7453 (92) 8137 (93) 8383 (95)
2.56 1.53 1.81 1.29 1.11 1.10 0.95 0.83 0.62 0.64 0.11 0.11 0.04
98.7 43.1 22.9 12.39 8.68 5.92 4.06 2.92 2.15 1.41 1.04 0.93 0.9
friction and restriction of the solvent motions increases at lower temperatures [20]. 3.2. Solvent viscosity effect
Intensity (u.a.)
To study the effect of the viscosity and hydrogen-bonding interactions of the medium on the deactivation processes of excited HAN, we measured the fluorescence spectrum and lifetime in different alcohols (Fig. 3). Table 2 lists the values
10 4
10
Counts
To get the non-radiative rate constants of K* (knr1 ) and KR* (knr2 ), we used the data of Tobita et al. [9] for HAN at 77 K (mean fluorescence lifetime ¼ 7.8 ns and global emission quantum yield ¼ 0.23 give a radiative lifetime of 34 ns). Thus, assuming that the radiative time for K* and KR* are similar we calculated knr1;2 using sf ¼ 1= ðknr þ kr Þ. Arrhenius-plot of ln knr1;2 vs. 1/T gives the activation energy and the pre-exponential (frequency) factor for non-radiative decays of K* (Ea1 ¼ 0.96 0.20 kcal/mol, A1 ¼ 9.7 1010 s1 ) and KR* (3.4 0.2 kcal/mol, A2 ¼ 2.3 1012 s1 ) (Fig. 2b). Based on the previous assignment of these barriers, twisting motion of the protonated acetyl group has a lower energy barrier and a lower frequency term (A2 /A1 ¼ 24) than those of the out-of-plane (dragging) motion of the naphthalene skeleton. Consistent with this explanation (large amplitude motions of the naphthalene ring) is the Arrhenius plot (Fig. 2b) behavior when the liquid–glass transition temperature of MC (90 K) is reached [18]. Below this temperature, the viscosity of MC undergoes a large increase (g88 K ¼1013 cP) and the slope of variation of free volume holes with the temperature exhibits a significant change [19]. The linearity of both plots in the 298–130 K range shows that the non radiative processes of K* and KR* in MC are controlled by hydrodynamics factors. While below 130 K the deactivation of KR* is rather controlled by free volume factors because the large solute motions decrease drastically at the same time that the
3
1-Octanol
1200 800
Methanol
400 0 420
10 2
480 540 600 Wavelength / nm
1-Octanol IRF
Methanol
10
1
0
0.5
1.0
1.5
2.0
Fig. 3. 480 nm and magic-angle emission decays of HAN in methanol and 1-octanol at room temperature when excited at 393 nm. IRF is the instrument response function, 65 ps. Inset: steady-state emission spectra (excitation at 365 nm) in methanol, ethanol, 1-butanol, 1-hexanol and 1-octanol at the same optical density at 365 nm (O.D.365 ¼ 0.55) at 298 K.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
763
Table 2 Fluorescence lifetimes (s1;2 ), pre-exponential factors (between brackets) and non-radiative rate constants (knr1;2 ) of HAN in the used solvents at 480 nm Solvent
s1 (ps) (%)
s2 (ps) (%)
(knr1 ) (1010 s1 )
(knr2 ) (109 s1 )
sD (ps)
g (cP)
f(; n)
Methanol Ethanol 1-Butanol 1-Hexanol 1-Octanol Methyl cyclohexane Dioxane Tetrahydrofurane Acetonitrile
34 32 40 53 63 39 44 47 51
142 180 194 225 283 101 133 214 181
2.94 3.12 2.50 1.88 1.58 2.38 2.27 2.12 1.96
6.98 5.52 5.12 4.41 3.50 9.87 7.49 4.64 5.49
56 139 482 974 1350
0.59 1.22 2.93 5.06 8.56 0.67 1.30 0.55 0.38
0.71 0.67 0.61 0.55 0.50 0.00 0.03 0.44 0.71
(33) (31) (33) (32) (29) (22) (14) (7) (9)
(67) (69) (67) (68) (71) (78) (86) (93) (91)
We also show the values of the viscosity (g) at 298 K, and Debye relaxation time (sD ) of the used alcohols.
of lifetimes, non-radiative rate constants (knr ) and few parameters of the used alcohols. The knr values were calculated assuming a radiative time constant of 34 ns as explained in the previously part. Clearly, a decrease of knr1;2 of the phototautomers when the viscosity (gÞ of the medium increases is observed (Table 2 and inset of Fig. 3). This indicates the presence of a non-radiative process sensitive to the viscosity, like a twisting motion. This is the major non-radiative channel of the produced phototautomers. In these solvents, all the emission decays were fitted using a double exponential function. Furthermore, we observed that the contribution (pre-exponential factor) of the short-time component (assigned to K*) drastically decreases when gating the emission at longer wavelengths. In 1-butanol for example, the contribution shows the following changes: 59% (425 nm), 33% (460 nm), 9% (520 nm) and 3% (560 nm). This indicates that the emission of K* tautomer is mainly centered at the blue part of the spectrum, in full agreement with the nanocavities results [5–7]. Thus, the dependence of the lifetimes of K* and KR* on the viscosity (and polarity, vide infra) of the alcohol is a consequence of their coupling to conformational changes between the twisted configuration of these structures involving the acetyl group and the naphthalene molecular frame, respectively. From the Arrhenius plot using MC (Fig. 2b) we obtained the activation energy of the non-radiative processes. This activation energy is associated to an intrinsic barrier (E0 ) of the reaction and a solvent-dependent crossing barrier (via viscosity).
One has to carry out experiments at constant viscosity and changing the temperature to determine (E0 ). Such experiments are currently in progress. 3.3. Solvent friction and polarity effects To study the solvent friction, we examined the variation of knr with the Debye relaxation time (sD ) (Table 2, Fig. 4a). We used this parameter because for alcohols, it is associated to specific H-bonding interactions and it reflects the overall rotational motion of solvent molecules as a hydrogen-bonded cluster. The motion may require a breaking of the hydrogen bonds of the shell around the solute [21]. The non-radiative processes of K* and KR* involve rotational and torsion motions, and thus the longest sD is appropriate to analyze this behavior. The figure shows that solvent friction due to hydrogen-bonding interactions between solvent molecules and phototautomers play a significant role in the non-radiative processes of both structures. The friction might be influenced by the intermolecular H-bonds with the solvent molecules, controlling the internal rotation of the phototautomers. A similar behaviour has been observed studying 4-(hydroxymethyl)stilbene in alcohols [22]. Two further points can be drawn from the plots: (i) knr1;2 decreases when sD increases. Thus, a longer Debye relaxation time for solvation shell allows stronger fluorescence of the phototautomers. (ii) The dependence (proportional to the slope) is more significant for K* (knr1 ) than for KR* (knr2 ). For the later, the dependence is very weak.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765 C1
(1)
C4
DX
C4 C6
20
C8
(1)
10
20
C6 C8
10 (2)
0
400
800
0 1200
0.5
0.6
THF
AC
20
(2)
0
MC
C2
30 knr1,2 / 109 s-1
knr1,2 / 109 s-1
C1 C2
30
0.7
knr1,2 / 109 s-1
764
(1)
15 10 5 0 0.0
(2)
0.2
0.4
0.6
0.7
Fig. 4. (a) Non-radiative rate constants (knr1;2 ) of HAN in methanol (C1 ), ethanol (C2 ), 1-butanol (C4 ), 1-hexanol (C6 ) and 1-octanol (C8 ) as a function of Debye relaxation time (sD ). (b) Variation of non-radiative rate constants of HAN (knr1;2 ) in C1 , C2 , C4 , C6 and C8 2 with f ðe; nÞ ¼ e1 nn2 1 ; and n are the dielectric constant and refractive index, respectively. (c) Non-radiative rate constants (knr1;2 ) of eþ2 þ2 HAN in methyl cyclohexane (MC), tetrahydrofurane (THF), acetonitrile (AC), and dioxane (DX) as a function of f ð; nÞ. For a, b, and c, family (1) corresponds to (knr1 ) and family (2) corresponds to (knr2 ).
Therefore, the solvent friction on the radiationless processes is more important in K* before internal twisting of the protonated acetyl group. This suggests also that motion of the naphthalene ring does not play a major role in the deactivation process of K* and KR*. Otherwise we should see a similar influence of sD on knr2 variation. Solvent effect can be analyzed also by considering the reaction dielectric field factor parameter 2 e1 f ðe; nÞ ¼ eþ2 nn2 1 [23]. Fig. 4b shows the variation þ2 of knr with the value of f ð; nÞ. Using the polaritypolarizability p* scale [24] we obtained analogous results, but using ET (30) (involving H-bonding and polarity) [25] we have not found a good correlation. For both keto-type tautomers, we observed a linear correlation. Both proton transfer and rotation processes involve a charge redistribution and therefore a solvent polarity effect is expected. The calculated dipole moment of K* and KR* are similar 4.2 D [4]. However the slopes of knr vs. f ð; nÞ are different being that of K* the most pronounced one (Fig. 4b). This indicates that the emission of this tautomer in this family of solvents is more efficiently deactivated by solvent polarity than that of KR*. One of the deactivation channels is that of the protonated acetyl rotation leading to KR*, as said above. This behavior is because the activation energy for non-radiative processes of KR* (3.4 kcal/mol) is larger than that for K* (0.96 kcal/mol), and consequently KR* emission is less affected by the changes in the
polarity of the medium. However, in the series of alcohols, the viscosity of the medium and the Hbonding interaction with the solute play a crucial role in the deactivation process. Therefore, to reduce the H-bonding donating effect of the solvent, and working at lower and similar viscosities, we obtained knr1;2 in non-hydroxylic solvents. Table 1 contains the data and Fig. 4c shows a plot of knr1;2 vs. f ð; nÞ. Two points should be noted: (i) in this non-hydroxylic series, both slops are negative, contrary to the alcohols behavior. This reflects the role played by the H-bonds in the alcohol frictions, making slower the rupture of the clusters structure (solvation shell) around the dye. (ii) The dependencies for both (knr ) in this series of solvents are similar suggesting that the radiationless channel, sensitive to polarity, is of the same nature for both tautomers. This channel might be connected to the coupling of the p; p* and n,p* states mediated outof-plane motions (dragging) as said above [10]. Therefore this internal conversion should be more important for K* relaxation leading to shorter lifetimes of this structure as it is actually observed.
4. Conclusion We found the K* ! KR* reaction dependent on the viscosity, H-bonding and polarity of the medium. Solvent friction and polarity are influenced by H-bonding ability of the solvent. For the
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
alcohols family, knr increases with the solvent polarity. While for non H-bonding media, we observed the reverse situation. We believe that the knowledge of the above factors which control the photodynamics and spectroscopy of both keto phototautomer structures of HAN can help for the design and study of new molecular systems with potential technological applications such as molecular memories and molecular probes. Acknowledgements This work was supported by the Ministry of Science and Technology (Spain) and ÔConsejerıa de Ciencia y Tecnologıa de la Junta de Comunidades de Castilla-La ManchaÕ through projects MAT-2002-01829 and PAI-02-004, respectively.
References [1] A. Douhal, F. Lahmani, A. Zehnacker-Rentien, Chem. Phys. 178 (1993) 493. [2] A. Douhal, F. Lahmani, A.H. Zewail, Chem. Phys. 207 (1996) 477. [3] J.A. Organero, I. Garcıa-Ochoa, M. Moreno, J.M. Lluch, L. Santos, A. Douhal, Chem. Phys. Lett. 328 (2000) 83. [4] J.A. Organero, M. Moreno, L. Santos, J.M. Lluch, A. Douhal, J. Phys. Chem. A 104 (2000) 8424. [5] J.A. Organero, L. Santos, A. Douhal, in: A. Douhal, J. Santamaria (Eds.), Femtochemistry and Femtobiology: Ultrafast Dynamics in Molecular Science, World Scientific, Singapore, 2002, p. 225. [6] J.A. Organero, L. Tormo, A. Douhal, Chem. Phys. Lett. 363 (2002) 409.
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[7] J.A. Organero, A. Douhal, Chem. Phys. Lett. 373 (2003) 426. [8] C. Lu, R.-M.- R. Hsieh, I.-R. Lee, P.-Y. Cheng, Chem. Phys. Lett. 310 (1999) 103. [9] S. Tobita, M. Yamamoto, N. Kurahayashi, R. Tsukagoshi, Y. Nakamura, H. Shizuka, J. Phys. Chem. A. 102 (1998) 5206. [10] S. Lochbrunner, T. Schultz, J.P. Shaffer, M.Z. Zgierski, A. Stolow, J. Chem. Phys. 114 (2001) 2519. [11] S. Lochbrunner, K. Stock, V. DeWaele, E. Riedle, in: A. Douhal, J. Santamaria (Eds.), Femtochemistry and Femtobiology: Ultrafast Dynamics in Molecular Science, World Scientific, Singapore, 2002, p. 202. [12] J. Catalan, J.C. del Valle, J. Am. Chem. Soc. 115 (1993) 4321. [13] J. Catalan, J.L.G. de Paz, J. Phys. Chem. A 105 (2001) 7315. [14] J.A. Organero, A. Vargas Diaz, M. Moreno, J.M. Lluch, L. Santos, A. Douhal, J. Phys. Chem. A 105 (2001) 7317. [15] K. Tokumura, K. Nagaosa, Y. Ohta, R. Matsushima, Chem. Phys. Lett. 295 (1998) 516. [16] A.D. Roshal, J.A. Organero, A. Douhal, Chem. Phys. Lett. 379 (2003) 53. [17] K.K. Smith, K.J. Kaufmann, J. Phys. Chem 85 (1981) 2895. [18] B. Brocklehurst, R.N. Young, J. Phys. Chem. 103 (1999) 3809. [19] J. Bartos, J. Kristiak, J. Phys. Chem. B 104 (2000) 5666. [20] R.O. Loutfy, B.A. Arnold, J. Phys. Chem. 86 (1982) 4205. [21] S.K. Grag, C.P. Smyth, J. Phys. Chem. 69 (1965) 1294. [22] K. Wiemers, J.F. Kauffman, J. Phys. Chem. A 104 (2000) 451. [23] N. Mataga, T. Kubota, in: Moleuclar Interactions and Electronic Spectra, Mercel Dekker, New York, 1970, p. 371. [24] M.J. Kamlet, J.-L.M. Abboud, R.W. Taft, Prog. Phys. Org. Chem. 13 (1981) 485. [25] C. Reichardt, Chem. Rev. 94 (1994) 2319.
Temperature and solvent effects on the photodynamics of 10 -hydroxy-20-acetonaphthone J.A. Organero, A. Douhal
*
Departamento de Quımica Fısica, Secci on de Quımicas, Facultad de Ciencias del Medio Ambiente, Campus Tecnol ogico de Toledo, Avenida Carlos III, S.N., Universidad de Castilla-La Mancha, 45071 Toledo, Spain Received 4 September 2003; in final form 10 October 2003 Published online: 4 November 2003
Abstract We present studies of temperature viscosity and polarity effects on the photodynamics of 10 -hydroxy-20 -acetonaphthone. We observed a striking temperature effect on the non-radiative decays and which explains the low emission quantum yield of the formed keto-type phototautomers. The thermal activation energy is lower for the keto structure (0.96 kcal/mol) than for the keto rotamer one (3.4 kcal/mol). For the alcohols family of solvents, the keto-type tautomer is more sensitive to solvent response than the keto rotamer. However for non-hydroxylic media the low influence of solvent polarity is similar for both keto type structures. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction In previous reports, we have studied the photophysics of 10 -hydroxy-20 -acetonaphthone (HAN) using laser fluorescence spectroscopy under jet-cooled molecular beam conditions, quantum chemistry at both S0 and S1 states, and in solution nanocavities [1–7]. The results show that the most stable conformer of HAN (enol structure, E) is that having an intramolecular H-bond between the OH and COCH3 group, and electronic excitation of E induces through or over a small energy barrier an excited-state intramolecular
*
Corresponding author. Fax: +34-925-268840. E-mail address: [email protected] (A. Douhal).
proton transfer (ESIPT) reaction leading to a ketotype tautomer (K*) (Scheme 1). This structure subsequently may lead to a rotamer (KR*) though a twisting motion of C–C bond linking the now protonated acetyl group and the naphthalene frame [4]. Femtosecond experiment in gas phase at room temperature performed by Cheng group found that the time constant for shifting the proton within E* to rapidly equilibrate with the generated keto type K* structure is in the picosecond time scale (60–85 ps), in agreement with the gas phase jet-cooled molecular beam conclusion [8]. Earlier, nanosecond–microsecond time-resolved experiments in solution carried out by Tobita et al. [9] showed a long lived ground keto tautomer after relaxation of K*. To explain the result, the authors
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.10.039
760
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
O H
O C
O CH3
Proton
H O C
O CH3
Transfer
HO C
Twisting
CH3
Motion
Scheme 1. Molecular structures of enol (E*), keto (K*) and keto rotamer (KR*) of 10 -hydroxy-20 -acetonaphthone (HAN). The arrows indicate the involved motion at the S1 state.
2. Experimental HAN (from Aldrich) were used as received. All the solvents (spectrograde quality) were used as received and these purity was checked before used. Steady-state emission spectra were recorded on Perkin–Elmer (LS 50B) spectrophotometer. Timeresolved emission measurements were done using a picosecond time-correlated single-photon counting spectrophotometer described earlier [6]. For measurements of emission decays at different temper-
atures we used a Cryostat (Oxford OptistatDN) with a digital controller ITC 601. Decays and anisotropy data were analysed using the FluoFit software package. Exponential decay functions were convoluted with the experimental response function (65 ps) and fitted to the experimental decay. The quality of the fits were characterized in terms of the reduced v2 value and the distribution of residuals.
3. Results and discussion 3.1. Temperature effect The emission spectrum of HAN in methyl cyclohexane (MC, an inert solvent) at 298 K (Fig. 1) is broad with the maximum at 480 nm and a shoulder at 450 nm. Upon lowering the temper-
Normalized intensity (a.u.)
suggested the involvement of a structural change of K* to produce a metastable keto rotamer KR*. Stolow and co-workers found a 30 ps component in the femtosecond transient of excited (excess energy 2500 cm1 ) HAN in molecular beam, which it was assigned to an internal conversion processes enhanced by a close lying n,p* state [10]. Lochbrunner et al. [11] using a femtosecond pump–probe technique observed that the ESIPT of HAN occurs in 30 fs. However, reports by Catalan et al. [12,13] did not recognize the occurrence of ESIPT reaction in HAN [14]. Recently, we have reported on experiments using a-, b- and c-cyclodextrins (CDÕs) as caging media of HAN and concluded that K* may suffer a twisting motion in few tens of ps along the naphthyl-COCH3 bond yielding to a nanosecond-living tautomer which emits at the red side of the K* spectrum. This photoisomerization is largely affected by the size of the nanocavity and stochiometry of the complex [6,7]. In continuation to the previous efforts for a better understanding of HAN fast dynamics, we report here studies of temperature and solvent effects on its photophysics.
0.5
0.0 400
440
480
520
560
600
Fig. 1. Steady-state emission spectra of lM HAN in methyl cyclohexane (MC) at 298 (dotted line) and 77 K (solid line) when excited at 380 nm.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
ature to 77 K, the emission exhibits a structured band with maxima at 440 and 480 nm. Besides these peaks, other blue (420 nm) and green (500 nm) shoulders appear at this temperature. The structural difference between both spectra when lowering the temperature is interpreted in terms of the involvement of a twisting motion in the formed phototautomer at S1 , in agreement with previous findings [4–10]. A study of 30 ,40 benzo-20 -hydroxychalcone, a comparable system, at different temperatures has reached a similar conclusion [15]. To obtain more information about the effect of the temperature on the photodynamics of HAN in MC when exciting at 393 nm (almost without excess energy at E*), we gated the emission at different wavelengths and at different temperatures (80–298 K). Fig. 2 shows the emission decays at 480 nm. To begin with, the decay at 298 K could not be fitted to a single exponential function and the fitting needed a biexponential with time constants of 39 ps (22%) and 101 ps (78%). The shortest time, assigned to K*, might be considered as a limit value for the twisting motion of this structure to produce KR* which has the longest time constant. Femtosecond experiments showed that the time constant of the proton transfer in E* is 30 fs [11]. Under the assumption of equilibrium between K* and KR*, and using 5 ns as a fluorescence lifetime value for KR* at room temperature (similar to that observed in caging nanocavities [6,7]), we obtained 660 ps for the conversion of K* to KR* using the Birks kinetics model and proceeding as in a previous contribution [16]. This time is clearly longer than 90 ps, the fluorescence decay time of K* deduced from the same supposed equilibrium scheme. Therefore, in this solvent at room temperature we suggest that K* and KR* structures are not equilibrated. Lowering the temperature to 80 K, the global emission decays show a remarkable change. At 480 nm for example, the lifetime values change from 39 ps to 2.16 ns (4%) and from 101 ps to 8.38 ns (96%) assigned for K* and KR*, respectively (Table 1). The presence of efficient non-radiative processes at higher temperatures like those due to conformational motions of the phototautomers is the reason of short lifetimes at room temperature [5–7]. We
761
Fig. 2. (a) 480 nm and magic-angle emission decays of HAN in MC at 298, 260, 240, 220, 210, 200, 190, 180, 170, 160, 150, 130, 100, 80 K when excited at 393 nm. (b) Arrhenius plot (solid line) of the non-radiative rate constants (knr1;2 ) of HAN in MC. Ea1 and Ea2 are the activation energies for the non-radiative processes of K* and KR*, respectively. The dotted line indicates the deviation from the Arrhenius behaviour.
identify these motions as twisting motion along the naphtyl-COCH3 bond, torsion and out-of-plane (dragging) motion of the naphthalene molecular frame in K* and in KR*, respectively. Theoretical results suggested a loss of aromaticity of the naphthalene ring and a lower conjugation of the twisted and protonated acetyl group in KR* [4]. Similar emission deactivation processes with a dependency on the viscosity and polarity of the medium have been observed in methyl salicylate, a molecule similar to HAN and which suffers a proton transfer at the S1 state [17].
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Table 1 Fluorescence lifetimes (s1;2 ), pre-exponential factors (between brackets) and non-radiative rate constants (knr1;2 ) of HAN in MC at different temperatures Temperature (K)
s1 (ps) (%)
s2 (ps) (%)
(knr1 ) (1010 s1 )
(knr2 ) (108 s1 )
298 260 240 220 210 200 190 180 170 150 130 100 80
39 (22) 65 (23) 55 (29) 77 (21) 90 (19) 90 (20) 105 (19) 119 (18) 160 (15) 156 (12) 873 (8) 893 (7) 2156 (4)
101 (78) 230 (77) 431 (71) 788 (79) 1114 (81) 1608 (80) 2294 (81) 3103 (82) 4078 (85) 5860 (88) 7453 (92) 8137 (93) 8383 (95)
2.56 1.53 1.81 1.29 1.11 1.10 0.95 0.83 0.62 0.64 0.11 0.11 0.04
98.7 43.1 22.9 12.39 8.68 5.92 4.06 2.92 2.15 1.41 1.04 0.93 0.9
friction and restriction of the solvent motions increases at lower temperatures [20]. 3.2. Solvent viscosity effect
Intensity (u.a.)
To study the effect of the viscosity and hydrogen-bonding interactions of the medium on the deactivation processes of excited HAN, we measured the fluorescence spectrum and lifetime in different alcohols (Fig. 3). Table 2 lists the values
10 4
10
Counts
To get the non-radiative rate constants of K* (knr1 ) and KR* (knr2 ), we used the data of Tobita et al. [9] for HAN at 77 K (mean fluorescence lifetime ¼ 7.8 ns and global emission quantum yield ¼ 0.23 give a radiative lifetime of 34 ns). Thus, assuming that the radiative time for K* and KR* are similar we calculated knr1;2 using sf ¼ 1= ðknr þ kr Þ. Arrhenius-plot of ln knr1;2 vs. 1/T gives the activation energy and the pre-exponential (frequency) factor for non-radiative decays of K* (Ea1 ¼ 0.96 0.20 kcal/mol, A1 ¼ 9.7 1010 s1 ) and KR* (3.4 0.2 kcal/mol, A2 ¼ 2.3 1012 s1 ) (Fig. 2b). Based on the previous assignment of these barriers, twisting motion of the protonated acetyl group has a lower energy barrier and a lower frequency term (A2 /A1 ¼ 24) than those of the out-of-plane (dragging) motion of the naphthalene skeleton. Consistent with this explanation (large amplitude motions of the naphthalene ring) is the Arrhenius plot (Fig. 2b) behavior when the liquid–glass transition temperature of MC (90 K) is reached [18]. Below this temperature, the viscosity of MC undergoes a large increase (g88 K ¼1013 cP) and the slope of variation of free volume holes with the temperature exhibits a significant change [19]. The linearity of both plots in the 298–130 K range shows that the non radiative processes of K* and KR* in MC are controlled by hydrodynamics factors. While below 130 K the deactivation of KR* is rather controlled by free volume factors because the large solute motions decrease drastically at the same time that the
3
1-Octanol
1200 800
Methanol
400 0 420
10 2
480 540 600 Wavelength / nm
1-Octanol IRF
Methanol
10
1
0
0.5
1.0
1.5
2.0
Fig. 3. 480 nm and magic-angle emission decays of HAN in methanol and 1-octanol at room temperature when excited at 393 nm. IRF is the instrument response function, 65 ps. Inset: steady-state emission spectra (excitation at 365 nm) in methanol, ethanol, 1-butanol, 1-hexanol and 1-octanol at the same optical density at 365 nm (O.D.365 ¼ 0.55) at 298 K.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
763
Table 2 Fluorescence lifetimes (s1;2 ), pre-exponential factors (between brackets) and non-radiative rate constants (knr1;2 ) of HAN in the used solvents at 480 nm Solvent
s1 (ps) (%)
s2 (ps) (%)
(knr1 ) (1010 s1 )
(knr2 ) (109 s1 )
sD (ps)
g (cP)
f(; n)
Methanol Ethanol 1-Butanol 1-Hexanol 1-Octanol Methyl cyclohexane Dioxane Tetrahydrofurane Acetonitrile
34 32 40 53 63 39 44 47 51
142 180 194 225 283 101 133 214 181
2.94 3.12 2.50 1.88 1.58 2.38 2.27 2.12 1.96
6.98 5.52 5.12 4.41 3.50 9.87 7.49 4.64 5.49
56 139 482 974 1350
0.59 1.22 2.93 5.06 8.56 0.67 1.30 0.55 0.38
0.71 0.67 0.61 0.55 0.50 0.00 0.03 0.44 0.71
(33) (31) (33) (32) (29) (22) (14) (7) (9)
(67) (69) (67) (68) (71) (78) (86) (93) (91)
We also show the values of the viscosity (g) at 298 K, and Debye relaxation time (sD ) of the used alcohols.
of lifetimes, non-radiative rate constants (knr ) and few parameters of the used alcohols. The knr values were calculated assuming a radiative time constant of 34 ns as explained in the previously part. Clearly, a decrease of knr1;2 of the phototautomers when the viscosity (gÞ of the medium increases is observed (Table 2 and inset of Fig. 3). This indicates the presence of a non-radiative process sensitive to the viscosity, like a twisting motion. This is the major non-radiative channel of the produced phototautomers. In these solvents, all the emission decays were fitted using a double exponential function. Furthermore, we observed that the contribution (pre-exponential factor) of the short-time component (assigned to K*) drastically decreases when gating the emission at longer wavelengths. In 1-butanol for example, the contribution shows the following changes: 59% (425 nm), 33% (460 nm), 9% (520 nm) and 3% (560 nm). This indicates that the emission of K* tautomer is mainly centered at the blue part of the spectrum, in full agreement with the nanocavities results [5–7]. Thus, the dependence of the lifetimes of K* and KR* on the viscosity (and polarity, vide infra) of the alcohol is a consequence of their coupling to conformational changes between the twisted configuration of these structures involving the acetyl group and the naphthalene molecular frame, respectively. From the Arrhenius plot using MC (Fig. 2b) we obtained the activation energy of the non-radiative processes. This activation energy is associated to an intrinsic barrier (E0 ) of the reaction and a solvent-dependent crossing barrier (via viscosity).
One has to carry out experiments at constant viscosity and changing the temperature to determine (E0 ). Such experiments are currently in progress. 3.3. Solvent friction and polarity effects To study the solvent friction, we examined the variation of knr with the Debye relaxation time (sD ) (Table 2, Fig. 4a). We used this parameter because for alcohols, it is associated to specific H-bonding interactions and it reflects the overall rotational motion of solvent molecules as a hydrogen-bonded cluster. The motion may require a breaking of the hydrogen bonds of the shell around the solute [21]. The non-radiative processes of K* and KR* involve rotational and torsion motions, and thus the longest sD is appropriate to analyze this behavior. The figure shows that solvent friction due to hydrogen-bonding interactions between solvent molecules and phototautomers play a significant role in the non-radiative processes of both structures. The friction might be influenced by the intermolecular H-bonds with the solvent molecules, controlling the internal rotation of the phototautomers. A similar behaviour has been observed studying 4-(hydroxymethyl)stilbene in alcohols [22]. Two further points can be drawn from the plots: (i) knr1;2 decreases when sD increases. Thus, a longer Debye relaxation time for solvation shell allows stronger fluorescence of the phototautomers. (ii) The dependence (proportional to the slope) is more significant for K* (knr1 ) than for KR* (knr2 ). For the later, the dependence is very weak.
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765 C1
(1)
C4
DX
C4 C6
20
C8
(1)
10
20
C6 C8
10 (2)
0
400
800
0 1200
0.5
0.6
THF
AC
20
(2)
0
MC
C2
30 knr1,2 / 109 s-1
knr1,2 / 109 s-1
C1 C2
30
0.7
knr1,2 / 109 s-1
764
(1)
15 10 5 0 0.0
(2)
0.2
0.4
0.6
0.7
Fig. 4. (a) Non-radiative rate constants (knr1;2 ) of HAN in methanol (C1 ), ethanol (C2 ), 1-butanol (C4 ), 1-hexanol (C6 ) and 1-octanol (C8 ) as a function of Debye relaxation time (sD ). (b) Variation of non-radiative rate constants of HAN (knr1;2 ) in C1 , C2 , C4 , C6 and C8 2 with f ðe; nÞ ¼ e1 nn2 1 ; and n are the dielectric constant and refractive index, respectively. (c) Non-radiative rate constants (knr1;2 ) of eþ2 þ2 HAN in methyl cyclohexane (MC), tetrahydrofurane (THF), acetonitrile (AC), and dioxane (DX) as a function of f ð; nÞ. For a, b, and c, family (1) corresponds to (knr1 ) and family (2) corresponds to (knr2 ).
Therefore, the solvent friction on the radiationless processes is more important in K* before internal twisting of the protonated acetyl group. This suggests also that motion of the naphthalene ring does not play a major role in the deactivation process of K* and KR*. Otherwise we should see a similar influence of sD on knr2 variation. Solvent effect can be analyzed also by considering the reaction dielectric field factor parameter 2 e1 f ðe; nÞ ¼ eþ2 nn2 1 [23]. Fig. 4b shows the variation þ2 of knr with the value of f ð; nÞ. Using the polaritypolarizability p* scale [24] we obtained analogous results, but using ET (30) (involving H-bonding and polarity) [25] we have not found a good correlation. For both keto-type tautomers, we observed a linear correlation. Both proton transfer and rotation processes involve a charge redistribution and therefore a solvent polarity effect is expected. The calculated dipole moment of K* and KR* are similar 4.2 D [4]. However the slopes of knr vs. f ð; nÞ are different being that of K* the most pronounced one (Fig. 4b). This indicates that the emission of this tautomer in this family of solvents is more efficiently deactivated by solvent polarity than that of KR*. One of the deactivation channels is that of the protonated acetyl rotation leading to KR*, as said above. This behavior is because the activation energy for non-radiative processes of KR* (3.4 kcal/mol) is larger than that for K* (0.96 kcal/mol), and consequently KR* emission is less affected by the changes in the
polarity of the medium. However, in the series of alcohols, the viscosity of the medium and the Hbonding interaction with the solute play a crucial role in the deactivation process. Therefore, to reduce the H-bonding donating effect of the solvent, and working at lower and similar viscosities, we obtained knr1;2 in non-hydroxylic solvents. Table 1 contains the data and Fig. 4c shows a plot of knr1;2 vs. f ð; nÞ. Two points should be noted: (i) in this non-hydroxylic series, both slops are negative, contrary to the alcohols behavior. This reflects the role played by the H-bonds in the alcohol frictions, making slower the rupture of the clusters structure (solvation shell) around the dye. (ii) The dependencies for both (knr ) in this series of solvents are similar suggesting that the radiationless channel, sensitive to polarity, is of the same nature for both tautomers. This channel might be connected to the coupling of the p; p* and n,p* states mediated outof-plane motions (dragging) as said above [10]. Therefore this internal conversion should be more important for K* relaxation leading to shorter lifetimes of this structure as it is actually observed.
4. Conclusion We found the K* ! KR* reaction dependent on the viscosity, H-bonding and polarity of the medium. Solvent friction and polarity are influenced by H-bonding ability of the solvent. For the
J.A. Organero, A. Douhal / Chemical Physics Letters 381 (2003) 759–765
alcohols family, knr increases with the solvent polarity. While for non H-bonding media, we observed the reverse situation. We believe that the knowledge of the above factors which control the photodynamics and spectroscopy of both keto phototautomer structures of HAN can help for the design and study of new molecular systems with potential technological applications such as molecular memories and molecular probes. Acknowledgements This work was supported by the Ministry of Science and Technology (Spain) and ÔConsejerıa de Ciencia y Tecnologıa de la Junta de Comunidades de Castilla-La ManchaÕ through projects MAT-2002-01829 and PAI-02-004, respectively.
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