- Email: [email protected]
Confinement effects on the photorelaxation of a proton-transfer phototautomer
Chemical Physics Letters 373 (2003) 426–431 www.elsevier.com/locate/cplett
Confinement effects on the photorelaxation of a proton-transfer phototautomer 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 7 March 2003; in final form 14 April 2003
Abstract We report on emission studies of excited phototautomers of 10 - hydroxy-20 -acetonaphthone in aqueous solutions of a-cyclodextrin (a-CD). Caging by a-CD provokes a 40 nm blue shift of the emission, and increases the emission lifetime from 90 ps to 1 ns. The rotational time of the caged phototautomers varies from 50 to 180 ps, while that of global rotational time of 1:2 complex (HAN:(a-CD)2 ) is 950 ps. These results are explained on the basis of confinement effects on the twisting motion of the formed phototautomers within the cavities provided by one or two CD. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The study of confinement effects on the behaviour of molecules provides valuable information on the nature of the environment (space domain) and dynamical properties (time domain) of the system [1–9]. Among the simplest systems for these studies are the inclusion complexes of dyes with cyclodextrins (CDs) [1–13]. The results show a strong relationship between space, frequency and time domains of the ultrafast properties of the nanosystem. Recently, we have reported on the photoisomerization of 10 -hydroxy-20 -acetonaphthone (HAN) encapsulated in the cavities of b- and c-CD [12,13]. *
Corresponding author. Fax: +34-925-268-840. E-mail address: [email protected] (A. Douhal).
In agreement with previous studies [14–18], we observed the occurrence of an excited-state intramolecular proton-transfer (ESIPT) reaction in the first electronically excited-state of enol (E*) form giving birth to a keto (K*) phototautomer structure. Experiments using b- and c-CD as caging media have shown that after the ultrafast protontransfer process, K* may suffer a subsequent twisting motion in few tens of picoseconds (ps) along the naphthyl–COHCH3 bond yielding to a nanosecond-living keto rotamer (KR*). This one emits at the red side of the K* spectrum, according to previous calculations [14]. The photoisomerization of K* is largely affected by the size of the nanocavity [12,13]. In continuation to the previous studies using b- and c-CD we have examined the behaviour of HAN caged in the smaller cavity offered by a-CD (Fig. 1).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00626-2
J.A. Organero, A. Douhal / Chemical Physics Letters 373 (2003) 426–431
427
Fig. 1. Molecular structures for equilibrium inclusion complexes between 10 -hydroxy-20 -acetonaphthone (HAN) and a-cyclodextrin (a-CD).
2. Experimental HAN and a-CD (from Aldrich) were used as received. Steady-state absorption and emission spectra were recorded on a Varian (Cary E1) and Perkin–Elmer (LS 50B) spectrophotometers, respectively. Time-resolved measurements were done exciting at 393 nm (40 ps, 20 MHz, 0.8 mW) using a time-correlated single-photon counting picosecond spectrophotometer (FluoTime 200, PicoQuant) described earlier [13]. Decay and anisotropy data were analysed using the FluoFit software package (PicoQuant). 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. All the measurements were done at 296 1 K and using fresh solutions.
3. Results and discussion Fig. 2 shows the emission spectra of HAN in neutral water and in aqueous solutions of a-CD. In pure neutral water, the dye exhibits emission of phototautomers (K* and KR*) at 490 nm [13] and a global fluorescence quantum yield u0 ¼ ð4:0 0:5Þ 103 . Upon a-CD addition, the fluorescence intensity gradually increases due to
Fig. 2. Change of emission spectrum of HAN (105 M) upon addition of a-CD. The dashed spectrum corresponds to the emission of the complex at [a-CD]0 ¼ 110 mM, and which was deduced as Ic ¼ I aI0 , where Ic , I and I0 are the emission intensity of the complex, with and without a-CD, respectively, and a (0.03) is the fraction of non-caged HAN. The inset is the fluorescence quantum yield variation with [a-CD]0 and best fit (solid line) following Eq. (1). Excitation wavelength 365 nm and T ¼ 293 K.
the formation of a more fluorescent structure, and a new maximum at 460 nm appears as a result of the formed complexes. Based on previous studies [12,13] these complexes are molecular entities of HAN trapped into a-CD cavity. The blue shift is explained in terms of hindrance in rotational
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motion of the formed phototautomer (K*) in these complexes. The polarity of the medium does not play a significant role in the position of the emission spectrum [13]. The binding constants and stoichiometry of the inclusion complexes (Fig. 1) were estimated using the change of the overall fluorescence quantum yield (u) of each sample upon addition of a-CD. The variation was then analysed assuming the formation of 1:1, 1:2 or 2:2 HAN:a-CD inclusion complexes [19]. We found that the equation (Eq. (1)) that correctly fits the experimental data corresponds to an 1:2 stoichiometry, u¼
u0 þ K1 u1 ½a-CD þ K1 K2 u2 ½a-CD 1 þ K1 ½a-CD þ K1 K2 ½a-CD
2
2
;
ð1Þ
where K1 and K2 are the equilibrium or binding constants of 1:1 and 1:2 complexes, respectively (Fig. 1). u0 , u1 and u2 are the fluorescence quantum yields of the HAN in pure neutral water, of 1:1 and 1:2 complexes, respectively. The best fit (inset of Fig. 2) gives K1 ¼ 2 1 M1 , K2 ¼ 273 100 M1 , u1 ¼ ð3:7 0:3Þ 102 and u2 ¼ ð3:9 0:1Þ 102 . The apparent equilibrium constant ðKapp ¼ K1 K2 Þ between free and 1:2 complex is 546 M2 . Taking into account the obtained values of K1 and Kapp , the corresponding variation of standard free energy at 293 K to 1:1 (DG0293 (1:1)) and 1:2 (DG0293 (1:2)) complexation processes, relatively to free HAN are DG0293 ð1Þ ¼ 1:7 1:2 kJ mol1 and DG0293 ð2Þ ¼ 15:5 4:0 kJ mol1 , respectively. The DG0293 ð2Þ ¼ DG0293 (1:2) ) DG0293 (1:1) ¼ 13:8 4:0 kJ mol1 is the free energy gain upon addition of the second nanocavity to 1:1 complex (Fig. 1). Using the values of equilibrium constants and taking into account that [CD]0 [HAN]0 (110 mM 102 mM), we constructed the emission spectrum of the 1:2 complex of the most a-CD concentrated solution (dashed spectrum in Fig. 2). The spectrum presents a structure with two maxima at 460 and 490 nm. Its blue shift relatively to the spectrum of HAN without a-CD is due to the confinement effect on the relaxation routes involving the rotational motion of COHCH3 group of the formed K* phototautomers. The spectrum presents a shape similar to that of HAN in methylcyclohexane at 77 K, and
which was correlated with those observed in jet cold molecular beam (maximum at 426 and 452 nm) [15]. At an [a-CD]0 ¼ 30 mM, 64% of initial HAN is free, 4% is complexed with one a-CD and 32% is complexed with two a-CD. When [aCD]0 ¼ 110 mM these values change to 13% (free), 3% (1:1 complex) and 84% (1:2 complex), respectively. For 2-naphthol:a-CD system, K€ ohler et al. [20] found K1 ¼ 250 and K2 ¼ 35 M1 at 298 K for 1:1 and 1:2 entities, respectively. Our results show that the 1:2 HAN:(a-CD)2 complex is significantly more stable than 1:1 complex. Compared to the parent compound (2-naphthol) the internal H-bond between OH and COCH3 groups in HAN plays a stabilizing role on the stability of 1:2 complex relatively to 1:1 entity. To obtain information on the fluorescence dynamics of the nanostructures, we measured the fluorescence decays of HAN in presence of different a-CD concentrations (Fig. 3). In neutral water, we observed a bi-exponential decay with two components having time constants of 90 10 ps (99%) and 5 1 ns (1%), assigned to K* and KR*
Fig. 3. Fluorescence decays at 480 nm of HAN (105 M) in neutral water and in presence of 15, 22, 27, 35, 45, 80 and 110 mM of a-CD. Inset: normalized time-resolved emission spectra of HAN in presence of 22 mM of a-CD and gated at 75, 300, 500 ps and 1 ns. Excitation at 393 nm and emission at 480 nm (magic angle), the instrumental response function (IRF) is 65 ps.
J.A. Organero, A. Douhal / Chemical Physics Letters 373 (2003) 426–431
tautomers, respectively [13]. Although the contribution of nanosecond component is very small, its presence is clear in the decay. In presence of a-CD, a tri-exponential function was needed to obtain an accurate fit (Fig. 3). For example at [a-CD]0 ¼ 110 mM, the obtained lifetimes are: 90 10 ps (29%), 1.0 0.1 ns (70%) and 5 1 ns (0.5%). For all the a-CD solutions, we found that the contribution of the fastest component (90 ps) in the decay decreases gradually from 99% using pure neutral water to 29% when [a-CD]0 ¼ 110 mM (saturation). The contribution of the 1 ns component changes from 2% to 70% when [a-CD]0 was varied from 3 to 110 mM. We assigned this nanosecond component to the lifetime of 1:2 complexes. The contribution of the longest component (5 ns) was maintained with a small population and is assigned to the lifetime KR*. The results suggest that the 1:1 complex has a lifetime similar to that of the free HAN in water (90 ps). The COHCH3 group of K* photoproduced in 1:1 complex does not probably suffer the confinement effect. Thus, the enol structure of HAN in 1:1 entity is not totally included into the CD cavity (Fig. 1). Comparable behaviour has been observed in the nanostructures (1:1 and 1:2 complexes) of 2- naphthol and methyl orange complexed to CD [20,21]. To get more insight into the spectral relaxation of the confined geometries, we recorded time-resolved emission spectra (TRES) of a HAN:a-CD solution (Fig. 3). Position and shape of the gated emission spectrum depend on the time of observation. At 75 ps, the emission is similar to that of K* and KR* upon excitation of E in water (max 490 nm) and an ultrafast ESIPT reaction in this structure [12,13]. While at times longer than 300 ps, the maximum appears at 460 nm and the emission originates from the phototautomers photoproduced inside the CD cavity (at this time mostly from 1:2 complexes). The blue shift in the TRES (1350 cm1 – 16 kJ mol1 ) is due to the hindrance in isomerization channel of K* that would lead to KR* inside the complexes. Therefore, the COHCH3 rotation in K* to yield its rotamer KR* involves an energy gain of 16 kJ mol1 , close to the obtained energy gap (24 kJ mol1 ) between these structures in gas phase using theoretical calculations [14].
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To elucidate the dynamics of the orientational motion of the guest and of the host–guest system, we obtained the rotational relaxation times (/) by time-resolved fluorescence anisotropy rðtÞ measurements (Fig. 4). To begin with pure water and tetrahydrofurane (THF), rðtÞ fits to a mono-exponential function with time constant / 70 10 ps and 35 0 ps, respectively. Comparable rotational times have been obtained for similar dyes in size [8,22]. For HAN in a saturated solution of aCD (110 mM), a double-exponential function with time constants /1 ¼ 50–180 15 and /2 ¼ 950 50 ps were needed. The value of the shortest one (/1 ) and its populations change with the gated region: 180 (24%), 130 (13%), 95 (12%) and 50 (26%) ps when the emission is collected at 430, 480, 520, 560 nm, respectively. The variation suggests the existence of several rotors (or conformers) that contribute to this component. In presence of 110 mM of a-CD, 13% of HAN is free, 3% is forming 1:1 complex and 84% is involved in 1:2 complexes. Stokes–Einstein–Debye hydrodynamic theory [23,24], and under a stick boundary condition, predicts an overall rotational time of 210 ps for the 1:1 complex, very close to the experimental value
Fig. 4. Anisotropy decays at 480 nm of HAN in neutral water ( ), in presence of 110 mM a-CD ( ) and in THF (NNNN). Single- and double-exponential fittings were used for the best fit (solid lines) to obtain the rotational relaxation time (/). The initial time (t ¼ 0) for the anisotropy decay is taken at the intensity maximum of IRF.
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of 180 ps at 430 nm. The rotational time for the guest into 1:1 and 1:2 entities will depend on the degree of freedom of COHCH3 group. A time (few tens of picoseconds as found in THF, 35 ps) component is expected for a free rotation of the guest. The relatively small value of /1 and its decrease to 50 ps at longer wavelengths to a value comparable to free HAN (70 ps) and in THF (35 ps), suggest that at the red side of spectrum the short component is largely governed by the phototautomers of HAN in water and by the internal rotation of the guest in the complexes. The longer component ð/2 950 ps) was found independent on the emission wavelength, and it is assigned to the overall rotational time of the 1:2 complex. The theoretical value predicted by the hydrodynamic theory under a stick boundary condition for this rotor having a shape and a volume similar to those of two linked a-CD, 600 ps, is in the same order of magnitude of the experimental one, /2 950 ps. The initial values of anisotropy (rð0Þ 0:25–0:35) for HAN in water, THF and in presence of a-CD (110 mM) are different from the ideal value (0.4) for a situation where the transition dipole moments of the absorbing and emitting entities are parallel. This is due to the proton-transfer reaction in E* (absorbing entity) generating K* and KR* (the emitting tautomers) where large electronic changes have occurred. Following the values of rð0Þ the changes are larger in water and THF than in nanocavities, in agreement with a restricted photorelaxation due to the confinement.
4. Conclusion The results indicate that the size (space domain) of the cavity of the host (1 or 2 a-CD) governs the photodynamics (time domain, from picosecond to nanosecond timescale) and spectroscopy (spectral domain, shift by 40 nm) of the nanostructure. The results might be relevant for designing size-tuned fluorescent nanostructures like those involved in nanophotonics, and could be used for a better understanding of the behaviour of medicines and drugs used in phototherapy or in neuromuscular blockers [25].
Acknowledgements This work was supported by the Ministry of Science and Technology (Spain) through the projects PB98-0310, MAT-2002-01829 and ÔConsejerıa de Ciencia y Tecnologıa de la Junta de Comunidades de Castilla-La ManchaÕ through the project PAI-02-004.
References [1] A.H. Zewail, J. Phys. Chem. A 104 (2000) 5660. [2] A. Douhal, F. Amat-Gerri, A.U. Acu~ na, Angew. Chem. Int. Ed. Engl. 36 (1997) 1514. [3] See, for example, the special issue of Chem. Rev. 98 (1998) No. 5. [4] A. Douhal, T. Fiebig, M. Chachisvilis, A.H. Zewail, J. Phys. Chem. A 102 (1998) 1657. [5] M. Chachisvilis, I. Garcia-Ochoa, A. Douhal, A.H. Zewail, Chem. Phys. Lett. 293 (1998) 153. [6] A. Douhal, in: F.C. De Schryver, S. De Feyter, G. Schweitzer (Eds.), Femtochemistry, Wiley–VCH, Weinheim/New York, 2001, p. 267. [7] I. Garcıa-Ochoa, M.A. Dıez L opez, M.H. Vi~ nas, L. Santos, E. Martınez Ataz, F. Amat-Guerri, A. Douhal, Chem. Eur. J. 5 (1999) 897. [8] D.P. Zhong, A. Douhal, A.H. Zewail, Proc. Natl. Acad. Sci. USA 97 (2000) 14052. [9] D.P. Zhong, S.K. Pal, A.H. Zewail, Chem. Phys. Chem. 2 (2001) 11867. [10] P. Choppinet, L. Jullien, B. Valeur, Chem. Eur. J. 5 (1999) 3666. [11] P. Bortolus, G. Marconi, S. Monti, B. Mayer, G. K€ ohler, G. Grabner, Chem. Eur. J. 6 (2000) 1578. [12] 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. [13] J.A. Organero, L. Tormo, A. Douhal, Chem. Phys. Lett. 363 (2002) 409. [14] J.A. Organero, M. Moreno, L. Santos, J.M. Lluch, A. Douhal, J. Phys. Chem. A 104 (2000) 8424. [15] A. Douhal, F. Lahmani, A. Zehnacker-Rentien, Chem. Phys. 178 (1993) 493. [16] S. Tobita, M. Yamamoto, N. Kurahayashi, R. Tsukagoshi, Y. Nakamura, H. Shizuka, J. Phys. Chem. A 102 (1998) 5206. [17] C. Lu, R.-M.-R. Hsieh, I.-R. Lee, P.-Y. Cheng, Chem. Phys. Lett. 310 (1999) 103. [18] S. Lochbrunner, K. Stock, V. De Waele, E. Riedle, in: A. Douhal, J. Santamaria (Eds.), Femtochemistry and Femtobiology: Ultrafast Dynamics in Molecular Science, World Scientific, Singapore, 2002, p. 202. [19] G. Pistolis, A. Malliaris, Chem. Phys. Lett. 310 (1999) 501.
J.A. Organero, A. Douhal / Chemical Physics Letters 373 (2003) 426–431 [20] H.-R. Park, B. Mayer, P. Wolschann, G. K€ ohler, J. Phys. Chem. 98 (1994) 6158. [21] M. Takei, H. Yui, Y. Hirose, T. Sawada, J. Phys. Chem. A 105 (2001) 11395. [22] N. Balabai, B. Linton, A. Napper, S. Priyadarshy, A.P. Sukharevsky, D.H. Waldeck, J. Phys. Chem. B 102 (1998) 9617.
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[23] C. Hu, R. Zwanzig, J. Chem. Phys. 60 (1974) 4354. [24] J.S. Baskin, A.H. Zewail, J. Phys. Chem. A 105 (2001) 3680. [25] A. Bom, M. Bradley, K. Cameron, J.K. Clark, J. van Egmond, H. Feilden, E.J. MacLean, A.W. Muir, R. Palin, D.C. Rees, M.-Q. Zhang, Angew. Chem. Int. Ed. Engl. 41 (2002) 265.
Confinement effects on the photorelaxation of a proton-transfer phototautomer 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 7 March 2003; in final form 14 April 2003
Abstract We report on emission studies of excited phototautomers of 10 - hydroxy-20 -acetonaphthone in aqueous solutions of a-cyclodextrin (a-CD). Caging by a-CD provokes a 40 nm blue shift of the emission, and increases the emission lifetime from 90 ps to 1 ns. The rotational time of the caged phototautomers varies from 50 to 180 ps, while that of global rotational time of 1:2 complex (HAN:(a-CD)2 ) is 950 ps. These results are explained on the basis of confinement effects on the twisting motion of the formed phototautomers within the cavities provided by one or two CD. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The study of confinement effects on the behaviour of molecules provides valuable information on the nature of the environment (space domain) and dynamical properties (time domain) of the system [1–9]. Among the simplest systems for these studies are the inclusion complexes of dyes with cyclodextrins (CDs) [1–13]. The results show a strong relationship between space, frequency and time domains of the ultrafast properties of the nanosystem. Recently, we have reported on the photoisomerization of 10 -hydroxy-20 -acetonaphthone (HAN) encapsulated in the cavities of b- and c-CD [12,13]. *
Corresponding author. Fax: +34-925-268-840. E-mail address: [email protected] (A. Douhal).
In agreement with previous studies [14–18], we observed the occurrence of an excited-state intramolecular proton-transfer (ESIPT) reaction in the first electronically excited-state of enol (E*) form giving birth to a keto (K*) phototautomer structure. Experiments using b- and c-CD as caging media have shown that after the ultrafast protontransfer process, K* may suffer a subsequent twisting motion in few tens of picoseconds (ps) along the naphthyl–COHCH3 bond yielding to a nanosecond-living keto rotamer (KR*). This one emits at the red side of the K* spectrum, according to previous calculations [14]. The photoisomerization of K* is largely affected by the size of the nanocavity [12,13]. In continuation to the previous studies using b- and c-CD we have examined the behaviour of HAN caged in the smaller cavity offered by a-CD (Fig. 1).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00626-2
J.A. Organero, A. Douhal / Chemical Physics Letters 373 (2003) 426–431
427
Fig. 1. Molecular structures for equilibrium inclusion complexes between 10 -hydroxy-20 -acetonaphthone (HAN) and a-cyclodextrin (a-CD).
2. Experimental HAN and a-CD (from Aldrich) were used as received. Steady-state absorption and emission spectra were recorded on a Varian (Cary E1) and Perkin–Elmer (LS 50B) spectrophotometers, respectively. Time-resolved measurements were done exciting at 393 nm (40 ps, 20 MHz, 0.8 mW) using a time-correlated single-photon counting picosecond spectrophotometer (FluoTime 200, PicoQuant) described earlier [13]. Decay and anisotropy data were analysed using the FluoFit software package (PicoQuant). 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. All the measurements were done at 296 1 K and using fresh solutions.
3. Results and discussion Fig. 2 shows the emission spectra of HAN in neutral water and in aqueous solutions of a-CD. In pure neutral water, the dye exhibits emission of phototautomers (K* and KR*) at 490 nm [13] and a global fluorescence quantum yield u0 ¼ ð4:0 0:5Þ 103 . Upon a-CD addition, the fluorescence intensity gradually increases due to
Fig. 2. Change of emission spectrum of HAN (105 M) upon addition of a-CD. The dashed spectrum corresponds to the emission of the complex at [a-CD]0 ¼ 110 mM, and which was deduced as Ic ¼ I aI0 , where Ic , I and I0 are the emission intensity of the complex, with and without a-CD, respectively, and a (0.03) is the fraction of non-caged HAN. The inset is the fluorescence quantum yield variation with [a-CD]0 and best fit (solid line) following Eq. (1). Excitation wavelength 365 nm and T ¼ 293 K.
the formation of a more fluorescent structure, and a new maximum at 460 nm appears as a result of the formed complexes. Based on previous studies [12,13] these complexes are molecular entities of HAN trapped into a-CD cavity. The blue shift is explained in terms of hindrance in rotational
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motion of the formed phototautomer (K*) in these complexes. The polarity of the medium does not play a significant role in the position of the emission spectrum [13]. The binding constants and stoichiometry of the inclusion complexes (Fig. 1) were estimated using the change of the overall fluorescence quantum yield (u) of each sample upon addition of a-CD. The variation was then analysed assuming the formation of 1:1, 1:2 or 2:2 HAN:a-CD inclusion complexes [19]. We found that the equation (Eq. (1)) that correctly fits the experimental data corresponds to an 1:2 stoichiometry, u¼
u0 þ K1 u1 ½a-CD þ K1 K2 u2 ½a-CD 1 þ K1 ½a-CD þ K1 K2 ½a-CD
2
2
;
ð1Þ
where K1 and K2 are the equilibrium or binding constants of 1:1 and 1:2 complexes, respectively (Fig. 1). u0 , u1 and u2 are the fluorescence quantum yields of the HAN in pure neutral water, of 1:1 and 1:2 complexes, respectively. The best fit (inset of Fig. 2) gives K1 ¼ 2 1 M1 , K2 ¼ 273 100 M1 , u1 ¼ ð3:7 0:3Þ 102 and u2 ¼ ð3:9 0:1Þ 102 . The apparent equilibrium constant ðKapp ¼ K1 K2 Þ between free and 1:2 complex is 546 M2 . Taking into account the obtained values of K1 and Kapp , the corresponding variation of standard free energy at 293 K to 1:1 (DG0293 (1:1)) and 1:2 (DG0293 (1:2)) complexation processes, relatively to free HAN are DG0293 ð1Þ ¼ 1:7 1:2 kJ mol1 and DG0293 ð2Þ ¼ 15:5 4:0 kJ mol1 , respectively. The DG0293 ð2Þ ¼ DG0293 (1:2) ) DG0293 (1:1) ¼ 13:8 4:0 kJ mol1 is the free energy gain upon addition of the second nanocavity to 1:1 complex (Fig. 1). Using the values of equilibrium constants and taking into account that [CD]0 [HAN]0 (110 mM 102 mM), we constructed the emission spectrum of the 1:2 complex of the most a-CD concentrated solution (dashed spectrum in Fig. 2). The spectrum presents a structure with two maxima at 460 and 490 nm. Its blue shift relatively to the spectrum of HAN without a-CD is due to the confinement effect on the relaxation routes involving the rotational motion of COHCH3 group of the formed K* phototautomers. The spectrum presents a shape similar to that of HAN in methylcyclohexane at 77 K, and
which was correlated with those observed in jet cold molecular beam (maximum at 426 and 452 nm) [15]. At an [a-CD]0 ¼ 30 mM, 64% of initial HAN is free, 4% is complexed with one a-CD and 32% is complexed with two a-CD. When [aCD]0 ¼ 110 mM these values change to 13% (free), 3% (1:1 complex) and 84% (1:2 complex), respectively. For 2-naphthol:a-CD system, K€ ohler et al. [20] found K1 ¼ 250 and K2 ¼ 35 M1 at 298 K for 1:1 and 1:2 entities, respectively. Our results show that the 1:2 HAN:(a-CD)2 complex is significantly more stable than 1:1 complex. Compared to the parent compound (2-naphthol) the internal H-bond between OH and COCH3 groups in HAN plays a stabilizing role on the stability of 1:2 complex relatively to 1:1 entity. To obtain information on the fluorescence dynamics of the nanostructures, we measured the fluorescence decays of HAN in presence of different a-CD concentrations (Fig. 3). In neutral water, we observed a bi-exponential decay with two components having time constants of 90 10 ps (99%) and 5 1 ns (1%), assigned to K* and KR*
Fig. 3. Fluorescence decays at 480 nm of HAN (105 M) in neutral water and in presence of 15, 22, 27, 35, 45, 80 and 110 mM of a-CD. Inset: normalized time-resolved emission spectra of HAN in presence of 22 mM of a-CD and gated at 75, 300, 500 ps and 1 ns. Excitation at 393 nm and emission at 480 nm (magic angle), the instrumental response function (IRF) is 65 ps.
J.A. Organero, A. Douhal / Chemical Physics Letters 373 (2003) 426–431
tautomers, respectively [13]. Although the contribution of nanosecond component is very small, its presence is clear in the decay. In presence of a-CD, a tri-exponential function was needed to obtain an accurate fit (Fig. 3). For example at [a-CD]0 ¼ 110 mM, the obtained lifetimes are: 90 10 ps (29%), 1.0 0.1 ns (70%) and 5 1 ns (0.5%). For all the a-CD solutions, we found that the contribution of the fastest component (90 ps) in the decay decreases gradually from 99% using pure neutral water to 29% when [a-CD]0 ¼ 110 mM (saturation). The contribution of the 1 ns component changes from 2% to 70% when [a-CD]0 was varied from 3 to 110 mM. We assigned this nanosecond component to the lifetime of 1:2 complexes. The contribution of the longest component (5 ns) was maintained with a small population and is assigned to the lifetime KR*. The results suggest that the 1:1 complex has a lifetime similar to that of the free HAN in water (90 ps). The COHCH3 group of K* photoproduced in 1:1 complex does not probably suffer the confinement effect. Thus, the enol structure of HAN in 1:1 entity is not totally included into the CD cavity (Fig. 1). Comparable behaviour has been observed in the nanostructures (1:1 and 1:2 complexes) of 2- naphthol and methyl orange complexed to CD [20,21]. To get more insight into the spectral relaxation of the confined geometries, we recorded time-resolved emission spectra (TRES) of a HAN:a-CD solution (Fig. 3). Position and shape of the gated emission spectrum depend on the time of observation. At 75 ps, the emission is similar to that of K* and KR* upon excitation of E in water (max 490 nm) and an ultrafast ESIPT reaction in this structure [12,13]. While at times longer than 300 ps, the maximum appears at 460 nm and the emission originates from the phototautomers photoproduced inside the CD cavity (at this time mostly from 1:2 complexes). The blue shift in the TRES (1350 cm1 – 16 kJ mol1 ) is due to the hindrance in isomerization channel of K* that would lead to KR* inside the complexes. Therefore, the COHCH3 rotation in K* to yield its rotamer KR* involves an energy gain of 16 kJ mol1 , close to the obtained energy gap (24 kJ mol1 ) between these structures in gas phase using theoretical calculations [14].
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To elucidate the dynamics of the orientational motion of the guest and of the host–guest system, we obtained the rotational relaxation times (/) by time-resolved fluorescence anisotropy rðtÞ measurements (Fig. 4). To begin with pure water and tetrahydrofurane (THF), rðtÞ fits to a mono-exponential function with time constant / 70 10 ps and 35 0 ps, respectively. Comparable rotational times have been obtained for similar dyes in size [8,22]. For HAN in a saturated solution of aCD (110 mM), a double-exponential function with time constants /1 ¼ 50–180 15 and /2 ¼ 950 50 ps were needed. The value of the shortest one (/1 ) and its populations change with the gated region: 180 (24%), 130 (13%), 95 (12%) and 50 (26%) ps when the emission is collected at 430, 480, 520, 560 nm, respectively. The variation suggests the existence of several rotors (or conformers) that contribute to this component. In presence of 110 mM of a-CD, 13% of HAN is free, 3% is forming 1:1 complex and 84% is involved in 1:2 complexes. Stokes–Einstein–Debye hydrodynamic theory [23,24], and under a stick boundary condition, predicts an overall rotational time of 210 ps for the 1:1 complex, very close to the experimental value
Fig. 4. Anisotropy decays at 480 nm of HAN in neutral water ( ), in presence of 110 mM a-CD ( ) and in THF (NNNN). Single- and double-exponential fittings were used for the best fit (solid lines) to obtain the rotational relaxation time (/). The initial time (t ¼ 0) for the anisotropy decay is taken at the intensity maximum of IRF.
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of 180 ps at 430 nm. The rotational time for the guest into 1:1 and 1:2 entities will depend on the degree of freedom of COHCH3 group. A time (few tens of picoseconds as found in THF, 35 ps) component is expected for a free rotation of the guest. The relatively small value of /1 and its decrease to 50 ps at longer wavelengths to a value comparable to free HAN (70 ps) and in THF (35 ps), suggest that at the red side of spectrum the short component is largely governed by the phototautomers of HAN in water and by the internal rotation of the guest in the complexes. The longer component ð/2 950 ps) was found independent on the emission wavelength, and it is assigned to the overall rotational time of the 1:2 complex. The theoretical value predicted by the hydrodynamic theory under a stick boundary condition for this rotor having a shape and a volume similar to those of two linked a-CD, 600 ps, is in the same order of magnitude of the experimental one, /2 950 ps. The initial values of anisotropy (rð0Þ 0:25–0:35) for HAN in water, THF and in presence of a-CD (110 mM) are different from the ideal value (0.4) for a situation where the transition dipole moments of the absorbing and emitting entities are parallel. This is due to the proton-transfer reaction in E* (absorbing entity) generating K* and KR* (the emitting tautomers) where large electronic changes have occurred. Following the values of rð0Þ the changes are larger in water and THF than in nanocavities, in agreement with a restricted photorelaxation due to the confinement.
4. Conclusion The results indicate that the size (space domain) of the cavity of the host (1 or 2 a-CD) governs the photodynamics (time domain, from picosecond to nanosecond timescale) and spectroscopy (spectral domain, shift by 40 nm) of the nanostructure. The results might be relevant for designing size-tuned fluorescent nanostructures like those involved in nanophotonics, and could be used for a better understanding of the behaviour of medicines and drugs used in phototherapy or in neuromuscular blockers [25].
Acknowledgements This work was supported by the Ministry of Science and Technology (Spain) through the projects PB98-0310, MAT-2002-01829 and ÔConsejerıa de Ciencia y Tecnologıa de la Junta de Comunidades de Castilla-La ManchaÕ through the project PAI-02-004.
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