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Effect of β-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate
Spectrochimica Acta Part A 57 (2001) 1819– 1828 www.elsevier.com/locate/saa
Effect of b-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate Ayman Ayoub Abdel-Shafi * Department of Chemistry, Faculty of Science, Ain Shams Uni6ersity, Abbassia 11566 Cairo, Egypt Received 19 October 2000; accepted 18 January 2001
Abstract The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous b-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of b-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of b-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the b-CD cavity with an association constant of 1915 M − 1 using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of b-CD. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluorescence; Inclusion complexes; b-Cyclodextrin; Naphthol derivatives; Benesi-Hildebrand; 1H NMR
1. Introduction Reaction dynamics and mechanisms of proton transfer have received the attention and efforts of chemists for a long time [1]. It is well known that molecules showing excited state intramolecular * Present address: Department of Chemistry, Loughbourough University, Loughbourough, Leicestershire, LE11 3TU, UK. Tel.: + 44-1509-222579; fax: +44-1509-223925. E-mail address: [email protected] (A.A. AbdelShafi).
proton transfer (ESIPT) are useful in lasing systems as laser dyes [2,3], in high energy radiation detectors [4,5], as materials for protecting against UV radiation damage [6], as photochromic materials [7], as molecular energy storage systems [8], and as fluorescent probes [9]. Cyclodextrins (CDs) are water soluble cyclic oligosaccharides composed of D-(+ ) glucopyranose units. a, b and g-CDs, which are made up of six, seven, and eight D-(+ ) glucopyranose units, are the most common members of the CD family. These CDs are shaped like a truncated cone with
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 4 0 3 - 6
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a hollow, hydrophobic cavity and have an internal diameter of 4.7– 8.3 A, [10,11] in which a large variety of organic and inorganic guest compounds can be incorporated. The chemical reactivity and the spectroscopic properties of the guest molecules are modified as a result of the inclusion. Recently, Bortolus et al. [12] have shown that the dual fluorescence of 2- and 3-hydroxybiphenyl in water, due to intermolecular proton transfer of the excited molecules to solvent, is affected by the inclusion of these derivatives in a, b and g-CD. The extent of proton transfer depends on the nature of the CD. In all cases they shown that 1:1 inclusion complexes are formed with the exception of 3-hydroxybiphenyl-a-CD where 2:2 complexes are formed. A decrease in the rate of deprotonation was observed upon the inclusion of 1- and 2-naphthol in CD [13– 17]. The decrease of the deprotonation rate of 2-naphthol in the presence of a-CD was accounted for by the formation of 2:1 (host:guest) complex [17]. Stam et al. [18] have studied the complexation of 2-naphthol by b-CD and the effect of added short linear alcohols on the association constants. These authors showed that the association constant decreases linearly with an increasing number of carbon atoms in the chain of the alcohol, which they attributed to the competition between 2-naphthol and alcohol for the b-CD cavity. Hamai [19] has examined the formation of 1:1 inclusion complexes of 2-anilino-6-naphthalenesulfonic acid, 1chloronaphthalene, and azulene with poly-b-CD. However, in case of 1-chloronaphthalene 2:2 inclusion complexes are formed due to the self association of the 1:1 complexes as evident from the observation of the excimer emission of 1choronaphthalene. Sugiura et al. [20] have investigated the 1:1 inclusion complexation of cresols, phenol, nitrophenols and 2-naphthol with b-CD. Harada et al. [21] have revealed that 2-ptoluidinylnaphthalene-6-sulfonate inclusion complexes with poly(acryloyl-b-CD) changes from 2:1 to 1:1 as the concentration poly(acryloyl-b-CD) decreases. It is well known that the excited state proton transfer reactions of aromatic compounds (e.g. naphthols) occurs effectively to water molecules. The proton produced by deprotonation in the
excited state of such compounds are trapped by water clusters [22] and the rate of deprotonation is very sensitive to any changes in the structure of water clusters. The increase of naphthol derivatives acidity upon photoexcitation was first demonstrated by Fo¨ rster in 1949 [23]. Proton dissociation from 1and 2-naphthol was found to depend on a specific water structure [24–27]. Robinson et al. [28] suggested that the formation of water cluster in these weak acid systems is determined by reorientational motions of the neighbouring solvent. In addition, Douhal et al. [29] have shown that the ultrafast excited intramolecular proton transfer processes is markedly slowed inside the CD cavities. The intermolecular proton transfer processes are also found to be markedly affected by CD, lipids, and microemulsions [11,13,14,17,30,31]. Although a large number of systems are capable of forming inclusion complexes with CDs, the changes in the photophysical properties on encapsulation are, in many cases too small to provide any meaningful and reliable information on the micro-heterogeneity of the CD or the location of the probe. This necessitates further studies of the complexation processes involving probes whose photophysical properties are sensitive to any small changes in its environment. In the present study we are examining the effect of solvent and inclusion by b-CD on proton dissociation from a more complicated systems where intramolecular hydrogen bonding takes place.
2. Experimental The potassium salt of 1-napthol-2-sulfonate (1NOH-2-S) was purchased from Kodak and purified as in literature [32]. Purity was checked by UV and fluorescence spectroscopy. b-CD, acetonitrile, ethanol, 1,4-dioxane and cyclohexane were obtained from Aldrich and used as received. Doubly distilled water was used. Concentrations of 1-NOH-2-S were around 2–3 ×10 − 5 M, while those of CDs were around 0.1 mM to maximum solubility. Absorption spectra were recorded on a Perkin Elmer 330 spectrophotometer. Steady state
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fluorescence measurements were obtained using a Perkin Elmer 650– 10S spectrofluorophotometer. All measurements were carried out at 25°C. 1H NMR spectra were recorded on a 400 MHz Bruker Avance DPX spectrometer. All solutions were nitrogen purged before measurements since quenching of the excited singlet state by oxygen were found to be efficient [33]. The fluorescence quantum yields in different solvents were measured relative to naphthalene in cyclohexane as a standard with a fluorescence quantum yield (Ff) of 0.23 [34]. The refractive index of the solution was reported to change by only about 1% upon going from pure water to 7 mM solution of b-CD, and corrections to the fluorescence yields for refractive index were not necessary [35].
3. Results and discussion Absorption and fluorescence spectral data of 1-NOH-2S in different solvents are listed in Table 1. Changing the solvent polarity has a negligible effect on the wavelength of maximum absorption or fluorescence. Fluorescence quantum yields are approximately constant of about 0.509 0.03 except in case of water where the yield drops to 0.1. Fig. 1 shows the fluorescence emission in acetonitrile and water. The fluorescence spectra of 1NOH-2-S in all organic solvents are similar to that in acetonitrile. The excited state of 1-NOH*2-S gives two emission bands in water, one due to the neutral molecule (1-NOH*-2-S) emission centred at : 362 nm and a broad structureless band at 460 nm due to emission from the anion form 1-NO−*-2-S (see Fig. 1). Upon changing to organic solvents, the anion emission disappears completely and only the neutral molecule emission is observed which is hardly showing any solvent emission wavelength dependence. Krishnan et al. [36] have shown for 1-NOH-2-S that upon deuteration, the emission quantum yield of the neutral species increases, but no spectral shift is observed. They also reported that with decreasing pD, the emission intensity of the neutral species remain constant while the intensity of the anion form decreases.
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Since 1-NOH-2S has a pKa of 9.58, essentially all ground state (1-NOH-2-S) molecules will be fully protonated for a solution pHB 7. Irradiation of such a solution produces a substantially more acidic (pK *a of 1.58 [32]) electronically excited state (1-NOH*-2-S), which transfers a proton to water producing an electronically excited anion (1-NO−*-2-S). As expected from simple thermodynamic arguments such as Fo¨ rster cycle [37,38], the emission from the anionic form (1NO−*-2-S) is at lower energies than that of the neutral species (1-NOH*-2-S). This spectral difference allows the two species to be distinguished Table 1
Solvent
uabs (nm)
u max (nm) f
Ff
m (l mol−1cm−1)
Water Acetonitrile Ethanol Dioxane Cyclohexane
328 328 329 329 329
455 362 363 363 363
0.10 0.50 0.51 0.48 0.47
3399 4720 4280 4595 4212
Fig. 1. The fluorescence emission spectra of 1-NOH-2-S in acetonitrile ( —) and water (· · ·).
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Fig. 2. The fluorescence emission spectra of 1-NOH-2-S in dioxane-water mixed solvents.
and independently monitored. It has also been found that the decay time of the (1-NOH*-2-S) emission and the rise time of (1-NO−*-2-S) emission are equal indicating that the excited anionic species in formed from the excited neutral species [36]. Table 1 shows that there is no apparent correlation between the photophysical properties of this molecule and these solvent parameters (with water as an exceptional case). Fig. 2 shows the emission from the neutral species in dioxane where addition of water to dioxane shows that the intensity of the anion form increases and subsequently the rate of deprotonation. Massad and Huppert [39] have shown that the first step after excitation by a short light pulse is an intramolecular proton transfer from the hydroxy group to the adjacent sulfonate group. Subsequently, at a slower rate, the proton is transferred to the solvent. The intramolecular proton transfer is mediated by water molecules. In comparison to 1-naphthol, the intramolecular hydrogen bonding in the sulfonate derivatives sterically reduces the extramolecular proton disso-
ciation and recombination rates. In all cases proton dissociation is controlled by reorientation motions of the adjacent water and requires a common (H2O)4 9 1 cluster as the proton acceptor [32]. Therefore, addition of dioxane to water solution of 1-NOH-2-S disturbs the water cluster necessary for proton transfer and consequently the rate of dissociation. The presence of intramolecular hydrogen bonding in 1-NOH-2-S causes this molecule to behave somewhat differently than 1-naphthol and 2-naphthol [32,36]. The effect of CDs on the fluorescence spectra of 1-NOH-2-S is more pronounced than the corresponding effect on the absorption spectra. The presence of b-CD leads to a twice enhancement in the fluorescence quantum yield (Fig. 3). An increase of the band intensity at 362 nm is observed with concomitant decrease of the band intensity at 460 nm. This can be attributed to the decrease of deprotonation rate from 1-NOH*-2-S which may be attributed to the formation hydrogen bonding with b-CD and/or the change in water cluster structure of (H2O)4 9 1 necessary for proton transfer process. The changes in the fluorescence of 1-NOH-2-S on addition of increasing concentrations of b-CD are shown in Fig. 3. An isoemessive point can be seen at 410 nm. The data in Fig. 3 can be treated using the Benesi-Hildebrand [40] equation for 1:1 binding model (Eq. (1)) or 2:1 model (Eq. (2)) 1/(F0f − Ff)= 1/(Ff − Fcomplex) + 1/(Ff − Fcomplex)K[b-CD]
(1)
1/(F − Ff)= 1/(Ff − Fcomplex) 0 f
+ 1/(Ff − Fcomplex)K[b-CD]2
(2)
where, F is the fluorescence quantum yield in the absence of b-CD, Ff is the observed fluorescence quantum yield in the presence of b-CD, Fcomplex is the fluorescence quantum yield of the 1-NOH-2-bCD complex and K is association constant. According to Eq. (1), a plot of 1/(F0f − Ff) versus 1/[b-CD], a good straight line is observed (Fig. 4) from which K was calculated to be 1915 M − 1. No linear plot was observed by plotting 1/(F0f −Ff) versus 1/[b-CD]2 according to Eq. (2) and hence the possibility of formation of a 2:1 complex is ruled out. The linearity of the plot indicate that 0 f
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Fig. 3. Effect of b-CD on the emission spectrum of 2 × 10 − 5 M of 1-NOH-2-S. [b-CD] varies from 0.1 to 8 mM. Inset shows a plot of fluorescence quantum yield vs. [b-CD].
1:1 inclusion complex is formed between 1-NOH2-S and b-CD. The value of the association constant obtained from Eq. (1), is comparable to that obtained for the inclusion of 2,3-N-butylnaphthalimide in b-CD (1915 vs. 2000 M − 1, respectively) which suggest a tight fit of 1-NOH-2-S within the b-CD cavity [41]. Proton nuclear magnetic resonance (1H NMR) spectroscopy has proved to be a useful tool in the study of CD inclusion complexes [42– 51]. Nuclear magnetic resonance (1H NMR) spectroscopy provides an effective means of assessing the dynamic interaction site of b-CD with that of the guest molecules. The basis of information gained from NMR spectroscopy is located in the shifts, loss of resolution, and broadening of signals observed for the host and guest protons [42– 52]. The resonance assignment of the protons of b-CD (Fig. 5a) are well established [45,48–52] and consists of six types of protons: the H-1 doublet at l 5.06 ppm, the H-3 triplet at l 3.94 ppm, a strong unresolved broad peak consisting of l H-5 and H-6 at l 3.87 – 3.85 ppm, the H-2 appearing as two doublets centered at l 3.64 ppm and l 3.67
Fig. 4. The Benesi-Hildebrand plot of 1/(F0f −Ff) vs. 1/[b-CD]. 1-NOH-2-S concentration 2 × 10 − 5 M.
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Fig. 5. 1H NMR spectra in D2O of (a) b-CD, (b) b-CD containing 1-NOH-2-S.
ppm, and the H-4 triplet at d 3.58 ppm. The chemical shift of b-CD protons reported by different authors [42– 48] are very close to those reported in this work 90.05 ppm. The H-3 and H-5 protons are located in the interior of the CD’s cavity, and it is, therefore, likely that the interaction of the host with the b-CD inside the cavity will affect the chemical shifts of the H-3 and H-5 protons. This, in turn, can provide a rationale for inclusion processes. The addition of 1-NOH-2-S to b-CD causes an upfield shift of b-CD spectrum by about 0.08 ppm (Fig. 5b). More specifically, in the presence of 1-NOH-2-S, the H-5 and H-6 resonances appear somewhat more resolved with respect to that in pure b-CD. A new doublet appears shifted upfield by about 0.12 ppm that is more likely to be assigned to H-5 [52]. A similar upfield shift of 0.12 ppm was observed for the H-3 proton. The upfield chemical
shift observed for H-3 and H-5 protons suggest the inclusion inside the cavity. The upfield chemical shift observed for H-3 and H-5 protons of b-CD was observed earlier by Wang and Eaton [52]; an observation from which they concluded the inclusion of p-nitroaniline in the cavity and exclude the possibility of complexation to the exterior of the b-CD ring. On the other hand, 1-NOH-2-S has two types of protons: four doublets (H%-1, H%-2, H%-3, and H%-6), and two triplets (H%-4 and H%-5). Assignments of 1-NOH-2-S protons signals were made based on both 1H– 1H correlation spectroscopy (COSY) (Fig. 6a) and 1H NMR (Fig. 7a) spectra. Figs. 6a and 7a show three doublets at l 8.28, 7.87 and 7.49 ppm and a multiplet at l 7.56–7.66 ppm. The COSY spectrum of 1-NOH-2-S (Fig. 6a) shows three correlation peaks for the interactions between the three doublet protons and the
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multiplet. The assignment of 1-NOH-2-S peaks are as follows (see numbering in Fig. 7): the H%-1, H%-3, and H%-6 appear as doublets located at l 7.87, 7.49, and 8.28 ppm respectively, while the multiplet at l 7.56 – 7.66 ppm includes the peaks for the protons H%-2, H%-4, and H%-5. Upon the
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addition of 1-NOH-2-S to the solution of b-CD, the multiplet peak located at l 7.56–7.66 ppm splits to give a clear doublet at l 7.65 ppm with a difficulty to determine any shift from the multiplet and a broad singlet. The doublet is assigned to H%-2 as H%-1 correlates to the same value of l at
Fig. 6. 1H – 1H COSY spectra in D2O of (a) 1-NOH-2-S, (b) b-CD containing 1-NOH-2-S.
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Fig. 7. 1H NMR spectra in D2O of (a) 1-NOH-2-S, (b) b-CD containing 1-NOH-2-S.
7.65 of the pure 1-NOH-2-S (Fig. 6a). H%-3 and H%-6 protons show correlation with the multiplet at about l 7.66 (Fig. 6a). Addition of 1-NOH-2-S to the solution of bCD results in a very small upfield shift for the protons H%-2, H%-3 and H%-6 of 1-NOH-2-S. The doublet of H’-1 of pure 1-NOH-2-S located at l 7.87 shows an upfield shift of about 0.11 ppm concomitant with broadening and loss of resolution. In addition, the multiplet of pure 1-NOH-2S located at l 7.56 – 7.66 ppm splits to show a doublet at l 7.65 ppm and a broad singlet. The broad singlet is assigned to H%-4 and H%-5 protons, where they show, upon inclusion, upfield shift by about 0.08 ppm and overlapping with H%-2 and H%-3 proton signals (Fig. 7b). COSY measurements for the inclusion complex of 1NOH-2-S turned to be complicated as it becomes difficult to determine the correlation between the protons especially those involved the binding due
to their broadening and their overlap with other peaks (Fig. 6b). Therefore, it is clear that protons H%-1, H%-4 and H%-5 protons, with highest chemical upfield shift upon inclusion, are those involved in the binding upon inclusion. It is widely believed that benzene derivatives fit well in a-CD ring while substituted naphthalenes best fit b-CD ring because of size matching [53]. The high upfield chemical shift observed for b-CD interior protons (H-3 and H-5), together with those observed for the guest molecule (H%-1, H%-4 and H%-5 of 1-NOH-2-S) upon inclusion confirm the equatorial inclusion of 1-NOH-2-S into b-CD cavity with the sulfonate group stays outside the cavity (see Fig. 8). Self-association of the 1:1 inclusion complexes of naphthalene and a number of its derivatives in b-CD has been reported previously [54–59]. The upfield chemical shift observed for the exterior protons of b-CD by about 0.08 ppm namely for
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Fig. 8.
the protons H-1, H-2 and H-6 and probably H-4 upon the inclusion of 1-NOH-2-S point to the possibility of self-association of the 1:1 inclusion complexes.
4. Conclusions The fluorescence quantum yield of 1-NOH-2-S does not change much with solvents. In aqueous solution, the fluorescence quantum yield drops to 0.1. Proton transfer to solvent was intermediated by intramolecular proton transfer to the adjacent sulfonate group. In aqueous solution two emission bands are observed one due to the neutral species 1-NOH*-2-S and the broad structurless band due to the anionic form 1-NO*−-2-S. In organic solvents, only emission due to the neutral species is observed. Inclusion of 1-NOH-2-S in b-CD cavity led to an enhancement in the fluorescence of the neutral species and disappearance of the emission of the anionic form. 1H NMR studies confirm the inclusion and gave an information of the orientation of the guest in b-CD cavity. Self association of the 1:1 inclusion complexes is also considered.
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Effect of b-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate Ayman Ayoub Abdel-Shafi * Department of Chemistry, Faculty of Science, Ain Shams Uni6ersity, Abbassia 11566 Cairo, Egypt Received 19 October 2000; accepted 18 January 2001
Abstract The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous b-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of b-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of b-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the b-CD cavity with an association constant of 1915 M − 1 using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of b-CD. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluorescence; Inclusion complexes; b-Cyclodextrin; Naphthol derivatives; Benesi-Hildebrand; 1H NMR
1. Introduction Reaction dynamics and mechanisms of proton transfer have received the attention and efforts of chemists for a long time [1]. It is well known that molecules showing excited state intramolecular * Present address: Department of Chemistry, Loughbourough University, Loughbourough, Leicestershire, LE11 3TU, UK. Tel.: + 44-1509-222579; fax: +44-1509-223925. E-mail address: [email protected] (A.A. AbdelShafi).
proton transfer (ESIPT) are useful in lasing systems as laser dyes [2,3], in high energy radiation detectors [4,5], as materials for protecting against UV radiation damage [6], as photochromic materials [7], as molecular energy storage systems [8], and as fluorescent probes [9]. Cyclodextrins (CDs) are water soluble cyclic oligosaccharides composed of D-(+ ) glucopyranose units. a, b and g-CDs, which are made up of six, seven, and eight D-(+ ) glucopyranose units, are the most common members of the CD family. These CDs are shaped like a truncated cone with
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 4 0 3 - 6
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a hollow, hydrophobic cavity and have an internal diameter of 4.7– 8.3 A, [10,11] in which a large variety of organic and inorganic guest compounds can be incorporated. The chemical reactivity and the spectroscopic properties of the guest molecules are modified as a result of the inclusion. Recently, Bortolus et al. [12] have shown that the dual fluorescence of 2- and 3-hydroxybiphenyl in water, due to intermolecular proton transfer of the excited molecules to solvent, is affected by the inclusion of these derivatives in a, b and g-CD. The extent of proton transfer depends on the nature of the CD. In all cases they shown that 1:1 inclusion complexes are formed with the exception of 3-hydroxybiphenyl-a-CD where 2:2 complexes are formed. A decrease in the rate of deprotonation was observed upon the inclusion of 1- and 2-naphthol in CD [13– 17]. The decrease of the deprotonation rate of 2-naphthol in the presence of a-CD was accounted for by the formation of 2:1 (host:guest) complex [17]. Stam et al. [18] have studied the complexation of 2-naphthol by b-CD and the effect of added short linear alcohols on the association constants. These authors showed that the association constant decreases linearly with an increasing number of carbon atoms in the chain of the alcohol, which they attributed to the competition between 2-naphthol and alcohol for the b-CD cavity. Hamai [19] has examined the formation of 1:1 inclusion complexes of 2-anilino-6-naphthalenesulfonic acid, 1chloronaphthalene, and azulene with poly-b-CD. However, in case of 1-chloronaphthalene 2:2 inclusion complexes are formed due to the self association of the 1:1 complexes as evident from the observation of the excimer emission of 1choronaphthalene. Sugiura et al. [20] have investigated the 1:1 inclusion complexation of cresols, phenol, nitrophenols and 2-naphthol with b-CD. Harada et al. [21] have revealed that 2-ptoluidinylnaphthalene-6-sulfonate inclusion complexes with poly(acryloyl-b-CD) changes from 2:1 to 1:1 as the concentration poly(acryloyl-b-CD) decreases. It is well known that the excited state proton transfer reactions of aromatic compounds (e.g. naphthols) occurs effectively to water molecules. The proton produced by deprotonation in the
excited state of such compounds are trapped by water clusters [22] and the rate of deprotonation is very sensitive to any changes in the structure of water clusters. The increase of naphthol derivatives acidity upon photoexcitation was first demonstrated by Fo¨ rster in 1949 [23]. Proton dissociation from 1and 2-naphthol was found to depend on a specific water structure [24–27]. Robinson et al. [28] suggested that the formation of water cluster in these weak acid systems is determined by reorientational motions of the neighbouring solvent. In addition, Douhal et al. [29] have shown that the ultrafast excited intramolecular proton transfer processes is markedly slowed inside the CD cavities. The intermolecular proton transfer processes are also found to be markedly affected by CD, lipids, and microemulsions [11,13,14,17,30,31]. Although a large number of systems are capable of forming inclusion complexes with CDs, the changes in the photophysical properties on encapsulation are, in many cases too small to provide any meaningful and reliable information on the micro-heterogeneity of the CD or the location of the probe. This necessitates further studies of the complexation processes involving probes whose photophysical properties are sensitive to any small changes in its environment. In the present study we are examining the effect of solvent and inclusion by b-CD on proton dissociation from a more complicated systems where intramolecular hydrogen bonding takes place.
2. Experimental The potassium salt of 1-napthol-2-sulfonate (1NOH-2-S) was purchased from Kodak and purified as in literature [32]. Purity was checked by UV and fluorescence spectroscopy. b-CD, acetonitrile, ethanol, 1,4-dioxane and cyclohexane were obtained from Aldrich and used as received. Doubly distilled water was used. Concentrations of 1-NOH-2-S were around 2–3 ×10 − 5 M, while those of CDs were around 0.1 mM to maximum solubility. Absorption spectra were recorded on a Perkin Elmer 330 spectrophotometer. Steady state
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fluorescence measurements were obtained using a Perkin Elmer 650– 10S spectrofluorophotometer. All measurements were carried out at 25°C. 1H NMR spectra were recorded on a 400 MHz Bruker Avance DPX spectrometer. All solutions were nitrogen purged before measurements since quenching of the excited singlet state by oxygen were found to be efficient [33]. The fluorescence quantum yields in different solvents were measured relative to naphthalene in cyclohexane as a standard with a fluorescence quantum yield (Ff) of 0.23 [34]. The refractive index of the solution was reported to change by only about 1% upon going from pure water to 7 mM solution of b-CD, and corrections to the fluorescence yields for refractive index were not necessary [35].
3. Results and discussion Absorption and fluorescence spectral data of 1-NOH-2S in different solvents are listed in Table 1. Changing the solvent polarity has a negligible effect on the wavelength of maximum absorption or fluorescence. Fluorescence quantum yields are approximately constant of about 0.509 0.03 except in case of water where the yield drops to 0.1. Fig. 1 shows the fluorescence emission in acetonitrile and water. The fluorescence spectra of 1NOH-2-S in all organic solvents are similar to that in acetonitrile. The excited state of 1-NOH*2-S gives two emission bands in water, one due to the neutral molecule (1-NOH*-2-S) emission centred at : 362 nm and a broad structureless band at 460 nm due to emission from the anion form 1-NO−*-2-S (see Fig. 1). Upon changing to organic solvents, the anion emission disappears completely and only the neutral molecule emission is observed which is hardly showing any solvent emission wavelength dependence. Krishnan et al. [36] have shown for 1-NOH-2-S that upon deuteration, the emission quantum yield of the neutral species increases, but no spectral shift is observed. They also reported that with decreasing pD, the emission intensity of the neutral species remain constant while the intensity of the anion form decreases.
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Since 1-NOH-2S has a pKa of 9.58, essentially all ground state (1-NOH-2-S) molecules will be fully protonated for a solution pHB 7. Irradiation of such a solution produces a substantially more acidic (pK *a of 1.58 [32]) electronically excited state (1-NOH*-2-S), which transfers a proton to water producing an electronically excited anion (1-NO−*-2-S). As expected from simple thermodynamic arguments such as Fo¨ rster cycle [37,38], the emission from the anionic form (1NO−*-2-S) is at lower energies than that of the neutral species (1-NOH*-2-S). This spectral difference allows the two species to be distinguished Table 1
Solvent
uabs (nm)
u max (nm) f
Ff
m (l mol−1cm−1)
Water Acetonitrile Ethanol Dioxane Cyclohexane
328 328 329 329 329
455 362 363 363 363
0.10 0.50 0.51 0.48 0.47
3399 4720 4280 4595 4212
Fig. 1. The fluorescence emission spectra of 1-NOH-2-S in acetonitrile ( —) and water (· · ·).
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Fig. 2. The fluorescence emission spectra of 1-NOH-2-S in dioxane-water mixed solvents.
and independently monitored. It has also been found that the decay time of the (1-NOH*-2-S) emission and the rise time of (1-NO−*-2-S) emission are equal indicating that the excited anionic species in formed from the excited neutral species [36]. Table 1 shows that there is no apparent correlation between the photophysical properties of this molecule and these solvent parameters (with water as an exceptional case). Fig. 2 shows the emission from the neutral species in dioxane where addition of water to dioxane shows that the intensity of the anion form increases and subsequently the rate of deprotonation. Massad and Huppert [39] have shown that the first step after excitation by a short light pulse is an intramolecular proton transfer from the hydroxy group to the adjacent sulfonate group. Subsequently, at a slower rate, the proton is transferred to the solvent. The intramolecular proton transfer is mediated by water molecules. In comparison to 1-naphthol, the intramolecular hydrogen bonding in the sulfonate derivatives sterically reduces the extramolecular proton disso-
ciation and recombination rates. In all cases proton dissociation is controlled by reorientation motions of the adjacent water and requires a common (H2O)4 9 1 cluster as the proton acceptor [32]. Therefore, addition of dioxane to water solution of 1-NOH-2-S disturbs the water cluster necessary for proton transfer and consequently the rate of dissociation. The presence of intramolecular hydrogen bonding in 1-NOH-2-S causes this molecule to behave somewhat differently than 1-naphthol and 2-naphthol [32,36]. The effect of CDs on the fluorescence spectra of 1-NOH-2-S is more pronounced than the corresponding effect on the absorption spectra. The presence of b-CD leads to a twice enhancement in the fluorescence quantum yield (Fig. 3). An increase of the band intensity at 362 nm is observed with concomitant decrease of the band intensity at 460 nm. This can be attributed to the decrease of deprotonation rate from 1-NOH*-2-S which may be attributed to the formation hydrogen bonding with b-CD and/or the change in water cluster structure of (H2O)4 9 1 necessary for proton transfer process. The changes in the fluorescence of 1-NOH-2-S on addition of increasing concentrations of b-CD are shown in Fig. 3. An isoemessive point can be seen at 410 nm. The data in Fig. 3 can be treated using the Benesi-Hildebrand [40] equation for 1:1 binding model (Eq. (1)) or 2:1 model (Eq. (2)) 1/(F0f − Ff)= 1/(Ff − Fcomplex) + 1/(Ff − Fcomplex)K[b-CD]
(1)
1/(F − Ff)= 1/(Ff − Fcomplex) 0 f
+ 1/(Ff − Fcomplex)K[b-CD]2
(2)
where, F is the fluorescence quantum yield in the absence of b-CD, Ff is the observed fluorescence quantum yield in the presence of b-CD, Fcomplex is the fluorescence quantum yield of the 1-NOH-2-bCD complex and K is association constant. According to Eq. (1), a plot of 1/(F0f − Ff) versus 1/[b-CD], a good straight line is observed (Fig. 4) from which K was calculated to be 1915 M − 1. No linear plot was observed by plotting 1/(F0f −Ff) versus 1/[b-CD]2 according to Eq. (2) and hence the possibility of formation of a 2:1 complex is ruled out. The linearity of the plot indicate that 0 f
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Fig. 3. Effect of b-CD on the emission spectrum of 2 × 10 − 5 M of 1-NOH-2-S. [b-CD] varies from 0.1 to 8 mM. Inset shows a plot of fluorescence quantum yield vs. [b-CD].
1:1 inclusion complex is formed between 1-NOH2-S and b-CD. The value of the association constant obtained from Eq. (1), is comparable to that obtained for the inclusion of 2,3-N-butylnaphthalimide in b-CD (1915 vs. 2000 M − 1, respectively) which suggest a tight fit of 1-NOH-2-S within the b-CD cavity [41]. Proton nuclear magnetic resonance (1H NMR) spectroscopy has proved to be a useful tool in the study of CD inclusion complexes [42– 51]. Nuclear magnetic resonance (1H NMR) spectroscopy provides an effective means of assessing the dynamic interaction site of b-CD with that of the guest molecules. The basis of information gained from NMR spectroscopy is located in the shifts, loss of resolution, and broadening of signals observed for the host and guest protons [42– 52]. The resonance assignment of the protons of b-CD (Fig. 5a) are well established [45,48–52] and consists of six types of protons: the H-1 doublet at l 5.06 ppm, the H-3 triplet at l 3.94 ppm, a strong unresolved broad peak consisting of l H-5 and H-6 at l 3.87 – 3.85 ppm, the H-2 appearing as two doublets centered at l 3.64 ppm and l 3.67
Fig. 4. The Benesi-Hildebrand plot of 1/(F0f −Ff) vs. 1/[b-CD]. 1-NOH-2-S concentration 2 × 10 − 5 M.
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Fig. 5. 1H NMR spectra in D2O of (a) b-CD, (b) b-CD containing 1-NOH-2-S.
ppm, and the H-4 triplet at d 3.58 ppm. The chemical shift of b-CD protons reported by different authors [42– 48] are very close to those reported in this work 90.05 ppm. The H-3 and H-5 protons are located in the interior of the CD’s cavity, and it is, therefore, likely that the interaction of the host with the b-CD inside the cavity will affect the chemical shifts of the H-3 and H-5 protons. This, in turn, can provide a rationale for inclusion processes. The addition of 1-NOH-2-S to b-CD causes an upfield shift of b-CD spectrum by about 0.08 ppm (Fig. 5b). More specifically, in the presence of 1-NOH-2-S, the H-5 and H-6 resonances appear somewhat more resolved with respect to that in pure b-CD. A new doublet appears shifted upfield by about 0.12 ppm that is more likely to be assigned to H-5 [52]. A similar upfield shift of 0.12 ppm was observed for the H-3 proton. The upfield chemical
shift observed for H-3 and H-5 protons suggest the inclusion inside the cavity. The upfield chemical shift observed for H-3 and H-5 protons of b-CD was observed earlier by Wang and Eaton [52]; an observation from which they concluded the inclusion of p-nitroaniline in the cavity and exclude the possibility of complexation to the exterior of the b-CD ring. On the other hand, 1-NOH-2-S has two types of protons: four doublets (H%-1, H%-2, H%-3, and H%-6), and two triplets (H%-4 and H%-5). Assignments of 1-NOH-2-S protons signals were made based on both 1H– 1H correlation spectroscopy (COSY) (Fig. 6a) and 1H NMR (Fig. 7a) spectra. Figs. 6a and 7a show three doublets at l 8.28, 7.87 and 7.49 ppm and a multiplet at l 7.56–7.66 ppm. The COSY spectrum of 1-NOH-2-S (Fig. 6a) shows three correlation peaks for the interactions between the three doublet protons and the
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multiplet. The assignment of 1-NOH-2-S peaks are as follows (see numbering in Fig. 7): the H%-1, H%-3, and H%-6 appear as doublets located at l 7.87, 7.49, and 8.28 ppm respectively, while the multiplet at l 7.56 – 7.66 ppm includes the peaks for the protons H%-2, H%-4, and H%-5. Upon the
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addition of 1-NOH-2-S to the solution of b-CD, the multiplet peak located at l 7.56–7.66 ppm splits to give a clear doublet at l 7.65 ppm with a difficulty to determine any shift from the multiplet and a broad singlet. The doublet is assigned to H%-2 as H%-1 correlates to the same value of l at
Fig. 6. 1H – 1H COSY spectra in D2O of (a) 1-NOH-2-S, (b) b-CD containing 1-NOH-2-S.
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Fig. 7. 1H NMR spectra in D2O of (a) 1-NOH-2-S, (b) b-CD containing 1-NOH-2-S.
7.65 of the pure 1-NOH-2-S (Fig. 6a). H%-3 and H%-6 protons show correlation with the multiplet at about l 7.66 (Fig. 6a). Addition of 1-NOH-2-S to the solution of bCD results in a very small upfield shift for the protons H%-2, H%-3 and H%-6 of 1-NOH-2-S. The doublet of H’-1 of pure 1-NOH-2-S located at l 7.87 shows an upfield shift of about 0.11 ppm concomitant with broadening and loss of resolution. In addition, the multiplet of pure 1-NOH-2S located at l 7.56 – 7.66 ppm splits to show a doublet at l 7.65 ppm and a broad singlet. The broad singlet is assigned to H%-4 and H%-5 protons, where they show, upon inclusion, upfield shift by about 0.08 ppm and overlapping with H%-2 and H%-3 proton signals (Fig. 7b). COSY measurements for the inclusion complex of 1NOH-2-S turned to be complicated as it becomes difficult to determine the correlation between the protons especially those involved the binding due
to their broadening and their overlap with other peaks (Fig. 6b). Therefore, it is clear that protons H%-1, H%-4 and H%-5 protons, with highest chemical upfield shift upon inclusion, are those involved in the binding upon inclusion. It is widely believed that benzene derivatives fit well in a-CD ring while substituted naphthalenes best fit b-CD ring because of size matching [53]. The high upfield chemical shift observed for b-CD interior protons (H-3 and H-5), together with those observed for the guest molecule (H%-1, H%-4 and H%-5 of 1-NOH-2-S) upon inclusion confirm the equatorial inclusion of 1-NOH-2-S into b-CD cavity with the sulfonate group stays outside the cavity (see Fig. 8). Self-association of the 1:1 inclusion complexes of naphthalene and a number of its derivatives in b-CD has been reported previously [54–59]. The upfield chemical shift observed for the exterior protons of b-CD by about 0.08 ppm namely for
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Fig. 8.
the protons H-1, H-2 and H-6 and probably H-4 upon the inclusion of 1-NOH-2-S point to the possibility of self-association of the 1:1 inclusion complexes.
4. Conclusions The fluorescence quantum yield of 1-NOH-2-S does not change much with solvents. In aqueous solution, the fluorescence quantum yield drops to 0.1. Proton transfer to solvent was intermediated by intramolecular proton transfer to the adjacent sulfonate group. In aqueous solution two emission bands are observed one due to the neutral species 1-NOH*-2-S and the broad structurless band due to the anionic form 1-NO*−-2-S. In organic solvents, only emission due to the neutral species is observed. Inclusion of 1-NOH-2-S in b-CD cavity led to an enhancement in the fluorescence of the neutral species and disappearance of the emission of the anionic form. 1H NMR studies confirm the inclusion and gave an information of the orientation of the guest in b-CD cavity. Self association of the 1:1 inclusion complexes is also considered.
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