6158
J . Phys. Chem. 1994,98, 6158-6166
Excited-State Proton Transfer of 2-Naphthol Inclusion Complexes with Cyclodextrinst Hyoung-Ryun Park,* Bernd Mayer, Peter Wolscbann, and Gottfried Kiihler’ Institut fur Theoretische Chemie und Strahlenchemie, University of Vienna, Wahringerstrasse 38, A - 1090 Vienna, Austria Received: November 15, 1993; In Final Form: April 17, 1994”
Ground- and excited-state acid-base properties of 2-naphthol inclusion complexes with a-,8-,and some substituted 8-cyclodextrins were studied. Ionization constants, determined by steady-state and time-resolved optical spectroscopy techniques, are related to the complexation constants of the two protolytic forms: as the standard free energy of inclusion is larger for the molecular form than for the ion, this aromatic acid becomes less acidic upon complexation. Excitation effects relative to the ground state are discussed with respect to spectral shifts. In the case of a-cyclodextrin as host, 1:2 guest:host complexation suppresses excited-state deprotonation. The variation of the microenvironment and its implications on dissociation and recombination rates are discussed. The experimental results are interpreted on the basis of structural models for these complexes, obtained from molecular modeling techniques.
Introduction The study of inclusion complexes of smaller molecules in macromolecular cavities of appropriate size are interesting developments in supramolecular chemistry.’ Cyclodextnns(CDx) are a-(1,4)-linkedglucopuranose rings forming truncated coneshaped compounds, and a wide variety of organic molecules can be complexed in the hydrophobic interior.2 This property leads to an abundant applicability of CDx’s in different fields of analytical and synthetic chemistry. Thenumber of theD(+)-glucopuranoseunits, Le. six for a-CDx, seven for 8-CDx, and eight for y-CDx, defines the width of the central cavity and the flexibility of the compound. The narrow rimof the truncated cone bears primary, the wider rim secondary, OH-groups.2 The stabilization of the complex is provided by Van der Waals forces, hydrogen bonding, decrease of strain energy, and release of high-energy water from the cavity. Inclusion modifies reactivity and the spectroscopic properties of the guest molecule. The spectroscopic and photochemical behaviors of such complexes found considerable interest, and changes in fluorescence and electronic absorption properties, in excimer and exciplex formation, in excited-state protolytic equilibria and photoisomerization were reviewed in recent ~ e a r s . 3Ground~ and excitedstate proton transfer was studied in detail,5q9-14as variation of the microenvironment of the organic guest causes large changes in the deprotonation rates and in the pathway of proton transfer. Such systems were also considered as models for the influence of a lipophilic environment on proton-transfer ~r0perties.l~ Studies on the prototropic reactions established that the deprotonation rate of CDx inclusion complexes either increases, e.g. for heterocyclic compounds like carbazole13 or for aminopyrene,14 or it decreases in comparison to the corresponding free molecules as was observed for 1- and 2-naphtho15JlJ2J4 or naphthylamines.l3 No general explanation of these results was found. Participation of the cooperative flip-flop proton transfer of the CDx’s -OH groups found recently in molecular dynamics studies16 was evoked to explain the doubling of the protonation rate for nonflexible carbazole.I3 On the other hand, the decrease in the deprotonation rate of excited 2-naphthol was ascribed to
* Author to whom correspondence should be addressed.
This work is dedicated to Prof. Nikola Getoff on occasion of his 70th birthday. !On leave from the Department of Chemistry, Chonnam National University, Kwang-ju 500-757, South Korea. Abstract published in Advance ACS Abstracrs, May 15, 1994. f
Q
0022-3654/94/2098-6158%04.50/0
the protection of the probe moiety toward OH- attack by the hydrophobic environment within the CDx ~ a v i t y . I~ JThese results suggested different conformations for 2-naphthol complexed by a-and &CDx, once the polar - O H group and once the aromatic moiety of 2-naphthol embedded in the cavity. For a substituted CDx, a 1:2 guest:host complex of 1-naphtholwas postulated from absorption spectroscopic studies.” Structural information for CDx complexes was obtained recently from NMR studies,’*but molecular modeling techniques and molecular dynamics also proved useful to gain information on geometrical conformations and the solvation of CDx’s and their complexes.19 Some structural information is available from measurements of induced circular dichroism, as its magnitude and direction are directly dependent on the relative orientation of the guest’s transition dipole with respect to the asymmetric groups of the host.2031 Different orientationsresult for 2-naphthol complexed with a-and fl-CDx.1130,21 Additional thermodynamic studies showed that the negativeenthalpyof complexation is widely cancelled by a negative entropy term, revealing the important entropic effects on such hydrophobic interactions.10921 In the present work we studied the protolytic properties of 2-naphthol complexes with various CDx’s by a combination of spectroscopicand time-resolved studies with the aim of drawing a full kinetic and energetic scheme for the ground- and excitedstate processes. 2-Naphthol might be considered as a general model compound as excited-stateprotolytic equilibria were studied in great detail.z2-24 In particular, spectroscopic shifts and thermodynamic parameters were related to obtain information on the importance of various contributions to the energetics of the process causing changes of the protolytic equilibria upon complexation. Detailed structural data were estimated by molecular modeling techniques.
Experimental Section 2-Naphthol (p.a., Merck, Darmstadt) was purified by sublimation under reduced pressure. a-and B-CDx (research grade, Serva, Heidelberg) and hydroxyethyl-8-CDx and hydroxypropyl&CDx (molar substitution 0.6; Roquette, Lestrem) were used as received. Purity was checked by UV and fluorescence spectroscopy. Triply distilled water was used for the preparation of all solutions. Concentrations of 2-naphthol were around 1 X 10-4 M, thoseof CDx’s in the range 1 X 10-4M tomaximum solubility. pH values were adjusted with HCl or NaOH (both p.a., Merck, Darmstadt; pH-meter PHM85, radiometer, and a standard glass electrode). 0 1994 American Chemical Society
Excited-State Deprotonation of 2-Naphthol Complexes
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6159 TABLE 1: Equilibrium Constants K' for Inclusion Complex Formation of 2-Naphthol and 2-Naphtholate with Some B-Cyclodextrins and the p& Values of the Complexes at 25 OC.
300
340
380
nm
Figure 1. Effect of the association of aqueous 2-naphthol (1.34 X 10-4 M) with j3-cyclodextrinon the absorption spectra at pH 7 (1 and 2) and at pH 12 (3 and 4) in the absence and presence of 5 X lW3 M &cyclodextrin, respectively.
K,' M-I K,i M-1 pH36.5 p H 1 1 1 . 7 pKi(1) pKi(2) j3-cyclodextrin 590 & 50 180 f 30 10 & 0.1 9.9 f 0.1 HO-ethyl-fl320 f 50 cyclodextrin HO-propyl-fl- 700 f 100 150 f 50 10.2 & 0.2 10.0 f 0.1 cyclodextrin a-cyclodextrin Kdi = 250 50 9f2 Knzi= 35 i 5 a pKi values were obtained from relation 5 (column 1) and measured by titration (2). pKf of free 2-naphthol is 9.52.2'
that the system can be described as a mixture of independently absorbing and reacting species. In this case the optical density od,, of the solution is a sum of contributions by free molecules (indexj) and various inclusion complexes (index i), e.g. 1:l or 1:2 (guest:host) (tvalues are the respectiveextinctioncuefficients):
A
+ 2B + A-B + B * B*A.B
(1)
n
I
,
3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 nm
Figure 2. Effect of the association of aqueous 2-naphthol (1.34 X 10-4 M, pH 7) with a-cyclodextrin on the absorption spectra. The arrow indicates increasing host concentration: 0.0,0.01,0.02,0.033,and 0.0657 M.
Absorption spectra were recorded on a Perkin-Elmer 330 or Lamda 16 and a Hitachi U3501 spectrophotometer at 25 OC. The steady-state and time-resolved fluorescence equipment and the procedure for correcting the spectra and determining absolute quantum yields and lifetimes were described previ0usly.2~ Molecular mechanics studies were performed using the MM287 force field of AllingerZ6on an IBM ES/9021-720 and an IBM RISC System/6000-375 computer. Minimizationwas performed by the steepest descent and Newton-Raphson algorithms. More details will be given elsewhere. Results Optical Absorption Spectra. The absorption spectrum of 2-naphthol in aqueous solution is characterized by a poorly structured band around 330 nm, normally referred to as lLb in Platt's nomenclature, and a second bandaround 280 nm (1La).22-24 In a basic medium, both bands shift to considerably longer wavelengths (350 and 290 nm) because of protolytic dissociation above pK = 9.5.22-24 Figure 1 shows the effects of the addition of 5 X 10-3 M j3-cyclodextrin to aqueous 2-naphthol at pH = 5.5 and pH = 11.7: theextinction coefficients increase, and thespectra shift bathochromically. The results are in good agreement with reported spectra in slightly acidified solutions, pH 5." Figure 2 reports 2-naphthol absorption spectra at increasing a-cyclodextrin concentrations. A small but clearly observable effect appears below a host concentration of 0.01 M: similar to @CDx addition, the absorbtivity increases and the maximum shows a small red shift. At higher concentrations, however, a new band appears red-shifted by more than 7 nm versus the case for pure water. Above 0.05 M a-CDx, the absorptivity around 327 nm decreases, whereas that at 335 nm still increases. In basic solutions, however, the spectra are only slightly affected by the addition of up to 0.6 M a-CDx. Association constants K = [2NCDx]/[2N] [CDx] for the formation of ground-state inclusion complexes were obtained from the concentration dependence of absorption spectra, assuming
The concentrationscfand ctare related to theaddedconcentrations co and cdo of guest and host, respectively, by the equilibrium constants K1 and K2
and the initial conditions, that the sum of the respective concentrations is constant. od,, as a function of the cyclodextrin concentrationcanbefitted to theexperimentaldataod,at various wavelengths by a least-squares method: N
E(.&j= 1
%#t) j
-
'
minimum
(4)
The sum is taken over all measured host concentrations. This leads to a set of linear equations for tf and el. The association constants K f were determined by an iterative algorithm. Association constants and the spectra of the inclusion complexes (Le. the ti values versus wavelength) were elucidated. For P-CDx the experimental data fit a 1:1 model. In the case of a-CDx as host, results are incompatible with a simple 1:l association model, and the formation of 1:2 guest:host complexes must be considered, as at wavelengths around the maximum of the 1:l complex the optical density decreases when the host concentration exceeds 0.05 M. The results obtained are given in Table 1, and the respective spectra for the inclusion complexes are given in Figure 3. The red shift of the spectrum and the increase of absorptivity generally observed for the complexes can be compared to analogous spectral changes as water is replaced by an alcohol or an ether as solvent (see Figure 3). Such solvent effects are comparable to those of 1:l inclusion; shifts in the 1:2 complex with a-CDx are, however, much larger. Association constants of 2-naphthol with B-CDx are in good agreement with those reported by Yorozu et al.11 and are considerably larger than for the naphtholate ion in a basic medium (see Table 1). In basic a-CDx solutions, only formation of 1:l complexes was observed. These results contrast recent observations for phenols in a basic aqueous medium where K is larger for the phenolate ion than for phenol.'O A reduced binding power observed at very high sodium hydroxide concentrations (pH >
6160 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
-
26004
I
5
300
310
320
330
340
360 nm
Figure 3. Absorption spectra of 2-naphthol in water (1) and of the 1:l inclusion complex with 8-cyclodextrin (2) and the 1:2 complex with a-cyclodextrin(3) obtained by the deconvolution procedure described in
the text. For comparison the 2-naphthol absorption spectra in ethanol (4) and diethyl ether ( 5 ) are also given.
SCHEME 1 Kn NOH + CDx
NO-
&
+ CDx
NOH'CDx
NO -.CDx
Park et al.
Fluorescence Spectra. The variation of the 2-naphthol fluorescence spectra upon addition of HO-ethyl-j3-CDx and of a-CDx to a neutral aqueous 2-naphthol solution are shown exemplarily in Figure 4a and 4b, respectively. The solutions were excited at the isospestic point near 310 nm, ensuring that free 2-naphthol molecules and thecomplexes areexcitedaccording to their relative concentrations. Adiabatic protolytic dissociation in the excited state leads to the observationof fluorescencefrom both theneutral naphthol molecule (350 nm) and the naphtholate ion (420 nm) at p H values quite below the ground-state pK.22-24 Upon CDx addition, the short-wavelength emission increases at the expense of anion emission, thus indicating decreasing dissociation efficiency upon complexation. The results are in good agreement with the &CDx complexation effects reported previously.s~l1The appearance of an isoemissive point a t 388 nm and at 392 nm (less well defined) with the addition of j3-CDx and a-CDx, respectively, results from a ground-state equilibrium between free molecules and inclusion complexes; release of naphthol during the excitedstate lifetime of the inclusion complexes should then be negligible. The variation in the position of the isoemissive point is the consequence of different emission spectra for inclusion complexes with different hosts, Le. different dissociation yields and absolute fluorescence yields of the complexed molecules. The fluorescence maxima are compiled in Table 2. Fluorescence shifts are generally much smaller than absorption shifts, and emission spectra narrow on complexation; the half-band width (HBW) becomes similar to that obtained for pureorganicsolvents. Spectral shifts between acid and base are correlated to the excited-state deprotonation efficiency by the Fiirster cycle,22 which yields the following relationship:
Ka
13), which was attributed to deprotonation of the cyclodextrins or to coinclusion of the hydroxide ion, should be of minor importance below pH 5 12.10927 Effects of ionic strength on the association constant were investigated for naphthol complexation with O-CDx by adding 1 M KC1: Ki increases to reach 1130 f 100 M-I. In all other cases ionic strength was kept near zero. The decrease in the association constant of the basic form complicates essentially determinations of the ground-state pK values of inclusion complexes by titration. For 0.01 M j3-CDx solutions, titration results have to be corrected for partial dissociation of the complex in a basic medium (see Table 1). pKi values could not be estimated for complexes with a-CDx. Such an increase in the protolytic dissociation constants can be rationalized by Scheme 1, which yields the following relation for the respective equilibrium con~tants:~.28
5 =5 Ki Ka
which gives pK, = pK,
+ log
pKivalues calculated from this relationship are included in Table 1, and they agree reasonably well with those estimated experimentally. In comparison to those for the naphthols, pK values for phenols should decrease on complexation because of the larger association constant of the anion. Absorption properties of 2-naphthol under different environmental conditions and of its inclusion complexes are compiled in Table 2. The energy of the absorption transition is highest for pure water but decreases in less polar solvents like ethanol and dioxane. This effect was related to the hydrogen-bonding properties of these solvents.29 The spectrum of the j3-CDx complex resembles closely that of 2-naphthol in a binary alcohol-water mixture containing 50-60 vol % organic cosolvent. The spectrum of the 1:2 complex with a-CDx is, however, a t much lower wavenumbers than solutions in dioxane or cyclohexane.
An asterisk refers to the excited state and the upper index G to the ground state; & and $, are the wavenumbers of the 0 4 transitions of the conjugated acid-base pair. According to a suggestion made by Grabowski et al.,30 the arithmetic mean of the wavenumbers of absorption and emission maxima (Table 2) was taken to approximate the electronic transition energies &and ik. So obtained pK* values are compiled in Table 3; that for aqueous 2-naphthol is in good agreement with published data.22-24 The pK* of the 8-CDx complex results 0.56 pH units above that of the pure aqueous solutions. Most of this increase, 0.4 units, is due to the reduced acidity in the ground state, and only -0.15 pH units result from a small increased spectral shift of 4 can be estimated, and this renders the occurrence of excited-state dissociation improbable. Fluorescence spectra were measured at 0.012 M j3-CDx and 0.05 M a-CDx as a function of pH between pH = 0.5 and 7.The emission intensity of the neutral 2-naphthol molecule is maximal around pH = 1 and decreases slightly below, because of quenching by hydrochloric acid. Respective quenching constants K ~ = v kqHClr,, where kqHC1 is the quenching rate constant and r , the lifetime of excited 2-naphthol or of the respective complex, were KSV= 0.19 f 0.02 M-1 for 0.012 M j3-CDx and KSV = 0.17 f 0.02 M-l for 0.5 M a-CDx. For free 2-naphthol a quenching constant KSV= 0.18 was reported p r e v i ~ u s l y .kqHC1 ~ ~ values were calculated using the lifetime data given in Table 5. They decrease slightly on complexation (Table 5 ) , and quenching effects are smaller than 2% a t pH = 1. The fraction of anionic fluorescence, Z8(anion), on the total emission was calculated by fitting spectra obtained a t pH < 1 to the high-energy region of the respective composite spectra; integrating the difference gives then the respective intensity of
Excited-State Deprotonation of 2-Naphthol Complexes
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6161
TABLE 2 Spectroscopic Parameters for the 2-Naphthol Inclusion Complexes with Aqueous Cyclodextrins. The Same Data in Some Pure Solvents Are Given for Comparison' LA (nm) ~~
A ;
(103 cm-1)
cmu(M-l cm-I)
~
water, pH 0.5 water, pH 12 a-cyclodextrin, 1:1 complexb a-cyclodextrin, 1:2 complexb 0.06 M a-cyclodextrin, pH 12 8-cyclodextrinb 0.012 M 6-cyclodextrin, pH 12 0.01 M y-cyclodextrin, pH 6.5 water 60 v/v % ethanol ethanol dioxane cyclohexane
+
327.2 346 328 334.6 346 329 347 328.5 329.8 33 1 332 328.5
30.55 28.90 30.49 29.89 28.90 30.38 28.82 30.40 30.32 30.21 30.12 30.44
1670 2960 1820 2200 3450 2050 3330 1760 1880 21 10 2350
353 420
28.33 23.81
3260
356 420 354.3 420 353
28.09 23.8 28.22 23.8 28.33
2978
356 355 349
28.09 28.17 28.65
3074 2879
3050 3250
a XA and XF are the wavelengths (nm) and ;A and ;F the wavenumbers (100cm-I) of the long-wavelength absorption and the fluorescence band, respectively; em (M-I cm-I) is the extinction coefficient a t the maximum and HBW (lo00 cm-I) the fluorescence half-bandwidth. Absorption data for the inclusion complexes were obtained from deconvolved spectra (see text) measured at pH 6.5; fluorescence spectra were measured at pH 0.9.
*
TABLE 4 Fluorescence Quantum Yield qf and (IF' of 2-Naphthol and 2-Naphtholate Inclusion Complexes, Respectively, and the 2-Naphthol Excited-State Dissociation Yield qd aqueous solution 8-CD a-CD, 1:l incl. compl. a-CD, 1:2 incl. compl.
BFe Bf 0.24 0.260 0.24 A O.Olb 0.29 f 0.03b 0.22 f O.Olb 0.28 f 0.03b 0.40 f 0.02b
a Errors for absolute fluorescence yields *lo%. in aqueous solution.
qd
0.38 f 0.02 0.17 f 0.02 0.23 f 0.04
