Journal of Photochemistry

and Photobiology,

A:

Chemistry,

52 (1990)

199 - 204

199

EFFECT OF INCLUSION OF CYCLODEXTRIN ON EXCITED STATE PROTON TRANSFER: CARBAZOLEyCYCLODEXTRIN NITIN CHATTOPADHYAY*, and MIHIR CHOWDHURY Department of Science, (Received

TAPAS

CHAKRABORTY,

of Physical Chemistry, Indian Association Jadavpur, Calcutta 700032 (India) February

28, 1989;

in revised form October

ASHIS

NAG

for the Cultivation 16, 1989)

Summary The excited state proton transfer reaction of carbazole was investigated in the presence of y-cyclodextrin (r-CD) using steady state and time-resolved fluorometry. The deprotonation rate was enhanced for the carbazole-y-CD inclusion complex compared with free carbazole, but the rate of reverse protonation was unaffected.

1. Introduction In recent years cyclodextrins (CDs) have received considerable attention [l, 21. They are interesting microvessels for appropriately sized molecules and the resulting supramolecules serve as excellent miniature models of enzyme-substrate complexes. CDs with different cavity diameters have been used advantageously to sequester guests on the basis of size, e.g. simple benzene derivatives fit easily within o-CD, and larger aromatics can be accomodated within p- or -/-CD according to their molecular dimensions. The reduced polarity and the restricted space provided by the CD cavity markedly influence a number of photophysical and/or photochemical processes [ 3 - 121. Shizuka and coworkers [8, 91 have demonstrated that prototropic reactions are affected by inclusion of the substrate molecules in CD cavities. Their fluorescence studies have revealed that ,B-naphthol forms 1 :l inclusion complexes with CY-, p- and y-CDs with different association constants. The probe molecule is packed very loosely in a T-CD cavity, whereas it is tightly packed in a P-CD cavity. The rate of proton dissociation is markedly decreased as a consequence of inclusion. *Present address: West Bengal, India. lOlO-6030/90/$3.50

Department

of Chemistry,

Kanyapur

Polytechnic,

@ Elsevier Sequoia/Printed

Asansol7

13304,

in The Netherlands

200

We have studied the excited state proton transfer reaction of carbazole (CAZL) for quite some time [13 - 171. In this work, we examine the effect of inclusion in y-CD on the well-characterized excited state prototropic reaction of CAZL using steady state and time-resolved fluorometric measurement s.

2. Experimental

details

CAZL was purified as described previously [ 131. Analytical grade y-CD was obtained from Fluka and was used as received. Sodium hydroxide, ammonium hydroxide and ammonium chloride (Ranbaxy, analytical reagent) were used without further purification. Triply-distilled water was used as solvent as in earlier studies [13, 141. All the solutions were freshly prepared just before the experiments and degassing of the solutions was found to be unnecessary.

3. ResuIts and discussion The solubility of CAZL in water increases in the presence of y-CD; the optical density (at 290 nm) of a 5 mM T-CD solution saturated with CAZL is about 2.5 times the optical density of a CAZL-saturated water solution, indicating interaction between CAZL and y-CD. Fluorescence anisotropy measurements of neutral CAZL (Yemiseon = 360 nm) further indicate that CAZL is included in T-CD. The polarization ratio (11,--Il)/(Ill + 11) is 0.32 (excitation wavelength, 295 nm) for a saturated aqueous solution of CAZL in the absence of y-CD, but is 0.37 in the presence of 5 mM y-CD. The increase in the polarization ratio in the presence of y-CD cannot be ascribed to a shortening of the lifetime on addition of y-CD (see later); it obviously reflects the restricted rotational relaxation of excited CAZL within the CD cavity (the rotation of the large CAZL-y-CD supracomplex is comparatively slow). However, our search for an isosbestic point in the absorption spectrum and for a CAZL-T-CD complex of definite stoichiometry has yielded negative results. No change occurs in the absorption spectrum on increasing the pH, which is consistent with the complete lack of acidic behaviour of CAZL in the ground state. As in the absorption spectrum, the fluorescence spectrum shows little change on addition of ?-CD at pH 7. However, significant differences are seen in the fluorescence spectra at higher pH*. Figure 1 shows the variation in the fluorescence spectrum of CAZL on addition of 5 mM -y-CD at pH 12.0. At this pH, emission occurs from both the neutral and anionic forms ?Since the rate of recombination of the guest molecule and CD and the rate of dissociation of the inclusion complex are slow, it can be reasonably assumed that no dissociation of the inclusion complex and no association between CAZL and y-CD occur in the photoexcited state within the lifetime of CAZL or the corresponding anion [8].

201

3

1 320

360

LOO

Wavelength

440 (nm)

LB0

-

Fig. 1. Fluorescence spectra of aqueous solutions of CAZL at pH 12.0 in the absence (a) and presence (b) of 5 mM y-CD (Yexcitation = 295 nm). (It was checked that the addition of 5 mM y-CD did not change the overall pH of the solution.)

of CAZL, and the neutral to anion emission intensity ratio decreases when y-CD is added. Th is clearly indicates that the apparent excited state prototropic equilibrium is shifted towards the anionic form on inclusion in CD. This shift could either be due to an increase in the deprotonation rate or a decrease in the rate of reverse protonation. Quenching rates were determined to establish which of these two cases applies. The variation in luminescence from the neutral and anionic forms of CAZL at various concentrations of added NaOH and NH,OH in the presence of 1 mM y-CD is shown in Figs. 2 and 3. By application of the Stern-Volmer equation to the neutral fluorescence, the rate constants k, of deprotonation were determined in the presence and absence of 1 mM y-CD as described previously [ 141. The deprotonation rate constant k, is doubled (for both NH,OH and NaOH addition) in the presence of y-CD. The reverse protonation rate was only evaluated in the case of NH,OH, following the same method and monitoring the anion luminescence of CAZL [ 141. The results (Table 1) indicate that k, is unaffected by y-CD. This is somewhat surprising. Therefore we carried out time-resolved studies to measure the deprotonation and protonation rate constants directly. The time-resolved decay curves were analysed by the methods described previously [13]. Although the anion shows growth and decay as a function of time and can be analysed by two exponentials with equal (but opposite in sign) pre-exponential coefficients, the decay of the neutral species is virtually monoexponential since the pre-exponential factor of the second exponential term is very small [ 131. The possible presence of

202

320

1

I

360

400

Wavelength

440

(nm)

480

320

-

360

400

Wavelength

440 (nm)

480

-

Fig. 2. Fluorescence spectra of CAZL solutions in the presence of 1 mM y-CD as a funcf are 0.0, 0.005, tion of NaOH concentrations. NaOH concentrations in spectra a __f 0.01, 0.02, 0.04 and 0.06 M respectively (Yexcitation = 295 nm). Fig. 3. Fluorescence spectra of CAZL solutions in the presence of 1 mM y-CD as a function of NH40H concentration. The NH40H concentrations in spectra a h are 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 M respectively (Yexcitation = 295 nm).

TABLE

1

Rate constants

obtained

Base

from steady state and time-resolved

NaOH NH40H

Time resolved

Steady state 121 T-CD y-CD y-CD y-CD

absent present absent present

1.0 2.0 3.0 6.4

x x x x

1o1O 1o’O lo8 10’

studies

kz

kl

5.3 x lo6 5.5 x lo6

9.0 1.7 3.7 7.0

k2 x x x x

lo9 1o’O lo8 lo8

8.5 8.0 4.3 4.6

x x x x

lo6 lo6 lo6 lo6

multiple complexes has not been taken into account. Table 2 lists a set of 71 values monitored at 360 nm. In the absence of a base, the lifetime of neutral CAZL is greater in the presence of y-CD than in its absence; however, at higher pH, a faster decay of the y-CD complex occurs. These unprocessed data illustrate fairly convincingly that T-CD only has an effect on the

203

prototropic equilibrium, and not on the lifetimes of the species involved. The rate constants h, and k, determined from the observed TV and r2 values are shown in Table 1. TABLE

2

Fluorescence and presence Concentration

decay times of neutral CAZL at different of 1 mM y-CD (Yemission = 360 nm) of NaOH

0.00 0.005 0.01 0.02 0.04 0.06

NaOH concentrations

in theabsence

71 (ns) Y-CD absent

y-CD present

10.0 5.7 4.6 3.1 2.0 2.5

10.3 5.6 3.8 2.2 1.3 0.9

Both the steady state and time-resolved studies demonstrate that the rate of deprotonation, in the excited state proton transfer reaction of CAZL, is doubled when the substrate molecule is included in y-CD. The reverse protonation rate is unaffected. Similar observations have been made for indole and diphenylamine [ 181. However, the results are opposite to those observed for Z-naphthylamine [18] and 2-naphthol 18, lo]. As mentioned earlier, these workers [S, IO] noticed a suppression in the proton dissociation process which was ascribed to protection from OH- attack by the hydrophobic micro-environment of the CD cavity. Two tentative explanations can be offered for the unexpected increase in the deprotonation rate of CAZL on addition of y-CD. Firstly, geometric distortion (twisting or bending) of the encaged molecule enforced by the ?-CD cavity and/or charge delocalization over the guest-host supramolecule may affect the pK* value of CAZL in the excited state. However, no appreciable change in the excitation and emission spectra of the molecules occurs at pH 7, indicating that any distortion is minor and is probably localized at the >NI-I centre. The second suggestion is that the pK* value of CAZL is not changed, but the hydroxyl groups of they-CD cavity somehow assist the attack of the OHgroup to deprotonate the heterocyclic compound. Saenger and coworkers [19, 201 have observed that the OH groups in CD are organized in a circular pattern and have suggested the possibility of bidirectional flip-flop hydrogen f----f H-----O* - -H. If a hydrogen bond migration bonding, e.g. H* 00 -H mechanism exists, a proton dissociated from excited CAZL could be abstracted by bases outside the cavity through a cooperative mechanism the deprotonation process. Figure 4 via OH groups, thus facilitating shows a schematic representation of this cooperative deprotonation process. l

l

204

Y0 H/

‘0.

s F--l-l

..-H

t

0

*!ko

-0 --_

o,H..-“,H..-o

Fig. 4. A schematic

diagram of cooperative

proton

transfer in a CD cavity.

Acknowledgments project (CE-2) sponsored This work was carried out as an Indo-US jointly by the National Bureau of Standards, U-S A. and the Department of Science and Technology, Government of India. References V. T. D’Souza and M. 1;. Bender, Act. Chem. Res., 20 ( 1987) 246. V. Ramamurthy and D. F. Eaton, Act. Chem. Res., 21 (1988) 200. G: S. Cox and N. J. Turro, J. Am. Chem. Sot., 106 (1984) 422. T. Tarozu, M, Hoshino and M. Imamura, J. Phys. Chem., 86 (1982) 4426. P. Bortulos and S. Monti, J. Phys. Chem., 91 (1987) 5046. D. W. Armstrong, F. Nome, L. A. Spino and T. D. Golden, J. Am. Chem. Sot., 108 (1986) 1418. 7 M. P. Eastman, B. Freiha, C. C. Hsu and C. A. Cheng, J. Phys. Chem., 92 (1988) 1682. M. Hoshino, M. Imamura and H. Shizuka, J. Phys. Chem., 86 (2982) 8 T. Yorozu, 4422. T. Fuju, T. Kobayashi, H. Ohtani and M. Hoshino, BulI. 9 H. Shizuka, M. Fukushima, Chem. Sot. Jpn., 58 (1985) 2207. 10 D. F. Eaton, Tetrahedron, 43 (1987) 1551. 11 A. Nag and K. Bhattacharyya, Chem. Phys. Lett., 151 (1988) 474. 12 A. Nag, R. Dutta, N. Chattopadhyay and K. Bhattacharyya, Chem. Phys. Left., 157 (1989) 83. 13 A. Samanta, N. Chattopadhyay, D. Nath, T, Kundu and M. Chowdhury, Chem. Phys. Lett., 121 (1985) 507. 14 N. Chattopadhyay and M. Chowdhury, J. Photochem., 38 (1987) 301. 15 N. Chattopadhyay, R. Dutta and M. Chowdhury, J. Photochem. Photobiol., A: Chem., 47 (1989) 249. 16 N. Chattopadhyay, A. Samanta, T. Kundu and M. Chowdhury, J. Photochem. Photobiol., A: Chem., 48 (1989) 61. 17 N. Chattopadhyay, R_ Dutta and M. Chowdhury, Bull. Assoc. Kinet. India, in the press. 18 N. Chattopadhyay, T. Chakraborty and M. Chowdhury, unpublished results. 19 W. Saenger, C. Betzel, B. Hingerty and G. M. Brown, Nature (London), 296 (1982) 582. 20 W. Saenger, in J. L. Attwood, J. E. Davies and D. D. MacNicol (eds.), Inclusion Compounds, Vol. 2, Academic Press, London, 1984, p. 231.