Journal of Molecular Structure 570 (2001) 145±152
www.elsevier.com/locate/molstruc
Photoluminescence of 2-(2 0-hydroxy-5 0 -methylbenzoyl)-1,5diphenylpyrrole in aqueous and b-cyclodextrin environments: manifestation of open and closed conformers Pradipta Purkayastha, Nitin Chattopadhyay* Department of Chemistry, Jadavpur University, Calcutta 700 032, India Received 24 November 2000; revised 9 January 2001; accepted 9 January 2001
Abstract Steady-state ¯uorometric studies have been performed on 2-(2 0 -hydroxy-5 0 -methylbenzoyl)-1,5-diphenylpyrrole (HMBDPP) in aqueous and aqueous b-cyclodextrin (b-CD) environments at ambient temperature. The ¯uorophore mostly shows a single emission in aqueous solution. Addition of b-CD to the aqueous solution of the ¯uorophore results in the development of another emission band at higher energy. The difference in the ¯uorometric behaviour is assigned to a remarkable change in the polarity of the microenvironment within the supramolecular structural environment compared to that of the bulk aqueous phase. Semi-empirical calculation (AM1-SCI) rules out the possibility of intramolecular proton transfer reaction in any of the S0, S1 and T1 states of the ¯uorophore. It is proposed that HMBDPP exists mostly in the intermolecularly hydrogenbonded form (open conformer) in aqueous solution while within b-CD environment, it is the intramolecularly hydrogen-bonded form (closed conformer) that predominates. q 2001 Elsevier Science B.V. All rights reserved. Keywords: b-Cyclodextrin; Supramolecular complex; Fluorescence; Closed conformer; Open conformer; Hydrogen bond
1. Introduction The understanding of the solute±solvent interaction and its effect on the spectroscopic properties of ¯uorophores is an important aspect of research relating to solvatochromic properties of the organic solutes. Much attention has been directed, from both experimental and theoretical points of view, to the ground and excited state proton transfer of hydrogen bonded molecules [1±8]. Depending on the structure of the probe molecule and the environment, two types of * Corresponding author. Present address: Department of Chemistry, Coimbra University, Coimbra 3049, Portugal. Tel.: 1351239-852-080; fax: 1351-239-827-703. E-mail address:
[email protected] (N. Chattopadhyay).
excited state proton transfer reactions are dealt with. They are the intermolecular proton transfer (ESPT) and the intramolecular proton transfer (ESIPT) [9± 12]. An intramolecular hydrogen bond between the donor and the acceptor atoms of a probe in a suitable non-polar environment facilitates intramolecular proton transfer [9]. The transfer of a proton between two groups of an aromatic molecule causes large electronic and structural rearrangements, which is associated with signi®cant changes in molecular geometry, dipole moment and, often, large ¯uorescence shifts. As a consequence, dynamics of such processes strongly depend on the nature of the solvent, specially, its ability towards hydrogen bond formation [9]. The proton transfer reaction of an intramolecularly hydrogen-bonded molecule
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00484-7
146
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
is in¯uenced remarkably by the kind of substitution on the aromatic ring or side groups as well as the nature of the environment [3]. Several spectroscopic studies have shown that the compounds like o-hydroxybenzaldehyde, o-amidophenol, methylsalicylate, 3-hydroxy¯avone and related molecules are hydrogen bonded to the solvent when the solvent is a proton acceptor [6,8,13] giving an open structure to the molecular system. Nagaoka et al. [14] have made elaborate investigations on the dynamic processes of the excited states of o-hydroxybenzaldehyde and o-hydroxyacetophenon by emission and picosecond transient spectroscopy. They have established the existence of the closed (intramolecularly hydrogen bonded form) and the open conformers (intermolecularly hydrogen bonded form in hydrogen bonding solvents) for different molecular systems in solvents of different hydrogen bonding ability [14]. Spectroscopic studies of certain interesting ¯uorophores in the present context have also been made in microheterogeneous environments like cyclodextrins (CDs) [15,16]. CDs are linked glucopyranose rings forming a doughnut-shaped compound [17]. They are interesting microvessels for appropriately sized molecules. CDs with different cavity diameters have been used advantageously to sequester guests on the basis of size. The reduced polarity and the restricted space provided by the CD cavity markedly in¯uence a number of photophysical and/or photochemical processes [18±22]. In the present paper, we discuss the ¯uorometric properties of a new organic compound, viz. 2-(2 0 hydroxy-5 0 -methylbenzoyl)-1,5-diphenylpyrrole (HMBDPP), in aqueous and aqueous b-CD environments. The molecular structure of HMBDPP (vide supra for the molecular structure) with one hydroxyl and a nearby sCyO group re¯ects the potential of the molecular system to act as an open and/or closed conformer depending on the nature of the environment. It is also interesting to examine the viability of the molecular system towards intramolecular proton transfer reaction in the ground and/or photoexcited states. Our experimental observations reveal that the probe forms a supramolecular complex with the CD. Furthermore, within the CD cavity, the ¯uorophore feels a microenvironment much less polar compared
to the bulk aqueous phase. It has been proposed that the probe molecule prefers to remain as the closed conformer within the CD environment in contrast to the open conformer in the aqueous environment. Potential energy curves (PEC) have been simulated using semi-empirical (AM1-SCI) method [23] to see the possibility of intramolecular proton transfer reaction of the probe. The simulated potential energy curves in different electronic states, viz., S0, S1 and T1, dictate that intramolecular proton transfer reaction (IPT) is not feasible for the probe in any of the three states. 2. Experimental The ¯uorophore, 2-(2 0 -hydroxy-5 0 -methylbenzoyl)-1,5-diphenylpyrrole (HMBDPP) was received as a kind gift from Prof. A.K. Mallik, of the Organic Chemistry Section of our department. The compound was puri®ed through column and recrystallisation ®rst from petroleum ether and ®nally from ethanol. The purity of the ¯uorophore was checked by thin layer chromatography before going through the ¯uorometric investigations. b-cyclodextrin (b-CD), procured from Sigma, was used as-received. UV spectroscopic grade solvents, viz. n-heptane and ethanol (E. Merck, Germany) were used without further processing. Triply distilled water was used for the experiments in aqueous medium. The absorption and ¯uorescence spectra were recorded on Shimadzu MPS 2000 spectrophotometer and Spex Fluorolog Spectro¯uorimeter, respectively. The relative quantum yields were measured from the area under the emission curves taking quinine sulphate in 0.1 N sulphuric acid as standard
f 0:54 [24]. Although ab initio calculations involving extended basis sets with extensive con®guration interaction (CI) have been successful in explaining structures, energetics and reactivities of small molecules in different electronic states, such reports are still limited in number for large molecular systems. However, semi-empirical molecular orbital methods have established their wide utilities in this respect. The methods provide acceptable approximations to give results close to the experimental ®ndings [7,25±28]. For the present calculations, we have used the commercial
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
147
Fig. 1. Absorption spectra of HMBDPP in aqueous b-CD solutions. In different solutions, b-CD concentrations are: (a) 0; (b) 1.20; (c) 1.85; (d) 2.50; (e) 3.15 and (f) 3.80 mM. Concentration of HMBDPP is 8 £ 1025 M:
software package Hyperchem 5.01 obtained from Hypercube Inc., Canada. The geometry of the molecule was optimised in the ground state using the AM1 method. For the excited states, we have adopted the AM1-SCI method, whereby we have considered all the con®gurations (around 130 con®gurations) within an energy window of 11 eV from the ground state, for the single electron transitions only. The calculations yielded the energy (Eg) and dipole moment (m g) in the ground state and the transition energies (DEi!j) to different electronic states. DEi!j corresponds to the excitation of an electron from the orbital f i (occupied in the ground state) to the orbital f j (unoccupied in the ground state). The total energy of the excited state (Ej) was then calculated as Ej Eg 1 DEi!j : The CI wave functions were used to calculate the dipole moments of the excited states of the molecular species. The reliability of our method of calculation is already established for different molecular systems [7,27,28]. Even for the present system, the calculation gives the lowest energy absorption at 341.1 nm corresponding to the S0 ±S1 transition. The experiment, as will be discussed in Section 3, yields the ®rst absorption band at around 340 nm in very good agreement with the calculation. Solvent stabilisation of the different energy states has been calculated from the solvation energies based on Onsager's continuum model [29]. Assuming that
the solute molecule, having a dipole moment m i in the ith electronic state, to be fully solvated, the solvation energy is given by DEsolv
2m2i
1 2 1 a3
21 1 1
where 1 is the bulk dielectric constant of the solvent and a the cavity radius. The cavity radius is taken as half the maximum molecular length for the optimised molecular geometry, which for HMBDPP is calcuÊ. lated to be 6.71 A 3. Results and discussion Fig. 1 shows the absorption spectra of HMBDPP in aqueous solution as a function of b-CD concentration. In aqueous solution, the absorption spectrum consists of two bands appearing at around 280 and 340 nm. With increasing CD concentration, the intensity at 340 nm decreases whereas the 280 nm band intensity increases giving an isosbestic point at ,300 nm. This indicates an interaction between the ¯uorophore (HMBDPP) and b-CD in the solution and the isosbestic refers to 1:1 complex formation between the probe and the CD. Although we could not determine the equilibrium constant for the complexation process from the absorption studies, the same has
148
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
Fig. 2. Normalised ¯uorescence spectra of HMBDPP as a function of b-CD concentration. (a)±(f) refer to 0, 1.20, 1.85, 2.50, 3.15 and 3.80 mM b-CD concentrations in the aqueous CD solutions. Concentration of HMBDPP is 8 £ 1025 M: The ¯uorescence spectrum of HMBDPP in nheptane solution is given in the inset.
been determined from the ¯uorescence study (vide supra). When excited at the isosbestic wavelength of the absorption spectrum, the aqueous solution of HMBDPP gives a broad emission with a maximum at ,415 nm (Fig. 2) with only a shoulder at ,355 nm. The ¯uorescence quantum yield was determined to be 8 £ 1023 : The nature of the emission spectrum of the ¯uorophore is quite similar in other protic solvents e.g. ethanol. Considering the protic character of the solvents, we assign the principal emission at ,415 nm, tentatively, to the excited species of the open conformer that is intermolecularly hydrogenbonded with the solvent molecule. Addition of bCD to the aqueous solution modi®es the emission spectrum and the 355-nm band develops remarkably (Fig. 2). Development of this band with the addition of b-CD establishes that the ¯uorophore, within the CD cavity, experiences a microenvironment appreciably different from that of the bulk aqueous phase. The equilibrium constant for the complex formation between HMBDPP and b-CD has been determined following the method of double reciprocal plot [30] the basic principle of which is outlined here. Assuming a 1:1 complex formation (supported by the existence of the isosbestic point in the absorption
spectra and to be con®rmed by the linearity of the ®nal double reciprocal plot), the equilibrium constant for the process HMBDPP 1 b-CD Y complex is given by K
complex : HMBDPPb-CD
1
Denoting [complex] as CC, [HMBDPP] as CH and [bCD] as CCD, one can write K
CC : CH CCD
2
The ¯uorescence quantum yield of a solution of the ¯uorophore containing b-CD can be written as
f
fH IH 1 fC IC IH 1 IC
3
where f is are the ¯uorescence quantum yields for the individual ith species and Iis are the fraction of light absorbed by the ith species. Again IH 1 C H H IC 1C CC
4
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
149
Fig. 3. Double reciprocal plot for HMBDPP in aqueous b-CD solution (for details, see text).
Combining Eqs. (2)±(4), one obtains f 2 fH 21 fH 21 21 fC f 1H 21 1 C 21 : fH fH K 1C CCD
5
The experimental data yielded a straight line (Fig. 3) when
f 2 fH =fH 21 was plotted against 1/CCD substantiating the one-to-one complexation between the ¯uorophore and b-CD. Since addition of b-CD to the HMBDPP solution does not change the absorbance at the excitation wavelength (isosbestic point of
Fig. 4. Optimised structure of HMBDPP.
the absorption spectra), we assume 1H 1C : The association constant thus comes out to be 2000 mol 21 dm 3 giving a value of 4.5 kcal mol 21 for the free energy of formation of the complex. A reasonably large value re¯ects that at least a part of the ¯uorophore is well included inside the b-CD cavity to form a host±guest supramolecular complex. As already mentioned, in aqueous b-CD environment, the 355 nm emission band of HMBDPP develops remarkably. In non-polar solvents, like nheptane, the emission spectrum of HMBDPP has a maximum at around 350 nm (inset of Fig. 2) with a shoulder at around 415 nm. The spectral similarity and the proximity of the ¯uorescence band positions of the probe embedded within b-CD with that of the probe in n-heptane solution establishes that the b-CD cavity environment is much less polar than the bulk water. However, the large shift in the ¯uorescence energy (,4500 cm 21) is not probably due to the solvent stabilisation. We assign the 350±355 nm emission of the ¯uorophore in apolar and/or CD environments, tentatively, to the closed conformer that is intramolecularly hydrogen bonded. We believe that in solution, the closed and the open conformers of HMBDPP are in equilibrium. In the protic (and/or hydrogen bonding) solvents, the equilibrium shifts to favour the open conformer while in the non-polar (and/or non-hydrogen bonding) solvents, it moves towards the closed conformer. The possibility that the 355-nm emission may originate from the
150
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
Fig. 5. Simulated potential energy curves for the intramolecular proton transfer process of HMBDPP in S0, S1 and T1 states in vacuum (v) and in aqueous solution (s).
¯uorophore through intramolecular proton transfer reaction in ground or excited electronic states is ruled out from the theoretical study discussed later in this section. Fig. 4 shows the optimised geometry of HMBDPP obtained through semi-empirical (AM1) calculation. Ê The optimised structure gives a distance of 6.17 A Ê between between C9 and H18 and a distance of 6.66 A H26 and H23 (Fig. 4). The cavity diameter of b-CD is Ê , the depth being around 8.0 A Ê [31]. around 7.8 A Thus, the calculation reveals that there is enough space within the b-CD cavity to include, at least, a
part of the ¯uorophore, the effect of which is re¯ected in its absorption as well as emission spectra. To rule out the possibility of the 355 nm emission to correspond to the species formed through the ground or excited state intramolecular proton transfer (IPT) reaction, we have simulated the potential energy curves (PEC) for the IPT process for the molecular system in S0, S1 and T1 states. Fig. 5 represents the plot of such PECs for HMBDPP in vacuum and in water solvent as a function of the reaction coordinate. The distance (R16±23) between the hydrogen (H23) of the hydroxyl group and the oxygen (O16) of the carbonyl
Table 1 Calculated binding energies (in kcal/mol) for the normal and tautomeric forms of HMBDPP in S0, S1 and T1 states in vacuum (with the corresponding dipole moments in debye within parentheses) and in different solvents Medium
Dielectric constant
Species
E(S0) (kcal/mol)
E(S1) (kcal/mol)
E(T1) (kcal/mol)
Vacuum
±
Normal Tautomer
25273.72 (3.00) 25258.48 (3.93)
25191.04 (3.43) 25189.13 (5.23)
25219.10 (4.57) 25216.32 (3.80)
n-Heptane
2.0
Normal Tautomer
25273.89 25258.78
25191.26 25189.65
25219.50 25216.59
Ethanol
24.3
Normal Tautomer
25274.12 25259.18
25191.57 25190.36
25220.04 25216.96
Water
78.5
Normal Tautomer
25274.14 25259.21
25191.59 25190.41
25220.08 25216.99
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
group (to which the hydrogen might get attached to form the tautomer) has been considered as the reaction coordinate for the process. For the generation of the potential energy curves for the intramolecular proton transfer process, we have optimised the geometry with various preset values of the reaction coordinates. Table 1 presents the energies (in kcal/mol) of the normal and the tautomeric (had there been proton transfer) forms of HMBDPP along with the dipole moments (in debye within parentheses) in the S0, S1 and T1 states. The energies of the solvated species in n-heptane, ethanol and water are also included in the table. The reaction enthalpies (kcal/mol) and the activation energies (kcal/mol) for the intramolecular proton transfer process for the molecular system in vacuum and in different solvents have been calculated from the potential energy curves and are presented in Table 2. The table shows that in all the three states considered here, the IPT process for the ¯uorophore is endothermic. There is also a reasonably high activation barrier for the reaction although the barrier height is minimum in the S1 state. Hence, the reaction is not favourable both from the thermodynamic (reaction enthalpy) as well as kinetic (activation energy) considerations. 4. Conclusions
151
Table 2 Calculated activation energies (Eact, in kcal/mol) and reaction enthalpies (DH, in kcal/mol) for the intramolecular proton transfer reaction of HMBDPP in different electronic states (S0, S1 and T1) in different environments Medium
Energy states
Eact (kcal/mol)
DH (kcal/mol)
Vacuum
S0 S1 T1
25.95 14.07 22.35
15.23 1.91 2.79
n-Heptane
S0 S1 T1
25.80 14.00 22.43
15.11 1.61 2.91
Ethanol
S0 S1 T1
25.60 13.89 22.57
14.95 1.21 3.08
Water
S0 S1 T1
25.58 13.88 22.57
14.93 1.18 3.09
Acknowledgements Financial support from Council of Scienti®c and Industrial Research and Department of Science and Technology, Govt. of India, is gratefully acknowledged. Thanks are due to Prof. A.K. Mallik for his generous gift of the compound HMBDPP and to Prof. S.C. Bera for his kind interest in the work.
The present experiment leads to propose the following points:
References
1. The ¯uorophore interacts with b-cyclodextrin forming a 1:1 supramolecular inclusion complex with the CD. 2. In aqueous medium, HMBDPP remains mainly in its open conformer form and it is intermolecularly hydrogen bonded to the solvent molecule. 3. The ¯uorophore experiences a less polar environment within the CD cavity where it remains principally in the intramolecularly hydrogen-bonded closed conformer form. 4. Simulation of the potential energy curves reveals that the intramolecular proton transfer reaction is not favourable, both thermodynamically as well as kinetically, for the HMBDPP system in any of the S0, S1 and T1 states.
[1] L.G. Arnaut, S.J. Formosinho, J. Photochem. Photobiol. A 75 (1993) 1. [2] U. Nagashima, S. Nagaoka, S. Katsumoto, J. Phys. Chem. 95 (1991) 3532. [3] K.K. Smith, K.J. Kaufmann, J. Phys. Chem. 85 (1981) 2895. [4] A. Grabowska, J. Sepiol, C. Rulliene, J. Phys. Chem. 95 (1991) 10493. [5] M.A. Rios, M.C. Rios, J. Phys. Chem. A 102 (1998) 1560. [6] S. Nagaoka, N. Hirota, K. Yoshihara, E.L. Kochang, H. Ewamura, J. Am. Chem. Soc. 106 (1984) 6913. [7] P. Purkayastha, N. Chattopadhyay, Phys. Chem. Chem. Phys. 2 (2000) 203. [8] D. Guha, A. Mandal, S. Mukherjee, J. Lumin. 90 (2000) 7 (and references therein). [9] S.J. Formosinho, L.G. Arnaut, J. Photochem. Photobiol. A: Chem. 75 (1993) 21. [10] K. Das, N. Sarkar, A.K. Ghosh, D. Majumdar, D.N. Nath, K. Bhattacharyya, J. Phys. Chem. 98 (1994) 9126.
152
P. Purkayastha, N. Chattopadhyay / Journal of Molecular Structure 570 (2001) 145±152
[11] P.T. Chou, M.L. Martinez, W.C. Cooper, D. McMorrow, S.T. Collins, M. Kasha, J. Phys. Chem. 96 (1992) 5203. [12] F. Gai, L. Rich, J.W. Petrich, J. Am. Chem. Soc. 116 (1994) 735. [13] M. Yanagimachi, N. Tamai, H. Masuhara, Chem. Phys. Lett. 201 (1993) 115. [14] S. Nagaoka, N. Hirota, M. Sumitani, K. Yoshihara, J. Am. Chem. Soc. 105 (1983) 4220. [15] E.L. Roberts, J. Dey, I.M. Warner, J. Phys. Chem. 100 (1996) 19681. [16] S. Kundu, S.C. Bera, N. Chattopadhyay, Ind. J. Chem. A 34 (1995) 55. [17] M.L. Bender, M. Komiyama, Cyclodextrin Chemistry, Springer, Berlin, 1978 (chap. 2). [18] V.T. D'Souza, M.L. Bender, Acc. Chem. Res. 20 (1987) 146. [19] G.S. Fox, N.J. Turro, J. Am. Chem. Soc. 106 (1984) 422. [20] P. Bortolus, S. Monti, J. Phys. Chem. 91 (1987) 5046. [21] N. Chattopadhyay, T. Chakraborty, A. Nag, M. Chowdhury, J. Photochem. Photobiol. A 52 (1990) 199.
[22] N. Chattopadhyay, J. Photochem. Photobiol. A: Chem. 58 (1991) 31. [23] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [24] J.V. Morris, M.A. Mahaney, J.R. Huber, J. Phys. Chem. 80 (1976) 969. [25] T. Arthen-Engeland, T. Bultmann, N.P. Ernsting, M.A. Rodriguez, W. Thiel, Chem. Phys. 163 (1992) 43. [26] B. Dick, J. Phys. Chem. 94 (1990) 5752. [27] P. Purkayastha, P.K. Bhattacharyya, S.C. Bera, N. Chattopadhyay, Phys. Chem. Chem. Phys. 1 (1999) 3253. [28] P. Purkayastha, S.C. Bera, N. Chattopadhyay, J. Mol. Liq. 88 (2000) 33. [29] C.J.F. BoÈttcher, Theory of Electronic Polarization, vol. 1, Elsevier, Amsterdam, 1983. [30] S. Kundu, S.C. Bera, N. Chattopadhyay, Ind. J. Chem. A 37 (1998) 102. [31] S. Li, W.C. Purdy, Chem. Rev. 92 (1992) 1457.
