Solvent-dependent host–guest complexation of two homologous merocyanines by a water-soluble calix[8]arene: Spectroscopic analysis and structural calculations

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Journal of Molecular Structure 846 (2007) 49–54

Solvent-dependent host–guest complexation of two homologousmerocyanines by a water-soluble calix[8]arene: Spectroscopic

analysis and structural calculations

Andrea Lodi a, Monica Caselli a, Alessandro Casnati b, Fabio Momicchioli a,Francesco Sansone b, Davide Vanossi a, Glauco Ponterini a,*

a Dipartimento di Chimica, Universita di Modena e Reggio Emilia and INSTM, via Campi 183, 41100 Modena, Italyb Dipartimento di Chimica Organica e Industriale, Universita di Parma and INSTM, V.le G. P. Usberti 17/A, 43100 Parma, Italy

Received 11 October 2006; received in revised form 28 December 2006; accepted 8 January 2007Available online 21 January 2007

Abstract

The sulfonated calixarene I8C12 acts as a host for homologous merocyanines Mc1 and Mc2 in organic solvents, exhibiting neitherselectivity towards the guest dyes nor solvent dependence of the complexation equilibria. In water, on the contrary, only the lower homo-logue, Mc1, is solubilized in the presence of the calixarene. A combination of UV–visible and fluorescence spectroscopic and photophysi-cal analysis and MD structural simulation of the calixarene-dye complexes was employed to account for the observations, and suggeststhat a radical change in the complexation mode occurs upon moving from an organic to an aqueous environment.� 2007 Elsevier B.V. All rights reserved.

Keywords: Inclusion complexes; Merocyanines; Calixarenes

1. Introduction

In the early 1990s, after about a decade of intensiveresearch, calixarenes were recognized to possess the proper-ties – ability to complex both metal ions and organic mol-ecules, selectivity, ease of large-scale preparation – to beclassified as the third generation of ‘supramolecules’, fol-lowing cyclodextrins and crown ethers [1]. Studies of com-plexation by calixarenes received strong impetus from thepreparation of water-soluble hosts obtained by introducingcharged or neutral hydrophilic groups in the para positions(‘upper rim’) [2]. Among these, calix[n]arenes carrying sul-fonate groups on the ‘upper rim’ and n-alkyl groups on the‘lower rim’ (phenolic –OR groups) proved particularly effi-cient in complexing, sometimes selectively, organic mole-cules in water [3–15]. This approach to solubilization and

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doi:10.1016/j.molstruc.2007.01.025

* Corresponding author. Tel.: +39 053 2055084; fax +39 059 373543.E-mail address: [email protected] (G. Ponterini).

transport of organic molecules in water takes advantageof hydrophobic interactions involving the guest and then-alkyl chains of the host [6], with complex formation oftencharacterized by an overall entropy gain [9,12].

Calixarenes have also been extensively employed tobuild-up mono- and multilayers with a number of potentialapplications [16]. p-Sulfonated calixarenes, being polya-nions, can be alternated with polycations to make multilay-ers by electrostatic adsorption [17]. This technique offersadvantages over other approaches for obtaining thin films,such as Langmuir–Blodgett deposition or the build-up ofcovalently bound self-assembled layers [18]. Obviously,neutral molecules need to be complexed by multichargedcarriers in order to be included in electrostatically self-as-sembled multilayers (ESAMs). To the best of our knowl-edge, the use of p-sulfonated, O-alkylated calixarenes forincluding water-insoluble organic molecules into ESAMshas been reported only once [19]. We have decided to fur-ther explore the potential of these calixarenes in the build-up of ESAMs containing neutral molecules by trying to

SO3Na

OC12H25

8

Mc2Mc1

16C

N

S

N

S S

O O CH2COOH

S

N

SCHCH

N

S

C18 H37H33

CH2COOH

I8C12

Scheme 1.

50 A. Lodi et al. / Journal of Molecular Structure 846 (2007) 49–54

include in such films dyes of the families of merocyaninesand ketocyanines. The second-order optical properties ofthese push–pull molecules (containing electron donatingand electron accepting groups) have been the subject ofextensive theoretical and experimental investigation [20–30]. So, ESAMs incorporating such dyes may be taken intoconsideration as nonlinear optical materials. The obviouspre-requisite for the inclusion of a water-insoluble dye inan ESAM is its complexation with a suitable multichargedcarrier. In this paper, we show how moving from an organ-ic solvent to water drastically changes the ability of calixa-rene I8C12 to act as a host for merocyanines Mc1 and Mc2,which differ essentially in the length of the chromophoricchain (formulas are shown in Scheme 1). While both dyesare complexed by the calixarene with similar associationconstants in organic solvents, only the lower analogue,Mc1, is complexed, and thereby dissolved, in water. Aninterpretation of the spectroscopically monitored behav-iour is attempted with the aid of simple MD simulationsin the absence of solvent (giving qualitative informationon the structures in low-polarity organic media) as wellas taking into account the expected structural modifica-tions undergone by the hydrophobic lower rim of the calix-arene in an aqueous environment.

2. Experimental

Merocyanines Mc1 and Mc2 were obtained, respective-ly, from Imation s.p.a., Ferrania, Italy, and fromHayashibara Biochemical Laboratories, Inc., Okayama,Japan. Octasulfonato-octadodecyloxy-calix[8]arene (I8C12)was synthesized according to the procedure used forhexasulfonato-hexadodecyloxy-calix[6]arene [4] startingfrom octasulfonato-calix[8]arene. Pure compound I8C12

was obtained by crystallization from methanol/water andthen water. Elem. Anal. for C152H232Na8O32S8 · 8H2O:required C, 60.96; H, 8.35; found: C, 60.82; H, 8.51. Mp

>360 �C; 1H NMR (D2O, 300 MHz, rt) d 7.5 (16H, ArH,br s), 3.6 (16H, ArCH2Ar, br s), 3.44 (16H, OCH2, t,J = 7 Hz), 1.64 (16H, OCH2CH2, m), 1.42 (144H,OCH2CH2(CH2)9CH3, br s), 1.03 (24H, CH3 t, J = 7 Hz);13C {1H} NMR (DMSO-d6, 75 MHz, rt) d 13.5 (CH3),22.0 (CH2CH3), 25.9, 28.5, 28.9, 29.3, 29.5, 29.7, 31.3(OCH2(CH2)5CH2, ArCH2Ar), 72.7 (OCH2), 125.9(ArHm), 132.3 (ArHo), 142.2 (ArHp), 155.6 (ArHi). All sol-vents (dichloromethane (DCM), ethanol and dimethylsulf-oxide (DMSO)) were of spectroscopic grade and werechecked to have negligible emissions in the experimentalconditions adopted. The same held for deionized water(Millipore, Milli-Q, resistivity of 18.2 MX cm). Spectrawere recorded on a Varian Cary 100 spectrophotometerand a Spex Jobin-Yvon Fluoromax2 spectrofluorometer.Fluorescence quantum yields (UF) were determined relativeto quinine bisulfate in 1 N H2SO4 (UF = 0.546 [31]) forMc1 and to acridine orange HCl in ethanol (UF = 0.46[32]) for Mc2. All spectroscopic measurements were carriedout at 18–20 �C.

3. Computational

The analysis of the structures of the host–guest calixa-rene–merocyanine complexes was tackled at an empiricallevel using a force field in the framework of classical molec-ular mechanics/dynamics (MM/MD). The force fieldadopted here to investigate the structure of I8C12–Mc1and I8C12–Mc2 complexes was the Universal Force Field(UFF) [33,34] contained in the Cerius2 simulation package.After setting up reasonable 3D model structures for thetwo merocyanines and the calixarene, a calculation of theatomic point charges was made at the SCF PM3 level forMc1 and Mc2 whereas, because of the large number ofatoms, the charge-equilibration method [35] was employedfor I8C12. Using the UFF, the geometries of Mc1, Mc2 andI8C12 were fully optimised with a standard local BFGS

0.05

0.10

0.15

0 5 100

2

4

10-6

/c 0

10-3/

A

A

A. Lodi et al. / Journal of Molecular Structure 846 (2007) 49–54 51

(quasi-Newton) method [36]. For I8C12, in order to locatethe global minimum, an MD procedure in the microcanon-ical (NVE) ensemble was performed for 5000 steps (1000equilibration steps at 300 K plus 4000 acquisition stepswith a time step of 0.001 ps) followed by a local BFGSoptimization technique. A similar approach was employedwith the calixarene–merocyanine complexes. This compu-tational strategy ensured a quite complete sampling ofthe calixarene–merocyanine PES thus providing the mainminima corresponding to stable structures. All calculationswere performed in the gas phase and without consideringpolarization effects.

350 400 450 500 5500.00

nm

Fig. 2. Absorbance changes of Mc1 in DCM (1.7 · 10�6 mol l�1) uponaddition of I8C12 (total concentrations, c0, for the three curves reported: 0,6 · 10�6, 1.25 · 10�5 mol l�1). Insert: Benesi–Hildebrand plot (DA is theabsorbance increase, Mc1 concentration was 5.8 · 10�7 mol l�1).

4. Results and discussion

Merocyanines Mc1 and Mc2 are well soluble in mostorganic solvents (absorption spectra are shown in Fig. 1),but they are not appreciably soluble in water. In ethanol–water mixtures rich in water, both dyes undergo formationof H-aggregates, likely dimers [37], as shown in Fig. 1 forMc1. Addition of excess I8C12 to solutions of both dyesin DCM, ethanol and DMSO did not result in significantspectral shifts or changes in the colour-band shape.Instead, in all cases, it caused an increase of absorbancethat exhibited a saturating behaviour (e.g., absorbancesincreased by about 25% and 7% for Mc1 and Mc2 inDCM, respectively; some data for Mc1 are reported inFig. 2). Changes in the UV–visible absorption spectra ofguest molecules upon complexation with calixarenes arecommonplace. Spectral shifts were attributed to changesin the polarity of the environment and/or to host–guestspecific interactions [3,9,38]. As yet, we have found no con-vincing explanation of the observed hyperchromism with-out spectral shift.

Evidence of dye complexation in organic solvents wasalso obtained from fluorescence observations. Maximum

350 400 450 500 550 6000.0

0.5

1.0

Mc2Mc1MH M}

A

nm

Fig. 1. Normalized absorption spectra of Mc2 in DCM and Mc1 in DCM(dots), 1:15 ethanol–water (dash–dot) and in water with I8C12

1.7 · 10�4 mol l�1 (solid line, absorbance at maximum = 0.99). M,monomer; H, H aggregate.

emission wavelengths were around 460 and 550 nm forMc1 and Mc2 and were almost the same in the three sol-vents. As was found for absorption, emission spectra werevery little modified both in shape and position by additionof calixarene. But the emission intensity, i.e., its quantumyield UF, increased by about 10% upon calixarene additionfor both molecules in DCM (for free Mc1 and Mc2, UF

were 1.0 · 10�3 and 2.1 · 10�3, respectively). More sensi-tive to complexation was the fluorescence anisotropy, r.As expected upon formation of host–guest complexes,which are bulkier than the free guest and therefore undergoa slower rotational diffusion, r increased from 0.15 to 0.31in the case of Mc1 in DCM and from 0.26 to 0.31 for Mc2in acetone (for free Mc2 in DCM r was already 0.31, nearits maximum experimental value [39]).

Analysis of the absorbance change as a function of theadded amount of calixarene, according to the standardBenesi–Hildebrand approach [40], yielded associationconstants of 3.0 · 105 and 1.5 · 105 mol�1 dm3 in DCMand 2.0 · 105 and 1.0 · 105 mol�1 dm3 in ethanol for Mc1and Mc2, respectively. Both merocyanines are thereforecomplexed by I8C12 in organic solvents, with associationconstants very similar to each other and very little sol-vent-polarity dependent.

MD simulations provided results consistent with thesefindings as well as insight into the structures of the com-plexes. Using the molecular geometries obtained for Mc1,Mc2 and I8C12 we built up two notably different startingstructures (sketched in Scheme 2 for the Mc1/I8C12 adduct)for the simulation of the host–guest complexes. In one case(a-type arrangement) the merocyanine chromophore wasplaced on the side of the SO3

� groups of the calixarene,whereas it was placed near the end of the I8C12 alkyl chainsin the b-type arrangement.

In both cases, the MD simulation resulted in a deepinclusion of the merocyanine alkyl chain inside thecalixarene –OR hydrophobic cavity. The four calixarene–

CH

2 CO

OH

N

SS

ON S

CH

2CO

OH

N

SS

ONS

a

b

Scheme 2.

Table 1Interaction energies (Eint, kJ mol�1) of the calixarene–merocyaninescomplexes in Figs. 3 and 4

Structure Mc1 Mc2

a �359.4 �341.8b �438.5 �541.4

52 A. Lodi et al. / Journal of Molecular Structure 846 (2007) 49–54

merocyanine complex structures obtained from MD andlocal geometry optimisation are shown in Figs. 3 and 4.Their relative stability is measured by the interaction ener-gies, Eint (Table 1), calculated as the difference between thehost–guest complex energy and the sum of the energies ofthe two isolated molecules. Structures of type b turn outto be decidedly more stable than the a-type ones. The

Fig. 3. a-type structures of Mc1/I8C12 (left) and Mc2/I8C12 as

Fig. 4. b-type structures of Mc1/I8C12 (left) and Mc2/I8C12 as

differences in Eint mainly arise from stronger van der Waalsinteractions between the long alkyl chains of the merocya-nines and the host cavity in b-type relative to a-type struc-tures. We may therefore suppose that, in organic solvents,Mc1 and Mc2 preferably adopt structures of type b withI8C12 and that the greater stability of Mc2/I8C12 relativeto Mc1/I8C12 (second row of Table 1) be partly attributableto the longer alkyl chain borne by Mc2. This assumption isconsistent with the previously described spectroscopicobservation. In particular, in a-type structures, the dyechromophores would experience strongly polarizing elec-trostatic interactions with the eight, very near SO3

� groups,resulting in spectral shifts upon complexation which werenot observed.

Formation of b-type host–guest complexes is not howev-er expected to occur in water. In this solvent, in fact, thecalixarene alkyl chains self-assemble under the influence

obtained by classical MD followed by local optimization.

obtained by classical MD followed by local optimization.

400 450 5000.0

0.5

1.0

H2O-EtOH 15:1Mc1-I

8C

12EtOH

A,I f /

a.u.

/ nm

Fig. 5. Normalized absorption and emission spectra of Mc1 in ethanol(EtOH), ethanol–water 1:15 (ca. 10�6 mol l�1, maximum absorbance ca.0.015 for a 1 cm optical path) and of the complex with I8C12 in water.

A. Lodi et al. / Journal of Molecular Structure 846 (2007) 49–54 53

of hydrophobic interactions and form a closed ‘hydropho-bic lid’ [6]. As a result, the only way for a guest to gainaccess into the host is through the hydrophilic upper rim:only inclusion complexes of type a (Scheme 2) can beobtained. Such complex rearrangements of the long alkylchains cannot, of course, be accounted for by our MDsimulations which did not include the solvent. NMRspectroscopy is the most appropriate and widely employedtechnique to investigate the structure and the binding prop-erties of calixarenes (for recent references, see [15,41–43]).However, we could not resort to NMR in this case becauseof the low concentrations of I8C12 attainable in water, notlarger than ca. 2 · 10�4 mol l�1, and the one-order-of-mag-nitude lower concentrations of the complex (see below).The investigation of the calixarene–merocyanine interac-tion in water was therefore based on UV–visible absorptionand emission spectroscopic and photophysical evidence.Upon addition of I8C12 at concentrations ranging from10�5 to 1.7 · 10�4 mol l�1 to a suspension of Mc1 in water,the dye was solubilized in a constant 1:12 mol ratiowith total added calixarene (the host–guest complexconcentration was evaluated spectrophotometrically (seeFigs. 1 and 5) assuming an extinction coefficient of thecomplex equal to that of the dye in ethanol). On the con-trary, no Mc2 absorption was detected in water in the sameconditions. In this concentration range, I8C12 does notaggregate into large micelles [19] but rather behaves as a‘unimolecular micelle’, i.e., it provides a water-free regionto host hydrophobic guests [3,4,6]. The Mc1/I8C12 absorp-tion spectra in water (Figs. 1 and 5) clearly show the dye tobe monomeric in the dissolved complexes. Furthermore,the maximum occurs at 432 nm, nearer to the absorptionmaximum featured by Mc1 in ethanol (426 nm) than inthe 1:15 ethanol–water mixture (ca. 440 nm for themonomer band, likely blue-shifted by the pronounceddimer band around 410–425 nm, see Fig. 5).

A similar trend is exhibited by the emission maxima:454, 460 and ca. 480 nm (the latter figure is made uncertain

by the overlap of the monomer with the stronger H-aggre-gate emission with maximum around 495–500 nm, seeFig. 5). This spectroscopic behaviour suggests that, in thecomplex, the chromophoric region of Mc1 interacts witha polar organic environment, rather than an aqueousone. The emission spectrum confirms that Mc1 is mono-meric in the complex. Its quantum yield was found to be2.6 times that of Mc1 in DCM, i.e., more than two timeslarger than that of the complex in DCM: this differencesupports the different nature of the complexes in the twosolvents.

The selectivity of the solubilising effect of I8C12 towardsMc1 strongly suggests that an inclusion complex of type a(Scheme 2), rather than an aspecific, ‘external’ adduct, isformed by the two in water [44]. In fact, only the smallerMc1 chromophoric unit satisfies the strict geometricalrequirements for inclusion of the guest within the host cav-ity formed by the aromatic rings; to do likewise, Mc2would have to undergo a highly energetically demandingdeformation of the conjugated chromophoric unit. Thespectroscopic/photophysical observations reported (con-stant dissolved dye/total calixarene mole ratio, absorptionand emission maximum positions, absence of dye H-aggre-gation, increase of fluorescence quantum yield) provideconsistent support for this conclusion.

5. Conclusions

Complexation of both merocyanines Mc1 and Mc2 bythe water-soluble calixarene I8C12 takes place readily inorganic solvents. The equilibrium constants for such pro-cess are very little dependent on both the guest and the sol-vent natures. In addition, minor changes in thespectroscopic and photophysical properties of Mc1 andMc2 (apart from a pronounced increase in the fluorescenceanisotropy) occur upon complexation in several organicsolvents. These features are consistent with the complexesassuming structures of the b-type, as predicted by MD sim-ulations (Fig. 4), in which the merocyanine chromophoresdangle out of the host ‘basket’ and the driving force tocomplexation resides in van der Waals interactions mainlyinvolving the long alkyl chains of the guests interpenetrat-ing those of the host. In water solubilization of Mc1 byI8C12 becomes specific, as the only slightly bulkier Mc2remains insoluble even in the presence of a large excessof calixarene. Spectroscopic and photophysical evidencesuggests that Mc1 is complexed as a monomer and the cal-ixarene acts as a ‘unimolecular’ micelle. However, in thecomplex the guest chromophore experiences an organicrather than an aqueous environment. These observations,combined with the known tendency of the hydrophobicportion of the calixarene to form a closed pocket, demanda qualitative change in the structural and energetic featuresof complexation with respect to the behaviour in organicsolvents. This qualitative difference is likely attributableto formation in water of complexes of the a-type (Scheme

54 A. Lodi et al. / Journal of Molecular Structure 846 (2007) 49–54

2) in which only the smaller Mc1 chromophore can fit intothe host cavity.

In conclusion, spectroscopic and photophysical mea-surements, combined with MD structural simulations, haveprovided a reasonable explanation for the deep changes inthe complexation of merocyanines Mc1 and Mc2 by calix-arene I8C12 observed upon moving from organic to aque-ous environments, including a selective solubilization ofthe smaller guest molecule in water.

Acknowledgement

This work was supported by the Grant FIRBRNBE01P4JF from the Italian Ministry of Educationand Research (MIUR).

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