Organic &Biomolecular Chemistry

PAPER

Cite this: Org. Biomol. Chem., 2013, 11,1318

Received 23rd May 2012,Accepted 14th December 2012

DOI: 10.1039/c2ob27164h

www.rsc.org/obc

Analysis of guest binary mixtures by tert-butylcalix[6]-arene using host memory of previously bound guests†

Goulnaz D. Safina,a Marat A. Ziganshin,a Aidar T. Gubaidullinb andValery V. Gorbatchuk*a

A new principle of quantitative and qualitative analysis of binary organic mixtures is offered, which is

based on an ability of calixarene receptor for specific polymorphic transitions related to the composition

of the analyzed guest mixture. The ability of tert-butylcalix[6]arene to remember selectively some guests

bound from headspace both of pure liquids and their binary mixtures is used. The image of guest

mixture remains written in metastable polymorphs of host after partial or complete guest elimination

from clathrates. The memory was read using differential scanning calorimetry as the enthalpy of exother-

mic polymorphic transition of host collapse. This enthalpy monotonously changes with the variation of

guests’ ratio in mixture, unlike the enthalpies of endothermic pseudopolymorphic transitions of guest

release. So, the composition of volatile binary mixture can be estimated using only one receptor and only

one its parameter even in absence of preferential binding from a binary mixture of guests. This is an

example of a genuine molecular recognition.

Introduction

A search of new experimental approaches giving molecularrecognition of small neutral molecules is of major importancefor a number of applications including medical diagnosticsand odor detection.1,2 A usual key-to-lock recognition schemerequires a substrate (guest) to have at least two specificallyoriented functional groups capable of H-bonding or donor–acceptor interaction with receptor (host) having complemen-tary structure.3,4 The substrate, which fits to such lock, is pre-ferentially bound. However, only moderate selectivity isobserved in many cases, especially in mixtures, which is a truetest of molecular recognition.5,6 Partial improvement can bereached by using receptor arrays.7 Still, this approach givespoor results for recognition of relatively inert compounds.

To reach higher selectivity, clathrate-forming hosts, likecalixarenes, were tried, which offer a variety of possibilities.3

Being a little more selective than rubbery polymers,3 crystallinereceptors have problems with guest-binding irreversibility,host thermal history8,9 and mutual influence of the guests in amixture on each others’ binding.10–12 This makes their

preferential binding ability not good enough for recognitionpurposes. So, some other properties of calixarenes may beused. Thus, two-step formation of clathrates gives a genuinemolecular recognition of benzene in mixtures with its closehomologues just by number and position of guest-bindingsteps in sensor experiment.13

Another promising property of some calixarenes is theirability for induced polymorphism.14,15 If this polymorphism isinduced by a previously bound guest, it may be veryselective16–19 being equivalent to host memory of a particularguest compound eliminated from host–guest clathrate. Afterclathrate decomposition, tert-butylcalix[6]arene,16 adamantyl-calix[4]arene18 and some thiacalix[4]arene derivatives17,19 canform loose metastable polymorphs, Fig. 1a, which parameters(enthalpy and temperature of exothermic collapse) stronglydepend on the guest molecular structure. Thus, an eliminatedguest leaves its image, or imprint, in the guest-free host phase.The written memory can be read in a DSC experiment. Onlyfew guests can be remembered in such a way, while mostothers do not induce any memory, Fig. 1b.

In the present work, the applicability of this memory-basedapproach was studied for binary mixtures of guests with closemolecular structure. One of two guests in each studied mixturecan be remembered by studied host, tert-butylcalix[6]arene (1),after formation and decomposition of single clathrate, whilethe other one does not have such property, Fig. 1. With thepotential ability of both guests entering the same clathratecrystal, Fig. 1c, the problem is to find, which trait is dominant:that of remembered guest or of non-remembered one? Such

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ob27164h

aKazan Federal University, A.M. Butlerov Institute of Chemistry, Kremlevskaya, 18,

420008 Kazan, Russia. E-mail: Valery.Gorbatchuk@ksu.ru; Fax: +7 843 2337416;

Tel: +7 843 2337309bA.E. Arbuzov Institute of Organic and Physical Chemistry, Akad. Arbuzova 8,

420088 Kazan, Russia

1318 | Org. Biomol. Chem., 2013, 11, 1318–1325 This journal is © The Royal Society of Chemistry 2013

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mutual influence creates a possibility for detection of com-pounds in mixtures, with the host keeping an image not onlyof a target guest but also of its concentration in the presenceof other guests. This can give a new principle of molecularrecognition of a substrate without an ordinary key-to-lockmechanism and preferential binding.

Experimental sectionMaterials

tert-Butylcalix[6]arene (1) purchased from Fluka (No. 19723)was purified from nonvolatile impurities using multiple recrys-tallization from toluene, and from volatile impurities by

heating in vacuum of 100 Pa during 30 min at 230 °C and thenfor 6 h at 200 °C. This gives a thermally stable form of host 1(α-phase).16 The purity of the calixarene 1 powder was checkedas described elsewhere.18 Purified guests20 had at least 99.5%purity.

The samples of mixed clathrates (7–17 mg) were preparedin the aluminum crucibles (40 μl) by the saturation of calix-arene 1 powder (α-phase) with vapors created by a large excess ofbinary liquid mixtures of volatile compounds (100 μl). Equili-bration was performed in hermetically sealed vials of 15 ml for72 h at 298 K. In this equilibration process, liquid mixtureremained mostly unevaporated, and its composition did notessentially changed due to evaporation and binding. Alsosingle clathrates of calixarene 1 with benzene, cyclohexane,carbon tetrachloride and chloroform were prepared by the satu-ration of host with guest vapors at relative vapor pressure ofP/P0 = 1, as described elsewhere.18 Here, P is partial vaporpressure of guest, and P0 is its saturated vapor pressure.

Methods

Simultaneous thermogravimetry and differential scanningcalorimetry (TG/DSC) combined with evolved gas analysis bymass spectrometry (MS) were performed for clathrate samplesusing thermoanalyzer STA 449 C Jupiter (Netzsch) coupledwith quadrupole mass-spectrometer QMS 403 C Aeolos asdescribed elsewhere.18

The TG/DSC/MS experiment for these samples began after15–20 min of their equilibration in argon flow (75 ml min−1)inside the oven of thermoanalyzer at room temperature. Ineach experiment, the temperature rate was 10 K min−1. Com-positions of mixed and intermediate clathrates were calculated

Fig. 1 Memory-based molecular recognition of volatile guests: (a) positive response to guest remembered by host; (b) negative response to non-rememberedguest; (c) positive response to guest mixture of remembered and non-remembered guests with the last one having dominant influence on the host packing.

Organic & Biomolecular Chemistry Paper

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by comparison of mass loss and peak area ratios on the thermo-gravimetric curve and ion thermogram, respectively, on thefirst and second steps of guests elimination. The guest con-tents values were determined with 2% error but no less than0.02 mol of guest per 1 mol of host. The error of the enthalpydetermination for polymorphic and pseudopolymorphic tran-sitions is 1 kJ·mol−1 for host 1.

All powder X-ray diffraction data (XRPD) were collected onBruker D8 Advance diffractometer equipped with Vantec linearPSD, using Cu Kα radiation (40 kV, 40 mA). Room temperaturedata were collected in the Bragg–Brentano mode with a flat-plate sample. The samples of host 1 clathrates with cyclo-hexane and most clathrates with c-C6H12–C6H6 mixtures werelightly grounded and loaded into a standard sample holder.Other clathrate samples were studied on the flat silicon plate.X-ray patterns were recorded in the 2θ range in 0.008° steps,with a step time of 0.5 s.

Thermodynamic activity P/P0 of benzene dissolved in poly-ethyleneglycol PEG-400 was determined by headspace GCanalysis as described elsewhere.21

Results and discussion

To study the concentration effect of guest mixture on the prop-erties of host saturation products, α-phase of calixarene 1 wasequilibrated with headspace of binary liquids: CCl4 (1)/CHCl3(2) and C6H6 (1)/c-C6H12 (2). In each mixture, host 1 canremember only component (1) after its elimination fromclathrate.16

The saturation products were studied using simultaneousTG/DSC/MS analysis. The most representative TG and DSCcurves from these experiments are shown separately in Fig. 2and 3. Full data are given in ESI.† The obtained parameters ofthe studied systems are given in Tables 1 and 2, including thecomposition of clathrates S, mass loss Δm in each step ofclathrate decomposition, enthalpy of the second guest elimi-nation step, ΔH, and enthalpy of polymorphic transition, ΔHcol.

Both enthalpies are given per 1 mole of host. End points ofthe first decomposition step and temperatures of DTG-peaksare given in ESI.†

Each studied pair of guests has a concentration range,where their saturation product with 1 has two exothermicpeaks in DSC curve: at Tcol = 186 ± 3 and 253 ± 3 °C for C6H6/c-C6H12, and at Tcol = 193 ± 3 and 249 ± 3 °C for CCl4–CHCl3mixture, Fig. 3. This range of liquid concentration is 0–22 vol%of c-C6H12 in C6H6 and 0–8 vol% CHCl3 in CCl4. The secondexothermic peak on each DSC curve is not analyzed here beingtoo small for quantitative estimations (−1/−4 kJ·mol−1, ESI†).Besides, the enthalpy of the first guest elimination step is notconsidered in the present work, because of the big error ofdetermination.

Once temperatures of host collapse, Tcol, do not depend sig-nificantly on the guest mixture composition, Fig. 3, the valuesTcol may be used for qualitative recognition of benzene ortetrachloromethane. Other guests remembered by 1 have thehigher Tcol values.

16

The enthalpy of the first exothermic peak of host collapse,ΔHcol, changes with concentration of saturating mixtures,φ (vol%), in a sigmoidal way, Fig. 4. This shape of ΔHcol con-centration dependence is similar to the sigmoidal shape ofvapor sorption isotherms by calixarenes and other clathrateforming hosts,13,16,18,21–23 which indicates a phase transitionof clathrate formation. Respectively, one can conclude thepresence of two solid phases near the half-wave points of theplots on Fig. 4.

The enthalpy of the second guest elimination step, ΔH, mayhave a more complex dependence on the concentration ofsaturating mixtures. First, ΔH values increase with the increaseof component (2) contents but in a wider concentration rangethan ΔHcol value does. Then, for the C6H6–c-C6H12 mixture, nosignificant change of ΔH value takes place, while for the CCl4–CHCl3 mixture, a decrease of this value is observed, Fig. 4. So,the analysis of ΔHcol concentration dependence is a simplerproblem.

The shape of TG curves obtained complies with the con-clusion made above on the phase transition. Below the

Fig. 2 TG curves for products of calixarene 1 saturation with headspace ofliquid (a) cyclohexane, benzene and their mixtures, (b) chloroform, tetrachloro-methane and their mixtures. Curves are marked by φ (vol%) values for(a) c-C6H12 and (b) CHCl3 in guest mixtures.

Fig. 3 DSC curves for products of calixarene 1 saturation with headspace ofliquid (a) cyclohexane, benzene and their mixtures, (b) chloroform, tetrachloro-methane and their mixtures. Curves are marked by φ (vol%) values for(a) c-C6H12 and (b) CHCl3 in guest mixtures.

Paper Organic & Biomolecular Chemistry

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concentrations of saturating mixtures, corresponding to thehalf-wave points of ΔHcol vs. φ plots, Fig. 4, the saturation pro-ducts have the coinciding initial parts of TG curves, Fig. 2,which are differing only by the end points of the first guestelimination step (ESI†). Above φ values of these half-wavepoints, TG curves have coinciding parts corresponding to thesecond step of guest loss. So, below the half-wave concen-tration, the saturation product behaves as a clathrate with com-ponent (1), benzene or tetrachloromethane. Above this point, abehavior similar to that of clathrate with component (2), cyclo-hexane or chloroform is observed.

To check if this phase transition is really caused by com-ponent (2) additive, not by decrease of component (1) activity,we studied a product of host 1 saturation by excess of benzene

Table 1 The data of simultaneous TG/DSC/MS analysis of clathrates prepared by saturation of calixarene 1 with vapors of single and mixed cyclohexane andbenzene

φ (c-C6H12) (vol%) Δma (%) SBzb Sc-Hex

b ΔHcol (kJ·mol−1) ΔH (kJ·mol−1)

0 18.64 (18.64) 2.86 (2.86) 0 −36 02 18.67 (18.67) 2.81 (2.81) 0.05 (0.05) −35 05 18.42 (17.98) 2.68 (2.63) 0.13 (0.10) −35 26 18.32 (17.93) 2.63 (2.59) 0.15 (0.13) −34 18 17.85 (16.19) 2.39 (2.30) 0.19 (0.14) −34 410 16.69 (15.05) 2.19 (2.06) 0.29 (0.17) −30 912 16.02 (13.82) 2.00 (1.84) 0.35 (0.19) −28 1314 14.76 (11.42) 1.74 (1.51) 0.39 (0.15) −23 2116 11.92 (6.18) 1.10 (0.77) 0.55 (0.10) −12 3618 10.28 (2.96) 0.78 (0.36) 0.60 (0.05) −6 4820 9.43 (1.12) 0.62 (0.15) 0.63 (0.00) −2 5422 9.96 (2.26) 0.64 (0.26) 0.69 (0.05) −4 6124 8.87 0.42 0.73 0 6928 9.12 0.41 0.78 0 7430 9.49 0.43 0.81 0 8050 10.08 0.37 0.96 0 77100 13.51 (3.64) 0 1.81 (0.49) 0 79

a In brackets, the mass loss on the first decomposition step is given. b In brackets, molar amount of benzene (SBz) or cyclohexane (Sc-Hex) isshown, which is lost by 1 mol of host in the first decomposition step.

Table 2 The data of simultaneous TG/DSC/MS analysis of clathrates prepared by saturation of calixarene 1 with vapors of single and mixed chloroform andtetrachloromethane

φ (CHCl3) (vol%) Δma (%) SCCl4b SCHCl3

b ΔHcol (kJ·mol−1) ΔH (kJ·mol−1)

0 37.53 (36.32) 3.80 (3.68) 0 −30 42 37.57 (36.09) 3.66 (3.52) 0.19 (0.18) −28 64 37.17 (33.76) 3.51 (3.18) 0.30 (0.28) −20 226 36.85 (32.56) 3.39 (2.99) 0.39 (0.35) −17 268 31.44 (23.18) 2.59 (1.87) 0.40 (0.35) −7 4612 29.68 (20.53) 2.27 (1.50) 0.51 (0.44) −3 5615 25.10 (14.13) 1.77 (0.91) 0.45 (0.37) −1 5920 19.37 (6.04) 1.17 (0.24) 0.45 (0.31) 0 6856 15.38 (2.40) 0.54 (0) 0.79 (0.23) 0 7270 14.72 (2.09) 0.35 (0) 0.96 (0.20) 0 69100 12.63 (6.98) 0 1.18 (0.65) 0 55

a In brackets the mass loss on the first decomposition step is given. b In brackets, molar amount chloroform (SCHCl3) or tetrachloromethane(SCCl4) is shown, which is lost by 1 mol of host in the first decomposition step.

Fig. 4 Dependence of (○) clathrate decomposition enthalpy, ΔH, for 2nd step,and (●) enthalpy of the exothermic host 1 collapse, ΔHcol, after the 1st step ofclathrate decomposition on the composition (in vol%) of the saturating liquidmixtures of guests: (a) cyclohexane (2) + benzene (1), and (b) chloroform (2) +tetrachloromethane (1). The lines are drawn to guide the eye.

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vapor at relative vapor pressure P/P0 = 0.79 and 298 K. ThisP/P0 value corresponds to benzene activity in its liquid mixturewith 24 vol% of c-C6H12, which is the end point of phase tran-sition range, Fig. 4, Table 1. Benzene activity coefficient in thismixture is γ = 1.24 In this alternative experiment, host 1powder was equilibrated with vapor of benzene solution inPEG-400, and then benzene activity in the equilibrated systemwas determined using headspace GC analysis. Thermogram ofthus prepared sample is given in Fig. 5.

The TG and DSC curves of this sample indicate that it doesnot differ much from a single-guest clathrate of benzene. Ahigh negative enthalpy of host collapse (ΔHcol = −32 kJ·mol−1)is observed which is only 11% lower than that of saturatedclathrate. Besides, the TG curve shows a one-step guestremoval from 1·2.92C6H6 clathrate, which also indicates a com-plete host saturation. So, the change in sample properties atvariation of saturating mixture concentration within the phasetransition range is caused not by reduction of guest (1) activity,but by the presence by unremembered guest (2). So, the con-centration dependence of host collapse enthalpy may be usedfor quantitative estimation of guest contents in the analyzedmixture.

Another problem is the nature of phases, participating inphase transition near a half-wave point on Fig. 4. To makeclear whether these phases are mixed clathrates, not the mech-anical mixtures of two single clathrates, the thermal behaviorof the next two clathrate samples was compared. The first onewas a mechanical mixture of two single clathrates preparedseparately by saturation of 1 (α-phase) with pure vapors ofchloroform and tetrachloromethane. This sample contains53% (w/w) of 1·3.80CCl4 and 47% of 1·1.18CHCl3. The secondsample was prepared by saturation of host 1 (α-phase)with vapor mixture of chloroform and tetrachloromethanecreated by 20 vol% solution of CHCl3 in CCl4. This mixturegives nearly the same guest ratio in solid host phase afterequilibration as in the first sample. The results of thermalanalysis for these samples are shown in Fig. 6. Thermo-grams of pure clathrates of 1 with CCl4 and CHCl3 are givenin ESI.†

These data, Fig. 6, perform a non-equivalence of thestudied samples. The mechanical mixture of clathrates showsan independent release of guests, and each resulting

thermogram is close to a simple sum of thermograms for puresingle guest clathrates from separate experiments, Fig. 6a.Exothermic peak on DSC curve (ΔH = −18 kJ·mol−1) is near ahalf of that for pure 1·3.80CCl4 clathrate, Table 2. Additivethermal behavior was also observed for mechanical mixture of1·2.86C6H6 and 1·1.81c-C6H12 clathrates, ESI.†

The thermogram of the second sample prepared by host 1saturation with CCl4–CHCl3 vapor mixture, Fig. 6b, is essen-tially different from those of mechanical mixture, Fig. 6a. Thehost memory for tetrachloromethane is completely absent inthe second sample according to the DSC curve, which per-forms the absence of exothermic transition. The TG curveresembles that of pure 1·1.18CHCl3 clathrate, ESI,† where themost amount of guest is evolved with DTG peak at 230 °C. Ionthermograms and TG curve indicate simultaneous release ofboth guests in two steps. The first step is at 105 °C, which islower than those of single clathrates with both guests. Themost of tetrachloromethane leaves the product of 1 saturationwith guest mixture at 237 °C, Fig. 6b, which corresponds to themain decomposition step of single clathrate with chloroform1·1.18CHCl3, ESI.† So, this mixed clathrate sample behaves asa single phase having no significant traces of single clathratewith CCl4.

A specific nature of this mixed clathrate is confirmed by itsX-ray powder diffractogram, Fig. 7a, sample 2, which is closerto that of single 1·1.18CHCl3 clathrate, sample 1, and is essen-tially different from diffractogram of single-guest clathrate1·3.80CCl4, sample 4, despite the mixed clathrate containsmore tetrachloromethane than chloroform, Table 2. Besides,diffractograms of samples 1 and 2 also have some noticeabledistinctions at 2θ = 5° and in the range from 16 to 19°. Thesample 3, prepared by saturation of 1 with mixture of 6 vol%CHCl3 in CCl4, has a large reflex at 2θ = 5.2°, which is observedfor mixed clathrate 2, but not for single clathrates (sample 1and 4). So, formation of mixed clathrates with CCl4/CHCl3

Fig. 5 The data of TG/DSC/MS analysis for host 1 sample saturated withexcess of benzene (m/z = 78) vapor having a reduced relative vapor pressure ofP/P0 = 0.79 at 25 °C.

Fig. 6 The data of TG/DSC/MS analysis (a) for mechanical mixture of host 1clathrates with CHCl3 and CCl4 and (b) for product of host 1 saturation by head-space of chloroform solution (20 vol%) in tetrachloromethane at 25 °C. Ion ther-mograms of CCl4 (m/z = 117) and CHCl3 (m/z = 83) are shown.

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mixture gives at least one new phase, not coinciding withphases of corresponding single clathrates. This conclusion isin agreement with non-monotonous change of shape both ofDSC and TG curves at the variation of guests’ ratio in theirmixture, Fig. 2b and 3b.

The packing of mixed clathrates of host 1 changes moremonotonously with variation of guest ratio in C6H6–c-C6H12

saturating mixture, Fig. 7b. The samples 3 and 4 prepared bysaturation of 1 with mixtures having cyclohexane contents≥15 vol% are practically isomorphous to the single clathrate1·1.81c-C6H12, sample 5. The sample 2 prepared by saturationof host 1 with mixture of 10 vol% c-C6H12 in benzene givesmixed clathrate phase having packing very close to that ofsingle clathrate 1·2.86C6H6, sample 1. So, the formation ofonly 2 clathrate phases may be concluded for this ternarysystem.

The dependence of guest mixture composition Z in satur-ation product on guest contents X in saturating binary mix-tures indicates a moderate selectivity of 1 for cyclohexane in itsmixture with benzene, and a low selectivity for CCl4/CHCl3pair, Fig. 8. The values Z and X are guest molar fraction in amixture bound by host and in initial binary liquid, respect-ively. The observed selectivity is not much different from thatof other clathrate-forming hosts studied elsewhere.25–29

Comparison of concentration dependencies of ΔHcol andZ(CHCl3) values on the concentration of chloroform in thesaturating binary liquid given in Fig. 4b and 8b, respectively,performs the possibility of molecular recognition of binarymixture even without preferential host–guest binding.

However, the obtained ion thermograms show a strongmutual influence of bound guests on their release parameters.This is a change of guest distribution between clathratedecomposition steps depending on the composition of thesaturating mixture, Tables 1 and 2. In each studied system,both bound guests are released simultaneously in one or twosteps, despite essential difference in elimination temperaturesobserved for respective single-guest clathrates. Within the tran-sition range 5–22 vol% of c-C6H12 (2) in C6H6 (1) and 2–20 vol%of CHCl3 (2) in CCl4 (1), each ion thermogram has 2 peaks

at temperatures, corresponding to the main decompositionsteps of single clathrates with components of each mixture,ESI.†

Above and below the transition range, both bound guestsare released by scenarios, which were set up by single clath-rates with components (2) and (1), respectively. For example, atcyclohexane concentrations in benzene ≤16 vol%, most ofbound benzene leaves clathrate at the first step with a peak at129 °C, Fig. 9a, corresponding to the peak of its release fromthe single clathrate, Fig. 2a, ESI.† Below 12 vol% of c-C6H12 inthis saturating mixture, also most cyclohexane is eliminated atthis step, Table 1. At the 24 vol% and higher concentrations,cyclohexane becomes a major component in solid phase,Table 1, and both guests are simultaneously released in onestep at much higher temperature of 230 °C, Fig. 9b, which cor-responds to a second step of cyclohexane elimination from itspure clathrate with 1, Fig. 2a, ESI.† So, the sample preparedusing this guest mixture may be considered as homogeneousenough, which is in agreement with the XRPD data describedabove, Fig. 7.

The samples prepared by saturation of 1 with CCl4 (1) +CHCl3 (2) mixture have even more cooperative dependence oftheir thermal behavior on the presence of component (2),chloroform. Already 8 vol% of chloroform in the initial saturat-ing mixture is enough to move a significant amount of bound

Fig. 7 X-ray powder diffractograms for products of calixarene 1 saturation with headspace of (a) liquid chloroform (1) and tetrachloromethane (4), mixtures with20 (2) and 6 (3) vol% of chloroform in tetrachloromethane; (b) liquid benzene (1) and cyclohexane (5), mixtures with 10 (2), 15 (3) and 50 (4) vol% of cyclohexanein benzene.

Fig. 8 Selectivity of guest binding by host 1 from binary vapor mixtures: (a)C6H6–c-C6H12, (b) CCl4–CHCl3.

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tetrachloromethane, 0.72 mol per mol host 1, to the seconddecomposition step at 222 °C, which corresponds to DTG peakof 1·1.18CHCl3 single clathrate (230 °C), Fig. 9c, ESI.†

However, in this sample, only 0.05 mol of CHCl3 leavesclathrate on the second step. Moreover, for chloroform con-tents in saturating mixture of 20 vol%, this guest and tetra-chloromethane are mutually exchanged by their steps ofelimination from mixed clathrate, Fig. 6b. Most of tetrachloro-methane leaves this clathrate at higher temperature of 237 °C,while most of chloroform is released at the first step withDTG peak at 105 °C. So, by heating this sample to 205 °C,one may prepare 1·0.88CCl4·0.07CHCl3 clathrate, losing tetra-chloromethane on 80 °C above the point of this guest releasefrom its single clathrate with 1. In this case, the tempera-ture of remembered guest elimination is defined by un-remembered guest, most of which had gone already at lowertemperature.

Conclusions

An ability of tert-butylcalix[6]arene to remember selectively pre-viously bound guests can be effectively used for molecularrecognition of such compounds in binary mixtures of guests,where the second guest is not remembered but still plays anactive role wiping the host memory of the first one. This pro-perty may be used for quantitative estimation of mixture

composition even in the absence of preferential binding of itscomponents.

While the guest release from tert-butylcalix[6]arene satu-rated by a mixture of 2 guests gives up to 4 pseudopolymorphicand polymorphic transitions, such behavior may be used alsoto increase the stability of guest encapsulation by host. Thecomplex mutual influence of 2 bound guests on their releasepoints may cause the formation of an intermediate clathratewith a given guest where only a small additive of the secondguest is present. The thermal stability of such clathrate wasfound to be much higher than that formed by saturation ofhost with pure guest.

Acknowledgements

This work was supported by Russian Foundation for BasicResearch (grant no. 11-03-01215-a) and the Russian Ministry ofScience and Education (State Contract no. 16.552.11.7008).

Notes and references

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