Role of the conformational freedom of the skeleton in the complex formation ability of resorcinarene derivatives toward a neutral phenol guest

Article abstract of DOI:10.1021/jp111145d

The interaction of phenol guest molecules with 2-methylresorcinarene and its methylene-bridged cavitand derivative has been investigated in methanol. The host molecules were selected according to the flexibility of their cavities by varying the conformational freedom of the molecular skeleton prior to molecular association. The results show stronger host - phenol interactions when the host molecule possesses a rigid molecular skeleton (i.e., cavitand) compared to that of the flexible resorcinarene with phenol. Although the enthalpy change associated with the molecular interactions was found to be the same in both cases, higher negative entropy change was obtained when the resorcinarene interacted with the phenol molecules at room temperature. As a result, stronger host - guest complexes are formed at room temperature when the host molecules, possessing a rigid molecular skeleton, participated in the complex formation. Furthermore, since the higher entropy change results in higher temperature-dependence of the interactions, the stability of the complexes formed with the flexible resorcinarene is smaller at higher temperature. These results highlight that the decreasing flexibility of the host molecular skeleton itself can determine the entropy change during the complexation process; therefore, the temperature dependence of the complex stabilities highly depends on the flexibility of the host's molecular skeleton. This information might contribute to the development of selective and sensitive sensor molecules toward phenol derivatives. ? 2011 American Chemical Society.

Full text of DOI:10.1021/jp111145d

ARTICLE
pubs.acs.org/JPCB
Role of the Conformational Freedom of the Skeleton in the Complex
Formation Ability of Resorcinarene Derivatives toward a Neutral
Phenol Guest
Sꢀandor Kunsꢀagi-Mꢀatꢀe,*,† Zsolt Csꢀok,‡ Koichi Iwata,§ Erzsꢀebet Szꢀasz,† and Lꢀaszlꢀo Kollꢀar‡
†Department of General and Physical Chemistry and ‡Department of Inorganic Chemistry, University of Pꢀecs, Ifjꢀusꢀag 6,
H-7624 Pꢀecs, Hungary
§Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588 Japan
S Supporting Information
b
ABSTRACT: The interaction of phenol guest molecules with
2-methylresorcinarene and its methylene-bridged cavitand de-
rivative has been investigated in methanol. The host molecules
were selected according to the flexibility of their cavities by
varying the conformational freedom of the molecular skeleton
prior to molecular association. The results show stronger
hostꢀphenol interactions when the host molecule possesses a
rigid molecular skeleton (i.e., cavitand) compared to that of the
flexible resorcinarene with phenol. Although the enthalpy change associated with the molecular interactions was found to be the
same in both cases, higher negative entropy change was obtained when the resorcinarene interacted with the phenol molecules at
room temperature. As a result, stronger hostꢀguest complexes are formed at room temperature when the host molecules,
possessing a rigid molecular skeleton, participated in the complex formation. Furthermore, since the higher entropy change results in
higher temperature-dependence of the interactions, the stability of the complexes formed with the flexible resorcinarene is smaller at
higher temperature. These results highlight that the decreasing flexibility of the host molecular skeleton itself can determine the
entropy change during the complexation process; therefore, the temperature dependence of the complex stabilities highly depends
on the flexibility of the host’s molecular skeleton. This information might contribute to the development of selective and sensitive
sensor molecules toward phenol derivatives.
’ INTRODUCTION
In our previous works, the effects of the cavity shape19,20 and
also the solvent effect21 on the stability of calixarene, phenol or
calixarene, benzotrifluoride complexes were determined. The
effects of the solvent permittivity were examined in detail for
primary alcohols.22 The dependence of the molecular interac-
tions on the electron density of the guest molecules highlighted
the intricate behavior of the complex formation between species
possessing aromatic moieties.23,24 In particular, the solvent itself
can significantly modify the interactions due to the entropy term
associated with the thermodynamic equilibrium. This property
becomes more complex when the solvation shell of the interacted
species is built up by the molecules of protic solvents.25,26 The
overall entropy change of the interactions is determined as a
competitive effect between the entropy change of the interacting
species and the entropy change of the solvent molecules.
Calixarenes1 and the related resorcinarenes2 are members of a
fascinating class of macrocycles which have potential applications in
sensor chemistry. This property is due to the ability of these
molecules to form complexes with both cations and anions and also
with neutral molecules. Calixarenes are widely used as electro-
chemical,3 fluorescent sensors,4 and their sensing potential ranges
from toxic metals5 to proteins.6 Resorcinarenes were shown to
possess good sensitivity toward aliphatic alcohols,7 while the structu-
rally more rigid cavitands have been successfully employed in the
detection of gases and organic vapors.8,9 Recent studies show their
application on deposited films10 and on silicon grafting.11
These molecules show considerable photoluminescence (PL)
properties according to their aromatic moieties.12 Their PL proper-
ties show characteristic changes during the interactions with other
species, which makes the examination of these molecular interac-
tions possible by the highly sensitive PL methods.13,14 Several
factors that control the sensitivity and the selectivity characters of
these host molecules have been studied during the last decades.15ꢀ18
These investigations revealed that compounds with appropriate host
properties can be developed. That is, the type and strength of
hostꢀguest interactions can be finely tuned by the systematic
variation of the molecular skeleton.
In this work, the particular role of the entropy change of two
host molecules has been investigated: the conformationally
flexible methylresorcinarene (1a) and its methylene-bridged
cavitand analogue (1b). In the cavitand 1b, the rigid molecular
Received: November 22, 2010
Revised:
March 1, 2011
Published: March 15, 2011
r
2011 American Chemical Society
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Figure 1. Complexation ability of host resorcinarene 1a and cavitand 1b toward phenol were investigated in this work.
cavity cannot change significantly during the molecular interac-
tions, while in the resorcinarene 1a significant decreases the
freedom of the molecular skeleton that takes place during the
interactions with the phenol guest. According to our previous
investigations, PL methods and van’t Hoff theory are applied to
determine the thermodynamic parameters associated with the
formation of the hostꢀguest complexes.
temperature:
ΔH 1 ΔS
ð2Þ
ln K ¼ ꢀ
þ
R T
R
The enthalpy change can be obtained from the slope, while the
entropy change can be determined from the intercept of the
line fitted to the experimental data.
’ MATERIALS AND METHODS
’ RESULTS
Resorcinarene 1a27 and cavitand 1b28 were synthesized ac-
cording to literature procedures (see Figure 1). Phenol 2 (p.a.
grade) and methanol were purchased from Merck (Germany)
and from Panreac (Spain), respectively, and were used without
further purification.
Photoluminescence Measurements. A highly sensitive
Fluorolog τ3 spectrofluorometric system (Jobin-Yvon/SPEX)
was used to investigate the photoluminescence (PL) spectra
of different solutions. For data collection, a photon counting
method with 0.2 s integration time was used. Excitation and
emission bandwidths were set to 1 nm. A 1 mm layer thickness
of the fluorescent probes with front face detection was used to
eliminate the inner filter effect.
In order to investigate the interaction of the hosts (1a and 1b)
with guest (2), 10ꢀ3 M stock solutions were prepared in
methanol. The stoichiometry of the complexes was checked by
the Job’s method: samples were prepared as mixture of
the appropriate stock solutions of 1 and 2 with 1:9, ...9:1 molar
ratios. The PL spectra were recorded at 5 different temperatures
within the range from 288.16 to 308.16 K with 5 K steps.
The data evaluation method was described earlier.19ꢀ24 How-
ever, because of the fact that in this particular case the complexes
formed possess 1:1 stoichiometry (see below), the Benesiꢀ
Hildebrandt method was applied for data evaluations.29 Accord-
ing to the BenesiꢀHildebrand equation, if the complex formed
possesses 1:1 stoichiometry, the plot of the reciprocal PL change
against the reciprocal concentration of one of the reactant should
show a straight line. Therefore, the equilibrium constant can be
determined by the linear fitting of these data points using the
following equation:
Figure 2 shows the PL spectra of the solutions of hosts (1a and
1b) both in the absence and in the presence of guest phenol 2
molecules. Excitations at 280 and 380 nm were used for these
studies, and the emission spectra were recorded within the
290ꢀ700 nm spectral range. Both host molecules show emission
peaks at 305 and 460 nm using the appropriate excitations at 280
and 380 nm, respectively. The shapes of the emission bands are
the same for both 1a and 1b. However, while the emission
spectra at 305 nm are nearly the same for both macrocycles,
much higher emission intensity of the flexible resorcinarene (1a)
was obtained at 460 nm using the 380 nm excitation (Figure 2).
The presence of phenol guest molecules changes the peak
intensity of the host molecules at 305 nm due to the considerable
emission of phenol molecules at the same wavelength. Since the
intensities of the emission peaks observed at 460 nm show a
significant decrease in the presence of phenol, this peak was used
to determine the molecular interactions.
The application of the BenesiꢀHildebrand method assumes
1:1 stoichiometry of the complexes formed during the molecular
interaction. The Job’s method was applied to prove the 1:1
stoichiometry of the hostꢀguest complexes. Stock solutions of
10ꢀ3 M of host molecules and 10ꢀ3 M of phenol were used for
these studies as described in the Materials and Methods. Figure 3
shows the Job’s plot of mixtures formed from the solutions of 1a
and 1b with the solutions of phenol 2. The temperature
dependence of this plot validates the 1:1 stoichiometry of the
complexes through the temperature range of the examinations
(288.16 K ...308.16 K).
The thermodynamic parameters of the interactions between
the hosts 1a or 1b and phenol 2 molecules were determined as
follows. The stability constants of the hostꢀguest complexes
were determined at five different temperatures. Then, with
application of the van’t Hoff theory, the logarithm of the stability
constants was plotted against the reciprocal temperature and a
straight line was fitted to these data (Figure 4a,b). According to
the results of the Job’s method, the 1:1 stoichiometry of the
complexes results in the linear dependence on the Benesiꢀ
Hildebrand plot. The stabilities of the complexes are decreased
at elevated temperatures. Overall, the complexes formed with
1a resorcinarene show smaller stability constants at all tempera-
tures than the phenol complexes of 1b cavitand. The thermo-
dynamic parameters were determined by the van’t Hoff theory,
1
1 1 1
1
¼
þ
ð1Þ
I ꢀ I0
½aꢁ ½pꢁ K ½aꢁ
where [p] and [a] represents the analytical concentration of
phenol 2 and hosts 1a or 1b, respectively. I is the intensity of the
samples containing both the reactants, while I0 is the PL intensity
of the host’s solution in the absence of phenol.
The temperature dependence of the equilibrium constants
were employed to determine the enthalpy and the entropy
change of the association reaction using the van’t Hoff theory.
According to the van’t Hoff equation, the logarithm of the
equilibrium constants was plotted against the reciprocal
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Figure 2. PL spectra of hosts 1a (left) and 1b (right) in the absence and in the presence of 2 phenol. Spectra were recorded at 298.16 K.
Figure 3. Job’s plot of the interaction of hosts 1a and 1b with phenol 2 molecules. Stock solutions (10ꢀ3 M) of the hosts (1a and 1b) and guest (2) were
prepared in methanol, then the mixture of the appropriate stock solutions of 1 and 2 with 1:9, ...9:1 molar ratios were prepared. PL spectra of such
samples were recorded in 5 different temperatures within the range from 288.16 to 308.16 K with 5 K steps (see Table 1). Differences between the
intensities of the mixture and the pure host solutions measured at λ = 460 nm were divided by the PL intensities of the pure hosts and plotted against the
molar fraction of the host. Temperatures associated with the curves are increased from the top down.
and the results are plotted in the insets of Figure 4a,b. Accord-
ingly, quite the same enthalpy change obtained for the 1aꢀ2 and
1bꢀ2 complexes; they are ΔH = ꢀ47.4 ( 0.7 kJ molꢀ1 and
ΔH = ꢀ47.11 ( 1.1 kJ molꢀ1, respectively. However, very
different entropy changes are associated with the molecular
interactions. They are ΔS = ꢀ123 J Kꢀ1 molꢀ1 for the 1aꢀ2
complexes, and ΔS = ꢀ86 J Kꢀ1 molꢀ1 for the 1bꢀ2 complexes.
These results show that for the 1aꢀ2 complexes, the ordering
of the system is much higher after the molecular association than
in the 1bꢀ2 complexes. Our explanation for these properties is
the following: after the formation of the hostꢀguest complexes,
the skeletons of both 1a and 1b are fixed, i.e., the shape of the
cavity molecule cannot change anymore after the inclusion of
the phenol guest. However, in the case of resorcinarene 1a,
the shape of the molecular skeleton is determined only by
the tetrahedral distortion of the carbon bonds in the ethylidene
(CH3CH) bridge between the aromatic walls. In contrast,
the skeleton of 1b cavitand is further reinforced by a second,
methylenedioxy (OCH2O) bridge. As a result, cavitand 1b
has much more rigid structure compared to resorcinarene 1b.
According to the complex formation with phenol molecules, the final
state of the complexes has a similar entropy content to the initial
states, resulting in a higher decrease of the entropy when the more
flexible resorcinarene 1a forms complexes with phenols. The appro-
priate free enthalpies calculated at room temperature are ꢀ10.7 kJ
molꢀ1 and ꢀ21.5 kJ molꢀ1 for 1aꢀ2 and 1bꢀ2 complexes,
respectively. This phenomenon shows the presence of weaker
1aꢀ2 complexes compared to the 1bꢀ2 complexes at room
temperature. Therefore, 1b cavitand possesses a higher sensitivity
toward phenol around room temperature.
Taking into account the potential applications of these molecules
in chemical sensors, the results described here have considerable
consequences on the temperature dependence of the stability of the
complexes formed. Because of the fact that the stability is determined
by the free enthalpy change and since the temperature dependence of
the free enthalpy is determined by the entropy term within a narrow
temperature range, the devices prepared from 1a resorcinarene will
show higher temperature-dependence and less sensitivity at about
room temperature, compared to the devices based on the 1b
cavitands.
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Figure 4. BenesiꢀHildebrand plot of the interaction of hosts 1a and 1b with phenol 2 molecules. PL spectra of 10ꢀ3 M of 1a (left) or 1b (right) were
recorded in the presence of 2 guest, then the 1/(I ꢀ I0) were plotted against the reciprocal concentration of 2. (I and I0 are the PL intensities measured at
λem = 460 nm of the 1 þ 2 samples and the pure 1, respectively.) Temperatures associated with the curves are increased from the top down. Van’t Hoff
plots of the stability constants were inserted as insets.
Table 1. Temperature-Dependence of the Stability Constants
Associated with the Interaction of Hosts 1a or 1b with Phenol 2
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’ ASSOCIATED CONTENT
S
Supporting Information. Raw data of the PL intensities
b
measured during the interaction of hosts resorcinarene 1a or
cavitand 1b with the phenol 2 guest molecules and statistics
about the data evaluation plotted in Figure 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ ACKNOWLEDGMENT
Financial support by the Japan-Hungarian Intergovernmental
S&T Cooperation Programme (Project No. JP-14/2009) and
Science, Please! Research Team on Innovation (Grant SROP-
4.2.2./08/1/2008-0011) are gratefully acknowledged.
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