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The oligomer of 1,2,3-trihydroxybenzene with benzaldehyde
Reactive & Functional Polymers 55 (2003) 319–332 www.elsevier.com / locate / react
The oligomer of 1,2,3-trihydroxybenzene with benzaldehyde Jyotirekha G. Handique, Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology, Guwahati 781 039, India Received 1 August 2002; accepted 1 January 2003
Abstract A linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene was synthesised and characterized. The oligomer has a quinonic end group that leads to a solvatochromic property and interacts with a variety of amines. In aprotic medium the oligomer has two non-equivalent intra-molecular H-bonded quinonic states arising from keto–enol tautomerism, one having the carbonyl and olefinic group at the 1,4-disposition (X) whereas in the other the carbonyl and olefinic groups are at the 1,2-disposition (Y). Thus, in acetonitrile both forms exist and the X and Y states have their characteristic absorption maximum at 500 and 427 nm, respectively. In protic medium a highly ordered structure with the hydrophobic and hydrophilic part projecting apart produces a structure (Z) having inter-molecular hydrogen-bonding with solvent which has a characteristic absorption at 475 nm. Non-stoichiometric amounts of various primary, secondary and tertiary amines interact with the oligomer providing stability to either of the forms or generating an anion by which the absorption maximum changes. For example, the oligomer interacts with t-butylamine, and this interaction changes the absorption maximum to 566 nm. The solution on further treatment with mineral acid (HCl) gives a new absorption maximum at 475 nm with a distinct color change. The absorptions can be changed back and forth by alternate addition of hydrochloric acid and t-butylamine. However, the characteristic changes that take place in the optical spectra of the oligomer in acetonitrile with different amines are highly substrate-dependent. For example the amines like ammonia, ethylamine, iso-propylamine, n-butyl amine, t-butyl amine, ethylenediamine, and pyridine cause loss of absorption at 427 and 500 nm and lead to growth of new absorption at 566 nm; whereas amines like benzidine, o-diaminobenzene, m-iodoaniline, m-nitroaniline, and p-hydroxyaniline enhance the original absorbance at 427 nm at the cost of absorption at 500 nm without leading to a new absorption. Some amines like 1-naphthylamine, 1,7-diaminonaphthalene, p-iodoaniline, and p-methoxyaniline increase the absorbances at 427 and 500 nm, whereas aniline, o-methylaniline, m-methylaniline, and diphenylamine do not significantly affect the spectra of the oligomer. The electrochemical study of the oligomer with 1,4-phenylenediamine shows that redox potentials of the bound state of the oligomer with 1,4-phenylenediamine shifts from the unbound state, for example, 1,4-phenylenediamine with the oligomer has two quasireversible couples at E1 / 2 at 259 and 822 mV in contrast to the two reversible couples of free 1,4phenylenediamine at 334 and 863 mV, respectively. 2003 Elsevier Science B.V. All rights reserved. Keywords: Condensation oligomer; 1,2,3-Trihydroxybenzene; Benzaldehyde; Optical-switching; Amine-recognition; Solvatochromicity
*Corresponding author. Tel.: 191-361-690-321; fax: 191-361-690-762. E-mail address: [email protected] (J.B. Baruah). 1381-5148 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S1381-5148(03)00020-8
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1. Introduction Polyphenols with intervening carbon atoms have wide applications in analytical [1–9], biological [10,11] and material chemistry [12– 14]. The ability to recognize different guest molecules by the cyclic counterpart of such polyphenols, namely calixarenes, is well documented [15–20]. Depending on the number of phenolic units in the cyclic counterpart they can have different cavity sizes to accommodate guest molecules [21–24]. The conformational flexibility coupled with molecular recognition has been used for studying calixarenes as allosteric proton pumps [25]. Calixarenes also recognize ions [26–29] and can have p-cation interactions [30,31]. The calixarenes attached to quinonic units have interesting electrochemical and photochemical properties [32–34]. The interaction of calixarenes with C 60 and other derivatives of C 60 have broadened their application to material science [35]. Calixarenes having a macrocyclic part attached to the cyclic backbone act as ionophores [36]. The application of calixarenes as models to mimic dinuclear metallo-phosphodiesterase for phosphatediester cleavage has proved to be useful [37–39]. Calixarenes containing antibody mimics also have great potential [40]. Although the resurgence of application of cyclic oligomers of phenol and aldehydes (calixarenes) is vibrant, in contrast to this, analogous study on the open chain counterparts have not emerged as rapidly as that of the cyclic counterparts. The reason for this is the limited synthetic methods to overcome the multiple product formation during synthesis of open chain polyphenolic compounds and also their insolubility in common solvents [41–44]. Although there are several articles in which synthetic linear polyphenolic compounds having oxygen as the intervening atom are prepared through oxidative coupling reactions, studies on their physicochemical aspects lag far behind [45–47]. There are also interesting reports in which phenolic compounds with weak Hbonded or charge transfer interactions are built
to construct macromolecules having interesting properties [48]. The cyclic oligomer of 1,2,3trihydroxybenzene has been found to show liquid crystal behavior with potential application in optoelectronics [49]. The pioneering work on resorcinarene has established the aggregated structure and their association with their ammonium ion [50,51]. We have identified a linear oligomer of 1,2,3-trihydroxybenzene with benzaldehyde that has good solubility in common organic solvents for this study. The general formula of the oligomer covered in this study is shown in Fig. 1. The physicochemical property of the linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene (I) has several important aspects to be studied. For example, linear oligomers should also have the analogous property of a proton pump like their cyclic counterparts. The calix-
Fig. 1. Structural features of the linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene.
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arenes have less conformational flexibility while recognizing guests. The scope for reorganizing itself to bind multiple binding sites of a guest is also expected to be greater in a linear oligomer. Recognition of guest molecules by linear oligomers would throw light on the drug–substrate binding, detection and extraction of heavy metal ions [52,53] etc. The inter-molecular H-bonding interaction as well as intra-molecular interaction / s may also lead to stacks of aromatic rings of the oligomers to give structures having analogy to the base pairing in nucleotides [54– 57]. The oligomer derived from 1,2,3-trihydroxybenzene would have some analogy to the naturally occurring polyphenolic compounds containing 1,2,3-trihydroxybenzene unit that plays a crucial role in recognizing and binding guest substrates and also contributes to the coloration of natural pigments [58,59]. A few more important aspects in linear polyphenolic systems are the reversible binding of proline residues of peptides, the role as one electron oxidant and the ability as catalyst to hydrolyse a peptide containing a proline residue [60].
2. Experimental The 1 H NMRs were recorded on a 400 MHz Bruker or Varian spectrometer. The cyclic voltammograms were recorded with an electrochemical analyzer, CHI 660A, with three electrode systems comprising of Ag /AgCl reference electrode and two platinum electrodes as working and auxiliary electrodes. The measurements were done in dry acetonitrile (HPLC grade, distilled over CaH 2 ) with tetrabutylammonium perchlorate as supporting electrolyte. The EMF values are with reference to ferrocene as standard. The mass spectra were recorded with, as matrix, a-cyano-4-hydroxycinnamic acid using a Kratos PC-Kompact mass spectrometer. The UV–visible spectra were recorded by dissolving the stipulated amount of the substrate / s in appropriate solvent using a Hitachi UV–visible spectrometer U-2001. The interac-
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tions of amines were studied by recording visible spectra of the oligomer in acetonitrile solution. In each case definite amounts of substrate were dissolved in acetonitrile and added in aliquots with a microsyringe. The electronic spectral data on absorbance at each 0.1 nm interval were collected and used for simulation purposes with the aid of the IGOR software package on an IBM PC. For simulation purposes the matrices were constructed directly from the data of a normal Gaussian shape spectra or from the data of extrapolated peaks constructed by addition of the mirror image of one half of the spectra in the case of overlapping peaks. The matrices were constructed to find the component of each species with the aid of data available from the literature [3]. In these cases the package MATLAB was used in an IBM Pentium PC to obtain the relative contribution of each state.
2.1. Condensation reaction of 1,2,3 -trihydroxybenzene with benzaldehyde A solution containing benzaldehyde (1060 mg, 10 mmol), 1,2,3-trihydroxybenzene (1260 mg, 10 mmol) and oxalic acid (100 mg, 0.8 mmol) in dry ethanol (10 cm 3 ) was refluxed for 3 h. A viscous brick red solution was obtained along with a very small amount of white solid. The reaction mixture was filtered, the residue was discarded and the solvent was removed under reduced pressure to obtain a paste. The paste was then washed with ice-cold water (23 5 cm 3 ). A brick red viscous gel was obtained; the paste obtained after decantation of water was further dried under vacuum to obtain a brick red solid. Yield: 930 mg. Elemental analysis: calcd for C 13 H 10 O 3 ?1.25 H 2 O; C 65.95; H, 5.34, found C, 66.54, H 5.66; MALDI mass: (m /e) 2244.3, 2036.7, 1821.3, 1604.2, 1625.3, 1391.5, 1373.4, 1177.7, 1085.3, 981.8, 872.7 (100% intensity). Mn , Mw (THF) 1209, 1252. 1 H NMR 400 MHz (DMSO-d 6 ) 5.2–8.2 (m); IR (KBr) 3369 (bs), 2950 (w), 2900 (w), 1688 (m) 1623 (s), 1495 (s), 1464 (s), 1286 (s), 1207 (s),
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1068 (s), 1012 (s), 953 (w), 750 (m), 702 (s) cm 21 .
3. Results and discussion
3.1. Structural features of the oligomer The oligomer of 1,2,3-trihydroxybenzene and benzaldehyde (Fig. 1) was prepared by acidcatalysed condensation reaction. The literature on condensation reactions of an aldehyde with phenolic compounds suggests that the calix formation is the thermodynamically favorable process [27]. However, the possible ring opening of a cyclic counterpart as well as the termination of a chain in such oligomerisation reactions were not discussed in these reports. The reaction procedure reported earlier for formation of calixarenes from 1,2,3-trihydroxybenzene with aldehydes involves a long reaction time and drastic reaction conditions [48]. The reaction conditions investigated in the present report were different than the reported procedure for cyclic counterparts. We observed consistently reproducible results on the same type of product from several independent experiments. The MALDI mass spectra of the oligomer have the base peak that appears at 872.7 (m /e) corresponding to [–(C 6 H 5 )CH– C 6 H 4 O 3 ] 3 C 6 H 4 O 3 C 6 H 5 CH(OH) showing that it might have been released from a linear chain of the oligomer. It has the highest m /e peak at 2244.3 corresponding to a unit comprising of [–(C 6 H 5 )CH–C 6 H 4 O 3 ] 10 C 6 H 5 CH–, which is probably formed by association of smaller units comprising of five units of the structures shown in Fig. 1. This assumption is on the basis of the observation of the Mn value as 1209 in contrast to the higher molecular ion peak in MALDI mass. The GPC showed a single set of peaks having an Mn /Mw value close to unity indicating that the average chain length is half of the molecular ion peaks obtained in mass spectroscopy and the discrepancy arises from the association of the oligomer in solution. Since
the base peak occurs at 872.7 the oligomer is comprised of linear tetra- or pentamers. The 1 H-NMR integration of the signal of the end phenyl group when compared to the rest of the non-exchangeable protons, shows that the integration fits approximately to a pentameric structure. The oligomer also has m /e peaks at 2036.7, 1821.3, 1604.2, 1391.5, 1178.7 corresponding to the loss of a –C 6 H 4 O 3 C 6 H 5 CH– unit from the highest mass ion of the parent oligomer. The observation of an OH unit attached to the CH-bond in the base peak suggests that the oligomer has an OH end group that may be present from the incompletely dehydrated end group. The oligomers also have water of crystallization and the presence of such water molecules in the oligomer is reflected in elemental analysis as well as in thermogravimetry. The thermogravimetry study suggests that on heating there is a loss of 8.5% weight in the region of 80– 200 8C from the oligomer. This loss occurs due to the loss of interstitial water molecules present in the system. This observation also suggests that the oligomer has a tendency to strong hydrophilic interaction. Such hydrophilic interaction and self-aggregation is reported for the resorcinarene [50,51]. The IR spectra of each of these oligomers shows strong and broad peaks for the OH groups centering in the region 3369 21 cm . The appearance of weak peaks at 2950 and 2900 cm 21 are attributed to strong hydrogen bonding present in the system. NMR spectroscopy helps in ascertaining the conformation and end group analysis. The 1 HNMR spectra of the oligomers are complicated in the aromatic region but the temperature-dependent study shows definite changes in the conformation. The 1 H-NMR spectra of the oligomer in two different solvents, namely methanol-d 4 and DMSO-d 6 , were recorded with a view to show the possibility of solvent effect on the structure. In addition to this, the methanol-d 4 with labile deuterium will exchange the OH groups and make the spectra less complicated. The 1 H-NMR spectra of the oligo-
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Fig. 2. 1 H-NMR in methanol-d 4 of I (a) at 260 8C, (b) at 30 8C.
mer in methanol-d 4 (Fig. 2a) as well as DMSOd 6 has two triplets and a doublet at a lower field that are assigned to the aromatic ring at the end of the oligomer backbone (Fig. 2). The integration value supports that they arise from the end group which would be a phenyl group attached to a methylene carbon as shown in Fig. 1. The product under consideration from the condensation reaction of benzaldehyde with 1,2,3-trihydroxybenzene is open chain as the termination of oligomerisation occurs due to dehydrated quinonic end group formation (Fig. 1); that occurs due to the nature of the reaction where a strong acid is used as catalyst for oligomerisation. In the case of DMSO-d 6 the phenolic OH groups of the oligomer overlap with the end group signals but use of methanol-d 4 allows the exchange of these protons with deuterium thereby making the end group show clear non-overlapping signals. The 1 H-NMR signals of the aromatic region of the oligomers are not clearly differentiated in 1D proton NMR. But the 2D-HOMO-COSY of the sample in two different solvents has clear differentiation in the correlation showing that the systems are in different conformations in
two different solvents. The presence of a differentiable aromatic ring at the end group in the oligomer is clear from the correlation of the signals at 7.4, 7.6 and 8.0 ppm. In this oligomeric system the peak separations of individual H are difficult because of the possibility of interacting and non-interacting conformational states, nevertheless the observation of highly symmetric spectra suggests orderliness and that regular conformational patterns are present in the system. The 2D-HOMO-COSY NMR recorded in methanol-d 4 clearly shows the correlation of the protons of the aromatic ring of the end group. The spread of correlation in methanol-d 4 shows that it spreads over a much broader region and it proves inter-molecular H-bonding interaction with the solvent. But the correlation in dimethyl sulphoxide is narrow suggesting the system has lesser flexibility of the conformers. The h 1 Hj 13 C hetero-COSY shown in Fig. 3 leaves no ambiguity on the structure. The assignment of each peak of the oligomer (I) is marked in Fig. 3. The proton NMR spectra of the oligomer in methanol-d 4 were recorded at two different temperatures, at 30 8C and at 260 8C, and are
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Fig. 3. The h 1 Hj 13 C-hetero-COSY NMR of (methanol-d 4 at 30 8C) the oligomer.
shown in Fig. 2a and b. The signals from the phenyl group attached to the main chain that appears as a multiplet in the region of 6.6–7.2 ppm are narrowed on cooling and the signals in the region of 5.4–6.4 are greatly effected in which the majority of the multiplets convulse to a set of multiplets spreading from 5.7 to 6.2 ppm. Since these signals originate from the hydrogen attached to the bridging carbon between two 1,2,3-trihydroxybenzene building blocks, cooling hinders the conformational changes and this forces the oligomer to have a rigid geometry. The proton signal of water of methanol-d 4 originally present at 4.75 ppm shifts to 5.6 ppm, and appears with the multiplets of the C–H proton and the proton from the 1,2,3-trihydroxybenzene unit. This effect clearly indicates the hydrophilic interaction of the compound with water molecules in solvent and interstitial water of crystallization present in the oligomer. The changes are thus effected by aggregation of the oligomer on cooling. The shifting of the water signals as well the increase in the sharpness of the aromatic phenyl signals
suggests the aggregation of oligomer with water. The two-dimensional NMR spectra clearly show the presence of a phenyl end group but there are two possibilities for the end groups (Eq. (1))
(1)
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It can be an OH-terminated chain as shown in Eq. (1) or it may be an olefinic group formed via elimination of water. Understanding of this is provided by the 13 C-NMR spectrum of the oligomer, which obviously depends on the solvents. When methanol-d 4 is used as solvent, in addition to the signals arising from the backbone there is no signal in the carbonyl region, but it has a signal at 107.7 ppm with very low intensity compared to other signals. The signal at 107.7 ppm is assigned to the carbon at the end group as vinylic –CH carbon in the quinonic structure. The intensity of this signal is reduced in DMSO-d 6 , however a new signal appears at 161.1 ppm which may occur due to the conformational changes that may occur at the backbone of the oligomer which may lead to deshielding. Since the compound has conformational flexibility as shown from the 2D NMR and the temperature-dependent proton NMR spectra, we prefer to assign this peak to
Scheme 1.
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an end group carbon of vinylic carbon (assigned as * in Eq. (1)) that is formed from elimination of water during the condensation reaction. Similar information on the 13 C signals occurring at 109.2 and at around 160 ppm in cyclic calix-4arene prepared from resorcinol with acetaldehyde on the variation of conformation by changing pH has already been reported [25].
3.2. Solvatochromicity of the oligomers As discussed in the previous section there is a considerable amount of changes in the shape of the aromatic protons of the oligomer upon variation of solvent. Encouraged by this we could obtain further information on such interaction with the aid of visible spectroscopy. The oligomer has a single absorption maximum in the visible region at 475 nm in methanol. The absorption is assigned to a p–p* transition occurring in the quinonic structure that is intermolecularly H-bonded to methanol (Scheme 1). Such an interaction would make the hydroxy group and the carbonyl group present at the 1 and 3 positions have extended delocalisation (structure Z in Fig. 1). However, the compound can have the intra-molecular H-bonded states X and Y as depicted in structure X and Y in Fig. 1 in aprotic medium. This is reflected in the visible spectra of the compound in aprotic medium in which the oligomer has two absorptions at 427 and 500 nm. The 427 nm absorption is assigned to the structure Y and the peaks at 500 nm are assigned to a state X. Thus, the effect of solvent on the intra-molecular and inter-molecular H-bonded structures could be ascertained by using mixed solvent systems. The process leading to solvatochromicity can be represented by Scheme 1. For this purpose we studied the electronic spectra of the oligomer with two mixed solvent systems, namely acetonitrile–methanol and dimethyl sulphoxide– methanol. The changes that occur on addition of methanol to an acetonitrile solution of the oligomer are shown in Fig. 4. Only one absorbance results from the methanolic solution
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Fig. 4. The solvatochromicity of the oligomer with composition of acetonitrile and methanol (1–5) 0:1; 1:5; 1:2; 1:1; 1:0.
with a normal Gaussian shape. Two distinct absorption maxima from acetonitrile solution of the oligomer could be sorted out by extrapolating the mirror image of the part. By standard matrix construction for each absorption it could be possible to find out the composition of the three states in different solvent compositions and these are tabulated in Table 1. The results support that a greater amount of hydrophilic solvent such as methanol generates the state Z through hydrophilic interaction of the solvent with state X and Y. This calculation is done on the basis of the assumption that in methanol only the Z component prevails and in acetonitrile / aprotic solvent the X and Y components prevail; all of the states have equal extinction coefficient. It is noteworthy that the sequence of addition of protic and aprotic solvent makes a Table 1 The ratio of X, Y, Z present in the mixed solvent Solvent composition
Ratio of solvent
Ratios of X
Y
Z
CH 3 OH:CH 3 CN
5:1 2:1 1:1 5:1 2:1 1:2
0.10 0.16 0.20 0.22 0.20 0.32
0.14 0.20 0.26 0.20 0.27 0.35
0.76 0.64 0.54 0.58 0.52 0.33
CH 3 OH:DMSO
difference in the visible spectra, i.e. the two absorptions at 427 and 500 nm in aprotic medium can be shifted to absorption at 475 nm by addition of protic solvent. The reverse is not true of the absorbance at 475 nm in methanol which once developed can be switched back to 427 and 500 nm by adding acetonitrile.
3.3. The effect of different amines on the electronic spectra of the oligomer The oligomers are soluble in a wide variety of common solvents such as acetonitrile, methanol and ethanol. Schneider et al. have used the recognition of tetra-alkylammonium salts to construct an allosteric proton pump from calixarene derivatives [25]. Rebek et al. have shown cation aggregation in resorcinarene [51]. With a similar philosophy we studied the visible spectra of different oligomers in the presence of different amines. The absorption maxima of the oligomer in acetonitrile occur at 427 and 500 nm. The two absorption maxima of the oligomer change significantly when treated with organic amines. However, the oligomers do not require a stoichiometric amount of amine to change the color of the oligomer but the requirement is dependent on the nature of the amines. Based on
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Fig. 5. UV–visible spectra of a solution of I (3 mg) in acetonitrile (3 cm 3 ) on addition of different amines with: (a) (1) 0 cm 3 , (2) to (7) addition of 1.5310 27 mmol of ethylenediamine in each aliquot; (b) (1) 0 mg, (2) 2.7310 22 mmol, (3) 5.5310 22 mmol, (4) 8.3310 22 mmol and (5) 11.1310 22 mmol of benzidine.
the effect on the absorption spectrum of the oligomer, the amines are categorized into four types. Some amines cause loss of absorptions at 427 and 500 nm with growth of a new absorption maximum at 566 nm, another type of amines increases the absorbance at 427 nm without generating a new absorption, some other amines enhance the absorbance at 427 and 500 nm whereas some amines do not significantly effect the absorption spectra of the oligomer. Fig. 5a illustrates the growth of absorption at 566 nm on addition of ethylenediamine. In Fig. 5b the effect of benzidine on the visible spectra is shown. The interaction of 1,2-phenylenediamine with the oligomer results in growth of absorption at 500 nm but slightly effects the absorption at 427 nm. The changes of visible spectra of the oligomer by different amines are listed in Table 2. Increase in absorptions at both the absorptions due to some amines is due to stabilization of the interaction of the amine with hydroxy groups with the 1,2,3 hydroxy-benzene unit without disrupting the intra-molecular H-bonded structures X and Y. Some amines like aniline cause no disruption of the hydrogen bonding nor form an ion pair in the system. The bifunctional rigid amine benzidine is not strong enough to form an ion pair but interacts with the oligomer backbone to form intramolecular hydrogen bonding. Benzidine has
two aromatic rings intervening between the amine groups and the two ends of the amines are at a greater distance than in an analogous rigid diamine, namely 1,4-phenylenediamine. However, the molecule benzidine has a size that confines itself into the space between the alternate 1,2,3-trihydroxybenzene rings of the oligomers (Fig. 6). Such an interaction can lock the Y form and facilitates transformation of the X form to Y leading to decrease in absorbance at 500 nm and increase in absorbance at 427 nm. This binding causes stability of the form Y in Fig. 1 and thereby shows enhancement of absorption intensity at 427 nm while due to such interaction the 1,2-disposition of the carbonyl group to the olefinic group becomes favorable. This can also be understood in terms of the CPK model shown in Fig. 6 that shows end-to-end interaction of benzidine with two
Fig. 6. A CPK model of benzidine with a part of the oligomer (red balls are O-atoms, blue are N-atoms).
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Table 2 The list of amines that effect the absorption spectrum of the oligomer a Growth of lmax (new) at 566 nm
a
Growth of lmax at |427 nm
Growth of lmax at 427 as well as 500 nm
Insignificant change on lmax
Parentheses show pKa values [74] in water at 25 8C.
alternate pyrogallol units. The closed packed structure between the oligomer and benzidine suggests that an amine of the right size can lead to the self-aggregation of the oligomer which can control the optical properties of the end group. The cavitands prepared from functionalised resorcinarene have been found to assemble with water molecules [19]. Obviously the amines with high pKa require a lower concentration (of the order of 1 / 10,000 with respect to the oligomer) of amine to reach the absorption maximum as it directly causes the anion generation at the end group and such a salt in turn can self-aggregate to give a quinonic character on other moieties. It is also observed that the amount of amine required to achieve the maximum absorption is largely dependent on the amine under consideration. For example, the ethylenediamine requires a concentration ratio of oligomer to amine in a molar ratio of 1:0.00012, whereas in the case of imidazole it is
1:42.6. The observation of such a property is probably due to the ability of the amine to generate an anion in the quinonic structure as well as its participation in the hydrogen bonding with the main skeleton. The effect resembles the plasmon type of electronic transition in nanomaterials [62–65]. The chemistry discussed so far has some differences from conventional acid–base chemistry of quinonic structures, as the amines having comparable pKa values may behave in different ways. For example, the pKa of aniline is 4.60, benzidine 4.66, but they have different implications on the electronic spectra (refer to Table 2). Pyridine has a pKa of 5.23 whereas p-methyl aniline has a pKa of 5.10 but they have very different effects on electronic spectra. The basicity is one key factor for the observed electronic spectra but the conformational stabilization due to the interaction with the oligomer back-bone is one of the prime factors as in all
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cases the isosebestic point observed varies and in some cases isosebestic points are not observed. 1,2,3-Trihydroxybenzene itself can interact with organic amines such as t-butylamine. Addition of t-butylamine in aliquots to a solution of 1,2,3-trihydroxybenzene shows growth at 267 nm and after the ratio of 1,2,3-trihydroxybenzene to t-butylamine becomes 1:1 another absorption maximum at 333 nm grows on subsequent addition. Quantitative measurement of amine addition gives the amount required to reach the maximum absorption corresponding to 1:1 molar ratio, suggesting that in the first stage an ion-pair of t-butylamine with one equivalent proton takes place followed by the next ion pairs. Potentiometric titration also supports the result that the acidity of two sets of protons is different. But this study on the oligomer clearly indicates that the chemistry of the oligomer under consideration is vastly different from the acid–base chemistry of 1,2,3-trihydroxybenzene as the oligomer does not show clear differentiation on the visible spectra to generate distinguishable anionic states. The literature suggests that suitably designed calixarenes can recognize anions also [27]. Effects of the anions of the organic ammonium salts on the bound state with quaternary salts are tested with a set of anions. The visible spectra of the quaternary ammonium salt of tbutylamine with perchlorate, chloride, bromide, and sulphate anions do not show significant shifts in the absorption spectrum. The variation of anions on the quarternary salts shows a slight difference in the new lmax to the extent of 1 or 210 nm precluding the interaction of an anion. Treatment of the oligomer with benzidine and perchloric acid, respectively, gives rise to absorption at 475 nm whereas a similar state is also generated from benzidine and hydrochloric acid with the oligomer. This shows that the different states created by different anions of the organic ammonium salts of diamine with a rigid frame has a less significant effect on binding. The effect on the visible spectra of the oligomer
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in acetonitrile on addition of ammonium salt is due to the hydrogen bonding that causes delocalisation in the end group to have a greater delocalisation between the keto–enol form.
3.4. Design of chemically driven reversible optical switches The supramolecular interaction of a dye molecule can lead to photo-switching through p–p interactions [5,61]. Since our system can possibly have a quinonic structure on generation of an anion, we set out to study design of optical switching through molecular recognition and anion aggregation [66]. The oligomer on interaction with t-butylamine (traces) leads to strong absorption at 566 nm and this absorption diminishes on addition of hydrochloric acid, and a new absorption maximum at 475 nm is observed. On addition of excess t-butylamine, this absorption is lost and the absorption at 566 nm develops (Fig. 7a). This cycle can be reversed and we have tested the reversibility of such a cycle six times without degradation of the oligomer. This can be considered as a switch as there is a reversible change that takes place without an appreciable effect on the volume. The different states are created with traces of amines and upon addition of acid another aggregated state is created. The hydroxyl groups of the end group containing the quinonic unit of the oligomer initially form anions with amine, followed by anion aggregation showing growth in the absorbance at 566 nm. A new state is generated on treatment with acid and this returns to the state generated from interaction with ammonium cation. It generates a state equivalent to X where hydrogen bonding occurs through ammonium cation rather than the alcohol. This state has absorbance at 475 nm. An identical state to the one generated from treatment with amine followed by mineral acid can also be generated through interaction of the oligomer with quaternary ammonium salts directly. Thus, the new state that grows at 475 nm occurs from the H-bonding with ammonium
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Fig. 7. Visible spectra of a solution of the oligomer (3 mg) in acetonitrile (3 cm 3 ) with alternate treatment with amine and acid. (a) With (1) none, (2) 0.5 ml of tert.-butylamine, (3)–(5) 1 ml of HCl (1 M) in each aliquot to solution (2), (6) 1 ml of tert.-butylamine to (5); (b) with (1) none, (2) 5 mg of benzidine, (3) 5 ml of HClO 4 (70%) was added to (2).
salts as evident from an independent study in which first tertiary butyl amine was converted to ammonium salt with hydrochloric acid followed by addition of the oligomer in acetonitrile. The chemically driven optical switch can be designed to have transformation between X and Z states also. For example, benzidine allows growth of absorption at 427 nm of the oligomer which can be switched to 475 nm by addition of perchloric acid (Fig. 7b).
3.5. Electrochemical study Cyclic polyphenolic compounds such as calixarenes on appropriate functionalisation are expected to pave the way for designing a redoxswitch with an analogy to enzymes [67–72]. To study the electrochemistry of the calixarenes an electroactive site in the calixarenes must be present in the ring. The electrochemistry of the quinones with different hosts is well established [69]. The polyphenolic compounds are antioxidant and participate in one electron transfer processes [59,60]. Another advantage of studying electrochemical properties in a confined medium is to provide a supramolecular environ-
ment with a close analogy to biological systems. Since we are dealing with linear chains that recognize amines we have studied the electrochemical behavior of an electroactive aromatic diamine, namely 1,4-phenylenediamine. This electrochemical study has another facet as aromatic amines provide a way to the synthesis of polyanilinic compounds and also would provide information to design a redox switch [72]. The 1,4-phenylenediamine can be transformed into benzenoid units and thus can throw light on proton transfer and redox properties of the amine units [73]. As mentioned earlier, 1,4phenylenediamine is recognised by the oligomer and also can have benzene–benzenoid structures on electrochemical oxidation and reduction reactions (Scheme 2). The 1,4-phenylenediamine has two reversible redox cycles E1 / 2 at 334 mV and E1 / 2 at 863 mV due to the formation of cationic radicals. The first reversible cycle with E1 / 2 at 334 mV is due to a cationic radical, this radical in the second reversible cycle at E1 / 2 863 mV transforms to a diimine. The overall redox reactions of 1,4-phenylenediamine in the presence of the oligomers are not effected but positions of E1 / 2 of the original reversible
Scheme 2.
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cycles are shifted from the original positions and also become quasireversible. The cyclic voltammogram of 1,4-phenylenediamine with the oligomer shows the usual redox reactions of 1,4-phenylenediamine (Scheme 2), but their shape suggests that the first electrochemical step (Scheme 2) becomes quasireversible and shifts from the original value. The second redox step occurs but to a very less significant extent. The results suggest that the redox properties of the 1,4-phenylenediamine change and the second redox step in these processes is inhibited supporting the role of the oligomer in stabilizing a cationic radical.
Acknowledgements The authors thank the Council of Scientific and Industrial Research, New Delhi for financial support. The authors also thank Prof. P. Balaram of the Indian Institute of Science, Bangalore for the mass spectra and Sophisticated Instrumentation Facility, Bangalore and M / s Varian India for the NMR spectra. The authors thank one of the referees for suggestions.
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The oligomer of 1,2,3-trihydroxybenzene with benzaldehyde Jyotirekha G. Handique, Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology, Guwahati 781 039, India Received 1 August 2002; accepted 1 January 2003
Abstract A linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene was synthesised and characterized. The oligomer has a quinonic end group that leads to a solvatochromic property and interacts with a variety of amines. In aprotic medium the oligomer has two non-equivalent intra-molecular H-bonded quinonic states arising from keto–enol tautomerism, one having the carbonyl and olefinic group at the 1,4-disposition (X) whereas in the other the carbonyl and olefinic groups are at the 1,2-disposition (Y). Thus, in acetonitrile both forms exist and the X and Y states have their characteristic absorption maximum at 500 and 427 nm, respectively. In protic medium a highly ordered structure with the hydrophobic and hydrophilic part projecting apart produces a structure (Z) having inter-molecular hydrogen-bonding with solvent which has a characteristic absorption at 475 nm. Non-stoichiometric amounts of various primary, secondary and tertiary amines interact with the oligomer providing stability to either of the forms or generating an anion by which the absorption maximum changes. For example, the oligomer interacts with t-butylamine, and this interaction changes the absorption maximum to 566 nm. The solution on further treatment with mineral acid (HCl) gives a new absorption maximum at 475 nm with a distinct color change. The absorptions can be changed back and forth by alternate addition of hydrochloric acid and t-butylamine. However, the characteristic changes that take place in the optical spectra of the oligomer in acetonitrile with different amines are highly substrate-dependent. For example the amines like ammonia, ethylamine, iso-propylamine, n-butyl amine, t-butyl amine, ethylenediamine, and pyridine cause loss of absorption at 427 and 500 nm and lead to growth of new absorption at 566 nm; whereas amines like benzidine, o-diaminobenzene, m-iodoaniline, m-nitroaniline, and p-hydroxyaniline enhance the original absorbance at 427 nm at the cost of absorption at 500 nm without leading to a new absorption. Some amines like 1-naphthylamine, 1,7-diaminonaphthalene, p-iodoaniline, and p-methoxyaniline increase the absorbances at 427 and 500 nm, whereas aniline, o-methylaniline, m-methylaniline, and diphenylamine do not significantly affect the spectra of the oligomer. The electrochemical study of the oligomer with 1,4-phenylenediamine shows that redox potentials of the bound state of the oligomer with 1,4-phenylenediamine shifts from the unbound state, for example, 1,4-phenylenediamine with the oligomer has two quasireversible couples at E1 / 2 at 259 and 822 mV in contrast to the two reversible couples of free 1,4phenylenediamine at 334 and 863 mV, respectively. 2003 Elsevier Science B.V. All rights reserved. Keywords: Condensation oligomer; 1,2,3-Trihydroxybenzene; Benzaldehyde; Optical-switching; Amine-recognition; Solvatochromicity
*Corresponding author. Tel.: 191-361-690-321; fax: 191-361-690-762. E-mail address: [email protected] (J.B. Baruah). 1381-5148 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S1381-5148(03)00020-8
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1. Introduction Polyphenols with intervening carbon atoms have wide applications in analytical [1–9], biological [10,11] and material chemistry [12– 14]. The ability to recognize different guest molecules by the cyclic counterpart of such polyphenols, namely calixarenes, is well documented [15–20]. Depending on the number of phenolic units in the cyclic counterpart they can have different cavity sizes to accommodate guest molecules [21–24]. The conformational flexibility coupled with molecular recognition has been used for studying calixarenes as allosteric proton pumps [25]. Calixarenes also recognize ions [26–29] and can have p-cation interactions [30,31]. The calixarenes attached to quinonic units have interesting electrochemical and photochemical properties [32–34]. The interaction of calixarenes with C 60 and other derivatives of C 60 have broadened their application to material science [35]. Calixarenes having a macrocyclic part attached to the cyclic backbone act as ionophores [36]. The application of calixarenes as models to mimic dinuclear metallo-phosphodiesterase for phosphatediester cleavage has proved to be useful [37–39]. Calixarenes containing antibody mimics also have great potential [40]. Although the resurgence of application of cyclic oligomers of phenol and aldehydes (calixarenes) is vibrant, in contrast to this, analogous study on the open chain counterparts have not emerged as rapidly as that of the cyclic counterparts. The reason for this is the limited synthetic methods to overcome the multiple product formation during synthesis of open chain polyphenolic compounds and also their insolubility in common solvents [41–44]. Although there are several articles in which synthetic linear polyphenolic compounds having oxygen as the intervening atom are prepared through oxidative coupling reactions, studies on their physicochemical aspects lag far behind [45–47]. There are also interesting reports in which phenolic compounds with weak Hbonded or charge transfer interactions are built
to construct macromolecules having interesting properties [48]. The cyclic oligomer of 1,2,3trihydroxybenzene has been found to show liquid crystal behavior with potential application in optoelectronics [49]. The pioneering work on resorcinarene has established the aggregated structure and their association with their ammonium ion [50,51]. We have identified a linear oligomer of 1,2,3-trihydroxybenzene with benzaldehyde that has good solubility in common organic solvents for this study. The general formula of the oligomer covered in this study is shown in Fig. 1. The physicochemical property of the linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene (I) has several important aspects to be studied. For example, linear oligomers should also have the analogous property of a proton pump like their cyclic counterparts. The calix-
Fig. 1. Structural features of the linear oligomer of benzaldehyde with 1,2,3-trihydroxybenzene.
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arenes have less conformational flexibility while recognizing guests. The scope for reorganizing itself to bind multiple binding sites of a guest is also expected to be greater in a linear oligomer. Recognition of guest molecules by linear oligomers would throw light on the drug–substrate binding, detection and extraction of heavy metal ions [52,53] etc. The inter-molecular H-bonding interaction as well as intra-molecular interaction / s may also lead to stacks of aromatic rings of the oligomers to give structures having analogy to the base pairing in nucleotides [54– 57]. The oligomer derived from 1,2,3-trihydroxybenzene would have some analogy to the naturally occurring polyphenolic compounds containing 1,2,3-trihydroxybenzene unit that plays a crucial role in recognizing and binding guest substrates and also contributes to the coloration of natural pigments [58,59]. A few more important aspects in linear polyphenolic systems are the reversible binding of proline residues of peptides, the role as one electron oxidant and the ability as catalyst to hydrolyse a peptide containing a proline residue [60].
2. Experimental The 1 H NMRs were recorded on a 400 MHz Bruker or Varian spectrometer. The cyclic voltammograms were recorded with an electrochemical analyzer, CHI 660A, with three electrode systems comprising of Ag /AgCl reference electrode and two platinum electrodes as working and auxiliary electrodes. The measurements were done in dry acetonitrile (HPLC grade, distilled over CaH 2 ) with tetrabutylammonium perchlorate as supporting electrolyte. The EMF values are with reference to ferrocene as standard. The mass spectra were recorded with, as matrix, a-cyano-4-hydroxycinnamic acid using a Kratos PC-Kompact mass spectrometer. The UV–visible spectra were recorded by dissolving the stipulated amount of the substrate / s in appropriate solvent using a Hitachi UV–visible spectrometer U-2001. The interac-
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tions of amines were studied by recording visible spectra of the oligomer in acetonitrile solution. In each case definite amounts of substrate were dissolved in acetonitrile and added in aliquots with a microsyringe. The electronic spectral data on absorbance at each 0.1 nm interval were collected and used for simulation purposes with the aid of the IGOR software package on an IBM PC. For simulation purposes the matrices were constructed directly from the data of a normal Gaussian shape spectra or from the data of extrapolated peaks constructed by addition of the mirror image of one half of the spectra in the case of overlapping peaks. The matrices were constructed to find the component of each species with the aid of data available from the literature [3]. In these cases the package MATLAB was used in an IBM Pentium PC to obtain the relative contribution of each state.
2.1. Condensation reaction of 1,2,3 -trihydroxybenzene with benzaldehyde A solution containing benzaldehyde (1060 mg, 10 mmol), 1,2,3-trihydroxybenzene (1260 mg, 10 mmol) and oxalic acid (100 mg, 0.8 mmol) in dry ethanol (10 cm 3 ) was refluxed for 3 h. A viscous brick red solution was obtained along with a very small amount of white solid. The reaction mixture was filtered, the residue was discarded and the solvent was removed under reduced pressure to obtain a paste. The paste was then washed with ice-cold water (23 5 cm 3 ). A brick red viscous gel was obtained; the paste obtained after decantation of water was further dried under vacuum to obtain a brick red solid. Yield: 930 mg. Elemental analysis: calcd for C 13 H 10 O 3 ?1.25 H 2 O; C 65.95; H, 5.34, found C, 66.54, H 5.66; MALDI mass: (m /e) 2244.3, 2036.7, 1821.3, 1604.2, 1625.3, 1391.5, 1373.4, 1177.7, 1085.3, 981.8, 872.7 (100% intensity). Mn , Mw (THF) 1209, 1252. 1 H NMR 400 MHz (DMSO-d 6 ) 5.2–8.2 (m); IR (KBr) 3369 (bs), 2950 (w), 2900 (w), 1688 (m) 1623 (s), 1495 (s), 1464 (s), 1286 (s), 1207 (s),
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1068 (s), 1012 (s), 953 (w), 750 (m), 702 (s) cm 21 .
3. Results and discussion
3.1. Structural features of the oligomer The oligomer of 1,2,3-trihydroxybenzene and benzaldehyde (Fig. 1) was prepared by acidcatalysed condensation reaction. The literature on condensation reactions of an aldehyde with phenolic compounds suggests that the calix formation is the thermodynamically favorable process [27]. However, the possible ring opening of a cyclic counterpart as well as the termination of a chain in such oligomerisation reactions were not discussed in these reports. The reaction procedure reported earlier for formation of calixarenes from 1,2,3-trihydroxybenzene with aldehydes involves a long reaction time and drastic reaction conditions [48]. The reaction conditions investigated in the present report were different than the reported procedure for cyclic counterparts. We observed consistently reproducible results on the same type of product from several independent experiments. The MALDI mass spectra of the oligomer have the base peak that appears at 872.7 (m /e) corresponding to [–(C 6 H 5 )CH– C 6 H 4 O 3 ] 3 C 6 H 4 O 3 C 6 H 5 CH(OH) showing that it might have been released from a linear chain of the oligomer. It has the highest m /e peak at 2244.3 corresponding to a unit comprising of [–(C 6 H 5 )CH–C 6 H 4 O 3 ] 10 C 6 H 5 CH–, which is probably formed by association of smaller units comprising of five units of the structures shown in Fig. 1. This assumption is on the basis of the observation of the Mn value as 1209 in contrast to the higher molecular ion peak in MALDI mass. The GPC showed a single set of peaks having an Mn /Mw value close to unity indicating that the average chain length is half of the molecular ion peaks obtained in mass spectroscopy and the discrepancy arises from the association of the oligomer in solution. Since
the base peak occurs at 872.7 the oligomer is comprised of linear tetra- or pentamers. The 1 H-NMR integration of the signal of the end phenyl group when compared to the rest of the non-exchangeable protons, shows that the integration fits approximately to a pentameric structure. The oligomer also has m /e peaks at 2036.7, 1821.3, 1604.2, 1391.5, 1178.7 corresponding to the loss of a –C 6 H 4 O 3 C 6 H 5 CH– unit from the highest mass ion of the parent oligomer. The observation of an OH unit attached to the CH-bond in the base peak suggests that the oligomer has an OH end group that may be present from the incompletely dehydrated end group. The oligomers also have water of crystallization and the presence of such water molecules in the oligomer is reflected in elemental analysis as well as in thermogravimetry. The thermogravimetry study suggests that on heating there is a loss of 8.5% weight in the region of 80– 200 8C from the oligomer. This loss occurs due to the loss of interstitial water molecules present in the system. This observation also suggests that the oligomer has a tendency to strong hydrophilic interaction. Such hydrophilic interaction and self-aggregation is reported for the resorcinarene [50,51]. The IR spectra of each of these oligomers shows strong and broad peaks for the OH groups centering in the region 3369 21 cm . The appearance of weak peaks at 2950 and 2900 cm 21 are attributed to strong hydrogen bonding present in the system. NMR spectroscopy helps in ascertaining the conformation and end group analysis. The 1 HNMR spectra of the oligomers are complicated in the aromatic region but the temperature-dependent study shows definite changes in the conformation. The 1 H-NMR spectra of the oligomer in two different solvents, namely methanol-d 4 and DMSO-d 6 , were recorded with a view to show the possibility of solvent effect on the structure. In addition to this, the methanol-d 4 with labile deuterium will exchange the OH groups and make the spectra less complicated. The 1 H-NMR spectra of the oligo-
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Fig. 2. 1 H-NMR in methanol-d 4 of I (a) at 260 8C, (b) at 30 8C.
mer in methanol-d 4 (Fig. 2a) as well as DMSOd 6 has two triplets and a doublet at a lower field that are assigned to the aromatic ring at the end of the oligomer backbone (Fig. 2). The integration value supports that they arise from the end group which would be a phenyl group attached to a methylene carbon as shown in Fig. 1. The product under consideration from the condensation reaction of benzaldehyde with 1,2,3-trihydroxybenzene is open chain as the termination of oligomerisation occurs due to dehydrated quinonic end group formation (Fig. 1); that occurs due to the nature of the reaction where a strong acid is used as catalyst for oligomerisation. In the case of DMSO-d 6 the phenolic OH groups of the oligomer overlap with the end group signals but use of methanol-d 4 allows the exchange of these protons with deuterium thereby making the end group show clear non-overlapping signals. The 1 H-NMR signals of the aromatic region of the oligomers are not clearly differentiated in 1D proton NMR. But the 2D-HOMO-COSY of the sample in two different solvents has clear differentiation in the correlation showing that the systems are in different conformations in
two different solvents. The presence of a differentiable aromatic ring at the end group in the oligomer is clear from the correlation of the signals at 7.4, 7.6 and 8.0 ppm. In this oligomeric system the peak separations of individual H are difficult because of the possibility of interacting and non-interacting conformational states, nevertheless the observation of highly symmetric spectra suggests orderliness and that regular conformational patterns are present in the system. The 2D-HOMO-COSY NMR recorded in methanol-d 4 clearly shows the correlation of the protons of the aromatic ring of the end group. The spread of correlation in methanol-d 4 shows that it spreads over a much broader region and it proves inter-molecular H-bonding interaction with the solvent. But the correlation in dimethyl sulphoxide is narrow suggesting the system has lesser flexibility of the conformers. The h 1 Hj 13 C hetero-COSY shown in Fig. 3 leaves no ambiguity on the structure. The assignment of each peak of the oligomer (I) is marked in Fig. 3. The proton NMR spectra of the oligomer in methanol-d 4 were recorded at two different temperatures, at 30 8C and at 260 8C, and are
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Fig. 3. The h 1 Hj 13 C-hetero-COSY NMR of (methanol-d 4 at 30 8C) the oligomer.
shown in Fig. 2a and b. The signals from the phenyl group attached to the main chain that appears as a multiplet in the region of 6.6–7.2 ppm are narrowed on cooling and the signals in the region of 5.4–6.4 are greatly effected in which the majority of the multiplets convulse to a set of multiplets spreading from 5.7 to 6.2 ppm. Since these signals originate from the hydrogen attached to the bridging carbon between two 1,2,3-trihydroxybenzene building blocks, cooling hinders the conformational changes and this forces the oligomer to have a rigid geometry. The proton signal of water of methanol-d 4 originally present at 4.75 ppm shifts to 5.6 ppm, and appears with the multiplets of the C–H proton and the proton from the 1,2,3-trihydroxybenzene unit. This effect clearly indicates the hydrophilic interaction of the compound with water molecules in solvent and interstitial water of crystallization present in the oligomer. The changes are thus effected by aggregation of the oligomer on cooling. The shifting of the water signals as well the increase in the sharpness of the aromatic phenyl signals
suggests the aggregation of oligomer with water. The two-dimensional NMR spectra clearly show the presence of a phenyl end group but there are two possibilities for the end groups (Eq. (1))
(1)
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It can be an OH-terminated chain as shown in Eq. (1) or it may be an olefinic group formed via elimination of water. Understanding of this is provided by the 13 C-NMR spectrum of the oligomer, which obviously depends on the solvents. When methanol-d 4 is used as solvent, in addition to the signals arising from the backbone there is no signal in the carbonyl region, but it has a signal at 107.7 ppm with very low intensity compared to other signals. The signal at 107.7 ppm is assigned to the carbon at the end group as vinylic –CH carbon in the quinonic structure. The intensity of this signal is reduced in DMSO-d 6 , however a new signal appears at 161.1 ppm which may occur due to the conformational changes that may occur at the backbone of the oligomer which may lead to deshielding. Since the compound has conformational flexibility as shown from the 2D NMR and the temperature-dependent proton NMR spectra, we prefer to assign this peak to
Scheme 1.
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an end group carbon of vinylic carbon (assigned as * in Eq. (1)) that is formed from elimination of water during the condensation reaction. Similar information on the 13 C signals occurring at 109.2 and at around 160 ppm in cyclic calix-4arene prepared from resorcinol with acetaldehyde on the variation of conformation by changing pH has already been reported [25].
3.2. Solvatochromicity of the oligomers As discussed in the previous section there is a considerable amount of changes in the shape of the aromatic protons of the oligomer upon variation of solvent. Encouraged by this we could obtain further information on such interaction with the aid of visible spectroscopy. The oligomer has a single absorption maximum in the visible region at 475 nm in methanol. The absorption is assigned to a p–p* transition occurring in the quinonic structure that is intermolecularly H-bonded to methanol (Scheme 1). Such an interaction would make the hydroxy group and the carbonyl group present at the 1 and 3 positions have extended delocalisation (structure Z in Fig. 1). However, the compound can have the intra-molecular H-bonded states X and Y as depicted in structure X and Y in Fig. 1 in aprotic medium. This is reflected in the visible spectra of the compound in aprotic medium in which the oligomer has two absorptions at 427 and 500 nm. The 427 nm absorption is assigned to the structure Y and the peaks at 500 nm are assigned to a state X. Thus, the effect of solvent on the intra-molecular and inter-molecular H-bonded structures could be ascertained by using mixed solvent systems. The process leading to solvatochromicity can be represented by Scheme 1. For this purpose we studied the electronic spectra of the oligomer with two mixed solvent systems, namely acetonitrile–methanol and dimethyl sulphoxide– methanol. The changes that occur on addition of methanol to an acetonitrile solution of the oligomer are shown in Fig. 4. Only one absorbance results from the methanolic solution
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326
Fig. 4. The solvatochromicity of the oligomer with composition of acetonitrile and methanol (1–5) 0:1; 1:5; 1:2; 1:1; 1:0.
with a normal Gaussian shape. Two distinct absorption maxima from acetonitrile solution of the oligomer could be sorted out by extrapolating the mirror image of the part. By standard matrix construction for each absorption it could be possible to find out the composition of the three states in different solvent compositions and these are tabulated in Table 1. The results support that a greater amount of hydrophilic solvent such as methanol generates the state Z through hydrophilic interaction of the solvent with state X and Y. This calculation is done on the basis of the assumption that in methanol only the Z component prevails and in acetonitrile / aprotic solvent the X and Y components prevail; all of the states have equal extinction coefficient. It is noteworthy that the sequence of addition of protic and aprotic solvent makes a Table 1 The ratio of X, Y, Z present in the mixed solvent Solvent composition
Ratio of solvent
Ratios of X
Y
Z
CH 3 OH:CH 3 CN
5:1 2:1 1:1 5:1 2:1 1:2
0.10 0.16 0.20 0.22 0.20 0.32
0.14 0.20 0.26 0.20 0.27 0.35
0.76 0.64 0.54 0.58 0.52 0.33
CH 3 OH:DMSO
difference in the visible spectra, i.e. the two absorptions at 427 and 500 nm in aprotic medium can be shifted to absorption at 475 nm by addition of protic solvent. The reverse is not true of the absorbance at 475 nm in methanol which once developed can be switched back to 427 and 500 nm by adding acetonitrile.
3.3. The effect of different amines on the electronic spectra of the oligomer The oligomers are soluble in a wide variety of common solvents such as acetonitrile, methanol and ethanol. Schneider et al. have used the recognition of tetra-alkylammonium salts to construct an allosteric proton pump from calixarene derivatives [25]. Rebek et al. have shown cation aggregation in resorcinarene [51]. With a similar philosophy we studied the visible spectra of different oligomers in the presence of different amines. The absorption maxima of the oligomer in acetonitrile occur at 427 and 500 nm. The two absorption maxima of the oligomer change significantly when treated with organic amines. However, the oligomers do not require a stoichiometric amount of amine to change the color of the oligomer but the requirement is dependent on the nature of the amines. Based on
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Fig. 5. UV–visible spectra of a solution of I (3 mg) in acetonitrile (3 cm 3 ) on addition of different amines with: (a) (1) 0 cm 3 , (2) to (7) addition of 1.5310 27 mmol of ethylenediamine in each aliquot; (b) (1) 0 mg, (2) 2.7310 22 mmol, (3) 5.5310 22 mmol, (4) 8.3310 22 mmol and (5) 11.1310 22 mmol of benzidine.
the effect on the absorption spectrum of the oligomer, the amines are categorized into four types. Some amines cause loss of absorptions at 427 and 500 nm with growth of a new absorption maximum at 566 nm, another type of amines increases the absorbance at 427 nm without generating a new absorption, some other amines enhance the absorbance at 427 and 500 nm whereas some amines do not significantly effect the absorption spectra of the oligomer. Fig. 5a illustrates the growth of absorption at 566 nm on addition of ethylenediamine. In Fig. 5b the effect of benzidine on the visible spectra is shown. The interaction of 1,2-phenylenediamine with the oligomer results in growth of absorption at 500 nm but slightly effects the absorption at 427 nm. The changes of visible spectra of the oligomer by different amines are listed in Table 2. Increase in absorptions at both the absorptions due to some amines is due to stabilization of the interaction of the amine with hydroxy groups with the 1,2,3 hydroxy-benzene unit without disrupting the intra-molecular H-bonded structures X and Y. Some amines like aniline cause no disruption of the hydrogen bonding nor form an ion pair in the system. The bifunctional rigid amine benzidine is not strong enough to form an ion pair but interacts with the oligomer backbone to form intramolecular hydrogen bonding. Benzidine has
two aromatic rings intervening between the amine groups and the two ends of the amines are at a greater distance than in an analogous rigid diamine, namely 1,4-phenylenediamine. However, the molecule benzidine has a size that confines itself into the space between the alternate 1,2,3-trihydroxybenzene rings of the oligomers (Fig. 6). Such an interaction can lock the Y form and facilitates transformation of the X form to Y leading to decrease in absorbance at 500 nm and increase in absorbance at 427 nm. This binding causes stability of the form Y in Fig. 1 and thereby shows enhancement of absorption intensity at 427 nm while due to such interaction the 1,2-disposition of the carbonyl group to the olefinic group becomes favorable. This can also be understood in terms of the CPK model shown in Fig. 6 that shows end-to-end interaction of benzidine with two
Fig. 6. A CPK model of benzidine with a part of the oligomer (red balls are O-atoms, blue are N-atoms).
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Table 2 The list of amines that effect the absorption spectrum of the oligomer a Growth of lmax (new) at 566 nm
a
Growth of lmax at |427 nm
Growth of lmax at 427 as well as 500 nm
Insignificant change on lmax
Parentheses show pKa values [74] in water at 25 8C.
alternate pyrogallol units. The closed packed structure between the oligomer and benzidine suggests that an amine of the right size can lead to the self-aggregation of the oligomer which can control the optical properties of the end group. The cavitands prepared from functionalised resorcinarene have been found to assemble with water molecules [19]. Obviously the amines with high pKa require a lower concentration (of the order of 1 / 10,000 with respect to the oligomer) of amine to reach the absorption maximum as it directly causes the anion generation at the end group and such a salt in turn can self-aggregate to give a quinonic character on other moieties. It is also observed that the amount of amine required to achieve the maximum absorption is largely dependent on the amine under consideration. For example, the ethylenediamine requires a concentration ratio of oligomer to amine in a molar ratio of 1:0.00012, whereas in the case of imidazole it is
1:42.6. The observation of such a property is probably due to the ability of the amine to generate an anion in the quinonic structure as well as its participation in the hydrogen bonding with the main skeleton. The effect resembles the plasmon type of electronic transition in nanomaterials [62–65]. The chemistry discussed so far has some differences from conventional acid–base chemistry of quinonic structures, as the amines having comparable pKa values may behave in different ways. For example, the pKa of aniline is 4.60, benzidine 4.66, but they have different implications on the electronic spectra (refer to Table 2). Pyridine has a pKa of 5.23 whereas p-methyl aniline has a pKa of 5.10 but they have very different effects on electronic spectra. The basicity is one key factor for the observed electronic spectra but the conformational stabilization due to the interaction with the oligomer back-bone is one of the prime factors as in all
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cases the isosebestic point observed varies and in some cases isosebestic points are not observed. 1,2,3-Trihydroxybenzene itself can interact with organic amines such as t-butylamine. Addition of t-butylamine in aliquots to a solution of 1,2,3-trihydroxybenzene shows growth at 267 nm and after the ratio of 1,2,3-trihydroxybenzene to t-butylamine becomes 1:1 another absorption maximum at 333 nm grows on subsequent addition. Quantitative measurement of amine addition gives the amount required to reach the maximum absorption corresponding to 1:1 molar ratio, suggesting that in the first stage an ion-pair of t-butylamine with one equivalent proton takes place followed by the next ion pairs. Potentiometric titration also supports the result that the acidity of two sets of protons is different. But this study on the oligomer clearly indicates that the chemistry of the oligomer under consideration is vastly different from the acid–base chemistry of 1,2,3-trihydroxybenzene as the oligomer does not show clear differentiation on the visible spectra to generate distinguishable anionic states. The literature suggests that suitably designed calixarenes can recognize anions also [27]. Effects of the anions of the organic ammonium salts on the bound state with quaternary salts are tested with a set of anions. The visible spectra of the quaternary ammonium salt of tbutylamine with perchlorate, chloride, bromide, and sulphate anions do not show significant shifts in the absorption spectrum. The variation of anions on the quarternary salts shows a slight difference in the new lmax to the extent of 1 or 210 nm precluding the interaction of an anion. Treatment of the oligomer with benzidine and perchloric acid, respectively, gives rise to absorption at 475 nm whereas a similar state is also generated from benzidine and hydrochloric acid with the oligomer. This shows that the different states created by different anions of the organic ammonium salts of diamine with a rigid frame has a less significant effect on binding. The effect on the visible spectra of the oligomer
329
in acetonitrile on addition of ammonium salt is due to the hydrogen bonding that causes delocalisation in the end group to have a greater delocalisation between the keto–enol form.
3.4. Design of chemically driven reversible optical switches The supramolecular interaction of a dye molecule can lead to photo-switching through p–p interactions [5,61]. Since our system can possibly have a quinonic structure on generation of an anion, we set out to study design of optical switching through molecular recognition and anion aggregation [66]. The oligomer on interaction with t-butylamine (traces) leads to strong absorption at 566 nm and this absorption diminishes on addition of hydrochloric acid, and a new absorption maximum at 475 nm is observed. On addition of excess t-butylamine, this absorption is lost and the absorption at 566 nm develops (Fig. 7a). This cycle can be reversed and we have tested the reversibility of such a cycle six times without degradation of the oligomer. This can be considered as a switch as there is a reversible change that takes place without an appreciable effect on the volume. The different states are created with traces of amines and upon addition of acid another aggregated state is created. The hydroxyl groups of the end group containing the quinonic unit of the oligomer initially form anions with amine, followed by anion aggregation showing growth in the absorbance at 566 nm. A new state is generated on treatment with acid and this returns to the state generated from interaction with ammonium cation. It generates a state equivalent to X where hydrogen bonding occurs through ammonium cation rather than the alcohol. This state has absorbance at 475 nm. An identical state to the one generated from treatment with amine followed by mineral acid can also be generated through interaction of the oligomer with quaternary ammonium salts directly. Thus, the new state that grows at 475 nm occurs from the H-bonding with ammonium
330
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Fig. 7. Visible spectra of a solution of the oligomer (3 mg) in acetonitrile (3 cm 3 ) with alternate treatment with amine and acid. (a) With (1) none, (2) 0.5 ml of tert.-butylamine, (3)–(5) 1 ml of HCl (1 M) in each aliquot to solution (2), (6) 1 ml of tert.-butylamine to (5); (b) with (1) none, (2) 5 mg of benzidine, (3) 5 ml of HClO 4 (70%) was added to (2).
salts as evident from an independent study in which first tertiary butyl amine was converted to ammonium salt with hydrochloric acid followed by addition of the oligomer in acetonitrile. The chemically driven optical switch can be designed to have transformation between X and Z states also. For example, benzidine allows growth of absorption at 427 nm of the oligomer which can be switched to 475 nm by addition of perchloric acid (Fig. 7b).
3.5. Electrochemical study Cyclic polyphenolic compounds such as calixarenes on appropriate functionalisation are expected to pave the way for designing a redoxswitch with an analogy to enzymes [67–72]. To study the electrochemistry of the calixarenes an electroactive site in the calixarenes must be present in the ring. The electrochemistry of the quinones with different hosts is well established [69]. The polyphenolic compounds are antioxidant and participate in one electron transfer processes [59,60]. Another advantage of studying electrochemical properties in a confined medium is to provide a supramolecular environ-
ment with a close analogy to biological systems. Since we are dealing with linear chains that recognize amines we have studied the electrochemical behavior of an electroactive aromatic diamine, namely 1,4-phenylenediamine. This electrochemical study has another facet as aromatic amines provide a way to the synthesis of polyanilinic compounds and also would provide information to design a redox switch [72]. The 1,4-phenylenediamine can be transformed into benzenoid units and thus can throw light on proton transfer and redox properties of the amine units [73]. As mentioned earlier, 1,4phenylenediamine is recognised by the oligomer and also can have benzene–benzenoid structures on electrochemical oxidation and reduction reactions (Scheme 2). The 1,4-phenylenediamine has two reversible redox cycles E1 / 2 at 334 mV and E1 / 2 at 863 mV due to the formation of cationic radicals. The first reversible cycle with E1 / 2 at 334 mV is due to a cationic radical, this radical in the second reversible cycle at E1 / 2 863 mV transforms to a diimine. The overall redox reactions of 1,4-phenylenediamine in the presence of the oligomers are not effected but positions of E1 / 2 of the original reversible
Scheme 2.
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cycles are shifted from the original positions and also become quasireversible. The cyclic voltammogram of 1,4-phenylenediamine with the oligomer shows the usual redox reactions of 1,4-phenylenediamine (Scheme 2), but their shape suggests that the first electrochemical step (Scheme 2) becomes quasireversible and shifts from the original value. The second redox step occurs but to a very less significant extent. The results suggest that the redox properties of the 1,4-phenylenediamine change and the second redox step in these processes is inhibited supporting the role of the oligomer in stabilizing a cationic radical.
Acknowledgements The authors thank the Council of Scientific and Industrial Research, New Delhi for financial support. The authors also thank Prof. P. Balaram of the Indian Institute of Science, Bangalore for the mass spectra and Sophisticated Instrumentation Facility, Bangalore and M / s Varian India for the NMR spectra. The authors thank one of the referees for suggestions.
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