MultisensingAnionRecognitionwiththiourebased.doc

精品論文multisensing anion recognition with thioure basedreceptor functionalized with tetrathiafulvalene unit 1chang su, liying gu, bingzhu yin*key laboratory of organism functional factors of the changbai mountain, yanbian university, ministry of education, yanji 133002, p.r. cabstracta series thiourea/urea-based anion receptors functionalized with tetrathiafulvalene(1-4) were prepared. among them, 4 displays the high sensitive multisensing for acetate anion over various anions in acetonitrile with dramatic changes in uv-vis spectra , cyclic voltammogram and 1h nmr spectra. conformation change induced by anion complexing of 4 caused the improvement of ict channel to enhance the charge transfer not noly from thiourea moiety but also from ttf unit to electron-defect benzene ring. ttf unit plays a synergic charge transfer role for trifluoromethyl group in the ict process. dft calculation supported the hypothesis of conformational change and ict channel improvement.keywords: tetrathiafulvalene; anion receptor; thiourea; multisensing; recognition1introductionin recent years, the field of anion recognition associated with the signaling of reversible anion binding using synthetic chemosensors, stimulated by simmons and parks report in 1968 1, has witnessed increasing popularity and considerable efforts have been made to design chemosensors for anions since anions play a fundamental role in a wide range of chemical and biological processes 2-6. one of the most co-mmom modes of signal transduction is electrochemical-based changes in the sensor incurred by association of the analyte with the receptor 7. the tetrathiafulvalene is a strong - electron donor and an ideal redox-active sensing unit and its oxidation to the radical cation (ttf+) and dication(ttf2+) occurring sequentially and reversibly in a desirable range, but there are only a few example of anion receptor incorporating a ttf unit that are capable of multisensing so far 8-14.it is well known that urea/thiourea functional groups are among the most popular binding motifs currently being used to prepare neutral anion-binding receptors because of they provide two hydrogen-bond donor groups that point in same direction and which are spaced appropriately to interact with a range of anionic substrates 15-21. it is well performanced that the n-benzamidothioureas showed a dramatically increased anion affinity compared with n-benzoylthioureas and n-phenylthioureas due to anion binding induced n-n conformational change and the resultant intramolecular charge transfer (ict) in the anion binding complex 22-28. in order to explore the role of ttf unit in ict process, we synthesized four n-(6,7-dibutylthio- tetrathiafulvaleno2,3-c-pyrrole-2-amido)-n-arylurea/thiourea deriva-tives (1-4) (fig. 1) and eveluated their anion binding affinity.1we thank the national natural science foundation of china (grant 20662010) , the specialized research fund for the doctoral program of higher education ( grant 20060184001) and the open project of state key laboratory of supramolecular structure and materials for financial support.-6-2results and discussionamong these receptors, only the receptor 4, as the increased acidity of the nh bonds result from that more acidic thiourea than urea and the intense electron-withdrawing character of trifluoromethyl groupbus s sbus s sn tslda/clcoomethf, 75%bus s sbus s sn tsmeonameoh/thf5 6 coo me89%bus s sbus s s222h n nhh on hetoh, 95%bus s snhbus s srncxdmf7 coo me8 conhnh2bus s busssnh snh x ohn1:x = o ,r = h(68%) hn2:x = o ,r = cf3 (74%) r3:x = s, r =h (77%)4:x = s, r = cf3 (80%)fig 1 synthesis route of 1-429-30, displays the multisensing for acetate anion over various anions in acetonitrile with dramatic changes in uv-vis spectra , cyclic voltammogram and 1h nmr. the conformation change induced by anion complexing of 4 caused the improvement of ict channel which enhanced the charge transfer not noly from thiourea moiety but also from ttf unit to electron-defect benzene ring. as expected, ttf unit plays a synergic charge transfer role for trifluoromethyl group in the ict process.first of all, we investigated the effects of various anions on the absorption spectrum of receptor 4. addition of aco-, as tetrabutylammonium salts to solutions of compound 4 in acetonitrile resulted in dramatic changes in the uv-vis spectra of the receptor with an increase as well as red-shift of the absorbance in the328 nm and a disappearance of absorbance in the 237 nm region, respectively (fig. 1). the appearance of three isosbestic points at approximately 225, 250 and 297 nm, respectively, point to the formation of well-defined anion binding complexes which are later shown to be in 1:1 stoichiometry by job plots. as compared with to 4, no any spectral variation was observed with the precursor carbohydrazide 8 which have three nh bond but no thiourea moiety. on the basis of the known thiourea-aco- hydrogen-bonding mode 15-21, 31-36, this might point to the hydrogen-bonding interaction between thiourea moiety in 4 and aco- through 1:1 mode. further evidence for this 1:1 hydrogen bonding interaction was obtained from ms data. in the esi mass spectra of the 4-aco- mixture in acn we observed species of m/z value 984.0 (4:aco- + bu4n+ + h+).-in the same conditions, the nearly similar but smaller changes were found in the presence of f- and h2po4 , whereas hso4-, cl- and br- hardly induced any spectral changes (fig. 2, left). according to uv-vis titration experiments, the fitted binding constants 37 between 4 and acetate, fluoride and dihydrogen phosphatewere calculated to be 3.47106 m-1 (r 0.998), 1.78 106 m-1 (r 0.998), and 4.37104 m-1 (r 0.997), respectively and are too small for br-, cl-, and hso4- to be accurately determined (fig.2, right). it isobvious that the neutral receptor 4 shows a high selectivity in acetonitrile for aco- over other anions, and inabsorbsence0.40.40 1 2 3 4 5equiv. of aco-absorbsence0.3aco-f-h po-2 4hso-0.20.24br-cl-0.0250300350400450wavelength (nm)0.10 1 2 3 4 5equiv. of anionfig. 2 absorption spectra of 4 in acn in the presence of aco-. 4 = 2 10-5 mol l-1. aco- = 0 -10 10-5 mol l-1. inset: variation of absorbance at 350 nm versus equivalents of acetate (left); plots of absorbance of 4 at 350 nm as a function of anion concentration. 4 = 2.0 10-5 mol l-1 and anion = 0 10 x 10-5 mol l-1. all anions exist in the form of the n-bu4n+ salt.particular, its binding constant to both aco- and f- are close to that (106 mol-1) of a positively charged thiourea-based receptor 38. it is worthy of not that the binding constant of receptor 4 is same order with the n-(4-nitrobenzoamide)-n-(4-nitrophenyl)thiourea 39, without regard to the intense donor character of ttf unit. to the best of our knowledge, the receptor 4 displays one of the strongest aco- binding affinity recorded so far for a tetrathiofulvalene-based anion receptor 11, 13. however, in the cases of receptor 1-3, no significant changes were observed in not either uv-vis and cv or 1h nmr spectra, indicating that these receptors are not interacting significantly with the putative anionic guests in neutral condition.the presented reportors were designed to permit the detection of anions by binding-inducecd changes in the electrochemically properties of the ttf unit. thus, to explore the possibility of 4 as an electrochemical anion sensor, cyclic voltammetry (cv) studies were conducted under presence of aco- in acn /dmso (9:1,v/v). the progressive addition of aco- to the solution of 4 caused clear modifications in the cyclic voltammetry (figure 3, left), from which a significant anodic shift of the first wave of the ttf unit which accom-panied the appearance of a new redox system at 0.24 v was observed (e1 = 0.066v). this new peak, at e1ox = 0.24 v, is ascribed to oxidation of the complex between the receptor 4 and aco-. it is important to point out that the first oxidation potential of the ttf unit occurs not cathotic shift but anodic shift. in general, anion binding of a recepter increases electron density of molecule, this should impels cathotic shift of the redox wave to occur. we propose that these changes are consistent with the anion binding to the thiourea moiety through hydrogen bonding where need to change the conformation of receptor 4 to setisfy syn orientation of thiourea protons 40-41. this conformational change improved the ict channel which enhanced electron transfer from ttf unit to electron-defect benzene ring. as a result, the ttf unit in in complex of 4 with aco- hard to occur oxidation by comparison with neutral receptor,which induced the anodic shift of first oxidation wave. along with the continuous addition of aco-, the peak current of this new wave increased gradually, while the peak current of the original wave decreased gradually and becomes almost balanceable when 1 equiv of aco- was added. it implies also that thestoichiometry of 4 and aco- in the complex is 1:1.38 2.0 eq.37i / a36 1.5 eq.350 3433-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6equiv. of aco-i / a2040601.25 eq.1.00 eq.0.75 eq.0.50 eq.0.25 eq.0.00 eq.0.0 0.2 0.4 0.60.8e / v12 11 10 9 8 7ppmfig.3 cyclic voltammograms of the receptor 4 (10-3 mol l-1) recorded in dmso/acn(1:9 v/v) at 298 k with n-bu4npf6 (0.1 mol l-1) as the supporting electrolyte in the presence of increasing amounts n-bu4noac (left); stack plot of 1h nmr spectra of 4 in the presence of increasing amounts of n-bu4noac recorded in acetone-d6/cd3cn (1:9 v/v)( right).the formation of the binding complex between receptor 4 and aco- is further evident in 1h nmrspectroscopic experiments in cd3cocd3/cd3cn (19 v/v) (figure 3, right). upon addition of aco- , the1h nmr spectrum of 4 displayed dramatic changes. in the addition process of aco-, the all signals of four n-h protons were disappeared gradually. when upon addition of 0.5 equiv aco-, the signals of three n-h at 10.06, 8.44 and 8.29 ppm were disappeared completely except for the signal of n-h of pyrrole ring at9.08 ppm. fortunately, the progressive addition of aco- to the solution of 4 caused to appear again the threen-h signals which had disappeared, while the signal of n-h of pyrrole ring was disappeared completely. when about 1 equiv of aco- is added, the three amide signals shifted significantly downfield ( =1.25,1.39 and 0.85 ppm, respectively), but no futher changes occurred after the addition of 1 equiv of aco-.it implies the formation of a stable 1:1 complex. but the disappea-rance of three amide n-h in the process of addition of aco- made us impossible to evaluate bingding affinity from 1h nmr data.dft calculations at the mpw1pw91/6-31g(d) level supported the idea of conformational change and ict channel improvement.13 in the uncomplexed form 4, the dihedral angle between ttf unit and benzene ring is about 30 and electron density is localized on the ttf moieties in homo. in the acetate complex of4, ttf unit is almost coplanar with the terminal benzene ring, where ict channel is improved, and its homo possess more delocalized structure on thiourea and benzene moieties ( fig. 4). indeed, the delocalization of electron in homo of the complex induced the anodic shift of first oxidation wave of ttf unit.3conclutionin conclution, we have presented a highly sensitive receptor 4 for both aco- and f- with the multisensing44-aco-4 eh = -5.21 ev4-aco- eh = -0.96 evfig. 4 dft (mpw1pw91/6-31g(d)-minimized structures of uncom-plexed 4 and complex 4-aco- (left). homos of 4 (top) and 4:aco- (bottom) as caculated on the level of mpw1pw91/6-31g(d) (right).function by photical, electrochemical and nmr means, whose anion sensing take place under intramolecular charge transfer mechanism. the receptor 4, to the best of our knowledge, displays one of the strongest aco- binding affinity recorded so far for a ttf-based anion receptor. we are currently investigating this unique features in greater detail.references1c. h. park, h. e. simmons, 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