1,4-Diarylpentiptycenes (1a-e) were synthesized from 1,4-dichloro- or 1,4-difluoro-2,5-diarylbenzene derivatives by double base-promoted dehydrohalogenation to give corresponding arynes, which in the presence of anthracene undergo cycloaddition providing 1,4-diarylpentiptycenes in moderate overall yields. The resulting 1,4-diarylpentiptycenes show fluorescence modulated by the 1,4-aryl residues. The fluorescence is quenched in the presence of vapors of nitroaromatic compounds suggesting potential application in sensing of explosives

Anzenbacher, Pavel, Jr.

Palacios, Manuel A.

Zyryanov, Grigory V.

English

Bowling Green State University: ScholarWorks@BGSU

Bowling Green State University ScholarWorks@BGSU Chemistry Faculty Publications Chemistry 9-2008 Simple Molecule-based Fluorescent Sensors For Vapor Detection Of Tnt Grigory V. Zyryanov Manuel A. Palacios Pavel Anzenbacher Jr. Bowling Green State University, pavel@bgsu.edu Follow this and additional works at: https://scholarworks.bgsu.edu/chem_pub  Part of the Chemistry Commons Repository Citation Zyryanov, Grigory V.; Palacios, Manuel A.; and Anzenbacher, Pavel Jr., "Simple Molecule-based Fluorescent Sensors For Vapor Detection Of Tnt" (2008). Chemistry Faculty Publications. 86. https://scholarworks.bgsu.edu/chem_pub/86 This Article is brought to you for free and open access by the Chemistry at ScholarWorks@BGSU. It has been accepted for inclusion in Chemistry Faculty Publications by an authorized administrator of ScholarWorks@BGSU. Simple Molecule-Based FluorescentSensors for Vapor Detection of TNTGrigory V. Zyryanov, Manuel A. Palacios, and Pavel Anzenbacher, Jr.*Center for Photochemical Sciences and Department of Chemistry, Bowling Green StateUniVersity, Bowling Green, Ohio 43403paVel@bgsu.eduReceived June 7, 2008ABSTRACT1,4-Diarylpentiptycenes (1a-e) were synthesized from 1,4-dichloro- or 1,4-difluoro-2,5-diarylbenzene derivatives by double base-promoteddehydrohalogenation to give corresponding arynes, which in the presence of anthracene undergo cycloaddition providing 1,4-diarylpentiptycenesin moderate overall yields. The resulting 1,4-diarylpentiptycenes show fluorescence modulated by the 1,4-aryl residues. The fluorescence isquenched in the presence of vapors of nitroaromatic compounds suggesting potential application in sensing of explosives.Host materials that form inclusion complexes with smallmolecules are the subject of considerable interest owingto potential applications in the preparation of sensors andmaterials for selective sequestration of substrates ofinterest. Growing attention is currently devoted to studyingsupramolecular complexes of a wide variety of lipophilichosts for electroneutral species that include calixarenes1and related macrocycles, cyclodextrins,2 cyclovera-trylenes,3 cryptophanes,4 cucurbiturils,5 and their deriva-tives.The interest in pentiptycene receptors was sparked bythe discovery that iptycenes bind nitroaromate explosives6used in landmines and improvised explosive devices.7 Thematerials successfully used for detection of nitroaromatesare fluorescent polymers with the ability to form com-plexes with nitroaromates while changing their emission.Examples are polyphenyleneethynylenes with integratedpentiptycene receptors,8a polyphenylenebutadiynylenes,8bpolyacetylenes,8c and polymetalloles8d such as polysiloles.8eAs a part of our work on polythiophene conductivity-basedsensors9 we decided to investigate electropolymerizablepentiptycene receptors carrying thiophene moieties to bedeposited as electrochemical sensors for TNT.Pentiptycene derivatives are most frequently synthesizedby the Clar synthesis, which involves double Diels-Alder(1) Calixarenes 2001; Asfari, Z., Bohmer, V., Harrowfield, J., Vicens,J, Eds.; Kluwer Academic Publishers: Dordrecht, 2001.(2) ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies,J. E., MacNicol, D. D., Vo¨gtle, F., Reinhoudt, D. N., Lehn, J.-M., Eds.;Volume 3: Cyclodextrins; Szejtl, J., Osa, T., Volume Eds.; Pergamon-Elsevier Science: Oxford, 1996.(3) ComprehensiVe Supramolecular Chemistry; Volume 6: Solid-statesupramolecular chemistry; MacNicol, D. D., Toda, F., Bishop, R., VolumeEds.; Pergamon-Elsevier Science, Oxford, 1996; pp 281-303.(4) Collet, A. Tetrahedron 1987, 43, 5725.(5) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew.Chem., Int. Ed. 2005, 44, 4844.(6) (a) Thomas, S. W., III.; Joly, G. D.; Swager, T. M. Chem. ReV. 2007,107, 1339. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120,5321. (c) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.(7) (a) Forensic InVestigation of Explosions, 1st ed.; Beveridge, A., Ed.;Forensic Science Series; Taylor & Francis: London, 1998. (b) Containingthe Threat from Illegal Bombings: An Integrated National Strategy forMarking, Tagging, Rendering Inert, and Licensing ExplosiVes and TheirPrecursors; National Academy Press: Washington DC, 1998.(8) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000,100, 2537. (b) Zhao, D.; Swager, T. M. Macromolecules 2005, 38, 9377.(c) Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K. S. Langmuir 2001,17, 7452. (d) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am.Chem. Soc. 2003, 125, 3821. (e) Sohn, H.; Calhoun, R. M.; Sailor, M. J.;Trogler, W. C. Angew. Chem., Int. Ed. 2001, 40, 2104.(9) Aldakov, D.; Anzenbacher, P., Jr. J. Am. Chem. Soc. 2004, 126,4752.ORGANICLETTERS2008Vol. 10, No. 173681-368410.1021/ol801030u CCC: $40.75  2008 American Chemical SocietyPublished on Web 07/26/2008cycloaddition of an acene to benzoquinone.10 The resultingpentiptycene quinone 211 may then be reacted with organo-metallic reagents such as lithium acetylides to yield 1,4-substituted derivatives 3 according to Scheme 1, which wasutilized by Swager to generate poly-(phenyleneethynylene)scomprising pentiptycene moieties.6 Unfortunately, there isno literature describing a similar approach using aryllithiumsor arylmagnesium halides.This pattern of limited reactivity of both iptycene-benzoquinone carbonyls was also observed by Yang etal.,12,13 who synthesized pentiptycenes with middle ringsubstituents (described here as 1,4-positions based on thep-benzoquinone).14 Yang et al. successfully attached a varietyof substituents to 1,4-positions, however, carbon substituentsproved difficult to introduce to both 1,4-positions. Notableexceptions are the acetylene derivatives introduced followingthe outline in Scheme 1.Originally, we believed the lack of a direct couplingmethod to be due to the limited availability of the 1,4-dihaloderivatives.12,15,16 However, Yang et al.12,13 were able tosuccessfully perform a Suzuki-Miyaura17 coupling on1-alkoxy-4-iodopentiptycene in 72% yield. In an effort tocircumvent the problem of limited availability of 1,4-dihaloderivatives, we synthesized the corresponding 1,4-triflate 4,which we hoped would react in Pd0-catalyzed reactions suchas the Stille,18 Suzuki-Miyaura,16 and Negishi19 couplings(Scheme 2). The reduction of quinone 2 to the correspondinghydroquinone form20 followed by reaction with triflicanhydride in pyridine at 0 °C gave the corresponding bis-triflate 4 in a quantitative yield (>99%). Triflate 4 is a stablecrystalline compound that can be prepared at a multigramscale. The coupling procedures, however, failed to yield 1,4-diarylpentiptycenes 1 in yields exceeding 9%. The startingmaterial 4 was recovered regardless of the coupling methodor reaction conditions.Simultaneously with the above approach, we attemptedto construct the 1,4-diarylpentiptycene using the aryne-anthracene [4 + 2] cycloaddition approach.21 Toward thisend we considered several strategies to generate a 1,4-diarylbisaryne precursor 5 to be reacted with anthracene.Numerous methods of generating benzyne are known fromthe literature,22 starting, for example, from haloaromatesusing sodium amide or other strong bases, 1,2-dihaloarenes,23from arenediazonium ions derived from anthranilic acid,242-(trimethylsilyl)phenyl triflates, aminotriazoles,25,26 andaryl[2-(trimethylsilyl)phenyl]iodonium triflates,27 etc. Com-pared to the double dehydrohalogenation, these methods ofgenerating aryne are often carried out using mild conditionsand furnish moderate to high yields of corresponding arynes.However, the preparation of the hexasubstituted benzeneprecursors requires multiple-steps syntheses that with excep-tion of tetrahalo-1,2-diarylbenzenes in our hands did not(10) (a) Clar, E. Chem. Ber. 1931, 64, 1676. (b) Cadogan, J. I. G.; Hall,J. K. A.; Sharp, J. T. J. Chem. Soc. C. 1967, 1860. (c) Cadogan, J. I. G.;Harger, M. J. P.; Sharp, J. T. J. Chem. Soc. B. 1971, 602.(11) Zhu, X.-Z.; Chen, C.-F. J. Org. Chem. 2005, 70, 917.(12) Yang, J. S.; Ko, C.-W. J. Org. Chem. 2006, 71, 844.(13) Yang, J. S.; Yan, J. L. Chem. Commun. 2008, 1501.(14) Hart, H.; Shamouilian, S.; Takehira, Y. J. Org. Chem. 1981, 46,4427.(15) Williams, V. E.; Swager, T. M. Macromolecules 2000, 33, 4069.(16) Rare Sonogashira coupling of 1,4-diiodopentiptycene was reportedby: Williams, V. E.; Yang, J. S.; Lugmair, C. G.; Miao, Y. J.; Swager,T. M. Proc. SPIE 1999, 3710 (Pt. 1), 402.(17) (a) Miyaura, N. In Metal-Catalyzed Cross-Coupling Reactions; DeMeijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004; p 41. (b)Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020.(18) Mitchell, T. N. In Metal-Catalyzed Cross-Coupling Reactions; DeMeijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004; p 125.(19) Negishi, E.-I.; Dumond, Y. In Handbook of OrganopalladiumChemistry for Organic Synthesis; Negishi, E.-I., Ed.; Wiley: New York,2002; Vol. 1, p 229.(20) Yang, J.-S.; Lee, C.-C.; Yau, S.-L.; Chang, C.-C.; Lee, C.-C.; Leu,J.-M. J. Org. Chem. 2000, 65, 871.(21) (a) Hoffman, R. W. Dehydrobenzene and Cycloalkynes; AcademicPress: New York, 1967. (b) Gilchrist, T. L. In The Chemistry of FunctionalGroups; Supplement C, Chapter 11; Patai, S.; Rappoport, Z.; Eds.; Wiley,Chichester, 1983. (c) Hart, H. In The Chemistry of Triple-Bonded FunctionalGroups; Supplement C2, Chapter 18; Patai, S. ; Ed.; Wiley, Chichester,1994.(22) Pellisier, H.; Santenelli, M. Tetrahedron 2003, 59, 701.(23) Shahlai, K.; Acquaah, S. O.; Hart, H. Organic Syntheses 2004, 10,678; Organic Syntheses 1998, 75, 678.(24) Buxton, P. C.; Heaney, H. J. Chem. Soc., Chem. Commun. 1973,545.(25) Campbell, C. D.; Rees, C. W. J. Chem. Soc. (C) 1969, 742.(26) Hart, H.; Ok, D. J. Org. Chem. 1986, 51, 979.(27) (a) Kitamura, T.; Yamane, M.; Inoue, K.; Todaka, M.; Fukatsu,N.; Meng, Z.; Fujiwara, Y. J. Am. Chem. Soc. 1999, 121, 11674. (b)Kitamura, T.; Yamane, M. J. Chem. Soc. Chem. Commun. 1995, 9, 983.Scheme 1. Synthesis of 1,4-Substituted PentiptycenesScheme 2. Alternative Synthesis of 1,4-Diaryl PentiptycenesaaX-ray ORTEP representation of 4 (displacement ellipsoids are scaledto the 50% probability level).3682 Org. Lett., Vol. 10, No. 17, 2008result in overall yields exceeding 40% of the precursor forthe preparation of diaryl bisaryne 5. Because we are targetinglow-cost precursors for the sensor polymers, we decided tofocus on a more straightforward method that would utilizefew synthetic steps and would be easy to scaleup.For this reason, we decided to investigate the simpledehydrohalogenation/Diels-Alder cycloaddition of an in situgenerated aryne to anthracene. There are literature precedents,albeit not for diarylpentiptycenes, suggesting that this reactionwould proceed. For example, Hart28 described a Bu-Lipromoted reaction of 3,6-substituted 1,2,4,5-tetrabromoben-zene derivatives in the presence of anthracene to yieldpentiptycene, 1,4-dimethylpentiptycene, and 1,4-dimethoxy-pentiptycene in 26, 14, and 21% yield, respectively. Anearlier report by Cadogan29 describes the reaction of stericallycrowded 1,4-dibromo-2,5-di-t-butylbenzene and tBuOK asa base in the presence of anthracene, which yielded t-butylaryl ethers as well as a pentiptycene side-product, albeit in∼2% yield.First, we explored the reaction conditions described byCadogan28 using 3-halo-1,4-bis(thien-2-yl)benzene and po-tassium t-butoxide in the presence of anthracene (Scheme3). Reaction condition tuning allowed to generate thecorresponding di(thien-2-yl)triptycene 7 in up to 87% yield.The identity of 7 was confirmed by spectroscopic methodsas well as X-ray crystallography.Similar conditions were then used to synthesize 1,4-diarylpentiptycenes 1a-e. While the previous reaction was carriedout using 3-bromo- or 3-iodo-1,4-bis(thien-2-yl)benzene, for thesynthesis of 1a-e we decided to use the 3,6-dichloro congener.While the chloride is certainly not as good a leaving group asbromide or iodide, the preparation of the starting 1,4-diaryl-2,5-dichlorobenzenes 9a-e in photonic purity30 using appropri-ate arylboronic acid16 or aryl-tin derivative17 to give thecorresponding dichloro-derivatives 9a-e was easier. In the caseof 1,4-diaryl-2,5-dibromobenzenes prepared from 1,4-dibromo-2,5-diiodobenzene using Pd0 coupling procedures, the presenceof 1-5% of difficult to separate side-products was observed.31Following the arylation of 1,4-dibromo-2,5-dichlorobenzene 8the 1,4-diaryl-2,5-dichlorobenzenes 9a-e were subjected todouble t-BuOK-promoted dehydrohalogenation/cycloadditionto anthracene present in excess (∼5 equiv) in the reactionmixture. Pentiptycenes 1a-e were purified by column chro-matography and the pure products were obtained in respectableyields of 20-56%. Similarly, sodium amide also furnishedexpected pentiptycenes, albeit in a lower amount. The synthesisof 1,4-diarylpentiptycenes 1a-e is shown in Scheme 4.Thus far, we have prepared 1,4-diarylpentiptycenes 1a-e,and it seems that the reaction is fairly general: both arenesand heteroarenes may be used.Additionally, we explored the flexibility of this syntheticprotocol. The regioselectivity in the coupling step is deter-mined by the preferential oxidative addition (C-I > Br >Cl) of the haloarene to the Pd0-catalyst. The combination ofI and Br substituents gave the expected 1,4-diaryl-2,5-dibromobenzene, which underwent dehydrohalogenation-cycloaddition smoothly to give 1 in slightly higher (∼5-10%)yields compared to the dichloro-compound 8. Following therecently described double dehydrofluorination of 1,4-dif-luoro-2,5-dimethoxybenzene, which was also utilized in theDiels-Alder reaction32 we also tested the difluoro-analogsof 9. Here too, the 1,4-diaryl-2,5-difluorobenzenes underwentdehydrohalogenation-cycloaddition, with only slightly di-minished yields compared to the chloro-congeners. A step-wise dehydrohalogenation using BuLi at -78 °C as describedin the literature31 resulted in only small (<5%) improvement.It seems that the sterical demands of the 1,4-aryl substituents(28) Hart, H.; Shamouilian, S.; Takehira, Y. J. Org. Chem. 1981, 46,4427.(29) Cadogan, J. I. G.; Harger, M. J. P.; Sharp, J. T. J. Chem. Soc. 1971,602.(30) Photonic materials were purified to obtain satisfactory results usingMS/NMR/elemental analyses, but also single-exponential luminescence lifetimedecays. Even small amounts (<1%) of brigthly fluorescent terphenyl-typeimpurities would invalidate the later spectroscopic measurements.(31) These side-products were observed even when the reaction wascarried out at low temperature (30-45° C). According to MS spectra theseminor products originated from deiodination and coupling to m-position(i.e., result of C-Br oxidative addition).(32) Morton, G. E.; Barrett, A. G. M. J. Org. Chem. 2005, 70, 3525.(34) Zhao, D.; Swager, T. M. Macromolecules 2005, 9377.(33) Thomas, S. W.; Amara, J. P.; Bjork, R. E.; Swager, T. M. Chem.Commun. 2005, 4572.Scheme 3. Base-Promoted Reaction of3-Halo-1,4-bis(thien-2-yl)benzene (X ) Br, Cl)aaX-ray ORTEP representation of 7 (displacement ellipsoids are scaledto the 50% probability level).Scheme 4. Synthesis of 1,4-Diaryl PentiptycenesOrg. Lett., Vol. 10, No. 17, 2008 3683rather than their electronic properties, the nature of theleaving group or the method generating the aryne determinethe reaction yield.The ability of 1,4-diarylpentiptycenes to act as TNTsensors was studied in solution and in the solid statefollowing Swager’s procedures33,34 utilizing fluorescence asa measure of the pentiptycene receptor-nitroaromate interac-tion. The fluorescence quantum yields were estimated asfollows: 1a (10%, 4.03 ns), 1b (19%, ND), 1c (15%, 0.3ns), 1d (∼25%, 0.8 ns), 1e (20%, 0.82 ns) using anthraceneas a standard. Fluorimetric titration of 1a-e by variousnitroaromates such as nitrobenzene (NB), 2,4-dinitrotoluene(2,4-DNT), trinitrotoluene (TNT), 1-nitronaphthalene (NN),and 2,3-dimethyl-2,3-dinitrobutane taggant (DMDNB) indichloromethane showed an expected decrease in fluores-cence intensity (Figure 1). Fits of the first linear part of theStern-Volmer plots allowed estimating apparent quenchingconstants to be in the range of 1000-3300 M-1 in the caseof TNT. These values of quenching constants are comparableto the ones found for amplifying polymers.8aTo estimate the ability of 1a-e to sense nitroaromatevapors, we used a glass slide with round wells (1.3 mmdiameter; 0.8 mm deep; 4 × 3 wells, 3 compounds in 4replicas), and the sensors 1d and 1e were cast in the wellsas a polyurethane solution in THF. Tetraphenylporphyrin(TPP) was used as an internal standard.Upon illumination with black light (∼360 nm), TPP as wellas 1d and 1e showed emission in the solid state. The order offluorescence intensity from the slides in Figure 2, that is, beforebeing exposed to equilibrium vapors of DNT (0.18 ppm) orTNT (7.70 ppb), was 1e > TPP > 1d. The glass slide andsensor films were imaged before and after exposition to DNTor TNT vapors. Within 30 min the sensors fluorescence wasquenched (Figure 2). Comparison of all slides shows the redTPP fluorescence did not change, presumably because theelectron excited to the TPP LUMO level does not possess theenergy to reduce the nitroaromates.8a 1d and 1e, on the otherhand, displayed DNT and TNT-mediated fluorescence quench-ing suggesting an interaction between 1d and 1e and DNT orTNT.Given the difference in the LUMO-levels of the twosensors as well as the quenchers it is not entirely surprisingthat the sensor films show a different degree of quenchingfor each vapor analyte. The fluorescence of the sensor filmswas largely restored when the films were washed by a streamof clean air. This suggests that these easy-to-make sensorfilms may be used in differential (array-based) sensors fornitroaromates.35In summary, we have developed a simple method forsynthesis of 1,4-diarylpentiptycenes in two steps from 1,4-dibromo-2,5-dichlorobenzene that involves double dehydro-halogenation followed by aryne cycloaddition to anthracene.The yield of the respective 1,4-diarylpentiptycenes is moder-ate considering that four consecutive steps are involved(20-56% overall for four steps, i.e., 60-85% per step).Moreover, the synthesis is easy to perform from inexpensivestarting materials. Preliminary fluorescence quenching ex-periments aimed at evaluation of 2,4-DNT and TNT interac-tion with 1,4-diarylpentiptycenes 1d and 1e showed a highdegree of fluorescence quenching characterized by apparentquenching constant in the range of 1000-3300 M-1 for TNT.Solution-cast polyurethane films doped with 1d and 1eshowed similar quenching patters in the solid state. Takentogether these observations suggest that 1,4-diarylpentip-tycenes could be of potential interest for the fabrication ofsensors for explosives. The preparation of such materials anddevices is underway.Acknowledgment. Financial support from BGSU (TIEgrant to P.A.), the NSF (EXP-LA #0731153 to P.A.) andMcMaster Endowment to M.A.P. is acknowledged.Supporting Information Available: Experimental pro-cedures and spectroscopic data for 8a-e, 1a-e, X-ray datafor 3 and 6. This material is available free of charge via theInternet at http://pubs.acs.org.OL801030U(35) Germain, M. E.; Knapp, M. J. J. Am. Chem. Soc. 2008, 130, 5422.Figure 1. (Left) Emission quenching observed with 1e (5.0 µM inCH2Cl2) upon DNT addition. (Right) Stern-Volmer plots for 1eand various nitroaromates. Excited at 325 nm (abs. maximum).Figure 2. (Top) Photograph of the sensor slide before and afterexposure to DNT and TNT in their equilibrium vapors at 22 °C,and finally upon recovery in the stream of clean air. (Bottom)Integrated light output from the sensors represented as a quasi-3Dgraph (X-Y-Intensity) allows for signal quantitation.3684 Org. Lett., Vol. 10, No. 17, 2008

Simple Molecule-based Fluorescent Sensors For Vapor Detection Of Tnt

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Simple Molecule-based Fluorescent Sensors For Vapor Detection Of Tnt

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