Bhowmik, Benoy B

Nandy, Papiya

Paul, Tapan K

English

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I , "W, I j,Ji lit. 11iIf11l,IIIfi.JI1'IU, I 'i1 HII·,i ",.11 1""", ~ Iw II 1'1 i 1 i 1::1IndianJournalof ChemistryVol. 26A, November1987,pp.902-905ExciplexesofPolycyclicAromaticHydrocarbonswithHalomethanesBENOY B BHOWMIK*, TAPAN K PAUL &PAPIYA NANDYDepartmentsof Chemistry&Physics,JadavpurUniversity,Calcutta700032Received13October1986;revised12January1987;accepted12February1987The fluorescencespectraof polycyclicaromatichydrocarbonssuchasanthracene,1,2-benzanthracene,2 3-ben-zanthraceneand1,2,5,6-dibenzanthracenequenchedby halomethanes,revealshiftin fluorescenceband.Both Stern-Volmerconstantsfor fluorescencequenchingandfluorescencebandshiftsarelinearlyrelatedtothedifferenceof ioni-zationpotentialandfirstexcitedsingletenergyof aromatichydrocarbons.Fig. I-Fluorescence spectraof anthracenein differentsolventsat25°C(Solventsare:(1)heptane,(2)CHzClz, (3)CHCl3 and(4)CC14. Concentrationof anthraceneis 1.067x 10- 4 moldm- 3 inallcases]Resultsand DiscussionThe fluorescencespectraof anthracenein differ-entsolventsat 25°Carepresentedin Fig. 1;otheraromatichydrocarbonshowedsimilarbehaviourinnitrogenatmosphereandtoitwasaddedameasuredvolumeof heptaneto getthe requiredconcentra-tion.All solutionswerealwayspreparedafreshinthedarktoavoidphotochemicalchange.Fluorescencespectraof thesesolutionswerere-cordedon a Perkin-ElmerMPF-44B spectfofluor-imeterusingfusedsilicacell of 1 cm opticalpathlength.WAVElENGTH (nml2IIIII,I,,III 3,,IIII--,/'380.•'e~,..~:e.!!>-t-o;;ZUlt-~UlUZ~UJa:o::>-'u.Materialsand MethodsAromatic hydrocarbons (Aldrich Chemicals,USA), listedin Table1,werepurifiedbyrecrystalli-zationfrombenzene.Spectrograde(BDH) halome-thanesandheptanewereusedassuch;exceptchlor-oformwhichwaspurifiedby standardmethod.Be-causeof the sparingsolubilityof thesehydrocar-bonsinheptane,excepthatof anthracene,thesolu-tions of thesearomatichydrocarbonswere pre-paredby dissolvingthesein benzene.An aliquotfrombenzenesolutionwasdriedin thedarkunderRecentlywe havereported1theformationof con-tactchargetransfer(CT) pairinheptanesolventandthermodynamicandspectrophotometricpropertiesof the 1:1(CT) pairsin heptanesolution,betweendifferentpolycyclicaromatichydrocarbons,namelyanthracene,1,2-benzanthracene(tetraphene),2,3-benzanthracene(tetracene)and1,2,5,6-diabenzan-thracene(asdonors)andhalomethanessuchascar-bontetrachloride,chloroformanddichloromethane(asacceptors).There areseveralreports2-12of flu-orescencequenchingof aromatichydrocarbonsbyhalomethanesin solutionaswellasin gasphaseandof photoreactionsinvolvingexciplexesof reactantspecies.Eiectrondonor-acceptor(EDA) interactionbetweentwodissimilarmoleculeswhenoneof themis in electronicallyexcitedstategivesriseto familiarexciplexemission.In themajorityof reportedcases,polycyclicaromatichydrocarbonsactedaselectronacceptorsandtheelectrondonormoleculesinvar-iably containedone or moreheteroatomslike ni-trogenor oxygent3-17andoneofthe lonepairelec-tronwastransferedto theexcitedaromatichydro-carbonmoleculeduringexciplexformation.To un-derstandEDA propertiesof thesepolycyclicaro-matichydrocarbonsin theexcitedstateinteractionwith halomethanes,the fluorescencespectraofthesearomatichydrocarbonsin thepresenceof hal-omethaneshavenowbeenstudied.90211II" *""." IBHOWMIK etal: EXCIPLEXES OF POLYCYCLIC AROMATIC HYDROCARBONSTable1-FluorescenceMaximaofAromaticHydrocarbonsinDifferentSolventswithIonizationPotentials(ID) andFirstExcitedSingletEnergies(SI) ofAromaticHydrocarbonsAromatic l*~ V max(cm- I)insolventt0hydrocarbon(e.V)(eV)HeptaneCHzClzCl3Cl4Anthracene7.123.32250924814752691(280)342)403Tetraphe e35245403 06315179(2Tetracene6 6463198 21952739815Dibenza thracene41028821157*Valuesof 10 and5,aretakenfromrefs.20and19respectively.tFiguresin parenthesesrepresentspectralshiftson goingfromheptanetohalomethane.Table2-Stem-VolmerQuenchingConstantsof Tetra-cene-HalomethaneandCarbonTetrachloride-Aromatic 600HydrocarbonSystemsat25°CTetracene(4.108x 10-5) Carbontetrachloride(0.21-1.04)400200-\;2I'?<I0.1640.1060.049Ksv(dm3mol-I)0.641HalomethaneCHzClz(0.31-1.56)CHCl3(0.25-1.25)CCl4(0.21-1.04)Ksv Aromatic(dm3mol-I) hydrocarbon0.106 Anthracene(9.56x 10-5)0.147 Tetraphene(5.87x 10-5)0.164 Tetracene(4.108x 10-5)Dibenzanthracene(4.11x 10-5)Figuresin parenthesesindicateconcentrationinmoldm-3.Fig. 2-Correlation betweenthespectralshift(Av) of fluoresc-encebandsof aromatichydrocarbonsin differenthalomethanesand(Io - 51) of aromatichydrocarbons[Eachcurverepresentsfouraromatichydrocarbonswithonehalometbane:CHzClz (Q),CHCI3 (A)andCCI4(0)]fluorescencefor aromatichydrocarbonsaloneandin thepresenceof halomethane(A) respectively;[A]is the molar concentrationof halomethane.Theplotsof +[1+fagainst[A]areall linear(Figs3and4),fromtheslopesof whichKsv valuesarecalculated.The valuesof Ksv of differentaromatichydrocar-bonswithcarbontetrachlorideaswellasof differ-enthalomethaneswith tetraceneare presentedinTable2.The red-shiftin thefluorescencespectraof aro-matichydrocarbonsin halomethanesin additiontothenormalquenchingmaybedueto theformationthesesolvents.The fluorescencebandpositionsofthearomatichydrocarbonsin differentsolventsare·recordedin Table1.It is evidentfromTable1 thatthefluorescencebandof allthearomatic hydrocar-bonsundergoeshifton goingfromheptanetohal-omethane.This shiftlinearlyincreaseswithincreas-ingionizationpotentialof halomethanebutit is notrelatedto the ionizationpotentialof aromatichy-drocarbon.Howevera good linearrelationshipisobtainedbetweenspectrashift(~v)anddifferencein ionizationpotentialandfirstexcitedsingletener-gy(Io- 51)ofaromatichydrocarbon(Fig.2).By measuringthe decreasein relativequantumyieldsof aromatichydrocarbons(excitedatthelon-gestwavelengthabsorptionbandin heptane)atvar-yingconcentrationsof halomethanesinheptane,theStem-Volmerquenchingconstant,Ksvhasbeencal-culatedusingtherelation18(1).+~- =1+Ks,,{A] ... (1)+fwhere~fandcj>f arethe,relativequantumyieldsofo3·8 4·0In-SlleVI4·2 4·4903I, lii~1 j HJ'liH, •••~i.riJll'IUI wli"i 1,,01, I n Ilh II l,j'I:'1INDIAN J. CHEM., VOL. 26A,NOVEMBER 1987'·3'·2&.•...0& '.'o 0·4 011 '·2[H"'LOMETH"'NES]1.6;'-0-~Fig. 3-SpectrofIuorimetric determinationof Stern-Volmerquenchingconstants(Ksv) of tetracene-halomethanesin hep-taneat 25°C[Halomethanesare:(1)CH2CI2, (2)CHCl3 and(3)CCI4• Concentrationof tetracene:4.108x 10- 5 mol dm- 3 andconcentrationrangeofhalomethanes:0.2-1.5moldm- 3]o 0·4 0·8[c C14]12 1·6of exclplexes,i.e. complexesbetweenthe excitedaromatichydrocarbonsandthehalomethanemole-culeswhichbreakup shortlyafterformationeitherwith theemissionof thecharacteristicspectrumorthroughnonradiativepathway.If this emissionisdueto exciplexformation,onewouldexpectarela-tionshipof thetyperepresentedbyEq. (2),betweenthefrequency(vf) of emissionandothermolecularparameters.19In Eq. (2) ID andSl arerespectivelytheionizationpotentialandthefirstsingletexcitedstateenergyofaromatichydrocarbons,EA is theelectronaffinityofhalomethanesand A representsthevariousotherinteractionterms.As theplotof vf againstID- Sl orIDisnonlinear,theemissionbandsarethered-shift-ed normalfluorescencebandsdueto exciplexfor-mationandnotduetoexciplexemissionoremissionfrom ground stateCT complexes.The exciplexemissionis expectedat longerwavelengthwhichisnotobservedin thesesystems.The increasein red-shiftoffluorescenceband,A v withincreasein ioni-zationpotentialof halomethanesaswellaswithde-creasingmagnitudeof (ID - Sl) of aromatichydro-carbonssuggeststhathalomethanesactaselectronacceptorsand aromatichydrocarbonsas electrondonors.The donorcapacitiesof aromatichydrocar-hvf=(ID- Sl)- EA - A ... (2)Fig. 4-Spectrofluorimetric determinationof Stern-Volmerquenchingconstants(Ksv) of aromatichydrocarbon-CCl4inheptaneat 25°C [Aromatichydrocarbonswith concentration(mol dm-3): (1) anthracene(9.56x 10-5), (2) tetraphene(5.8.1x to-5), (3)tetracene(4.108x 10-5), (4)dibenzanthracene(4.11x to-5). ConcentrationrangeofCC14: 0.21-1.04moldm- 3in allcases]bons follow the order: anthracene>tetra-cene>tetraphene>dibenzanthracene;whiletheac-ceptor capacities of halomethanes are inthe order: carbontetracWoride>chloroform>di-cWoromethane.This supportedby the corres-pondingvalues of Stern-Volmerquenchingcon-stantsof thesesystems..[he Stem-Volmerquenchingconstantsaremea-suredatlow concentrationsof halomethanes,inceat higherconcentrations,a curvaturein Stern-Vol-mer plots is obtained.The curvatureindicatesground state complexes9and the formation ofgroundstatecomplexesin thesesystemsis verifiedbythestudiesof absorptionspectral.AcknowledgementWearegratefultoProf.Jeffrey1.Zink ofUnivers-ityof California,Los Angeles,USA for thegenerousgiftof somearomatichydrocarbons.References1 BhowmikB B & PaulT K, SpectrochimActa, 38A (19&2)713;40A(1984)606.904"11 '''j"l~jBHOWMIK etal.:EXCIPLEXES OF POLYCYCLIC AROMATIC HYDROCARBONS2 OsterG K, ActaPhysPolon,26(1964)435.3 Oster G K & Kallmann,InternationalLuminescenceSym-posium,Munich, (1965),eds N. Riehl & H. Kallman,Thiemig,Munich (1966).4 LeiteM SS C &NaqviK R, ChemPhysLeu,4 (1969)35.5 WareW R & LewisC,] chemPhys,57 (1972)3546.6 Klein J, PlazanetV P & LaustriatG,] chimPhysPhysichimBio~67 (1970)302.7 DasguptaG & Philips D, ] chemSoc FaradayII, (1972)2003.8 Soperstein&LevinE,] chemPhys,62 (1975)3560.9 Encinas M V, Rubio M A & Lissi E A, ] PholOchern,18(1982)137.10 KhwajaH A, SemlukG P & UngerI, Can] Chern,60( 13)(1982)1767.11 Rohatigi-MukherjeeK K, Fundamentalsofphotochemistry(WileyEasternLimited,NewDelhi),1986.12 SelvrajanN, PanickerM M, VaidyanathanS & Ramakrish-nanV, Indian] Chern,18A(1979)23.13 RehmD & WellerA, Zphys Chern,(NF), 69 (1970)183.14 NakajimaA, Bull chemSoc]apan,42 (1969)3409.15 MatagaN, OkadaT & EzumiK, MolecPhys,10 (1966)201.16 Knibbe H, Rehm D & WellerA, Z phys Chem(NF), 56(1967)95,99.17 Kar PurkayasthaA & BasuS, Indian] Chem,20A (1981)389.18 Stem0 &VolmerM,PhysikZ, 20 (1919)183.19 SlifkinM A, Nature,200 (1963)767.20 MatsenF A,] chemPhys,24 (1956)602.905

Exciplexes of Polycyclic Aromatic Hydrocarbons with Halomethanes

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Exciplexes of Polycyclic Aromatic Hydrocarbons with Halomethanes

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