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Published on Web Date: August 18, 2010 r2010 American Chemical Society 2618 DOI: 10.1021/jz1009295 | J. Phys. Chem. Lett. 2010, 1, 2618–2621 pubs.acs.org/JPCL Photoinduced Electron Transfer in a Prototypical Mulliken Donor-Acceptor Complex: C 2 H 4 333 Br 2 Lisa George, Laura Wittmann, Aimable Kalume, and Scott A. Reid* Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881 ABSTRACT We report a new dual nozzle late mixing scheme for the trapping and interrogation of prereactive donor-acceptor complexes that is used initially to examine photoinduced electron transfer in the prototypical Mulliken donor- acceptor (halogen-bonded) π-complex, C 2 H 4 333 Br 2 . Excitation into the intense charge-transfer band of the complex leads exclusively to the anti conformer of the single reaction product, 1,2-dibromoethane, in agreement with the Mulliken theory of electron transfer. SECTION Dynamics, Clusters, Excited States T he electron-transfer archetype for chemical reactivity underlies important processes in diverse areas ran- ging from biochemistry to solar cell development to nanomaterials and molecular electronics, 1- 11 and donor- acceptor organizations are widely exploited in all of these areas. As originally suggested by Mulliken, 12 electron transfer in these assemblies involves initial formation of a donor- acceptor complex (D 333 A), which exhibits an intense charge- transfer transition, representing the transfer of a single elec- tron from the highest occupied molecular orbital (HOMO) of the donor to the lowest unoccupied MO (LUMO) of the acceptor. It is widely accepted that photoexcitation of the complex generates the ion radical pair (D þ333 A -• ), which can react irreversibly to products or regenerate the complex via back electron transfer, 1 and these processes have, for selected donor-acceptor pairs, been followed in solution using ultrafast spectroscopy. 13-16 The prototypical Mulliken D 333 A complexes are also model systems for exploring halogen bonding 17 and involve donors such as ethylene (C 2 H 4 ) with acceptors such as the dihalogen Br 2 , which is the textbook example of electrophilic bromina- tion of an olefin. Isolation and subsequent charge-transfer excitation of the complex is an appealing route to probe the reaction mechanism; however, this has proven difficult. Thus, the complex has been detected and structurally characterized in a supersonic expansion using microwave spectroscopy 18 and, following an initial report of the IR spectrum in a low- temperature mixture of ethylene and bromine, 19 was trapped in an inert (Ar) matrix at 10 K using a continuous co- condensation approach. 20 Excitation of the isolated complex in the ultraviolet (λ > 300 nm) led to the formation, in nearly equal yield, of the anti and gauche conformers of the single reaction product, 1,2-dibromoethane, 20 which was attributed to a radical addition mechanism. In a recent study of the laser photolysis of matrix-isolated 1,2-dibromoethane, we observed the C 2 H 4 333 Br 2 complex as a primary photoproduct and recorded the UV/visible spec- trum of the complex for the first time. 21 The position of the charge-transfer band (λ max = 237 nm) is in excellent agree- ment with theoretical predictions, and we find that excitation at λ > 300 nm does not access this band but rather transitions localized on the Br 2 chromophore (Figure 1). The dominance of the radical mechanism in this case is then understood; as illustrated in Scheme 1a, cleavage of the Br-Br bond leads to bromine atom attack on the double bond, forming the bromoethyl radical, which, following our recent report, 21 has a classical ground-state structure. Cis or trans addition of the second bromine atom to the radical center, occurring with equal preference, then leads to the anti or gauche conformers of 1,2-dibromoethane. In contrast, charge-trans- fer excitation should initiate electron transfer, 1 Scheme 1b, leading through a bridged bromonium ion intermediate to the anti conformer of the reaction product. This Letter describes the development of a new method for isolation of the complex in high yield and reports initial studies of the charge-transfer photochemistry of this textbook Mulliken complex. Our experiments utilized a dual pulsed nozzle late mixing scheme in combination with matrix isolation, Figure 2a. Apart from the late mixing source, which is similar in design to sources reported for molecular beam studies, 22,23 the appa- ratus used in these experiments has been described in detail in earlier publications. 21,24,25 The source utilized two solenoid pulsed valves, which produced 1 ms duration pulses at a variable repetition rate and variable delay, controlled by a pulse/delay generator (SRS DG535). Three different experi- ments were conducted in an attempt to isolate the C 2 H 4 333 Br 2 complex, and the results are illustrated in Figure 2b, which displays a region of the infrared (IR) spec- trum near the ethylene monomer ν 12 band (CH 2 scissor) . The spectral region shown is convenient for monitoring Received Date: July 8, 2010 Accepted Date: August 13, 2010
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  • Published on Web Date: August 18, 2010

    r 2010 American Chemical Society 2618 DOI: 10.1021/jz1009295 |J. Phys. Chem. Lett. 2010, 1, 2618–2621

    pubs.acs.org/JPCL

    Photoinduced Electron Transfer in a PrototypicalMulliken Donor-Acceptor Complex: C2H4 3 3 3Br2Lisa George, Laura Wittmann, Aimable Kalume, and Scott A. Reid*

    Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881

    ABSTRACT We report a newdual nozzle latemixing scheme for the trapping andinterrogation of prereactive donor-acceptor complexes that is used initially toexamine photoinduced electron transfer in the prototypical Mulliken donor-acceptor (halogen-bonded) π-complex, C2H4 3 3 3Br2. Excitation into the intensecharge-transfer band of the complex leads exclusively to the anti conformer ofthe single reaction product, 1,2-dibromoethane, in agreement with the Mullikentheory of electron transfer.

    SECTION Dynamics, Clusters, Excited States

    T he electron-transfer archetype for chemical reactivityunderlies important processes in diverse areas ran-ging from biochemistry to solar cell development tonanomaterials and molecular electronics,1-11 and donor-acceptor organizations are widely exploited in all of theseareas. As originally suggested by Mulliken,12 electron transferin these assemblies involves initial formation of a donor-acceptor complex (D 3 3 3A), which exhibits an intense charge-transfer transition, representing the transfer of a single elec-tron from the highest occupied molecular orbital (HOMO) ofthe donor to the lowest unoccupied MO (LUMO) of theacceptor. It is widely accepted that photoexcitation of thecomplex generates the ion radical pair (Dþ• 3 3 3A

    -•), whichcan react irreversibly to products or regenerate the complexvia back electron transfer,1 and these processes have, forselected donor-acceptor pairs, been followed in solutionusing ultrafast spectroscopy.13-16

    TheprototypicalMullikenD 3 3 3Acomplexesare alsomodelsystems for exploring halogen bonding17 and involve donorssuch as ethylene (C2H4)with acceptors such as the dihalogenBr2, which is the textbook example of electrophilic bromina-tion of an olefin. Isolation and subsequent charge-transferexcitation of the complex is an appealing route to probe thereactionmechanism; however, this has proven difficult. Thus,the complex has been detected and structurally characterizedin a supersonic expansion using microwave spectroscopy18

    and, following an initial report of the IR spectrum in a low-temperaturemixture of ethylene and bromine,19was trappedin an inert (Ar) matrix at 10 K using a continuous co-condensation approach.20 Excitation of the isolated complexin the ultraviolet (λ>300 nm) led to the formation, in nearlyequal yield, of the anti and gauche conformers of the singlereaction product, 1,2-dibromoethane,20 which was attributedto a radical addition mechanism.

    In a recent study of the laser photolysis of matrix-isolated1,2-dibromoethane, we observed the C2H4 3 3 3Br2 complex asa primary photoproduct and recorded the UV/visible spec-trum of the complex for the first time.21 The position of the

    charge-transfer band (λmax = 237 nm) is in excellent agree-ment with theoretical predictions, and we find that excitationat λ>300nmdoesnot access this bandbut rather transitionslocalized on the Br2 chromophore (Figure 1). The dominanceof the radical mechanism in this case is then understood; asillustrated in Scheme 1a, cleavage of the Br-Br bond leads tobromine atom attack on the double bond, forming thebromoethyl radical, which, following our recent report,21

    has a classical ground-state structure. Cis or trans addition ofthe second bromine atom to the radical center, occurringwith equal preference, then leads to the anti or gaucheconformers of 1,2-dibromoethane. In contrast, charge-trans-fer excitation should initiate electron transfer,1 Scheme 1b,leading through a bridged bromonium ion intermediate tothe anti conformer of the reaction product. This Letterdescribes the development of a new method for isolationof the complex in high yield and reports initial studies of thecharge-transfer photochemistry of this textbook Mullikencomplex.

    Our experiments utilized a dual pulsed nozzle late mixingscheme in combinationwithmatrix isolation, Figure 2a. Apartfrom the late mixing source, which is similar in design tosources reported for molecular beam studies,22,23 the appa-ratus used in these experiments has been described in detailin earlier publications.21,24,25 The source utilized two solenoidpulsed valves, which produced 1 ms duration pulses at avariable repetition rate and variable delay, controlled by apulse/delay generator (SRS DG535). Three different experi-ments were conducted in an attempt to isolate theC2H4 3 3 3Br2 complex, and the results are illustrated inFigure 2b, which displays a region of the infrared (IR) spec-trum near the ethylene monomer ν12 band (CH2 scissor).The spectral region shown is convenient for monitoring

    Received Date: July 8, 2010Accepted Date: August 13, 2010

  • r 2010 American Chemical Society 2619 DOI: 10.1021/jz1009295 |J. Phys. Chem. Lett. 2010, 1, 2618–2621

    pubs.acs.org/JPCL

    complex formation since the complex shows two absorp-tions in this region that are infrared-inactive in free ethylene.

    In our initial experiments, trace (i), the timing of the C2H4/Ar and Br2/Ar pulses was adjusted for maximum overlap, sothat both gas pulses were simultaneously present in themixing channel. This resulted in the complete thermal reac-tion of the ethylenemonomer, as evidenced by the loss of thecorresponding bands in the IR spectrum, and the appearanceof two broad bands in the IR that we tentatively ascribe toformation of polymeric products. In this case, a thermalreaction occurs in the mixing channel prior to expansionand subsequent deposition.

    In a second set of experiments, we delayed the C2H4/Arand Br2/Ar pulses by 1/2 of the inverse repetition rate of theexperiment (167ms), so that a “sandwich”matrix comprisedof alternating layers of C2H4 and Br2 in Ar was deposited.Following annealingof thematrix to 33Kand recooling to 5K,the C2H4 3 3 3Br2 complex was formed, but in very small yield[trace (ii) in Figure 2b]. Presumably, the diffusion length of theC2H4 monomer is insufficient to afford transport to a Br2matrix site. From the integrated IR intensities and calculated(MP2/aug-cc-pVTZ) IR spectrum of C2H4 and the C2H4 3 3 3Br2complex, we estimate that the ratio of monomer to complexin this case is >50:1.

    In the final set of experiments, we slightly delayed (by1ms) the Br2/Ar pulse in order to limitmixing in the gas phase

    but ensure that the pulses arrived at the cold window in rapidsuccession. As shown in Figure 2b (trace iii), this resulted in amuch larger yield of the desired complex, even withoutannealing. The observed spectrum of the complex is in goodagreement with theory and previous work.20,21 Due to thevery small shift in the ν12 band of the monomer uponcomplex formation (Figure 2), we used the stronger ethylene

    Figure 1. UV/visible spectrum of the C2H4 3 3 3Br2 complex. Thecalculated (TDCAM-B3LYP/aug-cc-pVQZ) spectrum is shown asthe stick spectrum.

    Scheme 1. Radical (a) and Charge-Transfer (b) Mechanisms for Reaction of the C2H4 3 3 3Br2 Complex

    Figure 2. (a) Schematic of the matrix isolation apparatus with adual nozzle late mixing source. (b) Infrared spectra for threedifferent experiments, as described in the text. The calculated(unscaled MP2/aug-cc-pVTZ) spectra of C2H4 and the C2H4 3 3 3Br2complex are shown.

  • r 2010 American Chemical Society 2620 DOI: 10.1021/jz1009295 |J. Phys. Chem. Lett. 2010, 1, 2618–2621

    pubs.acs.org/JPCL

    ν7 band (CH2 wag), which displays a sizable frequency shiftupon complexation,20 to estimate the ratio of monomer tocomplex, determined to be∼2:1. In comparison, the ratio ofcomplex to thermal reaction product was ∼10:1 under thesame conditions. It is possible that this ratio could be furtherimproved by fine-tuning the pulse delay.

    The charge-transfer photochemistry of the complexwasprobed by laser irradiation (λ = 266 nm) of a C2H4 3 3 3Br2sample prepared according to the developed protocol. Theresulting difference IR spectrum, shown in Figure 3, de-monstrates the loss of bands assigned to the C2H4 3 3 3Br2complex and the growth of bands assigned to a singleproduct, the anti conformer of 1,2-dibromoethane. Thus,in the matrix environment, excitation into the charge-transfer band of the isolated complex leads to the exclusiveformation of anti-1,2-dibromoethane via the electron-transfer mechanism shown in Scheme 1b. Specifically,electron transfer following charge-transfer excitation leadsto formation of the ion radical pair (C2H4

    þ•3 3 3Br2

    -•). Thebreakup of the Br2

    -• anion radical and subsequent fastreaction of Br• with C2H4

    þ• leads to a bromonium ionintermediate, which rapidly reacts with Br- in the matrixcage to produce the final product. The bridged structure ofthe bromonium ion,26-32 which blocks cis addition, is thenresponsible for the conformational preference in thephotoproduct.

    The excitationwavelength (266 nm) used in this studywaschosen to allow efficient population of the charge-transferband while avoiding excitation of the photoproduct. We havepreviously shown21 that photolysis ofmatrix-isolated anti-1,2-dibromoethane at 220 nm produces a significant yield of thegauche conformer. The absence of this conformer is furtherevidence that secondary photolysis of the photoproduct doesnot occur.

    In conclusion, we have developed a new scheme for thetrapping and interrogation of prereactive donor-acceptorcomplexes that was used to examine photoinduced electrontransfer in the prototypicalMulliken donor-acceptor (halogenbonded) π-complex, C2H4 3 3 3Br2. Excitation into the intensecharge-transfer band of the complex leads exclusively to theanti conformer of the reaction product, in agreement withboth Mulliken theory and the product distribution of thethermal reaction.19 This work opens new opportunities fordirect observation of ultrafast electron transfer in this bench-mark system, andweplan to use this approach to studyothermodel halogen-bonded systems.

    AUTHOR INFORMATION

    Corresponding Author:*Towhom correspondence should be addressed. E-mail: [email protected]

    ACKNOWLEDGMENT The authors gratefully acknowledge thevaluable contributions of Prof. Rajendra Rathore. Support of theNational Science Foundation (Grant CHE-0717960), the Donors ofthe Petroleum Research Fund of the American Chemical Society(Grant 48740-ND6), the NSF Teragrid project (Grant TG-CHE100075), and the NSF funded Pere cluster at Marquette isacknowledged.

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