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
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,
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
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
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
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
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
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
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
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
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
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
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
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
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.
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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