C O M M U N I C A T I O N S
Chart 2 Structures of Radical Intermediates (3 and 4) Formed after
Photolysis of 1 and 2
changes similar to those resulting from an electronic excitation of
1,4-bis(phenylethynyl)benzene to the S1 state as observed by time-
resolved Raman spectroscopy12 or electronic excitation of Pt
acetylide complex to the T1 state as measured by TRIR.21
In summary, our TRIR measurements demonstrate that a free
electron conjugated with the phenylacetylene core is substantially
delocalized. The fingerprint shift of acetylene bond absorption
evidences extensive transmission of free electron density throughout
2
the π-system in metastable B1 state of aroyloxyl radical. Further
details will appear in the full paper.
Acknowledgment. We thank Dr. John Cable for fruitful
discussions. D.E.P. is grateful to the McMaster Endowment for
support in the form of a Fellowship.
phenylene-ethynylene chromophores is too short13 to be observed
on the time scale of the TRIR experiment. Thus, the kinetic data16,17
allow unambiguous assignment of the 2112 cm-1 transient to the
absorption of aroyloxyl radicals 3 and 4 (see Chart 2).
Supporting Information Available: Synthesis and characterization
of compounds 1 and 2; kinetic details of TRIR experiments; ground-
state FTIR of 1 and 2; DFT calculations details on phenylacetylen-
ecarbonyloxyl radicals. This material is available free of charge via
The presence of the two functional groups in 1 could lead to
formation of diradical species due to stepwise cleavage of both
oxygen-oxygen bonds of both peroxide moieties. The rapid
absorption of two photons can be expected at the high energy fluxes
of pulsed laser excitation. To establish if biradical or monoradical
transient species are formed after photolysis of 1, we found the
amount of carbon dioxide formed relative to amount of the starting
peroxyester decomposed within one laser pulse. Although TRIR
produced a significant systematic error and an accurate value of
this ratio could not be obtained, a more definitive way to prove
structure of the radical was to compare the TRIR spectra of 1 with
monofunctionalized peroxyester 2 which can produce only mono-
radical species. The similarity of TRIR spectra as a result of
photolysis of 1 and 2 unambiguously assigns the transient species
to monoradicals 3 and 4 respectively.
References
(1) Center for Photochemical Sciences Contribution #558.
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V. J. Phys. Chem. B 1998, 102, 941.
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(13) Beeby, A.; Findlay, K.; Low, P. J.; Marder, T. B J. Am. Chem. Soc. 2002,
124, 8280.
(14) The extensive delocalization of free electron increases the lifetime of
produced radical species sufficiently to be detected with available transient
techniques. Time resolution of the current TRIR instrument is ca. 30 ns.
For more details on experimental setup see ref 15.
(15) For detailed description of the TRIR setup see: Fedorov, A. V.; Danilov,
E. O.; Merzlikine, A. G.; Rodgers, M. A. J.; Neckers, D. C. J. Phys. Chem.
A 2003, 107, 3208.
Significant IR absorption at 2112 cm-1 in 3 and 4 suggests this
infrared transition to be more allowed in the radical species
compared to the parent. While the weak IR absorption correspond-
ing to the internal acetylene vibration mode in 2 was observed
around 2218 cm-1, no IR absorption in this spectral region was
observed in 1. The lowered symmetry of the radicals leads to an
increase in the transition dipole moment of the acetylene vibration.
In addition to the increased intensity, the energy of the -CtC-
vibration in 3 and 4 decreased by about 100 cm-1 relative to that
of 1 and 2. The decrease in the IR vibration frequency can be
predicted by DFT analysis (B3LYP 6-31G*) of aroyloxyl radical
(observed ν-CtC- ) 2112 cm-1; calculated18 ν-C≡C- ) 2126 cm-1
See SI for details).
.
The DFT analysis reveals that this shift takes place only in a
2
2
(16) Aroyloxyl radicals generally do not react with oxygen as opposed to
carbon-centered radicals. For example see: Chateauneuf, J.; Lusztyk, J.;
Ingold K. U. J. Am. Chem. Soc. 1988, 110, 2877.
state possessing B1 symmetry. A B2 state lying in the proximity
of the B1 does not show such a shift.19 Nonadiabatic population
of the higher-energy metastable B1 state observed by TRIR is
2
(17) The concentration of oxygen in chloroform (∼10-2 M) was sufficient to
quench carbon-centered radicals (kq ≈ 107 M-1 s-1), assuming diffusion-
controlled quenching reaction. For reaction rates of carbon-centered
radicals with oxygen, see: CRC Handbook of Organic Photochemistry;
Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989.
(18) Scaled by a factor of 0.96.
2
anticipated after excitation of 1 and 2 by the 355-nm laser pulse.
Steady-state EPR experiments of dinitrene diradical species have
demonstrated no delocalization of free electrons through the
phenylene-ethynylene bridge,20 contrary to transient experiments
allowing the probe of transient species immediately after the laser
flash in their nonequilibrated high-energy states.
(19) The energy of the 2B1 state is 25.7 kcal/mol higher than that of 2B2. ν-C≡C-
(2B2) ) 2217 cm-1; ν-C≡C- (2B1) ) 2126 cm-1
(20) Serwinski, P. R.; Lahti, P. M. Org. Lett. 2003, 5, 2099.
(21) Cooper, T. M.; Blaudeau, J.-P.; Hall, B. C.; Rogers, J. E.; McLean, D.
G.; Liu, Y.; Toscano, J. P. Chem. Phys. Lett. 2004, 400, 239.
Interestingly, the presence of free electron conjugated with the
central phenylene-ethynylene chromophore induces structural
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