terminator reaction that provides a good detection element.
The two elements must be compatible and must operate in
an appropriate time domain for ET reactions. In this work,
we demonstrate laser flash photolysis (LFP) methods that
are designed for direct kinetic studies of EET in the
nanosecond and microsecond time ranges in organic and
biological molecules.
Two methods for electron injection are photoinduced
electron transfer (PET) from a polycyclic aromatic compound
and reduction from a photochemically generated R-hydroxy
radical. Back electron-transfer reactions are possible in the
former case but not in the latter, and Giese, Carell, and co-
workers used R-hydroxy radicals for electron injection into
DNA.16 We used R-hydroxy phenyl ketones as precursors
in intra- and intermolecular electron transfer (ET) studies.
For detection in intramolecular studies, we employed an
ultrafast cyclopropylcarbinyl radical ring-opening reaction,24
which has been used for neutral radicals25 and radical
cations26 and which was expected to maintain its fast
character in a radical anion version.27
Figure 1. Time-resolved spectra. (A) LFP of 1 and 9-fluorenone
in 1:1 water-acetonitrile. (B) LFP of 1 and 4b in 1:1 water-
acetonitrile with 0.05 N NaOH. The insets show kinetic traces (gray)
and fits (black) at one wavelength.
gave the cyclohexanone radical anion (3) that was a stronger
reducing agent.32 Ketyl 3 reduced several carbonyl com-
pounds, and the results demonstrate the limits for fast electron
injection reactions from this class of initiators. For example,
aldehyde 4a and phenyl ketone 4b were reduced and
subsequently ring opened to give UV-detectable diphenyl-
alkyl radicals 5 (λmax ) 335 nm) (Scheme 2, Figure 1B),
Intermolecular ET studies employed 1-phenacylcyclohex-
anol (1) as the radical precursor (Scheme 1). Photolysis of
Scheme 1
Scheme 2
1 with 355 nm laser light gave the 1-hydroxycyclohexyl
radical (2) in a fast cleavage reaction (k ≈ 1 × 109 s-1)28
with φ ≈ 0.4.29 In neutral protic solvent, radical 2 reduced
9-fluorenone to the ketyl radical anion with a rate constant
of k ≈ 1 × 108 M-1 s-1. The spectrum of the fluorenone
ketyl radical is shown in Figure 1A. Radical 2 did not reduce
simple carbonyl compounds, but deprotonation of 2 (pKa ≈
12.5)30,31 in 1:1 water-acetonitrile containing 0.05 N NaOH
whereas the corresponding methyl ketone (4; R ) CH3) did
not react with 3.
Second-order rate constants for electron transfer from 2
or 3 to the various acceptors were determined from the
observed rate constants for a series of reactions under pseudo-
first-order conditions using eq 1, where kobs is the observed
rate constant, k0 is a background rate constant, kET is the
second-order electron-transfer rate constant, and [sub] is the
concentration of substrate. Kinetic results for reactions of
acceptors with 3 and the acceptor reduction potentials32-34
are listed in Table 1.
(18) Ito, T.; Rokita, S. E. Angew. Chem., Int. Ed. 2004, 43, 1839-1842.
(19) Ito, T.; Rokita, S. E. J. Am. Chem. Soc. 2004, 126, 15552-15559.
(20) Schiemann, O.; Feresin, E.; Carl, T.; Giese, B. ChemPhysChem
2004, 5, 270-274.
(21) Cai, Z. L.; Sevilla, M. D. Top. Curr. Chem. 2004, 237, 103-127.
(22) Kaden, P.; Mayer-Enthart, E.; Trifonov, A.; Fiebig, T.; Wagenknecht,
H. A. Angew. Chem., Int. Ed. 2005, 44, 1637-1639.
(23) Voityuk, A. A. J. Chem. Phys. 2005, 123.
kobs ) k0 + kET[sub]
(1)
(24) Newcomb, M.; Johnson, C. C.; Manek, M. B.; Varick, T. R. J. Am.
Chem. Soc. 1992, 114, 10915-10921.
Figure 2 shows a plot of the second-order rate constants
for reactions of radical anion 3 vs the reduction potentials
of oxidants. For the series aldehyde 4a, phenyl ketone 4b,
acetophenone, benzophenone, and 9-fluorenone, the plot is
linear (solid circles). The rate constant for reduction of easily
(25) Newcomb, M.; Tanaka, N.; Bouvier, A.; Tronche, C.; Horner, J.
H.; Musa, O. M.; Martinez, F. N. J. Am. Chem. Soc. 1996, 118, 8505-
8506.
(26) Horner, J. H.; Bagnol, L.; Newcomb, M. J. Am. Chem. Soc. 2004,
126, 14979-14987.
(27) Tanko, J. M.; Gillmore, J. G.; Friedline, R.; Chahma, W. J. Org.
Chem. 2005, 70, 4170-4173.
(28) Ruhlmann, D.; Fouassier, J. P.; Schnabel, W. Eur. Polym. J. 1992,
28, 287-292.
(32) Lilie, V. J.; Beck, G.; Henglein, A. Ber. Bunsen Phys. Chem. 1971,
75.
(33) Pal, H.; Mukherjee, T.; Mittal, J. P. Radiat. Phys. Chem. 1994, 44,
603-609.
(34) Barwise, A. J. G.; Gorman, A. A.; Leyland, R. L.; Smith, P. G.;
Rodgers, M. A. J. J. Am. Chem. Soc. 1978, 100, 1814-1820.
(29) Jockusch, S.; Landis, M. S.; Freiermuth, B.; Turro, N. J. Macro-
molecules 2001, 34, 1619-1626.
(30) Laroff, G. P.; Fessenden, R. W. J. Phys. Chem. 1973, 77, 1283-
1288.
(31) Xu, L.; Newcomb, M. J. Org. Chem. 2005, 70, 9296-9303.
1838
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