J . Org. Chem. 1996, 61, 2647-2656
2647
Mesolytic Scission of C-C Bon d s a s a P r obe for P h otoin d u ced
Electr on Tr a n sfer Rea ction s of Qu in on es
Przemyslaw Maslak* and William H. Chapman, J r.
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
Received November 28, 1995X
Photoinduced electron transfer reactions of chlorinated benzoquinones are investigated using
bibenzylic donors that undergo rapid fragmentation upon oxidation. The fragmentation rates and
the quantum yields are used to probe the dynamics of back-electron transfer (BET) in two types of
radical ion pairs. The triplet ion pairs formed by interception of excited state quinones give products
with high quantum yields. The singlet ion pairs formed by irradiation of the charge-transfer (CT)
complexes between the quinones and the donors undergo reactions with significantly lower efficiency.
The advantage of the first method (triplet quenching) over the CT-irradiation depends on the
energetics of BET. It is large for reactions with relatively small ∆Get for BET and it decreases for
reactions with more negative ∆Gbet. The indirectly obtained rates of BET are in excellent agreement
with literature data for similar, but unreactive systems, and the rates of C-C bond scission in
radical cations generated in these systems are consistent with the thermodynamics of these
processes.
Photoinduced electron transfer (PET) between neutral
donors and acceptors leads to generation of ion pairs.1
Dynamics of these ions determines the efficiency of many
photoreactions. There are two main methods to generate
the radical ions. One method involves deposition of light
energy into acceptor or donor molecules, converting them
into potent redox reagents that undergo electron transfer
Once generated, the ion pairs undergo a multitude of
-3
processes that include change of solvation status (from
CIP to SSIP to free ions) and intersystem crossing that
are superimposed on the competition between BET and
product-forming reactions. All these processes often
occur on similar time scales. The understanding of all
these elementary processes that constitute PET systems
(
ET) with ground state substrates.1,2 In the second
has increased dramatically in recent years,
1-5
especially
method the ground-state charge-transfer (CT) complexes
are irradiated leading to promotion of an electron within
the complex.3 The method of generation presets the stage
for the competition between the energy-wasting back
electron transfer (BET) and product-forming processes
for organic molecules. Kinetic data obtained by time
resolved techniques and refinements of theory allow
chemists to make at least semiquantitative predictions
about systems not studied previously.
7
We present here an investigation of PET systems
testing such predictions. In our design, a simple unimo-
lecular C-C bond scission in radical cations (mesolytic
4
of radical ions, such as proton transfers, adduct forma-
tions, rearrangements, or fragmentations.5 Usually, in
the first method (where one of the components serves as
sensitizer) solvent separated ion pairs (SSIPs) are formed
8
cleavage ) serves as a model of a product-forming reac-
tion, and provides, at the same time, an internal probe
(
especially in polar solvents) with spin multiplicities
depending on the nature of the excited-state quenched
singlet or triplet). On the other hand, according to the
9,10
of PET dynamics.
The experimental system selected
(
(7) (a) Marcus, R. A. J . Chem. Phys. 1956, 24, 966. (b) Marcus, R.
6
Mulliken theory, irradiation of CT complexes gives, at
least initially, contact ion pairs (CIPs) in a singlet state.
A. Annu. Rev. Phys. Chem. 1964, 15, 155. (c) Marcus, R. A.; Sultin, N.
Biochem. Biophys. Acta 1985, 811, 265. (d) Kakitani, T.; Matsuda. N.;
Yoshimori, A.; Mataga, N. Prog. React. Kinet. 1995, 20, 347.
(
8) Maslak, P.; Narvaez, J . N. Angew. Chem., Int. Ed. Engl. 1990,
29, 283.
(9) (a) Maslak, P.; Chapman, W. H., J r. Tetrahedron 1990, 46, 2715.
(b) Maslak, P.; Chapman, W. H., J r. J . Org. Chem. 1990, 55, 6334. (c)
Maslak, P.; Chapman, W. H., J r. J . Chem. Soc., Chem. Commun. 1989,
1809.
X
Abstract published in Advance ACS Abstracts, April 1, 1996.
(
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(
2
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(4) See for example: (a) Chanon, M.; Rajzmann,M.; Chanon, F.
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(
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(
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0
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