Multiple-Photon Chemistry in the Laser-J et
J . Org. Chem., Vol. 62, No. 11, 1997 3729
Ta ble 2. La ser -J et P h otolyses of Na p h th yl Eth er 1 in a
1:4 Mixtu r e of Ca r bon Tetr a ch lor id e a n d Meth a n ol
To identify the transients produced from ether 1, the
model compounds 1-methylnaphthalene, 1-(hydroxymeth-
yl)naphthalene (4a ), and 1-(ethoxymethyl)naphthalene
(4b) were studied. Instead of bond homolysis to radical
products, these compounds are expected to generate
triplet states on irradiation. With acetonitrile as solvent,
in all cases spectra were observed characterized by peaks
at ca. 240, 395, and 420 nm with depletion of the OD at
290 nm, at which the parent compounds absorb. The
transients could be effectively quenched by O2 with
quantitative recovery of the starting material. These
observations are interpreted in terms of formation of the
triplet states of these naphthalene derivatives, followed
by relaxation (with or without O2) to the respective
ground states (eq 1). Therefore, by analogy it is con-
entry
laser powera (W)
product ratiob 4c/3a
1
2
3
3.0
3.5
4.0
1.9
3.4
4.6
a
c ) 3.00 × 10-3 M, 100-µm capillary, flow rate of 1.0 mL/min,
all UV lines (333, 351, and 364 nm), 1 cycle. b 1H NMR spectros-
copy, error 5% of stated value.
ether 4c may arise also from methanolysis in the dark
or photosensitized methanolysis of 1-(chloromethyl)-
naphthalene (3a ) produced from the ether 1, it was
necessary to run control experiments in the laser-jet
photolyses. Fortunately, the methanolysis of 1-(chloro-
methyl)naphthalene (3a ) in the dark and the photolysis
of a 1:1 mixture of the ether 1 and 1-(chloromethyl)-
naphthalene (3a ) gave only low amounts (ca. 3%) of the
1-naphthylmethyl methyl ether (4c) product.
Np(S0) 9
hν8 Np(S1) <20 ns8 Np(T1) f Np(S0) (1)
cluded that in the case of ether 1 the peaks at 241, 395,
and 419 nm are due to the triplet of ether 1, an
assignment that is in agreement with the known spec-
trum of the unsubstituted naphthalene triplet.11a In this
respect, it is interesting that even immediately after the
pulse (20 ns) there was no evidence for the triplet of the
benzophenone chromophore in the ether 1, although this
moiety also absorbs light at 248 or 308 nm. This
indicates rapid (k g 5 × 107 s-1) and efficient intramo-
lecular energy transfer from the benzophenone to the
naphthalene moiety. Such intramolecular energy migra-
tion is expected to be energetically downhill, since for
comparison, the triplet energy (ET) of 4-methoxyben-
zophenone is 64.9 kcal/mol12 versus 60.8 kcal/mol13-16 for
1-methylnaphthalene.
Tim e-Resolved Sp ectr a l Stu d ies of th e Tr a n sien ts
by La ser -F la sh P h otolysis a n d P u lse-Ra d iolysis
a n d Con tr ol Exp er im en ts. On photolysis of a deoxy-
genated 0.2 mM solution of ether 1 in acetonitrile with
the 20-ns pulses of 248-nm laser light, a spectrum was
observed after the pulse that was characterized by peaks
at ca. 241, 336, 366, 395, and 419 nm and a broad
absorption between 450 and 700 nm (see Figure 1). In
the 280-310 nm region, in which the ether 1 possesses
a strong absorption band, the optical density decreased
(negative ∆OD) immediately after the excitation pulse.
Thereafter, the negative signal partly decreased (which
is assigned to the recovery of the ground-state parent
ether 1 by the decay of its triplet), with a rate similar to
that of the decay of the positive signals at 241, 395, or
419 nm (see below).
Whereas the peaks at 241, 395, and 419 nm were fully
developed after the excitation pulse and started to
decrease in intensity thereafter, those at 336 and 366 nm
showed “delayed” formation, which extended over 100 ns
(k ≈ 2 × 107 s-1).10 The decay of the absorptions at 241,
395, and 419 nm and the recovery of the absorption at
280-310 nm followed a first-order rate law with about
equal rates (k ≈ 7 × 105 s-1). In contrast, the decays at
336 and 366 nm followed second-order kinetics. When
O2 was admitted to the solution, at all wavelengths the
decay of the transient absorptions was accelerated, from
which it may be concluded that the transient species have
radical or triplet character. The rate constant for quench-
ing of the ether 1 triplet by O2 was determined as 2 ×
109 M-1 s-1, a value that is characteristic for reaction of
triplet ketones.
To identify the peaks at 336 and 366 nm observed on
photolysis of ether 1 at 248 nm, 1-(bromomethyl)naph-
thalene in acetonitrile was photolyzed. The spectrum
observed at the end of the pulse showed depletion at 290
nm and peaks at 337 and 366 nm, but none at 395 or ca.
420 nm and only very weak absorption at ca. 240 nm.
These results are explained in terms of 1-naphthylmethyl
•
radical (1-Np CH2 ) formation by photoinduced homolysis
of the C-Br bond. The identification of the radical was
•
confirmed by producing the 1-Np CH2 radical in an
alternative way, i.e., by allowing 1-NpCH2Br or 1-NpCH2-
Cl in 2-propanol as solvent to react with radiation-
chemically generated solvated electrons. As expected, the
observed spectrum was independent of the nature of the
halogen and displayed the same peaks as in the case of
the photolyses. The 336- and 366-nm peaks obtained in
the 248-nm photolysis of ether 1 are, therefore, due to
the 1-Np CH2• radical generated under these conditions.
Consequently, in addition to triplet formation, the 248-
nm photolysis of ether 1 also leads to C-O bond homoly-
sis to afford the naphthylmethyl and phenoxy radicals.
By measuring the optical density at 336 nm as a function
Solutions of the ether 1 in acetonitrile were also
photolyzed at 308 nm, and the spectrum observed was
the same as that with 248-nm light, except for the peaks
at 336 and 366 nm, which were absent. The decay rates
at 241, 395, and 419 nm and those for the recovery at
280-310 nm were the same as on excitation at 248 nm.
At 10-20 µs after the 308-nm pulse, all the absorption
changes had essentially disappeared and the recovery of
parent as monitored at 280-310 nm was more or less
quantitative, which indicates that the effects of excitation
with 308-nm light are to a large extent reversible, in
contrast to excitation with 248-nm light.
(11) (a) Steenken, S.; Warren, C. J .; Gilbert, B. C. J . Chem. Soc.,
Perkin Trans. 2 1990, 335-342. (b) Heilbronner, E.; Hornung V.;
Pinkerton, F. H.; Thames, S. F. Helv. Chim. Acta 1972, 55, 289-302.
(12) Leigh, W. J .; Arnold, D. R. J . Chem. Soc., Chem. Commun. 1980,
406-408.
(13) Murov, S. L. Handbook of Photochemistry, 3rd ed.; Marcel
Dekker, Inc.: New York, 1973; p 18.
(14) Workentin, M. S.; J ohnston, L. J .; Wayner, D. D. M.; Parker,
V. D. J . Am. Chem. Soc. 1994, 116, 8279-8287.
(15) (a) Baciocchi, E.; Bartoli, D.; Rol, C.; Ruzzziconi, R.; Sebastiani,
G. V. J . Org. Chem. 1986, 51, 3587-3593. (b)Trahanovsky, W. S.;
Brixius, D. W. J . Am. Chem. Soc. 1973, 95, 6778-6780.
(16) Walling, C.; El-Taliawi, G. M.; Amarnath, K. J . Am. Chem. Soc.
1984, 106, 7573-7578.
(10) The “delayed” formation of the absorption in the wavelength
range 310-380 nm (see Figure 1) is an artifact, caused by the decay
of the fluorescence of the naphthalene moiety (k ≈ 2 × 107 s-1) in this
region.
Adam, Waldemar
