Three-color three-laser photochemistry of di(p-methoxyphenyl)methyl chloride

Article abstract of DOI:10.1021/jp027002l

Three-color three-laser photochemistry of di(p-methoxyphenyl)methyl chloride ((p-CH₃OC₆H₄)₂CHCl = An₂CHCl) was studied by three-step excitation using 308-, 355-, and 495-nm lasers with delay times of 100 ns to 3 μs. Di(p-methoxyphenyl)methyl radical (An₂CH) was produced together with An₂CH in the excited state (An₂CH*) and di(p-methoxyphenyl)methyl cation (An₂CH⁺) in quantum yields of 0.09, 0.12, and 0.12, respectively, after a laser flash during 308-nm laser (first laser) photolysis of An₂CHCl in acetonitrile. Excitation of An₂CH with a 355-nm laser (second laser) resulted in formation of transient absorption of An₂CH* and An₂CH⁺ and fluorescence of An₂CH* with a peak at 550 nm. The formation of An₂CH⁺ from An₂CH requires two-photon energy at 355 nm and proceeds by resonant two-photon ionization (RETPI) of An₂CH through sequential excitation of An₂CH*. Excitation of An₂CH⁺ with a 495-nm laser (third laser) produced fluorescence with a peak at 560 nm. Although the fluorescence of An₂CH⁺ was also observed without the second laser excitation because of the initial formation of An₂CH⁺ during the first 308-nm laser photolysis, the fluorescence intensity of An₂CH⁺ increased approximately 1.2 times with the second 355-nm laser excitation of An₂CH. Therefore, the second laser excitation can perform the conversion of An₂CH to An₂CH⁺ through RETPI within the laser flash duration, and the fluorescence intensity of An₂CH⁺ can be controlled by the second irradiation.

Full text of DOI:10.1021/jp027002l

4778
J. Phys. Chem. A 2003, 107, 4778-4783
ARTICLES
Three-Color Three-Laser Photochemistry of Di(p-methoxyphenyl)methyl Chloride
Michihiro Hara, Sachiko Tojo, and Tetsuro Majima*
The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki,
Osaka 567-0047, Japan
ReceiVed: September 16, 2002; In Final Form: February 12, 2003
Three-color three-laser photochemistry of di(p-methoxyphenyl)methyl chloride ((p-CH3OC6H4)2CHCl )
An2CHCl) was studied by three-step excitation using 308-, 355-, and 495-nm lasers with delay times of 100
ns to 3 µs. Di(p-methoxyphenyl)methyl radical (An2CH•) was produced together with An2CH• in the excited
state (An2CH•*) and di(p-methoxyphenyl)methyl cation (An2CH+) in quantum yields of 0.09, 0.12, and 0.12,
respectively, after a laser flash during 308-nm laser (first laser) photolysis of An2CHCl in acetonitrile. Excitation
of An2CH• with a 355-nm laser (second laser) resulted in formation of transient absorption of An2CH•* and
An2CH+ and fluorescence of An2CH•* with a peak at 550 nm. The formation of An2CH+ from An2CH•
requires two-photon energy at 355 nm and proceeds by resonant two-photon ionization (RETPI) of An2CH•
through sequential excitation of An2CH•*. Excitation of An2CH+ with a 495-nm laser (third laser) produced
fluorescence with a peak at 560 nm. Although the fluorescence of An2CH+ was also observed without the
second laser excitation because of the initial formation of An2CH+ during the first 308-nm laser photolysis,
the fluorescence intensity of An2CH+ increased approximately 1.2 times with the second 355-nm laser excitation
of An2CH•. Therefore, the second laser excitation can perform the conversion of An2CH• to An2CH+ through
RETPI within the laser flash duration, and the fluorescence intensity of An2CH+ can be controlled by the
second irradiation.
Introduction
Z-isomer with the second laser excitation at 480 or 532 nm.
Ouchi et al. have reported that two-step homolytic cleavages
of two C-Br or C-Cl bonds proceed to give a biradical during
two-color two-laser photolyses of 1,8-bis(bromomethyl)- and
1,8-bis(chloromethyl)naphthalenes using the first 193-, 248-, or
308-nm and second 351-nm excimer lasers.3 It has been reported
by Redmond et al. that hydrogen abstraction of a triplet ketone
proceeds to give ketyl radical during the first 308-nm laser flash
photolysis of substituted benzophenone, and that the ketyl radical
is excited to give fluorescence during the second 337- or 532-
nm laser flash photolysis.4 It has been also reported by Goez et
al. that photoinduced electron transfer (PET) between ketones
such as 4-carboxybenzophenone and 1,5-anthraquinonedisul-
fonate and donor molecules occurs to give the radical anion
and cation during the first 308-nm laser flash photolysis, and
electron detachment from the radical anion proceeds to give
the ketone and solvated electron during the second 387.5-nm
laser flash photolysis.5,6 Wang et al., Johnston et al., McGimpsey
et al., and Goez et al. have reported that the excited singlet or
triplet state molecules are generated during the first 266-, 308-,
or 355-nm laser flash photolysis of anthracene, diphenylamine,
(4′-(phenanthrene-9-carboxylate)methyl-4-methylnaphthoate)-
biphenyl, and dimethylaminobenzonitrile and Ru(bpy)32+, re-
spectively, and that the radical cations are produced during the
second 355-, 480-, 532-, or 590-nm laser flash photolysis of
the excited-state molecules.7-11 It has been reported by Wang
et al. that resonant two-photon ionization (RETPI) of all-trans-
1,6-diphenyl-1,3,5-hexatriene (t-DPH) proceeds to give the
Multilaser chemistry is defined as chemistry involving multi-
excitation with more than two lasers, in which short-lived
species such as excited molecules and reactive intermediates
are generated by the first excitation of starting molecules, and
used as starting molecules for the second excitation. Short-lived
species generated by the second excitation are used as starting
molecules for the third excitation. In other words, photochem-
istry of short-lived species can be investigated by the multistep
excitation method using more than two lasers. Properties of
lasers such as the monochromaticity, fine directivity, high
intensity, and short-pulse duration are certainly used in the
multilaser photochemistry in which selective excitation of a
short-lived molecule, spatially selective excitation, time control
of irradiation, and excitation at high laser fluence are carried
out.
As the simplest multilaser chemistry, two-laser chemistry of
short-lived species such as excited molecules and reactive
intermediates in liquid phase has been extensively investigated
by various groups. For example, Miranda et al. have reported
that heterolytic cleavage of C-Cl bond proceeds to give
R-chloroalkylbenzyl cation and chloride ion during the first 266-
nm laser flash photolysis of dichlorodiphenylalkanes,1,2 followed
by dehydrochlorination of the cation to give R-E-allylbenzyl
cation, and that the R-E-allylbenzyl cation isomerizes to the
* To whom correspondence should be addressed. Telephone: Japan
+6-6879-8495. FAX: Japan +6-6879-8499. E-mail: majima@
sanken.osaka-u.ac.jp.
10.1021/jp027002l CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/24/2003

Photochemistry of Di(p-methoxyphenyl)methyl Chloride
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4779
SCHEME 1: Three-Laser Photochemistry of A using
the Three-Color Three-Laser Photolysis Method
t-DPH radical cation during the first 308- or 355-nm laser flash
photolysis of t-DPH, and that t-DPH radical cation isomerizes
to the cis-isomer radical cation during the second 590-nm laser
flash photolysis.12 It has been reported by Itoh et al., Brewer et
al., and Fuket et al. that the ground-state tautomer is excited to
give the equilibrated excited-state tautomers during the first 248-,
308-, or 355-nm laser flash photolysis of 3-hydroxyflavone,13-15
3-hydroxychromone,13 7-hydroxyquinoline,16,17 7-hydroxyiso-
flavone,18 7-hydroxy-1-indanone,19 7-azaindole dimer,20 2-(2-
hydroxyphenyl)benzoxazole,21,22 and 7-azaindole/1-azacarba-
zole,23 and that fluorescence of two excited-state tautomers was
observed during the second dye-laser flash photolysis.
In the two-step, two-laser flash photolysis, short-lived species
such as excited molecules and reactive intermediates are
generated in liquid phase from one-photon excitation, photolysis,
PET, and RETPI during the first laser flash photolysis.
Photochemistry such as isomerization and ionization of short-
lived species occurs through the excited state during the second
laser flash photolysis. Although the two-laser chemistry has been
extensively investigated, three-color three-laser chemistry in
liquid phase has never been reported. The first laser (hν1)
excitation of starting molecule (A) produces short-lived species
(B) through A in the excited state (A*), the second laser (hν2)
excitation of B produces another short-lived species (C) through
B in the exited state (B*), and the third laser (hν3) excitation of
C produces another short-lived species or causes photochemistry
of C through C in the exited state (C*) (Scheme 1). The delay
times of hν2 and hν3 from hν1 and hν2, respectively, depend on
the formation rates and stabilities of B and C.
In the present work, three-step three-laser photochemistry of
di(p-methoxyphenyl)methyl chloride ((p-CH3OC6H4)2CHCl )
An2CHCl) has been investigated by three-color three-laser flash
photolysis with delay times of 100 ns to 3 µs. Homolysis and
heterolysis of the C-Cl bond within a laser flash duration
(picosecond to nanosecond) are well-known in photochemistry
of An2CHCl in various solvents.24,25 Both di(p-methoxyphenyl)-
methyl radical (An2CH•) and di(p-methoxyphenyl)methyl cation
(An2CH+) are generated during the first 308-nm laser irradiation.
Then, An2CH• is irradiated by the second 355-nm laser to give
An2CH+ through RETPI of An2CH•, and then An2CH+ is
irradiated by the third 495-nm laser to give the excited An2-
CH+.
Figure 1. Transient absorption spectrum observed at 100 ns after a
flash during 308-nm laser flash photolysis of An2CHCl (1.9 × 10-2
M) in acetonitrile. This spectrum has been analyzed by three peaks at
348, 385, and 510 nm assigned to An2CH• (∆O.D.348 ) 0.16), An2-
CH•* (∆O.D.385 ) 0.045), and An2CH+ (∆O.D.510 ) 0.49), respectively.
450) synchronized with the laser flash. The intensity of monitor
light source was detected using a photomultiplier (Hamamatsu
R928). The electric signal from the photomultiplier was
digitalized by oscilloscope, and transported to a personal
computer with the interface RS 232C. Transient absorption
spectra were measured by a multichannel analyzer fitted with
an image intensifier with 30 ns gate width (Hamamatsu C4500),
and transported to a personal computer. The samples were
prepared in a transparent rectangular cell made by quartz (1.0
× 1.0 × 4.0 cm, optical-path length of 1.0 cm) at room
temperature, and were degassed with Ar for 15 min. The
concentration (1.9 × 10-2 M) of An2CHCl was chosen to have
the absorbance of 1.0 at 308 nm of the excitation laser
wavelength. The first laser beam passed through an aperture
with a diameter of 1.0 cm which was the widest among three
laser beams. The second laser diameter was wider than that of
the third laser. The third laser irradiation spot was wider than
the divergence of the monitor light.
γ-Radiolysis was carried out using a 60Co γ source (dose,
2.6 × 102 to 1.0 × 103 Gy) at 77 K. Optical absorption spectra
were taken at 77 K by a spectrophotometer and multichannel
photodetector, while fluorescence spectra were measured at 77
K by a fluorescence spectrophotometer. The fluorescence
lifetime measurement was carried out by single photon counting
method.
Results
1. Transient Absorption Measurements. One-Laser Pho-
tolysis. The transient absorption spectrum with three peaks at
348, 385, and 510 nm was observed at 100 ns after a flash from
a XeCl laser at 308 nm with 4.5 × 10-2 J pulse-1 during laser
flash photolysis of An2CHCl in acetonitrile (Figure 1). This
spectrum has been analyzed by three peaks at 348, 385, and
510 nm assigned to An2CH• (∆O.D.348 ) 0.16), An2CH•*
(∆O.D.385 ) 0.045), and An2CH+ (∆O.D.510 ) 0.49), respec-
tively.24,25 The time profiles of ∆O.D. at 348, 385, and 510 nm
(Figures 2 (2) and 2 (3)) show the decay of An2CH• and
formation of An2CH•* and An2CH+, respectively, during 308-
Experimental Section
An2CHCl was prepared by reactions of di(p-methoxyphenyl)-
methanol with HCl and methylene chloride (CH2Cl2) in the
presence of anhydrous CaCl2, according to the reported method,24
and purified by fractional distillation. Acetonitrile (Wako) as a
solvent for laser flash photolysis was dried over calcium hydride
(CaH2), and purified by distillation for several times.
Three-color three-laser flash photolysis experiment was
carried out using 308-nm laser flash from a XeCl excimer laser
(Lambda Physics, Compex102; pulse width of 25 ns fwhm) as
the first laser, the third harmonic oscillation (355-nm) of an
Nd3+:YAG laser (Quantel Brilliant Q-switch laser; pulse width
of 5 ns fwhm) as the second laser, and 495-nm flash from an
and 355-nm laser flash photolyses of An2CHCl. ∆O.D.348 )
0.005, ∆O.D.385 ) 0.0035, and ∆O.D.510 ) 0.025 in Figure 2
(3) were much smaller than those in Figure 2 (2), indicating
that formation yields of An2CH•, An2CH•*, and An2CH+ with
the 355-nm excitation were much lower than those with the
308-nm excitation. This is due to lower photon numbers
absorbed by An2CHCl with the 355-nm excitation compared
with the 308-nm excitation, because the excitation wavelength
at 355 nm was at the longer wavelength edge of the absorption
OPO laser (Continuum Surelite) pumped by a Q-switched Nd3+
:
YAG laser (Continuum Surelite) as the second or third laser.
The monitor light source was a 450 W Xe lamp (Osram XBO-

4780 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Hara et al.
Figure 3. Transient fluorescence spectrum of An2CH• observed
immediately after the second 355-nm laser flash and at 3 µs after the
first 308-nm laser flash during the two-color two-laser photolysis of
An2CHCl in acetonitrile.
formation of ∆O.D.385 within the laser flash, ∆O.D.385 decreased
rapidly in the time scale of 100 ns and then decreased according
to the time profile without the second 355-nm laser excitation.
The rapid decay of ∆O.D.385 was consistent with the recovery
of ∆O.D.348 (Figure 2 (A) (1)).
Figure 2 (C) (1) shows the time profile of ∆O.D.510 assigned
to An2CH+. Formation of An2CH• occurred in the time scale
shorter than a few tens of nanoseconds after the first 308-nm
laser flash, and formation of An2CH+ (∆∆O.D.510 ) 0.061) was
observed with excitation of the second 355-nm laser flash. No
change of the time profiles of ∆O.D. at 348, 380, and 510 nm
was observed during the first 308- or 355-nm laser irradiation
and the second 495-nm laser irradiation with the delay time of
200 or 100 ns, respectively.
These results were essentially similar to those reported by
Faria and Steenken using the first 248-nm and second 308-nm
lasers.26 Since An2CHCl in acetonitrile has the ground-state
absorption at 308 nm and An2CH• has an absorption peak at
348 nm, both An2CHCl and An2CH• can be excited with
irradiation at 308 nm. Therefore, selective excitation of An2CH•
can be performed with laser photolysis at 355 nm.
Figure 2. Time profiles of transient absorption at 348 (A), 385 (B),
and 510 nm (C) assigned to An2CH•, An2CH•*, and An2CH+,
respectively, during the three-color three-laser (first 308-, second 355-,
and third 495-nm laser) photolysis of An2CHCl (1), and one-laser
photolysis at 308 nm (2), 355 nm (3), and 495 nm (4) in acetonitrile at
room temperature. The delay times between the first and second lasers
and between the second and third lasers were 100 ns. Three arrows
show the timing of three laser flashes. No change was observed for
the transient absorption with the third 495-nm laser excitation.
Three-Laser Photolysis. Figure 2 (1) shows the time profiles
of ∆O.D. at 348, 380, and 510 nm. The delay times of the
second 355-nm laser irradiation after the first 308-nm laser
irradiation and of the third 495-nm laser irradiation after the
second 355-nm laser irradiation were both 100 ns. The transient
behaviors of An2CH•, An2CH•*, and An2CH+ were same as
those during two-laser photochemistry with the first 308-nm
and second 355-nm laser irradiation, since An2CH•, An2CH•*,
and An2CH+ did not form during the third 495-nm laser
irradiation.
peak. No transient absorption was observed during 495-nm laser
photolysis of An2CHCl. Half-lifetimes of the decay of An2CH•,
An2CH•*, and An2CH+ were 500, 110, and 260 ns, respectively.
Although the decay rate constants of diphenylmethyl radical
and cation have been reported by Bartl et al.,24 no data are
available for An2CH• and An2CH+. Only decay rate constant
of An2CH•* has been found to be 5.8 × 106 s-1 from the
transient absorption and fluorescence measurements of
An2CH•*,26 which is in the same order of that calculated from
the half-lifetime of 110 ns observed in the present study.
Two-Laser Photolysis. Figure 2 (A) (1) shows the time profile
of ∆O.D.348. The delay time of the second 355-nm laser flash
was 100 ns after the first 308-nm laser irradiation. Formation
of An2CH• occurred in the time scale shorter than a few tens of
nanoseconds after the first 308-nm laser flash as described
above. Bleaching of An2CH• (∆∆O. D.348 ) 0.04) and recovery
of An2CH• were observed within a laser flash and in the time
scale of 100 ns, respectively, after the second 355-nm laser
excitation.
2. Transient Fluorescence Measurements. Figure 3 shows
the transient fluorescence spectrum with a peak at 550 nm and
characteristic vibrational bands at the longer wavelength
observed immediately after the second 355-nm laser flash and
at 3 µs after the first 308-nm laser flash during the two-color
two-laser photolysis of An2CHCl in acetonitrile. The spectrum
was assigned to the fluorescence of An2CH•*.27 Figure 4A shows
the transient fluorescence spectra with a peak at 560 nm
observed immediately after the third 495-nm laser irradiation,
respectively, during the three-color three-laser photolysis of
An2CHCl in acetonitrile. The spectrum was assigned to the
fluorescence of excited An2CH+ (An2CH+*) as described below.
The lower intensity fluorescence spectrum drawn by dotted line
(Figure 4A) was observed without the second 355-nm laser
irradiation. The fluorescence intensity of An2CH+ increased
approximately 1.2 times with the second 355-nm laser excitation
Figure 2 (B) (1) shows the time profile of ∆O.D.385 assigned
to An2CH•* which formed in the time scale shorter than a few
tens of nanosecond after the first 308-nm laser irradiation.
Formation of An2CH•* (∆∆O.D.385 ) 0.009) was also observed
with excitation of the second 355-nm laser flash. After the

Photochemistry of Di(p-methoxyphenyl)methyl Chloride
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4781
XeCl excimer laser at 308 nm (hν308) (eq 1), An2CHCl* causes
the homolytic and heterolytic cleavages of the C-Cl bond within
a laser flash duration (30 ns) to give An2CH• + Cl• (eq 2) and
An2CH+ + Cl- (eq 3), respectively, while An2CH•* forms
subsequently from excitation of An2CH• within the laser flash
duration (eq 4). From the formation quantum yields and
formation of An2CH•* via two-photon excitation, An2CHCl*
decomposes and deactivates in 45 and 55% yields, respectively.
Generation of An2CH+ within the laser flash duration may be
also explained by fast electron transfer between An2CH• and
Cl• in the contact radical pair (eq 5). We cannot distinguish
two mechanisms of the formation of An2CH+, initial formation
(eq 3) or stepwise formation (eqs 2 and 5), since the stepwise
processes (eqs 2 and 5) must proceed within the laser flash
duration of 30 ns. Direct and stepwise formation mechanisms
of diphenylmethyl cations with or without substituents on the
phenyl rings such as methyl, methoxyl, chloro, fluoro, and others
have been discussed by Steenken et al. during 248-nm laser
photolysis of the corresponding diphenylmethyl halides in
several solvents.24
Figure 4. Transient fluorescence spectra for An2CH+ observed
immediately after the third 495-nm laser flash during three-color three-
laser photolysis of An2CHCl in acetonitrile (solid line with higher
intensity) (A). The dotted line with lower intensity was observed without
the second 355-nm laser irradiation (A). The difference fluorescence
spectrum (B) obtained from the solid line subtracted by the dotted line.
The delay times between the first and second lasers and between the
second and third lasers were both 3 µs.
of An2CH•. Figure 4B shows a difference spectrum between
two fluorescence spectra.
An2CHCl + hν308 f An2CHCl*
An2CHCl* f An2CH• + Cl•
An2CHCl* f An2CH+ + Cl-
An2CH• + hν308 f An2CH•*
An2CH•/Cl• f An2CH+ + Cl-
(1)
(2)
(3)
(4)
(5)
To assign the fluorescence of An2CH+, we examined γ-ra-
diolysis of n-butyl chloride rigid matrix containing An2CHCl
at 5.0 × 10-3 M at 77 K where An2CH+ was generated from
dissociation of chlorine atom from An2CHCl radical cation. The
absorption and fluorescence spectra of An2CH+ were observed
with peaks at 510 and 530 nm, respectively, and were similar
to those observed for An2CH+ in acetonitrile at room temper-
ature in the present study. The fluorescence lifetime of An2CH+*
in n-butyl chloride rigid matrix at 77 K was measured to be 4.2
ns by single photon counting method.
After the second 355-nm laser irradiation (hν355), the transient
absorption bands of An2CH•* and An2CH+ at 385 and 510 nm,
respectively, increased, while the band of An2CH• at 348 nm
was bleached. The increases of ∆O.D.385 and ∆O.D.510 and
decrease of ∆O.D.348 were found to be ∆∆O.D.385 ) 0.0058,
∆∆O.D.510 ) 0.046, and ∆∆O.D.348 ) -0.045, respectively.
The increase of An2CH+ concentration and decrease of An2CH•
concentration were calculated to be ∆[An2CH+] ) 4.0 × 10-7
and ∆[An2CH•] ) -9.6 × 10-7 M, respectively, using the ꢀ
values. The yield of An2CH+ was 42% from the excitation of
An2CH• at 355 nm. Since formation of An2CH+ occurs through
RETPI involving sequential two-step excitation as discussed
later, it is reasonably assumed that An2CH•* is produced in the
remaining 58% yield, ∆[An2CH•*] ) 5.6 × 10-7 M. Thus, ꢀ385
Discussion
It has been reported by Steenken et al. that homolytic and
heterolytic cleavages of the C-Cl bond proceed to give An2CH•
+ chlorine atom (Cl•) and An2CH+ + chloride ion (Cl-) during
laser flash photolysis of An2CHCl.24 The formation of An2CH•*
has been also found to occur through sequential excitation of
An2CH• within a laser flash duration. We examined the 308-
nm laser flash photolysis of An2CHCl in acetonitrile and found
essentially same results with formation of An2CH• ([An2CH•]
) 3.4 × 10-6 M, the absorption coefficient (ꢀ) at 352 nm, ꢀ352
) 4.7 × 104 M-1 cm-1),24 An2CH•* ([An2CH•*] ) 4.6 × 10-6
M, ꢀ385 ) 3.2 × 103 M-1 cm-1) (see below), and An2CH+
([An2CH+] ) 4.5 × 10-6 M, ꢀ507 ) 1.1 × 105 M-1 cm-1 24
)
in
of An2CH•* was calculated to be 1.0 × 104 M-1 cm-1
.
the ratio of 27:37:36 immediately after the 308-nm laser
irradiation. The emission spectrum of An2CH•* with the
characteristic structure around 550-700 nm was also observed
during the laser photolysis. The numbers of 308-nm laser
photons absorbed by An2CHCl was calculated to be 3.7 × 10-5
Einstein L-1 from absorbance ) 1.0 of the An2CHCl solution
at 308 nm with the path length of the laser beam (1.0 cm) in
the sample solution using chemical actinometry of T-T
absorption of the triplet benzophenone at 532 nm in benzene
(ꢀT ) 6250 M-1 cm-1).28 Therefore, the formation quantum
yields of An2CH•, An2CH•*, and An2CH+ were found to be
Since the increase of ∆O.D.385 and bleach of ∆O.D.348
occurred within a laser flash at 355 nm, An2CH• in the ground
doublet state (D0) is excited to An2CH•* in the excited doublet
state (Dn, n > 2)24,25,29 with the second 355-nm laser irradiation
(eq 6). From the transient absorption peak at 385 nm and
transient fluorescence peak at 550 nm assigned to An2CH•*,
the Stokes shift of An2CH• was found to be 165 nm. The D0 f
D1 absorption is a forbidden process similar to that for
diphenylmethyl radical,24,25,29 while the D0 f Dn absorption and
D0 r D1 fluorescence are allowed processes with high transition
probabilities. Therefore, the apparent large Stokes shift can be
understood.
0.092, 0.12, and 0.12 from ∆O.D.348, ∆O.D.385, and ∆O. D.510
,
respectively. Those values for An2CH• and An2CH+ have been
reported by Steenken et al. to be 0.24 and 0.31, respectively,
during 248-nm laser photolysis,24 while that of An2CH•* has
not been reported.
Thus, An2CH•* (Dn) decayed very fast to the lowest excited
doublet state of An2CH•* (D1) via nonradiative deactivation (eq
7) at the rate constant of approximately 1011 s-1 which was
estimated from the energy gap of An2CH•* (Dn)-An2CH•* (D1)
according to the relationship between the energy gap of the
singlet excited state (S1) and ground state (S0) and the rate
On the basis of the results, An2CHCl is excited to An2CHCl
in the singlet excited state (An2CHCl*) with irradiation of a

4782 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Hara et al.
constant of the internal conversion of S0 r S1.30,31 The recovery
of the transient absorption of An2CH• (D0) at 348 nm was
consistent with the decay of An2CH•* (D1) at 385 nm in the
time scale of 100 ns. An2CH•* (D1) decayed to An2CH• (D0)
with fluorescence (eq 8) or via nonradiative deactivation with
a half-lifetime of 110 ns. An2CH•** (Dn′, n′ > 2) was generated
from excitation of An2CH•* (D1) with the second 355-nm laser
flash (eq 9). Since An2CH+ was generated during the second
355-nm laser irradiation, ionization of An2CH•** occurs to give
An2CH+ (eq 10). The photon energy at 355 nm is 3.5 eV, and
the oxidation potential of Ph2CH• is estimated to be 5.8 eV from
ionization potential (I. P.) of 7.3 eV32,33 and solvation energy
of 1.5 eV for the solvated electron in acetonitrile. The oxidation
potential of An2CH• must be lower than that for Ph2CH• because
of the electron donating methoxyl groups. Therefore, RETPI
of An2CH• with two-step excitation involving An2CH•* (D1)
and An2CH•** (Dn′) is energetically possible and occurs within
a laser flash as shown in eq 10
Figure 5. Energy diagram for three-color three-laser photochemistry
of An2CHCl in acetonitrile.
fluorescence at 560 nm within a laser flash duration. This is
consistent with the lifetime of 4.2 ns for An2CH+* (see above).
An2CH• (D0) + hν355 f An2CH•* (Dn)
An2CH•* (Dn) f An2CH•* (D1)
(6)
(7)
An2CH+ + hν495 f An2CH+*
An2CH+* f An2CH+ + hνf560
(11)
(12)
An2CH•* (D1) f An2CH• (D0) + hνf550
An2CH•* (D1) + hν355 f An2CH•** (Dn′)
An2CH•** (Dn′) f An2CH+ + es-
(8)
On the basis of the experimental results, the energy diagram is
suggested for the three-color three-laser photolysis of An2CHCl
(Figure 5). Since three intermediates of An2CH•, An2CH•*, and
An2CH+ formed immediately after a laser flash, the homolytic
and heterolytic cleavages to An2CH• + Cl• (eq 2) and An2CH+
+ Cl- (eq 3), respectively, occurred together with sequential
excitation of An2CH• to An2CH•*. Electron transfer between
An2CH• and Cl• may be involved for the formation of An2CH+.
The concentration of An2CH+ was 3.4 × 10-6 M just before
(9)
(10)
The numbers of 355-nm laser photons absorbed by An2CH•
was calculated to be 1.5 × 10-6 Einstein L-1 from ∆O.D.355
)
0.16 for An2CH• with the laser beam path (1.0 cm) using T-T
absorption of triplet benzophenone.34 Therefore, quantum yields
of the bleaching of An2CH• (D0) and formations of An2CH•*
(D1) and An2CH+ were found to be 0.62, 0.36, and 0.26 from
∆∆O.D.348, ∆∆O.D.385, and ∆∆O.D.510, respectively. From the
formation quantum yields and formation of An2CH+ via RETPI
of An2CH•, excitation of An2CH• generates An2CH•* and
An2CH+ in 36 and 26% yields, respectively, and deactivates in
38% yield.
the second 355-nm laser flash, while that was 3.9 × 10-6
M
immediately after the second 355-nm laser flash. Therefore, the
ratio of the concentrations of An2CH+ before and after the
second 355-nm laser flash was approximately 1:1.2.
The yield of An2CH+ from RETPI of An2CH• and fluores-
cence intensity of An2CH+ increased with the second 355-nm
laser excitation of An2CH•. The fluorescence intensity of
An2CH+ increased approximately 1.2 times with the second 355-
nm laser excitation of An2CH•. Thus, fluorescence yield of An2-
CH+ was enhanced by the second 355-nm laser irradiation. The
present study is the first example to control the intermediates
and reaction pathways using the three-color three-laser pho-
tolysis in solution.
The concentrations of An2CH• (D0), An2CH•* (D1), and
An2CH+ were 3.4 × 10-6, 3.3 × 10-6, and 3.4 × 10-6 M,
respectively, with the ratio of 34:33:34 at 100 ns after 308-nm
laser flash and just before the second 355-nm laser irradiation.
The decreased concentration of An2CH• (D0) and increased
concentrations of An2CH•* (D1) and An2CH+ were 9.6 × 10-7
,
5.6 × 10-7, and 4.0 × 10-7 M in the ratio of 50:29:21,
respectively, during the second 355-nm laser photolysis. There-
fore, the concentrations of An2CH• (D0), An2CH•* (D1), and
An2CH+ were 2.5 × 10-6, 4.2 × 10-6, and 3.9 × 10-6 M in
the ratio of 24:40:37, respectively, immediately after the second
355-nm laser flash with the delay time of 100 ns after the first
308-nm laser flash. The ratio of concentrations of An2CH• (D0):
An2CH•* (D1): An2CH+ changed from 34:33:34 to 24:40:37
during the second 355-nm laser flash photolysis. This indicates
that excitation of An2CH• (D0) gives An2CH•* (D1) and An2CH+
in the ratio of 1.4:1.
According to Scheme 1A-C, in the three-color three-laser
photochemistry of An2CHCl are An2CHCl, An2CH•, and
An2CH+, respectively. The first laser (hν308) excitation of
An2CHCl produces An2CH• through An2CHCl*, the second
laser (hν355) excitation of An2CH• produces An2CH+ through
RETPI of An2CH• involving An2CH•*, and the third laser (hν495
)
excitation of An2CH+ gives fluorescence of An2CH+. Control
the intermediates and reaction pathways is accomplished in this
reaction, although three intermediates of An2CH•, An2CH•*, and
An2CH+ are generated during the first 308-nm laser excitation
of An2CHCl.
The lifetime of An2CH•** (Dn′) has been reported to be in
the femtosecond to picosecond range.26 No change of the
transient absorbance was observed during the third 495-nm laser
photolysis (hν495) of An2CH+ (eq 11), while the fluorescence
of An2CH+ (hνf560) was observed with a peak at 560 nm (eq
12). This result clearly demonstrates that An2CH+* is generated
with excitation of An2CH+ at 495 nm and that An2CH+* emits
Characteristics of the multiccolor multilaser flash photolysis
involving the three-color three-laser flash photolysis should be
further discussed from the viewpoint of controlling the inter-
mediates and reaction pathways. It is essentially impossible to
activate selectively an intermediate and to cause a selective
reaction of the intermediates in thermal chemistry and one-laser
chemistry. However, selective activation of an intermediate can

Photochemistry of Di(p-methoxyphenyl)methyl Chloride
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4783
(8) Johnston, L. J.; Redmond, R. W. J. Phys. Chem. A 1997, 101, 4660.
(9) McGimpsey, W. G.; Samaniego, W. N.; Chen, L.; Wang, F. J. Phys.
Chem. A 1998, 102, 8679.
(10) Goez, M.; Zubarev, V. Angew. Chem., Int. Ed. Engl. 2001, 40,
2867.
be easily accomplished by the molecule-selective and time-
specific excitation during the multiccolor multilaser photolysis,
as shown in the three-color three-laser photochemistry of
An2CHCl. We can point out possibilities of the multiccolor
multilaser flash photolysis to generate new reactive intermedi-
ates, reactions, and chemical species at a given time during the
reaction pathway, and to change a reaction pathway to other
ones. In addition, spatial selectivity of lasers allows us to apply
the multiccolor multilaser photolysis method to biological and
material science and technology such as photodynamic therapy35,36
and photopolymerization37-39 which will be developed by time-
and spatial-control of photochemical reactions with respect to
the reactive intermediates.
(11) Goez, M.; Schiewek, M.; Musa, M. H. O. Angew. Chem., Int. Ed.
Engl. 2002, 41, 1535.
(12) Wang, Z.; Mcgimpsey, W. G. J. Phys. Chem. 1993, 97, 5054.
(13) Itoh, M.; Fubwara, Y. J. Phys. Chem. 1983, 87, 4558.
(14) Itoh, M.; Adachi, T. J. Am. Chem. Soc. 1984, 106, 4320.
(15) Brewer, W. E.; Studer, S. L.; Standiford, M.; Chou, P. T. J. Phys.
Chem. 1989, 93, 6088.
(16) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1983, 105,
4828.
(17) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1984, 106,
850.
(18) Itoh, M.; Hasegawa, K.; Fujiwara, Y. J. Am. Chem. Soc. 1986, 108,
5853.
Conclusions
(19) Nishiya, T.; Yamauchi, S.; Hirota, N.; Fujiwara, Y.; Itoh, M. J.
Am. Chem. Soc. 1986, 108, 3880.
(20) Tokumura, K.; Watanabe, Y.; Udagawa, M.; Itoh, M. J. Am. Chem.
Soc. 1987, 109, 1346.
(21) Itoh, M.; Fujiwara, Y. J. Am. Chem. Soc. 1985, 107, 1561.
(22) Brewer, W. E.; Martinez, M. L.; Chou, P. T. J. Phys. Chem. 1990,
94, 1915.
(23) Fuket, K.; Kaya, K. J. Phys. Chem. 1989, 93, 614.
(24) Bartl, J.; Steenken, S.; Mayr, H.; Mcclelland, R. A. J. Am. Chem.
Soc. 1990, 112, 6918.
The three-color three-laser chemistry in solution was dem-
onstrated for the first time by the three-color three-laser flash
photolysis of An2CHCl in which the delay times of the second
355-nm laser irradiation after the first 308-nm laser irradiation
and of the third 495-nm laser irradiation after the second 355-
nm laser irradiation were both 100 ns. The first 308-nm laser
excitation of An2CHCl produced An2CH• through An2CHCl*,
the second 355-nm laser excitation of An2CH• produced An2-
CH+ through RETPI of An2CH• involving An2CH•*, and the
third 495-nm laser excitation of An2CH+ gave fluorescence of
An2CH+. The present work is the first example to control the
intermediates and reaction pathways using the three-color three-
laser photolysis method in solution.
(25) Das, P. K. Chem. ReV. 1993, 93, 119.
(26) Faria, J. L.; Steenken, S. J. Phys. Chem. 1993, 97, 1924.
(27) Bromberg, A.; Schmidt, K. H.; Meisel, D. J. Am. Chem. Soc. 1985,
107, 83.
(28) Carmichael, I.; Helman, W. P.; Hug, G. L. J. Phys. Chem. Ref.
Data 1987, 16, 239.
(29) Bromberg, A.; Schmidt, K. H.; Meisel, D. J. Am. Chem. Soc. 1984,
106, 3056.
(30) Siebrand, W.; Williams, D. F. J. Chem. Phys. 1968, 49, 1860.
(31) Turro, N. J. Modern Molecular Photochemistry; University Science
Book: Sausalito, 1991.
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Scientific Research from Ministry of Education,
Culture, Sports, Science and Technology of Japan.
(32) Harrison, A. G.; Lossing, F. P. J. Am. Chem. Soc. 1960, 82, 1052.
(33) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys.
Chem. Ref. Data 1977, Suppl. 1, 6.
References and Notes
(1) Miranda, M. A.; Perez-Prieto, J.; Font-Sanchis, E.; Konya, K.;
Scaiano, J. C. J. Phys. Chem. A 1998, 102, 5724.
(2) Miranda, M. A.; Font-Sanchis, E.; Perez-Prieto, J.; Scaiano, J. C.
J. Org. Chem. 2002, 67, 1162.
(3) Ouchi, A.; Koga, Y.; Adam, W. J. Am. Chem. Soc. 1997, 119, 592.
(4) Redmond, R. W.; Scaiano, J. C.; Johnston, L. J. J. Am. Chem. Soc.
1992, 114, 9768.
(5) Goez, M.; Zubarev, V.; Eckert, G. J. Am. Chem. Soc. 1998, 120,
5347.
(6) Goez, M.; Zubarev, V. J. Phys. Chem. A 1999, 103, 9605.
(7) Wang, Z.; Weininger, S. J.; Mcgimpsey, W. G. J. Phys. Chem.
1993, 97, 374.
(34) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 54.
(35) Dougherty, T. J.; Lawrence, G.; Kaufman, J. H.; Boyle, D.;
Weishaupt, K. R.; Goldfarb, A. J. Natl. Cancer Inst. 1979, 62, 231.
(36) Dougherty, T. J.; Gomer, C. J.; Henderson, B. W.; Jori, G.; Kessel,
D.; Korbelik, M.; Moan, J.; Peng, Q. J. Natl. Cancer Inst. 1998, 90, 889.
(37) Belfield, K. D.; Ren, X. B.; Van Stryland, E. W.; Hagan, D. J.;
Dubikovsky, V.; Miesak, E. J. J. Am. Chem. Soc. 2000, 122, 1217.
(38) Strickler, J. H.; Webb, W. W. Opt. Lett. 1991, 16, 1780.
(39) Borisov, R. A.; Dorojkina, G. N.; Koroteev, N. I.; Kozenkov, V.
M.; Magnitskii, S. A.; Malakhov, D. V.; Tarasishin, A. V.; Zheltikov, A.
M. Appl. Phys. B 1998, 67, 765.