Solvent effects on the formation and decay of an exciplex between the lowest excited singlet state of 9,10-dibromoanthracene and ground-state amine (N,N-dimethylaniline or triethylamine)

Article abstract of DOI:10.1021/jp962554n

Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state [DBA*(S₁)] of 9,10-dibromoanthracene (DBA) and ground-state amine in acetonitrile (CH₃CN) and ethanol (EtOH) containing N,N-dimethylaniline (DMA) [and n-heptane (HP) containing DMA or triethylamine (TEA)]. Only in CH₃CN-DMA, however, can decomposition of the DBA-DMA exciplex into the DBA radical anion (DBA^.-) and the DMA radical cation be seen, indicating very small generation of DBA^.- from the DBA-amine exciplex formed in EtOH-DMA and HP-amine (DMA or TEA). Interestingly, no DBA-TEA exciplex is formed in CH₃CN and EtOH containing TEA but nanosecond laser photolysis reveals the existence of DBA^.- in the former solvent. Furthermore, the rate of DBA → 9-bromoanthracene debromination upon steady-state photolysis in CH₃CN-TEA is 1 order of magnitude smaller than that in CH₃CN-DMA but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine. This suggests that diffusion-controlled quenching of DBA*(S₁) by TEA in CH₃CN and EtOH gives rise to the formation of a nonemissive short-lived encounter complex or ion pair. It thus be concluded that generation of DBA^.- from the DBA-amine exciplex or the encounter complex (or the ion pair) is affected by the dielectric constant of a pure solvent.

Full text of DOI:10.1021/jp962554n

J. Phys. Chem. 1996, 100, 18431-18435
18431
Solvent Effects on the Formation and Decay of an Exciplex between the Lowest Excited
Singlet State of 9,10-Dibromoanthracene and Ground-State Amine (N,N-Dimethylaniline or
Triethylamine)
Toshihiro Nakayama, Tetsuya Hamana, Pallabi Jana,† Seiji Akimoto,‡ Iwao Yamazaki,‡ and
Kumao Hamanoue*
Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
ReceiVed: August 19, 1996X
Picosecond laser photolysis reveals the formation of an exciplex between the lowest excited singlet state
[DBA*(S1)] of 9,10-dibromoanthracene (DBA) and ground-state amine in acetonitrile (CH3CN) and ethanol
(EtOH) containing N,N-dimethylaniline (DMA) [and n-heptane (HP) containing DMA or triethylamine (TEA)].
Only in CH3CN-DMA, however, can decomposition of the DBA-DMA exciplex into the DBA radical
anion (DBA•-) and the DMA radical cation be seen, indicating very small generation of DBA•- from the
DBA-amine exciplex formed in EtOH-DMA and HP-amine (DMA or TEA). Interestingly, no DBA-
TEA exciplex is formed in CH3CN and EtOH containing TEA but nanosecond laser photolysis reveals the
existence of DBA•- in the former solvent. Furthermore, the rate of DBA f 9-bromoanthracene debromination
upon steady-state photolysis in CH3CN-TEA is 1 order of magnitude smaller than that in CH3CN-DMA
but 2 or 3 orders of magnitude greater than those in EtOH and HP containing amine. This suggests that
diffusion-controlled quenching of DBA*(S1) by TEA in CH3CN and EtOH gives rise to the formation of a
nonemissive short-lived encounter complex or ion pair. It thus be concluded that generation of DBA•- from
the DBA-amine exciplex or the encounter complex (or the ion pair) is affected by the dielectric constant of
a pure solvent.
Introduction
reaction of XA*(S1) with amine; (2) since the lowest excited
triplet state [XA*(T1)] of XA in the absence of amine was
populated via the indirect XA*(S1) f XA*(Tn) f XA*(T1)
intersystem crossing through an adjacent higher excited triplet
state [XA*(Tn)],13-15 amine-assisted population of XA*(T1) was
ascribed to the intersystem crossing from 1(XA-amine)* to its
triplet exciplex [3(XA-amine)*] followed by rapid decomposi-
tion into XA*(T1) and amine.
In order to understand the solvent effect on the formation
and decay of an exciplex or a nonemissive short-lived encounter
complex (or ion pair) which generates the radical anion of 9,10-
dibromoanthracene (DBA) as an intermediate for amine-assisted
DBA f 9-bromoanthracene debromination, the present paper
deals with quenching of the lowest excited singlet state [DBA*
(S1)] of DBA by amine (DMA or TEA) in acetonitrile (CH3-
CN), ethanol (EtOH), and n-heptane (HP) studied by picosecond
laser photolysis as well as steady-state photolysis.
It was proposed that the intermediates for the photoinduced
dehalogenation of aromatic halo compounds in the presence of
amine were the radical anions of halo compounds generated by
decomposition of exciplexes formed between the lowest excited
singlet states of halo compounds and ground-state amine.1-8
By means of nanosecond laser photolysis and steady-state
photolysis, we thus studied photoinduced debromination of
haloanthracenes (XA, i.e., the 9-bromo, 9,10-dibromo, 9-chloro,
and 9,10-dichloro compounds) in acetonitrile containing N,N-
dimethylaniline (DMA) or triethylamine (TEA).9-12 Although
no absorption bands responsible for the lowest excited singlet
state [XA*(S1)] of XA and an exciplex [1(XA-amine)*] of
XA*(S1) with amine could be detectable, following scheme
(Scheme 1) was proposed:
SCHEME 1
Experimental Section
DBA (Aldrich) was recrystallized three times from EtOH,
and GR-grade DMA and TEA from Wako were refluxed over
calcium hydride and distilled under a nitrogen atmosphere. The
spectral-grade solvents used were CH3CN (Dojin), EtOH
(Nacarai), and HP (Dojin); although HP was used without
further purification, CH3CN and EtOH were dried using
molecular sieves 3A (Wako). All experiments were performed
at room temperature, and the sample solution was degassed by
several freeze-pump-thaw cycles or deoxygenized by bubbling
of Ar gas.
i.e., (1) the intermediate for photoinduced dehalogenation of
XA was its radical anion (XA•-) generated by decomposition
of (XA-amine)* which is formed by a diffusion-controlled
1
The emission spectra due to DBA*(S1) and its exciplex with
amine [1(DBA-amine)*] were recorded using a Hitachi MPF-4
spectrofluorometer, and the changes in the emission intensities
with time were measured by a single-photon-counting method
using a picosecond Ti:sapphire laser.16 For the picosecond
* Author to whom correspondence should be addressed.
† On leave from the Department of Spectroscopy, Indian Association
for the Cultivation of Science, Jadavpur, Calcutta 70032, India.
‡ Department of Chemical Engineering, Faculty of Engineering, Hokkaido
University, Sapporo 060, Japan.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02554-3 CCC: $12.00 © 1996 American Chemical Society

18432 J. Phys. Chem., Vol. 100, No. 47, 1996
Nakayama et al.
Figure 2. Plots of DM(t)/DM(max) [at 620 nm (O)] and DE(t)/DE(max)
[at 685 nm (b)] against time obtained in (a) CH3CN and (b) EtOH
containing 1 M DMA. The solid and dashed curves are the best-fit
absorbances calculated assuming a Gaussian intensity function (fwhm
) 7 ps) for both the excitation and probing light pulses, and using the
biexponential concentration functions given by eqs 1 and 2, respectively;
the rate constants k1 and k2 are listed in Table 2.
Figure 1. Transient absorption spectra obtained by picosecond laser
photolysis of DBA in (a) CH3CN and (b) EtOH containing 1 M DMA.
double-beam absorption spectroscopy, a mode-locked Nd:glass
laser was used;17 the excitation light pulse was the third
harmonic [λ ) 351 nm with a full width at the half-maximum
intensity (fwhm) of 7 ps] and the probing light pulse was
generated by focusing the fundamental (λ ) 1054 nm) into D2O.
The transient absorption spectra due to DBA*(S1) and 1(DBA-
amine)* were recorded using two polychromator-image sensor
detector systems.
TABLE 1: Rate Constants (k1, k2) Determined from
Biexponential Changes of Absorption and Emission
Intensities with Time Obtained for DBA*(S1) and
1(DBA-Amine)* in Solvents Containing 1 M Amine (DMA
or TEA)
from absorption
from emission
Steady-state photolysis of DBA in the presence of 0.04 M
amine (DMA or TEA) was carried out using the 404.6-nm
monochromatic light selected from a USH-500D super-high-
pressure mercury lamp. Then, using a Hitachi 200-20 spectro-
photometer, the relative rate of DBA f 9-bromoanthracene
debromination was determined by measurements of the absorb-
ance decrement of DBA at 402 nm.
solvent
k1/1010 s-1 k2/109 s-1 k1/1010 s-1 k2/109 s-1
CH3CN-DMA
EtOH-DMA
HP-DMA
4.2 ( 1.5
7.7 ( 3.5
4.5 ( 1.3
1.3 ( 0.2
1.5 ( 0.1
1.5 ( 0.1
1.4 ( 0.1
3.6 ( 0.5
4.2 ( 0.5
7.7 ( 1.2
4.5 ( 0.8
1.3 ( 0.1
1.5 ( 0.1
1.5 ( 0.1
1.4 ( 0.1
3.6 ( 0.5
HP-TEA
The time-dependent intensity changes of bands BM [DM(t)/
DM(max), open circles] and BE [DE(t)/DE(max), closed circles]
shown in Figure 2 can be reproduced by the solid and dashed
curves, respectively. These best-fit curves are calculated as
follows; (1) a Gaussian intensity function (fwhm ) 7 ps) is
assumed for both the excitation and probing light pulses; (2) at
time t, the concentration [CM(t)] of a transient species respon-
sible for band BM and that [CE(t)] of another transient species
responsible for band BE are expressed by biexponential functions
given by eqs 1 and 2, respectively,
Results
Formation and Decay of an Exciplex between the Lowest
Excited Singlet State of DBA in CH3CN-DMA, EtOH-
DMA, and HP-Amine. Figure 1 shows the transient absorp-
tion spectra obtained by picosecond laser photolysis of DBA
in CH3CN and EtOH containing 1 M DMA. Clearly, band BM
with an absorption maximum at 620 nm decreases with time
accompanied by the increase and then the decrease of band BE
with an absorption maximum at 685 nm. Although a similar
result is obtained in HP containing 1 M amine (DMA or TEA),
band BE observed in the presence of TEA has an absorption
maximum at 680 nm. With regard to the spectral profile and
the position of absorption maximum, band BM is very similar
to the absorption bands due to the lowest excited singlet states
of anthracene and its several derivatives such as the 9-acetoxy,
9-phenyl, 2,9-diacetoxy, 2,9-diacetoxy-3-methoxy, and 2,9-
diacetoxy-3-methoxy-10-phenyl compounds.18 In the absence
of amine, furthermore, only band BM is observed and its decay
rate constant (kM) is found to be equal to the fluorescence
lifetime (kf), i.e., kM ) (7.7 ( 1.2) × 108 s-1 and kf ) (7.7 (
0.6) × 108 s-1 in CH3CN, kM ) (7.1 ( 1.0) × 108 s-1 and kf
) (7.1 ( 0.5) × 108 s-1 in EtOH, and kM ) (8.3 ( 1.4) × 108
s-1 and kf ) (8.3 ( 0.7) × 108 s-1 in HP. Hence, band BM is
assigned to the singlet-singlet (S′ r S1) absorption originating
from the lowest excited singlet state [DBA*(S1)] of DBA.
CM(t) ) c1 exp(-k1t) + c2 exp(-k2t)
CE(t) ) c3[-exp(-k1t) + exp(-k2t)]
(1)
(2)
where k1 and k2 are the rate constants listed in Table 1, and c1,
c2, and c3 are found to be positive experimental constants.
As shown in Figure 3, addition of a high concentration of
DMA (1-2 M) in CH3CN or EtOH causes the appearance of a
broad emission band at wavelengths longer than those for the
fluorescence band [the monomer emission band due to
DBA*(S1)]. A similar result is also obtained in HP containing
0.5-1 M DMA or 1-2 M TEA. In these cases, the intensity
of the monomer emission band decreases continuously but that
of the broad emission band increases at first and then decreases
with time. Furthermore, the time-dependent intensity changes
of emission bands can be reproduced by their calculation using

Exciplex between DBA*(S1) and Ground-State Amine
J. Phys. Chem., Vol. 100, No. 47, 1996 18433
Figure 4. Plots of k1 + k2 against DMA (O) or TEA (b) concentration
Figure 3. Monomer and exciplex emission bands observed for DBA
in (a) CH3CN and (b) EtOH containing 1 or 2 M DMA. The spectral
intensities are normalized at the emission peaks indicated by arrows.
in CH3CN, EtOH, and HP.
TABLE 2: Rate Constants for the Formation (kq) and
1
Decay (k′, kE) of (DBA-Amine)*
SCHEME 2
solvent
kq/1010 M-1 s-1
k′/109 s-1
kE/109 s-1
CH3CN-DMA
EtOH-DMA
HP-DMA
3.8 ( 0.2
2.7 ( 0.1
3.7 ( 0.2
0.91 ( 0.03
2.7 ( 0.3
11 ( 2
2.1 ( 0.2
1.0 ( 0.1
4.8 ( 0.5
40 ( 4
7.3 ( 0.7
5.1 ( 0.4
HP-TEA
the rate constants kq and kE′ ) k′ + kE can be obtained from
the slope and the intercept of eq 6, respectively; kM ()kf) is the
decay rate constant of DBA*(S1) obtained previously in the
absence of amine. By measurements of the time-dependent
intensity changes of monomer and exciplex emissions at various
amine concentrations, therefore, the rate constants (k1 and k2)
are determined and a plot of k1 + k2 against DMA (open circles)
or TEA (closed circles) concentration gives a straight line (cf.
Figure 4). As listed in Table 2, however, the quenching rate
constants (kq) thus obtained in CH3CN, EtOH, and HP contain-
ing DMA are 2.0-4.4 times greater than those estimated from
the viscosities (η) of solvents20 using the Debye-Smoluchowski
equation (kD ) 8RT/3000η) given for a diffusion-controlled
reaction, i.e., kD ) 1.9 × 1010 M-1 s-1 in CH3CN, 6.1 × 109
M-1 s-1 in EtOH, and 1.7 × 1010 M-1 s-1 in HP at 25 °C.
Since kq obtained in HP-TEA is 0.5 times smaller than kD, no
explanation can be given for the difference between kq and kD.
2. The intensity ratio (I0/IA) of monomer emissions in the
absence (I0) and presence (IA) of amine should be expressed
by
the biexponential concentration functions of transient species
responsible for the monomer emission band and the broad
emission band given by eqs 1 and 2, respectively. Naturally,
as listed in Table 1, the rate constants (k1 and k2) thus obtained
are found to be equal to those obtained for the time-dependent
intensity changes of transient absorptions.
If quenching of DBA*(S1) by amine gives rise to the
formation of an exciplex [1(DBA-amine)*] (cf. Scheme 2),19
the concentrations of DBA*(S1) and 1(DBA-amine)* at time t
can be expressed by eqs 3 and 4, respectively:
1
[DBA*(S1)]t
[(k + kM - λ2) exp(-λ1t) +
λ1 - λ2
(λ1 - k - kM) exp(-λ2t)] (3)
k
[1(DBA-amine)*]t
[-exp(-λ1t) + exp(-λ2t)]
λ1 - λ2
(4)
where the rate constants λ1 and λ2 are given by
λ1,2 ) (1/2) [k + k′ + kE + kM (
I0/IA) λ1λ2/kM(k′ + kE)
{(k + kM - k′ - kE)2 + 4kk′}1/2] (5)
) 1 + kq[amine] (1 - γ)/kM
) 1 + s′[amine]
(7)
Hence, one can conclude that eqs 1 and 2 are identical with
eqs 3 and 4, respectively; i.e., both the absorption band (BE)
shown in Figure 1 and the broad emission band shown in Figure
where γ ) k′/(k′ + kE) is the efficiency for repopulation of
DBA*(S1) from 1(DBA-amine)*. Hence, I0 and IA are
calculated by integration of the corresponding fluorescence
spectra over wavenumbers and I0/IA is plotted against amine
concentration. Typical examples obtained in CH3CN-DMA
are shown in Figure 5, where (a) the fluorescence spectrum due
to the emission of DBA*(S1) decreases with increasing DMA
concentration and (b) a plot of I0/IA (open circles) against DMA
concentration gives a straight line with a slope s′ indicated.
Similar results are obtained in EtOH-DMA and HP-amine
1
3 are responsible for (DBA-amine)*.
In order to determine the rate constants for the formation (kq)
and decay (k′, kE) of 1(DBA-amine)* shown in Scheme 2, the
following measurements and calculations are performed.
1. Since the sum of rate constants λ1 and λ2 is expressed by
λ1 + λ2 ) k1 + k2
) kq[amine] + k′ + kE + kM
(6)

18434 J. Phys. Chem., Vol. 100, No. 47, 1996
Nakayama et al.
Figure 6. Plot of I0/IA (O) against TEA concentration in (a) CH3CN
and (b) EtOH. s is the slope of a straight line drawn for I0/IA.
Figure 5. (a) Intensity decrease of the monomer emission band upon
addition of DMA and (b) plot of I0/IA (O) against DMA concentration
in CH3CN. In (b), s′ is the slope of a straight line drawn for I0/IA.
with the following slopes of straight lines: s′ ) kq(1 - γ)/kM
) 29.3 M-1 in EtOH-DMA, 33.9 M-1 in HP-DMA, and 9.0
M-1 in HP-TEA. Hence, a combination of γ ) k′/(k′ + kE)
derived from s′ with kE′ ) k′ + kE gives the decay rate constants
1
(k′ and kE) of (DBA-amine)* as listed in Table 2. Although
the decay rate constants obtained in CH3CN-DMA and HP-
amine are on the order of 109 s-1, those obtained in EtOH-
DMA are on the order of 1010 s-1; no reason for this discrepancy
is presently understood.
Quenching of the Lowest Excited Singlet State of DBA
by TEA in CH3CN and EtOH. No emission band due to
1(DBA-TEA)* can be seen in CH3CN and EtOH containing
e4 M TEA, although such an emission band is observed in
neat TEA. Also, no exciplex absorption (band BE) is observed
by picosecond laser photolysis of DBA in CH3CN and EtOH
containing 1 M TEA, and band BM decreases with time
following a single-exponential function. In neat TEA, however,
both bands BM and BE can be seen and the rate constants
Figure 7. Absorption spectral change upon steady-state photolysis of
DBA and plot of Dt/D0 against photolysis time in CH3CN containing
0.04 M DMA.
obtained are k1 ) 1.0 × 1011 s-1 and k2 ) 2.5 × 109 s-1
.
quenching of DBA*(S1) by TEA can be attributed to a diffusion-
controlled reaction. In fact, the rate constant [kA ) 2.2 × 1010
s-1 in CH3CN-TEA (1 M) or 5.3 × 109 s-1 in EtOH-TEA (1
M)] obtained for the decay of DBA*(S1) with time is almost
equal to that estimated by kcal ) kM + kq[TEA], i.e., 2.4 × 1010
s-1 in CH3CN-TEA (1 M) or 5.6 × 109 s-1 in EtOH-TEA (1
M).
Relative Rates of Amine-Assisted DBA f 9-Bromoanthra-
cene Debromination upon Steady-State Photolysis. As shown
in Figure 7a, steady-state photolysis of DBA in CH3CN-DMA
(0.04 M) causes the decrease of reactant absorption peaks (R1,
R2, and R3) with time accompanied by the increase of product
absorption peaks (P1, P2, and P3) which are identical with those
of 9-bromoanthracene.9 Measuring the 402-nm absorbance of
DBA at photolysis times 0 (D0) and t (Dt), Dt/D0 is plotted
against photolysis time as shown in Figure 7b. Since similar
results are obtained in CH3CN-TEA, EtOH-amine and HP-
amine, the relative rate (VR) of debromination is estimated by
dividing the initial slope of absorbance decrement obtained in
a given solvent by that obtained in CH3CN-DMA (cf. Table
3).
Although the decay rate constant of band BM, i.e., kA ) 2.2 ×
1010 s-1 in CH3CN-TEA (1 M) or 5.3 × 109 s-1 in EtOH-
TEA (1 M), is found to be equal to that obtained from the
emission decay curve, kA is very large compared with the decay
rate constant (kM ) 7.7 × 108 s-1 in CH3CN or 7.1 × 108 s-1
in EtOH) obtained in the absence of TEA.
Hence, the decay rate constant (kA) of DBA*(S1) in CH3CN
or EtOH containing e1 M TEA can be expressed by
kA ) kM + kq[TEA]
(8)
Then, the intensity ratio (I0/IA) of monomer emissions obtained
in the absence (I0) and presence (IA) of TEA is given by
I0/IA ) 1 + kq[TEA]/kM
(9)
This indicates that the slope (s ) 29.2 M-1 in CH3CN-TEA
or 6.5 M-1 in EtOH-TEA) of the straight line shown in Figure
6 should be equal to kq/kM. A choice of kM ) kf ) 7.7 × 108
s-1 in CH3CN or 7.1 × 108 s-1 in EtOH obtained in the absence
of TEA gives the quenching rate constants of kq ) 2.3 × 1010
M-1 s-1 in CH3CN-TEA and 4.7 × 109 M-1 s-1 in EtOH-
TEA which are comparable with those (kD) estimated previously
using the Debye-Smoluchowski equation, i.e., kD ) 1.9 × 1010
M-1 s-1 in CH3CN and 6.1 × 109 M-1 s-1 in EtOH. Hence,
Discussion
Figures 1a and 2a indicate that the disappearance of band BE
in CH3CN-DMA (1 M) results in the existence of a residual

Exciplex between DBA*(S1) and Ground-State Amine
J. Phys. Chem., Vol. 100, No. 47, 1996 18435
1(DBA-amine)* or an encounter complex (or an ion pair) into
DBA•- and the amine radical cation is affected by the dielectric
constant of a pure solvent. This is supported by the well-known
fact in radiation chemistry,25-29 where organic halides are
frequently used as effective electron scavengers and a so-called
dissociative electron attachment reaction occurs; i.e., a solvated
electron in polar solvents easily attaches to the organic halides
generating the radical anions followed by dissociation into the
dehalogenated neutral radicals and the halogen anions.
TABLE 3: Relative Rates (VR) of DBA f
9-Bromoanthracene Debromination in Solvents Containing
0.04 M Amine (DMA or TEA) and Dielectric Constants (E)
of Pure Solvents
VR (relative)
solvent
DMA
TEA
ꢀa
CH3CN
EtOH
HP
1.0
2.9 × 10-1
3.7 × 10-3
4.4 × 10-3
37.5
24.6
1.92
5.8 × 10-3
4.4 × 10-4
a Reference 24.
Acknowledgment. This work was supported by a Grant-
in-Aid for Priority-Area-Research on Photoreaction Dynamics
from the Ministry of Education, Science, Sports and Culture of
Japan (No. 06239101).
absorption band. In EtOH-DMA (1 M) and HP-amine (1 M
DMA or TEA), however, no residual absorption band can be
seen after the disappearance of band BE (cf. Figures 1b and
2b). Since we have confirmed that this residual absorption band
is identical with the absorption band of the radical anion
(DBA•-) observed at the end of nanosecond pulse excitation of
References and Notes
(1) Ohashi, M.; Tsujimoto, K.; Seki, K. J. Chem. Soc., Chem. Commun.
1973, 384.
1
DBA in CH3CN-DMA (1 M),11 decomposition of (DBA-
amine)* into DBA•- and the amine radical cation in EtOH-
DMA (1 M) and HP-amine (1 M) may be extremely slow
compared with that in CH3CN-DMA (1 M). In fact, nano-
second laser photolysis of DBA in the former solvents gives
rise to no appreciable appearance of the absorption band
responsible for DBA•-. As shown in Figure 1, the intensity of
a weak band (with an absorption maximum at ∼475 nm) in
CH3CN-DMA is greater than that in EtOH-DMA; although
this band is very similar to that of the DMA radical cation,21-23
no such a band is observed in HP-amine. Since a charge-
transfer character (ø) sometimes enhances the absorption
intensity, ø of 1(DBA-amine)* may decrease in the order of ø
(in CH3CN-DMA) > ø (in EtOH-DMA) . ø (in HP-amine)
and this may be a cause for observation of the absorption band
due to DBA•- [generated from 1(DBA-DMA)*] only in
CH3CN-DMA (1 M) upon picosecond laser photolysis.
By nanosecond laser photolysis of DBA in CH3CN-TEA
(1 M), the absorption band of DBA•- can be clearly seen with
an absorbance of ∼0.03.11 Since this absorbance is nearly equal
to the baseline fluctuation ((0.02 absorbance unit) of the present
picosecond laser photolysis system,17 no observation of the
absorption band due to DBA•- by picosecond laser photolysis
of DBA in CH3CN-TEA (1 M) may be reasonable. For DBA
in EtOH-TEA (1 M), however, no absorption band of DBA•-
can be seen even by nanosecond laser photolysis. Also no
emission originating from 1(DMA-TEA)* can be seen in both
CH3CN and EtOH containing TEA as stated previously. In spite
of these circumstances, Table 3 indicates that the relative rate
(VR) of DBA f 9-bromoanthracene debromination upon steady-
state photolysis in CH3CN-TEA is 1 order of magnitude smaller
than that in CH3CN-DMA but 2 or 3 orders of magnitude
greater than those in EtOH and HP containing amine; however,
VR in EtOH-TEA is nearly equal to that in EtOH-DMA.
All the facts stated above suggests that diffusion-controlled
quenching of DBA*(S1) by TEA in CH3CN and EtOH gives
rise to the formation of an encounter complex or an ion pair;
i.e., this short-lived complex with no detectable emission and
absorption intensities is different from an exciplex proposed in
Scheme 1. Since the dielectric constant (37.5) of CH3CN is
much greater than those of EtOH (24.6) and HP (1.92),24 it can
be concluded that generation of DBA•- is essential for amine-
assisted debromination of DBA and that decomposition of
(2) Tsujimoto, K.; Tasaka, S.; Ohashi, M. J. Chem. Soc., Chem.
Commun. 1975, 758.
(3) Bunce, N. J.; Pilon, P.; Ruzo, L. O.; Sturch, D. J. J. Org. Chem.
1976, 41, 3023.
(4) Bunce, N. J.; Kumar, Y.; Ravanal, L.; Safe, S. J. Chem. Soc., Perkin
Trans. 2 1978, 880.
(5) Chittim, B.; Safe, S.; Bunce, N.; Ruzo, L.; Olie, K.; Hutzinger, O.
Can. J. Chem. 1978, 56, 1253.
(6) Davidson, R. S.; Goodwin, J. W. Tetrahedron Lett. 1981, 22, 163.
(7) Saeva, F. D. Topics Current Chem. 1990, 156, 61.
(8) Fulara, J.; Latowski, T. Polish J. Chem. 1990, 64, 369.
(9) Hamanoue, K.; Tai, S.; Hidaka, T.; Nakayama, T.; Kimoto, M.;
Teranishi, H. J. Phys. Chem. 1984, 88, 4380.
(10) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K. J. Phys. Chem.
1992, 96, 10297.
(11) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K.; Otani, A. J.
Photochem. Photobiol. A: Chem. 1993, 69, 305.
(12) Nakayama, T.; Ibuki, K.; Hamanoue, K. Proc. Indian Acad. Sci.
(Chem. Sci.) 1993, 105, 567.
(13) Tanaka, M.; Tanaka, I.; Tai, S.; Hamanoue, K.; Sumitani, M.;
Yoshihara, K. J. Phys. Chem. 1983, 87, 813.
(14) Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H. Bull. Chem.
Soc. Jpn. 1983, 56, 1851.
(15) Hamanoue, K.; Nakayama, T.; Ikenaga, K.; Ibuki, K. J. Photochem.
Photobiol. A: Chem. 1993, 74, 147.
(16) Ohta, N.; Tamai, N.; Kuroda, T.; Yamazaki, T.; Yamazaki, I. Chem.
Phys. 1993, 177, 591.
(17) Ushida, K.; Nakayama, T.; Yuhara, Y.; Ito, M.; Hamanoue, K.;
Teranishi, H.; Matsui, T.; Nagamura, T. Radiat. Phys. Chem. 1989, 34,
465.
(18) Szczepan´ski, J.; Heldt, J. Z. Naturforsch. 1985, 40a, 849.
(19) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: New York, 1970; Chapter 7.
(20) Weast, R. C. Handbook of Chemistry and Physics, 62nd ed.; CRC
Press: Boca Raton, FL, 1981.
(21) Hamil, W. H. Radical Ions; Kaiser, E. T., Kevan, L. Eds.; Wiley:
New York, 1968; Chapter 9.
(22) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier:
Amsterdam, 1988.
(23) Iwamura, H.; Eaton, D. F. Pure Appl. Chem. 1991, 63, 1003.
(24) Riddick, J. A.; Bunger, W. B. Organic SolVents, 3rd ed.; Techniques
of Chemistry 2; Wiley-Interscience: New York, 1970.
(25) Arai, S.; Tagawa, S.; Imamura, M. J. Phys. Chem. 1974, 78, 519.
(26) Neta, P.; Behar, D. J. Am. Chem. Soc. 1980, 102, 4798; J. Am.
Chem. Soc. 1981, 103, 103.
(27) Behar, D.; Neta, P. J. Phys. Chem. 1981, 85, 690; J. Am. Chem.
Soc. 1981, 103, 2280.
(28) Kigawa, H.; Takamuku, S.; Toki, S.; Kimura, N.; Takeda, S.;
Tsumori, K.; Sakurai, H. J. Am. Chem. Soc. 1981, 103, 5176.
(29) Hamanoue, K.; Kimoto, M.; Nakayama, T.; Teranishi, H.; Tagawa,
S.; Tabata, Y. Radiat. Phys. Chem. 1984, 24, 445.
JP962554N