4-(9-Anthryl)aniline. 1. Intramolecular charge-transfer state formation in solution

Article abstract of DOI:10.1021/jp970332z

4-(9-Anthryl)aniline (AA) was synthesized for the first time, and its absorption and fluorescence spectra, as well as its fluorescence lifetime, were measured in various solvents. The absorption spectra of AA are nearly independent of the solvent polarity

Full text of DOI:10.1021/jp970332z

5228
J. Phys. Chem. A 1997, 101, 5228-5231
4
-(9-Anthryl)aniline. 1. Intramolecular Charge-Transfer State Formation in Solution
†
Seonkyung Lee, Koji Arita, and Okitsugu Kajimoto*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kitashirakawa-Oiwakecho,
Sakyo-ku, Kyoto 606-01, Japan
Kohei Tamao
The Chemical Research Institute of Kyoto UniVersity, Gokasyou, Uji, Kyoto 611, Japan
X
ReceiVed: January 24, 1997; In Final Form: May 15, 1997
4-(9-Anthryl)aniline (AA) was synthesized for the first time, and its absorption and fluorescence spectra, as
well as its fluorescence lifetime, were measured in various solvents. The absorption spectra of AA are nearly
independent of the solvent polarity while the fluorescence spectra and its lifetimes are strongly dependent on
the solvents. The dipole moment of the fluorescent state was evaluated to be 15.8 D in the solvent polarity
range f(ꢀ,n) ) 0.45∼0.8. The large dipole moment, together with the largely red-shifted fluorescence and its
long radiative lifetime, demonstrates the formation of the intramolecular charge-transfer state.
1. Introduction
equilibrium torsional angle of ADMA, but found that the LIF
spectra is much too congested to deduce the torsional potential.
In the present series of study, we have for the first time
Upon photoexcitation, a group of compounds called “twisted
intramolecular charge-transfer (TICT)” molecules form the
charge-transfer state whose donor and acceptor parts are
perpendicular with each other. Such a TICT state is formed
synthesized and studied 4-(9-anthryl)aniline (AA), which has
the same structure as ADMA except for the methyl groups being
substituted by hydrogen atoms. Because of the reduced number
of internal rotors comparing with ADMA, AA is expected to
show sharp separate peaks in its LIF excitation spectra, which
help us to deduce detailed information about the torsional
potential. We have first measured the absorption and fluores-
cence spectra together with the fluorescence lifetimes in several
polar solvents to assure the CT state formation, then studied
the structure and torsional potential of AA in a free jet, and
finally observed the CT state formation of AA-polar molecule
complexes in a free jet. This paper presents the first part of
the study, the synthesis of AA and its CT state formation in
solution.
1-3
only in polar solvents such as alcohol, acetone, and acetonitrile.
These molecules attract renewed attention because of the recent
development in the detailed analysis of internal rotation under
supersonic jet conditions. We have studied the structure and
dynamics of these compounds in a supersonic jet since 1986
and have reported the results for (N,N-dimethylamino)benzoni-
trile,4 3,5-dimethyl-4-(N,N-dimethylamino)benzonitrile, 9,9′-
,5
6
7-9
10
bianthryl (BA), and 4-(9-anthryl)dimethylaniline (ADMA).
The potential around the twisting angle was found to be quite
important in determining the behavior of the TICT state
formation.
Among the molecules forming a TICT state in polar solvents,
BA and ADMA are known to give a sharp contrast in their
dynamic behavior of the TICT state formation. In the case of
BA, the time dependence of the charge-transfer (CT) fluores-
cence can be analyzed simply in terms of the two-state model
2
. Experimental Section
.1. Synthesis of 4-(9-Anthryl)aniline. Since 4-(9-anthryl)-
2
aniline has been synthesized for the first time, the synthetic
procedure will be given in detail. The total scheme of the
synthesis was
1
1
including only the locally excited (LE) and the CT states. In
contrast, for ADMA, the rise and decay curve of the CT
fluorescence is known to be quite complex.12 One has to assume
many intermediate states between the LE and the CT states or
use several lifetimes for the curve fitting. In addition, the
transient absorption spectra cannot be expressed by a linear
1
3
combination of the absorption of the LE and the CT states.
The reason for such a big difference between these two
molecules, BA and ADMA, has not been fully analyzed yet
though a large number of related data have been published. In
this respect, determination of the torsional potential and the study
of isolated solvated clusters of these compounds in a supersonic
jet are important. Some years ago, we measured the laser-
induced fluorescence (LIF) spectra of BA and determined the
torsional potential. In the S0 state, two anthryl moieties are
perpendicular with each other, while in the S1 state the
equilibrium angle is 68°.7 We also attempted to determine the
p-Bromoaniline (8.61 g, 50 mmol) was reacted with n-BuLi
in hexane, 1.70 M, 62 mL, 105 mmol) in tetrahydrofuran (THF)
(
for 2 h at -70 °C and subsequently with Me3SiCl (14 mL, 110
mmol) for 0.5 h at the same temperature under nitrogen
atmosphere. After being stirred for an additional 1 h at room
temperature, the reaction mixture was filtrated and condensed
under reduced pressure. Distillation from the mixture (89-91
†
Present address: Electronics Research Laboratory, Corporate Technol-
ogy Center, Matsushita Electronics Corporation, Takatsuki, Osaka 569,
Japan.
°
C/0.7 mmHg) gave disilylated compound 1 in 68% yield (10.8
g, 34 mmol).
X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5639(97)00332-0 CCC: $14.00 © 1997 American Chemical Society

4-(9-Anthryl)aniline. 1.
J. Phys. Chem. A, Vol. 101, No. 29, 1997 5229
A Grignard reagent (0.37 M, 58 mL, 21.1 mmol) prepared
from compound 1 and Mg in THF were added to a THF (50
mL) solution of 9-bromoanthracene (4.97 g, 19.3 mmol) and
NiCl2(PPh3)2 (0.19 g, 0.3 mmol) at 0 °C under a nitrogen
atmosphere. This mixture was refluxed for 1 h. After the
confirmation of completion of the reaction by thin-layer
chromatography, HCl (2 N, 90 mL) was added dropwise to the
mixture at 0 °C, and the mixture was stirred for 0.5 h at room
temperature. After extraction with ether, the organic layer was
washed with 1 N NaOH several times and dried over MgSO4.
The resulting mixture was filtered and condensed under reduced
pressure. Recrystallization from ethanol afforded 3.05 g (11
mmol) of 4-(9-anthryl)aniline in 57% yield as yellow crystals:
1
mp 132-133 °C; H NMR (CDCl3) δ 3.83 (brs, 2H), 6.86-
6
7
1
1
.94 (m, 2H), 7.16-7.24 (m, 2H), 7.28-7.49 (m, 4H), 7.73-
.81 (m, 2H), 7.98-8.06 (m, 2H), 8.45 (s, 1H); 13C (CDCl3) δ
14.97, 125.00, 125.03, 126.07, 127.08, 128.27, 128.57, 130.60,
Figure 1. Absorption spectra of AA in hexane (s) and acetonitrile
(- - -).
31.45, 132.15, 137.38, 145.66; MS m/e (relative intensity) 269
+
(M , 100). Anal. Calcd for C20H15N (269.346): C, 89.19; H,
5
.61; N, 5.20. Found: C, 89.24; H, 5.57; N, 5.15.
2.2. Absorption and Fluorescence Measurements. Ab-
sorption spectra were taken with a Shimadzu UV-240 spectro-
photometer. Fluorescence spectra were recorded with a Shi-
madzu RF-502A spectrofluorophotometer. Fluorescence grade
hexane, cyclohexane, benzene, chloroform, dichloromethane,
and acetonitrile and spectrograde ethanol were used without
further purification. All solutions were deaerated by repeated
freeze-pump-thaw cycles.
2.3. Lifetime Measurements. Fluorescence lifetimes were
determined by the time-correlated single-photon counting
method using a mode-locked Ti:sapphire laser (Spectra-Physics,
Tsunami model 3950) which was pumped by a continuous wave
argon ion laser (Spectra-Physics, Beamlock 2060). The laser
pulse was frequency-doubled (383 nm) with a LiIO3 crystal and
used for exciting the sample. The full width at half-maximum
(
fwhm) of the instrument response function for the laser system
Figure 2. Absorption spectra of BA, AA, ADMA, and ATMA in
cyclohexane, showing that the peak width is related to the molecular
flexibility in the torsional motion.
was about 100 ps.
Samples were placed in quartz tubes, deaerated, and sealed.
A Hamamatsu R2809-02u microchannel plate photomultiplier
tube was used for detection. Data were collected typically up
by Wortmann et al.,15 the shape of the absorption spectrum
strongly reflects the torsional potential of BA both in the ground
and excited states. In the case of BA, the ground state torsional
potential has a very narrow valley around 90° (perpendicular
orientation) because of the large repulsive force between the
bulky anthryl moieties. The Franck-Condon region of the
excited state is therefore narrow, and the absorption spectrum
becomes sharp. In addition, the depth of the double-minimum
torsional potential in the excited state is known to be small,
which limits the energy range of the excitation from the ground
state.
In contrast, the repulsive force between the aniline and
anthracene moieties in AA and ADMA is rather small compared
with the force between the two anthryl moieties of BA. This
makes the torsional motion more flexible, and the potential curve
around 90 degrees becomes much flatter, or even makes the
equilibrium position deviate from the perpendicular position in
the ground state. Consequently, the Franck-Condon region in
the excited state becomes much wider and thus the peak width
of the absorption spectrum also becomes wider. If one attaches
methyl substituents to the 3,5-positions of benzene ring to limit
the flexibility in the torsional motion, the absorption peak again
gets sharper as demonstrated by the spectrum of 4-(9-anthryl)-
3,5-dimethyl-N,N-dimethylaniline (ATMA). In this way we can
infer the flatness of the torsional potential in the cases of ADMA
and AA from the observed broadness of the absorption spectrum.
Such a difference in the flexibility of torsional motion strongly
3
to 6 × 10 counts in the peak channel.
Fluorescence spectra were carefully measured before and after
laser excitation, and it was confirmed for all solutions that no
detectable photochemical decomposition occurred except chlo-
roform and dichloromethane.
3
. Results and Discussion
.1. Absorption Spectra. The absorption spectra of AA
3
taken in nonpolar hexane and polar acetonitrile are shown in
Figure 1. The absorption spectrum of AA is almost the same
as anthracene and ADMA with intense vibronic peaks due to
n
0
14
the anthracene 6 vibration. It appears, therefore, that the
excitation occurs into a locally excited state where the an-
thracene moiety is excited. The shift in the absorption
maximum by solvent polarity is very small, i.e., ca. 1 nm
between these solvents, indicating that the degree of stabilization
by these solvents is similar in both the ground and excited states.
Complete similarity of the absorption spectra in the vibronic
structure and the wavelength between these nonpolar and polar
solvents indicates that the absorption occurs in both cases to
the same excited state.
The width of the absorption peak is worth discussing in
relation to the torsional potential around the phenyl-anthryl
bond. As shown in Figure 2, the fwhm of the absorption peak
is smaller for BA than for ADMA and AA. As demonstrated

5230 J. Phys. Chem. A, Vol. 101, No. 29, 1997
Lee et al.
Figure 3. Fluorescence spectra of AA in various solvents: [AA] )
-
5
1
.0 × 10 M at room temperature.
Figure 4. Solvent effects on the fluorescence band maxima (νmax) of
AA (b), ADMA (O), and BA (0). The solvent parameter is f(ꢀ,n) )
affects the distribution of the torsional angle of BA or AA
molecules in solution and eventually causes a significant
difference in the dynamic behavior of the electron-transfer
reaction.
2
2
{
2(ꢀ - 1)/(2ꢀ + 1) - (n - 1)/(2n + 1)}, where ꢀ and n are the
dielectric constant and refractive index of the solvent, respectively.
though the points are rather scattered. By the straight-line
analyses of the plots in the range f(ꢀ,n) ) 0.45 ∼ 0.8, the dipole
moment was evaluated to be about 18.4 and 15.8 D for ADMA
and AA, respectively, taking the cavity radius in Onsager’s
reaction field theory as 5 Å. The smaller dipole moment
observed for AA is due to the smaller electron-donating power
of the amino group compared with the dimethylamino substitu-
ent in ADMA.
3
.2. Dispersed Fluorescence Spectra and the CT State
Formation in Polar Solvents. The dispersed fluorescence
spectra in various solvents are shown in Figure 3. The spectrum
in nonpolar hexane shows a vibronic structure. In polar
acetonitrile, however, the spectrum is devoid of vibronic
structure and largely red-shifted. Among the solvents studied,
the vibronic structure was observed in hexane and cyclohexane,
whereas only the red-shifted structureless fluorescence was
detected in benzene, chloroform, dichloromethane, ethanol, and
acetonitrile. Two noticeable features in these fluorescence
spectra of AA are worth discussing. First, the drastic change
in the shape of the fluorescence spectra in polar solvents
indicates that the fluorescent state is completely different from
the originally excited LE state. Second, the increasing Stokes’
red shift with increasing solvent polarity implies that AA in
the fluorescent state has a large dipole moment. This fluorescent
state, possessing a large dipole moment, is most likely the
intramolecular charge transfer state. One can conclude from
the results of the absorption and the fluorescence experiments
that AA is first excited to the LE state and then, if it is in polar
media, transfers to the CT state.
3
.3. Energy Level of the CT State: Comparison between
BA and AA. Although we use the Lippert-Mataga equation
to estime the excited state dipole moment, it is important to
realize that the equation is not appropriate when the nature of
the excited state changes gradually from an LE-like state to a
TICT-like state with increasing polarity of solvents. Similar
to the trend reported for ADMA, the plot for AA cannot be
represented by a single straight line but is curved or represented
by the superposition of several lines of different slopes. The
convex shape in the plot suggests that the charge-transfer
character of the emitting state, and hence the dipole moment,
increases with increasing solvent polarity.
The value νCT(0), i.e., the hypothetical CT fluorescence
maximum in the gas phase, can be obtained from the (ordinate)
intercept of the plot in Figure 4, though it contains some
uncertainty. Then, one can roughly estimate the energy gap
between the LE and the CT states of an isolated molecule. For
AA, the hypothetical emission maximum obtained from the plot
The difference in the fluorescence maximum between non-
polar hexane and polar acetonitrile was about 5000 cm for
AA, while in ADMA, the difference was 7000 cm . This fact
suggests that the dipole moment of ADMA is larger than that
of AA. In order to estimate the dipole moment of the CT state,
the fluorescence band maxima are plotted in Figure 4 against
-
1
-
1
-1
is about 27400 cm . Using the observed absorption maximum
-
1
of 26100 cm , the energy gap is estimated to be about 1300
-
1
cm ; the CT state is only slightly higher in energy than the
LE state. Such a small gap may facilitate the mixing of these
two states even in a nonpolar solvent. Interestingly, in Figure
2, all three compounds having an amino group exhibit the long
tail toward longer wavelength, suggesting the possible appear-
ance of the CT state beneath the strong LE absorption even in
nonpolar cyclohexane.
solvent polarity parameters according to the Lippert-Mataga
equation.16
2
µe
νCT ) ν (0) -
f(ꢀ,n)
(1)
CT
3
hca
with
On the other hand, for BA, the CT state of an isolated
molecule is known to have a higher energy compared to that in
AA and ADMA. Actually, from the plot for BA in Figure 4,
2
2
(ꢀ - 1)
(n - 1)
f(ꢀ,n) )
-
(2)
2
-1
(2ꢀ + 1)
one can obtain the energy gap of about 8200 cm using the
(2n + 1)
intercepts of the two separate lines for the LE and the CT states.
It is quite likely that the difference in the energy gap between
AA and BA plays an important role for the drastic difference
of the dynamic behavior in the CT state formation. However,
such a low-lying CT state has not been observed in the solution
phase. This is probably due to the small transition moment to
the CT state and also due to the broad absorption spectra which
makes the discrimination between the two transitions difficult.
where νCT and νCT(0) denote the emission maximum of each
solution and the hypothetical gas-phase emission maximum in
wavenumbers, µe is the excited state dipole moment, a is the
Onsager radius of the solute, and ꢀ and n are the dielectric
constant and the refractive index of the solvent, respectively.
The dipole moment of the CT state can be estimated from the
slope of the straight line in the region of higher solvent polarity,

4-(9-Anthryl)aniline. 1.
J. Phys. Chem. A, Vol. 101, No. 29, 1997 5231
TABLE 1: Fluorescence Lifetimes of AA in Various
Solvents
3
-1
solvent
ν
max, 10 cm
lifetime, ns
n-hexane
24.4
24.1
22.2
21.7
19.4
19.4
3.2
3.2
(4)
cyclohexane
chloroform
dichloromethane
ethanol
7.6
13.2
acetonitrile
TABLE 2: Comparison of the Fluorescence Lifetimes (ns)
among AA, ADMA, and BA
solvent
AA
ADMAa
BAb
n-hexane
ethanol
acetonitrile
3.2
7.6
13.2
2.1
8
33
35
31.4
a
Values taken from ref 14. b Values taken from ref 20.
polarity.18,19 The polarity dependence in the lifetime of AA is
in accord with the above tendency and therefore supports the
formation of the CT state in polar solvent.
Figure 5. Fluorescence decay curves monitored at the fluorescence
maxima of AA in various solvents excited at 383 nm.
In order to unambiguously show the presence of such a CT
state, the detailed spectroscopic observation of an isolated
molecule in a free jet and its analysis in terms of the electronic
structure and torsional potential must be necessary.
In conclusion, the above mentioned three types of features
of the fluorescent state in polar solventsslarge dipole moment,
largely red-shifted fluorescence, and its long radiative lifetimes
confirm that 4-(9-anthryl)aniline forms the intramolecular
charge-transfer state in polar solvent. In addition, from the
width of the absorption spectrum and the trend in the solvato-
chromic fluorescence shift, the characteristics of AA are quite
similar to that of ADMA.
3.4. Lifetimes of the LE and the CT States. The solutions
of AA in various solvents were excited with picosecond laser
pulses at 383 nm. The fluorescence rise and decay curves were
measured at selected emission wavelengths. The rise time at
the fluorescence maximum in polar solvents was too rapid to
deduce the rate of electron transfer; the electron-transfer reaction
was completed within 20 ps. Typical decay curves observed
at the fluorescence maximum are shown in Figure 5. The
emission at shorter wavelength (ca. 410 nm) mainly originates
from the LE form whereas the emission at longer wavelength
comes from the CT state. All the observed decay curves were
well fitted by single-exponential functions. The decay curve
in nonpolar hexane gave a lifetime of 3.2 ns, while that in polar
acetonitrile was 13.2 ns. In chloroform and dichloromethane,
fluorescence spectra changed with time of exposure to pico-
second laser pulses, though it did not show any significant
change by repeated measurements with an ordinary fluorometer.
AA seems to react slowly with chloroform and dichloromethane
upon irradiation by intense laser pulses. For chloroform, the
measurement was carried out within a short period where the
change due to the slow reaction can be neglected.
Acknowledgment. This work is partially supported by the
Grani-in-Aid (number 06453022) from the MInistry of Educa-
tion, Science and Culture.
References and Notes
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J.; Baumann, W. NouV. J. Chim. 1979, 3, 443.
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1
(
(
3) Rettig, W.; Chandross, E. A. J. Am. Chem. Soc. 1985, 107, 5617.
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(
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(7) Yamasaki, K.; Arita, K.; Kajimoto, O.; Hara, K. Chem. Phys. Lett.
986, 123, 277.
8) Kajimoto, O.; Yamasaki, K.; Arita, K.; Hara, K. Chem. Phys. Lett.
1
1
1
1
(
The decay curve of AA in ethanol measured at its fluores-
cence maximum has two components: a fast decay component
with a lifetime of 40 ps and a slow decay component with a
lifetime of 7.6 ns. The reason for the presence of the fast decay
component is not certain at this stage. However, such a fast
decay component was also observed by Anthon et al. for BA
986, 125, 184.
(9) Honma, K.; Arita, K.; Yamasaki, K.; Kajimoto, O. J. Chem. Phys.
991, 94, 3496.
10) Kajimoto, O.; Hayami, S.; Shizuka, H. Chem. Phys. Lett. 1991,
(
77, 219.
(11) Kang, T. J.; Kahlow, M. A.; Giser, D.; Swallen, S.; Nagarajan, V.;
Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 6800.
(12) Siemiarczuk, A.; Ware, W. R. J. Phys. Chem. 1987, 91, 3677.
17
in alcohol solutions and by Siemiarczuk and Ware for ADMA
in 2-propanol.13 Since the fast component of the fluorescence
decay is only seen in hydrogen-bonding solvent, some process
including excited-state hydrogen bonding may be responsible
for this fast decay. The lifetimes in various solvents are listed
in Table 1.
(13) Okada, T.; Mataga, N.; Baumann, W.; Siemiarczuk, A. J. Phys.
Chem. 1987, 91, 4490.
(14) (a) Okada, T.; Fujita, T.; Mataga, N. Z. Phys. Chem. N. F. 1976,
1
01, 57. (b) Okada, T.; Fujita, T.; Kubota, M.; Masaki, S.; Mataga, N.;
Ide, R.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1972, 14, 563.
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1991, 95, 6371.
16) (a) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1955,
(
(
One can see from Table 1 that the lifetime increases with
increasing solvent polarity. This trend is in agreement with the
previous results for ADMA and substituted anthracenes; Table
2
8, 690. (b) Pasman, P.; Rob, F.; Verhoeven, J. W. J. Am. Chem. Soc.
1982, 104, 5127.
(17) Anthon, D. W.; Clark, J. H. J. Phys. Chem. 1987, 91, 3530.
(18) Herbich, J.; Kapturkiewicz, A. Chem. Phys. 1993, 170, 221.
2
compares the lifetimes of the fluorescent states among AA,
(
19) Wang, S.; Cai, J.; Sadygov, R.; Lim, E. C. J. Phys. Chem. 1995,
9, 7416.
20) Nakagima, N.; Murakawa, M.; Mataga, N. Bull. Chem. Soc. Jpn.
1976, 49, 854.
ADMA, and BA in typical nonpolar and polar solvents.
Generally, the lifetime of the CT state is longer than that of the
LE state and becomes further longer with increasing solvent
9
(