Tamkang Journal of Science and Engineering, Vol. 6, No. 1, pp. 37-42 (2003)
The Protonation and Hydrogen Bonding Interaction in
N,N-dimethyl-4-(phenylimino-methyl)-aniline System
Tong-Ing Ho1, Tai-Chen Lee, Jinn-Hsuan Ho and Shun-Li Wang2
1
Department of Chemistry
National Taiwan University
Taipei, Taiwan 106, R.O.C.
E-mail: [email protected]
2
Department of Chemistry
National Chiayi University
Chiayi, Taiwan 600, R.O.C.
Abstract
The influence of hydrogen bonding (HB) and protonation on an
intramolecular charge transfer (ICT) compound, such as
N,N-dimethyl-4-(phenylimino-methyl)-aniline (NMe2-4PmA) in the
ground and excited state with two protic solvents 2,2,2-trifluoroethanol (TFE) and 2,2,2-trichloroethanol (TCE) is investigated. Mono
HB and mono protonation complexes with TFE and TCE both in the
ground state as well as in the excited state are all identified. TCE
shows stronger protonation as well as hydrogen bonding ability in the
ground state of NMe2-4PmA. In the excited state both TCE and TFE
show similar behavior in the hydrogen bonding and protonation
ability.
Key Words: Hydrogen Bonding, Intramolecular Charge Transfer (ICT),
Protonation
1. Introduction
The study of donor and acceptor groups
linked through electron conducting double
bonds or triple bonds has become an
important topic in photonics. We [1-4] have
studied the influence of hydrogen bonding
(HB) and protonation on a series of intramolecular charge transfer (ICT) systems such
as
p-N,N-dimethylamino-2-styrylnapthalene (2-StN-NMe2 ), p-N,N-diethylamino-2styrylnapthalene (2-StN-NEt2 ), p-N,N-dimethylamino-2-styrylquinoline
(2-StQ-NMe2 )
and p-N,N-diethylamino-2-styrylquinoline (2StQ-NEt2 ) (Scheme 1), The steric effect of
HB on ICT compounds is studied in different
protic solvents. It is found that a steric and
ICT effect will weaken the HB ability of the
N,N-diethylamino site and enhance the HB
ability of the quinoline site in 2-StQ-NEt2 .
There is excited state proton transfer (ESPT)
in the HB complex of 2-StQ-NMe2 . We have
also observed excited state deprotonation
(ESDP) process in the double protonation
from of 2-StQ-NMe2 in 2,2,2-trichloroethanol
(TCE). It is also possible to separate the HB
and protonation interactions by using another
strong base. To compare the difference of two
strong hydrogen bonding donors [5,6], TCE
and 2,2,2-trifluoroethanol (TFE) in their
bonding behavior toward ICT systems
become most interesting and is the main
purpose of our study. In the report, we would
like to extend our research to the interaction
of external protic solvents on the photophysical behavior of N,N-dimethyl-4-(phenylimino-methyl)-aniline (NMe2 -4PmA) which
is also a donor-acceptor system linked with a
carbon nitrogen double bond and it is
37
38
Tong-Ing Ho et al.
N
N
N
2-StQ-NMe2
2-StN-NMe2
N
N
N
2-StN-NEt2
2-StQ-NEt2
N
N
NMe2-4PmA
Scheme 1
reported that this molecule with strong ICT
interaction [7,8].
2. Experimental
2.1 Material
Compound N,N-dimethyl-4-(phenyliminomethyl)-aniline (NMe2-4PmA) were prepared by
condensation procedure. 4-Dimethylaminobenzaldehyde, aniline and excess magnesium
sulfate in dried toluene were reacted in room
temperature for 24h. The solid product was
purified by recrystallized from ethanol.1H NMR
(200 MHz, TMS, CDCl3) δ 8.31 (s, 1H), 7.80 (d,
J=8.4 Hz, 2H) 7.41-7.14 (m, 5H), 6.742 (d, J=8
Hz, 2H), 3.06 (s, 6H).
All the solvents were of Uvasol grade from
Merck or spectrophotometric grade from
ACROS and were used as received.
2.2 Method
UV-visible absorption spectra were
recorded on a Hitachi U-2000 spectrophotometer
and fluorescence spectra were obtained with a
Hitachi F-3000 fluorescence spectrophotometer.
3. Results and Discussion
The title compound N,N-dimethyl-4(phenylimino-methyl)-aniline (NMe2-4PmA) is
very sensitive to hydrochloric acid that in the
presence of little HCl caused the decomposition
by hydrolysis. The absorption spectra of
NMe2-4PmA in different solvents are shown in
Figure 1. In acetonitrile (Figure 1(a)), the charge
transfer band (CT) occurred at 353 nm. In TFE
and TCE, the CT bands are red shifted to 435.5
nm and 445.5 nm respectively (Figure 1(c), 1(d)).
This red shift (almost 80 nm) is quite different as
compared to our previous observation on
2-StN-NMe2 and 2-StQ-NMe2 systems [1]. In
the presence of TCE and TFE, the absorption
maxima of 2-StN-NMe2 have blue shifted while
a different behavior was observed for
2-StQ-NMe2 system. For the 2-StN-NMe2
system, there is only one basic site, the NMe2
site which can be hydrogen bonded, HB
interaction at this site will produce a blue shift in
the absorption maximum and solvents with a
stronger HB ability will show a longer shift. For
2-StQ-NMe2 there are two basic sites, that is
NMe2 and quinoline sites, HB interaction at the
NMe2 site will cause a blue shift, while HB
interaction at the quinoline site will produce a
red shift. TCE produces a red shift in the
absorption maximum of 2-StQ-NMe2 because of
HB interaction at the quinoline site. The
absorption maximum for NMe2-4PmA is also
red shift to 438 nm (Figure 1(b)) in a strong
protic acid trifluoroacetic acid. This red shift can
be ascribed as the protonation of the acceptor
site, the imino nitrogen site of NMe2-4PmA
(Scheme 2). Thus TCE and TFE both become
strong proton donors and produce the protonated
form at the imino nitrogen due to the strong
basicity of the imino nitrogen. To understand the
pure HB interaction of NMe2-4PmA with TFE
and TCE, the absorption maxima of NMe24PmA in TFE or TCE which were mixed with
acetonitrile in the presence of a quantity of
triethylamine are listed in Table 1. The presence
of large amount of triethylamine will reduce the
concentration of proton and show little change in
solvent medium. From Table 1 and Figure 2 the
absorption maxima of NMe2-4PmA are all
red-shifted with both TFE and TCE content.
Since the HB interaction at the acceptor site will
produce red shift in the absorption spectra. Thus
it is due to the HB interaction at imino nitrogen
site only. The HB interaction is stronger for TCE
since it caused larger red shift (359 nm vs. 366.5
nm). Even TFE can not result in the HB
interaction at the donor NMe2 site, which is
quite different from the 2-StQ-NMe2 system.
Only solvents of strong HB ability and small in
size like TFE can result in the formation of
double HB interaction in 2-StQ-NMe2 and
The Protonation and Hydrogen Bonding Interaction in N,N-dimethyl-4-(phenylimino-methyl)-aniline System
produce a blue shift in the absorption maxima.
Obviously the stronger imino nitrogen
predominates in the HB interaction and prevents
the HB interaction at the NMe2 site. Compare
the absorption maxima in Table 1 and Figure 1,
it is clear that protonation at the imono nitrogen
results in large red shift of the absorption
maxima than the hydrogen bonding interaction
at the same site. Figure 3 indicates the emission
spectra of NMe2-4PmA in different solvents.
Figure 2(a) shows the emission maximum at
393.6 nm and 438.6 nm in pure acetonitrile. The
emission maximum red shifted to 488.8 nm,
482.6 nm and 489.2 nm for trifluoroacetic acid
(Figure 3(b)), TFE (Figure 3(c)) and TCE
(Figure 3(d)) respectively. This indicates that the
emission from the monoprotonated form (at the
imino site only). As for the HB in the excited
state of NMe2-4PmA, the emission spectra are
recorded in the presence of large quantity of
strong base (triethylamine). The spectra are
shown in Figure 4. It indicated that the emission
maxima of the mono HB complex are at 445.5
nm and 442.2 nm for TFE and TCE respectively.
Again there is no much difference in energies
between the two HB complexes in the excited
state.
TFE shows larger δc value than TCE
according to Taft’s definition. However, TCE
has higher hydrogen bonding acidity according
to Catalán definition. There is some
N
N
discrepancies between the definition of
hydrogen-bonding donor and hydrogen-boding
acidity [9-13]. Our previous studies has shown
that TCE has a greater tendency to cause
protonation and create a larger blue shift in the
absorption maximum for 2-StN-NMe2 system
and TFE is a stronger hydrogen bonding donor
than TCE. In our present study, it has been
observed that TCE shows stronger protonation
as well as hydrogen-bonding ability in the
ground state of NMe2-4PmA. In the excited state
both TCE and TFE shows similarity in the
protonation and hydrogen bonding ability.
The absorption and emission spectra of
NMe2-4PmA in pure CF3COOH are shown in
Figure 5. There are two extinct absorption
maxima at 341 nm and 437 nm. Compared to the
neutral absorption maximum of 353 nm in
CH3CN, there is about 12 nm blue shift. Thus
the absorption at 341 nm can be ascribed to the
double protonation form of NMe2-4PmA. The
438 nm peak can be ascribed to the
monoprotonation form as compared with Figure
1(b). The emission maximum at around 411 nm
is quite different with the excited state
monoprotonated form (emission maximum at
488.8 nm, Figure 3(b)). And it is also different
from the excited HB complex (emission
maximum at 445 nm, Figure 4(b)). Thus we
assign the emission from 411 nm is due to the
emission of the double protonated form.
N
Hydrogen-bonding
BH
N
H B
NMe2-4PmA
BN
N
N+
Protonation
BH
NMe2-4PmA
Scheme 2
N
H
39
40
Tong-Ing Ho et al.
ABS
2.0
0.0
400
300
200
500
Wavelength (nm)
Figure 1. The absorption spectra of N,N-dimethyl-4-(phenylimino-methyl)-aniline NMe2-4PmA) in different solvents, (a)
2.5 x 10-5 M NMe2-4PmA in CH3CN, λmax = 353 nm, ABS = 0.730; (b) 2.5 x 10-5 M NMe2-4PmA with 1.25 x
10-4 M CF3COOH, λmax = 438 nm, ABS = 1.506; (c) 2.5 x 10-5 M NMe2-4PmA in CF3CH3OH, λmax = 435.5
nm, ABS = 1.107; (d) 1.5 x 10-5 M NMe2-4PmA in CCl3CH2OH, λmax = 445.5 nm, ABS = 0.765
Table 1. The absorption maxima of N,N-dimethyl-4-(phenylimino-methyl)-aniline (NMe2-4PmA) in different solvents
mixed with CH3CN which include excess triethylamine.
Absorption max (nm)
V/V0
[CF3CH 2OH],
0%
353.0
[CF3CH2OH],
20%
355.0
[CF3CH2OH],
40%
357.0
[CF3CH2OH],
60%
358.0
[CF3CH2OH],
80%
359.5
[CF3CH2OH],
100%
359.0
[CCl3CH2OH],
0%
353.0
[CCl3CH2OH],
20%
355.5
[CCl3CH2OH],
40%
359.0
[CCl3CH2OH],
60%
362.0
[CCl3CH2OH],
80%
364.0
[CCl3CH2OH], 100%
366.5
The Protonation and Hydrogen Bonding Interaction in N,N-dimethyl-4-(phenylimino-methyl)-aniline System
41
Relative Intensity
c
b
a
Wavelength (nm)
Figure 2. The emission spectra of N,N-dimethyl-4(phenylimino-methyl)-aniline (NMe2-4PmA)
in different solvents, (a) 2.5 x 10-5 M
NMe2-4PmA in CH3CN, λfl = 393.6 nm, λfl’
= 438.6 nm, I = 4.384, I’=3.554; (b) 2.5 x
10-5 MNMe2-4PmA with 1.25 x 10-4 M
CF3COOH, λfl = 488.8 nm, I = 16.44; (c) 2.5
x 10-5 M NMe2-4PmA in CF3CH3OH, λfl =
482.6 nm, I = 29.9; (d) 1.5 x 10-5 M
NMe2-4PmA in CCl3CH2OH, λfl = 489.2 nm,
I = 174.6
Wavelength (nm)
Figure 4. The emission spectra of N,N-dimethyl-4(phenylimino-methyl)-aniline (NMe2-4PmA)
in different solvents with enough triethylamine, (a) 2.5 x 10-5 M NMe2-4PmA in
CH3CN, λfl = 393.6 nm, λfl’ = 438.6 nm;(b)
2.0 x 10-5 M NMe2-4PmA with 1.2 x 10-2 M
triethylamine in CF3CH3OH, λfl = 445.4 nm;
(c) 2.0 x 10-5 M NMe2-4PmA with3.6 x 10-2 M
triethylamine in CCl3CH2OH, λfl = 442.2 nm
c
Relative Intensity
d
b
300
a
400
400
500
Wavelength (nm)
450
500
550
600
Figure 5. The absorption (a) and emission (a*) spectra
of N,N-dimethyl-4-(phenylimino-methyl)aniline (NMe2-4PmA) in pure CF3COOH
Wavelength (nm)
4. Conclusion
Figure 3. The absorption spectra of N,N-dimethyl-4(phenylimino-methyl)-aniline (NMe2- 4PmA)
in different solvents with enough triethylamine, (a) 2.0 x 10-5 M NMe2-4PmA in
CH3CN, λmax= 353 nm, ABS = 0.673; (b) 2.0
x 10-5 MNMe2-4PmA with 1.2 x 10-2 M
triethylamine in CF3CH3OH, λmax = 359.5
nm, ABS = 0.682; (c) 2.0 x 10-5 M
NMe2-4PmA with 3.6 x 10-2 M triethylamine
in CCl3CH2OH, λmax = 366.5 nm, ABS =
0.729
The emission and absorption spectra of
NMe2-4PmA in neutral and protic solution TFE
and TCE are recorded. The absorption and
emission maxima due to the mono HB complex,
monoprotonation complex at the acceptor site (the
imino site) from TFE and TCE are all well
assigned. The double protonated form can only be
produced using strong trifluoroacetic acid.
Emission from this doubly protonated complex is
also observed. There is no excited state proton
transfer in the HB complex. The excited state
deprotonation from the doubly protonated form
42
Tong-Ing Ho et al.
was not observed. TCE shows stronger protonation
as well as hydrogen bonding ability in the ground
state of NMe2-4PmA. In the excited state both TCE
and TFE show similar behavior with regard to their
protonation and hydrogen bonding ability. The
comparison of NMe2-4PmA system with other
donor-acceptor system 2-StN-NMe2, 2-StQ-NMe2
with regards to their interactions with protonation
and HB interaction is most interesting.
Acknowledgment
We are grateful to the National Science Council of
the Republic of China for financial support.
References
[1]
Wang, S. L.; Ho, T. I.; Spectrochim. Acta A
2001, 57, 361.
[2] Wang, S. L.; Ho, T. I.; Chem. Phys. Lett.
1997, 268, 434.
[3] Wang, S. L.; Lee, T. C.; Ho, T. I. J.
Photochem. Photobiol. A: Chem. 2002, 151,
21.
[4] Wang, S. L.; Ho, T. I.; J. Photochem.
Photobiol. A: Chem. 2000, 135, 119.
[5] Kamlet, M. J.; Abbound, J. L. M.; Abrahan,
M. H.; Taft, R. W. J. Org. Chem. 1983, 48,
2877.
[6] Catalan, J.; Couto, A.; Gomez, J.; Saiz., J. L.;
Laynez, J.; J. Chem. Soc., Perkin Tran. 1992,
2, 1181.
[7] El-Bayoumi, M. A.; El-Asser, M.;
Abdel-Halim, F. J. Am. Chem. Soc. 1971, 93,
586.
[8] El-Bayoumi, M. A.; El-Asser, M.;
Abdel-Halim, F. J. Am. Chem. Soc. 1971, 93,
590.
[9] Abboud, J. L. M.; Sraidi, K.; Abraham, M. H.;
Taft, R. W. J. Org. Chem. 1990, 55, 2230.
[10] Abraham, M. H.; Grellier, P. L.; Prior, D. V.;
Taft, R. W.; Morris, J. J.; Taylor, P. J.;
Laurence, C.; Berthelot, M.; Doherty, R. M.;
Kamlet, M. J.; Abboud, J. L. M.; Sraidi, K.;
Guiheneuf, G. J. Am. Chem. Soc. 1988, 110,
8543.
[11] Molina, M. T.; Bouab, W.; Esseffar, M.;
Herreros, M.; Notario, R.; Abboud, J. L. M.;
Mo, O.; Yanez, M.; J. Org. Chem. 1996, 61,
5485.
[12] Abraham, M. H.; Grellier, P. L.; Prior, D. V.;
Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem.
Soc., Prekin Trans. 1989, 2, 699.
[13] Kolling, O. W. J. Phys. Chem. 1992, 96,
1729.
[14] Wang, S. L.; Ho, T. I.; Spectrochim. Acta A
2001, 57, 361.
[15] Wang, S. L.; Ho, T. I.; Chem. Phys. Lett.
1997, 268, 434.
[16] Wang, S. L.; Lee, T. C.; Ho, T. I. J.
Photochem. Photobiol. A: Chem. 2002, 151,
21.
[17] Wang, S. L.; Ho, T. I.; J. Photochem.
Photobiol. A: Chem. 2000, 135, 119.
[18] Kamlet, M. J.; Abbound, J. L. M.; Abrahan,
M. H.; Taft, R. W. J. Org. Chem. 1983, 48,
2877.
[19] Catalan, J.; Couto, A.; Gomez, J.; Saiz., J. L.;
Laynez, J.; J. Chem. Soc., Perkin Tran. 1992,
2, 1181.
[20] El-Bayoumi, M. A.; El-Asser, M.;
Abdel-Halim, F. J. Am. Chem. Soc. 1971, 93,
586.
[21] El-Bayoumi, M. A.; El-Asser, M.;
Abdel-Halim, F. J. Am. Chem. Soc. 1971, 93,
590.
[22] Abboud, J. L. M.; Sraidi, K.; Abraham, M. H.;
Taft, R. W. J. Org. Chem. 1990, 55, 2230.
[23] Abraham, M. H.; Grellier, P. L.; Prior, D. V.;
Taft, R. W.; Morris, J. J.; Taylor, P. J.;
Laurence, C.; Berthelot, M.; Doherty, R. M.;
Kamlet, M. J.; Abboud, J. L. M.; Sraidi, K.;
Guiheneuf, G. J. Am. Chem. Soc. 1988, 110,
8543.
[24] Molina, M. T.; Bouab, W.; Esseffar, M.;
Herreros, M.; Notario, R.; Abboud, J. L. M.;
Mo, O.; Yanez, M.; J. Org. Chem. 1996, 61,
5485.
[25] Abraham, M. H.; Grellier, P. L.; Prior, D. V.;
Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem.
Soc., Prekin Trans. 1989, 2, 699.
[26] Kolling, O. W. J. Phys. Chem. 1992, 96,
1729.
Manuscript Received: Dec. 24, 2002
and Accepted: Feb. 10, 2003