Intramolecular charge transfer with N-benzoylaminonaphthalenes. 1-Aminonaphthalene versus 2-aminonaphthalene as electron donors

Article abstract of DOI:10.1039/b210106h

N-(substituted-benzoyl)-1-aminonaphthalenes and N-(substituted-benzoyl)-2-aminonaphthalenes (1-NBAs and 2-NBAs) with varied substituents at the para- or meta-position of benzoylphenyl ring were prepared to probe the difference between 1-aminonaphthalene (1-AN) and 2-aminonaphthalene (2-AN) as electron donors, using benzanilide-like charge transfer as a probe reaction. An abnormal long-wavelength emission was found for all of the prepared aminonaphthalene derivatives in cyclohexane and was assigned to the CT state by the observation of a substantial red shift with increasing solvent polarity or with increasing electron-withdrawing ability of the substituent. The CT emission energies were found to follow a linear relationship with the Hammett constant of the substituent and the value of the linear slope for 1-NBAs (-0.45 eV) was higher than that of 2-NBAs(-0.35 eV), the latter being close to that of the aniline derivatives (BAs, -0.345 eV). This pointed to a higher extent of charge separation in the CT state of 1-NBAs in which a full charge separation was established by the reduction potential dependence of the CT emission energy with a linear slope of -1.00. The possible contribution of the difference in the steric effect and the electron donating ability of the donors in 1-NBAs and 2-NBAs was ruled out by the observation that the corresponding linear slopes of benzoyl-substituted BAs remained unchanged when para-, meta-, ortho-, or ortho, ortho-methyls were introduced into the aniline moiety. It was therefore concluded that 1-AN enhanced the charge transfer in 1-NBAs and the proximity of its ¹L_a and ¹L_b states was suggested to be responsible. Results showed that the charge transfers in 1-NBAs and 2-NBAs were not the same and 1-AN and 2-AN as electron donors were different not only in electron donating ability but in shaping the charge transfer pathways as well.

Full text of DOI:10.1039/b210106h

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Intramolecular charge transfer with N-benzoylaminonaphthalenes.
1-Aminonaphthalene versus 2-aminonaphthalene as electron donors
Xuan Zhang,a Chun-Hua Liu,a Li-Hong Liu,a Fang-Ying Wu,a,b Lin Guo,a Xiang-Ying Sun,a,c
Chao-Jie Wangd and Yun-Bao Jiang*a
a Department of Chemistry and the MOE Key Laboratory of Analytical Sciences,
Xiamen University, Xiamen 361005, China
b Department of Chemistry, Nanchang University, Nanchang 310047, China
c Department of Environmental Science and Engineering, Huaqiao University,
Quanzhou 362011, China
d Department of Chemistry and the State Key Laboratory for Physical Chemistry of Solid
Surfaces, Xiamen University, Xiamen 361005, China
Received 14th October 2002, Accepted 10th January 2003
First published as an Advance Article on the web 29th January 2003
N-(substituted-benzoyl)-1-aminonaphthalenes and N-(substituted-benzoyl)-2-aminonaphthalenes (1-NBAs and
2-NBAs) with varied substituents at the para- or meta-position of benzoyl phenyl ring were prepared to probe
the difference between 1-aminonaphthalene (1-AN) and 2-aminonaphthalene (2-AN) as electron donors, using
benzanilide-like charge transfer as a probe reaction. An abnormal long-wavelength emission was found for all of
the prepared aminonaphthalene derivatives in cyclohexane and was assigned to the CT state by the observation
of a substantial red shift with increasing solvent polarity or with increasing electron-withdrawing ability of the
substituent. The CT emission energies were found to follow a linear relationship with the Hammett constant of the
substituent and the value of the linear slope for 1-NBAs (Ϫ0.45 eV) was higher than that of 2-NBAs (Ϫ0.35 eV),
the latter being close to that of the aniline derivatives (BAs, Ϫ0.345 eV). This pointed to a higher extent of charge
separation in the CT state of 1-NBAs in which a full charge separation was established by the reduction potential
dependence of the CT emission energy with a linear slope of Ϫ1.00. The possible contribution of the difference in the
steric effect and the electron donating ability of the donors in 1-NBAs and 2-NBAs was ruled out by the observation
that the corresponding linear slopes of benzoyl-substituted BAs remained unchanged when para-, meta-, ortho-,
or ortho,ortho-methyls were introduced into the aniline moiety. It was therefore concluded that 1-AN enhanced the
charge transfer in 1-NBAs and the proximity of its 1La and 1Lb states was suggested to be responsible. Results showed
that the charge transfers in 1-NBAs and 2-NBAs were not the same and 1-AN and 2-AN as electron donors were
different not only in electron donating ability but in shaping the charge transfer pathway as well.
Introduction
The photophysics of 1-aminonaphthalene (1-AN) derivatives
has attracted much recent attention1–7 since a notable radiation-
less internal conversion (IC) occurs with 1-dialkylamino-
naphthalene,2–6,8,9 which leads to increased quantum yield and
prolonged lifetime with increasing solvent polarity. It was pro-
posed that the IC occurred via vibronic coupling of the proxim-
ate S1(1Lb) and S2(1La) states (the proximity effect), in which S1
is the emissive state involving intramolecular charge transfer
Scheme
1
Molecular structures of 1-NBAs, 2-NBAs and BAs.
(ICT).2–6,8,9 Such a fast IC process was not observed for the
corresponding 2-aminonaphthalene (2-AN) derivatives and was
ascribed to the relatively large energy gap between S1(1Lb) and
S2(1La), ∆E(S1, S2).8,9 In these investigations the amino group in
aminonaphthalenes was considered as the electron donor while
the naphthyl moiety as the electron acceptor. It thus remains to
be of interest to compare 1-AN and 2-AN as electron donors.
Charge transfer with benzanilide has been investigated since
1970’s10–20 by its dual fluorescence in non-polar solvent, in
which aniline was defined as the electron donor and benzoyl as
the electron acceptor. We recently showed that with benzoyl-
substituted benzanilides (BAs, Scheme 1) the CT emission
energies varied linearly with the Hammett constants of the
substituents at the benzoyl moiety [eqn. (1)]:20
Substituents X’s are (a) p–OC2H5, (b) p–CH3, (c) H, (d) p–F, (e) p–Cl,
(f ) p–Br, (g) m–Cl, (h) m–NO2, and (i) p–NO2, respectively. The
numbers shown in the structures were only used to define the dihedral
angles.
CT emission, σ is the Hammett constant of the substituent, ρ is
the reaction constant defined in the classic Hammett equation,
and R and T are the gas constant and absolute temperature.
The linear slope indicated the charge transfer extent in the CT
state while the intercept indexed the electron donating ability of
the un-substituted aniline moiety.20 We therefore decided to
employ the CT reaction in BAs to probe the difference of 1-AN
and 2-AN as electron donors by preparing N-benzoylamino-
naphthalene derivatives with substituent at the benzoyl moiety
and examining the substituent effects on the charge transfer
fluorescence.
hνmax(CT) = Ϫ2.303RTρσ ϩ E0,0 Ϫ δErep ϩ constant (1)
In the present paper we report the synthesis and the fluores-
cence and absorption spectra in cyclohexane of N-(substituted-
benzoyl)-1-aminonaphthalenes and 2-aminonaphthalenes
In eqn. (1) E0,0 is the (0,0)-transition energy, δErep is the repul-
sion energy of the Franck-Condon ground state reached after
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 2 8 – 7 3 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 AM1 calculated ground-state structural parameters
1-NBAs
2-NBAs
a
a
Dihedral angles/Њ
C1–C2–N3–C4
Dipole moment/D
Dihedral angles/Њ
C1–C2–N3–C4
Dipole moment/D
X
N3–C4–C1Ј–C2Ј
N3–C4–C1Ј–C2Ј
p–OC2H5
p-CH3
H
p–F
p–Cl
25.8
25.9
25.8
26.4
26.3
26.4
26.2
27.3
27.4
40.9
42.7
43.4
42.6
43.3
43.8
43.5
40.7
45.4
3.43
3.71
3.43
2.73
2.82
2.79
2.53
3.19
4.61
9.8
8.3
9.6
11.4
11.1
11.2
10.1
12.2
12.0
41.5
43.3
44.6
43.2
44.1
44.6
43.0
41.2
45.5
3.53
3.66
3.34
2.42
2.55
2.49
2.41
3.18
4.22
p–Br
m–Cl
m–NO2
p–NO2
a The atom numbers were assigned as shown in Scheme 1.
(1-NBAs, 1 and 2-NBAs, 2, Scheme 1). We found all of the
prepared amide derivatives, as benzanilides, emitted CT
fluorescence in cyclohexane and the CT emission energy varied
linearly with the Hammett substituent constant with the
linear slope higher for 1-NBAs in which a full charge separation
in the CT state was established. We showed that the charge
transfer in 1-NBAs was more enhanced than in 2-NBAs and 1-
AN and 2-AN were different not only in their electron donating
abilities.
meter using a 1-cm quartz cell. All spectra were measured at
a sample concentration of ca. 10Ϫ5 M at room temperature
(298 K).
The reduction potentials of 1-NBAs were measured by cyclic
voltammetry on BAS-200 electrochemical analyzer (BAS, US)
in a three-electrode configuration. The glass–carbon disc was
employed as the working electrode, SCE as the reference
electrode, and platinum wire as the counter electrode. The CV
diagrams in ethanol–water (1:1, v/v) solutions were taken
under N2 by a scan rate of 0.1 V sϪ1 over the potential range of
ϩ1.0 V to Ϫ3.0 V. 0.25 M of (n-C4H9)4NBr was used as the
supporting electrolyte.
Experimental
Ground-state geometrical structures were optimized by semi-
empirical AM1 method using MOPAC (version 97.0) programs
in Alchemy 2000 software package,22 and the single-point
calculations were performed by ab initio Hartree–Fock method
using STO-3G basis sets in Gaussian 94 program,23 denoted as
HF/STO-3G//AM1.
N-(substituted-benzoyl)-1-aminonaphthalenes and 2-amino-
naphthalenes (1-NBAs and 2-NBAs, Scheme 1) and the
corresponding aniline derivatives (Scheme 2) were synthesized
by the reaction in the presence of anhydrous Na2CO3 in
diethyl ether of aminonaphthalenes or anilines with substituted
benzoyl chlorides which were in situ prepared from corre-
sponding benzoic acids and SOCl2. All compounds were
purified by repeated recrystallization from DMF/H2O (10:1,
v/v) for 1-NBAs or from acetone for 2-NBAs and aniline
derivatives and were confirmed by IR (Nicolet Avatar FT-IR
360) and 1H NMR (Varian Unityϩ 500MHz, DMSO-d6, TMS)
data. Solvents used for fluorescence and absorption spectra
measurements were purified just before use and were checked
to have no fluorescent impurity at the employed excitation
wavelength.
Results and discussion
To avoid the steric effect introduced by the substituent, only
para- and meta-substituted NBAs were synthesized. The single-
point calculations indicated that cis-NBAs were ca. 3 kcal
molϪ1 higher in total energy than trans-NBAs, revealing that
NBAs exist preferentially in trans-conformation, as BAs
did.17,20 The ground state structures of all trans-NBA deriv-
atives were optimized with the AM1 method and the dihedral
angles and the dipole moments were compiled in Table 1. It
follows from Table 1 that the benzoyl substitution does not
result in an obvious change in the ground-state structures
within the same series, whereas the dihedral angle ЄC1–C2–
N3–C4 of 1-NBAs is much higher than that in 2-NBAs, sug-
gesting the steric effects of the substitution ortho to the amino
group and/or of the peri-H at C-8 in 1-NBAs.
Fig. 1 shows the absorption spectra of 1-NBAs and 2-NBAs
in cyclohexane. Obviously, the absorption spectra of 1-NBAs
and 2-NBAs were much similar to those of 1-AN and 2-AN,
and the main absorption bands could be ascribed to the 1La
1
1A and Lb
1A transitions, respectively.8,9 The absorption
of 1-NBAs peaked at 307 nm was found well mixed with that
of the 1Lb 1A transition that appeared as a shoulder at
around 325 nm. With 2-NBAs, however, the broad absorption
Scheme 2 Molecular structures of the methyl-substituted benzanilides
1
Corrected fluorescence spectra were measured on a Hitachi
F-4500 fluorescence spectrometer with excitation wavelength
of 310 nm for 1-NBAs, 300 nm for 2-NBAs and 280 nm
for BAs derivatives by a xenon lamp of 150 W. The slits
for both monochromators were 5 nm and a scan rate of
240 nm minϪ1 was chosen. Fluorescence quantum yields
were measured using quinine sulfate as standard (0.546 in
0.5 M H2SO4, [ref. 21]). The absorption spectra were taken on
Beckman DU-7400 or Varian Cary 300 UV-Vis spectrophoto-
at ca. 298 nm originating from the Lb
1A transition was
1
obviously separated from the La
1A absorption that lied at
shorter wavelength (Fig. 1 and Table 2). It was noted that the
position and shape of the absorption spectra within 1-NBAs
and 2-NBAs series, respectively, were hardly affected when
substituents were varied from electron-donating to electron-
withdrawing groups at the para- or meta-positions of the
benzoyl moiety (Fig. 1 and Table 2). The nitro-derivatives,
as exceptions, had absorption tails extending to longer
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 2 8 – 7 3 2
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Table 2 Spectroscopic data for 1-NBAs and 2-NBAs in cyclohexane
Comp.
λabs/nm
ε/MϪ1 cmϪ1
λflu/nm
Fluorescence quantum yield
1a
1b
1c
1d
1e
1f
1g
1h
1i
2a
2b
2c
2d
2e
2f
2g
2h
2i
307
307
307
305
307
308
308
304
293/330
298
298
298
297
299
303
300
308
303
16115
17127
21095
23523
17251
12798
11962
14876
9966/6304
15621
11038
13071
8129
480
502
514
508
532
536
546
0.0029
0.0025
0.0019
0.0020
0.0016
0.0013
0.0010
0.0003
<0.0001
0.0027
0.0044
0.0044
0.0053
0.0060
0.0044
0.0025
0.0012
0.0014
a
—
—
a
476
490
500
496
514
518
530
550
555
6616
8789
13315
8321
5372
a Too weak to be estimated accurately.
The fluorescence spectra of 1-NBAs and 2-NBAs in cyclo-
hexane are shown in Fig. 2 in which it was seen that all of the
NBA derivatives exhibited an abnormal long-wavelength fluor-
escence at around 500 nm and parts of them showed a very
weak emission at about 370 nm. The short-wavelength band
positions were noted to be close to those of 1-AN (373 nm) and
2-AN (372 nm) in n-hexane24 and could result from the LE
state or impurity that still remains to be elucidated, as also
with benzanilide derivatives reported previously.11–20 The long-
wavelength fluorescence of 1-NBAs and 2-NBAs were found
to shift to the red with increasing electron withdrawing ability
of the substituent at the benzoyl moiety, and was therefore
Fig. 1 Absorption spectra of (a) 1-NBAs and (b) 2-NBAs in
cyclohexane
wavelength, probably indicating the occurrence of the ground
state charge transfer, in particular with 1-NBAs (Fig. 1). The
weak dependence of the absorption spectra on the substitution
(Table 2) reflected the similarity of the locally excited (LE) state
structure within the same series because of the similar ground-
state structure suggested by the quantum mechanical calcu-
lations (Table 1). It is therefore not risky to assume that both
E0,0 and δErep remain constant within 1- and 2-NBA series,
respectively, which means that eqn. (1) would hold for 1-NBAs
and 2-NBAs for clarifying the charge transfer character as has
been done with BAs.20
Fig. 2 Normalized fluorescence spectra of (a) 1-NBAs and (b) 2-
NBAs in cyclohexane
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 2 8 – 7 3 2
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assigned to the intramolecular charge transfer (ICT) state with
aminonaphthalene and benzoyl moiety being electron donor
and acceptor, respectively, as has been done similarly with
BAs.16–20 The CT nature was further supported by the observ-
ation of a substantial red shift of the long-wavelength emission
with increasing solvent polarity, for example, the ICT emission
of 1-NBA shifted from 514 nm in pure cyclohexane to 522, 546
and 558 nm, respectively, when 1, 5 and 20%(v/v) of di-
ethylether was introduced into cyclohexane. Importantly, linear
free energy correlations between the CT emission energies
and the Hammett constants25 of the substituents, as given in
eqn. (1),20 were indeed found with 1-NBAs and 2-NBAs series,
respectively (Fig. 3). The value of the linear slope with 1-NBAs
(Ϫ0.45 eV, γ = Ϫ0.9583, n = 7) was found to be higher than that
with 2-NBAs (Ϫ0.35 eV, γ = Ϫ0.9425, n = 7), and the latter was
close to that of the aniline derivatives (Ϫ0.345 eV).20 This
observation indicated that the charge transfer in 1-NBAs and
2-NBAs be different, with the latter being similar to that
of BAs. The higher slope value suggested a higher charge
separation extent in the CT state of 1-NBAs than in that of
2-NBAs.20 This difference in the linear slopes for 1-NBAs and
2-NBAs could in principle result from the differences in the
electron-donating abilities of 1-AN and 2-AN as electron
donors and/or the steric effects in 1-NBAs due to the C-8
peri-H and the substitution ortho to amino group that are
absent in 2-BNAs. The lower intercept in the linear correlation
of 1-NBAs (Fig. 3) is actually indicative of the higher electron
donating ability of 1-AN,20 which is in agreement with the
known data of lower oxidation potential of 1-AN than of
2-AN.24
Fig. 4 The linear correlations of the CT emission energies of p-MBAs,
m-MBAs, o-MBAs and o,o-DMBAs in cyclohexane against the
Hammett substituent constant
the electron donating ability of the electron donor (aniline in
BAs, m-methylaniline in m-MBAs and p-methylaniline in
p-MBAs of enhancing electron donor, also suggested by
decreasing intercept from 2.54,20 2.49 to 2.45 eV, Fig. 4) and in
the steric effect (p-MBAs, m-MBAs, o-MBAs and o,o-DMBAs
of increasing steric hindrance around the aniline amino group)
did not result in obvious difference in the linear slope for the
correlation between the CT emission energy and Hammett
substituent constant. It was therefore made clear that the
difference in the linear slopes for 1-NBAs and 2-NBAs was not
due to the steric effect in 1-NBAs and the difference in the
electron donating ability of the donor. This means that 1-AN
and 2-AN, as electron donors, differ not only in their electron
donating abilities. The observation that the charge transfer
in 2-NBAs was similar to that in BAs pointed to the similarity
of 2-AN as electron donor to aniline. With 1-NBAs a higher
extent of charge separation in the CT state was shown by
a higher value of linear slope (Ϫ0.45 eV, Fig. 3), suggesting
that 1-AN enhanced the charge transfer reaction in 1-NBAs.
The charge separation extent in the CT state of 1-NBAs was
further evaluated by correlating the CT emission energies
with their reduction potentials. A linear dependence was found
with a slope of Ϫ1.00 (γ = Ϫ0.9229, n = 6, Fig. 5), which indi-
cated a full charge separation in the CT state of 1-NBAs, as
assumed in exciplexes in nonpolar solvents by the Weller
[eqn. (2)].26
hνCT = EDox ϪEAred Ϫ δErep ϩ 0.15 eV
(2)
Fig. 3 The linear correlation between the CT emission energies in
cyclohexane of (1) 1-NBAs and (2) 2-NBAs and the Hammett
substituent constant
To clarify these factors, several series of benzanilide deriv-
atives (Scheme 2) were prepared and the charge transfer
reactions were similarly investigated. In those benzanilide
derivatives methyl substituents were introduced into the aniline
moiety for modifying the electron donating ability of the aniline
donors (para- and meta-methyl, p-MBAs and m-MBAs,
Scheme 2) and for introducing steric hindrance (ortho-methyls,
o-MBAs and o,o-DMBAs, Scheme 2). The steric effect in the
latter two series was shown by the dihedral angle ЄC1–C2–N3–
C4, obtained from AM1 calculations, that increased from an
average of 8.7 and 9.3Њ for p-MBAs and m-MBAs to 18.5Њ for
o-MBAs and 62.5Њ for o,o-DMBAs.
It is significant to note that similar linear slopes (Ϫ0.35 eV)
were found for the correlation between the CT emission
energies and the Hammett constants for all of the four series
of aniline derivatives (Fig. 4) and were also close to those
of BAs (Ϫ0.345 eV)20 and 2-NBAs (Ϫ0.35 eV, Fig. 3). This
observation clearly suggested that both the differences in
Fig. 5 Plots of the CT emission energies of 1-NBAs in cyclohexane
against the reduction potentials of (1a) Ϫ2.218, (1b) not obtained, (1c)
Ϫ2.081, (1d) Ϫ2.110, (1e) Ϫ1.963, (1f ) Ϫ2.061 and (1g) Ϫ1.944 V.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 2 8 – 7 3 2
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In a discussion of the internal conversion in 1-AN and 2-AN
derivatives the vibronic coupling of the S1(1Lb) and S2(1La)
states has been invoked to be an important promoting
factor.2–6,8,9 This vibronic coupling has also been suggested as a
reaction coordinate for the ICT in p-dimethylaminobenzonitrile
(DMABN) and its derivatives.27,28 We therefore tentatively
assumed that the enhanced charge transfer observed for 1-NBAs
than for 2-NBAs (Fig. 3) resulted from an enhanced vibronic
coupling of the proximate S1(1Lb) and S2(1La) states in 1-AN
than in 2-AN in which the two excited states are well separ-
ated8,9 which was also indicated in the absorption spectra of 2-
NBAs (Fig. 1b). This finding showed that 1-AN and 2-AN as
electron donors were different in shaping the charge transfer
pathway. It hence deserves to point out that the benzanilide-like
CT reaction could indeed be employed to probe the difference
between 1-AN and 2-AN as electron donors and might be
extended to other aromatic amine donors.
Province under grant No.D0220001, and by the Ministry of
Education (MOE) of China. We also thank the German
VolkswagenStiftung for partial support through grant No. I/77
072.
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A
systematic study of N-(substituted-benzoyl)-1-amino-
naphthalenes (1-NBAs) and N-(substituted-benzoyl)-2-amino-
naphthalenes (2-NBAs) with substituents at the para- or
meta-position of the benzoyl phenyl ring were carried out to
identify the difference of 1-AN and 2-AN as electron donors
using benzanilide-like charge transfer as a probe reaction. All
of the derivatives were found to emit a Stokes-shifted charge
transfer emission at around 500 nm in non-polar solvent cyclo-
hexane. While the absorption showed very weak dependence on
the substitution, the long-wavelength CT emission strongly
shifted to the red with increasing electron-withdrawing ability
of the substituent. The CT emission energies were found to
correlate linearly with the Hammett substituent constants,
and a higher slope was found for 1-NBAs (Ϫ0.45 eV) than for
2-NBAs (Ϫ0.35 eV) which was very close to that of the aniline
derivatives, benzanilides(Ϫ0.345 eV),20 which clearly pointed to
a higher charge separation extent in the CT state of 1-NBAs in
which a full charge separation was established by the linear
dependence of the CT emission energy upon reduction poten-
tial that gave a slope of Ϫ1.00. A comparative investigation of
the charge transfer in benzoyl-substituted BAs with methyl-
substitution at the aniline moiety showed that the corre-
sponding linear slope remained almost unchanged when para-,
meta-, ortho-, or ortho,ortho-methyls were introduced into the
aniline moiety (Ϫ0.35 eV). The possible contributions of the
differences in the electron donating abilities of 1-AN and 2-AN
and in the steric effect due to the C-8 peri-H and the ortho-
substitution to the amino group in 1-NBAs were then ruled out.
It was suggested that the enhanced charge transfer in 1-NBAs
was due to the proximity of the 1La and 1Lb states in 1-AN that
allows for a better vibronic coupling important for the charge
transfer reaction. It was therefore shown that 1-AN and 2-AN
as electron donors were different not only in electron donating
ability but also in shaping the charge transfer coordinate.
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Acknowledgements
This work has been supported by the NSF of China through
grants No.29975023 and No.20175020, the NSF of Fujian
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 7 2 8 – 7 3 2
732