Dual fluorescence of naphthylamines in alkaline aqueous solution

Article abstract of DOI:10.1016/S0009-2614(01)00973-3

Dual fluorescence was observed with N-(1-naphthyl)aminoacetate (1-NAA) in aqueous solution of pH 13.0 in the presence of cationic surfactants, cetyltrimethylammonium bromide (CTAB) and chloride (CTAC), below and after the critical micelle concentration (CMC). Similar dual fluorescence was also found with 1- and 2-naphthylamine (1-NA, 2-NA), N-(2-naphthyl)aminoacetate (2-NAA) and (1-naphthyl)ethylenediamine (1-NEDA), in the presence and absence of the cationic surfactants, but not with N,N-disubstituted 1- and 2-NAs. We concluded that the dual fluorescence was due to the excited-state deprotonation of the amino group in these NAs. The pK_a^>s of the dual fluorescent NAs were estimated to be around 14 from the dual fluorescence pH titrations. No clear correlation was found for pK_a^> with the amino substitution and the presence of cationic micelle.

Full text of DOI:10.1016/S0009-2614(01)00973-3

12 October 2001
Chemical Physics Letters 346 *2001) 423±429
www.elsevier.com/locate/cplett
Dual ¯uorescence of naphthylamines in
alkaline aqueous solution
Li-Hua Ma, Zhen-Chang Wen, Li-Rong Lin, Yun-Bao Jiang *
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005, China
Received 12 July 2001
Abstract
Dual ¯uorescence was observed with N-*1-naphthyl)aminoacetate *1-NAA) in aqueous solution of pH 13.0 in the
presence of cationic surfactants, cetyltrimethylammonium bromide *CTAB) and chloride *CTAC), below and after the
critical micelle concentration *CMC). Similar dual ¯uorescence was also found with 1- and 2-naphthylamine *1-NA, 2-
NA), N-*2-naphthyl)aminoacetate *2-NAA) and *1-naphthyl)ethylenediamine *1-NEDA), in the presence and absence
of the cationic surfactants, but not with N,N-disubstituted 1- and 2-NAs. We concluded that the dual ¯uorescence was
due to the excited-state deprotonation of the amino group in these NAs. The pKaÃs of the dual ¯uorescent NAs were
estimated to be around 14 from the dual ¯uorescence pH titrations. No clear correlation was found for pKaà with the
amino substitution and the presence of cationic micelle. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
has shown that NAs have an emissive state of
charge transfer character [8±11]. Enhancement in
the acidity might be expected for NAs as `acids',
which would make it possible to observe the ex-
cited-state acid±base reaction. It is known that
substitution at the reaction center, an electron-
withdrawing group in particular [12], and the
presence of cationic micelle [13] might enhance the
ground-state proton dissociation reaction. There-
fore we ®rst prepared N-*1-naphthyl)aminoacetate
*1-NAA, Scheme 1) that has an electron-with-
drawing group substituted at the amino moiety
and its carboxylic group could ensure a good
binding to cationic micelle. We examined the ¯u-
orescence of 1-NAA in both cetyltrimethylammo-
nium bromide *CTAB) and chloride *CTAC)
solutions of varying pH and observed dual ¯uo-
rescence at high pH. The investigations were then
extended to other NA derivatives *Scheme 1) in
order to clarify the origin of the dual ¯uorescence,
The research on the acid±base equilibrium of
naphthylamines *NAs) has mainly focused on the
protonation at lower pH [1±3], whereas the acid
dissociation of the NA has been limited. A possible
reason could be the high ground-state pKa of the
amines, for example, the pKa of NH3 is 35 and of
aniline is 25 [4]. Large di€erences in pK *often 6
units or more) of organic acids or bases have been
observed upon excitation to the excited singlet
state [5]. In the case of phenol and aniline [6,7], for
example, it was reported that they were stronger
acids in the excited state than in the ground state.
Recent interest in the photophysics of NAs con-
cerning the peculiar internal conversion character
* Corresponding author. Fax: +86-592-218-5662.
E-mail address: ybjiang@xmu.edu.cn *Y.-B. Jiang).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 * 0 1 ) 0 0 9 7 3 - 3

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L.-H. Ma et al. / Chemical Physics Letters 346,2001) 423±429
Scheme 1. Chemical structures of NAs and relative naphthalene derivatives.
1
which led to the conclusion that the dual ¯uores-
cence observed with NAs and its N-monosubsti-
tuted derivatives was due to the excited-state acid±
base reactions with the NAs as acids. The pKaÃs
were estimated and their relationship with molec-
ular structure and the presence of cationic micelle
were discussed.
ing with absolute ethanol. H NMR *D2O, 500
MHz): d 8.259±8.244*1H, d), 7.918±7.902*1H, d),
7.573±7.535*3H, m), 7.439±7.423*1H, t), 7.081±
7.067*1H, d), 3.896*4H, s). 2-NADA was synthe-
sized from 2-NAA and chloroacetic acid in a
similar way and characterized by IR.
MNAA was prepared by methylization of 1-
NAA by dimethyl sulfate at room temperature.
The product was subject to silica gel column
chromatography eluting with petroleum ether and
2. Experimental
1
ethyl acetate *10:1). H NMR *CDCl3, 500 MHz):
1-NAA was prepared by re¯uxing 1-naphthyl-
amine *1-NA) and chloroacetic acid in aqueous
sodium hydroxide solution [14]. The product was
repeatedly recrystallized from absolute ethanol. 1H
NMR *DMSO-d6, 500 MHz): d 12.50*COOH±H),
8.122±8.106*1H, d), 7.779±7.762*1H, d), 7.449±
7.421*2H, m), 7.279±7.248*1H, t), 7.146±7.130*1H,
d), 6.352±6.337*1H, d), 3.979*2H, s), 3.348*s, N±
H). N-*2-naphthyl)aminoacetate *2-NAA) was
prepared by re¯uxing 2-naphthylamine *2-NA)
with chloroacetic acid in a similar way and char-
acterized by IR.
d 8.205±8.818 *1H, d), 7.788±7.772*1H, d), 7.513±
7.329*4H, m), 7.329±7.132*1H, d), 3.886*2H, s),
3.683*3H, s), 2.980*3H, s).
DMAN was purchased from Fluka and puri®ed
by TLC prior to use. 1-NA, 2-NA, *1-naph-
thyl)ethylenediamine *1-NEDA) *AR, Shanghai),
CTAB*AR, Shanghai) and CTAC *Tokyo Kasei)
were used as received. Twice de-ionized water was
further distilled in the presence of KMnO4. The
solution pH was adjusted by NaOH.
Corrected ¯uorescence emission and excitation
spectra were recorded on Hitachi F-4500 ¯uores-
cence spectrophotometer under excitation wave-
length of 320 nm by a Xenon lamp of 150 W.
Absorption spectra were taken on Beckman
1-NADA was synthesized by further reaction of
1-NAA with chloroacetic acid. The product was
subject to silica gel column chromatography elut-

L.-H. Ma et al. / Chemical Physics Letters 346,2001) 423±429
425
DU-7400 absorption spectrophotometer using a 1-
cm quartz cell. IR spectra were measured on
micelle concentration *CMC) of CTAB*9 :2 Â 10À4
M) and level o€ after the CMC, which is clearly
shown in the inset of Fig. 1. The excitation spectra
obtained by monitoring emission at 425 and 545
nm, respectively, were similar, indicating that the
425 and 545 nm emission have the same excitation
origin. This means that the emissive state for the
545 nm emission was generated from the emissive
state for the 425 nm emission, and therefore the
appearance of the dual ¯uorescence was due to an
excited-state reaction. Similar observations were
made for 1-NAA in alkaline CTAC solutions in
which the intensity ratio was found higher than
that in CTABsolution.
1
Nicolet Avatar FT-IR 360 spectrophotometer. H
‡
NMR spectra were measured on a Varian Unity
500 MHz NMR spectrometer. All experiments
were carried out at room temperature ꢀ25 Æ 3 °C).
3. Results and discussion
3.1. Fluorescence spectra of 1-NAA
Fig. 1 shows the ¯uorescence spectra of 1-NAA
in pH 13.0 alkaline solutions of varying CTAB
concentration. In pH 13.0 aqueous solution with-
out CTAB, 1-NAA emitted a single band ¯uores-
cence peaked at 447 nm. Upon addition of CTAB
into the solution, the emission was quenched and
blue-shifted, and a wide and structureless emission
at longer wavelength of 545 nm appeared. The
latter band did not shift as CTABconcentration
was increased. The intensity ratio of the long-
wavelength emission to the short-wavelength
emission, Ilw=Isw, was found to increase with in-
creasing CTABconcentration before the critical
3.2. Origins for the dual ¯uorescence of 1-NAA
The observation of dual ¯uorescence for an-
ionic ¯uorophore in the presence of a cationic
surfactant, such as 4-*1-pyrene) butyric acid in
CTABor CTAC solution, has been reported and
the dual ¯uorescence was ascribed to the excimer
formation because of the aggregation of anionic
¯uorophores in aqueous solution induced by
CTABor CTAC [15,16]. The dual ¯uorescence of
Fig. 1. Fluorescence spectra of 1-NAA in pH 13.0 aqueous solution as a function of CTABconcentration. Inset shows the variations
of the short-wavelength band position and Ilw=Isw of 1-NAA with CTABconcentration. `lw' means long-wavelength and `sw' means
short-wavelength. [1-NAA] is 3:0 Â 10À5 M. Curves 1±10 show the spectra with CTABconcentration varying from 0 to 1 :5 Â 10À3 M:
*1) 0 M, *2) 1:0 Â 10À4 M, *3) 2:0 Â 10À4 M, *4) 3:0 Â 10À4 M, *5) 4:0 Â 10À4 M, *6) 5:0 Â 10À4 M, *7) 6:0 Â 10À4 M, *8) 8:0 Â 10À4 M,
*9) 1:0 Â 10À3 M, and *10) 1:5 Â 10À3 M.

426
L.-H. Ma et al. / Chemical Physics Letters 346,2001) 423±429
1-NAA in alkaline CTABsolutions looked like the
phenomenon mentioned above. We noted, how-
ever, that Ilw=Isw increased with CTABconcentra-
tion below the CMC and leveled o€ beyond CMC
*inset in Fig. 1), which is di€erent from the 4-*1-
pyrene) butyric acid/CTABsystem in which the
intensity ratio decreased at higher CTABconcen-
tration [16]. Also, the intensity ratio was found,
instead of increasing as expected from the excimer
mechanism, to decrease with increasing 1-NAA
concentration *Fig. 2). It thus appeared that the
dual ¯uorescence observed with 1-NAA in CTAB
and CTAC solutions was not due to excimer for-
mation, though the aggregation of 1-NAA in the
presence of CTABwas suggested by the appre-
ciable red-shift in the absorption spectra *not
shown). The fact that no dual ¯uorescence was
found from similar series of solutions at pH 11±13,
when 1-NAA existed in the anionic form [17] as at
pH > 13, suggested that the observation of dual
¯uorescence at highly alkaline CTABsolution
might relate to the excited-state acidic dissociation
of the amine moiety in 1-NAA. Meanwhile, we
indeed observed a continuous blue-shift of the 1-
NAA ¯uorescence in ethanol±water mixtures of
increasing ethanol content, which pointed to the
charge transfer character of the emissive state [18].
An excited-state charge transfer promoted acidic
dissociation reactions could then be assumed as
the origin for dual emission. We thus extended our
investigation to other 1-NA derivatives and 2-NA
in the presence and absence of cationic surfactants.
We found that these NA derivatives with at least
one amino hydrogen, such as 1-NA, 1-NAA, 1-
NEDA, 2-NA, 2-NAA *Scheme 1), showed dual
¯uorescence in highly alkaline solutions both in
the presence and absence of CTABor CTAC, see
Fig. 2. The concentration e€ect on the intensity ratio of 1-NAA
in pH 13.0 aqueous CTABsolutions.
Fig. 3. Fluorescence spectra of 1-NA as a function of pH; [1-NA] is 1:0 Â 10À5 M.

L.-H. Ma et al. / Chemical Physics Letters 346,2001) 423±429
427
3.3. pH titration of NAs' dual ¯uorescence
for example, in Fig. 3 the ¯uorescence spectra of 1-
NA as a function of pH, whereas N,N-disubsti-
tuted NAs, such as 1-NADA, MNAA, 2-NADA
and DMAN *Scheme 1), did not show dual ¯uo-
rescence. It was therefore evident that the dual
¯uorescence observed with NAs in highly alkaline
solution originated from the excited-state acid±
base reaction, with the short-wavelength emission
corresponding to the acid form of the amines while
the long-wavelength emission to the dissociated
`base' amines. Table 1 compiled the detailed data
for the emission band positions of both the acid
and base forms of the investigated NAs.
In order to evaluate the excited-state pKaÃ, the
pH titrations of the dual ¯uorescence were carried
out. Fig. 4 shows the intensity ratio, Ilw=Isw, in the
absence and presence of CTABmicelle, as a
function of pH for the dual ¯uorescent NAs, from
which pKaÃs were estimated from the in¯ection
points and tabulated together with those in CTAC
micelle in Table 1. It was found that in the pres-
ence of CTABor CTAC *not shown) micelle, the
pH titration pro®les were similar to those in the
absence of micelle. Several signi®cant features,
Table 1
The maximum emission wavelengths, intensity ratio of the long-wavelength to short-wavelength emission and the pKaÃs of the dual
¯uorescent NAs in aqueous solution and in CTAB, CTAC micelles
Water
CTABa
CTACa
NAs
pKaÃ
k1
*nm)
k2
*nm)
Ilw=Isw
pKaÃ
k1
*nm)
k2
*nm)
Ilw=Isw
pKaÃ
b
c
d
b
c
d
b
c
d
k1
*nm)
k2
*nm)
I
lw=Isw
1-NA
1-NAA
545
545
447
447
447
410
3.18
0.99
1.38
2.99
14.6
14.7
13.9
13.7
545
545
550
528
414
425
419
404
22.0
1.56
14.2
14.7
13.8
13.6
547
545
551
533
420
423
421
420
79.0
1.72
7.31
13.6
14.3
14.2
13.8
13.7
1-NEDA 545
2-NA 535
5.08
4.56
a CTABand CTAC concentrations are 1 :5 Â 10À3 M.
b Emission maximum of the base form.
c Emission maximum of the acid form.
d Taken at pH 14.5.
Fig. 4. pH titration curves for the dual ¯uorescent NAs in the absence *a) and presence *b) of 1:5 Â 10À3 M CTAB.

428
L.-H. Ma et al. / Chemical Physics Letters 346,2001) 423±429
however, deserved to be pointed out. In CTABor
CTAC micelle, the emission from the acid form is
in general shifted to the blue, which is indicative of
the charge transfer character of the emissive state,
whereas the emission from the base form did not
show appreciable spectral shift which means that
the excited base form is of low dipole moment. The
presence of CTABor CTAC micelle decreased the
minimum pH for observing the dual ¯uorescence,
for example, in aqueous solution the minimum pH
for observing 1-NAA dual ¯uorescence was 13.6
while in the presence of 1:0 Â 10À3 M CTAB, this
pH decreased to 13.0. The presence of CTABor
CTAC micelle also enhanced the base to acid
emission intensity ratio *see Table 1), which could
be due to the shift of excited-base equilibrium and/
or the better protection of the excited base form.
The pKaà values presented in Table 1 showed
that, within the experiment accuracy, the presence
of CTABor CTAC micelle did not change the
pKaÃ, suggesting that the excited-state acid±base
equilibrium was not a€ected by the micelle. This is
in clear di€erence from that observed for ground-
state pKa [13]. It was hence assumed that the mi-
celles provided better shielding of the base form
that lead to higher intensity ratio in the micelle
compared to that in aqueous solution. It was also
noted from Table 1 that the pKaÃs were around 14,
both in aqueous solution and in micellar media,
inspite of the di€erent substituents at the amino
group. This is quite di€erent from the case in the
ground-state acid±base dissociation, in which a
decrease in pKa could be observed when the acid
center is substituted by an electron-withdrawing
group, see for example, the variations of pKa of N-
substituted anilines [12].
cationic micelle could be analyzed similarly, since
the cationic micelle could enhance the dissociation
as observed for that in the ground state [13], but
also suppress the charge transfer because of the
less polar micellar environment.
4. Conclusions
We observed both in the presence and absence
of CTABand CTAC micelles in highly alkaline
solutions the dual ¯uorescence of 1- and 2-NAs
and their N-monosubstituted derivatives, but not
with N,N-disubstituted derivatives. We concluded
that the dual ¯uorescence was due to the excited-
state acidic dissociation of these NAs, which is
facilitated by the excited-state charge transfer. The
pKaÃs were estimated from the dual ¯uorescence
pH titration curves as around 14 and showed no
clear correlation with the substitution at the amino
group and the presence of cationic micelles. We
showed that the excited acid form of the dual
¯uorescent NAs had a dipole moment higher than
that of the ground state, while that of the excited
base form had a dipole moment similar to that of
the ground state. This is, to the best of our
knowledge, the ®rst report to show the excited-
state deprotonation of NAs and the emissions of
the involved forms.
Acknowledgements
Supports were received from the National
Natural Science Foundation of China by grant
No. 29975023, the Natural Science Foundation
of Fujian Province, China, by grant No.
D990002, and the Ministry of Education
*MOE), China.
We observed from the solvatochromic investi-
gations that all the dual ¯uorescent NAs had a
locally excited state of charge transfer character.
The substitution of an electron-withdrawing group
at the amino nitrogen would, on the one hand,
enhance the acidic dissociation, whereas on the
other hand suppress the electron transfer that
would be unfavorable for the acidic dissociation.
As a consequence, the charge transfer promoted
excited-state acid±base equilibrium showed no
clear correlation with the substitution at the reac-
tion center. The weak relevance of the pKaÃs to the
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