Fluorescent properties of 9-anthracenecarboxamides

Article abstract of DOI:10.1023/B:RUBI.0000049777.07961.11

A number of new 9-anthracenecarboxamides are synthesized in order to create new fluorescent probes for studying biological systems. The parameters of their fluorescence in organic solvents of various polarities are investigated, and possible mechanisms of

Full text of DOI:10.1023/B:RUBI.0000049777.07961.11

Russian Journal of Bioorganic Chemistry, Vol. 30, No. 6, 2004, pp. 586–591. Translated from Bioorganicheskaya Khimiya, Vol. 30, No. 6, 2004, pp. 649–655.
Original Russian Text Copyright © 2004 by Boldyrev, Molotkovsky.
Fluorescent Properties of 9-Anthracenecarboxamides
1
I. A. Boldyrev and Jul. G. Molotkovsky
Shemyakin–Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences,
ul. Miklukho-Maklaya 16/10, Moscow, 117997 Russia
Received December 25, 2003; in final form, March 25, 2004
Abstract—A number of new 9-anthracenecarboxamides are synthesized in order to create new fluorescent
probes for studying biological systems. The parameters of their fluorescence in organic solvents of various
polarities are investigated, and possible mechanisms of internal quenching of fluorescence of these compounds
are discussed. One of the compounds, 4-ethoxycarbonylphenylamide of 9-anthracenecarboxylic acid, is shown
to be a promising basis for the development of a new fluorescent probe.
Key words: 9-anthracenecarboxamides, fluorescence
1
INTRODUCTION
RESULTS AND DISCUSSION
We describe here the fluorescence parameters (posi-
tions of emission maxima and quantum yields in a num-
ber of organic solvents) for methyl (11-(9-anthroyl-
amino)undecanoate (VII) [3] and five aromatic amides
(VIII)–(XII) (see scheme). The derivatives (VIII)–
(XII) were obtained by one method, the reaction of 9-
anthroyl chloride with the corresponding amine (I)–
(VI) in the presence of an HCl acceptor. The following
AC derivatives with an electron donor group in aryl-
amide substituent were synthesized by this way:
4-methylphenylamide (VIII), 4-methoxyphenylamide
(IX), 2-dodecanoylaminophenylamide (X), and iso-
meric 4-dodecanoylaminophenylamide (XI); 4-ethoxy-
carbonylphenylamide (XII) with an electron acceptor
group in arylamide residue was also obtained. The 2-
and 4-dodecanoylaminophenylamines (IV) and (V),
synthons necessary for the synthesis of diamides (X)
and (XI), were obtained by monoacylation of 1,2- and
1,4-phenylenediamines, respectively.
9-Anthracenecarboxylic acid and its esters (mainly
9-anthroyloxyfatty acids) have received wide accep-
tance as fluorescent probes for studying biological and
2
model membranes. Up to now, properties of these
compounds have been much studied [1, 2]. Aliphatic
amides of AC received no application as fluorescent
probes probably due to their low quantum yields in
most cases. For example, we found [3] that the quantum
yield of 11-(9-anthroylamino)undecanoic acid is 1–3%
in various media. Both aliphatic and aromatic amides of
AC have scarcely been studied. Their photophysical
properties are interesting, because the parameters of
their fluorescence are in a complex dependence on the
medium polarity and the fluorophore conformation [4,
5]. The mechanisms of relaxation of the excited state of
AC aromatic amides attract attention [5] (see below). In
addition, these substances and their analogues may be
applied in fluorescent sensors for the determination of
a number of ions [5, 6]. Our preliminary results indicate
that anthracene-9-carboxamides may be prospective
probes for studying biopolymers and membrane sys-
tems, since the emission parameters of some of them
(position of maximum, quantum yield, etc.) largely
depend on the polarity of medium and pH value. The
properties of fluorescent probes available to researchers
are far from optimal; new claims are being laid to them;
and, therefore, the need in new probes of various types
does not decrease [7–9].
Note that 9-anthroyl chloride has rather low acylat-
ing ability toward low basic aromatic amines. Long
reaction times (or heating) and the use of N-ethyldiiso-
propylamine for binding HCl were necessary to achieve
satisfactory yields in this case. Triethylamine and pyri-
dine were unsatisfactory as the HCl-binding agents:
their use resulted in low yields of target products and
polar side products probably due to the amine acylation
with 9-anthroyl chloride to form quaternary ammonium
salts and products of their further degradation.
This study is undertaken in order to comparatively
study some fluorescent properties of an AC aliphatic
amide and a number of AC aromatic amides.
Structures of the synthesized 9-anthroylamide
derivatives were confirmed by UV, mass, and 1H NMR
spectra. The signal of anthracene H10 (singlet at δ 8.4–
8.7 ppm) is characteristic of 9-anthroyl residue [5]. UV
spectra of all the AC amides display an intensive
short-wave peak at 250–270 nm (ε 6–7 × 104 å–1 cm–1)
and a triplet at 340–350, 360–370, and 380–390
1
Corresponding author; phone/fax: +7 (095) 330-6601; e-mail:
jgmol@ibch.ru
2
Abbreviations: AC, 9-anthracenecarboxylic acid; DBU, 1,8-diaza-
bicyclo[5.4.0]undec-7-ene; PET, photoinduced electron transfer;
and TICT, twisted internal charge transfer.
1068-1620/04/3006-0586 © 2004 MAIK “Nauka /Interperiodica”

FLUORESCENT PROPERTIES OF 9-ANTHRACENECARBOXAMIDES
587
+ RNH
COCl
CONH R
2
(I)–(VI)
(VII)–(XII)
Compound
R
(I), (VII)
C11H23COOMe
Me
(II), (VIII)
(III), (IX)
(IV), (X)
OMe
NHCOC11H23
NHCOC11H23
COOEt
(V), (XI)
(VI), (XII)
Scheme.
(ε 5−9 × 103 å–1 cm–1) characteristic of 9-anthroyl
derivatives [5, 10]; the corresponding maxima are
clearly visible in the excitation spectra of (VII), (XI),
and (XII) (see Fig. 1).
Unlike 9-anthroyl esters [2], the emission spectra of
AC amides (table, Fig. 1) do not show the same
unequivocal dependence of the positions of their fluo-
rescence maxima and quantum yields (Φ) on the polar-
ity of medium. Obviously, all the amides have their
quantum yields lower in polar solvents than in apolar
ones; this effect is more pronounced for aromatic
amides (VIII)–(XII) than for aliphatic (VII). In this
case, aromatic substituents at the amide nitrogen atom
The fluorescence spectra of (VII)–(XII) were
recorded for their solutions in media of various polari-
ties: apolar (hexane, chloroform), polar aproton (aceto-
nitrile), and polar proton (methanol). The excitation
and emission spectra of the most interesting three very actively participate in the processes of fluorophore
excitation and relaxation of the excited state, which is
evident from the table. The aromatic amides with the
electron donor substituents (VIII)–(XI) display sub-
stantially lower Φ values than the aliphatic amide (VII)
or the aromatic (XII) with an electron donor substitu-
ent. The AC esters exhibit a clear dependence of the
position of emission maximum on the polarity of
medium (the more the polarity the more the shift of the
maximum to the long-wave area and the less the quan-
tum yield [2]), whereas the AC amides do not demon-
strate such a direct dependence. Undoubtedly, this is a
consequence of a complex character of the interaction
amides (VII), (XI), and (XII) are shown in the figure.
The most important parameters of the emission spectra
of all the studied compounds are summarized in the
table.
A close similarity of the absorbance and excitation
spectra of all the studied substances is readily explained
by the fact that, in the main state, the carbonyl group of
anthroyl residue is oriented perpendicularly to the
anthracene ring and its electrons are not conjugated
with the aromatic system (see [5] and references
therein).
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 30 No. 6 2004

588
BOLDYREV, MOLOTKOVSKY
I, rel. units
6
(a)
5
4
3
2
1
1
3
×2
5
4
2
0
6
×15
(b)
5
4
3
2
1
0
3
1
4
5
2
120
×12
100
80
1
3
(c)
×15
5
4
60
2
40
20
0
300
350
400
450
nm
500
550
600
Fig. 1. The (1, 2) excitation and (3, 4, 5) emission spectra of AC amides (a) (VII), (b) (XI), and (c) (XII). The concentration of
substances is approximately 20 µM and temperature 20°C. The spectra are measured in (1, 3) hexane, (4) acetonitrile, and (2, 5)
methanol. The emission spectra are recorded at λ (a, b) 410 and (c) 440 nm; all the excitation spectra, at λ 365 nm.
em
ex
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 30 No. 6 2004

FLUORESCENT PROPERTIES OF 9-ANTHRACENECARBOXAMIDES
589
Maxima of fluorescence and quantum yields of N-anthroyl derivatives in media of various polarities
Solvent
Substance
hexane
chloroform
acetonitrile
methanol
384.5, 407, 431
------------------------------------
9
391, 413.5, 454.5
-----------------------------------------
15
409.5, 458
------------------------
5
386.5, 407.5, 432.5
---------------------------------------------
5
(VII)
(VIII)
(IX)
408, 435
--------------------
0.2
416.5, 436.5
-----------------------------
0.1
409, 433
--------------------
0.07
408, 432
--------------------
0.07
409, 444
--------------------
0.02
414.5, 437
------------------------
0.01
410, 436
--------------------
0.005
408, 431
--------------------
0.005
408, 430
--------------------
0.5
416.5, 439
------------------------
0.4
409.5, 434
------------------------
0.3
451
----------------
<0.001
(X)
408, 463, 480, 497.5
------------------------------------------------
5
392, 413, 461.5
------------------------------------
0.4
388, 409, 455
--------------------------------
0.3
387.5, 408, 434
------------------------------------
0.3
(XI)
438.5
------------
18
456
--------
9
442.5
------------
1
438
--------
1
(XII)
Note: The wavelengths of emission maxima (the most intensive is given in boldface) are listed in numerator; the quantum yields (%), in
denominator. The excitation wavelength is 365 nm.
of amide grouping and the substituent at it with the fluorophore state occurs due to the photoexcited
basic (anthroyl) fluorophore.
unshared electron pair at the nitrogen atom in the side
chain at fluorophore. However, until now, the PET
mechanism is only ascribed to the fluorescence quench-
ing of aromatic fluorophores with an amine group in
side chain [12]. We can presume that the same mecha-
nism is also inherent in the photophysical processes in
the case ofAC amides under consideration. The follow-
ing observation could confirm this presumption. An
addition of DBU, a strong base, up to the concentration
of 1 mM to a 20 µM solution of (VII) in acetonitrile
(spectrum 4, Fig. 1a) results in significant changes in
the fluorescence spectrum of the solution: it becomes
structured, its maximum is shifted to short-wave area,
and it becomes practically identical to the spectrum in
methanol (5). A similar, but a weaker, effect of DBU
was also observed in the case of other AC amides (data
not shown). This effect can be attributed to the change
in the charge on the amide nitrogen atom under the
action of DBU; amide group is known to have the prop-
erties of both a weak acid and a weak base [13]. How-
ever, this effect may also be explained within the
frames of TICT mechanism; the choice could obviously
be made after the further thorough studies.
It is accepted [5] that carbonyl remains perpendicu-
lar to fluorophore in the photoexcited state I of AC
amide. The emission from this state is short-wave and
resembles that from anthracene (see spectrum 3 in
Fig. 1a or spectrum 5 in Fig. 1c). The subsequent relax-
ation into the excited state II leads to the conjugation of
the carbonyl with the anthroyl fluorophore; this state
has a longer-wave light emission (like that of spectrum
4 in Fig. 1a), if an aromatic substituent is present at the
NH group. Its conjugation through the amide bond
additionally shifts the emission maximum to the red
side of spectrum (see spectrum 3 in Fig. 1c). The con-
jugation degree of the amide grouping bearing an aro-
matic substituent with the anthroyl fluorophore may be
different, and therefore, the emission spectrum of the
AC amides may be structured to a lesser degree than
that of anthracene or even be completely unstructured.
In the case of aromatic amides, one more excited state
with the separation of charges of fluorophore and aro-
matic substituent is formed due to the rotation of C(O)–
NH bond by the TICT mechanism. The recovery from
this state into the main state is not emissive; i.e., a
quenching of fluorescence occurs and the TICT struc-
tures are stabilized by polar solvents with a low viscos-
ity [5, 11]. It is highly probable that the fluorescence
quenching by the TICT mechanism is the cause of low
quantum yields of (VIII)–(XI), particularly, in acetoni-
trile and methanol (see the table).
Among theAC amides described here, (XII) attracts
a particular interest. It displays the highest quantum
yield in comparison with the other AC amides, and its
fluorescence maximum and the form of spectrum little
depend on the polarity of medium, whereas its Φ value
largely depends on this parameter (see the table and
Fig. 1c). Obviously, this effect is due to the electron-
acceptor action of ethoxycarbonyl group, which
The observed dependences of the positions of λem
and Φ values on the AC amide structures are probably
determined (at least, partially) by another mechanism, reduces the participation of the electrons of phenylene
PET. According to PET, the quenching of the excited ring in the quenching of anthroyl fluorescence. On the
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 30 No. 6 2004

590
BOLDYREV, MOLOTKOVSKY
other hand, the interaction of solvent dipoles with ester A, B); mp 93–94°C (from chloroform–hexane); MS,
1
m/z: 290 [M]+; H NMR (CDCl3): 0.89 (3 H, t, J 7.1,
CH3), 1.28 [16 H, broad m, (CH2)8CH3], 1.76 (2 H, m,
COCH2CH2), 2.41 (2 H, t, J 7.6, COCH2), 3.86 [bs,
(NH2)], 6.81 (2 H, d, J 7.1, arom. H), 7.07 (1 H, t, J 7.8,
arom. H), 7.10 (1 H, bs, NH), and 7.19 (1 H, d, J 8.3,
arom. H).
group leads to a decrease in the electron-acceptor effect
and an increase in the quenching presumably by the
TICT mechanism (see the table). Amide (XII) has sat-
isfactory chemical and photostabilities: its alcoholic
solution does not undergo a marked degradation in the
dark at room temperature for more than 15 days (TLC)
and irradiation of its hexane solution with the excitation
light in a fluorimeter (see the Experimental section) for
1 h does not change its emission spectrum.
We believe that the parameters of (XII) (fluores-
cence parameters, the dependence of Φ value on the
polarity of medium, stability, and a convenient method
of synthesis) allow its use for the design of fluorescent
probes suitable for studying biological systems.
4-Dodecanoylaminophenylamine
(V)
was
obtained as described for (IV) by a reaction of a solu-
tion of 1,4-phenylenediamine dihydrochloride (0.7 g,
3.9 mmol) and N-ethyldiisopropylamine (1 ml) in chlo-
roform (25 ml) with dodecanoyl chloride (0.22 g);
chromatographically homogeneous cream-white pow-
der; yield 20%; Rf 0.5 (80 : 17 : 1 chloroform–ethyl ace-
tate–methanol; A, B); mp 150–153°C (sintered at about
140°ë, from chloroform–hexane); MS, m/z: 290 [M]+.
EXPERIMENTAL
Anthroylamides (general procedure). 9-
Anthracenecarboxylic acid chloride (1.5 equiv) was
added to a stirred solution of an aromatic amine or a
derivative (IV) or (V) (0.2–1 µmol) and N-ethyldiiso-
propylamine (3–5 equiv) in dry chloroform (5–15 ml).
The mixture was stirred until the complete dissolution,
allowed to stand for 2 days, diluted with ethyl acetate,
treated with water, and intensively stirred for 6 h. Then
it was twice washed with water and a saturated NaCl
solution (10 ml each), and dried with Na2SO4. The tar-
get substance was isolated from the extract by column
chromatography on silica gel in a step gradient of ethyl
acetate in benzene or chloroform (99 : 1 to 9 : 1) mon-
itoring the separation by TLC (detection A–C). In this
manner, there were obtained:
We used dodecanoyl chloride, DBU, N-ethyldiiso-
propylamine, and ethyl p-aminobenzoate from Fluka
(Switzerland), 9-anthracenecarboxylic acid and 1,4-
phenylenediamine dihydrochloride from Merck (Ger-
many), and the other reagents and solvents from Rea-
khim (Russia). Dry chloroform was obtained by distil-
lation over phosphorus pentoxide, other solvents were
used after the purification by usual procedures. Kiesel-
gel 60 (Merck, Germany) was used for column chroma-
tography. Precoated plates with fluorescent indicator
(Kieselgel 60 F254) and without the indicator (Kieselgel
60) were from (Merck, Germany). Substance spots
were detected by phosphomolybdic acid (A), ninhydrin
(B), and by visualization under UV light (C).
Anthracene-9-carboxylic acid chloride was obtained by
boiling of the acid with thionyl chloride excess in chlo-
roform containing 0.05% DMF; mp 95–98°ë
(decomp., from chloroform–toluene). Methyl 11-(9-
anthroylamino)undecanoic acid (VII) was synthesized
as described previously [3].
9-Anthracenecarboxylic acid 4-methylphenyla-
mide (VIII); yield 40%; Rf 0.5 (19 : 1 benzene–ethyl
acetate, A, C); mp 202–205°ë (with sublimation, from
chloroform–methanol); MS, m/z: 311 [M]+, 205
1
[ë14ç9ëé]+; H NMR (CDCl3): 2.40 (3 H, s, CH3),
7.26 (2 H, d, J 8.6, arom. H), 7.53 (4 H, m, arom. H),
7.64 (1 H, bs, NH), 7.67 (2 H, d, J 8.6, arom. H), 8.05
(2 H, d, J 8.1, arom. H), 8.19 (2 H, d, J 8.1, arom. H),
and 8.54 (1 H, s, arom. H).
Mass spectra were measured under the electron
impact ionization (70 eV) on a SSQ-710 instrument
(Finnigan MAT, United States); UV spectra, on a
Ultraspec II spectrophotometer (LKB, Sweden); fluo-
rescence spectra, on a Hitachi F-4000 spectrofluorime-
9-Anthracenecarboxylic acid 4-methoxypheny-
lamide (IX); yield 35%; Rf 0.4 (19 : 1 benzene–ethyl
acetate, A, C); mp 219–222°ë (from chloroform–meth-
1
ter (Japan); and H NMR spectra (δ, ppm, relative
Me4Si, spin coupling constants, J, Hz), on a Bruker
1
anol); MS, m/z: 327 [M]+, 205 [ë14ç9ëé]+; H NMR
WM 500 spectrometer (Germany).
(CDCl3): 3.77 (3 H, s, CH3), 6.89 (2 H, d, J 10.9, arom.
H), 7.43 (4 H, m, arom. H), 7.51 (1 H, bs, NH), 7.60
(2 H, d, J 10.9, arom. H), 7.95 (2 H, d, J 10.6, arom. H),
8.09 (2 H, d, J 10.6, arom. H), and 8.44 (1 H, n,
arom. H).
2-Dodecanoylaminophenylamine (IV). A solution
of dodecanoyl chloride (0.22 g, 1 mmol) in chloroform
(1 ml) was added in five portions at 5-min intervals to a
solution of 1,2-phenylenediamine (0.52 g, 4.2 mmol)
and N-ethyldiisopropylamine (0.2 ml) in dry chloro-
form (10 ml). The solution was kept for 12 h, diluted
with ether (100 ml), washed with water (3 × 15 ml),
dried with Na2SO4, and evaporated. Chromatography
9-Anthracenecarboxylic acid 2-dodecanoylami-
nophenylamide (X); yield 70%; Rf 0.55 (83 : 15 : 2
benzene–ethyl acetate–acetic acid, A, C); mp
151−153°ë (from chloroform–methanol); MS, m/z:
494 [M]+, 476 [M – H2O]+, 311 [M – C11H23CO]+, 293
on a silica gel column eluted with 5
10% ethyl ace-
tate in chloroform led to pure (IV) as a white powder
1
[M – C11H23CO – H2O]+, 205 [ë14ç9ëé]+; H NMR
homogeneous according to TLC; yield 152 mg (52%);
Rf 0.7 (80 : 17 : 1 chloroform–ethyl acetate–methanol; (CDCl3): 0.89 (3 H, t, J 6.8, CH3), 1.22 [16 H, bm,
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 30 No. 6 2004

FLUORESCENT PROPERTIES OF 9-ANTHRACENECARBOXAMIDES
591
(CH2)8CH3], 1.55 (2 H, m, COCH2CH2), 2.32 (2 H, t, J where A and Ast are the optical absorbances, and I and
7.5, COCH2), 7.31 (2 H, m, arom. H), 7.51 (4 H, i, Ist are the integral intensities of fluorescence of the sam-
arom. H), 7.56 (1 H, m, NH), 7.63 (1 H, m, NH), 8.03 ple and the standard, respectively; the quantum yield of
(2 H, d, J 7.8, arom. H), 8.13 (2 H, d, J 7.8, arom. H), standard Φst = 0.2 in methanol [14].
8.34 (1 H, s, arom. H), 8.45 (1 H, s, arom. H), and 8.52
(1 H, s, arom. H).
ACKNOWLEDGMENTS
9-Anthracenecarboxylic acid 4-dodecanoylami-
nophenylamide (XI); yield 45%; Rf 0.5 (83 : 15 : 2
benzene–ethyl acetate–acetic acid, A, C); mp 195–
200°C (sintered at ~160°ë, from chloroform–metha-
This work was supported by the Russian Foundation
for Basic Research, project nos. 03-04-48420 and
02-04-48287.
1
nol); MS, m/z: 494 [M]+, 205 [ë14ç9ëé]+; H NMR
(CDCl3/ëD3OD): 1.06 (3 H, t, J 6.4, ëç3), 1.45 (16 H,
bm, (ëç2)8ëç3)), 1.90 (2 H, m, OCH2CH2), 2.55 (2 H,
t, J 7.5, OCH2), 7.66–7.72 (5 H, bm, arom. H + NH),
7.78 (2 H, d, J 8.1, arom. H), 7.94 (2 H, d, J 8.1, arom.
H), 8.22 (2 H, d, J 8.2, arom. H), 8.29 (2 H, d, J 8.2,
arom. H), and 8.71 (1 H, s, arom. H).
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standard:
IAst
Ist A
--------
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Φ = Φst
,
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RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 30 No. 6 2004