The strong fluorescence of the new fluorophores could be
ascribed mainly to two factors: one is the strong rigidity, and the
other is the efficient intramolecular charge transfer (ICT) nature—
they contained both strongly electron-withdrawing groups and
electron-donating groups along the axis of the p-conjugation
systems.
The strongly fluorescent C derivatives might be potent
fluorophore candidates for biological applications. Although their
spectra have not entered the near infrared range, they have
relatively long wavelengths compared with most available
flurophores in the UV-vis range, e.g. fluorescein (lflo # 520 nm),
BODIPY (lflo # 520 nm), 4-amino-1,8-naphthalimide (lflo
30 nm), amino-NBD (lflo # 550 nm), Cy3(lflo # 570 nm),
#
5
tetramethylrhodamine (lflo # 570 nm), etc. They are smaller
molecules than other long-wavelength fluorophores, and so they
might not suffer from aggregation so much. Their optical
properties are stable in different chemical environments and acidic
media, they are electronically neutral unlike those cation or anion-
type fluorophores, and these properties facilitate cellular mem-
brane permeation. The most outstanding advantage is the
simplicity of synthesis or derivation at room temperature. This is
valuable for their competition with other fluorophores, because the
Fig. 1 Normalized absorption and emission spectra of C1 in actonitrile.
Table 1 Spectroscopic data of C1–C3 in three solvents
a
W
synthetic method involved the parent compound E which is very
H
electron-deficient and readily undergoes S Ar reactions under
N
Solvent
labs/nm log e
lflo/nm
t/ns
Dichloromethane 574
4.84
4.90
4.94
4.10
4.11
4.24
4.73
4.83
4.81
592
597
596
591
596
597
591
596
597
0.59 7.65
0.58 8.34
0.57 8.13
0.81 7.65
0.95 8.25
0.88 7.96
0.64 7.17
0.62 7.91
0.60 7.47
very mild conditions without a conventional leaving-group like a
halide atom.
C1 Acetononitrile
Ethanol
574
577
Dichloromethane 570
ab
Yi Xiao, Fengyu Liu, Xuhong Qian* and Jingnan Cui
a
ab
a
C2 Acetononitrile
Ethanol
574
577
a
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, 158 Rd. Zhongshan, P.O. Box 89, Dalian, 116012, P. R.,
China. E-mail: xhqian@dlut.edu.cn; Fax: +86-411-83673488;
Tel: +86-411-83673466
Dichloromethane 574
C3 Acetononitrile
Ethanol
575
577
b
Shanghai Key Laboratory of Chemical Biology, East China University
a
Determined by comparison with rhodamine B in ethanol (W 5 0.49,
according to ref. 12).
of Science and Technology, Shanghai, 200237, P. R., China
Notes and references
Table 1. These fluorophores show high fluorescence quantum
yields (0.55–0.95), moderate Stokes shifts (18–23 nm), relatively
long fluorescence lifetimes (7.2–8.4 ns) and relatively long-
wavelength absorption and emission with maxima around 575
and 595 nm respectively. The spectra were not greatly influenced
by solvents of differing polarities: both the absorption and the
emission spectra of compounds C1–C3 showed small spectral
shifts (just several nanometres). The fluorescence quantum yields
of each compound were of similarly high values, and also the
lifetime values did not change greatly in the three solvents. When
small amounts of water (,10% volume, if more, precipitation
would appear) were added to the acetonitrile solution, the spectral
properties of C1–C3 were not influenced greatly, either. These data
indicate that the new fluorophores have stable spectral properties.
Due to the insolubility in aqueous media, the influence of pH on
the photophysical properties could not be recorded accurately.
Instead, similar studies were carried out in organic solution. When
solutions of compounds C1–C3 in ethanol, acetonitrile or
dichloromethane, were acidified carefully with small amounts of
trifluoroacetic acid, neither any apparent spectral shift, nor
fluorescence quench could be observed which indicated that the
spectral properties of C1–C3 are insensitive to acidic media, unlike
the often used fluorescein-type fluorophores whose fluorescence
would be quenched greatly in acidic solution.
{
Selected characterization data.
1
E: mp 275–277 uC; H NMR (400 MHz, [D ]DMSO): d 5 8.71–8.69 (d,
6
J 5 8.0 Hz, 1H), 8.67–8.65 (d, J 5 7.6 Hz, 1H), 8.64–8.62 (d, J 5 8.0 Hz,
1H), 8.42–8.40 (d, J 5 7.6 Hz, 1H), 8.04–8.08 (t, J 5 8.0 Hz, 1H), 7.99–7.95
13
t, J 5 7.8 Hz, 1H); C NMR (100 MHz, [D
(
6
]DMSO): d 5 177.48, 138.26,
137.73, 134.40, 132.72, 131.82, 131.37, 128.91, 127.94, 127.37, 126.13,
122.22, 119.72, 113.82, 113.38; IR (KBr) n/cm : 2231, 1643, 1577; ESI-
MS: m/z 5 253, (M + Na) ; HRMS: m/z calcd. for C15
found 230.0477.
21
+
16 2
H N 230.0480,
1
C1: mp .300 uC; H NMR (400 MHz, [D
6
]DMSO): d 5 9.60 (br s,
–NH–, 1H), 8.95–8.93 (d, J 5 7.6 Hz, 1H), 8.60–8.58 (d, J 5 7.2 Hz, 1H),
.98–7.96 (d, J 5 8.8 Hz, 1H), 7.88–7.92 (t, J 5 7.8 Hz, 1H), 7.04–7.02 (d,
J 5 9.2 Hz, 1H), 3.60–3.59 (br s, –NHCH CH –, 2H), 1.75–1.71 (m,
NHCH CH CH –, 2H), 1.47–1.43 (m, –CH CH CH CH , 2H), 0.94–
0.98 (t, J 5 7.2 Hz, 3H); C NMR (100 MHz, [D ]DMSO): d 5 176.45,
7
2
2
–
2
2
2
2
2
2
3
13
6
155.79, 138.66, 132.35, 130.99, 129.63, 127.98, 126.95, 125.49, 121.97,
116.12, 114.33, 111.30, 108.13, 103.95, 43.42, 30.05, 19.68, 13.66; IR (KBr)
21
2
n/cm : 3284, 2217, 1619, 1562, 1529; ESI-MS: m/z 5 300, (M 2 H) ;
HRMS: m/z calcd. for 301.1215, found 301.1223.
19 15 3
C H N O
1 R. P. Haugland, Handbook of Fluorescent Probes and Research
Products, Molecular Probes, Inc., Eugene, OR, USA, 9th edn., 2002;
A. C o´ mez-Hens and M. P. Aguilar-Caballos, Trends Anal. Chem., 2004,
23, 127; A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J.
M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
Rev., 1997, 97, 1515.
W. E. Moemer, Acc. Chem. Res., 1996, 583; W. E. Moemer and
M. Orrit, Science, 1999, 283, 1670; S. Weiss, Science, 1999, 283, 1676.
J. E. Whitaker, R. P. Haugland and F. G. Prendergast, Anal. Biochem.,
1991, 194, 330; O. S. Wolfbeis, Mikrochim. Acta, 1992, 108, 133.
2
3
2
40 | Chem. Commun., 2005, 239–241
This journal is ß The Royal Society of Chemistry 2005