2
Tetrahedron Letters
Figure 1. The fluorescence spectra of fluorene and Compounds 1-5
in CH CN
3
Sensing of nitroaromatic compounds
Compound 5 (sensor) is highly soluble in some organic
solvents such as acetonitrile, methanol. We take the acetonitrile
as the solvent for sensing study. The sensor exhibits strong
emission peak at 367 nm (λex = 280 nm).
Scheme 1. Synthetic Routes of compound 5
compound 2, the dihedral angle between two benzene rings (C7,
C8, C9, C10, C11, C12; C7A, C8A, C9A, C10A, C11A, C12A)
and the five-member ring (C7, C12, C13, C12A, C7A) are both
.0 ( 0.0 ) °. X-ray single crystal study shows that compound 3
Figure S1B) is crystallized in a monoclinic system with P2 /c
space group. The dihedral angle between benzene rings (C11,
C12, C13, C14, C15, C16; C18, C19, C23, C20, C21, C22) and
the five-member ring (C14, C15, C17; C19, C18) are 2.1 (0.2) °
and 1.8 (0.2) °, which are larger than compound 2. And the
dihedral angle between benzene ring (C3, C4, C5, C6, C7, C8)
and the fluorene moiety is 43.6 ( 0.1 )°, showing that C-C single
bond distorting between benzene ring and the fluorene moiety is
strong.
The changes in luminescence intensity with the increasing
addition of nitroaromatic compounds (up to 100 ppm) in the
sensor are shown in Figure 2 and Figure S6, respectively. In the
current study, the NACs used are nitrobenzene (NB), 4-
nitrotoluene (4-NT), 4-nitrobenzaldehyde (4-NBA), 1,3-
dinitrobenzene (DNB), 2-nitrophenol (2-NP), 3-nitrophenol (3-
NP), 4-nitrophenol (4-NP), 2,4,6-trinitrophenol (TNP), and 2,4,6-
trinitro toluene (TNT) (Figure 3) to judge the selectivity of
sensing towards nitroaromatic compounds. The most effective
quencher is TNP with QP (quenching percentage = (I − I)/I ×
0
(
1
0
0
100%, I and I are luminescence intensity of sensor before and
0
after exposure to the nitro compound) of 97.91%. Other NACs
can weaken the photo luminescent intensity of sensor to different
level. The order of quenching efficiency is TNP > 2-NP > 4-
NBA ≈ NB ≈ TNT ≈ 4-NP > 3-NP > 4-NT > DNB. In all the
cases, the initial fluorescence intensity of the sensors decreased
considerably upon gradual addition of the NACs. The rate of
quenching, which is essentially related to the Stern - Volmer
constant (KSV), revealed that TNP has faster fluorescence
quenching over the other nitroaromatic compounds. To better
understand this fact, we need to calculate the Stern-Volmer
constants of the quenching process.
Fluorescence measurements
The emission spectra of the fluorene and the compounds 1-5
were shown in Figure 1 and the luminescent spectra of them were
analyzed under excitation wavelength (280 nm) with 400V and
slit width (5:5). For compounds 1 and 2, almost no fluorescence
responds were observed.
wavelength was 313 nm for fluorene, 436 nm for compound 3,
08 nm for compound 4, 380 nm for compound 5. Compared
While, the maximum emission
4
with fluorene, there are fluorescence quenching phenomenon
occurring in compounds 1, 2, 3 and the red shift in the maximum
emission wavelength for compounds 3, 4, 5.
All of compounds 1, 2 and 3 are containing bromine atoms.
22
In this condition of containing halogen atoms, the excited state
of the compounds and heavy atom form the composite in a ratio
of 1:1. The degree of spin orbit coupling in this compound is
much larger than normal compound. This excited state complex
has
a strong charge transfer property, normally is the
fluorescence quenching process. In compounds 3, 4 and 5,
fluorene moiety is conjugated with benzene ring after the Suzuki
coupling reaction of bromination derivatives of fluorene, which
make the fluorescence enhance and a red shift occurs in the
maximum emission wavelength but compound 3 still has the
heavy atom effect. At the same time, the dihedral angle between
two benzene rings and the five-member ring increased, showing
that the fluorene molecular skeleton is distorted by the effect of
benzene rings. Compared with 3, compound 4 has two benzene
rings leading to distorted structure which made the conjugated
effect weak and blue shift. Finally, the compound 4 turned into
compound 5 by hydrolysis reaction, which made the fluorescence
intensity decrease because it contains the carboxyl group which
is an electron-withdrawing group.
Figure 2. Reduction of the fluorescence emission intensity of the
sensor upon gradual addition of TNP