Z.-M. Zhang et al. / Tetrahedron Letters 57 (2016) 1917–1920
1919
Table 1
Optical and electrochemical properties of compounds 5 and 6
ox
onset
HOMOc (eV)
d
Compounds
k
abs (nm)
a
k
em (nm)
b
U
Film
F
(%)
T
g
/T
d
(°C)
E
LUMO (eV)
g
E (eV)
Solution
Film
5
6
334
343
464
491
28.8
44.1
142/485
152/427
1.14
1.03
À5.46
À5.35
À2.19
À2.32
3.27
3.03
a
b
c
À6
2
 10 M in THF solution.
Film drop-casted on quartz plate.
HOMO levels were determined using the following equations: HOMO = Àe(Eonset À 0.48 V) À 4.8 eV, where the value 0.48 V is for FOC versus SCE electrode.
d
Estimated from the onset of the absorption spectra: 1240/konest
.
Figure 2. (a) PL spectra of compound 5 in THF/water mixtures with different water fractions (f
w
). Inset: photo of compound 5 in THF/water mixtures (f
w
= 0 and 90%) under
is the PL
UV lamp illumination. Excitation wavelength: 370 nm. (b) Plot of (I/I
0
À 1) values versus the compositions of the THF/H O mixtures of compound 5, where I
2
0
intensity in pure THF solution.
fluorescence intensity of this emission peak increases sharply with
increasing the water fractions until the maximum emission inten-
sity is reached in mixtures with 60% water content. A further
increase of water leads to a quick decline of the emission intensity.
When the water fraction is 70%, the PL intensity decreases to a
minimum value and the emission peak is red-shifted with the
value of 18 nm. However, after 70% water content, the fluorescence
intensities of compound 5 increase again with the increasing water
fraction, and the wavelength of 90% fraction is 488 nm. The unu-
sual changes of the PL intensities of compound 5 indicate there is
a complicated competition between ACQ and AIE as in Figure 2b.
Actually, similar phenomenon was also shown by other com-
compound 5 is due to the aggregation phase transformation from
crystal to amorphous state and the increase in particle sizes. The
further emission increase when the water fraction above 70% can
be ascribed to the decrease in particle sizes.
The PL spectra of compound 6 in solutions with various water/
THF ratios are given in Figure 3a. Similar to common AIE com-
pounds, when the f
w
values are small, the emission of the com-
pound is very weak. Along with the increasing water fractions,
the emission intensities increase and reach the highest emission
efficiency in 90% water content. TEM study of compound 6 in
60%, 70% and 90% water contents (Fig. S3) indicates that only
amorphous particles formed. In addition, we measured the average
diameter of the particles in 70% and 90% water contents by DLS
(Fig. S4). Particles formed in 90% water content have much smaller
average diameter (428 nm) than that in 70% water content
(606 nm), which is consistent with their emission behavior. It is
obvious that the formation of the aggregated state enhances the
emission of the compound 6, showing distinct AIE property.
The thermal properties of compounds 5 and 6 were analyzed by
thermo-gravimetric (TGA) and differential scanning calorimetry
(DSC) analyses, and are given in Figure S5. Both the compounds
2
7,28
pounds,
but the researchers only speculated the possible rea-
To make it clear, transmission electron microscopic
sons.2
9–32
(
TEM) and dynamic light scattering (DLS) of compound 5 in differ-
ent water content were measured. In Figure S1, the TEM images
show that in 60% THF/water mixture compound 5 is packed in
obvious crystalline aggregates and in 80% and 90% water contents,
it aggregates into amorphous particles. During the formation pro-
cess of crystal phase, molecules may adjust their conformation
3
3
by twisting their phenyl rings to fit into the crystalline lattices.
While the molecules in amorphous state aggregate with random
style and thus have a more planar conformation. This molecular
conformation decides that molecules in amorphous phase are
d
show high thermal decomposition (T ) temperatures with the val-
ues of 485 and 427 °C. The thermal stability of 6 is lower than that
of compound 5 because the ethylidene in 4,5,9,10-tetrahydropy-
easier to form intermolecular
p–p
interactions, leading to a red-
g
rene unit is easy to decompose. The (T ) of Compounds 5 and 6
shifted emission and a decrease of emission compared to those
in its crystal state. Hence, during the process of competition
between ACQ and AIE, molecular crystal state is more beneficial
to AIE than that of amorphous phase. The other important factor
which can affect the emission efficiency of the compound is the
particle size. Generally, particles with larger size weaken the emis-
sion due to that only the molecules on the surface of the nanopar-
ticles can emit fluorescence. The DLS results (Fig. S2) show that the
average diameters of 60%, 70%, and 90% water contents of com-
pound 5 are 301, 528 and 374 nm, respectively. Therefore, the
quick decrease in emission from 60% to 70% water fractions for
show relatively high glass transition temperatures with T
of 142 and 152 °C, respectively.
The electrochemical properties were investigated by cyclic
voltammetry (CV) measurements of the two compounds in anhy-
g
values
À4
drous dichloromethane with the concentration of 1.0 Â 10 M.
The corresponding data are summarized in Table 1. On the basis
ox
of the onset potentials of oxidation (Eonset), the HOMO energy
levels were calculated with the values of À5.46 and À5.35 eV using
the equation HOMO = ÀeEonset À 4.32 eV. The band gaps were
derived from the absorption edge in the absorption spectra with
the values of 3.27 eV for 5 and 3.03 eV for 6. The LUMO values of