H. Dai, Z.-B. Cai, Q.-X. Lou et al.
Tetrahedron 96 (2021) 132359
which indicates that the bromo groups have relatively strong
electron-withdrawing ability. The chemical shifts of the protons in
the methoxy groups change from 3.87 ppm (2MOHA) to 3.96 ppm
(1MOBI), which reveals that benzimidazole is a good electron
acceptor.
dihexylamino donors can improve the light-absorbing ability.
Compared 1MOBI with 2MOHA and 3BrBI with 4BrHA, respec-
abs
max
tively, the values of
l
are red-shifted in the same solvent as the
terminal groups change from benzimidazole to dihexylamino. For
abs
max
example, the
l
values change from 403 nm (for 1MOBI) to
In the FI-IR spectra, the absorption peaks at 1181, 1366e1383,
and 1600e1601 cmꢁ1 belong to the stretching vibration of eCeN,
]CeN, and C]N, respectively. The characteristic bands appearing
at 2852-2958 cmꢁ1 are assigned to the eCeH stretching vibration,
which is particularly strong in 2MOHA and 4BrHA due to the ex-
istence of the dihexylamino groups. The characteristic absorption
peaks at 1046 and 1206e1208 cmꢁ1 are attributed to the CeOeC
symmetric stretching vibration and CeOeC asymmetric stretching
vibration. The CeH out-of-plane bending vibration of the trans
olefinic groups is observed at 957e969 cmꢁ1, which further cer-
tifies the E-configurations of the CeC double bonds.
Besides, the HRMS spectra were used to further verify the
structures of the compounds. In the case of all four compounds, the
signals of [MþH]þ and [Mþ2H]2þ were detected. That's probably
because the benzimidazole ring and the dihexylamino group have
relatively strong alkalinity.
428 nm (for 2MOHA), and from 362 nm (for 3BrBI) to 434 nm (for
4BrHA) in THF. It can be attributed to the strong electron-donating
ability of the dihexylamino groups, which is beneficial to reduce the
abs
max
absorption energy and finally cause the red-shift of
l
.
In order to further analyze and understand the absorption
spectra, the theoretical studies based on the TDDFT calculations
were performed using the Gaussian 09 program with the non-local
abs
density function of B3LYP/6-31G(d) basis set. The calculated
l
max
values are given in Table 2, whose values are 443 (1MOBI), 454
(2MOHA), 429 (3BrBI), and 460 (4BrHA) nm, respectively, which
are attributed to the transition from the highest occupied molecular
orbital (HOMO) to the lowest unoccupied molecular orbital
calculated HOMO-LUMO energy gaps (DE) change in the following
order: 2.928 (4BrHA) < 2.970 (2MOHA) < 3.064 (1MOBI) < 3.181
abs
max
(3BrBI) eV, showing the same tendency with the
l
. For 3BrBI,
the electron clouds of the HOMO are distributed on the whole
molecule, whereas in the LUMO, the density of electron clouds in
the terminal benzimidazole acceptors reduces obviously. For
1MOBI, 2MOHA, and 4BrHA, the electron transition from the
HOMO to the LUMO could lead to obvious charge transfer from the
oxygen atoms in the methoxy donors and the nitrogen atoms in the
2.2. One-photon absorption and TDDFT calculations
The linear optical properties of the target compounds in
different solvents are listed in Table 1. Their UVevisible absorption
sorption band, while the other three target compounds show two
dihexylamino donors to the assembled stilbene
bridge.
p-conjugated
absorption
bands.
The
high-energy absorption
bands
(332e360 nm) can be attributed to the
p-p* transition of the
assembled stilbene, whereas the low-energy absorption bands
(403e443 nm) can be ascribed to an ICT process. With increasing
solvent polarity from toluene to DMSO, the maximal absorption
2.3. One-photon excited fluorescence
abs
peak positions (
l
) of 1MOBI and 3BrBI change slightly ( 4 nm),
max
As shown in Fig. 4, all of four target compounds exhibit dual
emission behavior in slightly polar toluene, which can be attributed
to the locally excited (LE) state emission and the ICT state emission,
p-A-p
respectively. For the A0- -A0 structure compound (3BrBI), the
LE state emission predominates in toluene, however, in the solvents
of medium to high polarity (THF, DMF, and DMSO), the ICT state
emission plays a leading role. As a contrast, the LE state emission of
1MOBI, 2MOHA, and 4BrHA remain dominant with a rise in the
polarity of solvents, while the ICT state emission degenerates into a
shoulder and even disappear. These can be explained by the in-
crease of twisted intramolecular charge transfer (TICT) effect arise
from the strong solute-solvent interaction. As can be seen in Fig. 5,
abs
while 2MOHA and 4BrHA show the variation of the
l
is
max
10e13 nm. This phenomenon indicates that the polarity of solvents
have a slightly deeper influence on the transition energy of the
compounds with dihexylamino donor. The maximum molar ab-
sorption coefficients (ε) of four compounds can be sequenced as
2MOHA > 4BrHA > 3BrBI > 1MOBI, which reveals that the
Table 1
Linear optical properties of the target compounds.
OPEF
max
(nm)
abs
Compound
Solvent
(cmꢁ1
l
l
max
(molꢁ1 L cmꢁ1
)
)
(nm)
the maximal OPEF peak positions (lOmPaExF) of the target compounds
can be sequenced as 4BrHA > 2MOHA > 1MOBI > 3BrBI, which is
1MOBI
Toluene
THF
DMF
DMSO
Toluene
THF
404
403
404
406
427
428
432
437
358
362
358
360
430
434
440
443
4.22
5.43
7.56
3.60
12.67
12.70
12.08
5.35
6.54
8.05
7.92
3.96
8.36
11.19
8.87
4.74
451
453
457
461
481
486
504
514
431
450
456
460
495
511
544
550
2580
2739
2871
2939
2629
2788
3307
3428
4731
5402
6003
6039
3054
3472
4345
4392
0.87
0.69
0.45
0.40
0.94
0.75
0.49
0.47
0.41
0.29
0.16
0.11
0.52
0.37
0.28
0.23
abs
max
OPEF
values display
max
corresponding with the sequence of
l
. The
l
solvent polarity dependencies and exhibit bathochromic shifts of
different degrees with an increase in solvent polarity, which is also
accompanied by an increase in Stokes shifts (Dn). One rational
explanation is that the increase of the polarity of solvents can
enhance the dipole-dipole interaction between the solute and the
solvent, which leads to a lower energy level for the excited state
[17]. We can conclude from Table 1 that the co-existence of the
2MOHA
3BrBI
DMF
DMSO
Toluene
THF
DMF
DMSO
Toluene
THF
DMF
DMSO
bromo group and the dihexylamino group can enhance the sol-
4BrHA
OPEF
max
vatochromic behavior. The
l
of 4BrHA is located at 495 nm in
toluene and red-shifted to 550 nm in DMSO while the color of the
solution changes from green to orange-yellow.
a
Maximum linear absorption wavelength, c ¼ 1 ꢀ 10-5 mol L-1
Maximum molar absorption coefficient.
.
In this paper, equation (1) was used to calculate the fluorescence
b
c
quantum yields (F) of four target compounds in different solvents,
Maximum one-photon excited fluorescence wavelength, c ¼ 1 ꢀ 10ꢁ6 mol Lꢁ1
.
and fluorescein in 0.1 mol Lꢁ1 sodium hydroxide aqueous solution
d
e
Stokes shift.
Fluorescence quantum yield.
(F
¼ 0.9 [18]) or quinine sulfate in 0.5 mol Lꢁ1 sulfuric acid
3