Synthesis, characterization and third-order nonlinear optical behaviour of three novel ethyne-linked donor/acceptor chromophores

Article abstract of DOI:10.1016/j.dyepig.2021.109235

Three novel ethyne-linked donor/acceptor chromophores, BAB, BAS and SAS were synthesized. The results of BAB in femtosecond and nanosecond Z-scan at 532 nm reveal that the transition from saturable absorption (SA) to reverse saturable absorption (RSA), wh

Full text of DOI:10.1016/j.dyepig.2021.109235

Dyes and Pigments 188 (2021) 109235
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis, characterization and third-order nonlinear optical behaviour of
three novel ethyne-linked donor/acceptor chromophores
,
,**
Yuehua Yuana, Wenfa Zhoub, Maozhong Tiana *, Jiangtao Songa, Yunfeng Baia, Feng Fenga
,
,
***
Yinglin Songb
a School of Chemistry and Chemical Engineering, Shanxi Datong University, Datong, 037009, China
b Department of Physics, Harbin Institute of Technology, Harbin, 150001, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Three novel ethyne-linked donor/acceptor chromophores, BAB, BAS and SAS were synthesized. The results of
BAB in femtosecond and nanosecond Z-scan at 532 nm reveal that the transition from saturable absorption (SA)
to reverse saturable absorption (RSA), while that of BAS and SAS display RSA. The femtosecond Z-scan results for
all compounds at 600 nm–800 nm indicate the obvious RSA, which can be mainly attributed to the two-photon
absorption (TPA). The TA results demonstrate that all compounds display strong excited-state absorption and
long lifetime, and the evolution of TA spectra for these compounds reveals the relaxation process from the local
excited-state (LE) to charge transfer state (CTS). With different conjugation extent and intramolecular charge
transfer (ICT), the nonlinear response and excited-state dynamics of these chromophores could be dramatically
modulated. The threshold of BAS/DCM solution excited at 532 nm with nanosecond pulse was 0.631 J/cm2. The
results indicate that these compounds may be the potential candidates for future application in optical limiting.
Ethyne-linked chromophores
Third-order nonlinear optical properties
Z-Scan
Transient absorption spectra
Excited-state dynamics
1. Introduction
unit as an intensive acceptor have received great deal of interest because
of their high nonlinearities when linked to some donors [30–32]. On the
Nonlinear optical (NLO) materials have stimulated great enthusiasm
of many researchers around the world in recent years owing to their
diverse applications in optical switching [1–3], signal processing [4,5],
optical communication [6,7], optical sensing [8,9], and optical
computing [10,11], and so forth. Compared to their inorganic counter-
parts, organic materials are more suitable NLO material owing to their
fast nonlinear optical responses [12–14], large third-order optical non-
linearities [15–17], their relatively low cost, ease of fabrication, and
more flexible to optimize the required nonlinear optical property by
altering the functional groups on the organic molecules [18–21]. Thus,
nonlinear optical properties in organic systems are under active study
[22–27]. It has been found that strong nonlinearities in organic mate-
other hand, thiophene is one of the most widely used electron donors for
π
-conjugated systems due to their electrical properties, environmental
stability, and non-linear optical properties [33–35]. Motivated by this,
we synthesized three new molecules based on thiophene and 2,1,3-ben-
zothiadiazole, in which the donor moieties consisted of the ((2-ethyl-
hexyl)oxy)benzene and thiophene units, while the benzothiadiazole
acted as the electron acceptor. In this paper, three new molecules were
fabricated, which can be easily obtained via Sonogashira crosscoupling
and protection/deprotection technique of functional groups such as a
terminal ethyne. The characterization of three new compounds and their
photophysical properties were analyzed. Their nonlinear absorption
(NLA) responses were investigated by means of femtosecond and
nanosecond open aperture Z-scan. The evolutions of their excited-state
dynamics for these compounds were analyzed via the femtosecond
transient absorption (TA) measurements. The threshold of BAS/DCM
solution was obtained through the optical limiting (OL) experiment. Our
results indicate that all compounds are excellent NLO materials and the
rials usually arise from intensive
metric polarizability in this kind of material [28]. In general, the
strength of donor and acceptor, nature of the -conjugated spacers have
significant effects on the nonlinear properties of the molecules [29].
Recently -conjugated molecules bearing the 2,1,3-benzothiadiazole
π-electron delocalization and asym-
π
π
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: tmzhong2002@163.com (M. Tian), feng-feng64@263.net (F. Feng), ylsong@hit.edu.cn (Y. Song).
https://doi.org/10.1016/j.dyepig.2021.109235
Received 13 December 2020; Received in revised form 3 February 2021; Accepted 9 February 2021
Available online 16 February 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
conjugation extent and/or intramolecular charge transfer (ICT) can
modulate their NLO response.
bromothiophene (1.29 g, 5.0 mmol), compound 3 (1.2 g, 5.0 mmol), Pd
(PPh3)2Cl2 (50 mg, 17.4 mmol), and cuprous iodide (25 mg, 0.11 mmol)
was suspended in dry tetrahydrofuran (30 mL) and triethylamine (30
mL), and the resulting solution was stirred and heated at 60 ◦C for 10 h.
The solvent was evaporated and the crude product chromatographed
directly with dichloromethane: light petroleum (1:80), affording a yel-
low solid 8 (1.43 g, 73%). To a solution of compound 8 (1.43 g, 3.6
mmol) in the mixture of THF and methanol was added 497 mg of K2CO3.
The reaction mixture was stirred for 1 h at room temperature and the
solvent and K2CO3 were removed. The residue was purified by column
chromatography over silica using dichloromethane: light petroleum
(1:80) mixtures as eluent to give compound 9 (956 mg, 78%). 1H NMR
(400 MHz, CDCl3) δ 7.46 (d, J = 8.8 Hz, 2H), 7.16 (d, J = 3.8 Hz, 1H),
7.08 (d, J = 3.8 Hz, 1H), 6.89 (d, J = 8.8 Hz, 2H), 3.97–3.77 (m, 2H),
3.38 (s, 1H), 1.75 (m, 1H), 1.58–1.39 (m, 4H), 1.38–1.31 (m, 4H),
0.95–0.91 (m, 6H). 13C NMR (100 MHz, CDCl3) δ 160.0, 133.1, 133.1,
131.0, 125.8, 122.7, 114.8, 114.2, 94.4, 82.1, 80.8, 77.5, 70.7, 39.4,
30.6, 29.2, 23.9, 23.2, 14.2, 11.2. MALDI-TOF Calcd for C22H24OS [M]
336.1548; Found 336.1544.
2. Experimental
2.1. Materials and instruments
All reagents were obtained from commercial suppliers, and were of
analytical reagent grade, and used without further purification. Solvents
were distilled by using standard methods. All reactions were performed
under an argon atmosphere. TLC analysis was performed on silica gel
plates and flash column chromatography was conducted over silica gel
(mesh 200–300), both of which were obtained from the Qingdao Ocean
Chemicals.
NMR spectra in CDCl3 solution were recorded on Bruker ARX400
1
spectrometer operating at 400 MHz for H and 100 MHz for 13C. Tet-
ramethylsilane (TMS) was used as the internal standard and chemical
shifts (δ) are given in ppm relative to TMS. Matrix-assisted laser
desorption/ionization reflectron time-of-flight (MALDI-TOF) mass
spectrometry was performed on a BrukerBiflex III mass spectrometer.
Fluorescence emission spectra were recorded at room temperature with
a HITACHI F2500 spectrophotometer with a 1 cm standard quartz cell.
UV–vis absorption spectra were obtained on PerkinElmer Lambda 35
UV/Vis spectrophotometer.
Synthesis of compound BAB. A mixture of compound 6 (400 mg,
1.03 mol), PdCl2(PPh3)2 (30 mg, 0.042 mmol) and CuI (15 mg, 0.079
mmol) was suspended in dry tetrahydrofuran (20 mL) and triethylamine
(20 mL). The resulting mixture was stirred for 8 h and worked up. The
solvent was evaporated and the crude product was purified by column
chromatography over silica using light petroleum:EtOAc (10:1) mix-
tures as eluent to give pure BAB as a red solid (263 mg, 66%). 1H NMR
(400 MHz, CDCl3) δ 7.84 (d, J = 7.4 Hz, 2H), 7.74 (d, J = 7.4 Hz, 2H),
7.60 (d, J = 8.6 Hz, 4H), 6.92 (d, J = 8.6 Hz, 4H), 3.89 (d, J = 5.8 Hz,
4H), 1.74 (dd, J = 12.0, 5.8 Hz, 2H), 1.57–1.38 (m, 8H), 1.36–1.33 (m,
8H), 0.96–0.90 (m, 12H). 13C NMR (100 MHz, CDCl3) δ 160.5, 154.8,
154.4, 134.5, 133.8, 131.8, 119.4, 114.9, 114.2, 99.6, 84.5, 81.30, 80.5,
70.8, 39.5, 30.6, 29.2, 24.0, 23.2, 14.2, 11.2. FT-IR (thin film on KCl,
cmꢀ 1): 3444, 2963, 2926, 2856, 2195, 1598, 1535, 1510, 1460, 1365,
1290, 1251, 1162, 1030, 886, 827, 631, 536. MALDI-TOF Calcd for
2.2. Synthesis and characterization
Compound 1 [36], compound 2 [37], compound 4 [38] and com-
pound 7 [39] were synthesized according to the literature. All physical
data agreed with reported literature values.
Synthesis of compound 5. A mixture of compound 2 (3.6 g, 12.5
mmol), toluene (100 mL) and potassium hydroxide (700 mg, 12.5
mmol) was heated under argon in an oil bath held at 110 ◦C for 30 min.
The mixture was allowed to cool to room temperature and the solvent
was removed. The residue was purified by column chromatography over
silica using dichloromethane: light petroleum (1:80) mixtures as eluent
to give compound 3 as an oil (2.3 g, 80%). A mixture of compound 3
(1.2 g, 5.0 mmol), compound 4 (1.5 g, 5.0 mmol), Pd(PPh3)2Cl2 (50 mg,
17.4 mmol), and cuprous iodide (25 mg, 0.11 mmol) was suspended in
dry tetrahydrofuran (30 mL) and triethylamine (30 mL), and the
resulting solution was stirred and heated at 60 ◦C for 10 h. The solvent
was evaporated and the crude product was separated by column chro-
matography with elution in dichloromethane/light petroleum (1:100) to
afford a yellow solid 5 (1.5 g, 70%). 1H NMR (400 M, CDCl3, 298K) δ
7.69 (d, J = 7.4 Hz, 1H), 7.65 (d, J = 7.4 Hz, 1H), 7.58 (d, J = 8.8 Hz,
2H), 6.90 (d, J = 8.8 Hz, 2H), 3.87 (d, J = 6.1 Hz, 2H), 3.01 (s, 1H),
1.68–1.78 (s, 7H), 1.55–1.38 (m, 4H), 1.28–1.36 (m, 4H), 1.00–0.86 (m,
6H). 13C NMR (100 MHz, CDCl3, 298 K) δ160.3, 154.5, 154.4, 133.6,
132.7, 131.9, 117.9, 116.1, 114.8, 114.3, 102.0, 98.2, 84.2, 78.2, 70.8,
65.8, 39.4, 31.5, 30.6, 29.2, 23.9, 23.1, 14.2, 11.2. MALDI-TOF Calcd for
C
48H46N4O2S2, [M] 774.3062; found 774.3063.
Synthesis of compound BAS. To a solution of compound 6 (400 mg,
1.03 mmol) in acetone (20 mL) was added NBS (195 mg, 1.1 mmol) and
AgNO3 (30 mg, 0.18 mmol) at room temperature with magnetic stirring.
After 2 h, the reaction mixture was diluted with hexanes (20 mL) and
filtered off the crystals formed. The filtrate was concentrated under
reduced pressure and passed through a pad of silica gel using hexane as
an eluent. The filtrate was collected and evaporated under reduced
pressure to afford a pure compound 10 (393 mg, 82%). To a stirred
solution of compound 10 (300 mg, 0.64 mmol) and compound 9 (215
mg, 0.64 mmol) in tetrahydrofuran (20 mL) and Et3N (20 mL) were
added PdCl2(PPh3)2 (30 mg, 0.04 mmol) and CuI (15 mg, 0.08 mmol)
under an argon flow at room temperature. The solution was stirred at
room temperature for 8 h. The solvent was evaporated under reduced
pressure, and the mixture was purified by column chromatography over
silica with light petroleum:EtOAc (6:1) mixtures as eluent to obtain pure
BAS as a brown solid (133 mg, 29%). 1H NMR (400 MHz, CDCl3) δ 7.50
(d, J = 8.8 Hz, 4H), 7.24 (d, J = 3.8 Hz, 2H), 7.11 (d, J = 3.8 Hz, 2H),
6.91 (d, J = 8.8 Hz, 4H), 3.87 (d, J = 5.9 Hz, 4H), 1.81–1.75 (m, 2H),
1.57–1.39 (m, 16H), 1.02–0.99 (m, 12H). 13C NMR (100 MHz, CDCl3) δ
160.4, 160.0, 154.7, 154.3, 134.7, 133.9, 133.6, 133.1, 131.7, 131.2,
127.4, 122.1, 119.0, 114.9, 114.7, 114.1, 114.0, 99.4, 95.4, 84.3, 81.2,
80.7, 79.9, 78.7, 77.8, 70.7, 39.3, 30.5, 29.1, 23.9, 23.0, 14.1, 11.1. FT-
IR (thin film on KCl, cmꢀ 1): 3444, 3083, 3038, 2964, 2926, 2856, 2540,
2209, 2142, 1889, 1787, 1604, 1567, 1529, 1501, 1460, 1388, 1295,
1251, 1174, 1141, 1105, 1036, 998, 833, 732, 682, 612, 534. MALDI-
TOF Calcd for C46H46N2O2S2, [M] 722.3001; found 722.3001.
C
27H30N2O2S [M] 446.2028; Found 446.2034.
Synthesis of compound 6. A mixture of compound 5 (1.2 g, 2.7
mmol), toluene (50 mL) and potassium hydroxide (151.2 mg, 2.7 mmol)
was heated under argon in an oil bath held at 110 ◦C for 30 min. The
mixture was allowed to cool to room temperature and the solvent was
removed. The residue was purified by column chromatography over
silica using dichloromethane: light petroleum (1:60) mixtures as eluent
to give 6 as a light brown oil (873 mg, 87%). 1H NMR (400 MHz, CDCl3,
298 K) δ 7.76 (d, J = 7.4 Hz, 1H), 7.71 (d, J = 7.4 Hz, 1H), 7.59 (d, J =
8.8 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 3.87 (d, J = 6.0 Hz, 2H), 3.66 (s,
1H), 1.72–1.75 (m, 1H), 1.56–1.38 (m, 4H), 1.38–1.23 (m, 4H),
0.89–0.95 (m, 6H). 13C NMR (100 MHz, CDCl3, 298 K) δ160.4, 154.7,
154.3, 133.7, 131.7, 118.8, 115.3, 114.8, 114.2, 98.6, 84.8, 84.1, 79.4,
70.8, 39.4, 30.6, 29.2, 23.9, 23.1, 14.2, 11.2. MALDI-TOF Calcd for
Synthesis of compound SAS. A mixture of compound 9 (400 mg,
1.19 mmol), PdCl2(PPh3)2 (30 mg, 0.042 mmol) and CuI (15 mg, 0.079
mmol) was suspended in dry tetrahydrofuran (20 mL) and triethylamine
(20 mL). The resulting mixture was stirred for 8 h and worked up. The
solvent was evaporated and the crude product was purified by column
C
24H24N2OS [M] 388.1609; Found 388.1603.
Synthesis of compound 9. A mixture of 2-(trimethylsilyl)ethynyl-5-
2

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Scheme 1. Synthesis of intermediates. Reagents and conditions: (a) K2CO3, ACN, 2-ethyl-1-bromohexane, reflux, 91% yield; (b), (d) 2-methylbut-3-yn-2-ol,
PdCl2(PPh3)2, THF/Et3N, CuI. Compound 2: 53% yield. Compound 4: 35% yield; (c) and (f) KOH, toluene, reflux. Compound 3: 80% yield. Compound 6: 87%
yield; (e) PdCl2(PPh3)2, THF/Et3N, CuI, 70% yield.
chromatography over silica using light petroleum:EtOAc (8:1) mixtures
as eluent to offer pure SAS as a yellow green solid (313 mg, 79%). 1H
NMR (400 MHz, CDCl3) δ 7.44 (d, J = 8.7 Hz, 4H), 7.21 (d, J = 3.8 Hz,
2H), 7.08 (d, J = 3.8 Hz, 2H), 6.87 (d, J = 8.7 Hz, 4H), 3.86 (d, J = 5.8
Hz, 4H), 1.76–1.70 (m, 2H), 1.56–1.24 (m, 16H), 1.01–0.80 (m, 12H).
13C NMR (100 MHz, CDCl3) δ 160.0, 134.5, 133.1, 131.2, 127.1, 122.3,
114.7, 114.0, 95.3, 80.8, 78.6, 70.7, 39.4, 30.5, 29.1, 23.9, 23.1, 14.1,
11.1. FT-IR (thin film on KCl, cmꢀ 1): 3437, 3096, 2952, 2926, 2856,
2540, 2206, 2135, 1889, 1781, 1604, 1566, 1522, 1503, 1383, 1295,
1250, 1174, 1143, 1104, 1029, 998, 833, 801, 732, 682, 612, 543.
MALDI-TOF Calcd for C44H46O2S2, [M] 670.2939; found 670.2944.
response function was estimated to be around 250 fs. The wavelength of
pump beam was tuned to 400 nm.2.4. DFT calculation.
All the density functional theory (DFT) calculations were performed
with the Gaussian 16 program [41] package with Becke’s
three-parameter hybrid functional and Lee-Yang-Parr’s gradient cor-
rected correlation functional (B3LYP) level of theory using the 6-311G
basis set.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes of intermediates and target compounds are
shown in Schemes 1 and Scheme 2. Sonogashira crosscoupling were
carried out to offer BAB, BAS, and SAS in a yield of 66%, 29%, 79%，
respectively (see Experimental section for details). These new com-
pounds were characterized by the 1H NMR, 13C NMR (see Supplemen-
tary data) and mass spectrometry.
2.3. Nonlinear optical experiments
The NLAs were measured by open aperture Z-scan method [40] with
multiple laser sources: an optical parametric amplifier (OPA, Light
Conversion, ORPHEUS) pumped by a mode-locked Yb: KGW-based fiber
laser (1030 nm, 190 fs, 20 Hz). The excitation wavelengths were tuned
from 532 nm to 800 nm 4 ns (FWHM) pulse of 532 nm were extracted
from a Q-switched Nd: YAG laser (Surelite II, Continuum). The low
repetition rate (10 Hz in nanosecond pulse) can effectively eliminate the
influence of thermally induced nonlinearities. The spatial and temporal
distributions of the pulses were all nearly Gaussian profiles. These
compounds dissolved in dichloromethane (DCM) solvent were con-
tained in 2 mm quartz cell for measurement, respectively. The concen-
tration of BAS/DCM and SAS/DCM was 6.92 × 10ꢀ 4 mol/L and 7.45 ×
10ꢀ 4 mol/L, respectively, while that of BAB/DCM was 3.23 × 10ꢀ 4
mol/L. The experimental setup used for OL measurement was same as
that for Z-scan. The sample with concentration of 3.46 × 10ꢀ 4 mol/L in
5 mm quartz cell was placed at focus position for measurement. The
linear transmittance of BAS/DCM in OL experiment was 72%.
3.2. Photophysical properties
The UV–vis absorption and fluorescence emission spectra of BAB,
BAS and SAS are shown in Fig. 1 (a) and (b), respectively. The con-
centration of compounds BAB, BAS and SAS are all fixed at 1 × 10ꢀ 6
mol/L in DCM. As depicted in Fig. 1 (a), the main absorption peaks of
BAB are located at 315 and 462 nm, respectively. The BAS exhibited
three distinct absorption peaks in the range of 301–338, 338–396 and
396–540 nm. Wherein higher energy peaks observed at 315 nm for BAB
and at 328 and 358 nm for BAS, correspond to the π–π* electronic
transitions in the molecules, while a band at lower energy region
(358–552 nm) for BAB and a band (396–540 nm) for BAS correspond to
Since the TA spectrum experiment records the evolution of absorp-
tive spectrum of samples within 1.7 ns delay time after excitation, the
ultrafast relaxing processes of the samples could be revealed. The laser
source used in this experiment is same as that used in femtosecond open-
aperture Z-scan. A supercontinuum white light generating from funda-
mental frequency (1030 nm) passing through sapphire crystal was used
as probe beam while the output wavelength from OPA was used as pump
beam. The repetition rate of laser pulse was 6 kHz. The instrument
the ICT, leading to the extension of π–conjugation. In the case of
molecule SAS the observed peak at 392 nm could be attributed to π–π*
electronic excitations.
As shown in Fig. 1 (b), the fluorescence spectra of BAB, BAS and SAS
showed emission bands at about 534 nm, 558 nm and 530 nm respec-
tively when they were selectively excited at 460 nm, and the peak of the
spectrum of BAS also has a red shift compared with that of SAS corre-
sponding the UV–vis absorption spectrum of them.
3

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Scheme 2. Synthesis of compounds BAB, BAS and SAS. Reagents and conditions: (a) PdCl2(PPh3)2, trimethylsilylacetylene, Et3N, CuI, 29% yield; (b) Compound 3,
PdCl2(PPh3)2, Et3N, CuI, 73% yield; (c) K2CO3, MeOH/THF, 78% yield; (d) NBS, AgNO3 and acetone, 82% yield; (e), (f), (g) PdCl2(PPh3)2, Et3N, CuI. BAB: 66% yield,
BAS: 29% yield, SAS: 79% yield.
3.3. Femtosecond open aperture Z-scan and transient absorption spectrum
the input intensity increases. This phenomenon is commonly referred to
as the saturable absorption (SA). On the contrary, when sample excited
by laser pulses displays a unimodal curve with its valley locates at zero
position (focus point), it means the transmittance of the sample de-
creases when the input intensity increases. This phenomenon is
commonly referred to as the reverse saturable absorption (RSA). The
observed NLA of compounds was evaluated to originate from the solu-
tion molecule because the pure solvent displayed no NLA in the exper-
imental conditions.
Femtosecond open aperture Z-scan was conducted to investigate the
ultrafast NLA of three compounds. The excitation wavelengths were
tuned from 532 nm to 800 nm. The linear transmittances for BAS and
BAB at 532 nm are 72% and 73%, respectively, while they exhibit high
linear transmittance (BAS ≥ 97%, BAB ≥ 97%) at 600 nm–800 nm. SAS
displays high linear transmittance (SAS ≥ 98%) at all measurement
wavelengths. It is widely known that when sample excited by laser
pulses displays a unimodal curve with its peak locates at zero position
(focus point), it means the transmittance of the sample increases when
The experimental results of 532 nm for three compounds are dis-
cussed first, as shown in Fig. 2. With the increase of input intensity, it
4

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Fig. 1. (a) UV–Vis absorption spectra of compounds BAB, BAS and SAS in DCM. (b) The fluorescence spectra of compounds BAB, BAS and SAS in DCM. [BAB] =
[BAS]¼ [SAS] = 1 M.
μ
Fig. 2. Open aperture Z-scan curves of BAB/DCM (a), BAS/DCM (b) and SAS/DCM (c) at 532 nm under 190 fs excitation. The circles are the experimental results.
The solid lines are the theoretical fitting curves.
RSA. When excited at 532 nm (2.33 eV), electrons in the ground state
Table 1
will be excited to the excited state. The transition energies (between S0
Parameters of femtosecond open-aperture Z-scan experiments at 532 nm.
and S1) of the three compounds (BAB, BAS, SAS) were calculated to be
Sample
Wavelength (nm)
I0 (GW/cm2)
Is (GW/cm2)
α2(10ꢀ 2 cm/GW)
2.275, 2.271, and 2.64 eV. Hence, we consider that TPA is the main
mechanism for SAS at 532 nm. However, since 532 nm locates at the
edge of the absorption band of BAB and BAS (see UV–Vis), one-photon
induced excited-state absorption is considered as the mechanism of
nonlinear absorption for them. The transition from SA to RSA of BAB/
DCM could be modeled via saturable intensity (Is) and effective
nonlinear absorptive coefficient (α2), respectively [42]. The total ab-
BAB
532
4.05 ± 0.42
20.2 ± 1.5
40.5 ± 2.1
20.2 ± 1.5
40.5 ± 2.1
20.2 ± 1.5
40.5 ± 2.1
6.1 ± 0.4
0
12.8 ± 1.3
13.2 ± 1.1
7.42 ± 0.63
8.16 ± 0.79
5.34 ± 0.54
6.74 ± 0.65
BAS
SAS
/
/
sorption (α) of the sample could be expressed as:
1
can be seen that the NLA of SAS and BAS display RSA while the NLA of
BAB exhibits the characteristic of transforming from SA to RSA. When
excited by femtosecond pulse, two-photon absorption (TPA) and
excited-state absorption (ESA) are commonly mentioned as the causes of
α
=
α
+ α2I
/
01 + I
Is
Where α0 represents linear absorptive coefficient and I is the input
Fig. 3. Z-scan open aperture curves of BAB/DCM (a), BAS/DCM (b) and SAS/DCM (c) at various wavelengths under 190 fs excitation. The circles are the exper-
imental data. The solid lines are the theoretical fitting.
5

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
linear transmittance was measured at 600 nm–800 nm and the femto-
second laser pulse was used as light source. As such, the measured NLA is
mainly related to TPA. The experimental results are fitted by Sheik
Bahae’s theory [40]. The parameters are summarized in Table 2. The
experimental error was about 10%.
Table 2
Parameters of NLA at different wavelengths extracted from femtosecond open-
aperture Z-scan.
Wavelength
(nm)
I0 (GW/
BAB
BAS
SAS
cm2)
α2(10ꢀ 2 cm/
α2(10ꢀ 2 cm/
α2(10ꢀ 2 cm/
It is known that organic molecules may exhibit both TPA and TPA
induced ESA processes under off-resonant excitation [43,44]. Herein,
the TA measurement was conducted to further interpret the TA dynamic
changes of the molecules under femtosecond laser excitation. The evo-
lution associated difference spectra (EADS) for compounds are plotted in
Fig. 4. The changes in the absorption intensity at specific wavelengths
obtained from transient absorption spectra experiments are represented
by changes in optical density (ΔOD), which can be expressed as follows:
GW)
GW)
GW)
600
700
800
47.8 ±
2.6
3.92 ± 0.36
4.74 ± 0.49
1.52 ± 0.16
1.94 ± 0.19
1.86 ± 0.18
2.01 ± 0.22
3.28 ± 0.29
3.86 ± 0.37
1.33 ± 0.15
1.84 ± 0.18
1.58 ± 0.17
1.64 ± 0.16
1.93 ± 0.18
2.33 ± 0.22
1.45 + 0.17
1.77 ± 0.19
/
63.7 ±
3.3
46.8 ±
2.4
70.2 ±
3.5
53.7 ±
2.7
ΔOD = ꢀ lg(T/T0)
71.6 ±
3.5
/
Here, T is the transmittance of the sample after pumping, and T0 is
the linear transmittance of the sample. The positive signal represents
RSA, and the negative signal represents SA. For BAB/DCM (Fig. 4(a)&
(d)), two signal bands appear near the zero delay, namely, a negative
signal band centered at 500 nm and a positive signal band between 560
nm and 755 nm. Then the amplitude of spectra gradually increases. The
positive signal reaches its maximum value near 0.5 ps. The absorption
spectrum evolves into a negative signal band of 470 nm–595 nm and a
positive signal band of 595 nm–755 nm. As the delay time become
longer，The amplitude of the positive signal band of the absorption
spectrum decreases while the amplitude of the negative signal band
increases. In addition, the absorption bands exhibit the red shift. After
more than a dozen of picosecond, the spectrum evolves into a negative
signal band centered at 560 nm and a positive signal band in the range
intensity. The results of BAS/DCM and BAB/DCM can be fitted by Sheik
Bahae’s theory [40]. The numerical fitting results of open aperture
Z-scan at 532 nm are list in Table 1.
As shown in Fig. 3, all compounds exhibit obvious RSA at 600
nm–800 nm. The photon energies of excitation were calculated to be
2.067, 1.771, and 1.55 eV, respectively. Electrons in the ground state
will be excited to the excited state, which has the closest energy when
simultaneously absorbing two or more photons. The transition energies
(between S0 and S1) of the three compounds (BAB, BAS, SAS) were
calculated to be 2.275, 2.271, and 2.695 eV, respectively. So, it can be
confirmed that TPA dominates the RSA at 600–800 nm. In addition, high
Fig. 4. The TA spectrum of (a) BAB/DCM, (b) BAS/DCM and (c) SAS/DCM. Several absorptive spectra of (d) BAB/DCM, (e) BAS/DCM and (f) SAS/DCM at selected
delay time are displayed.
6

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Fig. 5. The kinetic traces of several selected wavelengths for (a) BAB/DCM, (b) BAS/DCM and (c) SAS/DCM following femtosecond laser excitation at 400 nm. Inset
graphs show early stage dynamics of the same (within the first 20 ps). Circles are experimental data. The solid lines represent theoretical fitting.
from 625 nm to 755 nm. Finally，the amplitude of spectra for BAB/
DCM attenuated according to this waveform. The negative signal band
ranging from 470 nm to 625 nm can be attributed to ground state
bleaching (centering at 470 nm) and stimulated emission (SE) (centering
at 560 nm). The evolution processes of the absorption spectra for BAB/
DCM show that the particles under photoexcitation have different
relaxation processes. Through global analysis [45] (see Fig. 5(a)), we
obtain four dynamic processes and the lifetimes are about 0.25 ps, 2.1
ps, ~587.8 ps and ~7.8 ns, respectively. As will be discussed below
regarding the quantum chemical calculations, the HOMO-LUMO tran-
sitions of all molecules are clearly accompanied by ICT, which is ex-
pected to activate the NLO response. We consider that the optical
nonlinearity originates from the layout of the excited-state particles to
the charge-transfer state (CTS) [46]. The first content is unable to be
distinguished as the time resolution of our experiment is 0.25 ps. So we
assign the ultrafast process of 0.25 ps to the internal conversion in the
local excited state (LE). The next component with a 2.1 ps lifetime
corresponds to the vibrational relaxation of LE. The last two lifetimes are
the ICT process of particles from LE to the CTS (~587.8 ps) and the
process of relaxation from CTS to the ground state (~7.8 ns).
nm band. After that, the amplitude of spectra for BAS/DCM attenuated
according to this waveform. After dozens of picoseconds, it evolves into
a negative signal band in the range of 470 nm–495 nm and a positive
signal band centered at 520 nm, 610 nm and greater than 755 nm,
respectively. As the increase of delay time, the amplitude of the ab-
sorption spectrum gradually increases. The absorption spectrum evolves
into a negative signal band of 470 nm–495 nm and a positive signal band
centered at 590 nm near 1700 ps. Due to the limitation of experimental
conditions, the change of absorption spectrum with a delay time greater
than 1700 ps cannot be accurately measured. The spectral changes of
BAS/DCM mean that ESA does not originated from the same excited-
state [47,48]. The negative signal before 500 nm can be owing to
ground state bleaching while the generation and disappearance of the
negative signal ranging from 500 nm to 590 nm could be caused by the
combination of SE and ESA. Through global analysis (see Fig. 5(b)), we
obtain four similar dynamic processes and the lifetimes are about 0.25
ps, 5.6 ps, ~956.8 ps and ~12 ns, respectively.
For SAS/DCM (As shown in Fig. 4(c)&(f)), a negative signal band of
470 nm–500 nm and a positive signal band of 500 nm–755 nm appear
near zero delay time. As the delay time increases, the amplitude of the
absorption spectrum gradually increases. Near 0.3 ps, the amplitude of
absorption spectrum is stable, which evolved into a negative signal band
centered at 485 nm and a positive signal band centered at 745 nm. The
amplitude of spectra for SAS attenuated according to this waveform. But
the spectral form changes after dozens of picoseconds, evolving into a
positive signal band centered at 675 nm and 745 nm while the negative
signal band gradually weakens. The absorption peak of positive signal
For BAS/DCM(Fig. 4(b)&(e)), a negative signal band centered at 480
nm and a positive signal band centered at 650 nm appear near zero
delay. Then the amplitude of the absorption spectrum gradually in-
creases, and the absorption peak displays red-shifted. The amplitude of
the absorption spectrum reaches the maximum near 0.5 ps. The ab-
sorption spectrum evolves into a negative signal band in the range from
470 nm to 585 nm and a positive signal in the range from 585 nm to 755
7

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Fig. 6. Open aperture Z-scan curves of BAB/DCM (a), BAS/DCM (b) and SAS/DCM (c) at 532 nm under 4 ns excitation. The circles are the experimental data. The
solid lines are the theoretical fitting.
relevant to the conjugation extent and ICT.
Table 3
Open aperture Z-scan data of 4 ns for BAB/DCM, BAS/DCM and SAS/DCM
3.4. Nanosecond open aperture Z-scan
solutions.
Sample
Wavelength
(nm)
I0 (10ꢀ 2 GW/
Is (10ꢀ 2 GW/
α2(102 cm/
cm2)
cm2)
GW)
To investigate the NLO properties about three samples under
different pulse excitation, we perform Z-scan experiments using 4 ns
laser pulses at 532 nm to evaluate the NLA response of three compounds.
The Z-scan curves of the three compounds at different incident in-
tensities are shown in Fig. 6. The results of nonlinear absorption pa-
rameters measured are summarized in Table 3. It can be seen that there
is no obvious response found in Z-scan for SAS/DCM under 4 ns ex-
periments indicates neglectable nonlinear absorption at 532 nm while
the NLA of BAB/DCM display the transition from SA to RSA and that of
BAS/DCM exhibit RSA. As discussed in femtosecond Z-scan, the mech-
anism of RSA for BAB/DCM and BAS/DCM was one-photon induced
excited-state absorption while that for SAS/DCM was mainly TPA. When
excited with nanosecond laser, ground state electrons are continuously
promoted to excited state during the nanoseconds laser pulse, more
electrons are maintained in excited-state comparing to femtosecond
case, thereby ESA is enhanced. It should be noted that TPA needs certain
intensity to occur, which may be the reasonable explanation that SAS/
DCM has no obvious nonlinear response. Since the samples used in ex-
periments are solutions and the solution is uniformly distributed as well
as the molecules are chaotic, they have no directionality. Under the
excitation of low repetition (10 Hz) laser without external force (such as
BAB
BAS
SAS
532
1.4 ± 0.20
26.5 ± 2.12
1.4 ± 0.20
26.5 ± 2.12
26.5 ± 2.12
1.6 ± 0.3
0
0.323 ± 0.33
3.68 ± 0.52
2.42 ± 0.25
/
/
/
band stabilizes at 670 nm near 100 ps, and the negative signal band
disappears. Finally, the amplitude of spectra for SAS/DCM attenuated
according to this waveform. Similarly, the generation and disappear-
ance of the negative signal is caused by the combination of ground state
bleaching and ESA. Through global analysis (see Fig. 5 (c)), four similar
dynamic processes are obtained and the lifetimes are about 0.25 ps, 1.3
ps, ~40.5 ps and ~5.8 ns, respectively. Unlike BAB/DCM and BAS/
DCM, there is no SE in the TA spectrum of SAS/DCM, and the intensity of
ESA of ASA/DCM is greater than that of BAB/DCM and BAS/DCM in the
visible range. However, the lifetime of excited-state for BAS/DCM is the
longest among the three samples. Obviously, the results of TA spectra for
three derivatives imply that the excited-state relaxation processes
change dramatically with the molecule structure change, which may be
Fig. 7. DFT optimized structure and distribution of frontier molecular orbitals in BAB, BAB and SAS (in the ball-and-stick representation, carbon, oxygen, nitrogen
and sulfur atoms are colored in gray, red, blue and yellow, respectively).
8

Y. Yuan et al.
Dyes and Pigments 188 (2021) 109235
Table 4
Comparison with reported optical limiting materials.
Sample
Pulse
Wavelength
(nm)
T0
Threshold
(J/cm2)
Ref.
(%)
BAS/DCM
4 ns
7 ns
532
72
0.631
This
work
49
0.2 wt% c-dot
doped LC
532
–
1.23
GO with silver NPs
H-shaped
5 ns
7 ns
532
532
–
1.56
50
51
70–74
>3.14
Thiophene-based
oligomers
excited-state absorption.
4. Conclusions
In summary, three ethyne-linked donor/acceptor compounds, BAB,
BAS and SAS, were designed and synthesized based on the large
nonlinear response of chromophores and the effective electron transfer
of structure. Their third-order nonlinear optical properties were inves-
tigated using Z-scan technique and transient absorption spectra at
femtosecond regime. Results show that the NLA of BAB exhibits the
characteristic of transforming from SA to RSA，while that of SAS and
BAS only display RSA. The excited-state relaxation processes change
dramatically with the molecule structure change, which may be relevant
to the conjugation extent and ICT. In nanosecond pulse, the threshold of
BAS/DCM was 0.631 J/cm2. These results illustrate that these chro-
mophores may be potential candidates for future application in optical
limiting.
Fig. 8. The OL performance of BAS/DCM as the function of the input fluence at
532 nm with nanosecond excitation.
a magnetic field, electric field, etc), the orientation of the molecule
cannot be induced. As the result, the contribution of thermal effect can
be neglected in the above results.
3.5. Quantum chemical calculation
In order to obtain further insight into the differences of the photo-
physical properties of compounds BAB, BAS and SAS, quantum chemi-
cal calculation of density functional theory (DFT) at b3lyp/6–311+g(d,
p) level has been performed for them. The frontier molecular orbitals
distributions along with optimized structures of BAB, BAS and SAS were
displayed in Fig. 7.
Author statement
Under supervision by Maozhong Tian, Yinglin Song, and Feng Feng,
Yuehua Yuan, Jiangtao Song and Maozhong Tian performed the syn-
thesis of all compounds and data analysis. Wenfa Zhou and Yinglin Song
investigated nonlinear optical (NLO) properties of three compounds and
analysis. Yunfeng Bai performed UV–vis absorption and fluorescence
emission spectra. Feng Feng performed DFT calculation. All authors read
and contributed to the manuscript.
Obviously, the distribution of the highest occupied molecular orbital
(HOMO) is highly localized on the molecular backbone connected by the
alkyne group for three compounds, while that of lowest unoccupied
molecular orbital (LUMO) is mainly drifted toward the benzothiazole
moiety for BAB and BAS, and drifted toward the thiophene for SAS. This
reflects that the ICT comes from the ((2-ethylhexyl)oxy)benzene subunit
to benzothiazole one via the alkyne bridge. The HOMO-LUMO gap (2.29
eV) of compound BAB and that (2.33 eV) of compound BAS are less than
that of compound SAS 2.77 eV, which prove that both BAB and BAS
exhibit larger conjugation extent than SAS. The charge-transfer in-
teractions between donor and acceptor moieties are relatively strong
and has higher hyperpolarizabilities [13]. This is consistent with the
above Z-Scan experimental results and interprets that the remarkable
NLO responses under photoexcitation may be related to the conjugation
extent and the excited absorption from charge transfer for BAB and BAS.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors thank the financial support of 1331 project of Shanxi
Province, the Applied basic Research Program of Shanxi Province
(No.201801D121039), Key Research and Development Project of
Datong (No. 2019019), the Science Research Foundation of Shanxi
Datong University (No.2018K1). Research topic of postgraduate edu-
cation reform (No.2020YJJ299 for Shanxi Province, 20JG03 for Shanxi
Datong University).
3.6. Optical limiting
From above analysis, BAS/DCM exhibits superior RSA in nanosecond
pulse, which means it may display excellent OL response. To evaluate
the OL property of BAS/DCM, its OL experiments was conducted under
the excitation of nanosecond pulse. Fig. 8 displays the variation of the
transmittance of BAS/DCM as a function of the input laser fluence at
excitation wavelength of 532 nm, and the transmittance of BAS/DCM
decrease with increasing input fluence. The threshold of OL defined as
the input fluence when the transmittance decreases to half of the initial
value is one of properties of the materials. The OL threshold of BAS/
DCM is 0.631 J/cm2, which is superior to reported materials [49–51].
The results are summarized in Table 4. Based on analysis in Z-scan, the
OL property of BAS/DCM can be attributed to one-photon induced
Appendix B. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109235.
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