Optical limiting effects of cyano substituted distyrylbenzene derivatives

Dyes and Pigments 134 (2016) 368e374

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Optical limiting effects of cyano substituted distyrylbenzene derivatives Juan Du a, c, Na Xie b, Xiaodong Wang b, **, Li Sun b, Yuxia Zhao a, *, Feipeng Wu a a

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, P. O. Box 919-988, Mianyang 621900, PR China c Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 June 2016 Received in revised form 24 July 2016 Accepted 27 July 2016 Available online 28 July 2016

Two liquid cyano substituted distyrylbenzene derivatives (P2 and P4) have been synthesized by incorporating two tetraethylene glycol groups into the prototype scaffolds of 2,5-bis(4-(diethylamino)styryl) terephthalonitrile (P1) and 2,2'-(1,4-phenylene)bis(3-(4-(diethylamino)phenyl)acrylonitrile) (P3), respectively, for increasing their solubilities in liquid and solid substrates. The linear photophysical properties, optical/thermal stabilities and optical limiting behaviors of P1eP4 have been investigated. Results show that both P2 and P4 have significant optical limiting behaviors on 800 nm laser pulses and high thermal stabilities. However, only P4 whose cyano groups substituted on the vinylene bond exhibits good optical stability, while P2 whose cyano groups substituted on the central phenylene ring presents gradual photobleaching under illumination. Through doping P4 into a solid epoxy resin, effective optical limiting devices to 800 nm pulsed laser are fabricated. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Optical limiting Two-photon absorption Cyano substituted distyrylbenzene derivative 800 nm laser Optical/thermal stability

1. Introduction With the fast development and wide application of 800 nm short pulsed lasers in many fields, such as 3D micro-fabrication, high resolution biological fluorescence imaging, and photodynamic therapy, it is urgent to develop optical limiting materials or devices for this wavelength to protect human eyes or optical detectors from intense light-induced damages. Several different mechanisms can lead to optical limiting behaviors, such as reverse saturable absorption (RSA), two-photon absorption (2PA), nonlinear refraction and optically induced scattering [1]. Among them, 2PA is considered as one of the most ideal approaches due to its advantages of nearly 100% transmission at low incident intensity and fast temporal response [2]. Recently, many organic compounds showing strong 2PA have been reported [3e6]. For an organic molecule, theoretically its 2PA cross-section (s2 ) is positively related to the transition dipole moment between the initial state

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.dyepig.2016.07.035 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

and the final state. Thus, generally a molecule with a large p conjugation skeleton and a strong intramolecular charge transfer (ICT) absorption band may achieve a large s2 value [7]. However, the linear ICT absorption band of such a molecule will appear in the long wavelength region. It brings a contradiction to seek a molecule with a large s2 value and a short cutoff wavelength simultaneously. For a selected molecule, a high solubility in liquid or solid substrates is required to fabricate optical limiters with significant optical limiting behaviors. Moreover, a good optical/thermal stability is another key requirement for optical limiting materials. Due to the mutual checks and balances among the above factors, though many 2PA materials were reported up to now, which were rarely investigated for the optical limiting application on 800 nm pulsed laser. Cyano substituted distyrylbenzene derivatives have been reported showing large s2 values around 800 nm [8,9]. However, due to the rigid molecular framework, their solubilities in common organic solvents are very limited. In this work, we selected 2,5bis(4-(diethylamino)styryl)terephthalonitrile (P1) and 2,2'-(1,4phenylene)bis(3-(4-(diethylamino)phenyl)acrylonitrile) (P3) as two prototype scaffolds to synthesize two novel derivatives (P2 and P4) with improved solubilities by incorporating two tetraethylene glycol (TEG) groups into the terminal ethyl groups of P1 and P3, respectively. The efficiency of this strategy has been proven in our

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previous work on improving another compound's solubility without affecting its optical/thermal stability [10]. Furthermore, the effect of cyano substitution sites on optical limiting behaviors and optical/thermal stabilities was investigated. And a significant difference shows in their optical stabilities. Only P4 whose cyano groups substituted on the vinylene bond exhibits good thermal and optical stabilities simultaneously. Solid optical limiters by doping P4 into a solid epoxy resin were fabricated to evaluate the application potential on 800 nm pulsed lasers. 2. Experimental 2.1. Synthesis The molecular structures of studied compounds P1eP4 are illustrated in Fig. 1. Detailed synthetic routes and structure characterization data are presented below (Scheme 1). 4-(ethyl(2-hydroxyethyl)amino)benzaldehyde (1), triethylene glycol p-toluenesulfolnate (2) and 1,4-bis(diethylphosphorylmethyl)-2,5-dicyanobenzene (5) were synthesized according to the literature procedures [11e14]. 4-(Diethylamino)benzaldehyde (4) and 1,4-benzenediacetonenitrile (6) were purchased from TCI (Shanghai) Development Co., Ltd. and used as received. All the other A.R. grade regents and solvents were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd and used after dry with common methods. 1H NMR and 13C NMR were recorded on a Bruker DPX 400 and 100 MHz, reference to TMS or residual Chloroform-d. High-resolution mass spectroscopy (HRMS) was performed on a FTMS-Bruker APEX IV. 2.1.1. 4-(ethyl(2,5,8,11-tetraoxatridecan-13-yl)amino)benzaldehyde (3) Ground potassium hydroxide powder (5.60 g, 0.1 mol) was added to anhydrous tetrahydrofuran, stirred at room temperature for 30 min, the solution of triethylene glycol p-toluenesulfolnate (2) (3.50 g, 11 mmol) and 4-(ethyl(2-hydroxyethyl)amino)benzaldehyde (1) (2.43 g, 10 mmol) in tetrahydrofuran were added to the reaction flask dropwise and successively, the resulting solution stirred at room temperature for 5 days till no 1 could be checked out by TLC. Afterwards, the potassium hydroxide was got rid via centrifugal sedimentation, and tetrahydrofuran was removed on a rotary evaporator, and the crude product was purified by column chromatography on silica gel using petroleum ether/acetone (V/ V ¼ 3/1) as eluent to get flavescent oil, yield 1.86 g (55%). 1H NMR (400 MHz, CDCl3) d 7.24 (t, J ¼ 6.2 Hz, 2H), 6.55 (d, J ¼ 9.0 Hz, 2H), 3.69e3.57 (m, 15H), 3.55 (dd, J ¼ 5.7, 3.6 Hz, 2H), 3.46 (t, J ¼ 6.3 Hz, 2H), 3.38 (s, 3H), 1.13 (t, J ¼ 7.0 Hz, 3H). HRMS (ESI): m/z Calcd. For [MþH]þ: 340.2046, Found: 340.1215. 2.1.2. 2,5-bis(4-(diethylamino)styryl)terephthalonitrile (P1) Under a nitrogen atmosphere, potassium tert-butoxide (0.25 g, 2.2 mmol) was added to a dry tetrahydrofuran solution containing

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1,4-bis(diethylphosphorylmethyl)-2,5-dicyanobenzene (5) (0.428 g, 1 mmol) and 4-(diethylamino)benzaldehyde (4) (0.39 g, 2.2 mmol). After 30 min of stirring in ice bath, tetrahydrofuran was removed on a rotary evaporator, the crude product was dissolved in dichloromethane and washed by distilled water for three times, the separated organic phase was dried over anhydrous sodium sulfate. After removing the solvent, further purification was carried out by column chromatography on silica gel with a petroleum ether/ dichloromethane (V/V ¼ 1/4) eluent, orange-red solid was obtained, yield 0.35 g (75%). 1H NMR (400 MHz, CDCl3) d 7.95 (s, 2H), 7.46 (d, J ¼ 8.2 Hz, 4H), 7.20 (d, J ¼ 15.8 Hz, 2H), 7.11 (d, J ¼ 16.2 Hz, 2H), 6.67 (d, J ¼ 8.1 Hz, 4H), 3.48e3.34 (m, 8H), 1.27e1.14 (m, 12H). 13 C NMR (100 MHz, CDCl3): d 148.5, 138.6, 134.5, 129.1, 122.9, 116.5, 114.0, 111.5, 44.5, 12.7. HRMS (ESI): m/z Calcd. For [MþH]þ: 475.2783, Found: 475.2857. 2.1.3. 2,5-bis(4-(ethyl(2,5,8,11-tetraoxatridecan-13-yl)amino) styryl)terephthalonitrile (P2) There was the similar synthetic way with P1. Compound 5 (0.428 g, 1 mmol), 3 (0.848 g, 2.5 mmol) and potassium tertbutoxide (0.281 g, 2.5 mmol) were stirred in dry tetrahydrofuran for 30 min in ice bath. The pre-processed crude product was purified by column chromatography on silica gel with a ethyl acetate eluent, red oil was obtained, yield 0.53 g (66%). 1H NMR (400 MHz, CDCl3) d 7.95 (s, 2H), 7.45 (d, J ¼ 8.7 Hz, 4H), 7.21 (d, J ¼ 16.0 Hz, 2H), 7.12 (d, J ¼ 16.0 Hz, 2H), 6.71 (s, 4H), 3.73e3.52 (m, 32H), 3.47 (dd, J ¼ 14.0, 6.9 Hz, 4H), 3.38 (s, 6H), 1.20 (t, J ¼ 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3): d 148.7, 138.6, 134.4, 129.0, 123.1, 117.5, 116.7, 114.0, 111.7, 72.1, 71.0, 68.0, 59.0, 50.1, 45.4, 12.3. HRMS (ESI): m/z Calcd. For [MþH]þ: 799.4568, Found:799.4643. 2.1.4. 2,2'-(1,4-phenylene)bis(3-(4-(diethylamino)phenyl) acrylonitrile) (P3) Potassium tert-butoxide (11.2 mg, 0.1 mmol) was added to an absolute ethyl alcohol solution containing 1,4-phenylenediacetonitrile (6) (0.156 g, 1 mmol) and 4 (0.39 g, 2.2 mmol), then stirred and refluxed for 3 h. After cooling to the room temperature, the solvent was removed on a rotary evaporator, and the crude product was recrystallized from a petroleum ether/trichloromethane mixture to give aurantius power, yield 0.30 g (63%). 1H NMR (400 MHz, CDCl3) d 7.86 (d, J ¼ 8.9 Hz, 4H), 7.66 (s, 4H), 7.42 (s, 2H), 6.70 (d, J ¼ 8.2 Hz, 4H), 3.44 (q, J ¼ 7.1 Hz, 8H), 1.22 (t, J ¼ 7.1 Hz, 12H). 13C NMR (100 MHz, CDCl3): d 149.4, 142.3, 135.1, 131.8, 125.8, 119.5, 111.3, 103.0, 44.6, 12.7. HRMS (ESI): m/z Calcd. For [MþH]þ: 475.2783, Found: 475.2858. 2.1.5. 2,2'-(1,4-phenylene)bis(3-(4-(ethyl(2,5,8,11tetraoxatridecan-13-yl)amino)phenyl)acrylonitrile) (P4) Synthetic way was referred to compound P3. To a mixture of compound 6 (0.156 g, 1 mmol) and 3 (0.848 g, 2.5 mmol) in absolute ethyl alcohol was added potassium tert-butoxide (11.2 mg, 0.1 mmol), then stirred and refluxed for 3 h. Removed solvent and

Fig. 1. Molecular structures of P1eP4.

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Scheme 1. The synthetic routes of P1eP4.

purified through column chromatography on silica gel using petroleum ether/ethyl acetate (V/V ¼ 1/4) as the eluent to give the red oil, yield 0.24 g (30%). 1H NMR (400 MHz, CDCl3) d 7.65 (d, J ¼ 8.4 Hz, 4H), 7.53 (dd, J ¼ 14.3, 8.5 Hz, 8H), 6.77 (d, J ¼ 7.4 Hz, 4H), 3.73e3.52 (m, 32H), 3.48 (dd, J ¼ 13.9, 6.9 Hz, 4H), 3.37 (s, 6H), 1.20 (t, J ¼ 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3): d 149.7, 142.2, 135.1, 131.7, 125.7, 122.2, 119.3, 111.4, 103.2, 72.0, 70.5, 68.7, 59.0, 50.0, 45.7, 12.1. HRMS (ESI): m/z Calcd. For [MþH]þ: 799.4568, Found: 799.4621. 2.2. Preparation of solid optical limiter The mould used to prepare solid devices was composed of two pieces of optical glass and a polytetrafluoroethylene (PTFE) gasket with thickness of 10 mm. The optimized optical limiting material was dissolved into liquid epoxy resin, and then aliphatic amine working as curing agent was added (20 wt%) and stirred evenly. After degassing under vacuum for 30 min, the mixture was injected into the fore-mentioned mould through a syringe filter (0.45 mm), and the sample was placed vertically for 2 days at room temperature till curing completely. Finally, open the mould, the solid epoxy resin device with high optical quality could be obtained with a 10 mm optical path. 2.3. Characterization All measurements in solution were performed in DMF (chromatographic grade). UVeVis absorption and linear transmittance spectra were obtained by a Hitachi U-3900 diode array spectrophotometer with 10 mm path length quartz cuvettes. The data of four compounds in diluted (1  105 M or 1  106 M) and concentrated solutions (saturated solutions for P1 and P3, 0.055 M for P2 and P4) were recorded to investigate the impact on linear photophysical properties caused by different structures and solubilities. Their fluorescence emission spectra were collected on a Hitachi F-4500 fluorescence spectrometer and absolute fluorescence quantum yields were recorded on a steady state & transient

state fluorescence spectrometer FLS 920. The optical limiting properties of P1eP4 in DMF solutions and solid samples doped by P4 were determined by the nonlinear transmission method [15]. 800 nm fs laser pulses was utilized as the light source. The setup in this experiment is shown in Fig. S1 (Supporting Information). A Ti: Sapphire regenerative amplifier system (Spitfire Fe1K) was used to pump an optical parameter amplifier (OPA-800C), providing the excitation beam with pulse duration of ~130 fs, repetition rate of 1 kHz, pulse energy of 10e550 mJ, and beam size of ~7 mm. The transmitted intensity was plotted as a function of the incident intensity. The experimental data was fitted with equation (1), and the corresponding 2PA cross section s2 can be calculated by equation (2).

I ¼ T0 lnð1 þ bLI0 Þ=bL

(1)

hnb ¼ s2 NA d0  103

(2)

Where T0 is the linear transmittance independent of I0, I is the transmitted intensity, I0 is the incident intensity, b is the 2PA coefficient, L is the thickness of the sample cell, hn is the energy of incident photon, s2 is 2PA cross section, NA is the Avogadro constant, d0 is the molar concentration of the sample. The optical stabilities of P1eP4 were characterized by the photobleaching rate [16]. Primarily, the diluted solutions for four compounds were prepared with the same absorption at 473 nm and filled in 10 mm quartz cuvettes, and then irradiated the samples using a 473 nm semiconductor solid-state laser (MBL-473 nmIII-30 mW, Changchun New Industries Optoelectronics Tech. Co., Ltd) for over 1 h. The laser power density was ~78 mW/cm2. The UVeVis spectra of all samples were recorded every 10 min under irradiation. The ln(A/A0) was plotted as a function of irradiation time and the slope obtained by linear fitting was the photobleaching rate of compound, where A0 and A were the maximum absorption before and after irradiation, respectively. The thermal stabilities of P1eP4 were studied by thermogravimetric analysis (TGA, Q50 thermogravimetric analyzer) with a heating rate of 10  C/min from 30  C to 600  C under a continuous nitrogen purge (60 mL/min).

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3. Results and dicussion 3.1. Synthetic strategy The intermediate 4-(ethyl(2,5,8,11-tetraoxatridecan-13-yl)amino)benzaldehyde (3) based on 4-(diethylamino)benzaldehyde (4) containing tetraethylene glycol group was prepared by reaction between 4-[(2-Hydroxyethyl)methylamino] benzaldehyde (1) and triethylene glycol p-toluenesulfolnate (2) under mild condition. P1 and P2 were obtained by Horner-Wadsworth-Emmons (HWE) reaction of 4 and 3 with 1,4-bis(diethylphosphorylmethyl)-2,5dicyanobenzene (5), respectively. P3 and P4 were synthesized using Knoevenagel condensation reaction of 4 and 3 with 1,4benzenediacetonenitrile (6), respectively. 3.2. Linear photophysical properties The saturated concentrations of P1 and P3 in DMF are 0.013 M and 0.003 M, respectively, both of which exhibit more than 99.0% linear transmittances at 800 nm, while the solubilities of P2 and P4 could be significantly improved to over 0.075 M via incorporating TEG groups into the skeletons of P1 and P3. In order not only to balance the performance of high linear transmittances and significant optical limiting behaviors but also to avoid the interference from the optical instability of samples induced by their linear absorption on measurement results, the linear optical losses of all samples were controlled within 1% in our experiment. Thus, the concentrations of P2 and P4 were selected as 0.055 M. The sample solutions were prepared at concentration of 1  105 M in DMF for UVeVis spectra and 1  106 M for fluorescence emission spectra. Fig. 2 presents the linear absorption, transmittance and fluorescence spectra of P1eP4 in solution using DMF as solvent. Weak absorption around 310 nm for P1, P2 and 325 nm for P3, P4 is assigned to a localized aromatic p / p* transition, whereas the other intense absorption at wavelength in the 350e600 nm range

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for P1, P2 and 340e550 nm range for P3, P4 are attributed to the intramolecular charge transfer (ICT) transitions for their D-p-A-p-D structures. Comparing the ICT peaks of P3, P4 with those of P1, P2, respectively, a ~40 nm hypsochromic shift is found, owing to the stronger distortion of p-backbone and smaller distance between the electron acceptors and donors of P3 and P4. Furthermore, the ICT absorption bands of P2 and P4 slightly shift to shorter wavelength compared with P1 and P3, respectively, which proves that the electron donating capability of the terminal ethyl groups is decreased a little after modified by TEG groups. The UVeVis spectra reveal no linear absorption beyond 600 nm in all diluted solutions of P1eP4. However, the absorb edges of ICT transition bands obviously shift toward longer wavelength in their concentrated solutions, which could be result from their intermolecular aggregation under high concentration. This viewpoint can be further supported by the larger red-shift of P2 and P4 compared with P1 and P3, respectively. Fortunately, their absorptions are all cutoff before 800 nm, ensuring their high linear transmittances at 800 nm (>99.0%). P2 and P4 present the similar fluorescence emission bands in shape to P1 and P3 with slightly decreased intensity. Furthermore, P3 and P4 exhibit shorter stoke's shift, which cyano substitution is on the double bond, than compounds P1 and P2, with cyano substituted on the central phenylene ring. The absolute fluorescence quantum yields ( FD ) are 0.301, 0.294, 0.025 and 0.021 for P1eP4, respectively. The much lower FD values of P3 and P4 are beneficial to forbid strong up-conversion fluorescence emission or even laser, which is profitable for protecting sensors or detectors from the secondary opto-damage. 3.3. Optical limiting behaviors in solutions Fig. 3 shows the optical limiting behaviors in concentrated solutions of P1eP4. In the measurement, Rhodamine B in methanol (0.05 M) was used as the reference. The solid lines are the

Fig. 2. Linear absorption spectra, fluorescence emission spectra (excited at maximum absorption wavelength), and linear transmittance spectra of compound P1eP4, dashed and dotted lines (FL) represent diluted solutions and solid lines represent concentrated solutions.

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Fig. 3. Output intensity versus incident intensity of P1eP4 under 800 nm fs pulse laser, the dots are experimental data, the solid lines are theoretical fitting curves using 2PA theory.

Fig. 4. (a) ln(A/A0) of P1eP4 versus irradiation time of a 473 nm laser, linear fitting parameter is the photobleaching rate constant, (b) the TGA curves of four compounds (10  C/min under nitrogen purge 60 mL/min).

Table 1 Photophysical properties and optical/thermal stabilities of P1eP4. Compound P1 P2 P3 P4 a b c d e f g

labs (nm)a 488 485 449 442

lfl (nm)b 606 604 534 534

Dy (cm1)c 3990 4062 3545 3898

εmax  104 (M1 cm1)a 6.55 6.22 6.10 5.78

FDe

T (%)d 100.0 (0.013 M) 99.7 (0.055 M) 99.8 (0.003 M) 99.2 (0.055 M)

0.301 0.294 0.025 0.021

V (min1)f 3.2 2.7 5.1 6.8

   

3

10 103 105 105

Td ( C)g 340 358 353 381

Linear absorption maximum (labs) and peak molar extinction coefficient (εmax). Fluorescence emission maximum. Stoke's shift in DMF solutions. Linear transmittance at 800 nm wavelength. Absolute fluorescence quantum yield. Photobleaching rate. The degradation temperature.

theoretical curves fitted by Equation (1). It is revealed that at very low optical power, the output intensity represents linear correlation to the input intensity. With the increase of input intensity, the linear relationship is deviated and gives rise to a typical optical limiting behavior. In addition, most of experimental points fall on the fitted curves, demonstrating the optical limiting mechanism is typical 2PA under the fs laser. 2PA coefficients (b) and 2PA cross sections (s2 ) of these four compounds have been obtained by fitting and then calculated. The relevant results are summarized in Table 2. Clearly, the b values of P2 and P4 are nearly twice and 9 times of those of P1 and P3 respectively. The much better optical limiting performances of P2 and P4 are obviously from their significantly improved solubilities. Nevertheless, it need to be noticed that the s2 value for a single molecule decreases, which may due to the intermolecular aggregation under high concentration.

Table 2 Nonlinear properties of P1eP4. Compound d0 (mol/L)a b (cm/GW)b s2 (GM)c Eth (mW)d Tmin (%)e FOMf P1 P2 P3 P4 a

0.013 0.055 0.003 0.055

0.3687 0.7370 0.0730 0.6595

1170 553 1003 494

480 170 >500 170

49.4 30.7 77.3 31.1

2.02 3.25 1.29 3.13

Solution concentration. Two-photon absorption coefficient. c Two-photon cross section; 1 GM ¼ 1  1050 cm4 s/photon. d Limiting threshold (the input intensity where nonlinear transmittance decreases to 50%). e Minimum nonlinear transmittance. f Figure of merit (FOM). b

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Fig. 5. Solid devices doped with various ratios of compound P4.

Fig. 6. (a) Linear transmittance spectra of solid devices with various doping ratios. (b) Output intensity versus incident intensity of solid devices with various doping ratios under 800 nm fs pulse laser, the dots are experimental data, the solid lines are theoretical fitting curves using 2PA theory.

Furthermore, three other important parameters obtained from this measurement for evaluating optical limiting behaviors of P1eP4 are also listed. The limiting thresholds (determined using the input intensity where nonlinear transmittance decreases to 50%) for P2 and P4 in concentrated solutions are both 170 mW, which are quite lower than those of P1 and P3. The values of minimum nonlinear transmittance of P2 and P4 can be as low as 30.7% and 31.1%, while P1 and P3 exhibit much higher values of 50.9% and 77.3%. Accordingly, P2 and P4 show higher figure of merit (FOM) [17] than P1 and P3. All of these data confirms that P2 and P4 have much more remarkable optical limiting effects than P1 and P3.

3.4. Optical/thermal stability Besides the optical limiting ability, the optical/thermal stabilities of materials are also critical for the optical limiting application. For organic compounds, the optical stability can be characterized by the photobleaching rate. The UVeVis absorption spectra of P1eP4 were recorded with different irradiation time under a 473 nm laser (~78 mW/cm2). As shown in Fig. S2 (Supporting Information), there is an obvious photobleaching for P1 and P2, while the absorption spectra of P3 and P4 show almost no variation within 60 min. The variation of absorption peak versus the irradiation time is depicted in Fig. 4(a) and the photobleaching rates obtained by linear fitting of P1eP4 are summarized in Table 1. The results illuminate that the terminal TEG chains scarcely weaken the optical stability of the

3.5. Optical limiting behaviors of solid devices In order to obtain optical limiters with desirable portability for laser protection, P4 was selected to fabricate solid optical limiters due to its good comprehensive performance. Samples with various doping ratios (0 wt%, 0.2 wt%, 1.0 wt% and 2.0 wt%) of P4 in epoxy resin were fabricated (shown in Fig. 5). The favourable compatibility between P4 and matrix ensures that the linear transmittances of these solid devices are all above 90.0%, as depicted in Fig. 6(a). Fig. 6(b) displays the power limiting behaviors of these solid devices under 800 nm fs laser pulses. There is a perfect agreement between the fitting curves and experimental data. Using Rhodamine B in methanol as the reference, the fitting parameters (b) for these three samples (doped ratios as 0.2 wt%, 1.0 wt% and 2.0 wt%) are 0.1021 cm/GW, 0.3626 cm/GW and 0.5713 cm/GW, respectively (Table 3). For the blank sample (epoxy resin without doping), there is a linear relationship between the output and input intensity and the fitting slope is in conformity with its linear transmission. Taking off the reflection losses of two surfaces from these solid devices, the optical loss of the epoxy resin matrix is very limited, which indicates that these fabricated solid devices have large potentials on laser protection under 800 nm pulse laser. 4. Conclusion

Table 3 Linear transmittance and 2PA coefficient of solid devices. Doping ratio

Blank (0 wt%)

0.2 wt%

1.0 wt%

2.0 wt%

T0 (%)

91.6 e

90.6 0.1021

90.0 0.3626

90.0 0.5713

b (cm/GW)

prototype scaffold, and P4 exhibits much better optical stability than that of P2. The thermal degradation temperatures (Td) of P1eP4 are 340  C, 358  C, 353  C and 381  C, respectively, which indicates that they all have good thermal stability.

Two liquid distyrylbenzene derivatives (P2 and P4) with different cyano substitution sites on a distyrylbenzene conjugated bridge have been synthesized by incorporating two TEG groups into their prototype scaffolds of P1 and P3, respectively. The solubilities

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of both P2 and P4 are increased significantly. Their concentrated solutions present obvious optical limiting property under 800 nm fs laser pulses with good thermal stabilities. However, only P4 exhibits good photostability and low fluorescence quantum yield owning to the steric hindrance on the vinylene bond and larger distortion of p-backbone, while a rapid photobleaching happens under illumination for P2, whose cyano groups are substituted on the central phenylene ring. Solid devices with 10 mm optical length and high linear transmittances have been fabricated through doping P4 into an epoxy resin matrix. The experimental results indicate these solid devices could be potential optical limiters aiming at 800 nm pulsed laser. Acknowledgement We thank the support of NSAF project (No. U1230123). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.07.035 References [1] Tutt LW, Boggess TF. A review of optical limiting mechanisms and devices using organics, fullerenes, semiconductors and other materials. Prog Quant Electron 1993;17:299e338. [2] Lin T-C, Lee Y-H, Huang B-R, Hu C-L, Li Y-K. Two-photon absorption and effective optical power-limiting properties of small dendritic chromophores derived from functionalized fluorene/oxadiazole units. Tetrahedron 2012;68: 4935e49. [3] Wang D, Wang X, Zhou G, Xu X, Shao Z, Jiang M. Two-photon absorptioninduced optical properties of a new lasing dye in two solvents. Appl Phys B 2001;73:227e31.

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