Polyfluorenylacetylene for near-infrared laser protection: polymer synthesis, optical limiting mechanism and relationship between molecular structure and properties

College of Material Science and Engineering Chemical Fibers and Polymer Materials, China. E-mail: hongyaoxu@163.com School of Chemistry and Chemical Enginee friendly Polymer Materials of Anhui Provinc College of Chemistry and Bioengineering, China. E-mail: syg@dhu.edu.cn Department of Physics, Department of Chem Science Drive 3, Singapore, 117542, Singapo † Electronic supplementary information ( M1–M3, M5 and P1–P3, P5; C NMR spe UV-vis absorption and FL emission spect dash dot) in THF solutions; open-apertur pulse energy in THF solution. See DOI: 10 Cite this: RSC Adv., 2017, 7, 53785


Introduction
With the rapid development of new laser technology, laser protection in recent years is believed to be promising for both civilians and the military. 1These works have mainly focused on the visible range, where the nonlinear transmission phenomena are mainly dependent on multiphoton absorption (MPA) or reverse saturable absorption (RSA), nonlinear scattering.The materials designed for laser protection involve graphene families, [2][3][4] single-walled and multi-walled carbon nanotubes, [5][6][7][8][9] organic dyes, 10 inorganic materials, 11,12 p-electron conjugated systems such as fullerene 13,14 and porphyrins, phthalocyanines, [15][16][17][18] and organometallic complexes [19][20][21] as optical limiting materials for protecting human eyes and sensitive optical sensors from laser damage.Accordingly, to obtain an ideal optical limiting implement, some major factors should be taken into account such as fast response speed, low transmittance at high intensities and high transmittance at low intensities, temporal response and broad spectral characteristics. 22For the classical benchmark, porphyrins, phthalocyanines and fullerenes can also provide excellent optical limiting properties.However, their narrow excited-state absorption (ESA) spectral regions and poor optical transparency characteristics restricted practical applications in broadband optical limiting properties.4][25][26][27][28][29][30][31][32] Specically, metal-based materials are premeditated as novel generation optical limiting materials displaying a high transparent characteristic in the visible spectral region. 33,34However, to the best of our knowledge, only a few papers reported the laser protection on the near-infrared spectral region has been investigated.4][45][46][47][48][49][50][51][52] It was found that the incorporation of nonlinear optical chromophore as a pendant group into polyacetylene backbone has endowed polyacetylenes with novel optical limiting properties.The optical limiting properties were mainly inuenced by the molecular structure of substituted chromophore group and electron interaction between polyacetylene backbone and substituted chromophore.The interaction generally increases with the decrease of space length between substituted chromophore group and polyacetylene backbone. 45The functional polyacetylene containing long conjugated chromophore with C]C double bond as p electron conjugation bridge exhibited better optical limiting property than that with C]N or N]N double bond as p-electron conjugation bridge under the same linear transmittance. 47,51Their optical limiting mechanism was attributed to the reverse saturated absorption (RSA) of molecules.
Thus, it was reported that polyacetylenes may be a potential near-infrared laser protection materials. 53However, to our knowledge, the polyacetylenes with optical limiting properties for near-infrared optical region have seldom been reported.In particular, the relationship between the structure and nearinfrared optical limiting properties of the molecules, so far, has not been understood.It is known that uorene is a well near-infrared TPA chromophore. 54,55It is expected that the incorporation of uorene-based chromophores as the pendant groups into polyacetylene could endow the polyacetylenes with novel optical properties.Therefore, in the work, we designed and synthesized a series of novel polyacetylenes that contain uorene pendant with different conjugation length and terminal substituent such as electron withdrawing or electron donating groups.At the same time, the alkyl chains at nine positions extending away from the conjugated p-system of uorene was introduced for improving the solubility of resultant functional polyacetylenes in common organic solvents and provide good compatibility with other matrices. 56The relationship between near-infrared optical limiting properties and molecular structure and the optical limiting mechanism were investigated in detail.

Instrumentation
FTIR spectra of KBr disks were recorded with a Thermo Nicolet 5700 spectrometer at room temperature, 32 scans were collected at a resolution of 1 cm À1 . 1 H NMR (400 MHz) and 13 C NMR (100 MHz) were recorded on a Bruker DMX-400 spectrometer utilizing tetramethylsilane (TMS, 0.00 ppm) as the internal standard in chloroform-d (CDCl 3 ) at room temperature.Elementary analyses were conducted on Vario EL-III elementary analysis apparatus.UV-vis spectra were recorded on a Shimadzu UV-265 spectrometer using a 1 cm square quartz cell.Fluorescence spectra were obtained on a Shimadzu RF-5301PC spec-trouorimeter equipped with a 450 W Xe lamp and a timecorrelated single-photon counting card.Weight-average (M w ) and number-average (M n ) molecular weights, and polydispersity index (PDI, M w /M n ) were determined by gel permeation chromatograph (GPC) using a Waters 510 HPLC equipped with a Rheodyne 7725i injector with a stand kit, a set of styragel columns (HT3, HT4, and HT6; molecular weight range 10 2 -10 7 ), a column temperature controller, a Waters 486 wavelength tunable UV-vis detector, a Waters 410 differential refractometer, and a system DMM/scanner possessing an 8-channel scanner option.All polymer solutions were prepared in THF (ca. 2 mg mL À1 ) and ltered through 0.45 mm PTFE syringe-type lters before injected into the GPC system.THF was used as the eluent at a ow rate of 1.0 mL min À1 .The column temperature was maintained at 30 C and the working wavelength of the UV detector was set at 254 nm.A set of mono-disperse polystyrene standards (waters) was used for calibration purposes.Differential scanning calorimetry (DSC) was performed on a TA Instruments Q2000 equipped with a liquid nitrogen cooling accessory (LNCA) unit under a continuous nitrogen purge (50 mL min À1 ).The scan rate was 10 C min À1 within the temperature range 30-250 C. Samples (4-6 mg) were weighed and sealed in aluminum pans.Thermogravimetric analysis (TGA) was carried out using a NETZSCH Instruments 449F3 thermogravimetric analyzer with a heating rate of 20 C min À1 from 20 to 800 C under a continuous nitrogen purge (100 mL min À1 ).Samples (15-25 mg) were loaded in alumina pans.The thermal degradation temperature (T d ) was dened as the temperature of 5% mass loss.

Polymerization
Scheme 2 all the polymerization reactions and manipulations were performed under puried nitrogen using Schlenk techniques either in vacuum-line system or an inert-atmosphere glove box, except for the purication of the polymers, which were done in open air.Typical procedure is given below: 1.0 mmol of the monomer was added into a baked 20 mL Schlenk tube with a side arm.The tube was evacuated under vacuum and then ushed with dry nitrogen three times through the side arm.Three milliliters of dioxane was injected into the tube to dissolve the monomer.The catalyst solution was prepared in another tube by dissolving 4.6 mg (0.01 mmol) [Rh(nbd)Cl] 2 and 2.02 mg (0.02 mmol) Et 3 N in 2 mL of dioxane, which was transferred to the monomer solution using a hypodermic syringe.The reaction mixture was stirred at 60 C under nitrogen for 4 h.The mixture was then diluted with 5 mL of dioxane and added dropwise to 200 mL of methanol under stirring.The precipitate was centrifuged and redissolved in THF.The THF solution was added dropwise into 200 mL of methanol to precipitate the polymer.The dissolution-precipitation process was repeated three times, and the nally isolated precipitant was dried under vacuum at 30 C to a constant weight. Poly
All polymerization reactions were carried out under nitrogen atmosphere using a standard Schlenk vacuum-line system.We rst attempted to polymerize these monomers (M1-M5) using the classical metathesis catalysts, such as WCl 6 -Ph 4 Sn, TaCl 5 -Ph 4 Sn and MoCl 5 -Ph 4 Sn.However, only trace polymeric products were obtained.On the contrary, [Rh(nbd)Cl] 2 /Et 3 N catalyst effectively works for these monomers and the results are listed in Table 1.It was found that P1 and P2 were partly insoluble in dioxane, only giving molecular weight of the soluble part of the polymers (Table 1, entry 1 and 3).However, the yield of P1 0 and P2 0 was increased by using THF as the polymerization solvent (Table 1, entry 2 and 4).P1 and P2 exhibited the low polymerization yield in dioxane, which may be mainly originated from the enwrapped effect of insoluble polymer to prevent further polymerization of monomers.Moreover, the polymerization yield of P2 in dioxane or P2 0 in THF was almost lower than that of corresponding P1 or P1 0 , hinting that polymerization activity of monomer M2 may be lower than that of M1.The result exhibited that polymerization activity of functionalized acetylene monomer decreased with the increase of conjugated chain length of substituted chromophore group.P3, P4, and P5 were Scheme 1 Synthetic routes of monomer molecules M1-M5.soluble in dioxane and gave dark red polymers with high molecular weight (M w ¼ 9 Â 10 4 -2.5 Â 10 5 ) in high polymerization yield (Table 1, entry 5, 6, 7), and P5 obtained the highest molecular weight and yield, hinting that the longer exible alkyl side chain on uorene was benecial to polymerization of monomer.

Solubility
It's reported that PAs have weak solubilities in common solvents. 25However, when the styryl/stilbene-uorene pendant with exible alkyl group as side tailor was incorporated into the PA backbone chain, all the polymers were readily soluble in various common organic solvents such as chloroform, toluene, THF, and 1,2-dichloroethane.

Structural characterization of the polymers
All the polymers and corresponding monomers were characterized by standard spectroscopic methods, from which satisfactory analysis data were obtained (see the Experimental section for details).An example of the FTIR spectrum of P4 is given in Fig. 1.For comparison, the FTIR spectrum of its corresponding monomer M4 is also given in the same gure.As can be seen from Fig. 1, M4 clearly exhibits two characteristic absorptions at 3319 and 2104 cm À1 , respectively, corresponding to the ^C-H and C^C stretching vibrations in the monosubstituted acetylene.The acetylenic absorption bands disappear in the spectrum of P4 and the characteristic vibration intensity of aromatic ring is signicantly enhanced, suggesting that the triple bond in the monomer has changed to double bond in the polymer.Similar results (Fig. S1-S4 †) were found in other monomers (M1-M3 and M5) and their polymers (P1-P3 and P5), suggesting that the triple bonds of these monomers have been transferred to double bond polyacetylene backbone.
Fig. 2 shows the 1 H NMR spectra of M4 and P4 in chloroform-d, respectively.The absorption peak of the acetylene proton in M4 at 3.17 ppm as a singlet peak, completely disappears in the spectrum of its polymer P4.Meanwhile, all the corresponding characteristic absorption peaks of monomer M4 signicantly were widened in the spectra of P4 with a widened and enhanced peak at 7.02 ppm corresponding to the olen protons and aromatic protons absorption, further proving that the M4 has been polymerized to yield the functional polyacetylene P4.The results are consistent with that of FTIR spectra.Alternatively, no absorptions are found in the cis-olen absorption region of 5.20-6.20 ppm, suggesting that these polymers possess a predominantly trans conformation. 59,60The high trans content may result from the strong steric hindrance of the pendants. 61ompared to that of M4, the signals at 77.2 and 84.8 ppm corresponding to the acetylenic carbons disappear in the 13 C NMR spectrum of P4 (Fig. S9 †), and the relevant absorption peak intensity of the carbon atoms of the aromatic pendants in polymers is signicantly enhanced because of the absorption overlapping effect of the olen carbon atoms of the polyacetylene backbone.Thus, the 1 H NMR and 13 C NMR spectra data further conrm that the acetylene polymerization has taken place.The similar results were found in polymerization of other monomers.The satisfactory spectral data are given in the ESI (Fig. S10-S12 †).

Linear optical properties
The linear optical properties of the polymers and corresponding monomers were investigated by UV-vis absorption and uorescence in THF solutions at 20 C in extremely diluted solution (2 Â 10 À5 M) and these spectra are depicted in Fig. 3.It can be seen from Fig. 3 that the maximum absorption wavelengths of P1-P5 are located at 400, 406, 388, 385 and 385 nm, respectively, which are assigned to the p-p* electronic transitions of the corresponding conjugated styryl/stilbene-uorene chromophores.Obviously, the maximum absorption wavelengths of the polymers changes at the different terminal substituents or the length of conjugated systems, but aren't inuenced by the length of alkyl exible chain.The absorption band of the nitrosubstituted polymers P1 and P2 shows signicantly red-shis, which results from the larger p-electron delocalization and  This journal is © The Royal Society of Chemistry 2017 stronger dipolar effect of NO 2 group.P2 with longer conjugated chromophore substituent shows larger red-shi at maximum absorption wavelength than P1.The absorption band of the methyl-substituted polymers P4 and P5 with the different alkyl exible chain exhibit the almost same maximum absorption wavelength.Although the polymers and their corresponding monomers show nearly the same l max , the polymers show signicant broad backbone absorption at longer wavelength than 420 nm, which result from the double contribution of conjugated double-bond backbones of polyacetylene and enhancement conjugation effect between the double-bond backbones of the polyacetylenes and styryl/stilbene-uorene pendant groups.These results further support that the monomers have been converted to polymers.
An interesting feature for the polymers P1-P5 is that they all possess luminescent properties upon photo irradiation while PA itself is nonluminescent. 62When excited at relative maximum absorption wavelength in THF solution, P1-P5 emit uorescence with maximum emitting peak at around 515 nm, 541 nm, 453 nm, 450 nm and 447 nm respectively, which are higher than that of the corresponding monomers (Fig. 3).The red-shi of uorescence emitting peak probably results from a strong pendant-pendant intermolecular interaction and/or intramolecular interaction between styryl/stilbene-uorene pendants and the polyacetylene conjugation main chain.Similar phenomena are found by Balcar. 63

Optical limiting properties and mechanism
Many optical limiting materials for visible optical region have been investigated.However, the optical limiting materials for the near-infrared optical region were rarely reported.Thus, we investigated the near infrared optical limiting properties of resultant polymers by a mode-locked Ti:sapphire laser (Quantronix, IMRA) at 780 nm using 450 femtosecond laser pulses at 1 kHz repetition rate.For avoiding the inuence from thermally induced scattering effects, the optical limiting properties of the polymers in THF solution were measured at low unit concentration (1 mM).Fig. 4 shows the optical limiting properties of P1-P5 in THF and the results are summarized in Table 1S.† For comparison, the optical limiting property of polyphenylacetylene (PPA) at the same condition is also made.The results were shown in Fig. 4. At low incident irradiance, the transmittances of all the polymer solutions remain unchanged with the incident irradiance obeying the Beer-Lambert law.However, the transmittances of the solutions start to deviate from the linear transmission with a further increase in the incident irradiance, and a nonlinear relationship is observed between the output and input irradiance, while that of PPA still rises linearly because of the laser-induced photolysis of the polyacetylene chains, 68 suggesting that the incorporation of conjugated styryl/stilbene-uorene group into polyacetylene backbone has endowed the resultant polyacetylenes with novel laser protection characteristics at 780 nm for 450 femtosecond pulses laser.It is also found that the limiting thresholds of the resultant polymers, which are dened as the incident irradiance uence where the transmittance starts to deviate from linearity, are signicantly inuenced by molecular structures.As seen in Table 1S, † polymer P5 (34.9 GW cm À2 ) with longer tail alkyl chain length in side position of uorene pendant exhibits better optical limiting properties than P4 (47.6 GW cm À2 ).Moreover, polymer P3 shows the better optical property when the terminal methyl group of stilbene-uorene pendant of P4 is replaced by methoxy group.The limiting threshold of P2 (16.7 GW cm À2 ) is better than P1 (21.4 GW cm À2 ), which should be attributed to the larger electron conjugation of chromophoric moiety in P2.It can be found that P2 shows the best optical limiting properties (lowest optical limiting thresholds) among all the polymers, owing to the longer conjugated chromophore substituent and larger dipolar effect of strong electron acceptor NO 2 .
The optical limiting mechanisms of organic compounds are oen based on TPA or RSA.Generally, TPA based optical limiting effect can be yielded in principle under the laser irradiation of picosecond or shorter pulses, while RSA is achieved on a nanosecond or longer time scale, owing to the different excited state lifetimes involved in a multilevel energy process. 64n this work, the polymers are excited by 450 femtosecond laser pulses at 780 nm.Therefore, we consider that the optical limiting properties of the polymers may mainly originate from TPA.
For investigation of optical limiting mechanism, we performed open-aperture Z-scans with the same laser.The Z-scan experiments of the polymer solution were carried out at 780 nm using femtosecond laser pulses (450 fs, 1 kHZ).In theory, the normalized transmittance for the open aperture can be written as 65 Tðz; s ¼ 1Þ ¼ where q 0 (z) ¼ a 2 I 00 (t)L eff /(1 + z 2 /z 0 2 ) with b is the TPA coefficient, I 0 is the intensity of laser beam at focus (z ¼ 0), L eff ¼ [1 À exp(Àa 0 L)]/a 0 is the effective thickness (a is the linear absorption coefficient, L is the sample thickness, z 0 is the Rayleigh range of the laser beam, and z is the sample position).The TPA cross-section s 2 is calculated by 55 where N is the number of molecules per cm 3 and hn is the excitation photon energy.The TPA cross-section s 2 is expressed in Göppert Mayer (GM) units, in which 1 GM ¼ 1 Â 10 À50 cm 4 s per photon.The Z-scan data (or the transmittance as a function of z position) were normalized to the linear (small-signal) transmittance.The nonlinear-optical signal from the solvent was negligible, compared to the signals from the fullerene derivatives.The Z-scan results conrmed that the samples had surprisingly good photostability, which was veried by indifference between the linear absorption spectra measured before and aer intense laser irradiation in the Z-scans.In addition, the TPA coefficients b, could be extracted from the best tting between the Z-scan theory and the data, and the TPA cross sections were then calculated from the denition (2).
As thermally induced scattering effects and exited-states absorption caused by high irradiance may inuence the accuracy of overall TPA cross sections measured, we undertook the Z-scans of P1-P5 in THF with different irradiances.As an example, the results of P2 are shown in Fig. 5. Aer calculation from denition (2), it is found that the TPA cross sections (s 2 ) of the polymers are independent of laser irradiance and remain nearly constant across the irradiance range of interest, indicating the observed nonlinear absorption is induced from a pure third-order nonlinear process (Table 1S †).Other nonlinear mechanisms such as thermally induced scattering and excited-state absorption, in particular singlet excited-state absorption, are negligible in our experiments.The similar phenomena (Fig. S14-S17 †) were also observed in other polymers.Furthermore, it is found that TPA cross sections of the polymers P1-P5 are obviously larger than those of PPA. 42Thus, the attachment of styryl/stilbene-uorene moieties into the PA main chain has endowed the resultant polyacetylenes with enhanced the nonlinear optical properties signicantly, which may be attributed to the enhanced conjugation effect between substituent uorene pendants and conjugated backbone of polyacetylene.The similar results were also found in oxadiazole-  based or azobenzene-based chromophore functionalized polyacetylenes. 45,49ig. 6 shows the TPF emission spectra of all polymers (P1-P5) in solid form.The maximum TPF peaks of P1-P5 located at 562 nm, 584 nm, 512 nm, 506 nm and 506 nm, respectively.Comparing the TPF curves of P1-P5 with those of one-photon uorescence of P1-P5 in THF (Fig. 3), red shis can be identi-ed, supporting that the re-absorption effects are involved in the nonlinear process in the solid polymers.It is well known that the uorescence emission intensity presented a square dependence of the uorescence on the input intensity to prove the TPA nature.When the sample is represented by TPA without stimulated emission, self-quenching, or any dark states inuencing the emission intensity, the signal intensity (I uor ) of the detected uorescence from the sample cell is given by 55

Thermal property
The thermal stability of the resulting polymers was evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere.
It is well known that poly(1-alkyne)s such as poly(1-butyne) and poly(1-hexyne), are so unstable that even the isolation process of the polymer products from the polymerization reactions leads to degradation. 59,62However, as shown in Fig. 7, all the uorenecontaining polyacetylenes exhibit good thermal stability.T d (dened as the temperature of 5% weight loss) for P1 is as high as 357 C, while T d for P2 with longer conjugated pendant is at 327 C. When para-position of stilbene-uorene substituted by different groups, the resulting polymers showed T d at 407, 397 and 345 C for P3, P4 and P5, respectively, which are much higher than that for PPA (225 C). 55 Thus, the incorporation of the rigid styryl/stilbene-uorene groups into polyacetylenes signicantly improves the thermal stability of resultant polyacetylenes.The enhancement of the thermal stability may be due to the "jacket effect" of the styryl/stilbene-uorene pendants.Namely, the alternating-double-bond backbone of the polymers may be surrounded by a rigid "jacket" formed through the strong intra-and interchain molecular electronic interaction of the styryl/stilbene-uorene groups, shielding the polymer main chains from the thermal attack.8]68,69

Conclusions
In summary, a series of uorene-containing functionalized polyacetylenes with different molecular structure was successfully prepared using [Rh(nbd)Cl] 2 -Et 3 N as the catalyst.The incorporation of conjugated styryl/stilbene-uorene group into polyacetylene backbone has endowed the resultant polyacetylenes with novel near-infrared laser protection.The optical limiting properties for near infrared laser protection are inuenced by their molecular structures.The functional polyacetylene with longer conjugation structure and stronger  terminal acceptor NO 2 group exhibits better optical limiting properties.The investigation of optical limiting mechanism shows that the optical limiting properties are mainly originated from two-photon absorption of the polymeric molecules.In addition, the thermal stabilities of the resultant polymers are signicantly improved due to "jacket effect" resulting from strong intra and inter molecular electronic interaction between polyacetylenes main chain and substituted chromophoric pendants.This work provides an important foundation for design of new materials for preparing soluble functional polyacetylenes with near-infrared laser protection and high thermal properties.

Fig. 3
Fig.3Normalized UV-vis absorption and FL emission spectra of the monomers (solid line) and corresponding polymers (dash dot) in THF solutions (spectra of M5 and P5 were showed in Fig.S13 †).

Fig. 5
Fig. 5 Open-aperture Z-scans of P2 at the different pulse energy in THF solution.Solid lines are the theoretical fit for two-photon absorption.

I
fluor f I 0 2 bl 0 (3) here I 0 is the input light intensity, l 0 and b are the optical path length and TPA coefficient of the medium, respectively.Therefore, Fig. 6(a) and (b) showed the two-photon uorescence (TPF) spectra at different input light intensities and the log-log plots of the signal intensity (I uor ) of the detected uorescence vs. the input light intensity (I 0 ) of P1-P5, respectively.The results displayed that the log-log plots of the detected TPF intensity are linear with the input light intensity, and the slopes are approximately 2, further conrming that the optical limiting properties are mainly attributed to the molecular TPA absorption of resultant polyacetylenes. 66