Synthesis and Characterisation of Photo-Cross-Linkable Liquid Crystalline Poly ( n-[ n-flurobenzoylstyryloxy ] alkylmethacrylate ) s and Their Fluorescence Lifetime Properties

This paper reports a study on photo-cross-linkable polymer containing pendant chalcone moiety exhibiting liquid crystalline as well as fluorescence lifetime properties in detail. The photoresponsive polymers were prepared, and their structure has been characterized by H-NMR, C-NMR, andUV-Visible spectroscopy.Thephoto-cross-linking behavior of polymers has been studied byUV-Visible and fluorescence spectroscopy. UV spectral studies revealed that the polymers follow 2π+2π cyclo addition reactions when they undergo photo-cross-linking under the influence of UV-light. Number and weight average molecular weight of the polymers were determined by Gel Permeation Chromatography (GPC) and polydispersity index value near to 1.5. The thermal and thermooxidative stability of the polymers were determined by Thermogravimetric Analysis (TGA). Thermal transitions were studied by DSC, and presence of mesophases was identified at 147 and 126C by hot stage polarized light optical microscopy (HPOM). Fluorescence lifetime measurements using the time-correlated single photon counting (TCSPC) method reveal that the average lifetime values decrease from 5.94 ns to 5.32 ns on UV-irradiation were discussed in detail.


Introduction
Liquid crystalline polymers have generated considerable interest in recent years, and the photo-cross-linkable LCPs have driven special attention if they contain both mesogen and photoactive groups in their structure [1][2][3][4][5].The former incorporates LC properties to the polymer, and the later facilitates cross-linking of the chain under the influence of UV radiation.These classes of polymers are very useful in fabricating anisotropic networks, information storage devices [6,7] and nonlinear optical devices [8].Many research articles reported photochemical and liquid crystalline behaviours of these polymers using UV-Visible spectral studies and polarised optical microscopic characterisations.In addition to these studies, fluorescence lifetime measurement has been used as a new instrumental technique to support photoreactive behaviour of photo-cross-linkable liquid crystalline polymers.
Fluorescence lifetime measurements encompass tremendously large fields of science.Since the mid-19th century, nearly every great breakthrough in chemistry and physics has aided the development of fluorescence lifetime techniques, and a growing number of discoveries in biology and medicine owe their existence to fluorescence lifetime.A variety of fluorescence detection methods are available for lifetime measurements but the advent of time-correlated single photon counting (TCSPC) [9,10] has simplified data collection and enhanced quantitative photon counting.
This paper reports newer root for the synthesis of poly(n-[n  -flurobenzoylstyryloxy] alkylmethacrylate)s; the photocross-linking and liquid crystalline behaviour of polymers have been well characterised by UV and HPOM studies.We believe that there is no report in recent years on fluorescence lifetime study in combination with UV and liquid crystalline mesophase transition studies about photo-cross-linkable liquid crystalline polymer containing pendant chalcone moiety.This research work may kindle significant scientific work and practical contribution with respect to the development of unique photo-cross-linkable liquid crystalline polymeric materials.

Experimental
2.1.Materials.4-Fluorobenzaldehyde and 4-hydroxyacetophe none were purchased from Spectrochem Chemicals.4-bromobutanols, 6-bromoheaxnol, methacryloyl chloride were purchased from Aldrich Chemicals.Ethanol, tetrahydrofuran, ethylacetate, chloroform, and diethylether were purchased from Merck, and all the solvents were distilled as per standard methods.Thin Layer Chromatography (TLC) technique was carried out on Merck aluminium plates with 0.2 mm silica gel.Anhydrous sodium sulphate was used to dry all organic extracts.AIBN was recrystallised using 1 : 1 methanol and chloroform.

Measurements.
The FT-IR spectra of the polymers were recorded on Perkin Elmer FT-IR Spectrometer RXI.The specimen was prepared in the pellet form using KBr. 1 H-NMR spectroscopic measurement was recorded with Bruker MSC 300 spectrometer.Thermal stability of polymers was investigated by TGA using NETZSCH STA 409 C/CD.The number average and weight average molecular weight of the polymer were determined by PL-GPC 650.Glass transition temperature of polymer was measured from Differential Scanning Calorimeter (DSC) NETZSCH.DSC.204.The photo-cross-linking studies have been done by Perkin Elmer Lambda 35 UV-Visible Spectrometer.The fluorescence spectrum of the polymer has been recorded in FluroMax 2.0.The texture of the prepared sample was studied by Euromax polarizing microscope equipped with a Linken HFS91 heating stage.The sample was prepared by a small quantity of the material being melted between two thin glass cover slips to get uniform film and anisotropic behavior observed by heating as well as cooling with Toshiba digital camera.

Fluorescence Lifetime Measurements.
Lifetime measurements were made using time-correlated single photon counting system (TCSPC, HORIBA JOBIN YUVON IBH, UK) by exciting the sample using 280 nm Nano-LED (pulse width: <1 ns) and 460 nm Nano-LED (pulse width: >1 ns), a fast response red sensitive PMT (Hamamatsu Photonics, Japan) detector.The fluorescence emission was collected to 90 degree from the path of the light source.The electrical signal was amplified by a TB-02 pulse amplifier (Horiba) fed to the constant fraction discriminator (CFD, Phillips, The Netherlands).The first detected photon was used as a start signal by a time-to-amplitude converter (TAC), and the excitation pulse triggered the stop signal.The multichannel analyzer (MCA) recorded repetitive start-stop signals from the TAC and generated a histogram of photons as a function of timecalibrated channels (55.7 ps/channel) until the peak signal reached 1000 counts.The instrument response function was obtained using a Rayleigh scatter of Ludox-40 (40 wt.% suspension in water; Sigma-Aldrich) in a quartz cuvette at 280 nm excitation and 460 nm excitation.Decay analysis software (DAS6 v6.0, Horiba) was used to extract the lifetime components.The goodness of fit was judged by the chisquare values, Durbin-Watson parameters, as well as visual observations of fitted line, residuals, and autocorrelation functions.

Synthesis of Pendant Chalcone (Scheme 1).
The pendant chalcone compound HPFSK was synthesized according to the reported literatures [11,12].The obtained products were purified by recrystallisation in ethanol and then identified using 1 H, 13 C-NMR, and FT-IR spectra.

Hydroxyphenyl-4 󸀠 -flurostyryl Ketone (HPFSK).
In a three-necked flask equipped with a mechanical stirrer and dropping funnel, a solution of NaOH (8 g) in distilled water (40 mL) was added to 4-hydroxyacetophenone (6.80 g, 0.05 mol) in 50 mL of ethyl alcohol.The reaction was cooled using an ice bath (10-15 ∘ C).A solution of 4fluorobenzaldehyde in 50 mL of ethyl alcohol was then added dropwise with constant stirring, and the temperature was not allowed to exceed 25 ∘ C.After 12 h, the reaction mixture was neutralized with 2 M HCl to isolate the product.The yellow coloured solid product was filtered and washed several times with ice-cold water.The crude product was recrystallized from methanol into a yellow crystalline product HPFSK (Scheme 1).

2.5.
4-[4  -Flurobenzoylstyryloxy]butyl Methacrylate (FBSOBMA) (M1) (Scheme 2).In a two-necked round bottom flask, chalcone (HPFSK) (3 g, 0.0123 mol) was dissolved in 100 mL of DMF and stirred well, and K 2 CO 3 (3.39g, 0.0246 mol) and a pinch of KI were added into the above solution and allowed stiring for 30 minutes in an oil bath.The prepared 4-bromobutanol (1.6 mL) was added dropwise into the above mixture and allowed stirring for 24 h at 90 ∘ C. The product yellow solid formed was poured into water, filtered, and dried.The crude product 4-hydroxybutyloxystyryl-4  -flurophenyl ketone obtained was recrystallised from ethanol-water mixture (50 : 50) (yield 65%, 2.5 g).4-Hydroxybutyloxystyryl-4-flurophenyl ketone (2 g, 6.3 mmol) and 1.5 mL of triethylamine were dissolved in ethylmethylketone (150 mL).The above mixture was cooled between 0 to −5 ∘ C, and methacryloyl chloride (2 mL in 20 mL of EMK) was added drop wise for an hour with constant stirring and cooling.The reaction mixture was stirred for another 6 hours at room temperature, and the precipitated ammonium salt was filtered off.After drying over anhydrous sodium sulphate, EMK was evaporated using rotary evaporator.The crude monomer product was purified by column chromatography using ethyl acetate/n-hexane (2 : 8 v/v) as eluent.The monomer FBSOBMA (Figure 2) obtained was pale yellow coloured solid (yield 75%).

Synthesis of Polymers (Scheme 2). The polymers poly(4-[4 󸀠
-flurobenzoylstyryloxy] butyl methacrylate) (P1) and poly(6-[4  -flurobenzoylstyryloxy] hexyl methacrylate) (P2) were synthesised by free radical polymerisation.The free radical polymerizations of monomer M1 and M2 were carried out using AIBN as initiator as shown in the schematic representation (Figure 2).The predetermined quantities of monomers were taken with AIBN (5% weight of monomer) in polymerization tube and dissolved with 20 mL of dry tetrahydrofuran (THF).The above solutions were degassed by N 2 gas atmosphere for 10-15 minutes.Then, the polymerization tubes were kept at 70 ∘ C for 48 hours and subsequently then poured into methanol to precipitate the polymer.The polymers obtained were separated by filtration and purified by repeated reprecipitation from chloroform into methanol and then dried in vacuum.The yield obtained was 70%.

Photoreactive Measurements
The photoreactivity of polymers was studied by dissolving the samples in chloroform, irradiated with UV-light at 254 nm using photoreactor, and kept at a distance of 10 cm from the light source for different time intervals.After each irradiation period, the UV spectra were recorded using Perkin Elmer scanning spectrometer.The rate of disappearance of double bond in photosensitive group was followed by the expression,

Rate of conversion
where   ,   , and  ∞ are absorption intensities due to the >C=C< group after the irradiation time  = 0,  = , and  = ∞ (maximum irradiation time), respectively.

Results and Discussions
where   and   are absorption intensities due to the >C=C< group after the irradiation time  = 0 and  =  respectively.The UV spectral changes during photo-cross-linking and photoconversions of polymers are shown in Figures 7 and 8.In the polymers P1 and P2, the pendant chalcone unit and polymeric backbone are linked by flexible methylene spacer units.The photo-cross linking rate of P2 was slightly faster than P1.This rapid cross-linking may be attributed to the spacer unit between the photosensitive group and polymer backbone which provide more flexibility and free movement for the side chain which accelerate the increased rate of cross linking [14].
The photolysis studies of various ethylene spacer containing polymers imparted that the rate of photo-cross-linking depends on the length of the methylene chain so the polymer P7 and P8 follow this trend Hexamethylene > Butamethylene.
(  Figure 9 denotes that the rate of disappearance of the C=C of photoreactive groups is slightly fast in P2 than P1 and shows 100% photoconversion, since they have smaller substituents in their pendant unit and there is no intramolecular crosslinking can be formed by the dimerization of adjacent chalcone groups.decreases as the time of irradiation increases.The decrease in intensity is due to 2 + 2 cycloaddition which leads to cyclobutane ring formation by destroying  electron conjugation.From the figures it is noticed that P1 shows gradual decrease in the intensity, whereas P2 shows sudden decrease.Fluorescence spectral changes during photo-crosslinking of polymers P1 and P2 are shown in Figures 10 and 11.Both polymers P1 and P2 were excited at the wavelength of 348 nm, and they were irradiated with UV-light of 254 nm at various time intervals.The decrease in fluorescence intensity was observed till the completion of photo-cross-linking of polymers which is noticed from their decrease in intensity or disappearance of emission peaks.The emission of both polymers P7 and P8 occurred at 430 nm, since they contain similar functional groups and spacer units.

Morphological Study of Photo-Cross-Linking Polymers.
The SEM technique can give high resolution images which enables the visualization of morphological information without losing any accuracy during analysis.The synthesized photo-cross-linkable liquid crystalline polymers were irradiated with UV-light of 254 nm for 30 minutes.The virgin polymers (P1 and P2) and photo-cross-linked polymers (P1 and P8) were characterised by HITACHI Scanning Electron Microscope (SEM) S-3400N model to understand the morphology of both virgin and photo-cross-linked polymers.The SEM images of both virgin and photo-cross-linked polymers shown in Figures 12(a) and 13(b).As observed from the SEM images of Figures 12(b) and 13(b), the photo-crosslinked polymer sample confirms loosely held dispersion of polymeric materials, while virgin polymer exhibits compact stringent dispersion of polymeric surface as described by Mathur and Kumar [15].
It is inferred from the SEM images that all the virgin polymers P1-P8 have irregular-shaped flakes in their lattice which packed on one over the other in nondirectional manner, and they all have hard and crystal-like surface.But, SEM images of photo-cross-linked polymers from P1 to P8 show uniform size of polymer flakes which were arranged regularly.
It can be clearly observed from the SEM images that their surfaces after photo-cross-linking were smoothed well, and all the irregular crystal with rough surface has been changed into brightened smooth surface.The smoothness in the polymer surface may be due to photodimerisation of polymers.When two polymer molecules undergo cyclobutane ring formation during photo-cross-linking, one polymer molecule bounds to the other through cyclobutane ring.This structural interaction may lead to smoothness and regular or ordered arrangement of polymer lattice after UV-treatment.

Liquid Crystalline Properties of Polymers.
The development of photosensitive media based on liquid crystalline compounds for data recording, optical storage, and reproduction is one of the most rapidly developing areas in the physical chemistry of low molecular mass and polymer liquid crystals [16].The rigidity of the mesogenic core, the flexible spacer length, and terminal units highly influence the molting temperature, mesophase temperature, and even molecular arrangement.
In some polymers, they are taking the effect of mesogen and spacer together; a polymer having rigid mesogen and shorter spacer should show the higher transition temperature [17].The phase transition temperature and mesophase of the polymers are studied by traces of DSC thermogram and HPOM images.Generally in the DSC thermogram, at the highest transition temperature there will be an endotherm corresponding to the transition from LC phase to isotropic phase.The transition in some cases from crystal to liquid crystal is marked by more than one endotherm.When such multiple curves were observed, the one having the highest temperature is attributed to crystal-to-mesophase transition.
The DSC thermograms of Polymers P1 and P2 are shown in the Figure 14 which shows two endotherms observed in the heating scan and in the annealed samples.In general, for the first lowest transition temperature which occur just after the glass transition temperature is melting endotherm, and the second highest transition temperature attributed to nematic phase to isotropic mesophase transition.The polymers P1 and P2 show crystal-to-nematic phase transition at temperatures at 105 ∘ C and 97 ∘ C, respectively.The polymers  The  2 values which are known as the fitting parameters determining the fine fit for triexponential decay are found to be <1.3, and average lifetime ⟨⟩ is calculated using the following equation [18]:  given in Table 2.The polymer solutions were prepared in the concentration range 10-20 mg/L using chloroform.The lifetime measurements of two polymers were determined at nonirradiative and UV-irradiative condition at different time intervals.Since the number of molecules with electron donor or withdrawing groups affects fluorescence only partially [19], the polymers show very little lifetime variations before and after UV-irradiations.In both polymers P1 and P2, lifetime values are decreasing after UV-irradiation at different time intervals 0, 150, and ∞ seconds.
In general, rotation of the part of the molecule participating in fluorescence is the most trivial process of the nonirradiative energy loss and typically occurs in the excited state.Considering the molecules of the bond order in ground state equal to 2, upon excitation the electron from the bonding orbital is promoted to the excited state orbital  producing bond order 1.Such change in the bond order transforms the rigid frame work formed by the double bond to a flexible system of single bond, leading to twisting of molecule around a C-C bond causing subsequent cis/trans isomerisation [20].Since rotation around the double bonds contributes to a decrease in lifetime, it is logical to suggest that any restriction of rotation, such as rigid environment of the molecules, would marginalize the role of nonradiative path way and lead to subsequent increase of fluorescence lifetime.Molecules capable of undergoing an electron transfer process possess strong electron donating and occasionally electron withdrawing group [21].Among withdrawing groups, only nitro group has been used successfully as quenchers other withdrawing groups may affect a little fluorescence lifetime.For example, tetranitrofluorescence has lifetime of 2.4 ns compared with 4.0 ns for fluorescence.
The polymers poly(4-[4  -flurobenzoylstyryloxy] butyl methacrylate) (P1) and poly(6-[4  -flurobenzoylstyryloxy] hexyl methacrylate) (P2) fluorescence lifetime values before irradiation were 5.94 and 6.05.The molecules capable of undergoing an electron transfer possess strong electron donating and occasionally electron withdrawing group.The exceptions are however numerous, and a number of molecules with electron donor or electron withdrawing group affect fluorescence lifetime partially.So the electronegative nature of flurogroup substitution at the 4th position of both polymers P1 and P2 might not be affected by the fluorescence lifetime values.Nevertheless, the two polymers showed restriction to free rotation after photodimerisation, and slight polarisation of ester linkage predominates into further decrease of fluorescence lifetime up to infinite photodimerisation.In the polymers P1 and P2 they show  2 values near to 1.1 or 1 which indicate all the fits are monoexponential and show the goodness of fitting parameters.

Conclusions
The photos-cross-linkable, liquid crystalline polymers P1 and P2 were synthesized by free radical polymerization in THF using AIBN as initiator.The synthesized polymers have been characterized by H 1 -NMR, C 13 -NMR, and UV-Vis spectral studies.The TGA analysis clearly indicates that the polymers show 50% weight loss near to 400 ∘ C which exhibit good characteristics of thermal and thermo-oxidative stability.The polydispersity index (PDI) values 1.50 and 1.51 obtained from GPC indicate that polymerization was terminated by free radical combination.The photo-cross-linking and fluorescence lifetime study of polymers show their indispensable importance in photoresist applications.The liquid crystalline property of the polymers was identified from DSC and confirmed by HOPM images at 147 ∘ C and 126 ∘ C. From the average lifetime values 5.32 ns and 4.77 ns at infinite UVirradiation on both P1 and P2 reveal that the photo physical behavior of polymers using the time-correlated single photon counting (TCSPC) method.Thus, the synthesized polymers exhibit both photoresponsive as well as liquid crystalline property, and they might be useful in optical data recording and nonlinear optical (NLO) applications.

Table 1 :
Thermogravimetric Analysis (TGA) and liquid crystalline properties data of polymers (P1 and P2).∘ C) at weight loss (%)  ( ∘ C)   ( ∘ C)   ( ∘ C) Δ =   −   ( ∘ C) 4.3.Thermal Properties.The Thermogravimetric Analysis (TGA) of prepared polymers was measured under nitrogen atmosphere in the temperature ranges 30-700 ∘ C in order to investigate the thermal stability.The TGA data are illustrated in Table1.The data in Table1and Figure6indicate that the homopolymers decompose at higher temperature, and they 4.4.Photo-Cross-Linking Studies.The photo-cross-linking studies were carried out to study the changes which occurred in the polymer during UV irradiation to confirm photoresist nature of polymer.The polymer solution was prepared in the concentration range of 10-20 mg/L using chloroform.It was irradiated with UV-light of 254 nm; the photo-cross-linking ability of the polymer was followed by the rate of disappearance of the C=C bond of photosensitive group in the UV spectrum.When the polymers irradiated with UV light of 254 nm, they undergo 2 + 2 cycloaddition and form photodimers as shown in Scheme 3. The absorption )