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 1H-NMR, 13C-NMR, and UV-Visible spectroscopy. The photo-cross-linking behavior of polymers has been studied by UV-Visible and fluorescence spectroscopy. UV spectral studies revealed that the polymers follow
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 [
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) [
This paper reports newer root for the synthesis of poly(n-[n′-flurobenzoylstyryloxy] alkylmethacrylate)s; the photo-cross-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.
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.
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. 1H-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.
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 time-calibrated 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 chi-square values, Durbin-Watson parameters, as well as visual observations of fitted line, residuals, and autocorrelation functions.
Synthesis of HPFSK.
The pendant chalcone compound HPFSK was synthesized according to the reported literatures [
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 4-fluorobenzaldehyde 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
FT-IR (KBr pellet, cm−1); 1497 (aromatic C=C); 1590 (olefinic CH=CH); 1650 (keto C=O); 3550 (Ar-OH). 1H-NMR (CDCl3,
1H-NMR spectrum of 4-hydroxyphenyl-4′-flurostyryl ketone (HPFSK).
Synthesis of monomers and polymers.
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 K2CO3 (3.39 g, 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
1H-NMR spectrum of (4-[4′-flurobenzoylstyryloxy] butyl methacrylate) (FBSOBMA) (M1).
1H-NMR (Figure
The monomer 6-[4′-flurobenzoylstyryloxy] hexyl methacrylate (FBSOHMA) (Figure
1H-NMR spectrum of (6-[4′-flurobenzoylstyryloxy] hexyl methacrylate) (FBSOHMA) (M2).
1H-NMR (Figure
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
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,
The photo-cross-linkable liquid crystalline monomers and polymers were prepared as shown in Scheme
13C-NMR spectrum of poly(4-[4′-flurobenzoylstyryloxy]butyl methacrylate) (FBSOBMA) (P1).
13C-NMR spectrum of poly(6-[4′-flurobenzoylstyryloxy]hexyl methacrylate) (FBSOHMA) (P2).
The number average and weight average molecular weight of polymers P1 and P2 were determined by PL-GPC650. The number average molecular weight (
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 Table
Thermogravimetric Analysis (TGA) and liquid crystalline properties data of polymers (P1 and P2).
S. no. | Polymer | Temperature (°C) at weight loss (%) |
|
|
|
|
Mesophase | |
---|---|---|---|---|---|---|---|---|
IDT (°C) | 50% | |||||||
P1 | Poly(FBSOBMA) | 160 | 287 | Nil | 105 | 147 | 42 | Nematic |
P2 | Poly(FBSOHMA) | 194 | 310 | Nil | 97 | 126 | 29 | Nematic |
Thermogravimetric Analysis of poly(4-[4′-flurobenzoylstyryloxy] butyl methacrylate) (P1) and poly(6-[4′-flurobenzoylstyryloxy] hexyl methacrylate) (P2).
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
Photodimerisation of polymers (P1 and P2).
The UV spectral changes during photo-cross-linking and photoconversions of polymers are shown in Figures
UV spectral changes during photo-cross-linking of poly(4-[4′-flurobenzoylstyryloxy]butyl methacrylate) (P7) at various time intervals in chloroform solution.
UV spectral changes during photo-cross-linking of poly(6-[4′-flurobenzoylstyryloxy]hexyl methacrylate) (P8) at various time intervals in chloroform solution.
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
Photoconversions on UV-irradiation for polymers P1 and P2.
The existence of photoresponsive behaviour of polymers can be evidenced by fluorescence spectra. The fluorescence intensity of polymer decreases as the time of irradiation increases. The decrease in intensity is due to
Fluorescence spectral changes during photo-cross-linking of polymers P1 and P2 are shown in Figures
Fluorescence spectral changes during photo-cross-linking of poly(4-[4′-flurobenzoylstyryloxy] butyl methacrylate) (P1) at various time intervals in chloroform solution.
Fluorescence spectral changes during photo-cross-linking of poly(6-[4′-flurobenzoylstyryloxy]hexyl methacrylate) (P2) at various time intervals in chloroform solution.
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
Scanning Electron Microscope image of (a) virgin polymer poly(FBSOBMA) (P1) and (b) photo-cross-linked polymer poly(FBSOBMA) (P1).
Scanning Electron Microscope image of (a) virgin polymer poly (FBSOHMA) (P2) and (b) photo-cross-linked polymer poly (FBSOHMA) (P2).
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.
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 [
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 [
The DSC thermograms of Polymers P1 and P2 are shown in the Figure
Differential scanning calorimetric thermogram of polymers poly(FBSOBMA) (P1) and poly(FBSOHMA) (P2).
Polarised optical micrograph of poly(FBSOBMA) (P1) showing nematic mesophase at 147°C.
Polarised optical micrograph of poly(FBSOHMA) (P2) showing nematic mesophase at 126°C.
The time-resolved fluorescence decays of polymers P1 and P2 have been monitored in chloroform. The polymers P1 and P2 were excited at 460 nm. The fluorescence lifetimes of both polymers after and before the UV-irradiation at wavelength of 254 nm have been measured and plotted in Figures
Fluorescence lifetime decay of polymers P1 and P2.
Polymers |
|
|
UV-irradiation time (sec) |
|
|
|
|
|
|
|
|
---|---|---|---|---|---|---|---|---|---|---|---|
0 | 3.07 | 0.4601 | 0.4287 | 0.3657 | 9.05 | 0.1742 | 5.94 | 1.15 | |||
P1 | 313 | 430 | 150 | 2.72 | 0.4417 | 0.4761 | 0.2899 | 7.63 | 0.2683 | 5.60 | 1.12 |
∞ | 2.40 | 0.4349 | 0.4469 | 0.2790 | 7.12 | 0.2862 | 5.32 | 1.06 | |||
| |||||||||||
0 | 3.19 | 0.4946 | 0.8282 | 0.3312 | 9.60 | 0.1742 | 6.05 | 1.18 | |||
P2 | 313 | 430 | 150 | 2.65 | 0.5065 | 0.6778 | 0.2663 | 7.23 | 0.2271 | 4.91 | 1.10 |
∞ | 2.61 | 0.4848 | 0.6154 | 0.2926 | 7.01 | 0.2226 | 4.77 | 1.09 |
(a) Lifetime decay of poly(4-[4′-flurobenzoylstyryloxy]butyl methacrylate) (P1) before UV-irradiation. (b) Lifetime decay of poly(4-[4′-flurobenzoylstyryloxy]butyl methacrylate) (P1) after 150 sec UV irradiation. (c) Lifetime decay of poly(4-[4′-flurobenzoylstyryloxy] butyl methacrylate) (P1) after infinite sec UV irradiation.
(a) Lifetime decay of poly(6-[4′-flurobenzoylstyryloxy]hexyl methacrylate) (P2) before UV-irradiation. (b) Lifetime decay of poly(6-[4′-flurobenzoylstyryloxy]hexyl methacrylate) (P2) after 150 sec UV irradiation. (c) Lifetime decay of poly(6-[4′-flurobenzoylstyryloxy] hexyl methacrylate) (P2) after infinite sec UV irradiation.
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 [
Molecules capable of undergoing an electron transfer process possess strong electron donating and occasionally electron withdrawing group [
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
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 H1-NMR, C13-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 UV-irradiation 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.
The authors are thankful to Professor (Dr.) S. Ganesan, Director of Students Affair, Anna University, Chennai, India, for providing fluorescence lifetime facility to carry out this research and sincerely thankful to Dr. T. Narasimhaswamy, Scientist, CLRI, Adyar, Chennai, for carrying out POM studies.