Fluorescent polyacrylate derivate from 4-biphenylmethanol with UV-green emission

ABSTRACT In this paper we report the synthesis of a new polyacrylate named poly(1,1ʹ-BP4MA) which is a derivate from 4-biphenylmethanol monomer. Poly(1,1ʹ-BP4MA) was obtained by solution and bulk polymerization techniques to yield polymers with high molecular weight and high solubility. The study of the optical properties showed that poly(1,1ʹ-BP4MA) is a fluorescent material with emission in the UV-green region and it has similar quantum yield to tryptophan.


Introduction
The short-wavelength radiation from solid state sources it is important for the construction of full colour displays because different phosphors can be excited by UV, violet or blue light to obtain light with higher wavelengths [1]. Further, short-wavelength emitter materials are interesting for construction of high density information storage devices [2].
However, it is difficult to obtain appropriate materials to fabricate OLEDs with efficient emission of UV or blue light. Some properties required for this are wide band gap, high quantum yield, high solubility and capability to form films with controlled morphology. To achieve the wide band gap it is necessary to confine the extent of conjugation imposing constraints in the molecular size or introducing a twist between conjugated rings. This implies the design of non-planar conjugated structures [3]. Despite poly(p-phenylenes) are still the most employed organic materials for short-wavelength emitters, they exhibit poor solubility in organic solvents and they are difficult to process [4]. Thus, it is necessary to design novel organic materials with UV-Blue emission.
However, there is not much report about satisfactory materials for UV and violet light emitters. Some examples of organic materials containing fluorene groups with emission around 400 nm have been reported recently [21]. An alternative for the obtaining of organic shortwavelength emitters are materials with biphenyl and terphenyl groups [22][23][24]. These groups contain conjugated aromatic rings connected by single C-C bonds with free rotation, which increase the value of band gap [25,26].
Previous works have reported the study of the spacial order in polymers containing biphenyl pendant groups. However, these polymers do not have any application until now [27,28].
Taking account this background, in this work we report the synthesis and characterization of a polyacrylate with biphenyl pendant groups. This polymer exhibits fluorescent emission in the range of 300 to 550 nm and wide band gap. Further, the polymer is highly soluble in different organic solvents and it can be obtained by solution or bulk polymerization techniques.

General experimental procedures
Reagents and solvents employed for the synthesis of monomer and polymers were purchased from Sigma-Aldrich Company. Acryloyl chloride was purified by distillation in presence of hydroquinone. Pyridine was distilled and dried over KOH under argon atmosphere. Tetrahydrofurane (THF) was distilled and dried with metallic sodium and benzophenone under argon atmosphere. 2,2ʹ-azobisisobutironitryle (AIBN) and benzoyl peroxide (BPO) were recrystallized in methanol. 4biphenylmethanol was employed as received.
FT-IR spectra were acquired with a Perkin-Elmer Frontier spectrophotometer with a spectral resolution of 4 cm −1 , samples were prepared as pellets with anhydrous KBr. Nuclear magnetic resonance (NMR) spectra were recorded with a Varian nuclear magnetic resonance 400 MHz spectrometer and it was operated at 400/100 MHz for 1 H and 13 C NMR respectively. Tetramethylsilane (TMS) was used as internal reference and samples were prepared with CDCl 3 as solvent. Molecular weight of polymer was obtained by gel permeation chromatography (GPC) with an ALLIANCE 2695 WATERS chromatographer with an UV WATTERS 2998 (Photodiode Array Detector) detector and two PLgel MIXED C lineal columns. Elemental analysis (C, H, N) was carried out with a Perkin-Elmer C, H, N analyser model 2400 using acetanilide as reference. UV-Vis absorption and fluorescence spectra in solution were acquired with with a Perkin-Elmer Lambda 2S and Lambda LS55 spectrophotometers. Samples were prepared as 10 mg dm −3 solutions employing chloroform, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), toluene and chlorobenzene as solvents. Spectra were recorded scanning between 200 and 800 nm at room temperature. Samples were placed in quartz cuvettes with optical length of 10 mm. Optical characterization in solid state was performed with a Perkin-Elmer Lambda 35 for UV-Vis absorption and Lambda LS55 spectrophotometer for fluorescence.
The optical band gap was calculated from the UV-Vis spectrum using the Planck equation: Where E g°p t corresponds to the optical band gap value between HOMO and LUMO orbitals expressed in eV; h is the Planck constant (4.14X10 −15 eV•s); and c represents the light velocity in the vacuum (3X10 8 m•s −1 ). λ is the intersection point of the tangent line to UV-Vis spectrum with x axis. The relative quantum yield of fluorescence (Φ) was calculated by a comparative method using the equation Where A is the absorbance in the UV-Vis spectrum, F is the area under the curve of the fluorescence spectrum, η is the refraction index of the solvent. The x and s subindices appoints the standard and sample respectively.
Samples were prepared as 2 mg•dm −3 solutions in chloroform. Tryptophan (Φ = 0.14) 10 −6 mol•L −1 in water was used as standard. The Stokes shift was calculated as the difference in nm between the spectral positions of λ max of absorption and λ max of emission.
The monomer (1,1ʹ-biphenyl)-4-ylmethyl acrylate (1,1ʹ-BP4MA) was synthetized by reaction nucleophilic acyl substitution reaction of 4-biphenylmethanol with acryloyl chloride as it is shown in the scheme 1.  (15,30 or 60 min). The percentage of insoluble polymer formed (gel content) was obtained by extracting the soluble part (sol fraction) from the cured monomer with acetone. After extraction, the gel content was dried to a constant weight. Gel content is taken as the weight percent of poly(1,1ʹ-BP4MA) obtained.

Results and discussion
3.1. Characterization of the 1,1ʹ-BP4MA monomer The monomer 1,1ʹ-BP4MA is an ester obtained by reaction of 4-biphenylmethanol with acryloyl chloride. The monomer was obtained as a yellow liquid miscible with different organic solvents (toluene, CHCl 3 , AcOEt, acetone, EtOH, DMF and DMSO) and it was characterized by FTIR and NMR spectroscopies. The most important evidence of formation of the new monomer in the FT-IR spectrum is the absorption band at 1735 cm −1 attributed to νC = O vibration of ester group. The presence of the double bond of alkene was confirmed by the absorption band at 1640 cm −1 assigned to νC = C vibration. Also, the spectrum showed the band of νC = C vibration of aromatic ring at 1600 cm −1 . The 1 H-NMR spectrum (Figure 1) showed three signals between 6.47 and 5.86 ppm attributed to protons of vinyl group. Signals due to aromatic protons were observed between 7.7 and 7.3 ppm. Finally, the single signal at 5.24 ppm was assigned to protons of methylene group adjacent to acrylate group. The 13   at 128.54 and 131.04 ppm were assigned to the carbon atoms of vinyl group.

Solution polymerization of 1,1ʹ-BP4MA
The homopolymer poly(1,1ʹ-BP4MA) was obtained as a yellow solid and it was characterized by FTIR and NMR.
Considering that the monomer 1,1ʹ-BP4MA is a liquid compound, the first evidence of polymerization of 1,1ʹ-BP4MA was the change of the physic state obtaining poly 1,1ʹ-BP4MA is a highly reactive monomer able to polymerized to yield polymers with high molecular weight. The molecular weight of poly(1,1ʹ-BP4MA) was determinate by GPC obtaining M n = 154,982 g•mol −1 , M w = 406,063 g•mol −1 and polydispersity of 2.62 ( Figure 3). Poly(1,1ʹ-BP4MA) is a linear polymer with high solubility in organic solvents such CHCl 3 , toluene, benzene, DMF and DMSO, which is useful for the obtaining of films of poly(1,1ʹ-BP4MA) by different techniques.

Bulk polymerization of 1,1ʹ-BP4MA
Bulk polymerization of 1,1ʹ-BP4MA was realized using two different thermal initiators: AIBN and BPO at 70 and 90°C respectively with different reaction time (15,30 or 60 min). After polymerization there was obtained a yellow and translucent solid with the mold shape. The cured polymer was treated with acetone to extract the not polymerized monomer and the gel content was calculated with the next equation % gel ¼ mass after extraction mass before extraction Ã 100 (4) Figure 4 shows the gel percent as function of the polymerization time for the 1,1ʹ-BP4MA with both thermal initiators. AIBN and BPO leaded to obtain gel percentages close to 100% after 60 min of reaction ( Figure 4). However, when polymerization is carried out with BPO, higher gel percent are obtained      Fluorescence spectra of poly(1,1ʹ-BP4MA) were acquired from 10 mg•dm −3 solutions in different solvents ant they are showed in the Figure 5(b). Spectra showed wide emission bands in the range of 275-550 nm which corresponds to UV-green region. The spectrum in chloroform showed λ max in 321 nm, which correspond to UV region. The spectra acquired in toluene, chlorobenzene and DMF are similar and λ max was observed at 319 nm, while the spectrum obtained in DMSO two maximum points at 321 and 370 nm. The Φ of poly(1,1ʹ-BP4MA) in chloroform is 0.12 and it is comparable with the quantum yield of fluorescence of tryptophan ( Figure 6). Nevertheless, the Φ of the poly(1,1ʹ-BP4MA) in toluene, chlorobenzene, DMSO and DMF was lower than Φ obtained in chloroform (Table 1), due to the low intensity of the emission band with the change of solvent. Poly(1,1ʹ-BP4MA) showed high values of Stokes Shift and they could be associated with the non-planar structure of biphenyl group which exhibit free rotation around C-C bond between aromatic rings. The results obtained from optical characterization are summarized in the Table 1.
Finally, the optical properties of poly(1,1ʹ-BP4MA) were studied in solid state. The UV-Vis spectrum shows three wide absorption bands at 258, 303 y 362 nm. While, the fluorescence spectrum shows a wide emission band in the range of 315 y 575 nm, which corresponds to UV-green region, with λ max at 382 nm. The UV-Vis and fluorescence spectra showed that the optical properties are preserved in solid state ( Figure 7).

Conclusions
In summary, 1,1ʹ-BP4MA is a highly reactive monomer which could be polymerized by different techniques to obtain polymers with high molecular weight and high solubility. Fluorescent polymers can be obtained by polymerization of 1,1ʹ-BP4MA and they have emission in the UV-green region of electromagnetic spectrum. The fluorescence quantum yield of the polymer was 0.12 showing similar properties to tryptophan. The polymers obtained from 1,1ʹ-BP4MA monomer could be easily applied in the construction of fluorescent thin layers and coatings.

Acknowledgments
The authors thank PRODEP for financial support.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the PRODEP.