Synthesis and Characterization of a Multifunctional Conjugated Polymer

This work reports the synthesis and characterization of a conjugated polymer based on fluorene and terpyridine, namely, poly[(9,9bis(3-((S)-2-methylbutylpropanoate))fluorene-alt-6,6-(2,2:6,2-terpyridin-6-yl)] (LaPPS71). The structure was characterized by Hand Cnuclearmagnetic resonance (NMR) andFourier-transform infrared (FTIR) spectroscopy.Themolarmasswasmeasured by gel permeation chromatography (GPC). As thermal characterization, the glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC). The polymer structure contains two sites capable of complexation with metallic ions, affording the possibility of obtainment of independent or electronically coupled properties, depending on the complexation site. The photophysical properties were fully explored in solution and solid state, presenting ideal results for the preparation of various metallopolymers, in addition to potential application as a metamaterial, due to the presence of the chiral center in the side chains of the polymer.


Introduction
In the last decades semiconducting polymers have emerged as promising structures with a wide range of photonic and electronic applications, such as flexible light-emitting displays, sensors for a variety of purposes, and solar panels, among many others [1,2].Their main advantages encompass the combination of semiconducting properties with the ease of processing, flexibility, and lightweight characteristics of polymer materials [2][3][4][5].Nowadays, a new class has emerged which apart from the mentioned features also add metallic properties to the system [6].This new class, the metallopolymers, are composed by a conjugated polymer backbone containing a metallic center that can occur in three main configurations, according to Wolf 's classification [7]: the metallic center is attached to a ramification of the main chain (Type I) coordinated directly to a ligand which is part of the repeating unit of the polymer (Type II) or still can be itself a monomeric unit of the polymer (usually as a co-monomer), (Type III).This way, metallic properties arise such as optical, magnetic, electrochromism and new paths for light emission, to mention the most relevant.The optical properties of these systems may or may not be a result of electronic coupling between the metal and the conjugated backbone, and each possibility offers a set of advantages [7][8][9][10].It is worth mentioning that in Type III ill-defined structures may result due to the possibility of chain growth in several directions that are associated with the coordination state of the metal, leading to ramifications and crosslinking [8].
In this paper we report on the synthesis and characterization of a precursor to a metallopolymer, which apart from adequate site for metal insertion, also bears a chiral center.This characteristic has been used to produce chiro-optical materials that represent a new class of polymers with very special behavior, giving rise to the obtainment of exceptional electromagnetic properties.The applications of chiral polymers are wide, allowing the manipulation of electromagnetic radiations for the achievement of superlenses, electromagnetic invisibility (invisibility cloak), and super-resolution in optical images [11,12], as examples.
The complexation with metals can be performed according to two paths, one uses the ester site or alternatively the terpyridine moiety.This last one is straightforward and can be accomplished as described elsewhere [9].The resulting complexed product belongs to Type II.The tridentate ligand is one of the most used for the preparation of lanthanide complexes [9,[15][16][17].Alternatively, a Type I could be also prepared by using the ester group pendant to the main chain.The ester site needs a conventional hydrolysis process to afford a carboxylic acid [18,19], providing the site for the insertion of the metal [20,21].

Equipment.
The structural characterization of the polymer was performed by 1 H and 13 C NMR spectroscopy, recorded on a Varian Inova-400 Instrument (399.65 MHz for 1 H, 100.40 MHz for 13 C) using polymer solutions containing deuterated chloroform (CDCl 3 ) and TMS as reference.The simulated data for 1 H NMR and 13 C NMR were obtained by ChemDraw Ultra 12.0 software.The FTIR spectra were acquired using KBr pellets with the BOMEM (Hartmann & Braun) MB-Series equipment scanning rate of 4000 -400 cm -1 and 24 scans/min.The molar mass of polymer was determined by gel permeation chromatography (GPC) using polystyrene as standards and THF as eluent at 1 mL/min flow rate in an Agilent 1100 liquid chromatography system with Plgel Mixed-B and Mixed-C columns.Thermal characterization was performed by DSC technique and was measured on a Netzsch DSC 204 F1.All samples were heated from 20 to 200 ∘ C in a nitrogen atmosphere and then cooled to 20 at 10 ∘ C min −1 with scanning of the 10 ∘ C min −1 and nitrogen flow 15 ml ⋅ min −1 .UV-Vis spectra were measured on a Shimadzu UV-3101PC spectrometer.Emission spectra were acquired with on a Shimadzu RF-5301PC spectrophotometer in THF solution.The UV-Vis and emission measurements were performed in THF solution.

Synthesis of the Polymer.
The LaPPS71 was synthesized via Suzuki coupling.The synthesis and structural characterization of the chiral monomer precursor were shown in an earlier work published by our group [13].The general procedure for the synthesis of LaPPS71 was the following: 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9bis(3-((S)-2-methylbutyl propanoate))fluorene (0.2 g, 0.285 mmol) and 6,6  -dibromo-2,2  :6,2  -terpyridine (0.123 g, 0.314 mmol) were added to a 2 mol ⋅ L -1 solution of K 2 CO 3 (12 mL) and toluene (36 mL).The mixture was subjected to argon atmosphere.After 30 minutes, tetrakis(triphenylphosphine)palladium(0) (0.02 g, 0.017 mmol) was added to the mixture and the temperature adjusted at 110 ∘ C. The reaction proceeded under constant stirring for 48 hours.After that, the organic phase was separated from the aqueous one and the product was extracted and washed three times with chloroform.The organic phase was separated from the aqueous one and the product was extracted and washed three times with chloroform.The product of the reaction was mixed with anhydrous MgSO 4 for retention of water traces.Subsequently, the product was purified in a chromatography column using silica as the stationary phase and as mobile phase a combination of dichloromethane/ethyl acetate (4:1) was used.At the end of the procedure the solvent was removed to give 141 mg of pale yellow solid with a 72.2 % yield. 1  For solid state studies, these films were prepared via spincoating at 1100 rpm using chloroform/chlorobenzene (10:1 v/v) solutions, at 10 mg ⋅ mL -1 , which were annealed at 25 ∘ C, 100 ∘ C, 150 ∘ C, 175 ∘ C, and 200 ∘ C, in a vacuum oven for 2 h.
Toluene, 100 ∘ C with density functional theory (DFT), using Becke, Three-Parameter, Lee-Yang-Parr functional B3LYP [22] functional, with 6-311G(d, p) split-valence basis set.Tetramer geometries were used because they represent satisfactorily the effective conjugation length for this kind of structure [23].The structure was submitted to a vibrational analysis, and no imaginary frequency modes were obtained.Based on the optimized structure, the electronic spectra calculation was performed using ZINDO/S (Zerner's Intermediate Neglect of Differential Overlap/Single) parametrization, using the first three singlet excited states, and the transition energies were weighted by the oscillator strength values (f) in the study of organic compounds [24].All simulations were performed using GAUSSIAN 09 program [25].

Results and Discussion
3.1.Synthetic Route.The synthesis followed the Suzuki crosscoupling polycondensation as described in detail above and is illustrated schematically in Figure 2.

Structural Characterization.
The molar mass of LaPPS71 was determined by GPC method, presenting values of M w = 18000 g ⋅ mol -1 and polydispersity (PDI) of 1.9.DSC runs gave for glass transition temperature the value of 98 ∘ C. The material presented good solubility in common organic solvents as chloroform, THF, and dichloromethane.Figure 3 shows the most significant FTIR absorptions, which can be associated with the stretching (]), asymmetric stretch (] as ), symmetric stretching (] s ), and out-of-plane deformation (  ).In the range 1426 and 1561 cm −1 , related to ](C ---C; C ---N) bond is attributed to the terpyridine groups [9,26,27].The band at 794 cm −1 is associated with the   (C-H) psubstituted ring of the fluorene group and 2947 cm −1 refers to ] as (C-H) sp 2 [28].All those bands are attributed to chemical species absorption in the aromatic region of the polymer structure.The strong band at 1730 cm −1 is characteristic of the ] as (C=O) of the ester group of the alkyls [29].There is also the ](C-O) bond at 1160 cm −1 and 2873 cm −1 attributed to the ] s (C-H) sp 3 of the polymer [26,29].These absorptions belong to the aliphatic chain of the structure.  H and 13 C NMR analyses of the polymer were also performed.Figure 4 shows the 1 H NMR spectrum of the structure, in which the numbers in the polymer chain of the spectrum correspond to the hydrogen atoms for the corresponding carbon atom.Table 1 shows the chemical shifts observed in the 1 H NMR spectrum, as well as the simulated and experimental integrations correlated to the polymer structure.The upfield region between 0.72 to 3.85 ppm refers to the hydrogen atoms bonded to the aliphatic carbon atoms from 1 to 7. The chemical shifts seen for the protons of the carbon atoms 5 and 6 are due to the inductive effect of the oxygen atom, strongly electronegative element.Hydrogen atoms of the aromatic chain absorb energy in a magnetic downfield with signs in the region between 7.88 and 8.87 ppm [26,28,[30][31][32].
The polymer structure was also confirmed by the 13 C NMR spectrum, shown in Figure 5, in which the singlet signals stand for the chemical shifts at 137.95 and 136.86 ppm, related to the carbon atoms 1 of the fluorene (see the polymer's structure in the figure).According to the simulated structure of the precursor monomer, the equivalent peaks to carbon atoms 1 would have chemical shifts at 130.9 ppm.The carbon atoms 2 highlighted in the terpyridine showed peaks in the range between 155.41 and 156.26 ppm of the spectrum, while those of the unpolymerized simulated structure possess chemical shifts at 143.1 ppm [26,27,32].A more detailed and accurate analysis of the IR and NMR data about conjugated polymers can be done as described in previous literature [33].

Photoluminescence Studies.
Photophysical studies were performed in THF solutions and film form.Figure 6 shows the absorption spectra in a range of concentrations at room temperature, in which the main bands were seen at 245 nm and 345 nm.For solutions, those bands can be related to the - * transitions, composed by the pyridine-fluorenepyridine chromophore, as supported by theoretical calculations.
Theoretical calculations were performed to study the nature of the electronic transitions involved.Table 2 brings the results obtained by the DFT calculations.
The calculated absorption spectra are characterized by two main contributions at 352 nm and 364 nm, associated with the 250 nm and 350 nm empirically observed.The transitions are mainly characterized as of the - * type, in which the main chromophore in all transitions is composed of pyridine-fluorene-pyridine unit.The first one (352 nm) is associated with the transition HOMO-2→LUMO, which occurs between orbitals presenting in the same atoms, while the other transition, HOMO-1→LUMO+1, takes place between different segments of the chain.Figure 7 brings the compositions of the frontier orbitals of LaPPS71 obtained by the calculations.It is noteworthy that more accurate results could have been obtained using the TD-DFT method instead of the semiempirical ZINDO/S [34].Nevertheless, the former was chosen because it is computationally more rapid, giving a satisfactory cost/benefit ratio, considering the quality of results required for the polymer characterization here.
It is possible to verify that the conjugation is confined to the segment pyridine-fluorene-pyridine.This is accounted  for the terpyridine unit mobility, which can adopt different conformations, breaking the chain conjugation.According to the literature [35] three different conformations can be assigned to this group: cis-cis, trans-trans and cis-trans which in turn would result in changes of the energy levels in the excited state, depending on the chemical environment.
The emission spectra run in THF solutions of various concentrations and in the solid state at room temperature are shown in Figure 8.The spectra are characterized by three bands: the first two show a well-defined profile peaking at 370 nm and 380 nm, whereas the third one covers the 390-500 nm range, depending on the concentration.The first band decreases in intensity and redshifts with increasing concentration.The opposite trend is observed for the second band: for 10 −6 , 10 −5 , and 10 −4 mol ⋅ L −1 solutions, the position of the transition does not change, but intensity increases substantially, indicating an increase in the concentration of emitting units.When the concentration attains 10 −3 mol ⋅ L −1 a redshift is clearly observed.This behavior can be attributed to the formation of intermolecular excited states brought about by - stacking, leading to the formation of new - excitation bands, consequently lowering the band gap [36,37].The decrease of the bands intensity with increases in solution concentration is accounted to the inner-filter effect (IFE), important phenomenon in fluorescence, which basically occurs due the reabsorption of the emitted light by the fluorophore itself, thus decreasing the intensity of emission [38].Additionally, the shoulder presented at 420 nm in all samples turns progressively more evident with concentration increases.In the solid state, the first band is suppressed, and the second one predominates, presenting a broadening on the shoulder, which seems to envelop various emitting states, corresponding to the various conformational states of the chain, mainly due to the terpyridine group.The emitting profile of LaPPS71 makes the polymer an excellent material for lanthanide complexation, especially Eu and Tb ions [9,39].The ligands associated with these ions generally are chromophores capable of sensitizing of the lanthanide light emission.-diketones, for example, absorb energy in a region of the ultraviolet-visible and then transfer efficiently to the lanthanide, via intersystem crossing process.This mechanism, called the "antenna effect" [39,40], could be explored here, since the polymer emits in the same region where the chromophore compound linked to the metal is absorbed [6][7][8].
The effect of thermal annealing was also studied to see how the photophysical properties would be affected using this treatment.The results are shown in Figure 9  in mind that this value is near to T g (98 ∘ C), and was attributed to the long range segmental motion acquired around this point, helping the chains to get close to each other and thus accommodating the - stacking state [13,41].It is noteworthy that the spectra profiles of absorption and emission did not change, indicating that the number of stacked chains increased, or the size of the stacks also increased, but their length (meaning the length of the associated packs) attained an equilibrium.Otherwise the conjugation length would be larger and a bathochromic shift would be observed.

Conclusions
The synthesis and characterization of a novel conjugated polymer based on fluorene and terpyridine were described.The photophysical properties of the material were exploited and their structure confirmed by NMR and FTIR spectroscopy.The structure is a promising conjugated polymer, providing the multifunctional properties of an emitting material, presenting at the same time the possibility of complexation with metallic ions in two configurations.Moreover, the chiral center in the polymer side chain could provide Advances in Condensed Matter Physics a useful material for several applications in the field of optoelectronics.

2. 4 .
Computational Methods.The optimized geometry and frontier levels of the tetramer structure of alternated fluorene and terpyridine copolymer (LaPPS71) were simulated

Figure 6 :Figure 7 :
Figure 6: Absorption spectra of LaPPS71 in solution and solid state.

Figure 8 :
Figure 8: Spectra of the light-emitting of the polymer LaPPS71 in different concentrations of solution and solid state ( exc = 345 nm).

Table 1 :
Chemical shifts of 1 H NMR of the LaPPS71.

Table 2 :
Calculation data for the electronic structure of the LaPPS71.
a oscillator strength; b major contribution.