Synthesis and characterization of low-molecular-weight π-conjugated polymers covered by persilylated β-cyclodextrin

The paper reports the preparation of a poly[2,7-(9,9-dioctylfluorene)-alt-5,5'-bithiophene/PS-βCD] (PDOF-BTc) polyrotaxane copolymer, through a Suzuki coupling reaction between the 5,5'-dibromo-2,2'-bithiophene (BT) inclusion complex with persilylated β-cyclodextrin (PS-βCD), and 9,9-dioctylfluorene-2,7-bis(trimethylene borate) (DOF) as the blocking group. The chemical structure and the thermal and morphological properties of the resulting polyrotaxane were investigated by using NMR and FT-IR spectroscopy, TGA, DSC and AFM analysis. The encapsulation of BT inside the PS-βCD cavity results in improvements in the solubility, as well as in different surface morphology and thermal properties of the PDOF-BTc rotaxane copolymer compared to its noncomplexed PDOF-BT homologue. In contrast, the number-average molecular weight (Mn) of PDOF-BTc rotaxane copolymer indicated lower values suggesting that the condensation reaction is subjected to steric effects of the bulkier silylated groups, affecting the ability of the diborate groups from the DOF molecule to partially penetrate the PS-βCD cavity.


Introduction
Organic materials with extended π-conjugation have attracted considerable attention in recent years as a new class of active organic materials for optoelectronic applications [1][2][3][4].Among the conjugated polymers, a number of poly(9-alkylfluorene)s (PFs) and poly(9,9-dioctylfluorene) (PDOF) polymers in particular have been the focus of much research as encouraging candidates for organic light-emitting diodes (OLEDs) due to their pure blue luminescence and high efficiency [5][6][7][8][9][10].The application of PFs and PDOF conjugated polymers is limited by unwanted side effects, such as aggregation and a wide emission band during operation [8].Taking into account the relevant photophysical properties of these organic compounds, new synthetic approaches were developed.Copolymerization of fluorene with thiophene, bithiophene or other aromatic comonomers is an alternative method for tuning the optical, electronic and thermal properties [11][12][13][14][15][16][17][18].However, as a consequence of intramolecular interactions, these synthetic methods are often accompanied by undesirable side effects influencing the optoelectronic properties, e.g., red shifting or lower fluorescence.
As a route to candidate materials for use in molecular devices, the construction of mechanically interlocked molecules, such as rotaxanes and polyrotaxanes, has attracted considerable attention [19][20][21][22][23].A rotaxane assembly comprises a macrocyclic component (host) encircling an axle (guest) through noncovalent interactions; bulky groups (also known as stoppers) are attached at the ends of the axle to prevent dethreading of the host.
In the past few years many authors have demonstrated that the encapsulation of conjugated polymers into macrocycle cavities plays an important role in the construction of diverse polymeric architectures.Moreover, the fabrication of mechanically interlocked molecules, such as polyrotaxanes, has been investigated as a method for the further improvement of thermal and electrooptical properties through the insulating backbones of conjugated polymers [20][21][22][23][24].The studies revealed an attractive approach to achieve a higher degree of control over molecular rigidity, prevention of aggregation, fluorescence efficiency, improved solubility, and surface-morphological properties of the resulting conjugated polyrotaxanes [5,11,21,23,[25][26][27][28][29][30][31].
The first step in the preparation of PDOF-BTc polyrotaxane is the threading of the BT monomer through the PS-βCD cavity to form the BTc inclusion complex.BTc obtained as a precipitate from a 2:1 mol/mol mixture of PS-βCD and BT in acetone was isolated, purified and characterized by 1 H NMR, as can be seen in Figure 1.The average number of PS-βCD macrocycles per BT unit was calculated by using the ratio of the integrated area of the peak assigned to the H from -CH 3 groups of PS-βCD (0.09-0.11 ppm, I H-CH3 ) and the proton peaks of BT (6.85-7.97ppm, I BT ), Figure 1.The average number of PS-βCD macrocycles per BT unit was calculated as (I BT /4)/(I H-CH3 /63) and found to be 0.95 (i.e., ca.95% coverage).
Not surprisingly, due to the presence of PS-βCD the rotaxane PDOF-BTc copolymer showed a marked contrast compared with PDOF-BT.The PDOF-BTc rotaxane copolymer was soluble (~10% by weight) in petroleum ether (after vortex stirring at room temperature for 15 minutes).The 1 H NMR spectrum of a soluble fraction in petroleum ether indicated a higher coverage with PS-βCD.From this spectrum a PS-βCD/BT molar ratio of about 1/1.5 was determined, as can be seen in Figure S1 in the Supporting Information File 1.The chemical structure of PDOF-BTc (insoluble part in petroleum ether) and PDOF-BT copolymers was proved by 1 H NMR and FTIR analysis.The infrared spectrum of the PDOF-BTc (Figure 2B) shows all the characteristic bands of PDOF-BT and additional bands located in the 748-1251 cm −1 region.Note that the peaks at 792 and 817 cm −1 , assigned to the C-H out-of-plane bending vibrations, and the peak at 880 cm −1 , assigned to the C-H in-plane vibrations of the aromatic rings, were at distinctly lower energy in PDOF-BTc as compared to the corresponding peaks of the non-rotaxane PDOF-BT copolymer.The coverage of the rotaxane copolymer with the macrocycle, i.e., the average number of PS-βCD macrocycles per repeating unit [23], was determined from the NMR spectral analysis and was calculated from the ratio of the integrated area of the peak assigned to the H from -CH 3 groups of PS-βCD (0.121-0.249 ppm, I H-CH3 ) and the proton peaks of BT (7.247-7.412ppm, I BT ), as shown in Figure 4.The average number of PS-βCD macrocycles per repeat unit was calculated as (I BT /4)/(I H-CH3 / 63) and found to be 0.32 (i.e., ca.32% coverage).The peaks corresponding to the fluorene copolymer chains in PDOF-BTc were down-field shifted by 0.03-0.08ppm, as compared to the noncomplexed PDOF-BT sample (Figure 4).The weight-average molecular weight (M w ) and numberaverage molecular weight (M n ), with the polydispersity index PDI (PDI = M w /M n ), of PDOF-BTc and PDOF-BT samples were determined by gel-permeation chromatography (GPC) analysis, using polystyrenes as standards and CH 2 Cl 2 as eluent.GPC data are listed in Table 1 and shown in Figure S2 and Figure S3 in the Supporting Information File 1.On the basis of the molecular weight, the numbers of repeat units for PDOF-BTc and PDOF-BT samples are 10 and 40, respectively.PDOF-BT non-rotaxane copolymer has higher molecular weights than the PDOF-BTc sample, an expected result owing to the lower accessibility of diborate to bromine groups encapsulated in the PS-βCD cavity.The higher PDI values of the PDOF-BTc copolymer compared to PDOF-BT can be attributed to variations in the average number of PS-βCD units per macromolecular chain (see incomplete coverage determined by 1 H NMR above).It is important to note that the lower molecular weights of PDOF-BTc rotaxane copolymer, which have not been observed for other polyrotaxanes [5,29,31,34], can be assigned to the contribution from the structure of the PS-βCD macrocycle, which provides a deeper insight into the blocking effect of silylated groups on the cross-coupling reaction.In order to obtain higher-molecular-weight polyrotaxanes the optimal reaction time has to be taken into consideration.The thermal stability of the inclusion-complex BTc, PDOF-BT and the PDOF-BTc copolymers was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as shown in Figure 5 and Figure 6, and the results are summarized in Table 2.The endothermic peak attributed to the melting temperature of BT (144 °C), as a result of a decomplexation process (17%) was present in the second heating scan of BTc (Figure 5).The presence of two broadening peaks accompanied by a shift to a lower melting temperature for BTc (133 °C, ΔH = 5.22 J/g) with a strong reduction of intensity could be attributed to a heating-favoured loosening of the  crystal forces of BT included in the amorphous PS-βCD cavities.
On the cooling cycle (not shown) the exothermal sharp peak attributed to the crystallization process of BTc was found at lower temperature (108 °C, ΔH = 7.66 J/g)) compared with noncomplexed BT (111 °C).DSC profiles of PDOF-BTc copolymer does not indicate T g or melting temperature in the 30-200 °C interval, as can be seen in Figure 6.These findings indicated their amorphous nature and rigid chains.
The effect of PS-βCD on the thermal stability of the samples was further supported by TGA in a nitrogen atmosphere, which revealed the stage of the degradation process, shown in Figure 6 and Figure S4 in the Supporting Information File 1.As one can see from Figure 6, BTc, PDOF-BT and PDOF-BTc samples present two thermal decomposition steps.For the PDOF-BT sample the decomposition process starts at 212 °C, while the degradation of its PDOF-BTc complex homologue begins at a higher temperature (341 °C).Thus, a stabilizing effect of the inclusion of the BT chain into the PS-βCD cavity for the first decomposition step is evidenced.The maximum degradation process of the samples is around 271 °C for BTc, 264 °C for PDOF-BT and 383 °C for PDOF-BTc, i.e., not much higher than the decomposition temperature of PS-βCD (380 °C).The TGA data revealed that the PDOF-BTc rotaxane copolymer is stable up to 300 °C.The improvement of the thermal stability provides an indication that the rotaxane forma-tion increases the stability of the PDOF-BT macromolecular chains.
The rotaxane copolymer has good solubility in common organic solvents, such as CHCl 3 , CH 2 Cl 2 , THF, and toluene, and allows the formation of homogeneous and transparent films.In order to analyse the influence on the surface morphological properties induced by using 3T as an end-capping reagent instead of bromobenzene [12], AFM experiments were also performed.The 2D AFM images of the top surface of both copolymer films, within the same scan areas of 2 × 2 μm 2 are shown in Figure 7a and Figure 7b (insets show the 3D images).They afforded the root-mean-square roughness of the formations (S q ) and the average roughness (µ) as well as the average heights (H a ), see Table 3.
The 3D AFM images of the complexed PDOF-BTc film (insets Figure 7b) show more favourable surface parameters in comparison to the noncomplexed copolymer (insets Figure 7a).PDOF-BT showed an agglomeration tendency and globular formations with S q and µ values of 12.43 and 9.83 nm, whereas the PDOF-BTc sample showed a uniform and smooth surface, covered with individual small, spherically shaped formations, with smaller S q and µ values of 5.29 and 3.70 nm.The average AFM heights of the PDOF-BT and PDOF-BTc decreased from 34.40 to 17.34 nm, respectively, as shown in Figure 7a' and Figure 7b' and documented in Table 3.
The trend toward a more uniform and smoother surface, already observed for rotaxane copolymers [12,23,27,31], continues for the PDOF-BTc polymer rotaxane, where a decrease of the S q and the value of the H a parameter (the first moment of the height distribution) indicated that the rotaxane architecture changed the morphology of PDOF-BT copolymer.These results could provide microscopic evidence for reduced interchain interactions, which allow the polyrotaxane chains to pack densely.A better quality of the film, with higher uniformity, flatness, and better mechanical properties, is always desirable for optoelectronic applications.
The optical properties of the PDOF-BT and PDOF-BTc compounds (not shown) recorded in CHCl 3 solution, as well as in thin film, upon excitation at the maximum absorption value (λ max = 380 nm) indicate no significant difference in the electronic absorption and emission spectra of both copolymers, consistent with previously reported results [12].The similarity between the absorption and fluorescence excitation spectra shows that major structural changes have not occurred, and that emissions arose from the compounds and not from the thienyl end-capping, as has been previously reported for PFO with perylene derivative as end-units [35].

Conclusion
The present study confirms that PS-βCD can be used as a host macrocycle molecule in the synthesis of a main-chain polyro-taxane with alternating fluorene-bithiophene moieties.Lower values of number-average molecular weight (M n ) of the rotaxane copolymer suggested that the condensation reaction is subjected to steric effects of the bulkier silylated groups from PS-βCD, thus requiring a longer reaction time.We have demonstrated that PS-βCD covered bithiophene-fluorene copolymer has beneficial effects on the solubility in nonpolar solvents, and on the thermal properties and surface characteristics.

Synthesis of the PDOF-BTc copolymer
To a three-necked flask, 0.602 g (0.202 mmol) of BTc and 0.112 g (0.2 mmol) of DOF was added.The flask was equipped with a condenser, evacuated, and filled with argon several times to remove traces of air.Degassed toluene (6 mL) was added as solvent into the flask and subsequently 4.8 mg (0.42 × 10 −2 mmol) of (Ph 3 P) 4 Pd(0), dissolved in 5 mL of degassed toluene, and 2 mL of 5 M Na 2 CO 3 solution were added.The mixture was vigorously stirred in the dark under an argon atmosphere for 72 h at 85-87 °C.An excess of 0.0113 g (0.02 mmol) of DOF dissolved in 3 mL of toluene was then added and the reaction was continued for 12 h in order to obtain the macromolecular chains terminated with borate units.Finally, 0.2 μL of 3BT was added as the end-capper of the copolymer chain, and the reaction was continued overnight.After cooling, the mixture was poured into the stirred mixture of methanol and deionised water (10:1).The fibrous solid obtained by filtration was solubilised in 20 mL of toluene, washed with water three times to completely remove the alkali solution, dried over anhydrous MgSO 4 , and concentrated by vacuum evaporation of the solvents.The residue was dissolved in a minimum volume of CHCl 3 (10 mL) and poured into 10 times the volume of stirred methanol, and then filtered, thoroughly washed with 5 mL of acetone, and dried under reduced pressure at 60 °C.PDOF-BTc was obtained in 0.125 g (approximately 23.0% yield) as a lightorange solid.After drying, the solid was immersed in petroleum ether, vortex stirred for 15 min, and then filtered.The precipitate was filtered and dried under vacuum.The soluble part in petroleum ether was concentrated by vacuum evaporation.Both fractions of PDOF-BTc were found to be also soluble in THF, CH

1 H
NMR spectra of PDOF-BT and PDOF-BTc (insoluble part in petroleum ether) are presented in Figure3and Figure4, respectively.The NMR spectrum of PDOF-BT shows characteristic peaks for both DOF and BT chains in good agreement with the proposed structures.

Figure 5 :
Figure 5: DSC curves of BTc and PDOF-BTc from second-heating DSC measurements.

Figure 7 :
Figure 7: High-resolution tapping-mode AFM images and cross-section plots (along the solid line in the images) of the PDOF-BT (a, a') and PDOF-BTc (b, b').

Table 1 :
The molecular weights of the polymers.

Table 2 :
Thermal properties.The onset temperature of the degradation process.bThetemperature of complete degradation process.cThemaximumdegradationtemperature.dThemass percentage loss recorded in each stage.e The amount of residue at the end of degradation process. a
a Root-mean-square roughness.b Average roughness.c Average heights.