Effects of molecular weight on the optical and electrochemical properties of EDOT-based π-conjugated polymers

Absolute molecular weight values obtained by gel permeation chromatography with multi angle light scattering of PEDOTF8 were approximately 65% of the relative molecular weight values obtained by gel permeation chromatography using polystyrene standards. Both light absorption and molecular weight measurements showed the effective conjugation lengths (absolute molecular weight <2600, relative molecular weight <4000, number of EDOT-F8 units: n < ca. 5 unit). The low molecular weight polymers showed higher energy absorption and fluorescence bands. Molecular weight also affected the electrochemical process of the polymer films. The high molecular weight PEDOTF8 (number average molecular weight >70000) showed a higher redox stability than the low molecular weight one. The two polymers with number average molecular weights of 70200 and 40000 retained 65% and 25% of the charge storage capacity after 100 electrochemical scans, respectively. Square-wave potential step absorptiometry measurements of the polymers with number average molecular weights of 70200 and 40000 showed that the optical contrasts remain stable after ca. 55 and three cycles, respectively. The high molecular weight polymer has a high electrochemical stability and would be a good material for optoelectronic devices.

The optical and electronic properties of π-conjugated polymers are relevant for many applications in the field of semiconductors [1][2][3] . EDOT, 3,4-ethylenedioxythiophene, is an attractive building unit for producing π-conjugated polymers 4 . The unit enhances the π-conjugation and p-doping through specific interactions and its excellent electron-donating property. Thus, PEDOT, poly(3,4-ethylenedioxythiophene-2,5-diyl), and EDOT-based alternating copolymers are high performance organic semiconductors and fluorophores 4 . For this reason, the focus is often placed on the optical (light absorption and fluorescence) and electrochemical (doping-dedoping) in the polymer film. Several parameters of the polymer affect its optoelectronic properties and device performance, such as the degree of polymerisation (molecular weight values) [5][6][7] and purity 8,9 . The molecular weight values of π-conjugated polymers play an important role in the performance of organic field effect transistors and organic solar cells [5][6][7] . The molecular weight values, including number average molecular weight (M n ) and weight average molecular weight (M w ), and polydispersity of the polymers are of central importance for their applications. Properties such as effective conjugation length and interpolymer coupling within the aggregates as well as the packing behaviour of the polymer chains affect the optoelectronic properties [10][11][12] . Low molecular weight π-conjugated polymers, which were insufficient effective conjugation lengths, offer higher energy light absorption (band gap) and fluorescence than high molecular weight ones 13,14 . Thus, low molecular weight fractions of the polydispersive polymers are critically affected to the optoelectronic properties.
Polycondensation is a standard technique for synthesising the π-conjugated polymers. Synthetic pathways of EDOT-based π-conjugated polymers have been developed through palladium-catalysed cross-coupling research 15,16 . Because the direct arylation reactions have higher atom efficiency and reactivity than the other cross-coupling reactions 17,18 , the direct arylation polycondensation of EDOT with dibromoarenes have received great interest in recent years [19][20][21][22] . High molecular weight EDOT-based π-conjugated polymers (M n > 80000) were successfully demonstrated by flow conditions 19 , microwave-assisted conditions 20 and chloride-promoted conditions 21,22 . The molecular weight values of poly(EDOT-alt-isoindigo) have been reported to affect light absorption properties 19 . Despite the investigation of this donor-acceptor polymer, the relationship between the molecular weight values and the light absorption properties of poly(EDOT-alt-fluorene) is still not fully understood. Poly(EDOT-alt-fluorene) shows a high FET hole mobility (1.2 × 10 −3 cm 2 V −1 s −1 ) and high OPV performance (PCE = 4%) depending on its molecular weight 23 . However, the effects of molecular weight on the fluorescence and electrochemical properties have not been investigated, which presents a challenging topic for understanding polymer properties. In this study, we investigate the light absorption, fluorescence and electrochemical properties of poly(EDOT-alt-fluorene) with different molecular weights.

Results and Discussion
Direct (C-H) arylation polycondensation of EDOT with 2,7-dibromo-9,9-dioctylfluorene (F8) was performed at previously optimised conditions: 1.0 mol% of Pd catalyst (Pd(OAc) 2 , PdCl 2 , PITS-Cl) 21,22 , 30 mol% of 1-adamantanecarboxylic acid, (1AdCOOH) and 3.0 eq. of potassium carbonate (K 2 CO 3 ) in dry N,N-dimethylacetamide (0.3 M) at 80-120 °C under argon for 5-120 min, giving poly(3,4-ethylenedioxytiophene -2,5-diyl-alt-9,9-dioctylfluorene-2,7-diyl), PEDOTF8, with different molecular weights (Fig. 1). To systematically evaluate the relationship between properties and molecular weights of PEDOTF8 different measurements were performed ( Fig. 2 and Table 1). The molecular weights of PEDOTF8 samples were determined by gel permeation chromatography (GPC). The M n and M w /M n values of the polymers were 1800-95000 and 1.3-4.1, respectively (Fig. 2a). The measurement using GPC and polystyrene standards showed relative molecular weights (M n and M w ), which are not the true (absolute) values because rigid (rod) segments of π-conjugated polymers imitate a large hydrodynamic radius that does not correspond to the molecular weight of polystyrene, which has coil  segments. Thus, the average number of repeating unit (n) cannot be determined by the relative molecular weight from GPC. Moreover, calculating the n value by end-group determination using 1 H NMR spectroscopy is difficult because the polymer was synthesised via polycondensation. Absolute molecular weight values of the polymers are required for determining n value. To estimate the values of absolute molecular weight, GPC with multi angle light scattering 24, 25 was performed (Table 1). Absolute molecular weight (M n and M w ) values of PEDOTF8 were approximately 65% of the relative molecular weight values (Fig. 2b). This result shows that the rigid π-conjugated polymer segments cause a larger hydrodynamic radius than the coil segments in polystyrene.
Optical properties of the PEDOTF8 polymers are displayed in Fig. 3. UV-vis absorption bands of PEDOTF8 in dichloromethane were shifted to a lower energy for increasing M n values (Fig. 3a). The band in the film state also showed a bathochromic shift depending on the molecular weight (Fig. 3b). The samples with high M n values (70200, 80000 and 95000) showed similar absorption bands in both conditions ( Fig. 3a and b), displaying a sufficient effective conjugation length of PEDOTF8. The low molecular weight fractions of PEDOTF8, which were of insufficient effective conjugation lengths, were not present in these samples. Thus, the low molecular weight PEDOTF8 samples (M n = 1800-58500) include fractions of insufficient effective conjugation lengths (Fig. 2a, right). Because PEDOTF8 is a low crystallinity (highly amorphous) material, the redshift of absorption band from solution to film is very small 20 . Compared with PEDOTF8 (M n = 95000), PEDOTF8 (M n = 1800) showed a large redshift from solution to film, indicating that the low molecular weight fractions in PEDOTF8 (M n = 1800) form a domain for inter-polymer interaction (Fig. 3c). Fluorescence spectra of PEDOTF8 in dichloromethane also exhibited different bands depending on the molecular weights (Fig. 3d). The lowest molecular weight sample (M n = 1800) showed a low energy fluorescence band peaking at 515 nm (green-coloured fluorescence). However, the high molecular weight samples (70200, 80000 and 95000) showed fluorescence bands peaking at 549 nm (yellow-coloured fluorescence).
Low molecular weight fractions (elution time <12.2 min) were polymers with insufficient effective conjugation lengths (Fig. 2a). The relative molecular weight value of the fraction eluted at 12.2 min was 4000 (Fig. 2c). The absolute molecular weight of this PEDOTF8 was 2600. The number of EDOT-F8 units (n) of the MW was estimated at ca. 5 units. The fractions with an absolute molecular weight below 2600 (relative molecular weight <4000) and less than or equal to five EDOT-F8 units showed dramatically reduced optical properties (Fig. 2a). Therefore, the effective conjugation length of PEDOTF8 is n > 6 units of EDOT-F8.
The film forming ability and toughness of polymers depend on the length of polymer segment (molecular weight) and crystalinity 26 . Therefore, the film forming ability and toughness of amorphic PEDOTF8 depend on the molecular weight. Free-standing films of PEDOTF8 were prepared by casting PEDOTF8/toluene solution (10 g/L). The film of high molecular weight PEDOTF8 (M n = 70200) showed tough films suitable for bending and rolling (Fig. 4). However, the molecular weight PEDOTF8 (M n < 40000) produced brittle films.
Cyclic voltammetry measurements of PEDOTF8 films on electrode evaluated the doping-dedoping process of counter ions. The process causes a volume change, destroying and detaching the films from the electrode or electrochemical quenching 27,28 . Reversible current responses of cyclic voltammograms indicate the high electrochemical stability (or toughness) of the films. A high molecular weight polymer shows toughness for volume change because of the entanglement among polymer segments. A three-electrode system equipped with the glassy carbon working electrode, platinum counter electrode and Ag/AgCl as a reference electrode was used in an electrolytic solution of 0.1 M Bu 4 NBF 4 /CH 3 CN. High molecular weight samples (M n = 70200 and M n = 95000) showed similar p-and n-doping-dedoping profiles at both oxidation and reduction ( Fig. 5a and b). However, the profiles of the lower molecular weight sample (M n = 40000) were different and showed irreversible n-dopingdedoping (Fig. 5c). Moreover, the cyclic voltammogram of a low molecular weight polymer film showed irreversible response for n-doping-dedoping (Fig. 5d). The electrochemical stability of the polymer film was examined by continuous redox cycling 28 . p-Doping-dedoping profiles of the 1 st and 100 th scans, and plots of peak current on scan cycles are displayed in Fig. 5e-h. By comparing the charge storage capacity of the two curves of polymers with M n = 70200 and M n = 40000, 65% (Fig. 5e) and 25% (Fig. 5g) of the capacities were retained after 100 electrochemical scans, respectively, indicating that the high molecular weight PEDOTF8 film has a higher electrochemical stability than the low molecular weight PEDOTF8 and would be a good material for electrochromic devices. Chain ends of polymers often cause decomposition by external stimulation such as chemicals, thermal heating and photo-irradiation 29,30 . To our knowledge, the electrochemical decomposition of the polymers has not been reported. However, in most of the polymers, inner chains are higher stability than chain ends. The electrochemical stability of PEDOTF8 probably depends on the chain end (terminal group) of the polymer segment. Low molecular weight PEDOTF8 in film possesses higher content of the terminal groups than high molecular weight PEDOTF8. The spectroelectrochemical changes of PEDOTF8 were examined by recording the UV-vis absorption spectra at different applied potentials. The drop-cast-coated thin films on an indium-tin-oxide-coated polyethylene terephthalate plate together with the reference Ag/AgCl and counter Pt electrodes were placed in a 1 cm cell. The UV-vis absorption spectra were measured in CH 3 CN containing 0.1 M (n-C 4 H 9 ) 4 NBF 4 at 25 °C. The spectra for the polymer (M n = 70200) film are illustrated in Fig. 6a. In the neutral state, the polymer film exhibited only one major π-π* absorption band at 450 nm, producing a yellow colour. Upon progressive oxidation (<1.0 V) of the polymer film, the visible absorption depleted with a concomitant formation of low energy charge carriers, as indicated by the presence of a new band in the lower energy region around 700 nm. This is derived form the generation of polaron 31,32 . The more oxidation in higher potential generally induces the new bands of bipolaron at different wavelength with decreasing of the band of polaron. Above a potential threshold of >1.0 V, the new bands were observed in the visible and near infrared regions around 600 and 980 nm, respectively, and the intensity at 700 nm decreased. The polymer film revealed a dark green colour in the fully oxidised state, because of the slight tailing of the absorption across the visible region.  The stabilities of the polymer films were evaluated by monitoring the absorbance changes upon repeated redox cycling between the potentials of 1.3 and 0.0 V with a residence time of 30 s. Figure 6b shows the changes during the first four cycles of the devices in both the visible (430 and 650 nm) and near infrared (1000 nm) regions. The PEDOTF8 (M n = 70200) film exhibited long-term stabilities compared with low molecular weight PEDOTF8 films. The optical contrasts fall steadily at the initial stages and remain stable after ca. 55 repeated cycles (Fig. 6c). The spectral changes in the oxidation process from the neutral state were fully reversible. The switching time was evaluated as 90% of the full switch, because it was difficult to visually perceive any further colour changes beyond this point. The results revealed that the switching times of the colouring and bleaching processes for PEDOTF8 (M n = 70200) film were 8.1 and 4.9 s, respectively (Fig. 6d). The polymer film quickly switched between the neutral states. The electrochromic colour change of PEDOTF8 (M n = 40000) film was slow for the colouring and bleaching processes (Fig. 6e). These results were due to the unstable charge carrier states of the fully oxidised polymer backbone and low toughness of the film.

Conclusion
In conclusion, we successfully demonstrated the effect of molecular weight on the optical and electrochemical properties of PEDOTF8. Absolute molecular weight values obtained by GPC with multi angle light scattering measurements of PEDOTF8 are approximately 65% of the relative molecular weight values obtained by GPC using polystyrene standards. The incorporation of low molecular weight fractions (absolute M W < 2600, relative M W < 4000, number of EDOT-F8 unit: n < ca. 5 unit) in the polymer affected the absorption and fluorescence properties. The high molecular weight PEDOTF8 samples showed a higher redox stability of the electrochemical doping-dedoping process of polymer films than the low molecular weight ones, which contributes to the development of a high performance electrochromic device. The molecular weight of the EDOT-based polymers is very important for optoelectronics.
Elemental analyses were performed on a Thermo Finnigan Flash EA1112 CHN-O analyser. GPC analyses were performed by a Toso GPC system (HLC-8220), using tetrahydrofuran as the eluent after calibration with polystyrene standards. GPC-MALS measurements were taken in THF at 25 °C (column temperature: 40 °C) using a Dawn EOS instrument (Ga-As laser, 1⁄4690 nm) to evaluate absolute M n and M w of the polymers. The specific refractive index increment (q n /q c ), which is necessary for the analysis of GPC with multi angle light scattering, was measured with an Otsuka Electric DRM-3000 (1⁄4633 nm) at 25 °C. The measured q n /q c of PEDOTF8 in THF was 0.5935 cm 3 g −1 . UV-vis absorption spectra were obtained on an Ocean Optics USB4000-XR1 fibre spectrometer with DH2000-BAL tungsten halogen light source. Fluorescence spectra were obtained on an Ocean Optics USB4000 fibre spectrometer with PX-2 pulsed xenon light source. Cyclic voltammetry measurements were performed by ALS 611. A three-electrode system equipped with the glassy carbon or indium-tin-oxide-coated polyethylene terephthalate electrode, platinum counter electrode and Ag/AgCl as a reference electrode was used in an electrolytic solution of acetonitrile containing 0.1 M tetraethylammonium tetrafluoroborate.