Preparation of P(Fl-Ar) derivatives and their electrochemical propertiesIn accordance with the reported literature, P(Fl-Ar) derivatives were prepared via direct C–H arylation polycondensation between 2,7-dibromo-9,9-dioctylfluorene and another arylene unit [38, 39]. We prepared five types of P(Fl-Ar) derivatives based on the availability of monomers, structures, and electron densities, where Ar represents 3,4-ethylenedioxythiophene (EDOT), 3,4-dimethoxythiophene (DMeOTh), 3,4-diethylthiophene (DEtTh), 3,4-dihexylthiophene (DHexTh), or 1,2,4,5-tetrafluorobenzene (F4). The electrochemical properties of the P(Fl-Ar) derivatives were investigated via cyclic voltammetry (CV) measurements in an MeCN solution, which contained tetrabutylammonium hexafluorophosphate (Bu4NPF6) or AlCl3 (Fig. S1). Because there was Lewis acidic AlCl3, the oxidation onset potential of P(Fl-Ar) derivatives in an AlCl3/MeCN solution shifted in the negative direction from that observed in a Bu4NPF6/MeCN solution. This trend is consistent with the effects of AlCl3 on the homopolymers [37]. The oxidation onset potential of P(Fl-F4) in an AlCl3/MeCN solution could not be determined because P(Fl-F4) appeared to be oxidized outside the potential window of the AlCl3/MeCN solution. Table 1 summarizes the polycondensation results and oxidation onset potentials of the P(Fl-Ar) derivatives.Table 1 P(Fl-Ar) precursorsAnodic chlorination of P(Fl-Ar) derivativesAnodic chlorination of P(Fl-Ar) was performed under identical conditions to the anodic chlorination of PFO in our previous report (Fig. 2) [37]. P(Fl-Ar) films were prepared on a platinum (Pt) plate (2 cm × 2 cm) with a 5-mg/mL CHCl3 solution. The polymer-coated Pt plate was used as a WE for the reaction. Figure 2 shows the reaction profile as a function of the amount of charge passed through the electrode. The 1H NMR-based degree of chlorination was calculated by comparing the signals derived from the aromatic protons at 7.6–7.9 ppm and the signals derived from the η-methylene protons and terminal methyl protons of the side chains of the fluorene units (i.e., –CH2CH3) at approximately 0.8 ppm. The total number of protons of P(Fl-DHexTh) observed at approximately 0.8 ppm is different from that of other P(Fl-Ar) derivatives because the protons of the two methyl groups of the side chains of DHexTh had overlapping signals.Fig. 2Traces of the degree of chlorination of the P((Fl-Cl)-Ar) derivatives as a function of charge passed. The degree of chlorination of P(Fl-Ar) derivatives was determined via 1H NMRThe P(Fl-Ar) derivatives containing 3,4-dialkoxy-substituted thiophene units (i.e., EDOT or DMeOTh) were dissolved during the reaction. Hence, the degree of chlorination of the obtained polymers was estimated by collecting the polymer films that remained on the electrode after the reaction. The P(Fl-EDOT) film began to dissolve in the electrolyte after the passage of 1 F/mol of charge, whereas the P(Fl-DMeOTh) film immediately began to dissolve in the electrolyte at the initial stage of anodic chlorination. As a result, the degree of chlorination of P(Fl-EDOT) increased with increasing amount of charge passed, whereas that of P(Fl-DMeOTh) hardly changed for 1 F/mol and 2 F/mol of charge passed. With 2 F/mol of charge passed, the degree of chlorination of P((Fl-Cl)-EDOT) was approximately 0.3. The difference in anodic chlorination between P(Fl-EDOT) and P(Fl-DMeOTh) was presumably due to the difference in molecular weights (Table 1). The dissolution of the polymer films during the reaction hampered efficient electron transfer between polymer film and working electrode, which was believed to decrease the degree of chlorination. P(Fl-EDOT) has a much greater molecular weight than P(Fl-DMeOTh), which increases the tolerance of P(Fl-EDOT) to dissolution during the reaction and results in a greater degree of chlorination. This trend in the effects of the molecular weight on the efficiency of the ePF is consistent with our previous work [40].The P(Fl-Ar) derivatives containing 3,4-dialkylthiophene units (i.e., DEtTh or DHexTh) were successfully chlorinated while maintaining their film state after passing 12 F/mol of charge. The degree of chlorination of these polymers gradually increased with increasing amount of charge passed, which follows the same trend as the reported anodic chlorination of CPs [24, 25, 37]. With 12 F/mol of charge passed, the degree of chlorination of the P((Fl-Cl)-Ar) derivatives was approximately 0.4. However, when the amount of charge passed increased, three new signals gradually appeared at approximately 5.1 ppm (labeled as $), 5.8 ppm (labeled as #b), and 6.4 ppm (labeled as #a) in the 1H NMR spectra. Figure 3a shows the stacked partial 1H NMR spectra of P(Fl-DHexTh) with the corresponding regions after the anodic chlorination with different amounts of charge. The P((Fl-Cl)-DEtTh) samples had similar 1H NMR spectra to the P((Fl-Cl)-DHexTh) samples. These signals were due to the unexpected side reactions at the α-carbons of the side chains of the thiophene units. Signal $ was derived from the chlorination of the α-carbons (Cα-Th–Cl) [41]. The pair of signals # was derived from the formation of carbon–carbon double bonds (C = C) between α-carbon and β-carbon, whose stereochemistry was trans [42, 43]. These side reactions at the side chains of the thiophene units were not observed in the anodic chlorination of poly(3-hexylthiophene) (P3HT) under identical electrolysis conditions [37]. The degree of chlorination of the α-carbons hardly increased after the passage of 12 F/mol of charge. Negligibly few C = C bonds formed when 1 F/mol of charge was passed. Meanwhile, the degree of C = C bond formation gradually increased with increasing charge passage. When 12 F/mol of charge was passed, the degree of C = C bond formation was approximately 0.3, which is comparable to the desirable anodic sp2-chlorination.Fig. 3a Stacked partial 1H NMR spectra of P(Fl-DHexTh) and P((Fl-Cl)-DHexTh) samples obtained from the anodic chlorination with different amounts of charge passed. The degree of side reactions (i.e., C = C bond formation and C–Cl bond formation) is also presented. b Chemical structure and molecular orbitals corresponding to the β-LUMO of (Fl-DEtTh)2 in the one-electron oxidation state. The molecular orbital surface iso value is 0.04 (UωB97XD/6-31 G + (d,p)). c Plausible reaction mechanism of the anodic chlorination of P(Fl-DHexTh)To gain insight into the reaction mechanism, density functional theory (DFT) was used to investigate P(Fl-DHexTh) and P(Fl-DEtTh) in the one-electron oxidation state. The anodic substitution reaction is often discussed in terms of the lowest unoccupied molecular orbital (LUMO) of the substrate in the one-electron oxidation state, i.e., β-LUMO [32, 44,45,46]. We used the dimer unit of P(Fl-DEtTh) ((Fl-DEtTh)2) as a model structure for the DFT calculations. To reduce the calculation cost, the side chains of the fluorene units in (Fl-DEtTh)2 were changed from octyl units to methyl groups. Figure 3b shows the molecular structure of (Fl-DEtTh)2 and a diagram of its β-LUMO in the one-electron oxidation state. The β-LUMO coefficient of (Fl-DEtTh)2 in the one-electron oxidation state was mainly localized at the thiophene ring between two fluorene rings, which indicates that the thiophene moieties in both P(Fl-DHexTh) and P(Fl-DEtTh) served as more electron-deficient units than the fluorene moieties during the anodic chlorination. We also investigated the β-LUMO of a tetramer unit of 3-butylthiophene ((3-BuTh)4) in the one-electron oxidation state as a model molecule of homopolymers, such as poly(3-hexylthiophene) (P3HT) (Fig. S3a). Compared with (Fl-DEtTh)2, the β-LUMO coefficient of (3-BuTh)4 in the one-electron oxidation state was delocalized along its main chain, which suggests that the side reactions observed in the anodic chlorination of P(Fl-DHexTh) and P(Fl-DEtTh) were due to the localization of β-LUMO at the thiophene moieties. In addition, the chlorine atoms that were introduced into the fluorene units were expected to promote the localization of the β-LUMO coefficient on the thiophene units because of their electron-withdrawing nature. Thus, such side reactions were also facilitated by an increase in degree of chlorination of the precursor polymers. The gradual increase in the signals derived from the side reactions supports this idea (Fig. 3a). Based on the above discussion, a plausible reaction mechanism for the anodic chlorination of P(Fl-DHexTh) and P(Fl-DEtTh) is described in Fig. 3c. The localization of β-LUMO at the thiophene ring during the anodic chlorination induces deprotonation at the α-position, followed by a C = C bond formation via β-elimination or a C–Cl bond formation via α-chlorination.P(Fl-F4) was not electrochemically chlorinated under the present reaction conditions, as determined by the absence of changes in the 1H and 19F NMR spectra of P(Fl-F4) before and after the anodic chlorination (Fig. S2). Since the oxidation potential of P(Fl-F4) was greater than the potential window of the AlCl3/MeCN system (Fig. S1), the anodic oxidation of the electrolyte preferentially proceeded and resulted in no chlorination of P(Fl-F4) under the present reaction conditions.Characterization of chlorinated P(Fl-Ar) derivativesChlorinated P(Fl-Ar) derivatives such as P((Fl-Cl)-EDOT), P((Fl-Cl)-DEtTh) and P((Fl-Cl)-DHexTh) were characterized via NMR analyses, SEC measurements, X-ray photoelectron spectroscopy (XPS) measurements, and energy dispersive X-ray spectroscopy (EDS) analyses. To characterize the P((Fl-Cl)-Ar) derivatives, anodic chlorination of P(Fl-Ar) derivatives was performed under identical reaction conditions to those in Fig. 2 with either four 2-F/mol charge passed (Ar = EDOT) or three 12-F/mol charge passed (Ar = DEtTh or DHexTh). After each reaction, the obtained P((Fl-Cl)-Ar) samples, which remained in the film state on the electrode, were mixed. The degree of chlorination of the mixed samples (m-P((Fl-Cl)-Ar)), as determined by 1H NMR, was 0.30 (Ar = EDOT), 0.41 (Ar = DEtTh), and 0.47 (Ar = DHexTh).The regioselectivity of the anodic chlorination of P(Fl-Ar) derivatives was investigated according to a previous report on the anodic chlorination of PFO [37]. Unfortunately, the chlorine-substituted carbon on the fluorene rings was not determined, presumably because the m-P((Fl-Cl)-Ar) derivatives had a lower degree of chlorination than PFO (1.00 degree of chlorination); thus, the regioselectivity of the anodic chlorination was not clarified in the current study. Based on the DFT calculations for the model dimer unit in the one-electron oxidation state (Fig. 3b) and the literature of Swager and Song [47], chlorine would be replaced by hydrogen at the 1- or 3-position of a fluorene ring due to the electron-withdrawing nature of charged thiophene units at the 2-position of fluorene units. In addition, the anodic C–H chlorination of PFO [37] and anodic C–H phosphonylation of P(Fl-Ar) [32] selectively provided 3-position-functionalized fluorene moieties. Therefore, we speculate that the anodic C–H chlorination of P(Fl-Ar) derivatives also provides 3-chlorinated fluorene units.Next, XPS analyses of the m-P((Fl-Cl)-Ar) derivatives were conducted to confirm the valence state of sulfur and formation of carbon–chlorine (C–Cl) bonds (Fig. 4). m-P((Fl-Cl)-Ar) films on an indium tin oxide (ITO) substrate were prepared with m-P((Fl-Cl)-Ar) in CHCl3 and used for the XPS measurements. Interestingly, the S 2p XPS profiles of the m-P((Fl-Cl)-Ar) samples provided different results depending on the incorporated arylene units. An additional peak appeared at approximately 169 eV in the XPS spectra for the S 2p peak of m-P((Fl-Cl)-EDOT), which resulted from the sulfone formed by the oxidation of a sulfur atom in a thiophene ring (Fig. 4a) [32, 48, 49]. The XPS-based atomic ratio of sulfide (–S–) to sulfone (SO2) was determined to be 85:15. In contrast, the valence of sulfur in both m-P((Fl-Cl)-DEtTh) and m-P((Fl-Cl)-DHexTh) did not change, since the observed binding energy of S 2p was almost identical to the reported value for thiophene-containing CPs (Fig. 4b and 4c) [32, 37, 50]. DFT calculations for the (Fl-EDOT) dimer units in the one-electron oxidation state revealed that the β-LUMO coefficient was relatively localized at the EDOT unit (Fig. S3b). Since P(Fl-EDOT) has no α-methylene carbons on the thiophene rings, the sulfur atom was oxidized, which generated sulfone during the anodic chlorination. The Cl 2p XPS profiles of the m-P((Fl-Cl)-Ar) derivatives show two peaks derived from C–Cl bonds, which indicates the formation of C–Cl bonds via anodic chlorination. Two minor peaks in the m-P((Fl-Cl)-DEtTh) sample were due to the remaining Cl− ions [51]. The XPS-based atomic ratio of C–Cl bonds to Cl– ions was determined to be 85:15 (Fig. 4b). The amount of chlorine atoms introduced into the m-P((Fl-Cl)-Ar) samples was also evaluated via EDS measurements. The EDS measurements could not distinguish the chlorine atoms derived from C–Cl bonds from those arising from Cl− ions; thus, the atomic ratio of chlorine derived from C–Cl bonds to sulfur (ClC–Cl/S) was calculated using the XPS-based atomic ratio of ClC–Cl/ClCl− (ClC–Cl/ClCl− = 85:15). The atomic ratio of ClC–Cl/S for all m-P((Fl-Cl)-Ar) samples was close to their 1H NMR-based degrees of chlorination: ClC–Cl/S = 0.25 (Ar = EDOT), 0.46 (Ar = DEtTh), and 0.45 (Ar = DHexTh). These results suggest that the chlorination of the α-carbons in the side chains of the thiophene units was negligible, which is consistent with the weak signals from chlorinated α-methine, as shown in Fig. 3a.Fig. 4XPS spectra of the S 2p and Cl 2p regions: a m-P((Fl-Cl)-EDOT), b m-P((Fl-Cl)-DEtTh), and c m-P((Fl-Cl)-DHexTh). The numbers in the spectra are the atomic ratio of sulfide (–S–) to sulfone (SO2) or the atomic ratio of C–Cl bonds to Cl– ionsFinally, the effects of the anodic chlorination on the molecular weights of the m-P((Fl-Cl)-Ar) derivatives were investigated via SEC measurements. Figure S4 displays the SEC traces of the precursor polymers and m-P((Fl-Cl)-Ar) derivatives. The SEC chromatographic peaks of the P(Fl-Ar) derivatives (Ar = DHexTh or DEtTh) and m-P((Fl-Cl)-Ar) derivatives were observed at almost identical retention time, which indicates that the molecular weight of the precursor polymers was maintained without degradation and further propagation of their polymer main chain during the reaction. Meanwhile, m-P((Fl-Cl)-EDOT) had a broader SEC chromatogram toward longer retention time than P(Fl-EDOT), presumably due to the generation of sulfones in the EDOT moieties. The generated sulfone moieties increased the polarity of P(Fl-EDOT), which changed its hydrodynamic volume and decreased the SEC-based molecular weight of P((Fl-Cl)-EDOT).Optoelectronic properties of chlorinated polymersFinally, the effects of anodic chlorination on their optoelectronic properties were investigated (Table 2). The electrochemical properties of the m-P((Fl-Cl)-Ar) samples were evaluated via CV measurements in 0.1-M Bu4NPF6/MeCN (Fig. S5). The oxidation onset potential of the m-P((Fl-Cl)-Ar) samples shifted in the positive direction from that of the pristine precursors because of the electron-withdrawing nature of the substituted chlorine atoms [25, 37].Table 2 Optoelectronic properties of the P(Fl-Ar) derivatives and m-P((Fl-Cl)-Ar) samplesThe optical properties of the P(Fl-Ar) samples and m-P((Fl-Cl)-Ar) samples were also investigated. Figure 5 shows the ultraviolet/visible (UV/Vis) absorption spectra and fluorescence (FL) spectra of the P(Fl-Ar) derivatives and m-P((Fl-Cl)-Ar) samples in CHCl3 solutions and in the film state. The introduced chlorine atoms and aforementioned side reactions hardly affected the absorption properties of the P(Fl-Ar) derivatives because the P(Fl-Ar) derivatives and m-P((Fl-Cl)-Ar) samples had almost identical absorption spectra and absorption maximum wavelengths. The m-P((Fl-Cl)-Ar) samples had different emission properties from the precursor polymers, particularly in the film state. Compared with the pristine P(Fl-Ar) derivatives, all chlorinated P(Fl-Ar) exhibited a lower fluorescence quantum yield (ΦFL) in both CHCl3 and the film state. Interestingly, the maximum emission wavelength of m-P((Fl-Cl)-Ar) (Ar = DEtTh and DHexTh) was longer than that of the original polymers in both CHCl3 solution and the film state, which was accompanied by a change in emission color of P(Fl-Ar) in the film state (Ar = DEtTh and DHexTh) (Fig. 5b and 5c). The precursor polymers showed light blue emission under UV irradiation, whereas the chlorinated polymer showed light green emission under UV irradiation. According to the report by Inagi, Fuchigami, and coworkers, the emission wavelength of poly(9,9-dioctylfluorene-alt-thiophene-2,5-diyl) was blueshifted by introducing chlorine atoms into the thiophene units [24, 52]. The present study demonstrated the opposite trend (i.e., a redshift of the emission wavelength), which suggests that this redshift resulted from the partial charge transfer from thiophene units to chlorinated fluorene units and the formation of a C = C bond between α-carbon and β-carbon of the side chains of the thiophene units [53, 54].Fig. 5UV/Vis absorption spectra (solid lines) and FL spectra (dotted lines) of P(Fl-Ar) (black lines) and m-P((Fl-Cl)-Ar) (red lines) in CHCl3 (upper) and in the film state (lower): a Ar = EDOT, b Ar = DEtTh, and c Ar = DHexTh. Photographs of P(Fl-Ar) (upper) and m-P((Fl-Cl)-Ar) (lower) in the film state under UV irradiation (λ = 365 nm): d Ar = EDOT, e Ar = DEtTh, and f Ar = DHexTh
