New Type of Donor-Acceptor Through-Space Conjugated Polymer

We report the synthesis and properties of a novel through-space conjugated polymer with a [2.2]paracyclophane skeleton. The obtained polymer possessed donor (fluorene) and acceptor (2,1,3-benzothiadiazole) segments that were alternately π-stacked in proximity via the [2.2]paracyclophane moieties. The good overlap between the emission peak of the donor unit (fluorene) and the CT band of the acceptor unit (2,1,3-benzothiadiazole) caused fluorescence resonance energy transfer, and the visible green light emission from the acceptor unit was observed.

The use of [2.2]paracyclophane as a monomer for the synthesis of a conjugated polymer enables the development of π-stacked structures of various π-electron systems.Such through-space conjugated polymers can be expected to transfer charge and/or energy effectively via the throughspace interaction.Here, we report the synthesis of a new type of donor-acceptor through-space conjugated polymer with fluorene as a donor component and 2,1,3-benzothiadiazole as an acceptor component.The obtained polymer comprises donor and acceptor π-electron systems that are alternately π-stacked in proximity and held by covalent bonds, while the construction of π-stacked donor-acceptor systems has been achieved by a supramolecular approach [46][47][48][49][50].

Experimental Section
2.1.General. 1 H and 13 C NMR spectra were recorded on a JEOL JNM-EX400 instrument at 400 and 100 MHz, respectively.The chemical shift values were expressed relative to Me 4 Si as an internal standard.FTIR spectra were obtained on a Perkin-Elmer 1600 spectrometer.High-resolution mass spectra (HRMS) were obtained on a JEOL JMS-SX102A spectrometer.Analytical thin-layer chromatography (TLC) was performed with silica gel 60 Merck F 254 plates.Column chromatography was performed with Wakogel C-300 silica gel.Gel permeation chromatography (GPC) was carried out on a TOSOH 8020 (TSKgel α-3000 column) instrument using CHCl 3 as an eluent after calibration with standard polystyrene samples.Recyclable preparative highperformance liquid chromatography (HPLC) was performed in Japan Analytical Industry Co. Ltd., Model 918R (JAIGEL-2.5Hand 3H columns) using CHCl 3 as an eluent.UV-Vis absorption spectra were obtained on a Shimadzu UV3600 spectrometer.Photoluminescence spectra were obtained on a HORIBA Jobin Yvon FluoroMax-4 luminescence spectrometer.For cyclic voltammetry, a polymer thin film was obtained by spin-coating from a toluene solution on an indiumtin-oxide (ITO) coated-glass electrode.Cyclic voltammetry (CV) was carried out on a BAS CV-50W electrochemical analyzer in CH 3 CN containing 0.1 M Et 4 NBF 4 with a glassy carbon working electrode, a Pt counter electrode, an Ag/Ag + reference electrode, and ferrocene (Fc/Fc + ) as an external standard at a scan rate of 100 mV/s.Thermogravimetric analysis (TGA) was made on a Seiko EXSTAR 6000 instrument (10 • C/min).Elemental analyses were performed with an Elementar Analysensysteme varioMICRO V1.5.8 system using the CHN mode or performed at the Microanalytical Center of Kyoto University.

Results and Discussion
The synthesis of monomer 3 is outlined in Scheme 2. The treatment of the excess amount (>2.5 equivalent) of pseudo-p-dibromo[2.2]paracyclophane 1 with fluorene diboronic acid ester 2 in the presence of a catalytic amount of Pd(OAc) 2 and 2-dicyclohexylphosphino-2 ,6 -dimethoxybiphenyl (S-Phos) in THF with aqueous K 3 PO 4 [56,57] afforded the corresponding bis(pseudop-bromo[2.2]paracyclophanyl)fluorene 3 in 10% isolated yield.The purification of monomer 3 by column chromatography using SiO 2 and recyclable HPLC resulted in this low isolated yield (10%).The 1 H NMR spectrum of monomer 3 exhibited two peaks at around 2.1 ppm (approximately 1:1), as shown in Figure S1; these peaks were assigned to the methylenes of octyl groups at the 9-position of fluorene.This result suggests that the existence of two isomers was attributed to the two diastereomers (racemi and meso) derived from two planar chiral [2.2]paracyclophane units in monomer 3.
Polymer P1 was synthesized by the palladium-catalyzed polymerization of 3 and 4,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-2,1,3-benzothiadiazole 4, as shown in Scheme 3. The palladium-catalyzed coupling reaction of monomers 3 and 4 was carried out to obtain the corresponding through-space conjugated polymer P1; the polymerization results are listed in Table 1.An appropriate catalytic system was critically important for achieving successful polymerization.The standard Suzuki-Miyaura coupling reaction [56] with a Pd(PPh 3 ) 4 catalyst and aqueous K 2 CO 3 in toluene at 80 • C for 96 hours was ineffective for polymerization.Polymer P1 was obtained in 14% isolated yield with a number-average molecular weight (M n ) of 3300 and a weight-average molecular weight (M w ) of 3700 by GPC analysis (CHCl 3 , polystyrene standards, Run 1 in Table 1).Pd(OAc) 2 with S-Phos catalytic system considerably increased the catalytic activity for the polymer synthesis as well as the monomer synthesis to afford polymer P1 in 68% isolated yield, and the M n and M w were estimated to be 17000 and 49000, respectively (Run 2 in Table 1).
The obtained polymer P1 is a new type of donor-acceptor conjugated polymer in which a donor π-electron system and an acceptor π-electron system are linked alternately via the through-space interaction in the single polymer main chain (Scheme 3).The structure of P1 was confirmed by 1 H and 13 C NMR spectra (Figures S3 and S4 in Supplementary Material).Polymer P1 was highly soluble in common organic solvents such as THF, CHCl 3 , CH 2 Cl 2 , toluene, and DMF.In addition, it could be processed into a thin film by casting or spin-coating from toluene solution, and it was found to be air stable in solution and in the solid state.The thermal stability of P1 was evaluated by carrying out thermogravimetric analysis (TGA) under air (Figure S9 in Supplementary Material).The TGA results showed that P1 exhibited good thermal stability with a 10% weight loss and temperature at 408 • C.
In order to elucidate the optical properties of polymer P1, we designed and prepared model compounds M1 and M2.These compounds M1 and M2 represent the donor and acceptor unit layers of P1, respectively, as shown in Scheme 4. Figure 1 shows UV-Vis absorption spectra and photoluminescence spectra of M1, M2, and P1 in diluted CHCl 3 (1.0 ×10 −5 M for UV and 1.0 ×10 −6 M for photoluminescence).As shown in Figure 1(a), the typical π-π * transition band of a fluorene compound was observed at around 300 nm, and blue emission was observed at 364 nm with a vibrational structure.
As shown in Figure 1(b), M2 exhibited a broad absorption band at around 360 nm, which was attributed to a charge-transfer (CT) band from the benzene to the thiadiazole moieties, in addition to a π-π * transition band of xylyl-phenylene-xylyl backbone at around 310 nm.When the CT band at 360 nm was excited, the photoluminescence spectrum of M2 showed a maximum peak at 467 nm.The shape and the peak top of the photoluminescence spectrum of M2 were independent of the excitation wavelength (308 nm and  360 nm), indicating that the spectrum is characteristic of the benzothiadiazole moiety.
As shown in Figure 1(c), polymer P1 exhibited a UV spectrum with two absorption bands at around 330 nm and 400 nm.According to the UV-Vis absorption spectra of M1 and M2 (Figures 1(b) and 1(c), resp.), it was observed that the spectrum of P1 comprises the π-π * bands of M1 and M2 segments and the CT band of the benzothiadiazole moiety.In contrast, the absorption spectrum of P1 exhibited a red shift of approximately 50 nm in comparison with the absorption spectra of M1 and M2, because of the through-space conjugation.As in the case of M2, the photoluminescence spectrum of P1 exhibited a broad peak at around 505 nm.The photoluminescence spectra obtained at excitation wavelengths of 290 nm and 410 nm were identical, as shown in Figure 1 it was confirmed that the benzothiadiazole segments were the emitting species (Figure 2).We confirmed that this concentration (1.0 ×10 −6 M) was sufficiently diluted to avoid intermolecular interactions according to the concentration effect of the photoluminescence spectra (Figures S11) due to peak shift saturation.These results and the good overlap between the emission peak of M1 and the CT band of M2 suggest the occurrence of fluorescence resonance energy transfer (FRET) [58] from the donor-fluorene segments to the acceptor-benzothiadiazole segments.
Figure 3 shows the photoluminescence spectrum of a mixture of compounds M1 and M2 (concentration of each compound: 1.0 ×10 −6 M)in a diluted CHCl 3 solution excited at 300 nm.Emissions from both M1 and M2 were observed at 361 nm and 467 nm, respectively.Increasing the concentration from 1.0 ×10 −6 M to 1.0 ×10 −4 M resulted in an increase in the intensity of emission from M2 due to the intermolecular interaction.The closely π-stacked structure of alternate donor-fluorene and acceptor-benzothiadiazole segments in the polymer main chain caused FRET.The absolute photoluminescence quantum efficiency (Φ PL ) of P1 was calculated to be 0.49, which was lower than that of M2 (Φ PL = 0.75).The solvent effect on the photoluminescence of P1 was examined, and the Φ PL in more polar solvents such as DMF was 0.42 (Figure S12).This result implies that photoexcited electron transfer as well as energy transfer causes a decrease in the photoluminescence quantum efficiency.Incidentally, the Commission Internationale de L'Eclairage (CIE 1931) coordinates (x,y) of P1 were (0.2827, 0.5192) in solution and in the thin film, indicating visible green light emission.
The HOMO and LUMO energy levels of polymer P1 were estimated from the cyclic voltammogram as well as UV-Vis absorption spectrum.The cyclic voltammogram was obtained by fabricating thin films on an ITO glass electrode in CH 3 CN solution of 0.1 M Et 4 NBF 4 using a three-electrode cell with a Pt counter electrode, an Ag/Ag + reference electrode, and ferrocene (Fc/Fc + ) as an external standard.Figure 4 shows the cyclic voltammogram of P1 at a scan rate of 100 mV/s.The oxidation process of P1 resulted in an onset peak at approximately 0.7 V; in the cathodic scan, the onset reduction potential was observed at approximately -1.3 V (versus Fc/Fc + ).The HOMO and LUMO energy levels of 5 were roughly estimated to be -5.5 eV and -3.5 eV, respectively.The bandgap energy was approximately 2.0 eV, which was in agreement with the optical band gap energy (approximately 2.2 eV) estimated from the absorption spectrum of a thin film made of P1 (onset λ = 575 nm, as shown in Figure S10 in Supplementary Material).On the other hand, density functional theory (DFT) calculations at the B3LYP/6-31G * level were carried out for model compounds.As can be seen in Figure S13 in Supplementary Material, the HOMO and LUMO of the polymer comprise the fluorene unit and the benzothiadiazole unit, respectively.Thus, it can be reasonably concluded that both of the electron transfer and the energy transfer from the fluorene unit to the benzothiadiazole unit occurs in the polymer chain (vide supra).

Conclusion
In summary, we successfully synthesized a novel throughspace conjugated polymer with a [2.2]paracyclophane skeleton.The obtained polymer possessed donor and acceptor segments that were alternately π-stacked in proximity via the [2.2]paracyclophane moieties.The polymer was soluble in common organic solvents, and a homogeneous thin film was readily obtained by casting or spin-coating techniques.
The conjugation length of the polymer was extended by the through-space interaction.The polymer exhibited green emission with Φ PL of 0.49 and CIE coordinates of (0.2827, 0.5192) in a diluted solution.This emission was attributed to the benzothiadiazole moieties; in other words, the benzothiadiazole moieties emitted due to FRET even when the fluorene moieties were excited.The polymer exhibited oxidation and reduction potentials at 0.7 V and -1.3 V (versus Fc/Fc + ), respectively.Finally, it should emphasized that the polymer is a novel donor-acceptor conjugated polymer that combines the donor and acceptor units alternately through π-π stacking and not through a bond.Further studies on the synthesis of through-space conjugated polymers containing the donor and acceptor units at each polymer chain end are currently in progress.