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Introduction In the fields of polymer and material chemistry, conjugated polymers (Skotheim et al., 2006) are an important class of polymers for next-generation optoelectronic materials due to their intriguing conductivity, photoluminescence, electroluminescence, and liquid crystallinity. Numerous conjugated polymers have been synthesized thus far, and a variety of unique conjugation systems have been incorporated into conjugated polymer backbones. One of the current research topics in this field focuses on the construction of layered and/or π-stacked structures. Layered π-electron systems are commonly found in both nature and artificial materials; for example, light-harvesting antenna complexes in photosynthetic systems, such as chlorophylls and bacteriochlorophylls, consist of layered π-electron systems. In optoelectronic materials, charges are delocalized in some layers and transferred from one electrode to the opposite one through the layered π-electron systems. Despite the importance of the layered structures of the π-electron systems, the synthesis of polymers comprising layered-aromatic rings and π-electron systems in a single polymer chain has rarely been studied (Morisaki & Chujo, 2006; Morisaki & Chujo, 2008a; Morisaki & Chujo, 2009d; Nakano, 2010). To achieve the construction of the desired layered structure, xanthene, anthracene, and naphthalene compounds can be employed as scaffolds. The rotary motion of two aromatic units substituted at the 4,5-positions of xanthene, 1,8-positions of anthracene, and 1,8- positions of naphthalene is restricted due to steric hindrance, leading to a face-to-face structure. Thus, this review presents a summary of the syntheses and properties of a new class of aromatic ring-layered polymers, as well as oligomers containing xanthene, anthracene, and naphthalene scaffolds. Due to the vast number of studies on xanthene-, anthracene-, and naphthalene-based face-to-face dimeric systems, the polymers and oligomers (i.e., three or more face-to-face aromatic rings) are drawn. 2. Xanthene-based polymers The 4,5-positions of xanthene can be readily functionalized by the treatment of xanthene compounds with alkyllithium reagents to yield the corresponding 4,5-dilithiated xanthenes due to the lithium-oxygen interaction, as shown in Scheme 1 (Morisaki & Chujo, 2005). The reaction of the 4,5-dilithiated xanthenes with halogens such as iodine results in the formation of diiodoxanthene derivative 1, which is used in the palladium-catalyzed coupling reactions. Optoelectronics - Materials and Techniques 236 Scheme 1. Synthesis of monomer 1 Scheme 2. Synthesis of polymers P1-P3 entry Mono-ethylarene Feed ratio x : y : z Polymer Yield (%) M n (calcd.) M n (found by 1 H NMR) 1 10 : 9 : 2 P1a 79 5956 4100 2 5 : 4 : 2 P1b 65 3082 3100 3 3 : 2 : 1 P1c 50 1933 2100 4 10 : 9 : 2 P2a 70 5896 7500 5 5 : 4 : 2 P2b 50 3022 4200 6 3 : 2 : 1 P2c 65 1872 2600 7 10 : 9 : 2 P3a 76 5786 5750 8 5 : 4 : 2 P3b 59 2912 3000 9 3 : 2 : 1 P3c 56 1762 1700 Table 1. Polymerization results As shown in Scheme 2, the Sonogashira-Hagihara coupling (Tohda et al., 1975; Sonogashira, 2002) of diiodoxanthene 1 with either a diethynylarene, such as pseudo-p- diethynyl[2.2]paracyclophane 2, or mono-ethynylarenes 3–5 proceeded smoothly to produce [2.2]paracyclophane-layered polymers (Morisaki et al., 2008b; Morisaki et al., 2009b). The results are summarized in Table 1. In the presence of mono-ethynylarenes 3–5, aromatic groups were introduced as end-capping units. Polymers P1, P2, and P3 possess [2.2]paracyclophane, anthracene, and nitrobenzene as the end-capping units, respectively. Their molecular weights were controlled by the molar ratios of the monomers, as shown in Table 1. For example, in the case of a molar ratio (x:y:z) of 9:10:2, the number average molecular weights (M n ) of P1a, P2a, and P3a were 4100, 7500, and 5750 (entries 1, 4, and 7), respectively, which were calculated from their respective 1 H NMR spectra. Synthesis of Aromatic-Ring-Layered Polymers 237 Fig. 1. UV-vis absorption (UV) spectra of polymers P1a-c in CHCl 3 (1.0 × 10 –5 M). Fig. 2. (A) UV-vis absorption spectra in CHCl 3 (1.0 × 10 –5 M) and photoluminescence (PL) spectra in CHCl 3 (5.0 ×10 –7 M) of P1a. (B) UV-vis absorption spectra in CHCl 3 (1.0 × 10 –5 M) and PL spectra in CHCl 3 (5.0 × 10 –7 M) of P2b. As shown in the UV-vis absorption spectra (in CHCl 3 , 1.0 × 10 –5 M) of polymers P1a–c (Figure 1), there were π–π* absorption bands at around 290 and 330 nm. The absorption spectra of P1a–c were independent of the number of the layered [2.2]paracyclophanes. It is reported that neighboring [2.2]paracyclophane units in the polymer backbone have sufficient free space according to X-ray crystallographic analysis of the model compound (Morisaki et al., 2009a). Therefore, π-π interactions among [2.2]paracyclophane units in a single polymer chain are considered to be weak in the ground state. The optical properties of polymers P1a (M n = 4100) and P2b (M n = 4200) were compared. Figures 2A and 2B show the UV-vis absorption and emission spectra of polymers P1a and P2, respectively. As shown in Figure 2A (see also Figure 1), the π–π* band of the layered [2.2]paracyclophane units was observed in the spectrum of P1a, whereas a sharp absorption peak at around 270 nm and a broad absorption peak at around 400 nm appeared in the Optoelectronics - Materials and Techniques 238 spectrum of P2b (Figure 2B). These new absorption bands were derived from the anthracene units at the polymer P2b chain ends. Polymer P1a emitted blue light with a peak at around 400 nm after excitation at 334 nm (Figure 2A), which was attributed to emission from the layered [2.2]paracyclophane moieties. P2b exhibited a quite different photoluminescence spectrum with a peak at around 450 nm with a vibrational structure on the excitation wavelength of 334 nm (Figure 2B). This excitation wavelength excited only the layered [2.2]paracyclophane moieties because the end-capping anthracene units do not have an absorption band around 334 nm. Thus, P2b emitted from the terminal anthracenes instead of emitting from the layered [2.2]paracyclophanes. As shown in Figures 2A and 2B, the emission peak of the layered cyclophane units (at 400 nm in Figure 2A) efficiently overlapped with the absorption band of the anthracene moieties (at around 400 nm in Figure 2B). Time-resolved photoluminescence spectra of P2b are shown in Figure 3; these spectra indicate that emission from the cyclophane units decreased while that from the anthracene units increased. These results suggest that fluorescence resonance energy transfer (FRET) (Förster, 1946) from the cyclophane units to the end-capping anthracenes occurs. Fig. 3. Time-resolved PL spectra of polymer P2b in CHCl 3 . The UV-vis absorption spectra of the nitrobenzene-end-capped polymers P3a–c (in CHCl 3 , 1.0 × 10 –5 M) are shown in Figure 4A. These spectra exhibited broad absorption bands around 400 nm in addition to the π–π* transition band of the layered [2.2]paracyclophanes; the absorbance around 400 nm increased as the M n value decreased. This absorption band arises from the polymer chain ends that contain nitrophenyl groups as the concentration of the end-capping nitrophenyl groups increased with a decreasing M n value. As can be expected, the emission from the [2.2]paracyclophane moieties was quenched by the introduction of the nitrophenyl units at the polymer chain ends due to the good overlap between the emission peak of the [2.2]paracyclophane moieties and the absorption band of the nitrophenyl moieties. As shown in Figure 4B, the photoluminescence peak intensities and the photoluminescence quantum efficiencies of P3a–c decreased relative to those of P1a. The end-capping nitrophenyl groups of P3a–c effectively quenched the photoluminescence from the layered [2.2]paracyclophane moieties by FRET. It is reported that the end-capping nitrophenyl group quenched photoluminescence 1.0 × 10 4 times more effectively than the addition of nitrobenzene to a P1a solution. [...]... ligand (Scheme 4) (Barder et al., 2005), and the successive addition of pmethoxyphenyl boronic acid 8 and p-nitorophenyl boronic acid 9 afforded the corresponding polymers P8 and P9 in 37% and 40% isolated yields with Mn values of 6100 and 6250, respectively 242 Optoelectronics - Materials and Techniques Scheme 4 Synthesis of polymers P8 and P9 Figure 7A shows the photoluminescence spectra of P8 and. .. with diiodocarbazoles 4 and 5 The results of polymerization are summarized in Table 2 Polymers P4 (Fernandes et al., 2010) and P5 (Morisaki et al., 2009e) comprise 2,7-substituted and 3,6-substituted carbazoles, and their Mn values were calculated to be 2500 and 2300 (Table 2), respectively Scheme 3 Synthesis of monomer 3 and polymers P4 and P5 240 Optoelectronics - Materials and Techniques entry 1 2... 3600 (Arnold et al., 198 8) The synthetic procedure was modified as follows: the treatment of bis(2-octyl)ferrocene 43 with NaN(SiMe3)2 and FeCl2 provided the ferrocene-stacked polymer P 19 with an Mn of 18400 (Nugent & Rosenblum, 199 3; Rosenblum et al., 199 5; Hudson et al., 199 9) The addition of CoCl2 and NiBr2 instead of FeCl2 yielded the corresponding ferrocene/cobaltocene-stacked and ferrocene/nickelocene-stacked... dimers and trimers of perylene-3,4 :9, 10-bis(dicarboximide)s J Phys Chem A, Vol 112, No 11, (March 2008) 2322-2330, ISSN 10 89- 56 39 Grazulevicius, J V.; Strohriegl, P.; Pielichowski, J & Pielichowski, K (2003) Carbazolecontaining polymers: synthesis, properties, and applications Prog Polym Sci., Vol 28, No 9, (September 2003) 1 297 -1353, ISSN 00 79- 6700 Hudson, R D A.; Foxman, B M & Rosenblum, M ( 199 9) Organometallics,... Lett., Vol 6, No 3, (March 197 7) 301-302, ISSN 0366-7022 Kuroda, M.; Nakayama, J.; Hoshino, M.; Furusho, N & Ohba, S ( 199 4) Synthesis and properties of a-oligothiophenes carring three cofacially oriented thiophene rings through peri positions of naphthalene Tetrahedron Lett., Vol 35, No 23, (June 199 4) 395 7- 396 0, ISSN 0040-40 39 Milstein, D & Stille, J K ( 197 8) A general, selective, and facile method for... Chujo, Y (2009c) Synthesis and properties of oligophenylene-layered polymers Macromol Rapid Commun., Vol 30, No 13, (July 20 09) 1 094 -1100, ISSN 1022-1336 Morisaki, Y & Chujo, Y (2009d) Synthesis of π-stacked polymers on the basis of [2.2]paracyclophane Bull Chem Soc Jpn., Vol 82, No 9, (September 20 09) 10701082, ISSN 00 09- 2673 Morisaki, Y.; Fernandes, J A.; Wada, N & Chujo, Y (2009e) Synthesis and properties... in 13 adopt a relatively planar structure to extend its conjugation length (Figure 9) Scheme 5 Synthesis of polymer P10 Fig 8 UV spectra of polymer P10 and compound 13 in CHCl3 (1.0 × 10–5 M) 244 Optoelectronics - Materials and Techniques Fig 9 Structures of polymer P10 and compound 13 Terthiophene-, quarterthiophene- and quinquethiophene-layered polymers were synthesized by the coupling reaction of... polymers (Morisaki et al., 2009c) by modified Suzuki-Miyaura coupling (Miyaura et al., 197 9; Miyaura & Suzuki, 199 5) In this case, tert-butyl groups were not introduced into the xanthene skeleton because of their steric bulk and dodecyl groups were substituted at the 9- position of xanthene in order to improve the solubility of the polymers The reaction of monomers 6 and 7 (10 :9) was carried out in the... acid chlorides and organotin compounds catalyzed by palladium J Am Chem Soc., Vol 100, No 11, (May 197 8) 3636-3638 Miyaura, N.; Yamada, K & Suzuki, A ( 197 9) A new stereospecific cross-coupling by the palladium-catalyzed reaction of 1-alkenylboranes with 1-alkenyl or 1-alkynyl halides Tetrahedron Lett., Vol 20, No 36, (September 197 9) 3437-3440, ISSN 004040 39 Miyaura, N & Suzuki, A ( 199 5) Palladium-catalyzed... Vol 95 , No 7 (July 199 5) 2457-2483, ISSN 00 09- 2665 Morin, J F.; Leclerc, M.; Ades, D & Siove, A (2005) Polycarbazoles: 25 years of progress Macromol Rapid Commun., Vol 26, No 11, (May 2005) 761-778, ISSN 1022-1336 Morisaki, Y & Chujo, Y (2005) Construction of benzene ring-layered polymers, Tetrahedron Lett Vol 46, No 15, (April 2005) 2533-2537, ISSN 0040-40 39 258 Optoelectronics - Materials and Techniques . Scherf, U. ( 199 9). Ladder-type materials. J. Mater. Chem., Vol .9, No .9, (September 199 9) pp.1853-1864, ISSN 095 9 -94 28. Segalman, R. A.; McCulloch, B.; Kirmayer, S. & Urban, J. J. (20 09) . Block. Miller, R. D. ( 199 8). Colorfast Blue- Light-Emitting Random Copolymers Derived from Di-n-hexylfluorene and Anthracene. Adv. Mater., Vol.10, No.13, (September 199 8) pp .99 3 -99 7, ISSN 093 5- 96 48. Klăerner,. H. & Miller, R. D. ( 199 9). Exciton Migration and Trapping in Copolymers Based on Dialkylfluorenes. Adv. Mater., Vol.11, No.2, (February 199 9) pp.115-1 19, ISSN 093 5 -96 48. Lamansky, S.; Djurovich,