Ligand Exchange Reaction for Controlling the Conformation of

Jan 22, 2018 - Control of the conformation of polymers can be achieved by the ligand exchange reaction of optically active poly(phenyleneethynylene) 1...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Ligand Exchange Reaction for Controlling the Conformation of Platinum-Containing Polymers Yu Miyagi,† Takahiro Ishida,† Manabu Marumoto,† Natsuhiro Sano,‡ Tatsuo Yajima,† and Fumio Sanda*,† †

Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan ‡ R&D Division, Nippon Chemical Industrial Co., LTD., 9-11-1 Kameido, Koto-ku, Tokyo 136-8515, Japan S Supporting Information *

ABSTRACT: Control of the conformation of polymers can be achieved by the ligand exchange reaction of optically active poly(phenyleneethynylene) 1′ containing −Pt(PPh3)2− moieties in the main chain. Polymer 1′ was reacted with 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), and 1,4-bis(diphenylphosphino)butane (dppb) to give the corresponding polymers 2′, 3′, and 4′ containing −Pt(dppe)−, −Pt(dppp)− , and −Pt(dppb)− moieties in the main chain, respectively. Polymers 1′ and 2′ exhibited negligibly small circular dichroism (CD) signals in THF, indicating the absence of regulated chiral structures, while polymers 3′ and 4′ exhibited strong CD signals in THF. The dynamic light scattering (DLS) analysis of the polymer solutions indicated that polymer 3′ formed a chirally regulated one-handed helix intramolecularly bridged with dppp, and polymer 4′ formed aggregates intramolecularly and/or intermolecularly bridged with dppb.

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chain ends of Pt-polyynes.14,15 The HOMO−LUMO gaps and hole mobilities of Pt-polyynes have been examined regarding the π-conjugation length of arylene units,16,17 utilization of functional arylene units such as carbazole,18 fluorene,18,19 fluorenone,19 ferrocenylfluorene,20 and metal porphyrins,21 πconjugation interruption with heteroatoms,22 and the zigzag structure of the arylene linkages.23 Pt-polyynes are utilized as key components of metallacycles and metallacages.24 Both cis and trans configurations are possible at the Pt centers of Pt-acetylide complexes. There are several examples of cis-Pt-acetylide complexes with monophosphine ligands,25,26 while there is no report on cis-Pt-polyynes with monophosphine ligands. Pt-polyynes ligated with PBu3, the most commonly employed monophosphine ligand because of the versatility and stability of its complexes, adopt the trans configuration at the Pt centers. Employment of a diphosphine ligand, dppe [Ph2P−(CH2)2−PPh2], enables a cis-Pt configuration due to geometrical restrictions,27,28 but Pt-polyynes with monophosphine ligands likely to adopt the trans configuration due to the thermodynamic favorableness. Although the configuration of the precursor (PtL2Cl2) is cis, Pt-polyynes transform into the trans configuration, presumably by isomerization during the dehydrochlorination process.

INTRODUCTION Artificial polymers with controlled conformation attract much attention because of their regulated alignment of substituents, leading possibly to advanced functions in a manner similar to biological polymers such as peptides and proteins. Existing efforts to control conformation utilize noncovalent intra/ intermolecular interactions, like ionic, hydrogen bonding, dipole, and van der Waals interactions. Meanwhile, metalcontaining polyynes attract much attention because of their excellent photoelectric properties, including photo-electroluminescence, nonlinear optical properties, the photovoltanic effect, and semiconductivity.1−4 Group 10 elements, nickel, palladium, and platinum (Pt), are commonly employed for metal polyynes because of the easy preparation and high stability. Pt-containing polyynes [−Pt(L2)−CC−Ar−C C−]n (L = ligand, Ar = arylene) were first synthesized in the 1970s by the dehydrochlorination of platinum(II) chloride complexes (PtL2Cl2) and diethynylarylenes (H−CC−Ar− CC−H) catalyzed with copper.5−8 Pt-polyynes were expected to exhibit lyotropic liquid crystallinity based on their rigid-rod-like structures. Recent research concerning Ptcontaining polyynes has mainly focused on the photoelectric properties, including experimental and theoretical studies of delocalization in the singlet/triplet excited states9−11 and radical ionic states,12 solar cells fabricated from blends of Ptpolyynes and a fullerene,13 and electron transfer between the main chain and naphthalene diimide units incorporated at the © XXXX American Chemical Society

Received: October 21, 2017 Revised: December 27, 2017

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DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The ligand exchange reaction is a useful synthetic method for preparing metal complexes with various ligands, wherein the ligands coordinated to a metal center are exchanged with other ligands to give the corresponding metal complexes. There are various reports of the exchange reactions of PPh3 ligands for trialkylphosphines because PPh3 has a low coordination ability compared to PR3 (R = alkyl). Other examples include replacement of ligands with diphosphines to form stable chelated structures.29−34 Ligand exchange reactions are also applicable to the control of double-helical structures of Ptcontaining dimers.35,36 We have reported the synthesis of Ptcontaining polymers ligated with PBu3 as well as the conformation and photoluminescence properties.37,38 At the moment, there is no report concerning the ligand exchange reactions of Pt-polyynes. Since cis/trans-geometries and ligand structure remarkably affect the photoelectric properties and shapes of Pt-polyynes, one can expect to gain control of the conformation and properties utilizing the ligand exchange reaction. To investigate this premise, in the present study we investigated the synthesis of novel Pt-containing optically active polymers 2′−4′ by ligand exchange of 1′ (Scheme 1) and the

Scheme 2. Ligand Exchange Reaction of 1 with dppe, dppp and dppba

Scheme 1. Ligand Exchange Reaction of Polymer 1′ with Ph2P(CH2)mPPh2 (m = 2: dppe; m = 3: dppp; m = 4: dppb)

a Conditions: [1]0 = [diphosphine] = 10 mM in CDCl2CDCl2 or CHCl3 at 25 °C.

investigation of the conformation. This paper reports a novel methodologyligand exchange reaction of Pt-containing polymersfor controlling the conformation of polymers.

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RESULTS AND DISCUSSION Ligand Exchange Reaction of Compound 1. Prior to the ligand exchange of polymer 1′, the reactions of dppe, dppp, and dppb with compound 1 (Scheme 2) were performed as models to simplify the analysis. First, the ligand exchange reaction of 1 with dppe was carried out and monitored by 1H and 31P NMR spectroscopies (Figures S1 and S2). After 5 min, the 1H NMR peaks at 6.17 ppm and the 31P NMR peak at 19.0 ppm assignable to 1 completely disappeared. Simultaneously, new 1H and 31P NMR peaks appeared at 6.97 and 42.6 ppm, respectively, and the peaks were intact after 48 h. The coupling constant of the 31P NMR peak was 2300 Hz, indicating the cis configuration of the P−Pt−P moiety.33 These results indicate that cis-Pt complex 2 was directly formed by the ligand exchange reaction of 1 with dppe, without going through a trans dimeric complex 2dimer. The structure of 2 was confirmed by single crystal X-ray analysis as shown in Figure 1. The P− Pt−P angle of 2 was 85.44°. Next, the ligand exchange reaction of monomer 1 with dppp was carried out. Figure 2 depicts the 1H NMR spectra of 1 after the addition of dppp measured in CDCl2CDCl2 over a period of 0−122 h at room temperature. After 5 min, signals a and c at 6.19 and 7.78 ppm of 1 completely disappeared, while signals a′, b′, and c′ at 6.45, 7.18, and 7.65 ppm appeared instead. As time passed, the intensities of signals a′−c′ gradually decreased, while signals a″ and c″ appeared simultaneously, and the intensities gradually increased. A similar trend was observed in

Figure 1. ORTEP drawings of 1, 2, 3dimer, and 3 (50% probability ellipsoids).

the 31P NMR spectra as shown in Figure 3. After 5 min, signal y at 19.3 ppm (J = 2663 Hz), assignable to a trans phosphine, completely disappeared, while signal y′ at 11.8 ppm (J = 2594 Hz) assignable to another trans phosphine and signal z at −4.9 ppm assignable to PPh3 appeared.33 As time progressed, the intensity of signal y′ gradually decreased, while signal y″ at −6.0 ppm (J = 2216 Hz) appeared simultaneously, and the intensity gradually increased. These 1H and 31P NMR spectroscopic changes indicate that the ligand exchange reaction consisted of two steps unlike the one-step reaction with dppe. In the first step, compound 1, with PPh3 ligands in the trans configuration, rapidly reacted with dppp to form chelate compound 3dimer with the trans configuration accompanied by the complete release of PPh3. In the second step, 3dimer gradually isomerized B

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Time courses of the transformation from 3dimer into 3 monitored in CDCl3 (blue) and CDCl2CDCl2 (red) (c = 0.01 M) at 25 °C by 1H NMR spectroscopy.

In benzene-d6, the progress of reaction was confirmed by 31P NMR spectroscopy, but the conversion was not clearly determined. Polar solvents with larger electron-donating ability coordinate to a metal center to form metastable intermediates,39−44 commonly leading to acceleration of ligand exchange reactions, and the isomerization of 1 may also have involved metastable intermediate formation. It is assumed that the smaller molecular size (van der Waals volume) of CDCl3 (75.3 Å3) compared to CDCl2CDCl2 (107.8 Å3)45 contributes to accelerating the transformation from 3dimer into 3 via metastable intermediates ligated with solvent molecules, judging from their very close donor number46 and polarity parameters (ET): 64.7 for CHCl3 and 64.3 for CHCl2CHCl2.47 The ligand exchange reaction of compound 1 with dppp was further carried out at various temperatures ranging from 25 to 75 °C in CDCl2CDCl2. Figure 5 depicts the time courses of the

Figure 2. 1H NMR (400 MHz) spectroscopic change of a mixture of compound 1 and dppp over a period of 0−122 h measured in CDCl2CDCl2 (c = 0.01 M) at 25 °C.

Figure 3. 31P NMR (162 MHz) spectroscopic change of a mixture of compound 1 and dppp over a period of 0−122 h measured in CDCl2CDCl2 (c = 0.01 M) at 25 °C. Figure 5. Time courses of the transformation from 3dimer into 3 monitored in CDCl2CDCl2 (c = 0.01 M) at 25−75 °C by 1H NMR spectroscopy.

into the cis configuration to give 3 as shown in Scheme 2. The first step was very fast and was completed within 5 min. The second step was slow and was incomplete even after 122 h. The ligand exchange reaction of monomer 1 with dppp was also monitored by 1H NMR spectroscopy measured in CDCl3. The reaction also consisted of two steps: rapid dimerization followed by isomerization into a monomeric cis compound. As shown in Figure 4, the isomerization took place much faster in CDCl3 than that in CDCl2CDCl2, and was almost completed after 1440 min. The ligand exchange reaction was also monitored in benzene-d6 and toluene-d5. In both cases, solvent-insoluble mass precipitated soon after the reaction was initiated.

transformation from 3dimer with trans configuration into 3 with the cis configuration monitored by 1H NMR spectroscopy. The conversion (isomerization) rate was increased by raising the temperature. The isomerization was completed after 1000 min at 75 °C. A linear relationship was observed in the first-order plot (Figures S3−S6). The activation energy Ea and frequency factor A were calculated to be 55 kJ/mol and 2000 s−1, respectively, from the Arrhenius plot (Figure S7). The reaction was also monitored by SEC eluted with CHCl3 (Figure 6). Monomer 1 was eluted at a retention time of 31.5 C

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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exergonic (ΔH = −39.5 kJ/mol, ΔG = −53.5 kJ/mol at 25 °C), presumably due to the tight bidentate coordination with dppp compared to flexible monodentate coordination with PPh3. πStacking between the phenylene moieties possibly contributes to enthalpic stabilization of 3dimer. It is likely that the entropy gain by the release of PPh3 exceeds the entropy loss by the formation of chelated cyclic 3dimer, resulting in the positive entropy (ΔS) in the first step. The second step was endothermic (ΔH = +20.3 kJ/mol) but exergonic (ΔG = −12.6 kJ/mol at 25 °C). The cis configuration is commonly energetically unfavorable compared to the trans-counterpart in transition metal complexes,33 resulting in the unfavorable ΔH of 3 compared to 3dimer in the present case. The loss of πstacking is also the possible reason for the positive ΔH. On the contrary, the entropy gain by the transformation from conformationally constrained 3dimer into conformationally more flexible monomeric 3 leads to the favorable ΔG. Consequently, the rapid reaction rate of 1 with dppp to give 3dimer and slower conversion rate from 3dimer into 3 were well supported by the ΔG values calculated by the DFT method. The single crystals of 3dimer were successfully obtained by storing the reaction mixture in a refrigerator at −20 °C just after mixing 1 and dppp. As shown in Figure 1, the X-ray data clearly confirm that 3dimer has two Pt centers bridged by two dppp ligands adopting the trans configuration, while 3 has one Pt center bridged by one dppp ligand adopting the cis configuration. Interestingly, 3 does not adopt a symmetrical conformation but a tilted conformation. The P−Pt−P angle of 3 was 92.59°, which was 7.15° larger than that of 2, as was reasonably explained by the longer propylene chain of 3 compared to the ethylene chain of 2. As described in detail above, compound 1 with the trans configuration reacted with dppp to give 3 with bent cis configuration via 3dimer with the trans configuration. It is likely that the degree of conjugation changes during the reaction. The UV−vis spectroscopic change during the reaction was monitored as shown in Figure 8. Compound 1 showed a λmax

Figure 6. SEC charts (eluent: CHCl3; detector: RI) of a mixture of compound 1 and dppp in CHCl3 (c = 10 mM) at 25 °C over a period of 0−24 h.

min (Mtop = 600, polystyrene calibration). The peak completely disappeared after 10 min, while a new peak assignable to a higher molecular weight trans compound, i.e., 3dimer, appeared at 30.9 min (Mtop = 1900). The peak at 32.9 min (Mtop = 200) was assignable to PPh3 released by the exchange reaction of 1 with dppp. The intensity of the peak of 3dimer gradually decreased, while another peak appeared at 31.5 min (Mtop = 600) after 4 h, and the intensity gradually increased. These 1 H/31P NMR spectroscopic and SEC analyses indicate that the ligand exchange reaction proceeded as illustrated in Scheme 2. The first step is the rapid ligand exchange reaction of 1 with dppp accompanied by the release of PPh3 to give 3dimer. The second step is a slow isomerization from 3dimer with the trans configuration into 3 with cis configuration. The relative enthalpies (ΔH) and Gibbs free energies (ΔG) of 1, 3dimer, and 3 were determined by DFT calculations. As described in Figure 7, the first step was largely exothermic and

Figure 8. UV−vis spectra of a mixture of compound 1 and dppp over a period of 0−24 h measured in CHCl3 (c = 0.02 mM) at room temperature.

at 361 nm (εmax = 44 884) assignable to the trans H−CC− 1,4-C6H4−CC−Pt−CC−C6H4−CC−H chromophore. After 5 min, the λmax of the sample solution was observed at 358 nm (εmax = 40 174). It seems that the λmax and εmax values of 1 and 3 dimer are not very different because both their configurations are trans. The absorbance of the peak at 358

Figure 7. Potential map for the ligand exchange reaction of compound 1 with dppp followed by isomerization. The ΔH and ΔG (at 25 °C) values were calculated by the DFT method [B3LYP/6-31G*(C, H, P)LANL2DZ (Pt)]. D

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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with a coupling constant of 2316 Hz,33 indicating the cis-P− Pt−P configuration in the main chain (Figure 9).

nm gradually decreased to zero after 24 h. Simultaneously, the absorbance at around 330 nm gradually increased, and finally, a peak assignable to the cis-Pt(−CC−1,4-C6H4−CC−H)2 chromophore appeared at 329 nm. The λmax was blue-shifted by the isomerization from trans into cis, indicating the longer conjugation of the linear trans structure compared to that of bent cis structure as predicted by the difference in conjugation efficiency between linear and bended molecules. The conjugation should be expanded through the Pt center. Thus, the isomerization pathway from trans 1 into cis 3 via 3dimer having a trans-dimeric structure was successfully clarified by 1 H/31P NMR and UV−vis spectroscopies, SEC, single crystal X-ray analysis, and also DFT calculations. Further, the ligand exchange reaction of 1 with dppb was carried out (Scheme 2) under conditions similar to those with dppe and dppp. After 5 min, the 1H NMR peaks at 6.17 ppm and 31P NMR peak at 19.0 ppm assignable to 1 completely disappeared (Figures S8 and S9). Simultaneously, new 1H and 31 P NMR peaks appeared at 6.53 and 9.79 ppm, respectively, and the peaks were intact after 24 h. In the SEC measurement, a higher molecular weight peak assignable to a dimeric compound was detected (Figure S10). Judging from the coupling constant of the 31P NMR peak, another trans compound, 4dimer, was obtained by the reaction with dppb, and 4dimer did not isomerize into a dppb-chelated Pt complex with the cis configuration in this reaction. After leaving the reaction mixture another 24 h, a white precipitate was observed. The precipitate was insoluble in common organic solvents including CHCl3, CHCl2CHCl2, THF, and DMF and exhibited an IR spectroscopic pattern very similar to that of 4dimer (Figure S11). It seems that the longer butylene chain of dppb did not allow the formation of a dppb-chelated cis-Pt complex but, rather, that 4dimer with the trans configuration, which was gradually transformed into 4″ as illustrated in Scheme 2. Ligand Exchange Reaction of Polymer 1′. As described, the ligand exchange reactions of compound 1 with dppe, dppp, and dppb were carried out, and the behavior was characterized by various measurements. Next, the ligand exchange reaction of polymer 1′ with the same diphosphines was carried out in CHCl2CHCl2 to obtain polymers 2′−4′ (Scheme 1 and Table 1). The 31P NMR spectra of the polymers were measured in CDCl2CDCl2 (Figure S12 and Table 1). Polymer 1′ showed a triplet 31P NMR peak at 19.3 ppm with a coupling constant of 2684 Hz,33 indicating the trans configuration of the P−Pt−P moieties in the main chain, as shown in Figure 9. On the other hand, polymer 2′ showed a triplet 31P NMR peak at 42.6 ppm

Figure 9. Possible structures of polymers 1′, 2′, and 3′. Polymer 4′ seems to contain trans platinum parts intramolecularly and/or intermolecularly bridged between the polymer main chains.

Polymer 3′ showed a 31P NMR peak with a coupling constant of 2547 Hz at 12.2 ppm assignable to the trans P−Pt− P ligated with dppp, as illustrated in Figure 9. A peak assignable to starting polymer 1′ remained, indicating the incomplete transformation of 1′ into 3′. It should be noted that compound 1 reacted with dppp to form 3dimer with the trans configuration first and then transformed into 3 with the cis configuration as described above. On the other hand, no such dimeric species was detected in the reaction of 1′ with dppp, likely because the polymer main chain is less mobile than the monomer. Polymer 4′ did not show a clear 31P NMR peak. Compound 1 reacted with dppb to form 4dimer and then 4″ with the trans configuration as illustrated in Scheme 2. It is therefore assumed that 4′ possibly contains trans platinum parts intramolecularly and/or intermolecularly bridged between the polymer main chains. These bridging parts induce intra/intermolecularly assembled structures, resulting in the broad 31P NMR signals (Figure S12). Unfortunately, it was therefore not possible to determine the P−Pt−P configuration in the main chain of 4′. The Mn of 2′ was lower than that of prepolymer 1′ (Table 1),

Table 1. Ligand Exchange Reaction of Polymer 1′ with dppe, dppp, and dppba polymer

Mnb

Đb

Mtopb

chemical shift of 31P NMR signalc (ppm)

J(Pt−P)c (Hz)

1′ 2′ 3′ 4′

15000 5800 5700 7700d

1.9 1.8 1.9 2.4d

39000 7400 8300 9700d

19.3 42.6 12.2 −e

2684 2316 2547 −e

a

Conditions: [1′]0 = [diphosphine]0 = 10 mM in CHCl2CHCl2 at 25 °C for 48 h under argon. bHexane-insoluble part. The polymers were recovered quantitatively. The Mn, Đ, and Mtop values were estimated by SEC eluted with DMF (10 mM LiBr), polystyrene calibration. c Measured in CDCl2CDCl2. dThe CDCl2CDCl2-soluble part was separated from the CDCl2CDCl2-insoluble part to measure SEC. e Could not be determined. E

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although 2′ consists of a cis configurated Pt center, leading to a compact structure compared with that of the trans counterpart. On the contrary, 3′ and 4′ seem to form chirally regulated structures. The CD/UV−vis spectra and DLS of the polymers were further measured in various solvents (Figures S13−S16 and Table 2). No particle with an average diameter larger than 10

presumably due to the reduction of hydrodynamic radius upon transformation from the extended trans P−Pt−P− into the folded cis counterpart. The smaller hydrodynamic radius causes the SEC retention time to be longer, giving an apparently lower Mn. The Mn values of 3′ and 4′ were also lower than that of 1′. In these cases, the formation of intra/intermolecularly bridged P−Pt−P− moieties seems to be the reason for the reduction of hydrodynamic radius (and apparent decrease in Mn) upon transformation from 1′ into 3′ and 4′. The CD and UV−vis spectra of polymers 1′−4′ were measured to obtain information about their conformation (Figure 10). The polymers showed UV−vis absorption bands

Table 2. λmax and Hydrodynamic Diameters of Polymers 1′− 4′ hydrodynamic diameterb (nm)

polymer

λmaxa (nm)

1′ 2′ 3′ 4′

380 345 367 370d

THF

THF/ MeOH = 1/9

THF/ hexane = 1/9

THF/ toluene = 1/9

THF/ DMF = 1/9

9.0 5.3 4.4 −e

43 14 13 −e

−c 630 1020 −e

−c 1500 300 −e

−c −c −c −e

Measured in THF (c = 0.02 mM) at 20 °C. bMeasured by DLS (c = 0.02 mM) at 25 °C. cNot observed. dMeasured in CHCl2CHCl2 (c = 0.02 mM) at 20 °C. eCould not be measured due to insolubility.

a

nm was observed in THF solutions of polymers 1′, 2′, and 3′, indicating the absence of polymer aggregates. On the other hand, particles with an average diameter of 28 nm were observed in a CHCl2CHCl2 solution of polymer 4′ (Table S1). It should be noted that 3′ and 4′ exhibited analogous CD signals as mentioned above; i.e., these two polymers form analogous chirally regulated structures. It is assumed that both 3′ and 4′ adopt predominantly one-handed helical structures, and 3′ exists unimolecularly, while molecules of 4′ assemble into particles. The Pt centers of 3′ seem to be bridged by dppp intramolecularly, leading to fixation of the helical structure. On the other hand, the Pt centers of 4′ seem to be bridged intermolecularly as well as intramolecularly, leading to fixation of the helical structure and formation of particles with an average diameter of 28 nm. Polymer 1′ showed intense CD signals when toluene or MeOH was added (Figure S13 and Table 2). Polymer 2′ showed weak signals when toluene was added (Figure S14 and Table 2). These results indicate that 1′ and 2′ form chirally regulated structures when toluene or MeOH is added. The CD spectroscopic measurement and DLS analysis of 3′ and 4′ were performed in various solvents (Figures S15 and S16, Table 2). In THF and THF/toluene = 1/9, 3′ showed analogous CD and UV−vis spectroscopic patterns. In THF/toluene = 1/9, the presence of particles with a diameter of 300 nm was detected. Polymer 3′ seems to be assembled by adding toluene, keeping the helix structure. By adding MeOH and DMF, the intensities of the CD signals slightly decreased. These results indicate that the chiral helical structure of 3′ was slightly affected by solvents. On the other hand, polymer 4′ showed monosignated signals and the highest λmax, which was 3−6 nm red-shifted in THF/MeOH and THF/ DMF compared with that in THF. The chiral secondary structure of 4′ was affected to a greater extent than 3′. It is likely that the smaller solvent effect on 3′ comes from the smaller mobility of 3′ intramolecularly bridged by dppp with a shorter −(CH 2 ) 3 − chain compared to 4′ intra- and intermolecularly bridged by dppb with a longer −(CH2)4− chain. The CD and UV−vis spectra of polymers 3′ and 4′ were measured at various temperatures as shown in Figure 11 to

Figure 10. CD and UV−vis spectra of polymers 1′, 2′, and 3′ measured in THF and polymer 4′ measured in CHCl2CHCl2 (c = 0.02 mM) at 20 °C.

around 300−420 nm. The λmax of initial polymer 1′ was at 380 nm, while cis polymer 2′ showed a λmax at 345 nm, blue-shifted by 35 nm from that of 1′. The trans/cis configuration of the Pt center apparently affected the conjugation length of the polymer main chain. The λmax’s of 3′ and 4′ with trans configurations appeared around 367−370 nm, 10−13 nm blueshifted comparted to 1′, also with trans configuration, indicating that the conjugation length is affected by the phosphine ligands. The conjugation of 1′ with triphenylphosphine ligand is longer than those of 3′ and 4′ with dppp and dppb ligands, presumably due to the larger contribution from the π-electrons of the triphenylphosphine (three phenyl groups) compared to dppp and dppb (two phenyl groups). The intra/intermolecular bridged structures of 3′ and 4′ possibly decrease the planarity, also leading to the shorter conjugation length compared to 1′. No significant difference was observed between 3′ and 4′. Polymers 1′ and 2′ showed negligibly small CD signals in the absorption range of the main chain, while polymers 3′ and 4′ showed clear bisignated CD signals assignable to exciton coupling in the absorption range of the main chain. Polymer 1′ seems to adopt a nonregulated extended conformation. The triphenylphosphine ligands with trans configuration around the Pt center do not lead the polymer to form a foldamer. Polymer 2′ also seems to adopt a chirally nonregulated conformation, F

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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Figure 11. CD and UV−vis spectra of (a) polymer 3′ measured in THF (c = 0.02 mM) and (b) polymer 4′ measured in CHCl2CHCl2 (c = 0.02 mM) ranging from −10 to 60 °C.

check the effect of temperature on the chiroptical properties. The CD and UV−vis absorption intensities of 3′ gradually decreased by raising the temperature. At 60 °C, the intensity of the CD signal of 3′ at around 400 nm decreased to 61% of the value at −10 °C. On the other hand, the degree of decrease for the CD and UV−vis absorption intensities of 4′ by raising the temperature was smaller than the case of 3′. At 60 °C, the intensity of the CD signal of 4′ around 400 nm decreased to 74% of that at −10 °C. Thus, the temperature dependence of the CD and UV/vis spectra of 4′ was smaller than that of 3′. It is likely that the larger conformational stability of 4′ is caused by the intermolecular bridging as well as intramolecular bridging suggested by the DLS measurement as listed in Table S1. Molecular modeling was carried out to obtain information about the polymer structures after ligand exchange. Figure 12 shows a possible conformer of a 12-mer model for polymer 3′ bearing trans platinum configurations, in which the platinum atoms are bridged with dppp ligands at the ith and (i + 6)th units. Because of the large molecular size of the 12-mer model, a more simplified model having no substituents bridged with H2P(CH2)3PH2 was first constructed and the geometry was optimized by the DFT method. Next, all substituents were attached to the simplified model, and the geometries were optimized by the two-layer ONIOM method,48−50 in which the polymer backbone and H2P(CH2)3PH2 were placed in the higher layer assigned to DFT calculation, while all the substituents were placed in the low layer assigned to MM calculation with universal force field to save CPU time. Finally, the geometries were fully optimized by the DFT method. Thus, the possible structure of polymer 3′ having trans platinum centers bridged with dppp ligands was obtained, as shown in Figure 12. A 12-mer model for polymer 2′ having cis platinum centers with dppe ligands was also constructed in a similar fashion (Figure S17). In this case, each dppe coordinates to only one platinum atom with a cis configuration, similar to 2. Figure 13 shows the CD and UV−vis spectra of polymer 3′ simulated by TD-DFT [B3LYP/6-31G*(C, H, N, O, P)LANL2DZ (Pt)]. A 12-mer model without substituents was used for the simulation because a model with all substituents was too large to perform the TD-DFT calculation. The conjugated main chain bridged with diphosphine ligands is poorly mobile, while nonconjugated substituents are largely

Figure 12. Top and tilt views of a 12-mer model for polymer 3′. The geometries were optimized by the DFT method [B3LYP/6-31G*(C, H, N, O, P)-LANL2DZ (Pt)].

Figure 13. TD-DFT [B3LYP/6-31G* (C, H, N, O, P)-LANL2DZ (Pt)] simulated CD and UV−vis spectra of a 12-mer model of polymer 3′ without substituents (nstates = 40, half-width wavelength of Gaussian distribution = 20 nm).

mobile due to lack of restriction by noncovalent interaction such as hydrogen bonding and π-interaction. It is therefore likely that the freely movable substituents do not appreciably affect the CD absorption of polymer 3′. In fact, the simulated CD spectrum of the simple model agreed well with the G

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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formation of 3dimer with trans configuration by the rapid exchange of PPh3 of 1 with dppp. The second step was the isomerization of 3dimer into 3 with the cis configuration. The reaction pathway could be clearly explained by the change of ΔG of each step calculated by the DFT method. The reaction of 1 with dppb gave 4dimer, followed by transformation into polymer 4″. No transformation from 4dimer into a dppbchelated Pt complex with the cis configuration took place. Thus, the behavior of the exchange reaction of 1 was dramatically different depending on the methylene chain length of the diphosphines. Namely, dppe with a short −(CH2)2− chain selectively gave cyclic cis complex 2 in one step. On the contrary, dppp with a medium −(CH2)3− chain gave cyclic trans complex 3dimer, which gradually transformed into cyclic cis complex 3. Dppb with a long −(CH2)4− chain also gave cyclic trans complex 4dimer, which did not transform into cyclic cis complex 4, transforming instead into trans polymer complex 4″. These results suggest the possibility of control over the conformations of the corresponding polymers by the length of methylene chains of diphosphine ligands. In fact, the ligand exchange reaction of polymer 1′ with dppe, dppp, and dppb gave the corresponding polymers showing completely different properties. Prepolymer 1′ and dppe-based polymer 2′ exhibited negligibly small CD signals in THF, indicating the absence of chirally regulated structures. On the other hand, dppp- and dppb-based polymers 3′ and 4′ exhibited intense CD signals in THF, assignable to the main chain chromophores and indicating the formation of chirally regulated structures likely stabilized by diphosphine bridges. The DLS analysis indicated that dppp-based 3′ adopted a predominantly one-handed helical conformation, and dppb-based polymer 4′ formed chiral aggregates. Thus, we could successfully control the conformation and assembled structures of the polymers by the ligand exchange reaction, based on the configuration of the Pt center in conjunction with intra/intermolecular diphosphine bridges between the Pt centers in the polymer main chain. We believe that the present study provides a new strategy for controlling the geometry, conjugation length, and conformation and assembled structures of metal-containing polymers.

experimental results, with both a positive intense and a negative weak CD signal, as shown in Figures 10 and 13. Thus, the simulated CD and UV−vis spectra provide supporting evidence for the folded right-handed helical conformer (foldamer) of 3′ shown in Figure 12. There should be no π-interaction between the phenyleneethynylene moieties judging from the helix pitch (10 Å). It is therefore considered that the bisignate CD pattern of 3′ does not come from exciton coupling,51 which is often observed in helically folded poly(phenyleneethynylene)s with helix pitch around 4 Å52 but, rather, from chiral twisting of the phenyleneethynylene main chain. The solid state morphology of the polymers was analyzed using TEM. Particles with diameters ranging from 100 to 500 nm were observed in the sample of polymer 1′ as shown in Figure 14, and particles with diameters smaller than 100 nm

Figure 14. TEM images of samples of polymers 1′−3′ fabricated on quartz plates from THF solutions and a sample of polymer 4′ fabricated on a quartz plate from a CHCl2CHCl2 solution (2 mg/mL).

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were observed in the samples of polymers 2′ and 3′. On the other hand, no such particles were observed in the sample of polymer 4′. Polymers 1′−3′ seem to assemble to form particles, while 4′ does not form assemblies. Instead, it forms a homogeneous film, presumably because the intermolecularly bridged structure prevents the molecules of 4′ from forming particles. These results indicate that the bridging ligands remarkably affect the assembled structures of the polymers in the solid state as well as the conformation in the solution state.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02249. Figures S1−S17 and Table S1 (PDF) CIF files of 1, 2, 3dimer, and 3; mol2 files of 2′ and 3′ (ZIP)

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CONCLUSIONS In this paper, we have demonstrated the ligand exchange reaction of Pt-containing optically active conjugated polymer 1′ with dppe, dppp, and dppb and the effect of phosphine ligands on the conformation and assembled structures. We have investigated the mechanistic aspects of the ligand exchange reaction of compound 1 as a model system for the polymer reaction by 1H/31P NMR and UV−vis spectroscopies, SEC, single crystal X-ray analysis, and DFT calculations. The ligand exchange reaction of 1 with dppe gave no trans dimer, instead giving 2 with cis configuration by the rapid exchange of PPh3 with dppe. On the contrary, two steps were detected in the ligand exchange reaction of 1 with dppp. The first step was the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.S.). ORCID

Fumio Sanda: 0000-0002-1113-4771 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by a Grant-in-Aid for Scientific Research (B) (16H04158), a Grant-in-Aid for H

DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX

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Challenging Exploratory Research (16K14011), and a Grant-inAid for progress of research in graduate course, 2014−2016 from Kansai University. The authors are grateful to Dr. Seiji Watase and Dr. Masashi Nakamura at Osaka Municipal Technical Research Institute for measuring 31P NMR spectra as well as Prof. Kenneth B. Wagener and Dr. Kathryn R. Williams at the University of Florida for their helpful suggestions and comments.

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DOI: 10.1021/acs.macromol.7b02249 Macromolecules XXXX, XXX, XXX−XXX