Letter pubs.acs.org/macroletters
Poly(cyclopentadienylene ethynylene)s: Breaking Conventional Polyenyne Motifs Md. Mahbubur Rahman, Xiaoxin Zhao, Jaren Harrell, Lei Chen, and Agostino Pietrangelo* Department of Chemistry, Rutgers University−Newark, 72 Warren Street, Newark, New Jersey 07102, United States S Supporting Information *
ABSTRACT: Conjugated polyenynes comprising dienyne constituents have been synthesized for the first time, expanding the architectural scope of this unique polymer class. The poly(cyclopentadienylene ethynylene)s (PCE)s are soluble in common organic solvents, a rare feature among polyenynes, and possess physicochemical properties that are influenced by the structure of their solubilizing groups. A comparative analysis between the PCEs and other soluble polyenynes, poly(arylene ethynylene)s, and a hybrid of both polymer classes show among other characteristics that the inclusion of cyclopentadienes into the conjugated backbone significantly reduces electronic transition energies while completely suppressing photoluminescence.
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comparative analysis, a PCE derivative bearing more sterically demanding geminal dipentyl groups was also synthesized. The results of our investigation show that solubilizing group geometry has a significant effect on polymer molecular weight and optical absorption while having less influence on electrochemistry as characterized by cyclic voltammetry (CV). Moreover, the properties of our PCEs are markedly different than those of other soluble polyenynes, poly(arylene ethynylene)s, and a novel hybrid of both polymer classes, suggesting that the dienyne mode of conjugation can be used as a design tool to manipulate the properties of π-conjugated materials for potential organic device applications. While PDAs are prepared primarily by irradiation-induced topochemical polymerization23,24 (and, more recently, by acyclic enediyne metathesis),2 the PCEs prepared in this work were synthesized by Pd-catalyzed Sonogashira crosscoupling of bifunctional cyclopentadiene-based diynes and organotriflates according to Scheme 1a. First, 5,5-disubstituted1,3-cyclopentadiones 3MeCy-1 and Dipentyl-1 were synthesized by Lewis acid-catalyzed geminal acylation25 upon reacting commercially available 1,2-bis((trimethylsilyl)oxy)cyclobutene with rac-3-methylcyclohexanone and 6-undecanone (i.e., dipentyl ketone), respectively. Subsequently, the diones were converted to the organotriflate comonomers 3MeCy-2 and Dipentyl-2 upon treatment with potassium bis(trimethylsilyl)amide (KHMDS) and Comins’ reagent (ClPyN(Tf)2). Indeed, the 1H NMR spectra of 3MeCy-2 (Figures 1a and S6) and Dipentyl-2 (Figure S10) are consistent with their structures,
onjugated polyenynes are classified by backbones consisting entirely of sp- and nonaromatic sp2-hybridized carbon atoms, lending them an extraordinarily rare status among π-conjugated (macro)molecules. Over several decades, the synthesis of polyenynes has been an active research area as their structural topologies and (opto)electronic properties lend them potential applicability in materials science.1−10 However, despite intensive research efforts, only a handful of polyenyne motifs have been realized (e.g., poly(diacetylene) PDA,11−14 iso-poly(diacetylene) iso-PDA,15 poly(triacetylene) PTA,16−19 and iso-poly(triacetylene) iso-PTA, Chart 1),20 principally due to a dearth of suitable precursors and preparatory methods that have confined polyenynes primarily to enyne- and enediynebased carbon hybridization sequences. Consequently, these limitations have created an extremely challenging research frontier that aims to access new polyenyne motifs that are essential to gaining a better understanding of how alternative modes of π-electron delocalization affect their physicochemical property profiles, information that is critical to establishing material design criteria. In this regard, we report on the synthesis and characterization of poly(cyclopentadienylene ethynylene)s (PCE)s, polyenyne congeners of the popular poly(p-phenylene ethynylene) polymer class. To the best of our knowledge, PCEs are not only the first polyenynes to be constructed from dienyne-based repeat units, but also the only solution processable π-conjugated copolymers to rely solely on functionalized cyclopentadienes for solubility.21,22 Specifically, spirocyclic 3-methylcyclohexyl substituents were selected to introduce a stereorandom and regiorandom microstructure to the PCE backbone, a feature that is anticipated to disrupt intermolecular packing arrangements that impede room temperature dissolution into common organic solvents. For a © XXXX American Chemical Society
Received: March 29, 2017 Accepted: May 31, 2017
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DOI: 10.1021/acsmacrolett.7b00238 ACS Macro Lett. 2017, 6, 632−636
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ACS Macro Letters Chart 1. Common Polyenynes and PCE
Scheme 1. Synthetic Route to PCEs and PPECEs
Figure 1. 1H NMR spectra of (a) 3MeCy-2, (b) 3MeCy-4, and (c) 3MeCy-PCE (CDCl3, 500 MHz). 3MeCy-2 and 3MeCy-4 are drawn as a single enantiomer.
Next, diynes 3MeCy-4 and Dipentyl-4 were prepared by coupling the appropriate organotriflates with 2 equiv of ethynyltrimethylsilane to afford 3MeCy-3 and Dipentyl-3, respectively, followed by protodesilylation under alkaline conditions. Strong IR absorption bands at ∼2084 and 3306 cm−1 confirmed the presence of −CC− and CC−H moieties in the dienediyne comonomers (Figures S27 and S32).26 Unlike the 1H NMR spectrum of Dipentyl-4 (Figure S29), that of 3MeCy-4 (Figures 1b and S24) exhibits resonance
with the former possessing olefinic resonances (HA and HA′) at about 6.05 and 5.99 ppm that arise from its asymmetry. Regardless of solubilizing group structure, the UV/vis absorption spectra of the organotriflates are near identical (Figures S8 and S12), each possessing an absorption maximum (λmax) at 260 nm (3MeCy-2, ε = ∼4740; Dipentyl-2, ε = ∼4580 (L mol−1 cm−1)) that is attributed to the π−π* transition of the cyclopentadiene scaffold. 633
DOI: 10.1021/acsmacrolett.7b00238 ACS Macro Lett. 2017, 6, 632−636
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ACS Macro Letters signal pairs at 6.65 and 6.53 ppm and 3.47 and 3.41 ppm that are assigned to the asymmetric olefinic (HA and HA′) and acetylenic (HB and HB′) protons, respectively. As is the case with the organotriflates, the UV/vis absorption spectra of the dienediyne comonomers (Figures S26 and S31) are nearly identical, each dominated by a narrow absorption band with a λmax at 319 nm (3MeCy-4, ε = ∼11890; Dipentyl-4, ε = ∼12680 (L mol−1 cm−1)) and a low-energy shoulder at 332 nm that is ascribed to vibronic coupling phenomena that are common among polyenynes and polyynes.27,28 Use of the 5,5-substituted cyclopentadiene-based comonomers enabled room temperature Pd-catalyzed Sonogashira polycondensation polymerizations (Scheme 1a) to afford soluble dark purple solids after purification. The GPC chromatograms of 3MeCy-PCE and Dipentyl-PCE (Figures S33 and S34) are predominantly monomodal, showing number-average molecular weights Mn of 25.1 kDa (Đ = 2.2, THF, polystyrene standards) and 5.4 kDa (Đ = 2.2, THF, polystyrene standards), respectively, suggesting that the sterically demanding pentyl groups impede carbon−carbon cross-coupling as a result of steric congestion around the catalyst center. IR spectra of the PCEs (Figures S35 and S36) possess a common band at about 2130 cm−1 that is ascribed to the disubstituted acetylene moieties of the polymer chains,9 while the absence of bands at about 3300 cm−1 indicates that CC−H end-groups are not present to a significant extent. Unlike the 1H NMR spectrum of Dipentyl-PCE (Figure S38) that exhibits a single broad olefin resonance at 6.64 ppm, that of 3MeCy-PCE (Figures 1c and S37) exhibits multiple peaks (designated as HA) in the range of 6.54 to 6.71 ppm that arise from the regiorandom and stereorandom arrangement of the 3methylcyclohexyl groups along the copolymer backbone. Regarding their thermal properties, 3MeCy-PCE exhibits moderate thermal stability by thermogravimetric analysis (TGA), as indicated by its 95% weight retainment at approximately 229 °C (Figure S39). The same specification was measured at 157 °C for Dipentyl-PCE, a result that may be attributed to its larger weight fraction of end-groups that promote premature thermal decomposition. The second heating cycle of the differential scanning calorimetry thermograms of 3MeCy-PCE and Dipentyl-PCE (Figures S40 and S41) revealed glass transitions at approximately 139 and 6 °C, respectively. The glass transition temperature is substantially lower in the latter and is in agreement with the oligomeric form of Dipentyl-PCE and the steric bulk associated with having geminal dipentyl groups on each cyclopentadiene ring. The normalized UV/vis absorption spectra of the PCEs dissolved in o-dichlorobenzene (ODCB) and as thin films are shown in Figure 2. The solution-phase spectra of 3MeCy-PCE and Dipentyl-PCE each exhibit a single broad absorption band with a λmax at 541 nm (ε = ∼20060 L mol−1 cm−1) and 483 nm (ε = ∼22370 L mol−1 cm−1), respectively, with the former being, to the best of our knowledge, the lowest energy absorption maximum reported to date among soluble polyenynes. For comparison, PDAs bearing solubilizing undecyl (Mn = ∼16 kDa, GPC in CHCl3, Đ = 1.9, polystyrene standards) and 4-tert-butylphenyl (Mn = ∼10 kDa, GPC in CHCl3, Đ = 1.4, polystyrene standards) groups exhibit a λmax of 430 and 500 nm in dichloromethane (DCM), respectively.2 To elucidate the origin of the red-shift between the 3MeCy-PCE and Dipentyl-PCE λmax, low-molecular weight 3MeCy-PCE (Mn = ∼4.9 kDa, Đ = 2.1) was prepared by cross-coupling the appropriate comonomers under dilute conditions. Indeed, the
Figure 2. Normalized UV/vis absorption spectra of 3MeCy-PCE (red solid, solution; red dash, thin film) and Dipentyl-PCE (blue solid, solution; blue dash, thin film).
λmax of this copolymer (∼524 nm) was red-shifted by approximately 41 nm with respect to Dipentyl-PCE of comparable molecular weight, indicating that the difference in absorption features arises from the identity of the solubilizing group and not the length of the chain. Note that the relatively small difference (∼17 nm) between the λmax of high and low molecular weight 3MeCy-PCE is not particularly surprising as Tykwinski has shown that both the absorption maximum and cutoff of an iso-PDA heptamer and nonamer are near identical, with saturation of electronic delocalization having been reached at the stage of the former.15 Compared to their solution-phase spectra, those obtained from PCE thin films exhibit lower energy onsets of absorption (λonset) and a broadening of the full width at half-maximum, features that are indicative of polymer aggregation in the solid state. Both the λmax of 3MeCy-PCE (λmax = 524 nm (2.37 eV)) and Dipentyl-PCE (λmax = 478 nm (2.59 eV)) are slightly blueshifted compared to solution suggesting that the bulky solubilizing groups induce packing arrangements that promote nonplanar conformations. Indeed, an identical observation was observed from a 4-tert-butylphenyl substituted PDA system reported by Qin and co-workers.2 The optical band gap of 3MeCy-PCE in ODCB was estimated from the λonset (∼632 nm) to be about 1.96 eV, a value that was verified by calculating the frontier orbital energy difference (∼1.90 eV) of a 3MeCy-PCE model trimer using density functional theory at the B3LYP/6-31G* level. Here, methylcyclohexyl groups were replaced with cyclohexyls to simplify the calculations by omitting regiochemical and stereochemical considerations. The calculations also predict a minimum energy conformation that is planar (Figure 3a) with the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO; Figure 3b,c) delocalized over the entire conjugated scaffold, a topology that is ideal for charge-carrier transport. Next, the electrochemical properties of our PCEs as thin films were investigated by CV using tetrabutylammonium hexafluorophosphate (0.1 M in anhydrous acetonitrile) as electrolyte and silver wire as a quasi-reference electrode.29 The onset potentials of oxidation and reduction for 3MeCy-PCE 634
DOI: 10.1021/acsmacrolett.7b00238 ACS Macro Lett. 2017, 6, 632−636
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ACS Macro Letters
Figure 3. (a) Side view of an energy minimized structure of a 3MeCy-PCE trimer (B3LYP/6-31G*). Top view of (b) LUMO and (c) HOMO distributions across the trimer.
Chart 2. Poly(arylene ethynylene)s and PPECE
identical λonsets (∼527 nm), indicating that solubilizing group structure has little effect on their optical absorption, a feature that is in contrast to the two PCEs. Moreover, the λmax of 3MeCy-PCE and Dipentyl-PCE in THF (Figure S48) are redshifted (3MeCy-PCE, λmax = 531 nm; Dipentyl-PCE, λmax = 472 nm) along with their λonsets of 588 and 611 nm, respectively. Taken together, the results show conclusively that the cyclopentadiene constituents are responsible for the low energy electronic transitions of PCEs, a characteristic supported further when comparing these results against absorption data of other poly(arylene ethynylene)s (Chart 2) such as poly(2,5-bis(n-octyloxy)-p-phenylene ethynlyene)31 PPE (λmax = ∼440 nm, THF; Mn = ∼12.1 kDa, Đ = 1.8, polystyrene standards), poly(2,5-bis(n-dodecyloxy)-p-phenylene ethynlyene-alt-p-phenylene ethynylene)32 PPEPE (λmax = 414 nm, THF; Mn = ∼24.4 kDa, THF, polystyrene standards), poly(p-phenylene ethynlyene-alt-2,5-thienylene ethynylene)33 PPETE (λmax = 414 nm, THF; Mn = ∼26.5 kDa, Đ = 1.4, polystyrene standards), and poly(3-hexylthiophene ethynylene)34 PTE (λmax = 440 nm, THF; Mn = ∼116.4 kDa, Đ = 1.9, polystyrene standards). It is also worth noting that both PPECEs exhibit photoluminescence, with emission spectra (Figure S53) displaying near identical peak maxima at 533 nm and profiles that are common among poly(arylene ethynylene)s. On the contrary, the PCEs were found to be nonemissive, a feature that is in accordance with solid-state blue phase PDA35 and poly(acetylene).36 In addition to differences in photophysics, C10-PPECE and Ethylhexyl-PPECE exhibit enhanced thermal stability with ∼95% weight retainment measured at approximately 345 and 261 °C (Figure S51), respectively,
(Figure S42) are observed at 0.43 and −1.96 V, while the corresponding values for Dipentyl-PCE (Figure S43) are 0.57 and −1.99 V versus ferrocene/ferrocenium (Fc/Fc+). On the basis of the ferrocene reference energy that is 4.8 eV below the vacuum level,30 the HOMO levels of 3MeCy-PCE and Dipentyl-PCE are estimated to be about −5.23 and −5.37 eV, while their LUMO levels are −2.84 and −2.81 eV, respectively. Taken together, the electrochemical band gaps of 3MeCy-PCE and Dipentyl-PCE were estimated to be 2.39 and 2.56 eV, respectively, and are consistent with their UV/vis absorption maxima energy measured in the solid state. Finally, two poly(p-phenylene ethynylene)/ poly(cyclopentadienylene ethynylene) hybrid copolymers (PPECEs, Chart 2) were targeted to examine the effects of replacing every second cyclopentadiene repeat unit in PCEs with 2,5disubstituted phenylene congeners commonly found in PPEs. Toward this end, PPECEs with linear dodecyloxy (C10-PPECE, Mn = ∼20.2 kDa, Đ = 3.3, THF, polystyrene standards) and ethylhexyloxy (Ethylhexyl-PPECE, Mn = ∼33.0 kDa, Đ = 2.5, THF, polystyrene standards) side chains were prepared by copolymerizing the appropriate 1,4-diethynylbenzene (C10DEB and Ethylhexyl-DEB) with 1,4-bis[(trifluoromethanesulfonyl)oxy]5,5-dimethylcyclopenta-1,3diene Dimethyl-1 (Scheme 1b). Note that a less sterically demanding organotriflate was used in lieu of 3MeCy-2 and Dipentyl-2 in order to minimize congestion between bulky solubilizing groups. Indeed, the normalized UV/vis absorption spectra of the two PPECEs dissolved in THF (Figure S48) exhibit comparable absorption maxima (C10-PPECE, λmax = 468 nm; Ethylhexyl-PPECE, λmax = 463 nm) with near 635
DOI: 10.1021/acsmacrolett.7b00238 ACS Macro Lett. 2017, 6, 632−636
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ACS Macro Letters
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results that are attributed to introducing aromatic character into the polymer backbone. In solution, both PCEs and PPECEs exhibit good stability when exposed to air and light with absorption maxima hypsochromically shifting a maximum of 2 nm and absorption intensities that decrease a maximum of 6% over a period of 6 h (Figures S54−57). In summary, bifunctional cyclopentadiene-based diynes and organotriflates were copolymerized by Pd-catalyzed Sonogashira cross-coupling to afford poly(cyclopentadienylene ethynylene)s, the first polyenynes to be prepared with nonconventional dienyne-based repeat units. The results of our investigation show that solubilizing group structure has a marked effect on polymer molecular weight and optical absorption profiles when compared to other soluble polyenynes and poly(arylene ethynylene) analogs. Substituting every second cyclopentadiene-based repeat unit in PCEs with 2,5disubstituted phenylenes afforded photoluminescent copolymers with higher energy absorption profiles indicating that the cyclopentadiene motif has an overall influence of reducing the energy of the optical band gap while suppressing photoluminescence. Efforts to evaluate the performance of PCEs as semiconductors in organic electronic devices and to identify their nonemissive electronic states are currently underway.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00238.
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Synthetic details and characterization data (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Agostino Pietrangelo: 0000-0003-3573-5368 Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Rutgers University for financial support and the NSF for funds used to purchase our Bruker ASCEND 500 MHz spectrometer (NSF MRI 1229030).
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REFERENCES
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DOI: 10.1021/acsmacrolett.7b00238 ACS Macro Lett. 2017, 6, 632−636
