Effects of Side-Chain Topology on Aggregation of Conjugated

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Effects of Side-Chain Topology on Aggregation of Conjugated Polymers Brenden McDearmon,†,‡ Eunhee Lim,† In-Hwan Lee,‡,⊥ Lisa M. Kozycz,†,§ Kathryn O’Hara,† P. Isaac Robledo,∥ Naveen R. Venkatesan,†,‡ Michael L. Chabinyc,*,†,‡ and Craig J. Hawker*,†,‡,§ †

Materials Department, ‡Materials Research Laboratory, §Department of Chemistry and Biochemistry, and ∥Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States ⊥ Department of Chemistry, Ajou University, Suwon 16499, Korea S Supporting Information *

ABSTRACT: Controlling interchain interactions in conjugated polymers is critical to the development of high performance materials. These interchain interactions are dictated by the aggregation and self-assembly of conjugated polymers in solution and from processing steps, such as thermal annealing, in the solid state. Herein, a macrocyclic benzodithiophene building block for conjugated polymers is developed, and the properties of the resulting polymers are compared to analogous acyclic derivatives. The properties of small molecule macrocyclic BDT compounds show the influence of the side-chain substitution on the thermodynamic forces of self-assembly. Comparison of the optical properties of conjugated polymers with macrocyclic and acyclic side-chains in solution and the solid state reveals the ability of the macrocyclic side-chain to modify the structure of aggregates. Grazing incidence wide-angle X-ray scattering shows that the macrocyclic polymers can remain ordered in the solid state while having higher photoluminescence yields than the acyclic derivatives.

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stacking and decrease this enthalpic driving force.13 Modification of the side-chain structure in conjugated polymers has been used in a variety of ways to study the impact on their physical properties. For example, branching of side-chains can lower the melting temperature of polythiophenes, allowing crystallization to be studied at temperatures well below decomposition.14−16 The use of polar side-chains can enhance solubility,17 modify interactions with ions,18 and impact selfassembly.19 The structure of side-chains can also modify electronic properties by changing the intermolecular electronic coupling between chains.16,20,21 In further evolving this concept, the addition of macrocyclic side-chains to block one or both sides of a conjugated polymer’s aromatic backbone (Scheme 1) may be a powerful strategy for tuning optical properties.22−28 This side-chain design has been found to decrease aggregation, resulting in an increase in the photoluminescence of these conjugated polymers which in turn increases performance for light-emitting devices.29 Previous studies have also made use of macrocycle side-chains to provide the capability for sensing by inducing optoelectronic changes in the backbone upon interactions with analytes.30−32 Disrupting π-stacking with macrocyclic side-chains provides the ability to examine how interchain interactions contribute to

INTRODUCTION The molecular design of conjugated polymers controls the performance of organic electronic devices ranging from transistors1,2 to light-emitting diodes3 and solar cells.4−6 The design of the backbone helps to set the optical properties and electronic levels of the polymer and the intramolecular transport of charge carriers. In order for charge carriers to transit the macroscopic distances associated with a device, however, charge carriers must hop between chains.7 Frequently, it is the charge carrier’s ability to hope between chains that limits or dictates the performance of devices such as thin film transistors or solar cells.1,8 Therefore, it is important to consider the impact of molecular design on the interchain interactions that are present in solid films. Side-chains play an important role in regulating interchain interactions.9 Without side-chains, the solubility of conjugated polymers is poor and solution processing not practical. However, this increase in solubility by the addition of sidechains comes at a cost because the side-chains are typically made of insulating groups, such as alkyl chains, and their steric interactions can interfere with interchain electronic coupling. Balancing these effects to control interchain electronic interactions is key to developing high performance materials.10 The side-chains of conjugated polymers are typically linear or branched alkyl groups that serve to increase the entropy of mixing in solution.9,11,12 These entropic effects also help to overcome the enthalpic drive of aggregation and crystallization due to π−π interactions. Bulky side-chains can disrupt π© XXXX American Chemical Society

Received: January 25, 2018 Revised: March 16, 2018

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DOI: 10.1021/acs.macromol.8b00176 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Schematic of (a) Side-View of π-Stacked Polymer Chains with Linear Side-Chains and (b) Polymers with Disrupted π-Stacking Due to Macrocyclic Side-Chains

procedure at synthetically useful concentrations (>10 mM) (Figure 1a). The C10 bridge length was chosen based on its use

the optoelectronic properties of conjugated polymers. Here we show how a macrocyclic benzodithiophene (BDT) building block and its linear acyclic analogue can be used to control properties when compared to the corresponding conjugated polymers with traditional backbone structures. Differing only by the connectivity of the side-chains, and the two fewer hydrogen atoms that the macrocyclic connectivity demands, the effect of this disruption of π-stacking on the thermodynamics, solution aggregation, and solid-state packing for a series of small molecule model compounds and conjugated polymers can be compared relative to their more traditional linear acyclic analogues. These studies lend insight into how molecular scale structure impacts microscale morphology, resulting in different macroscopic material properties.

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Figure 1. (a) Synthesis of macrocyclic BDT using phase separation conditions at 10 mM concentration. (b) Three separate views of the single crystal X-ray structure of macrocyclic BDT showing one π-face blocked. (c) Synthesis of acyclic BDT using standard conditions. (d) Three separate views of the single crystal X-ray structure of acyclic BDT showing both π-faces open.

EXPERIMENTAL SECTION

Details pertaining to the synthesis, purification, and crystal growth of the small molecule compounds are given in the Supporting Information. Single crystal X-ray structures were visualized using VESTA.33 The details of synthesis and characterization of polymers are also provided in the Supporting Information. Variable temperature UV−vis absorption measurements of the polymers were conducted at a concentration of 0.01 mg/mL in either chlorobenzene for the P(BDT-2T) and P(BDTTPD) series or 1,2dichlorobenzene for the P(BDTTBTDT) series. Samples were first heated and then allowed to cool to room temperature before being heated from 30 to 120 °C in increments of 10 °C, allowing 10 min for thermal equilibration at each temperature before each absorption spectrum was recorded. For variable concentration experiments, the concentrations were adjusted such that the absorption maximum was 0.1, 0.3, or 0.6 au for each measurement. Each of these sample concentrations was then sparged for 30 s with argon before measurement of its photoluminescence spectrum. Solid-state measurements were conducted on films that had been spun-cast at 2000 rpm for 30 s from 20 mg/mL solutions of either hot chlorobenzene for the P(BDT-2T) and P(BDTTPD) series or 1,2dichlorobenzene for the P(BDTTBTDT) series. Films for optical measurements were cast on quartz substrates and were then annealed overnight at 230 °C under nitrogen. Photoluminescence spectra were obtained from films that had been drop-cast from 10 mg/mL solutions of either hot chlorobenzene for the P(BDT-2T) and P(BDTTPD) series or 1,2-dichlorobenzene for the P(BDTTBTDT) series. These films were then annealed at 230 °C for 1 h under nitrogen. Films for X-ray scattering experiments were prepared similarly to the spun-cast samples for optical absorption measurements, except using silicon substrates with a native oxide.

previously in analogous hydroquinone-based macrocycles.34−36 While cyclization reactions typically require either careful slow dropwise addition or extreme dilution of less than 1 mM,34−36 the two-phase method here allows for significantly higher concentrations and consequently easier synthesis of sufficient quantities for incorporation in polymers. The acyclic BDT analogue was synthesized using standard conditions (Figure 1c). The macrocyclic and acyclic BDT compounds are pseudoisomers differing only in the connectivity of the sidechains and the two fewer hydrogen atoms that this connectivity necessitates. Single crystal X-ray structures of the macrocyclic BDT and acyclic BDT are shown in Figures 1b and 1d. The macrocycle is observed to block one π-face of the macrocyclic BDT molecule whereas both π-faces of the acyclic BDT molecule remain open. The unit cell from the single crystal structure of the macrocyclic BDT molecule comprises ten independent molecules with small variations in the conformation of the macrocycle, whereas the acyclic derivative only shows a single conformation with C2 symmetry. The average C−C bond length and C−C−C bond angle in the side-chains for the acyclic BDT were 1.52 Å and 114° as compared to 1.51 Å and 114° for the macrocyclic BDT, which indicates that the C−C bonds in the macrocycle are not substantially strained. The alkyl substitution has a pronounced effect on both the solid-state packing structure and the melting thermodynamics of the macrocyclic BDT derivative relative to the acyclic BDT derivative. The macrocyclic BDT molecules pack in a less symmetric arrangement in single crystals than the acyclic BDT molecules that pack in a herringbone structure in the single crystal (Figure 2a,b). Despite this lack of symmetry, the

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RESULTS AND DISCUSSION Synthesis of Macrocyclic Benzodithiophene Compounds. Synthesis of the macrocyclic benzodithiophene (BDT) compound was conducted using a phase separation B


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Figure 2. (a) Single crystal X-ray structure of macrocyclic BDT showing the packing structure. (b) Single crystal X-ray structure of acyclic BDT showing the herringbone-type packing structure. Hydrogens are omitted for clarity.

Figure 3. (a) Synthesis of π-extended macrocyclic and acyclic TBDTT derivatives. (b) Three separate molecular views of the single crystal Xray structure of macrocyclic TBDTT showing partial obstruction of one of the π-faces. (c) Three separate views of the single crystal X-ray structure of the acyclic TBDTT showing both π-faces fully open.

macrocyclic BDT is seen to pack with a larger crystal density of 1.336 g/cm3 versus 1.269 g/cm3 for the acyclic derivative. Differential scanning calorimetry (DSC) measurements show that the macrocyclic derivative has both a lower entropy and enthalpy change upon melting than the acyclic derivative (Table 1). Based on the single crystal X-ray structure, this

axis and the macrocycle of each molecule alternating in an up or down position (Figure 4a−c). These tubules then pack side by side along their long axes in a hexagonal structure. The acyclic TBDTT derivative shows a slightly larger packing density of 1.235 g/cm3 versus 1.219 g/cm3 for the macrocyclic derivative. Once again, the macrocyclic derivative is seen to have a lower enthalpy and entropy of melting relative to the acyclic derivative (Table 1). This is consistent with the macrocycle decreasing π-stacking to lower the melting enthalpy and the topological constraint of the macrocycle decreasing the change in the number of accessible microstates for the sidechains to adopt upon melting which lowers the melting entropy. Together, these small molecule studies highlight the impact of the macrocycle on disrupting π-stacking and impacting the underlying thermodynamics of self-assembly by reducing both the enthalpy and entropy of melting. Synthesis of BDT-Based Polymers. A series of conjugated polymers incorporating either the macrocyclic or acyclic BDT unit were then synthesized according to Figure 5. The P(BDT-2T) series has the exact repeat unit of the TBDTT small molecule model compounds studied above. Similar P(BDT-2T) polymers have been used previously in solar cell devices.37,38 The P(BDTTBTDT) series have a repeat structure also containing the TBDTT unit along with benzothiadiazole (BTD) as an acceptor. This backbone structure has been used for donor polymers in BHJ solar cells.39,40 This results in a donor−acceptor polymer with a lower linear density of BDT units along the backbone. The P(BDTTPD) series incorporates the BTD unit as a comonomer with thieno[3,4-c]pyrrole-4,6-dione (TPD). This series has the highest linear density of BDT units along the backbone, and P(BDTTPD) polymers are an extensively studied class of solar cell materials.41−45 All of these polymers include conventional linear or branched alkyl side-chains on the comonomer. Where possible, samples with differing molecular weight for each polymer series were fractionated and collected to provide control for any molecular weight effects on their physical properties (Supporting Information). Together, these materials establish a diverse library of conjugated polymers that build on the small molecule studies and enable a comparison of

Table 1. Mass Density, ρ, for the Macrocyclic and Acyclic BDT and TBDTT Derivatives Determined from X-ray Crystal Structures and Melting Entropy ΔSm, Enthalpy ΔHm, and Temperature Tm for the Macrocyclic and Acyclic BDT Derivativesa compound macrocyclic-BDT acyclic-BDT macrocyclicTBDTT acyclic-TBDTT a

ρ (g/cm3)

ΔSm (J/(mol K))

ΔHm (kJ/mol)

Tm (oC)

1.336 1.269 1.219

39.7 102 92.4

13.5 33.9 31.7

73 63 70

1.235

146

46.6

42

The melting entropy was calculated via ΔSm = ΔHm/Tm.

decrease in enthalpy is likely due to a decrease in π−π interactions in the macrocyclic BDT derivative. This interpretation is further evidenced by the fact that the macrocyclic derivative has a denser packing structure which would seemingly imply a larger enthalpic change on melting for the macrocyclic BDT under the assumption that the intermolecular forces are spherically symmetric. The smaller change in entropy on melting for the macrocyclic derivative is likely due to the fact that the topological constraint of the macrocyclic side-chains inherently decreases the number of degrees of freedom for the macrocycle relative to the more flexible and topologically unconstrained linear side-chains of the acyclic derivative. Extending the π-face of the BDT core by substitution with 3hexylthiophene allowed for further confirmation of the impact of side-chain topology on the solid-state packing structure and melting thermodynamics for these materials (Figure 3a). As with the BDT derivatives, the single crystal X-ray structure of the macrocyclic TBDTT derivative shows that the macrocycle partially blocks one side of the molecules π-face whereas both of the π-faces of the acyclic derivative are open (Figure 3b,c). The macrocyclic TBDTT derivative packs in a hexagonal “tubule structure” with the molecules aligned along their long C


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Figure 4. (a) Single crystal X-ray packing structure of macrocyclic TBDTT showing hexagonally packed tubules. (b) Top view of a single tubule of cyclic TBDTT showing π-stacked 3-hexylthiophenes. (c) Side view of a single tubule of TBDTT showing alternation of the macrocyclic loop up and down along the long axis of the tubule.

Figure 5. (a) Synthesis of macrocyclic and acyclic P(BDT-2T) derivatives with a repeat unit equivalent to the cyclic and acyclic TBDTT small molecules. (b) Synthesis of macrocyclic and acyclic P(BDTTBTDT) derivatives where the repeat structure is that of the cyclic and acyclic TBDTT small molecules as the donor group and benzothiadiazole as the acceptor group. (c) Synthesis of macrocyclic and acyclic P(BDTTPD) derivatives.

the effects of side-chain topology across a range of polymer backbones. Side-Chain Structure Controls Aggregation in Solution. Often, aggregation and the resulting structure of a conjugated polymer in solution dictate the structure and morphology of these materials once deposited into thin films.4,9,46,47 The aggregation in solution of these BDT-based polymers was studied using a combination of variable temperature absorption spectroscopy and variable concentration absorption and photoluminescence spectroscopy. Because the polymer backbone is the same for both the macrocyclic and acyclic derivatives, the differences in spectroscopic properties are determined by the differences in aggregation and interchain interactions. The effects of aggregation on the spectroscopic properties of conjugated polymers can be understood through a model based on H- or J-aggregates.47−49 This model was originally developed to explain the differences in the spectroscopic properties of small molecule dyes upon aggregation. Aggregates wherein the absorption spectra blue-shifts and the photoluminescence quenches upon aggregation were dubbed Haggregates, and aggregates wherein the absorption spectra redshifts and photoluminescence was maintained were dubbed J-

aggregates. The spectroscopic shift is due to the relative orientation of transition dipoles of the aggregated chromophores and their through-space coupling. Aggregates with coplanar transition dipoles are termed H-aggregates, and those with collinear dipoles are J-aggregates. In the HJ-aggregate model of conjugated polymers, the polymer backbone is viewed as a series of J-aggregates and interchain interactions are viewed as H-aggregates with many polymers exhibiting mixed HJ aggregate behavior.48 Assignment of H-type versus J-type aggregation in conjugated polymers takes into account both absorption and emission because planarization of the conjugated backbone can result in a red-shifted absorbance even in the case of H-type aggregation.47 Many conjugated polymers exhibit order−disorder transitions in solvent below room temperature, e.g., P3HT and MEH-PPV, while others such as PCPDTBT have transitions at elevated temperatures. 47,49,50 To study the nature of aggregation as a function of the side-chain structure and backbone, variable temperature UV−vis absorption experiments were conducted. Spectroscopic measurements were performed at 0.01 mg/mL in chlorobenzene for the P(BDT2T) and P(BDTTPD) series and 1,2-dichlorobenzene for the P(BDTTBTDT) series (Figure 6). Our results show specific D


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Figure 6. Variable temperature absorption measurements for 0.01 mg/mL samples. (a) Acyclic P(BDT-2T) in chlorobenzene. (b) Acyclic P(BDTTBTDT) in 1,2-dichlorobenzene. (c) Acyclic P(BDTTPD) in chlorobenzene. (d) Macrocyclic P(BDT-2T) in chlorobenzene. (e) Macrocyclic P(BDTTBTDT) in 1,2-dichlorobenzene. (f) Macrocyclic P(BDTTPD) in chlorobenzene.

Figure 7. Variable concentration absorption and photoluminescence measurements for (a) acyclic P(BDT-2T) in chlorobenzene excited at 475 nm, (b) acyclic P(BDTTBTDT) in 1,2-dichlorobenzene excited at 472 nm, (c) acyclic P(BDTTPD) in chlorobenzene excited at 472 nm, (d) macrocyclic P(BDT-2T) in chlorobenzene excited at 475 nm, (e) macrocyclic P(BDTTBTDT) in 1,2-dichlorobenzene excited at 472 nm, and (f) macrocyclic P(BDTTPD) in chlorobenzene excited at 472 nm.

differences for all three polymers with the macrocyclic substitution changing the nature of aggregation for each. All-Donor Polymer P(BDT-2T). Aggregation of P(BDT-2T) in solution can be observed by comparison of the acyclic and macrocyclic derivatives. Acyclic P(BDT-2T) aggregates at room temperature in chlorobenzene as evidenced by vibronic peaks at 535 and 576 nm with an absorption onset of 663 nm at 30 °C. The vibronic features in the acyclic P(BDT-2T) spectra are lost upon heating in what appears to be an order−disorder transition with an isosbestic point at 491 nm, similar to observations for regioregular P3HT.49 The absorbance at 120 °C is comparable to the room temperature absorbance reported for a P(BDT-2T) polymer with larger alkyl substituents

(ethylhexyl on BDT and dodecyl on the thiophene rings) in chloroform.37 In contrast, the peak absorbance of the macrocyclic derivative is at 586 nm at room temperature and blue-shifted relative to the acyclic derivative. The macrocyclic P(BDT-2T) has a weak vibronic shoulder near the onset and an apparent isosbestic point upon heating at 475 nm with a shoulder around 562 nm, suggesting a structural transition. The normalized spectra of the macrocyclic and acyclic derivatives converge to the same shape with increasing temperature, which we interpret as the absorption of noninteracting disordered chains at high temperature. A comparison of the spectra as a function of concentration allows further insight into the differences in aggregation E


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Donor−Acceptor Polymer P(BDTTBTDT). The acyclic P(BDTTBTDT) shows an order−disorder transition in solution with isosbestic points at 573, 467, and 407 nm due to the donor−acceptor structure of the backbone where there are two singlet bands in the spectrum. In addition, a shoulder is present at 668 nm for the lowest energy band that is potentially a vibronic band. A previously reported P(BDTTBTDT) derivative with octyl-dodecyl substitution on BDT and bare thiophene units yields a comparable spectra in chlorobenzene, suggesting that it aggregates similarly to the derivative here.39 An order−disorder transition above room temperature has also been observed in PCPDTBT, a different benzothiadiazole containing donor−acceptor polymer.50 In contrast, the macrocyclic derivative here does not show any evidence of aggregation. The absorption onset at room temperature is at 674 nm, which is blue-shifted relative to that of the acyclic polymer at 747 nm. The variable temperature absorption spectra for the macrocyclic derivative showed no isosbestic points, and the hypsochromic progression with increased temperature is characteristic of increased torsional defeats along the backbone with increasing temperature. The normalized spectra of the macrocyclic and acyclic derivatives converge at 120 °C with the acyclic derivative showing only a small tail around 650 nm indicative of minor residual aggregation. These results clearly demonstrate that the observed spectroscopic differences between the macrocyclic and acyclic derivative are indeed due to aggregation effects which are lost at higher temperatures. We have deconvoluted the spectrum of the acyclic P(BDTTBTDT) into the aggregated and disordered form using both the high temperature results and the data for the temperature-dependent macrocyclic derivative (Figure S6). The latter strategy can account for changes due to temperature-dependent torsional disorder assuming the two backbones behave comparably. There are only small differences with the two methods due to the weak temperature dependence of the spectra of the macrocyclic compound. The acyclic derivative is ≈30% aggregated at room temperature in solution with the transition to increasing disaggregation beginning at ≈70 °C. The absorption spectra of the acyclic P(BDTTBTDT) derivative show the development of a slight vibronic shoulder with increasing concentration whereas the macrocyclic derivative does not show any evidence of aggregation. Concentration-dependent spectra of P(BDTTBTDT) are consistent with the interpretation of the temperature-dependent spectra. The intensity of photoluminescence of the acyclic derivative was significantly reduced at all concentrations relative to the macrocyclic derivative (Figure 7). The emission of the acyclic derivative reveals a pronounced red-shifting with increased concentration whereas that of the macrocyclic derivative does not change. Instead, the macrocyclic derivative displays a single peak with a maximum of 695 nm in its photoluminescence spectrum that increases in intensity with increasing concentration as expected for a larger number of emitters. The concentration-dependent absorption and photoluminescence for the acyclic P(BDTTBTDT) derivative are similar in character to the well-characterized donor−acceptor polymer PCPDTBT, which has been shown to form disordered HJ-aggregates in solution.50 These results further shows that the acyclic P(BDTTBTDT) derivative aggregates in solution with little or no aggregation observed for the macrocyclic P(BDTTBTDT) derivative.

between acyclic and macrocyclic P(BDT-2T) (Figure 7). The variable concentration absorption spectra for the acyclic P(BDT-2T) are strongly red-shifted with increasing concentration and show the emergence of vibronic structure. This behavior is consistent with the presence of aggregates determined from the temperature-dependent spectra. In contrast, the macrocyclic derivative showed a weaker redshifting and emergence of a small shoulder around 575 nm in the absorption spectra as the concentration was increased. The differences in the spectra as a function of concentration aid in the interpretation of the temperature-dependent measurement (vide inf ra). The amount of aggregation in solution for both derivatives of P(BDT-2T) can be estimated from the temperature-dependent spectra. The difference in oscillator strength of the disordered and ordered chains of P(BDT-2T) was calculated by the method used in the literature for P3HT (details in Supporting Information).51 By spectral deconvolution using the hightemperature spectra of each version to represent the fully disordered chain, the fraction of aggregates at room temperature was nearly 50% for the acyclic derivative (Figure S6) and begins to decrease near 80 °C. The ratio of the first two absorption peaks (A0−0/A0−1) of the aggregated population decreased with increasing temperature, as expected for increasing disorder using Spano’s model for H-aggregation.51 A similar analysis yields only ≈25% of a longer conjugation length population for the macrocyclic derivative. The spectra of both derivatives at low concentration are nearly identical but are red-shifted relative to the high temperature spectra at higher concentration (Figure S7). This observation implies either that both derivatives have a comparable aggregated form at low concentration or that the temperature-dependent change in the absorption spectra of the macrocyclic derivative is due to changes in conjugation length due to the conformation of the backbone. An alternative analysis uses the temperaturedependent spectra of the macrocyclic derivative to model a population of chains that is not strongly aggregated, but with a varying conjugation length (see Supporting Information). The fraction of aggregates for the acyclic derivative is not substantially altered with this model, but the detailed structure of the aggregate is different. The photoluminescence spectra showed significantly different behavior for the two derivatives (Figure 7). The spectra of acyclic P(BDT-2T) showed quenched emission that red-shifted with increasing concentration. The maximum PL intensity for the lowest concentration was at 575 nm, which then shifted to 705 nm at the highest concentration. These observations are consistent with the formation of aggregates with more interchain interactions and increased conjugation length with increasing concentration.47 The vibronic structure, red-shifting absorbance, and photoluminescence quenching seen in the acyclic derivative with aggregation are similar to that seen in regioregular P3HT which is known to exhibit H-type aggregation.48,49,49 The spectra at high concentration still display emission from the aggregates and disordered chains. For macrocyclic P(BDT-2T), the intensity of photoluminescence increased with increasing concentration, and the primary and secondary maxima were found at 590 and 635 nm, respectively. The lack of fluorescence quenching for the acyclic derivative suggests that the interchain interactions are weaker in the aggregates than for the acyclic derivative consistent with the separation of the chains afforded by the macrocycle ring. F


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Macromolecules Donor−Acceptor Polymer P(BDTTPD). In contrast to the other two polymers, P(BDTTPD) does not show a simple order−disorder transition with temperature. Neither the macrocyclic nor the acyclic P(BDTTPD) derivatives show isosbestic points upon heating, and both have vibronic peaks in the absorption spectra. This indicates that the conjugation length does not significantly change with temperature. Both the macrocyclic and acyclic P(BDTTPD) derivatives show only a small change in their absorbance profiles with increasing temperature, and these profiles do not converge to the same trace within the observed temperature range. The acyclic derivative with an absorption onset of 659 nm is red-shifted relative to the macrocyclic derivative with an onset of 629 nm. The spectra show little change with concentration as well. For both polymers the ratio A0−0 /A0−1 is >1, consistent with an Jaggregate structure. Interestingly, the ratio of intensities of the A0−0 to A0−1 peaks is seen to increase with increasing molecular weight for both the macrocyclic and acyclic P(BDTTPD) derivatives (Figure S9). This has been observed previously in diketopyrrollopyrole−thienothiophene polymers and may be a result of an increase in the persistence length with increasing molecular weight.52 The ratio decreases with temperature for the acyclic derivative which is more consistent with H-aggregate behavior. In contrast, one would expect the ratio to increase as the exciton width increases for a J-aggregate. Comparable behavior has been seen for PCPDTBT where the full assignment of all vibrational modes contributing to the excitation has been taken to suggest strongly disordered and mixed H/J-aggregates with the ratio of the two lowest energy peaks being greater than 1.50 Full characterization requires further investigation of the spectral assignment and is under investigation. Together, thiese observations suggest that both of the P(BDTTPD) derivatives have different detailed structure in solution although their optical spectra are relatively similar. The photoluminescence spectra do not show apparent quenching as a function of concentration for either side-chain structure (Figure 7). The spectrum of the acyclic derivative has a small peak at 635 nm, a primary maximum at 700 nm, and a small shoulder that appears at 750 nm at higher concentrations. The photoluminescence spectrum for the macrocyclic derivative has a maximum at 673 nm with a shoulder at 745 nm. The slightly higher emission from the acyclic derivative suggests that it is possible that there are interchain interactions for the acyclic derivative, but it is difficult to rule out other conformational effects. It is clear from these variable temperature and variable concentration spectroscopic measurements that introduction of the macrocyclic BDT unit decreases aggregation and increases photoluminescence. This shows that in the acyclic BDT derivatives there are stronger interchain interactions, whereas in the macrocyclic BDT derivatives intrachain interactions dominate the optical properties. This is consistent with the macrocyclic derivatives disrupting π-stacking and restricting interchain interactions. Solid State Ordering and Side-Chain Structure. Optical spectroscopy revealed that the structure of the side-chains control aggregation in the polymers synthesized herein with the macrocyclic derivatives being markedly less aggregated. Driven by these insights, we further examined how the structure in solution impacts the solid-state structure. Through systematic comparison of the absorption/emission spectra coupled with grazing incidence X-ray scattering (GIWAXS) from solid films

of the three systems, we observed that macrocyclic substitution also provides a means to control solid-state properties. All of the macrocyclic derivatives have blue-shifted absorption spectra and increased photoluminescence intensity relative to their acyclic analogues. This observation suggests a reduced intermolecular interaction between chains. On the basis of the single crystal structure of macrocyclic BDT and DFT calculations, we can estimate the closest stacking distance between polymer chains incorporating the macrocycle. The distance from the plane of the BDT ring to the carbon atoms in the middle of the macrocycle is ≈4.6 Å with a slightly larger distance for the hydrogen atoms at this position of ≈4.9 Å. The minimum separation of chains for a packing geometry with the macrocyclic facing the open face of a neighboring BDT unit is ≈8 Å (Figure S10). We can also consider the possibility of structures where the repeat units in the polymer are all oriented such that the macrocycles are on the same face of the conjugated backbone. This conformation could lead to structures where there are pairs of π-stacked chains separated from other pairs by the macrocycle (Figure S10). Mixtures of these two types of structures would lead to disorder as the chains aggregate in the solid state and cause broadening or the absence of scattering from the stacking of the backbones. All-Donor Polymer P(BDT-2T). The optical absorption spectra of both derivatives of P(BDT-2T) have characteristics of H-aggregates (Figure 8). Absorption measurements showed

Figure 8. Photoluminescence (dashed) and absorbance (solid) spectra for macrocyclic (orange) and acyclic (blue) (a) P(BDT-2T), (b) P(BDTTBTDT), and (c) P(BDTTPD). The samples for photoluminescence were drop-cast, and the films for absorbance were spuncast. The samples were annealed at 230 °C for 1 h under nitrogen to remove solvent.

that the acyclic P(BDT-2T) had an absorption onset of 639 nm with a primary maximum at 542 nm and a vibronic peak at 581 nm. These peaks are only slightly red-shifted relative to the solution measurements. Spectral deconvolution shows about 40% aggregation in the film, a small shift relative to the solution value near 48%. The onset of absorption for the macrocyclic P(BDT-2T) derivative was 604 nm with a primary maximum at 506 nm and a vibronic shoulder at 555 nm. These solid-state absorption measurements are similar to the solution measurements of the macrocyclic P(BDT-2T) derivative, and the estimated concentration of aggregates is ≈20%. The photoluminescence of the acyclic derivative reveals similar trends with red-shifts relative to the solution spectra with a maximum at 715 nm having a difference in maxima of only 10 nm (Figure 8). The intensity of photoluminescence from the macrocyclic derivative at its maximum is only about a factor of 1.5 greater than that of the acyclic derivative. Both P(BDT-2T) polymers have ordered structures in thin films. GIWAXS shows that ordered domains in acyclic P(BDT2T) have a lamellar structure similar to many alkylated conjugated polymers with an edge-on orientation with respect G


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Macromolecules to the substrate (Figure 9). The in-plane π-stacking peak is at 3.71 Å (q = 1.693 Å−1), and the out-of-plane alkyl staking

absorption onset in the solid state of 709 nm relative to the 674 nm maximum in the solution measurements consistent with a red-shift expected upon an increase in conjugation length. Both polymers show red-shifted emission relative to solution with significantly higher PL intensity from the macrocyclic derivative consistent with the weaker interaction between chains as expected (Figure 8). The difference in the shape of the emission suggests substantially different solid-state ordering for the two P(BDTTBTDT) derivatives. GIWAXS shows that the macrocyclic P(BDTTBTDT) has glassy ordering relative to acyclic P(BDTTBTDT) (Figure 10)

Figure 9. GIWAXS patterns (a, b) and line cuts (c) for the acyclic and macrocyclic all-donor polymer P(BDT-2T).

distance is 16.9 Å. The GIWAXS pattern for the macrocyclic P(BDT-2T) derivative includes many scattering peaks, suggesting that the chains are well-ordered. In the out-ofplane direction, a feature at a spacing of 11.9 Å is observed and can be attributed to the separation of backbones by the linear alkyl side-chains on the bithiophene comonomer unit (Figure S10). The in-plane scattering shows two progressions: one with a d-spacing of 15.8 Å with 3 orders of diffraction observed and the other with a d-spacing of 9.9 Å with 2 orders of diffraction observed. The length of the repeat unit calculated by DFT at the B3LYP/6-31G* level of theory is 16.2 Å. The two progressions can be fit with a unit cell where the b- and c-axes (the interchain separation and the periodicity of the repeat respectively) are b = 10.1 Å and c = 16.2 Å, and the angle between them α = 77°. In such a cell, the backbones of neighboring chains along the c-axis would be translated by ≈3 Å with respect to each other. Without detailed molecular modeling, e.g., molecular dynamics simulations, we cannot use the X-ray data to support particular detailed structures, but the scattering provides a starting point for such simulations. It is clear however that the polymer chains in the macrocyclic polymer are well-separated in the ordered regions of the film consistent with the higher PL intensity. Donor−Acceptor Polymer P(BDTTBTDT). The aggregation of solid films of P(BDTTBTDT) reflects their aggregation in solution. The acyclic P(BDTTBTDT) derivative shows only a small red-shift in its absorption in the solid state at 758 nm relative to 747 nm in solution (Figure 8). By spectral decomposition, the amount of aggregated acyclic P(BDTTBTDT) is near 22% in the solid state, very close to the value in solution. During deposition, ordering is not substantially increased indicating the critical role of solution structure. The macrocyclic derivative has a red-shifted

Figure 10. GIWAXS patterns (a, b) and (c) line cuts for the acyclic and macrocyclic donor−acceptor polymer P(BDTTBDTT).

with the acyclic P(BDTTBTDT) being the least ordered of the three acyclic polymers. The weak X-ray scattering is consistent with the small percentage of aggregates in the films and reveals an alkyl stacking distance of 15.3 Å and π-stacking distance of 3.61 Å. The crystallites in the film are weakly textured with a preference for an edge-on orientation. The macrocyclic derivative has very weak scattering with only a broad feature in the out-of-plane direction (Figure 10). Both the macrocyclic and acyclic polymers show this broad feature near q = 1.63 (d = 3.85 Å−1), suggesting it is due to disordered domains of both derivatives. While this peak could be assigned to stacking of the chains, it is unlikely because it is observed for both derivatives, and the macrocyclic derivative cannot have such a close contact. The increased emission from the macrocyclic derivative can therefore be understood as weaker electronic coupling of the chains in the solid state due to its glassy structure. Donor−Acceptor Polymer P(BDTTPD). Both derivatives of P(BDTTPD) showed comparable optical properties in solution but have substantially different emission in the solid state. The emission maximum for the macrocyclic derivative shifts significantly from 673 to 765 nm in going from solution to the solid state. The general trend of a blue-shift in the optical absorption for the macrocyclic derivative relative to the acyclic polymer is again observed (Figure 8). Linear side-chains on the TPD unit in P(BDTTPD) derivatives have been previously H


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Macromolecules

(Figure S10). There is also a broad scattering feature at qxy = 1.415 Å−1, which likely represents scattering between functional groups (≈4.4 Å). Significantly, this data shows that there are ordered domains in films of macrocyclic P(BDTTPD).

shown to have a red-shifted absorption edge relative to polymers with bulkier branched substituents.43,45 In contrast to the comparable photoluminescence in solution where both polymers have comparable emission intensity, the macrocyclic derivative has stronger emission in the solid state. The solidstate photoluminescence spectrum for the acyclic derivative is lower in intensity and much broader than that of the macrocyclic derivative. The acyclic derivative’s photoluminescence spectrum is seen to broaden out, and its emission maximum is seen to red-shift from 700 to 748 nm in going from solution to the solid state. This observation suggests that the macrocycle is modifying the proximity of the backbones in the solid state. The packing model inspired by GIWAXS analysis of the two polymers is consistent with the hypothesis that the backbones have differing intermolecular electronic coupling due to the side-chains (Figure 11). The acyclic P(BDTTPD) packs in a

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CONCLUSIONS Understanding and controlling the interchain interactions of semiconducting polymers is critical to the development of high performance devices. Interchain interactions are driven by enthalpic π−π stacking that are countered by the entropic and steric effects of side-chains that induce solubility. We developed a topologically constrained macrocyclic BDT derivative that restricts π-stacking and does not directly affect the electronic structure of the conjugated polymer backbone. Differences in spectroscopic and other material properties were shown to be a direct result of differences in aggregation and interchain interactions. The macrocyclic BDT building block is seen to decrease interchain interactions in conjugated polymers and modify their aggregation in solution, leading to enhanced photoluminescence in both solution and the solid state. These results illustrate that the incorporation of macrocyclic sidechains into conjugated polymers is an excellent strategy for testing and measuring the impact of these interchain interactions. Quantitative comparison of the properties of such polymers should provide greater understanding of the properties of conjugated polymers helping to guide the design of future materials.

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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/acs.macromol.8b00176. Synthetic details, NMR spectra, GPC traces, DSC thermograms, thermogravimetric plots, characterization of other molecular weight polymer fractions, diffusion ordered spectroscopy (DOSY) NMR, and GIWAXS line cuts (PDF) Crystallographic information for macrocyclic-BDT (CIF) Crystallographic information for acyclic-BDT (CIF) Crystallographic information for macrocyclic-TBDTT (CIF) Crystallographic information for acyclic-TBDTT (CIF)

Figure 11. GIWAXS patterns (a, b) and (c) line cuts for acyclic and macrocyclic P(BDTTPD).

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lamellar structure similar to previously reported derivatives with differing alkyl substitution.42,43,45,53 The layered structure has an alkyl spacing of 21.2 Å (q = 0.297 Å−1 for the (100) peak), and a π-stacking peak is observed at 3.6 Å (q = 1.745 Å−1). The anisotropy of the scattering features shows that the crystallites in the film are oriented in an edge-on manner. The macrocyclic derivative has weaker scattering peaks with those present being assigned based on the structure of the polymer. In the out-ofplane direction, an alkyl spacing of 14.7 Å can be assigned based on peaks at q = 0.427 Å−1 and q = 0.852 Å−1. This spacing is shorter than the value for the acyclic derivative. An in-plane progression can be observed at qxy = 0.54 Å−1, qxy = 1.07 Å−1, and qxy = 1.628 Å−1 with a d-spacing of 11.6 Å. The length of the repeat unit in the backbone calculated from a DFT model is ≈12 Å, consistent with this spacing. While the peak at qxy = 1.628 Å−1 is also comparable to the distance of πstacked polymer chains, the photoluminescence spectra suggest that the chains are not strongly coupled, and additionally the size of the macrocycle again prevents such close stacking

AUTHOR INFORMATION

Corresponding Authors

*(M.L.C.) E-mail [email protected]. *(C.J.H.) E-mail [email protected]. ORCID

Michael L. Chabinyc: 0000-0003-4641-3508 Craig J. Hawker: 0000-0001-9951-851X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported through the NSF DMR 1436263. The authors also acknowledge the use of UCSB MRL Shared Experimental Facilities which are supported by the MRSEC Program of the NSF under Award No. DMR 1720256. BM would like to thank the DoD and ASEE for funding through the NDSEG Fellowship program. EL received support from a I


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National Science Foundation Graduate Research Fellowships (DGE-1144085). LMK is grateful for an Elings Prize Postdoctoral Fellowship. PIR received partial support through LSAMP program of the National Science Foundation under Award no. DMR-1102531. Single crystal X-ray analysis was performed by Dr. Guang Wu at the UCSB X-ray Analysis Facilities. GIWAXS experiments were performed at the Stanford Synchrotron Radiation Lightsource (beamline 11-3). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515.

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