Bandgap Engineering of Conjugated Materials with Nonconjugated

Article pubs.acs.org/Macromolecules

Bandgap Engineering of Conjugated Materials with Nonconjugated Side Chains Robert H. Pawle, Ankita Agarwal, Stephanie Malveira, Zachary C. Smith, and Samuel W. Thomas, III* Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States S Supporting Information *

ABSTRACT: Controlling the optical properties of conjugated materials, especially their bandgaps, is critical to nearly all of applications of these materials. The most prevalent strategy involves changes to the structures of conjugated backbones, while side chains are generally reserved for imparting solubility. This paper, using a series of donor−acceptor conjugated oligo- and poly(arylene−ethynylene)s that have terephthalate units as the electron-deficient unit, demonstrates examples of how the structures of side chains that are not formally part of the conjugated backbone can have significant effects on bandgaps of these materials. In organic solution, changing alkoxy substituents on the terephthalate unit yields changes in absorbance onsets of, in some cases, greater than 20 nm; the position of absorbance spectra of these materials correlates with the Taft σ* values of the ester alkoxy groups, consistent with the side chains inductively altering the electron-accepting nature of the terephthalate ring. This structure−property relationship persists in the solid state. These results indicate that synthetically simple side-chain substitutions of formally nonconjugated groups may be useful in rational design of the optoelectronic properties of conjugated materials in both solution and the solid state.

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narrowed bandgaps for donor−acceptor copolymers.21−23 These popular and generally applicable strategies enable the design and preparation of organic materials that absorb at wavelengths in the red and infrared regions of the electromagnetic spectrum.24−27 The generation of ultralow bandgap organic materials is important both fundamentally and in applications such as photovoltaics. In emissive devices and sensors, however, control over bandgap, but not necessarily its minimization, is critical. A common strategy to tune the bandgaps of conjugated materials within a specific subset, such as poly(arylene−ethynylene)s (PAEs),28−32 poly(phenylene−vinylene)s,8,33 or polythiophenes34,35 is to increase the HOMO energy by addition of electron-donating heteroatoms such as nitrogen, sulfur, or oxygen as substituents and decrease the LUMO energy by addition of electron-withdrawing substituents such as fluoro, perfluoralkyl, perfluoroaryl, cyano, nitro, or carboxy groups directly to the conjugated backbone,36−39 Alternating these electron-rich and electron-poor units in oligomers and polymers yields derivatives with donor−acceptor character. Beyond releasing or withdrawing electron density from the main chain π-system, substituents on conjugated polymers can serve different purposes:40,41 appropriately designed side chains impart functionality such as solubility,42 analyte recognition and binding,42−44 reactivity,45 and photoactivity46 to materials. Side chains can also modify solution-state optical properties by extension of the formal π-system along another structural

INTRODUCTION This paper reports the use of nonconjugated side chains to control the optical properties of donor−acceptor π-conjugated materials. Over the past 30 years, conjugated polymers (CPs) have become critical components of a variety of electronic devices.1−4 Key advantages of CPs over their inorganic counterparts include structural tunability, solution processability, and mechanical flexibility.5 Even with the large body of work that is already available, with new structures reported consistently, there remains demand for a more complete understanding of the properties of these important materials.6 Key to the properties of a material is the energy gap between its valence band (HOMO) and conduction band (LUMO).7,8 Many approaches exist for tuning the energies of the HOMO and LUMO in conjugated materials.9−13 A fundamental example is the dependence of bandgap on the length of the conjugated backbone: as the length of the π-system increases, the HOMO energy increases and the LUMO energy decreases. The number of repeat units at which the effect reaches an asymptote is known as the effective conjugation length and depends strongly on chemical structure.14 Beyond controlling the degree of extension of the π-system, common strategies include (i) enforcing the coplanarity of rings and multiple bonds through covalent bonding15,16 (e.g., fluorenes and polycyclic aromatic systems) or noncovalent interactions (e.g., hydrogen bonds or other electrostatic interactions),17−19 (ii) decreasing the aromaticity of the monomer units and increasing the quinoidal character of the conjugated backbone,20 and (iii) alternating electron-rich donor units with electron-poor acceptor units: the donor units have high HOMO levels while the acceptor units have low LUMO levels, resulting in © 2014 American Chemical Society

Received: February 16, 2014 Revised: March 13, 2014 Published: March 27, 2014 2250

dx.doi.org/10.1021/ma5003547 | Macromolecules 2014, 47, 2250−2256

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axis40,47,48 or by introduction of energy transfer acceptors.49,50 Of these uses of side chains on CPs, imparting solubility is the most common.51 This property is achieved through alkyl substituents that are either bound directly to the backbone or through linkages such as alkoxy, amino, or carboxy groups. Being aliphatic, the structures of these solubilizing side chains are generally not a critical parameter in the electronic structure of isolated polymer chains, although their structures often affect the solid-state bandgaps of conjugated materials through directing the packing of the materials.52,53 Herein, we report a series of donor−acceptor PAEs and oligo(arylene−ethynylene)s (OAEs), members of an important class of conjugated materials,40,54 in which changing the side chains of terephthalate monomer units, but not altering the formally conjugated CP backbone, affects solution- and solidstate optical properties significantly by perturbing donor− acceptor character. Furthermore, we show that such side-chain design can yield precise tuning of the bandgaps of these materials.

indices (PDI) between 1.5 and 6.0 as determined by gelpermeation chromatography versus narrow PDI poly(styrene) standards in THF. Figures 1 and 2 show the absorbance and fluorescence spectra of polymers P1−P4 in dichloromethane and in thin

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RESULTS AND DISCUSSION Chart 1 shows the first series of poly(phenylene−ethynylene)s, P1−P4, described in this study. Each polymer comprises Chart 1. Structures and Molecular Weights of Conjugated Polymers P1−P4

Figure 1. Electronic absorbance spectra of conjugated polymers P1− P4 in dichloromethane (top) and as thin films (bottom).

electron-poor terephthalates alternating with electron-rich pdialkoxyphenylenes. Benzoate esters are known to show electron-accepting behavior in donor−acceptor conjugated polymers.32,55 The alkoxy component of the ester, the structure of which is readily installed through straightforward esterification reactions, is a useful design feature for imparting functionality.35,42,45,46 Esterification of previously reported 2,5diiodoterephthaloyl chloride yielded the desired diiodoterephthalate monomers.42 Sonogashira polymerization of these monomers with 1,4-dialkoxy-2,5-diethynylbezene yielded polymers P1−P4. Typical polymerizations yielded number-average molecular weights on the order of 104 g/mol and polydispersity

Figure 2. Fluorescence spectra of polymers P1−P4 in dichloromethane (top) and as thin films (bottom).

films; Table 1 summarizes their properties. Despite there being no difference in the connectivity of formally π-conjugated atoms for these polymers, the positions of absorbance and emission spectra of these polymers in dichloromethane solution 2251


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Table 1. Optical Properties of P1−P4 (All λ in nm) P1 P2 P3 P4

λmax(abs) CH2Cl2

λonset(abs) CH2Cl2

log(ε)max CH2Cl2

λmax(em) CH2Cl2

ϕF CH2Cl2

λmax(abs) film

λonset(abs) film

λonset(em) film

454 459 472 469

506 514 519 528

4.6 4.6 4.6 4.4

498 505 509 523

0.85 0.62 0.58 0.52

485 499 515 515

528 544 551 565

492 512 510 519

differ significantly: P1, which bears alkyl solubilizing groups on the terephthalate ring consistent with a typical design for imparting solubility, shows higher energy transitions than P2− P4 reflected in absorbance maxima (18 nm, between P1 and P3), emission maxima (25 nm, between P1 and P4), and absorbance onset (22 nm, between P1 and P4). The only significant structural differences between these polymers are the structures of the alkoxy groups of the esterseither alkyl, fluorinated alkyl, or 2-nitrobenzyl groupson every other phenylene ring. Differences in degree of polymerizations of these materials does not explain this trend: minimal bandgap decreases are observed after 10 repeat units in PAEs, which is below the DP of all polymers described here.14 In addition to studies in solution, we measured electronic absorbance and fluorescence spectra of spun-cast thin films of these polymers after annealing at 60−70 °C under vacuum. Polymers P1−P4 display a bathochromic shift and change in shape of absorbance upon transition from solution phase to thin films. These changes in spectral shape are characteristic of PPEs: the primary absorption band in solution generally shows a bathochromic shift in addition to the development of a sharp band on the red edge of the spectrum; agglomeration-induced planarization and aggregation from interchain π-system overlap cause the observed changes.56−59 As in solution, the optical spectra of P2, P3, and P4 display clear bathochromic shifts from those of P1. In examining the emission spectra of the films, we compared their high-energy onsets to at least partially account for the effects of agglomeration and aggregation; the solid-state emission onsets of P2, P3, and P4 are 18−27 nm larger than that of P1. We initially found these trends, which demonstrate that atoms not directly conjugated to the backbone of the chromophore can modulate the bandgaps of conjugated materials significantly, surprising in that it represents a different approach to that usually taken for altering the bandgaps of conjugated materials. Uncertainties in the cause of the effect, however, arose from the inherent heterogeneities of polymeric samples: differences in molecular weight distributions, differences in the extent of diyne linkages, and potential differences in propensity for planarization and aggregation in the solid state made it challenging to reach definitive conclusions regarding the role of different substituents. From the perspective of understanding the effect of chemical structure on properties of conjugated materials, conjugated oligomers offer the advantages that while their electronic structures can mimic those of analogous polymers, pure samples are homogeneous, have well-defined conjugation lengths, and typically have good solubility in common organic solvents.14,46 We therefore designed the series of oligomers O1−O8, illustrated in Chart 2, to study this trend without the potential for interference by the heterogeneities of polymer samples. The structures of oligomers O1−O6 model the donor−acceptor electronic structures of conjugated polymers P1−P4: each has a central terephthalate ring with two p-anisyl terminal rings, prepared using Sonogashira coupling between

Chart 2. Structures of Conjugated Oligomers O1−O8

the appropriately substituted p-diiodoterephthalate and 4methoxyphenylacetylene. As in P1−P4, the only differences between the structures of these conjugated oligomers are the structures of the ester side chains. The esters of O1−O6 have butoxy, 4,4,4-trifluorobutoxy, benzyloxy, 3,3,3-trifluoropropoxy, 4-butoxy-2-nitrobenzyloxy, and 2,2,2-trifluoroethoxy substituents, respectively, each with distinct electron-withdrawing character. Table 2 summarizes the relevant optical properties of O1− O8 in dichloromethane, while Figure 3 displays the absorbance and emission spectra of O1−O6 (the spectra of O7−O8 are in the Supporting Information). The optical spectra of these compounds are generally similar in shape: each absorbance spectrum consists of two bands with maxima of approximately 320 nm and between 374 and 395 nm, while each emission spectrum is one broad band with maximum between 452 and 482 nm. These spectra are similar in shape and spectral position to other reported three-ring donor−acceptor phenylene− ethynylene oligomers.29 Consistent with the trends we 2252


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Table 2. Optical Properties of O1−O8 in Dichloromethane (All λ in nm) O1 O2 O3 O4 O5 O6 O7 O8

λmax(abs)

λonset(abs)

λmax(em)

ϕF

374 379 381 383 384 392 368 365

439 445 447 449 452 463 404 399

452 460 461 465 469 482 399 393

0.36 0.33 0.44 0.44 0.20 0.54 0.70 0.74

Figure 4. Correlation of oligomer absorbance with Taft σ* values of ester substituents on central terephthalate ring.

spectra of esters and the Taft σ* constant of alkoxy substituents of esters is known.61 We therefore attribute the effect that these different side chains have on optical spectra of our polymers and oligomers in solution to substituent-dependent changes in the electronwithdrawing nature of the terephthalate moiety. Our conclusion is consistent with the well-known design principle of decreasing the bandgaps of CP by increasing their donor−acceptor character.11 Also consistent with this conclusion is our observation that compounds that lack this donor−acceptor design do not show this strong dependence of bandgap on the structures of nonconjugated substituents. In O7−O8, the central ring has electron-donating substituents rather than electron-withdrawing substituents. In this case, replacing the butoxy substituent on O7 with the trifluoropropoxy substituent on O8 results in a small hypsochromic shift (see Supporting Information) instead of a bathochromic shift, as we had observed for the analogous comparison between donor− acceptor polymers O1 and O4. To gain greater insight into the effect of structures of different ester alkoxy groups on both the geometries and electronics of these materials, we performed geometry optimizations of compounds O1, O4, and O6 at the DFT level of theory using the B3LYP functional and the 6311G(d,p) basis set. We used ethyl chains to approximate the butyl chains of O1 to reduce computation time. As shown in Figure 5, the lowest energy geometries for these molecules were nearly identical, with highly coplanar backbones. In all three of these donor−acceptor compounds, and in agreement with reported observations on other donor−acceptor systems, the HOMOs are distributed evenly across most of the π-systems, while the LUMOs are more concentrated on the electron-poor terephthalate rings and onto the ester groups. The bandgaps predicted by DFT calculations are 0.20−0.25 eV greater than those observed experimentally. Nevertheless, and most importantly with respect to our experimental results, the trend in calculated HOMO−LUMO gaps for these compounds at this level of theory tracks closely with the experimentally observed trend: the calculated bandgap for O1 is 0.17 eV larger than that of O6, with O4 having an intermediate calculated bandgap (2.95 eV). Furthermore, a significant decrease in LUMO energy from O1 to O4 to O6, which correlates with increasing electron-withdrawing character of the ester alkoxy

Figure 3. Electronic absorbance (top) and fluorescence emission (bottom) spectra of O1−O6 in dichloromethane.

observed with the optical spectra of P1−P4, the spectra of oligomers containing benzylated or fluorinated esters (O2− O6) showed bathochromic shifts from the spectra of O1, which increased in magnitude from O2−O6. The difference in absorbance maxima between O1 and O6 is 17 nm (0.15 eV), while the difference in emission maxima between O1 and O6 is 30 nm (0.17 eV). Onsets of absorbance, which give the optical bandgaps of these compounds, followed an analogous trend, with a difference of 24 nm (0.14 eV) between O1 and O6. Figure 4 shows that there is a positive correlation between the lowest energy absorbance maximum of oligomers O1−O4 and O6 with the respective Taft σ* 60 value for its corresponding ester alkoxy substituent. The Taft equation is a linear free energy relationship that correlates the hydrolysis rate of esters with the structures of their acyl components and contains separate substituent constants for electronic (σ*) and steric (ES) effects. The σ* substituent constants, which we use in this analysis, also correlate strongly with the pKa values of alcohols61 and are thus a measure of the ability of substituent on an alkyl groupas opposed to the substituent on an aromatic ring as is the case with Hammett substituent constantsto stabilize negative charge. In addition, correlation between spectral shifts of the carbonyl stretch in the infrared 2253


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Figure 5. DFT optimized geometries and frontier molecular orbitals for conjugated oligomers O1 and O6, as calculated using the B3LYP functional and the 6-311G(d,p) basis set.

groups, is the principal source of the trend in bandgaps. That this same trend occurs when the oligomers have highly coplanar backbones refutes the competing hypothesis that the experimentally observed trend in bandgaps is due to compounds with more donor−acceptor character having, on average, a more coplanar conjugated π-system. Lastly, these theoretical calculations also capture the importance of the donor−acceptor arrangement with respect to the observed substituent effects, as geometry-optimized structures of “alldonor” oligomers O7 and O8 showed only a 0.02 eV difference in bandgap at this level of theory, also consistent with our experimental observation. As a demonstration that this effect is more general than this specific class of PPEs, we prepared analogous benzodithiophene-containing polymers, P5 and P6 (Chart 3), the analysis

Figure 6. Electronic absorbance spectra of conjugated polymers P5 and P6 in dichloromethane (top) and as thin films (bottom).

that even in the presence of polymer−polymer interactions in solution, side-chains not formally conjugated to the πconjugated main chains can influence bandgap according to the same qualitative trend observed for P1−P4.

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Chart 3. Benzodithiophene−Terephthalate PAE Copolymers P5 and P6

CONCLUSION The main conclusion of this paper results from structure− property studies of formally nonconjugated side chains on conjugated oligomers and polymers: side-chain groups not formally part of the conjugated network can have significant effect on the bandgaps of conjugated systems. We attribute the trend observed here to the inductively electron-withdrawing nature of, for example, fluorinated groups increasing the donor−acceptor character of these materials, yielding bathochromic shifts of optical spectra. As a result, the optical spectra of members of this class of conjugated materials are both tunable and quantitatively predictable by appropriate selection of side chains. Such tuning is important in applications such as light-emitting devices, for which control over emission color is critical. In addition, the reliability and broad structural scope of esterification reactions give this approach the possibility of being highly generalizable for the tuning of optoelectronic properties. Although the shifts observed are modest, we believe these results are important for future rational design and tuning of conjugated materials, as they offer a strategy complementary to the more typical approach of direct alteration of the conjugated backbone.

of which illustrates that the control of bandgap demonstrated in P1−P4 can be extended to other classes of PAEs (Figure 6). P5 and P6 show a similar structure−property relationship in dichloromethane solution to that described above: the absorbance maxima and onsets of P6 are red-shifted from those of P5 by 13 and 22 nm, respectively. We attribute the lowest energy band in both spectra to intermolecular interactions of the polymers in solution, as relatively sharp peaks with higher energy shoulders characterize the absorbance spectra of P5 and P6. Upon dilution, the higher-energy shoulders become slightly more prominent relative to the lower energy peaks (Supporting Information). In the absorbance spectra of thin films, the lower energy peak dominates the absorbance spectra of both P5 and P6, with P6 having an onset of absorbance 20 nm red-shifted from P5. This demonstrates

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis and characterization for all new molecules and polymers; absorbance and emission spectra of O7−O8; absorbance spectra of P5−P6 at different concentrations; emission spectra of P5−P6 in dichloromethane and in films; 2254


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NMR spectra of all compounds synthesized. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.W.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. This work was also supported by an NSF CAREER award (DMR-1151385) and Tufts University. We also thank Prof. Yu-Shan Lin (Tufts University) for assistance with DFT calculations.

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