Carpenter's Rule Folding in RigidâFlexible Block ... - ACS Publications

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Carpenter’s Rule Folding in Rigid−Flexible Block Copolymers with Conjugation-Interrupting, Flexible Tethers Between Oligophenylenevinylenes Jeffrey M. Lucas,† Joelle A. Labastide, Lang Wei, Jonathan S. Tinkham,‡ Michael D. Barnes,* and Paul M. Lahti* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Rigid−flexible segmented block copolymers were synthesized and characterized as 4.5-oligophenylenevinylene chromophores tethered by flexible, conjugationinterrupting 1,2-ethanedioxy or 1,4-butanedioxy units. The flexible tethers allow the possibility of collapsed order chromophore assemblies within individual polymers by chain folding at specific sites much like an old fashioned, folding carpenter’s rule. Our results indicate that using a short, flexible tether in a rigid−flexible segmented copolymer can result in collapsed rodlike structures as signaled by strongly quenched photoluminescence, even after thermal annealing. Such ability to “program” folding and tertiary structure in conjugated copolymers is important for solid-state organic light emitting materials and understanding of organic chromophore self-assembly.

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have been observed by Barnes and co-workers19 in oriented single-molecule nanostructures. Karasz and co-workers studied 2.5- and 3.5-oligophenylenevinylene type rigid−flexible segmented copolymers as blue16,17 and green18 OLED emitters. For blue emission, they used systems with structure PV2.5Fn, where 2.5 is the degree of phenylenevinylene (PV) oligomerization, and F is a flexible polyethylenedioxy linker unit of (2n+2)-atoms to link the PV units (Scheme 1). Systems with varying length tethers were amorphous in the solid state, with little difference in emissive behavior. In more recent studies, Vanden Bout and co-workers compared20,21 the solution and solid film electronic spectral behaviors of oPV chromophores to those of corresponding segmented copolymers connected by flexible tethers in structures PV2.5PE, PV4.5PE, PV6.5PE, PV2.5DOO, PV4.5DOO, PV6.5DOO (Scheme 1, TE for pentaethylene tether, DOO for dioxyoctane tether). Some of these copolymers have long chromophores that should favor πstacking, save for stack inhibition by the large, branched ethylhexyloxy side arm groups attacked to each phenylene group of the oPV unit. Both of the flexible tether units are both moderately long with 10 atoms in the backbone, allowing substantial freedom of motion for chromophores in a polymer chain. The solution phase behaviors of the systems were very similar to one another. But, solid film studies showed evidence that the longer oPV systems had more electronic coupling

INTRODUCTION Control over the morphological characteristics of organic molecular and polymeric materials is critical for their use in electronic devices.1−6 For example, contemporary organic photovoltaic active layer film morphologies are tuned at mesoscales and nanoscales by solution processing and thermal annealing of active layer constituents, where the morphology of the active layer is often determined by arrested kinetics of phase separation and domain crystallization.7−11 The resultant morphology is often disordered, with crystalline domains separated by amorphous regions. Also, reproducing the sizescale of the crystalline domains often requires carefully controlled conditions, which contributes significantly to device pathologies in organic optoelectronics. Organic electronic materials based on conjugated polyphenylenevinylene (PPV) type polymers typically are structurally rigid but frequently have structural defects that affect average conjugation length.12−15 In one strategy to retain chromophoric purity, enhance processability, and avoid changes in luminescence behavior by encouraging amorphous solidstate film formation, segmented alternating block copolymers have been tested16−18 in organic light-emitting diode (OLED) devices. The nonconjugated flexible linkers that connect chromophoric units have enough flexibility to allow chromophore self-organization during film casting, with segments of the copolymer collapsing like an old-style, folding carpenter’s rule to give self-assembled, folded lamellar regions of chromophore stacking. Such collapsed structures were proposed by Barbara and co-workers13 to be associated with large single-(polymer) molecule polarization anisotropies, and © 2015 American Chemical Society

Received: March 9, 2015 Revised: May 12, 2015 Published: June 30, 2015 8010

DOI: 10.1021/acs.jpca.5b02295 J. Phys. Chem. A 2015, 119, 8010−8020

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The Journal of Physical Chemistry A

synthetic procedures for new compounds and synthetic intermediates are given in the Supporting Information. Basic Characterization. 1H NMR spectra were obtained at 400 MHz in solution using a Bruker Avance 400 instrument: peak positions are reported in ppm downfield of tetramethylsilane. UV−vis absorption spectra were obtained in 1 cm path length quartz cuvettes (solution) or on borosilicate glass plates (solid film), using a Shimdazu UV-2600PC spectrometer. Photoluminescence (PL) spectra were obtained using a PTI QM-30 fluorimeter. Solution samples were sonicated before electronic spectra were obtained. Polymer molecular weight and dispersity (Đ) analysis was done by solution gelpermeation chromatography (GPC) in 1,2,4-trichlorobenzene at 135 °C using a Polymer Laboratories PL-220 hightemperature GPC instrument calibrated against polystyrene standards. Differential scanning calorimetry (DSC) was carried out using a TA Instruments DSC 2910. Dynamic light scattering (DLS) measurements were obtained using a Malvern Nano Zetasizer using sonicated 0.01 mM polymer solutions in acetonitrile that were passed through a 0.45 μm PTFE filter disk. Luminescence quantum yields in spectrograde chloroform were determined by a literature procedure22 at an excitation wavelength of 420 nm using external standard coumarin-6 (ethanol, quantum yield = 0.78): a solvent refractive index correction was made as part of the determination. Melt-cast films of some solid organic samples were made by placing a small amount of powder onto a borosilicate glass plate and heating on a hot plate until melting was complete. The melt-cast film was then cooled on a metal plate. Drop-cast films were made by dropping the analyte solution (0.01 mM in chloroform) onto a borosilicate glass plate. The films were then allowed to dry slowly under a bell jar flushed with nitrogen gas. Annealing of drop-cast films was conducted by placing on a preheated hot plate at 120 °C for 2 min. New Compounds. M0. 1H NMR (CDCl3) δ: 0.97 (t, 6H, J = 6 Hz); 1.41 (m, 8H); 1.56 (m, 4H); 1.89 (m, 4H); 3.90 (s, 6H); 3.96 (s, 12H); 4.10 (t, 4H, J = 6 Hz); 7.09 (m, 4H); 7.16 (m, 8H); δ 7.54 (m, 10H). UV−vis (CHCl3; nm[ε = M−1cm−1]), 422[57,800]. PL (CHCl3, excit =420 nm; nm [φ = quantum yield]: 482 [φ = 0.58]). FT-IR (neat, cm−1) 2957 (aliph C−H), 2853 (OCH2−H), 1579 (CC), 1126 (C−O), 958 (trans CH out-of-plane bend). Mp = 174−175 °C (d). DSC: 65−80 (broad, weak endotherm). MS (EI) molecular ion peak m/z = 866.53, calculated for C56H66O8 m/z = 866.48. P2. 1H NMR (CDCl3) δ: 0.95 (t, 6 H, J = 6 Hz), 1.41 (br m, 8 H), 1.58 (br m, 4 H), 1.89 (m, 4 H), 3.91 (m, 12 H), 4.08 (br t, 4 H, J = 6 Hz), 6.75 (s, 4 H), 7.10 (m, 8 H), 7.52 (m, 10H). UV−vis (CHCl3; nm [ε = M−1cm−1]), 425[78,800]. PL (CHCl3, excit =420 nm; nm [φ = quantum yield]: 484 [φ = 0.53]). FT-IR (neat, cm−1) 2924 (aliph C−H), 2853 (OCH2− H), 1579 (CC), 1123 (C−O), 956 (trans CH out-ofplane bend). DSC: 60−70 °C (endotherm), 70−85 °C (shoulder endotherm). GPC (CHCl3 eluent, vs polystyrene standards): M̅ n = 3889; Đ = 2.65. P4. 1H NMR (CDCl3) δ: 0.94 (t, 6 H, J = 6 Hz), 1.40 (br. m, 8 H), 1.56 (br m, 4 H), 1.88 (br m, 4 H), 1.97 (br. m, 4 H), 3.89 (m, 12 H), 4.07 (t, 4 H, J = 6 Hz), 6.76 (s, 4 H), 7.10 (m, 8 H), 7.52 (m, 10H). UV−vis (CHCl3); nm [ε = M−1cm−1]), 418[58,000]. PL (CHCl3, excit = 420 nm; nm [φ = quantum yield]: 482 [φ = 0.44]). FT-IR (neat, cm−1) 2928 (aliph C−H), 2857 (OCH2−H), 1578 (CC), 1123 (C−O), 955 (trans  CH out-of-plane bend). DSC: 60−80 °C (endotherm). GPC

Scheme 1. Examples of Luminescent Rigid−Flexible Segmented Copolymers Based on Oligophenylenevinylenes with Conjugation-Interrupting Flexible Tethers

between chromophores, and that the dioxyoctane-linked systems had more folding and interchromophore interactions than did the pentaethylene-linked systems. The long flexible linker systems in the Vanden Bout work allow for folding and assembly of elongated linear chromophores, but the side-chain branched-substituent modification should disrupt chromophore stacking. In contrast, we wanted to gauge the effects upon photophysical behavior and possible chromophore aggregation of using shorter flexible linkers that act more like a simple pivot or that mimic a simple defect in an otherwise conjugated linear polymer chain. Shorter linkers with more limited degrees of conformational freedom should limit the opportunities for randomly amorphous association of the polymer chains, because a higher percentage of the possible conformer space should fold chromophores into proximity. In this contribution, we describe the synthesis, static, and timeresolved photophysical characterization, and transmission electron microscopy (TEM) study of 4.5-oPV chromophores linked by flexible 1,2-ethandioxy and 1,2-butandioxy units, shown in Scheme 1 as P2 and P4, respectively. The corresponding monomeric chromophore M0 was also synthesized for comparison to the block copolymers. We found quite substantial effects of the linker length, as well as chromophore aggregation effects.

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EXPERIMENTAL SECTION Materials and Procedures. Solvents and reagents were purchased from Fisher Scientific or Sigma-Aldrich and used as received unless stated otherwise. Tetrahydrofuran was distilled from sodium-benzophenone under nitrogen just before use. Microwave assisted reactions were carried out using a CEM STAR Synth microwave reactor, using CEM disposable microwave reaction vessels capped with silicone/PTFE caps. Microwave pressure limits were set to 300 PSI, and the “max power” cooling option was used for all reactions. Detailed 8011


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The Journal of Physical Chemistry A Scheme 2. Synthetic Routes to M0, P4, and M1a

a

(i) Dibromoalkane, K2CO3, DMF [n = 1 (86%), n = 2 (83%)]; (ii) CH3PPh3Br, nBuLi, THF [n = 2, 4 (83%]; (iii) bromohexane, K2CO3, CH3CN (85%); (iv) paraform, HBr, HOAc (77%); (v) P(OEt)3 (100%); (vi) 4-bromobenzaldehyde, NaH, THF (94%); (vii) Pd(OAc)2, P(o-tolyl)3, Et3N, DMF [M1 (30%), P2 (33%), P4 (36%)].

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(CHCl3 eluent, vs polystyrene standards): M̅ n = 2858; Đ = 1.33. Time-Resolved Spectroscopy. Time-correlated singlephoton count (TCSPC) measurements were conducted using a fluorescence microscope outfitted with a 100×/1.4 N.A. immersion oil objective, configured for epi-illumination. The 440 nm excitation was sourced from a pulsed photodiode laser (PicoQuant PDL LDH-P-C-440B) with a 10 MHz repetition rate. Data were collected through a 480 nm long pass filter using a precision avalanche photodiode (IDquantique) that is registered to the microscope confocal spot, and a TCSPC module (PicoQuant PicoHarp 300). TCSPC histograms were analyzed using Symphotime, and normalized to their respective maxima for comparison, using a literature procedure.23 Transmission Electron Microscopy (TEM). TEM images were collected using a JEOL JEM-2000FX transmission electron microscopy. In a typical experiment, one drop of freshly prepared 0.01 mM chloroform solvent polymer dispersion was deposited on a carbon-coated copper grid (Electron Microscopy Sciences) and left to dry overnight. Some samples were subjected to an additional annealing process of 2 min at 120 °C. Subsequent sample investigations were performed at room temperature using an accelerating voltage of 200 kV.

RESULTS Synthesis. Scheme 2 shows the syntheses of monomeric control compound M0 and the corresponding, conjugatednonconjugated, block copolymers P2 and P4. For the polymers, flexible linker units were synthesized by linking two syringaldehyde groups with the appropriate 1,n-dibromoalkane (n = 2, 4) by Williamson etherification, followed by Wittig vinylation of the terminal aldehyde groups to give compounds 1 and 2, respectively. A central oPV unit 1,4-bis(4bromostyryl)-2,5-dihexyloxybenzene with solubilizing straightchain hexyloxy side chains (3) was made by alkylating hydroquinone, bromomethylating, functionalizing to the bis(phosphonate ester) groups under Arbuzov conditions, and Horner-Emmons coupling with 4-bromobenzaldehyde: the latter two steps in this sequence gave superior product formation in less time under microwave conditions instead of conventional heating. Compound 3 was then coupled with 1 and 2 by microwave-assisted Heck-type polymerization to give polymers P2 and P4, having GPC molecular weights of ∼3500 (Đ = 2.65, degree of polymerization 4−5) and ∼3000 (Đ = 1.33, degree of polymerization 3−4), respectively. Although these are not high polymers, they have the alternating block copolymeric structure needed to assess effects of the controlled conjugation-interrupting flexible units. 8012


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Figure 1. Absorption spectra for M0 (), P2 (− −), and P4 (- -) in chloroform (left) and acetonitrile (right). Analogous plots for toluene are given in Supporting Information Figure S1. See also Table 1.

Figure 2. PL spectra for M0 ( and △), P2 (− −), and P4 (- - -) in chloroform (left) and acetonitrile (right), obtained using 0.01 mM solution concentrations with excitation at 420 nm for both. Analogous plots for toluene are given in Supporting Information Figure S1. The right-hand plot includes M0 spectra for samples that were () and were not (△) subjected to sonication. See also Table 1.

Table 1. Solvent Effects on Static Absorbance and Emission Spectral Maximaa M0 P2 P4

ABS λmax [ε], PhMe

ABS λmax [ε], CHCl3

ABS λmax [ε], MeCN

PL λmax [nm], PhMe

PL λmax [nm], CHCl3

PL λmax [nm], MeCN

424 [43600] 426 [69400] 418 [44000]

422 [57800] 425 [78800] 418 [58000]

418 [40800] 418 [15800] 414 [23300]

485 (510) 483 (514) 480 (511)

482 (511) 484 (514) 482 (511)

484, 507b 481 (510) 480 (510)

For absorbance (ABS) spectra, λmax = nm, ε = L mol−1 cm−1. All PL spectra obtained for 0.01 mM concentration solutions using a 420 nm excitation wavelength; weaker shoulder PL peak positions are given in parentheses. bThe longer wavelength peak is higher intensity for an unsonicated solution; otherwise, the shorter wavelength peak is higher intensity.

a

Absorption and Luminescence Spectra. Figures 1−2 show the static absorption and PL emission spectra for M0, P2, and P4 under similar conditions. The similarity of these spectra is consistent with expectations for the main π → π* transitions arising from the same oPV chromophore, with little or no perturbation from conformational twisting or from interchromophore association. The absorption measurements (Figure 1) all show strong overlapping bands at about 350 and 418−425 nm with lineshapes that are typical for oPVs. The small absorption solvatochromic blue shifts (≤6 nm, ≤42 meV; Table 1 and Supporting Figure S1) from nonpolar toluene (PhMe) to polar acetonitrile (MeCN)rather than a red shift, as more typically anticipated for π → π* transitionscan be attributed to specific solvent−solute interactions24 of acetonitrile with polar methoxy/alkoxy substituents in the structures. The absorption spectral lineshapes in chloroform do not shift or change significantly with dilution up to 64-fold (Supporting

Information Figure S2). Finally, the measured molar absorptivities in acetonitrile for P2 and P4 are anomalously low by comparison to the control, M0. Static PL spectra (Figure 2) for 420 nm excitation all show highest energy λmax = 480−485 nm bands with poorly resolved, lower energy vibronic structure. The PL spectra used to compile Table 1 are given in the Supporting Information, Figure S1. The quantum yields for M0, P2, and P4 in chloroform (CHCl3) are 58%, 53%, and 44%. While all other photoluminescence and absorption spectra of the compounds studied appear spectroscopically as J-type aggregatestypical of PPV systemsthe PL spectrum of M0, obtained immediately after dissolution by brief hand-agitation in acetonitrile is unique in that it shows25 the spectral characteristics of an H-type (cofacial coupling) aggregate. The unsonicated M0 spectrum (open triangles in Figure 2b) reproducibly shows highest intensity at 507 nm rather than at 8013


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decay rates in a study of similar (albeit shorter conjugation length) alkoxy-substituted oPVs, including a segmented copolymer.27 This process predominates under most conditions, being slightly slower and more predominant in polar MeCN than in the less polar solvents. The faster τ1 PL decay process is more predominant in PhMe and CHCl3 for P2 (almost half), but almost absent for P4. We attribute the fast decay process to involve chromophores in close proximity to other chromophores (as in solvated aggregates), in a manner that causes them to relax more quickly to the ground state. This assignment is supported by additional findings described below. Overall, the presence or absence of the nonconjugating linker to create a segmented copolymer does not greatly change the solution phase photophysical behaviors of M0, P2, and P4, except for the nonequilibrium aggregation of unsonicated M0 in acetonitrile. But, solid-state behavior was much more influenced by the nonconjugated tether structure. Solid film absorption spectra of solid films drop-cast from chloroform (Figure 3) have maxima at 450 nm for M0, 450 nm for P2, and 435 nm for P4. The samples all have similar PL spectra, a featureless maximum at 550 nm (Supporting Information Figure S5). Annealing the samples at 120 °C for 5 min causes the absorption maxima for the systems to blue-shift by 28, 27, and 9 nm for M0, P2, and P4, respectively. The annealing greatly reduces the originally strong solid film PL intensity of pristine M0, somewhat reduces the solid film PL intensity of P2, and renders P4 nonemissive (even the original P4 sample emits only weakly, as shown in Supporting Information Figure S6−S7). ATR-IR spectroscopy shows no postannealing formation of carbonyl bands to indicate oxidative cleavage of oPV units in any of M0, P2, and P4. Therefore, the annealinginduced changes in the spectra strongly suggest chromophore movement from a kinetically trapped, pristine distribution of sites, to more thermodynamically favorable arrangements with different ensemble photophysical behavior. DSC shows endotherms in the 60−85 °C range for all of M0, P2, and P4, possibly from side chain annealing. Given the identifiably different conformation-based behaviors seen by Buratto and coworkers28 in single-molecule spectroscopy of isolated oPV systems similar in structure to those in the present study, annealing-based changes in ensemble solid film behavior due to relative intermolecular movement assisted by conformational re-equilibration (including side chain motion) seem quite reasonable. TCSPC measurements were therefore conducted on the same drop-cast films used to obtain the static spectral

484 nm, along with a significant decrease in integrated photoluminescence intensity. However, if the M0 solution is sonicated for several minutes, the more typical PL spectrum with maxima at 480 nm was obtained. The difference is attributable to chromophore aggregation in the unsonicated samples; this is also consistent with the reduced emission intensity before sonication. Dynamic light scattering studies show P2 and P4 to give ∼900 nm diameter particles (see Supporting Information Figure S3), while M0 gives much smaller particles ∼250 nm in diameter. Despite the particle formation, P2 and P4 show no strong changes in PL spectral line shape in acetonitrile. The changes in M0 static PL therefore are reasonably attributed to H-aggregates that are expected23,25,26 to suppress the intensity of the higher energy (0−0 transition) portion of the spectrum. Further evidence given below is consistent with molecular stacking in solid M0. Time-Resolved Photoluminescence Studies. Timeresolved photoluminescence lifetime studies in toluene (PhMe), chloroform (CHCl3), and acetonitrile (MeCN) for 0.01 mM solutions of M0, P2, and P4 all show decay behavior fitting two major decay processes, one with τ1 ≈ 0.14−0.55 ns and the other with τ2 ≈ 1.1−1.3 ns, as shown in Table 2: the Table 2. Solvent Effects on Solution TRPL Decay for M0, P2, and P4a M0 P2 P4 M0 P2 P4 M0 P2 P4

PhMe

CHCl3

MeCN

τ1 [ns] (A1)

τ2 [ns]/(A2)

0.399 0.641 0.164 0.397 0.200 0.548 0.151 0.141 0.152

1.161 1.279 1.067 1.224 1.332 1.230 1.324 1.298 1.275

(0.327) (0.457) (0.150) (0.195) (0.307) (0.159) (0.140) (0.108) (0.110)

(0.671) (0.534) (0.838) (0.798) (0.690) (0.816) (0.859) (0.876) (0.877)

a

All TRPL experiments carried out using 0.01 mM concentration solutions with a 440 nm excitation wavelength, monitoring PL decay at >480 nm. A1 and A2 are fractional contributions to decay functions fitted to data in Figure S4.

actual PL decay curves are given in Supporting Information Figure S4. Mathematical inclusion of a third decay exponential gives less than 3% of a longer decay time process. The τ2 ∼ 1 ns process is attributable to excited state decay of well-solvated individual oPV chromophores, based on comparison to PL

Figure 3. Solid-state absorption spectra for M0 (left), P2 (middle), and P4 (right) dropcast films: pristine (□) and after (△) 5 min of annealing at 120 °C. Spectra are normalized on the ordinate. 8014


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by TEM, which shows distinct differences between the samples, and in particular shows changes when pristine samples are thermally annealed. Pristine M0 shows layered structure in striated domains clustered throughout the film (Figure 4). The presence of strong electron diffraction (ED) rings indicates substantial crystallinity; a number of the striated domain regions are single crystalline or nearly so, since they produce well-defined patterns of ED spots, rather than the rings seen in thicker regions. After annealing, M0 samples show fewer ED rings, but with strengthened ring intensity; the regions showing discrete ED spots show simplified ED patterns. TEM micrographs show the pristine P4 films also to form sheet-like layers (Figure 5), but without well-defined striations like those in M0. The P4 samples show multiple, grainy ED rings indicating significant crystallinity with some orientation; after annealing, the ED rings become faint and diffuse, with a few ED spots indicating remnant crystalline domains. This indicates that P4 becomes glassy and less crystalline upon annealing. TEM shows that annealed sample layers contract into isolated foam-like ovoids with some irregular striations. Pristine P2 TEM shows a foamy microstructure (Figure 5) that totally lacks the stacked sheet mesostructures of pristine M0 and P4 films. Despite this, ED of pristine P2 shows welldefined rings without much granularity. After annealing, the ED rings become more intense with strong granularity, indicating an increased degree of crystallite formation. The ED pattern trend postannealing is the qualitatively opposite to that in P4. While the majority “foamy-film” and featureless regions remain in P2 postannealing, a few small regions of layering were found that look similar to those in M0 and P4. Analysis of ED rings for the various samples yielded the dspacings shown in Table 4. The d-spacing in M0 decreases by 0.2 Å postannealing, so the M0 chromophore stacking arrangement changes significantly in association with the quenching of its solid film PL by annealing. By contrast, dspacing in the segmented copolymers essentially is not changed by annealing, despite changes in ED ring intensities under those conditions. The copolymer structures therefore seem to undergo only changes in the amount of interchromophore assembly, whether interchain or intrachain folding, whereas M0 actually undergoes overall changes in interchromophore stacking. Notably, the strong film PL in pristine M0 is almost completely quenched by annealing. We interpret these results to mean that any disorganized M0 emitting chromophores and/or those in a crystalline but PL-permissive environment shift and assemble upon annealing into a closer-spaced

measurements, both before and after thermal annealing. The same triexponential model used for the solution TRPL was used to analyze the film TCSPC data. The results are summarized in Table 3, and the decay curves are given in Table 3. TCSPC Decay Behavior for Dropcast Films of M0, P2, and P4, Pristine and Annealeda M0 P2 P4 M0 P2 P4

pristine

annealed

τ1 [ns] (A1)

τ2 [ns]/(A2)

nil 0.31 0.22 0.22 0.31 0.21

1.45 1.50 1.34 1.33 1.48 1.35

(0.849) (0.935) (0.930) (0.848) (0.954)

(0.999) (0.145) (0.064) (0.068) (0.146) (0.046)

a All TCSPC experiments obtained using dropcast films excited at 440 nm wavelength, monitoring PL photon counts at >480 nm. Annealed samples were heated at 120 °C for 5 min. A1 and A2 are fractional contributions to decay functions fitted to data in Figure S8.

Figure S8 of the Supporting Information. The control M0 film PL decay behavior is very different from that in P2 and P4. Pristine M0 PL decay is essentially monoexponential with τ2 = 1.45 ns, but only 6.8% of this process remains after annealing; instead 93% of decay arises from a fast decay process with τ1 = 0.22 ns. The dramatic change indicates that the emitting chromophores undergo major changes in structure or (more likely) local environment. We attribute the 1.45 ns decay to relaxation of M0 chromophores in sites having little interchromophore interaction, and the fast decay process to relaxation of structurally interactingpossibly cofacially πstackedchromophores. Pristine P2 has essentially biexponential TCSPC PL decay behavior: a major 84.9% component of fast decay with τ1 = 0.31 ns, plus significant 14.5% contribution from a τ2 = 1.50 ns process whose rate corresponds to noninteracting chromophore behavior, by analogy to the M0 assignment. Interestingly, there is essentially no change in the decay rates or their relative contributions to P2 postannealing PL decay. Pristine P4 has qualitatively similar behavior to P2, but more (93.5%) of the τ1 fast decay process: unlike the case in P2, annealing further increases the already large contribution of fast decay. Morphology. The differences in solid state photophysical behavior among M0, P2, and P4 presumably arise from morphological differences. In an effort to probe these differences, the pre- and postannealed films were examined

Figure 4. TEM micrographs of pristine (left) and annealed (right) M0 thin films. Areas outlined in red indicate regions from whence the corresponding electron diffractogram was measured. 8015


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Figure 5. Transmission electron micrographs of P2 and P4 drop-cast films, pristine (left) and annealed (right). Areas outlined in red gave the electron diffractograms shown.

completely quenched PL with the same d-spacing of 4.8 Å that is observed in annealed M0, whereas P2 with its substantially larger (and annealing-resistant) d-spacing of 5.4 Å remains substantially luminescent. In comparing the solid film static and time-resolved PL results with the TEM/ED results, it is important to remember that the PL assays only monitor emitting sites in the solid films, while TEM and ED reflect results from all examined portions of the films. Polymer films are well-known to exhibit different PL behavior in different regions, depending on local domain nanostructure.29,30 Microscopy shows the solid films in this study (see Supporting Information Figure S7) to have regions

Table 4. Ring-to-Center d-Spacings in Angstroms, from Electron Diffractograms of M0, P2, and P4, before and after Annealinga pristine film annealed film

M0

P2

P4

5.0 4.8

5.4 5.4

4.8 4.8

a All d-spacing values were calibrated against a gold standard. Annealing was carried out for 5 min at 120 °C.

arrangement that quenches emission. This relationship between d-spacing and PL quenching can be extrapolated to the copolymer samples. Pristine and annealed P4 shows poor to

Scheme 3. Oligophenylenevinylenes Exhibiting Crystallographic Intermolecular Herringbone or Cofacial H-Aggregate Stacking

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Figure 6. B97D/6-31G(d,p) optimized, folded, cofacial conformer for a model of P4 (left) compared to an optimized model dimer of M0 (right). The central-ring hexyloxy group was replaced by a methoxy group for the computations. An analogous model of P2 failed to converge on a folded structure. Hydrogen atoms are omitted in some views for clarity.

Therefore, the annealed TCSPC results show loss of the isolated chromophore emission decay process in M0, to give essentially only the fast relaxation PL decay associated with interchromophore association.

of bright PL emission surrounded by darker regions: the density of emitting sites on this scale is qualitatively consistent with the naked-eye intensities of PL in pristine and postannealed samples. The microscopic level change in the M0 sample is particularly dramatic, with loss of fairly large emitting regions after annealing, consistent with qualitative observation. There are few examples of single crystal analysis for longer oPV chains, but these provide useful insight. Oligophenylenevinylenes that lack large side-chain substituents favor herringbone PPV-type stacking, for example the 4.5-oligophenylenevinylene GOZHOS31 in Scheme 3. Perfluorination of end group phenyl groups induces32 brickwork J-type stacking in QERWUF, due to a special supramolecular effect33−35 for solids that can form fluoroarene/arene interactions. Methoxy substituents on the end units of an oPV do not disrupt the tendency to herringbone packing in36 the 3.5-oPV HR3.5 (see Supporting Information crystal structure and Figure S9). But, addition of straight-chain alkyl or alkoxy groups to the interior phenylene units of oPVs favor cofacial π-system stacking, as exemplified by ABOWET37 and OJUVUK38 (see Supporting Information Figures S10−S11) from the Cambridge Structure Database; these are appropriate model, rigid conjugated systems with interior long-chain substitution on the phenylene rings, but no substituents directly on the alkene units. Although we so far have been unable to obtain single crystals of M0, if it behaves like the structural analogues that were just described, its interior side chains should favor formation of cofacial π-stacking. If so, this in turn would favor solid state exciton hopping and other PL quenching mechanisms,39−41 and would fit the solid film PL behavior of annealed M0. Pristine M0 films have crystalline domains but also enough disordered and/or larger d-spacing domains to show film PL with TCSCP decay rates very similar to those from isolated chromophores. M0 does not have the flexible tether conformational restrictions of the segmented copolymers, so disordered M0 domains become better ordered by annealing (consistent with ED results) and previously emitting chromophores can assemble into π-stacks having rapid excited state relaxation.

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DISCUSSION The results described above show readily detectable differences in the photophysical and morphological behavior of M0, P2, and P4, that are related to the differences between self-assembly of free monomeric chromophores, versus folding and assembly in tightly tethered (P2) or more loosely tethered (P4) chains of chromophores. While chromophore self-assembly is driven by the desire of the rigid oPVs to stack, the nature of the stacking is reinforced or hindered both by side chains on the chromophore, and by chain-wide conformational flexibility of the tether units. P2 has short flexible tether groups that limit flexibility within even short chains of these rigid-flexible block copolymers. Various attempts to geometry-optimize a folded P2 dyad structure using a B97D/6-31G(d,p) density functional level of theory with the Gaussian 09 program42 failed to yield a stable structure. The B97D functional43 was used because of its inclusion of longer range dispersive interactions by Grimme’s formulation. The tether group in P2 is apparently too short to give intrachain folding in this structure. Of course, interchain πstacks are still possible, but the experimental observation of static PL in pristine and in annealed P2 does not support pervasive formation of PL-quenching cofacial π-stacked chromophores, although the TCSPC results indicate that enough stacking of some sort occurs to sufficient extent for a fast relaxation process (of similar rate to fast relaxation in annealed M0) to dominate. The ED results indicate that already-quenched, orderly domains of P2 become more orderly upon annealing, but domains that lack interchromophore quenching contacts largely do not reorganize into quenching geometries: as a result, these chromophore-disordered, emitting domains remain after annealing to dominate the TCSPC decay results. 8017


Article

The Journal of Physical Chemistry A Assuming a lack of chain flexibility limits P2 in its ability to achieve folded or interchain cofacial or similar quenching geometries between chromophores, P4 should have long enough linkers to permit chain folding. The same level of computational modeling that does not support P2 dyad folding, does find cofacial dyad folding in P4, as well as cofacial dimer formation in a pair of free M0 monomers (see Figure 6; pictures generated using GaussView44). The cofacial stack geometries were similar minima with no negative vibrational modes in the frequency analysis. The experimental evidence is consistent with cofacial monomer organization in P4. From ED results, pristine P4 has some crystallinity in pristine drop-cast films, but after annealing loses much of that crystallinity. P4 is poorly photoluminescent in both pristine and annealed films, consistent with the P4 tether allowing easy formation of PLquenching geometries. Notably, P4 has the lowest solution phase PL quantum efficiency of the systems in this study, suggesting extra quenching even in dilute solution conditions. The computations for P4 suggest that chain folding to quench PL can readily occur. Precipitation of (at least partly) precollapsed chains would explain the low static PL in pristine P4 films, as well as the fact that pristine P4 films already exhibit TCSPC relaxation behavior that is nearly the same as that of postannealed M0 films. Annealing decreases crystallinity in P4 films according to EDprobably due to glassing disorder and further increases PL quenching. The results are consistent with initial formation of numerous folded, intrachain chromphore contacts that quench PL, with some disordering between chains. Annealing gives better interchromophore, PLquenching contacts on a short-range local scale (possibly in both folded chains and for localized interchain contacts) despite loss of longer range order. It is useful to compare results for M0, P2, and P4 to those for copolymers in the Vanden Bout work, which used long, branched side chain substitution on every monomer unit, and considerably longer tethers than we used. Most of the absorption and PL behavior in these segmented copolymers resembled that of monomeric chromophores, but there was some variation in solid state PL behavior as a function of tether to indicate that greater length or flexibility −O(CH2)nO− linked systems folded better than −(CH2)n− linked systems. This would give more interchromophore interaction within a chain, consistent with solid film PL spectral line shape changes that were observed. Single-molecule emission spectral histograms and molecular dynamics simulations indicated21 that (despite the branched side chains) longer oPV chromophores induced a greater tendency for chromophore self-assembly, including intramolecular chain folding of the type that we have termed carpenter’s rule folding in this study. Our study reveals even modest degrees of oligomerization like those in M0, P2, and P4 are sufficient to induce significant amounts of carpenter’s rule folding in flexible/rigid rod copolymers, so long as the flexible tethers are long enough to allow some freedom of motion for chromophore intrachain organization. The flexible tethers do not have to be particularly long to achieve this, based on our analysis of results for P4. By comparison, in Karasz’ and co-workers’ study17 of PV2.5 type segmented copolymers (Scheme 1), systems with −O(CH2)nO− tethers having n = 2,4,6,8 all gave the same PL spectrum without major quenching or indications of crystallinity in powder X-ray diffraction studies. If the chromophores in the Karasz studies form either intra- or interchain local close contacts, they seem more likely to be

herringbone contacts rather than cofacial, based on comparison to HR3.5 in Scheme 3. Herringbone contacts would not favor PL quenching, consistent with the efficacy of PV2.5 segmented copolymers as blue emitting OLED layers.16,17 By comparison, the systems in the present study show quenching. While we lack direct crystallographic evidence regarding interchromophore interactions in M0, P2, and P4, the sum of computational modeling, luminescence quenching, electron diffraction d-spacings, and comparisons to close structural analogues makes a strong case that these chromophores pack similarly to PPV-type oligomers that have long, straight-chain alkyl or alkoxy substitution.

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CONCLUSIONS The present work shows that a segmented copolymer design strategy does not invariably prevent interchromophore association for rigid-rod oligophenylenevinylenes. Even use of branched side chain chromophores, like those in previous studies, still allows chromophore interaction, so long as the flexible tether units allow chromophore movement. Conversely, use of some short linkers can actually constrain interchromophore interaction, by inhibiting intrachain folding. It is plausible that other rigid-rod chromophores with sufficiently long aspect ratios might act similarly. Tuning segmented copolymer structures with interior solubilizing substituents (or not), with varying length flexible tethers, and by varying the chromophore length can clearly give a range of behavior from eliminating cofacial π-stacks that are bad for OLEDs, to encouraging cofacial stacks for charge transport applications where interchromophore electronic interaction is desirable. This variation in behavior is important to factor into design of structurally related, conjugated−nonconjugated systems for luminophore usage.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthetic procedures; UV−vis and PL spectra, normalized TRPL decay curves for M0, P2, and P4 in different solvents; dynamic light scattering, solid film PL, photographs of luminescing films, TRPL decay curves of films of M0, P2, P4; chromophore packing diagrams of compounds in Scheme 3; computational summaries for M0 dimer, folded P4, extended P4 structures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b02295.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.M.L.). *E-mail: [email protected] (M.D.B.). Present Addresses †

J.M.L.: Tokyo Electron Ltd., Albany, NY 12203 USA. J.S.T.: Colorado School of Mines, Golden, CO 80401 USA.

‡

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The majority of this work was supported by the US Department of Energy (Program Manager: Larry Rahn) under award DE-FG02-05ER15695. Electron microscopy studies (L.W.) were supported by Polymer-Based Materials for Harvesting Solar Energy, an Energy Frontier Research 8018


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Center, funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award DE-SC0001087.

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