Impact of backbone fluorination on nanoscalemorphology and excitonic coupling in polythiophenesZhongjian Hua,1,2, Ryan T. Hawsb,3, Zhuping Feic, Pierre Bouffletc, Martin Heeneyc, Peter J. Rosskyb,and David A. Vanden Bouta,1

aDepartment of Chemistry, University of Texas at Austin, Austin, TX 78712; bDepartment of Chemistry, Rice University, Houston, TX 77251; and cDepartmentof Chemistry, Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom

Edited by Michael L. Klein, Temple University, Philadelphia, PA, and approved April 5, 2017 (received for review December 16, 2016)

Fluorination represents an important strategy in developing high-performance conjugated polymers for photovoltaic applications.Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT)and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplifiedmodel materials, using single-molecule/aggregate spectroscopy andmolecular dynamic simulations, to elucidate the impacts of backbonefluorination on morphology and excitonic coupling on the molecularscale. Despite its high regioregularity, regioregular P3EHT exhibits arather broad distribution in polymer chain conformation due to thestrong steric hindrance of bulky ethylhexyl side chains. This confor-mational variability results in disordered interchain morphologyeven between a few chains, prohibiting long-range effective in-terchain coupling. In stark contrast, the experimental and moleculardynamic calculations reveal that backbone fluorination of F-P3EHTleads to an extended rod-like single-chain conformation and hencehighly ordered interchain packing in aggregates. Surprisingly, theordered and close interchain packing in F-P3EHT does not lead tostrong excitonic coupling between the chains but rather to dominantintrachain excitonic coupling that greatly reduces the molecularenergetic heterogeneity.

fluorinated polythiophenes | photophysics | single-molecule spectroscopy |morphology | organic electronics

Morphology and excitonic coupling are among the mostimportant factors dictating the functions and performance

of conjugated polymers (CPs) in a variety of optoelectronic ap-plications (1–4). To tune and optimize the morphological andoptoelectronic properties of CPs, chemical structure modifica-tion of the backbone and side chains is one of the main and mosteffective approaches (5–7). It has been revealed that the posi-tion, size, and length of side chains dramatically affect themorphology and optoelectronic properties of CPs. In the effortsto develop high-performance photovoltaic CPs, the introductionof fluorine atoms onto aromatic comonomers, i.e., the fluori-nation of polymer backbone, offers a very appealing strategyto tune the electronic properties, morphology, and photochem-ical stability (6–8). It is proposed that the inherent electron-withdrawing nature of fluorine atoms lowers the highest energyoccupied molecular orbital energy levels, thereby increasing theopen-circuit voltage in photovoltaics. In addition, the strong elec-tronegativity might induce F–S and F–H interactions and poten-tially modifies the molecular conformation and intermolecularorganization (6). Furthermore, it has been shown that fluorinationoften leads to enhanced thermal and oxidative stability (9–11). Givena combination of these desirable properties, fluorine-containingCPs have led to most of the best-performing polymer-based solarcells to date (12–15).For typical CPs, the primarily created exciton is Coulombically

bound upon photoexcitation mainly due to the low dielectricconstant and strong electron–phonon interaction in polymers.The interchain and intrachain morphology not only determinesthe ultrafast exciton delocalization and relaxation pathways butalso affects the longer timescale behavior including exciton mi-gration, recombination, dissociation, and charge transfer (1, 16–18).

Therefore, in an endeavor to design high-performance CPs, aninvestigation into how the backbone fluorination affects polymermorphology and exciton properties is warranted (6). Comparedwith structurally complicated fluorine-containing donor–acceptorpolymers, fluorinated poly(3-alkylthiophenes) (P3AT) represent asimplified and prototypical system to interrogate the effects offluorination. Recently, we have reported the manipulation of theP3AT backbone by incorporating fluorine in the vacant 4-positionto make poly(3-alkyl-4-fluoro)-thiophenes (7). It was revealed thatbackbone fluorination appeared to promote aggregation or short-range ordering in thin films but frustrate long-range ordering(crystallization). Despite the reduced long-range crystallinity, thin-film transistors of F-P3EHT exhibited better electrical perfor-mance than their nonfluorinated analogs. Nevertheless, funda-mental and critical knowledge about the influence of fluorinationon single-chain conformation, intermolecular morphology, andexcitonic coupling on a molecular basis is still lacking. Single-molecule/aggregate spectroscopy in conjunction with nanoscaleaggregates fabricated via a solvent-vapor-annealing (SVA) tech-nique has proven to be an unprecedented and elegant approach inelucidating complex relations between morphology and photo-physics in CPs (2, 19), while circumventing ensemble averagingeffects due to bulk measurements and morphological heteroge-neity in bulk samples (2, 19, 20).

Significance

Conjugated polymers (CPs) are some of the most attractiveorganic semiconductors for flexible optoelectronic device ap-plications. The structural and optoelectronic properties of CPsare dependent on both the conformation of individual polymerchains and interactions between chains. Chemists have soughtto control these interactions by creating more planar polymersthat can pack tightly together and share strong electronic in-teractions. Single-molecule spectroscopy (SMS) studies of afluorinated CP show that backbone fluorination does, in fact,lead to more planar polymer chains and highly ordered ag-gregates. Interestingly though, the electronic interactions arefound to extend along individual chains rather than betweenchains. The studies demonstrate how SMS can test hypothesesabout structure–property relationships in large molecularsystems.

Author contributions: Z.H., R.T.H., M.H., P.J.R., and D.A.V.B. designed research; Z.H. andR.T.H. performed research; Z.F. and P.B. contributed new reagents/analytic tools; Z.H. andR.T.H. analyzed data; and Z.H., R.T.H., M.H., and D.A.V.B. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence may be addressed. Email: dvandenbout@cm.utexas.edu orzjhu05@gmail.com.

2Present address: Center for Integrated Nanotechnologies, Materials Physics and Applica-tions Division, Los Alamos National Laboratory, NM 87544.

3Present address: Cray Inc., St. Paul, MN 55101.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1620722114/-/DCSupplemental.

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In the present contribution, we investigate the impacts of fluo-rination of the polythiophene backbone on nanoscale morphologyand excitonic coupling, using simple model polythiophenes, namely,poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophenes) (F-P3EHT) (Fig. 1A) (see SI Materials andMethods for details). To avoid the complexity of intramolecularfolding-induced interchain interactions, we use polymers with a lownumber average molecular mass of 13 kDa, for which there is nodetectable folding of the polyalkylthiophene chain according to ourprevious studies (2, 21, 22). Fluorescence excitation polarizationexperiments and molecular dynamics (MD) simulations reveal thatbackbone fluorination dramatically changes the polymer chainconformation from disordered coil-like for P3EHT into extendedrod-like for F-P3EHT. Our photon correlation data, fluorescencetransients, and spectral data obtained for SVA-assemblednanoscale aggregates reveal that the modified single-chain con-formation due to fluorination surprisingly limits efficient interchaincoupling. However, the high order in the molecular aggregates of theF-P3EHT leads to a narrowing of the distribution of polymer chainenergies such that the collection of chains shows less heterogeneitythan the individuals.

Results and DiscussionAbsorption and Emission of Bulk Solution and Film. The absorptionand emission spectra of chloroform solutions and films ofP3EHT and F-P3EHT are shown in Fig. 1B. In solution, themain absorption peak of F-P3EHT (3.0 eV) is blue-shifted byabout 0.20 eV with respect to that of P3EHT (2.8 eV). Similarly,the photoluminescence spectrum of F-P3EHT is blue-shiftedby 0.15 eV compared with P3EHT. The spectral blue shift ofF-P3EHT relative to P3EHT could be attributed to a combina-tion of a more twisted backbone conformation as a result of thebulkier fluorine in F-P3EHT (as we will show later in MD Sim-ulations) and/or the electron withdrawing of F atom (7). Goingfrom solution to film, P3EHT exhibits a red shift (0.17 eV) in theabsorption. Furthermore, the absence of clear vibronic structuresin the low-energy range of the film spectrum (23) indicates un-favorable interchain interactions in solid-state P3EHT, despiteits high head-to-tail (HT) regioregularity of 97%. This observa-tion implies a destructive effect of the bulky ethylhexyl sidechains on interchain interaction in P3EHT.Despite the blue-shifted absorption spectrum of F-P3EHT in

solution relative to P3EHT, F-P3EHT exhibits a large red shiftfrom solution to film. In addition, distinct vibronic structureswith a strong 0–0 transition appear in the F-P3EHT film

spectrum. These stark changes in F-P3EHT suggest that there isa dramatic variation in the F-P3EHT chain conformation fromsolution to solid state, presumably from a random coil to anextended chain conformation. This conformational variationcould be due to a combined effect of planarization of individualchains upon going from solution to solid state (due to a smallrotational barrier that will be discussed in MD Simulations) andinterchain packing interaction. The fluorescence spectrum ofF-P3EHT film exhibits a Stokes shift of ∼0.12 eV, which issmaller than ∼0.20 eV of typical P3HT films (H-aggregate) butlarger than ∼0.06 eV of P3HT J-aggregate nanofibers (24, 25).To provide more in-depth understanding of the morphologicaland photophysical properties while avoiding the heterogeneity ofbulk samples, we will focus on single chains and controllably self-assembled molecular aggregates in what follows.

Morphology of Single Chains and Aggregates. To examine howfluorination affects the morphology of single-polymer chains andaggregates, fluorescence excitation polarization analysis was per-formed (see SI Materials and Methods for details). In these ex-periments, the fluorescence intensity traces were modulated with arotating linearly polarized excitation light and then fitted withIðαÞ  ∝   1+M cos2ðα−ϕÞ, where α is the excitation polarizationangle and ϕ is the polarization angle at maximum absorption (2,26, 27). The modulation depth, M, represents the anisotropy ofthe absorption (excitation) tensor projected on the x−y plane ofthe laboratory frame and is related to the morphological order ofindividual molecules or aggregates. Fig. 2 A and B presents the Mdistribution histograms of single chains of P3EHT and F-P3EHT,respectively, embedded in poly(methyl methacrylate) (PMMA)matrix. Although P3EHT investigated in the present work has ahigh HT regioregularity of 97%, the M distribution is spreadbroadly from 0 to 1, meaning single-chain conformations rangefrom highly disordered to highly ordered, with a mean M valueof 0.53. This result is similar to that observed for regiorandom P3HTwith a regioregularity of 60% (2, 28). The highly disordered single-chain conformations of P3EHT are assumed to be due to a twistedbackbone as a result of the strong steric hindrance of branched andbulky ethylhexyl side chains, which will be further elucidated by thetheoretical simulations shown below. Interestingly, the fluorination

S n

S n

F

P3EHT

F-P3EHT

A B Solution

Film

Fig. 1. (A) Chemical structures of P3EHT and F-P3EHT. (B) Absorption andfluorescence spectra for solutions (Top) and films (Bottom) of P3EHT (13 kDa,black) and F-P3EHT (13 kDa, red). The solid and dashed curves are for ab-sorption and fluorescence spectra, respectively.

A

B

D

E

single chains solvent-vapor-annealing aggregates

C

Fig. 2. Distribution histograms of fluorescence excitation modulationdepth, M, for single chains of P3EHT (A) and F-P3EHT (B). (C) Formation ofaggregates using the SVA approach. (D and E) M histograms for SVA-assembled aggregates of P3HT and F-P3EHT, respectively.

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of P3EHT thiophene rings significantly enhances the ordering ofsingle chains. As displayed in Fig. 2B, the M values for F-P3EHTsingle chains are much higher than that of P3EHT with a largemajority over 0.60 and a mean of 0.72. Apparently, fluorine atomsubstitution at the vacant 4-position in thiophene rings in F-P3EHT greatly alleviates the disorder caused by the ethylhexylside chains and renders more extended backbone conformation.The effect of backbone fluorination on the nanoscale inter-

chain morphology was then explored by studying small aggre-gates, which were assembled using the previously reported SVAtechnique in conjunction with the wide-field imaging (2, 19).Briefly, during SVA a concentrated single-chain sample inPMMA is swollen with a mixed organic solvent vapor of chlo-roform (good solvent) and acetone (poor solvent) (Fig. 2C).Here, chloroform is capable of dissolving both PMMA and CPchains, whereas acetone is a poor solvent for CPs but still candissolve PMMA. The diffusion of CP molecules in the swollenand softened PMMA causes the aggregation of CP chains thatusually can be described by an Ostwald ripening mechanism. Byusing mixed solvents of chloroform and acetone with a vaporratio of 38/62 (volume ratio of 55/45), we constructed P3EHTand F-P3EHT aggregates composed of a few chains. For detaileddescriptions about the aggregate fabrication by the SVA, pleaserefer to SI Materials and Methods and our recent work in ref. 2.Fig. 2 D and E demonstrates theM distribution histogram of (5 ±1)-chain P3EHT aggregates and (4 ± 1)-chain F-P3EHT aggre-gates, respectively. Highly disordered interchain morphology isobserved for P3EHT aggregates as evidenced by the dominantlow-M values below 0.5 with a mean near 0.32 (Fig. 2D). Thebulky side chains together with disordered single-chain configu-ration lead to unfavorable packing, even between a few P3EHTchains. In stark contrast to P3EHT, F-P3EHT aggregates exhibitan enhanced extent of ordering evidenced by a higher mean-Mvalue of 0.70 (Fig. 2E). In addition, the M distribution is alsonarrower in comparison with single chains. This indicates that inF-P3EHT aggregates the conformation of the constituting sin-gle chains becomes ordered upon packing. The different observa-tions between these two polymers suggest that extended chains ofF-P3EHT as a result of F substitution facilitate packing betweenF-P3EHT chains despite the bulky side chains. The result obtainedfor the nanoscale aggregates is in agreement with our previousdata, which revealed short-range ordering in F-P3EHT film (7).

MD Simulations. To further elucidate the role of fluorine atomsubstitution in impacting polymer chain morphology, we per-formed MD simulations with a classical potential. Please refer toSI Materials and Methods for details. For P3EHT, we use theoptimized potentials for liquid simulations-all atom (OPLS-AA)model of DuBay et al. with standard OPLS description of theside chain (29). Modeling of the F-P3EHT required modifyingboth the partial charges and the dihedral potential due to theinclusion of the fluorine atom. The partial charges were takenfrom the charges from electrostatic potentials using a grid-basedmethod (CHELPG) calculations on dimers and trimers (TableS1), a common method for fitting partial charges (30). The di-hedral potential was fitted using the second-order Møller–Plessetperturbation theory (MP2) potential surface from the previouscalculation (Figs. S1 and S2 and Table S2). For the MD simula-tion, 30-mers of P3EHT and F-P3EHT were simulated in a box ofchloroform. Fig. 3A demonstrates the representative simulatedstructures of the 30-mers of P3EHT and F-P3EHT. By visual in-spection, the F-P3EHT adopts more elongated configurations thanP3EHT. This is verified by calculation of the end-to-end distanceof the polymer during the simulation where the F-P3EHT end-to-end distance is more than 50% larger than for the P3EHT (Fig.S3). The greater elongation observed for the F-P3EHT comparedwith the P3EHT is also consistent with the modulation depthprofiles observed in the single-molecule samples where the

F-P3EHT consistently shows more anisotropic conformations asdisplayed in Fig. 2 A and D. Despite the elongation, the backbonedihedral distribution shown in Fig. 3B reveals that the F-P3EHT isdecidedly nonplanar in solution with a broad dihedral angleθ-distribution and a maximum at 75°. In contrast, P3EHT ex-hibits a slightly skewed conformation as we observed for P3HT(26), although with a moderate preference for cis configurations.As shown in Fig. 1B for absorption and emission spectra,

F-P3EHT is blue-shifted in solution but red-shifted in film relativeto P3EHT. From the above simulation we think that F-P3EHT isless planar along the backbone than P3EHT in solution; thisobservation is consistent with the blue-shifted electronic transi-tions. In neat films, the spectrum can red-shift as a combinedeffect of the relatively flat dihedral potential surface of individualchains and the interchain packing facilitated by the more elongatedinitial structure of F-P3EHT. This is supported by the Raman datafor films in our previous work (7). Additionally, F-P3EHT hasa more ordered modulation depth profile than P3EHT (Fig. 2) forboth single chains and aggregates while having blue- and red-shiftedabsorption spectra, respectively.

Excitonic Coupling. Interchain excitonic coupling was examined byperforming fluorescence intensity time trace and photon corre-lation measurements on individual aggregates constructed withthe SVA technique. Please refer to SI Materials and Methods fortechnique details. Note that the fluorescence transients weretaken in the presence of a random generation of photochemicalquenchers at high laser excitation power density, while photoncorrelation data were simultaneously collected until detectablephotodegradation occurred. For P3EHT aggregates composed of5 ± 1 chains, ∼65% of the aggregates exhibit gradual or approxi-mately stepwise (type I) photodegradation behavior, whereas ∼35%show random and large intensity fluctuation, i.e., photoblinking orphotobleaching (type II). Fig. 4A presents two exemplary fluores-cence transients, shown as black and red curves, respectively, forthese two types of aggregates. Corresponding photon correlationhistograms taken for the same aggregates with a pulsed 473-nm laserat a repetition rate of 20 MHz are displayed in Fig. 4B. The numberof emitters, N, in individual aggregates can be estimated by com-paring the integrated area of the central peak at time 0 to the av-erage area of the side peaks. It should be noted that to correctlyestimate N the photon correlation analyses were performed before

P3EHT F-P3EHT

A

B

Fig. 3. (A) Representative structures of single chains taken from the explicitsolvent trajectory. (B) Backbone dihedral distributions from the MD simu-lations of the 30-mers of P3EHT and F-P3EHT.

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photodegradation. If N is significantly smaller than (close to) thenumber of chains estimated from SVA experiment, then we can de-duce that there is efficient (no or limited) interchain coupling. For thetype I aggregate with the transient shown in Fig. 4A, N is calculated tobe ∼4.0 (Fig. 4B). Generally, P3EHT aggregates with type I degrada-tion behavior have a broad distribution of N from 2 to 9, but with adominant population around 2, which is smaller than themean numberof chains in the aggregates (Fig. 4C). This observation suggests a widevariety of interchain morphology and interaction for these types ofaggregates, ranging from aggregates with several domains in which fewchains are locally coupled, to those with individual component chainsacting as discrete emitters. For the aggregates with large intensityfluctuation (type II, red trace in Fig. 4A), the central peak at correlationtime 0 is greatly depressed relative to the side peaks, characteristic ofphoton antibunching as displayed in Fig. 4B (red). This type of ag-gregate usually contains only one or two emitters according to photoncorrelation measurements. We think that in these type II aggregatesthere exists a high ordering of interchain morphology either throughoutthe whole aggregates or within very few local domains. This orderedmorphological characteristic facilitates effective interchain excitoniccoupling or energy transfer––hence the observation of blinking behav-ior and photon antibunching as a signature of a single emitter in someof the type II aggregates.Given that the F-P3EHT aggregates have a fairly high in-

terchain ordering (Fig. 2E) and close interchain stacking distanceof ∼3.8 Å (7, 31), one would anticipate an effective interchainexcitonic coupling signified by fluorescence blinking and photonantibunching. Contrary to this expectation, typical (4 ± 1)-chainF-P3EHT aggregates exhibit a stepwise degradation. As shownfor the aggregate in Fig. 4D, approximately four steps of deg-radation are observed and the N estimated from the photoncorrelation measurement for the same aggregate is 3.3 (Fig. 4E).For ∼70 aggregates examined, a mean N value of 3.5 is obtainedas displayed in Fig. 4F. These results unambiguously suggest theabsence of efficient excitonic coupling between F-P3EHT chainsregardless of the high ordering and close packing betweenpolymer chains. In conjugated polymers, excitonic coupling be-tween adjacent chromophoric segments can occur not only alongsingle-polymer chains but also between closely packed polymerchains. Prior studies on the excitonic coupling in CPs haverevealed that there exists a competition between the interchain

and intrachain coupling (2, 32, 33). With extended chain con-formation, the intrachain coupling is improved; this weakens theinterchain coupling because the interaction between the ex-tended chains scales inversely with conjugation length (34). Wethink that the limited interchain excitonic coupling in F-P3EHTaggregates is primarily caused by the strong and dominantintrachain coupling due to its relatively extended backbone whenthe chains are packed. Our results show that although the fluo-rination of P3EHT backbone extends the backbone in the solidstate and promotes interchain morphological order, the resultantstructural features significantly restrain the excitonic couplingbetween chains. The observation made here is of importance in

A

ED

C

F

B

Fig. 4. Representative fluorescence intensity transients (A), photon correlation functions (B), and distribution histogram of number of emitters N (C), forP3EHT aggregates. (D–F) Corresponding data for F-P3EHT aggregates. For fluorescence trajectories in A and D, the background counts rate is ∼50 counts per100 ms. The inset values of N in B and E are for respective photon correlation data shown in the panel.

A

B

C

D

Fig. 5. Ensemble fluorescence emission spectra of single chains (green) andaggregates (red) for P3EHT (A) and F-P3EHT (C). The histograms at thebottom of A and C present the 0–0 transition energy, E0, distribution forsingle chains (green) and aggregates (red). B and D exhibit a typical spec-trum for single chain (green) and aggregate (red) of P3EHT and F-P3EHT,respectively. Corresponding fits (black) of the Franck–Condon model or theH-aggregate model are overlaid on the spectra in B and D.

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the rational design of potential fluorinated polymers for specificoptoelectronic applications. For instance, the backbone fluori-nation on one hand improves the backbone and interchainmorphology and therefore enhances the charge transport (7, 33).On the other hand, the resultant extended chain conformationturns out to be detrimental to the interchain excitonic coupling,which would therefore limit the 3D exciton delocalization anddiffusion demanded for photovoltaic applications.

Fluorescence Spectra. Next, we interrogated fluorescence emissionspectral features to provide more detailed understanding about theconsequence of fluorine substitution on the photophysics of CPs.Fig. 5A shows the ensemble spectra and the 0–0 transition energy(E0) distributions of single chains and aggregates of P3EHT.Compared with the spectrum of single chains, the spectrum ofaggregates is red-shifted, probably due to a slightly increasedconjugation length (i.e., planarity) of the chains upon chainpacking in aggregates. The broad E0 distribution for both singlechains and aggregates of P3EHT is indicative of their energeticinhomogeneity, consistent with their broad morphological vari-ations. As exhibited in Fig. 5B, the spectra of single chains andaggregates of P3EHT can be satisfactorily fit with a single Franck–Condon progression (25, 35). Although two types of fluorescencetransient behavior have been detected for P3EHT aggregates (Fig.4A), we did not find two types of aggregate emission spectra. Thisis probably due to similar electronic structure of P3EHT chainsin the aggregates and the weak excitonic coupling between poly-mer chains.With respect to F-P3EHT, from single chains to aggregates, a

red shift of 0.1 eV in emission with a slight variation in vibronicstructures is observed. Despite the ordered single-chain confor-mation, the F-P3EHT single chains are energetically heteroge-neous. As displayed in Fig. 5C, single F-P3EHT chains exhibit abroad distribution in values of E0, from 2.1 to 2.4 eV, analogousto morphologically disordered P3EHT chains. This observationis presumably due to a variety of chromophore sizes as a result ofthe relatively small interring dihedral angle in F-P3EHT as wediscussed above. Most notably, however, the E0 distribution ofF-P3EHT aggregates is much narrower than that of single chains.One might think that this could result from energy funneling tolow-energy sites in aggregates. However, this seems unlikelybecause photon antibunching, or at least a smaller number ofemitters than composing chains, should be expected in this case,but these were not observed in the photon correlation mea-surement (Fig. 4 E and F). Instead, we believe that the narrowerdistribution of E0 implies that when packed together in the ag-gregates, individual chains act as discrete emitters that are allnearly identical in energy. Whereas often we assume that in-terchain interaction in the solid state leads to an array of inter-actions and broad distribution in electronic energies, in the caseof the F-P3EHT aggregates the interactions between the chainssignificantly reduce the heterogeneity by increasing the intra-chain coupling along each chain.Although the emission spectra of F-P3EHT single chains can

be fit with a single Franck–Condon progression (Fig. 5D), such afit does not work well for the spectra of F-P3EHT aggregates andfilm. The dampened 0–0 emission in the aggregate and filmspectra signifies the formation of H aggregation. The “H-like”emission of F-P3EHT aggregates and film seems contradictory tothe “J-like” absorption of film (Fig. 1B). However, such an asym-metry actually has been demonstrated both theoretically and ex-perimentally. First, according to Spano’s model, the 0–0/0–1 ratioin absorption is related to the exciton bandwidth, whereas the 0–0/0–1 ratio in emission is related to the exciton coherence (that is, it ismore sensitive to the subtle competition between the intrachain andinterchain coupling) (32). Second, compared with the absorption,the emission undergoes fast dynamic processes. Paquin et al. (36)and Parkinson et al. (37) both have revealed a dynamic evolution

from an initial vibrationally hot excited state (low symmetry) to ageometrically relaxed state (high symmetry) (i.e., dynamic torsionalplanarization of the backbones) for P3HT, which results in a dy-namic and strong decrease of the relative 0–0 intensity within∼10 ps. With time-resolved absorption measurements, Ade and co-workers have reported the interchain coupling involves a dynamicprocess evolving from J-like to H-like within a few hundreds ofpicoseconds for P3HT in polyethylene oxide matrix* (38).In the H-aggregate model (25, 36), the 0–0 transition from the

first excited state to the ground state is symmetry forbidden andthe emission can be described by a modified Franck–Condonprogression:

PðZωÞ∝ n3ðZωÞ3e−S"αΓðZω−E0Þ+

Xm=1

Sm

m!Γ

"Zω−

�E0 −mEp

�#,

where α is the scaling factor that quantifies the amplitude of the0–0 transition, n is the refraction index of the surrounding envi-ronment, m is the vibration level, Γ is the Gaussian line-widthfunction that represents the inhomogeneously broadened spec-tral line of the vibronic replica, E0 and Ep represent the energyof the 0–0 transition and carbon–carbon stretching vibration(∼0.18 eV), respectively, and S denotes the Huang–Rhys factor(which represents the coupling strength between the electronictransition and a phonon mode). Fig. 5D displays a typical spec-trum of a single F-P3EHT aggregate fit with the H-aggregatemodel. The obtained Huang–Rhys factor for aggregates is 0.7 ±0.2, which is consistent with 0.7 obtained for the F-P3EHT filmand is much smaller than the value of 1.3 ± 0.2 obtained forF-P3EHT single chains. This suggests an increase in the intrachaincoupling and that the HJ-aggregate model, in which the intra-chain and the interchain coupling are treated equally, is moreaccurate in describing the F-P3EHT (36). There are several keyfindings that support this view. First, as shown in Fig. 1B, the 0–0 band in the F-P3EHT film absorption spectrum is much stron-ger than that in typical P3HT films (25, 36), implying a strongerintrachain coupling in F-P3EHT that is consistent with the as-sertion of the extended F-P3EHT polymer chains. Second, theI0–0/I0–1 ratio in F-P3EHT film emission is about 0.75 accordingto Fig. 1, which is ∼2–3× higher than what is typically measuredin H-like P3HT films (25, 36). In addition, the Stokes shift of F-P3EHT lies in between that of the H-like P3HT film/aggregatesand the much more highly ordered J-like P3HT nanofibers(24). These two observations suggest weaker interchain cou-pling in F-P3EHT than in the H-like P3HT films, which is inalignment with the limited exciton coupling in F-P3EHT de-duced from the photon correlation measurement. Collectively,we think the spectral data of F-P3EHT aggregates and film canbe reasonably described by the HJ-aggregate model and supportthe conclusion of a more extended F-P3EHT backbone even withthe H-like emission.

ConclusionThe influence of backbone fluorination in conjugated polymerson morphology and excitonic coupling at the molecular andnanoscale levels were studied with two prototypical poly-thiophenes, P3EHT and F-P3EHT. Due to the presence of bulkyethylhexyl side chains in P3EHT, there is a broad variety ofmorphology for both single chains and nanoscale aggregates.Because of the morphological variability in P3EHT aggregates,the excitonic coupling between polymer chains is widely distributedbut generally not efficient, i.e., from localized coupling between

*Gautam B, et al., APS March Meeting 2014, March 3–7, 2014, Denver, CO, abstr BAPS.2014.MAR.T20.2.

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only a few chains to strongly limited. Fluorescence excitation po-larization experiments in combination with MD revealed thatbackbone fluorination greatly extends the single-chain conformationof P3EHT. Consequently, the fluorinated single chains can assembleto form aggregates with a high extent of interchain ordering. De-spite the ordered and close interchain packing in F-P3EHT, theinterchain excitonic coupling is surprisingly inhibited. This is at-tributed to the dominant and competitive coupling along F-P3EHTchains due to fluorine-induced extended backbone. Furthermore,the highly ordered packing in F-P3EHT surprisingly leads to areduction in the energetic heterogeneity of the individual chains asthe intrachain coupling is increased. This provides the unusualresult that the ensemble of chains in the aggregate has a narrowerdistribution of energies than the corresponding distribution of in-dividual polymer chains. Our study pinpoints the importance ofthorough and systematic efforts to understand the role that struc-ture and morphology play in determining how inter- and intrachainexcitonic coupling affect the electronic properties of conjugatedpolymers for optoelectronic applications.

Materials and MethodsThe synthesis of P3EHT and F-P3EHT was reported in our previous work (7).The crude polymers were further fractionated to obtain a molecular mass of13 kDa and dispersity of 1.1. Single-molecule samples were spin cast fromdiluted chloroform solutions containing 2 wt % PMMA on clean coverslips toform PMMA films with conjugated polymer chains embedded inside. Theaggregates were prepared via the SVA technique with a vapor ratio of 38/62of solvent mixture of chloroform and acetone that was detailed previously(2). Single-molecule/aggregate spectroscopy was performed on a typicalwide-field and laser scanning confocal microscope (2). In order to do MDsimulations, we first performed electronic structure oligomer optimizationsand classical force-field parameter derivation. MD simulations included a 30-mer of F-P3EHT or P3EHT in explicit chloroform solvent. All simulations werecalculated with Gromacs v4.6.7 (39) using the OPLS-AA force field (40). Moredetails of all materials, methods, and procedures can be found in SI Materialsand Methods.

ACKNOWLEDGMENTS. This work was supported by National ScienceFoundation Grant CHE-1310222 (to D.A.V.B.), National Science FoundationGrant CHE-1362381(to P.J.R.), Robert A. Welch Foundation Grant F-0019 (toP.J.R.), and a doctoral training grant through the Engineering and PhysicalSciences Research Council (to M.H.).

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