Absorption and Emission of Bulk Solution and Film.

The absorption and emission spectra of chloroform solutions and films of P3EHT and F-P3EHT are shown in Fig. 1B. In solution, the main absorption peak of F-P3EHT (3.0 eV) is blue-shifted by about 0.20 eV with respect to that of P3EHT (2.8 eV). Similarly, the photoluminescence spectrum of F-P3EHT is blue-shifted by 0.15 eV compared with P3EHT. The spectral blue shift of F-P3EHT relative to P3EHT could be attributed to a combination of a more twisted backbone conformation as a result of the bulkier fluorine in F-P3EHT (as we will show later in MD Simulations) and/or the electron withdrawing of F atom (7). Going from solution to film, P3EHT exhibits a red shift (0.17 eV) in the absorption. Furthermore, the absence of clear vibronic structures in the low-energy range of the film spectrum (23) indicates unfavorable interchain interactions in solid-state P3EHT, despite its high head-to-tail (HT) regioregularity of 97%. This observation implies a destructive effect of the bulky ethylhexyl side chains 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 shift from solution to film. In addition, distinct vibronic structures with a strong 0–0 transition appear in the F-P3EHT film spectrum. These stark changes in F-P3EHT suggest that there is a dramatic variation in the F-P3EHT chain conformation from solution to solid state, presumably from a random coil to an extended chain conformation. This conformational variation could be due to a combined effect of planarization of individual chains upon going from solution to solid state (due to a small rotational barrier that will be discussed in MD Simulations) and interchain packing interaction. The fluorescence spectrum of F-P3EHT film exhibits a Stokes shift of ∼0.12 eV, which is smaller than ∼0.20 eV of typical P3HT films (H-aggregate) but larger than ∼0.06 eV of P3HT J-aggregate nanofibers (24, 25). To provide more in-depth understanding of the morphological and photophysical properties while avoiding the heterogeneity of bulk samples, we will focus on single chains and controllably self-assembled molecular aggregates in what follows.

Morphology of Single Chains and Aggregates.

To examine how fluorination affects the morphology of single-polymer chains and aggregates, fluorescence excitation polarization analysis was performed (SI Materials and Methods for details). In these experiments, the fluorescence intensity traces were modulated with a rotating linearly polarized excitation light and then fitted with I(α) ∝ 1+M cos2(α−ϕ), where α is the excitation polarization angle and ϕ is the polarization angle at maximum absorption (2, 26, 27). The modulation depth, M, represents the anisotropy of the absorption (excitation) tensor projected on the x−y plane of the laboratory frame and is related to the morphological order of individual molecules or aggregates. Fig. 2 A and B presents the M distribution 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 a high HT regioregularity of 97%, the M distribution is spread broadly from 0 to 1, meaning single-chain conformations range from highly disordered to highly ordered, with a mean M value of 0.53. This result is similar to that observed for regiorandom P3HT with a regioregularity of 60% (2, 28). The highly disordered single-chain conformations of P3EHT are assumed to be due to a twisted backbone as a result of the strong steric hindrance of branched and bulky ethylhexyl side chains, which will be further elucidated by the theoretical simulations shown below. Interestingly, the fluorination of P3EHT thiophene rings significantly enhances the ordering of single chains. As displayed in Fig. 2B, the M values for F-P3EHT single chains are much higher than that of P3EHT with a large majority over 0.60 and a mean of 0.72. Apparently, fluorine atom substitution at the vacant 4-position in thiophene rings in F-P3EHT greatly alleviates the disorder caused by the ethylhexyl side chains and renders more extended backbone conformation.

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Fig. 2.

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

The effect of backbone fluorination on the nanoscale interchain morphology was then explored by studying small aggregates, which were assembled using the previously reported SVA technique in conjunction with the wide-field imaging (2, 19). Briefly, during SVA a concentrated single-chain sample in PMMA is swollen with a mixed organic solvent vapor of chloroform (good solvent) and acetone (poor solvent) (Fig. 2C). Here, chloroform is capable of dissolving both PMMA and CP chains, whereas acetone is a poor solvent for CPs but still can dissolve PMMA. The diffusion of CP molecules in the swollen and softened PMMA causes the aggregation of CP chains that usually can be described by an Ostwald ripening mechanism. By using mixed solvents of chloroform and acetone with a vapor ratio of 38/62 (volume ratio of 55/45), we constructed P3EHT and F-P3EHT aggregates composed of a few chains. For detailed descriptions about the aggregate fabrication by the SVA, please refer to SI Materials and Methods and our recent work in ref. 2. Fig. 2 D and E demonstrates the M distribution histogram of (5 ± 1)-chain P3EHT aggregates and (4 ± 1)-chain F-P3EHT aggregates, respectively. Highly disordered interchain morphology is observed for P3EHT aggregates as evidenced by the dominant low-M values below 0.5 with a mean near 0.32 (Fig. 2D). The bulky side chains together with disordered single-chain configuration lead to unfavorable packing, even between a few P3EHT chains. In stark contrast to P3EHT, F-P3EHT aggregates exhibit an enhanced extent of ordering evidenced by a higher mean-M value of 0.70 (Fig. 2E). In addition, the M distribution is also narrower in comparison with single chains. This indicates that in F-P3EHT aggregates the conformation of the constituting single chains becomes ordered upon packing. The different observations between these two polymers suggest that extended chains of F-P3EHT as a result of F substitution facilitate packing between F-P3EHT chains despite the bulky side chains. The result obtained for the nanoscale aggregates is in agreement with our previous data, which revealed short-range ordering in F-P3EHT film (7).

MD Simulations.

To further elucidate the role of fluorine atom substitution in impacting polymer chain morphology, we performed MD simulations with a classical potential. Please refer to SI Materials and Methods for details. For P3EHT, we use the optimized potentials for liquid simulations-all atom (OPLS-AA) model of DuBay et al. with standard OPLS description of the side chain (29). Modeling of the F-P3EHT required modifying both the partial charges and the dihedral potential due to the inclusion of the fluorine atom. The partial charges were taken from the charges from electrostatic potentials using a grid-based method (CHELPG) calculations on dimers and trimers (Table S1), a common method for fitting partial charges (30). The dihedral potential was fitted using the second-order Møller–Plesset perturbation theory (MP2) potential surface from the previous calculation (Figs. S1 and S2 and Table S2). For the MD simulation, 30-mers of P3EHT and F-P3EHT were simulated in a box of chloroform. Fig. 3A demonstrates the representative simulated structures of the 30-mers of P3EHT and F-P3EHT. By visual inspection, the F-P3EHT adopts more elongated configurations than P3EHT. This is verified by calculation of the end-to-end distance of 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 compared with the P3EHT is also consistent with the modulation depth profiles observed in the single-molecule samples where the F-P3EHT consistently shows more anisotropic conformations as displayed in Fig. 2 A and D. Despite the elongation, the backbone dihedral distribution shown in Fig. 3B reveals that the F-P3EHT is decidedly nonplanar in solution with a broad dihedral angle θ-distribution and a maximum at 75°. In contrast, P3EHT exhibits a slightly skewed conformation as we observed for P3HT (26), although with a moderate preference for cis configurations.

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Fig. 3.

(A) Representative structures of single chains taken from the explicit solvent trajectory. (B) Backbone dihedral distributions from the MD simulations of the 30-mers of P3EHT and F-P3EHT.

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Fig. S1.

Results of the dihedral potential fit for F-P3EHT dimer structures produced by the relaxed dihedral scan. The unfitted OPLS potential uses the same dihedral potential as the P3EHT model, but with F-P3EHT partial charges. The fit OPLS potential includes dihedral terms to resolve the discrepancy between the unfit potential surface and electronic structure results.

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Fig. S2.

Atom labels used for interpreting the partial charges of F-P3EHT listed in Table S1. Unlabeled atoms indicate that partial charges were not modified in the MD simulations.

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Fig. S3.

Distribution of end-to-end length for the 30-mers of P3EHT and F-P3EHT. The distance was calculated using the sulfur atom coordinates from the terminal monomer units on the 30-mer chains.

As shown in Fig. 1B for absorption and emission spectra, F-P3EHT is blue-shifted in solution but red-shifted in film relative to P3EHT. From the above simulation we think that F-P3EHT is less planar along the backbone than P3EHT in solution; this observation is consistent with the blue-shifted electronic transitions. In neat films, the spectrum can red-shift as a combined effect of the relatively flat dihedral potential surface of individual chains and the interchain packing facilitated by the more elongated initial structure of F-P3EHT. This is supported by the Raman data for films in our previous work (7). Additionally, F-P3EHT has a more ordered modulation depth profile than P3EHT (Fig. 2) for both single chains and aggregates while having blue- and red-shifted absorption spectra, respectively.

Excitonic Coupling.

Interchain excitonic coupling was examined by performing fluorescence intensity time trace and photon correlation measurements on individual aggregates constructed with the SVA technique. Please refer to SI Materials and Methods for technique details. Note that the fluorescence transients were taken in the presence of a random generation of photochemical quenchers at high laser excitation power density, while photon correlation data were simultaneously collected until detectable photodegradation occurred. For P3EHT aggregates composed of 5 ± 1 chains, ∼65% of the aggregates exhibit gradual or approximately stepwise (type I) photodegradation behavior, whereas ∼35% show random and large intensity fluctuation, i.e., photoblinking or photobleaching (type II). Fig. 4A presents two exemplary fluorescence transients, shown as black and red curves, respectively, for these two types of aggregates. Corresponding photon correlation histograms taken for the same aggregates with a pulsed 473-nm laser at a repetition rate of 20 MHz are displayed in Fig. 4B. The number of emitters, N, in individual aggregates can be estimated by comparing the integrated area of the central peak at time 0 to the average area of the side peaks. It should be noted that to correctly estimate N the photon correlation analyses were performed before photodegradation. If N is significantly smaller than (close to) the number of chains estimated from SVA experiment, then we can deduce that there is efficient (no or limited) interchain coupling. For the type I aggregate with the transient shown in Fig. 4A, N is calculated to be ∼4.0 (Fig. 4B). Generally, P3EHT aggregates with type I degradation behavior have a broad distribution of N from 2 to 9, but with a dominant population around 2, which is smaller than the mean number of chains in the aggregates (Fig. 4C). This observation suggests a wide variety of interchain morphology and interaction for these types of aggregates, ranging from aggregates with several domains in which few chains are locally coupled, to those with individual component chains acting as discrete emitters. For the aggregates with large intensity fluctuation (type II, red trace in Fig. 4A), the central peak at correlation time 0 is greatly depressed relative to the side peaks, characteristic of photon antibunching as displayed in Fig. 4B (red). This type of aggregate usually contains only one or two emitters according to photon correlation measurements. We think that in these type II aggregates there exists a high ordering of interchain morphology either throughout the whole aggregates or within very few local domains. This ordered morphological characteristic facilitates effective interchain excitonic coupling or energy transfer––hence the observation of blinking behavior and photon antibunching as a signature of a single emitter in some of the type II aggregates.

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Fig. 4.

Representative fluorescence intensity transients (A), photon correlation functions (B), and distribution histogram of number of emitters N (C), for P3EHT aggregates. (D–F) Corresponding data for F-P3EHT aggregates. For fluorescence trajectories in A and D, the background counts rate is ∼50 counts per 100 ms. The inset values of N in B and E are for respective photon correlation data shown in the panel.

Given that the F-P3EHT aggregates have a fairly high interchain ordering (Fig. 2E) and close interchain stacking distance of ∼3.8 Å (7, 31), one would anticipate an effective interchain excitonic coupling signified by fluorescence blinking and photon antibunching. Contrary to this expectation, typical (4 ± 1)-chain F-P3EHT aggregates exhibit a stepwise degradation. As shown for the aggregate in Fig. 4D, approximately four steps of degradation are observed and the N estimated from the photon correlation measurement for the same aggregate is 3.3 (Fig. 4E). For ∼70 aggregates examined, a mean N value of 3.5 is obtained as displayed in Fig. 4F. These results unambiguously suggest the absence of efficient excitonic coupling between F-P3EHT chains regardless of the high ordering and close packing between polymer chains. In conjugated polymers, excitonic coupling between adjacent chromophoric segments can occur not only along single-polymer chains but also between closely packed polymer chains. Prior studies on the excitonic coupling in CPs have revealed that there exists a competition between the interchain and intrachain coupling (2, 32, 33). With extended chain conformation, the intrachain coupling is improved; this weakens the interchain coupling because the interaction between the extended chains scales inversely with conjugation length (34). We think that the limited interchain excitonic coupling in F-P3EHT aggregates is primarily caused by the strong and dominant intrachain coupling due to its relatively extended backbone when the chains are packed. Our results show that although the fluorination of P3EHT backbone extends the backbone in the solid state and promotes interchain morphological order, the resultant structural features significantly restrain the excitonic coupling between chains. The observation made here is of importance in the rational design of potential fluorinated polymers for specific optoelectronic applications. For instance, the backbone fluorination on one hand improves the backbone and interchain morphology and therefore enhances the charge transport (7, 33). On the other hand, the resultant extended chain conformation turns out to be detrimental to the interchain excitonic coupling, which would therefore limit the 3D exciton delocalization and diffusion demanded for photovoltaic applications.

Fluorescence Spectra.

Next, we interrogated fluorescence emission spectral features to provide more detailed understanding about the consequence 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 of aggregates is red-shifted, probably due to a slightly increased conjugation length (i.e., planarity) of the chains upon chain packing in aggregates. The broad E0 distribution for both single chains and aggregates of P3EHT is indicative of their energetic inhomogeneity, consistent with their broad morphological variations. As exhibited in Fig. 5B, the spectra of single chains and aggregates of P3EHT can be satisfactorily fit with a single Franck–Condon progression (25, 35). Although two types of fluorescence transient behavior have been detected for P3EHT aggregates (Fig. 4A), we did not find two types of aggregate emission spectra. This is probably due to similar electronic structure of P3EHT chains in the aggregates and the weak excitonic coupling between polymer chains.

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Fig. 5.

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

With respect to F-P3EHT, from single chains to aggregates, a red shift of 0.1 eV in emission with a slight variation in vibronic structures is observed. Despite the ordered single-chain conformation, the F-P3EHT single chains are energetically heterogeneous. As displayed in Fig. 5C, single F-P3EHT chains exhibit a broad distribution in values of E0, from 2.1 to 2.4 eV, analogous to morphologically disordered P3EHT chains. This observation is presumably due to a variety of chromophore sizes as a result of the relatively small interring dihedral angle in F-P3EHT as we discussed above. Most notably, however, the E0 distribution of F-P3EHT aggregates is much narrower than that of single chains. One might think that this could result from energy funneling to low-energy sites in aggregates. However, this seems unlikely because photon antibunching, or at least a smaller number of emitters than composing chains, should be expected in this case, but these were not observed in the photon correlation measurement (Fig. 4 E and F). Instead, we believe that the narrower distribution of E0 implies that when packed together in the aggregates, individual chains act as discrete emitters that are all nearly identical in energy. Whereas often we assume that interchain interaction in the solid state leads to an array of interactions and broad distribution in electronic energies, in the case of the F-P3EHT aggregates the interactions between the chains significantly reduce the heterogeneity by increasing the intrachain 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 a fit does not work well for the spectra of F-P3EHT aggregates and film. The dampened 0–0 emission in the aggregate and film spectra signifies the formation of H aggregation. The “H-like” emission of F-P3EHT aggregates and film seems contradictory to the “J-like” absorption of film (Fig. 1B). However, such an asymmetry actually has been demonstrated both theoretically and experimentally. First, according to Spano’s model, the 0–0/0–1 ratio in 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 is more sensitive to the subtle competition between the intrachain and interchain 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 a geometrically relaxed state (high symmetry) (i.e., dynamic torsional planarization of the backbones) for P3HT, which results in a dynamic 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 dynamic process evolving from J-like to H-like within a few hundreds of picoseconds 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 and the emission can be described by a modified Franck–Condon progression:P(ℏω)∝n3(ℏω)3e−S[αΓ(ℏω−E0)+∑m=1Smm!Γ[ℏω−(E0−mEp)],where α is the scaling factor that quantifies the amplitude of the 0–0 transition, n is the refraction index of the surrounding environment, m is the vibration level, Γ is the Gaussian line-width function that represents the inhomogeneously broadened spectral line of the vibronic replica, E0 and Ep represent the energy of 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 electronic transition and a phonon mode). Fig. 5D displays a typical spectrum of a single F-P3EHT aggregate fit with the H-aggregate model. The obtained Huang–Rhys factor for aggregates is 0.7 ± 0.2, which is consistent with 0.7 obtained for the F-P3EHT film and is much smaller than the value of 1.3 ± 0.2 obtained for F-P3EHT single chains. This suggests an increase in the intrachain coupling and that the HJ-aggregate model, in which the intrachain and the interchain coupling are treated equally, is more accurate in describing the F-P3EHT (36). There are several key findings that support this view. First, as shown in Fig. 1B, the 0–0 band in the F-P3EHT film absorption spectrum is much stronger than that in typical P3HT films (25, 36), implying a stronger intrachain coupling in F-P3EHT that is consistent with the assertion of the extended F-P3EHT polymer chains. Second, the I_0–0/I_0–1 ratio in F-P3EHT film emission is about 0.75 according to Fig. 1, which is ∼2–3× higher than what is typically measured in H-like P3HT films (25, 36). In addition, the Stokes shift of F-P3EHT lies in between that of the H-like P3HT film/aggregates and the much more highly ordered J-like P3HT nanofibers (24). These two observations suggest weaker interchain coupling in F-P3EHT than in the H-like P3HT films, which is in alignment with the limited exciton coupling in F-P3EHT deduced from the photon correlation measurement. Collectively, we think the spectral data of F-P3EHT aggregates and film can be reasonably described by the HJ-aggregate model and support the conclusion of a more extended F-P3EHT backbone even with the H-like emission.