Rational Design of New Conjugated Polymers with Main Chain Chirality for Efficient Optoelectronic Devices: Carbo[6]Helicene and Indacenodithiophene Copolymers as Model Compounds

The unique properties of conjugated polymers (CPs) in various optoelectronic applications are mainly attributed to their different self‐assembly processes and superstructures. Various methods are utilized to tune and control CP structure and properties with less attention paid to the use of chirality. CPs with main chain chirality are rare and their microscopic and macroscopic properties are still unknown. In this work, the first experimental results are provided along these lines by synthesizing a series of racemic and enantiopure CPs containing statistical and alternating carbo[6]helicene and indacenodithiophene moieties and evaluating their microscopic (optical, energy levels) and macroscopic properties (hole mobilities, photovoltaic performance). It is demonstrated that a small statistical insertion of either the racemic or enantiopure helicene into the polymer backbone finely tunes the microscopic and macroscopic properties as a function of the statistical content. The microscopic properties of the enantiopure versus the racemic polymers with the same helicene loading remain similar. On the contrary, the macroscopic properties, and more interestingly those between the two enantiomeric forms, are altered as a function of the statistical content. Once incorporated into a solar cell device, these chiral CPs display better performance in their enantiopure versus racemic forms.


Introduction
Conjugated polymers (CPs) are seen as excellent candidate materials for the ever expanding applications of opto(bio)electronics, activity. [2]2h-j] Despite such broad ranging applications, only a handful of CPs possessing chirality through enantiopure organic building blocks in the main chain polymer backbone have been synthesized, adequately characterized and incorporated into optoelectronic devices.This prevents the in-depth understanding of their photophysical properties, microstructures and device performance toward the design of next generation efficient chiral CPs. [7]n this context, a judicious choice of chiral building block for the preparation of main chain chiral CPs may be the use of helicenes, [10] since they are fully -conjugated helical molecules that possess semi-conducting activity and good chiroptical properties, such as strong electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). [10,11]12b,d,e] Furthermore, these previously reported polymers possess low molecular weights, closer to small oligomers. [13]Due to the rigid structure of the helicene core, the corresponding polymers (oligomers) exhibit low solubility in common organic solvents which limits their processability for the preparation of high-quality thin films, hindering the fabrication of efficient electronic devices.
Given this, it remains synthetically challenging to develop helicene-based chiral CPs possessing high molecular weight, good solubility and processability in the appropriate quantity to accomplish a complete structure/optoelectronic properties/device performances relationships study; this aspect is still missing from the scientific community.
In this work, we present the efficient synthesis of a series of solution processable high molecular weight CPs containing either racemic or enantiopure helicenes in the polymer backbone, which allowed us to perform a detailed microscopic-macroscopic study.More specifically, we first evidenced how the addition of a low proportion of racemic helicene into the polymer backbone can fine tune the optical properties, energy levels, and charge transporting properties of the pristine CPs, as well as impact their performances when implemented in organic photovoltaic (OPV) devices as electron donors.We then performed similar studies on the enantiopure CPs and compared the enantiopure to the racemic materials with respect to their optical properties, energy levels and charge transporting properties.We also compared enantiopure versus racemic materials in OPV devices, highlighting the role of self-assembly on the macroscopic properties.To the best of our knowledge, this work represents the first reported use of chiral CPs based on main-chain chirality in a solar cell device.

Synthesis and Properties Characterization of the Racemic Polymers
A series of polymers containing statistical amounts of 4,4,9,9-tetrakis(4-hexylphenyl)−4,9-dihydro-s-indaceno[1,2b:5,6-b']dithiophene IDT6 and Hel were synthesized from 2, 15-dibromo-[6]helicene (Hel-Br) and the corresponding dibrominated and distannylated derivatives of IDT6 using Stille aromatic cross-coupling polymerization conditions (see Synthesis section of the SI).Derivatives of IDT are known to provide wide bandgap polymers with excellent performance in devices such as OPVs [14] and OFETs. [15]They can be synthesized in a straightforward manner including a wide range of functionalities.Typical hexyl side chains were chosen on IDT to balance good solubility and packing, and to compensate for the low solubility of the helicene moiety.
The Hel-Br molar loading fraction (x) was selected to range from 10 to 50%, with 10% increments, through control of IDT6-Br stoichiometry in the reaction mixture.The homopolymer pIDT6 was prepared as a reference polymer.The racemic and enantiopure (P)-and (M)−2, 15-dibromo-carbo[6]helicene (P/M)Hel-Br monomers were prepared through a classical oxidative photocyclization reaction, following previously reported procedures, followed by chiral HPLC resolution (see SI part). [16]As shown in Table 1, the increase of the helicene loading in the reaction mixture (from 10 to 50%) had a direct impact on the reaction time required to generate a polymer molecular weight that would allow for sufficient processabilty.While the reaction was stopped after 8 min in the case of the homopolymer, it required one day (24 h) for the pIDT6-Hel49 polymer.
After purification, the resulting polymers were collected from the chloroform fraction using a Soxhlet extraction.The average molecular weight per number (M n ), the average molecular weight per weight (M w ) and the polydispersity (D = M w /M n ) of these polymers were estimated by Gel Permeation Chromatography (GPC, see Figure S2, Supporting Information).Increasing the helicene loading in the reaction mixture led to polymers with lower molecular weight, despite the longer reaction times used (see Table 1).This indicates the lower reactivity of the helicene monomer in comparison to the IDT6 moiety.We mainly attribute this to the steric hindrance of 2, 15-dibromo-carbo[6]helicene, which negatively impacts the kinetics of the cross-coupling reaction and slows down the overall polymerization.For instance, racemic polymers IDT6-Hel1 and IDT6-Hel11 displayed a M w of 83.5 and 13.4 kDa, respectively (which corresponds to 92  [12b,d,e] and general structure of the polymers developed in the present work.and 16 repeating units).Nevertheless, the increased reaction times enabled us to obtain high molecular weight IDT6-based CPs containing helicenes.It is especially noticeable that the molecular weight obtained for polymers pIDT6-Hel1 and pIDT6-Hel6 and their processability (all the newly prepared polymers were found readily soluble in common chlorinated solvents) are comparable to state-of-the-art (non-chiral) polymers in the literature.
The polymers were analyzed by 1 H-NMR spectroscopy to assess the exact proportion of helicene unit incorporated into the polymeric backbone.From Figure S1 (Supporting Information), it is clear that by increasing the 2,15-dibromo-carbo[6]helicene   HelBr reaction loading from 10 to 50%, the intensity of the multiplet at 7.90-8.35ppm, typical of the helicene core, gradually increases.Interestingly, the thiophene proton peak at 6.28 ppm is influenced by the neighboring effect of the helicene.Furthermore, the signal at 2.55 ppm (corresponding to the methylene of the IDT6 alkyl chains, see Part 3 -NMR in the SI for the full spectra) starts to split, which indicates a dissymmetric environment for these protons due to the presence of the helicene moiety.The helicene and methylene signals were used to evaluate the actual IDT6:Hel ratio in each polymer, which was found to be different from the monomer ratio in the reaction (see Table 1).Note that this NMR methodology gives approximate values since the slow relaxation times observed in NMR measurements for polymers may affect the peak intensities.As an illustrative example, the loading of 10% helicene gives access to a polymer incorporating ≈1% helicene and displaying a M w of 83.5 kDa.It was observed that only pIDT6-Hel49 has a feed ratio equal to the observed one in the backbone, though it is also the lowest molecular weight polymer of this study.Overall, the sterically hindered Hel-Br unit shows low reactivity and tends to slow down the polymer growth following its incorporation into the polymer chain, while the presence of the IDT6-Br co-monomer is key to achieve high molecular polymers.
The UV-Vis absorption spectra of the synthesized polymers (Figure 2a) were recorded in chloroform solution at room temperature (c ≈ 10 −5 m); note that all  and Δ values in this work are given in units of m −1 .cm−1 .The pIDT6, as well as the polymers containing low helicene ratios (pIDT6-Hel1 and pIDT6-Hel6), exhibit a structured band with a maximum ≈558 nm and a well-resolved vibronic side band at 522 nm, characteristic of the rigid planar structure of the IDT6 unit. [17]Interestingly, when the helicene percentage increases, a blue-shift, and a broadening of the absorption bands with lower intensity are observed ( = 39 950 for pIDT6-Hel1 versus 13 100 for pIDT6-Hel49).This confirms that the incorporation of the helicene into the polymer chain has a strong impact on the overall polymeric structure.Concomitantly, new bands between 400-500 nm appear, originating from the helicene monomer, with increased loading of the helicene units.Overall, the polymers pIDT6-Hel11 and pIDT6-Hel21 containing 11% and 21% of helicene, respectively, are the most balanced in terms of optical response, with observable absorption contributions from both Hel and IDT6.No significant differences were observed for polymer absorption between the solution (Figure 2a) and the solid-state (Figure 2b).This indicates a lack of any significant supramolecular arrangement or aggregation-induced effect, which was further supported by thermally induced absorption spectra (Figure S3, Supporting Information).Indeed, the thermally induced absorption spectra do not show any differentiation in the absorption profiles, even at 105 °C in oDCB.The polymer bandgap was estimated based on the solid-state absorption data (Table 1). [18]It is interesting to note that the absorption offset, and thus the bandgap, remains the same throughout the whole series, i.e., ≈2.1 eV (except the pIDT6-Hel49 for which a 0.3 eV difference is observed).The following trends evident: statistical insertion of the helicene monomer results in the same absorption maxima and bandgap as pure pIDT6 up to 11% loading.Broadening of the absorption spectra with a blue shift of the absorption maxima is observed up to 21% loading, but without a change in bandgap.A significant blue shift and simultaneous widening of the bandgap are observed at 49% loading.These results can be explained by considering how the different compositions and main chain positioning of the helicene in the polymer backbone affects intramolecular charge transfer (ICT).
The emission properties of the polymers were also studied both in solution and as thin films (Table 1; Figures S4 and S5, Supporting Information).In solution, upon excitation at 365 nm, an efficient energy transfer from helicene to the IDT occurs.The characteristic vibronic feature of the pIDT6 is shown in the helicene-based IDT CPs up to 11% helicene content (pIDT6-Hel1 and pIDT6-Hel11, with the emission wavelength  max PL within the range of 567-571 nm).The same vibronic feature is also present in the pIDT6-Hel49 but blue shifted, with  max PL at 491 nm (Figure S4, Supporting Information).Interestingly, the pIDT6-Hel21 exhibits slightly blue shifted  max PL (559 nm) with an additional small peak at 491 nm, as observed for pIDT6-Hel49.The photoluminescence quantum yields (PLQY) also present an interesting trend.Upon increasing the helicene content up to 11%, the PLQY gradually increases from 30% to 45% and then gradually decreases to 33% and 16% for the pIDT6-Hel21 and pIDT6-Hel49, respectively.Passing from solution to the solid state, polymer emission is much less intense, where only the pIDT6-Hel21 and pIDT6-Hel49 exhibit a photoluminescence signal (see Figure S5, Supporting Information), with  max PL of 660 and 605 nm for the pIDT6-Hel21 and pIDT6-Hel49, respectively.As in solution, efficient energy transfer from helicene to the IDT occurs for pIDT6-Hel21 and pIDT6-Hel49, whereas the aggregation of the IDT seems to be much stronger with lower helicene loadings resulting in significant photoluminescence quenching.
The E HOMO of the CPs levels were determined by Ambient pressure Photoemission Spectroscopy (APS) using a Kelvin probe (see APS part in the SI) [19] whereas the E LUMO levels were estimated by deducing the bandgap obtained from solid-state absorption spectra on the E HOMO levels.To our surprise, the introduction of even 1% of Hel has a strong impact on the E HOMO values (−5.08 eV for pIDT6 vs −5.17 eV for pIDT6-Hel1) which can be attributed to the deeper E HOMO level and the weaker electron donating nature of Hel as compared to IDT (Figure S7, Supporting Information).As shown in Table 2 and in Figure 2c, the energy levels and bandgaps do not change significantly for pIDT6-Hel1/pIDT6-Hel6 which contain below 10% loading of the helicene.As the content of the helicene increases to 11% and 21% for pIDT6-Hel11 and pIDT6-Hel21, respectively, the E HOMO level further decreases by ≈0.2 eV (−5.32 and −5.36 eV, respectively), without altering the optical bandgap.Finally, for the pIDT6-Hel49 the E HOMO strongly decreases to ≈−5.6 eV with an increase in bandgap.Thus, for this specific series of polymers a small amount of helicene up to 21% can decrease the E HOMO which can be beneficial on the open circuit voltages (V oc values) in OPVs, without negatively impacting the optical bandgap (only a small blue shift on the absorption maxima).
To correlate the increased composition of the helicene in the backbone of the pristine CPs with a macroscopic property, we performed a detailed study on their charge transporting properties by space charge limited (SCLC) and results are in Table 1 and in Figure 2d.Since these polymers will be employed as electron donor polymers in OPVs we focused our attention on hole mobility (μ h ).Polymers pIDT6 and pIDT6-Hel1 exhibit a higher μ h which approaches 10 −4 cm 2 .V −1 s −1 (9.04 × 10 −5 cm 2 .V −1 s −1 for pIDT6 and 8.32 × 10 −5 cm 2 .V −1 s −1 for pIDT6-Hel1), which is similar to the high-performance CPs reported in the literature. [20]The fact that the pIDT6-Hel1 presents similar μ h to pIDT6 demonstrates that 1% of Hel does not prevent the pIDT6-Hel1 from adopting a similar self-assembly process as pIDT6.As the presence of helicene increases (6% and 11%) in the polymer backbone, pIDT6-Hel6 and pIDT-Hel11 maintain μ h in the order of 10 −5 cm 2 .V −1 s −1 , whereas the further increase of the helicene content to 21% reveals a significant decrease of the μ h by two orders of magnitude (4.8 × 10 −7 cm 2 .V −1 s −1 for pIDT6-Hel21).Since there is not a large difference between the M w of the pIDT-Hel11 and pIDT-Hel21 this significant decrease in μ h can be attributed to the increased content of the helicene.Finally, pIDT-Hel49 presents the lowest μ h which is in the order of 10 −9 cm 2 .V −1 s −1 .This likely arises from the very low M w , resulting in poor thin film quality, as well as the additional helicene content.

Synthesis and Properties Characterization of the Enantiopure Polymers
Having analyzed in detail the whole series of racemic helicene IDT CPs with the purpose of studying the effect of main chain helicene loading on CP microscopic and macroscopic properties (μ h and OPV performances), we decided to generate enantiopure analogs of pIDT6-Hel1 and pIDT6-Hel11.The (P) and (M) enantiomers of 2,15-dibromo-carbo[6]helicene, namely PHel and MHel, were readily obtained using HPLC over a chiral stationary phase.The same procedures as for the racemic polymers were used to synthesize and characterize the corresponding enantiopure pIDT6-Hel1 and pIDT6-Hel11 (see Part 2 -Synthesis in the SI).The obtained enantiopure polymers share most of the features of their racemic comparators, such as the same helicene loading in the backbone (1% and 11% respectively, as checked by 1 H NMR) and similar molecular weights (Table 2).
To verify that the obtained polymers did not undergo a loss in enantiopurity through the cross-coupling reaction, ECD spectroscopic analyses were carried out (see Figure 3a-d).The racemic helicene CPs are, unsurprisingly, ECD-silent, whereas the enantiopure samples have almost mirror-image responses.The signals of pIDT6-(P/M)Hel1 in solution (Figure 3a) and solid state (Figure 3c) were found noisy and of low intensity (Δ values below 100 m −1 .cm−1 in the helicene absorption region in solution) and the helicene loading was found not sufficient to induce chirality to the IDT chromophore (no ECD-active band in the IDT absorption region).
The situation is totally different for pIDT6-(P/M)Hel11 both in solution (Figure 3b) and in solid state (Figure 3d) where the ECD response is not solely from the helicene unit. [21]Indeed, pIDT6-PHel11 in Figure 3b shows two strong negative bands between 250-300 nm (|Δ| up to 200-300 m −1 .cm−1 ) attributable to the -* transitions within the helicene.Two additional bands of lower intensity appear at longer wavelengths, a positive one at 330 nm (Δ ∼ +105 m −1 .cm−1 ) and a negative one at 360 nm (Δ ∼−27 m −1 .cm−1 ).A set of three intense positive bands, from 390 to 570 nm, are also observed (Δ up to 180 m −1 .cm−1 ).Interestingly, this ECD signature, which increases with the Hel content, corresponds to the absorption region of the IDT6 unit, thus indicating transfer of the chiroptical response from the chiral helicene to the achiral IDT6 unit.For the (M)-enantiomer, higher Δ values were observed.Note that dissymmetry factors in ab-sorption and emission are in the typical range of chiral organic materials (g abs up to 1.4 × 10 −3 in the helicene part and 1.0 × 10 −3 for the IDT6 region, see Figure S9).The same profile is also present in the solid state with lower intensity and slightly increased noise.
Samples of pIDT6-(P/M)Hel11 were further examined at the Diamond synchrotron beamline number 23 for advanced chiroptical analyses (Figure 3e). [22]The two enantiopure polymers as thin films were found to have an ECD profile very similar to the solution and solid state (Figure 3b,d), suggesting intrachain chirality as the source of chiroptical activity.It is interesting to note that some increase in the ECD signal was observed upon heating, especially in the 260 and the 360 nm region of the spectra.This may possibly originate from the presence of the helicene in the backbone that constrains the polymeric chain movements.
To correlate the different structure of the enantiopure pIDT6-(P/M)Hel1 and pIDT6-(P/M)Hel11 samples in the solid state with a macroscopic property, we measured their hole mobilities (μ h ) by SCLC (Table 2 and Figure 3f).The (P) and (M) enantiomers of pIDT6-Hel1 exhibit very similar μ h of 3.28 × 10 −5 cm 2 .V −1 s −1 and 4.01 × 10 −5 cm 2 .V −1 s −1 , respectively (Figure S10, Supporting Information), which is also very similar to the racemic comparator.On the contrary, the (P) and (M) enantiomers of pIDT6-Hel11 exhibit distinct μ h (Figure 3f) .The pIDT6-PHel11 .exhibits μ h of 5.62 × 10 −5 cm 2 .V −1 s −1 that is slightly higher than the racemic analogue (4.30 × 10 −5 cm 2 .V −1 s −1 ), whereas the pIDT6-MHel11 exhibits μ h of 7.59 × 10 −6 cm 2 .V −1 s −1 , which is one order of magnitude lower.Therefore, it is evident that upon insertion of 1% Hel (either in racemic or enantiopure forms) the obtained μ h are similar and close to the homopolymer.This indicates that addition of 1 Hel moiety over 100 indacenodithiophene units does not significantly affect the self-assembly process of the polymers and all the 1% polymers as thin films will result to similar μ h (in agreement to the GPC findings of Figure S8a, Supporting Information).Then increasing the insertion of Hel to 11% in the polymer backbone, the obtained μ h are not similar even for the enantiopure derivatives.This shows now that the addition of 1 Hel moiety every ten indacenodithiophene units (and especially the positioning/sequence within the repeat unit as they are statistical polymers) plays a more important role on the self-assembly process of the polymers (again in agreement with the GPC findings in Figure S8b, Supporting Information)

Organic Photovoltaic (OPV) characterization of the racemic and enantiopure polymers
To continue further the detailed microscopic to macroscopic relationship study on both the racemic and chiral CPs, we examined their performance in heterojunction OPVs (Figure 4a,b).As a non-fullerene acceptor, the well-studied IO4Cl was utilized given its wide bandgap and favorable E HOMO level, which provides the required offset of ≈0.5 eV [23] for exciton dissociation (Figure S11, Supporting Information) for most of the synthesized racemic (pIDT6-Hel1, pIDT6-Hel6, pIDT6-Hel11) and enantiopure polymers (pIDT6-(P/M)Hel1, pIDT6-(P/M)Hel11).
We fabricated OPVs in a conventional architecture (Figure S11, Supporting Information) based on ITO/PEDOT/active layer/Na-DPO/Ag.We first measured the current density-voltage (J-V) characteristics of blends comprising of the reference polymer IDT6 and the racemic polymers pIDT6-Hel1 -pIDT6-Hel6 -pIDT6-Hel11 under AM1.5G illumination (Figure 4a).As summarized in Table 3, pIDT6:IO4Cl-based solar cells delivered a power conversion efficiency (PCE) up to 4.1%, together with an open-circuit voltage (V oc ) of 1.02 V, a short-circuit current density (J sc ) of 7.92 mA.cm −2 and a fill factor (FF) of 51.7%.Interestingly, pIDT-Hel1-based devices showed similar performance (PCE of 4%), whereas higher loading of helicene leads to reduced efficiency, mainly attributed to lower FF values (Figure 4b; Figure S12, Supporting Information).
We then turned our attention to the different performances of racemic and enantiopure pIDT6-Hel1-based devices.We first verified the ECD response of the enantiopure blends and found no ECD response in the absorption region of the IO4Cl unit, i.e., ≈600-700 nm (see Figure 4c,d).Figure 4e shows the J-Vs under one sun illumination of pIDT6-Hel1:IO4Cl,

Figure 1 .
Figure 1.Chemical structures of chiral conjugated covalent polymers based on the carbo[6]helicene moiety described in the literature[12b,d,e]  and general structure of the polymers developed in the present work.

Figure 2 .
Figure 2. a) Experimental absorption spectra of pIDT6-Hel(x) polymers in CHCl 3 solutions (c = 10 −5 m) at room temperature; b) Experimental normalized absorption spectra in solid-state spin-coated films of pIDT6-Hel(x) polymers at r.t.; c) Energy levels of the racemic pIDT6-Hel(x) polymers stemming from APS and ss-UV-Vis; d) Hole mobilities of the racemic pIDT6-Hel(x) polymers from SCLC measurements.

Figure 4 .
Figure 4. a) J-V curves of the cells based on racemic polymers, b) box plot of the PCE of all the solar cells studied.c,d) ECD (top) and UV-Vis absorption spectra (bottom) of the thin-films blends of pIDT6-(P/M)Hel1:IO4Cl (c) and pIDT6-(P/M)Hel11:IO4Cl (d).e,f) J-V curves of the cells based on racemic and enantiopure polymers on 1% helicene loading (e) and on 11% helicene loading (f).

Table 2 .
Critical parameters obtained for the enantiopure pIDT6

Table 3 .
Average and best performances of pIDT6-Hel(x) cells.