Surface-induced enantiomorphic crystallization of achiral fullerene derivatives in thin films

The chirality of organic semiconductors is important for various applications in optoelectronics and spintronics. Here, we propose a new strategy to induce structural chirality in achiral organic semiconductors in thin films. Enantiomeric fullerene derivatives (S)-pSi and (R)-pSi, which have oligo(dimethylsiloxane) as a low-surface-energy moiety, were synthesized and used as surface-segregated monolayers (SSMs) in spin-coated films of several achiral fullerene derivatives. Upon thermal annealing, the presence of the chiral SSMs led to the crystallization of the fullerenes in the films as an SSM-induced crystal phase at lower temperatures. The crystallized films showed circular dichroism ascribed to the fullerene absorption, the sign and the intensity of which depended on the handedness of the SSM molecules and the film thickness, respectively. These results indicate that the achiral fullerene derivatives in the films were induced by the SSMs to crystallize into enantiomorphic crystals. Our approach to inducing chirality in organic thin films is compatible with many device applications.


Introduction
Chirality is a basic property in nature and is observed across all length scales, from elemental particles to macroscopic objects. 1 Chiral molecules with le-and right-handed chemical structures linked by mirror symmetry (enantiomers) are of fundamental importance in the life sciences and medicine. [2][3][4][5] In addition, in assemblies of molecules, the symmetry of the packing structure is the key to expressing the chirality of bulk materials as a whole and enantiomeric structures can even be formed from achiral molecules. Chiral materials are useful for advanced applications in nonlinear optics, 6,7 stereospecic chemistry, [8][9][10][11] and spintronics [12][13][14][15] owing to their interactions with electromagnetic waves, molecules, and electronic spin, respectively. For example, chiral lms can be used in optoelectronic devices that emit or detect circularly polarized light. 16 Chiral induction, where an assembly of achiral species acquires structural chirality by interacting with a chiral species, provides a way to create chirality in condensed matter. 17,18 Chiral induction has been reported in self-assembled structures in solution, [19][20][21] crystallization from solution, [22][23][24][25] and liquid crystals. [26][27][28][29] For thin lm form of the materials which is relevant to the applications, chiral dopants can induce structural chirality in lms of achiral semiconducting polymers. [30][31][32] However, the mixing of the dopants affects the lm structure and interferes with the properties of the organic semiconductors. The application of chiral stimuli to the materials, such as irradiation with circularly polarized light, can also induce enantiomorphic structures, [33][34][35] but the enantiomorphic effects are generally weak. To our knowledge, chiral induction during crystallization has not been reported for thin-lm organic semiconductors.
Thin-lm organic semiconductors with high structural order are critical components in organic electronic devices. We have been developing surface-segregated monolayers (SSMs) for modifying the surface of organic semiconductor lms and controlling lm structures. [36][37][38][39][40][41][42] SSMs can be prepared using a blend solution of a base organic semiconductor and a surface modier that consists of a semiconducting part and a moiety with low surface energy, such as uoroalkyl or oligosiloxane chains. This molecular design drives spontaneous segregation of the modiers to the surface as a monolayer with a preferred molecular alignment during coating to minimize the total energy of the system. The phenomeon can be understood in an analogous way to the Langmuir adsorption of surfactants at liquid/air interfaces. 43 Recently, we discovered that SSMs based on a fullerene derivative (e.g., pSi in Fig. 1c) induce the crystallization of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) inside the lm from the surface to give PCBM lms with an unprecedented high crystallinity aer thermal annealing. 42 Structural analysis revealed that the crystal structure of PCBM was induced by the SSMs. Importantly, the SSM-induced crystal structure in the PCBM lm belonged to a non-centrosymmetric space group and the unique axis was aligned in the vertical direction of the lms, reecting the directed crystal growth from the surface. These ndings led us to the hypothesis that crystalline PCBM lms can acquire structural chirality through crystallization from the surface.
In this study, we synthesized new chiral surface modiers: (S)-pSi and (R)-pSi (Fig. 1a). Lactic acid was used as the chiral building block to connect the fullerene part and the oligosiloxane moiety with low surface energy. Both the compounds were expected to function as surface modiers to form SSMs in the PCBM lms, as in the case of pSi/PCBM. The crystalline materials inside the lms were investigated for other achiral fullerene derivatives with higher crystallinity ([6,6]-thienyl-C 61butyric acid methyl ester (ThCBM) and [6,6]-phenyl-C 61 butyric acid ethyl ester (PCBE), Fig. 1b). We hypothesized that the (S)-pSi and (R)-pSi SSMs induce the chirality of the crystal packing structure of the achiral fullerene derivatives inside the lms through surface-induced crystallization (Fig. 2).

Formation of SSM
The surface segregation behavior of (S)-pSi in the fullerene derivative lms was systematically investigated by X-ray photoelectron spectroscopy (XPS) according to our previous method. 36,38,40,42 The concentration of the fullerene derivative in the blend solutions was xed as 10 mg mL À1 and the concentrations of the chiral surface modiers were varied from 0 to 1.75 mg mL À1 . The spin-coated lms were thermally annealed under N 2 at 150 C for 30 min before XPS measurements. The Si/ C atomic ratios on the lm surface were calculated from the intensities of the Si 2p and C 1s peaks in the XPS spectra.   In as-cast films of (S)-pSi/PCBM, the (S)-pSi SSM forms with the preferential molecular orientation, and (b) during thermal annealing, the (S)-pSi SSM induces the crystallization of PCBM inside the film with a single-handed helicity along the vertical direction to form (c) the crystallized PCBM films with the enantiomorphic structure after thermal annealing. concentration of (S)-pSi in the solutions. The Si/C ratios depended linearly on the concentration of (S)-pSi in the lower concentration region up to 1.5 mg mL À1 and become constant at above that. This behavior is similar to previous studies on SSM formation 42 and suggests that the surface energy of (S)-pSi is low enough for all the added (S)-pSi molecules to adsorb to the air/liquid interface during spin-coating and remain on the lm surface. The Si/C ratio at the saturation point was close to that of pSi/PCBM (7.5%), at which the surface coverage with pSi is estimated to be higher than 90%. 42 These results indicate that (S)-pSi was segregated to the surface of the lms of PCBM, ThCBM, and PCBE to form the SSMs (Fig. 2). This was further conrmed by the results of XPS depth proles and angleresolved XPS (Fig. S1-S3 †). The depth proles showed that the high surface coverages with (S)-pSi SSMs can be maintained with a xed surface modier concentration (1.5 mg mL À1 ) in the solutions while the total lm thicknesses can be independently controlled with the concentrations of the fullerene derivatives for the bulk of the lms. The thicknesses of the oligosiloxane layers on the surface were estimated as 1.18-1.25 nm by the angle-resolved XPS analysis based on a bilayer model. The concentration of the chiral surface modiers close to the saturation point (1.5 mg mL À1 ) was used in subsequent experiments to ensure the maximum SSM coverage.
The surface Si/C ratios of the lms with an (S)-pSi concentration of 1.5 mg mL À1 aer annealing at different temperatures (120-160 C) were measured by XPS (Fig. S4 †). The Si/C ratios showed very little change aer annealing, regardless of the temperature, indicating that all the (S)-pSi molecules had already segregated to the lm surface of the as-cast lms and that thermal annealing did not substantially change the density or the molecular orientation of (S)-pSi at the surface. Fig. 4a shows the out-of-plane X-ray diffraction (XRD) patterns of the pure PCBM lm aer annealing at different temperatures. There were no clear peaks in the patterns for the as-cast PCBM lms and the PCBM lms annealed below 140 C, indicating an amorphous structure. Aer annealing above 150 C, the patterns showed peaks at 10.9 and 17.5 , which is consistent with the reported unsolved structure of PCBM lms formed through spontaneous cold crystallization. 44,45 Fig. 4b shows XRD patterns of (S)-pSi/PCBM lms aer thermal annealing at different temperatures. The characteristic diffraction peaks at 3.9 , 7.8 , 11.7 , 15.6 , and 19.5 appeared aer annealing above 140 C, suggesting that the crystal phase formation was induced by the SSM, as reported for pSi/PCBM, and the peaks can be assigned to 002, 004, 006, 008 and 0010, respectively. 42 The peak intensities of the surface-induced phase reached a maximum aer annealing at 150 C and increasing the annealing temperature to 160 C did not change the intensities. 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements of the crystallized lms showed similar diffraction patterns for the (S)-pSi/PCBM and pSi/PCBM lms (Fig. S5 †). These results indicate that the (S)-pSi SSM induced the PCBM crystallization in a similar way to the pSi SSM. 42 The pure (S)-pSi lm aer annealing at 150 C showed only halos in the GIWAXS pattern ( Fig. S5a †), suggesting that it was amorphous.

Crystallization induced by SSM
The crystallization behavior of ThCBM and PCBE in the lms was different from that of PCBM. The pure ThCBM lms aer thermal annealing showed the same diffraction peaks as the (S)-pSi/PCBM lm at 3.9 , 7.8 , 11.7 , 15.6 , and 19.5 when the annealing temperature was above 140 C (Fig. 4c). In addition, an extra peak appeared at 17.4 , which could not be assigned to the reported crystal structure of the SSM-induced phase. This extra peak may have the same origin for the peak of PCBM lm aer the cold crystallization (17.5 ). This result indicates that pure ThCBM lms underwent spontaneous cold crystallization with a packing motif similar to that of the SSM-induced phase of PCBM with possible mixed crystal phases. The (S)-pSi SSM at the lm surface lowered the lowest crystallizing temperature of ThCBM from 140 to 130 C (Fig. 4d). The diffraction patterns of the (S)-pSi/ThCBM lms were similar to that of the pure ThCBM lm, although the extra peak at 17.4 was not observed. In addition, the (S)-pSi/ThCBM lms had higher intensities and sharper peaks compared with the pure ThCBM lms, most notably at an annealing temperature of 150 C. These results suggest that even though there was no drastic change in the crystal structure, as for (S)-pSi/PCBM, the (S)-pSi SSMs induced the crystallization of ThCBM into the pure phase at lower temperatures and increased the lm crystallinity. The 2D GIWAXS measurements conrmed that the packing motifs of ThCBM in (S)-pSi/ThCBM and PCBM in (S)-pSi/PCBM were similar ( Fig. S5c and f †).
The (S)-pSi/PCBE lm showed crystallization behavior similar to that of the (S)-pSi/ThCBM lm. The pure PCBE lm started to crystallize aer annealing at 140 C, and a high diffraction peak intensity was observed aer annealing above 150 C, indicating high crystallinity (Fig. 4e). Fig. 4f shows the XRD patterns of (S)-pSi/PCBE lms aer annealing at different annealing temperatures. The (S)-pSi/PCBE lm showed a lower crystallization transition temperature (130 C) compared with that for the pure PCBE lm (140 C). In contrast to ThCBM, the diffraction peak intensities of the pure PCBE lm were higher than those of (S)-pSi/PCBE, suggesting that the SSM did not improve the crystallinity of PCBE, even though it lowered the crystallization temperature. The 2D GIWAXS patterns of the PCBE and (S)-pSi/PCBE lms were similar to those of the pSi/ PCBM, (S)-pSi/PCBM, and (S)-pSi/ThCBM lms (Fig. S5 †), indicating that all these lms had similar packing motifs.
Atomic force microscopy (AFM) was performed to investigate the morphology of the crystallized fullerene lms with and without the SSMs. The as-cast (S)-pSi/PCBM lm was very at with a root-mean-square roughness (R q ) of 0.20 nm (Fig. S6 †). Aer annealed at 150 C for 30 min, although the pure PCBM lm had a very at surface with R q of 0.31 nm (Fig. 5a), the (S)-pSi/PCBM lm had larger grains with a larger R q of 4.3 nm (Fig. 5b). This could reect the differences in the crystal phases and the crystallinity, as revealed by XRD and GIWAXS. The pure ThCBM lm had a grained surface structure (Fig. 5c) because the lm was crystallized by thermal annealing. The (S)-pSi/ ThCBM lm had a similar surface structure, but with larger polygonal grains that were more distinct (Fig. 5d). The R q values of ThCBM and (S)-pSi/ThCBM were 0.49 and 1.86 nm, respectively. The pure PCBE lm showed large square grains on the surface (Fig. 5e) owing to the high crystallinity of the lms and the tetragonal crystal structure. The (S)-pSi/PCBE lm had similar square grains with a higher surface roughness (Fig. 5f). The R q values of PCBE and (S)-pSi/PCBE were 1.1 and 4.1 nm, respectively.

Induced chirality
Circular dichroism (CD) spectroscopy was used to investigate the induced chirality in the fullerene derivative lms with (S)-pSi and (R)-pSi SSMs. The lms for the CD measurements were prepared by spin coating on fused quartz substrates. Aer spincoating, the lms were annealed at 150 C for 30 min to crystallize the fullerene derivatives. All the PCBM, ThCBM, and PCBE lms with (S)-pSi and (R)-pSi SSMs showed clear cotton effects in the CD spectra (Fig. 6). The signs of the signals were inverted depending on the enantiomeric structure of the surface modier. The signal positions in the CD spectra were correlated with the strong UV-vis absorption bands of PCBM, ThCBM, and PCBE at around 220 and 270 nm (Fig. S7 †). To exclude the possible effects of optical anisotropy of the lms on the CD signals, the samples were rotated to the incident light axis or ipped the faces in the CD measurements (Fig. S8 †). The operations did not change the CD signal intensity, and linear dichroism (LD) signals remained silent regardless of the rotation angle. These results indicated that the CD activities are originated from the chiral structure in the lm, not the optical anisotropy. [46][47][48] Before thermal annealing, the (S or R)-pSi/ PCBM, (S or R)-pSi/ThCBM, and (S or R)-pSi/PCBE lms were amorphous and completely silent in the CD spectra (Fig. S9 †). These results indicate that the chirality of the lms appeared during crystallization under thermal annealing. In addition,  This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 4702-4708 | 4705 chloroform solutions of (S)-pSi and (R)-pSi (Fig. S10a †) and the pure amorphous lms of (S)-pSi and (R)-pSi aer annealing at 150 C for 30 min showed no signal in CD spectra (Fig. S10b †). These control experiments suggest that the CD signals observed in the lms with the SSMs did not originate from the chiral surface modiers themselves, but from the crystallized achiral fullerene derivatives (PCBM, ThCBM, and PCBE) inside the lms. Notably, no CD signal was observed when a 1 : 1 mixture of (S)-pSi and (R)-pSi was used as surface modier to crystallize fullerene lm (Fig. S11 †).
To conrm the source of chirality in the fullerene lms, the thickness dependence of the CD spectra was investigated. The (S)-pSi/PCBM lms were prepared with a xed (S)-pSi concentration of 1.5 mg mL À1 and PCBM concentrations of 5, 10, 12.5, 15, and 20 mg mL À1 to maintain the surface coverage with (S)-pSi SSMs while the total lm thickness increased. The lms were spin-coated from the blend solutions and annealed at 150 C for 30 min, and the thicknesses of the lms were measured by surface prolometry. Fig. 7a shows the CD spectra of (S)-pSi/PCBM lms with different lm thicknesses and Fig. 7b shows the ellipticity of the peak at 230 nm plotted as a function of the thickness. The ellipticity increased almost linearly as the thickness of the lms increased from 22 to 102 nm. The absorbance of the lms had a linear relationship with the thickness of the lms in this range, indicating that the lms had a uniform density and structure in the vertical direction (Fig. S12b †). These results conrmed that the CD signals originated from the crystallized PCBM inside the lms, not from the surface or the interfaces between the samples and the substrates. In addition, the good linearity suggests that the enantiomorphic crystals grew from the top to the bottom of the lms, with a thickness of at least 100 nm.
The thickness dependence for (S)-pSi/ThCBM was also investigated in a similar way. Fig. 7c shows the CD spectra of these lms, and the ellipticity of the peaks at 230 nm in the CD spectra were plotted as a function of lm thickness (Fig. 7d). The CD intensity increased linearly as the lm thickness increased from 28 to 100 nm. For (S)-pSi/PCBE lms, due to the low solubility of PCBE, only 5 and 10 mg mL À1 PCBE with 1.5 mg mL À1 (S)-pSi blend solutions were used. Nevertheless, a clear increase in CD intensity was also observed for (S)-pSi/ PCBE lms when the lm thickness increased from 27 to 49 nm (Fig. 7e and f).
The dependence of the ellipticity on the thickness can give information on the factors that affect the chiral induction from the SSMs. The slopes of the linear tting for the q-thickness relationship were 0.038, 0.071, and 0.131 mdeg nm À1 for (S)-pSi/ PCBM, (S)-pSi/ThCBM, and (S)-pSi/PCBE, respectively. This difference may be related to the crystallinity of the fullerene derivatives inside the lms; the order of the crystallinity is PCBE > ThCBM > PCBM, as determined from the intensity of the XRD patterns in Fig. 4. The fraction of the crystallized domains or the disorder of the crystal structures inside the lms could affect the intensity of the induced CD signals.
To further conrm the origin of the crystallization and the chirality, the surface layer of the as-cast (S)-pSi/PCBM lm was removed by reactive-ion etching with O 2 plasma and the resulted lm was subsequently annealed at 150 C for 30 min (Fig. S13 †). XRD results showed that the lms did not crystalize into the SSM-induced crystal phase aer annealing, but into the ordinary crystal phase observed for the pure PCBM lms. The lms showed no CD signal. This control experiment provides the strong evidence that the crystallization and the chirality of the fullerene lm was induced from the surface monolayer of (S)-pSi.
Our previous structural analysis on pSi/PCBM lms based on GIWAXS patterns gave the crystal structure with the space group of I 4c2 that is non-centrosymmetric but not chiral. Since the chiral (S)-pSi/PCBM crystal structure must belong to the chiral space group, but gives the identical GIWAXS pattern as pSi/ PCBM lms, there are two possible situations: (1) the crystallization induced by (S)-pSi SSM lowers the crystal symmetry from I 4c2 into I4 or I422, or (2) pSi/PCBM and (S)-pSi/PCBM lms have the same crystal structure with the space group of I4 or I422. In either case, I4 or I422 structure could show the additional diffraction peaks on the GIWAXS patterns resulting from disappearance of the c-glide symmetry of I 4c2 space group. Unfortunately, however, the information from GIWAXS patterns of the 2-D random lms is limited and detailed analysis on the patterns did not show any evidence of the lowered symmetry in either (S)-pSi/PCBM or pSi/PCBM lms. This is possibly due to a pseudo c-glide symmetry between crystallographically independent PCBM molecules and/or the lack of the electron density contributing to the corresponding reections. In the case of (2), the pSi/PCBM lms have chiral crystal domains but they are 1 : 1 mixture of the two enantiomorphic structures. In this case, these domains may be observable by advanced imaging methods such as CD imaging. At this stage, we cannot conclude which situation is real, and further investigation on the actual crystal structure of the chiral (S)-pSi/PCBM lm is necessary.

Conclusions
We demonstrated that chiral SSMs of (S)-pSi and (R)-pSi induced the formation of enantiomorphic crystal structures of achiral fullerene derivatives in thin lms. This is the rst report of the surface-induced chirality of organic semiconductors in lms during crystallization. This study provides a strategy to prepare chiral crystalline organic lms and offers a better understanding of chiral induction. In addition, the resulting thin-lm chiral materials may have interesting applications in nonlinear optics and spintronic devices.

Materials
PCBM was purchased from Solenne BV (Netherlands). ThCBM and PCBE were purchased from ATR Company (Japan). The other chemicals and solvents were purchased from FUJIFILM Wako Pure Chemical (Japan), Sigma-Aldrich (USA), or TCI Chemicals Co. (Japan). All reagents were used as received, unless otherwise indicated. The synthesis and characterizations of the surface modiers (S)-pSi and (R)-pSi are described in the ESI. † All moisture-or air-sensitive reactions were carried out under a nitrogen atmosphere by standard Schlenk techniques. Column chromatography was conducted using silica gel with a particle diameter of 20-40 mm.

Sample preparation
Silicon wafers and quartz glass substrates were cleaned by sequential ultrasonication in detergent solution, water, acetone, and 2-propanol. The substrates were dried, and then subjected to UV-O 3 treatment. The spin-coating solution was prepared by dissolving the fullerene derivative (5-20 mg) and surface modiers (0-1.75 mg) in chloroform (1 mL). The solution was spin-coated on the substrates at 2500 rpm for 30 s under N 2 . The lms were thermally annealed on a hotplate under N 2 for 30 min.
Characterization 1 H and 13 C NMR spectra were recorded on a 300 MHz spectrometer (JNM-AL300, JEOL, Japan). Data are reported as chemical shi in ppm (d), multiplicity, coupling constant (Hz), integration, and assignments. High-resolution mass spectrometry was performed on a mass spectrometer (JMS-T100GCV, JEOL). XPS was performed with an X-ray spectrophotometer (PHI 5000 Versa Probe II, ULVAC-PHI, Japan). Monochromated Al Ka (1486.6 eV) radiation was used in all XPS measurements. The C 1s (285 eV), Si 2p (102 eV), and O 1s (532 eV) peaks were used in the characterizations. To obtain the XPS depth prole, each sample was etched with Ar + ion at an acceleration voltage of 500 V with an etching rate of approximately 0.25 nm s À1 . The CD and UV-vis absorption spectra of lms on quartz glass substrates were recorded on a spectropolarimeter (J-820, JASCO, Japan) and spectrophotometer (V-670, JASCO), respectively. The lm thickness was measured with a surface prolometer (Dektak 6 M, ULVAC-PHI). XRD measurements were performed on an X-ray diffractometer (Smartlab, Rigaku, Japan) with Cu Ka radiation (l ¼ 0.154 nm). GIWAXS measurements were conducted at beamline BL46XU of SPring-8, Japan. The irradiation wavelength for GIWAXS was l ¼ 0.10002 nm (energy: 12.398 keV) and the incident angle was xed at 0.12 . AFM images were obtained with a scanning probe microscope (5400, Agilent Technologies, USA) in tapping mode.

Conflicts of interest
The authors declare no conict of interest.