Perfect isolation of π -conjugated molecules on inorganic surfaces with [1]rotaxane structure for enhancing electrical properties

46 π -Conjugated molecules have been utilized to functionalize inorganic surfaces to form organic – inorganic hybrid materials. However, the intrinsically strong π–π interaction results in undesirable aggregations on 48 the inorganic surface, thereby disturbing the charge transfer through the organic – inorganic interface. In this 49 study, a new strategy was developed using insulated π -conjugated molecules bearing a [1]rotaxane structure, where the π - conjugation was covered with covalently linked permethylated α -cyclodextrins. Aggregation- free immobilization was achieved on an inorganic surface by using insulated molecules to suppress 52 intermolecular interaction. In the presence of these insulated molecules, the hybrid interface displayed 53 excellent interfacial electrical properties. Moreover, the functionalized hybrid surface was utilized as an 54 electrocatalyst to produce hydrogen peroxide using a Co(II) – chlorin complex, wherein the catalytic 55 efficiency was improved dramatically by utilizing insulated molecules as bridging moieties at the interface. 56 These results demonstrate that the insulation of π -conjugated molecules is a powerful strategy for modifying 57 inorganic surfaces.


59
Immobilization of organic molecules onto inorganic surfaces synergistically integrates the features 60 of both components, resulting in exceptional material properties. 1,2 In particular, π-conjugated molecules, 61 which possess unique optical and electrical properties, have been utilized for diverse organic-inorganic 62 hybrid devices such as memory devices, solar cells, and biosensors. [3][4][5] Particularly, in the case of electrical 63 devices, the high molecular modification density and high charge-transfer efficiency at the interface, which 64 are governed by the morphology of the surface organic components, are critical to the device 65 performance. 6,7 Conventional surface modifications of inorganic materials using π-conjugated molecules 66 have been achieved via self-assembled monolayers (SAMs), 8 covalent bond formations, 9,10 and 67 depositions. 11,12 However, these classical methods are only suitable for simple and limited conjugated 68 molecules (i.e., unexpanded π-conjugation) to provide uniform and ordered hybrid interfaces. Although π-69 expanded conjugated molecules offer high functionalities for such electrical devices, they readily form 70 objectionable aggregation on surfaces, owing to their strong π-π interaction (Fig. 1a). [13][14][15] This aggregation 71 induces disordering and large protrusions on the surface; therefore, rather than direct injection into the 72 inorganic materials, unanticipated charge transfer between the adjacent molecules occurs on the surface of 73 these materials. This decreases the charge-transfer efficiency of the material. Thus, π-expanded conjugated 74 molecules for electrical devices result in undesirable aggregation, limiting the improvement of the 75 functional device performance. Paving the optimum conductive pathways at the interface by inhibiting π-76 aggregation is therefore critical to improve the material performance, especially for electrical devices with 77 π-expanded conjugated molecules. 78 To suppress π-aggregation, an isolated environment is typically created around π-expanded 79 conjugated molecules on the material surface by utilizing aliphatic compounds. Mixed SAM methods, 80 where the conjugated molecules are diluted in a non-conjugated molecular monolayer, provide an isolated 81 environment around the π-conjugation. 16-18 However, such methods require a low π-conjugation 82 composition ratio, which is disadvantageous for electric devices. On the other hand, increasing the 83 concentration of π-conjugated molecules induces the formation of aggregation structures on the surface of 84 the material. Similarly, some π-conjugated systems bearing bulky side chains to suppress π-π interaction 85 afford uniform surface monolayers. 19,20 However, such ordered systems are attributed to the appropriate 86 surface molecular interaction, and further π-expansion would also result in their aggregation. Overall, 87 except for low-concentration immobilization, most π-expanded aromatic compounds cannot be 88 immobilized on surfaces without aggregation. Accordingly, high-density uniform immobilization of π-89 expanded functional conjugation at the hybrid interface remains a challenging target. controlled interface at the molecular scale provides high-performance electrical devices via efficient charge 120 transfer through each π-conjugated backbone. In this study, we synthesized an insulated conjugated 121 molecule to achieve independent immobilization on inorganic substrates. The molecule comprises 122 phenylene-ethynylene (PE)-based π-conjugation covered by a linked permethylated α-cyclodextrin (PM α-123 CD) as the shielding unit, and displays high solubility in organic solvents together with a deep cavity (Fig.  124 1c). As an anchoring moiety to the substrates, phosphonic acid was directly introduced at the end of the 125 conjugated section to allow strong interaction and to transfer the charge to various metal oxide substrates 9,42 126 such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), which are utilized as electrodes in 127 electrical devices. The insulation effects were investigated to clarify the high-degree independency and 128 excellent electrical properties on the hybrid interface compared to those on the uninsulated counterparts, 129 owing to the inhibition of intermolecular interactions by three-dimensional encapsulation. In addition, the 130 hybrid system based on [1]rotaxanes was applied to electrocatalysis, revealing marked improvement in the 131 catalytic efficiency. The results demonstrated that enhanced charge-transfer and catalytic efficiencies were 132 achieved because the [1]rotaxane strategy efficiently isolated the π-conjugated molecules to inhibit π-π 133 interaction. This strategy shows great potential for application to various electric devices. 134 135

137
The immobilization behavior of cyclic-insulated molecules was investigated using insulated 138 conjugated molecule 3. Precursor 1 bearing PM α-CD and terminal ethyl-protected phosphonic acid was 139 prepared according to previously reported procedures. 43 Moreover, 1 was converted into the corresponding 140 insulated structure 2 quantitatively by hydrophobic-hydrophilic interactions in a heated (60 °C) high-141 polarity solvent (MeOH/H2O = 1:1). This was followed by deprotection of the ethyl ether groups in 1 and 142 2 using trimethylsilyl bromide (TMSBr) and triethylamine (TEA) in dichloromethane (CH2Cl2) to obtain 143 the uninsulated and insulated phosphonic acid-derived molecules 3' and 3, respectively, (Fig. 2a). The high 144 activation barrier for the threading/dethreading transformations 44 was confirmed in previous works [45][46][47][48] and 145 hence, heating the solvent mixture was essential for converting 1 to 2 and vice versa. Thus, these structures 146 displayed kinetic stability at ambient temperature. The high kinetic stability at room temperature allowed 147 the selective preparation of 3' and 3 via deprotection of the ethyl ether groups in 1 and 2, respectively. In 148 the 1 H NMR spectra (Fig. 2b), the chemical shifts in the aromatic region of 3 were clearly shifted downfield 149 relative to those of the uninsulated counterpart 3', owing to deshielding by the threading structure. 46 The 150 loss of ethyl proton signals in the aliphatic region confirmed successful deprotection without any unwanted 151 changes to the insulated/uninsulated structures. In addition, insulated molecule 3 was kinetically stable even 152 in low-polarity solvents, in which the uninsulated structures are thermodynamically favored. Indeed, the 153 insulation remained intact after storage in CHCl3 at room temperature for one day. 154

155
The insulation effects on the molecular morphologies of 3' and 3 after their immobilization on the 156 metal oxide surface were next investigated. To establish whether the surface protrusions were created by 157 the immobilized species or the surface substrate itself, commercially unavailable single-crystalline ITO 158 substrates with ultra-flat surfaces (root mean square roughness, <0.2 nm), fabricated by pulsed laser 159 deposition on single crystal YSZ plate, were utilized. 49,50 Compared to those of amorphous ITO, the 160 individual heights of 3' and 3 on the single-crystalline ITO surfaces were easily and correctly analyzed in 161 detail using atomic force microscopy (AFM). The crystalline ITO substrates were modified by dipping into 162 a low-concentration (50 μM) MeOH solution of 3' or 3. The low concentration prevented high-density 163 adsorption of the conjugated molecules on the ITO surface, thereby enabling the analysis of the individual 164 height of each adsorption species by AFM. Consequently, the AFM image revealed that the single-165 crystalline ITO substrate, which showed a regular step-and-terrace structure, similar to that reported by 166 Ohta et al., 49,50 was modified by the numerous protrusions of insulated conjugated molecule 3 (Fig. 2c). The 167 height distribution of this molecule on the single-crystalline ITO surface mostly ranged from 0.9 to 1.9 nm 51 168 ( Fig. 2e), which approached the calculated length of 3 (1.7 nm; Fig. S2). This indicated that the observed 169 structure was the attached single molecule. The formation of huge protrusions, owing to undesirable 170 aggregation of the uninsulated conjugated molecule 3' on the single-crystalline ITO surface, was observed 171 (Figs. 2d and 2f). In the histogram of immobilized 3' (Fig. 2f, inset), the height frequency exceeded 2 nm, 172 which was considered as aggregation, in more than 10% of the graph. This indicated that for uninsulated  (2) 203 where ip is the peak current and ν is the scan rate. In addition,there was negligible change in the Γ value of 204 immobilized 4 when the potential sweep was repeated 25 times (Figs. S4a and S4b). This indicated that the 205 modified surface prepared utilizing the insulated molecule 4 displayed significant redox durability. 206 In the cyclic voltammograms, the separation between the anodic and cathodic peaks of the redox-207 active species immobilized on the electrode surface is designated as 0 mV under ideal reversible 208 conditions. 55 At a scan rate of 100 mV s -1 , the peak-to-peak separation was 3 mV for both immobilized 4 209 ( Fig. 3a) and immobilized 4' (Fig. S3b). This value is small enough to be in excellent agreement with the 210 ideal value for the reversible response of a surface-adsorbed species. Moreover, the value of the full width 211 at half maximum (FWHM) in the cyclic voltammograms was utilized to assess the immobilized molecules 212 at the point of the electrostatic interaction with the neighboring species, 53 i.e., as the repulsion forces were 213 dominant, the redox peak was wider than the ideal width (90.6/n mV at 25 °C, n = 1 for 4 and 4'), while 214 when the attraction forces were dominant, the redox peak became narrower than the ideal width. 56 Moreover, 215 while the FWHM value of immobilized 4' (77 mV) was smaller than the ideal value ( Fig. S3b), that of 216 immobilized 4 (88 mV) approached the ideal value (Fig. 3a). The smaller FWHM of 4' on the ITO surface 217 was attributed to the electrostatic interaction between neighboring molecules due to unwanted aggregation. 218 On the other hand, the intermolecular interaction between the molecules of 4 on the ITO surface was 219 efficiently inhibited by the [1]rotaxane structure, providing isolated π-conjugated cores even at the organic-220 inorganic interface. 221 The increase in peak-to-peak separation with faster scan rates was attributed to the slow charge 228 transfer between the ITO and immobilized redox-active species. 57,58 To investigate the insulation effects on 229 the charge transfer between the ferrocene unit and ITO electrode, the variation in the peak-to-peak 230 separation potentials ( Fig. 4a; ∆E = Ep -E°′, where Ep is the peak potential and E°′ is the formal 231 potential) with increasing scan rate was determined to construct the trumpet plots (Fig. 4b). 57,59,60 While the 232 peak-to-peak separation of uninsulated 4' occurred at log(ν) = 0.6 (ν = 4 V s -1 ), insulated 4 did not display 233 any significant peak-to-peak separation, even at a high scan rate of log(ν) = 2 (ν = 100 V s -1 ). This indicated 234 that the rate of charge transfer between the ITO electrode and 4 increased, owing to the insulated structure. In our previous study on the single molecular conductance of insulated molecules, the conductance of single 236 insulated conjugation was lower than that of the corresponding uninsulated molecule. This occurred 237 because the π-conjugated backbone was twisted, owing to insulation by PM α-CD. 61 In this study, however, 238 the insulated structure improved the charge-transfer efficiency relative to the uninsulated counterpart, 239 indicating that the insulation enhanced the charge transfer at the interface rather than on the π-conjugated 240 core. The highly efficient charge transfer between 4 and the ITO electrode were attributed to the inhibition 241 of π-π interaction and intermolecular charge transfer. Indeed, the intermolecular charge transfer between 242 the bare π-conjugated cores of uninsulated 4' decreased the charge-transfer efficiency. Thus, appropriate π-243 aggregation inhibition at the organic-inorganic interface utilizing [1]rotaxanes structures is a promising 244 strategy for developing exceptional electrical devices, owing to the high charge-transfer efficiency. shift of anodic peak potential (0.01⇨100 V s -1 ) shift of anodic peak potential (0.01⇨100 V s -1 )

Surface Functionalization with Metalloporphyrin 250
The isolated surface π-conjugations were effectively applied to electrical devices based on organic-251 inorganic hybrid materials. To provide functionalities on the hybrid materials, we focused on the 252 immobilization of metal complexes through metal-ligand coordination, which have been widely utilized 253 for dyes and surface inorganic catalysis. 62,6362 In particular, the modification of metalloporphyrin analogs 254 onto metal oxide surfaces have been effectively applied to artificial photosynthesis and electrosynthesis 255 devices. 64,65 In the current study, the 4-pyridyl group was introduced as a coordination point into insulated 256 and uninsulated structures to obtain 5 and 5', respectively (Scheme S4). To evaluate the potential of 5 for 257 application to surface engineering, we first investigated the complexation behavior between 5 (or 5') and 258 Rh III Cl(OEP) (OEP: octaethylporphyrin), both in the solution system and on the surface system. In the 259 former, the coordination between compound S12 66 and Rh III Cl(OEP) was confirmed from the 260 characteristic up-field shifts observed for complexation in the 1  corresponded to that observed in the Rh III Cl(OEP)-S12 solution (Fig. 5b). The XPS survey spectra of 273 Rh III Cl(OEP)-5/ITO also showed Rh 3d peaks at 310 and 315 eV (Fig. 5c). These results strongly support 274 the formation of Rh III Cl(OEP)-5 complexes with ITO as the hybrid interface. According to the integral 275 areas of the Rh 3d3/2 and P 2p peaks, which were normalized by that of the In 3d peak derived from the ITO 276 substrate, the ratio of Rh-porphyrin-to-insulated 5 in Rh III Cl(OEP)-5/ITO was 1:1 (Table S4). On the other 277 hand, that between Rh-porphyrin and 5' in Rh III Cl(OEP)-5'/ITO was 1.7:1, because of the non-278 independence of the uninsulated molecule on the surface. According to the space filling model obtained by 279 calculation (Fig. S10), the molecular size of Rh III Cl(OEP) was near-identical to the diameter of PM α-CD. Co II (Ch), a μ-1,2-peroxo dinuclear structure 68 is formed, which carries on the four-electron-reduced 301 dioxygen, producing H2O as a by-product. 69 In this study, the high-degree independency and outstanding 302 electrical properties of a [1]rotaxane-based hybrid system were applied to the Co II (Ch)-catalyzed 303 electrochemical device, to prevent the formation of μ-1,2-peroxo dinuclear structures and enhance the 304 charge-transfer efficiency. Insulated 5 and uninsulated 5' coordinated to Co II (Ch) were immobilized on 305 FTO to form Co II (Ch)-5/FTO 70 (Fig. 5a) and Co II (Ch)-5'/FTO electrodes through the Langmuir-Blodgett 306 (LB) technique at the same surface pressure, respectively. Both metal complexes formed a solid condensed 307 monolayer on the FTO surface according to the pressure-area (π-A) isotherms (Fig. S12). In addition, the 308 Soret and Q bands of Co II (Ch) were observed in the differential absorption spectra of modified FTO in 309 both Co II (Ch)-5/FTO and Co II (Ch)-5'/FTO (Fig. S13). 310 During the electrocatalytic synthesis, the Co II (Ch)-5/FTO system showed the best H2O2 production 311 efficiency (Fig. 5d). In contrast, the Co II (Ch)/FTO system, which directly deposited the catalyst on the FTO 312 surface by the LB technique, exhibited the lowest production efficiency. The H2O2 production of the 313 Co II (Ch)/FTO system declined after 1 h because of catalyst desorption, owing to the structure without 314 phosphonic acid-based-anchoring portion. In addition, the catalyst films formed a μ-1,2-peroxo dinuclear 315 structure on the surface which, as previously mentioned, yielded H2O as a byproduct, thereby decreasing 316 the selectivity. Unlike the Co II (Ch)/FTO system, the Co II (Ch)-5'/FTO system contained an anchoring 317 portion to improve the adsorption stability. Although Co II (Ch)-5'/FTO exhibited a similar adsorption 318 density to that of its insulated counterpart in line with the absorption spectra (Fig. S14), the performance of 319 the Co II (Ch)-5'/FTO system was inferior. According to the previously described charge-transfer 320 experiment, the uninsulated structure of 5' was detrimental to charge transfer. This was suggested to be one 321 of the reasons for the lower production efficiency compared to that of the Co II (Ch)-5/FTO system. In 322 addition, because of the dependence on the uninsulated structure, the uninsulated Co II (Ch)-5' complex 323 formed a monolayer with intrinsically loose alignment on the FTO surface, while part of the Co II (Ch) 324 portion formed a μ-1,2-peroxo dinuclear structure, decreasing the selectivity. Consequently, improvement 325 in the production efficiency of the Co II (Ch)-5/FTO system should be considered independently of the 326 insulated surface structure and charge transfer improvement. 327

328
In summary, [1]rotaxane molecules bearing a phosphonic acid-derived PE as the conjugated 329 backbone and PM α-CD as a protective macrocycle were immobilized on metal oxide surfaces via a wet 330 process. AFM and CV analyses revealed the insulation effects on the hybrid system of the [1]rotaxanes. 331 The insulated molecules were immobilized on the metal oxide surfaces in ideal state without aggregation 332 and displayed high charge-transfer efficiency at the interface, as compared to their uninsulated counterparts. 333 The high advantages of insulation were applied to electrocatalysis as electrical hybrid devices. The 334 [1]rotaxane system was utilized for the platform to introduce organic functionalities on the inorganic 335 electrode. The catalytic efficiency of the cobalt chlorin complex was markedly improved by utilizing 336 insulated molecules. These results indicate the importance of the [1]rotaxane strategy in isolating molecules 337 from unfavorable molecular interactions, even at the hybrid interface, providing excellent performance in 338 electrical hybrid devices. This methodology possesses high potential to upgrade the performances of 339 existing electrical devices based on π-conjugated hybrid systems to outstanding devices, by improving the 340 independency and charge-transfer efficiency of the π-conjugated molecules at the hybrid interface. The 341 [1]rotaxane strategy can be considered as a general and versatile method for interfacial control, even for 342 other types of hybrid junctions in addition to phosphonic acid and tin oxides, which would efficiently solve 343 the conventional problems observed in hybrid interfaces composed of π-conjugated molecules and 344 inorganic materials.

Synthesis of 4'
420 Under a nitrogen, S8 (29.8 mg, 17.2 μmol) was dissolved in TEA/CH2Cl2 (0.15/2 mL), and TMSBr (44 μL, 421 346 μmol) was added into the solution. The reaction mixture was stirred at room temperature for 12 h. After 422 this reaction completed, the solvent was removed by vacuum distillation and 5 mL of methanol was added 423 to the resulting crude. After the dissolved crude was stirred at room temperature for another 12 h, the solvent 424 was evaporated by vacuum. The residue was dissolved in CHCl3 and washed by dilute aqueous HCl solution. 425 The organic layer was separated and dried over MgSO4, then the solvent was removed in vacuo. 426   123.93, 114.58, 112.78, 100.62, 100.25, 100.18, 100.16, 100.13 Under a nitrogen, S6 (34.6 mg, 20 μmol) was dissolved in TEA/CH2Cl2 (0.17/2 mL), and TMSBr (77 μL, 440 606 μmol) was added into the solution. The reaction mixture was stirred at room temperature for 12 h. After 441 this reaction completed, the solvent was removed by vacuum distillation and 5 mL of methanol was added 442 to the resulting crude. After the dissolved crude was stirred at room temperature for another 12 h, the solvent 443 was evaporated by vacuum. The residue was dissolved in CHCl3 and washed by dilute aqueous HCl solution. 444 The organic layer was separated and dried over MgSO4, then the solvent was removed in vacuo. 445

Synthesis of 5'
457 Under a nitrogen, S13 (27.7 mg, 17.0 μmol) was dissolved in TEA/CH2Cl2 (0.23/2.5 mL), and TMSBr (71 458 μL, 554 μmol) was added into the solution. The reaction mixture was stirred at room temperature for 12 h. 459 After this reaction completed, the solvent was removed by vacuum distillation and 5 mL of methanol was 460 added to the resulting crude. After the dissolved crude was stirred at room temperature for another 12 h, the  working electrode (electrode area: 3.5 2 π mm 2 ), and a platinum wire was used as the counter electrode.