RESEARCH ARTICLE
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Insulated 𝝅-Conjugated Azido Scaﬀolds for Stepwise
Functionalization via Huisgen Cycloaddition on Metal Oxide
Surfaces
Qi-Chun Jiang, Tomohiro Iwai,* Morihiro Jo, Takuro Hosomi, Takeshi Yanagida,
Ken Uchida, Kazuki Hashimoto, Takashi Nakazono, Yusuke Yamada, Atsushi Kobayashi,
Shin-ya Takizawa, Hiroshi Masai, and Jun Terao*

In organic−inorganic hybrid devices, ﬁne interfacial controls by organic
components directly aﬀect the device performance. However, fabrication of
uniformed interfaces using 𝝅-conjugated molecules remains challenging due
to facile aggregation by their strong 𝝅-𝝅 interaction. In this report, a𝝅-conjugated scaﬀold insulated by covalently linked permethylated
𝜶-cyclodextrin moiety with an azido group is synthesized for surface
Huisgen cycloaddition on metal oxides. Fourier-transformed infrared (FT-IR)
spectroscopy and X-ray photoelectron spectroscopy conﬁrm the successful
immobilization of the insulated azido scaﬀold on ZnO nanowire arraysurfaces. Owing to the highly independent immobilization, the scaﬀold
allows rapid and complete conversion of the surface azido group in Huisgencycloaddition reactions with ethynyl-terminated molecules, as conﬁrmed by
FT-IR spectroscopy monitoring. Cyclic voltammetry analysis of modiﬁed
indium tin oxide substrates shows the positive eﬀects of cyclic insulation
toward suppression of intermolecular interaction between molecules
introduced by the surface Huisgen cycloaddition reactions. The utility of the
scaﬀold for heterogeneous catalysis is demonstrated in electrocatalytic
selective O2 reduction to H2 O2 with cobalt(II) chlorin modiﬁed ﬂuorine
doped tin oxide electrode and photocatalytic H2 generation withiridium(III) dye-sensitized Pt-loaded TiO2 nanoparticle. These results
highlight the potential of the insulated azido scaﬀold for a stepwisefunctionalization process, enabling precise and well-deﬁned hybridinterfaces.

Q.-C. Jiang, T. Iwai, M. Jo, S.-ya Takizawa, H. Masai, J. Terao
Department of Basic Science
Graduate School of Arts and Sciences
The University of Tokyo
3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan
E-mail: ciwai@g.ecc.u-tokyo.ac.jp; cterao@mail.ecc.u-tokyo.ac.jp
The ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202403717© 2024 The Author(s). Small published by Wiley-VCH GmbH. This is an
open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution andreproduction in any medium, provided the original work is properly citedand is not used for commercial purposes.

1. Introduction
Organic−inorganic hybrid interface plays
important roles in various electronic
devices, including solar cells,[1,2] lightemitting diodes,[3] and organic ﬁeld-eﬀect
transistors.[4,5] Hence, precise engineering of hybrid interface gains signiﬁcantattentions in recent years. In particular,
bottom-up modiﬁcation of the interface,where the organic components are ﬁnely
constructed on inorganic surface, provides precise interfacial structures andproperties, enabling fabrication of uniquedevices with complicated conﬁguration andanisotropic properties.[6–9] The bottom-up
modiﬁcation generally obtained via sequential processes as follows: Organic molecules
for subsequent modiﬁcations are depositedon the surface via physical or chemical
methods and then vertical elongations
from the hybrid substrates are conducted
through layer-by-layer methods at the interface, such as coordination,[10–12] electrostatic interaction,[13,14] and covalent-bondformation.[15–18] Thus, organic−inorganic
junction molecules could be the starting point for controlling the performance of the whole bottom-up process.

T. Hosomi, T. Yanagida
Department of Applied Chemistry
Graduate School of Engineering
The University of Tokyo
Bunkyo-ku, Tokyo 113-8656, JapanK. Uchida
Department of Materials Engineering
Graduate School of Engineering
The University of Tokyo
Bunkyo-ku, Tokyo 113-8656, JapanK. Hashimoto, Y. Yamada
Department of Chemistry and Bioengineering
Graduate School of Engineering
Osaka Metropolitan University
3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

DOI: 10.1002/smll.202403717

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Figure 1. a) Conceptual view of the problems in stepwise functionalization on organic−inorganic hybrid surfaces with 𝜋-conjugated azido scaﬀolds.
b) Utilization of the [1]rotaxane scaﬀold to enhance the conversion of the surface azido groups immobilized on metal oxides. c) Molecular structure of
an insulated 𝜋-conjugated azido scaﬀold used in this work.

In order to utilize the hybrid materials as optical and electrical devices, 𝜋-conjugated scaﬀolds between functional organic molecules and inorganic materials are promising due
to their high electron transfer eﬃciencies.[19,20] However, 𝜋conjugated molecules undergo facile aggregation on the surface due to their strong 𝜋–𝜋 interaction to form non-uniformed

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interfaces with disordered molecular orientation, resulting in
the degradation of optical and electrical properties through intermolecular charge/energy transfer among adjacent molecules(Figure 1a).[21–23] Furthermore, such undesired aggregation
could also disturb the bottom-up modiﬁcations due to sterichindrance around the reaction sites. Thus, ﬁne engineering

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of 𝜋-conjugated molecules on inorganic surface is eagerly
pursued.[24–26]
To address the problems of 𝜋-aggregation, encapsulation of
𝜋-guest molecules by electronically non-conductive cyclic hostmolecules has emerged as a promising strategy.[27–29] We recently
reported a strategy utilizing a [1]rotaxane structure in which conjugated backbone covered by covalently linked permethylated 𝛼cyclodextrin (PM 𝛼-CD) moiety (Figure 1c) was “insulated” fromthe external environment.[30] The covalent linkage enables to ensure eﬀective protection on the 𝜋-conjugated backbone without
dissociation of the macrocycle. This insulation strategy that suppresses intermolecular interaction was eﬀective in improving
the interfacial electrical properties between functional moleculeswith phosphonic acid anchors and metal oxide substrates, which
are widely utilized in the fabrication of electrical devices. Indeed,cobalt(II) chlorin (CoII (Ch)) complex coordinatively immobilized
on such aggregation-free interface enhanced electrocatalytic activity and selectivity in O2 reduction to H2 O2 .[30]In this context, we focus on versatile bottom-up modiﬁcationsof 𝜋-conjugated components from aggregation-free surface withthe [1]rotaxane structure,[30–33] as the next milestone in ﬁne surface modiﬁcations. For this purpose, the copper-catalyzed azidealkyne Huisgen cycloaddition reaction is promising for covalentsurface modiﬁcations due to its high reactivity, chemoselectivity, and mild reaction conditions.[34–36] The 𝜋-extended interfaces
through 1,2,3-triazole ring system can oﬀer unique optical andelectrical properties with high chemical stability. In the present
work, we describe details of the synthesis of an insulated 𝜋conjugated azido scaﬀold (Figure 1b,c) and its immobilization
on metal oxide surface. Owing to the high molecular independency, this scaﬀold allowed rapid and complete conversion of thesurface azido group in the Huisgen cycloaddition with ethynylterminated molecules, as conﬁrmed by Fourier-transformed infrared (FT-IR) spectroscopy monitoring using ZnO nanowire arrays as supports. Cyclic voltammetry (CV) analysis of modiﬁed
indium tin oxide (ITO) substrates showed the positive eﬀects ofthe cyclic insulation on suppressing intermolecular interactions
between molecules introduced by the surface Huisgen cycloaddition. The utility of this strategy was investigated in heterogeneous
catalysis such as electrocatalytic selective O2 reduction to H2 O2
with CoII (Ch) modiﬁed ﬂuorine doped tin oxide (FTO) electrodeand photocatalytic H2 generation with iridium(III) dye-sensitizedPt-TiO2 nanoparticle.

2. Results and Discussion
Figure 2a shows the synthetic route of the insulated 𝜋-conjugatedazido scaﬀold. The [1]rotaxane structure was prepared according to the previously reported synthetic procedures with slight

T. Nakazono, Y. Yamada
Research Center for Artiﬁcial Photosynthesis (ReCAP)
Osaka Metropolitan University
3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
A. Kobayashi
Department of Chemistry
Faculty of Science
Hokkaido University
North-10 West-8, Kita-ku, Sapporo 060-0810, Japan

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modiﬁcations.[30] Speciﬁcally, the Sonogashira coupling between
4-iodonitrobenzene derivative 1 bearing the PM 𝛼-CD unit anddiethyl (4-ethynylphenyl)phosphonate provided diphenylacetylene derivative 2 in 96% yield. Reduction of the nitro group in
2 to amine 3 followed by azidation with tBuNO2 and Me3 SiN3
gave 4 in high yields. Subsequently, in a highly polar solvent(MeOH/H2 O (1:2), 45 °C, overnight), the uninsulated derivative
4 was converted to insulated counterpart 5 via self-inclusion process driven by hydrophilic-hydrophobic interaction between thediphenylacetylene and the PM 𝛼-CD moieties.[37–39] The complete conversion from 4 to 5 was conﬁrmed by 1 H NMR spectra, wherein the aromatic protons of the diphenylacetylene moiety of 5 shifted downﬁeld. Moreover, strong nuclear Overhauser
eﬀect with PM 𝛼-CD inner protons of 5 in ROESY NMR spectra supports this insulation structure (Figure S42, Supporting
Information). Finally, deprotection of the ethyl ester in 5 with
Me3 SiBr/Et3 N followed by treatment with H2 O to ensure the[1]rotaxane structure yielded the insulated azido scaﬀold 6 bearing phosphonic acid anchor. A computational calculation of the
optimized structure of 6 shows the high cover ratio of the conjugated backbone by PM 𝛼-CD (Figure S55, Supporting Information). The insulation structure of 6 remain intact at room temperature not only in aqueous solvents such as pure D2 O andDMSO-d6 /D2 O (1:1) but also a less-polar solvent, CDCl3 (Figures
S45–S47, Supporting Information), in which the uninsulated
structure is thermodynamically favored; However, gradual dethreading of 6 occurs in MeOH-d4 (Figure S48, Supporting Information). Thus, by choosing appropriate solvents, 6 could be
utilized in the wet-process surface modiﬁcation without conformational change.
To characterize properties of insulated 𝜋-conjugated azido
scaﬀold 6 at the interface by spectroscopic analysis, metal oxide
single-crystalline nanowire array with a large surface area that allows for high detection intensity was employed as supports.[40,41]
As shown in Figure 3a, ZnO nanowires grown on silicon substrate were immersed into 1 mM CHCl3 solution of 6 for 3 h atroom temperature to obtain 6/ZnO-NW.[42] The FT-IR spectrumof 6/ZnO-NW shows the sharp azide absorption ≈2116 cm–1
(Figure 3b). The XPS spectra shown in Figure 3c–e suggest thepresence of N, O, P atoms originating from 6 on the substrate.
The double P 2p3/2 and 2p1/2 peaks at bonding energy 133.3 and134.1 eV, respectively, are characteristic of the immobilizationof phosphonic acid groups on the surface of metal oxides.[43–46]
The O 1s peak at bonding energy 533.7 eV corresponds to PM
𝛼-CD C–O bonds in 6 (Figure S50, Supporting Information).[47]These results indicate successful immobilization of 6 on ZnO
nanowires surface. This is also supported by the solid-state
31
P CP-MAS NMR investigation using ZnO nanoparticles, instead of ZnO nanowires, as supports (Figure S52, Supporting
Information).
To evaluate the insulated eﬀect in the surface Huisgen cycloaddition reactions, we traced surface azide conversions using FT-IR measurements. For the surface Huisgen cycloaddition
on ZnO nanowire arrays, reported procedures were followed.[48]Thus, the Huisgen cycloaddition between 6/ZnO-NW and N-(2propynyl)phthalimide (3 mM) was conducted in the presenceof [Cu(CH3 CN)4 ]BF4 (3 mM), tris[(1-benzyl-1H-1,2,3-triazol-4yl)]methylamine (TBTA, 3 mM) and sodium ascorbate (3 mM)at room temperature in DMSO/H2 O (1:1) solution (Figure 4a).

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Figure 2. a) Synthetic scheme of insulated 𝜋-conjugated azido scaﬀold 6. b) Aromatic region in the 1 H NMR spectra of (top) 4 and (bottom) 5 in CDCl3 .

Figure 4b shows FT-IR spectra of 6/ZnO-NW focusing on theazido group ≈2116 cm–1 before and after 40 min. More than 95%
of azido group was consumed within 3 min, while a new imide
peak appeared (≈1735 cm–1 , Figure S53, Supporting Information), indicating the proceeding of Huisgen cycloaddition.
In contrast, 7/ZnO-NW, which was prepared from an uncovered azido scaﬀold 7 bearing a triethylene glycol chain as a reference, exhibited a slower rate of the surface Huisgen cycloaddition
reaction, with 41% of the azido groups remaining unconsumed
even after 40 min. The low azide conversion can be attributed tosteric hindrance due to the close proximity of the surface azido
group, as previously reported in literatures.[49] This assumption
is also indicated by the shift of the azide peak on 7/ZnO-NW from

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2121 to 2116 cm−1 as the reaction progresses (Figure 4c), unlike
the case of 6/ZnO-NW. These diﬀerences are evidence that theinsulated structure is eﬀective for high independency of the surface azido group, thereby increasing the azide conversion rate.
To investigate the insulation eﬀects on electronic properties, a
redox active ferrocene (Fc) unit was introduced by Huisgen cycloaddition on azide-modiﬁed ITO on glass substrates. Speciﬁcally, the ITO substrates were modiﬁed by dipping into H2 O solution of 6, followed by the copper-catalyzed Huisgen cycloaddition with ethynylferrocene to form Fc-6/ITO. The uncovered reference substrate Fc-7/ITO was also prepared in the same mannerfrom 7 (Figure 5a). In the CV analysis at the scan rate of 500 mV
s–1 , both Fc-6/ITO and Fc-7/ITO showed single and reversible

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Figure 3. Characterization of 6/ZnO-NW; a) graphical images of insulated 𝜋-conjugated azido scaﬀold 6 immobilized on ZnO nanowires; b) FT-IR
spectrum of azide region of 6/ZnO-NW; XPS spectra of 6/ZnO-NW for c) N 1s, d) O 1s, and e) P 2p. In all XPS spectra, experimental data, ﬁtted data,
ﬁtted component peaks, and the background are represented by black unﬁlled circles, solid red lines, shaded regions, and solid black lines, respectively.

Figure 4. Scheme and spectroscopic trace of the surface Huisgen cycloaddition on ZnO nanowires: a) reaction scheme of the Huisgen cycloaddition
between the surface azido group (6/ZnO-NW or 7/ZnO-NW) and N-(2-propynyl)phthalimide. b) FT-IR spectral changes in the azide region on 6/ZnONW. c) FT-IR spectral changes in the azide region on 7/ZnO-NW. d) Time-course conversion rate of the surface azido group normalized to initial azide
amounts of 6/ZnO-NW and 7/ZnO-NW, respectively.

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Figure 5. a) Reaction scheme of modiﬁcation of scaﬀolds on ITO substrates following the Huisgen cycloaddition between the surface azido group andethynylferrocene. b) AFM images of 6/ITO (top) and 7/ITO (bottom). c) Representative cyclic voltammogram of Fc-6/ITO (top) and Fc-7/ITO (bottom)
at a scan rate of 500 mV s−1 . Conditions: 100 mM CH2 Cl2 solution of tetrabutylammonium hexaﬂuorophosphate.

oxidation peak ≈0.28 V versus Ag/Ag+ derived from ferrocene
moiety (Figure 5c).[30] The fact that oxidation peak current Ip
is proportional to the scan rate v supports the successful introduction of the ferrocene moiety on the ITO substrate (Figure
S57, Supporting Information).[50] The surface coverages concentration (Γ) of Fc-6/ITO was calculated to be 20±1 pmol/cm2 ,which was the same order to the theoretical maximum value(86 pmol/cm2 ), assuming that the PM 𝛼-CD is circular witha diameter of 1.5 nm and has a close-packed structure on thesubstrate without steric repulsion between adjacent cyclodextrin rings (Figure S55, Supporting Information).[51] On theother hand, Fc-7/ITO provided much higher Γ value to be
209±4 pmol/cm2 . This high loading is attributed to the formationof signiﬁcant aggregates on the azide-modiﬁed ITO substrate before the Huisgen cycloaddition, as conﬁrmed by atomic force microscopy (AFM) analysis (Figure 5b). The insulated Fc-6/ITO exhibited the full width at half maximum (FWHM, 126±4 mV)
value close to the ideal value with no interaction (90.6 mV,
25 °C),[50] compared to non-insulated Fc-7/ITO (147±1 mV).

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These results suggest that the insulated structure eﬀectively suppresses the electrostatic interactions between neighboring Fc
units. This assumption is also supported by the smaller peak-topeak separation of Fc-6/ITO (32±2 mV) compared to Fc-7/ITO(117±6 mV, i.e., faster charge transfer between the Fc moieties
and ITO substrates due to the insulation eﬀects).[52] Thus, the insulation junction molecule is promising for fabricating ﬁne hybrid interfaces in bottom-up process.
The utility of 6 as a scaﬀold on heterogeneous catalytic
system was investigated in electrocatalytic O2 reduction with
CoII (Ch),[53,54] in which the [1]rotaxane strategy works eﬀectively
for selective H2 O2 generation owing to the suppression of theformation of μ−1,2-peroxo dinuclear specie that causes undesired four-electron O2 reduction to H2 O.[30] CoII (Ch)-6/FTO was
fabricated through immobilization of 6 on FTO substrates followed by the surface Huisgen cycloaddition with an ethynyltethered CoII (Ch) complex (Figure 6a). The uncovered counterpart CoII (Ch)-7/FTO as a reference was also prepared from 7
in the same manner. The amount of CoII (Ch) on the modiﬁed

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Figure 6. a) Chemical structures of CoII (Ch)-6/FTO, CoII (Ch)-7/FTO, and CoII (Ch)-8/FTO. b) Balanced chemical equation of electrocatalytic O2 reduction
to H2 O2 with CoII (Ch). c) Summary of the electrocatalytic eﬃciency for O2 reduction to H2 O2 in CoII (Ch)-6/FTO, CoII (Ch)-7/FTO, and CoII (Ch)-8/FTO
as working electrodes. TON was calculated as [H2 O2 evolved (mol)]/[CoII (Ch) complex (mol)].

FTO substrates were determined using CV measurements. During the electrocatalytic O2 reduction performed in the aqueous
solution of HClO4 (pH 1.3) under applied potential of 0 V versus Ag/AgCl for 2 h (Figure 6b), the turnover number (TON)of CoII (Ch)-6/FTO (3520) was signiﬁcantly greater than that of
CoII (Ch)-8/FTO (462), in which CoII (Ch) was connected via coordination bond with pyridine-terminated insulated 𝜋-conjugated
scaﬀold,[30] highlighting the importance of covalent-bond linkage
to CoII (Ch). The high Faraday eﬃciency (FE) of Co(Ch)-6/FTO(92%) compared to CoII (Ch)-7/FTO (84%) suggested the eﬀective suppression of molecular aggregation at the interface basedon the insulation structure.
The positive eﬀects of 6 on electrode catalysis encouraged
its use as a scaﬀold of photosensitizer in photocatalytic H2
generation using Pt-cocatalyst-loaded TiO2 (Pt-TiO2 ) nanoparticle catalyst, in which electron injection from the excited
photosensitizer to TiO2 semiconductor is crucial.[55–58] In thiswork, a coumarin-based biscyclometalated Ir(III) complex was
adopted as photosensitizer owing to its strong visible-light absorbing property,[59–61] and introduced onto Pt-TiO2 nanoparticle surface.[62] Speciﬁcally, scaﬀold molecules (6 or 7) weregrafted on 5 wt.% Pt-TiO2 nanoparticle surface by stirring in
H2 O overnight. After centrifugation of 6 or 7-modiﬁed Pt-TiO2
nanoparticles, subsequent surface Huisgen cycloaddition with an
ethynyl-tethered coumarin-Ir(III) complex gave Ir-6/Pt-TiO2 andSmall 2024, 2403717

Ir-7/Pt-TiO2 , respectively (Figure 7a). X-ray ﬂuorescence (XRF)
analysis conﬁrmed the successful introduction of the Ir(III) complex on Pt-TiO2 nanoparticle surface (Figure S67, Supporting Information). The loading amounts of 6 and Ir(III) complex were
estimated by the ultraviolet-visible (UV–vis) absorption spectraof the supernatant solutions after each immobilization reaction(Figures S65 and S66, Supporting Information). H2 generation
experiments were performed in the presence of 0.1 M sodium
L-ascorbate as an electron donor in acetic acid buﬀer solution(pH 4.6) under irradiation using a Xe lamp through a cutoﬀ ﬁlter(>420 nm). The TONIr of Ir-6/Pt-TiO2 after 1 h was determined
to be 1300, which was higher than that with the uncovered counterpart Ir-7/Pt-TiO2 (TONIr 720) (Figure 7c). The enhanced performance using scaﬀold 6 is likely due to the suppression of photosensitizer aggregation-induced quenching, and/or rapid electron injection into the TiO2 semiconductor.[63] This assumption
is supported by results from the AFM and CV analysis for themorphologic and electronic properties on the modiﬁed ITO substrate as shown in Figure 5.

3. Conclusion
In summary, the insulated 𝜋-conjugated azido scaﬀold with phosphonic acid anchor was successfully synthesized and immobilized on metal oxide surfaces. The highly independent reaction

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Figure 7. a) Chemical structures of Ir-6/Pt-TiO2 and Ir-7/Pt-TiO2 . b) Balanced chemical equation of photocatalytic H+ reduction to H2 with Ir-Pt-TiO2 .
c) Photocatalytic H2 generation driven by Ir-6/Pt-TiO2 (red circles) and Ir-7/Pt-TiO2 (blue circles). Conditions: 4 μM of the Ir(III) complex in a 0.1 M
sodium L-ascorbate, CH3 COOH buﬀer (pH 4.6, Xe lamp, 𝜆 > 420 nm) under a nitrogen atmosphere. Turnover number with respect to the Ir(III)photosensitizer (TONIr ) was calculated as 2 × [H2 evolved (mol)]/[Ir(III) complex (mol)].

space rendered by the three-dimensional insulation structure
maximized the azide conversion rate in the surface Huisgen cycloaddition on ZnO nanowire arrays as supports, as monitored by
FT-IR spectroscopy. Furthermore, CV analysis on modiﬁed ITO
substrates shows eﬀective suppression of intermolecular interaction between the neighboring functional groups by the insulation. The applicability of the insulated 𝜋-conjugated azido scaffold toward electrocatalytic selective O2 reduction with CoII (Ch)and photocatalytic H2 generation with Ir(III)-sensitized Pt-TiO2
nanoparticle demonstrated the utility of this strategy. This work
provides a new approach for precise engineering of inorganic surface via bottom-up process.

Conﬂict of Interest
The authors declare no conﬂict of interest.

Data Availability Statement
The data that support the ﬁndings of this study are available in the
supplementary material of this article.

Keywords
bottom-up modiﬁcation, cyclodextrins, Huisgenorganic−inorganic hybrid materials, rotaxanes

Received: May 8, 2024
Revised: July 11, 2024
Published online:

Supporting Information
Supporting Information is available from the Wiley Online Library or fromthe author.

Acknowledgements
This research was partially supported by JSPS KAKENHI Grant Number
JP22K19022 (T.I.), JP22H02060 (J.T.), by JST CERST Grant Number JPMJCR19I2 (J.T., T.Y., K.U.), by the Joint Usage/Research Center for Catalysis Grant Number 22AY0135, 23AY0263, 23DS0433, 23DS0486, 24DS0635(T.I.), by Izumi Science and Technology Foundation (T.I.), and by Toshiaki Ogasawara Memorial Foundation (J.T.). The authors appreciate Prof.
Hitoshi Tamiaki (Ritsumeikan University) for his valuable comments on
preparation of cobalt(II) chlorin complexes. The authors thank Dr. Yoichi
Masui (The University of Tokyo) for solid-state NMR analysis, Yuki Narita(The University of Tokyo) for XPS analysis, and Dr. Nobutaka Yoshimura(Hokkaido University) for the preparation of Pt-TiO2 nanoparticles. The
authors also thank the group members of Dr. Hiromichi V. Miyagishi(Hokkaido University), Masaya Otani, Yuka Imanishi, and Hiroto Miyabe
for their support during the preliminary work.

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