Phase-selective modulation of TiO2 for visible light-driven C–H arylation: Tuning of absorption and adsorptivity

Article abstract of DOI:10.1016/j.mcat.2019.04.017

To understand and modify TiO₂ for organic photoreaction, two points are important: improving light absorption and retaining adsorption sites for organic reagents. Herein, we tuning the absorption and adsorption of TiO₂ by introducing

Full text of DOI:10.1016/j.mcat.2019.04.017

MolecularCatalysis471(2019)71–76
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Phase-selective modulation of TiO2 for visible light-driven CeH arylation:
Tuning of absorption and adsorptivity
Sora Baka,b, Sae Mi Leea,b, Hee Min Hwanga,c, Hyoyoung Leea,b,c,
⁎
a Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon, 16419, Republic of
Korea
b Department of Chemistry, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon, 16419, Republic of Korea
c Department of Energy Science, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon, 16419, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
To understand and modify TiO2 for organic photoreaction, two points are important: improving light absorption
and retaining adsorption sites for organic reagents. Herein, we tuning the absorption and adsorption of TiO2 by
introducing the defects on each phase of TiO2, called phase-mixed structures of ordered anatase (OA) and dis-
ordered rutile (DR) (OA/DR P25) and disordered anatase (DA) and ordered rutile (OR) (DA/OR P25). Disordered
structure of TiO2 broadened the absorption wavelength range including visible light, but changed surface
structure reduced the adsorptivity of organic reactant. Additionally, anatase and rutile phase of TiO2 has dif-
ferent surface properties and energy band structure, so keeping the crystalline anatase surface for adsorption was
important while introducing the TiO2. Thus, it is assumed that the phase combination of OA and DR can be the
best photocatalytic structure of TiO2 even without any supporting materials and/or co-catalysts. In fact, the
arylation yield of OA/DR P25 is the highest (63.4%), compared to those of untreated P25 (40.0%) and DA/OR
(20.1%), highlighting the potential of OA/DR P25 in various visible light-driven photocatalytic organic reac-
tions.
Adsorptivity
C-H activation
Photocatalysis
Selective disorder
Titanates
Introduction
photocatalytic activity without the requirement for additional materials
because defect sites serve hole-trapping sites that can absorb the visible
Light contains a large amount of energy that can facilitate chemical
synthesis.[1,2] TiO2 is a well-known photocatalytic semiconductor be-
cause of its high stability and low price; [2,3] however, it is not widely
used in organic reactions because of its broad band gap (3.2 eV for the
anatase phase and 3.0 eV for the rutile phase) [4–6]. The most well-
known photocatalytic structure is P25 TiO2, which comprises anatase
and rutile (7:3) phases of TiO2 [7–10]. Some studies have proposed
using TiO2 as a photocatalyst for new bond formation under light
[10–14] and TiO2 are modified to enhance its activity, generally using
co-catalysts or supporting materials. [14–18] To achieve TiO2-catalyzed
organic synthesis, two factors need to be considered: one is visible light
absorption, which is the main energy source for the reaction, [2,4,16]
and the other is the adsorptivity of organic molecules on the surface of
TiO2, as these generate radical intermediates via single electron
transfer. [13,19,20]
light and block charge recombination. [3,21–23] In addition, the dis-
order engineering can affect reactivity in the photocatalytic organic
reaction by changing the surface properties of TiO2, which affects the
adsorptivity of organic reactants on the surface of TiO2 [6]. However,
the effect of defect sites in photocatalytic organic reactions has not been
systematically investigated.
To investigate the photocatalytic role of defect sites on the ab-
sorption of visible light and adsorptivity of organic reactants according
to phase, phase-selectively disordered TiO2 nanomaterials comprising
disordered anatase or/and disordered rutile phases are required.
Previously, our group reported a phase-selective reduction method for
TiO2.[3,23] Li-ethylenediamine (EDA) treatment of TiO2 generated a
disordered rutile (DR) phase structure to yield a phase-mixed structure
of ordered anatase (OA) and DR from P25 TiO2, called OA/DR P25
TiO2, which displayed enhanced visible light absorption. In contrast,
Na-EDA-treated P25 TiO2 comprised disordered anatase (DA) from
pristine anatase phase and a phase-mixed structure of DA and ordered
Disorder engineering of TiO2 is a general method that can be used to
broaden the light absorption range of TiO2 and enhance its
⁎ Corresponding author at: Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, 2066 Seoburo,
Jangan-gu, Suwon, 16419, Republic of Korea.
E-mail address: hyoyoung@skku.edu (H. Lee).
https://doi.org/10.1016/j.mcat.2019.04.017
Received 11 January 2019; Received in revised form 3 April 2019; Accepted 23 April 2019
Availableonline06May2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

S. Bak, et al.
MolecularCatalysis471(2019)71–76
General procedure for TiO2 catalyzed C–H arylation of isoquinoline
Aniline (10 mmol) was dissolved in THF (5 mL) in a 50 mL round-
bottom flask and cooled to −5 °C. Keeping the low temperature, iso-
amyl nitrite (16 mmol) was added slowly and then boron trifluoride
diethyl etherate (20 mmol) was added drop-by-drop. The reaction was
stirred for 30 min at −5 °C. Aryldiazonium tetrafluoroborate (2) was
filtered off by washing with cold diethyl ether and dried in vacuo. 2
(0.5 mmol) and TiO2 catalyst (0.25 mmol) were added to a 20 mL vial.
Then, DMA (2.5 mL), isoquinoline (1, 1 mmol), and TFA (1 mmol) were
added. The mixture was stirred and irradiated using a 10 W CFL lamp
(distance 10 cm) for 48 h, and the reaction was monitored by TLC (n-
hexane/ethyl acetate). The crude product was dissolved in ethyl acetate
and washed with brine. After drying over anhydrous sodium sulfate, the
crude product was purified by column chromatography (n-hexane/
ethyl acetate). All yields were calculated after isolation.
Scheme 1. TiO2 catalyzed C–H arylation of isoquinoilne.
rutile (OR) (DA/OR P25 TiO2) from P25 TiO2 because Na-EDA treat-
ment induced disordering of the anatase phase only.
Herein, we report the visible light-driven photocatalized organic
reactions by using two opposite phase-selectively disordered P25 TiO2
nanoparticles (i.e., phase-mixed structures of OA/DR P25 TiO2 and DA/
OR P25 TiO2) that are allowed to modulate the absorption and ad-
sorption sites of TiO2, suggesting the best structure of TiO2 for photo-
catalysis applications. We chose C–H arylation of isoquinoline with aryl
diazonium salts to study (Scheme 1). Since defects in the DA and DR
phases can increase visible light absorption and the crystalline anatase
phase generally has better adsorptivity than the crystalline rutile phase,
therefore we expect that the combination of OA and DR, as present in
OA/DR P25 TiO2, should provide the best yields in visible light-driven
organic reactions.
General procedure to make a complex of TiO2 with aryl diazonium salt
In a 30 mL vial, TiO2 (1.5 mmol, anatase and rutile TiO2) and 2
(3.0 mmol) were dissolved in 15 mL of DMA and stirred for 48 h. For
purification of CTC, reacted TiO2 was filtered through a nylon mem-
brane filter (0.2 μm), washed with DMSO, deionized water, and diethyl
ether, and then dried.
Scale-up and recycling experiment
Experimental section
2a (3 mmol), OA/DR P25 (1.5 mmol), DMA (15 mL), 1 (6 mmol),
and TFA (6 mmol) were combined in a 30 mL vial. After photocatalysis
under optimized conditions, the catalyst was recovered via filtration
washing with deionized water and ethyl acetate. The washed catalyst
was re-used under identical conditions after adding the same amount of
starting materials and solvent.
Materials and general information
Unless otherwise noted, all reactions were conducted under a ni-
trogen atmosphere using anhydrous solvent. Commercially available
reagents were used without further purification. Thin layer chromato-
graphy (TLC) was performed and results were visualized using a UV
light. Photocatalytic reactions were carried out with a CFL-10 W lamp
obtained from Jungjin Technology. 1H and 13C NMR spectra were re-
corded in CDCl3 with tetramethylsilane (TMS) on a Bruker 500 MHz
spectrometer. Chemical shifts in 1H NMR spectra are reported in parts
per million (ppm) on the δ scale using the internal standard of residual
TMS (0 ppm). Data for 1H NMR are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet), coupling constant in Hertz (Hz), and integration. Data for
13C NMR spectra are reported in terms of chemical shift in ppm from
the central peak of CDCl3 (77.16 ppm). FT-IR spectra were measured
using a Bruker Vertex 70. GCeMS experiments were performed using a
Results and discussions
Photocatalytic activities of various TiO2 nanoparticles
To understand the photocatalysis of TiO2, we performed direct C–H
arylation of isoquinoline (1) with a aryl diazonium salt (2a) under ir-
radiation of a compact fluorescent lamp (CFL) as a visible light source
(Table S1). [25] Based on the previous paper which perform the ar-
ylation reaction of heteroarenes with the aryl diazonium salts using
TiO2 as photocatalyst [13], the aryl radical intermediate from the aryl
diazonium salt can be generated in alcohol conditions resulting high
product yield, but only trace amount of the arylated product was
formed in DMSO and DMF solvent. However, DMSO and DMF can
produce the higher product yield than alcohols in this condition. To
determine the effects of defect sites on the different phases of TiO2,
arylation yields were summarized for various structures of TiO2 cata-
lysts (pristine anatase, DA, pristine rutile, DR, P25, DA/OR P25, and
OA/DR P25 TiO2) (Table 1). There was no big change in diffuse re-
flectance spectroscopy (DRS), the X-ray powder diffraction (XRD), and
Raman peak for Li-EDA treated anatase TiO2 or Na-EDA treated rutile
TiO2 (Figs. S1, S2, and S3), therefore arylation using these compounds
was not investigated.
Rutile TiO2, which has a smaller band gap, showed a higher yield
than anatase phase (entries 1 and 3). The synergistic effect of the phase-
mixed structure explains the better photocatalytic activity of P25 TiO2
(entry 7).[7,9] Introduction of defects in any phase of the TiO2 surface
enhanced visible light absorption (Fig. S1), but it did not necessarily
translate to an improvement in arylation even though the defects
broadened the light absorption wavelength of TiO2 and suppressed
charge recombination.[3,21–23] Though OA and DR TiO2 absorbed the
most visible light (they appeared black), the yield of DA TiO2 was
smaller than that of pristine anatase TiO2 (entries 1–2) and there was no
VARIAN 4000 GCeMS. XPS were recorded using
a Thermo VG
Microtech ESCA 2000 with a monochromatic Al-Kα X-ray source at
100 W. X-ray diffraction (XRD) experiments were performed using a
Rigaku Ultima IV, and diffuse reflection spectroscopy (DRS) was per-
formed using a Jasco V-670 absorption spectrometer.
Preparation of TiO2 catalysts with various structures
Crystalline TiO2 was purchased from Sigma-Aldrich, and pristine
P25 from Degussa. Rutile and anatase phases of TiO2 were reduced after
Li- and Na-EDA treatment. EDA (100 ml) was added under a N2 atmo-
sphere to a 250 ml round-bottom flask with TiO2 powder (1 g) and
granular Li or Na (0.1 mol). The mixture was stirred vigorously at room
temperature for 10 days. To end the reaction, deionized water was
added slowly. The product was purified via centrifugation and washed
with deionized water and ethanol until it became neutral, and then
dried in a vacuum oven (80 °C). [3,23] In XPS result of the prepared
samples, there was no Li, Na, and N peak, indicating there is no residual
Li- and Na-EDA moieties [24].
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MolecularCatalysis471(2019)71–76
Table 1
Product yield of C–H arylation of isoquinoline using various TiO2 catalysts.a)
.
Entry
TiO2 catalyst
Isolation yield
1
2
3
4
5
6
7
8
9
Anatase
DA
Rutile
DR
Anatase + Rutile (7:3)
Anatase + DR (7:3)
P25
DA/OR P25
OA/DR P25
10.4%
Trace
30.5%
30.5%
10.6%
11.2%
40.0%
20.1%
63.4%
a)
1 mmol 1, 0.5 mmol 2a, 0.25 mmol TiO2, 1 mmol TFA, 2.5 ml DMA, irra-
diation with a 10 W CFL lamp from a distance of 10 cm for 48 h under a N2
atmosphere.
difference in the yield of DR in comparison to that of pristine rutile TiO2
(entries 3–4). Furthermore, after phase-selective reduction of P25 TiO2,
different results were obtained for the prepared samples (entries 8–9).
Although both of them absorbed more visible light than untreated P25
TiO2, only use of OA/DR P25 enhanced the yield which was comparable
to homogeneous Ru catalyst, [25] while DA/OR P25 deactivated the
arylation. The lack of enhancement in photocatalytic activity of dis-
ordered TiO2 indicated that visible light absorption is not the only
factor involved in enhanced catalytic activity.
Table S2 showed the results of the arylation using other common
heterogeneous and homogeneous photocatalysts. Among hetero-
geneous photocatalysts, ZnO could produce the target product with
yield of 38.6%, which was lower than P25 TiO2. Ru(bpy)3Cl2, which is
the homogeneous catalyst, showed the higher yield (61.8%) than P25
TiO2, because the homogenous photocatalyst usually showed higher
yield than the heterogeneous photocatalyst. Surprisingly, the yield of
the heterogeneous OA/DR P25 photocatalyst gave the best catalyst,
which was even higher than the homogeneous photocatalyst.
Fig. 2. Product yield of arylation using OA/DR P25 TiO2 (black solid circle)
and the maximum reflectance of OA/DR P25 TiO2 (blue open circle) obtained
by using different Li-EDA treatment times (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article).
power of 1 sun solar light that is much higher power than any other
light sources, only trace product was produced. Even though TiO2 could
well absorb the UV light, the UV light source was not proper for the
reaction of isoquinoline due to the photoionization.[26] It is assumed
that UV light of solar light may decompose the organic compounds
instead of a formation of carbon-carbon coupling bond. As a result, a
blue light gave the best yields for P25 (49.2%) and OA/DR P25
(74.2%), respectively.
Adsorptivity of aryl sources on TiO2 surface
A DA structure deactivated the reaction, which might have been due
to a change in the adsorptivity of the reactant. We therefore analyzed
the charge transfer complex (CTC) of TiO2 with the diazonium salt
according to phase (Fig. 1). Formation of the CTC increased in the
absorption of TiO2 in visible light range, [6,19,20,27,28] while ab-
sorption of the anatase phase was better than that of rutile phase in the
Based on the additional experiments under different light source
irradiations (Table S3), the reaction yields were increased as the power
of the CFL lamp increased from 5 to 22 W,. However, even though we
increased from 10 W to 22 W, the yields were similar, which meant that
over 10 W was enough power in our reaction system. Even under the
Fig. 1. a) DRS spectrum of anatase, rutile, and P25 TiO2 and their CTCs with 2a. b) FTIR spectra and c), d) XPS spectra of the CTCs of anatase and rutile TiO2.
73

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MolecularCatalysis471(2019)71–76
Chart 1. TiO2 catalyzed C–H arylation of isoquinoline. Substrate scope of aryl
diazonium salts under optimized condition.
Scheme 2. Proposed mechanism for photocatalytic C–H arylation of iso-
quinoline catalyzed by phase-mixed TiO2 under visible light.
band at 1615 cm-1 with several characteristic bands corresponding to
the aryl group, [31,32] and the peak intensity of anatase-CTC was
higher than that of rutile-CTC. This azo bond was also confirmed via
XPS of the N 1 s spectrum (˜ 400 eV), [33] and a higher N amount was
observed on the anatase TiO2 (1.36%) than the rutile TiO2 (0.94%).
These results are consistent with the observations in previous studies
that the anatase phase is an acceptable catalytic structure with higher
adsorption of organic compounds and a better electron transport layer
with a longer charge carrier lifetime than the rutile phase.[6,20] Ad-
ditionally, Raman results of various TiO2 structure also were shifted
after making the complex. Formation of CTC induced the shift of the
Raman peaks [30] and the peaks of anatase phase were more shifted
compared with rutile phase. Among the phase-mixed structure, Raman
peaks of bare P25 and OA/DR P25 were shifted, while there was no
peak in Raman result of DA/OR P25 because main phase (anatase) were
disordered.
Though defect sites enhanced visible light absorption, presence of a
disordered structure on the TiO2 surface decreased the adsorption of
organic molecules. [6,20] This suggests that the diazoether complex
(complex 4) as the intermediate structure that generates aryl radical
intermediates through single electron transfer (SET) [13,19,20] formed
preferentially on the surface of anatase TiO2. Thus the introduction of
defects in the anatase phase dramatically deactivated the reaction,
while DR TiO2 showed the same yield as rutile TiO2 [3,23].
Fig. 3. Scale-up (6X) and recycling test of OA/DR P25 TiO2 for the C–H ar-
ylation of isoquinoline using aryl diazonium salt.
Photocatalytic activities depending on reduction time
visible light region. FT-IR spectra of pristine anatase and rutile TiO2 had
only broad peak under 1000 cm−1 due to Ti-O bond, [29,30] while
those of the CTC showed additional peaks including an N]N stretching
In contrast, OA/DR P25 TiO2 enhanced the arylation yield com-
pared to P25 TiO2 and yield increased as Li-EDA treatment time was
extended (Fig. 2). A longer Li-EDA treatment time resulted in darker
74

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MolecularCatalysis471(2019)71–76
samples (Fig. S6). Isolated product yields of OA/DR P25-3, -5, -7, and
-10 TiO2 (P25 TiO2 obtained after Li-EDA treatment for 3, 5, 7, and 10
days, respectively) were 51.8, 57.2, 62.5, and 63.4%, respectively. After
7 days of treatment, light absorption and arylation yield were saturated.
These results indicated that DR affected light absorption, producing
electrons to generate radical intermediates and accelerating the reac-
tion. The active sites on the OA surface and the enhanced visible light
absorption and charge separation of the DR phase of OA/DR P25 TiO2
interacted synergistically to give the highest yield.
suggest that phase-selective disorder engineering based on the en-
hancement of light absorption and molecular adsorptivity can be ap-
plied to various visible light-driven photoreactions.
Acknowledgements
This work was supported by Institute for Basic Science (IBS-R011-
D1).
Appendix A. Supplementary data
Substrate scope
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.04.017.
C–H arylation of several diazonium salts with electron-rich and
-deficient substituents in different positions provided reasonable isola-
tion yields (Chart 1 ) and OA/DR P25 TiO2 showed better yields than
P25 TiO2 in all cases. As expected, the yield of meta- or ortho-sub-
stituted aryl diazonium salts (3h–j) was decreased relative to that of
para-substituted ones (3a, c, e) due to steric hindrance.
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Based on previous papers and our experimental results, a disordered
structure on TiO2 has contradictory effects on arylation (Scheme 2);
defects enhance visible light absorption and charge separation,
[3,21–23] but block the formation of the CTC to generate the radical
intermediates, which form mainly on the anatase phase. [13,25] Thus
OA/DR P25 TiO2, which had well-modulated defects, provided the best
yields due to selective reduction on the rutile phase while active sites
were still provided by the crystalline anatase surface. [13,25,34] While
the aryl diazonium salt predominantly made CTCs on the crystalline
anatase phase (4), the DR phase absorbed visible light, effectively
generating photo-excited electrons that flowed to the anatase phase
based on type-II band alignment. [7,9] Aryl radicals (5) generated
through SET then initiated the arylation process. [22,23] Protonated
isoquinoline 6 subsequently reacted with 5, making the radical cation
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product 8, which was mediated by TiO2 or the radical chain process for
5, and then deprotonation produced the desired product 3.
Conclusion
Several TiO2 samples, namely anatase, DA, rutile, DR, P25, and two
opposite phase-selectively disordered TiO2 samples (DA/OR P25 and
OA/DR P25 TiO2), were used as catalysts for the arylation under visible
light to tune the absorption and adsorption sites due to the contra-
dictory roles of defect sites, that is, expansion of absorption wave-
lengths including visible light but a decrease in adsorptivity of organic
molecules. The introduction of defect sites on anatase phase decreased
the activity while that of rutile phase did not decrease because the
crystalline anatase surface is main adsorption sites for organic reactant
compared with rutile phase. Thus, making disordered structure on ru-
tile phase enhanced the arylation yield due to improvement in visible
light absorption. As a result, the OA/DR P25 TiO2, which had OA as an
effective adsorption site and DR to enhance light absorption, provided
the most improved yield for the arylation. The OA/DR P25 TiO2 cata-
lyzed-arylation with various aryl sources provided high yields, showing
a good reusability as promising heterogeneous catalyst. These results
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