Assembling polydopamine on TiO2 for visible light photocatalytic selective oxidation of sulfides with aerial O2

Journal Pre-proofs Assembling Polydopamine on TiO2 for Photocatalytic Selective Oxidation of Sulfides with Aerial O2 Ji-Long Shi, Xianjun Lang PII: DOI: Reference:

S1385-8947(19)33047-5 https://doi.org/10.1016/j.cej.2019.123632 CEJ 123632

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Chemical Engineering Journal

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23 August 2019 26 November 2019 27 November 2019

Please cite this article as: J-L. Shi, X. Lang, Assembling Polydopamine on TiO2 for Photocatalytic Selective Oxidation of Sulfides with Aerial O2, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123632

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Assembling Polydopamine on TiO2 for Photocatalytic Selective Oxidation of Sulfides with Aerial O2 Ji-Long Shi a, and Xianjun Lang a,* a

Sauvage Center for Molecular Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China.

*To

whom

correspondence

should

be

[email protected]

1

addressed

(X.

Lang).

E-mail:

Abstract: The selective aerobic oxygenation of organic sulfides under mild conditions represents one of the most important organic transformations because of a wide variety of applications of sulfoxides. However, this reaction is very challenging to be implemented by conventional semiconductor photocatalysis. Herein, polydopamine (PDA) could be easily assembled onto TiO2 semiconductor to afford visible light absorbing TiO2@PDA with a large π-conjugated system of PDA. Subsequently, TiO2@PDA was used to implement aerobic oxidation reaction in CH3OH, relying on the improved stability of PDA due to a large π-conjugated system. Illuminated by 460 nm blue light LEDs, PDA could efficiently transfer electrons to the conduction band of anatase TiO2 which can activate aerial O2. Subsequently, visible light-promoted selective aerobic oxidation of sulfides into corresponding sulfoxides with the cooperation of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) on TiO2@PDA have been achieved eventually. This work suggests that the promise of assembling a large π-conjugated system on a semiconductor in establishing visible light photocatalytic selective organic transformations. Keywords: Polydopamine • Aerobic oxidation • Titanium dioxide • Visible light • Oxidation of sulfides

2

1. Introduction The selective oxidation of sulfides into their corresponding sulfoxides is one of the most important chemical transformations in the synthesis of chemically and biologically important compounds [1]. Therefore, environmentally benign selective oxidation of sulfides without overoxidation is much needed. Among all the oxidants, aerial O2 is an ideal reagent as it is nearly infinite and free, the same as visible light. Several very interesting visible light photocatalytic processes for oxidation of sulfides into sulfoxides with O2 have been reported [2-4]. There is diversified applications of sulfoxides in the synthesis of biomedical intermediates. Therefore, it is especially necessary to unravel photocatalytic aerobic oxidation of sulfides with high selectivities. However, this reaction is very challenging to be implemented by conventional semiconductor photocatalysis. Thus a strategy, such as assembling organic molecules on semiconductor, was deployed to address the unique challenges of this reaction. There is significant environmental implication to realize the selective aerobic oxidation of sulfides with aerial O2. On one hand, some organic sulfides like sulfur mustard is the prototypical substance of cytotoxic and vesicant chemical warfare agents whose aerobic oxidative elimination is of environmental importance [5]. On the other hand, as sulfoxides are essential chemical intermediates for a wide variety of molecules, the production of sulfoxides without the involvement of organic peroxides or toxic metal oxidants but aerial O2, can reduce the amounts of concomitant organic or inorganic pollutants at sources which has more substantial environmental 3

implications for processes of selective chemical transformations. Dopamine moieties are found in mussel glue protein that strongly bind to virtually all inorganic and organic surfaces in aqueous environments which is otherwise challenging for adhesives [6]. Moreover, there is strong tendency for the formation of polymer of PDA from dopamine precursors under oxidative conditions. Inspired by the molecular underpinning of adhesive proteins in mussels, it was found that PDA could be used as the multifunctional coating materials on the wide variety of substrates [7]. PDA incorporates many functional groups such as catechol, amines and imines which can be used as an initial point for covalent modification of metal oxide surfaces under mild conditions [8]. PDA is also known to reduce metal salts into metal nanoparticles via the catechol functional groups within a solution. Addition of dopamine solution into a metal oxide suspension can lead to the generation of a PDA shell with a measurable thickness on the metal oxides surfaces, commonly resting on the indicative biocompatible characteristics of PDA. In many biological and chemical processes, electron transfer reactions play vital and significant roles. In this regard, dopamine is a chemical messenger that ferries signals between neurons in brain cells [9], a fundamental electron transfer reaction. Therefore, it is quite plausible to construct visible-light-promoted electron transfer reaction by coating PDA on the surface of a metal oxide semiconductor. Metal oxide semiconductors have been extensively used in many energy and environment relevant redox processes such as water splitting, decomposition of organic pollutants and organic transformations [10-14]. Wide use of metal oxide semiconductor materials is 4

based on its exceptionally efficient photoactivity, high thermal, and mechanical stabilities [15-17]. The application of a protective PDA shell was an effective strategy for improving the compatibility and stability of photocatalysts. Integrating semiconductor with large π-system like graphene or polyimide has been proven successful in constructing visible light photocatalytic systems [18-20]. Meanwhile, significant efforts are being directed towards metal oxide@PDA core-shell nanoparticulate photocatalysts including Cu2O [21], ZnO [22] and Fe3O4 [23]. An effective strategy was offered to introduce Ag nanoparticles into ZnO by in situ PDA coating, which are separated from each other through a graphite-like multi-layered phase for improved photoelectrochemical properties [22]. It was reported a succinct method to synthesize Ag nanowires/Fe3O4@PDA nanocables, which exhibited outstanding photocatalytic activity for the reduction of methylene blue under visible light irradiation [23]. Above study showcases the usage of PDA as a multifunctional coatings on metal oxide semiconductor surfaces which can largely improve the optical properties of these materials by itself due to the presence of the π-π* electron transition. Apart from the above mentioned metal oxide semiconductors, TiO2 was considered as the most promising photocatalyst because of its high stability, nontoxicity, superior photochemical activity. But its large band gaps (3.0–3.2 eV) necessitate ultraviolet (UV) excitation to achieve photocatalytic applications [24-28]. PDA was used as a metal-free dopant to apprehend the narrowing of band gap of TiO2 by modifying a visible light absorbing thin layer. For example, it was unveiled that a 5

two-step modification scheme to prepare TiO2/PDA/glutaraldehyde coating and fibers which could increase the photocatalytic performance toward environment pollutants under visible light irradiation [29]. In addition, TiO2@PDA was successfully fabricated for the visible light photocatalytic degradation of organic dye [30]. Recently, we found several types of colorless organic molecules like catechol and others could be adsorbed onto TiO2 to form surface complexes which can be utilized to efficiently perform the selective aerobic oxidation of amines in CH3CN under visible light irradiation [31-34]. Nevertheless, TiO2 surface complexes could not be adopted to carry out the selective aerobic oxidation of organic sulfides because this reaction necessitates a redox active solvent, CH3OH, which could potentially destroy these molecular surface complexes. Therefore, we hypothesize improved stability of PDA is due to a large π-conjugated system that could help the selective formation of sulfoxides in a redox-active solvent of CH3OH. TEMPO is a well-established organocatalyst for a wide arrange of oxidative transformation. Integrating photocatalysis with TEMPO catalysis has recently been envisaged the new usage of TEMPO [35]. Importantly, the single electron shuttle between TEMPO and 2,2,6,6-tetramethylpiperidine-1-oxoammonium (TEMPO+) assuages the redox pressure placed upon PDA shell, therefore maintaining the durance of TiO2@PDA. Therefore, in this contribution, we describe a facile in situ synthesis of the TiO2@PDA photocatalyst by exploring both adhesive and electronic properties of

6

dopamine. TiO2@PDA possesses a large π-conjugated shell of PDA with better stability than catechol-TiO2 complex. Besides, in principle, the electron-rich amine group of PDA can provide extra electron in comparison with catechol. The selective oxidation of sulfides into sulfoxides with the cooperation of TiO2@PDA photocatalysis and TEMPO catalysis was achieved under irradiation of 460 nm blue LEDs. This work foreshadows embryonic selective aerobic oxidation reactions by TiO2 surface complex visible light photocatalysis.

2. Experimental section 2.1 General procedure for the photocatalytic oxidation of sulfides First, 3.75×10-3 mmol of dopamine hydrochloride, 50 mg of TiO2, 0.3 mmol of thioanisole and 0.015 mmol of TEMPO were added to 1 mL of CH3OH in a 10 mL Pyrex vessel. Afterwards, the reaction mixture was stirred for 30 min in dark to reach adsorption equilibrium. Aerial O2 was directly connected to the Pyrex vessel by a hole in the rubber septum. The reaction mixture was stirred magnetically at 1500 rpm and irradiated with 460 nm blue LEDs (Shenzhen Ouying Lighting Science and Technology Co., Ltd. China) at around 25 C. At the end of reaction, TiO2@PDA were separated from the reaction mixture by centrifugation and the products were analyzed quantitatively by gas chromatography equipped with a flame ionization detector (GC-FID, Agilent 7890B) using chlorobenzene as the internal standard. Conversion and selectivity for oxidation of sulfide to the desired sulfoxide were defined as follows: 7

Conv.[%] = [(C0-CS)/C0] × 100; Sel.[%] = [CSO/(C0 - CS)] × 100 where C0 is the initial concentration of sulfide, and CS and CSO are the concentrations of sulfide and sulfoxide at a certain time after the photocatalytic reaction. For conversions, the mean standard deviation was ±3%; for selectivities, the mean standard deviation is ±3%. The products were confirmed by the retention time comparison with that of standard samples and further verified by gas chromatography–mass spectrometry (GC-MS, Shimadzu GCMS-QP2010 Ultra). 2.2 Characterization of photocatalytic materials Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected on a JEOL JEM2012—FEF operated at 200kV. Powder X-ray diffraction (PXRD) measurement was carried out using a Rigaku/Miniflex600 diffractometer with filtered Cu Kα radiation, and the data were collected from 10° to 80°. The UV-visible absorbance of TiO2@PDA and TiO2 samples were collected on a UV visible spectrophotometer (Shimadzu UV 3600) equipped with a diffuse reflectance measurement accessory. BaSO4 was used as a reflectance standard. The FTIR spectra of photocatalysts were performed by a Nicolet iS10. The specific surface areas were determined by N2 physisorption by using an ASAP automated system and the Brunauer-Emmet-Teller (BET) method. Each sample was degassed under vacuum (<1×10-5bar) in the Micromertics system at 120 ℃ for 12 h prior to N2 physisorption. 8

2.3 EPR experimental procedures The EPR experiments was carried out on an electron paramagnetic resonance (EPR) spectrometer (JEOL, JES-FA300). The EPR tube was added with 0.3 mmol of thioanisole, 50.7 mg of TiO2@PDA, 0.3 mmol of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 1 mL of CH3OH. The signals of DMPO-·OOH were collected at 0 min, 2 min and 4 min respectively under air irradiated by a high-pressure Hg lamp with a filter to select the band of 460 nm in the light chamber of the EPR spectrometer. Next, 0.015 mmol of TEMPO was added to the same EPR tube under air and irradiated with the same light source to collect the signals in situ at 0 min, 2 min and 4 min respectively. See the Supplementary Data for further information regarding the experimental details.

3. Results and discussion 3.1 Synthesis and characterization of TiO2@PDA Previously, we have successfully implemented the visible light photocatalytic selective oxidation of sulfides into sulfoxides with O2 by amine-TiO2 complexes [36, 37]. The Lewis acid and base interaction between amine and TiO2 underpins the visible light activity at periphery of 400 nm visible light. When we carried out the same reaction under irradiation of 460 nm blue LEDs, no appreciable amount of sulfoxide could be afforded. Catechol-TiO2 can be efficient photocatalyst in performing the selective aerobic oxidation of amines in CH3CN with TEMPO as a 9

co-catalyst [31]. However, this complex turns out not to be so efficient for the photocatalytic selective aerobic oxidation of sulfides. Thus we assembled PDA on TiO2 with a large π-conjugated shell of PDA with better stability. TiO2@PDA was assembled by means of in situ polymerization of DA on the surface of TiO2 under 460 nm blue LED irradiation (Fig. 1). HRTEM images obviously confirmed the size and morphology of the TiO2 nanoparticles and the core-shell structured TiO2@PDA.

Fig. 1. Visible light-promoted formation of core-shell structured TiO2@PDA Fig. 2 shows the TEM images of the anatase TiO2 (Fig. 2, a) and TiO2@PDA samples (Fig. 2, b-d) prepared with different crystal phases of TiO2 nanoparticles, explicitly, anatase, rutile and anatase/rutile (P25) correspondingly. The PDA shell for TiO2 cannot be directly visualized by TEM due to the amorphous nature of the shell. Due to the small size of anatase TiO2 nanoparticles, the TEM image of anatase TiO2@PDA (Fig. 2, b and Figure S1, supplementary data) can only observe the intrinsic pattern of (101) facet lattice spacing of 0.350 nm for anatase TiO2. However, for much larger nanoparticle ones, close examinations by TEM show that there might be amorphous PDA species occurred as the shell around the TiO2 particles (Fig. 2, c 10

and d).

Fig. 2. TEM images of (a) pristine anatase TiO2, (b) TiO2@PDA with a core of anatase crystal phase (ST-01), (c) TiO2@PDA with a core TiO2 of rutile crystal phase, (d) TiO2@PDA with a core TiO2 of anatase/rutile crystal phase (P25). The PXRD characterizations (Fig. S2, supplementary data) of anatase TiO2 and TiO2@PDA agrees well with the TEM characterization in Figure 1, which did give further evidence of PDA on the surface of TiO2. Thus, more surface techniques were employed to gather evidence of PDA shell on TiO2 core. To further confirm the homogeneous dispersion of PDA on TiO2@PDA, we conducted energy-dispersive 11

X-ray spectroscopy (EDX) measurements (Fig. 3) using high-angle annular dark-field imaging of in a scanning transmission electron microscope. The mapping images revealed that the elements N, O, and Ti have a homogeneous distribution in the TiO2@PDA.

Fig. 3. HAADF-STEM and EDS elemental mapping images of TiO2@PDA

X-ray photoelectron spectroscopy (XPS) measurements were made on TiO2@PDA to inspect the signals of C 1s, O 1s, N 1s and Ti 2p (Figure 4). In Fig 4a, the C 1s region was fit with three peaks assigned to C=O (288.1 eV), C-N/C-O (286.1 eV) and CHx (284.5 eV) species, which could be explained the conversion of the 12

indolic compounds during the auto-oxidative and self-polymerization process of dopamine [38]. The O-C (532.0 eV), O=C (530.9 eV) and O-Ti (529.6 eV) peaks in Fig 4b were consistent with the results of O 1s, indicating the binding of catechol units onto TiO2 successfully. The N 1s region is fit with three peaks assigned to R-NH2 (amine groups), R-NH-R (indole groups), and =N-R (imino groups) functionalities are shown in Fig 4c, demonstrating PDA could be positively assembled onto TiO2 surface to afford TiO2@PDA. For the Ti 2p spectrum, it was deconvoluted by the Ti 2p1/2 and Ti 2p3/2 peaks at 464.4 eV and 458.5 eV, which were consistent with the reported results [39].

Fig. 4. High-resolution XPS spectra of C 1s (a), O 1s (b), N 1s (c), and Ti 2p (d). As we all know, neither dopamine nor TiO2 could absorb visible light. However, the PDA coating not only served as an adhesion interface to improve the bonding

13

force between core and shell but also acted as a photosensitizer to broaden the wavelength response range of TiO2. UV-vis spectroscopy reveals that the absorbance of TiO2@PDA overlaps with the light emitting spectrum of the blue LED (Fig. 5a). On the other hand, Fig. 5b shows the FT-IR spectra of pure TiO2, dopamine and TiO2@PDA. These spectral properties are in accord with the bonding configuration of chelating complexes through phenoxy groups on TiO2@PDA [40].

Fig. 5. (a) Diffuse reflectance UV-visible spectra of TiO2@PDA, TiO2, dopamine and relative spectrum distribution of the blue LED. (b) FTIR spectra of TiO2@PDA, TiO2 and dopamine.

3.2 Visible light photocatalytic selective aerobic oxidation of sulfides

A series of characterizations of Brunauer–Emmett–Teller (BET) specific surface areas for different crystal phase core materials of TiO2@PDA have been carried out and the results were listed Table S1. In dye-sensitized TiO2 photocatalytic selective aerobic oxidation of sulfides [41], we could improve the selectivities for sulfoxides by surrogating TiO2 (ST-01) with a newly prepared anatase TiO2 [42]. Nonetheless, assembling PDA on the same anatase TiO2 cannot yield comparable conversion of 14

thioanisole (entry 6, Table 1), suggestive of the sharp difference of dye-sensitized TiO2 and TiO2@PDA photocatalysis. With titanate substituting the role of anatase TiO2, no photocatalytic oxidation of thioanisole could be observed (entry 7, Table 1). Furthermore, we carried out the reaction with trifluoroethanol as a solvent, which is a more protic organic solvent. 7% conversion of thioanisole was obtained in 1 h. Thus trifluoroethanol was not an appropriate solvent for the present protocol. Table 1. The influence of different TiO2 on the visible light-promoted selective oxidation of thioanisole into methyl phenyl sulfoxide [a]

S + O2

O

dopamine hydrochloride, TiO2 TEMPO (5 mol%), air (1 atm)

S

3 W blue LEDs, CH3OH

Crystal phase

Conv.[%][b]

Sel.[%][b]

Entry

TiO2

1

TiO2 (ST-01)

Anatase

79

90

2

TiO2 (Alfa)

Anatase

13

99

3

TiO2 (P25)

Anatase/rutile

11

99

4

TiO2 (P90)

Anatase/rutile

23

99

5

TiO2

Rutile

2

99

6

TiO2

Anatase

27

99

7

H2Ti3O7

Titanate

<1

99

[a] Reaction conditions: thioanisole (0.3 mmol), TiO2 (50 mg), dopamine hydrochloride (3.75×10-3 mmol), TEMPO (0.015 mmol), 460 nm blue LEDs (3 W × 4), air (1 atm), CH3OH (1 mL), 1 h. [b] Determined by GC-FID using chlorobenzene as the internal standard, conversion of thioanisole, selectivity of methyl phenyl sulfoxide. 15

Hereafter, TiO2 is refered to anatase TiO2 (ST-01) unless otherwise stated. Next, the TiO2@PDA prepared from TiO2 (ST-01) was used for further photocatalytic investigation. Catechol-TiO2 complex was also underwent the same procedure for a comparison. The performance of these two different photocatalysts is shown in Fig. 6a. The selective oxidation of thioanisole was obtained in 79% by TiO2@PDA photocatalyst. In contrast, the oxidation efficiency of catechol-TiO2 complex could only reach 33% when exposed to visible light for 1 h. The activity of TiO2@PDA was almost twice to that by catechol-TiO2 complex under the same conditions, suggesting the PDA coating plays a positive role in improving the visible light photocatalytic perfermance. Invigorated by the positive information above, we curtained the reaction conditions for photocatalytic thioanisole oxidation using TiO2@PDA as the photocatalyst and TEMPO as the cocatalyst. A series of control experiments were performed (Fig. 6b). Control experiments established that light and photocatalyst were necessary, and omitting any one of these components did not result in the product (Fig 6b, a and i). The experiments with sole TiO2, sole dopamine or sole TEMPO gave low yield of products, suggesting that any part of them does not drive the reaction process individually (Fig. 6b, b-d). Without dopamine or TiO2, the reaction almost could not occur (Fig 6b, e-f), indicating dopamine and TiO2 played the core role in photocatalytic oxidation of thioanisole under visible light irradiation. Without TEMPO, the photocatalysis perfermance of TiO2@PDA for oxidatin of thioanisole

16

drastically fallen compared to that of standard condition, showing its synergistic effect in the photocatalytic reaction (Fig. 6b, g).

Fig. 6. (a) Photocataytic selective oxidation of thioanisole concentration versus reaction time by TiO2@PDA and catechol-TiO2. (b) Control experiments for the visible light-promoted selective oxidation of thioanisole by combining TiO2@PDA photocatalysis and TEMPO catalysis. a, blank reaction; b, TiO2 only; c, dopamine only; d, TEMPO only; e, without dopamine; f, without TiO2; g, without TEMPO; h, standard conditions; i, without blue LED irradiation; j, without O2. Reaction conditions: thioanisole (0.3 mmol), TiO2@PDA (50.7 mg, containing 50 mg TiO2), 3.75×10-3 mmol dopamine hydrochloride, TEMPO (0.015 mmol), 1 atm of air, CH3OH (1 mL), 460 nm blue LED irradiation (3 W × 4), 1 h, yields of methyl phenyl sulfoxide were determined by GC-FID.

The exceptional results described in recent report have recognized that metal-complex and organic-dye photocatalysts can be merged with TEMPO as a redox co-catalyst under visible light irradiation. Owing to the superior applicability of TEMPO and its derivatives in chemical conversion, we studied their influence on the visible light-promoted aerobic oxidation of thioanisole. TEMPO and its derivatives are as the synergistic photocatalyst for selective photocatalytic sulfides oxidation to

17

improve the selectivity of sulfoxides and the conversion of sulfides. TEMPO exhibited the highest synergistic effect, much higher than those of other 4-substituted derivatives in all tested cases, including HO-TEMPO, CH3-TEMPO, Oxo-TEMPO, NH2-TEMPO, CH3CONH-TEMPO and PhCOO-TEMPO (Fig. 7a). With the growing importance of O2 in chemical transformations, the merger of catalytic amounts of TEMPO with manganese and cobalt nitrate has produced very effective results in aerobic oxidation of sulfides to sulfoxides. Inspired by the positive result, we investigated the influence of the amount of TEMPO on the selective photocatalytic sulfides oxidation. As TEMPO was introduced to the reaction system, both the conversion rate of sulfide was dramatically increased and the selectivity of sulfoxide was greatly improved (Fig. 7b). First, the conversion rate of sulfide increases linearly with the change of the amount of TEMPO (Fig 7b, a-h). However, this expected effect is further reduced (Fig. 7b, g-h), which means that there may be potential saturation points in the collaborative system that TEMPO acts as an electron transfer intermediary getting peak load values (Fig 7b, g-h). The influences of different factors have been considered, including different LEDs (Table S2) and several

solvents

(Table

18

S3).

Fig. 7. (a) Influence of TEMPO derivatives on the selective oxidation of thioanisole with aerial O2 by TiO2@PDA photocatalysis. (b) Influence of the amount of TEMPO on the selective oxidation of sulfides with air by TiO2@PDA photocatalysis. Reaction conditions: (a) and (b): thioanisole (0.3 mmol), TiO2 (50 mg), dopamine hydrochloride (3.75 × 10-3 mmol), TEMPO (0.015 mmol), aerial O2, CH3OH (1 mL), 460 nm blue LEDs (3 W × 4), 1 h, yield of methyl phenyl sulfoxide was determined by GC-FID.

To endorse the general applicability of the method, various thioanisoles were subjected to the oxidation protocol (Table 2). Next, we inspected the performances of the photocatalytic selective aerobic oxidation of sulfides. A scale-up selective oxidation of thioanisoles was run under the conditions except that the amount of TiO2 was increased to 100 mg, and the desired product was obtained on a gram scale in 86% conversion, 81% selectivity (entry 1, Table 2). Thioanisole, a representative of sulfide, was altered to the corresponding methyl phenyl sulfoxide in 83% conversion, 87% selectivity (entry 2, Table 2). Having electron-donating substituent on thioanisoles (entries 3-5, Table 2) cuts the reaction time clearly and affords good selectivity of the oxidized products. In contrast, thioanisoles with the electron-withdrawing groups (entries 6-9, Table 2) earning much longer time to reach the same achievement as standard condition. In particular, the nitro substituent of thioanisole, was found to require much longer reaction time in completion of the reaction, which may be ascribed to the strong electron-withdrawing effect of -NO2 group (entry 10, Table 2). The photocatalytic selective oxidation of 2-methoxy-substituted thioanisole is almost comparable with that of thioanisole (entry 11 vs. entry 2, Table 2). However, the poor 19

reactivity of 2-chloro-substituted thioanisoles were observed (entry 12, Table 2), demonstrating both the electronic effect and steric hindrance could determine the reaction outcomes (entries 11-12, Table 2). The reactivity of the meta-substituent of thioanisoles was weakened, which was due to the weaker electron-donating ability compared to that of the same para-substitution group (entries 13-14, Table 2). Moreover, the low yield of diphenyl sulfoxide from oxidation of diphenyl sulfide, indicates the steric hindrance of diphenyl sulfide directly (entry 16, Table 2). When the phenyl group was replaced with the ethyl or benzyl groups, the effect of steric hindrance remained small (entries 15 and 17, Table 2). Even larger substrates like methyl 2-naphthyl sulfide reacted readily forming the corresponding sulfoxides under the standard conditions (entry 18, Table 2). Aliphatic sulfides could also be oxidized to the corresponding sulfoxides (entries 19-20, Table 2). Besides, apart from sulfide, benzyl alcohol was tested as a substrate under the present protocol. Only 3% conversion of benzyl alcohol was observed in 1 h, indicating it cannot be extended to the selective oxidation of alcohols. Table 2. Visible light-promoted selective oxidation of sulfides into sulfoxides by combining TiO2@PDA photocatalysis with TEMPO catalysis [a] S R

Entry 1[d]

+ O2

Substrate

R

3 W blue LEDs, CH3OH

Product S

O S

TiO2@PDA 5 mol% TEMPO, 1 atm air

O S

T(h)

Conv.[%][b]

Sel.[%][b]

TON[c]

6.0

86

81

275

20

O S

S

2

O S

S

3

H3CO

O S

4

O S

S

6

7

8

9

H 3C

O S

S F

O S

S Cl

O S

S Br

87

92

70

1.0

87

90

70

1.0

86

90

69

2.0

82

87

65

1.4

81

94

65

1.6

84

89

67

1.4

82

86

65

2.5

43

91

34

1.3

82

98

65

1.2

44

99

35

1.2

84

92

67

1.7

81

90

64

1.5

75

82

60

Br O S

S I

I O S

O 2N

O 2N

O S

OCH3

OCH3

Cl

Cl

O S

S

12

15

1.0

Cl

11

14

66

F

S

13

87

H 3C

S

10

83

H3CO S

5

1.2

H3CO

S

Cl

S

S

O S

H3CO

O S

Cl

O S

21

16

17

O S

S

O S

S

18

O S

S

19 20

O S

S

n-Bu

S

n-Bu

n-Bu

O S

n-Bu

2.0

28

90

22

2.0

87

62

70

1.2

84

90

67

0.8

85

92

68

1.2

84

93

67

[a] Reaction conditions: sulfide (0.3 mmol), TiO2 (50 mg), dopamine hydrochloride (3.75 × 10-3 mmol), TEMPO (0.015 mmol), 460 nm blue LEDs (3 W × 4), Aerial O2, CH3OH (1 mL). [b] Determined by GC-FID using chlorobenzene as the internal standard, conversion of sulfide, selectivity of sulfoxide. [c] Turnover numbers in terms of dopamine hydrochloride. [d] thioanisole (1.2 mmol), TiO2 (100 mg), TEMPO (0.06 mmol). n-Bu, n-butyl.

3.3 Kinetic studies and mechanistic investigation With the insights gained in both the reaction mechanism and process, the application of optimized reaction conditions to kinetic study became a viable protocol. Time-dependent experiments on the selective oxidation of para-substituted thioanisoles, including H, OCH3, CH3, F, Cl and Br groups, were conducted using TiO2@PDA to study the reaction kinetics (Fig. S2). Kinetic experiments revealed a zero-order rate equation with respect to the selective oxidation of sulfides. Besides, the reaction profiles for the photocatalytic oxidation of a series of 4-substuituted thioanisoles were collected (Fig. S3, supplementary data). The Hammett plot has been

22

obtained to study the key intermediate in the oxidation of sulfides during photocatalytic selective oxidation of various para-substituted thioanisoles by TiO2@PDA photocatalysis (Fig. 8a). A reasonable linear relationship was established between log (kX/kH) and Brown-Okamoto constant σ+, implying that a positive charged radicals was involved in the reaction pathway. To gain insight regarding the pivotal role of reaction solvents, relevant experiment data were obtained (Fig. 8b). When the CH3OH solution was replaced with CD3OD for the photocatalytic selective oxidation of thioanisole without altering the experimental parameters, the conversion of thioanisole descended sharply. If energy transfer of singlet oxygen (1O2) bring about the formation of sulfoxides, the oxidation rate in CD3OD solution should be much faster than that in CH3OH. Because the lifetime of 1O2 is significantly longer in a deuterated solvent. However, the experimental results failed to support this idea, which suggests that superoxide radical anion (O2•-), which reacts preferably with a S-centered radical cation for the accumulation of sulfide peroxide, was the only reactive oxygen species (ROS) in the photocatalytic selective oxidation of sulfides. Furthermore, the amount of protons in CH3OH played a key role in determining ultimate photocatalytic efficiency by terminating sulfide peroxide to sulfoxide.

23

Fig. 8. (a) Hammett plot for the photocatalytic selective oxidation of para-substituted thioanisoles. (b) Kinetic curves for the visible light-promoted selective oxidation of thioanisoles in air by combining TiO2@PDA photocatalysis and TEMPO catalysis in CH3OH and CD3OD.

To highlight the crucial role of generated reactive oxygen species (ROS) in the photocatalytic reaction, we executed the quenching experiments of ROS by adding different scavengers in the oxidation reaction of sulfide using TiO2@PDA (Table 3). When the O2 was replaced with N2, the reaction stopped immediately, indicating that the O2 played the core role in formation of superoxide radical anion (O2•-) on the conduction band of TiO2 (entry 1, Table 3). The fact that the addition of 0.2 equiv. of p-benzoquinone (p-BQ) led to almost complete inhibition of the photocatalytic oxidation reactions evidenced that the central ROS was most likely O2•-, given that p-BQ was well-known as an efficient scavenger for O2•- (entry 2, Table 3). With the singlet oxygen (1O2) scavenger 1,4-diazabicyclo [2.2.2] octane (DABCO) adding into the reaction mixtures, the yield of methyl phenyl sulfoxide increase to 85% (entry 3, Table 3), which can be explained there was no formation of 1O2 in this oxidation system. To verify the above hypothesis, we conducted additional control experiments with the aid of NaN3, a good scavenger of 1O2 generation. However, the oxidative 24

reaction of thioanisole proceeds very efficiently (entry 4, Table 3), further implying 1O

2

is not the central ROS. To get a deeper understanding about this protocol, the electron trapping

experiments were carried out. When 0.1 eq of AgNO3 or K2S2O4 as a competing electron acceptor to O2 was added to the standard reaction conditions, the oxidation of sulfides to sulfoxides was totally stifled, which indicated the core role of O2•- in achieving the oxidation of sulfides (entries 5-6, Table 3). According to above experiment results, superoxide radical anions (O2•-) were the sole pathway to exist in the photocatalytic selective oxidation of sulfides. Moreover, hydrogen peroxide (H2O2) was also applied as an oxidant for the present system. It was found that the visible photocatalytic oxidation of thioanisole disappeared completely, implying H2O2 was not involved in affording methyl phenyl sulfide. Table 3. ROS Quenching experiments for the visible light-promoted photocatalytic selective aerobic oxidation of thioanisole [a] Entry

Quencher

Equiv.

Roles

Yield [%][b]

1

N2

--

O2 replacement

0

2

p-BQ

0.2

O2•- scavenger

0

3

DABCO

0.1

1O

2

scavenger

85

4

NaN3

0.3

1O

2

scavenger

77

5

AgNO3

0.1

Electron scavenger

8

6

K2S2O4

0.1

Electron scavenger

4

[a] Reaction conditions: thioanisole (0.3 mmol), dopamine hydrochloride (3.75 × 10-3 mmol), TiO2 (50 mg), TEMPO (0.015 mmol), 460 nm blue LEDs (3 W × 4), air (1 atm), CH3OH (1 mL), 1 h. 25

[b] Determined by GC-FID using chlorobenzene as the internal standard, conversion of sulfide, selectivity of methyl phenyl sulfoxide.

Enlightened by these factors in mind, we presented that optimizing the initial O2 pressure to achieve highly efficient photocatalysis performances and appropriative ROS generation were needed for selective oxidation reactions. For a deeper understanding of the effect of ROS, the different amounts of initial O2 pressure on the selective oxidation of thioanisole were investigated (Fig. 9a). The methyl phenyl sulfoxide production gradually increased within 1.0 atm of initial O2 pressure. However, the conversion of thioanisole followed by the slackened and the selectivity of products slowly lowers at the range of 1.5 atm to 2.5 atm. This phenomenon can be ascribed to the fact that aerial O2 is suitable for photocatalytic selective oxidation of thioanisole, instead of additional O2 pressure. With an understanding of the procedures of reaction better, we set out to investigate the activation of ecb− with the aid of EPR spectroscopic techniques (Fig 9b). The EPR signal of 2 min irradiation got stronger compared than that of the dark condition, suggesting the ecb− was excited under visible light irradiation. The EPR signal became much stronger from 2 min to 4 min, indicating visible light induced the production of extra ecb−. The signal of EPR became much weaker after adding O2 to the reaction system for 1 min, which was accredited to the transfer of ecb− to O2. Comparison of different EPR results provided information about that visible light can effectively induce the generation of ecb−, which transferred finally to O2.

26

In situ EPR is a very suitable and convenient technique to determine the free radical species during photocatalytic processes [43]. In order to find the influence of ROS on application to photocatalysis, we investigated the behavior of TiO2@PDA in the photocatalytic system by means of EPR spectroscopy (Fig 9c). Generated from irradiated TiO2@PDA, the remaining signals of O2•- from 0 min to 4 min were compared by using DMPO, a spin trap commonly used for the detection of O2•-. The EPR signal strength abruptly enhanced after 2 min irradiation and continue to strengthen at 4 min, demonstrating O2•- was generated from irradiated TiO2@PDA complex in this system. Fig. 9d showed the variation of TEMPO signal during irradiation of TiO2@PDA. The trademark EPR signal of TEMPO became much weaker from 0 min to 4 min indicating its good synergistic photocatalytic effect due to its conversions to corresponding intermediates when exposed to visible light. A TEMPO+ salt, TEMPO+BF4- has been tested as a redox mediator. Almost no reaction could be observed with TEMPO+BF4-, suggesting that only transient TEMPO+ rather than stable TEMPO+ salt could be a mediator for oxidation of sulfide.

27

Fig. 9. (a) The influence of initial O2 pressure on the oxidation of thioanisole. (b) The EPR spectra of ecb− of the TiO2@PDA under visible light irradiation (77 K). (c) The EPR signals of superoxide radicals captured by DMPO during the TiO2@PDA photocatalysis. (d) The EPR signals of TEMPO during the TiO2@PDA photocatalysis.

Based on the above experimental findings, a plausible reaction mechanism for photocatalytic

selective

oxidation

of

sulfides

by

combining

TiO2@PDA

photocatalysis and TEMPO catalysis is showed in Scheme 1. Under 460 nm blue LED irradiation, the catechol group of PDA was oxidized into the corresponding quinone group, releasing electrons. These electrons travelled through every monomer of PDA by the π-π* electron transition which could injected into the conduction band of anatase TiO2 to react with aerial O2 adsorbed on the surface of anatase TiO2, forming 28

O2•-. Meanwhile, PDA was excited to an unstable intermediate PDA•+, which could regenerate to PDA by the transformation of TEMPO to TEMPO+. Thioanisole was transformed into S-centered radical cation with the aid of TEMPO+, which prefers to react with O2•- for the fruition of sulfide peroxide. The sulfoxide product would be obtained after combination sulfide peroxide and CH3OH. O2 NH O2 O

O

n

PDA O S

O

O PDA

S 2 H+

N

NH

O

O TEMPO

O

n S

+ N

H 2O

S

O TEMPO+

Scheme 1. A plausible mechanism of visible light-promoted selective aerobic oxidation of thioanisole by combining TiO2@PDA photocatalysis and TEMPO catalysis

4. Conclusions In summary, visible light-promoted selective oxygenation of sulfides by circumventing the shortcomings of conventional semiconductor photocatalysis for this unique reaction have been achieved. In this regime, TiO2 semiconductor was coated with a PDA layer that possesses a large π-conjugated system in comparison with catechol, giving rise to improved stability under more challenging oxidative conditions in a redox active protic solvent. Eventually, visible light-promoted 29

selective oxidation of sulfides into sulfoxides with aerial O2 by combining TiO2@PDA photocatalysis and TEMPO catalysis was achieved with compelling success. The strong synergistic consequence of TEMPO on the photocatalytic activity greatly contributes to high conversions of sulfides and makes PDA shell durable through the shuttle between TEMPO and TEMPO+. This work provides a novel strategy for fabrication of TiO2@PDA, as stable and highly active visible light photocatalysts with surface modification of semiconductors toward selective organic transformations. Acknowledgments Financial support from the National Natural Science Foundation of China (grant numbers 21773173 and 21503086), the Fundamental Research Funds for the Central Universities (grant number 2042018kf0212), and the start-up fund of Wuhan University were gratefully acknowledged. References: [1] M.C. Carreno, G. Hernandez-Torres, M. Ribagorda, A. Urbano, Enantiopure sulfoxides: recent applications in asymmetric synthesis, Chem. Commun. (2009) 6129-6144. [2] P.F. Zhang, Y. Wang, H.R. Li, M. Antonietti, Metal-free oxidation of sulfides by carbon nitride with visible light illumination at room temperature, Green Chem. 14 (2012) 1904-1908. [3] Z.M. Li, C. Liu, H. Abroshan, D.R. Kauffman, G. Li, Au38S2(SAdm)(20) Photocatalyst for One-Step Selective Aerobic Oxidations, ACS Catal. 7 (2017) 3368-3374.

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35

GRAPHIC ABSTRACT

S R

+ O2

O

TiO2@Polydopamine TEMPO (5 mol%) 3 W blue LEDs, CH3OH

36

S R

HIGHLIGHTS 1. TiO2 surface complex photocatalysis can carry out a reaction in a redox-active solvent. 2. Turnover number in terms of dopamine precursor can be as large as 275. 3. Visible light photocatalytic selective aerobic oxidation of sulfides into sulfoxides has been achieved on TiO2@PDA. 4. TEMPO could improve the endurance of TiO2@PDA photocatalyst. 5. The electron transfer between TEMPO and TEMPO+ accelerates the formation of sulfoxides.

37