Flavin Dibromide as an Efficient Sensitizer for Photooxidation of

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Flavin Dibromide as an Efficient Sensitizer for Photooxidation of Sulfides Can Dang,†,⊥ Lijuan Zhu,†,⊥ Huimin Guo,*,† Hongyu Xia,† Jianzhang Zhao,*,† and Bernhard Dick‡ †

State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, No. 2, Linggong Road, Dalian 116024, P. R. China ‡ Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstrasse 31, Regensburg 93053, Germany

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/26/18. For personal use only.

S Supporting Information *

ABSTRACT: Flavin derivatives (FLs) are the building blocks and functional groups within many enzymes that absorb strongly in the visible light region and are redox cofactors in a large number of biological processes. We directly attached Br atoms into the conjugated framework of FL to afford FL dibromide (DBFL) and expected the heavy atom effect of Br to facilitate the intersystem crossing of excited FLs to reach the triplet states for efficient sensitization of O2. Compared with FL (ε = 1.01 × 104 M−1 cm−1 at 441 nm), DBFL shows stronger absorption in the visible range (ε = 1.90 × 104 M−1 cm−1 at 450 nm). The singlet oxygen quantum yield of DBFL is enhanced from 55.3% in FL to 92.2% at the expense of decreased luminance quantum yield from 37.7% in FL to 5.5%, confirming that a large portion of the excited DBFL molecules evolves into triplet excited states. Both FL and DBFL were used in photosensitized oxidation of various sulfides to afford corresponding sulfoxides. DBFL exhibits a twofivefold performance enhancement with respect to FL in sensitizing O2 for photocatalytic oxidation. In addition, the oxidation of sulfides with DBFL was found efficient and led exclusively to sulfoxides, with no secondary oxidation products observed. Mechanistic investigations showed that both singlet oxygen and superoxide anion radical are formed as reactive oxygen species. The findings pave the way for design and application of novel organic sensitizers for photocatalytic oxidation. KEYWORDS: Photochemistry, Flavin, Heavy atom effect, Photocatalysis, Sulfide

■

INTRODUCTION Current synthetic chemistry is committed to finding efficient and low energy demanding protocols to tailor the structure of matter. Oxidation is a fundamental organic transformation that commonly involves formation of chemical bonds with oxygen or oxygen-containing species. O2 is the ideal reagent to perform the oxidation processes in a sustainable way for its abundant availability and nontoxicity.1 However, the ground state of O2 is a triplet spin-state while those of organic molecules are singlet spin-states. This difference makes direct reactions between organic molecules and O2 in their ground states spin-forbidden. To facilitate this process, a suitable catalyst, either organic or inorganic, may be used to convert ground state O2 into reactive oxygen species (ROS), including singlet oxygen (1O2), by spin-allowed photosensitization.2−6 Competing with this energy transfer, photoinduced electron transfer may also take place and stimulate the activation of both O2 and substrates and the formation of ROS or substrate radical ions.7 Oxidation of sulfides is a typical oxidation reaction and has attracted tremendous research attention recently.8 The oxidation of sulfides is an important chemical process and has significant application in production of medicine,9 precombustion desulfurization of fuels,10,11 innocent treatment of wastewater12,13 and chemical warfare agents,13,14 etc. Photocatalysis with visible light has been recognized as an © XXXX American Chemical Society

ecobenign procedure for synthesis of chemicals as bonds can be selectively activated and it requires less energy input. In the past, inorganic materials, such as TiO2, ZnO, etc., have been tested as photocatalysts for oxidation of sulfides. However, the poor absorption of these inorganic photocatalysts in the visible light region has limited their practical applications.15−17 Different from inorganic materials, the photoredox properties of organic sensitizers may be precisely tailored by rational design and synthetic approach.15 Recently, several organic sensitizers, such as N-methylquinolinium tetrafluoroborate,18 9,10-dicyanoanthracene,19 rose bengal,20,21 2,4,6-triphenylpyrylium tetrafluoroborate,22 4,4-difluoro-4-bora-3a,4a-diaza-sindancene (BODIPY),23 methyl blue,24 Eosin Y,25,26 etc., have been investigated for photocatalytic oxidation of sulfides. The major drawback of these organic sensitizer is the high oxidation potential of ROS, leading to overoxidation and competitive carbon oxidation, making the product containing not only sulfoxides but also sulfones and aldehydes, etc.6,27 Therefore, it is still demanding for efficient and selective production of sulfoxides in mild reaction condition via sulfide oxidation.28 Flavin derivatives (FLs), widely observed as the building blocks of redox-active coenzymes, are the reaction sites of Received: July 31, 2018 Revised: October 15, 2018 Published: October 16, 2018 A

DOI: 10.1021/acssuschemeng.8b03729 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of the DBFL (Upper) and FL (Lower)

Figure 1. UV−visible spectra of (a) FL and (b) DBFL in different solvents. c = 1 × 10−5 M, 20 °C. (c) UV−visible spectra of FL and DBFL in CH2Cl2.

boosting the performance in photocatalytic oxidation of sulfides.

many thermal and photoinduced processes and are tolerant to the variation of redox potentials.29−37 FLs have been used to catalyze oxidation reaction of many organic substrates, such as the oxidation of benzylalcohols, benzylamines, methylbenzenes, benzyl methyl ethers, etc.38−52 In these reaction, photoinduced charge transfer is commonly accepted as the dominant mechanism. On the other hand, FLs are also capable of sensitizing the production of singlet oxygen as ROS in many other cases.40,45,53,54 Mechanism investigations showed that the singlet oxygen is the major ROS in FLs-sensitized oxidation of unsaturated fatty acids and esters.54 FLs absorbs strongly in the visible light range. The λabs at longer wavelength is ∼450 nm and tunable by further functionalization.55−57 Recently, selective oxidation of sulfides sensitized by riboflavin and riboflavin tetraacetate (RFTA) in visible region was reported by Cibulka et al., and they proposed that the FLs-sensitized formation of singlet oxygen is the dominant mechanism.42,51,58,59 Though the catalytic performance remains to be enhanced further, nearly all the substrates can be converted chemoselectively to the corresponding sulfoxides within a reasonable time duration, independent of the types of sulfides.59,60 In this sense, high triplet quantum yield of the photosensitizer in radiation would be necessary to achieve enhanced photocatalytic performance. We recently attached FL to [Ru(bpy)3]2+ and observed the room temperature phosphorescence of FL, showing that the triplet quantum yield of FL can be enhanced as the intersystem crossing within the FL-decorated complex was facilitated by the Ru(II) center.61 In this work, we directly bonded Br to the conjugated framework of FL to afford DBFL. We expected that the heavy atom effect introduced by Br into DBFL can also facilitate the intersystem crossing to the triplet excited state, enhancing the triplet quantum yield of DBFL and by this

■

MATERIALS AND METHODS

The FL and DBFL were synthesized according to Scheme 1. Please see the Supporting Information for more details on the methods for characterization and theoretical calculations (Supporting Information Section 1) and synthesis (Section 2).

■

RESULTS AND DISCUSSION We attached Br atom to the conjugation system of FL to tailor its photophysical properties (see the Supporting Information for details). The structures and properties of FL and DBFL were further characterized to investigate their photophysical properties.62,63 In good agreement with reported results, the absorption of FL in the UV/vis range is constituted by two broad peaks at ∼350 and ∼450 nm, respectively (Figure 1a).49,64 We performed theoretical calculations to interpret the absorption spectra of FL and DBFL (Table 1, Figure 2, Table S1, and Figure S37). The DFT/TD-DFT results well reproduce the experimental finding for FL and DBFL absorption in the visible light region. For FL, the absorption above 400 nm (S0 → S1) involves electron transitions from MO 59 to MO 60 (Figure 2) contributed by states of π,π* symmetry delocalized on the conjugated framework of FL and the imide moiety and thus can be attributed to both π → π* transition within FL and also the n → π* involving the imide moiety. The absorption at ∼330 nm (S0 → S4) has contributions from transitions from MO 58 to MO 60 and from MO 59 to MO 61. According to the spatial distribution of the wave function of these states, this transition is also of π → π* and n → π* character. The UV/vis spectrum of DBFL also inherits these absorption features, with B


Research Article

ACS Sustainable Chemistry & Engineering

In this sense, the absorption of FL depends strongly on the solvent properties (Figure 1a).32 More specifically, the absorption of FL is influenced by the π−π stacking with toluene, the polarity, and hydrogen bonds formed with MeOH, DCM, and MeCN. This leads to slightly more significant absorption of FL in DCM. The solvent-dependent absorption feature is also observable in DBFL, and similar solventdependent trend is observed (Figure 1b). The emission of FL and DBFL was examined in various solvents (Figure 3). In toluene, both compounds show fluorescence in the range from 500 to 800 nm with a similar shape, featuring two peaks at 491 and 520 nm in FL. These two peaks are red-shifted at 504 and 530 nm in DBFL. A similar red shift was also observed for FL and DBFL in DCM solutions, where the λem,max values of FL in DCM are 500 and 522 nm, respectively, while those of DBFL are 514 and 532 nm, respectively. Even though Br also introduced a dipole within the conjugated framework of FL, the emissions of FL and DBFL are still significant in toluene and DCM, and the vibrational structures are apparent. In solution of the highpolarity solvents, like methanol and acetonitrile, the impact of solvation would be more significant and may lead to fast conversion of excited molecules within alternative pathways that compete with the radiative decay path and result in significant decreased emission intensity.32 The emission properties of FL and DBFL were also investigated with DFT/TD-DFT calculations (Figure 4). In the simulated emission spectra at 298 K, the emissions of FL are at 482 and 518 nm, respectively, while those of DBFL are at 500 and 538 nm, respectively (Figure 4, upper left). The calculated results well reproduce the features of the experimental emission spectra (Figure 3). Based on Kasha’s rule, the emission of excited chromophore molecules is mainly from its accessible lowest lying excited state. As the population of excited molecules in different excited states obeys Boltzmann statistics, the fluorescent emission spectra of a chromophore are mainly due to the emissive transition from its first singlet excited state (S1).65,66 The wavelengths and driving force of the possible emissions are determined by the energy difference between the S1 state and the energy of the chromophore molecule in the S1 structure on the ground state (S0) potential energy surface. The driving force for the nonemissive decay is governed by the difference in energy of S1 and S0 structures on the S0 energy surface and named as reorganization energy (Ereorg) hereafter. The calculated Ereorg values of FL and DBFL were then projected back to vibration modes on the corresponding S0 potential energy surfaces (Figure 4, lower left). Ereorg and also the nonemissive decay of FL are contributed mainly by vibration modes 7, 43, 47, 57, 58, and 59, while those for DBFL are contributed mainly by 10, 51, 59, 60, and 61 on the corresponding S0 surfaces. These vibration modes were vectorized into atomic motions (Figure 4, right). The vibration modes of FL that contribute dominantly to the Ereorg can be easily recognized in DBFL (Figure 4, lower left). These vibrations can be assigned as the stretching of CC and C N bonds and act as the main paths for nonemissive decay of excited FL and DBFL. It should also be noted that the introduction of Br does not give rise to additional nonemissive decay paths (Figure 4, right panel) and does not contribute to significant variation of Ereorg. The calculated Ereorg values for FL and DBFL are 1794.63 and 1673.78 cm−1, respectively. The slightly less significant Ereorg of DBFL suggests that the

Table 1. Major Electronic Transitions Involved in the UV− Vis Absorption of FL and DBFL transitions

energya

fb

compositionc

CId

FL: S0 → S1 FL: S0 → S4

3.03 eV/ 410 nm 3.75 eV/ 331 nm

0.2006

59 → 60

0.6947

0.1816

58 → 60

0.6797

59 → 61

0.1475

55 → 60 58 → 60

0.1572 0.1207

59 → 61

0.6596

0.2554

93 → 94

0.6960

0.2522

92 → 94

0.6842

93 → 95

0.1278

84 → 94

0.1186

87 → 94 89 → 94

0.1466 0.1031

92 → 94

0.1008

93 → 95

0.6540

92 → 95

0.4177

93 → 97

0.5543

FL: S0 → S9

DBFL: S0 → S1 DBFL: S0 → S4

DBFL: S0 → S11

DBFL: S0 → S12

4.90 eV/ 253 nm

2.91 eV/ 426 nm 3.61 eV/ 344 nm

4.69 eV/ 264 nm

4.91 eV/ 252 nm

0.7354

0.8261

0.0584

character π → π*, n → π* π → π*, n → π* π → π*, n → π* n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π* π → π*, n → π*

a

Only the major electronic transitions in the UV−vis range are presented. bOscillator strength. We only included transitions, the oscillator strength of which are more significant than 0.02. Please see the Supporting Information for the complete lists of possible transitions. cOnly the configurations with significant CI coefficients are shown. dThe CI coefficients are in absolute values.

two peaks in the range from 300 to 500 nm (Figure 1b). Similar groups of transitions were also observed for DBFL (Table 1). The absorption at ∼440 nm (S0 → S1) involves electron transitions from MO 93 to MO 94 contributed by delocalized states of π,π* symmetry and also the nonbonding orbitals on Br, so this transition is attributed to both π → π* transition within the FL moiety and also the n → π* transition involving the imide moiety and the Br orbitals. This also holds for the absorption at ∼340 nm. The similarity in the absorption implies that FL and DBFL should exhibit similar photophysical properties. As the introduction of Br atoms also extended the conjugation in the FL framework (Figure 2), the two major absorption peaks in the visible light region are red-shifted from 420 and 330 nm for FL to 440 and 340 nm for DBFL, respectively, and the absorption in the visible light region is also enhanced (Figure 1c and Table 1). In DCM, the molar extinction coefficient of DBFL is 1.90 × 104 M−1 cm−1 and is nearly doubled as compared with that of FL (1.01 × 104 M−1 cm−1, Figure 1c). The nearly doubled molar extinction coefficient of DBFL can thus be attributed to the introduction of Br. The absorption of FL and DBFL in toluene, CH2Cl2 (DCM), acetonitrile (MeCN), and methanol (MeOH) was also investigated (Figure 1a, b). Due to the special structure of FL, it can form various interactions with the solvent molecules. C


Research Article


Figure 2. Contour plots of the molecular states of FL and DBFL that contribute mainly to the transitions mentioned in Table 1. The isovalue is ±0.02 au. The C, O, N, I, and H atoms are plotted as gray, red, blue, brown, and white balls, respectively. Please see Figures S38 and S39 in the Supporting Information for more details.

conversion are the dominant decay processes. As kp is significantly smaller than kisc(T1−S0), nonradiative decay from T1 to S0 is dominant over phosphorescent emission. Compared with FL, the kf and kic of DBFL increase slightly. However, the kisc(S1−T1) increases to 3.23 × 105 s−1 and is increased by about 9-fold as compared with that of FL, showing that more excited DBFL will be populated to T1 via

nonemissive decay of DBFL would be slower as compared with FL. The DFT/TD-DFT calculated theoretical photoluminescence properties of FL and DBFL are summarized in Table 2. For FL, the kf and kic are 6.04 × 107 and 1.76 × 108 s−1, respectively, while kisc and kp are only 3.53 × 104 and 1.07 × 10−1 s−1, suggesting that fluorescent emission and internal D


Research Article


excited DBFL can evolve into the lowest triplet excited states required for sensitization of O2. The ΦΔ of DBFL is significantly higher than that of FL, showing that the population of excited DBFL in T1 is much higher than for FL, either due to a high triplet quantum yield or due to a prolonged triplet excited state lifetime or both. These are supported by the aforementioned TD/DFT calculations. We noticed that both FL and DBFL are outstanding in terms of ΦΔ in the solvents tested. The ΦΔ of FL in MeCN is already 55.3%, comparable to that of [Ru(bpy)3]2+ (57%). ΦΔ of DBFL in MeCN is up to 92.2% which is nearly doubled as compared with that of [Ru(bpy)3]2+ (57%). In addition to these, the ΦΔ values of DBFL in various solvents are much higher than those of FL. In this sense, both of them can in principle be used as photosensitizers to sensitize singlet oxygen. In a practical attempt, both FL and DBFL were used as sensitizer for photocatalytic oxidation of sulfides to afford corresponding sulfoxides. We first investigated the performance of DBFL for conversion of sulfides and found that 0.2 mmol thioanisole can be fully converted in 3 mL of DCM/ MeOH (v:v = 9:1) solution with 0.5 mol % DBFL within 500 min (entry 1, Table 4). For comparison, we examined the conversion of thioanisole with tetra-O-acetylriboflavin (RFTA) as a photosensitizer, and we noticed a 90% conversion at the same condition confirming at least the similar performance of DBFL and RFTA in DCM/methanol solution (entry 2, Table 4).57−60 Though high singlet oxygen quantum yields were observed in MeCN, this does not guarantee superior conversion of substrates. The performance of a photosensitizer is also strongly solvent-dependent as the solvability of both the sensitizer and substrate are important to keep the photooxidation homogeneous, and the solvents are also vital in the formation and evolution of reaction intermediates. Photocatalytic oxidation of sulfides was found sluggish in aprotic solvents.58 Previously, Cibulka and co-workers proposed that introduction of protonic solvent like methanol and ethanol will

Figure 3. Emission spectra of (a) FL and (b) DBFL in different solvents (optically matched solutions were used). λex = 435 nm, 20 °C.

intersystem crossing as facilitated by the heavy atom effect of Br and the T1 quantum yield would be increased by about 10fold in DBFL as compared with FL. At the same time, kisc(T1− S0) which corresponds to the nonradiative decay of DBFL in T1 decreases to 3.89 × 101 s−1 and is only about 1/3 that of FL (1.13 × 102 s−1). As mentioned before, nonradiative decay is the dominant decay path for FL, and the decreased kisc(T1−S0) suggests potentially longer triplet lifetime of DBFL as compared with FL. In this sense, DBFL is superior in promoting the photocatalytic processes using triplet excited state as compared with FL, where the fast ISC rates ensure the high triplet quantum yields, and the slow decay along the nonradiative decay path ensures longer triplet excited state lifetime for the photochemical events to take place. The photophysical properties of FL and DBFL are collected in Table 3. The emissions of FL and DBFL are all short-lived fluorescence emissions with lifetimes in the nanosecond range. According to fluorescence quantum yields, the emission is significantly weakened after introduction of Br into the conjugation framework of FL. The incorporation of Br leads to heavy atom effect that promotes the ISC by which the

Figure 4. Simulated emission spectra (upper left), reorganization energy projected onto the vibrational normal modes on ground state potential energy surfaces (lower left), and the vector presentation of atomic displacement for these normal modes for FL (upper right) and DBFL (lower right). E


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Calculated Theoretical Photoluminescence Properties of FL and DBFL at 298 K 298 K

kfa/s−1

kicb/s−1

FL DBFL

6.04 × 10 7.32 × 107 7

1.76 × 10 2.20 × 108 8

kisc(S1−T1)c/s−1

kisc(T1−S0)d/s−1

kpe/s−1

3.53 × 10 3.23 × 105

1.13 × 10 3.89 × 101

1.07 × 10−1 3.23 × 10−1

4

2

S1 → S0 emissive decay rate. bSn → S0 nonemissive decay rate. cS1 → T1 intersystem crossing rate. dT1 → S0 nonemissive decay rate. eT1 → S0 emissive decay rate. a

of only 78.9% was achieved after reaction at the same condition for 240 min (entry 6, Table 4). We noticed that without DBFL or light radiation or either of them may lead to nonobservable conversion of the substrate, suggesting the vital role of both DBFL and light radiation for this reaction (entries 7−9, Table 4). In parallel experiments, the same reaction protocol was used when FL was used as the photocatalyst. According to the product yield, the catalytic performance of DBFL is significantly higher than that of FL (Table 5). In accordance with the reaction yield, we noticed that when DBFL was used as the catalyst, substrates, including 4chlorothioanisole, 4-bromothioanisole, benzyl phenyl sulfide, and dibenzyl sulfide, can be fully converted into corresponding sulfoxides. This is already comparable or superior to other reported catalysts of this type, such as Rose Bengal,21 riboflavin, and tetra-O-acetylriboflavin,59 etc., that require prolonged reaction time or addition of promoters to achieve full conversion of these substrates. However, the conversions of 4-nitrothioanisole and cyclopropyl phenyl sulfide are still low even at a prolonged reaction time. This is because nitrobenzene is a known radical trap, and this also suggests a radical-based mechanism is involved in the reaction.21,67 Though the conversion of substrate with cyclopropyl group is only 80.6% within 1025 m (Table 5, entry 7), it is already superior to the reaction with a Pt(II) complex as the catalyst, reported by Casado-Sanchez et al., that fully converts the substrate in 2880 m.68 In addition to this, the cyclopropyl group is retained after the photocatalytic oxidation. According to final reaction mixture 1H NMR spectrum, only peaks of substrate and product were observed (Figure S33). In this sense, photocatalytic oxidation reactions with FL and DBFL are highly selective and are not capable of breaking the C−C bond even in a highly strained cyclopropyl ring. Experiments were also carried out to capture the ROS and probe the reaction mechanism for the photocatalytic oxidation of sulfides. Different from nonprotonic solvent, the reaction rate of photoxidation within protonic solvents would be higher as a proton may stabilize the reaction intermediate and accelerate the formation of singlet oxygen.18,60,67 Benzoquinone is commonly used as a selective scavenger to capture or remove O2−• radical. The photooxidation will be slowed down when benzoquinone is added to the reaction mixture where O2−• radical is the one of the reactive oxygen species.67,69,70 We noticed that, when only DCM is used as solvent, the reaction in such an aprotic solvent will be slowed down (Table 6, entry 1), showing that singlet oxygen is one of the ROS involved in the photocatalytic oxidation.59 However, the further addition of benzoquinone also results in significant decrease in the substrate conversion (Table 6, entry 3). Therefore, O2−• radical is also one of ROS during the photocatalytic oxidation. In both cases, with only DCM and with the addition of benzoquinone, the product yields are not quenched to zero suggesting the complexity of the reaction mechanism (Table 6, entries 1−3).50 To further confirm the mechanisms for DBFL-catalyzed photooxidation, parallel

Table 3. Experimental Photophysical Properties of FL and DBFL FL

DBFL

solvent

ΦF (%)a

τF (ns)b

ΦΔ (%)c

toluene CH2Cl2 MeCN MeOH toluene CH2Cl2 MeCN MeOH

60.0 46.2 37.7 32.3 10.4 5.8 5.5 3.6

7.82 6.42 7.10 6.05 1.23 0.68 0.73 0.49

28.6 37.4 55.3 25.9 41.5 44.0 92.2 34.8

a

Fluorescence quantum yields, calculated with that of [Ru(bpy)3]2+ in MeCN (Φp = 0.095) as a standard. bLuminescence lifetimes. cSinglet oxygen quantum yields, calculated using that of [Ru(bpy)3]2+ in MeCN as a standard (ΦΔ = 0.57). Please consult ref 59 for the spectra of [Ru(bpy)3]2+ in MeCN.

Table 4. Optimization of Experimental Conditions for Photooxidation of Thioanisolea entry

reaction condition

1 2 3

0.5 mol % DBFL, 500 min,110 W/m2 0.5 mol % RTFA, 500 min,110 W/m2 0.5 mol % DBFL, 500 min,110 W/m2, in ACN/water = 9/1 (v/v) (3.0 mol) 2 mol % DBFL, 500 min,110 W/m2 1 mol % DBFL, 500 min,110 W/m2 0.5 mol % DBFL, 240 min,110 W/m2 no DBFL, 500 min, 110 W/m2 0.5 mol % DBFL, 500 min, no light no DBFL, 500 min, no light

4 5 6 7 8 9

yield (%) 100 90 93 100 100 78.9 0 0 0

a

Reaction conditions: thioanisole (0.20 mmol) and catalyst were mixed in CH2Cl2/CH3OH = 9/1 (v/v) to form 3.0 mL of solution. The mixture was exposed to 30 W OLED light (110 W/m2), and TLC analysis was performed to detect the reaction, 20 °C. 1H NMR spectra were used to determine the product yields.

promote the photocatalytic oxidation using FL derivatives as catalysts.59,60 Inspired by the high singlet quantum yield of DBFL in MeCN, we also studied the usage of MeCN/H2O (v/ v = 9:1) as the potential solvent as suggested by previous investigations and found that the conversion is 93% (entry 3, Table 4), showing the comparable performance of DBFL for photooxidation of thioanisole in these systems.57−60 To ensure the solvability of aromatic sulfides, we selected the mixture of DCM where FL and DBFL show significant absorption and protonic methanol (DCM/CH3OH = 9/1 (v/v)) as solvent in the subsequent investigation. The absorptions of DBFL in DCM and DCM/MeOH were also examined, and we found the absorption spectra are identical, so there is no problem with the solubility of DBFL even in DCM/MeOH (Figure S37). The reaction protocols were then optimized using thioanisole as the model compound (Table 4). The optimized catalyst concentration was determined as 0.5 mol % (entries 1, 4−5, Table 4). The reaction time was set as 500 min as a yield F


Research Article

ACS Sustainable Chemistry & Engineering Table 5. Visible Light Promoted Aerobic Photocatalytic Oxidation of Various Sulfides with DBFL and FLa

a Reaction conditions: thioanisole (0.20 mmol), DBFL, and FL (0.5 mol %) were mixed with CH2Cl2/CH3OH = 9/1 (v/v) to form 3.0 mL of solution, the mixture was exposed to 30 W OLED light (110 W/m2), and TLC analysis was performed to detect the reaction, 20 °C. 1H NMR spectra were used to determine the product yields.

experiments (Table 6, entries 4−6) were carried out in methanol and deuterated MeOH. Without any scanvenger, we found a product yield of 9.0% in 180 m (Table 6, entry 4) while a decrease to 6.0% was observed when DABCO was introduced as selective scanvenger (Table 6, entry 5), confirming that 1O2 is one of the ROS.68 As it is well-known that oxidation with 1O2 can be accelerated using deuterated solvents;59 this ROS role of 1O2 was further confirmed by the increased product yield to 15.0% in reaction without DABCO in deuterated MeOH (Table 6, entry 6). For comparison, the mechanism of FL-catalyzed photooxidation was also investigated (Table 6, entries 7−10) and found similar to that of DBFL according to the decrease of product yield in existence of any scavengers. Namely, in DCM/MeOH solution without any scavengers, the product yield with FL is 13% in 180 m (Table 6, entry 7) while that with benzoquinone is decreased to 3% (Table 6, entry 8), confirming that the O2−• radical is one of the ROS. The existence of singlet oxygen as ROS during the reaction was confirmed by the reactions in methanol as the addition of DABCO decreases the product yield from 3% to nondetectable (Table 6, entries 9 and 10). These findings prove that both singlet oxygen and O2−• radical are ROS in the photocatalytic oxidation of sulfides.

Table 6. Visible Light Promoted Aerobic Oxidation of Sulfides with DBFL in Different Conditionsa entry

catalysts

1 2

DBFL DBFL

3

DBFL

4 5

DBFL DBFL

6

DBFL

7

FL

8

FL

9 10

FL FL

solvents DCM DCM/ CH3OH DCM/ CH3OH CH3OH CH3OH deuterated CH3OH DCM/ CH3OH DCM/ CH3OH CH3OH CH3OH

reaction time (m)

yield (%)

none none

500 500

22.9 100

50 mol % benzoquinone none 0.5 mol % DABCO none

500

14.8

180 180

9.0 6.0

180

15.0

none

180

13.0

50 mol % benzoquinone none 0.5 mol % DABCO

180

3.0

180 180

3.0 0.0

scavengers

a

Reaction conditions: Thioanisole (0.20 mmol) and catalyst (0.5 mol %) were mixed with solvents and scavengers to form a liquid solution of 3.0 mL. The mixture was exposed to 30 W OLED light (110 W/ m2). TLC analysis was used to detect the progression of the reaction, 20 °C. 1H NMR spectra were to determine the product yields.

■

CONCLUSIONS Using Br for introduction of heavy atom effect to facilitate the ISC to reach the triplet excited state of FL for photosensitizer G


Research Article


SOLVENT AND COMPARTMENTALIZATION EFFECTS. Chem. Rev. 1993, 93 (2), 699−723. (5) Campbell, A. N.; Stahl, S. S. Overcoming the ″Oxidant Problem″: Strategies to Use O-2 as the Oxidant in Organometallic C-H Oxidation Reactions Catalyzed by Pd (and Cu). Acc. Chem. Res. 2012, 45 (6), 851−863. (6) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen. Chem. Rev. 2005, 105 (6), 2329−2363. (7) Lopez, L. In Photoinduced electron transfer oxygenations; Springer: Berlin, Heidelberg, 1990; pp 117−166. (8) Bian, C. L.; Singh, A. K.; Niu, L. B.; Yi, H.; Lei, A. W. VisibleLight-Mediated Oxygenation Reactions using Molecular Oxygen. Asian J. Org. Chem. 2017, 6 (4), 386−396. (9) Caron, S.; Dugger, R. W.; Ruggeri, S. G.; Ragan, J. A.; Ripin, D. H. B. Large-scale oxidations in the pharmaceutical industry. Chem. Rev. 2006, 106 (7), 2943−2989. (10) Otsuki, J.; Okuda, N.; Amamiya, T.; Araki, K.; Seno, M. Directional photochemical processes with an amphiphilic anthracenorutheninm complex in dihexadecyl phosphate vesicles. Chem. Commun. 1997, No. 3, 311−312. (11) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W. H.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction. Energy Fuels 2000, 14 (6), 1232−1239. (12) Sorokin, A. B. Photocatalytic Degradation of Pollutants with Emphasis on Phthalocyanines and Related Complexes. In Photosensitizers in Medicine, Environment, and Security; Nyokong, T., Ahsen, V., Eds.; Springer: Dordrecht, The Netherlands, 2012; pp 433−467. (13) Marin, M. L.; Santos-Juanes, L.; Arques, A.; Amat, A. M.; Miranda, M. A. Organic Photocatalysts for the Oxidation of Pollutants and Model Compounds. Chem. Rev. 2012, 112 (3), 1710−1750. (14) Yang, Y. C.; Baker, J. A.; Ward, J. R. DECONTAMINATION OF CHEMICAL WARFARE AGENTS. Chem. Rev. 1992, 92 (8), 1729−1743. (15) Fukuzumi, S.; Ohkubo, K. Selective photocatalytic reactions with organic photocatalysts. Chem. Sci. 2013, 4 (2), 561−574. (16) Suzuki, K.; Jeong, J.; Yamaguchi, K.; Mizuno, N. Photoredox catalysis for oxygenation/deoxygenation between sulfides and sulfoxides by visible-light-responsive polyoxometalates. New J. Chem. 2016, 40 (2), 1014−1021. (17) Schultz, D. M.; Levesque, F.; DiRocco, D. A.; Reibarkh, M.; Ji, Y. N.; Joyce, L. A.; Dropinski, J. F.; Sheng, H. M.; Sherry, B. D.; Davies, I. W. Oxyfunctionalization of the Remote C-H Bonds of Aliphatic Amines by Decatungstate Photocatalysis. Angew. Chem., Int. Ed. 2017, 56 (48), 15274−15278. (18) Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M. F.; Guerra, M.; Lapi, A.; Liberali, P. Electron transfer and singlet oxygen mechanisms in the photooxygenation of dibutyl sulfide and thioanisole in MeCN sensitized by N-methylquinolinium tetrafluoborate and 9,10-dicyanoanthracene. The probable involvement of a thiadioxirane intermediate in electron transfer photooxygenations. J. Am. Chem. Soc. 2003, 125 (52), 16444−16454. (19) Baciocchi, E.; Crescenzi, C.; Lanzalunga, O. Photoinduced electron transfer reactions of benzyl phenyl sulfides promoted by 9,10-dicyanoanthracene. Tetrahedron 1997, 53 (12), 4469−4478. (20) Bonesi, S. M.; Torriani, R.; Mella, M.; Albini, A. The photooxygenation of benzyl, heteroarlymethyl, and allyl sulfides. Eur. J. Org. Chem. 1999, 1999 (7), 1723−1728. (21) Gu, X. Y.; Li, X.; Chai, Y. H.; Yang, Q.; Li, P. X.; Yao, Y. M. A simple metal-free catalytic sulfoxidation under visible light and air. Green Chem. 2013, 15 (2), 357−361. (22) Bonesi, S. M.; Fagnoni, M.; Albini, A. Photosensitized electron transfer oxidation of sulfides. A steady-state study. Eur. J. Org. Chem. 2008, 2008 (15), 2612−2620. (23) Li, W. L.; Xie, Z. G.; Jing, X. B. BODIPY photocatalyzed oxidation of thioanisole under visible light. Catal. Commun. 2011, 16 (1), 94−97.

applications, DBFL was synthesized and characterized for its photophysical properties as well as the performance in photooxidation of sulfides. The absorption DBFL in the visible light region is strong (ε = 1.90 × 104 M−1 cm−1 at 450 nm) and is nearly doubled with respect to FL (ε = 1.01 × 104 M−1 cm−1 at 441 nm). The heavy atom effect of Br enhances the ΦΔ from 55.3% to 99.2% at the expense of ΦF from 37.7% to 5.5%. Both compounds were used in photocatalytic oxidation of various sulfides to afford sulfoxides. As expected, DBFL exhibits a superior photocatalytic performance with respect to FL in photocatalytic oxidation. In parallel experiments, two−fivefold performance enhancements with respect to FL were observed. In addition to these, the photocatalytic oxidation with DBFL was found highly selective without overoxidation. Mechanistic investigations suggest that DBFL can effectively sensitize the formation of both singlet oxygen and superoxide anion radicals as ROS for photocatalytic oxidation of sulfides. The findings highlight the significant role of heavy atoms in photocatalysis and are vital for the design and application of novel organic sensitizers for photocatalytic oxidation.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03729. Structural characterization spectra of FL and FL derivatives; NMR spectra for the photooxidation products and TD/DFT calculations (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.G.). *E-mail: [email protected] (J.Z.). ORCID

Huimin Guo: 0000-0001-9283-7374 Jianzhang Zhao: 0000-0002-5405-6398 Bernhard Dick: 0000-0002-9693-5243 Author Contributions ⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, Nos. 21573034, 21771029, 11811530631, 21373036, and 21103015). The supercomputer time was provided by National Supercomputing Center in Guangzhou, China and the High Performance Computing Center at Dalian University of Technology.

■

REFERENCES

(1) Sheldon, R. A. Catalysis: The key to waste minimization. J. Chem. Technol. Biotechnol. 1997, 68 (4), 381−388. (2) DeRosa, M. C.; Crutchley, R. J. Photosensitized singlet oxygen and its applications. Coord. Chem. Rev. 2002, 233, 351−371. (3) Schweitzer, C.; Schmidt, R. Physical mechanisms of generation and deactivation of singlet oxygen. Chem. Rev. 2003, 103 (5), 1685− 1757. (4) Lissi, E. A.; Encinas, M. V.; Lemp, E.; Rubio, M. A. SINGLET OXYGEN O2(1-DELTA-G) BIMOLECULAR PROCESSES H


Research Article

ACS Sustainable Chemistry & Engineering (24) Madhavan, D.; Pitchumani, K. Reactions in clay media: photooxidation of sulfides by clay-bound methylene blue. Tetrahedron 2001, 57 (39), 8391−8394. (25) Guerrero-Corella, A.; Martinez-Gualda, A. M.; Ahmadi, F.; Ming, E.; Fraile, A.; Aleman, J. Thiol-ene/oxidation tandem reaction under visible light photocatalysis: synthesis of alkyl sulfoxides. Chem. Commun. 2017, 53 (75), 10463−10466. (26) Li, Y. M.; Wang, M.; Jiang, X. F. Controllable Sulfoxidation and Sulfenylation with Organic Thiosulfate Salts via Dual Electron- and Energy-Transfer Photocatalysis. ACS Catal. 2017, 7 (11), 7587− 7592. (27) Wojaczynska, E.; Wojaczynski, J. Enantioselective Synthesis of Sulfoxides: 2000−2009. Chem. Rev. 2010, 110 (7), 4303−4356. (28) Silvi, M.; Melchiorre, P. Enhancing The Potential Of Enantioselective Organocatalysis With Light. Nature 2018, 554 (7690), 41−49. (29) Walsh, C. Flavin Coenzymes - At The Crossroads Of Biological Redox Chemistry. Acc. Chem. Res. 1980, 13 (5), 148−155. (30) Walsh, C. Naturally-Occurring 5-Deazaflavin Coenzymes Biological Redox Roles. Acc. Chem. Res. 1986, 19 (7), 216−221. (31) Bruice, T. C. Mechanisms of Flavin Catalysis. Acc. Chem. Res. 1980, 13 (8), 256−262. (32) Heelis, P. F. the Photophysical and Photochemical Properties of Flavins (Isoalloxazines). Chem. Soc. Rev. 1982, 11 (1), 15−39. (33) Heelis, P. F.; Hartman, R. F.; Rose, S. D. Photoenzymic Repair of Uv-Damaged DNAA Chemist’s Perspective. Chem. Soc. Rev. 1995, 24 (4), 289. (34) Kaim, W.; Schwederski, B. Cooperation of metals with electroactive ligands of biochemical relevance: Beyond metalloporphyrins. Pure Appl. Chem. 2004, 76 (2), 351−364. (35) Kaim, W.; Schwederski, B.; Heilmann, O.; Hornung, F. M. Coordination compounds of pteridine, alloxazine and flavin ligands: structures and properties. Coord. Chem. Rev. 1999, 182, 323−342. (36) Murakami, M.; Ohkubo, K.; Fukuzumi, S. Inter- and Intramolecular Photoinduced Electron Transfer of Flavin Derivatives with Extremely Small Reorganization Energies. Chem. - Eur. J. 2010, 16 (26), 7820−7832. (37) Kaim, W.; Schwederski, B. Non-innocent ligands in bioinorganic chemistry-An overview. Coord. Chem. Rev. 2010, 254 (13−14), 1580−1588. (38) Svoboda, J.; Schmaderer, H.; Koenig, B. Thiourea-enhanced flavin photooxidation of benzyl alcohol. Chem. - Eur. J. 2008, 14 (6), 1854−1865. (39) Silva, E.; Edwards, A. M.; Pacheco, D. Visible light-induced photooxidation of glucose sensitized by riboflavin. J. Nutr. Biochem. 1999, 10 (3), 181−185. (40) Sikorska, E.; Sikorski, M.; Steer, R. P.; Wilkinson, F.; Worrall, D. R. Efficiency of singlet oxygen generation by alloxazines and isoalloxazines. J. Chem. Soc., Faraday Trans. 1998, 94 (16), 2347− 2353. (41) Schmaderer, H.; Hilgers, P.; Lechner, R.; Koenig, B. Photooxidation of Benzyl Alcohols with Immobilized Flavins. Adv. Synth. Catal. 2009, 351 (1−2), 163−174. (42) Megerle, U.; Wenninger, M.; Kutta, R. J.; Lechner, R.; Konig, B.; Dick, B.; Riedle, E. Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from subpico- to microseconds. Phys. Chem. Chem. Phys. 2011, 13 (19), 8869− 8880. (43) Massad, W. A.; Barbieri, Y.; Romero, M.; Garcia, N. A. Vitamin B(2)-sensitized photo-oxidation of dopamine. Photochem. Photobiol. 2008, 84 (5), 1201−1208. (44) Lechner, R.; Kuemmel, S.; Koenig, B. Visible light flavin photooxidation of methylbenzenes, styrenes and phenylacetic acids. Photochem. Photobiol. Sci. 2010, 9 (10), 1367−1377. (45) Fukuzumi, S.; Tanii, K.; Tanaka, T. Protonated Pteridine and Flavin Analogs Acting as Efficient and Substrate-Selective Photocatalysts in the Oxidation of Benzyl Alcohol Derivatives by Oxygen. J. Chem. Soc., Chem. Commun. 1989, No. 13, 816−818.

(46) Fukuzumi, S.; Yasui, K.; Suenobu, T.; Ohkubo, K.; Fujitsuka, M.; Ito, O. Efficient catalysis of rare-earth metal ions in photoinduced electron-transfer oxidation of benzyl alcohols by a flavin analogue. J. Phys. Chem. A 2001, 105 (46), 10501−10510. (47) D’Souza, V. T. Modification of cyclodextrins for use as artificial enzymes. Supramol. Chem. 2003, 15 (3), 221−229. (48) Cibulka, R.; Vasold, R.; Konig, B. Catalytic photooxidation of 4-methoxybenzyl alcohol with a flavin-zinc(II)-cyclen complex. Chem. - Eur. J. 2004, 10 (24), 6223−6231. (49) Korvinson, K. A.; Hargenrader, G. N.; Stevanovic, J.; Xie, Y.; Joseph, J.; Maslak, V.; Hadad, C. M.; Glusac, K. D. Improved FlavinBased Catalytic Photooxidation of Alcohols through Intersystem Crossing Rate Enhancement. J. Phys. Chem. A 2016, 120 (37), 7294− 7300. (50) Feldmeier, C.; Bartling, H.; Magerl, K.; Gschwind, R. M. LEDIlluminated NMR Studies of Flavin-Catalyzed Photooxidations Reveal Solvent Control of the Electron-Transfer Mechanism. Angew. Chem., Int. Ed. 2015, 54 (4), 1347−1351. (51) Dadova, J.; Kummel, S.; Feldmeier, C.; Cibulkova, J.; Pazout, R.; Maixner, J.; Gschwind, R. M.; Konig, B.; Cibulka, R. Aggregation Effects in Visible-Light Flavin Photocatalysts: Synthesis, Structure, and Catalytic Activity of 10-Arylflavins. Chem. - Eur. J. 2013, 19 (3), 1066−1075. (52) Muhldorf, B.; Wolf, R. Photocatalytic benzylic C-H bond oxidation with a flavin scandium complex. Chem. Commun. 2015, 51 (40), 8425−8428. (53) Sikorska, E.; Khmelinskii, I.; Komasa, A.; Koput, J.; Ferreira, L. F. V.; Herance, J. R.; Bourdelande, J. L.; Williams, S. L.; Worrall, D. R.; Insinska-Rak, M.; Sikorski, M. Spectroscopy and photophysics of flavin related compounds: Riboflavin and iso-(6,7)-riboflavin. Chem. Phys. 2005, 314 (1−3), 239−247. (54) Fukuzumi, S.; Tanii, K.; Tanaka, T. Flavin-Sensitized PhotoOxidation of Unsaturated Fatty-Acids. J. Chem. Soc., Perkin Trans. 2 1989, No. 12, 2103−2108. (55) Saleem-Batcha, R.; Stull, F.; Sanders, J. N.; Moore, B. S.; Palfey, B. A.; Houk, K. N.; Teufel, R. Enzymatic control of dioxygen binding and functionalization of the flavin cofactor. Proc. Natl. Acad. Sci. U. S. A. 2018, 115 (19), 4909−4914. (56) Frederick, R. E.; Mayfield, J. A.; DuBois, J. L. Regulated O-2 Activation in Flavin-Dependent Monooxygenases. J. Am. Chem. Soc. 2011, 133 (32), 12338−12341. (57) Iida, H.; Imada, Y.; Murahashi, S. I. Biomimetic flavin-catalysed reactions for organic synthesis. Org. Biomol. Chem. 2015, 13 (28), 7599−7613. (58) Jensen, R. L.; Arnbjerg, J.; Ogilby, P. R. Temperature Effects on the Solvent-Dependent Deactivation of Singlet Oxygen. J. Am. Chem. Soc. 2010, 132 (23), 8098−8105. (59) Dad’ova, J.; Svobodova, E.; Sikorski, M.; Konig, B.; Cibulka, R. Photooxidation of Sulfides to Sulfoxides Mediated by Tetra-OAcetylriboflavin and Visible Light. ChemCatChem 2012, 4 (5), 620− 623. (60) Nevesely, T.; Svobodova, E.; Chudoba, J.; Sikorski, M.; Cibulka, R. Efficient Metal-Free Aerobic Photooxidation of Sulfides to Sulfoxides Mediated by a Vitamin B-2 Derivative and Visible Light. Adv. Synth. Catal. 2016, 358 (10), 1654−1663. (61) Guo, H.; Zhu, L.; Dang, C.; Zhao, J.; Dick, B. Synthesis and photophysical properties of ruthenium(II) polyimine complexes decorated with flavin. Phys. Chem. Chem. Phys. 2018, 20 (25), 17504−17516. (62) Ji, S. M.; Guo, H. M.; Wu, W. T.; Wu, W. H.; Zhao, J. Z. Ruthenium(II) Polyimine-Coumarin Dyad with Non-emissive (IL)-I3 Excited State as Sensitizer for Triplet-Triplet Annihilation Based Upconversion. Angew. Chem., Int. Ed. 2011, 50 (36), 8283−8286. (63) Ji, S. M.; Wu, W. H.; Wu, W. T.; Guo, H. M.; Zhao, J. Z. Ruthenium(II) Polyimine Complexes with a Long-Lived (IL)-I-3 Excited State or a (MLCT)-M-3/(IL)-I-3 Equilibrium: Efficient Triplet Sensitizers for Low-Power Upconversion. Angew. Chem., Int. Ed. 2011, 50 (7), 1626−1629. I


Research Article

ACS Sustainable Chemistry & Engineering (64) Sichula, V.; Kucheryavy, P.; Khatmullin, R.; Hu, Y.; Mirzakulova, E.; Vyas, S.; Manzer, S. F.; Hadad, C. M.; Glusac, K. D. Electronic Properties of N(5)-Ethyl Flavinium Ion. J. Phys. Chem. A 2010, 114 (46), 12138−12147. (65) Kasha, M. Phosphorescence and the Role of the Triplet State in the Electronic Excitation of Complex Molecules. Chem. Rev. 1947, 41 (2), 401−419. (66) Niu, Y. L.; Peng, Q. A.; Deng, C. M.; Gao, X.; Shuai, Z. G. Theory Of Excited State Decays And Optical Spectra: Application To Polyatomic Molecules. J. Phys. Chem. A 2010, 114 (30), 7817−7831. (67) Bonesi, S. M.; Manet, I.; Freccero, M.; Fagnoni, M.; Albini, A. Photosensitized Oxidation Of Sulfides: Discriminating Between The Singlet-Oxygen Mechanism And Electron Transfer Involving Superoxide Anion Or Molecular Oxygen. Chem. - Eur. J. 2006, 12 (18), 4844−4857. (68) Casado-Sanchez, A.; Gomez-Ballesteros, R.; Tato, F.; Soriano, F. J.; Pascual-Coca, G.; Cabrera, S.; Aleman, J. Pt(Ii) Coordination Complexes As Visible Light Photocatalysts For The Oxidation Of Sulfides Using Batch And Flow Processes. Chem. Commun. 2016, 52 (58), 9137−9140. (69) Bonesi, S. M.; Fagnoni, M.; Albini, A. Photosensitized Electron Transfer Oxidation Of Sulfides: Structure and medium effect. J. Sulfur Chem. 2008, 29 (3−4), 367−376. (70) Bonesi, S. M.; Protti, S.; Albini, A. Reactive Oxygen Species (ROS)-vs Peroxyl-Mediated Photosensitized Oxidation of Triphenylphosphine: A Comparative Study. J. Org. Chem. 2016, 81 (23), 11678−11685.

J


Flavin Dibromide as an Efficient Sensitizer for Photooxidation of

Recommend Documents