catalysts

Article Surface-Doped Graphitic Carbon Nitride Catalyzed Photooxidation of Olefins and Dienes: Chemical Evidence for Electron Transfer and Singlet Mechanisms

Apostolos Chatzoudis 1 , Vasileios Giannopoulos 1, Frank Hollmann 2 and Ioulia Smonou 1,* 1 Department of Chemistry, University of Crete, University Campus-Voutes, 71003 Heraklion, Crete, Greece 2 Department of Biotechnology, Biocatalysis Group, Delft University of Technology, Van der Maasweg 9, 2629HZ Delft, The Netherlands * Correspondence: [email protected]; Tel.: +30-2810545010

 Received: 2 July 2019; Accepted: 25 July 2019; Published: 27 July 2019 

Abstract: A new photocatalytic reactivity of carbon-nanodot-doped graphitic carbon nitride (CD-C3N4) with and dienes, has been disclosed. We have shown that CD-C3N4 photosensitizes the oxidation of unsaturated substrates in a variety of solvents according to two competing mechanisms: the energy transfer via (1O ) and/or the electron transfer via superoxide (O ). The 2 ·−2 singlet oxygen, derived by the CD-C3N4 photosensitized process, reacts with alkenes to form allylic hydroperoxides (ene products) whereas with dienes, endoperoxides. When the electron transfer mechanism operates, cleavage products are formed, derived from the corresponding dioxetanes. Which of the two mechanisms will prevail depends on solvent polarity and the particular substrate. The photocatalyst remains stable under the photooxidation conditions, unlike the most conventional photosensitizers, while the heterogeneous nature of CD-C3N4 overcomes usual solubility problems.

Keywords: photocatalyst; photosensitizer; graphitic carbon nitride; singlet oxygen; photooxidation mechanism; ene products

1. Introduction

Graphitic carbon nitride (g-C3N4) is recently one of the most studied heterogeneous, metal-free photocatalysts. g-C3N4 is a polymeric material, composed of highly abundant elements such as carbon and nitrogen, thermally stable and nontoxic. Ever since the synthesis of this material in bulk quantities [1–3], a large number of reports concerning the photochemical properties and photocatalytic activity were published. Next, the demand to optimize the photocatalytic properties of g-C3N4 was inevitable. The relatively low band gap energy Eg of 2.7 eV and high conduction and valence bond positions of the photocatalyst [4] prompted many research groups to improve the photocatalytic activity of g-C3N4. This was achieved by doping the surface of the catalyst with a variety of metallic elements [3,5–11], oxides [12,13], sulfides [14] even graphene [15] and carbon nanotubes [16]. It is important to note here that structure-controlled g-C3N4 is a highly efficient photocatalyst for water splitting to produce hydrogen, in an internal quantum yield of 26.5% under visible light [17]. In a more recent article [18], the metal-free carbon-nanodot-doped graphitic carbon nitride (CD-C3N4) impressively photocatalyzed the water splitting via a two-electron mechanism. Although many studies concern the photochemical activity and spectroscopic data analysis of g-C3N4 as well as modification of its surface [19], applications to organic transformations are limited. For example, graphitic C3N4 as well as its surface-doped-materials, catalyze a few reactions; a) photoacetalization of aldehydes/ketones [20] b) the Friedel-Crafts reaction of benzene [21] c) oxidation of alcohols using transition metal doped

Catalysts 2019, 9, 639; doi:10.3390/catal9080639 www.mdpi.com/journal/catalysts CatalystsCatalysts 20192019,, 99,, 639x FOR PEER REVIEW 22 of 1212

of aldehydes/ketones [20] b) the Friedel-Crafts reaction of benzene [21] c) oxidation of alcohols using g-Ctransition3N4 [22 metal–25] d)doped oxidation g-C3N of4 [22–25] sulfides d) to oxidation sulfoxides of usingsulfides oxidized to sulfoxides g-C3N using4 (CNO). oxidized In addition, g-C3N4 selective(CNO). In oxidations addition, ofselective benzylic oxidations C-H bonds of utilizingbenzylic mesoporousC-H bonds utilizing g-C3N4 with mesoporousN-OH co g-C catalysts3N4 with have N- beenOH co reported catalysts [26 have–29]. been reported [26–29]. Herein,Herein, following our recent work in this fieldfield [[30],30], wewe reportreport aa newnew approachapproach thatthat utilizesutilizes carbon-nanodot-dopedcarbon-nanodot-doped g-C g-C3N34N(CD-C4 (CD-C3N43)N as4) a visibleas a lightvisible photocatalyst light photocatalyst for the direct for photooxidation the direct ofphotooxidation alkenes via the of alkenes intermediacy via the ofintermediacy singlet oxygen of sing tolet allylic oxygen hydroperoxides to allylic hydr asoperoxides well as the as well {4 + as2} cycloadditionthe {4 + 2} of singlet oxygenof singlet to oxygen hexacyclodiene. to hexacyclodiene. We also demonstrate We also demonstrate chemically chemically that in CD-C that3N in4 photosensitizedCD-C3N4 photosensitized reactions, electronreactions, transfer electron and transfer singlet and oxygen singlet mechanisms oxygen mechanisms are in competition, are in dependingcompetition, mostly depending on the mostly nature on of the nature substrate of andthe substrate the polarity and of the the polarity solvent. of In the addition, solvent. the In CD-Caddition,3N4 themediated CD-C3N photooxygenation4 mediated photooxygenation results werecompared results were with compared those derived with those using derived conventional using singletconventional oxygen singlet photosensitizers. oxygen photosensitizers. 1 1 SingletSinglet molecularmolecular oxygen oxygen ( (O1O2,2, ∆1Δg)g) plays plays an an important important role role in chemicalin chemical [31 ],[31], biological biological [32,33 [32,33]] and therapeuticand therapeutic processes processes [34,35 ],[34,35], as well as as inwell the as degradation in the degradation of food and of materials food and [36 materials–38]. Historically, [36–38]. 1 althoughHistorically,O2 althoughwas discovered 1O2 was more discovered than 80 yearsmore agothan [39 80,40 years], and ago until [39,40], the early and 1960s until wasthe early considered 1960s towas be considered a molecular to species be a molecular of passing species astrophysical of passing interest astrophysical and rather limitedinterest as and a research rather subject.limited as The a 1 pioneeringresearch subject. work byThe Foote pioneering and Wexler work [41 by,42 Foote], provided and Wexler strong [41,42], evidence provided for the strong formation evidence of O2 foras the 1 reactiveformation intermediate of 1O2 as the in reactive photosensitized intermediate processes in photosensi in solution.tized Today,processes the in use solution. of O2 asToday, a reagent the use in organicof 1O2 as synthesis a reagent has in organic received synthesis remarkable hasattention received [remarkable43,44]. attention [43,44].

2.2. Results and Discussion In the present work, the photocatalyst g-C N was prepared according to the procedure described In the present work, the photocatalyst3 g-C4 3N4 was prepared according to the procedure bydescribed Tang and by coworkersTang and [coworkers45], while the[45], carbon while nanodotsthe carbon via nanodots thermal decompositionvia thermal decomposition of sucrose [46 of]. The so produced nanodots were deposited by heating on the g-C N surface to provide the metal-free sucrose [46]. The so produced nanodots were deposited by heating3 4 on the g-C3N4 surface to provide - carbon-nanodot-dopedthe metal-free carbon-nanodot-doped g-C3N4 (CD C g-C3N43).N4 Details (CD-C3 onN4). the Details experimental on the experi conditionsmental asconditions well as the as spectroscopicwell as the spectroscopic characterization characterization are described are in described our previous in our work previous [30]. work [30]. PhotosensitizedPhotosensitized oxidations were performed in a 4 mL pyrexpyrex cellcell containingcontaining 0.09 MM solutionsolution ofof 1 - 2,3-dimethylbut-2-ene2,3-dimethylbut-2-ene ( ()1 in) ain variety a variety of solvents of solvents with 4 mgwith/mL 4 of themg/mL insoluble of the CD insolubleC3N4 photocatalyst. CD-C3N4 Whilephotocatalyst. bubbling While oxygen bubblin at 20g◦ C,oxygen the solution at 20 °C, was the irradiated solution was for air periodradiated of for 30 a min. period A 300 of 30 W min. xenon A lamp300 W was xenon utilized lamp as was the utilized visible lightas the source. visible The light photooxidation source. The photooxidation reaction was followed reaction bywas GC followed and/or 1 byH NMRGC and/or analysis 1H after NMR filtration analysis from after celite filtration followed from by reductioncelite followed of the allylicby reduction hydroperoxides of the allylic with triphenylphosphinehydroperoxides with to triphe the correspondingnylphosphine allylicto the alcohols.corresponding allylic alcohols. WeWe reportreport herehere thethe unprecedentedunprecedented photooxygenationphotooxygenation ofof alkenesalkenes utilizingutilizing CDCD--CC33NN44 as thethe photosensitizer.photosensitizer. The electron rich tetramethylethylene (1) was chosen as the singlet oxygen acceptor, acceptor, SchemeScheme1 1.. WithinWithin aa fewfew minutesminutes ofof irradiation,irradiation, thethe allylicallylic hydroperoxidehydroperoxide 1a (see(see GCGC chromatogramschromatograms S16,S16,S17 S17 and S18) waswas formedformed followed followed by by reduction reduction with with Ph Ph3P3P to to the the corresponding corresponding allylic allylic alcohol alcohol1b (GC1b (GC chromatogram chromatogram S19). S19).

Scheme 1. CD-C3N4 photosensitized oxidation of tetramethylethylene (1). Scheme 1. CD-C3N4 photosensitized oxidation of tetramethylethylene (1). The GC and/or 1H NMR analysis of product 1b, as can be seen in S2, showed identical results with thoseThe when GC typical and/or singlet 1H NMR oxygen analysis photosensitizers, of product such1b, as as can rose be Bengal seen in (RB) S2, or showed tetraphenyl-porphyrine identical results (TPP),with those were when utilized. typical The productionsinglet oxygen of allylic photosensiti hydroperoxideszers, such by as the rose sensitized Bengal photooxygenation(RB) or tetraphenyl- of alkenes,porphyrine the so(TPP), called were “ene orutilized. Schenck” The reaction, production was studied of allylic several hydroperoxides years ago, for itsby synthetic the sensitized [43,44], mechanisticphotooxygenation [47,48] of and alkenes, theoretical the so [49 called,50] point “ene ofor view.Schenck” Furthermore, reaction, was addition studied of aseveral small amountyears ago, of for its synthetic [43,44], mechanistic [47,48] and theoretical1 [49,50] point of view. Furthermore, 1,4-diazabicyclo[2.2.2]octane (DABCO), a well-known O2 quencher [51,52], into the reaction mixture, Schemeaddition1, of retarded a small completely amount of the 1,4-diazabicyclo[2.2.2]octane production of allylic hydroperoxides. (DABCO), This a well-known is an additional 1O2 indication quencher [51,52],1 into the reaction mixture, Scheme 1, retarded completely the production of allylic of the O2 involvement as the reactive intermediate in the above-mentioned reaction.

Catalysts 2019, 9, x FOR PEER REVIEW 3 of 12 Catalysts 2019, 9, x FOR PEER REVIEW 3 of 12 hydroperoxides.Catalysts 2019, 9, 639 This is an additional indication of the 1O2 involvement as the reactive intermediate3 of 12 inhydroperoxides. the above-mentioned This is anreaction. additional indication of the 1O2 involvement as the reactive intermediate in theThe above-mentioned kinetics of the reaction.CD-C3N4 photosensitized oxygenation of 1, Scheme 1, was examined in a The kinetics of the CD-C3N4 photosensitized oxygenation of 1, Scheme1, was examined in a varietyThe of kinetics solvents. of Reactionthe CD- Cprogress3N4 photosensitized at several conversions oxygenation was of easily1, Scheme monitored 1, was by examined GC analysis. in a variety of solvents. Reaction progress at several conversions was easily monitored by GC analysis. Diglymevariety of was solvents. used asReaction the internal progress standard. at several First, conversions assuming thatwas duringeasily monitoredthe photooxygenation, by GC analysis. the Diglyme was used as the internal standard. First, assuming that during the photooxygenation, the oxygenDiglyme concentration was used as remainsthe internal constant standard. while First, there assumingis a first-order that dependenceduring the photooxygenation, of the reaction rate the on oxygen concentration remains constant while there is a first-order dependence of the reaction rate on theoxygen tetramethylethylene concentration remains (1) concentration, constant while the followingthere is a first-order equation can dependence be applied: of kt the = − reactionln (1 − x) rate where on the tetramethylethylene (1) concentration, the following equation can be applied: kt = ln (1 x) where kthe, t, tetramethylethyleneand x define the rate (1 constant,) concentration, the irradiation the following time, equation and the canconversion be applied: of tetramethylethylene, kt =− −ln (1 − x) where k, t, and x define the rate constant, the irradiation time, and the conversion of tetramethylethylene, respectively.k, t, and x define However, the rate an constant, adequate the linear irradiation correlati time,on was and not the found. conversion Therefore, of tetramethylethylene, we simply plot the respectively. However, an adequate linear correlation was not found. Therefore, we simply plot conversionrespectively. of However, tetramethylethylene an adequate (1 )linear vs the correlati illuminationon was time, not asfound. shown Therefore, in Figure we 1. The simply plot plot(Figure the the conversion of tetramethylethylene (1) vs the illumination time, as shown in Figure1. The plot 1)conversion shows that of tetramethylethylenethe rate of product formation (1) vs the illuminationdepends on time,solvent as shownpolarity, in e.g.,Figure in 1.EtOAc, The plot is favored (Figure (Figure1) shows that the rate of product formation depends on solvent polarity, e.g., in EtOAc, is compared1) shows that to CCl the4 rateby roughly of product a factor formation of two. depend s on solvent polarity, e.g., in EtOAc, is favored favored compared to CCl4 by roughly a factor of two. compared to CCl4 by roughly a factor of two.

Figure 1. Conversion % vs irradiation time of CD-C3N4 sensitized photooxidation of Figure 1. Conversion % vs irradiation time of CD-C3N4 sensitized photooxidation of tetramethylethylene. tetramethylethylene.Figure 1. Conversion % vs irradiation time of CD-C3N4 sensitized photooxidation of tetramethylethylene.Next, we examined the regioselectivity of the present oxidation system with an alkyl trisubstituted ,Next, Scheme we 2examined. For this purpose,the regioselectivity the regiospecifically of the deuteriumpresent oxidation labeled alkene system2 waswith synthesized an alkyl trisubstituted(see S3,Next, S4, S5we andalkene, examined S6) andScheme wasthe used2.regioselectivity For as this an appropriatepurpos ofe, thethe substrate regiospecifically present for oxidation sensitized deuterium photooxygenationsystem labeled with analkene usingalkyl 2 wastrisubstituted synthesized alkene, (see SchemeS3, S4, S52. Forand thisS6) purposand wase, theused regiospecifically as an appropriate deuterium substrate labeled for sensitized alkene 2 two different catalysts, one conventional photosensitizer (TPP) and the photocatalyst CD-C3N4. It is wellphotooxygenationwas establishedsynthesized that (see using double S3, twoS4, bond S5differe and formation ntS6) catalysts, and via was singlet one used oxygenconventional as an ene appropriate reaction photosensitizer will substrate occur preferentially (TPP)for sensitized and the in photocatalystphotooxygenationthe most substituted CD-C using3N side4. It oftwo is the welldiffere alkyl established substitutednt catalysts, that double one double conventional bond bond [53]. formation Indeed, photosensitizer as via seen singlet in Table(TPP) oxygen1 and(entries enethe reactionphotocatalyst will occur CD-C preferentially3N4. It is well in established the most subs thattituted double side bond of theformation alkyl substituted via singlet double oxygen bond ene 1 and 2), both conventional (TPP) and CD-C3N4 photocatalysts upon visible light irradiation show, within[53].reaction Indeed, experimental will asoccur seen preferentially in error, Table identical 1 (entries in regioselectivity.the 1 andmost 2), subs bothtituted conventional For example, side of the(TPP)2a andalkyl and2b substituted CD-Cwhich3N can4 photocatalysts double be seen bond in S7 upon[53].and S8,Indeed, visible are the lightas seen main irradiation in products Table show,1 (entries (cis ewithinffect) 1 and [experiment53 2),], whileboth conventional theal error,E-methyl identical (TPP) group regioselectivity. and is CD-C highly3N unreactive.4 photocatalysts For example, This 2aupon and visible 2b which light can irradiation be seen show,in S7 and within1 S8, experimentare the mainal productserror, identical (cis effect) regioselectivity. [53], while theFor Eexample,-methyl result is again a strong evidence of O2 involvement, excluding the involvement of other reactive 1 group2aoxygen and is2b species highly which (ROS).unreactive. can be seen This in S7 result and S8,is again are the a strong main productsevidence (ofcis effect)O2 involvement, [53], while excluding the E-methyl the involvementgroup is highly of otherunreactive. reactive This oxygen result species is again (ROS). a strong evidence of 1O2 involvement, excluding the involvement of other reactive oxygen species (ROS).

Scheme 2. Sensitized Photooxidation of 2-(1,1,1-trideuteromethyl)-5-methylhex-2-ene (2 ). Scheme 2. Sensitized Photooxidation of 2-(1,1,1-trideuteromethyl)-5-methylhex-2-ene (2). Scheme 2. Sensitized Photooxidation of 2-(1,1,1-trideuteromethyl)-5-methylhex-2-ene (2). Table 1. Photooxidation of alkyl trisubstituted alkene 2 with TPP and CD-C3N4.

Table 1. Photooxidation of alkyl trisubstituted alkene 2 with TPP and CD-C3N4. Entry Photocatalyst Regioselectivities (%) b Entry Photocatalyst Regioselectivities (%) b

Catalysts 2019, 9, 639 4 of 12

Catalysts 2019, 9, x FOR PEER REVIEW 4 of 12 Table 1. Photooxidation of alkyl trisubstituted alkene 2 with TPP and CD-C3N4. Catalysts 2019, 9, x FOR PEER REVIEW 4 of 12 Conversion Irradiation time b Regioselectivities2a 2b (%) 2c Entry Photocatalyst Conversion(%)(%) a a Irradiation(min) Time (min) Conversion Irradiation time 2a 2b 2c 2a 2b 2c 1 TPP 100a 5 56 31 13 1 TPP 100(%) (min) 5 56 31 13 2 CD-C3N4 34 30 58 29 13 21 CD-C3NTPP4 34 100 30 5 58 56 2931 13 13 a, b Determined by 1H NMR analysis. 2 CD-C3N4 a, b Determined34 by 1H NMR analysis. 30 58 29 13 1 a, b 1 The second mode of 1O2 reactivityDetermined is the by{4 +H 2}NMR cycloadd analysis.ition to conjugated dienes to yield 1 endoperoxides.The second We mode demonstrate of O reactivity here, Scheme is the 3, {4 that+ 2}CD-C cycloaddition3N4 photosensitizes to conjugated the addition dienes of to singlet yield The second mode of 1O2 reactivity is the {4 + 2} cycloaddition to conjugated dienes to yield oxygenendoperoxides. to 1,3-cyclohexadiene We demonstrate (3) to here, produce Scheme efficiently3, that CD-CendoperoxideN photosensitizes 3a (Figure 2, theS9) additionin ambient of endoperoxides. We demonstrate here, Scheme 3, that CD-C3N4 photosensitizes3 4 the addition of singlet conditions. conditions.oxygensinglet oxygento 1,3-cyclohexadiene to 1,3-cyclohexadiene (3) to produce (3) to produceefficiently effi endoperoxideciently endoperoxide 3a (Figure3a 2,(Figure S9) in2 ,ambient S9) in conditions.ambient conditions. hv O CD-C3N4 / O2 O hv O CDCl3 CD-C3N4 / Oo2 O 30 min 0 oC 33aCDCl3 30 min 0 oC 33a Scheme 3. Photooxidation of 1,3-cyclohexadiene in CDCl3 with CD-C 3N4 as photocatalyst. Scheme 3. Photooxidation of 1,3-cyclohexadiene in CDCl3 with CD-C3N4 as photocatalyst. Scheme 3. Photooxidation of 1,3-cyclohexadiene in CDCl3 with CD-C3N4 as photocatalyst.

1 Figure 2. 1H-NMRH-NMR spectrum spectrum of of 3a3a..

1 Furthermore, apart from oxygenFigure photoactivation, 2. H-NMR spectrum the CD of-C 3a3N. 4 catalyst was tested for promoting Furthermore, apart from oxygen photoactivation, the CD-C3N4 catalyst was tested for promoting an electron transfer process. The proposed two mechanisms for the sensitized photooxygenation of an electron transfer process. The proposed two mechanisms for the sensitized photooxygenation of Furthermore, apart from oxygen photoactivation, the CD-C3N4 catalyst was tested for promoting organic substrates were initially classified classified by by Gollnick Gollnick [54] [54] as as Type Type I and Type II as shown below in organican electron substrates transfer were process. initially The classified proposed by two Gollnick mechanisms [54] as for Type the I sensitizedand Type photooxygenationII as shown below ofin Scheme 44.. Schemeorganic substrates4. were initially classified by Gollnick [54] as Type I and Type II as shown below in 3 Scheme 4. Type I O2 Radicals 2 Oxygenated Type I 3O products Radicals 2 Oxygenated hv ISC Substrate products Sens 1Sens 3Sens hv ISC Substrate _ Sens 1Sens 3Sens . Type II O2 by electron transfer Type II 2_ . O Type II 1O2 byby electronenergy transfer transfer O2 by energy transfer 1O by energy transfer SchemeScheme 4. 4. TypeType I I and and Type Type II phot photosensitizedosensitized mechanisms.2

Scheme 4. Type I and Type II photosensitized mechanisms.1 The two mechanisms, Type I and Type II as well as singlet oxygen (1O2) and superoxide (O·̄2̄ ) formation, are always in competition. Which of them will prevail depends on the nature of the The two mechanisms, Type I and Type II as well as singlet oxygen (1O2) and superoxide (O·̄2) formation, are always in competition. Which of them will prevail depends on the nature of the

Catalysts 2019, 9, 639 5 of 12

The two mechanisms, Type I and Type II as well as singlet oxygen (1O ) and superoxide (O ) 2 ·−2 Catalystsformation, 2019, are9, x FOR always PEER inREVIEW competition. Which of them will prevail depends on the nature5 of of the 12 photocatalyst, the solvent, the substrate’s nature and the concentration. In Type II mechanism photocatalyst, the solvent, the substrate’s nature and the concentration. In Type II mechanism singlet singlet oxygen is produced by energy transfer whereas superoxide anion (O −2) by electron oxygen is produced by energy transfer whereas superoxide anion (O·̄2) by electron· transfer transfer mechanism. To check the electron transfer ability of CD-C3N4 catalyst, the electron rich mechanism. To check the electron transfer ability of CD-C3N4 catalyst, the electron rich 2-(4-1 2-(4-methoxyphenyl)-3-methylbut-2-ene (4) was prepared (see S11). This substrate apart from O2 1 methoxyphenyl)-3-methylbut-2-eneacceptor is also a good electron donor. (4) Thewas resultsprepared from (see the S11). irradiation This substrate of aryl apart alkene from4 in O a2 variety acceptor of issolvents also a good and sensitizers, electron donor. Scheme The5 ,resu are shownlts from in the Table irradiation2. of aryl alkene 4 in a variety of solvents and sensitizers, Scheme 5, are shown in Table 2.

Scheme 5. SensitizedSensitized photooxygenation photooxygenation of aryl alkene 4.. Table 2. Regioselectivities and activities of various sensitizers in the photooxygenation of 4. Table 2. Regioselectivities and activities of various sensitizers in the photooxygenation of 4. Entry Photocatalyst Solvents 4a % a 4b % b 4c % c Conversion % a b c Entry Photocatalyst Solvents 4a % 4b % 4c % Conversion % d 1 TPP CCl4 76 24 - 100 d d 21 RBTPP CDCl3 CCl4 7076 24 30- -100 100 d 32 MBRB CH3CNCDCl3 6670 30 34- -100d 100 d 4 DCA CD3CN 61 27 12 87 3 MB CH3CN 66 34 - 100d 5 CD-C3N4 CDCl3 53 31 16 80 64 CD-C 3N4 DCA CD3CNCD3CN 5861 27 2712 1587d 70 7 CD-C3N4 Benzene 53 31 16 67 5 CD-C3N4 CDCl3 53 31 16 80 8 CD-C3N4 EtOAc 53 29 18 65 96 CD-C 3NCD4 -C3N4EtOH CD3CN -58 27 -15 10070 100 e 10 CD-C3N4 CDCl3 ---- 7 CD-C3N4 Benzene 53 31 16 67 f 11 no catalyst CDCl3 traces traces - <1 a, b c 1 d ene reaction8 products;CD-Celectron3N4 transferEtOAc products determined53 by29H-NMR 18 analysis; 5 min65 irradiation at 0 ◦C; e in the dark for 3 h, determined by 1H NMR and GC analysis; f irradiated without any photocatalyst. Samples after 0.5 h, 1.0 h and9 2 h wereCD analyzed-C3N4 by 1H NMREtOH and GC analysis.- - 100 100 10 CD-C3N4 CDCl3 - - - - e - For comparison11 reasons,no catalyst apart fromCDCl CD3 C3Ntraces4, four sensitizerstraces were- utilized<1 in f the above reaction (entries 1–4). Rose Bengal (entry 2, RB), TPP (entry 1) and methylene blue (entry 3, MB) are well studied a, b ene reaction products; c electron transfer products determined by 1H-NMR analysis; d 5 min singlet oxygen photosensitizers. However, 9,10-dicyano-anthracene (entry 4, DCA), an electron poor irradiation at 0 °C; e in the dark for 3 h, determined by 1H NMR and GC analysis; f irradiated without compound,any photocatalyst. sensitizes Samples the photooxygenation after 0.5 h, 1.0 h byand both 2 h were singlet analyzed oxygen by and 1H NMR electron and transfer GC analysis. mechanisms depending on the particular substrate and the polarity of the solvent [55]. ForIrradiation comparison of 4 reasons,in the presenceapart from of CD TPP,-C3N RB4, four or MBsensitizers as photosensitizers were utilized in aff theorded above the reaction allylic (entrieshydroperoxides 1–4). Rose4a Bengaland 4b (entryin a ratio 2, RB), of approximately TPP (entry 1) 7:3. and These methylene hydroperoxides blue (entry are 3, typicalMB) are singlet well studiedoxygen productssinglet oxygen whose ratiophotosensi 7:3 istizers. independent However, of solvent 9,10-dicyano-a polaritynthracene [47,56] or the(entry para-substitution 4, DCA), an electronof the phenyl poor ring.compound, However, sensitizes the DCA-photosensitized the photooxygenation oxygenation by both singlet of 4 in oxygen acetonitrile and (entryelectron 4), transferafforded mechanisms apart from thedepending ene products on the4a particularand 4b considerable substrate and amount the polarity of p-methoxy-acetophenone of the solvent [55]. (4c).Irradiation In this case, of the 4 resultsin the arepresence supported of TPP, by both RB electronor MB as transfer photosensitizers and singlet oxygenafforded mechanisms. the allylic hydroperoxides 4a and 4b in a ratio of approximately 7:3. These hydroperoxides are typical singlet Similarly, in the CD-C3N4 catalyzed photooxidation of 4 in non protic solvents, apart from the oxygenallylic hydroperoxides products whose4a ratioand 7:34b is, considerableindependent amountsof solvent of polarity4c in the [47,56] range or of the 15–18% para-substitution were obtained. of theThese phenyl results ring. are alsoHowever, similar the with DCA-photosensitized those derived earlier oxygenation from DCA photosensitized of 4 in acetonitrile oxygenation (entry 4), of affordeddiphenyl-ethylene apart from [55 the]. p-Methoxy-acetophenoneene products 4a and 4b considerable4c is most probably amount produced of p-methoxy-acetophenone from the cleavage of (4c). In this case, the results are supported by both electron transfer and singlet oxygen mechanisms. dioxetane 4d via the intermediacy of superoxide anion (O −2) through an electron transfer mechanism, Similarly, in the CD-C3N4 catalyzed photooxidation of 4 in· non protic solvents, apart from the allylic as shown in Scheme6. The formation of superoxide anion O −2 and not a hydroxyl has been hydroperoxides 4a and 4b, considerable amounts of 4c in the· range of 15–18% were obtained. These also recently documented [20]. We must note here that control experiments either with the CD-C3N4 resultsin dark are (entry also 10) similar or without with the those photocatalyst derived ea underrlier irradiationfrom DCA conditions photosensitized (entry 11)oxygenation did not show of diphenyl-ethyleneany detectable oxidation [55]. p-Methoxy-acetophenone products (entry 10) or small 4c is tracesmost probably observed produced by 1H NMR from and the GC cleavage analysis of dioxetane 4d via the intermediacy of superoxide anion (O·2̄ ) through an electron transfer mechanism, as shown in Scheme 6. The formation of superoxide anion O·2̄ and not a hydroxyl radical has been also recently documented [20]. We must note here that control experiments either with the CD-C3N4 in dark (entry 10) or without the photocatalyst under irradiation conditions (entry 11) did not show any detectable oxidation products (entry 10) or small traces observed by 1H NMR and GC analysis

Catalysts 2019, 9, 639 6 of 12

Catalysts 2019, 9, x FOR PEER REVIEW 6 of 12 (entry 11). It is interesting to note here that when this reaction was run in EtOH as the solvent (entry 9, Table(entry2 ),11). the It cleavage is interesting product to note4c was here exclusively that when this obtained reaction in 100% was run conversion. in EtOH Thisas the result solvent indicates (entry that9, Table under 2), thesethe cleavage conditions, product electron 4c was transfer exclusively is the obtained only operating in 100% mechanism conversion. (Scheme This result6). However, indicates that under these conditions, electron transfer is the only operating mechanism (Scheme 6). However, itthat is notunder clear these at the conditions, present why electron the irradiationtransfer is the of CDonly-C operating3N4 surface mechanism in EtOH, promotes(Scheme 6). the However, electron it is not clear at the present why the irradiation of CD-C3N4 surface in EtOH, promotes the electron transferit is not clear mechanism at the present leading why exclusively the irradiation to cleavage of CD products-C3N4 surface4c, (entry in EtOH, 9, Table promotes2). Most the probably, electron amongtransfer some mechanism other unknown leading exclusively at the time reasons,to cleavage the polarproducts protic 4c, solvent (entry ethanol9, Table stabilizes2). Most probably, favorably theamong pair some of radical other ionsunknown (anion at+ thecation) time favoringreasons, the dramatically polar protic the solvent electron ethanol transfer stabilizes path producing favorably the pair of radical ions (anion + cation) favoring dramatically the electron transfer path producing exclusivelythe pair of radical the superoxide ions (anion anion + cation) (O −2) asfavoring the reactive dramatically intermediate. the electron transfer path producing exclusively the superoxide anion (O·· 2̄ ) as the reactive intermediate. exclusively the superoxide anion (O·2̄ ) as the reactive intermediate.

Scheme 6. CD-C3N4 photosensitized oxidation of alkene 4 by electron transfer mechanism. Scheme 6. CD-C3N4 photosensitized oxidation of alkene 4 by electron transfer mechanism. Scheme 6. CD-C3N4 photosensitized oxidation of alkene 4 by electron transfer mechanism.

To assess further the extent of electron transfer efficiency of CD-C3N4 surface in a polar and To assess further the extent of electron transfer efficiency of CD-C3N4 surface in a polar and non non proticTo assess solvent, further the the photooxidation extent of electron of 1,1-di( transferp-anisyl)ethylene efficiency of CD (5)-C was3N4 performed surface in a (Scheme polar and7). non An protic solvent, the photooxidation of 1,1-di(p-anisyl)ethylene (5) was performed (Scheme 7). An electronprotic solvent, transfer the test photooxidation can be probed byof utilizing1,1-di(p-anisyl)ethylene substrate 5; an electron (5) was rich performed alkene, not(Scheme reactive 7). with An electron transfer test can be probed by utilizing substrate 5; an electron rich alkene, not reactive with singlet oxygen. Scheme7 shows the CD -C3N4 sensitized photooxygenation of 5 in acetonitrile. singlet oxygen. Scheme 7 shows the CD-C3N4 sensitized photooxygenation of 5 in acetonitrile. singlet oxygen. Scheme 7 shows the CD-C3N4 sensitized photooxygenation of 5 in acetonitrile.

SchemeScheme 7.7. Photooxidation ofof 55 withwith CD-CCD-C33N4 asas photocatalyst. photocatalyst. Scheme 7. Photooxidation of 5 with CD-C3N4 as photocatalyst. This reaction afforded exclusively and quantitatively the cyclic peroxide 5a (Figure3, S15), by This reaction afforded exclusively and quantitatively the cyclic peroxide 5a (Figure 3, S15), by cycloaddition of photochemically produced two radical cations of 5 and one super oxide anion. cycloaddition of photochemically produced two radical cations of 5 and one super oxide anion. Similar results under the same conditions have been reported previously [57,58], when DCA was the Similar results under the same conditions have been reported previously [57,58], when DCA was the photosensitizer. Furthermore, upon addition of a small quantity of 1,3,5-trimethoxybenzene, the above photosensitizer. Furthermore, upon addition of a small quantity of 1,3,5-trimethoxybenzene, the reaction, Scheme7, was retarded. This result supports an electron transfer mechanism considering that above reaction, Scheme 7, was retarded. This result supports an electron transfer mechanism the lower oxidation potential of a donor molecule (1,3,5-trimethoxybenzene) than the corresponding considering that the lower oxidation potential of a donor molecule (1,3,5-trimethoxybenzene) than potential of the competing electron donor alkene, [55], quenches the electron transfer pathway from the corresponding potential of the competing electron donor alkene, [55], quenches the electron the alkene to the sensitizer. Similar mechanism was published earlier by Ericksen and Foote [55], transfer pathway from the alkene to the sensitizer. Similar mechanism was published earlier by when in their case, electron deficient DCA was used as the photocatalyst and 1,1-diphenylethylene or Ericksen and Foote [55], when in their case, electron deficient DCA was used as the photocatalyst and tetramethylethylene were used as the substrates. 1,1-diphenylethylene or tetramethylethylene were used as the substrates.

Catalysts 2019, 9, x FOR PEER REVIEW 7 of 12

Catalysts 2019, 9, 639 7 of 12

Figure 3. 1H-NMR spectrum of 5a.

3. Materials and Methods Figure 3. 1H-NMR spectrum of 5a.

3.1.3. Materials General and Methods

3.1. GeneralAll solvents and tetramethylethylene (1) (2,3-dimethylbut-2-ene), 4-bromoanisole, 3-methyl-2-butanone, 1,3-cyclohexadiene (3) were purchased from Sigma-Aldrich (Munich- Germany)All solvents in the highest and tetramethylethylene purity and were used (1 without) (2,3-dimethylbut-2-ene), any purification. In 4-bromoanisole, addition, the photosensizers 3-methyl-2- 5,butanone, 10, 15, 20-tetraphenyl-21H,23H-porphyrine 1,3-cyclohexadiene (3) were purchased (TPP), from 9,10-dicyano-anthracene Sigma-Aldrich (Munich- (DCA), Germany) methylene in blue the (MB)highest and purity Rose and Bengal were (RB) used were withou purchasedt any purification. from Sigma-Aldrich. In addition, the photosensizers 5, 10, 15, 20- tetraphenyl-21H,23H-porphyColumn chromatographicrine separations (TPP), 9,10-dicyano-anthracene were carried out by a flash (DCA), chromatography methylene blue system (MB) using and silicaRose Bengal gel and (RB) hexane were/ethyl purchased acetate from or petroleum Sigma-Aldrich. ether/ethyl acetate solvent mixtures. For thin layer Column chromatographic separations were carried out by a flash chromatography system using chromatography (TLC), Merck silica gel (grade 60 F245, Merck & Co., Kenilworth, NJ, USA) was used. silicaThe gel and progress hexane/ethyl of the acetate photooxidation or petroleum reactions ether/ethyl was acetate monitored solvent by mixtures. gas chromatography For thin layer usingchromatography SHIMADZU (TLC), GC-2014 Merck silica gas gel chromatograph (grade 60 F245, Merck FID & detector, Co., Kenilworth, (HP-5 NJ, capillary USA) was column used. 30 mThe0.32 progress mm 0.25 of µthem, photooxidation 5% diphenyl and reactions 95% dimethylpolysiloxane) was monitored by and gas by chromatography1H-NMR. NMR spectra using × × wereSHIMADZU recorded atGC-2014 room temperature gas chromatograph on Bruker DPX-300 FID and Brukerdetector, Avance (HP-5 series 500.capillary Chemical column shifts 30m×0.32mm×0.25μm, 5% diphenyl and 95% dimethylpolysiloxane) and by13 1H-NMR. NMR spectra (δ) are reported in ppm relative to the residual solvent peak (CDCl3, δ: 7.26, CDCl3, δ: 77.0, CD3CN, δwere: 1.94), recorded and the at multiplicity room temperature of each signal on Bruker is designated DPX-300 by and the Bruker following Avance abbreviations: series 500. s, singlet;Chemical d, doublet;shifts (δ) t,are triplet; reported q, quartet; in ppm m, relative multiplet; to the br, broad.residual Coupling solvent constantspeak (CDCl (J)3 are, δ: quoted7.26, 13CDCl in Hz.3, δ: 77.0, CD3CN, δ: 1.94), and the multiplicity of each signal is designated by the following abbreviations: s, 3.2.singlet; General d, doublet; Photooxidation t, triplet; Method q, quartet; m, multiplet; br, broad. Coupling constants (J) are quoted in Hz. In a suitable 4 mL pyrex cell were initially added 0.5–2.0 mL of the solvent and the CD-g-C3N4 catalyst in concentration 4 mg mL 1. Next, the compound, which will be oxidized was added, using a 3.2. General Photooxidation Method − 25 µL syringe. The corresponding amounts were: 93 mM of tetramethylethylene (1), 12 10 3 mM × − of (E)-2-(1,1,1-trideuteromethyl)-5-methylhex-2-eneIn a suitable 4 mL pyrex cell were initially added (alkene 0.5–2.02), 130mL mMof the of solvent 1,3-cyclohexadiene and the CD-g-C (3) and3N4 110catalyst mM in of 1-methoxy-4-(3-methylbut-2-en-2-yl)benzeneconcentration 4 mg mL−1. Next, the compound, (aryl which alkene will4). be Also, oxidized the photosensitizers was added, using TTP, MB,a 25 DCAμL syringe. or RB were The corresponding used for control amounts experiments were: in 93 catalytic mM of amounts tetramethylethylene (0.5–1.0 mg). In(1), the 12x10 experiments−3 mM of which(E)-2-(1,1,1-trideuteromethyl)- include the quenchers; 1,4-diazabicyclo[2.2.2]octane5-methylhex-2-ene (alkene 2 (DABCO)), 130 mM or of 1,3,5-trimethoxybenzene, 1,3-cyclohexadiene (3) theyand were110 mM added of 1-methoxy-4-(3-methylbut-2-en-2-yl)benzene in the amount of 1% mole in proportion to the oxidizing(aryl alkene substance. 4). Also, The the test photosensitizers tube was then placedTTP, MB, into DCA an ice or bath RB inwere a pyrex used receptacle, for control at experiments a distance of approximatelyin catalytic amounts 10 cm from(0.5–1.0 the visiblemg). In light the sourceexperiments (variable which intensity include Xenon the bulb, quenchers; Cermax 3001,4-diazabicyclo[2.2.2]octane W). Oxygen was transferred (DABCO) inside the or test 1,3,5- tube bytrimethoxybenzene, a syringe undergentle they were andcontinuous added in the flow amount (gentle of bubbling). 1% mole The in proportion reaction was to usually the oxidizing left for 30substance. min., or 60The min. test Attube the was end, then the carbon placed nitride into an catalyst ice bath was in removed a pyrex byreceptacle, simple filtration at a distance through of

Catalysts 2019, 9, 639 8 of 12 cotton cloth and celite. In the cases where deuterated solvents were used, they were not evaporated, and the conversion was determined by 1H NMR analysis. In all the other cases with non-deuterated solvents, either they were evaporated under reduced pressure and the conversion was determined 1 by H NMR analysis using CDCl3 or with aprotic solvent the reaction progress was monitored by gas chromatography.

3.3. Synthesis of 2-(4-methoxyphenyl)-3-methylbut-2-ene (4) Synthesis of 2-(4-methoxyphenyl)-3-methylbutan-2-ol • Under dry nitrogen atmosphere, 220 mg of Mg turnings (9.00 mmol) were dissolved in dry diethyl ether (10 mL). Then, 0.8 mL (6.50 mmol) of 4-bromoanisole were initially dissolved in extra dry diethyl ether (4 mL) and the solution was added dropwise, while the mixture was stirred. In order to initiate the reaction, 1 granule of iodine was added to the mixture. The reaction was then heated at 35 C (reflux) for about 45 min. The reaction was then cooled to room temperature and ◦ 3-methyl-2-butanone (0.7 mL, 6.50 mmol) was added dropwise. After the addition of the ketone, the reaction was heated again to reflux (35 ◦C) for 30 min. The reaction mixture was quenched with distilled water (20 mL) and after addition of saturated NH4Cl (4 mL) the mixture was extracted with diethyl ether (2 15 mL). The combined organic layers were dried over MgSO and evaporated to × 4 dryness. Pure 2-(4-methoxyphenyl)-3-methylbutan-2-ol was obtained after silica gel chromatography 1 (petroleum ether/EtOAc v/v, 10/1), 250 mg. H NMR (CDCl3, 300 MHz) δ: 7.33 (d, 2H, J = 8.8 Hz), 6.86 (d, 2H, J = 8.8 Hz), 3.80 (s, 3H), 1.99 (m, 1H), 0.86 (d, 3H, J = 6.8 Hz), 0.82 (d, 3H, J = 6.8 Hz). Synthesis of aryl alkene 4 • 2-(4-methoxyphenyl)-3-methylbutan-2-ol (250 mg, 1.03 mmol) was heated neat with catalytic amount of I2 (1/2 granule) at 120 ◦C for 1 minute. Then, the reaction was cooled at room temperature and then extracted with diethyl ether (3 10 mL), distilled water (10 mL) and saturated Na S O × 2 2 3 (4 mL). The organic layers were dried over MgSO4 and the solvents were evaporated to dryness. Pure 2-(4-methoxyphenyl)-3-methylbut-2-ene was obtained after brief purification by a short pad of silica 1 with Hexane/EtOAc v/v, 50/1 with 20% overall yield (230 mg). H NMR (CDCl3, 300 MHz) δ: 7.05 (d, 2H, J = 8.7 Hz), 6.84 (d, 2H, J = 8.7 Hz), 3.81 (s, 3H), 1.94 (s, 3H), 1.80 (s, 3H), 1.60 (s, 3H). 13C NMR (CDCl3, 500 MHz) δ: 157.54, 137.68, 132.22, 129.40, 126.94, 113.26, 55.18, 22.07, 20.87, 20.57.

3.4. Synthesis of 2-(1,1,1-trideuteromethyl)-5-methylhex-2-ene (2) Alkene 2 was prepared (synthesis of similar deuterated alkenes have been reported [59]) by the following method: Wittig coupling of the stabilized ylide methyl(triphenylphosphoranylidene)propionate (1.64 g, 4.7 mmol) with 2-methylbutanal (0.39 g, 4.5 mmol) in dry CH2Cl2 (10 mL) at RT overnight, gave the E configuration of the corresponding ester in 94–96% isomeric purity by 1H NMR analysis. The isolated yield was 80% (0.56 g). 1H NMR (500 MHz, CDCl3) δ: 0.89 (d, J = 7.0 Hz), 1,72 (m, 1H), 1.75 (s, 3H), 2.02 (m, 2H), 3.69, (s, 3H), 6.76 (m, 1H). Reduction of the above ester (0.7 g, 4.4 mmol) with LiAlD4 (0.126 g, 3 mmol) in diethyl ether, at 0 oC, gave the corresponding dideuterio allylic alcohol, in 90% yield (0.52 g). 1H NMR (500 MHz, CDCl3) δ: 0.89 (d, J = 6.5 Hz, 6H), 1.65 (m, 1H), 1.66 (s, 3H), 1.91 (t, J = 7.0 Hz, 2H), 5.43 (td, J = 7.0, 1.5 Hz, 1H). Next, this allylic alcohol (0.5 g, 3.8 mmol) was brominated with Br2 (0.76 g, 4.75 mmol) and Ph3P (1.19 g, 4.55 mmol) in 10 mL of dry dichloromethane. The isolated yield of the allylic bromide was 1 50.6% (0.372 g, 1.92 mmol). H NMR (500 MHz, CDCl3) δ: 0.89 (d, J = 6.5 Hz, 6H), 1.65 (m, 1H), 1.74 (s, 3H), 1.91 (t, J = 7.5 Hz, 2H), 5.62 (td, J = 7.5 Hz, 1.5 Hz, 1H). Finally, reduction of the above allylic bromide (295 mg, 1.52 mmol) in dry diglyme (2mL) with LiAlD4 (23.8 mg, 0.57 mmol) in diglyme (2 mL) was left at RT overnight, followed by the usual quench with two drops of water. Finally, the reaction mixture was heated at 150 ◦C, for 1.5 h, and 75 mg of the alkene (E)-2 were distilled from diglyme. Catalysts 2019, 9, 639 9 of 12

1 Compound (E)-2: H NMR (500 MHz, CDCl3,) δ: 0.86 (d, J = 11.0 Hz, 6H), 1.55 (m, 1H), 1.56 (s, 3H), 1.85 (t, J = 7.0 Hz, 2H), 5.13 (t, J = 12.0 Hz, 1H).

3.5. Synthesis of 1,1-di(p-anisyl)ethylene (5) Alkene 5 was prepared by the following method: in a solution of bis(4-methoxyphenyl)methanone (0.87 g, 3.6 mmol) in dry THF (40 mL), MeLi (2 mL) was added at 0 ◦C. After 24 h the reaction mixture was quenched with a few drops of H2O. The reaction mixture was acidified with 3N HCl, and washed with NH4Cl and H2O solutions. After drying the organic layer, and evaporation of the solvent, 0.76 g 1 (87 %) of alkene 5 was isolated as a solid. H NMR (500 MHz, CDCl3,) δ: 3.83 (s, 6H), 5.29 (s, 2H), 6.87 (d, J = 9 Hz, 4H), 7.28 (d, J = 9 Hz, 4H).

4. Conclusions

In conclusion, we have shown that CD-C3N4 photocatalyst sensitizes the photooxidation of unsaturated compounds. With simple alkenes and dienes, the singlet oxygen mechanism operates efficiently producing the allylic hydroperoxides and endoperoxides respectively. In the case of aryl 1 alkenes, efficient electron donor molecules, both O2 and/or electron transfer mechanisms can be achieved. Which of these competing mechanisms will prevail depends on the nature of substrate and the protic-non protic solvents. Finally, the easy modification of graphitic carbon nitride to bulk amounts of CD-C3N4 shows the following advantages: a) photocatalyzes the oxidation of a variety of alkenes and dienes b) the reaction is heterogeneous, a simple celite filtration removes the catalyst which can be reused at least three times c) stability of the catalyst under the photooxidation conditions unlike most of the conventional photosensitizers and d) the heterogeneous nature of the present photocatalyst overcomes solubility problems.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/8/639/s1, 1H NMR spectra of compounds 1a and 1b: S2, 1H NMR spectrum of E- ester, precursor of alkene 2: S3, 1H NMR 1 spectrum of allylic alcohol-d2, precursor of alkene 2: S4, H NMR spectrum of allylic bromide-d2 precursor of alkene 2: S5, 1H NMR spectrum of alkene 2: S6, 1H NMR spectrum o 2a, 2b, 2c (TPP as the photosensitizer): S7, 1H 1 13 1 NMR spectrum of 2a, 2b, 2c (CD-C3N4 as the photocatalyst): S8, H and C NMR spectra of compound 3a: S9, H NMR spectrum of 2-(4-methoxyphenyl)-3-methylbutan-2-ol, precursor of alkene 4: S10, 1H and 13C NMR spectra 1 of aryl alkene 4: S11, H NMR (CD3CN) spectrum of 4a, 4b, 4c, products from the photooxidation of alkene 4: S12, 1 1 H NMR (CDCl3) spectrum of 4a, 4b, 4c, products from the photooxidation of alkene 4: S13, H NMR spectrum of alkene 5: S14, 1H and 13C NMR spectra of 5a after photooxidation of alkene 5: S15, GC Chromatogram of compound 1a in Acetonitrile: S16, GC Chromatogram of compound 1a in Ethyl acetate: S17, GC Chromatogram of compound 1a in Chloroform-d: S17, GC Chromatogram of compound 1a in Hexane: S18, GC Chromatogram of compound 1a in Carbon tetrachloride: S18, GC Chromatogram of compound 1b in Dimethyl sulfoxide: S19. Author Contributions: Conceptualization, I.S.; methodology, I.S., A.C., V.G.; investigation, A.C, V.G.; resources, I.S., F.H.; data curation, I.S, F.H. A.C. V.G.; writing—original draft preparation, IS., A.C.; writing—review and editing, I.S., F.H., A.C.; supervision, I.S.; project administration, I.S.; funding acquisition, I.S. Funding: This work was supported by the Special Account for Research Funds of University of Crete (KA 39432, SARF UoC). V. Giannopoulos, acknowledges the support by a scholarship of “State Scholarships Foundation” of the Operational Program for the Human Resources Development, Education and Lifelong Learning 2014-2020. Acknowledgments: We thank M. Orfanopoulos for his valuable comments and discussions. Wuyuan Zhang is acknowledged for providing useful information. Conflicts of Interest: “The authors declare no conflict of interest.” “The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results”.

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