Photocatalytic hydrogen production from acidic aqueous solution in BODIPY-cobaloxime-ascorbic acid homogeneous system

An Xie,Zhong-Hua Pan,Miao Yu,Geng-Geng Luo,*,Di Sun*

a School of Materials Science and Engineering,Xiamen University of Technology,Xiamen 361024,China

b School of Chemistry and Chemical Engineering,Shandong University,Ji ’nan 250100,China

cFujian Key Laboratory of Photoelectric Functional Materials,College of Materials Science and Engineering,Huaqiao University,Xiamen 361021,China

Key words:BODIPY dyes Photocatalytic H2 production Photosensitizer Cobaloxime Electron donor

ABSTRACT Iodinated boron dipyrromethene(BODIPY)dyes with 8-hydroxyl-quinoline or phenylamine moiety at the meso-position on the BODIPY core were used as efficient photosensitizers(PSs)of three-component light-driven production of H2 systemfrom acidic aqueous solution in conjunction with a cobaloxime[Co III(dmgH)2Py Cl](dmgH=dimethylglyoximate,and py=pyridine)as proton-reducing catalyst and ascorbic acid(H2A)as sacri fi cial electron donor.This is the fi rst example of BODIPYs as homogeneous hydrogen-generating PSs employed in the acidic aqueous conditions.That they are active in the acidic solutions and inactive in the basic conditions may indicate that the extent of competition between intramolecular and intermolecular electron transfer reactions exists.efficient bimolecular electron transferreaction between PS and molecular catalyst is needed to make H2 production,while the intramolecular electron transfer of PSmay curb H2 production.The resultsunderscore that the chemical modification of BODIPYs can be performed,thus allow ing for the transformation of acid and base conditions for the light-driven H2 production.

he splitting of w ater via Artificial photosynthesis(AP)is currently attracting immense attention from chemists because of the signi fi cance of the conversion of solar energy into chemically stored energy in the form of clean fuel hydrogen(H2)[1,2].In general,an efficient AP system for photochemical reduction of protons to H2,contains a photosensitizer(PS)for harvesting light energy and transforming it into electrochemical energy,and a proton-reducing catalyst as electron acceptor and catalyzing the proton reduction,and a sacri fi cial electron donor to allow the reductive half-reaction to be studied separately.As light harvesters desirable for AP systems,noble-metal-based complexes are most w idely employed.However,these complexes are limited by the high cost and low supply of noble metals.As an alternative to noble-metal complexes,organic dyes show potential candidatesas PSs due to their less extensive cost and readily adjustable structure and chemical properties[3,4].Among a few organic dyes,BODIPY(boron dipyrromethene,IUPAC name:4,4-di fl uoro-4-bora-3a,4a-diaza-s-indacene)compounds have been tested as cheap alternative light harvesters for light-driven proton reduction due to their high extinction coefficients and possibilities for more sophisticated PSs design[5–13].

However,similar to xanthenes dyes[14],these reported BODIPY compounds as PSs for light-driven hydrogen evolution in combination with the catalyst[CoIII(dmgH)2Py Cl](dmgH=dimethylglyoximate,and py=pyridine)and electron donors triethanolamine(TEOA)or triethylamine(TEA)are usually p erformed in the basic conditions[5–9,15–17].We w onder w hether BODIPY chromophores can be carried out in the acidic solutions only by modifying their substitution patterns.To this end,in the present work,BODIPY dyes having 8-hydroxylquinoline or phenylamine moiety at the meso-position on the BODIPY core were p repared.The d etailed m olecular structures of dyes are displayed in Fig.1.These BODIPYdyes were tested for investigating the structure-and-property relationship between the BODIPY structure and activity of visible-light-driven H2production as well as solution p H.

Fig.1.Structures of BODIPY PSs(B1–B6)and catalyst C1 employed in this study.

There BODIPYdyes were synthesized according to the[13_TD DIFF]previous literature procedures[18–23].The photophysical properties of the BODIPYs werefi rstly measured in deaerated dry acetontrile(MeCN)via UV–vis absorption and steady-state fluorescence spectroscopy(Fig.S1 in Supporting information),and the corresponding data are summarized in Table 1.As show n in Fig.S1a,the absorption spectrum of B1 with a group of 8-hydroxylquinoline-5-yl at themeso-position on the BODIPY core is of comparable shape as those of described BODIPY dyes[5–10],having a strong S0!S1(p!p*)transition centered at 501 nm and a weaker,broad S0!S2band centered at 370 nm.Upon introduction of iodine atomsat the pyrrole carbonsof the BODIPYcore in B2 showed S0!S1transition which experienced a red-shift of?36 nm compared to that of B1.The emission spectra of B1 and B2 were mirror images of the absorbance spectra with small Stokes’shifts in the range of 760–580 cm?1,implying there is little change in geometry or polarity between ground and excited states.The fluorescence quantum yield(Ffl)of B1 in deoxygenated acetonitrile solution at room temperature was found to be?0.30.In comparison,iodinated B2 showed substantial reduction of fluorescence quantum yield(Ffl=0.007).A signi fi cant decrease of Fflindicates that the presence of heavy atom Iw ill facilitate an efficient intersystem crossing ef fi ciency from the lowest singlet excited state to the triplet states,resulting in population of the longer lived triplet excited states from which electron transfer occurs.As show n in Fig.S1b,replacement of the 8-hydroxylquinoline substituent with a phenylamine moiety on the meso place on the BODIPYcore does not significantly alter the absorption and emission maxima of the dyes.Similarly,the addition of iodo atoms results in absorption and emission energies that are shifted to lower energy.Notably,iodinated B5 augmented the absorption compared with iodinated B2(e=86800 Lmol?1cm?1for B5 vs.e=79800 Lmol?1cm?1for B2).

The oxidation and reduction potentials of BODIPYs(B1-B2 and B5-B6)were further studied by cyclic voltammograms in dry MeCN using tetraburylammonium hexa fl uorophosphate(TBAP)as a supporting electrolyte,and the corresponding electrochemical data are also gathered in Table 1.As observed with other BODIPY PSs[5,6],within the electrochemical w indow of solvent MeCN,all these BODIPYs exhibited one-electron oxidation and reduction.Compared to non-iodinating BODIPYs(B1 and B6),iodinating BODIPYs(B2 and B5)with tw o iodo atom s on the 2,6-positions,exhibited less negative reduction and more positive oxidation potentials,indicating that iodinated compounds(B2 and B5)are easier to get one electron to be reduced than the corresponding counterparts(B1 and B6).This suggests the electron-de fi cient nature of iodo-substituted BODIPYs due to the presence of iodine atoms at the core of the pyrrole7].In addition,the electronde fi cient nature of iodo-substituted BODIPYs w ill be also easier to form radical anions in the presence of appropriate electron donors during the course of light-driven proton reduction.

Fig.2.(a)Photoinduced H2 evolution using H2A,TEOA or TEAas sacri fi cial electron donor with C1(2.5?10?3 mol/L)and B2(1.0?10?4 mol/L)in MeCN/H2O(1:1,v/v)after 6 h irradiation;(b)Effect of solution p H on the photocatalytic H2 production from a system composed of C1,B2 and H2A in MeCN/H2O 1:1.

In most of the hydrogen-producing systems reported using C1 asthe catalyst and BODIPYor xanthene dyesas the PSs,irreversible electron donors such as triethanolamine(TEOA)or triethylamine(TEA)were frequently used.However,w hen TEOA or TEA was initially chosen aselectron donorswith B2 serving asthe PSand C1 as the catalyst,no H2gas was detected by GCanalysis after 6 h of visible-light irradiation.Despite the negative results regarding H2generation in the neutral and basic aqueous solution,one electron reduced BODIPYspecies obtained from the excited state of BODIPY(BODIPY*)and electron donor is thermodynamically able to reduce the catalyst C1(D G=E(BODIPY??/BODIPY)?E(CoIII/CoII)=?0.65 eV),which encourages us to explore the three-component system in the acidic conditions.Ascorbic acid(H2A)is w idely used as an electron donor in acidic photocatalytic H2evolution system.

In an initial experiment,visible light(l>420 nm)irradiation of 10 m L of aqueous solution (VMeCN:VH2O=1:1)containing B2(1.0?10?4mol/L),C1(2.5?10?3mol/L)and H2A(0.1 mol/L)at p H 4,leads to hydrogen production(Fig.2a).Control experiments suggested that the system is inactive in the absence of any of the three components(catalyst C1,PS B2 or sacri fi cial donor H2A).The formation of Co colloids as possible catalytically active species in the course of the photocatalysis was ruled out on the basis of Hgpoisoning experiments.It should be noted that w hen using H2A as electron donor,the net reaction being driven photochemically can be expressed by the follow ing equation:H2A!A+H2.The formation of A and H2from H2A has been determined to be thermodynamically unfavorable by 0.41 V at p H 4.Thus,photocatalytic H2generation from H2A represents a photochemically driven upconversion of?20 kcal/mol.The visible light-driven H2production was dependent on the solution p H.A maximal rate for H2evolution was achieved at p H 5(Fig.2b),while lower amountsof H2were obtained at either lower or higher p H values,similar to that in other systems[24].This p Hdependent effect may be related to several factors,including the protonation behavior of the PS,and the equilibrium of H2A H++HA?(pKa=4.03 and 3.90 for H2Aand HA?,respectively).

Table 1Electrochemical,absorption and emission data.a,b

In order to investigate and compare the ability of B1-B2 and B5-B6 acting as PSs for H2evolution,a series of photolysis experiments were performed under the same reaction conditions in the presence of C1(2.5?10?3mol/L),PS(1.0?10?4mol/L),and H2A(0.1 mol/L)with continuous visible-light irradiation(l>420 nm).The optimal p H of the reaction system was kept at optimal value of 5.Fig.3 show s the time dependence of H2production in this catalytic system for the different BODIPYPSs.The plot reveals:(1)No observable amount of H2production was detected by GCfor B1 and B5.As reported in previous studies[7],compared with iodinated BODIPY dyes noniodinated chromophores B1 and B2 lack any appreciable hydrogen evolution under identical conditions due to the absence of the internal heavy atom effect and the long-lived photoexcited triplet state.The reason is that the diffusion-controlled bimolecular collision frequency in fluid solution is 109–1010Lmol?1s?1.As a consequence,the Stern-Volmer quenching constant(Ksv=kq?t0,w here kqis the bimolecular quenching constant and t0is the lifetime of the excited state of the PS(in ns))w ill be on the scale of 1–10 L/mol,which is too small to induce any efficient intermolecular electron transfer.(2)B2 and B6 show signi fi cant amounts of hydrogen.Iodination of BODIPY PSs(B2 and B5)increase the rate of intersystem crossing from the initially excited1pp*state via internal heavy atom effect,thus generating greater amounts of3pp*state.The excited triplet state tends to have a longer lifetime(in m s to ms)than the corresponding excited singlet state(in ns),thereby allow ing diffusion-controlled bimolecular interactions to occur more substantially.Although the triplet lifetimes for B5 and B2 were not measured due to our instrumental limitation,the analog of B5,not having the–NH2substituent,has a reported triplet excited-state lifetime of 57.1 m s in MeCN,which is>30000-fold longer than the corresponding singlet excited state.Furthermore,an additional iodide source such KIshowed no enhancement of H2production compared to the iodine-free system of C1/B1 or C1/B6,thus exclude the role of external heavy atom effect.(3)B5 and B2 gave 930 and 622 m mol of H2evolution after 6 h irradiation,respectively,corresponding to TONs of 93 and 62 with respective to the PS.Generally,multicomponent photocatalytic systems with different dyes alw ays exhibit different photocatalytic H2evolution behavior due to their different light-absorbing propertiesand longlived3pp*states generation capacities.When compared with B2 having one 8-hydroxyl-quinoline moiety,B5 with one phenylamine substituent shared a similar absorption and emission pro fi le,but augmented the absorption(e=86800 Lmol?1cm?1for B5 vs.e=79800 Lmol?1cm?1for B2).

After approximately 6 h of visible-light irradiation,the activity of H2evolution in the system of C1/B5/H2A ceased,indicating that at least one of the system components had been consumed.Addition of one equivalent of B5 to the catalytic systems after 6 h of irradiation partially restores the H2evolution activity(Fig.S2a,Supporting information),implying that the system is mainly limited by the decomposition of the PS and the catalyst.The photodegradation of the PS and the catalyst is also evidence by comparison of the follow ing absorption spectra recorded during the photolysis processes.The absorption spectra of the irradiated B5 solution show that 40 min irradiation did not produce fading of B5(Fig.S2b),indicating the high photostability of iodinated BODIPY[21].As show n in Fig.S2c,in the presence of only sacri fi cial donor,B5 bleaches very rapidly as a result of the instability of the formed radical anion B5??upon reductive quenching electron transfer between excited state of B5 and H2A.

Fig.3.Time courses of photocatalytic H2 evolution in PS/C1 systems with different BODIPY dyes(B1–B2 and B5–B6).Conditions:CH3CN-H2O(1:1,v/v)at p H 5 containing a BODIPYPS(1.0?10?4 mol/L),C1(2.5?10?3 mol/L)and H2A(0.1 mol/L)under visible light irradiation(300 W Xe lamp,l>420 nm).

The peak position was shifted?510 nm due to the cleavage of C-I bond.It has been documented that the halogen groups can be easily photobleached through reductive quenching[7].The absorption spectra of the irradiated B5+C1+H2A solution display that the decomposition of B5 becomes much slower(Fig.S2d)by the addition of C1 as the electron acceptor,which is attributed to the fact that the radical anion B5??may transfer an electron to C1 and regenerates B5.

As mentioned in the context,an interesting feature of the photogeneration of H2systems of C1/B2 and C1/B5 is that they keep active not in the basic solutions but in the acidic conditions.A possible,although still ambiguous,speculation may be rationalized from the structural analysis of BODIPYs.Taking the PS B5 for example,to minimize unfavorable steric clashes between the ortho-hydrogen atoms and the vicinal methyl groups,the orthogonal arrangement of the phenylamino and dipyrromethene fragments is anticipated in the B5.Thus,the rapid intramolecular electron transfer across the orthogonal structure w ill be favorable in the B5.In other w ords,the content of competition exists between intramolecular and intermolecular electron transfer in the basic and acidic conditions(Scheme S1 in Supporting information).specifically,in the basic conditions,the phenylamino unit may serve as electron transfer donor,rapidly quenching the singlet or triplet BODIPY*state intramolecularly and preventing electron transfer to the catalyst C1.The control experiment thus introduced an alternative reaction path forw ard electron transfer from phenylamino group to the excited state of BODIPY*followed by back electron transfer to ground state that prevents H2production.In contrast,once the phenylamino unit is protonated in acidic solutions,the path of intramolecular electron transfer is restricted and intermolecular electron transfer from dipyrromethene unit to the catalyst C1 is favorable.As a consequence,the activity of hydrogen production is restored.It should be noted that the[23_TD DIFF]current mechanism is only preliminary speculation on the basis of structure[24_TD DIFF]of BODIPYs.Further work including transient absorption spectroscopy and theoretical calculations [25_TD DIFF]should needed to shed light on the detailed mechanistic process.

To conclude,a series of BODIPYdyes with 8-hydroxyl-quinoline or phenylamine moiety at the meso-position on the BODIPY core were tested as PSs in the three-component homogeneous photocatalytic system containing the cobaloxime catalyst C1 and H2A as the electron donor in aqueous organic media.Only systems containing iodinated BODIPY chromophores were found to be active for H2production in the acidic aqueous solutions.This is the fi rst time for the BODIPYs as homogeneous hydrogen-generating PSs used in the acidic aqueous solutions.The results underscore that the chemical modification of PSs can be performed,thus allow ing for the transformation of acid and base conditions for the light-driven hydrogen production.

Acknow ledgments

We are grateful to the National Natural Science Foundation of China(Nos.21641011,21571115 and 21701133),the Program for New Century Excellent Talents in Fujian Province University and the Fujian Key Laboratory of Functional Materials and Applications(No.fma2017107),the Natural Science Foundation of Shandong Province(No.JQ201803),Young Scholars Program of Shandong University(No.2015WLJH24),and the Fundamental Research Funds of Shandong University(No.104.205.2.5)for fi nancial support of this work.

Appendix A.Supp lementary data

Supplementary data associated with this article can be found,in the online version,at https://doi.org/10.1016/j.cclet.2018.05.003.