6918 | Chem. Commun., 2017, 53, 6918--6921 This journal is©The Royal Society of Chemistry 2017

Cite this:Chem. Commun., 2017,

53, 6918

Efficient energy-level modification of novelpyran-annulated perylene diimides forphotocatalytic water splitting†

Ran Wang,a Gang Li,*a Andong Zhang,b Wen Wang,a Guanwei Cui,a Jianfeng Zhao,c

Zhiqiang Shi*a and Bo Tang *a

We design and synthesize four pyran-embedded perylene diimide

(PDI) compounds through a straightforward methodology. UV-driven

photocatalytic water splitting using the compounds as photocatalysts

demonstrates that the highest photocatalytic H2 evolution rate under

UV light is 0.90 mmol g�1 h�1, which paves the way towards organic

photoresponsive materials.

Hydrogen generated by photocatalytic water splitting is considereda potentially renewable energy source to replace traditionalfossil-fuel energy sources. Although significant progress has beenwitnessed in inorganic systems such as transition metal oxides andmetal sulfides,1–7 they are usually confined by their relativelylimited properties. The advantages of organic conjugated mole-cules, for example, accessibility, chemical versatility, and tunableproperties, have indeed attracted the interest of materials scientists.But there are few reports on photocatalytic water splitting usingorganic small molecules directly as photocatalysts.

In the past decades, polycyclic aromatic hydrocarbons(PAHs), regarded as the basic building blocks of grapheneand other carbon-rich nanostructures, have received intenseattention due to their unique electronic and photophysicalproperties and highly ordered structures in the solid state,which made them potential candidates for applications in

organic electronics.8–26 Among all members of the PAH family,perylene diimide (PDI) and its derivatives,27–33 synthesizedfrom economical starting materials, have rigid planar back-bones and an extended p-conjugation, which exhibited highcharge-carrier mobility, intense luminescence, extraordinarythermal and photochemical stability, appreciable and tunablevisible light absorption, strong self-assembly characteristics,and low-lying frontier molecular orbitals,34 all factors thatmake them well suited to adaptation as promising activeelements (electron transport materials) in organic photovoltaicdevices (OPVs),35–38 organic field effect transistors (OFETs),39–42

and organic light-emitting diodes (OLEDs).43 In order to furtherimprove the properties of the PDIs, developing core-extendedPDIs has been strongly sought. Typically, incorporating maingroup elements into extended-PDI frameworks has been exten-sively explored in the past decade.28,44–46 Alternatively, a largenumber of PDI derivative fused p-systems consisting of five-membered rings, such as sulfur-, selenium-, oxygen-, andN-heterocycles, have been realized by the implementation ofone or more bay regions.28,33,35,47,48 Although promising, weviewed these structures as being limited in structural diversityand large branched alkyl chains are required to ensure ade-quate solubility in organic solvents. Thus, in order to furtherexplore the potential of these ‘‘bay-fused’’ PDI derivatives, weenvision that the incorporation of a 2H-pyran ring (the so-called‘‘pyran-annulation’’) at the bay position would increase struc-tural diversity through the modulation of the substituted groupin the pyran synthon.

Herein, we described the synthesis, characterization, andphysical properties of four novel pyran-annulated perylenediimide compounds 1, 2, 3 and 4 through an efficient facileelectrophilic alkylation method one/two pyran rings are fusedonto a PDI core to extend the p-conjugation system, and wedemonstrated that the inclusion of a pyran ring improvedsignificantly the degree of solubility. The structural and photo-physical nature as well as redox properties have been fullyinvestigated using experimental techniques and theoretical

a College of Chemistry, Chemical Engineering and Materials Science, Collaborative

Innovation Center of Functionalized Probes for Chemical Imaging in Universities

of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of

Education, Institute of Materials and Clean Energy, Shandong Provincial Key

Laboratory of Clean Production of Fine Chemicals, Shandong Normal University,

Jinan, 250014, P. R. China. E-mail: ligang@sdnu.edu.cn, zshi@sdnu.edu.cn,

tangb@sdnu.edu.cnb Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. Chinac Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials

(SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing,

211816, P. R. China

† Electronic supplementary information (ESI) available: The synthetic detail,original 1H NMR, 13C NMR, MALDI-TOF, TGA, and FT-IR. See DOI: 10.1039/c7cc03682e

Received 11th May 2017,Accepted 5th June 2017

DOI: 10.1039/c7cc03682e

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calculations. Another photocatalytic water splitting using thesefour compounds as photocatalysts was also discussed.

The synthetic routes of monopyran-fused PDIs are depictedin Scheme 1. Compounds 1 and 2 were prepared via a one-stepcyclization reaction by N,N-dicyclohexyl-1-nitro-perylene-3,4,9,10-tetracarboxylic acid bisimide (PDI-NO2) with 2-nitro-propane anddiethyl malonate at room temperature, respectively. Then bothwere subjected to a classical nitration reaction in the presence offuming nitric acid in dichloromethane affording selectivelyimportant intermediates 1-NO2 and 2-NO2 in almost quantitativeyields. Subsequent further cyclic condensation reactions werecarried out and yielded the other two target products 3 and 4 ingood yields. The proposed mechanism is shown in the ESI.† Theas-synthesized compounds have been fully characterized by1H NMR, 13C NMR, FT-IR, and high-resolution mass spectro-metry. In the 1H NMR spectrum (see Fig. S8 in the ESI†) ofcompound 3, two singlet signals were observed at the low field.The chemical shifts of the two singlet signals at the low fieldwere 8.17 and 8.07 ppm.

We assigned these singlet resonances to protons Ha and Hb,respectively. Compared to compound 3, the correspondingaromatic proton signals for compound 4 (see Fig. S11, ESI†)observed at 8.49 and 8.32 ppm were shifted to a relatively lowerfield due to the electron-withdrawing effect of the two ethylacetate groups. The peaks at 5.03 ppm (m, 1H) of 3 and 5.02 ppm(m, 1H) of 4 in Fig. S8 and S11 (ESI†) were contributed from theprotons (namely, Hc) in cyclohexane groups adjacent to the Natom, which clearly confirmed that 3 and 4 had centrosymmetricstructures. The four compounds had a moderate solubility and candissolve in polar organic solvents such as dichloromethane (DCM),chloroform, tetrahydrofuran (THF), and dimethylformamide(DMF). The thermal stability of them was investigated by thermalgravimetric analysis (TGA) (see Fig. S19, ESI†). The curves mea-sured under nitrogen illustrated the remarkable stability againstdecomposition when the temperature exceeded 300 1C. The photo-stabilization of compounds 1–4 was also measured under sunlightfor 7 days; the NMR signals showed no obvious change. Such aremarkable thermal and light stability is very promising for itsfuture application in organic semiconductor devices.

The absorption and fluorescence spectra of the four com-pounds in a dichloromethane solution (1 � 10�5 mol L�1) are

shown in Fig. 1, and the data are listed in Table 1. Compound 1containing one pyran-fused component shows a strong absorp-tion band across a broad range from 400 to 625 nm; there werethree well-defined absorption peaks at 420, 520 and 577 nm(e = 69 400, 130 400, and 76 300 M�1 cm�1, respectively). Com-pound 3 possessing two pyran-fused segments shows a similarabsorption spectrum in the range of 450–650 nm, but themaximum absorption peak red-shifted to 606 nm in compar-ison with that of compound 1, which could be attributed to theintroduction of two pyran units resulting in the expansion ofthe p-conjugation system. The absorption spectra of 2 and 4 insolution (chloroform) are also shown in Fig. 1, which showedthree major bands in the range of 400–625 nm with low-energymaxima at 557 nm and 579 nm, respectively.

The emission spectra of compounds 1–4 in dichloro-methane solutions are shown in Fig. 2. The photoelectric dataare also summarized in Table 1. The maximum emission ofcompound 1 in DMF appeared at 610 nm, then it red-shifted to625 nm of compound 3 fused with two pyran rings. The trendsof absorption and emission spectra of compounds 2 and 4 weresimilar to those of compounds 1 and 3. The small Stokes shift(19–37 nm) between the absorption and emission bands of

Scheme 1 Synthetic routes of compounds 1, 2, 3 and 4. Reaction conditions:(a) 2-nitro-propane, NMP, K2CO3, room temperature; (b) diethyl malonate, NMP,K2CO3, room temperature; and (c) fuming HNO3 (65–68%), CH2Cl2, roomtemperature.

Fig. 1 Absorption spectra of 1–4 in dichloromethane solutions. Concen-tration: 1 � 10�5 M.

Table 1 Physical properties of compounds 1–4

Compounds labs (nm) e (mol�1 cm�1) lem (nm) jf (%)

1 577 43 200 610 42.152 557 38 700 595 45.753 606 46 640 625 39.094 579 42 800 605 38.09

Fig. 2 Fluorescence spectra of 1–4 in dichloromethane solutions.Concentration: 1 � 10�5 M.

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6920 | Chem. Commun., 2017, 53, 6918--6921 This journal is©The Royal Society of Chemistry 2017

compounds 1–4 was probably due to the rigid conjugatedstructure. The fluorescence quantum yield in a chloroformsolution was moderate (0.38–0.46) using fluorescein as thestandard (Ff = 0.85, 0.1 M NaOH).29

The electrochemical behaviors of all the fused PDI com-pounds were investigated by cyclic voltammetry (CV) in dichlor-omethane (vs. Ag/AgCl), shown in Fig. 3, and their redoxpotentials and energy levels are summarized in Table S2 (ESI†).The four compounds in DCM exhibited reversible reduction andoxidation waves. The onset reduction potentials (vs. Ag/AgCl)of the as-synthesized compounds were �0.88 V, �0.81 V,�1.18 V and �0.80 V, respectively. Compound 1 showed threereduction peaks and compound 2 showed four reduction peaks,implying their ability to accept at least three and four electrons,respectively. The LUMO energy levels (electron affinity) estimatedfrom the equation ELUMO = �[Ered(onset) + 4.4] eV21 are �3.52 eV,�3.59 eV,�3.22 eV and �3.6 eV, respectively. The onset oxidation

potentials (vs. Ag/AgCl) of the as-synthesized compounds were1.04 V, 1.08 V, 0.92 V and 1.19 V, respectively. The HOMO energylevels estimated from the equation EHOMO =�[Eox(onset) + 4.4] eV21

are shown in Table S2 (ESI†). So we can see that the bandgapsestimated from the equation Egap = Ered1� Eox1

14 shown in Table S2(ESI†) were similar to the optical band gap energies (Eopt

g ) derivedfrom the onset absorption edges in solutions.

In order to understand the electronic structure properties ofcompounds 1–4, a theoretical investigation was conductedthrough molecular simulation. The molecular geometries of1–4 in the ground state were fully optimized using densityfunctional theory (DFT) at the B3LYP/6-31G* level,49 and thefrontier molecular orbitals of 1–4 are shown in Fig. 4. Evidently,the HOMO and LUMO orbitals were more delocalized over thewhole molecules, indicating an effective p-conjugation in thefused systems. Furthermore, the calculated HOMO–LUMO gaps(see Table S3 in the ESI†) of 1 (2.36 eV) and 3 (2.29 eV) arereduced relative to 2 (2.42 eV) and 4 (2.32 eV); this finding canbe attributed to the peripheral fused dimethylpyran groups,which can stabilize the LUMO levels.

In order to investigate the photoelectric transfer properties ofthe as-synthesized compounds, the photocatalytic hydrogen evolu-tion activities of different samples were studied using them asphotocatalysts. The photocatalytic H2 evolution was performed ona XPA-7 photocatalytic reaction instrument. Table 2 shows theexperimental results of photocatalytic H2 evolution with differentas-prepared catalysts under the same experimental conditions. Asshown in Table 2, compound 4 presented a fairly higher photo-catalytic activity compared to those of compounds 1–3. Theaverage photocatalytic hydrogen evolution rate of compound 4was 0.90 mmol g�1 h�1, which was the highest value of a perylenediimide based photocatalyst under UV light,50,51 whereas those ofcompounds 1–3 were in the range of 0.010–0.085 mmol g�1 h�1.The excellent hydrogen production activity of compound 4 can beattributed to the following: (1) it showed a strong and broadabsorption in the visible spectral region, which can effectivelyutilize light energy and convert it into chemical energy. (2) Thereduction position of 4 was distinctly and sufficiently morenegative toward proton reduction potential52 compared with thoseof compounds 1–3, which was beneficial for rapid photogeneratedelectron injection from the LUMO level of compound 4 into wateron the surface effectively. In the irradiation of visible light(l Z 420 nm), compounds 1–3 showed very small photocatalyticH2 evolution, and the average photocatalytic hydrogen evolutionrate of compound 4 was 23.51 mmol g�1 h�1, which is also not soas that of irradiation under UV light (0.90 mmol g�1 h�1).

In conclusion, four novel pyran-fused perylene diimides havebeen successfully synthesized through one concise intramolecular

Fig. 3 Cyclic voltammetry curves of 1–4 in dichloromethane solutionscontaining 0.1 M Bu4NPF6 electrolyte. Scanning rate: 50 mV s�1.

Fig. 4 Wave functions for the HOMO and LUMO of compounds 1–4.

Table 2 Experimental results of photocatalytic H2 evolution from different catalysts

Samples H2(1)/(mmol g�1 h�1) H2(2)/(mmol g�1 h�1) H2(3)/(mmol g�1 h�1) Average H2/(mmol g�1 h�1)

1 0.017 0.015 0.016 0.0152 0.081 0.086 0.082 0.0853 0.012 0.011 0.009 0.0104 0.85 0.92 0.93 0.90

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electrophilic arene reaction. The as-prepared compounds havebeen fully characterized using conventional spectroscopy tech-niques. The HOMO, LUMO and energy gaps obtained usingexperimental CV values are close to those obtained using DFTcalculations. Interestingly, compound 4 presents a relativelyhigher photocatalytic hydrogen evolution activity compared tothe other three compounds because of reduced reductionpotential. Our study not only demonstrates that the perylenediimides can be used directly as a photocatalyst, but may alsohelp in paving the way towards the design and preparation ofnew types of photoresponsive materials for energy conversionrequirements.

Z. Shi acknowledges Shandong Province Natural ScienceFoundation (ZR2012BM012). B. Tang acknowledges the financialsupport from 973 Program (2013CB 933800). G. L. acknowledgesthe financial support from a start-up grant (Shandong NormalUniversity) and Shandong Province Natural Science Foundation(ZR2016BM24).

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