DOI: 10.1002/ajoc.201402131

Synthesis, Physical Properties, and Photocurrent Behavior of StronglyEmissive Boron-Chelate Heterochrysene Derivatives

Hailei Zhang,[a] Xiaozhong Hong,[b] Xinwu Ba,*[a] Bei Yu,[a] Xin Wen,[a] Sujuan Wang,[a]

Xuefei Wang,[c] Lei Liu,[a] and Jinchong Xiao*[a]

Introduction

Over the past decade, the synthesis of organic semiconduc-tors and exploitation of their various applications havebecome one of the most active research areas. The obtainedp-conjugated derivatives can be used in organic light-emit-ting diodes (OLEDs), field effect transistors (FET), photo-voltaic cells, and fluorescent probes.[1] For these applica-tions it is prefer to develop flexible, processable, thermallystable molecules with well-matched HOMO/LUMO bandgaps.

Recently, as an intriguing class of organic dyes, there isa growing interest in fluorescent boron complexes becauseof their potential application in photodynamic therapy, cat-alysis, and materials science.[2] In general, dipyrroboradia-zaindacenes have interesting optoelectronic properties.However, these compounds usually have small Stokesshifts, which results in self-quenching effect and restricts

the access to electroluminescent materials to a certainextent.[3] Boron-based b-diketonate complexes are anotherclass of fluorescent compounds which are directly synthe-sized from commercially available chemicals. For example,some of these b-diketonate-based compounds have beenexplored as imaging probes.[4] Although many derivativeshave been reported, there is still a driving force to makeboron-decorated molecules that have useful physical prop-erties.

Acenes, described as one kind of polycyclic aromatic hy-drocarbons (PAHs), encompass a series of linearly fusedbenzene units. First introduced by Clar, these compoundsare intensively studied in academia and industry at pres-ent.[5] For example, Kivelson and Chapman predict thatlarge polyacenes could be high-temperature superconduc-tors and ferromagnets.[6] Bendikov et al. also speculatedthat nonacene and other larger acene derivatives have anopen-shell biradical ground state.[7] In addition, some othergroups synthesized a series of polyacenes and investigatedtheir optoelectronic properties.[8] Chrysene, usually found insoot and smoke, has a mismatch structure compared withtetracene. In fact, chrysene has been scarcely used in organ-ic electronics. However, incorporating heteroatoms into thearomatic skeleton may give derivatives that have finelytuned electronic properties and would lead to major advan-ces in candidates for organic dyes.

We have prepared a heterochrysene derivative 2(Scheme 1). Although heterochrysene 1 was synthesized byHohaus and co-workers,[9] here we are more interested inits physical properties. Single crystal analysis of compound1 shows that boron atoms are twisted out of the plane of allcarbon and nitrogen atoms. Complex 1 and 2 emit stronggreen and yellow light in methylene chloride with highquantum yields, respectively. In addition, such a structuremotif also presents enhanced thermal and photochemical

Abstract: A BPh2-chelated hetero-chrysene derivative has been synthe-sized and characterized. Single crystalanalysis of its analogous BF2 complexshows that all carbon and nitrogenatoms are almost in a plane, while theboron atoms twist out of this planewith a torsion angle of 28.7968 mea-sured between plane O�B�N and

plane C1–C4–C4’. The BPh2 and BF2

compounds emit strong yellow andgreen light in methylene chloride withhigh fluorescence quantum yields, re-spectively. Thermogravimetric analysisresults indicate that the thermal sta-bility of both compounds is high. Inaddition, a photoswitching device wasfabricated based on a thin film of the

BF2 complex spin-coated onto single-walled carbon nanotubes, which pro-duces a steady and reproducible pho-tocurrent response.

Keywords: boron · heterochry-senes · organic dyes · organic elec-tronics · semiconductors

[a] Dr. H. Zhang, Prof. X. Ba, B. Yu, Dr. X. Wen, Dr. S. Wang, L. Liu,Prof. J. XiaoCollege of Chemistry and Environmental ScienceKey Laboratory of Chemcial Biology of Hebei UniversityBaoding 071002 (P. R. China)E-mail : baxw@mail.hbu.edu.cn

jcxiaoiccas@gmail.com

[b] Dr. X. HongCollege of Physics Science and TechnologyHebei UniversityBaoding 071002 (P. R. China)

[c] Prof. X. WangSchool of Chemistry and Chemical Engineering ofUniversity of Chinese Academy of SciencesBeijing 100049 (P. R. China)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/ajoc.201402131.

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stability, and thus the photocurrent behavior was alsotested.

Results and Discussion

The synthetic routes to compounds 1 and 2 are shown inScheme 2. Compound 4 was obtained through the self-con-

densation reaction of commercially available 2-pyridinecar-boxaldehyde (3) in the presence of acetic acid.[10] It waspredicted that when compound 4 was further treated withBF3·Et2O or BPh3 in anhydrous methylene chloride or tet-rahydrofuran at room temperature 1 or 5 and 2 or 6 wouldbe produced, respectively. In fact, 1 and 2 were obtained inhigh yields of 76 % and 55 %, respectively. This might be as-signed to the relatively larger stability of the as-formed six-membered rings in 1 and 2. Compound 1 is soluble in mostof organic solvents, such as methylene chloride, chloroform,tetrahydrofuran (THF), 1,2-dichlorobenzene (ODCB), andDMSO, which is favored for characterization through1H NMR and 13C NMR spectroscopy, and MALDI-TOFmass spectrometry. However, the solubility of BPh2 deriva-

tive 2 is low in these solvents. Until now, 1H NMR spectros-copy, MALDI-TOF mass spectrometry, and elemental anal-ysis were performed for 2, the results of which are consis-tent with the structure presented.

To determine the molecular spatial arrangement, a greensingle crystal of compound 1 suitable for X-ray crystallo-graphic analysis was obtained by slow evaporation of a mix-ture solution of methylene chloride/n-hexane. Unfortunate-ly, the as-obtained orange crystal of compound 2 was verysmall, and could not be analyzed. Compound 1 has a triclin-ic crystal system and its space group is P1. The unit cell pa-rameters are a=5.0631(7) �, b=7.9078(11) �, c=

7.9178(11) �, a =82.946(2)8, b=86.528(2)8, g= 74.036(2)8.As shown in Figure 1, the analogous chrysene is a planar

molecule. However, when heteroatoms were inserted intothis skeleton, one can notice that the boron atoms are bentout of the molecular plane. The as-formed torsion anglecalculated at O�B�N and C1�C4�C4� is 28.7968. Mean-while, the plane at F�B�F and plane at C1�C2�C4’ arenearly perpendicular (87.2998). It should be noted thatthese two molecules form slipped-parallel arrangements. Inaddition, it can be seen that the intermolecular distance in1 (3.30 �) is smaller than that in chrysene (5.07 �), whichnot only suggests that the introduction of heteroatom in-creases intermolecular interaction, which forms partial p–p

stacking, but also the packing arrangement should be bene-ficial for charge transport.[11]

The absorption and fluorescence spectra of 1 and 2 weremeasured in diluted methylene chloride solution as shownin Figure 2. The BF2 complex 1 presents a maximum ab-sorption peak at 410 nm, which can be ascribed to the p–p*transition of the chromophore. However, BPh2 complex 2has an obvious bathchromic-shift absorption band at468 nm. This is comparable with reported linear tetracenederivatives.[12] In addition, the high coefficient values (e=

25300 for 1 and 10700 for 2) are typical for BF2 and BPh2

complex derivatives.[13] The red-shift of compound 2 might

Scheme 1. Chemical structures of chrysene, 1, and 2.

Scheme 2. Synthetic routes to 1 and 2.

Figure 1. Single crystal structures of a) chrysene and b) 1 and c) andd) their corresponding packing models.

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be attributed to the extended p-conjugation by substitutionof the fluorine atoms with phenyl groups and the relativeelectron-donating ability of the phenyl units, which canform an intramolecular charge-transfer system in D-p-Asystem 2. When excited at 410 nm and 468 nm, the maxi-mum emission peaks of compounds 1 and 2 are at 486 nmand 573 nm, respectively.

Compound 4 in methylene chloride does not fluoresce.However, the as-obtained boron-chelated BF2 complex1 and BPh2 complex 2 are more rigid, which can reduce theenergy loss from nonradiative thermal vibrations.[14] Clearly,compound 1 (73 nm) and 2 (106 nm) have large Stokesshifts, the value of which for compound 1 is smaller thanthat of 2. The quantum yields (Ff) of compounds 1 and 2were calculated as 0.63 and 0.32, respectively, with 9,10-di-phenylanthracene (Ff = 0.95 in ethanol)[8b] and N,N’-di(2,6-diisopropylphenyl)-perylene-3,4:9:10-tetracarboxylic acidbisimide (Ff =1.00 in chloroform)[15] as the reference stand-ards.

The electrochemical behavior of compounds 1 and 2 wasinvestigated by cyclic voltammetry (CV) at room tempera-ture with tetrabutylammonium hexafluorophosphate(TBAPF6, 0.1 m) as a supporting electrolyte to test theirelectron affinity. Figure 3 shows the electrochemical re-sponse of compounds 1 and 2 in anhydrous methylene chlo-ride. BF2 complex 1 presents one oxidation peak (+1.29 V)and one reduction peak (�1.31 V). However, compound 2gives two oxidation peaks at + 0.79 V and + 1.20 V, and onereduction peak at �1.27 V, which indicates that the incorpo-ration of BPh2 does affect the electron affinity of the conju-gated skeleton. In comparison, BPh2 complex 2 has a nega-tively shifted oxidation wave and slightly positively reduc-tion wave, which is indicative of a better electron-donatingand electron-withdrawing ability. The HOMO–LUMO gapsof compounds 1 and 2 measured based on the half-waveredox potentials are 2.60 eV and 2.13 eV respectively, whichagrees well with the band gaps estimated by the UV/vis ab-sorption data (2.68 eV and 2.32 eV). The thermal properties

of complexes 1 and 2 were studied by thermogravimetricanalysis (TGA) as shown in Figure 4. The onset tempera-tures of weight loss (5%) of compounds 1 and 2 are 325 8Cand 206 8C, which indicates high thermal stability. Accord-ing to the UV/vis absorption spectra, CV, and thermal sta-bility, we believe that the as-prepared compounds can beused as active layer in organic electronics.[16]

To better understand the spectroscopic properties ofcompounds 1 and 2, density functional theory (DFT) calcu-lations were performed using the Gaussian 09 program, andthe results are depicted in Figure 5.[17] The electron density

of compounds 1 and 2 in the HOMO wavefunction is morelocalized on ethene-1,2-diol moiety, while the electron den-sity of the LUMO wavefunction delocalizes through theheterochrysene framework. Clearly, the F and Ph substitu-ents almost do not participate in the orbitals. Moreover, thedihedral angle calculated at O�B�N and C1�C4�C4’ is 308,which correlates well with that of the single crystal data(28.7968). In addition, taking the estimated band positions

Figure 2. UV/vis spectra of 1 (i) and 2 (ii) measured in CH2Cl2 ([c] =1�10�5

m), and fluorescence spectra of 1 (iii) and 2 (iv) in CH2Cl2. The exci-tation wavelengths of 1 and 2 were 410 nm and 468 nm, respectively.The inset shows the fluorescence images.

Figure 3. Cyclic voltammograms of 1 and 2 in anhydrous methylenechloride at a scan rate of 0.1 Vs�1.

Figure 4. TGA curves of 1 and 2 under a nitrogen atmosphere at a heat-ing rate of 10 8C min�1.

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of 1 and 2 and the corresponding band gaps into consider-ation, one can see that the theoretical calculations are con-sistent with the experimental results.

The photoswitching properties of compounds 1 and 2 de-posited on single-walled carbon nanotubes (SWCNT) wastested according to the reported method.[18] The photocur-rent was recorded at 14.4 mW cm�2 halogen lamp irradia-tion. As shown in Figure 6, a steady and reproduciblecathodic 4 mA/cm�2 photocurrent from the 1-SWCNT thinfilm was obtained by turning the light on and off. In gener-al, the photocurrent is larger than those in the reported re-sults,[18,19] which might be ascribed to high quantum yieldand efficient photoinduced electron-transfer. Clearly, therise curve of the photocurrent is very prompt and the fall issluggish, resulting from the trap-releasing current.[18] How-

ever, there was no photocurrent response from a 2-SWCNT, which might be assigned to the low solubility of 2in organic solvents. Thus the decoration of compound 2with bulky substituents and the optimization of device per-formance are needed.

Conclusions

In summary, we have successfully prepared a BPh2 complexheterochrysene derivative 2 through simple chelation ofpyridoin and boron derivatives. 1 and 2 emit strong greenand yellow light in methylene chloride, respectively, withhigh quantum yields. Moreover, these two compounds alsodisplay large Stokes shifts and good stability. In addition,the photocurrent from a thin film based on 1-SWCNT issteady and the response reproducible. Our primary studymight provide insight for the rational design and potentialapplications of the heteroacenes.

Experimental Section1H NMR and 13C NMR spectra were obtained on a Bruker AV600 spec-trometer using tetramethylsilane as the internal standard. Mass spectrawere recorded on Bruker Biflex MALDI-TOF/TOF. Elemental analysiswas performed on a Carlo Erba 1106 elemental analyzer. Electrochemi-cal behavior was investigated by cyclic voltammetry on an Ivium Plus II(Ivium Technologies, Holland) instrument equipped with a standardthree-electrode electrochemical cell in tetrabutylammonium hexafluoro-phate (0.1 m) under nitrogen. Thermogravimetric analysis was performedon a Pyris-6, (PerkinElmer, USA) operated under 20 mL min�1 nitrogenflow rate at the heating rate of 10 8C min�1. UV/vis absorption and fluo-rescence spectra were recorded on Shimadzu UV-2550 and RF-5301spectrophotometers, respectively.

Synthesis of 6,6,13,13-Tetrafluoro-6,13-dihydropyrido ACHTUNGTRENNUNG[1,2-c]pyrido-[1’,2’:3,4] ACHTUNGTRENNUNG[1,3,2]oxazaborinino ACHTUNGTRENNUNG[6,5-e] ACHTUNGTRENNUNG[1,3,2]oxazaborinine-7,14-diium-6,13-diuide (1)

A mixture of 2,2’-pyridoin (4, 100 mg, 0.47 mmol), boron trifluoride-ethyl ether complex (500 mL, 2.58 mmol) in anhydrous methylene chlo-ride (20 mL) was stirred at room temperature for 6 h under a nitrogenatmosphere. The solvents were then removed under reduced pressure.The as-prepared residue was re-dissolved in methylene chloride, andthen n-hexane was added. The precipitate was collected by centrifuga-tion. The crude product was further washed with diethyl ether threetimes to give the target compound 1 as a green solid (110 mg, 76%). FT-IR (KBr): 3085, 1616, 1561, 1483, 1451, 1321, 1274, 1247, 1145, 1117,1046, 929, 858, 787 cm�1. 1H NMR (600 MHz, [D6]DMSO, 298 K): d=

8.82 (d, J=6.0 Hz, 2H), 8.55 (t, J =6.0 Hz, 2 H), 8.2 (d, J=6.0 Hz, 2H),7.94 ppm (t, J=6.0 Hz, 2H); 13C NMR (150 MHz, [D6]DMSO, 298 K):d=147.28, 145.91, 145.12, 144.35, 142.27, 141.37, 132.53, 126.26, 124.69,123.97, 120.99, 120.29 ppm; MS (EI): C12H8B2F4N2O2, calcd [M]+ :310.07; found: 310. CCDC 974905 (1) contains the supplementary crys-tallographic data for this paper. These data can be obtained free ofcharge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of 6,6,13,13-Tetraphenyl-6,13-dihydropyridoACHTUNGTRENNUNG[1,2-c]pyrido-[1’,2’:3,4] ACHTUNGTRENNUNG[1,3,2]oxazaborinino ACHTUNGTRENNUNG[6,5-e] ACHTUNGTRENNUNG[1,3,2]oxazaborinine-7,14-diium-6,13-diuide (2)

A mixture of 2,2’-pyridoin (4, 50 mg, 0.23 mmol) and triphenyl borane(225 mg, 0.929 mmol) was stirred in anhydrous tetrahydrofuran (20 mL)at room temperature under nitrogen overnight. Then the solvent was

Figure 6. Photocurrent response of 1-SWCNT thin film upon the irradia-tion of 14.4 mW cm�1�2 white light; Vg =�0.2 V and Vds =2 V.

Figure 5. Wavefunctions for the HOMO and LUMO of 1 and 2.

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evaporated under reduced pressure. The as-obtained residue was furtherpurified by column chromatography on silica gel using methylene chlo-ride as eluent to afford 2 as an orange solid (70 mg, 55%). FT-IR (KBr):3069, 3010, 1608, 1561, 1470, 1306, 1223, 1196, 787, 744, 704 cm�1.1H NMR (600 MHz, [D6]DMSO, 298 K): d= 8.35–8.32 (m, 2H), 8.16 (d,J =8.4 Hz, 2 H), 8.03 (d, J=5.4 Hz, 2H), 7.69–7.67 (m, 2 H), 7.15–7.13(m, 12H), 7.07–7.05 ppm (m, 8 H); MS (MALDI-TOF): C36H28B2N2O2,calcd [M]+ : 542.23; found: [M+Na]+ : 565.1. Elemental analysis: calcd(%) for C36H28B2N2O2: C 79.74, H 5.20, N 5.17; found: C 79.22, H 5.25,N 5.11.

Acknowledgements

We thank the National Natural Science Foundation of China (21102031,21273006, and 91233107), the Natural Science Foundation of HebeiProvince (B2014201007), and the Introduction of Overseas StudentsFoundation of Hebei Province.

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Received: July 24, 2014Published online: && &&, 0000

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www.AsianJOC.org Xinwu Ba, Jinchong Xiao et al.

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Heterochrysenes

Hailei Zhang, Xiaozhong Hong,Xinwu Ba,* Bei Yu, Xin Wen,Sujuan Wang, Xuefei Wang, Lei Liu,Jinchong Xiao* &&&&—&&&&

Synthesis, Physical Properties, andPhotocurrent Behavior of StronglyEmissive Boron-Chelate Heterochry-sene Derivatives

Crysene point : A N,O-chelated boroncomplex 2 and its analogue 1 havebeen synthesized and characterized.Single crystal analysis of 1 shows thatall carbon and nitrogen atoms arealmost in a single plane. 1 and 2 emitgreen and yellow light in dichlorome-thane, respectively. The photocurrentresponse of a 1-single-walled carbonnanotube thin film is steady and repro-ducible.

Asian J. Org. Chem. 2014, 00, 0 – 0 � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim6&&

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