NIR-Absorbing Dye Based on BF2-Bridged Azafulvene Dimer as a

Letter Cite This: Org. Lett. 2018, 20, 5135−5138

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NIR-Absorbing Dye Based on BF2‑Bridged Azafulvene Dimer as a Strong Electron-Accepting Unit Hiroyuki Shimogawa, Yasujiro Murata, and Atsushi Wakamiya* Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

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S Supporting Information *

ABSTRACT: BF2-bridged azafulvene dimers designed to be strong electron-accepting units were selectively synthesized using a bulky base. Single-crystal X-ray diffraction analysis revealed that the high electron-accepting ability of this structure stems from the contribution of the π-conjugation mode of the azafulvene dimer upon formation of B−N coordination bonds. As a result of the low-lying LUMO energy of this electron-accepting unit, the corresponding D− A−D dye exhibits an intense NIR absorption band at 922 nm, which tails up to 1150 nm, while significant absorption bands in the visible region are absent. As a NIR dye this molecule exhibits moreover exceptional photostability and resistance to oxidation by atmospheric oxygen, even in dilute solution.

D

intramolecular B−N bond. Based on this observation, we designed a 2-fold BF2-bridged azafulvene dimer as a strongly electron-accepting building block (Figure 1). We synthesized 1

yes that absorb near-infrared (NIR) light have been investigated intensively in the context of a variety of applications, including photovoltaics, photodetectors, heat absorbers, and medical applications.1 NIR dyes can be small molecules2 and polymers3 with donor−acceptor (D−A) units, such as rylenes,4 cyanines,5 and the corresponding heteroatombridged analogues.6,7 In many applications, the photostability and resistance to oxidation of these NIR dyes are very important.1 NIR dyes usually have narrow HOMO−LUMO gaps on account of the destabilized HOMO level and are thus easily oxidized by atmospheric oxygen. A strong electronaccepting unit having low-lying LUMO levels should therefore be expected to lead to NIR dyes with improved air- and photostablility. The combination of an electron-accepting building block and electron-donating units can generate narrow HOMO−LUMO gaps while maintaining moderate HOMO levels.8 Intramolecular coordination bonds are effective at inducing strong electron-accepting properties. Wakamiya and Yamaguchi reported that boryl-substituted thienylthiazoles show much stronger electron-accepting properties than the parent skeleton due to the formation of the intramolecular B−N coordination bond.9 The methodology can be applied to a variety of CNcontaining molecular structures,6a,c,10,11 including stable boronbridged dyes, for example, BODIPY derivatives with N−B−N bridges7 as well as boron diketonato complexes with O−B−O bridges.12 Recently, numerous boron-bridged cyanine analogues have been developed as new building blocks for NIR dyes.6a,c,10c,k,12c Examining the structure and properties of these building blocks, we noticed that the origin of their strong electron-accepting ability appears to originate from the large contribution of the azafulvene resonance structure, a contribution that is further enhanced by the formation of the © 2018 American Chemical Society

Figure 1. Molecular design concept for a strongly electron-accepting BF2-bridged azafulvene dimer and structures of model compounds 1−3.

and 2 as model compounds and 3 as a model D−A−D dye composed of the electron-acceptor unit and triarylamines as electron-donor units. The structural, electrochemical, and photophysical properties of 1−3 were examined in detail. The stability of 3 was compared to the commercially available naphthalocyanine (Napht) NIR dye.5a,6c Following a typical protocol for the synthesis of BODIPY,7b dithienyl derivative 1 was synthesized from the borylation of dipyrrolylethanedione derivative 4 using i-Pr2EtN in THF at room temperature to afford 1 in only 29% yield together with Received: July 1, 2018 Published: August 17, 2018 5135

DOI: 10.1021/acs.orglett.8b02056 Org. Lett. 2018, 20, 5135−5138

Letter

Organic Letters large amounts of a complex mixture of unidentified products (Scheme 1). To try to improve the yield, various bases were Scheme 1. Synthesis of BF2-Bridged Azafulvene Dimers 1 and 1-5-mem

Figure 2. (a) ORTEP representation of the molecular structure of 1 (thermal ellipsoids set at 50% probability; only selected atoms are labeled); (b) possible resonance structures A and B.

bond alternations: C1−C2 [1.377(3) Å], C3−C4 [1.356(3) Å]. Meanwhile the C5−N1 [1.351(3) Å] bonds are much shorter than the C1−C1* [1.459(4) Å], C2−C3 [1.431(3) Å], C4−C5 [1.439(3) Å], and C2−N1 [1.396(3) Å] bonds. From this we can conclude that the electronic structure of 1 has a significant contribution from the π-conjugation mode B having azafulvene character. The crystal structures of 1-5-mem and 2 also show high planarity and a significant contribution from the π-conjugation mode of the azafulvene dimer (Figures S3−S4 and Table S1). The N1−B1−O1 bond angle around the coordinated tetrahedral boron center in 1-5-mem [100.0(2)°] is however much smaller than a typical tetrahedral bond angle (109.5°) as well as that in 1 [109.79(16)°]. The lower degrees of distortion in the BF2-bridged 6-membered ring of 1 should endow 1 with higher stability than 1-5-mem. In order to gain a deeper understanding of the effects of the BF2-bridges on the electronic structure in the central building block, DFT calculations (B3LYP/6-31G(d)) were conducted on model compounds 1′ and dipyrrolylethanedione 4′, in which the thienyl moieties were replaced with hydrogen atoms. Dipyrrolylethanedione 4′ can be represented by its tautomeric structures (Figure 3): One is well described as pyrrole and oxalyl groups in keto form (4′-keto), while the other exhibits an azafulvene dimer with enol form (4′-enol). Although 4′-enol is the energetically unfavorable tautomer, its electron-deficient azafulvene character lowers the LUMO level by 0.66 eV relative to that of 4′-keto. In BF2-bridged 1′, the formation of two B−N coordination bonds not only stabilizes the enol form but also induces a further decrease of the LUMO level by 0.93 eV, suggesting a substantial electron-accepting ability of the BF2-bridged azafulvene dimer structure. To experimentally confirm the electronic features of the BF2bridged azafulvene dimer building unit, we measured the cyclic voltammogram for 1−3 in CH2Cl2. The electrochemical data are summarized in Table 1. Dithienyl derivative 1 showed two reversible reduction waves at E1/2 = −0.48 and −0.89 V (vs Fc/ Fc+), which appear at much less negative potential than that of 4 (Epc = −1.60 V) (Table 1). The first reduction potential of 1 appears at a substantially more positive potential than those of typical n-type materials such as fullerene C60 (E1/2 = −0.98 V)13a or perylene diimide (E1/2 = −0.97 V).13b Dimesityl derivative 2 also showed two reversible reduction waves at E1/2 = −0.50 and −1.05 V, which is also much less negative compared to that of

Reaction conditions: base (4 equiv), BF3·OEt2 (6 equiv). Reaction conditions: base (46 equiv), BF3·OEt2 (60 equiv). Reduced amounts resulted in poor yield. cA mixture of 1 and 1-5-mem was obtained; ratio determined by 1H NMR. a

b

screened. Typical organic bases, such as DBU, triethylamine, and pyridine, furnished merely complex mixtures, whereas in the presence of inorganic bases, such as K2CO3 and K3PO4, no reactions were observed even in excess amounts. We subsequently focused on a bulky base, 2,6-di-tert-butylpyridine, which led to a significant increase of the yield (52%) of 1 having 6-membered-ring moieties. Another compound, the 5-membered-ring product 1-5-mem, was formed in 26% yield. Both structures 1 and 1-5-mem were determined by single-crystal Xray diffraction analysis (vide infra). A selective synthetic route for 1 or 1-5-mem was found after careful examination of the reaction conditions (Scheme 1): When the borylation was performed in dry CH2Cl2 instead of THF, 1-5-mem was obtained as a sole product in 71% yield. According to 1H NMR and TLC, 1-5-mem was found to gradually deborylate back to 4 in wet CH2Cl2 or even in dry THF. Conversely, 1 was found to be stable under such conditions. DFT calculations at the B3LYP/6-31G(d) level of theory suggested that 1-5-mem is less stable by +8.2 kcal/mol than 1 (Figure S1). 1 and 1-5-mem therefore represent the thermodynamically and kinetically controlled product, respectively. Thus, we expect that in THF solvent the formed 5-mem will be selectively deborylated to increase the yield of 1, and indeed, the borylation of 4 in refluxing THF afforded 1 as the sole product in 74% yield. In a similar manner, dimesityl derivative 2 and D−A−D dye 3 with 6-membered rings were synthesized from the corresponding dipyrrolrylethanedione derivatives in 62% and 83% yields, respectively (the details are shown in the Supporting Information (SI)). Single-crystal X-ray diffraction analysis revealed a highly planar structure for 1, evident from a torsion angle of 0.0° between the pyrrole rings, and the fact that the terminal thienyl rings adopt torsion angles of 2.5° relative to the pyrrole rings (Figure 2a). For compound 1, two resonance structures are possible: Dipyrrolylethanedione A with B−O coordination and azafulvene dimer B with B−N coordination as shown in Figure 2b. In the X-ray structure, the central structure of 1 exhibits large 5136

DOI: 10.1021/acs.orglett.8b02056 Org. Lett. 2018, 20, 5135−5138

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Organic Letters

Figure 4. (a) UV/vis/NIR absorption spectra of 1−3 in CH2Cl2 and films of 3. Photographic images of their CH2Cl2 solutions and the films of 3 are included. (b) Change of the absorption maxima after irradiation of NIR light in dilute degassed or nondegassed toluene.

Figure 3. Effect of introducing BF2-bridges into dipyrrolylethanedione; energy diagram for 1′ and 4′, calculated at the B3LYP/6-31G(d) level of theory.

The observed intense NIR absorption profile can be rationalized by DFT and TD-DFT calculations. DFT calculations (B3LYP/6-31G(d)) on model compound 3′ confirmed that the HOMO and LUMO are delocalized over the entire πconjugated skeleton (Figure S15). TD-DFT calculations on this optimized structure (CAM-B3LYP/6-31G(d)//B3LYP/631G(d)) suggest that the longest absorption in 3 is attributed to the HOMO → LUMO transition and exhibits a large oscillator strength ( f = 2.313). No solvatochromism was observed in the solutions of 3 (λabs = 907 nm in toluene and 902 nm in DMF, Figure S12). This was also confirmed by TDDFT calculations using the polarizable continuum model (PCM) at the CAM-B3LYP/6-31G(d) as shown in Table S3. In order to examine the photophysical properties in the solid state, the absorption spectra of spin-coated thin films of 3 were recorded (Figure 4a). Whereas the poly(methyl methacrylate) (PMMA) film doped with 1 wt % of 3 shows a spectrum (λabs = 914 nm) similar to that in solution, the neat film of 3 shows additional red-shifted absorption bands (λabs = 1066 and 1174 nm) due to intermolecular interactions. To assess the effectiveness of the BF2-bridged azafulvene dimer unit with significant electron-accepting ability as a NIR dye, the photostability of 3 was investigated under irradiation with NIR light (Xe lamp, 690−1100 nm, 1.3 × 104 lux) in degassed or nondegassed toluene (4 × 10−6 M) and compared to that of a commercially available naphthalocyanine (Napht) NIR dye5a,6c (Figure 4b). After irradiation for 1 h, the absorbance of the Napht dye (λabs = 865 nm) diminished to 27% in degassed toluene and to 6% in nondegassed toluene. In sharp contrast, the absorbance of 3 remained unchanged (97%), even after irradiation for 50 h in nondegassed toluene, clearly demonstrating the high photostability of 3 and its resistance to oxidation by atmospheric oxygen. We have designed and developed a 2-fold BF2-bridged azafulvene dimer and demonstrated that the formation of intramolecular B−N coordination bonds to dipyrrolylethanedione endows this structure with extraordinary electronaccepting properties. On the basis of this concept, a D−A−D dye was synthesized which exhibits a narrow HOMO−LUMO gap while maintaining a moderate HOMO level. This leads to

Table 1. Electrochemical Properties of 1−4a dye 1 2 3 4

Eoxb (V)

Eredb (V)

+0.21, + 0.54 +1.16c

−0.48, − 0.89 −0.50, − 1.05 −0.78, − 0.98 −1.60c

HOMOd (eV)

LUMOd (eV)

−5.26 −6.00

−4.68 −4.66 −4.40 −3.63

a

Measured in CH2Cl2 (1 mM); supporting electrolyte: [(n-Bu)4N][PF6] (0.1 M); scan rate: 100 mV s−1. bHalf-wave potentials (vs Fc/ Fc+). cIrreversible wave; Epc or Epa is shown. dEstimated by − (Eonset + 5.1 eV).14

bis(mesityl)-substituted BODIPY derivative (E1/2 = −1.40 V).8a These results demonstrate the significant electron-accepting ability of the BF2-bridged azafulvene dimer building unit. Whereas 1 and 2 show no oxidation waves in the potential window of CH2Cl2, D−A−D dye 3 with diarylaminothienyl groups exhibits two reversible oxidation waves (E1/2 = +0.21 and +0.54 V). In addition, 3 showed two reversible reduction waves at E1/2 = −0.78 and −0.98 V, which are slightly shifted to negative from 1 and 2 due to the effect of the electron-donating diarylaminothienyl groups. Nevertheless, the first reduction potential of 3 is still less negative compared to those of typical ntype materials (vide supra).13 The HOMO and LUMO levels estimated from these potentials are given in Table 1.14 Owing to the high electron-accepting ability of the BF2-bridged azafulvene dimer building unit, 3 exhibits significantly narrower HOMO− LUMO gap (0.86 eV) with a moderate HOMO level (−5.26 eV). In the UV/vis/NIR absorption spectra in CH2Cl2 (Figure 4a), the absorption band of 1 and 2 appears at λabs = 625 nm (log ε = 4.76) and 457 nm (log ε = 4.57), respectively. D−A−D dye 3 exhibits an intense NIR absorption at λabs = 922 nm (log ε = 5.06) tailing up to λedge = 1150 nm, reflecting the narrow HOMO−LUMO gap, while relatively little absorption in the visible region (400−700 nm) was observed. This is the ideal photophysical property for a selective NIR absorber. 5137

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high photostability of the molecule, and strong resistance to oxidation by atmospheric oxygen. Moreover, while there is a strong absorption band in the NIR region, absorptions in the visible region are largely absent. In line with this molecular design principle based on the development of strong electronaccepting building blocks, we believe that several more sophisticated NIR materials should be easily attainable, the development of which is currently in progress in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02056. Experimental details, spectra of new compounds, and Cartesian coordinates of optimized structures (PDF) Accession Codes

CCDC 1547424−1547426 and 1547497 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yasujiro Murata: 0000-0003-0287-0299 Atsushi Wakamiya: 0000-0003-1430-0947 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JST ALCA (Grant No. JPMJAL1603), JST COI, and JST ERATO (Grant No. JPMJER1302) and a Grant-in-Aid for Scientific Research (B) (JP26288093) and JSPS Core-to-Core Program. H.S. thanks the JSPS for a Research Fellowship for Young Scientists. Synchrotron single-crystal X-ray diffraction analyses were carried out at the SPring-8 beamline BL38B1 with the approval of JASRI (2015B1074). We thank Dr. K. Suzuki (Hamamatsu Photonics K. K.) and T. Handa and Prof. Y. Kanemitsu (Kyoto University) for the photophysical measurements. We appreciate Prof. L. T. Scott (Boston College and ERATO) for fruitful discussions.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b02056 Org. Lett. 2018, 20, 5135−5138