Stable Thiele's Hydrocarbon Derivatives Exhibiting Near-Infrared

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Stable Thiele’s Hydrocarbon Derivatives Exhibiting NearInfrared Absorption/Emission and Two-Step Electrochromism Yuta Okamoto, Masaru Tanioka, Atsuya Muranaka, Kazunori Miyamoto, Tetsuya Aoyama, Xingmei Ouyang, Shinichiro Kamino, Daisuke Sawada, and Masanobu Uchiyama J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11092 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Journal of the American Chemical Society

Stable Thiele’s Hydrocarbon Derivatives Exhibiting Near-Infrared Absorption/Emission and Two-Step Electrochromism Yuta Okamoto,† Masaru Tanioka,*,† Atsuya Muranaka,*, ‡ Kazunori Miyamoto,† Tetsuya Aoyama,‡ Xingmei Ouyang,‡ Shinichiro Kamino,§, ◊ Daisuke Sawada,§, ◊ and Masanobu Uchiyama*, †, ‡ †

Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Cluster for Pioneering Research (CPR), Advanced Elements Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan § Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, 1-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan ◊ Next-Generation Imaging Team, RIKEN Center for Biosystems Dynamics Research (BDR), 6-7-3 Minatojimaminamimachi, Chuo-ku, Kobe, Hyogo 650-0047, Japan ‡

Supporting Information Placeholder ABSTRACT: We report synthesis and characterization of

near-infrared (NIR)-absorbing/emitting Thiele’s hydrocarbon derivatives, in which four aryl groups are bridged to a quinodimethane skeleton. The quinoid structure of the bridged-tetra-aryl-p-quinodimethanes (BTAQs) was confirmed by spectroscopic, X-ray crystallographic, and computational methods. Although quinodimethane derivatives with a small HOMO–LUMO energy gap often exhibit biradical character, BTAQs showed no biradical character. Instead, they exhibited two-step near-infrared electrochromism. The donor/acceptor properties of the aryl groups were found to play a key role in the unique properties of BTAQs.

We report herein a new type of p-quinodimethanes (1Hex-3Hex), which we designate as bridged tetra-aryl-pquinodimethanes (BTAQs). They can be regarded as derivatives of Thiele’s hydrocarbon, but they do not show biradical character at all, regardless of the small HOMO–LUMO energy gap, and they exhibit intense near-infrared (NIR) absorption/fluorescence. Thiele’s hydrocarbon and related p-quinodimethanes are generally considered to exhibit two resonance structures, a quinoid structure and a biradical benzenoid structure (Chart 1).1 Due to this biradicaloid character, they are often labile, readily undergoing dimerization/decomposition.2 Thus, it is sometimes difficult to utilize p-quinodimethane derivatives as functional materials, even though they could have utility as functional dyes3 and organic semiconductors,4 for example. Organic molecules that strongly absorb/emit NIR light have many potential applications, including security inks,

bio-imaging, photodynamic therapy, photoimmuno therapy, and photovoltaic devices.5 Although various types of NIR-absorbing/emitting organic molecules have been developed so far, most of them suffer from relatively low stability and/or high synthetic cost. Breakthroughs in molecular design and synthetic strategy are therefore needed to develop superior NIR dyes. Chart 1. Structures of Thiele’s hydrocarbon and bridgedtetra-aryl-p-quinodimethanes (BTAQs)

Thiele’s Hydrocarbon

R N

R O

X2

O

X2

X1

O O X1

N R R

1Et : R = C2H5, X1,X2 = H 1Bu : R = n-C4H9, X1,X2 = H 1Pen : R = n-C5H11, X1,X2 = H 1Hex : R = n-C6H13, X1,X2 = H 2Hex : R = n-C6H13, X1,X2 = H, tBu 3Hex : R = n-C6H13, X1,X2 = Cl

Bridged-Tetra-Aryl-p-Quinodimethane (BTAQ)

In the course of our studies on the creation of new functional materials based on fluoran leuco dyes,6 we unexpectedly observed the formation of 1Hex. When isoaminobenzopyranoxanthene (iso-ABPX) was synthesized from 2-(4-dihexylamino-2hydroxybenzoyl)benzoic acid and hydroquinone using

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concentrated sulfuric acid as a solvent, a trace amount of 1Hex was detected by MALDI-TOF-MS. With the aim of improving the product yield, the reaction conditions were optimized using iso-ABPX as a starting material and yields of up to 17% were achieved (Schemes 1a and S1). We further found that 1Et, 1Bu, and 1Pen can be synthesized in 31, 16, and 13% yields, respectively, by onestep reaction from commercially available 2-(4dialkylamino-2-hydroxybenzoyl)benzoic acid and 1,4dimethoxybenzene (Scheme 1b). A tert-butyl-substituted derivative (2Hex)7 and a chloro-substituted derivative (3Hex) were also prepared by similar one-step reaction in 10% and 1% yields, respectively. All the products were characterized by NMR spectroscopy and high-resolution mass spectroscopy (see SI). Sharp 1H-NMR signals were observed for all BTAQs, suggesting that they have closed-shell structures in the ground state.

that of 1Hex. In the case of 1Pen, a methanol molecule is hydrogen-bonded to each carbonyl group, and the bond alternation is less clear (Figure 1c).

Scheme 1. Synthesis of BTAQsa

Figure 1. X-ray structures of 1Hex (a) and 1Pen (b). The thermal ellipsoids are scaled to the 40% probability level. Alkyl groups were omitted for clarity. (c) Selected bond distances of 1Hex and 1Pen (see Table S3 for detailed results).

1.0

a

The molecular structure of BTAQs was unambiguously determined by single-crystal X-ray diffraction analysis of 1Hex and 1Pen. As shown in Figure 1a, 1Hex consists of a Thiele’s hydrocarbon structure in which each aryl group is linked to the central 6-membered ring via an oxo or a carbonyl bridge. Clear bond alternation was seen in the p-quinodimethane moiety (the C1-C3’ and C2-C11 bond lengths were shorter than the C1-C2 and C2-C3 bond lengths), indicating that 1Hex adopts a quinoid structure.8 Although the π-skeleton is not planar due to steric repulsion between hydrogen atoms on the peripheral aryl groups, the p-quinodimethane moiety is planar, and the deviation from the averaged plane is within 0.015 Å. Intermolecular π-π stacking was observed between the benzoyl moieties (see Figure S10). The X-ray structure of 1Pen was essentially the same as

828 865

2Hex

0.5

10–5 ε (M–1 cm–1)

Reagents and conditions: (i) conc. H2SO4, 140°C, 4 h. (ii) conc. H2SO4, 75 to 100°C, 48 h; then 160°C, 2 h (6 h for 1Et). (iii) conc. H2SO4, 95 to 130 (150) °C, 78 (74) h for 2Hex (3Hex).

1.0

Fluorescence Intensity (a.u.)

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1Hex

0.5

1.0

874 919

3Hex

0.5 400

600

800 Wavelength (nm)

1000

1200

Figure 2. Electronic absorption (solid line) and fluorescence (broken line) spectra of 1Hex-3Hex in CHCl3 at room temperature. Figure 2 shows the electronic absorption and fluorescence spectra of BTAQs with two dihexylamino groups (1Hex-3Hex). All the compounds exhibited an intense absorption band in the 800–900 nm region. Their extinc-


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tion coefficients were over 8 ´ 104 M–1 cm–1. The peak position of the NIR absorption band was found to depend on the peripheral substituents, and the longestwavelength absorption band (874 nm) was observed for the tetrachloro derivative (3Hex). In contrast to open-shell singlet biradical p-quinodimethane derivatives,9 BTAQs do not show any weak absorption band at longerwavelength than the main band. All BTAQs exhibited NIR fluorescence beyond 850 nm. The fluorescence quantum yields (FF) of 1Hex, 2Hex, and 3Hex were 3.2, 4.9, and 2.0%, respectively. BTAQs with different alkyl lengths (1Et, 1Bu, and 1Pen) also exhibited NIR absorption/fluorescence (see Figure S11 and Table S4). Density functional theory (DFT) calculations were conducted to understand the unique properties of BTAQs. We first calculated the optimized structure of 1Et at the (U)B3LYP/6-31G(d,p) level. The closed-shell quinoid structure was calculated to be most stable, whereas the open-shell singlet biradical form was not obtained as a converged structure. Time-dependent density functional theory (TDDFT) calculations for the optimized structure of 1Et predicted an intense absorption band in the longwavelength region (labs = 710 nm, f = 0.71, see Figure S12). The absorption band corresponds to the HOMO®LUMO transition and is attributable to the π®π* transition, since the HOMO and LUMO are delocalized over the π-skeleton (Figure 3a). Similar results were obtained for other BTAQs. The π ® π* transition showed little intramolecular charge transfer character, since the contributions of the aniline/benzoyl unit to the HOMO/LUMO are large. The peak position of the NIR absorption band of BTAQs was dependent upon solvent polarity (see Figures S13 and S14).

Figure 3b shows the results of molecular orbital analysis of 1Et and model molecules. The HOMO-LUMO energy gap of 1Et was 1.74 eV10, which is much smaller than that of the parent p-quinodimethane (3.76 eV). The origin of the small energy gap of BTAQs can be explained by the donor/acceptor properties of the aryl groups. Model calculations clearly indicated that the aniline unit destabilizes the HOMO energy and the benzoyl unit stabilizes the LUMO energy. The observed differences in absorption/fluorescence wavelengths among BTAQ derivatives (1Hex-3Hex) are therefore related to the different electron-accepting properties of the benzoyl units: increase in the acceptor properties leads to a longer-wavelength shift. No biradical character was observed for BTAQs under our experimental conditions, despite the fact that the HOMO–LUMO energies were quite small. This is in sharp contrast to other low-band-gap quinodimethane derivatives, which often exhibit biradical character.9 BTAQs therefore have potential utility as stable NIR organic dyes. Indeed, no marked decrease in the NIR absorption of a toluene solution of 2Hex was observed over one month under ambient conditions, and 1Bu were found to be thermally stable in air up to at least 250°C (see Figures S15 and S16). These unexpected features are considered to be a consequence of the strong donor/acceptor properties. A zwitterionic form should contribute to the ground state of BTAQs, which leads to a negligible contribution of the biradical benzenoid form (Scheme 2). The X-ray structures of 1Hex and 1Pen support this interpretation: a relatively short C1–O1 distance (1.337(2) Å for 1Hex, 1.345(3) Å for 1Pen) in the oxo bridge is associated with the pyrylium structure of the zwitterionic form.11 Scheme 2. Resonance structures of BTAQs

Figure 3. (a) Frontier molecular orbitals of pquinodimethane (left) and 1Et (right). (b) HOMO and LUMO energy levels of p-quinodimethanes and 1Et. Calculations were performed at the B3LYP/6-31G(d,p) level.

The stability of BTAQs motivated us to further explore their unique properties. As most p-quinodimethane derivatives with extended π-conjugation show rich electrochemistry,2,4,12 cyclic voltammograms of BTAQs with hexyl groups were measured (see Figure S17). Compound 1Hex exhibited two reversible oxidation waves and two reversible reduction waves with half-wave potential (E1/2) of –0.21, 0.01 and –1.43, –1.72 V (vs Fc/Fc+) in an argon atmosphere. Interestingly, the shape of the cyclic voltamogram was retained after 50 potential cycles between –0.1 and +1.0 V (vs Ag/AgCl) in air, indicating that the radical cation (1Hex•+) and dication (1Hex2+) are


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highly stable (see Figure S18). As would be expected, the redox potentials were shifted to more positive values with increasing electron-accepting character of the benzoyl unit (see Table S5).

type semiconducting layer in the fabricated devices. The hole mobility was determined to be 1.6 ´ 10–3 cm2 V–1 s– 1 . In addition, photocurrent action spectra were investigated in vacuum-deposited films of 1Et. As shown in Figure 4d, photocurrent was observed in the NIR region by applying a voltage, indicating that the film of 1Et has a photocarrier generation ability upon NIR light irradiation, and can transport charge carriers. These carrier transport and photoresponse properties indicate that BTAQs would be useful as a NIR light-sensing material. In summary, novel NIR-absorbing/emitting Thiele’s hydrocarbon derivatives, BTAQs, were synthesized and their molecular structure and physicochemical properties were investigated. BTAQs uniquely do not show biradical character in the ground state because of the strong donor/acceptor properties of the aryl groups, and therefore are stable. We observed stable two-step electrochromism in the NIR region. We believe that BTAQs represent a new class of organic molecules with potential applications in various technologies utilizing NIR light. ASSOCIATED CONTENT

Figure 4. (a) Absorption spectra during stepwise electrochemical oxidation of 1Hex in CH2Cl2 with 0.1 M nBu4NClO4 in air. (b) Two-step equilibrium of 1Hex. (c) Transfer characteristics of a transistor made with 1Hex. (d) Near-infrared photocurrent in a film of 1Et at an applied voltage of 1 V. Schematic device structures are shown in insets of (c) and (d).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization data, computational details Crystal data for 1Hex and 1Pen (CIF)

AUTHOR INFORMATION Corresponding Author

[email protected] [email protected] [email protected] Funding Sources

Figure 4a shows the electronic absorption spectra during stepwise electrochemical oxidation of 1Hex in air. When a voltage corresponding to the first oxidation potential was applied, several intense NIR absorption bands emerged in the 800-1500 nm region. When a higher voltage was applied, a peak appeared in the 600850 nm region. These spectral changes can be assigned to the generation of 1Hex•+ and 1Hex2+ (Figure 4b and Table S6. TDDFT calculations reproduced well the observed spectral changes (see Figure S19 and Table S7). Since both 1Hex•+ and 1Hex2+ are easily formed and the switching is reversible in air, BTAQs are a good candidate for NIR electrochromic materials.13 We finally tested the charge transport and photoconductive properties of BTAQ films. Bottom-gate topcontact transistors with a spin-coated film of 1Hex were fabricated and the carrier transport properties were evaluated. The transfer characteristics shown in Figures 4c and S20 indicate that the film of 1Hex operates as a p-

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by JSPS KAKENHI(S) (No. 17H06173) and JSPS Grant-in-Aid for Scientific Research on Innovative Areas (No. 17H05430) (to M.U.). The authors thank Dr. Takashi Matsumoto (Rigaku) for X-ray analysis of 1Hex. RIKEN Integrated Cluster of Clusters (RICC) and HOKUSAI provided the computer resources for the DFT calculations. We also thank Ms. Natsumi Fukino (Okayama Univ.), Dr. Eiyu Imai (RIKEN), Ms. Asako Nakano (RIKEN) and Ms. Mieko Utsugi (RIKEN) for experimental support.

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pound and Process for Producing Same. WO 2017/038987 A1, March 9, 2017. The tert-butyl-substituted derivative (2Hex) was obtained as a mixture of three regioisomers. Montgomery, L. K.; Huffman, J. C.; Jurczak, E. A.; Grendze, M. P. The Molecular Structures of Thiele's and Chichibabin's Hydrocarbons. J. Am. Chem. Soc. 1986, 108, 6004–6011. (a) Sun, Z.; Huang, K.-W.; Wu, J. Soluble and Stable Heptazethrenebis(dicarboximide) with a Singlet Open-Shell Ground State. J. Am. Chem. Soc. 2011, 133, 11896–11899. (b) Sun, Z.; Lee, S.; Park, K. H.; Zhu, X.; Zhang, W.; Zheng, B. Hu, P.; Zeng, Z.; Das, S.; Li, Y.; Chi, C.; Li, R.-W.; Huang, K.-W.; Ding, J.; Kim, D.; Wu, J. Dibenzoheptazethrene Isomers with Different Biradical Characters: An Exercise of Clar’s Aromatic Sextet Rule in Singlet Biradicaloids. J. Am. Chem. Soc. 2013, 135, 18229–18236. (c) Su, Y.; Wang, X.; Li, Y.; Song, Y.; Sui, Y.; Wang, X. Nitrogen Analogues of Thiele’s Hydrocarbon. Angew. Chem. Int. Ed. 2015, 54, 1634–1637. (d) Shi, X.; Quintero, E.; Lee, S.; Jing, L.; Herng, T. S.; Zheng, B.; Huang, K.-W.; Navarrete, J. T. N.; Ding, J.; Kim, D.; Casado, J.; Chi, C. Benzothia-fused [n]Thienoacenequinodimethanes with Small to Moderate Diradical Characters: The Role of Pro-aromaticity versus Anti-aromaticity. Chem. Sci. 2016, 7, 3036–3046. The HOMO-LUMO gaps obtained by DFT calculations (1Et: 1.74 eV; 1Hex: 1.73 eV) were slightly overestimated compared to those derived from the absorption spectra (1Et: 1.51 eV; 1Hex: 1.48 eV) and the redox potential (1Hex: 1.44 eV). A similar tendency is seen in other π conjugated molecules (for example, see Takeuchi, Y.; Matsuda, A.; Kobayashi, N. Synthesis and Characterization of meso-Triarylsubporphyrins. J. Am. Chem. Soc. 2007, 129, 8271.). Minami, Y.; Tokoro, Y.; Yamada, M.; Hiyama, T. Facile Onepot Synthesis of Solid-state Luminescent Benzopyrylium Tetrafluoroborates Derived from Annulation of Aryl Silylethynyl Ethers with Alkynes. Chem. Lett. 2017, 46, 899–902. Rudebusch, G. E.; Zafra, J. L.; Jorner, K.; Fukuda, K.; Marshall, J. L.; Arrechea-Marcos, I.; Espejo, G. L.; Ortiz, R. P.; GómezGarcía, C. J.; Zakharov, L. N.; Nakano, M.; Ottosson, H.; Cadaso, J.; Haley, M. M. Diindeno-fusion of an Anthracene as a Design Strategy for Stable Organic Biradicals. Nature Chem. 2016, 8, 753–759. (a) Yen, H.-J.; Lin, H.-Y.; Liou, G.-S. Novel Starburst Triarylamine-Containing Electroactive Aramids with Highly Stable Electrochromism in Near-Infrared and Visible Light Regions. Chem. Mater. 2011, 23, 1874–1882. (b) Nie, H.-J.; Zhong, Y.-W. Near-Infrared Electrochromism in Electropolymerized Metallopolymeric Films of a Phen-1,4-diyl-Bridged Diruthenium Complex. Inorg. Chem. 2014, 53, 11316–11322. (c) Cui, B.-B.; Zhong, Y.-W.; Yao, J. Three-State Near-Infrared Electrochromism at the Molecular Scale. J. Am. Chem. Soc. 2015, 137, 4058– 4061. (d) Cui, B.-B.; Tang, J.-H.; Yao, J.; Zhong, Y.-W. A Molecular Platform for Multistate Near-Infrared Electrochromism and Flip-Flop, Flip-Flap-Flop, and Ternary Memory. Angew. Chem. Int. Ed. 2015, 54, 9192–9197.


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Stable Thiele's Hydrocarbon Derivatives Exhibiting Near-Infrared

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