Ovalene As a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its Π ─ Extension to Circumpyrene Xiushang Xu 1, Qiang Chen 2, and Akimitsu Narita 1,2＊

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Synthesis and Characterization of Dibenzo[hi,st]ovalene as a Highly Fluorescent Polycyclic Aromatic Hydrocarbon and Its π ─ Extension to Circumpyrene Xiushang Xu 1, Qiang Chen 2, and Akimitsu Narita 1,2＊

1＊ Organic and Carbon Nanomaterials Unit, Okinawa Institute of Science and Technology Graduate University

1919 ─ 1 Tancha,2＊ Onna ─ son, Kunigami ─ gun, Okinawa 904 ─ 0495, Japan Max Planck Institute for Polymer Research Ackermannweg 10, 55128, Mainz, Germany

(Received August 4, 2020; E ─ mail: [email protected])

Abstract: Polycyclic aromatic hydrocarbons (PAHs) with zigzag edges have attracted increasing attention for their unique optical and electronic properties. This account describes our synthetic approaches to dibenzo[hi,st]- ovalene (DBOV) as a novel PAH with a combination of armchair and zigzag edges and the elucidation of its unique optoelectronic and photophysical properties, such as strong red emission with a uorescence quantum yield of up to 0.89 and stimulated emission. Furthermore, DBOV demonstrated the so ─ called uorescence blinking that enables its application as a uorophore in single ─ molecule localization microscopy, which is one of the modern superresolution uorescence microscopy methods. The self ─ assembly of a DBOV derivative bearing two 3,4,5 ─ tris(dodecyloxy)phenyl groups was also investigated, showing the formation of helical columnar stacks. On the other hand, the regioselective bromination of DBOV was achieved, allowing the postsynthetic functionalization and modulation of the optoelectronic properties. Moreover, π ─ extension of the DBOV at the bay regions led to circumpyrene, the largest circumarene synthesized to date.

modulated by peripheral functionalization, 1a,7 heteroatom 1． Introduction 8 1g,9 doping, and the incorporation of nonhexagonal rings. Polycyclic aromatic hydrocarbons (PAHs) have attracted In 1995, Müllen and his coworkers demonstrated a facile renewed attention in recent decades due to their intriguing and synthesis of hexa ─ peri ─ hexabenzocoronene (HBC), which can tunable electronic, optical, and magnetic properties, which render be regarded as a hexagonal nanographene only with armchair them promising for applications in advanced optoelectronic edges, through oxidative cyclodehydrogenation of hexaphenyl- devices. 1 Large PAHs with sizes of over 1 nm are also called benzene. 10 Afterwards, they and others have achieved the syn- nanographenes or (nano)graphene molecules, whose chemical thesis of a number of extended armchair ─ edged PAHs, or structures can be regarded as nanoscale fragments of gra- nanographenes, by employing tailor ─ made oligophenylene phene. 2 In the eld of theoretical physics, such graphene frag- precursors. 1e,1f,6a,9c,11 For nanographenes with armchair edge ments have been intensively studied as graphene quantum dots structures, the energy gaps between the highest occupied mole- (GQDs), which are also structurally the same as large PAHs. 3 cular orbital (HOMO) and the lowest unoccupied molecular The synthesis of PAHs was pioneered by Scholl 4 and Clar 5 orbital (LUMO) are mainly dependent on size, and good cor- in the early 20th century, and the relationship between PAH relation is found between the gap and the number of carbon structures and their properties has been continually explored. 6 atoms in the aromatic core, showing decreased energy gaps as The size, symmetry, and edge structure are the key factors that the core becomes larger. 7b,12 Currently, the largest nanogra- de ne the chemical and physical properties of PAHs. 1b,6b In the phene synthesized consists of 222 sp 2 carbon atoms and has a graphene eld, two types of edge structures are predominantly relatively small HOMO ─ LUMO gap of 1.4 eV, as calculated by discussed, namely, armchair and zigzag edges, which corre- a density functional theory (DFT) method. 7b,13 spond to extensions of the so ─ called bay and L ─ regions, On the other hand, PAHs with zigzag edges, such as tetra- respectively (see Figure 1). The properties of PAHs can also be cene, 14 dibenzo[b,def]chrysene, 15 bisanthene, 16 ovalene, 17 and benzo[rst]pentaphene 18 (Figure 2), display intriguing properties, such as higher chemical reactivity, smaller HOMO ─ LUMO energy gaps, and higher uorescence quantum yields than other PAHs with similar sizes (i.e., number of carbon atoms) but without zigzag edges. 19 Moreover, in contrast to armchair ─ edged PAHs, some PAHs with zigzag edges show open ─ shell character. For example, Kubo and his coworkers reported the syntheses of teranthene 20 and quarteranthene, 21 revealing their open ─ shell ground state (Figure 3). On the other hand, Wu’s group pioneered the syntheses of zethrenes 22 with open ─ shell characters (Figure 2) and demonstrated a synthesis of a laterally extended heptazethrene in 2016. 23 In 2018, the long ─ awaited peritetracene was achieved indepen- 24 25 Figure 1. Schematic representation of edge types of PAH. dently by Feng et al. and Wu et al, exhibiting a moderate

1094 （ 96 ） J. Synth. Org. Chem., Jpn. biradical character and a small energy gap (1.1 eV). However, family, was rst reported by Diederich and his colleagues in the number of PAHs with zigzag edges reported in the litera- 1991, 32 and its soluble derivative was reported by Feng et al. in ture is still limited, and many of them suffer from low stability, 2018 through Diels ─ Alder cycloaddition at the bay regions of 24 obstructing their in ─ depth characterization and application. peritetracene (Figure 2). Nevertheless, circumarene larger To this end, in 2017, our group reported the synthesis of than circumanthracene had not been achieved before our dibenzo[hi,st]ovalene (DBOV) as a new PAH with a combina- group reported the synthesis of circumpyrene through a π ─ tion of zigzag and armchair edges. 26 DBOV exhibits strong red extension of DBOV. 33 uorescence and stimulated emission, which renders it an This account focuses on the synthesis and functionalization 26 interesting dye for light ─ emitting devices, including lasers, as of DBOVs and the investigation of their optoelectronic and well as for uorescence microscopy imaging. 27 photophysical properties. After establishing a scalable synthetic route towards DBOV derivatives, the regioselective bromination of DBOV was achieved, enabling postsynthetic functionalization at the bay regions and modulation of their optoelectronic properties. The self ─ assembly behavior of a DBOV derivative bearing two tridodecyloxyphenyl groups was also explored. Furthermore, π ─ extension at the bay regions, taking advantage of dibrominated DBOV, provided circumpyrene, which represents the largest circumarene synthesized to date. 2． Synthesis and Characterizations of DBOV In view of the in ─ depth studies on their intriguing properties and potential applications, stability is a crucial factor for PAHs with zigzag edges. Their low stability is mainly attributed to their potential open ─ shell character, which makes them sus- ceptible to oxidation, dimerization, and/or other possible reac- 19,34 tions with solvent or atmospheric molecules. While open ─ shell PAHs are highly interesting for spintronic and even 35 quantum information technology applications, closed ─ shell PAHs with long ─ wavelength absorption, strong uorescence and high chemical stability can be more useful for other applications, such as in (opto)electronics, 36 photonics, 37 and uores- 27,38 cence imaging. The stability and possible open ─ shell character of PAHs can be qualitatively assessed according to Clar’s 39 aromatic π ─ sextet rule. In general, a PAH with a given number of aromatic π ─ sextets is more stable than its isomers with fewer aromatic π ─ sextets. On the other hand, open ─ shell PAHs typically have more Clar’s π ─ sextets in the open ─ shell forms than in the closed ─ shell forms. For example, teranthene has 6 Clar’s π ─ sestets in the open ─ shell form in comparison to 3 Figure 2. Examples of PAHs with zigzag edges. Clar’s π ─ sextets in the closed ─ shell form (Figure 3a), which was experimentally demonstrated to indeed have the open ─ Most of the PAHs with zigzag edges thus far synthesized shell biradical character in the ground state. 20 Conversely, a have a combination of zigzag edges with armchair edges or bay stable PAH with zigzag edges can in principle be designed by 28 regions. Representative exceptions are pyrene, coronene, making the number of Clar’s π ─ sextets larger or the same in acenes, 19 anthanthrene, 29 and ovalene 17 (Figure 2), which can be considered classical examples of zigzag ─ edged PAHs without any bay region, which we call zigzag PAHs in this account. Very recently, Wu et al. demonstrated the syntheses and characterizations of a series of parallelogram ─ shaped PAHs with four ─ zigzag ─ edges, including peripentacenopentacene (Figure 2), which showed global aromaticity and moderate energy gaps. 30 Notably, in 2019, they successfully fabricated organic distributed feedback (DFB) laser devices using anthanthrene and several other parallelogram ─ shaped zigzag PAHs in the active layer. 31 Circumarene is a subclass of PAHs, structures with small central aromatic cores surrounded by one outer layer of annu- lene. The classical examples of circumarene are circumbenzene Figure 3. The resonance structures of teranthene (a) and DBOV (b) (coronene) and circumnaphthalene (ovalene) (see Figure 2). in closed ─ and open ─ shell forms with Clar’s π ─ sextets Circumanthracene, as the next member of the circumarene indicated with circles.

Vol.78 No.11 2020 （ 97 ） 1095 the closed ─ shell form compared to that in the open ─ shell form. fused bichrysene 3 with methine groups, thus forming zigzag To this end, DBOV has 4 Clar’s π ─ sextets in both the closed ─ edges. Therefore, the synthesis of DBOV was planned through and open ─ shell forms (Figure 3b), which suggests a more stable the introduction of formyl groups at the cove regions of 3 and 43 closed ─ shell character. an intramolecular Friedel ─ Crafts cyclization followed by 2.1 Initial Synthesis through Oxidative Cyclodehydrogenation dehydroxylation 44 (see Figure 5). The initial synthesis of DBOV was inspired by the achieve- The synthesis of DBOV 15a was started with the iodination ment of nanographenes with cove regions (see Figure 1) by of commercially available 4 ─ dodecylaniline (4) to give 4 ─ 40 Feng, Müllen, and their colleagues. Pioneering synthetic dodecyl ─ 2 ─ iodoaniline (5), followed by Sandmeyer bromina- works were reported by Chen, Liu, and their coworkers, who tion to afford 1 ─ bromo ─ 4 ─ dodecyl ─ 2 ─ iodobenzene (6) in 78% synthesized fused bichrysene 3 through PtCl 2─ catalyzed yield (Figure 5). Subsequently, Sonogashira coupling of 6 with 41 cycloaromatization of bis(biaryl)diyne 1 to give bichrysene 2, trimethylsilyl (TMS) ─ acetylene afforded 1 ─ bromo ─ 4 ─ dodecyl ─ followed by oxidative cyclodehydrogenation with 2,3 ─ dichloro ─ 2 ─ (TMS ─ ethynyl)benzene (7) in 96% yield, followed by boryla- 42 5,6 ─ dicyanobenzoquinone (DDQ)/CH 3SO 3H (Figure 4). tion with n ─ BuLi/triisopropyl borate and hydrolysis to obtain DBOV can be obtained by “bridging” the cove regions of the 4 ─ dodecyl ─ 2 ─ (TMS ─ ethynyl)benzeneboronic acid (8) in 94% yield. Subsequently, a Suzuki ─ Miyaura coupling of 8 and 7 ─ bromo ─ 2 ─ naphthaldehyde provided TMS ─ ethynylphenyl- naphthaldehyde 9 in 97% yield. A CuCl ─ mediated Glaser coupling of 9 then gave diaryldiacetylene 10a in 98% yield, which 41b,42 was subjected to a PtCl 2─ catalyzed cycloaromatization to afford bichrysene 11a in 48% yield. In contrast to the previous works by Chen and Liu et al. 42 and Feng and Müllen et al., 40 where fused bichrysene 3 was synthesized through the oxidative cyclodehydrogenation of bichrysene 2, fused bichrysene 14a with formyl groups could not be obtained by treating 11a under various Scholl conditions, presumably because of the electron ─ withdrawing effect of the formyl groups. Therefore, the formyl groups of 11a were

reduced with NaBH 4 and then protected with acetyl to electron ─ donating acetoxymethyl groups, affording 12a in 87% yield over two steps. 12a could then be cyclized to provide Figure 4. Synthesis of fused bichrysene 3 reported by Chen, Liu, 42 fused bichrysene 13a via oxidative cyclodehydrogenation using and their coworkers. DDQ: dichloro ─ 5,6 ─ dicyano ─ 1,4 ─ benzoquinone. DDQ and TfOH in 33% yield. The acetoxymethyl groups of

Figure 5. Synthesis of DBOV 15a. PTSA: p ─ toluenesulfonic acid; ACN: acetonitrile; THF: tetrahydrofuran; DMF: N,N ─ dimethylformamide; TEA: triethylamine; NBS: N ─ bromosuccinimide; 4 ─ DMAP: 4 ─ dimethylaminopyridine; PCC: pyridinium chlorochromate; Ac 2O: acetic anhydride; TfOH: triic acid.

1096 （ 98 ） J. Synth. Org. Chem., Jpn. fused bichrysene 13a were subsequently transformed back to The TIPS protecting group was removed with tetra ─ n ─ aldehyde groups to afford 14a in 70% yield over two steps. butylammonium uoride (TBAF) in 92% yield, which added Finally, the treatment of 14a with 2 ─ mesitylmagnesium one step compared with the initial synthetic route, and a subse- bromide, followed by BF 3·OEt 2─ catalyzed Friedel ─ Crafts reac- quent CuCl ─ mediated Glaser coupling gave diaryldiacetylene tion and oxidation by air, provided DBOV 15a in 24% yield 10b in 98% yield. Then, the formyl groups of 10b were reduced over two steps. The relatively low yield in these nal steps was with NaBH 4 and protected with acetyl to provide 19 in 87% considered to be due to the steric hindrance between the yield. Bichrysene 12b was prepared by the PtCl 2─ catalyzed dodecyl groups at the 5 and 13 positions of DBOV 15a and the cycloaromatization of 19, followed by oxidative cyclodehydro- mesityl groups introduced at the 6 and 14 positions (see genation with DDQ and TfOH at -78 ℃ to yield fused bichry- Figure 5). In this initial synthesis, the total yield of DBOV 15a sene 13b. Compound 13b was converted to fused bichrysene was only 1.5% over 14 steps in linear sequence from the com- 14b with two formyl groups, which was treated with 2 ─ mesityl ─ mercially available starting material 4. and phenylmagnesium bromide, followed by intramolecular 2.2 Second Synthesis of DBOV through Oxidative Friedel ─ Crafts cyclization and dehydrogenation to afford Cyclodehydrogenation DBOV derivatives 15b and 15c in 56% and 80% yields over two To scale up the synthesis, DBOV derivatives 15b and 15c steps, respectively. The relatively higher yield of 15c than 15b without dodecyl groups at the 5 and 13 positions were next was presumably due to the smaller steric hindrance of phenyl targeted to suppress the steric hindrance in the Friedel ─ Crafts groups compared with the mesityl groups in the Friedel ─ Crafts reaction in the nal steps (see Figure 6). 45 Boronic acid 16 was reaction. The yields in these nal steps were indeed signi - prepared from commercially available 2 ─ iodobromobenzene cantly improved from that for 15a (24%). However, the total 46 over 2 steps in 75% yield and used instead of its dodecyl ─ yields of DBOV 15b and 15c in this second synthesis route substituted counterpart 8, which reduced the number of steps were 1.6% and 2.2%, respectively, over 13 steps in linear by two. However, the TMS ─ protected analog of 16 was depro- sequence from the commercially available starting material, tected during the subsequent Suzuki ─ Miyaura coupling with which were not signi cantly higher than the 1.5% total yield in 7 ─ bromo ─ 2 ─ naphthaldehyde, leading to a mixture of different the rst synthetic route. products, in contrast to the same reaction of boronic acid 8 in 2.3 Improved Synthesis of DBOV via Photocyclization the rst synthesis. This complication was presumably due to an The low total yields of DBOV 15 in the rst and second increase in polarity upon removal of the dodecyl group from 8, synthetic routes could be mainly ascribed to the low yields in making it prone to enter the basic aqueous phase and become the Pt ─ catalyzed cycloaromatization and the Scholl reaction deprotected; thus, triisopropylsilyl (TIPS) ─ protected boronic steps as well as the necessity of transformation between the acid 16 was employed, affording 17 in 97% yield. formyl and acetoxymethyl groups before and after the Scholl reaction. To achieve more ef cient synthesis of DBOV derivatives, a sequence of cycloaromatizations with ICI, 41a,47 which allows simultaneous regioselective iodination, and subsequent 48 49 Pd ─ catalyzed cyclization was next considered. Diaryldi- acetylene 10b was thus reacted with ICl to provide iodinated bichrysene 20 in 76% yield (Figure 7). Pd ─ catalyzed cyclization of 20 was then attempted, but only deiodinated product was observed without the occurrence of cyclization. Interestingly, however, during thin ─ layer chromatography (TLC) analysis of a reaction mixture, the spot of starting material 20 gradually became red after irradiation with UV light. This observation indicated a photoinduced reaction of 20 to form a π ─ extended compound and prompted us to conduct the photocyclization of 20. Dehydrohalogenative photocyclization was reported by Henderson and Zweig in 1967 50 and by Sato et al. in 1970, 51 which enabled photocyclization without the use of oxidants. Recently, Morin and his colleagues synthesized a series of PAHs fused with either electron ─ rich (thiophene) or electron ─ poor (pyridine) rings through the photochemical cyclodehy- drochlorination of carefully designed aryl chlorides in high yields. 52 Moreover, Alabugin et al. demonstrated a sequence of ICl ─ promoted cycloaromatizations of bis(biaryl)acetylene precursors and subsequent photochemical cyclodehydroiodin- 49d ation to provide π ─ extended [5]helicene derivatives. These previous reports also suggested that photochemical cyclodehy- drohalogenation could be an ef cient alternative method for the synthesis of the key intermediate 14. Indeed, the photochemical cyclodehydroiodination of Figure 6. Synthesis of DBOV derivatives 15b and 15c. iodinated bichrysene 20 in acetone in the presence of triethyl- TBAF: tetra ─ n ─ butylammonium uoride. TFMSA: triuoromethanesulfonic acid. amine (TEA) gave fused bichrysene 14b in 86% yield

Vol.78 No.11 2020 （ 99 ） 1097 of 3.13 Å (Figure 8c). The CH ─ π interaction between the DBOV cores and the 4 ─ position of the 2,6 ─ dimethylphenyl groups was suggested by a distance of 3.25 Å, which might have facilitated the crystallization of DBOV 15e in comparison to DBOV 15b with mesityl groups. 2.4 Optoelectronic and Photophysical Properties of DBOV The UV/Vis absorption and uorescence spectra of DBOV 15a exhibit sharp absorption and emission peaks at 625 and 637 nm, respectively, with clear vibronic progressions and a small Stokes shift of 301 cm -1 (Figure 9a). 26 In comparison with fused bichrysene 14a with cove edges instead of zigzag edges, the absorption maximum of 15a is redshifted by 88 nm, and the molar extinction coef cient is enhanced by approximately ve times. The absolute photoluminescence quantum yield (PLQY) of 15a was determined to be 0.79, indicating the potential application of DBOV as a red emitter.

Figure 7. Synthesis of DBOV derivatives 15d i through a sequence of ICl ─ promoted cycloaromatization─ and photocyclization. MSA: methanesulfonic acid.

(Figure 7). 53 TEA was added as the base to trap the generated HI to prevent an intramolecular Friedel ─ Crafts reaction of 14b to give an insoluble diketone byproduct. The yield of 14b from 10b was thus improved to 65% over two steps from 4.2% over six steps in the previous synthetic route via the Scholl reaction. This simpli ed and ef cient synthetic route allowed Figure 9. (a) UV/Vis absorption and uorescence spectra of fused the preparation of the key intermediate 14b on a gram scale bichrysene 14a and 15a (10 -5 M in toluene for all measure- and the subsequent syntheses of a series of DBOV derivatives ments). Reproduced with permission 26. Copyright (2017) Wiley Library. (b) UV/Vis absorption and uorescence with different meso ─ substituents, namely, dodecyl (15c), aryl -5 spectra of DBOV derivatives 15d ─ h (10 M in toluene for (15d ─ f and 15h ─ i), and TIPS ─ ethynyl (15g) groups. The total all measurements) 53. (c) Fluorescence spectra of two single yields of 15c ─ i were in the range of 14 ─ 36% over 9 steps in lin- 15e molecules embedded in a Zeonex lm at 296 K (λ exc=561 nm) and 4.5 K (shown in the inset; ear sequence from the commercially available starting material, 53 λ exc=565 nm) . (d) Simpli ed energy level scheme to high- signi cantly better than the ～2% total yield in the rst and light the relevant photophysical transitions in 15e 53. second synthetic routes over 13 ─ 14 steps. Although DBOV derivatives 15a c from the rst and sec- DBOV 15b displays the longest ─ wavelength absorption ond syntheses could not be crystallized,─ the improved synthetic maximum at 609 nm (2.04 eV), which is 16 nm shorter than method enabled the exploration of a wider variety of DBOV that of DBOV 15a (635 nm; 1.98 eV), suggesting that the addi- derivatives with different substituents. A single crystal suitable tion of dodecyl chains at the 5 and 13 positions lowers the 45b for X ─ ray diffraction analysis could thus be obtained for optical energy gap by 0.06 eV. DBOV derivatives 15b and DBOV 15e with two 2,6 ─ dimethylphenyl groups, revealing the 15c h all show very similar absorption spectra with maxima in 53 ─ precise structure of the DBOV core (Figure 8a ─ b). Interest- the range of 607 ─ 611 nm (2.03 ─ 2.04 eV), which indicates that ingly, 15e exhibited a herringbone π ─ stacking motif in the the aryl and alkyl groups at the 6 and 14 positions have a negli- crystal, with a face ─ to ─ face distance between two DBOV cores gible inuence on the electronic properties of the DBOV core, most likely because of the large dihedral angles prohibiting π ─ conjugation for the aryl groups (Figure 9b). 53 For the redshift observed for 15a, there might also be an interaction between the neighboring dodecyl and mesityl groups affecting the dihedral angles of the latter and/or the planarity of the DBOV core. In contrast, the absorption maximum of DBOV 15h with TIPS ─ ethynyl groups is observed at 647 nm (1.92 eV), which is redshifted by approximately 40 nm (0.12 eV). This result dem- onstrates an ef cient π ─ conjugation between the DBOV core and the acetylene moieties and the consequent potential of Figure 8. Single ─ crystal structure of 15e: (a) front view; (b) side view; (c) packing pattern. 53 modulating the optoelectronic properties of DBOV by prop-

1098 （ 100 ） J. Synth. Org. Chem., Jpn. erly selecting the substituents. According to DFT calculations, the HOMO ─ LUMO energy gaps of 15d g are approximately 2.1 eV, while 15h has a smaller energy gap─ of 1.9 eV, which is in very good agreement with the experimental observations. DBOV derivatives 15d ─ g display strong red uorescence with maximum emission peaks located in the range of 611 ─ 617 nm (2.01 ─ 2.03 eV) with a high relative PLQY of 0.79 ─ 0.89 (Figure 9b). 53 The emission peak of DBOV 15h was redshifted to 650 nm, similar to the absorption spectra, while its PLQY was 0.67 and relatively lower than the values of the other Figure 10. (a,b) Ultrafast transient spectra of 15a in (a) solution and (b) a 1 wt% blend with PS. (c) Photoluminescence spectra derivatives, which can be ascribed to increased vibronic cou- of 15a in a 1 wt% blend with PS taken at laser power u- pling and enhanced intersystem crossing. ences below and above the ASE threshold. Reproduced 26 To further reveal the photophysical properties of DBOV, with permission. Copyright (2017) Wiley Library. single ─ molecule spectroscopy (SMS) of 15e was performed at room temperature (296 K) and 4.5 K by embedding 15e in a Further transient absorption studies comparing DBOV polymer matrix in collaboration with Thomas Basché and his derivatives 15a ─ c revealed that the lifetime of SE depends on colleagues 53. SMS enables to study the optical properties of the concentration and aggregation tendency because of the single molecules without intermolecular interactions, and the competition of the optical gain and intermolecular charge spectral broadening due to the vibronic coupling can be sup- transfer. 45b DBOV 15a with two mesityl and two dodecyl pressed by measurement at low temperatures, elucidating the groups thus demonstrated the longest SE lifetimes at lower purely optical transitions. 54 The high photostability of DBOV concentrations, while the less substituted 15b and especially is advantageous for the SMS to collect different information 15c displayed shorter SE lifetimes. during the measurements. The single ─ molecule uorescence The SE signal of DBOV 15a ─ c vanished in their neat lms spectrum of 15e at 296 K (Figure 9c) is very similar to the along with the aggregation ─ induced quenching of the uores- spectrum measured in solution (Figure 9b). In contrast, the cence. Nevertheless, the uorescence and SE in thin lms could single ─ molecule spectrum at 4.5 K displays a series of narrow be recovered by embedding 15a c in polystyrene (PS) vibronic transitions, providing evidence that the broad emis- (Figure 10b). Notably, ampli ed stimulated─ emission (ASE) sion bands observed in solution and for the single molecule at could be observed from lms containing 1 wt% 15a room temperature are due to the thermal broadening of several (Figure 10c) and 15b in the PS matrix. The ASE threshold was vibronic transitions. Furthermore, uorescence correlation also dependent on the aggregation tendency of the DBOV spectroscopy was performed to study the rates of the photo- derivatives, similar to the observation of the SE lifetimes, and physical transitions of DBOV 15e. At room temperature, a thus the threshold of 15a (60 μJcm -2) was lower than that of -2 three ─ level system with a singlet ground state (S 0), singlet 15b (180 μJcm ), and no ASE could be achieved for 15c. excited state (S 1), and triplet excited state (T 1) was applied, and These results indicate that the photophysical properties of the photophysical parameters of 15e could be determined as DBOV can be tuned by engineering the peripheral substituents, 8 -1 the uorescence decay rate k 21=1.5×10 s , intersystem cross- which is highly advantageous for their applications and makes 5 -1 ing (ISC) rate k 23=2.2×10 s , and triplet decay rate k 31 it indispensable to develop further synthetic methods to func- 3 1 =2.2×10 s - (Figure 9d). At 4.5 K, the zero ─ eld splitting of tionalize the DBOV core. the triple state (T 1) into triplet sublevels t xy and t z was consid- 2.5 Self assembly Behavior of DBOV with ered, and the ISC and triplet delay rates could be determined Tridodecyloxyphenyl─ Groups xy 4 -1 z 3 -1 xy 3 -1 as k 23=2.0×10 s , k 23=1.6×10 s , k 31=2.0×10 s , and PAHs show strong intermolecular π ─ π stacking, and their z 2 -1 k 31=4.4×10 s (Figure 9d). The triplet decay rate remained self ─ assembly behavior has been extensively explored, along almost unchanged at 296 and 4.5 K, while the ISC rate with the construction of various supramolecular structures by increased by more than one order of magnitude at 296 K. using different PAH cores and varying peripheral substitu- 57 Moreover, a single molecule of DBOV 15e showed high ─ con- ents. For example, columnar self ─ assemblies are observed for trast photon antibunching and single ─ photon emission, which PAHs with rigid cores that are substituted with exible ali- rendered DBOV of potential interest as a single ─ photon source phatic chains at the peripheral positions, displaying discotic for quantum information technology applications. 53,55 liquid crystalline (LC) properties. 7a,58 Triphenylene, perylene, The photophysical properties of DBOV 15a were also and HBC derivatives are representative examples, demonstrat- studied in detail by ultrafast transient absorption spectroscopy ing columnar supramolecular structures, which have also been 59 in collaboration with Francesco Scotognella and his col- studied for applications in eld ─ effect transistors. On the 26,45b,56 leagues. Transient absorption spectra of 15a in solution other hand, the self ─ assembly of PAHs on surfaces or at solid/ displayed i) a negative signal at approximately 450 nm, corre- liquid interfaces has been extensively investigated by scanning sponding to photoinduced absorption from S 1 to higher excited probe microscopy, demonstrating the formation of various states; ii) positive signals at 570 and 650 nm due to photo- two ─ dimensional (2D) supramolecular nanoarchitectures that bleaching, namely, the depletion of the ground state upon can be ne ─ tuned, for example, by the choice of PAH core, excitation; and iii) a positive signal at 695 nm, which could be peripheral substituents, and substitution pattern. assigned to the stimulated emission (SE) (Figure 10a). The The self ─ assembly behavior of DBOV was investigated by observation of SE indicated the potential of DBOV for appli- using 15i with two 3,4,5 ─ tris(dodecyloxy)phenyl groups at the cation to be optical gain materials, for example, in laser and meso ─ positions (see Figure 7) in collaboration with the labora- optical ampli ers. tories of Wojciech Pisula and Steven de Feyter. 60 Differential

Vol.78 No.11 2020 （ 101 ） 1099 scanning calorimetry (DSC) and polarized optical microscopy cules and determining their precise locations. By using uoro- (POM) revealed the melting of 15i into the LC phase upon phores with the so ─ called blinking properties, which switches heating to 158 ℃, and two ─ dimensional wide ─ and small ─ between on and off states under continues optical excitation, angle X ─ ray scattering (2D ─ WAXS and 2D ─ SAXS) analyses each uorescence image shows locations of different sets of the indicated self ─ assembly into columnar stacks arranged in a uorophore molecules without spatial overlaps, and tting of hexagonal lattice, with an intracolumnar π ─ π distance of the data provides the high ─ resolution images. With the view of 0.36 nm. After cooling the sample to the crystalline phase at applying DBOV as a uorophore for optical imaging, espe- 30 ℃, the molecular motion of 15i inside the supramolecular cially in SMLM, the blinking properties of 15b were investi- organization was reduced, demonstrating a higher long ─ range gated in collaboration with Xiaomin Liu, Mischa Bonn, and 27 order and slightly increased π ─ stacking distance of 0.37 nm. their colleagues. DBOV 15b indeed demonstrated blinking The intercolumnar arrangement changed from hexagonal to a with a blinking time of 87 ms, which is approximately 1.3 ─ fold square lattice, and helical intracolumnar packing with a rota- longer than that of Alexa 647 (69 ms), one of the most widely tion of 12° between adjacent molecules as well as a very long used uorophores for SMLM. Remarkably, the blinking of helical pitch of 5.55 nm were elucidated (Figure 11a). 15b could be observed under various environments, including under air and in a PS matrix, in stark contrast to Alexa 647, which needs a special buffer for blinking. Moreover, 15b displayed highly stable blinking behavior, showing comparable numbers of photons per blinking event after storing samples over several months. The applicability of 15b in the SMLM could also be con rmed by the visualization of nanoscale crevices on a glass surface, which agreed very well with the atomic force microscopy images. These results indicate the great potential of DBOV as a uorescent probe in imaging applications, especially in bioimaging. 3． Regioselective Bromination of DBOV

Figure 11. (a) Schematic illustration of the helical organization of After establishing the scalable synthetic method for DBOV 15i at 30 ℃. (b) High ─ resolution STM images superim- derivatives 15 with various meso ─ substituents and revealing posed on a tentative model of the molecular assembly. their highly intriguing optoelectronic and photophysical prop- Reproduced with permission. 60 Copyright (2019) Royal Society of Chemistry. erties as well as self ─ assembly behavior, direct postsynthetic substitution of the DBOV core was next considered. To this The 2D self ─ assembly behavior of 15i was studied at the end, meso ─ mesityl substituted DBOV 15b was employed, and 62 interface of highly oriented pyrolytic graphite (HOPG) and 1 ─ different bromination conditions were applied. The treatment phenyloctane by scanning tunneling microscopy (STM), which of 15b with bromine apparently resulted in the oxidation of revealed molecular layers with DBOV cores aligned in a row ─ 15b, and the use of NBS in a mixture of chloroform and N,N ─ like fashion (Figure 11b). Interestingly, 15i preferentially dimethylformamide led to no reaction. Nevertheless, the reac- formed molecular bilayers even in a dilute solution of 6×10 -6 tion of 15b with NBS in tetrahydrofuran successfully provided mol L -1, in contrast to the majority of previously studied dibrominated DBOV 21 in 79% yield (Figure 13). The struc- 1 PAHs, which predominantly showed self ─ assembled monolay- ture of 21 was con rmed by H NMR analyses in views of ers at solid ─ liquid interfaces. Moreover, the orientation of the disappearance of the doublet peak of 15b at 9.21 ppm and the DBOV cores was perpendicular to the row in the second layer, presence of 6 doublet peaks (Figure 14a ─ b), as well as by sin- but they were rotated by approximately 40° with respect to the row in the rst layer. These results indicated that the self ─ assembled structure of the second layer was affected by the rst layer and that the interactions between the molecules of 15i were stronger than the molecule ─ surface interactions. It should be noted that the layers above the second layer were probably removed during the STM measurements, as the STM tip needed to come closer to the HOPG to have suf cient tunneling currents. The self ─ assembly of 15i into well ─ organized columnar and 2D supramolecular structures supports the potential of such DBOV derivatives for optoelectronic device applications. 2.6 Application of DBOV in Superresolution Microscopy Superresolution uorescence microscopy, such as single ─ molecule localization microscopy (SMLM) and stimulated emission depletion (STED) microscopy, allows visualization with higher resolutions than conventional light microscopy, going beyond the optical diffraction limit. 61 The SMLM is Figure 13. Synthesis of DBOV derivatives 22 by the selective bro- typically based on the acquisition of thousands of uorescence mination of DBOV ─ Mes at the peripheral positions and images, detecting the emission from single uorophore mole- transition ─ metal ─ catalyzed cross ─ coupling reactions.

1100 （ 102 ） J. Synth. Org. Chem., Jpn. gle ─ crystal X ─ ray diffraction analysis that clearly revealed the the Diels ─ Alder cycloaddition of DBOV 15b with diphenyl- two bromo groups at the 3 ─ and 11 ─ positions (Figure 14c ─ d). acetylene was initially attempted, but there was no reaction Calculation of the Mulliken charge distributions on the DBOV even upon heating at 180 ℃ in o ─ dichlorobenzene for 24 h, core indicated higher electron density at the 3 ─ and 11 ─ posi- most likely because of the weak diene character of 15b tions, accounting for the selective bromination at these posi- (Figure 15). In an alternative approach, TIPS ─ ethynyl ─ substi- tions. tuted DBOV 22d was deprotected with TBAF to provide ethynyl ─ substituted DBOV 25 in 32% yield. Subsequently, the PtCl 2─ catalyzed cycloaromatization of the ethynyl groups of 25 afforded circumpyrene 26 in 46% yield. However, the poor solubility of 26 prohibited further characterization of its struc- 13 ture and properties, for example, by C NMR, single ─ crystal X ─ ray analysis, and cyclic voltammetry. On the other hand, as the second synthetic approach, the Pd ─ catalyzed direct benzannulation of 21 and diphenylacetylene was carried out, which provided benzo[bc]naphtho[2,1,8,7 ─ stuv ]ovalene (BNOV) 23 and circumpyrene 24 in 40% and 15% yields, respectively.

1 Figure 14. H NMR spectra of DBOV ─ Mes 15b (a) and 21 (b). Single crystallographic structure of 21 (c) front view and (d) side view. Reproduced with permission. 62 Copyright (2019) Wiley Library.

With dibrominated DBOV 21 in hand, further edge functionalization of DBOV at these bay positions was envis- aged. As an initial test, DBOV 21 was subjected to Suzuki ─ Miyaura coupling with three arylboronic acids and Sono- gashira coupling with TIPS ─ acetylene, which afforded 3,11 ─ diaryl ─ substituted DBOV 22a ─ c in 33 ─ 69% yield and 3,11 ─ bis(TIPS ─ ethynyl) ─ substituted DBOV 22d in 31% yield, respectively (Figure 13). The UV ─ Vis absorption spectra of 22a c were similar to those of DBOV derivatives 15 with ─ Figure 15. Synthesis of circumpyrenes 24 and 26. TABF: absorption maxima at 625 ─ 633 nm (1.96 ─ 1.98 eV), which were tetra ─ n ─ butylammonium uoride. redshifted by 10 ─ 18 nm (0.04 ─ 0.06 eV) in comparison to 15b. 1 The small redshift might be due to slightly extended π ─ conju- The H NMR spectrum of BNOV 23 exhibited ve new gation between the DBOV core and the aryl groups at the bay signals from the aromatic core due to the lower symmetry after positions. TIPS ─ ethynyl ─ substituted DBOV 22d displayed an fusing one C=C bond to DBOV 15b (Figure 16a). In contrast, absorption maximum at 646 nm (1.92 eV) with a larger redshift the 1 H NMR spectrum of circumpyrene 24 showed one singlet of 35 nm (0.10 eV) than that of 15b, similar to the observation peak and four sets of doublet peaks from the aromatic core, for DBOV 15h with TIPS ─ ethynyl groups at the meso ─ posi- and their chemical shifts were shifted down eld with respect to tions. Notably, 22a ─ c demonstrated an improved relative the corresponding peaks observed in the spectra of 15b and 23. PLQY of 0.91 ─ 0.97, which might be a result of limited inter- The structure of circumpyrene 24 was also demonstrated by molecular interactions by the additional bulky substituents as single ─ crystal X ─ ray diffraction analysis (Figure 16b ─ d). The well as distortion of the planarity of the DBOV core by bay plane ─ to ─ plane distance of 24 is 4.73 Å, indicating no π ─ π substitution. The PLQY of 22d was 0.69, similar to that of interactions (Figure 16d). The short distance between the CH 15h. This method paves the way towards the bay ─ position bonds on the phenyl rings and the core of the neighboring cir- functionalization of DBOV with various substituents for mod- cumpyrene molecules (2.75 Å) indicates the existence of CH ─ π ulation of the optoelectronic properties, as well as building up interactions, which can be responsible for the highly ordered complicated molecular systems incorporating DBOV units. packing mode in the single crystals (Figure 16d). Interestingly, the UV/Vis absorption spectra revealed that 4． Synthesis of Circumpyrene from Brominated DBOV the longest absorption wavelength of 23 (555 nm; 2.23 eV) was The bay regions of PAHs are often used for the further π ─ blueshifted by 56 nm in comparison with that of 15b (611 nm; extension of their π ─ conjugated structures, for example, by 2.03 eV) (Figure 17a), indicating an increase in the optical 63 intramolecular Friedel ─ Crafts cyclization, Diels ─ Alder cyclo- energy gap by 0.20 eV upon π ─ extension. This observation 64 65 addition, and Pd ─ catalyzed direct annulation. To this end, could be understood in terms of a decreased number of π ─ π ─ extension of DBOV through the introduction of two addi- electrons in the conjugation pathway of 23 according to the tional C=C bonds at its bay regions can lead to the long ─ anisotropy of the induced current density (ACID) calculations 33 awaited circumpyrene. For the synthesis of circumpyrene 24, and was also in line with an increased number of Clar’s π ─ sex-

Vol.78 No.11 2020 （ 103 ） 1101 were 0.79, 0.42, 0.11, and 0.17, respectively, which dropped drastically after fusing extra double bonds, highlighting the sensitive dependence of the optical properties on the PAH structure. Furthermore, cyclic voltammetry analyses of 15b, 23, and 24 allowed the estimation of the electrochemical energy gaps based on the onset potentials of the rst oxidation and reduction peaks to be 1.81, 2.06, and 2.16 eV, respectively (Figure 17c). This trend was consistent with the HOMO ─ LUMO energy gaps based on the DFT calculations (Figure 17d), as well as the optical energy gap estimated from the UV/Vis absorption spectra, which could be understood in terms of the decreased number of π ─ electrons in the conjugation pathway after fusing C=C bonds. 5． Conclusion The successful synthesis of DBOV 15, a new PAH with zigzag and armchair edges revealed its intriguing optical properties, including strong red uorescence, stimulated emission, and uorescence blinking under both air and polymer matrix, which enabled the application of 15 as a uorophore in SMLM imaging. Although the initial synthetic routes through oxida- 1 33 Figure 16. (a) H NMR spectra of 15b, 23, and 24. Single ─ crystal tive cyclodehydrogenation to prepare 14 provided 15 in total structure of circumpyrene 24: (b) front view; (c) side view; yields of only ～2% over 13 14 steps, a simpli ed and scalable 33 ─ (d) packing arrangement in the crystal. synthesis of DBOV 15 was developed based on the iodination ─ benzannulation of diyne 10b to give iodinated bichrysene 20, followed by photochemical cyclodehydroiodination to yield 14b on a gram scale. DBOV 15 could thus be obtained in a total yield of 14 ─ 36% over 9 steps, which also allowed the exploration of its postsynthetic substitution, leading to the regioselective bromination of DBOV at the 3 and 11 positions. Moreover, circumpyrene could be achieved as the largest synthesized circumarene to date through π ─ extension at the bay regions of the DBOV core via the transition ─ metal ─ catalyzed alkyne benzannulation of dibrominated DBOV 21. These results indicate the high potential of DBOV as a highly uorescent and stable PAH for optoelectronic, photonic, and imaging applications as well as the possibility of developing a wider variety of unprecedented PAH structures starting from DBOV.

Acknowledgments Figure 17. (a) UV/Vis absorption and (b) uorescence spectra of We acknowledge all of our distinguished collaborators and 15b, 23, 24, and 26 (10 -6 M in toluene for all measure- 33 dedicated colleagues who enabled the achievements described ments). (c) Cyclic voltammograms of 15b, 23, and 24 measured with 0.1 M tetra ─ n ─ butylammonium hexauo- in this article. We appreciate the nancial support from the rophosphate in dichloromethane ( ＊indicates the onset Okinawa Institute of Science and Technology Graduate Uni- 33 potential of these oxidation/reduction peaks). (d) versity, the Max Planck Society, the Marie Curie ITN project HOMO and LUMO energy levels and HOMO ─ LUMO energy gaps of 15b, 23, 24, and 26 calculated by DFT at iSwitch (GA No. 642196), and the ANR ─ DFG NLE grant 33 the B3LYP/6 ─ 31G(d,p) level. GRANAO by DFG 431450789. tets from 15b (four) to 16 ( ve). By comparison, circumpyrenes References 24 and 26 displayed distinct UV/Vis absorption patterns with 1） (a) Narita, A.; Wang, X. Y.; Feng, X. L.; Müllen, K. Chem. Soc. Rev. the longest ─ wavelength bands at 549 and 558 nm, respectively, 2015, 44, 6616. (b) Wang, X. Y.; Yao, X.; Müllen, K. Sci. China Chem. 2019, 62, 1099. (c) Xu, X. S.; Mullen, K.; Narita, A. Bull. Chem. Soc. which seemed to be forbidden transitions. This result agreed Jpn. 2020, 93, 490. (d) Chen, W. Q.; Yu, F.; Xu, Q.; Zhou, G. F.; very well with the theoretical prediction by Lischka et al., who Zhang, Q. C. Adv. Sci. 2020, 1903766. (e) Pozo, I.; Guitian, E.; Perez, reported that the longest ─ wavelength absorption band of cir- D.; Pena, D. Acc. Chem. Res. 2019, 52, 2472. (f) Ito, H.; Segawa, Y.; Murakami, K.; Itami, K. J. Am. Chem. Soc. 2019, 141, 3. (g) Mar- cumpyrene should arise from the forbidden HOMO ─ 1(H ─ quez, I. R.; Castro ─ Fernandez, S.; Millan, A.; Campana, A. G. Chem. 1)→LUMO (L) and H→L+1 transitions based on density Commun. 2018, 54, 6705. functional theory/multireference con guration interaction 2） Wang, X. Y.; Narita, A.; Müllen, K. Nat. Rev. Chem. 2017, 2, 0100. (DFT/MRCI) calculations. 66 In the orescence spectra, the 3） (a) Hu, W.; Huang, Y.; Qin, X. M.; Lin, L.; Kan, E. J.; Li, X. X.; Yang, C.; Yang, J. L. NPJ 2D Mater. Appl. 2019, 3, 1. (b) Sheng, W. maximum emission peaks were located at 614 nm (15b), D.; Korkusinski, M.; Guclu, A. D.; Zielinski, M.; Potasz, P.; Kadant- 563 nm (23), 555 nm (24), and 566 nm (26) with small Stokes sev, E. S.; Voznyy, O.; Hawrylak, P. Front. Phys. 2012, 7, 328. (c) Yan, shifts (Figure 17b). The absolute PLQYs of 15b, 23, 24, and 26 X.; Li, B.; Li, L. S. Acc. Chem. Res. 2013, 46, 2254. (d) Ritter, K. A.;

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Vol.78 No.11 2020 （ 105 ） 1103 K. J. Am. Chem. Soc. 2012, 134, 5876. (d) Pisula, W.; Tomović, Ž.; PROFILE Watson, M. D.; Müllen, K.; Kussmann, J.; Ochsenfeld, C.; Metzroth, T.; Gauss, J. J. Phys. Chem. B 2007, 111, 7481. Xiushang Xu received his Ph.D. degree at the 60） Chen, Q.; Zajaczkowski, W.; Seibel, J.; De Feyter, S.; Pisula, W.; Mül- Changchun Institute of Applied Chemistry, len, K.; Narita, A. J. Mater. Chem. C 2019, 7, 12898. Chinese Academy of Sciences under the su- 61） Pujals, S.; Feiner ─ Gracia, N.; Delcanale, P.; Voets, I.; Albertazzi, L. pervision of Professor Lixiang Wang in Janu- Nat. Rev. Chem. 2019, 3, 68. ary 2019. Since April 2019, he has been a 62） Chen, Q.; Wang, D.; Baumgarten, M.; Schollmeyer, D.; Müllen, K.; postdoctoral researcher in the Narita Unit at Narita, A. Chem. Asian J. 2019, 14, 1703. the Okinawa Institute of Science and Tech- 63） (a) Harvey, R. G.; Pataki, J.; Cortez, C.; Diraddo, P.; Yang, C. X. J. nology Graduate University (OIST). He was Org. Chem. 1991, 56, 1210. (b) Lungerich, D.; Papaianina, O.; Feo- also a guest scientist at the Max Planck Insti- fanov, M.; Liu, J.; Devarajulu, M.; Troyanov, S. I.; Maier, S.; Amsha- tute for Polymer Research (MPIP) in Mainz, rov, K. Nat. Commun. 2018, 9, 4756. Germany, for one year starting in April 2019. 64） Newman, M. S. J. Am. Chem. Soc. 1940, 62, 1683. His current research interest focuses on the 65） (a) Mandal, A. B.; Lee, G. H.; Liu, Y. H.; Peng, S. M.; Leung, M. K. bottom ─ up synthesis of novel nanocarbon J. Org. Chem. 2000, 65, 332. (b) Campo, M. A.; Huang, Q. H.; Yao, T. materials with atomically precise structures L.; Tian, Q. P.; Larock, R. C. J. Am. Chem. Soc. 2003, 125, 11506. (c) for the elucidation of their optical and elec- Ozaki, K.; Murai, K.; Matsuoka, W.; Kawasumi, K.; Ito, H.; Itami, tronic properties and applications for opto- K. Angew. Chem. Int. Ed. 2017, 56, 1361. electronic devices and bioimaging. 66） Shi, B. M.; Nachtigallova, D.; Aquino, A. J. A.; Machado, F. B. C.; Lischka, H. J. Chem. Phys. 2019, 150, 124302.

Qiang Chen obtained his Master’s degree at Nankai University in 2014 under the supervision of Professor Jianyu Zheng, where he

studied S NAr reactions of porphyrin. He then joined the group of Professor Klaus Müllen at the Max Planck Institute for Polymer Re- search in 2015, working in the subgroup of Dr. Akimitsu Narita, and obtained his Ph.D. degree in chemistry in 2019. Since December 2019, he has been a postdoctoral researcher in the same group. His current research focuses on the bottom ─ up syntheses of carbon ─ rich materials, including nanographene molecules and graphene nanoribbons, with well ─ de ned edge structures, as well as investigat- ing their optoelectronic properties and applications.

Akimitsu Narita studied chemistry at the University of Tokyo, where he received his Bachelor’s (2008) and Master’s (2010) degrees under the supervision of Professor Eiichi Nakamura. He then joined the group of Pro- fessor Klaus Müllen at MPIP and obtained his Ph.D. in Chemistry in 2014, granted by the Johannes Gutenberg University of Mainz. Since 2014, he has been a group lead- er at MPIP. In 2018, he joined OIST as an Assistant Professor (Adjunct) and became an Assistant Professor in 2020, leading the Or- ganic and Carbon Nanomaterials Unit. His current research focuses on the synthesis and characterization of large polycyclic aromatic hydrocarbons as nanographenes. He received the Chemical Society of Japan Award for Young Chemists for 2017.

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