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Enhancement of Antiaromatic Character via Additional Benzoannulation into Dibenzo[a,f]pentalene: Syntheses and Properties of Benzo[a]naphtho[2,1-f]pentalene and Dinaphtho[2,1-a,f]pentalene Akihito Konishi, Yui Okada, Ryohei Kishi, Masayoshi Nakano, and Makoto Yasuda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11530 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Journal of the American Chemical Society

Enhancement of Antiaromatic Character via Additional Benzoannulation into Dibenzo[a,f]pentalene: Syntheses and Properties of Benzo[a]naphtho[2,1-f]pentalene and Dinaphtho[2,1-a,f]pentalene Akihito Konishi,*,†,‡ Yui Okada,† Ryohei Kishi,§ Masayoshi Nakano,*,§ and Makoto Yasuda*,† † Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 5650871, Japan ‡ Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan § Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan ABSTRACT: Understanding the structure–property relationships in antiaromatic molecules is crucial to controlling their electronic properties and designing new organic optoelectronic materials. Dibenzo[a,f]pentalene, a structural isomer of dibenzopentalene, displays open-shell and antiaromatic character harmonization, which is not shared by the well-known isomer, dibenzo[a,e]pentalene. The next questions of interest concern the topological effects of the -extension on the harmonization of the open-shell and antiaromatic character in the dibenzo[a,f]pentalene -system. Herein, we describe the synthesis and characterization of the -extended (bis)annulated analogues, benzo[a]naphtho[2,1-f]pentalene 4 and dinaphtho[2,1-a,f]pentalene 5. The solid-state structures and the magnetic and optoelectronic properties characterized these -extended analogues as closed-shell antiaromatic molecules, in sharp contrast with dibenzo[a,f]pentalene 2. In these -extended analogues, the open-shell character was annihilated whereas the antiaromatic character was retained. The fusion of additional hexagons into 2 shifted the main 4n-conjugated circuit from a global to a local system. Further investigations into magnetic ring currents using gauge-including magnetically induced current (GIMIC) calculations suggested that an enhanced local paratropic ring current appeared in the pentalene core of 5. The preservation of the benzenoid character in the additionally fused hexagons confined the paratropicity to the pentalene subunit, and the inherent presence of an oquinoidal structure highlighted the 4n-electron delocalization on the pentalene unit. The antiaromaticity of 4 and 5 was characterized by their small HOMO–LUMO energy gap. Both experimental and computational results demonstrated that the [a,f]-type ring fusion of the pentalene core effectively enhanced the antiaromatic character compared with the [a,e]-type ring fusion in the reported bisannulated[a,e]pentalenes. The findings of this study could potentially be used for the rational design of optoelectronic devices based on novel antiaromatic molecules.

INTRODUCTION The concepts of aromaticity and antiaromaticity are fundamentally important to understanding and designing the physical and chemical properties of planar cyclic -conjugated systems.1–4 Non-alternant hydrocarbons that consist of several oddmembered rings, such as azulene, pentalene, and indacenes (Figure 1A),5–8 are an intriguing family for addressing the relationship between their -conjugation and (anti)aromaticity. They have peculiar electronic configurations and molecular orbital features that are not shared by alternant benzenoid hydrocarbons. A fusion of benzenoid rings to the non-alternant-systems impacts their electronic structures, which modulates the HOMO–LUMO gap and perturbations into their original (anti)aromatic nature.5,9–12

Figure 1. (A) Examples of non-alternant hydrocarbons and (B) molecular structures of the indenofluorene family, when isolable derivatives were first reported with ground state electronic configurations.

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The tunable narrow HOMO–LUMO gap in non-alternant hydrocarbons has fueled rich studies into the electronic properties of open-shell characters13–17 and into their potential use in functional organic optoelectronic materials.18–23 In this regard, the indenofluorene derivatives5,9,24 have recently received considerable attention (Figure 1B). Depending on the position and direction of the dibenzoannulation into the indacene core, the electronic configurations of these structural isomers in the ground state range widely from a closed-shell singlet25–31 to an open-shell singlet32 and triplet33 state. The -extension of the central part of indenofluorene or the change in the fused aromatic rings reduces the HOMO–LUMO energy gap34–37, enhances the open-shell22,38 or quinoidal39–41 character, and tailors the singlet–triplet energy gap.42 Investigations into the effects of benzoannulations of non-alternant -systems on their diatropic and/or paratropic character and electronic configurations have focused on the direct emergence of structural and electronic perturbations driven by annulations in smaller -systems.38,43 Pentalene is a small non-alternant hydrocarbon consisting of only 8 electrons. Due to the 8-antiaromatic character, pentalene readily dimerizes, even at –100 °C, without thermodynamic or kinetic stabilization44–46. Recently, the stable -extended pentalenes, dibenzo[a,e]pentalene47 1 and its derivatives, 10,12,48–56 have emerged as hot topics thanks to synthetic progress,57–59 including transition metalmediated annulation,60–64 Lewis acid-induced intramolecular coupling,65–69 and anionic or radical anionic trans-annulation.70– 74 The synthetic accessibility of dibenzo[a,e]pentalene derivatives has advanced our understanding of the antiaromaticity of the pentalene core modulated by the [a,e]-type ring fusions,10– 12 including the triplet aromaticity in the excited state.75 Furthermore, the high thermal stability and the amphoteric redox properties of dibenzo[a,e]pentalene derivatives have opened the door for functional chromophores.10,76–81 The dibenzo[a,e]pentalene-based materials have recently received significant attractiveness as semiconductors for filed-effect transistors.10,82–85 Very recently, our group reported another structural isomer of dibenzopentalene, dibenzo[a,f]pentalene86 2, which had been marginalized for many years due to its instability.87 The characterization of the dibenzo[a,f]pentalene derivative 2b enabled us to access an antiaromatic hydrocarbon in harmony with an open-shell character. The coexistence of antiaromaticity with open-shell character has been undefined in dibenzo[a,e]pentalene 1 and remains difficult to find in other non-alternant hydrocarbons.88–90 The [a,f]-type dibenzoannulation to the pentalene skeleton provides the following electronic features in the ground state (Figure 2B): (1) enhancement of the peripheral 16-delocalization, given by the resonance of the o-quinoidal substructure, and (2) the appearance of the singlet open-shell character induced by the trimethylenemethane (TMM) subunit with the recovery of an extra benzenoid ring. The energy balance between the destabilization of the singlet ground state induced by the 4n-electron delocalization and the stabilization of the triplet state derived from the TMM subunit in the openshell structure gives rise to the unusual electronic features. Our finding provides the fact that open-shell character and antiaromaticity should be inextricably linked together. In contrast to the numerous studies on the appearance of open-shell character associated with the recovery of local aromaticity in polycyclic hydrocarbons, a better understanding of the structure–property relationship between antiaromaticity and openshell character remains unexplored due to the lack of a suitably

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molecular series. Our diareno[a,f]pentalene framework could be an acceptable model to logically investigate this concerns. Further -extending dibenzo[a,f]pentalene with one or more benzene ring(s) provides the new insight into that how the harmonization of the antiaromaticity and open-shell character would be perturbed. Depending on the manner of the -extension, the exteriorization (or negation) of the only one character or the enhancement of the both characters would be logically achieved. Recently, Diederich’s group reported synthetic attempts toward the dinaphtho[1,2-a,f]pentalene derivative 3. Upon oxidative dehydrogenation of the precursor, the high reactivity of 3 toward cycloaddition with quinones, such as p-benzoquinone (BQ) or 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), hampered characterization of the fully conjugated product 3, which was hypothesized to have significant antiaromaticity based on theoretical calculations (Figure 2C).53 (A) Dibenzopentalenes R1

R1

R2

R2

[a,e]

R1

R1

R2

R2

[a,f]

1a: R1 = R2 = H

2a: R1 = R2 = H 2b: R1 = Mes, R2 = Me

(B) Resonance structures of 2a

TMM

16

TMM

(C) Synthetic attempt of 3 by Diederich's group (ref.53)

SiiPr3

SiiPr3

BQ

SiiPr3 H O H

3

O

(D) This work: -extended bisannulated[a,f]pentalenes 4 and 5 R1

R1

R1

R1

R2

4a: R1 = R2 = H 4b: R1 = Mes, R2 = Me

5a: R1 = H 5b: R1 = Mes

Figure 2. (A) Molecular structures of dibenzopentalenes 1 and 2. (B) Resonance structures of 2a. The peripheral 16 system in the closed-shell structure and the trimethylenemethane (TMM) subunit in the open-shell structures are highlighted by bold lines. The hexagonal rings in gray denote benzenoid rings. (C) Synthetic attempt toward 3 reported by Diederich’s group; BQ = benzoquinone, and (D) -extended bisannulated[a,f]pentalenes 4 and 5.

Herein, we report the synthesis and characterization of -extended (bis)annulated dibenzo[a,f]pentalene analogues, benzo[a]naphtho[2,1-f]pentalene 4 and dinaphtho[2,1-a,f]pentalene 5, composed of one or two 2,1-naphthoannulation(s) to the pentalene core in the [a,f] direction (Figure 2D). The compound 5 corresponds to a structural isomer of dinaphtho[1,2a,f]pentalene, which is the main framework of Diederich’s target 3. The compound 4 is a partially -extended analogue of 2 toward compound 5. The molecular geometries and physical properties of the synthesized 4b and 5b were characterized by


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means of X-ray crystallography, NMR spectroscopy, UV-visNIR spectroscopy, electrochemistry, and quantum chemical calculations. The obtained results of 4 and 5 indicated that the 2,1-naphthoannulation to the pentalene core in the [a,f] direction annihilated the open-shell character of dibenzo[a,f]pentalene 2 but enhanced the antiaromatic character of the 8-pentalene moiety.

RESULTS AND DISUCUSSIONS Synthesis. The syntheses of 4b and 5b, in which two mesityl groups were introduced to kinetically protect the reactive site, are shown in Scheme 1. For the standardization of the substituent effect on the NMR signals and bond length analyses with those of 2b, which was successfully characterized by the crystallographic analysis,86 a methyl group of 4b was introduced into the benzene ring. The methyl groups on the benzene rings would seemingly improve the crystalline nature of the product.60,86 Scheme 1. Synthetic route to 4b and 5ba)

ethyl malonate and the acetals 8/9 with catalytic amounts of indium(III) salts.91 The acetals 8/9 were prepared from the reported diarylketones 692/7.93 Considering that the desired compounds 4b/5b gain less aromatic stabilization than the dihydrogenated precursors 18/19, the conversion of the dihydrogenated precursors 18/19 into the fully -conjugated systems of 4b and 5b required extra attention. The oxidation of 18 was conducted through the dianion 4b2–. The dianion 4b2– was cleanly generated by the treatment of 18 with nBuLi in THF at –35°C according to Kitahara’s94 and our86 previous reports. Without the isolation of 4b2–, after confirming by NMR measurements, the subsequent two-electron oxidation of 4b2– with p-chloranil furnished 4b as a reddish-brown solid. Although the dianion 5b2– was also cleanly generated in the same fashion as the generation of 4b2–, the subsequent treatment of 5b2– with p-chloranil afforded complicated mixtures. As an alternative method, the treatment of 19 with DDQ, followed by a N2H4ꞏH2O quench, smoothly gave 5b as a reddish-purple solid, which was extremely different from the reported results for the synthetic attempts toward 3, which readily give the Diels–Alder adduct with DDQ. For the synthesis of 4b, the direct oxidative dehydrogenation of the precursor 18 with DDQ was tried, but this approach gave complex mixtures. Despite the kinetically protecting bulky mesityl groups, the deficit of one benzene ring in 4 compared to 5 failed to stabilize the final product or intermediates under the oxidative conditions of DDQ. Although compounds 4b and 5b are much more stable than 2b, a series of 1H NMR measurements of THF-d8 solutions of 4b and 5b revealed that exposure to air under indoor light at room temperature induced gradual decomposition (Figures S6 and S7). Half-lives of 3 days were observed for both 4b and 5b.

Open-shell Character.

a) DIBAL = diisobutylaluminum hydride, Mes = 2,4,6-trimethylphenyl, PPA = polyphosphoric acid, and DDQ = 2,3-dichloro5,6-dicyano-p-benzoquinone.

The dihydrogenated precursors 18/19 were synthesized according to our previous study on the dibenzo[a,f]pentalene derivative 2b.86 The diesters 10/11, which are key compounds for the syntheses of 18/19, were synthesized by the treatment of di-

The open-shell character of 4b and 5b was assessed by collecting 1H NMR spectra in THF-d8 at room temperature, as shown in Figure 3, along with the spectra of 2b recorded at – 100 °C. Sharp signals attributed to the main core protons of 4b and 5b were observed in the 1H NMR spectra, in contrast with the spectrum of 2b, which exhibited severe line broadening at room temperature due to the non-negligible contribution of the thermally excited triplet species.86 Variable temperature 1H NMR experiments conducted on 4b and 5b revealed no signal broadening of the aromatic protons, even at 100 °C (Figures S9A and S10A). Actually, ESR signals typical of triplet species were not detected in powdered samples of 4b and 5b at 420 K (Figures S9B and S10B). These observations illustrated that the -electrons strongly coupled to form a non-magnetic singlet ground state. The theoretical calculations supported the observed results (see also the discussions in the supporting information). At the LC-UBLYP/6-311G* level, the moderate diradical character (y) of 2a (y = 0.23) demonstrated that the ground state of 2a is an open-shell singlet state. In contrast, the indexes for 4a and 5a were negligible (4a: y = 0.00; 5a: y = 0.07), indicating that the ground states of 4a and 5a are closed-shell singlet states. Furthermore, the estimated singlet–triplet energy gaps (EST) of 4a and 5a were large (4a: EST = –8.20 kcal/mol; 5a: EST = –6.06 kcal/mol), which suggests that thermal populations of the triplet state around the room temperature are predicted to be negligibly small.



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Figure 3. Partial 1H NMR spectra (THF-d8) of (A) 2b (–100 °C, 400 MHz) and 2b2– (rt, 400 MHz), (B) 4b (rt, 600 MHz) and 4b2– (rt, 600 MHz), and (C) 5b (rt, 600 MHz) and 5b2– (rt, 600 MHz).

Table 1. Summary of the NICS(1) and HOMA valuesa)

NICS(1)

HOMA

NICS(1)

HOMA

NICS(1)

HOMA

ring

2a

2a2–

2b

ring

4a

4a2–

4b

5a

5a2–

5b

A

+27.2

–10.2

0.27

A

+20.4

–11.5

–0.18

A

+30.6

–12.5

0.18

A'

+23.8

–10.2

0.30

A'

+18.0

–11.6

–0.19

A'

+29.5

–12.5

0.27

B

+11.8

–9.26

0.79

B

+5.80

–9.23

–0.21

B

+8.34

–8.97

0.30

B'

+4.91

–9.26

0.70

B'

+1.28

–9.85

0.91

B'

–1.03

–8.97

0.65

C

–5.83

–8.10

0.89

C

–4.59

–8.62

0.88

C'

–6.03

–8.63

0.76

a) The NICS(1) values for 2a, 4a, 5a, 2a2–, 4a2–, and 5a2– were calculated at the GIAO-(U)B3LYP/6-311+G*//RB3LYP-D3/6-311G* level. The HOMA values for 2b, 4b, and 5b were calculated on the basis of the determined bond lengths by X-ray analysis.

The lowest-energy structures of 2b and 5b, which were estimated based on crystallographic analyses (Figure 5) and the computational optimizations of 2a and 5a (Table 1), assumed a CS-symmetric structure with bond length alternation (BLA); however, the observed NMR signals indicated that these molecules adopted a C2v-symmetric structure in the solution state. The rapid interconversion between the CS structures in a solution phase should have simplify the NMR signals; however, the 13 C NMR signals of 5b recorded at –100 °C retained the C2v symmetry (Figure S8).44,95 1

H NMR Spectroscopy and NICS(1) Calculation. A comparison of the 1H NMR spectra of 2b, 4b, and 5b revealed that the proton signals of main cores of 4b (5.47–6.74 ppm) and 5b (5.78–6.72 ppm) were shifted downfield compared to those of 2b (4.49–5.33 ppm). These downfield shifts for 4b and 5b suggested that the formally peripheral 4n-electron character, observed in 2b, was somewhat weakened by additional aromatic ring annulations into 2b (Figure 3). The nucleus-independent chemical shift (NICS) calculations (Table 1) supported the observed results. For 4 and 5, the outermost hexagons (rings C(C') represented in Table 1 and Figures 4 and 5) in the naphthalene units retained their aromatic character (NICS(1); 4a: –

5.83, 5a: –4.59(–6.03)). By contrast, the hexagons (rings B(B')) adjacent to the pentalene core showed weaker antiaromatic character (4a: +5.80(+1.28), 5a: +8.34(–1.03)) compared to 2a (+11.8(+4.91)), consistent with the downfield shifts of the H1 and H2 protons on the rings B(B') of 4b and 5b compared to those of 2b in the 1H NMR measurements. The variance of the NICS(1) values for rings B and B' of 5a is derived from the theoretically optimized structure assuming Cs-symmetry: At the structure, the ring B constitutes a quinoidal isobenzofulvene substructure, whereas the ring B' is a prat of an aromatic naphthalene unit. The effect of the paratropicity induced in 4b and 5b, however, were not neglected because the proton signals from the main cores of 4b and 5b were shifted upfield with respect to the normal aromatic region. The paratropicity in 2b, 4b, and 5b was quite obvious when comparing the 1H NMR signals from the main cores of 2b, 4b, and 5b with those of the aromatic dianions 2b2–, 4b2–, and 5b2– (Figure 3).96–101 The proton signals from the main cores of 2b2–, 4b2–, and 5b2– appeared in the range 6.27–8.21 ppm in the 1H NMR measurements, illustrating that these dianions exhibited (4n+2)-aromatic character. The NICS(1) calculations indicated that the aromatic characters in 2a2–, 4a2–, and 5a2– were comparable. All rings in 2a2–, 4a2–, and 5a2– were estimated to have similar negative NICS(1) values, indicating that global (4n+2)-aromatic character was induced (Table 1).


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Figure 4. (A) Magnetically induced current (MIC) density vector plots for the neutral and dianion species of 2a, 4a, and 5a. Diatropic currents rotate clockwise, whereas the paratropic currents rotate anticlockwise. The values in the color bar are given in a.u. The current vectors are scaled by a factor of 3 Å a.u.–1, where 1 a.u. = 100.63 nA T–1 Å–2. (B) Selected current strength (/nA T–1) of the peripheral C–C bonds of each system. Current strength was evaluated by integrating numerically the MIC densities on the bisection plane of each bond. The size of the arrow represents the relative strength of the current strength. Diatropic currents are denoted by red clockwise arrows with positive values, whereas paratropic currents are blue anticlockwise arrows with negative values.

The distributions of the negative charge in 2b2–, 4b2–, and 5b2– were estimated using the 13C NMR measurements (Tables S1– S3). With the help of 2D-HSQC and HMBC NMR spectroscopy, the 13C NMR signals from the carbon atoms bearing ring protons of the main core were assigned. The carbon atoms assigned to the same geometric positions in 2b2–, 4b2–, and 5b2– exhibited similar chemical shifts, indicating comparable -electron densities. The H1 protons on the rings B(B') were less affected by the shielding/deshielding effects of the mesityl groups and the adjacently fused hexagons. Thus, these H1 protons were placed in similar geometric and electronic environments within the series 2b, 4b, and 5b or 2b2–, 4b2–, and 5b2– (Tables S1–S3). The shielding effects of the pentalene moiety were assessed based on the upfield shifts in the H1 proton signals upon twoelectron oxidation of the dianions to the corresponding neutral pentalenes. Oxidation of 2b2– to 2b significantly shifted the H1 proton upfield ((1H) = –2.47 ppm), suggesting that the most pronounced paratropicity appeared in the hexagons of 2b, in good agreement with the large positive NICS(1) values in the rings B(B') of 2a (+11.8(+4.91)). The corresponding upfield shift in the H1 proton signals of 4b2–/4b, by contrast, was smaller ((1H) = –1.99 ppm), and the upfield shift in the H1 proton signals of 5b2–/5b ((1H) = –2.44 ppm) was comparable to that of 2b2–/2b, despite the less positive NICS(1) values in the rings

B(B') of 5a (+8.34(–1.03)) than in those of 2a. The observed upfield shift for 5b was affected by the paratropic ring current of the pentalene core, suggesting that a more enhanced paratropicity was induced in the pentalene core of 5 than in that of 2.

Gauge-Including Magnetically Induced Current (GIMIC) Calculations. A profound understanding of the (anti)aromatic nature of 2a, 4a, and 5a was provided by calculations of the magnetically induced ring current densities and ring current strengths using the gauge-including magnetically induced current (GIMIC) method,102–105 which is known to provide detailed information about current patterns, even in complicated -conjugated systems.105–108 Application of an external magnetic field perpendicular to the molecular plane (from the back to the front), induces diatropic (paratropic) ring currents that are visually represented as clockwise (anticlockwise) current patterns. A crucial advantage of the GIMIC method is that the ring current strengths can be quantitatively obtained by the numerical integration of the ring current densities passing the specific bonds of the studied molecule. A diatropic (paratropic) ring current strength is



assigned to a positive (negative) value. The calculation of current patterns and ring current strengths in polycyclic -conjugated systems can precisely characterize the (anti)aromatic features of the individual rings and distinguish the electron-delocalization pathways of the global and local -conjugated circuits.107,109,110 The magnetically induced current densities of the neutral and dianion species of 2a, 4a, and 5a are summarized in Figure 4. The GIMIC calculations perfectly supported the observed NMR results and NICS(1) calculations. The reconfigured ring currents upon oxidation of the dianion to a neutral species were visually demonstrated. Among the dianions 2a2–, 4a2–, and 5a2–,diatropic (clockwise) ring currents of ca. +11 to +15 nA/T uniformly flowed along peripheral pathways, indicating that these dianions behaved as global aromatic systems. By contrast, the neutral pentalenes 2a, 4a, and 5a exhibited paratropic (anticlockwise) ring currents that emerged preferably on the inner sides of the pentalene cores (Figures 4 and S13). The ring current patterns indicated the main canonical structures of these pentalene systems. The paratropic ring currents of 2a significantly expanded to the rings B and B' from the central pentalene core, indicating that the peripheral 16-antiaromatic character and the local 8-antiaromatic character contributed significantly to 2a. On the other hand, the paratropic ring currents of 4a and 5a were mainly distributed across the central pentalene units, and the influence on the adjacent hexagons was suppressed compared to 2a, illustrating that the local 8-antiaromatic character mainly appeared in the -extended pentalenes 4a and 5a. The fused outermost hexagonal rings (ring C(C')) of 4a and 5a retained their local aromatic character (Figures 4 and S12) The magnetic shielding effects observed in 4b and 5b originated from the balance of the induced ring currents between the 8antiaromatic pentalene unit and the fused aromatic rings.

X-ray Crystallographic Analysis and Bond Length Analysis. Single crystals suitable for X-ray crystallographic analysis were grown by slow evaporation of tetrahydrofuran/dimethyl sulfoxide (4b) and CH2Cl2/hexane (5b) in a nitrogen-filled glove box. Ortep drawings of 4b and 5b are shown in Figure 5. Two crystallographically independent molecules of 4b were present in the asymmetric unit, one of which is shown in Figure 5. No features distinguished the two molecules of 4b; therefore, the mean values of the two structures are reported in the following discussion (Figure S5). The X-ray crystallographic analyses illustrated that the main cores of 4b and 5b displayed planar structures, and the mesityl groups formed large dihedral angles (ca. 72° for 4b and ca. 78° for 5b) with the main core (Figures 5B and E). The antiaromatic characters of the central pentalene units in 4b and 5b were reflected in the molecular geometry. The solid state structures of 4b and 5b adopted CS-symmetry with significant BLA in the pentalene units, in good agreement with the theoretically optimized structures. The large BLAs in the pentalene cores of 4b and 5b are common features of reported antiaromatic pentalene derivatives50,51,111 and sharply contrast to the features of 2b (1.409(3)–1.458(3) Å)86, which was openshell in character. The degree of the BLA in the pentalene unit of 4b (1.362(6)–1.502(6) Å) was enhanced compared to that in 5b (1.397(6)–1.475(6) Å), indicating that -electron delocalization was lower in the pentalene moiety of 4b than in that of 5b.

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The harmonic oscillator model of the aromaticity (HOMA) values,112,113 which index the degree of bond alternation,114 also supported the observed features (HOMA values for rings A(A'): 2b: 0.27(0.30); 4b: –0.18(–0.19); 5b: 0.18(0.27)). The differences in the BLAs of the pentalene units in 4b and 5b were determined by differences in the -extension. In the unsymmetric -extension in 4, the outer hexagons C and B' structurally reveal a rather aromatic character (HOMA values: 0.91 and 0.89 in 4b). On the other hand, the inner hexagonal ring B exhibits a large degree of BLA (1.338(8)–1.502(6) Å, HOMA value: –0.21), which evokes the contribution of the isobenzofulvene substructure. The less aromatic character in ring B agreed with the slight inflow of the paratropic ring currents into the ring B, as demonstrated by the GIMIC calculations (Figure 4). These structural features, along with support from the 1H NMR observations (Figure 3) and the NICS(1) and GIMIC calculations (Table 1 and Figure 4), indicated that the main canonical structure of 4 was 4-A, with less antiaromatic character (Figure 6B), such that the two benzenoid rings relieved the 4n-antiaromatic characters found in 4-B and 4-C. By contrast, the symmetric -extensions in 5 effectively highlighted the 8-pentalene contributions to the main canonical structure of 5-A (or 5-A') (Figure 6C). The two symmetric benzenoid characters of the rings C(C') (HOMA value; 0.88(0.76)) derived from the two 2,1-naphthoannulations in the pentalene core effectively confined a substantial degree of the 8-antiaromatic character to the central pentalene -system of 5, in agreement with the observed 1H NMR chemical shifts (Figure 3) and the NICS(1) and GIMIC calculations (Table 1 and Figure 4).

Relationship between Antiaromatic Character and Benzoannulation(s) Notably, the paratropicity of the pentalene unit for 2a, 4a, and 5a is pronounced compared to that of the pristine pentalene itself (ring current strengths: –19.2 to –19.6 nA/T, see Table S5, and Figures S15–16). It is quite different from the -extended indacenes that exhibit weaker antiaromatic character than that of the pristine indacene.11,32 The ring current strengths, as well as the NICS(1) values, clearly indicated the effect of the hexagonal ring fusions on the enhanced antiaromaticity in the central pentalene core. The paratropic ring current strengths on the pentalene core increased in the following order: 4a (–22.8 to –20.1 nA/T), 2a (–30.6 to –26.9 nA/T), and 5a (–33.9 to –31.9 nA/T). We concluded that the additional aromatic ring annulation into 2 changed the dominant -conjugated system. In 2, the peripheral 16-antiaromatic and open-shell resonance structures contributed to the ground singlet state due to the presence of the o-quinoidal substructure and the recovery of an aromatic ring (Figure 6A).86 Fusion of the additional benzene ring(s) into 2 decreased the peripheral 4n-antiaromatic and open-shell characters, but the confined 8-antiaromatic character increased. Although fusion of the aromatic rings to an antiaromatic core usually suppressed its antiaromatic character, the regional preservation of aromatic stabilization in the outer hexagonal rings of 5 isolated the 8-pentalene character from the global conjugated system. The smaller 4n-conjugated circuit of 5 leads to the appearance of larger antiaromatic character than that of 2.


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Figure 5. X-ray structures of (A) top view and (B) side view of 4b; (D) top view and (E) side view of 5b. Thermal ellipsoids are drawn at the 50% probability level. Bond lengths (/Å) are presented in (C) for 4b and (D) for 5b with the representations of each ring.

A series of studies of diareno[a,e]pentalenes demonstrated that the larger bond order of the fused arenes bond played a significant role in enhancing the antiaromatic character of the central pentalene core because formal fixation of the double bonds on the pentalene core assisted the pentalene core in assuming an 8-system.10–12,51 Although compounds 4 and 5 were expected to display similar properties, the presence of an o-quinoidal structure (the 1,2-naphthoquinodimethane unit in 4 and 5) in diareno[a,f]pentalene characterized their antiaromatic character. The resonance or interconversion of the diareno[a,f]pentalenes, which relieved the frustrations of the quinoidal instability derived from the loss of an aromatic ring, encouraged the formal 8-electron delocalization on the pentalene units, giving rise to the 8-local antiaromatic character. The symmetric benzoannulation of 5, in which the two equivalent resonance structures (5A and A') equally shared the o-quinoidal unit, effectively confined the 4n-electron delocalization to the pentalene core while preserving the aromatic character in the outermost hexagons, leading to the appearance of strong paratropicity, unlike the [a,e]-type systems that do not include o-quinoidal structures (Tables S5 and S6, Figures S14–S16). The presence of the quinoidal subunits are frequently reminiscent of the appearance open-shell character as proaromatic units.13,14,16 For 4 and 5, however, the already-existing aromatic hexagons allows the formulation of an 8-conjugated circuit in the pentalene core,

which would override the cleavage of a -bond with the recovery of aromatic character of the quinoidal structure.



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Figure 7. UV/Vis/NIR absorption spectra of 2b (black), 4b (blue), and 5b (red) in CH2Cl2. The inset shows a magnified view. The background signals at 1700 nm arose from an overtone of the C–H vibrations of the solvent.

Table 2. Summary of the optoelectronic properties of 2b, 4b, and 5b.

Figure 6. Resonance structures of (A) 2a (B) 4a and (C) 5a. The hexagonal rings in gray denote benzenoid rings, and the 4n-conjugated circuits that contributed significantly to each canonical structure are highlighted in bold lines.

Optoelectronic Properties. The optoelectronic properties of 4b and 5b were investigated using electronic absorption spectroscopy and cyclic voltammetry. The electronic absorption spectra of 4b and 5b in CH2Cl2 exhibited moderate absorption bands at around 500 nm, together with weak broad bands having long tails that extended up to 1200 nm (Figure 7). The longest-wavelength absorption bands (4b: max = 808 nm; 5b:  max = 925 nm) showed higher energy shifts compared to that of 2b (max = 965 nm) but still reached the near-infrared. Notably, the tails of these bands exceeded 1200 nm (1.03 eV), indicating that the HOMO–LUMO gaps of these pentalene systems were narrow. No emission can be observed from 2b, 4b and 5b. The time-dependent (TD)-DFT calculations at the B3LYP/6-311+G* level indicated that the lowest-energy transitions in 4a and 5a could be ascribed to the forbidden HOMO–LUMO transition, in agreement with the observed results (Figure S17 and Tables S9 and S10). The weak and broad lowest-energy bands commonly observed in these bisannulated[a,f]pentalenes 2, 4, and 5, resulted from the inherent 4n-antiaromatic character10,12,50,51 of these pentalene systems rather than from the open-shell character found in 2.

The redox properties of 4b and 5b were examined by cyclic voltammetry. The results are summarized in Table 2. The cyclic voltammograms of 4b and 5b displayed two reversible and two irreversible redox waves (Figure 8). The large antiaromaticity of 4b and 5b was supported by the small HOMO–LUMO gaps. The difference between the first oxidation and reduction potentials was used to determine the electrochemical HOMO–LUMO gap of 1.68 eV for 4b and 1.41 eV for 5b, demonstrating that the HOMO–LUMO gaps in 4b and 5b were slightly expanded compared to that in 2b (1.34 eV) but remained sufficiently small compared to those of the reported diareno[a,e]pentalenes (ca. 2.0 eV).10,12,77,115 These experimental results indicated that the -expansion of pentalene core with the [a,f]-type ring fusions reduced the HOMO–LUMO energy gap more effectively than the [a,e]-type ring fusions.

Figure 8. Cyclic voltammograms of 2b (black), 4b (blue), and 5b (red) (V vs. Fc/Fc+, in 0.1 M nBu4NClO4/CH2Cl2, scan rate = 100 mV/s, room temperature).

CONCLUSION We revealed the effects of -extension(s) on the harmonization of the open-shell and antiaromatic characters of dibenzo[a,f]pentalene 2. Two -extended analogues having (bis)2,1-napthoannulation(s), benzo[a]naphtho[2,1-f]pentalene


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4, and dinaphtho[2,1-a,f]pentalene 5, were successfully synthesized and characterized. The (bis)annulation(s) into the dibenzo[a,f]pentalene 2 annihilated the open-shell character but focused the antiaromatic character on the pentalene core. We computationally examined the antiaromatic character of these -extended pentalenes. The fused benzene rings switched the antiaromatic character of these pentalenes from the peripheral 4n-character of 2 to the local 8-character of 5, which is a way to favor the enhancement of antiaromaticity over the emergence of open-shell character. The GIMIC calculations clearly demonstrated confinement of the paratropic ring currents to the central pentalene core induced by the additional fused hexagons. The confinement effect of the antiaromatic character was effective in the symmetric fusion pattern. The inherent presence of an oquinoidal substructure incorporated into the pentalene cores of these diareno[a,f]pentalene systems, which is not shared by the diareno[a,e]pentalenes, preserved the antiaromatic character, despite fusion of the aromatic rings. The narrow HOMO– LUMO gaps derived from the antiaromatic character provide attractive photophysical and electrochemical properties, such as absorptions that cover the visible to near-infrared region and amphoteric redox behaviors. These properties suggest the potential utility of these compounds as organic semiconducting materials. The present study not only introduces new pentalene-based chromophores, but also emphasizes the importance of topologically designed -extensions in global molecular structures. Our system is a model to control the balance between antiaromaticity and open-shell character. Thus, it broadens strategies supporting the design of other diareno[a,f]pentalene systems.

General Sekiyu Research & Development Encouragement Foundation for financial support. We thank Dr. N. Kanehisa (Osaka University) and Prof. I. Hisaki (Hokkaido University) for valuable advice regarding X-ray crystallography. We also thank Prof. S. Kuwabata, Dr. T. Uematsu, Prof. Y. Aso and Prof. Y. Ie (Osaka University) for assistance with electronic absorption measurements. Thanks are due to Dr. K. Inoue, (the Analytical Instrumentation Facility, Graduate School of Engineering, Osaka University) for assistance in obtaining the VT-NMR spectra. Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX Additional experimental, spectroscopic, and calculation data (PDF) Crystallographic data for 4b (CIF) Crystallographic data for 5b (CIF) Crystallographic data for 12Y (CIF)

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AUTHOR INFORMATION Corresponding Author

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* [email protected] * [email protected] * [email protected]

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ORCID

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Akihito Konishi: 0000-0002-3438-786X Ryohei Kishi: 0000-0002-6005-7629 Masayoshi Nakano: 0000-0002-3544-1290 Makoto Yasuda: 0000-0002-6618-2893

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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This work was supported by the JSPS KAKENHI Grants Number JP15H05848 in Middle Molecular Strategy, JP18H01977, JP18K19079 and JP18K14201. A.K. would like to thank the Tonen

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pentalene - ACS Publications - American Chemical Society

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