9,10-Dihydro-as-indacenodithiophenes: Isomers with an as-Indacene

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9,10-Dihydro-as-indacenodithiophenes: Isomers with an as-Indacene Core Hirokazu Miyoshi, Akino Nabe, Shreyam Chatterjee, and Yoshito Tobe J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03049 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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The Journal of Organic Chemistry

9,10-Dihydro-as-indacenodithiophenes: Isomers with an as-Indacene Core Hirokazu Miyoshi,† Akino Nabe,† Shreyam Chatterjee,‡ Yoshito Tobe*,†,‡,¶. † Division

of Frontier Materials Science, Graduate School of Engineering Science, Osaka University,

Toyonaka 560-8531, Osaka, Japan ‡ Nanoscience

and Nanotechnology Center, The Institute of Scientific and Industrial Research,

Ibaraki 567-0047, Osaka, Japan ¶ Department

of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu

30010, Taiwan [email protected] ABSTRACT Two isomers of 9,10-dihydro-as-indacenodithiophenes (DIDTs) and the corresponding diketones having an as-indacene core were synthesized. Their thermal, photophysical, and electrochemical properties were investigated, revealing that they depend on the direction of the fusion of the thiophene rings. For the DIDTs, the effect of the mode of ring fusion on the physical properties is discussed by comparison with the previously reported derivatives of DIDT isomers with an sindacene core. The observed difference between the HOMO/LUMO levels of the DIDT isomers is ascribed to the efficiency of π-conjugation, which depends on α- or β-linkage between the terminal thiophenes with the central benzene ring. In addition, the effect of the peripheral aromatic ring (thiophene or benzene) is elucidated by comparison with indeno[2,1-a]fluorene (DIF) bearing an asindacene core. The HOMO levels of DIDTs are significantly raised compared to that of structurally related DIF because of electron-donating character of the thiophene rings. For the DIDT diketones, structural effect due to the proximate carbonyl groups is discussed by comparison with the isomers with remote carbonyl groups. In diketones bearing proximate carbonyl groups, the LUMO levels are destabilized owing to antibonding interaction between the carbonyl oxygen atoms, resulting in approach of the LUMO and LUMO+1 energy levels.

INTRODUCTION Phenylene-based oligomers and polymers are widely employed as optoelectronic materials in organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs).1,2 Among them, dihydroindenofluorenes (DIFs; Figure 1)3–22 and DIF-based polymers23–31

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are particularly attractive for their ready accessibility, planarity of the π framework associated with high optoelectronic performance, and structural diversity due to the direction of the fusion of the 6-56-5-6 membered rings. As a result of different fusion modes, [1,2-a]-, [1,2-b]-, [2,1-a]-, [2,1-b]-, and [2,1-c]-DIF isomers are known. The latter characteristic leads to different photophysical properties as exemplified in a series of spiro-fluorene substituted DIF isomers by Poriel and co-workers.7–16 In addition, their diketo, bis(dicyanomethylene), and bis(dithiafulvenylene) derivatives of [1,2-b]-DIF, [2,1-b]-DIF and their polymers are shown to exhibit moderate to high charge mobility in OFET devices.31–38 Replacing the terminal benzene rings of DIFs with thiophene rings gives rise to dihydroindacenodithiophenes (DIDTs; Figure 1)40–43 which attract intense interest as alternatives of DIFs. In addition to higher HOMO levels of DIDTs than those of DIFs which allows modulation of charge carrier properties, versatile reactivity at the α-position of the terminal thiophene allows facile transformation to construct new π-conjugated systems for specific purposes such as co-polymers with various heterocyclic systems for OFETs44 and OPVs45–55 and unsymmetrically substituted spacers linking donor and acceptor units in dye-sensitized solar cells.56,57 Recently, n-type OFETs were fabricated using bis(dicyanomethylene) derivatives of DIDT isomers as active layers and their FET performance and stability were investigated.58 Thiazole-fused diketo-derivatives bearing the same framework were also synthesized and examined as n-type OFET active layers.59 Though the structural diversity of DIDTs increases significantly compared to DIFs due to variable position of the sulfur atoms in the two thiophene rings (see Figure S1 in Supporting Information),60 derivatives of only two s-indacene-based isomers, [1,2-b:5,6-b']-DIDT41–58 and [1,2-b:7,6-b']-DIDT,41,42,44 are hitherto known. In order to expand the structural diversity of DIDTs, we report here the synthesis of the parent compounds of as-indacene-based DIDTs, 9,10-dihydro-as-indaceno[2,3-b:7,6b']dithiophene (1) and 9,10-dihydro-as-indaceno[3,2-b:6,7-b']dithiophene (2) as well as the corresponding diketones 3, 4, and isomeric diketone 5 (Figure 1). In view of the reported synthesis of derivatives of DIFs and DIDTs, these compounds will serve as versatile starting materials of the dihydro-as-indacenodithiophene systems, because introduction of not only various alkyl/aryl groups at the methylene or carbonyl carbons to give thermal stability and solubility but also key functional groups at the α-position of the terminal thiophene rings to bestow functions is possible. In addition, the ketones will serve as precursors of fully conjugated as-indacenodithiophenes and their polymers.61–63 With the above-mentioned prospective, this paper reports the synthesis, thermal, photophysical, and electrochemical properties of 1–5. First, effect on the physical properties caused by replacing the peripheral benzene in indeno[2,1-a]fluorene (6)12 by thiophene in 1 and 2 is elucidated. It is deduced that the HOMO levels of 1 and 2 are significantly raised compared to that of


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6 because of electron-donating character of the thiophene rings. Whereas the LUMO level of 1 is higher than that of 6, that of 2 is slightly lower than that of 6. This leads to significant reduction of the HOMO-LUMO gap in 2. Then, effect of the mode of ring fusion on the physical properties is discussed by comparison of the properties of 1 and 2 with each other and with those of derivatives of DIDT isomers 7b (and 7a)41 and 8b (and 8a)41 with an s-indacene core. While the HOMO/LUMO levels of 1 are similar to those of 8a of meta-type ring fusion, those of 2 are nearly identical to those of 7a of para-type fusion mode. The difference between the HOMO/LUMO levels of 1 and 2 is ascribed to the efficiency of π-conjugation, which depends on α- (in 2) or β-linkage (in 1) between the terminal thiophenes with the central benzene ring. Interestingly, 2 exhibits fluorescence most efficiently among the known DIDTs. Finally, for diketones 3–5, their properties are compared to each other and with the known isomers 9b (and 9a)58 and 10b (and 10a),58 focusing on the steric effect of the proximate carbonyl groups in 3–5. The geometries of the DIDT frameworks of 3–5 are deformed, as judged from theoretically optimized structures, nucleus independent chemical shift (NICS), and harmonic oscillator model of aromaticity (HOMA), due to close proximity of carbonyl oxygen atoms. This leads to destabilization of their LUMO levels owing to antibonding interaction between the carbonyl oxygen atoms, resulting in not only increase of the HOMO-LUMO energy gaps of 3–5 compared to those of 9a and 10a but also approach of the LUMO and LUMO+1 energy levels of 3–5. In particular, because the LUMO and LUMO+1 levels of 4 are nearly degenerate, it exhibit a relatively intense lowest-energy absorption band compared to other DIDT diketones.

[2,1-a]-DIF 6

[1,2-b]-DIF

[2,1-b]-DIF

S

1

R

S

R'

R R [1,2-b:5,6-b']-DIT 7a R=R'=H 7b R=spiro-fluorene, R'=n-C6H13

R'

S

3 R

R'

S

R

S

S S

S

2 R

O

O

S

S

S

[2,1-c]-DIF

O

O

O

O S

S

[1,2-a]-DIF

R

4 O

R

S

R'

[1,2-b:7,6-b']-DIT 8a R=R'=H 8b R=spiro-fluorene, R'=n-C6H13

R

S

S

O

[1,2-b:5,6-b']-DIT 9a R=H 9b R= n-C6H13

5 R

R

O

O

S

S

R

[1,2-b:7,6-b']-DIT 10a R=H 10b R= n-C6H13

Figure 1. Chemical structures of DIFs including 6, DIDTs 1, 2, 7a/7b, 8a/8b, and diketones 3–5, 9a/9b and 10a/10b. RESULTS AND DISCUSSION



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Synthesis. Synthesis of 1–5 were performed as outlined in Scheme 1. Suzuki-Miyaura coupling reaction of dimethyl 3,6-dibromophthalate (12), obtained from 3,6-dibromo-o-xylene (11)64 by oxidation followed by esterification, with thiophene-3-ylboric acid65 or thiophene-2-ylboric acid66 gave 3,6-dithienylphthalates 13a and 14 in 99 and 98% yields, respectively. LAH reduction of 13a and 14 gave the corresponding alcohols 15 and 16 in 62 and 72% yields, respectively. Acidcatalyzed cyclization of 15 and 16 with TfOH afforded 1 and 2 in 42 and 96% yields, respectively, both as colorless solid. The lower yield of 1 may be due to its lability, because it gradually forms a base-line spot in TLC when kept in solution in the air, although both compounds are stable in solid state. In solution, 1 and 2 exhibit blue and bluish purple fluorescence, respectively. Direct cyclization of 13a with molten AlCl3-NaCl mixture at 140 °C gave diketone 3 in 72% isolated yield. The yield increase when Friedel-Crafts cyclization was conducted with diacid 13b obtained by alkaline hydrolysis of 13a (81% in two steps). Reaction of 13a with polyphosphoric acid (PPA) at 165 °C resulted in the formation of a mixture of 3, its isomer 5 derived by cyclization at β, β positions and most likely an less symmetrical isomer of α, β cyclization in an approximate ratio of 1 : 7 : 3 (by 1H NMR integration). The structure of the last isomer was assigned only based on the 1H NMR spectrum of the product mixtures (Figure S2). Because of the similar physical properties of the isomers it was difficult to separate the isomers by column chromatography and recycling GPC; only a small amount of 5 was isolated, and the isomer of α, β cyclization could never be isolated in pure form. Similar to the case for cyclization to the related DIF system,9 the ratio of these products changed depending on the temperature of reaction with PPA; at lower temperature (90 °C and 120 °C) only 3 was obtained in 9% and 22% yields, respectively, whereas 5 and the α, β cyclization product were not detected. On the other hand, attempts to obtain diketone 4 from diester 14 (PPA or H2SO4) resulted in the formation of complex mixture of products. Compound 4 was therefore obtained by permanganate oxidation of 2 in 12% yield. Scheme 1. Synthesis of 1–5a (d) or (e) (b)

(a) Br

Br 11

CO2Me

MeO2C

RO2C

3 or 3 + 5

CO2R S

S

HOH2C

13a R=Me, 13b R=H

S

(g) 1

(f)

Br

Br

CH2OH

S 15

12

MeO2C

(c)

CO2Me

S

HOH2C

CH2OH

(f)

S 14


(h)

(g) 2

S

S 16

4

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aReagents

and conditions: (a) (i) KMnO4, pyridine, H2O, reflux, (ii) dimethyl carbonate, H2SO4,

80 °C, 78% for two steps; (b) thiophen-3-ylboric acid, Pd(PPh3)4, aq. Na2CO3, toluene-EtOH, 75 °C, 99%; (c) thiophen-2-ylboric acid, Pd(PPh3)4, aq. Na2CO3, toluene-EtOH, 70 °C, 98%; (d) AlCl3, NaCl 140 °C, 72%, or (i) NaOH, H2O-EtOH, reflux, (ii) AlCl3, NaCl 140 °C, 81% for two steps; (e) PPA, 165 °C, mixture of 3, 5, and unsymmetrical isomer; (f) LAH, THF, 80 °C, 62% for 15 and 72% for 16; (g) TfOH, CH2Cl2, rt, 42% for 1 and 96% for 2; (h) KMnO4, pyridine, H2O, 100 °C, 12%. Thermal Properties. The thermal stability of 1–5 was investigated using differential scanning calorimetry (DSC) with a heating rate of 10 °C min−1 under N2. As shown in Figure S3, clear melting endothermic peaks are observed over 250 °C for compounds 1–4, whereas compound 5 shows no characteristic peak in the DCS curve. For 1 and 2, although sharp endothermic peaks corresponding to the melting points are observed in the first scan, there are no peaks in the cooling process as well as in second scan. The color of the samples of 1 and 2 changed from colorless to dark brown after melting and the materials were not recovered by washing with CDCl3, indicating thermal decomposition. On the other hand, both compounds 3 and 4 exhibit sharp, reversible endothermic peaks due to melting over 250 °C. The samples were recovered by dissolution in CDCl3. We also observed weight loss of up to 11% (for 3) and 12% (for 5) after heating presumably due to solvent evaporation. The weight loss, together with decomposition in the case of 1 and 2, explain the observed drift of the DCS curves. The DSC experiments thus reveal that compounds 3–5 have enough thermal stability to be used as building blocks in organic electronic applications. For 1 and 2, on the other hand, introduction of blocking groups is apparently necessary for applications. Photophysical and Electrochemical Properties. First, physical properties of 1 and 2 are described. Their physical properties are compared with those of DIF 612 with an as-indacene core and of DIDT isomers 7b41 and 8b41 to elucidate the effect of peripheral rings (benzene vs. thiophene) and the mode of ring fusion, respectively. Figure 2 shows UV-vis absorption and fluorescence spectra of 1 and 2 in THF and the corresponding data are summarized in Table 1 together with those reported for 6, 7b, and 8b. To examine the solvatochromic effects, absorption and fluorescence spectra were measured in cyclohexane and DCM (Figure S4, S5 and Table S1). However, little solvent effects are observed presumably because of small polarity in the ground and excited states of 1 and 2. The electrochemical behavior of DIDTs 1 and 2 was investigated by cyclic voltammetry (CV) in DCM solution containing 0.1 M tetrabultylammonium hexafluorophosphate (Bu4N·PF6) as a



supporting electrolyte. The ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an internal standard for the calibration of the potentials. The cyclic voltammograms are presented in Figure 3 and corresponding data are included in Table 1. No reduction waves of 1 and 2 are found within the potential window of the solvent (not shown), whereas irreversible oxidation waves are observed for both 1 and 2. The observed irreversibility indicates that the radical cations of the both compounds are kinetically unstable under electrochemical conditions, in contrast to the stable cationic species generated from 7b and 8b bearing substituents at both the methylene and α-thienyl positions. From the respective onset oxidation potentials (0.73 and 0.65 V, respectively) the HOMO energy levels were calculated as −5.13 eV and −5.05 eV for 1 and 2, respectively (Table 1). The experimental HOMO levels are in good agreement with the theoretical energy levels obtained from the DFT calculations described below (−5.17 eV and −5.06 eV, respectively, Table 2).

Figure 2. Absorption (ε × 104, left longitudinal axis) and fluorescence (normalized) spectra of 1 (black curves) and 2 (red curves) in THF. For both absorption and fluorescence spectra concentrations of 1 and 2 are 3.43 × 10−5 mol L−1 and 5.61 × 10−5 mol L−1, respectively. Excitation wavelengths for fluorescence spectra of 1 and 2 are 308 and 335 nm, respectively.

E onset

ox

Current (a.u.)

E onset

Current (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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: 0.73 V

ox

: 0.65 V

HOMO = 5.05 eV

HOMO = 5.13 eV

0.0

0.5

1.0

1.5

E (V) vs SCE

2.0 Paragon 2.5 Plus ACS 0.0 Environment 0.5

1.0

1.5

E (V) vs SCE

2.0

2.5

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Figure 3. Cyclic voltammograms for 1 (black curve) and 2 (red curve) in DCM with 0.1 M Bu4N·PF6 as a supporting electrolyte. No reduction peaks were observed. Table 1. Optical and Electrochemical Properties of 1, 2, 6, 7b, and 8b. Compd

λabs (nm)a

λem (nm)a

Φema

Eox / Eonsetox

Ered / Eonsetred

EHOMO

ELUMO

ΔEHOMO-

(V)

(V)

(eV)

(eV)

LUMO

1

281, 307, 323

349

0.12

Irrevb / 0.73

NOc

−5.13

–d

–d

2

334, 345, 352

360, 376

0.73

Irrevb / 0.65

NOc

−5.05

–d

–d

6e

307, 315, 322

326, 343,

0.6

1.31, 1.98 /

−2.6 / –f

−5.62

−1.94

3.68

0.80, 1.37,

−2.43, −2.75 /

−5.10

−2.09

3.01

1.65, 1.86 /

−2.31

−5.30

−1.92

3.38

–f

360 7bg

266, 340, 357,

383, 403

(eV)

0.26

376

0.70 8bg

261, 273, 293,

365, 373,

304, 321, 345,

380

0.33

0.99, 1.62,

−2.6, −2.8 /

1.99 / 0.9

−2.48

363 a In

THF unless otherwise stated. b Oxidation is irreversible. c Not observed. d Not determined due to

the absence of a reduction peak in CV. e Data taken from reference 12. f Data not reported. g Data taken from reference 41. Optical data are recorded in cyclohexane.

Table 2 compiles theoretical HOMO and LUMO energy levels, and HOMO-LUMO energy gaps of 1, 2, 6, and 7a and 8a calculated by DFT method with the B3LYP/6-311+G(d,p) level of theory. Compounds 7a and 8a are the unsubstituted parent compounds of 7b and 8b, respectively. The literature data for 7b and 8b are also included.41 The HOMO and LUMO diagrams of 1, 2, 6, 7a, and 8a are shown in Figure 4. Moreover, to see if the different mode of ring fusion would affect the stability and structure of DIDT isomers, the calculated energies by the DFT method and the parameters related to aromaticity, NICS(1) (nucleus independent chemical shift at 1 Å above the center of a ring)67,68 and HOMA (harmonic oscillator model of aromaticity)69,70 based on the optimized structures are compared for 1, 2, 7a, and 8a (Table S2). As seen in Table S2, though the calculated energies of β-linked isomers 1 and 8a are 1.8 and 0.4 kcal/mol larger than those of αlinked isomers 2 and 7a, there is virtually no difference in the geometries of the central benzene and terminal thiophene rings as judged from the aromaticity parameters which are nearly identical in all



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isomers. Table 2. Theoretical HOMO and LUMO Energy Levels, and HOMO-LUMO Gaps of 1, 2, 6 and 7a/7b and 8a/8b.a Compd

HOMO (eV)

LUMO (eV)

HOMO-LUMO gap (eV)

1

−5.47

−1.17

4.30

2

−5.38

−1.54

3.84

6

−5.70

−1.44

4.26

7a / 7bb

−5.34 / −5.11

−1.52 /−1.47

3.82 / 3.64

8a / 8bb

−5.53 / −5.23

−1.25 / −1.27

4.28 / 3.96

a DFT

calculations at the B3LYP/6-311+G(d,p) level. b Data taken from reference 40 for calculations

at the same level. S

S

S S

1

LUMO

1.17 eV

5.47 eV HOMO

S

S

6

2

LUMO

LUMO 1.54 eV

S

1.44 eV

LUMO

5.70 eV

HOMO

5.38 eV HOMO

S

8a

7a

LUMO 1.52 eV

5.34 eV HOMO

1.26 eV

5.53 eV HOMO

Figure 4. HOMO and LUMO of 1, 2, 6, 7a, and 8a for the geometries optimized by DFT calculations at the B3LYP/6-31G(d) level with orbitals shown with an isovalue of 0.04 (e bohr−3)1/2. In accordance with the theoretical HOMO/LUMO levels, 2 shows the first excitation absorption maximum at 352 nm which is lower in energy than that of 1 at 323 nm. Fluorescence maxima of 1 and 2 (excited at 308 nm and 335 nm, respectively) are observed at 349 and 376 nm with relatively narrow half-widths of 45 and 44 nm, respectively. The quantum yield of fluorescence of 2 (0.73) is much larger than that of 1 (0.12) being coincide with the extinction coefficients of the absorptions


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and the oscillator strength calculated by TD-DFT method at the B3LYP/6-31G(d) level (Table S3 and S4). Notably, solid material of 2 exhibits relatively intense fluorescence, which is recognized by naked eye, compared 1 whose emission cannot be recognized, when irradiated with a UV lamp. The details of solid fluorescence of 2 and its derivatives will be reported elsewhere. The smaller HOMOLUMO energy gap of 2 compared to that of 1 is due to higher HOMO and lower LUMO levels of 2 than those of 1. This is ascribed to more efficient π-conjugation in 2, in which the central benzene is linked at the α position of the terminal thiophenes, in contrast to cross conjugation in the β-linked isomer 1. Relevant effect of π-conjugation originating from the para- and meta-linkages in 7b and 8b has been reported.41 Compared to the structurally related [2,1-a]-DIF 6, which was reported to exhibit an absorption maximum at 322 nm and the shortest wavelength fluorescence maximum at 326 nm with a quantum yield of 0.60,11 DIDT 2 exhibits absorption and emission at longer wavelengths. On the other hand, DIDT 1 shows maxima at similar wavelengths to those of 6. As shown in Table 2, both HOMO and LUMO energies of 1 are increased compared to those of 6, presumably due to electron-donating effect of the thiophene rings of 1 on both HOMO and LUMO, resulting in a similar HOMO-LUMO gap to that of 6. In contrast, whereas the HOMO level of 2 is significantly raised compared to 6, the LUMO level is lowered, rendering the HOMO-LUMO gap narrower. This is attributed to electron-accepting effect of the sulfur atoms as can be seen in LUMO distribution (Figure 4) which spreads over the terminal thiophene rings. Similar HOMO raising/LUMO lowering effects of thiophene units are also observed in DIDT isomers 7b and 8b.41 DIDT isomers 7b and 8b are reported to show the lowest energy absorption maxima at 376 and 363 nm and fluorescence maxima at 403 nm and 380 nm (in cyclohexane), respectively, indicating that the nature of the linkages has a significant impact on the absorption and emission energies.41 Because it is demonstrated that the spiro-fluorene substituents decrease LUMO energy levels more efficiently than HOMO levels,12,41 direct comparison between the absorption and fluorescence maxima of 1 and 2 with those of 7b and 8b is not possible. Therefore, the theoretical HOMO/LUMO levels and their gaps of 1 and 2 are compared with those of theoretical data for unsubstituted 7a and 8a. As a result, the HOMO/LUMO energy levels of 1 and 8a are comparable, whereas those of 2 and 7a are similar to each other, though the terminal thiophene rings of 1 and 8a are linked at para and meta positions of the central benzene unit, respectively. The different bahavior of 1 and 2 is therefore ascribed to the efficiency of π-conjugated as described above. It is interesting to note that, in the isomers 1 and 2 with the different orientation of the thiophene rings, the fluorescence efficiency is remarkably different. Moreover, whereas the fluorescence quantum yield of 2 is comparable to that of DIF 6,12 those of DIDT isomers 7b and 8b are nearly half



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of those of 2 and 6.41 Although we do not understand the reason for these differences, these results suggest that the fluorescence efficiency is not solely determined by the presence or absence of sulfur atoms, but the mode of ring fusion and the location of sulfur atoms in the π-conjugated system which affect HOMO/LUMO distributions. Next, the physical properties of diketones 3–5 are discussed by comparing with those of isomeric diketones 9b and 10b.58 In contrast to the methylene-bridged DIDTs described above, the position of the carbonyl groups attached to the central benzene ring, ortho for 3–5 and para and meta for 9b and 10b, respectively, would affect not only the structures and energies of the diketones but also electronic properties including MO levels due to interaction between the carbonyl groups. More specifically, in diketones 3–5, the proximate carbonyl groups fixed in a planar molecular backbone would cause repulsive interaction between the oxygen atoms while no such interaction is expected in 9b and 10b. Therefore, the stabilities and aromaticity parameters of 3–5, and 9a and 10a, which are unsubstituted parent compounds of 9b and 10b, calculated based on the DFT method (B3LYP/6311+G(d,p) level) as listed in Table 3. Here the geometrical parameters, C=O bond length and O···O nonbonding distance, are also included. Table 3 clearly shows that ortho diketones 3–5 are less stable than para and meta isomers 9a and 10a. The difference is attributed to the steric compression around the proximate carbonyl groups in 3–5. The calculated C=O bond distances of 3–5 (1.207, 1.206, and 1.207 Å, respectively) are shorter than those of 9a and 10a (1.211 and 1.210 Å, respectively). This is experimentally supported from the higher frequencies of the C=O stretching bands of 3–5 in the IR spectra (1715, 1714, and 1712 cm−1, respectively) compared to those of 9a and 10a (both 1697 cm−1) and lower field 13C NMR chemical shifts of the former (181.8, 181.3, 182.4 ppm, respectively) than the latter (both 186.1 ppm). Moreover, the calculated O···O nonbonding distance of 3–5 (3.01 – 3.06 Å) are close to the sum of van der Waals radius of oxygen atom (1.52 Å),71 indicating steric compression between the facing oxygen atoms. It is worth mentioning that among the ortho-linked diketones 3–5, the aromaticity parameters (NICS(1) and HOMA) indicate that the benzene ring of 5 is the most aromatic whereas the thiophene ring of 5 is the least aromatic. This can be interpreted in terms of the fusion mode; in 5 the thiophene ring is cross-conjugated to both the central benzene ring and the carbonyl group and is not linearly conjugated to either of the π-system unlike 3 and 4 (see also Figure 1). The most pronounced aromaticity in the central benzene ring of 5 may be related to the relatively larger stability of 5 compared to 3 and 4. Table 3. Stability, Structure, and Aromaticity Parameters for 3–5, 9a, and 10a.


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Parameter

Compound 3

4

5

9a

10a

4.23

4.31

0.58

0.37

0

C=O bond length (Å)

1.207

1.206

1.207

1.211

1.210

O···O nonbonding distance (Å)

3.056

3.064

3.010

–

–

benzene

−6.7

−6.8

−8.8

−7.3

−6.9

thiophene

−8.7

−8.3

−7.9

−8.2

−8.5

benzene

0.778

0.828

0.927

0.950

0.943

thiophene

0.667

0.589

0.536

0.657

0.629

Relative stability (kcal/mol)a Structure around carbonyl

NICS(1)b

HOMAc

a

Relative energy of the DFT calculations at the B3LYP/6-311+G(d,p) level taking the energy of 10a

as the standard. b Nucleus independent chemical shift 1 Å above the center of the ring calculated by the GIAO method at the HF/6-311+G(d,p) level for the geometry optimized by the B3LYP/6311+G(d,p) level. c Harmonic oscillator model of aromaticity. UV-vis absorption spectra and cyclic voltammograms of 3–5 are shown in Figure 5 and 6, respectively. Diketones 3–5 did not show any fluorescence in solution. The experimental data are summarized in Table 4 with those reported for 9b and 10b. Theoretical HOMO, LUMO, and LUMO+1 energy levels, and HOMO-LUMO energy gaps of 3–5, 9a/9b, and 10a/10b are shown in Table 5. Figure 7 shows HOMO, LUMO, and LUMO+1 diagrams. In addition, to assign the electronic transitions, TD-DFT calculations for 3–5 were carried out, the results of which are included in Table S5–S7.



Figure 5. Absorption spectra of 3 (blue curve), 4 (green curve), and 5 (pink curve) in THF. Inset: Vertically enlarged spectra for long wavelength region. Concentrations of 3–5 are 1.21 × 10−5 mol L−1, 1.80 × 10−5 mol L−1, and 2.51 × 10−5 mol L−1, respectively.


Page 12 of 30

Page 13 of 30

1.21

-2.5

-2.0

-1.5

red

: 1.06 V

LUMO = 3.34 eV

-1.0

Current (a.u.)

Current (a.u.)

E onset

-0.5

0.0

E onset

ox

: 1.38 V

HOMO = 5.78 eV

0.0

0.5

1.0

E (V) vs SCE

-2.5

-2.0

-1.5

red

-1.0

-0.5

0.0

E onset

ox

2.5

: 1.26 V

HOMO = 5.66 eV

0.0

0.5

1.0

1.5

2.0

2.5

E (V) vs SCE

E (V) vs SCE

E onset

2.0

: 1.11 V

LUMO = 3.29 eV

Current (a.u.)

1.28

1.5

E (V) vs SCE

E onset

Current (a.u.)

red

: 1.16 V

LUMO = 3.24 eV 1.34

Current (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


E onset

ox

: 1.41 V

HOMO = 5.81 eV

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

E (V) vs SCE

Figure 6. Cyclic voltammograms for 3 (blue curve), 4 (green curve), and 5 (pink curve) in DCM with 0.1 M Bu4N·PF6 as a supporting electrolyte. Oxidation and reduction for 3 and 4 were done in separate cycles, whereas those for 5 were done in a same cycle.



Page 14 of 30

Table 4. Absorption and Electrochemical Properties of Diketones 3–5, 9b, and 10b. Compd

λabs (nm)a

Eox / Eonsetox (V)

Ered / Eonsetred (V)

EHOMO (eV)

ELUMO (eV)

ΔEHOMO-LUMO (eV)

3

404, 426, 496

Irrevb / 1.38

−1.21 /−1.06

−5.78

−3.34

2.44

4

421, 442, 505

Irrevb / 1.26

−1.28 / −1.11

−5.66

−3.29

2.37

5

366, 445

Irrevb / 1.41

−1.32 /−1.16

−5.81

−2.55

2.57

9bc

332, 743

1.33, 1.70 / 1.22

−0.95, −1.43 /

−5.62

−3.57

2.05

−5.75

−3.56

2.19

−0.83 10bc

350, 630

1.5, 2.1 / 1.35

−1.02, −1.53 / −0.84

a In

THF. b Irreversible. c Data taken from reference 58.

Table 5. Theoretical HOMO and LUMO Energy Levels, HOMO-LUMO Gaps of Diketones 3–5 and 9a/9b–10a/10b.a Compd

HOMO (eV)

LUMO (eV)

LUMO+1 (eV) HOMO-LUMO gap (eV)

3

−6.20

−2.97

−2.77

3.29

4

−6.11

−2.88

−2.84

3.24

5

−6.36

−2.94

−2.34

3.51

9a / 9bb

−6.12 / −5.80

−3.40 / −3.22

−2.19

2.74 / 2.58

10a / 10bb −6.28 / −5.93

−3.37 / −3.18

−2.17

2.92 / 2.75

a DFT

calculations at the B3LYP/6-311+G(d,p) level. b Data taken from reference 58 for calculations

done at the same level.


Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

O

S

O

O

O

O

O

S

O

O

S S

3

2.77 eV 2.97 eV

6.20 eV HOMO

S S

4

LUMO +1 LUMO

S

S

5

LUMO +1 2.84 eV 2.88 eV LUMO

LUMO +1 2.34 eV LUMO 2.94 eV

6.10 eV HOMO

O

9a

6.36 eV HOMO

LUMO +1 2.19 eV LUMO

3.40 eV

S

S

10a

LUMO +1

LUMO

6.12 eV HOMO

HOMO

2.17 eV

3.37 eV

6.28 eV

Figure 7. HOMO, LUMO, and LUMO+1 diagrams of 3–5, 9a, and 10a for the geometries optimized by DFT calculations at the B3LYP/6-311+G(d,p) level with orbitals shown with an isovalue of 0.04 (e bohr−3)1/2.

The UV-vis absorption spectra of 3–5 in THF showed weak absorptions due to forbidden HOMO → LUMO transitions at around 500 nm for 3 and 4 and 445 nm for 5. More intense absorptions due to HOMO → LUMO+1 transitions are observed at 404 and 426 nm for 3, 421 and 442 nm for 4, and 366 nm for 5. Whereas absorption spectra were not recorded in hexane due to extremely low solubility, spectra measured in DCM (Figure S5) did not show significant difference in the absorption, indicating little solvatochromic effect in diketones either. The weak absorption bands in 3–5 (496 nm, 505 nm, and 445 nm, respectively) correspond to the very weak oscillation strength (f = 0.0004 for 3 at 468 nm, 0.0000 for 4 at 500 nm, and 0.0001 for 5 at 453 nm) for the HOMO → LUMO transitions in TD-DFT calculations (Table S5–S7). Though the feature in absorption spectra of 3–5 are similar to those of the previously reported compounds 9b and 10b,57 which show weak absorption bands at long wavelength region (743 nm for 9b and 630 nm for 10b), it is worth mentioning that first excitation bands of 3–5 are significantly blue shifted compared to those of 9b and 10b. We also note that the LUMOs of 3–5 are not as much localized in the cyclopentadienone moieties as in the LUMOs of 9b and 10b,58 as well as those of 9a and 10a (Figure 7), in which two cyclopentadiene moieties are separated by a nodal plane bisecting the central benzene ring. The



Page 16 of 30

different LUMO levels and distributions are discussed in the following paragraph regarding the experimental (by CV) and theoretical MOs. In CV, both oxidation and reduction waves of diketones 3–5 were found within the potential window of the solvent. However, these diketones show irreversible oxidation wave in contrast to the reversible oxidations observed for 9b and 10b,58 indicating highly reactive character of radical cations generated from 3–5. From the respective onset oxidation potentials (1.38 V, 1.26 V, and 1.41 V for 3, 4, and 5, respectively) the HOMO energy levels are estimated as −5.78 eV, −5.66 eV, and −5.81 eV, respectively, which are comparable to those reported for isomers 9b and 10b (Table 4). In contrast, in the cathodic range, defined first reversible reduction waves with the maxima at –1.21 V, −1.28 V, and – 1.34 V are observed for 3–5, respectively, indicating the formation of kinetically stable anionic species under electrochemical conditions. However, in contrast to the spiro-fluorene substituted DIDT diketones 9b and 10b, the second reductions of 3–5 are irreversible, most likely due to the absence of protecting substituents. From the onset of the reduction potential at 1.06 V, 1.11 V, and 1.16 V, the LUMO levels are estimated as –3.34 eV, −3.29 eV, and −3.24 eV, respectively (Table 4). The most notable difference is the more positive reduction potentials of 3–5 (−3.23 V – −3.34 V) compared to those of 9b (−3.56 V) and 10b (−3.57 V), even taking the effect of spiro-fluorene substituents into account.12,41,58 Indeed, the theoretical LUMO energy levels of 3–5 (−2.88 eV – −2.97 eV) are higher than those of 9a (−3.40 eV) and 10a (−3.37 eV), in spite of the para-linkage between the central benzene and terminal thiophene rings in 3–5 as in 9a (Table 5). We attribute this to destabilization of LUMOs of 3–5 due to repulsive interaction between the carbonyl oxygen atoms described above. As shown in Figure 7, LUMOs of 3 and 5 and LUMO+1 of 4 locate significantly on the carbonyl moiety in an antibonding fashion, thereby destabilizing these orbitals. Such unfavourable interaction does not exist in the LUMOs of 9a and 10a because of the distant location of the carbonyl groups and the presence of a nodal plane. This explains the higher LUMO levels of 3–5 compared to those of 9a/9b and 10a/10b as elucidated from the experiments and theory. In the case of 4, the energy level of the Cs symmetric MO with a bonding interaction between the two carbonyl moieties becomes slightly lower than that of C2-symmetric MO. The former MO then becomes LUMO. The intense second absorption band of 4 (421 and 442 nm) compared to those of 3 and 5 can be ascribed to the mixing of LUMO and LUMO+1, which affects oscillation strength of the transitions including these orbitals.

CONCLUSION


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In summary, two as-indacene-based DIDT derivatives 1 and 2 and the corresponding diketones 3– 5 were synthesized efficiently and their thermal, photophysical, and electrochemical properties were investigated. The physical properties are compared not only within the same series of compounds 1 and 2 both bearing an as-indacene framework but also with structurally related DIF 6 with an asindacene core and DIDT isomers 7a/7b and 8a/8b of different fusion modes bearing an s-indacene core. The HOMO levels of 1 and 2 are significantly raised compared to that of 6 because of electrondonating character of the thiophene rings. On the other hand, the LUMO level of 2 is slightly lower than that of 6, leading to significant reduction of the HOMO-LUMO gap in 2. While the HOMO/LUMO levels of 1 turn out to be similar to those of 8a of meta-type ring fusion, those of 2 are nearly identical to those of 7a of para-type fusion mode. The different HOMO/LUMO levels between 1 and 2 is ascribed to the efficiency of π-conjugation, which depends on α- (in 2) or βlinkage (in 1) between the terminal thiophenes with the central benzene ring. In view of the high stability and moderate HOMO-LUMO gaps, as well as efficiency of fluorescence of compound 2, it presents suitable basis for their use as active layer in optoelectronic devices. On the other hand, the physical properties of diketones 3–5 are characterized by the proximity of the carbonyl oxygen atoms, destabilizing their LUMO energy levels due to antibonding interaction in the LUMOs in contrast to the s-indacene-based isomers 9a/9b and 10a/10b. As a results, not only the HOMOLUMO energy gaps of 3–5 are increase compared to those of 9a and 10a but also the LUMO and LUMO+1 energy levels of 3–5 become closer. In particular, because the LUMO and LUMO+1 levels of 4 are nearly degenerate, it exhibits a relatively intense lowest-energy absorption band compared to other DIDT diketones. EXPERIMENTAL SECTION General Considerations. Melting points were measured with a Yanagimoto Micro Melting Point Apparatus or a home-built hot-stage apparatus and are uncorrected. 1H and 13C NMR spectra were measured with a JEOL JNM-GSX-400 or a Bruker AVANCEIII-400 spectrometer (400 MHz for 1H, and 100 MHz for 13C) in the respective solvent indicated at 30 °C. When chloroform-d and acetoned6 were used as solvent, the spectra were referenced to residual solvent proton signals in the 1H NMR spectra (7.26 ppm for chloroform-d, and 2.05 ppm for actone-d6) and to the solvent carbons in the 13C

NMR spectra (77.0 ppm for chloroform-d, and 29.84 ppm for acetobe-d6). IR spectra were

recorded as KBr disks with a JASCO FT/IR-4100 spectrometer. Mass spectral analyses were performed on a JEOL JMS-700 spectrometer for the EI or FAB mode or a Thermo Scientific Orbitrap XL spectrometer for the APCI mode. UV-vis-NIR spectra were recorded on a SHIMAZU



Page 18 of 30

UV-3600 UV-vis-NIR spectrophotometer. Column chromatography and TLC were performed with Merck silica gel 60 (70–230 mesh ASTM) and Merck silica gel 60 F254, respectively. 3,6-Dibromoo-xylene (6),64 thiophene-3-ylboric acid,65 and thiophene-2-ylboric acid66 were prepared according to the literature methods. Other reagents were used as supplied. THF, CH2Cl2, and Et2O were dried by a Glass Contour solvent purification system. Electrochemical Studies. The electrochemical behavior of the compounds was investigated by cyclic voltammetry (CV) in DCM solution containing 0.1 M tetrabultylammonium hexafluorophosphate (Bu4·PF6) under a nitrogen atmosphere. The CV measurements were performed at room temperature (around 25 °C) using a BAS CV-620C voltammetric analyzer employing a platinum disk as the working electrode, platinum wire as the counter electrode, and Ag/AgNO3 as the reference electrode at a scan rate of 100 mV s−1. The ferrocene/ferrocenium (Fc/Fc+) redox couple was used as a standard for the calibration of the potentials. For 1–4, the measurement was done immediately after measuring the Fc/Fc+ couple (external standard). In the case of 5, the Fc/Fc+ couple was used as an internal standard, because we checked for 1–4 that the use of the internal standard did not affect the redox potentials significantly. The LUMO level was calculated as LUMO (eV) = −[Eonsetred (vs SCE) + 4.4] and the HOMO level as HOMO (eV) = −[Eonsetox (vs SCE) + 4.4], based on an SCE energy level of 4.4 eV relative to the vacuum.72 Spectroscopic Studies. The UV−vis absorption spectra were measured at room temperature (typically 25 °C) using a Shimadzu UV-3600 spectrophotometer using different solvents such as THF, DCM, and hexane. The fluorescence spectra were measured using a HORIBA Fluoromax-4 spectrometer in the photocounting mode with a Hamamatsu R928P photomultiplier. Fluorescence quantum yields were determined by the absolute method using a JASCO ILF-835 integration sphere equipped to a JASCO FP-8000 fluorometer or by a relative method using anthracene (Φ = 0.27, in 1.12 × 10−5 mol L−1 ethanol)73,74 as a standard. Measurements by the both methods gave virtually the same results. For UV−vis and fluorescence experiments, solvents with spectroscopic grade were used. Thermal Analysis. The DSC measurements were performed with a Shimadzu DSC-60 under purified nitrogen at a heating or cooling rate of 10 °C min−1. Materials Synthesis. Dimethyl 3,6-dibromophthalate (12). A mixture of 1164 (2.64 g, 9.99 mmol) and KMnO4 (6.34 g, 40.1 mmol) in pyridine (70 mL) and water (140 mL) was heated under reflux. Another 4 equiv. (40 mmol) of KMnO4 and water (20 mL) was added at every 30-min interval for six times and the mixture was further heated for 17 h. Since monitoring by TLC (CHCl3 / MeOH = 1 / 1) indicated the presence of monocarboxylic acid, additional KMnO4 (4 equiv.) and water (20 mL)


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were added and heating was continued for another 4 h. After cooling, the mixture was filtered through a pad of Celite and most of pyridine was evaporated under reduced pressure. The mixture was acidified with conc. HCl and extracted with ether. Removal of the solvent under reduced pressure gave a pale yellow solid (3.59 g), which was used for the next step without purification. The solid was heated with dimethyl carbonate (12.2 g, 136 mmol) and conc. H2SO4 (2 mL) at 80 C for 16 h. Another conc. H2SO4 (2 mL) was added and the mixture was heated for additional 22 h. After cooling, aqueous NaHCO3 solution was added and the mixture was extracted with CHCl3. The extract was washed with brine and dried over MgSO4. After removal of the solvent, the residue was subjected to silica gel chromatography (eluent: CHCl3) to give 12 (2.75 g, 7.81 mmol, 78% from 11) as a colorless solid; mp 78.580.5 C; 1H NMR (400 MHz, CDCl3)  7.53 (s, 2H), 3.92 (s, 6H); 13C{1H}

NMR (100 MHz, CDCl3)  165.6, 135.7, 135.3, 119.3, 53.1; IR (KBr) 1739, 1732, 1286,

1245, 1192, 1157, 965, 956, 842 cm−1; MS (EI) m/z 352.1 [M]+. HRMS (EI) m/z [M]+ Calcd for C10H8O479Br81Br:351.8769, found: 351.8765. Dimethyl 3,6-di(thiophen-3-yl)phthalate (13a). To a 500 mL three-necked flask was added 12 (3.52 g, 10.0 mmol) and thiophene-3-ylboric acid65 (4.22 g, 33.0 mmol) and flask was purged with Ar. Then toluene (100 mL), EtOH (100 mL), and 2 M aqueous solution of Na2CO3 (20 mL, 40 mmol) were added via syringes, and the solution was purged with Ar by bubbling the gas. Pd(PPh3)4 (592 mg, 512 mol) was added and the mixture was heated at 75 C for 11 h. After cooling, brine was added and the mixture was extracted with CH2Cl2. The extract was dried over MgSO4, and the solvent was evaporated under reduced pressure. The residue was subjected to silica gel chromatography (eluent: CHCl3) to afford 13a (3.55 g, 9.91 mmol, 99%) as a colorless solid. mp 161.5–162.0 C, 1H NMR (400 MHz, CDCl3)  7.52 (s, 2H), 7.37 (dd, J = 5.2, 3.2 Hz, 2H), 7.32 (dd, J = 2.8, 1.2 Hz, 2H), 7.14 (dd, J = 5.0, 1.2 Hz, 2H), 3.69 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3)  168.8, 139.8, 134.4, 131.8, 131.3, 127.9, 125.9, 123.0, 52.5; IR (KBr) 1740, 1709, 1246, 1203, 1159, 786 cm−1; MS (EI) m/z 358.2 [M]+. HRMS (EI) m/z [M]+ Calcd for C18H14O4S2: 358.0334, found: 358.0331. Anal. Calcd for C18H14O4S2: C, 60.32; H, 3.94. Found: C, 60.35; H, 3.60. Dimethyl 3,6-di(thiophen-2-yl)phthalate (14). Cross coupling of 12 (2.13 g, 6.06 mmol) and thiophene-2-ylboric acid66 (2.61 g, 20.4 mmol) was carried out as described above using Pd(PPh3)4 (0.355 g, 0.307 mmol), and 2 M Na2CO3 (12.5 mL, 25.0 mmol) in toluene (60 mL) and EtOH (40 mL) to afford 14 (2.14g, 5.96 mmol, 98%) as a colorless solid. mp 162.1–162.5 C, 1H NMR (400 MHz, CDCl3)  7.57 (s, 2H), 7.38 (dd, J = 5.0, 1.0 Hz, 2H), 7.10 (dd, J = 3.6, 1.2 Hz, 2H), 7.14 (dd, J = 5.0, 3.6 Hz, 2H), 3.73 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3)  168.3, 140.3, 132.6, 132.2, 131.9, 127.6, 126.9, 126.7, 52.7; IR (KBr) 1737, 1298, 1242, 827, 725, 708 cm−1. HRMS(APCI) m/z



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[M]+ Calcd for C18H14O4S2: 358.0334, found: 358.0326. [3,6-Di(thiophen-3-yl)-1,2-phenylene]dimethanol (15). To a solution of 13a (106 mg, 0.295 mmol) in THF (28 mL) was added LiAlH4 (36.6 mg, 0.964 mmol) and the mixture was heated under reflux for 4 h. Another LiAlH4 (60.2 mg, 1.59 mmol) was added and the mixture was heated for adDITional 21 h before monitoring with TLC (hexane / EtOAc = 4 / 1) indicated the disappearance of the starting material. After cooling, water (0.50 mL) and 15% aqueous NaOH solution (0.10 mL) were added and the mixture was filtered through a pad of Celite. The filtrate was washed with EtOAc and removal of the solvent under reduced pressure gave a yellow solid (0.161 g), which was subjected to silica gel chromatography (eluent: hexane / EtOAc = 3 / 1) to give 15 (64.1 mg, 0.212 mmol, 72%) as a colorless solid. mp 143.9-145.0 °C, 1H NMR (400 MHz, acetone-d6,) δ 7.59 (dd, J = 3.0, 1.2 Hz, 2H), 7.57 (dd, J = 4.8, 2.8 Hz, 2H), 7.39 (s, 2H), 7.37 (dd, J = 4.8, 1.2 Hz, 2H), 4.80 (d, J = 5.2 Hz, 4H), 4.51 (t, 2H); 13C{1H} NMR (100 MHz, acetone-d6) δ 142.6, 139.7, 137.8, 130.2, 126.2, 124.1, 60.5; IR (KBr) 3308, 997, 787, 661 cm−1; MS (FAB) m/z 302.1 [M]+; HRMS (FAB) m/z [M]+ Calcd. for C16H14O2S2: 302.0436, found 302.0442 (M+). [3,6-Di(thiophen-2-yl)-1,2-phenylene]dimethanol (16). Reduction of 14 (1.06 g, 2.96 mmol) was carried out as described above with LiAlH4 (530 mg, 14.0 mmol) in THF (195 mL) by heating at 80 °C for 2 h. The crude product (0.925 g) was purified by silica gel chromatography (eluent: hexane / EtOAc = 4 / 1) followed by washing the desired product with hexane to give 16 (554 mg, 1.83 mmol, 62%) as a pale yellow solid. mp 134.5–135.5 °C, 1H NMR (400 MHz, CDCl3) δ 7.46 (s, 2H), 7.39 (dd, J =5.4 Hz, 1.2 Hz, 2H), 7.23 (dd, J =3.6 Hz, 1.2 Hz, 2H), 7.13 (dd, J =5.2 Hz, 3.6 Hz, 2H), 4.89 (s, 4H); 13C{1H} NMR (100 MHz, CDCl3) δ 141.7, 139.1, 135.6, 130.9, 127.8, 127.6, 126.2, 60.7; IR (KBr) 3322, 995, 979, 827, 721, 703 cm−1. MS (FAB) m/z 302.07 [M]+; HRMS (FAB) m/z [M]+ Calcd. for C16H14O2S2: 302.0435, found 302.0433 (M+). as-Indaceno[2,3-b:7,6-b']dithiophene-9,10-dione (3). To a Schlenk tube was added 13a (358 mg, 0.998 mmol) and NaCl (2.01 g, 34.3 mmol). The mixture was heated at 100 C under reduced pressure to remove moisture, and Ar was introduced. After AlCl3 (8.04 g, 60.3 mmol) was added, the mixture was heated to 140 C around which temperature the mixture melted. After heating for 12 h, the mixture was cooled in an ice bath. The resulting solid was dissolved with 1N HCl and CHCl3. The aqueous solution was extracted with CHCl3. The combined extract was washed with saturated NaHCO3 solution followed by brine, and was dried over MgSO4. After evaporation of the solvent, the residue was subjected to silica gel chromatography (eluent: CHCl3). The desired product was washed with ether to afford ketone 3 (171 mg, 0.582 mmol, 58%) as a reddish orange solid. The ether washing was purified by chromatography to give additional amount of 3 (39.8 mg, 0.135


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mmol, 14%). The total yield was 72%. mp 245 C (turned brown after heating around the melting temperature), 1H NMR (400 MHz, CDCl3)  7.78 (d, J = 4.8 Hz, 2H), 7.12 (d, J = 4.8 Hz, 2H), 7.10 (s, 2H); 13C{1H} NMR (100 MHz, CDCl3)  181.8, 158.2, 139.9, 139.1, 138.0, 137.0, 122.3, 119.9; IR (KBr) 1715, 1239, 976, 755, 732, 710, 665 cm−1; MS (EI) m/z 294.2 [M]+. HRMS (APCI) m/z [M + H]+ Calcd. for C16H7O2S2: 294.9882, found 294.9880 (M+). A mixture of 13a (102 mg, 285 μmol) and NaOH (529 mg, 13.2 mmol) in water (20.0 mL) and EtOH (25.0 mL) was heated at reflux for 12 h. Additional NaOH (306 mg, 7.65 mmol and 1.06 g, 26.4 mmol) was added after 12 h and 15 h, respectively. After heating for additional 5 h, the mixture was cooled to room temperature and was acidified with conc. HCl. The mixture was extracted with ether and the extract was dried over Na2SO4. The crude product (13b, 104 mg) was used for the next step without purification. The solid was transferred to a Schlenk tube with NaCl (656 mg, 11.2 mmol) and the mixture was heated at 100 C under reduced pressure to remove moisture. After cooling, N2 gas was introduced to the tube. AlCl3 (2.29 g, 17.2 mmol) was added and the mixture was heated at 140 C for 4 h. The mixture was cooled in an ice bath and the contents of the tube was dissolved with ice-water and 2 N HCl. The mixture was extracted with CHCl3 and the extract was washed with brine and dried over MgSO4. Evaporation of the solvent gave essentially pure 3 (67.8 mg, 230 μmol, 81%). as-Indaceno[2,3-c:6,7-c']dithiophene-9,10-dione (5). To polyphosphoric acid (10.6 g) stirred by heating at 160 C in a Schlenk tube was added 13a (103 mg, 286 mol) and the mixture was heated at the same temperature for 1.5 h. The mixture was diluted with water while cooling in an ice bath and was extracted with CHCl3. The extract was washed with brine and dried over MgSO4. After evaporation of the solvent, the residue was subjected to silica gel chromatography (eluent: CHCl3) to give 37.6 mg (45%) a mixture of 3, 5, and unsymmetrical ketone. The latter product was obtained only as a mixture with 3 but not in pure state (Figure S2). The ratio of the ketone products was estimated to be ca. 1 : 7 : 3 by 1H NMR spectrum of a crude product. A small sample of 5 was isolated by the chromatography as an orange solid. No melting point was observed by 300 C. 1H NMR (400 MHz, CDCl3)  7.83 (d, J = 2.0 Hz, 2H), 7.55 (s, 2H), 7.17 (d, J = 2.0 Hz, 2H); 13C{1H} NMR (100 MHz, CDCl3)  182.4, 145.2, 141.6, 141.3, 139.8, 127.9, 125.9, 115.8. IR (KBr) 1712, 1496, 824, 764 cm−1. HRMS (APCI) m/z [M + H]+ Calcd. for C16H7O2S2: 294.9882, found 294.9881 (M+). 9,10-Dihydro-as-indaceno[2,3-b:7,6-b']dithiophene (1). To a solution of 15 (105 mg, 0.293 mmol) in CH2Cl2 (40.0 mL) a solution of CF3SO3H (450 µL, 5.06 mmol) dissolved in CH2Cl2 (5.0 mL) was added dropwise during 20 min at room temperature. After 5 min, saturated aqueous



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solution of NaHCO3 was added and the aqueous phase was extracted with CH2Cl2. The extract was washed with brine and dried over MgSO4. After evaporation of the solvent, the residue was subjected to silica gel chromatography (eluent: hexane) to give 1 (39.2 mg, 0.124 mmol, 42%) as a colorless solid. mp 199.9-200.3 °C (decomp), 1H NMR (400 MHz, CDCl3) δ 7.48 (s, 2H), 7.36 (d, J = 4.4 Hz, 2H), 7.29 (d, J = 4.8 Hz, 2H), 3.89 (s, 4H); 13C{1H} NMR (100 MHz, CDCl3) δ 148.0, 142.9, 142.7, 136.5, 128.8, 118.8, 117.9, 33.1; IR (KBr) 830, 717, 665 cm−1; MS (FAB) m/z 266.1 [M]+; HRMS (FAB) m/z [M]+ Calcd. for C16H10S2: 266.0224, found 266.0226 (M+). The vibrational splitting in the first excitation band in the UV-vis spectrum of 1 in THF (1537 cm−1) corresponds to the IR band at 1518 cm−1 due to C-C stretching of the aromatic rings. 9,10-Dihydro-as-indaceno[3,2-b:6,7-b']dithiophene (2). A reaction of 14 (102 mg, 0.284 mmol) in CH2Cl2 (45.0 mL) with CF3SO3H (450 µL) in CH2Cl2 (5.0 mL) was carried out as described above to give 2 (72.9 mg, 0.274 mmol, 96%) as a colorless solid. mp 203.5-204.5 °C (decomp), 1H NMR (400 MHz, CDCl3) δ 7.45 (s, 2H), 7.29 (d, J = 5.2 Hz, 2H), 7.12 (d, J = 4.8 Hz, 2H), 3.74 (s, 4H); 13C{1H} NMR (100 MHz, CDCl3) δ 146.4, 143.9, 142.4, 136.4, 126.8, 122.9, 117.7, 32.5; IR (KBr) 835, 800, 704, 646 cm−1; MS (FAB) m/z 266.2 [M]+; HRMS (FAB) m/z [M]+ Calcd. for C16H10S2: 266.0225, found 266.0223 (M+). The vibrational splitting in the first excitation band in the UV-vis spectrum of 1 in THF (1531 cm−1) corresponds to the IR band at 1537 cm−1 due to C-C stretching of the aromatic rings. as-Indaceno[3,2-b:6,7-b']dithiophene-9,10-dione (4). To a solution of 2 (50.2 mg, 0.189 mmol) in pyridine (10.0 mL) was added KMnO4 (298 mg, 1.89 mmol) and water (10.0 mL), and the mixture was heated at 100 °C for 2 h. After cooling, the mixture was filtered through a pad of Celite and the filtrate was washed with CH2Cl2. 1N HCl was added to the filtered solution, and the aqueous phase was extracted with CH2Cl2. The extract was washed with brine and dried over MgSO4. After evaporation of the solvent, the residue was subjected to silica gel chromatography (eluent: CH2Cl2) to give 4 (6.7 mg, 22.8 µmol, 12%) as a dark red solid. mp 233.8–243.0 °C, 1H NMR (400 MHz, CDCl3) δ 7.22 (d, J = 5.2 Hz, 2H), 7.19 (d, J = 5.2 Hz, 2H), 7.07 (s, 2H); 13C{1H} NMR (100 MHz, CDCl3) δ 183.3, 158.7, 142.7, 138.6, 135.3, 130.0, 122.3, 121.9; IR (KBr) 1714, 1230, 1106, 886, 831, 721, 649 cm−1; MS (FAB) m/z 295.04 [M]+; HRMS (FAB) m/z [M + H]+ Calcd. for C16H7O2S2: 294.9888, found 294.9889 (M+). SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Chemical structures for possible isomers of DIDTs, DSC curves of 1−5, electronic absorption spectra


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in different solvents of 1−5, fluorescence spectra of 1 and 2 in DCM, frontier molecular orbitals of 7a−10a, TD-DFT calculations of 1−5, NMR spectra of 1−5 and 12−16, Cartesian coordinates of theoretically optimized structures of 1−5 and 7a−10a AUTHOR INFORMATION Corresponding Author * Phone: +81 6 6879 8476. Fax: +81 6 6879 8479. E-mail: [email protected] ORCID Shreyam Chatterjee: 0000-0001-6378-3602 Yoshito Tobe: 0000-0002-1795-5829 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by JSPS KAKENHI Grant Numbers JP15K13643 and JP15H02164. References and Notes 1. Grimsdale, A. C.; Müllen, K. Oligomers and Polymers Based on Bridged Phenylenes as Electronic Materials, Macromol. Rapid Commun. 2007, 28, 1676–1702. 2. Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Synthesis of LightEmitting Conjugated Polymers for Applications in Electroluminescent Devices, Chem. Rev. 2009, 109, 897–1091. 3. Deuschel, W. Fluorenacene und Fluorenaphene. Synthesen in der Indeno-fluorenreihel). I. CisFluorenacen (Indeno-(2’,1’:2,3)-fluoren), Helv. Chim. Acta 1951, 34, 168–185. 4. Deuschel, W. Fluorènacene und Fluorènaphene. Synthèsen in der Indeno-Fluorenreihel). III. Exocis-Fluorenaphen (Indeno-(2’,l’:3,4)-fluoren, Helv. Chim. Acta 1952, 35, 1774–1776. 5. Chardonnens, L.; Laroche, B.; Sieber, W. Fluorénacènes et fluorénaphènes Synthèses dans la série des indéno-fluorènes, XVII. Dérivés méthylés du cis-fluorénacène, du trans-fluorénacène et du trans-fluorénaphène, Helv. Chim. Acta 1974, 57, 585–599. 6. Merlet, S.; Birau, M.; Wang, Z. Y. Synthesis and Characterization of Highly Fluorescent Indenofluorenes, Org. Lett. 2002, 4, 2157–2159. 7. Poriel, C.; Liang, J.-J.; Rault-Berthelot, J.; Barrière, F.; Cocherel, N.; Slawin, A. M. Z.; Horhant, D.; Virboul, M.; Alcaraz, G.; Audebrand, N.; Vignau, L.; Huby, N.; Wantz, G.; Hirsch L.



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9,10-Dihydro-as-indacenodithiophenes: Isomers with an as-Indacene

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