Exploring Indeno[2,1- c ]fluorene Antiaromatics with Unsymmetrical Disubstitution and Balanced Ambipolar Charge-Transport Properties

Unsymmetrically disubstituted antiaromatic indenofluorene (IF), in comparison to aromatic pentacene counterpart with unsymmetrical disubstitution, was rare in the literature until our recent report on indeno[1,2- b ]fluorene and indeno[2,1- a ]fluorene. Described herein is a straightforward access to unsymmetrically disubstituted indeno[2,1- c ] fluorenes bearing mesityl at one apical carbon and C 6 F 5 , 3,5-(CF 3 ) 2 C 6 H 3 , and CCSi i -Pr 3 at the other apical carbon, including 4-methoxy- phenyl/3,5-(CF 3 ) 2 C 6 H 3 push/pull substitution at the apical carbons with appreciable orbital density, and a previously unknown symmetrically C 6 F 5 -disubstituted [2,1- c ]IF. The electronic properties of the unsymmet- rical derivatives lie halfway in between the two symmetrical counterparts, while the 4-methoxyphenyl derivative showed the smallest HOMO – LUMO energy gap and near-infrared absorption with intramolecular charge transfer character. Single-crystal analyses showed 1D-co- lumnar stacks for the unsymmetrical motif with the C 6 F 5 units co-facially π -stacked with the IF core, whereas symmetrically C 6 F 5 -disubsti- tuted [2,1- c ]IF, with a low-lying LUMO, showed intermolecular π – π stacks between the IFs that resulted in good electron mobility ( µ e =8.66×10 − 3 cm 2 ·V − 1 ·s − 1 ) under space charge limited current measurements. Importantly, balanced ambipolar charge-transport behav- iour could be extracted for an IF series with symmetrical/unsymmetrical substitutions, in comparison to its π -contracted pentalene congener.


Introduction
Fully conjugated indeno[2,1-c]fluorene is a structural isomer of the five antiaromatic indenofluorene (IF) regioisom-ers. 1,2Studies on [2,1-c]IF revealed its slightly helical backbone with a para-quinoidal arrangement of the as-indacene subunit, and negligible diradical contribution in the singlet ground state, affording three stable symmetrically disubstituted derivatives 1, 2 and 3 (Figure 1b). 1 In comparison to the isomeric indeno[1,2-b]fluorene 2a with a para-quinodimethane (p-QDM) arrangement in the s-indacene subunit (shown in bold, Figure 1a), the as-indacene (shown in bold, Figure 1b) embedded [2,1-c]IF is less documented, 2e despite showing a smaller HOMO-LUMO energy gap 1 and potential application as an electron acceptor in bulk heterojunctions. 3ajority of the studies conducted with the [2,1-c]IF revealed that its electronic, redox, and solid-state properties can be tuned by elongating the pentacyclic backbone by quinoidal fluorenofluorene 4 modification (as-indacene subunit not retained) or by condensing the benzene unit in the form of benzo-fused [2,1-c]IF derivatives 4, 5 and 6, where the asindacene subunit is retained. 52e Lack of access to the unsymmetrically disubstituted IFs 1,2,5 is likely due to the limitation of a controlled nucleophilic addition 6 of aryl/ethynyl groups to the benzo-fused diketone precursor, which has hampered exploration of unsymmetrically disubstituted IFs.Lately, two unsymmetrically disubstituted IF regioisomers were reported by us with tunable antiaromaticity. 7We had reported a new synthesis approach to 1, 8 and further to a (4 n + 2)π s-indacenodifluorene. Inspired by the general route for unsymmetrically 6,13disubstituted pentacene molecules for various applications 9 and considering appreciable molecular orbital density at the apical carbons of 1 (Figure 1c) or related as-indacene 10 that suggested possibility to tune the electronic properties of [2,1-c]IFs by apical elongation, including the non-availability of unsymmetrically 5,8-disubstituted [2,1-c]IFs, we envisaged to extend our synthetic approach 8 as a general route to access the yet inaccessible unsymmetrically 5,8-disubstituted [2,1-c]IF scaffolds 7, 8, 9 and 10 (Figure 1d), and a previously unknown 1 symmetrically disubstituted [2,1-c]IF 17.
The aryl or ethynyl groups like C 6 F 5 , 11 [3,5-bis(trifluoromethyl)phenyl], 12 and [(triisopropylsilyl)ethynyl] 13 are chosen due to their proven ability to sterically protect the apical carbons and the electron-accepting nature. 14It was also anticipated that replacing weakly electron-donor mesityl of 8 with 4-methoxyphenyl for 10 may offer improved push-pull character.Additionally, the [2,1-c]IF scaffold was known 1 but its charge-carrier mobility was unknown, and it was speculated to have poor performance due to its U-shape hindering π-π stacks.2e Inspired by the reports of ambipolar charge transport of [1,2-b]IF 2f with symmetrical disubstitution, and use of [1,2-b]IF for single-molecule conductance studies, 2h herein, we report the syntheses of four unsymmetrical [2,1-c]IFs 7-10 and one symmetrical [2,1-c]IF 17 including their characterization by both experimental and computational 15 approaches, and the charge-carrier mobilities of 1, 7, 8 and 17 were experimentally investigated using the space charge limited current (SCLC) method.

Results and Discussion
C 6 F 5 -disubstituted [1,2-b]IF is an ambipolar material, 16 which motivated us to synthesize 17 as it was not reported earlier. 1 Treating our pre-synthesized aldehyde 11 8 with pentafluorophenylmagnesium bromide (C 6 F 5 MgBr) under ambient conditions afforded 12 (Scheme 1), which was converted to 13 using BF 3  Unlike our steric approach to synthesize [1,2-b]IF that required mesityl as a bulky substituent, 7 the current approach can be used to attach less bulky donors (e.g.4-methoxyphenyl) to afford push-pull dyes.Compound 10 was synthesized using the analogous synthesis approach (Scheme 3) that started with a nucleophilic addition of (4-methoxy-phenyl)magnesium bromide to aldehyde 11 to afford 25.Treating 25 with triflic acid at room temperature gave 26, which underwent a Suzuki cross-coupling reaction with (2formylphenyl)boronic acid to give 27 in 75 % yield.Nucleophilic addition of 3,5-(CF 3 ) 2 C 6 H 3 -MgBr to 27 followed by BF 3 ⋅OEt 2 treatment afforded the dihydro-precursor 29 in 61 % yield over two steps.Finally, treatment of 29 with excess of DDQ in toluene afforded 10 with donor and acceptor 14 substituents in moderate yield.
Single crystals for 7, 9 and 17 were grown successfully, and analyzed.The X-ray crystallographic analysis showed a nearly planar [2,1-c]IF backbone for 7 (Figure 2a, CCDC 2 117 444) with the outer benzenoid ring (near to C 6 F 5 ) twisted by 7.0°from the average plane of the cyclopenta[c] fluorene subunit.Compound 9 (Figure 2c, CCDC 2 117 445) packs in two independent arrangements with 12.9°and 13.6°twist angles, suggesting subtle P/M-like helicity with a very small 0.48 kcal/mol P/M-interconversion barrier (see SI). 1 The mesityl group is found to be near-orthogonally oriented to the [2,1-c]IF backbone (dihedral angle 86.9 -89.7°f or 7 and 9), while the other aryl/ethynyl groups suggest a  hydrogen bonding interaction measuring 2.457 Å (the value is less than the sum of the van der Waals radii of F and H = 2.67 Å) 17 with ∠C sp 2 -H⋯F = 137.8°,affording a herringbone-like packing arrangement (Figure S39) with a 1D columnar stack featuring the C 6 F 5 unit partly, co-facially, π-stacking (3.238 Å) with the outer ring of IF (Figure 2d).The hydrogen-bond distance and bond angle meet the geometrical requirements for intermolecular hydrogen bonding interactions. 18The H⋯F-C sp 2 hydrogen bond interaction is critical in the formation of a supramolecular assembly for calixphyrins 19 or antiaromatic isophlorins 20 ; however, from the perspective of diradicaloid polycyclic hydrocarbons, 11 including antiaromatic IFs, 2,16,21 H⋯F-C hydrogen bonding interactions received seldom attention.In comparison to 7, symmetrical 17 (CCDC 2 117 446) displayed weaker intermolecular F (3) ⋯H (8) -C (8) (2.595 Å, ∠C sp 2 -H⋯F = 124.0°)hydrogen bonding interactions in a 1D columnar arrangement featuring two IF backbones π-stacking at 3.574 Å, co-facially, at about a point of inversion (Figure 2e), implying strong intermolecular π-π interaction (Figure S40).
All new [2,1-c]IF derivatives displayed strong absorbance in the higher energy (UV-vis) region and a broad absorption in the lower energy (visible) region extending to 800 nm (Figure 3), except for 10 that has extended to 840 nm in the near-infrared (NIR) region.Symmetrical diarylation/ethynylation at the 5/8 positions to tune optoelectronic properties 1 seems logical by the calculated HOMO/LUMO plots (Figure 1c), which show considerable orbital density at the apical carbons.It is thus expected that unsymmetrical aryl/ aryl or aryl/ethynyl disubstitution could be a realistic step to fine-tune the optoelectronic properties.The source of the broad low energy absorption band was predicted by the time-dependent density functional theory (DFT) calculations as π→π* electronic transitions, originating from an admixture of the HOMO→LUMO and HOMO-1→LUMO transitions in the singlet closed-shell ground state (Table S1 -S5 in SI).The absorption profiles were also characteristically similar to the symmetrically 5,8-disubstituted [2,1-c]IF derivatives, 1 with 17 being the newest entry with λ abs max = 616 nm (theoretical λ max = 746 nm, oscillator strength (f) = 0.1056; Table S5).To our expectation, the absorption maxima for 8 are found to lie in between those of 1 and 2, and similarly for 9 (between 1 and 3) as well as 7 (between 1 and 17).
The red-shift in the absorption band (λ max = 610 nm, ε = 2780 M −1 • cm −1 ) with theoretical f = 0.1052 (719 nm, Table S1) for 7 in the visible region, in comparison to 1, can be attributed to some electronic communication between the C 6 F 5 and [2,1-c]IF π-rings due to the decrease in dihedral angle (observed for crystals).This observation is also supported by the DFT studies with a reduced dihedral angle ca.52 -53°for 7, in comparison to the orthogonally oriented mesityl group.The lower energy absorption maximum is further red-shifted for 8 (λ max = 615 nm, ε = 4170 M −1 • cm −1 ; f = 0.1197 at 724 nm, Table S2), suggesting a better π-delocalization between the 3,5-bis(trifluoromethyl)phenyl and [2,1-c]IF π-rings due to a further reduction of the dihedral angle (ca.44 -46°for 8 at B3LYP/6 -31 G(d,p)), including stronger electron-withdrawing effect.Further improvement in the electronic communication between the coplanar TIPSE and the [2,1-c]IF core in 9 is reflected by the more red-shifted absorption (λ max = 625 nm, ε = 1880 M −1 • cm −1 ; f = 0.1516 at 742 nm, Table S3) and a smaller optical HOMO-LUMO energy gap of 1.52 eV than that of 7 (E g opt = 1.56 eV) or 8 (E g opt = 1.55 eV).Compound 10 showed the smallest optical energy gap (E g opt = 1.47 eV) with a redshifted low-energy absorption band (λ max = 640 nm, ε = 4940 M −1 • cm −1 ; f = 0.1708 at 755 nm, Table S4), with the absorption tail reaching the NIR region.A weak but nonnegligible blue-shift (~10 nm) of the low-energy wavelength maximum for 10 was noticed when changing the solvent from polar CHCl 3 to non-polar hexane (Figure S36), suggesting an intramolecular charge transfer (ICT) character due to a better push-pull interaction between the 4-methoxyphenyl (electron donor) and 3,5-(CF 3 ) 2 C 6 H 3 (electron acceptor) 14 substituents through the as-indacene backbone.However, no obvious change in absorption profile was noticed when changing the solvent polarity for 8 (Figure S35), indicating negligible ICT interaction due to a weakly electron-donating mesityl group.Compounds 7, 8, 9, 10 and 17 are found to be non-emissive, a common trait for antiaromatic [2,1-c]IF. 22he unsymmetrical [2,1-c]IFs accept two electrons quasireversibly, and they are quasi-reversibly oxidized to a radical cation, and further, quasi-reversibly to the dications (Fig- ure 4, Table 1).It was noticed that if current was swept through the second reduction for 9 and 17, and second oxidation for 7, 8 and 9, new peaks (as bumps) appeared on their return during the cathodic and anodic scans.Such peaks were not observed if the current was not swept past the first reduction and oxidation potentials (Figure S37b in SI), indicating likely formation of some reactive species during the second reduction and oxidation.The first half-wave (E 1/2 ) oxidation potential, obtained from differential pulse voltammogram peak values (Figure S37a), at E 1/2 ox1 = 0.66 V (vs.ferrocene/ferrocenium (Fc/Fc + ) for 7 is the highest among the unsymmetrical motifs, and 8 and 9 appeared at E 1/2 ox1 = 0.63 V and 0.55 eV, respectively, while 10 showed the smallest value at E 1/2 ox1 = 0.43 V. Compound 9 appears to be a comparatively better electron acceptor with a low first halfwave reduction potential at E 1/2 red1 = −1.32V than 7 or 10 with E 1/2 red1 = −1.36V and 8 with E 1/2 red1 = −1.39V.The estimated electrochemical HOMO-LUMO energy gaps (E g ec ) are 1.54 eV for 10, 1.56 eV for 9, 1.75 eV for 8, and 1.76 eV for 7. Symmetrical 17 could be oxidized at a high potential of E 1/2 ox1 = 0.84 V and reduced at a much lower reduction potential of E 1/2 red = −1.16V with an estimated E g ec = 1.71 eV.The trend of optical and electrochemical energy gaps for 7, 8, 9, 10 and 17 is consistent with the theoretical HOMO-LUMO energy gaps (Table 1).The low-lying LUMO of 17 (−3.80eV) and improved solid-state packing interaction suggested its potential as a good electron transporter; however, the amphoteric redox waves 23 observed for these new IFs prompted us to investigate both hole and electron transport behaviors for a [2,1-c]IF series: 1, 7 and 17.Since reports suggest that possibly the device architecture in solar cells may not appropriately fit with the transistor mobility, 24 the Comp.more relevant 25 SCLC method was adopted by us to measure the charge-carrier mobility (vide infra), in order to find the potential for these new molecules as new semiconductors for organic electronic devices.The charge-carrier mobilities of compounds 1, 7, 8 and 17 were determined from the current-voltage characteristic of devices having hole (ITO/PEDOT : PSS/1, 7, 8 and 17/MoO 3 / Ag) and electron-only (ITO/SnO 2 /1, 7, 8 and 17/Al) architectures.The charge-carrier mobility is determined from fitting the J-V characteristics with a semi-log plot for compounds 1, 7, 8 and 17 using the modified Mott-Gurney equation given by Murgatroyd 26 : where J is measured as current density, ε o is the permittivity of free space (8.86 × 10 −14 F/cm), ε r is relative dielectric constant of the material, V is applied voltage, d is the thickness of the active layer of the material, µ is mobility, and γ is the fitting parameter representing the strength of the field dependence on mobility.The extracted mobilities of the compounds are summarized in Table 2.
The J -V characteristics for compounds 1, 7, 8 and 17 are shown in Figure 5. From the SCLC mobilities shown in Table 2, it is evident that the known compound 1, and the new compounds 7, 8 and 17, showed ambipolarity for charge-carrier transport where for both hole and electrons, the order of the observed charge-carrier mobility is same.Overall, for a device application, a material having balanced electron and hole mobility is preferred. 27In these reported compounds, the closeness order for the balanced ambipolar compound is found to be in the order 17 < 7 < 8 < 1, which seems reasonable considering appreciable intermolecular interactions observed in the solid state for 7 and 17 resulting in better charge-transport properties.On the other hand, compound 1 with bulkier mesityl substituents lacks any appreciable intermolecular π-π interactions, 1,2e unlike 17, affording relatively low SCLC mobilities than those of 17 or 7. Nonetheless, the charge-transport behavior of unsymmetrical 7 lies nearly halfway in between its symmetrical counterparts 1 and 17 (Table 2).The observed higher electron mobility for compound 17 can be attributed to its favorable energy level alignment in electron-only devices with alumi-num and a better intermolecular π-π interaction when compared with compounds 1 and 7.
It is worth noting that SCLC mobilities of organic semiconductor thin films are much lower in comparison to the field-effect mobility, usually within 10 −6 to 10 −4 cm 2 • V −1 • s − 1 . 28Although sulfur-embedded small polycyclic aromatic system was reported to show good SCLC hole mobility (8.72 × 10 −2 cm 2 • V −1 • s −1 ), 25a hydrocarbon motifs like antiaromatic diareno-pentalene showed low hole-carrier mobility (4.37 × 10 −4 cm 2 • V −1 • s −1 ) under the SCLC method. 29Considering [2,1-c]IF as a benzo-interpositioned 30 pentalene system, our studies on the symmetrical or unsymmetrical [2,1-c]IFs 1, 7, 8 and 17 π-systems revealed an order of magnitude greater charge-carrier mobility for both holes and electrons.Moreover, a balanced ambipolar chargetransport behavior could be extracted for the first time.This suggests that [2,1-c]IF-based antiaromatic systems could be  a useful candidate for applications in organic optoelectronic devices, and high-performance materials with proper structural modifications are in the scope of our future studies.

Conclusions
In summary, unsymmetrically 5,8-disubstituted [2,1-c]IFs 7, 8, 9 and a push-pull disubstituted 10, and a symmetrically 5,8-disubstituted [2,1-c]IF 17 were reported for the first time using a synthetic approach that has the potential to generate strongly polarized [2,1-c]IFs, as well as heteroatom-modified 31 isoelectronic [2,1-c]IF in the near future.X-ray crystallographic analyses revealed improved solidstate properties for 7 and 17, including shorter intermolecular F⋯H-C sp 2 hydrogen bonding interactions in unsymmetrical 7 (2.457Å, 137.8°) than that was observed for symmetrical 17 (2.595Å, 124.0°), and intermolecular π-π interactions.The electronic properties of the unsymmetrical 7, 8 and 9 lie halfway in between two symmetrically disubstituted [2,1-c]IF counterparts.The HOMO-LUMO energy gap of 10 was found to be the smallest due to the ICT character arising from the push-pull effect of apical substituents, implying possibilities to further lowering the energy gap by substituent modification.
Our report of convenient access to fully conjugated [2,1-c] IFs bearing desired substituents with balanced ambipolar charge-transport properties for 1, 7, 8 and 17 in the order of 10 −3 cm 2 • V −1 • s −1 could open the gateway to previously inaccessible apically extended antiaromatic materials for diverse applications in organic optoelectronics devices.To the best of our knowledge, this is the first systematic study of charge-carrier mobility of antiaromatic [2,1-c]IFs bearing symmetrical (1, 17) and unsymmetrical (7) disubstitution comprising two particular substituents (mesityl and C 6 F 5 ).
Our work distinctly showed the potential of [2,1-c]IF scaffolds as efficient charge-transporting materials, one of the fundamental requirements for efficient organic electronic devices, which is in contrast to earlier prediction.2e Further exploration of the [2,1-c]IF derivatives for optoelectronic applications with a focus on transistors and solar cells alongside the unsymmetrical [1,2-b]IF 7 regioisomers is currently being explored.

Experimental Section
General Information.All reagents and chemicals were obtained from commercial sources and used as received.Silica gel (100 -200 mesh) was used for column chromatography.NMR spectra, in solution, were recorded on a JEOL JNM ECS-400 spectrometer at 298 K.The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, and m = multiplet. 1H and 13 C chemical shifts (δ) are reported in ppm relative to the residual CHCl 3 ( 1 H: 7.26 ppm, 13 C: 77.16 ppm) reference.Single-crystal analyses were done on a CMOS-based Bruker D8 Venture PHOTON 100 diffractometer equipped with a IN-COATEC micro-focus source with graphite monochromated Mo Kα radiation (λ = 0.71 073 Å) operating at 50 kV and 30 mA.High-resolution mass spectra (HRMS) were recorded using electron spray ionization (ESI) methods on Waters (XEVO G2-XS QTOF) mass spectrometer.UV-vis-NIR absorption spectra were recorded as solution, on a JASCO V-770 spectrophotometer.Cyclic voltammetry measurements were performed in dry dichloromethane (DCM) at room temperature under a nitrogen atmosphere on a CHI-1110C instrument electrochemical analyzer with a three-electrode cell, using Bu 4 NPF 6 as the supporting electrolyte, Ag/AgCl as the reference electrode, Pt disk as the working electrode, and Pt wire as the counter electrode at 50 mV/s scan rate.The potential was externally calibrated against the ferrocene/ferrocenium couple.Melting points were determined using a BIBBY-SMP30 melting point analyzer.
Space charge limited current device fabrication.For evaluating hole and electron mobility of the compounds 1, 7, 8 and 17, hole-only and electron-only devices were fabricated in the ITO/PEDOT : PSS/1, 7, 8 and 17/MoO 3 /Ag and ITO/ SnO 2 /1, 7, 8 and 17/Al architecture, respectively.Initially, prepatterned ITO glass was substrates (12 Ω/□, Xin Yan Technology Limited, Hong Kong) were sequentially cleaned using 3 % HelmanexIII soap solution, deionized water, acetone, and isopropyl alcohol in an ultrasonicator for 20 min each, respectively.Substrates were then treated with 30 min exposure to UV ozone at 50 °C.For hole-only devices, ~30 nm PEDOT : PSS layer (Sigma Aldrich) was first spin-coated on cleaned ITO substrates (4000 rpm for 65 s) followed by annealing at 150 °C for 30 min.Further, ~100 nm thick (1500 RPM for 45 s) thin films of compounds 1, 7, 8 and 17 were spin-coated from a 10 mg/mL solution using chloroform as the solvent.Finally, hole-only devices were then completed with thermally evaporated 5 nm of MoO 3 and 100 nm of Ag on top of the active layer.For electron-only devices, at first, SnO 2 solution was prepared by diluting 15 % tin (IV) oxide colloidal solution (Alfa Aesar) in 1 : 6 by volume in deionized water.The SnO 2 solution was then spin-coated on cleaned ITO substrates at 4000 rpm for 65 s to obtain ~40 nm thick films.The spin-coated substrates were the annealed 180 °C for 1 h under ambient conditions.Furthermore, ~100 nm thick films of compounds 1, 7, 8 and 17 were spincoated from the same solution as used for the hole-only devices.Lastly, 120 nm of Al was thermally evaporated at a base pressure of 3 × 10 −6 mbar to complete the device.The active area of both hole-and electron-only devices was 6.6 mm 2 , calculated from the overlap area of patterned ITO and top contact.J -V characteristics were measured using a Keithley 2450 source measure unit.The dielectric constant of the compounds 1, 7, 8 and 17 was measured using highfrequency LCR meter ZM2376 with an applied oscillation level voltage of 1 V over the frequency range 20 Hz-1 MHz.

Figure 2
Figure 2 ORTEP drawing of a) 7, b) 17, and c) 9 (left: P-isomer; right: M-isomer) at 30 % probability level (hydrogen omitted), with the C=C bond distances (in Å) of p-quinodimethane unit are shown.The packing motifs for d) 7 and e) 17 showing the intermolecular F⋯H-C sp 2 hydrogen bonds (in Å) and their (∠C sp 2 -H⋯F) angles.

Figure 5 J
Figure 5 J -V characteristics in the semi-log plot used for extracting (a) hole and (b) electron mobilities using modified the Mott-Gurney SCLC equation for compounds 1, 7, 8 and 17.

Table 1
Summary of photophysical and electrochemical data for 7, 8, 9, 10 and 17 a