Herein we report the synthesis and characterization of a series of 6,12-diarylindeno[1,2-b]fluorenes (IFs). Functionalization with electron donor and acceptor groups influences the ability of the IF scaffold to undergo two-electron oxidation and reduction to yield the corresponding 18- and 22-π-electron species, respectively. A single crystal of the pentafluorophenyl-substituted IF can serve as an active layer in an organic field-effect transistor (OFET). The important finding is that the single-crystal OFET yields an ambipolar device that is able to transport holes and electrons.

Polycyclic hydrocarbons that possess extended p conjugation are of significant interest because of their potential use in optical and electronic devices such as light emitting devices, field-effect transistors, and photovoltaics.[1] While a majority of studies have focused on acenes such as pentacene and its derivatives (e.g., 1; Scheme 1), [2] these systems are susceptible to oxidative and photolytic degradation; [3] thus, there is a need for alternative, acene-like molecules. One avenue in this search has explored compounds containing five-membered rings, rather than the more traditional six-membered rings. Prime examples of such molecules are dibenzopentalene (2) and derivatives thereof, wherein the groups of Saito, Kawase, and Tilley have recently described improved methods for their construction. [4] Another attractive topology is the indeno[1,2-b]fluorene (IF) skeleton (e.g., 3), an acene analogue in which the B and D rings each contain one fewer carbon atom, thus making the 20-p-electron molecule formally antiaromatic. While the pentacyclic IF framework is common in the literature, nearly all examples bear substituents on the 6-and 12-positions, thus resulting in either cross-conjugation (e.g., ketones, exocyclic olefins) [5] or disrupted conjugation (e.g., disubstitution, spirofusion). [6] Of the four fully conjugated IFs known prior to 2011, three are rapidly oxidized by trace amounts of oxygen [7] and the other is poorly characterized.[8]Very recently we reported the synthesis of tetraalkynylated indeno[1,2-b]fluorenes (e.g., 4).[9] The compounds exhibited similar UV/Vis absorption profiles and slightly larger HOMO/LUMO energy gaps compared to those of 1 while maintaining potentially superior solution stabilities; however, the packing of 4 in the solid state resembled an expanded herringbone pattern, a motif often found in unsubstituted acenes. Since the steric bulk of the four interdigitated (triisopropylsilyl)ethynyl groups was the most likely cause for inhibiting a desirable "brick and mortar" p stacking, we sought to examine additional IF derivatives.[10]As a guide for experimental studies, we performed DFT calculations (B3LYP/6-311 + G** [11] using Gaussian 09) [12] on substituted IFs to determine the effect ethynylogation of 3 has on the HOMO (À5.53 eV) and LUMO (À3.03 eV) energy levels, and the energy gap (2.50 eV) of the IF core (Scheme 2, Table 1). Inclusion of the four ethynyl units in 5 significantly lowers the LUMO by approximately 0.5 eV while the HOMO remains unchanged, thus affording a gap energy of 1.97 eV. Inclusion of only two acetylenes at positions 5 and 11 (e.g., 6) slightly lowers the HOMO yet significantly raises the LUMO compared to 5, thus affording a net gap increase of 0.41 eV. If the two alkynes are located at positions 6 and 12, as in 7, the HOMO (À5.51 eV) is on par with that of 5 (À5.53 eV) and the LUMO is elevated slightly (À3.46 eV versus À3.56 eV), thus increasing the gap by only 0.08 eV. Similar to acenes, [2] these results illustrate that judicious positioning of the...

A new class of fully conjugated indenofluorenes has been synthesized and confirmed by solid-state structure analysis. These indeno[2,1-c]fluorene molecules, containing an antiaromatic as-indacene core (in red), possess high electron affinities and show a broad absorption that reaches into the near-IR region of the electromagnetic spectrum. All of the featured compounds reversibly accept up to two electrons. Their electronic properties make this class of compounds attractive for applications in organic electronic devices.

The synthesis and characterization of a series of quinoidal diindeno(thieno)thiophenes (DI[n]Ts) are reported. NIR absorption, deep LUMO energy levels and progressively tighter solid-state packing allude to organic materials applications.

The growing demand for flexible electronic devices and hydrogen storage materials has spurred a resurgence of interest in polyaryl hydrocarbons including graphene, acenes, fullerenes, polythiophenes, etc. Indenofluorenes are another polyaryl molecular scaffold that has shown utility in the organic and hybrid materials arena, with polymers incorporating the indeno[1,2-b]fluorene moiety being common in organic light emitting diodes. This review examines the syntheses and properties of the five distinct indenofluorene regioisomers, with a focus on small molecule applications in organic electronics of this intriguing and somewhat underexplored family of polyaryl hydrocarbons.

A sequential Sonogashira cross-coupling/Pd-mediated oxidative homocoupling strategy affords two-dimensional dehydrobenzoannulene trefoils containing different sizes of the central annulenic ring system. Use of these conditions instead of Cu-mediated homocoupling conditions yields a structural isomer possessing a triphenylene ([6]annulene) core. Noticeable differences in the absorption and emission spectra are observed depending upon the core unit.

SummaryWe report a new synthetic route to 5,11-disubstituted indeno[1,2-b]fluorene-6,12-diones that is amenable to larger scale reactions, allowing for the preparation of gram amounts of material. With this new methodology, we explored the effects on crystal packing morphology for the indeno[1,2-b]fluorene-6,12-diones by varying the substituents on the silylethynyl groups.

Emission from charge recombination between radical cations and anions of a series of regioisomeric 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes (bPEBs) substituted by various electron donor and/or acceptor groups was measured during pulse radiolysis in benzene (Bz). The formation of bPEB in the excited singlet state ((1)bPEB*) can be attributed to the charge recombination between bPEB(*+) and bPEB(*-), which are initially generated from the radiolytic reaction. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH(o)) for the charge recombination of bPEB(*+) and bPEB(*-) and excitation energy of (1)bPEB*. Since the degree of the pi-conjugation in the S(1) state and HOMO-LUMO levels of bPEB change with the substitution pattern of phenylacetylene groups on the central benzene ring and the various kinds of donor and/or acceptor group, the fine-tuning of the emission color and intensity of bPEB can be easily carried out during pulse radiolysis in Bz. For donor-acceptor-substituted bPEB, it was found that the difference in the charge transfer conjugated pathways between donor and acceptor substituents (linear-, cross-, and "bent"-conjugated pathways) strongly influenced the HOMO-LUMO energy gap.