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.

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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...

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The synthesis and characterization of four fully-conjugated indacenedithiophenes (IDTs) are disclosed. In contrast to anthradithiophenes, regioselective synthesis of both syn and anti isomers is readily achieved. Thiophene fusion imparts increased paratropic character on the central indacene core as predicted by DFT calculations and confirmed by 1H NMR spectroscopy. IDTs exhibit red-shifted absorbance maxima with respect to their all-carbon analogues and undergo two-electron reduction and one-electron oxidation.