Bradley D. Rose

6,12-Diarylindeno(1,2-b)fluorenes: Syntheses, Photophysics, and Ambipolar OFETs

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.

Electron-accepting 6,12-diethynylindeno[1,2-b]fluorenes: synthesis, crystal structures, and photophysical properties.

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. While a majority of studies have focused on acenes such as pentacene and its derivatives (e.g., 1; Scheme 1), these systems are susceptible to oxidative and photolytic degradation; 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.

Indeno[2,1-c]fluorene: A New Electron-Accepting Scaffold for Organic Electronics

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.

Fluoreno[4,3-c]fluorene: A Closed-Shell, Fully Conjugated Hydrocarbon

The synthesis and optoelectronic properties of 24 π-electron, formally antiaromatic 4,11-di-t-butyl-1,8-dimesitylfluoreno[4,3-c]fluorene (FF) are presented. The solid-state structure shows that the outer rings are aromatic, while the central four rings possess a bond-localized 2,6-naphthoquinone dimethide motif (in red). The biradical character of FF is assessed experimentally and computationally; the results of which implicate a closed-shell ground state.

Experimental and Computational Studies of the Neutral and Reduced States of Indeno[1,2-b]fluorene

This study examines the intrinsic structural and optoelectronic properties of the neutral indeno[1,2-b]fluorene skeleton as well as those of the corresponding anion radical and dianion. We report their characterization by analysis of solid-state structures and EPR, NMR, and absorbance spectra. Additionally, 20 popular density functional theory methods are used to evaluate their performance for predicting NMR chemical shifts, EPR hyperfine coupling constants, and low-energy transitions of the absorbance spectrum to act as a guide for future studies. This information can be used for a better understanding and tuning of the electron-transporting/accepting ability of the indenofluorene core for use in organic photovoltaics and organic field effect transistors.

Explorations of the Indenofluorenes and Expanded Quinoidal Analogues

Highly conjugated hydrocarbons have attracted interest for use as active materials in electronic devices such as organic field effect transistors (OFET) and organic photovoltaics (OPV). In this Account, we review our progress in synthesizing and studying a new class of small molecules for potential use as organic semiconductors. The idea originated from prior research on octadehydrodibenz[12]annulene, as the system can undergo double transannular cyclization to yield the indeno[1,2-b]fluorene skeleton. Subsequent functionalization afforded the first stable, well-characterized indeno[1,2-b]fluorene derivatives, albeit in minute quantities (tens of milligrams). The preparation of these formally antiaromatic compounds has since been optimized: the new synthetic routes utilize inexpensive starting materials, involve robust and high-yielding transformations, and are amenable to considerably larger scale reaction. We have since researched the chemical space of indeno[1,2-b]fluorenes and related quinoidal structures by substitution with a number of functional groups and by permutation of the indenofluorene scaffold. These modifications have allowed us to explore fundamental concepts such as biradical character and antiaromaticity, important considerations when tuning electronic properties to yield functional organic materials. Altering the outer rings by exchange of carbocycles for heterocycles or by inclusion of additional rings as part of the fully conjugated skeleton is one strategy we have examined. Fusing these different aryl groups to s-indacene revealed a dependence of the antiaromaticity of the indacene core upon the outer group. Computational analysis of a series of indeno[1,2-b]fluorene derivatives uncovered an array of different levels of antiaromaticity in the core of the indeno[1,2-b]fluorene derivatives, with one of the benzothiophene derivatives calculated to be as antiaromatic as the parent s-indacene itself. Conversely, we have prepared compounds with expanded cores, starting with the naphthalene-based fluoreno[4,3-c]fluorene, which was produced through a similar route as the indeno[1,2-b]fluorene, using a dione as the key intermediate. Similar to indeno[1,2-b]fluorene, fluoreno[4,3-c]fluorene showed a closed shell ground state, with no evidence of open shell character even upon heating to 170 °C. Increasing the size of the quinoidal core to three rings afforded a diindeno[b,i]anthracene (DIAn) derivative, a compound with a much more complex electronic picture. To produce DIAn, a new synthetic route was devised involving a Friedel-Crafts alkylation to form the five-membered ring and a DDQ oxidation to produce the final compound. DIAn displayed NMR signals that were broadened at room temperature and disappeared when heated, indicative of a molecule with significant biradical character. Extensive computational and experimental investigation verified the controllable expression of its biradical character, with DIAn best described with a ground state that lies between a closed shell compound and a open-shell singlet biradical, with ready access to a thermally populated triplet state.

Synthesis, crystal structures, and photophysical properties of electron-accepting diethynylindenofluorenediones.

A series of 6,12-bis[(trialkylsilyl)ethynyl]indeno[1,2-b]fluorene-5,11-diones has been synthesized. X-ray crystallographic analysis of these compounds reveals that triisopropylsilyl (TIPS) substitution on the alkyne terminus affords the largest number of intermolecular π-π interactions in the solid state. Conversely, use of trialkylsilyl groups smaller or larger than TIPS furnishes a variety of crystal-packing motifs that contain fewer π-π interactions. Electrochemical and photophysical data suggest that these molecules are excellent electron-accepting materials.

Scalable synthesis of 5,11-diethynylated indeno[1,2-b]fluorene-6,12-diones and exploration of their solid state packing

Summary We 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.

6,12-Bis[(tri-cyclo-hexyl-sil-yl)ethyn-yl]indeno-[1,2-b]fluorene.

The title compound, C60H76Si2, a formally anti-aromatic system containing 20-π electrons, contains a rare p-xylylene motif. This is displayed by the alternating short and long bonds. The outer rings possess nearly homogenous bond lengths. In the crystal, the molecules forms layers perpendicular to the c axis and within these layers there are two one-dimensional stacks with one stack that has a sp 2 carbon contact of 3.283 (6) Å, less than the sum of the van der Waals radii. The center of the molecule sits on an inversion center.