Electron‐Accepting 6,12‐Diethynylindeno[1,2‐b]fluorenes: Synthesis, Crystal Structures, and Photophysical Properties

Abstract

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...

Synthesis and properties of fully-conjugated indacenedithiophenes

“…Very recently, our group 9 and others 10 have begun to examine the isomers of indenofluorenes 11 (IFs, Fig. 2) as potential n-type materials due to their ability to reversibly accept two electrons.…”

“…Our initial studies showed that a range of electron-rich and electron-poor groups at the 2 and 8 positions of the indeno[1,2- b ]fluorene skeleton (e.g., 3a ) 9b had only a modest effect on the electronic properties of these compounds; however, we demonstrated subsequently that functionalizing the [1,2- b ]IFs with aryl groups at the 6 and 12 positions ( 3b ) 9c resulted in much greater variability in the redox properties of the molecules, with some displaying amphoteric behaviour. Tobe and our group reported, respectively, that [2,1- a ] isomer 4 10a and previously unknown [2,1- c ] isomer 5 9d also possessed excellent electrochemical properties with smaller HOMO-LUMO gaps than the [1,2- b ]IFs. Tobe et al very recently described unknown [2,1- b ] isomer 6 ; 10c however, the instability of the molecule may preclude its use in organic electronics.…”

Antiaromatic character of cycloheptatriene-bis-annelated indenofluorene framework mainly originated from heptafulvene segment

“…Based on these values, the HOMO-LUMO energy gap of 2 is calculated to be 1.23 eV. Interestingly, these E HOMO and E LUMO values are significantly different from those of IF(Me)-TA ( E HOMO  = −5.84 eV and E LUMO  = −3.99 eV) 30 . In order to understand the origin of this phenomenon, density functional theory (DFT) calculations of 2(H), IF, and BDA at the B3LYP/6-311 + G(d, p) level were performed.…”

A theoretical study of substituted indeno[1,2-b]fluorene compounds and their possible applications in solar cells

“…For this study were considered the anti-aromatic compounds; pentalene (1) [10,11,23], s-indacene (2) [10,23], indeno[1,2-b]fluorene (3) [7,24] and benzo[g]benzo [6,7]indeno[1,2-b]fluorene (4). Figure 2 shows how the frontier molecular orbital energies for these anti-aromatic molecules behave.…”