Dithienyl Acenedithiophenediones as New π‐Extended Quinoidal Cores: Synthesis and Properties

Abstract

We have synthesized two isomeric pairs of benzo- and naphthodithiophenediones with two flanking thiophenes and characterized them by single-crystal X-ray analysis, cyclic voltammetry, steady-state optical electronic absorption and emission spectroscopies, transient absorption spectroscopy, and vibrational spectroscopies with in situ spectroelectrochemistry techniques, and then compared them with the thieno[3,2-b]thiophene-2,5-dione counterpart that we previously reported. The results show that the central acenedithiophenedione cores have quinoidal conjugation with closed-shell character. The π-extension of the quinoidal core raises (lowers) the HOMO (LUMO) energy levels of the triads, resulting in the drastic reduction of their energy gaps from approximately 2.0 eV to 1.1 eV. Owing to the electron-withdrawing nature of the carbonyl terminal group at the quinoidal core, the triads have low-lying LUMO energy levels ranging from -3.9 eV to -4.3 eV, and can be regarded as strong electron-acceptor building units. Interestingly, the pairs of structural isomers have similar electronic structures in both the neutral and charged states despite the different shapes (linear and angular) and/or symmetry (C and C ) of the acenedithiophenedione cores.

Carbonyl-Terminated Quinoidal Oligothiophenes as p-Type Organic Semiconductors

“…The quinoidal oligothiophenes, 2TD, 3TD, 4TDa, and 4TDb were synthesized from the corresponding oligothiophenes by a one-pot reaction previously reported for the synthesis of quinoidal acenedithiophenediones ( Scheme 1 ) [ 30 , 31 ]. For the synthesis of 2TD, 4,4′-dihexylbithiophene was lithiated at the α-positions with n -butyllithium and borylated with triisopropyl borate, and the resulting diboronic ester was hydrolyzed with an aqueous ammonium chloride solution to in situ generate 2T-diboronic acid ( Scheme 1 ).…”

Quinoid‐Aromatic Resonance for Very Small Optical Energy Gaps in Small‐Molecule Organic Semiconductors: A Naphthodithiophenedione‐oligothiophene Triad System

“…The QnTs were synthesized according to our previous report (Scheme 1). [25] In the synthesis of Q2T, 5-(2-ethylhexyl)thiophen-2-yl groups were first introduced at the β-positions of the fused thiophenes of 1,6-diiodo-2,7-bis(trimethylsilyl)naphtho[1,2-b:5,6b']dithiophene by Migita-Kosugi-Stille cross-coupling, and this was followed by the treatment with tetrabutylammonium fluoride to remove the trimethylsilyl groups on the naphthodithiophene core, affording 1 a in 93 % yield. Then, the αpositions of the naphthodithiophene core were lithiated with nbutyllithium followed by borylation with triisopropyl borate to afford a diboronic ester.…”

“…[24] In addition, we have developed related isomers and π-extended analogues of the carbonyl-terminated thienoquinoid as acceptor units in the dithienyl donor-acceptor-donor (DÀ AÀ D) triads (Figure 1b). [25] Through the investigation of the electronic structures of the triads, π-extended naphtho[1,2b:5,6-b']dithiophene-2,7-dione was found to be the most highly electron-deficient unit, thus qualifying as a promising quinoidal acceptor unit that could offer efficient intramolecular donoracceptor interaction as well as low-lying E LUMO s in donoracceptor systems for developing NIR-absorbing organic semiconductors with very small optical energy gaps and air-stable ambipolar carrier transport properties.…”

Indandione‐Terminated Quinoids: Facile Synthesis by Alkoxide‐Mediated Rearrangement Reaction and Semiconducting Properties

“…Quinoidal compounds are of great interest because of their unique electronic, optical, and magnetic properties . Most of the reported quinoidal compounds are terminated by dicyanomethylene moieties, which stabilize the quinoidal scaffold by virtue of the strong electron‐withdrawing ability of the cyano groups . A commonly used method for accessing such compounds involves palladium‐catalyzed Takahashi coupling reaction between aromatic halides and dicyanomethanide anion followed by oxidation (Figure a), and various π‐conjugated quinoidal compounds have been synthesized by this approach.…”

“…A commonly used method for accessing such compounds involves palladium‐catalyzed Takahashi coupling reaction between aromatic halides and dicyanomethanide anion followed by oxidation (Figure a), and various π‐conjugated quinoidal compounds have been synthesized by this approach. However, their termini are difficult to functionalize, and they are known to be as important as the quinoidal core in regulating the electronic properties of these compounds . Another type of quinoidal compounds is end capped by the aryl groups,– and they can be synthesized by nucleophilic addition to a carbonyl group with subsequent reduction (Figure b).…”