Unusual stabilization of larger acenes and heteroacenes

Abstract

This review highlights recent developments concerning stabilization strategies of large (hetero-)acenes and discusses the resulting impact on the aromatic system.

Carbo[n]helicenes Restricted to Enantiomerize: An Insight into the Design Process of Configurationally Stable Functional Chiral PAHs

“…[5]Helicene is the borderline case, which displays partial configurational stability.I tc an be resolved into enantiomers, but an enantioenricheds ample racemizes within ac ouple of days at room temperature. In 1956, Lednicer et al achieved the optical resolution of [6]helicene by fractional crystallization with optically active (À)-2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid ((À)-TAPA). [35] This was the first time that the partial thermal racemization of [6]helicene was observed during the melting point determination.S everal years later in 1970, Stegemeyer et al obtained [36] enantioenriched [5]helicene by using the same methoda nd firstly determined the racemization rate constants, followingt he change in circulard ichroism signal in isooctane solution,a td ifferent temperatures between3 04 and 320 K. The estimated value of DG°(298) = 24.1 kcal mol À1 indicates the half-life of racemization (t 1/2 )t ob e2 9h in solution at 298 K. Twoy ears later, Martin et al reportedt he thermal racemization of [6]-, [7]-, [8]-, and [9]helicened issolved in naphthalene (6 %w /w) when heateda bove their melting points.…”

“…(ii)The second hypothesis, involving intramolecular double Diels-Alder reaction, was experimentally verified. For example, the double Diels-Alder reactiono f( M)-1,2,3,4-d 4 - [6]helicene after equilibration should give a1:1 mixture of the startingm aterial and (P)-1,2,13,14-d 4 - [6]helicene (Scheme 2). However, when (M)-1,2,3,4-d 4 - [6]helicenei nn aphthalene (10 %w /w) was heatedu nder reduced pressure at 286 8Cf or 70 min, the NMR spectrum of the crude mixture was identical to that of the initial material.…”

“…[2] These compounds are synthesized in an atomicallyp recise and structurally well-defined manner utilizing well-established synthetic organic chemistry tools. [3,4] Owing to their structurald iversity,c ommon PAHs (Figure 1) can be categorized [5] in as implified manner as acenes [6] (metafused benzene rings in one dimension), nanographenes [4] (benzene rings fused in two dimensions to give large nanometersized flakes or nanoribbons),a nd carbo[n]helicenes [7,8] (orthofused benzene rings in three dimensions).…”

“…[9] When compared with the linear analogs,acenes, carbo[n]helicenes exhibit distinctc hemical and physicalp roperties. [10] Whereas the former are excellent semiconductors and highly reactive, [6] the latter exhibit exceptional chiroptical properties and are very stable. Acenesw ith fiveo rm ore meta-fused benzene rings are reactive towards light and oxygen andtherefore cannotbeisolated under ambient conditions.…”

Synthesis of anthradithiophene containing conjugated polymers via a cross-coupling strategy

“…Pentacenes and their isoelectronic heteroacene analogues have long been investigated for their applications as optoelectronic materials in organic eld effect transistors, organic photovoltaics and organic light emitting diodes. [1][2][3][4][5] Despite pentacenebased materials having high hole carrier mobilities, their poor photostability and solution processability have limited their applications in conjugated polymer materials and have led to the investigation of alternative thiophene-fused polycyclic aromatic hydrocarbons. Anthradithiophenes (ADTs) in particular have been developed for their isoelectronic character with pentacenes.…”