Aromaticity is an important concept to understand the stability and physical properties of π-conjugated molecules. Recent studies on pro-aromatic and anti-aromatic molecules revealed their irresistible tendency to become diradicals in the ground state. Diradical character thus becomes another very important concept and it is fundamentally correlated to the physical (optical, electronic and magnetic) properties and chemical reactivity of most of the organic optoelectronic materials. Molecules with distinctive diradical character show unique properties which are very different from those of traditional closed-shell π-conjugated systems, and thus they have many potential applications in organic electronics, spintronics, non-linear optics and energy storage. This critical review first introduces the fundamental electronic structure of Kekulé diradicals within the concepts of anti-aromaticity and pro-aromaticity in the context of Hückel aromaticity and diradical character. Then recent research studies on various stable/persistent diradicaloids based on pro-aromatic and anti-aromatic compounds are summarized and discussed with regard to their synthetic chemistry, physical properties, structure-property relationships and potential material applications. A summary and personal perspective is given at the end.

Low band gap (E(g) < 1.5 eV) polycyclic hydrocarbons have become one of the most important types of materials for many applications, for example, as semiconductors in organic field effect transistors (OFETs), as light-harvesting dyes in organic solar cells and photodetectors, as near infrared (NIR) fluorescent probes in high resolution bio-imaging and bio-sensing, and as chromophores in non-linear optics. The benzenoid polycyclic hydrocarbons as nano-sized graphene fragments also serve as perfect model compounds to understand the fundamental structure-property relationship of graphene. The ground state of these molecules can be described as either a closed-shell or an open-shell structure on the basis of their molecular size and edge structure. In this review, a summary will be given on a series of low band gap polycyclic hydrocarbons about their synthesis, physical properties and material applications.

5452 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1 ) ≤ E (S 1 ). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exothermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However, early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pentacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso -positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Tuneable Singlet Exciton Fission and Triplet-Triplet Annihilation in an Orthogonal Pentacene DimerSteven Lukman , Andrew J. Musser , Kai Chen , Stavros Athanasopoulos , Chaw K. Yong , Zebing Zeng , Qun Ye , Chunyan Chi , Justin M. Hodgkiss , Jishan Wu , * Richard H. Friend , and Neil C. Greenham * Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from...

An efficient synthesis of 9,9-bis(2-ethylhexyl)fluorene oligomers up to the heptamer is reported, with repetitive Suzuki and Yamamoto coupling reactions employed in the synthesis. The key steps for preparation of the essential intermediates include Pd-catalyzed transformation of aryl bromides to aryl boronic esters (Miyaura reaction) and the application of the much higher reactivity of aryl boronic esters over aryl bromides in the Pd-catalyzed cross-coupling reaction with aryl diazonium salts. Variation of the UV/Vis absorption and photoluminescence characteristics with chain length is reported. Moreover, glass transition and liquid-crystal characteristics of the oligomers are described and compared with those of the polymer.

Acenes represent a series of molecules with intriguing physical and chemical properties for applications in organic electronics. Nevertheless, the stability and solubility issues associated with longer acenes are two major obstacles for their applications. In this Perspective, we summarize the major design principles for stabilizing acenes. A variety of stable acene based derivatives are included for discussion. Finally, we highlight some research areas where breakthroughs will be critical for the further development of acene based molecules and materials.