Can Strained Hydrocarbons Be “Forced” To Be Stable?

Many strained hydrocarbons are prone to isomerization, dimerization, and trimerization under normal laboratory conditions. Here we investigate a method to stabilize angle-strained cycloalkynes by applying a mechanical pulling force to the carbon atoms adjacent to the triple bond, which partially linearizes the CC–C bond angles. We discuss various methods of applying such pulling forces, including photoswitches and incorporation into additional strained macrocycles. We use the computational JEDI (Judgement of Energy DIstribution) analysis to quantify the distribution of energy in strained cycloheptyne and judge the change in stability upon application of an external force via isodesmic and homodesmotic reactions. We find that cycloheptyne can indeed be stabilized by external forces. However, the force generated by photoswitches during isomerization is too low to lead to a significant stabilization of the molecule. Hence, stronger forces are needed, which can be achieved by incorporating cycloheptyne into a second strained macrocycle

(Oligo-)Thiophene Functionalized Tetraazaperopyrenes: Donor–Acceptor Dyes and Ambipolar Organic Semiconductors

Tetraazaperopyrenes (TAPPs) have been functionalized with thiophene and terthiophene units of different architecture resulting in a variety of organic donor–acceptor (D–A) compounds. The influence of the connection of the thiophenes to the TAPP core on their structural, photophysical and electrochemical properties has been studied in detail by a combination of X-ray crystallography, UV–vis and fluorescence spectroscopy as well as cyclic voltammetry, which allowed the establishment of structure–property relationships. The HOMO–LUMO gap is significantly decreased upon substitution of the TAPP core with electron-donating thiophene units, the extent of which is strongly influenced by the orientation of the thiophene units. The latter also crucially directs the molecular packing in the solid. Linkage at the α-position allows both inter- and intramolecular N···S interaction, whereas linkage in the β-position prevents intramolecular N···S interaction, resulting in a less pronounced conjugation of the TAPP core and the thiophene units. The new TAPP derivatives were processed as semiconductors in organic thin-film transistors (TFTs) that show ambipolar behavior. The insight into band gap and structure engineering may open up new possibilities to tailor the electronic properties of TAPP-based materials for certain desired applications