Carbon nanotubes by design? Cyclo-para-phenylenes are monomers for the synthesis of armchair carbon nanotubes (see picture, right), and are also attractive fluorophores that display size-dependent emission properties. Geodesic polyarenes represent a realistic alternative for the rational design of carbon nanotubes through the chemical elongation of the hydrocarbon template (left).
Influencing the communication within a conjugated system as diphenylacetylene is a challenging subject in molecular electronics. Some examples of twisted tolanes are known, where high twists have been achieved in the solid state by steric encumbrance. The insertion of a spacer system is an alternative way to tailor rotation. Only a few examples of such tethered tolanes exist, and they all suffer from small twist angles (<35°). We report on tolanophanes containing a malonyl tether, where twist angles of almost 80° were reached. Long-lived phosphorescence (4 s at 77 K) was recorded, and quantum-chemical calculations were performed to confirm the experimental results.
The rotational motion of tolanes along their acetylene axis is not fully understood. What happens to the optical and electronic properties if the tolane backbone is forced into a twisted conformation? Several tethers were investigated to obtain tolanophanes, fixing the torsion angle of the two phenyl rings. X-ray crystal structures revealed tether-specific torsion angles in the solid state. The absorption, emission, and excitation spectra were recorded. Twisted tethered tolane conformers showed blue-shifted absorption; emission spectra were all torsionally independent and identical. The tethered tolanes were embedded in a rigid matrix by freezing to 77 K; well-resolved emission spectra were recorded for planar tolanes, but for twisted systems unexpectedly long-lived phosphorescence was observed. How is this triplet emission explained? Quantum chemical calculations (TDDFT/cam-B3LYP/6-31G*) of the unsubstituted tolane showed that intersystem crossing (ISC) is favored with large spin-orbit coupling, which occurs when the molecular orbitals are orthogonal to each other; this is the case at the crossing of S1/T7. Also, a small energy difference between singlet and triplet states is required; we found that ISC can favorably take place at four crossings: S1/T6, S1/T7, S1/T(8,9), S1/T10.
Kohlenstoffnanoröhren durch Design? Cyclo‐para‐phenylene sind Monomere für die Synthese von Lehnsessel‐Kohlenstoffnanoröhren (rechts im Bild) und zudem vielversprechende Fluorophore mit größenabhängigen Emissionseigenschaften. Geodäsische Polyarene bieten eine interessante Alternative für den rationalen Aufbau von Kohlenstoffnanoröhren durch chemische Verlängerung der gekrümmten Kohlenwasserstofftemplate (links).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.