title: Bridged tolanes: a twisted tale.
abstract: 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.
authors.affiliation: Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg , Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Menning
authors.forename: Maximilian
authors.initials: M
authors.lastname: Krämer
authors.forename: Andrew
authors.initials: A
authors.lastname: Duckworth
authors.forename: Frank
authors.initials: F
authors.lastname: Rominger
authors.forename: Andrew
authors.initials: A
authors.lastname: Beeby
authors.forename: Andreas
authors.initials: A
authors.lastname: Dreuw
authors.forename: Uwe H F
authors.initials: UH
authors.lastname: Bunz