作者:Sebastian Menning、Maximilian Krämer、Andrew Duckworth、Frank Rominger、Andrew Beeby、Andreas Dreuw、Uwe H. F. Bunz
DOI:10.1021/jo5010235
日期:2014.7.18
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 S-1/T-7. Also, a small energy difference between singlet and triplet states is required; we found that ISC can favorably take place at four crossings: S-1/T-6, S-1/T-7, S-1/T-8,T-9, S-1/T-10.