Bridged Tolanes: A Twisted Tale

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/T8,9, S1/T10.