Menning Sebastian, Krämer Maximilian, Duckworth Andrew, Rominger Frank, Beeby Andrew, Dreuw Andreas, Bunz Uwe H F
Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg , Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
J Org Chem. 2014 Jul 18;79(14):6571-8. doi: 10.1021/jo5010235. Epub 2014 Jun 25.
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.
关于托兰类化合物沿其乙炔轴的旋转运动尚未完全理解。如果将托兰主链强制扭曲成构象，其光学和电子性质会发生什么变化？研究了几种连接链以获得托兰环烷，从而固定两个苯环的扭转角。X射线晶体结构揭示了固态下连接链特异性的扭转角。记录了吸收、发射和激发光谱。扭曲的连接链托兰构象异构体显示出吸收峰蓝移；发射光谱在扭转方面均相互独立且相同。通过冷却至77K将连接链托兰嵌入刚性基质中；记录了平面托兰的分辨率良好的发射光谱，但对于扭曲体系，意外地观察到了长寿命磷光。如何解释这种三线态发射？对未取代托兰的量子化学计算（TDDFT/cam - B3LYP/6 - 31G*）表明，当分子轨道相互正交时，自旋 - 轨道耦合较大，有利于系间窜越（ISC）；在S1/T7交叉处就是这种情况。此外，单重态和三重态之间需要较小的能量差；我们发现ISC可以在四个交叉处顺利发生：S1/T6、S1/T7、S1/T(8,9)、S1/T10。
