The excited-state dynamics of an aniline–triazine electron donor–acceptor dyad with an alkyne spacer has been investigated using a combination of ultrafast broadband mid-IR and visible transient absorption and fluorescence spectroscopies. The transient IR data reveal the occurrence of an efficient alkyne to allene isomerization of the spacer with a time constant increasing from a few hundreds of femtoseconds to a few picoseconds with solvent viscosity. This process is faster than the vibrational cooling of the Franck–Condon excited state, indicative of nonequilibrium dynamics. The transient electronic absorption and fluorescence data evidence that this transformation is accompanied by a charge separation between the donor and the acceptor subunits. The allene character of the spacer implies an orthogonal orientation of the donor and acceptor moieties, similar to that proposed for twisted intramolecular charge-transfer states. Such states are often invoked in the excited-state dynamics of donor–acceptor dyads, but their involvement could never be unambiguously evidenced spectroscopically. The alkyne–allene isomerization involves not only a torsional motion but also a bending of the molecule due to the sp to sp² rehybridization of one of the alkyne carbon atoms. This twisted and rehybridized intramolecular charge transfer (“TRICT”) state decays back to the planar and linear alkyne ground state on a time scale decreasing from a few hundred to ten picoseconds upon going from weakly to highly polar solvents. The different solvent dependencies reveal that the dynamics of the allene buildup are controlled by the structural changes, whereas the decay is limited by the charge recombination step.

中文翻译：

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直接观察供体-受体二元体中的光化学炔-丙二烯反应以及扭曲和再杂交的分子内电荷转移态

结合了炔烃间隔基的苯胺-三嗪电子供体-受体二元组的激发态动力学已经结合超快宽带中红外和可见光瞬态吸收和荧光光谱法进行了研究。瞬态IR数据揭示了间隔物有效炔烃到丙二烯异构化的发生，时间常数随溶剂粘度从几百飞秒增加到几皮秒。这个过程比弗兰克-康登激发态的振动冷却要快，这表明存在非平衡动力学。瞬时电子吸收和荧光数据证明这种转变伴随着供体和受体亚基之间的电荷分离。间隔基的异位子特征暗示了供体和受体部分的正交取向，类似于针对扭曲的分子内电荷转移态所提出的。这种状态通常在供体-受体二元组的激发态动力学中被调用，但是它们的参与永远不可能在光谱学上得到明确证明。炔-丙二烯异构化不仅涉及扭转运动，而且还涉及由于sp到sp导致的分子弯曲。²炔碳原子之一的再杂化。当从弱极性溶剂变为高极性溶剂时，这种扭曲和再杂交的分子内电荷转移（“ TRICT”）状态会在几百皮秒到十皮秒的时间范围内衰减回到平面和线性炔基态。不同的溶剂依赖性揭示了丙二烯积累的动力学是由结构变化控制的，而衰变则受电荷重组步骤的限制。