Department of Physical Chemistry, University of Geneva , 30 Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland.
Institute of Applied Synthetic Chemistry, Vienna University of Technology , Getreidemarkt 9/163/MC, 1060 Vienna, Austria.
J Am Chem Soc. 2017 Nov 22;139(46):16885-16893. doi: 10.1021/jacs.7b09591. Epub 2017 Nov 13.
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 sp 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.
已使用超快宽带中红外和可见瞬态吸收和荧光光谱学的组合研究了具有炔烃间隔基的苯胺-三嗪电子给体-受体偶联物的激发态动力学。瞬态红外数据显示间隔基中的炔烃到丙二烯的异构化的发生，其时间常数随着溶剂粘度从几百飞秒增加到几皮秒。这个过程快于 Franck-Condon 激发态的振动冷却，表明存在非平衡动力学。瞬态电子吸收和荧光数据表明，这种转化伴随着供体和受体亚基之间的电荷分离。间隔基的丙二烯特性暗示了供体和受体部分的正交取向，类似于扭曲的分子内电荷转移态所提出的。这些状态经常在给体-受体偶联物的激发态动力学中被提及，但它们的参与在光谱学上从未被明确证明。炔烃-丙二烯异构化不仅涉及扭转运动，还涉及由于一个炔碳原子的 sp 到 sp 再杂化而导致的分子弯曲。这种扭曲和再杂化的分子内电荷转移（“TRICT”）态在从弱极性到强极性溶剂的过程中，通过从几百到十皮秒的时间尺度回到平面和线性炔烃基态。不同的溶剂依赖性表明，丙二烯构建的动力学受结构变化控制，而衰减受电荷复合步骤限制。
