Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene
Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene
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E / Z光可切换噻吩基乙烯的能量和构象异构体依赖性激发态弛豫
DOI：
10.1039/c9cp01226e
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发表时间：
2019
影响因子：
3.3
通讯作者：
Bragg, Arthur E.
中科院分区：
文献类型：
--
作者：
Young, Jamie D.;Honick, Chana R.;Zhou, Jiawang;Pitts, Cody R.;Ghorbani, Fereshte;Peters, Garvin M.;Lectka, Thomas;Tovar, John D.;Bragg, Arthur E.
双（联噻吩基）-1，2-二氰基乙烯（4 TCE）是一种光开关，其在吸收可见光后通过可逆的E/Z光异构化来操作。4 TCE的顺式至反式光异构化需要低于470 nm的激发，相对低效（量子产率< 5%）并且经由最低的三重态发生。我们目前的激发波长依赖（565-420 nm）的瞬态吸收（TA）的研究，以探测顺式到反式异构化，以确定转换效率低下的来源的物理。TA数据揭示了多个状态之间的开关构象和弛豫级联的贡献。在低激发能（>470 nm）下观察到的光谱动力学的快（~ 40 ps）和慢（~ 40 ps）分量很容易归因于两种构象异构体的失活;这一分配得到了计算的热布居和两种分子几何结构（PA和PB）的吸收强度的支持，其特征在于噻吩在乙烯键的相对两侧大致平行的偶极子。只有PB构象对三重态布居有贡献，顺式-4TCE的转换：PB的高能激发（<470 nm）涉及直接激发到S2，弛豫产生ISC活性S1几何构型（ISC QY 0.4-0.67，kISC = 1.6-2.6 × 10−9 s−1），这是三重态布居和异构化的门户。我们将低顺反异构化产率归因于非反应性PA构象的激发（75-85%损失）以及沿PB S2 → S1 → T1级联的沿着损失（10-20%损失）。相反，电环化被抑制的激发态的电子特性，以及不存在的热人口的反应性的“反平行”环构象。
Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (PA and PB) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the PB conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of PB involves direct excitation to S2, relaxation from which prepares an ISC-active S1 geometry (ISC QY 0.4–0.67, kISC ∼ 1.6–2.6 × 10−9 s−1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive PA conformer (75–85% loss) as well as loses along the PB S2 → S1 → T1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation.