Surface Hopping Dynamics for Azobenzene Photoisomerization: Effects of Packing Density on Surfaces, Fluorination, and Excitation Wavelength,The Journal of Physical Chemistry C

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Surface Hopping Dynamics for Azobenzene Photoisomerization: Effects of Packing Density on Surfaces, Fluorination, and Excitation Wavelength
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2020-11-19 , DOI: 10.1021/acs.jpcc.0c08052
Clemens Rietze ₁ , Evgenii Titov ₁ , Giovanni Granucci ₂ , Peter Saalfrank ₁

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Azobenzenes easily photoswitch in solution, while their photoisomerization at surfaces is often hindered. In recent work, it was demonstrated by nonadiabatic molecular dynamics with trajectory surface hopping [Titov et al., J. Phys. Chem. Lett.2016,7, 3591–3596] that the experimentally observed suppression of trans → cis isomerization yields in azobenzenes in a densely packed SAM (self-assembled monolayer) [Gahl et al., J. Am. Chem. Soc.2010, 132, 1831–1838] is dominated by steric hindrance. In the present work, we systematically study by ground-state Langevin and nonadiabatic surface hopping dynamics, the effects of decreasing packing density on (i) UV/vis absorption spectra, (ii) trans → cis isomerization yields, and (iii) excited-state lifetimes of photoexcited azobenzene. Within the quantum mechanics/molecular mechanics models adopted here, we find that above a packing density of ∼3 molecules/nm², switching yields are strongly reduced, while at smaller packing densities, the “monomer limit” is quickly approached. The UV/vis absorption spectra, on the other hand, depend on packing density over a larger range (down to at least ∼1 molecule/nm²). Trends for excited-state lifetimes are less obvious, but it is found that lifetimes of ππ* excited states decay monotonically with decreasing coverage. Effects of fluorination of the switches are also discussed for single, free molecules. Fluorination leads to comparatively large trans → cis yields, in combination with long ππ* lifetimes. Furthermore, for selected systems, also the effects of nπ* excitation at longer excitation wavelengths have been studied, which is found to enhance trans → cis yields for free molecules but can lead to an opposite behavior in densely packed SAMs.

中文翻译：

偶氮苯光异构化的表面跳变动力学：堆积密度对表面，氟化和激发波长的影响

偶氮苯很容易在溶液中发生光交换，而它们在表面的光异构化通常会受到阻碍。在最近的工作中，通过具有轨道表面跳变的非绝热分子动力学来证明[Titov等人，J。Phys。化学来吧 2016年，7， 3591-3596]，反式→顺异构化产率的实验观察到的抑制在偶氮苯在密集SAM（自组装单层）[GAHL等人，J. PM。化学 Soc。2010，132，[1831-1838]是位阻。在目前的工作中，我们通过基态朗格文和非绝热表面跳变动力学，堆积密度降低对（i）UV / vis吸收光谱，（ii）反式→顺式异构化收率和（iii）激发-激发的偶氮苯的寿命。在此处采用的量子力学/分子力学模型中，我们发现，在〜3个分子/ nm ²的堆积密度以上，转换产率会大大降低，而在较小的堆积密度下，会很快达到“单体极限”。另一方面，UV / vis吸收光谱取决于较大范围内的堆积密度（低至至少约1分子/ nm ²）。激发态寿命的趋势不太明显，但发现ππ*激发态的寿命随着覆盖率的降低而单调衰减。对于单个的自由分子，还讨论了开关的氟化作用。氟化会导致较大的反式→顺式产率，并具有较长的ππ*寿命。此外，对于选定的系统，还研究了更长激发波长下nπ*激发的影响，发现该作用可以提高自由分子的反式→顺式产率，但会导致紧密堆积的SAM中的行为相反。

更新日期：2020-12-03

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