Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways

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• 作者：Jiawang Zhou ; Xin Guo ; Howard E. Katz ; Arthur E. Bragg
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 26, 2015
• 年：2015
• 卷：137
• 期：33
• 页码：10841-10850
• 全文大小：564K
• ISSN：1520-5126

文摘

Mutual exclusivity in the nature of forward and reserve isomerization pathways holds promise for predictably controlling responses of photoswitchable materials according to molecular structure or external stimuli. Herein we have characterized the E/Z photoisomerization mechanisms of the visible-light-triggered switch 1,2-dithienyl-1,2-dicyanoethene (4TCE) in chlorobenzene with ultrafast transient absorption spectroscopy. We observe that switching mechanisms occur exclusively by relaxation through electronic manifolds of different spin multiplicity: trans-to-cis isomerization only occurs via electronic relaxation within the singlet manifold on a time scale of 40 ps; in contrast, cis-to-trans isomerization is not observed above 440 nm, but occurs via two rapid ISC processes into and out of the triplet manifold on time scales of 鈭? ps and 0.4 ns, respectively, when excited at higher energies (e.g., 420 nm). Observation of ultrafast ISC in cis-4TCE is consistent with photoinduced dynamics of related thiophene-based oligomers. Interpretation of the photophysical pathways underlying these isomerization reactions is supported by the observation that cis-to-trans isomerization occurs efficiently via triplet-sensitized energy transfer, whereas trans-to-cis isomerization does not. Quantum-chemical calculations reveal that the T₁ potential energy surface is barrierless along the coordinate of the central ethylene dihedral angle (胃) from the cis Franck鈥揅ondon region (胃 = 175掳) to geometries that are within the region of the trans ground-state well; furthermore, the T₁ and S₁ surfaces cross with a substantial spin鈥搊rbital coupling. In total, we demonstrate that E/Z photoswitching of 4TCE operates by multiplicity-exclusive pathways, enabling additional means for tailoring switch performance by manipulating spin鈥搊rbit couplings through variations in molecular structure or physical environment.