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Molecular Switching via MultiplicityExclusive E/Z Photoisomerization Pathways
Jiawang Zhou, Xin Guo, Howard E. Katz, and Arthur E Bragg
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 10 Aug 2015
Downloaded from http://pubs.acs.org on August 10, 2015
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Molecular Switching via Multiplicity-Exclusive E/Z Photoisomerization Pathways
Jiawang Zhou,† Xin Guo,‡ Howard E. Katz,†‡ and Arthur, E. Bragg†* †Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA ‡Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, USA
Abstract: 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-1,2dicyanoethene (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 timescale 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 timescales of ~2 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 (θ) from the cis Franck-Condon region (θ = 175°) to geometries that are within the region of the trans ground-state well; furthermore, the T1 and S1 surfaces cross with a substantial spin-orbital 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-orbit couplings through variations in molecular structure or physical environment.
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Photoswitchable molecules and materials that exhibit substantial changes in structure or functional properties upon wavelength-selective excitation are of considerable interest for numerous applications,1-3 including photochromism,4 memory storage,5 logic devices,6 molecular motors,7,8 mechanical manipulation,9-13 and light-triggered chemical sensitization14 and conductivity.15 Photoswitches that operate via large-scale structural changes, such as E/Z isomerization, are particularly attractive for manipulating the distance between chemical or biochemical moieties to interrogate the nature of their interactions.9-13,16-19 Photoswitching also has potential for controlling or patterning morphologies of aggregated organic materials.20,21
Desirable photoswitch characteristics include high conversion efficiency,22,23 robust fatigue resistance,24 and the feasibility for isomerization at red excitation wavelengths25-27 that can transmit through materials such as biological tissue.9 In addition, mutual exclusivity in the nature of the excited-state relaxation pathways that drive forward and reserve isomerization reactions can provide a handle for controlling the reversibility of a switch as desired for a particular application.28,29 On all counts, an understanding of photophysical dynamics and how they may be manipulated with structure are of prime importance for the synthetic design of photoresponsive materials.