Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

Abstract

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z-E photoisomerization mechanism and excitedstate decayd ynamics of two arylazopyrazole photoswitches. Tw oc hiral S 1 /S 0 conical intersections with associated enantiomeric S 1 relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. Forthe parent arylazopyrazole( Z8) both paths contribute evenly to the S 1 excited-state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S 1 relaxation path. To our knowledge,the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited-state relaxation.Photoswitchable compounds have many potential applications ranging from photopharmacology through optochemical genetics to data storage. [1,2] Azobenzenes are among the most studied photoswitchable compounds,s ince they are easily synthesized and highly stable. [3,4] Their high extinction coefficients and isomerization quantum yields enable repeated photoswitching with low-intensity light. Photoinduced isomerization leads to ar emarkable change of the shape and length of these azobenzenes,w hich has been exploited, for example,intuning the folding and unfolding of peptides and proteins. [5][6][7][8] Many azobenzenes have been explored experimentally and computationally in the past decade, [9][10][11][12][13][14][15][16][17][18][19][20][21][22] but there remain af ew drawbacks that limit their broad application. These include incomplete photoswitching because of overlapping absorbance of the two isomers and fast thermal rearrangement of the cis isomer (Z)b ack to the trans isomer (E). To improve overall performance,many groups have explored the properties of azobenzene variants by altering the substituents on the aromatic rings,f or example,i nb ridged and orthohydroxy azobenzenes. [23][24][25][26][27][28][29][30] Recently,n ovel classes of azoheterocycle photoswitches were reported, [31][32][33][34][35][36] especially arylazopyrazoles,w hich provide quantitative photoswitching and high thermal stability (ca. 1000 days). [35,36] Thea bsorption band maxima of the Eand Z-isomers of arylazopyrazoles are found to be well separated, which enables quantitative two-way photoswitching.[35] Furthermore,t he substitution on the heterocyclic ring offers photophysical and photochemical properties that cannot be accessed with azobenzenes.H owever,t he photoinduced isomerization mechanism and the dynamical behavior of these arylazopyrazoles are still unexplored at the atomistic level. Obvious questions that remain are: 1) Whether azobenzenes and arylazopyrazoles share similar photophysical and photochemical mechanisms and 2) whether they show analogous dynamical behavior (lifetimes,c onical intersections,p ath selectivities). Ad etailed mechanistic understanding will be helpful to further improve their performance through rational design. Motivated by ...

Potential‐Energy Surface and Dynamics Simulation of THBDBA: An Annulated Tetraphenylethene Derivative Combining Aggregation‐Induced Emission and Switch Behavior

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Diarylethenes with a Narrow Singlet–Triplet Energy Gap Sensitizer: a Simple Strategy for Efficient Visible‐Light Photochromism

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<i>Cis-Trans</i> Isomerization Reaction of Sulindac Induced by UV Irradiation in the Aqueous Media

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