From Light Absorption to Cyclization: Structure and Solvent Effects in Donor‐Acceptor Stenhouse Adducts

Abstract

Donor-acceptor Stenhouse adducts (DASAs) have emerged in the last years as novel reversible photoswitches characterized by the light-induced interconversion between a linear openchain isomer and a compact closed-ring isomer. Despite the considerable interest for potential applications (due to their changes in size, color and polarity), several steps of the photoswitching mechanism are still not completely understood, hence limiting the rational design of these compounds. Herein we propose a complete computational study of the switching mechanism by means of TD-DFT and CASPT2//CASSCF calculations. Special attention is paid to the excited state pathway, leading to a previously unknown general scenario with common features for different DASAs: After light absorption, a rotational energy barrier needs to be overcome in order to reach a conical intersection region with the ground state, whose topology and relative energy is investigated based on different solvents and acceptor moieties. Then, the evolution of the compounds in the ground state is considered by calculating different minima until the formation of the final closed form. Finally, the eventual reverse thermal path was also considered. We offer herein a complete mechanistic description which allows for an overall comparison with available experimental results.[a] Dr. C. García-Iriepa

Donor–Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character

“…[15] The solvatochromic shifts reveal the dipolar nature of the DASAs and provide as imple experimental methodt oh elp understand the switching properties of different DASA derivatives while also providing am ore accurate representation of the push-pull system than the XRD data. Feringa [28] and Marazzi [20] and their co-workers have both independently shown in theoretical studies that an increasing zwitterionic resonance contribution lowers the energy barrier of the thermal reversion between Aa nd A' (see Figure S3), whichi sc onsistent with our results. Thisi nhibits switching in more polar solvents, as thermal reversion outcompetes electrocyclization.T oe xtend this study to polymers, we analyzed absorbance shifts within polymer blends that have been shown to facilitatep hotoswitching.…”

“…[4] The synthetice ffort towardsD ASAs has been tightly coupled with extensive mechanistic studies by Feringa, Beves, Martinez, Marazzi, and others. [15][16][17][18][19][20][21] It was demonstrated that the actinic Z-E isomerization step is independento fs olventa nd concentration, and occurs on af emtoto nanosecond timescale. [18,22] This is followed by C3ÀC4 bond rotation and thermal 4p electrocyclization, leading to ringclosed cyclopentenone with a trans relationship betweent he C1 and C5 groups, which occurs on an ano-to millisecond timescale (see Figure S3 in the SupportingInformation).…”

π‐Bridge Substitution in DASAs: The Subtle Equilibrium between Photochemical Improvements and Thermal Control**

“… [37] As explained in the previous section, the nitro group seriously affects the electronic structure of the molecule. The computed results confirm the expectations: on one hand, D2‐Br‐A2 and D2‐CN‐A2 reach a planar S 1 minimum, typical of DASA photochemistry [28] . Indeed, a S 1 energy barrier along the torsion coordinate is expected to connect this minimum with a S 1 /S 0 conical intersection corresponding to a 90° twisted structure around the C g =C f photoisomerizable bond, constituting the first step toward cyclization.…”

“…Another key aspect of DASAs that is still poorly understood is the effect that the relative energy of each thermal isomer has on the overall photoswitching process. In contrast with other types of photoswitches, the complete mechanism for DASAs implies a photochemical step for the isomerization of a C=C double bond and several thermal steps that allow for the formation of the final structure, the closed form [16, 20, 28] . All these intermediates are in thermal equilibrium and could strongly affect the overall photoswitching process.…”

Redesigning donor–acceptor Stenhouse adduct photoswitches through a joint experimental and computational study

“…This photoswitching mechanism has been extensively investigated in several recent publications. [13][14][15][16][17][18][19][20] In DASAs, the donor and acceptor parts are mainly responsible for tuning the physical and chemical properties of the system. The first generation introduced in 2014 was designed with a secondary amine as a donor group and either Meldrum's acid (A1) or 1,3-dimethyl barbituric acid (A2) as the acceptor part.…”