Predicting photoactivity in dithienylethene crystalline solids

This commentary discusses the design of stimuli-responsive materials, specifically, light-responsive dithienylethene-based compounds. Recent progress in predicting photoactivity using a combination of theory and crystal structure landscape experiments is highlighted.

morphic and two structures were obtained.Through cocrystallization with multitopic carboxylic acids, four hydrogenbonded cocrystals were prepared.Six coordination polymers were synthesized with DTE as a ligand.Finally, seven solids were obtained from metal-organic framework syntheses using zinc, DTE and multitopic carboxylic acids.In total, 19 structures were used to form the basis of the structural landscape for this pyridyl-functionalized dithienylethene derivative.
Using established requirements for photoactivity in dithienylethene compounds (i.e.antiparallel geometry and interatomic distance less than 4.2 A ˚), the authors visually examined each DTE molecule within each solid-state structure.Within the 19 solids, 17 contained photoinactive DTE molecules and two contained photoactive DTE molecules.Considering the broad class of compounds prepared in the study, the work shows that obtaining photoactive conformations in the solid state is not trivial.
The authors conducted relaxed potential energy surface scans using a geometry-optimized DTE molecule, and the torsion angle [Fig.1(b), blue atoms] was rotated through AE1 increments to create a 360 analysis of the molecule with corresponding energies for each position.These calculations afforded four energetic minima overall, which correspond to four unique conformations, two antiparallel and two parallel.Since an antiparallel arrangement is required for photoactivity, the authors looked at the distance between the active carbons in the two minimum energy antiparallel conformations.One conformation had an interatomic distance of 3.45 A ˚, while the second was 5.19 A ˚; thus, only the former conformation would be expected to be photoactive.The authors mention that visual inspection of conformers can be time-consuming and somewhat subjective; thus, they sought to develop a simple method to determine conformer type and photoactivity potential unambiguously.
The first parameter they define is D active , which is simply the measured distance between the active carbons, and the distance should be less than 4.2 A ˚to be photoactive.The second parameter they define is D Me-Me , which is the distance between the two methyl groups on the thiophene rings [Fig.regions include: (I) antiparallel conformations with D active < 4.2 A ˚, (II) antiparallel conformations with D active > 4.2 A ˚and (III) parallel conformations.Moreover, when the experimentally obtained values from the 19 unique X-ray crystal structures are added to the plot, they also fall into the same three regions.The two synthesized DTE solids that were experimentally photoactive fall into region I, which is indeed the expected photoactive region [Fig.1(d)].
This work demonstrates the power of experiment and theory conducted in tandem for predicting properties in crystalline materials.The D active -D Me-Me analysis provides a straightforward way to predict photoactivity using only two distance measurements.Rapid and effective tools, such as the analysis method described, are valuable to the crystal engineering community.First, such tools are useful to understand why a given structure affords a property.Second, and where the tool shows effectiveness, is when it can be used with crystal engineering strategies to design materials that exhibit desired solid-state conformations and corresponding properties.Further application of this tool to DTE and other dithienylethene derivatives will aid in achieving photoactive conformations (and photoswitching) in the solid state.
1(b), right].D Me-Me will give values that are invariable with respect to structure inversion, i.e. enantiomers would afford identical values.Plotting the values of D active versus D Me-Me using the calculated structures affords a broken ellipse shape, with three distinct areas that are populated [Fig.1(c)].The

Figure 1 (
Figure 1 (a) General dithienylethene structure in open and closed form.(b) Pyridyl-functionalized dithienylethene derivative (DTE) highlighting atoms used in torsion angle measurements (blue), the distance between active carbons (D active ) and distance between methyl groups (D Me-Me ) (Mitchell et al., 2023).(c) Plot of D active versus D Me-Me showing locations of the three distinct and populated regions.(d) One of the two photoactive compounds obtained with DTE.