Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9‐Methylanthracene Single Crystals

Abstract

Photomechanical molecular crystals that expand under illumination could potentially be used as photonpowered actuators.I nt his study,w ef ind that the use of highquality single crystals of 9-methylanthracene (9MA)l eads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples,p resumably due to al ower concentration of defects.F urthermore,s imultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in asmooth expansion of 7% along the c-axis that is linearly correlated with reaction progress.The same expansion dynamics are highly reproducible across different single crystal samples.O rganic single crystals exhibit well-defined linear expansions during 100 %photoconversion, suggesting that this class of solid-state phase change material could be used for actuation.

Heterogeneous Photochemical Reaction on Optical Length Scale in Photomechanical Molecular Crystals

“…As can be seen, it showed a sigmoidal curve, suggesting that the photopolymerization reaction proceeds heterogeneously on molecular scale. This is very similar to the case of 9MA crystals, indicating that the presence of the photoproduct accelerated the next photoreactions of neighboring molecules 22 .…”

“…Furthermore, we focused on the change in the birefringence associated with the photopolymerization reaction. The birefringence change is very sensitive to the photochemical reactions and the change is proportional to the conversion in the photochemical reactions 22,27,28 .…”

“…This kind of reaction is called as a cooperative effect 21 . From such a point of view, recently, we have reported the correlation between the photomechanical response and the photochemical reaction dynamics in 9-methylanthracene (9MA) crystals by simultaneous observation of absorbance and crystal shape changes 22 . In this system, the absorbance changes corresponding to the photodimerization reaction of 9MA pairs followed a sigmoidal curve due to the cooperative effect on molecular scale.…”

Engineering Photomechanical Molecular Crystals to Achieve Extraordinary Expansion Based on Solid-State [2 + 2] Photocycloaddition

“…The first example of the large photomechanical expansion of organic molecular crystals was reported by Bardeen et al, who observed an average 8% expansion of 9- tert -butyl-anthracene ester (9TBAE) nanorods . Subsequent studies suggested that the 9TBAE nanorod expansion did not result from the molecular volume change but instead resulted from the rearrangement of molecular contents. , Recent studies have shown that molecular orientation is coherently correlated with photomechanical behavior and photochemical dynamics. , For our ( E )- NAAM hollow microcrystals, the significant molecular geometry and structural changes from parallel stacking monomeric pairs to butterfly-like photodimers satisfy the first prerequisite for expansion (Figure S19). Although there was no direct comparison between the crystal volume of the monomer and the photoreacted dimer, the formation of the highly distorted photodimer suggests that the crystal lattice might have experienced expansion during photodimerization.…”

“…From the perspective of crystal morphology, it seems that the unique hollow crystal structure is essential for such unusual expansion, as it leaves more void space for molecules to extend and distort. It has been reported that a neat single crystal would lead to more homogenous reaction kinetics than polycrystalline samples due to a lower concentration of defects . However, the unique interior cavities of ( E )- NAAM single crystals may help create an environment of high concentration of “defects”.…”