Effect of Alkyl Chain Length on Anthracene-Thiazolidinedione Molecular Crystal Motor Dynamics

Abstract

In a previous paper (J. Am. Chem. Soc. 2024, 146, 18836-18840), crystal microwires composed of (Z)-5-(anthracen-9-ylmethylene)-3-butylthiazolidine-2,4-dione (C4-ATD) were shown to undergo sustained oscillatory motion under constant 405 nm or solar illumination. In order to determine whether this oscillatory behavior can be generalized, the length n of the alkyl chain attached to the nitrogen of the thiazolidine-2,4-dione headgroup was varied from n = 0 to n = 8. Changing the alkyl chain length had little effect on the molecular properties, as measured by the molecules’ absorption spectra and Z→E reactivities in solution. Density functional calculations confirmed the electronic inertness of the alkyl sidechain. But when this family of molecules was crystallized, the length of the alkyl chain strongly impacted both the formation of reactive head-to-head versus nonreactive head-to-tail polymorphs and the crystal growth modes. n ≤ 5 derivatives all supported a head-to-head polymorph that enabled the Z↔E photoisomerization while growing in different morphologies, ranging from dendritic shapes to long, thin wires. Only the latter shape was suited for observing photomechanical oscillations, and we identified two new members of the ATD family (n = 0 and n = 2) that support robust oscillatory dynamics similar to those seen for the n = 4 derivative. The results illustrate how small changes in the molecular structure can change the crystal packing and growth kinetics that ultimately determine whether a molecular crystal exhibits photo-induced motor-like behavior.