From molecule to nanocrystalline: ultrafast dynamics of trans–cis isomerization of azobenzene derivatives

Abstract

Photoswitches are the most fundamental components of photonic integrated chips. The double-bond trans–cis photoswitching molecules are the most widely studied photochromic materials, but there are still numerous challenges in exploring their ultrafast photoisomerization dynamics. Here, three azobenzene-like derivatives (azo-derivatives: azo-dipyridine (AP), azobenzene (AB) and phenylazopyridine (PAP)) were selected to investigate the factors to modify the trans–cis isomerization dynamics systemically. In the solution, the photoisomerization rate of azo-derivatives from trans to cis is positively correlated with their dipole moments and their rate order AP < AB < PAP is consistent with the energy barrier obtained by potential energy surface scanning of excited states. When these azo-derivatives are assembled into nanocrystal in the suspension solutions, their isomerization rates will all accelerate, but the rate order is completely opposite to that of the solution phase. The crystal structure and energy decomposition analysis based on force field show that the weaker the intermolecular hydrogen bonding and the stronger the π–π stacking, the more favorable it is for trans–cis isomerization of azo-derivatives. This work reveals key factors that affect the trans–cis isomerization of azo-like derivatives, the dipole moment in solution, hydrogen bonding, and π–π stacking in the crystal, providing a theoretical basis for designing novel photoswitching molecular materials.