Photomodulated Optical Properties of Twisted Azobenzene Crystals

Crystal twisting, a spontaneous phenomenon estimated to occur in one-third of all molecular compounds, introduces mesoscale chirality into organic films. Film of twisted crystals exhibit circular birefringence (CB) and circular dichroism (CD) that arise from directional splaying of linearly anisotropic crystals as they twist about the growth direction. Here we explore the dynamic modulation of CB and CD in twisted crystal films comprising azobenzene photoswitches that undergo trans-cis photoisomerization under ultraviolet-blue light irradiation and cis-trans reisomerization upon green light irradiation or heat exposure. N,N-4-dimethylaminoazobenzene (commonly known as methyl yellow) in particular can undergo photoisomerization in the solid state. Upon photoisomerization from the trans to cis conformation, methyl yellow crystals exhibit a reduction in both linear birefringence (LB) and linear dichroism (LD) due to a reduction in crystallinity. Mueller matrix imaging, a complete polarimetry method to spatial map the LB, LD, CB, and CD of solid-state samples, revealed that a reduction in LB and LD of single methyl yellow crystals resulted in a reduction in the CB and CD of splayed stacks of the crystals. Furthermore, polarization-angle dependent absorption studies reveal the formation of an absorption band not previously reported. The ability to tune optical activity with external stimuli (e.g. UV light) will enable the design of chiroptical sensors, detectors, and waveguides sensitive to circularly polarized light.