Optical Activity and Optical Anisotropy in Photomechanical Crystals of Chiral Salicylidenephenylethylamines

Introducing chirality
into photomechanical crystals is beneficial
for the diversification of mechanical motion. Measurement of the chiroptical
and optical anisotropic properties of chiral crystals is indispensable
for evaluating photomechanical crystals. The platelike crystals of S- and R-enantiomers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenylethylamine
in enol form (enol-(S)-1 and enol-(R)-1) caused bending motion with twisting upon
ultraviolet (UV) light irradiation, due to shrinkage along the length
and width directions of the irradiated surface, based on the optimized
crystal structure of the photoisomerized trans-keto-(S)-1. By employing the generalized high-accuracy
universal polarimeter (G-HAUP), optical anisotropic (linear birefringence,
LB; linear dichroism, LD) as well as chiroptical (circular birefringence,
CB; circular dichroism, CD) spectra of both the enantiomeric crystals
on the (001) face were simultaneously measured before and under continuous
UV irradiation. The LD peak was observed at 330 nm in the negative
sign, derived from the π–π* transition of the intramolecularly
hydrogen-bonded salicylidenimino moiety. The CD spectra of the S and R crystals revealed the negative
and positive Cotton effect at 330 nm, respectively, and new peaks
appeared at 460 nm under UV light irradiation due to photoisomerization
to the S and R trans-keto isomers at around 10% conversion. The CB and CD spectra evaluated
by the HAUP measurement were opposite to those measured in the hexane
solution, as well as those simulated by quantum chemical calculation.
The dissymmetry parameter, g, of the enol-(S)-1 crystal along the c axis
(0.013) was approximately 10 times larger than the g values in the solution (0.0010) and by calculation (0.0016).