STUDIES OF MULTI-PHOTOCHROMIC LIQUID CRYSTALLINE DENDRIMERS

Abstract

The work described in this research entailed the preparation and studies of photochromic dendritic compounds with irradiation wavelength dependent properties and dendritic liquid crystalline compounds with photochromic response. Photochromic dendrons comprised of varying polar head groups and a dendritic shell of long alkyl chains were studied at both the air-water and air-solid interfaces. At the air-water interface, only an optimal balance between the areas of the polar head groups and the dendritic shells results in observable and significant photomechanical response. At the air-solid interface, all the dendrons formed smooth monolayers with photochromic response with the bulky dendritic shell prohibiting aggregation. Within the monolayers, the molecules are in a tilted molecular orientation. Thermotropic properties of small molecule and multi-photochromic dendrimers were investigated by differential scanning calorimetry, polarized optical microscopy, and powder X-ray diffraction. Dendritic azobenzene acids showed a propensity to form mesophases at higher generations. In the multi-photochromic dendrimers, the nonlinear tris(azobenzene)s formed hexagonal columnar liquid crystalline phases in the zeroth and first generation with a possible nematic phase observed in the second generation. The zeroth generation linear tris(azobenzene) formed a smectic phase with a tilt angle of 36° and a sandy mosaic texture. Bragg spacings in the low angle region of the X-ray diffraction of the first generation indicated a columnar hexagonal liquid crystalline phase whereas the second generation linear dendrimer has 1 sharp peak indicative of a possible nematic phase. Spectroscopic studies indicate 2 wavelengths of maximum absorption for the linear dendrimers at 350 nm and 313 nm respectively. However, the wavelength of irradiation did not impact the rates of thermal back isomerization. In comparing the linear and nonlinear series, the arrangement of the azobenzenes within the dendritic framework (radial vs linear) had no effect on the rates of back isomerization as all the rates were within an order of magnitude of each other. No generational effects were observed in either types of tris(azobenzene)s. The average activation energy for the linear series was 21.8 kcal/mol and 20.5 kcal/mol for the nonlinear. Isomerization of both linear and nonlinear dendrimers leads to smaller hydrodynamic volumes. The largest decrease in volume occurred in the zeroth generation in both series. The linear dendrimers decreased in hydrodynamic volume upon irradiation at both 331 nm and 350 nm with the greatest change occurring under 350 nm irradiation.