Molecular organization and optical switching of liquid-crystalline azobenzenes in monomolecular films

Abstract

The photoisomerization and thermal relaxation processes of three amphiphilic azobenzene derivatives (LCAn) showing liquid-crystalline properties were studied in solutions and in monomolecular films formed using Langmuir and Langmuir-Blodgett techniques. The well-controlled isomeric state of LCAn in chloroform during the UV irradiation allowed to prepare spreading solutions containing trans isomers, cis isomers or their mixture and to investigate molecular organization in the Langmuir films made of them. Surface pressure-mean molecular area isotherms and compression modulus-surface pressure dependences delivered information on a thermodynamic state, phase transitions and intermolecular interaction in the monolayers. LCAn in the trans state create densely-packed films, in which the photoisomerization do not occur due to the lack of free volume for the change of the molecules conformation and their strong tendency for the H-type aggregation. However, the reversible switching of the LCAn conformation during cyclic photoisomerization and thermal relaxation processes was observed in the LB films initially deposited onto quartz substrates in the cis state, where molecules are loosely-packed and exist in the monomeric form. Kinetics and rate constants of the photoisomerization and thermal relaxation were determined based on the recorded absorption spectra in order to obtain information on their efficiency and on the impact of the replacement of the alkyl chain attached in the LCAn para position with the alkyloxy chain. Our research show that LCAn are promising materials for application in optical memories, logic gates and holographic gratings.