Reversible phase transition and third-order nonlinearity of phthalocyanine derivatives

Two different liquid crystalline phases and the reversible phase transition between them have been found for the spin coated thin film of tetrakis(octylthio)phthalocyanines both with and without copper (II): Phase 1 of as-prepared films changed with time lapse at room temperature into phase 2 which had a new absorption maximum at the longer wavelength and the longer spacing of crystalline lattice, and the phase 2 returned to the phase 1 when heated up to 100 degree(s)C and cooled down to room temperature. The phase 2 gave several times larger (chi) (3) values than the phase 1 by THG measurements at the same resonant wavelengths. On the other hand, tetrakis(alkylthio)phthalocyanine with the alkylchain shorter than butyl and octakis(octylthio)phthalocyaninato copper (II) with symmetric molecular structure were found not to give the phase 2 and their (chi) (3) values were small, i.e., in the order of 10-12 esu. Thus, phthalocyanines that do not form a tight inclined stack but a loose dimeric aggregation due to permanent dipole and flexible alkyl chains are estimated to be better candidates for third-order nonlinear optics.