2ND-HARMONIC GENERATION FROM LANGMUIR-BLODGETT-FILMS OF CENTROSYMMETRIC SQUARAINES WITH HETEROCYCLIC DONOR GROUPS

Langmuir-Blodgett (LB) monolayers of squaraine dyes of general formula D-(C-4,O-2,)-D, where D is a heterocyclic donor, exhibit second-harmo nic generation (SHG) with, for example, chi(zzz),,((2))= 100-150 pm V- l at 1.064 mu m for 2,4-bis[(3,3-dimethylindolin-2-ylidene) methyl]squ araine. The behaviour is not an inherent property of the molecule and the second-order susceptibility is too high to arise from polarisation of the molecular film at the interface. Instead, the SHG is attribute d to noncentrosymmetric aggregation with intermolecular charge transfe r between the donor (heterocycle) and acceptor (C,O,) groups of adjace nt molecules playing a significant role and contributing to the suscep tibility. The nonlinear optical properties are discussed with referenc e to films of 1,4-bis(pyrrol-2-yl) squaraine derivatives which show ei ther very weak or no SHG. The behaviour is dependent upon the type of aggregation and the extent of the donor-acceptor interaction.