MULTIPLE AGGREGATION OF SURFACTANT SQUARAINES IN LANGMUIR-BLODGETT-FILMS AND IN DMSO WATER MIXTURES

The multiple aggregation behavior of surfactant squaraines, 1 [4-(distearylamino)phenyl-4'-(dimethylamino)phenylsquaraine], 2 [bis(4-(methylstearylamino)phenyl) squaraine], and 3 [4-(methylhexadecylamino)phenyl-4'-(dimethylamino)phenylsquaraine] in Langmuir-Blodgett (LB) films has been investigated. Squaraine 1 absorbs at 633 nm (monomer) in chloroform solution and forms a blue-shifted aggregate with lambda(max) at similar to 530 nm in LB films on glass. Upon heating the films at similar to 65 degrees C for 1-2 h, the blue-shifted aggregate rearranges to a red-shifted (660 mn, broad) aggregate. Both the blue-shifted and the red-shifted aggregates can be converted to another aggregate (690 nm, very sharp) when they are heated at similar to 105 degrees C for 10-20 min. It is concluded that this species is a J-aggregate of squaraine 1. Our results suggest that (1) both the blue-shifted and red-shifted aggregates are metastable and (2) the J-aggregate structure is the thermodynamically favored arrangement for 1 in a dry LB film on the glass substrate. Interestingly, the blue-shifted aggregate can be regenerated when either the J-aggregate or the red-shifted aggregate is treated with steam (water vapor at similar to 65 degrees C) for 2 h. Both 2 and 3 form red-shifted aggregates in the LB films on glass. Partial conversion of the freshly prepared red-shifted aggregate to the blue-shifted aggregate occurs when the LB films of 2 and 3 are steam-treated for 1-2 h. By comparing the limiting molecular areas of 1, 2, and 3 (similar to 52, similar to 60, and similar to 38 Angstrom(2), respectively), the cross-sectional areas of the squaraine chromophore in different orientations (verticle = 26 Angstrom(2)/molecule), and the Limiting area of a hydrocarbon chain (similar to 25 Angstrom(2)), We conclude that the multiple aggregation observed in the LB film of 1 is simply a foe area effect. To gain insight about the driving forces that lead to the multiple aggregation, we have studied the ''arrested crystallization'' of 1-3 in DMSO-water mixtures. Both hydrophobic interaction between the hydrocarbon chains and intermolecular charge-transfer interaction between the donor and acceptor groups in the squaraine chromophore are identified as drivers for the aggregation process. The exciton model previously advanced by Kasha and co-workers is used for a qualitative explanation of the spectral shifts observed in the different aggregates.