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

The multiple aggregation behavior of surfactant squaraines, 1 ylamino) phenyl-4'-(dimethylamino)phenylsquaraine], 2 [bis(4-(methylstearylamin o)phenyl) squaraine], and 3 cylamino)phenyl-4'-(dimethylamino)phenylsq uaraine] in Langmuir-Blodgett (LB) films has been investigated. Squara ine 1 absorbs at 633 nm (monomer) in chloroform solution and forms a b lue-shifted aggregate with lambda(max) at similar to 530 nm in LB film s 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, bro ad) aggregate. Both the blue-shifted and the red-shifted aggregates ca n 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 tha t this species is a J-aggregate of squaraine 1. Our results suggest th at (1) both the blue-shifted and red-shifted aggregates are metastable and (2) the J-aggregate structure is the thermodynamically favored ar rangement for 1 in a dry LB film on the glass substrate. Interestingly , the blue-shifted aggregate can be regenerated when either the J-aggr egate 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 agg regates in the LB films on glass. Partial conversion of the freshly pr epared 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 ar eas of the squaraine chromophore in different orientations (verticle = 26 Angstrom(2)/molecule), and the Limiting area of a hydrocarbon chai n (similar to 25 Angstrom(2)), We conclude that the multiple aggregati on observed in the LB film of 1 is simply a foe area effect. To gain i nsight 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 spe ctral shifts observed in the different aggregates.