Search for squaraine derivatives that can be sublimed without thermal decomposition

To search for sublimable squaraine (SQ) dyes for fabricating thin films under vacuum, we have synthesized a series of 2,4-bis[4-(N,N-dialkylamino)-2,6-dihydroxyphenyl]squaraines [SQ(OH)(4)] and studied their properties. The investigation of the behavior of their Langmuir films at the air-water interface revealed that SQ(OH)(4) molecules with branched N-alkyl groups such as sec-butyls and isobutyls have larger limiting areas and tend to form J-aggregates in the monolayers, whereas molecules with straight N-alkyl chains have smaller limiting areas and are apt to form H-aggregates. This behavior is attributable to the much larger steric hindrance of the branched N-butyl groups than that of the straight ones. The thermal stability of these dyes was investigated by differential thermal analysis (DTA) and thermogravimetry (TG), and their sublimation ability was evaluated through heating under vacuum. As a result, we verified that the four hydroxyls at the 2',6'-positions of the two phenyl rings significantly enhanced the thermal stability and the sublimation ability of an anilino SQ dye molecule, and the introduction of branched N-butyls further promoted the sublimation ability of the target SQ(OH)(4) molecules. These phenomena may be attributed to the intramolecular hydrogen bonding between the hydroxyls and the CO groups of the four-membered ring, and the much larger intermolecular repulsive force between branched N-alkyls, respectively. In particular, the SQ(OH)(4) dye with four N-isobutyls could be sublimed without any decomposition. These results suggest that SQ(OH)(4) molecule with branched N-butyls is the most effective structure for realizing a high sublimation ability. Furthermore, pure SQ dye thin films have been successfully fabricated by molecular beam deposition of the SQ(OH)(4) dye with four N-isobutyls. The vacuum-deposited thin films of such SQ dyes have potential applications in various fields such as electrophotography, solar energy conversion, optical recording, and nonlinear optics.