Alkyl substituent effects on J- or H-aggregate formation of bisazomethine dyes

Bisazomethine dyes with terminal alkyl substituents of different chain lengths (BAR: R = 1, 2, 3, 4, 5 and 6) were synthesized and deposited on a glass substrate to investigate the effect of the alkyl chain length on aggregate formation. Methyl- and ethyl-substituted bisazomethine dyes (BA1 and BA2) formed J-aggregates in thin films (ca. 50 nm), whereas, propyl-, butyl-, pentyl- and hexyl-substituted derivatives (BA3, BA4, BA5 and BA6) formed H-aggregates in thin films (ca. 50 nm). The aggregate formation of the BARs changed drastically between ethyl- and propyl-substituents (BA2/BA3). However, no remarkable changes were observed in the surface morphologies of BA2 and BA3 films. It is suggested that the critical determinant of aggregate formation of BAR is the molecular packing in the film, which depends on the chain length of the terminal alkyl substituent.

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► Bisazomethine dyes (BAR: R = 1–6) were synthesized and investigate the effect of the alkyl chain length on the aggregate formation.
► Methyl- and ethyl-substituted BARs (BA1 and BA2) formed J-aggregates.
► Propyl-, butyl-, pentyl- and hexyl-substituted BARs (BA3, BA4, BA5 and BA6) formed H-aggregates.