Phase transition, structure and color-transition behaviors of monocarboxylic diacetylene and polydiacetylene assemblies: The opposite effects of alkyl chain length

•Properties and structures of 7 diacetylenes(DA) and polydiacetylenes(PDA) are explored.•Shortening of alkyl tail of DA and PDA reduces phase/color transition temperature.•Shortening of alkyl segment near the carboxylic head group causes opposite results.•A drastic increase of phase/color transition temperature is detected in DA/PDA(2,10).•Strength of hydrogen bonding and molecular packing vary with the alkyl chain length.