Diacetylene derivatives exhibit solid-state polymerization to polydiacetylene initiated by UV light or γ-ray irradiation. The activation of the photopolymerization highly relies on the monomer diynes arrangement. Recently, we have demonstrated the first mechanoresponsive bisamide substituted diacetylenes (DAs) showing dramatic switching from light-inert to light-reactive state upon a given pressure. The origin of this unique phenomenon was apparently related to the pressure-sensitive crystalline transition in DAs, but the molecular mechanism remains elusive. To obtain the insight, we herein present a series of DAs with varying terminal alkyl spacer length, and their molecular structural effect on the intermolecular hydrogen bonding and steric repulsion. In pristine states, even-parity DAs were inactive upon UV irradiation (λ=254 nm) unless external pressure was given. By contrast, odd-parity DAs were easily polymerized upon UV irradiation without pressure application. However, the pressure-induced crystalline phase transition exhibiting photopolymerization was valid for all DAs regardless of their alkyl spacer length. A systematic investigation revealed that the terminal alkyl spacer length, especially its odd-even parity plays a key role to determine the intrinsic intermolecular hydrogen bonding nature of DA crystals and resultant molecular packing. In addition, the relevant thermochromic behaviour was also observed from photopolymerized polydiacetylenes.