Laser-intensity and irradiation-time dependence of blue- and red-phase polydiacetylene formation during femtosecond multiphoton polymerization

We investigated the effects of laser power density and irradiation time on the formation of blue- and red-phase polydiacetylene (PDA) by femtosecond multiphoton polymerization of 10,12-tricosadiynoic acid (TDA) thin films. By monitoring the absorbance changes at 638 and 514 nm, the temporal evolution of blue- and red-phase PDA during photopolymerization was evaluated separately. An intensity-time phase diagram constructed from these data revealed a trade-off between the degree of polymerization and blue-phase retention. Within the evaluated range, the condition yielding the maximum extent of polymerization while maintaining a blue-phase fraction of at least 80% was estimated to be a laser power density of 1.2 W/cm² and an irradiation time of 0.30 s. Translating the substrate at 0.54 cm/s reduced the effective irradiation time at each position and yielded a blue-phase fraction of 60.3% at 1.6 W/cm². Increasing the laser power density to 3.2 W/cm² decreased the blue-phase fraction to 47.3% but increased the deconvoluted blue-phase peak absorbance by a factor of 2.19. These results suggest that short-duration irradiation at a sufficiently high laser power density can promote the formation of blue-phase PDA while limiting its conversion to the red-phase.