Temperature and crystal size dependence of excitonic absorption for polydiacetylene microcrystals

The excitonic absorption of well defined polydiacetylene microcrystals was measured as a function of temperature and crystal size. Poly-DCHD microcrystals were prepared by a recently reported reprecipitation method, and crystal size was controlled by changing the concentration of diacetylene monomer solution injected into water. The excitonic absorption peak position was blue-shifted from 650 nm to 635 nm with decreasing crystal size from 120 nm to 30 nm. On the other hand, the peak position was red-shifted by 15 nm (300 cm^-1) upon decreasing the temperature from 273 K to 4 K. Narrowing of the half peak width was also observed to accompany this red-shift. These phenomena are considered to be related to the coupling interaction between the excitonic state and phonons depending on crystal size and temperature.