The temperature dependence of exciton absorption and Raman frequencies of polydiacetylene (PDA) isolated chains is studied for 3BCMU and 4BCMU chains dispersed in their respective monomer crystal matrix. While isolated 3BCMU chains are practically unstrained, and behave as typical high quality bulk PDA, isolated 4BCMU chains are under a compression that increases as T decreases and show a very different behaviour. A simple model of chain deformation and its effect on the exciton energy is proposed, which qualitatively accounts for all the results. (c) 2007 Elsevier B.V. All rights reserved.