  Highly luminescent isolated polydiacetylene (PDA) chains in a new diacetylene
crystal are studied. The diacetylene crystal structure and theoretical
calculations suggest that the chains are not planar and more strongly twisted
than any known PDA. Yet the chains behave as quasi perfect quantum wires,
showing that this behaviour is generic among PDA, provided crystal order is
high enough. The exciton energy, E0, is higher than in any other well ordered
PDA providing a relationship between E0 and twist angle. On the other hand the
exciton binding energy and Bohr radius are significantly affected as
demonstrated in electroabsorption experiments (and developed in a companion
paper).
