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, E 0 , is higher than in any other well ordered PDA providing a relationship between E 0 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).

We report on the first coherent control experiments on a purely electronic exciton state in an extended quasi-perfect organic quantum wire, a polydiacetylene chain isolated in the crystalline matrix of its own monomer. The time-integrated luminescence of a single wire is measured as the relative phase between two exciting sub-picosecond laser pulses is varied. From visibility functions the exciton dephasing time is extracted and its temperature dependence studied. Our work points the predominant role of thermalization upon the phase relaxation dynamics. By means of microscopic imaging spectroscopy we also show that despite local excitation coherent control is achieved on states delocalized over the chain at the micrometric scale.

Symposium on Modern Developmnets in Multiphysics Materials Simulations, Berlin, GERMANY, 2008International audienceThe exciton in single luminescent polydiacetylene (PDA) chains dispersed in their single crystal monomer matrix is studied. As reported these quantum wires have the properties of a perfect quasi-1D semiconducting system. The macroscopic spatial coherence of a single exciton state on a chain as a function of temperature is investigated using an interference experiment. The interference pattern contrast decreasing with increasing temperature is non zero up to 30 K. Experiments developed in order to observe the regime of formation of the stationary coherent exciton state are described. Within the time resolution of the experiment, it is concluded that the formation of this state is faster than similar to 5 picoseconds which confirms the assumption of a transient regime. Finally the first steps of the manipulation of phase locked excitonic wave-packets on a PDA wire are discussed. The experimental results measured in a weak exciton-photon coupling regime are compared to simulations obtained through the Bloch optical equations in the framework of a three-levels model. A direct estimate of the coherence time of the vibronic state photocreated in the present non resonant scheme is derived. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA. Weinhei