Twisted polydiacetylene quantum wire: Influence of conformation on excitons in polymeric quasi-one-dimensional systems

Abstract

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).

Stark effect and Franz-Keldysh effect of a quantum wire realized by conjugated polymer chains of a diacetylene 3NPh2

“…20 Absorption and luminescence spectra submitted in context with the present paper show a remarkable increase of the exciton transition energy to 2.4 eV, larger than in any other PDA. 21 We present here supplementary electroabsorption spectra which despite unexpected birefringence e¤ects con…rm not only the anticipated change of the electronic structure but show an excellent agreement of the spectra at the bandgap with the Franz-Keldysh e¤ect of a one-dimensional energy band.…”

“…The thermal broadening of the exciton and blue shift is shown elsewhere 21 and appears also in the electroabsorption spectrum. Fig.…”

“…Details of the structure and polymerization of 3NPh2 are given elsewhere. 20,21 Crystals were grown in the dark from a solution in acetone at 4 C reaching about 5 mm length along the a-axis and their thickness, 0.2 -0.4 mm, was measured by focusing a microscope on the front and back surface. The small initial polymer content was increased by -irradiation from a 137 Cs source with a dose of 5 krad/min.…”

“…The exciton peak of sample A is with 3 cm -1 about 100 times smaller than for polarization parallel to the chain and has smaller linewidth. 21 We therefore use the integrated absorption, S = 5.2 eV/cm and 0.027 eV/cm for polarizations parallel and perpendicular to the chain, respectively to derive an optical anisotropy close to 200. The sum rule of the oscillator strength f provides also an estimate of the polymer …”

Excitons in a perfect quasi‐1D organic quantum wire, an isolated polydiacetylene chain

“…At this temperature, the fluorescence quantum yield is around 0.35. The Bohr radius (~15 Å) and binding energy (~0.6 eV) of such a red exciton have been determined in two new luminescent polydiacetylenes [4,5].…”