Stark effect of one-dimensional Wannier excitons in polydiacetylene single crystals

Abstract

Electric-field-modulated reflectivity of three different polydiacetylenes---PTS, poly[2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)]; PFBS, poly[2,4-hexadiyne-1,6-diol-bis(p-fluorobenzene sulfonate)]; and DCHD, poly[1,6-di(n-carbazolyl)-2,4-hexadiyne]---has been measured and is analyzed with respect to the underlying mechanism for the observed sensitivity of \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ transitions to electric fields. Excitons of high oscillator strength and their vibronic satellites respond to fields along the polymer backbone by a large quadratic Stark shift, revealing a large polarizability for this direction. About 0.5 eV above the excitonic absorption edge, in a region of relatively low absorption, electroreflectance signals of different origin are observed, which, contrary to the excitonic signals, vary strongly in size among different specimens of the same composition. Line-shape analysis and its dependence on the field strength identify this signal as the Franz-Keldysh effect of free-electron states, the continuum of the excitons, and exclude an assignment to forbidden exciton transitions. The large polarizability of the excitonic states, which results from unusually strong coupling to their continuum, is consistent with a Wannier exciton extending over about ten conjugated bonds and with a small reduced mass of the order 0.1${\mathit{m}}_{0}$. The large binding energy (0.5 eV) and oscillator strength (f\ensuremath{\approxeq}0.6) of the excitons and their extremely strong coupling to continuum states are attributed to the one-dimensional character of the electron states.