Influence of disorder on the field-modulated spectra of polydiacetylene films

Abstract

Electroabsorption and electroreflectance spectra of disordered, optically isotropic polydiacetylene films (4-BCMU) have been measured with field perpendicular to the incident light. The modulated spectra are polarized reflecting the local anisotropy of \ensuremath{\pi}-conjugated polymers. Based on results on single crystals, the data are interpreted as quadratic Stark effect of excitonic transitions with the same values for polarizability and dipole moments as in single crystals, accounting only for inhomogeneous broadening and random orientation of polymer segments. Line-shape analysis reveals a large additional contribution proportional to the second derivative of the absorption spectrum. This part, absent in single crystals, is interpreted as a linear Stark effect due to permanent dipole moments arising from disorder-induced asymmetry of polymer segments. Isotropic distribution of the disorder-related dipoles leads to a quadratic dependence of the signal on field strength, different from the quadratic Stark effect only by the spectral line shape.