Structural, optical, and electrical properties of the organic molecular crystal 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate

We present dark conductivity, photoconductivity, and transient photoconductivity measurements of the organic salt 4-N,N-dimethylamino-$4^{\prime }-N^{\prime }$-methyl stilbazolium tosylate (DAST). By correlating these measurements to microscopic structural properties including both molecular orientation and intermolecular interactions, we have determined the charge-carrier generation and transport properties of DAST. The perfect alignment of the chromophores combined with the weak Van der Waals interactions $[\mathrm{CH}–\mathrm{HC}>2.5 \AA (1 \AA =0.1 \mathrm{nm})]$ determines a one-dimensional thermally activated conductivity with an activation energy of $0.34\pm 0.03 \mathrm{eV}$ along the chromophore chain. Photoconductivity that incorporates both positive and negative free charge carriers has been observed at a light energy of 1.6 eV, well below the first strong optical absorption band at 2.33 eV. That band is associated with intramolecular optical excitation. From transient photoconductivity data we found an upper limit of 3 ns to the free-carrier lifetime.