Optical Properties of Crystalline Pseudoisocyanine (PIC)
Abstract
Normal incidence reflectance spectra of different crystal faces of 1,1‘-diethyl-2,2‘-cyanine (pseudoisocyanine) bromide (PIC-Br) and chloride (PIC-Cl) single crystals have been analyzed by a polariton model. The spectra are anisotropic with wide bands of quasimetallic reflectance for two orthogonal polarizations of light due to directional dispersion of polariton resonances. The measured spectra were analyzed by Kramers−Kronig transformation and compared with theoretical spectra obtained by solving the wave equation. Because of the herringbone-like packing of PIC molecules in single crystal, the π → π* transition is split into two Davydov components with energy separation of about 30 meV. Most of the oscillator strength is concentrated in the fundamental exciton peaks, while a fine structure related to molecular vibrations is also present. Structural isomorphism of PIC-Br and PIC-Cl single crystals is reflected in identical reflection spectra. Glassy PIC-Br films of thicknesses between 5 and 150 nm were prepared by spin-coating. The films are built up by a network of randomly distributed J-aggregates. By treatment with humid air they become highly ordered, showing three differently colored crystalline domains in polarized light. The absorption spectra of the domains coincide with that of the (100), (−101), and (101) faces of single crystal. Surface force microscopy (SFM) reveals heterogeneous layers composed of aligned aggregates of about 0.5 μm width.