Optical properties of crystalline pseudoisocyanine (PIC)

Normal incidence reflectance spectra of different crystal faces of 1,1'-die thyl-2,2'-cyanine (pseudoisocyanine) bromide (PIC-Br) and chloride (PIC-Cl) single crystals have been analyzed by a polariton model. The spectra are a nisotropic with wide bands of quasimetallic reflectance for two orthogonal polarizations of light due to directional dispersion of polariton resonance s. The measured spectra were analyzed by Kramers-Kronig transformation and compared with theoretical spectra obtained by solving the wave equation. Be cause of the herringbone-like packing of PIC molecules in single crystal, t he pi --> pi* transition is split into two Davydov components with energy s eparation of about 30 meV. Most of the oscillator strength is concentrated in the fundamental exciton peaks, while a fine structure related to molecul ar vibrations is also present. Structural isomorphism of PIC-Br and PIC-CI single crystals is reflected in identical reflection spectra. Glassy PIC-Br films of thicknesses between 5 and 150 nm were prepared by spin-coating. T he films are built up by a network of randomly distributed J-aggregates. By treatment with humid air they become highly ordered, showing three differe ntly 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 layer s composed of aligned aggregates of about 0.5 mum width.