The optical properties of crystalline 1,1′-diethyl-2,2′-cyanine iodide

Optical properties of oriented single crystals of 1,1′-diethyl-2,2′-cyanine iodide (DCI) have been determined by Kramers–Kronig transformation of the reflectance in the visible region. The gross features of these spectra are understood in terms of simple exciton theory when account is taken of directional dispersion. Transmittance and reflectance measurements were also made on thin crystals. These spectra indicated that the thin crystals have a structure different from that of the large opaque crystals. Comparison of all of the crystal spectra with the so-called ’’J’’ and ’’H’’ aggregate bands of photographic importance suggests that these aggregate bands may be due to crystal transitions.