Polymer Spectra of a Cyanine Dye

In dilute aqueous solutions the dye 1,1′-diethyl-2,2′-cyanine chloride, a photographic sensitizer, has two broad molecular absorption bands with maxima at 4900 and 5230A. For a 10−2 molar water solution at room temperature, which is a gel, a new, intense, and exceedingly narrow absorption and fluorescence band has been found at 5730A by G. Scheibe, and by E. Jelley by other methods. The electric polarization of this band in both absorption and fluorescence is parallel to the long dye polymers formed in the gel. In the present investigation the occurrence and behavior of this ``P band'' were studied for thin dye films deposited on glass from solutions containing water. The P band shifts about 50A to greater wave-length, becomes broader and weaker, and finally disappears as the water is removed by pumping and/or by increasing the temperature. This behavior is reversible if water vapor is readmitted. The position of the P band enables a measurement of the humidity of the atmosphere surrounding the film. The effect of temperature from −195 to +100°C is described. The P band can be made to appear starting with water solutions as dilute as 10−5 M by freezing the water, locally concentrating the solution. For dye films, there is observed a second, weaker absorption band (called the PM1 band) at 5430A similar to the P band in structure and behavior but polarized perpendicular to the polymer chains and requiring a higher dye concentration before appearing. Reasons discussed in the paper indicate that the P band is due to single dye polymer chains while the PM1 band should belong to a system of coupled chains lying mutually parallel and forming threads. These threads were observed microscopically. A dehydrated film may contain a fraction of the dye molecules in monomeric form, another fraction (showing only the molecular bands much broadened) in an array anchored to the supporting surface such that the mere addition of water vapor immediately produces the polymer chains with the P and PM1 bands, and the remainder in crystalline form with a broad absorption band at 5650 and a broad fluorescence band at 6200A. Experiments undertaken to determine the number of water molecules per dye molecule necessary for polymerization were only sufficient to determine that the order of magnitude of the ratio is from 1 : 2 to 10 : 1. An attempt is made to interpret the absorption and fluorescence bands of the polymer chains by the hypothesis of exciton migration.