Photoluminescence properties of magnesium, chloroaluminum, bromoaluminum, and metal-free phthalocyanine solid films

Photoluminescence properties were measured for vacuum-deposited thin films of magnesium, chloroaluminum, bromoaluminum, and metal-fret phthalocyanines (MgPc, AlClPc, AlBrPc, and H2Pc). For MgPc, AlClPc, and AlBrPc, besides the as-deposited films, phase-transformed films having red-shifted absorption bands at approximately 800 nm were also prepared by a vapor treatment of acetone, dichloromethane, and ethanol, Fluorescence quantum yields of the films at room temperature were 10(-5)-10(-4). These values were much smaller than those of the corresponding monomers (>0.5), indicating that nonradiative relaxation is dominant in the solid films. Increase of the fluorescence intensity with decreasing temperatures was observed in all the samples, but the extent of the increase was at most as large as 10 times, even at the liquid helium temperature, indicating that nonradiative relaxation is still dominant. The spectral features were very different depending on the crystal phases and the materials. All the red-shifted films showed distinct emission bands located at 850-950 nm overlapped with the absorption edge. This feature was interpreted by the emission from the allowed lowest-lying exciton states. The as-deposited AlBrPc and H2Pc films showed broad emission bands at approximately 1000 and 900 nm, respectively, located away from the absorption edge. This feature was interpreted by the very weakly allowed transition from the forbidden lowest-lying exciton state to the vibronic sublevel of the ground state. In the as-deposited AlClPc film new emission bands appeared with decreasing temperature. From the corresponding change in the absorption spectrum, the appearance of the new bands was ascribed to the change of crystal packing.