Enhanced coloration of a methylene blue film on fused quartz by mechanical brushing and bleaching by solvent vapor

Broad visible absorption, peaked at ≈596 nm, of a methylene blue (MB) film prepared by dip coating on a quartz plate was found to increase significantly when the film surface was mechanically brushed. This coloration was nearly bleached by exposing the film to ethanol or water vapor. This treatment was reversible for 7-8 times. The enhanced coloration was ascribed to the destruction of the parallel alignment of the MB molecules in the original film by mechanical brushing. Spectroscopic and X-ray diffraction studies revealed that the ordered structure was restored by treatment with solvent vapor. Similar phenomena were observed with several thiazine and oxazine dyes.