OBSERVATION OF PHOTOCHROMIC REACTIONS IN SPIROPYRAN MONOLAYERS BY SURFACE-POTENTIAL MEASUREMENT

The monolayer films of photochromic spiropyran have been investigated by means of surface potential and optical absorption measurements. We found that UV light irradiation isomerises the spiropyran chromophore to zwitterionic photomerocyanine at the air-water interface and induces not only an optical absorption increase but a surface potential (DELTA-V) increase of about 500 mV. The thermal reversion process was also observed, using these techniques, to be fairly slow in the dark. In the DELTA-V - A (area) isotherm of the photomerocyanine monolayer, there exist three regions. In two of them, no detectable surface pressure is observed, while DELTA-V changes. The change in DELTA-V in the other region is mutually related to the surface pressure change and is caused by molecular orientational change.