KINETIC-ANALYSIS OF PHOTOCHROMIC SYSTEMS UNDER CONTINUOUS IRRADIATION- APPLICATION TO SPIROPYRANS

The kinetics of spiropyran photochromic systems in solution in a stirr ed batch reactor continuously irradiated with monochromatic light was studied by UV/visible spectrophotometry. The plots of absorbance vs ti me were analyzed, and the desired parameters (quantum yields, UV/visib le spectrum of the unstable photomerocyanine, ...) were extracted from an iterative computation which fitted the calculated curves to the ex perimental ones on the basis of a representative model of the reaction mechanism. The method was applied to spiro-[benzothiazoline-2',2-benz opyran] whose corresponding photomerocyanine has a lifetime of 100 s i n toluene; the two quantum yields of the direct process of photocolora tion and the reverse reaction of photobleaching could be determined al ong with the spectrum of the corresponding photomerocyanine. To demons trate the general nature of the photokinetic method, it was applied to the reaction of spiro[indoline-2',2-benzopyran], where the main photo chromic process is accompanied by a photodegradation. Despite this int erfering phenomenon, the photokinetic method could be used to extract the parameters of the main photochromic process. It also showed that t he photodegradation products catalyzed the thermal back-isomerization. The order of magnitude of the rate constant of this catalytic process and the quantum yields of photodegradation were estimated.