SUMMARYThe synthesis and spectrokinetic properties of photochromic spiropyrans in solution are reported. The most important results obtained during the last two decades, including the synthetic access to azaheterocyclic and non-azaheterocyclic spiropyrans are described, as each series confers its own particularities to the photocoloration. The influence of the structure (skeleton, heteroatoms, substituents) on the stability of the photomerocyanines is discussed in terms of electronic (correlation of Hammett type) and steric effects. The role of structural parameters and of solvent effects on the electronic distribution of the colored form is also emphasized.l "
H l s t O r W surveyAlthough the thermochromism of spirapyrans was noted in 1921, their photochromic properties (photocoloration in the indoline series) were not known prior to the observation by E. Fischer and Y.Hirshberg, in 1952 ' ; the same authors, and, independently, 0. Chaud6 and P. Rumpf discovered the photochemical reverse reaction. A few years later, Hirshberg's conception of a photochemical binary element for a computer memory 3'4 and the possibility of getting a variable density optical shutter initiated an intense research activity in this field in industrial and academic laboratories world-wide 536.
2The spiropyrans or spirochromenes and related compounds (e.g. spirooxazines) continue to arise a strong interest in connection with their applications in various fields such as : non-silver high resolution photography ', optical devices and variable transmission materials 8'9, photovoltaic and holographic systems, and biological applications (spectrophotometric probes, photocontrol of ion transport, micellar mede, polymer membranes ) . Some important articles on the thermochromism or photochromism of spiropyran systems appeared between 1948 and 1970 27-39, but the most comprehensive and important review was written by R.C. The spiropyrans or spirochromenes present two heterocyclic parts linked together by a common tetrahedral sp3 carbon atom (Fig. 1).-679 -
Q€ photomerocyanlnePhotochromic interconversion in the spitvpyran series and mesomeric structures of the photomerocyanine form.The reversion to the closed uncoioured form may be a fast process under thermal conditions (kA ) or by a photochemical pathway either by visible light absorption (hv 2) or also by U.V. absorption (hv ,) when the photochromic system is continuously irradiated " . Besides, prolonged Irradiation can lead to photodegradation which is promoted by atmospheric oxygen via radical processes 42. Application of the flash photolysis technique for the study of the photocoloration-photobleaching phenomenon is appropriate as it reduces the secondary effects of fatigue. The photochromism of spiropyrans has no systematic basis but rather depends on: The observation of photochromism depends also on the coloration intensity (optical density of the coloured form which is a function of the concentration). These parameters are quantitatively observable by spectrophoto met ric methods.Th...
