Photocoloration quantum yields

A0 is expressed by A0 = Sqf coiMCFIo (at low concentration), where k depends on the experimental conditions, Sqf is the molar absorptivity of the open form at Amax, co is the photocoloration quantum yield, and [CF]0 is the initial concentration of the closed form. All the photochromic parameters are very sensitive to the substitution on the naphthopyran ring. There is an important bathochromic shift of the absorption wavelength of the open forms (up to 88 nm) when a bithienyl entity is fixed at position 3. Moreover, the closed-form absorption spectra show minor but significant variations with a maximum shift of 20 nm. 

The various techniques of Raman spectroscopy have recently been applied to the spiropyrans, and this is discussed more extensively in Chapter 8 in Volume 2. BIPS and its 6-nitro-, 6-nitro-8-methoxy-, and 6-nitro-5,7-methoxy derivatives were studied by surface-enhanced (SERS) and resonance-enhanced Raman spectroscopy in neutral, acidic, and basic solutions. The complex results are too numerous to summarize here, but two unusual ones are that even in acid solutions the tested compounds gave the SERS spectra of the unprotonated open forms, and that an aged solid sample of 6-nitroBIPS gave results very different from a fresh sample. This was attributed to the existence of two enantiomers of the spiro form, which are slowly interconvertible in the solid state and which have greatly different photocoloration quantum yields.167... 

The quantum yields for photocoloration of spirobenzopyrans are collected in Ref. 1. Generally, the coloration quantum yields of spiroindolinobenzopyrans by UV irradiation (366 nm) at room temperature (15-25°C) are 0.1-0.7. Photobleaching quantum yields by visible light are very small (0.1) and less accurate, since both thermal and photobleaching occur simultaneously. 

Heller and Langan38 reported that the quantum yield for photocoloration (c) of fulgide 35 to the 7,7a-DHBF (36) in toluene was 0.20, and the E->c value appeared to be wavelength independent over the range 313-366 nm. Temperature (10-40°C) had little effect on the quantum yield for photocoloration. Furthermore, the cycles of photochromism of 35-36-35 did not affect the (E - c) value. These results showed that fulgide 35 is well suited for chemical actinometry in the near-UV and visible spectral region. 

A highly efficient photochromic process is essential for organic photochromic compounds used as optical recording materials.39,40 That means that the quantum yields for the photocoloring and bleaching reactions should be high, but the quantum yield for the side reactions should be as low as possible. In order to solve the problems mentioned earlier, extensive studies have been carried out, and some promising results have been obtained so far. 

The data currently available on the rates and quantum efficiency of the photocoloration reactions la — lb of perimidinespirocyclohexadienones and their analogs are rather scarce. The values of the quantum yield for the photoconversion of compound la under excitation at its long-wavelength absorption band were determined with the use of the Aberchrome 540 actinometer. These are given in Table 8.3. 

This work has allowed an evaluation of the molar absorptivity of the colored species and the quantum yields of photocoloration and photodecoloration. The examples given include a dihydroindolizine compound, a spiro[benzothiazoline-benzopyran], a 2//-chromcne, a spiro[indoline-benzopyran], and the dimer of the triphenylimidazolyl radical. 

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