Actinometers, photochemical experiments

The furyl fulgide 172 has found use as a stable, recyclable actinometer for conventional photochemical experiments in the 313-366 mn wavelength range, where 173 = 0.20 and is independent of temperature and concentration245. It has also been developed as an actinometer in one- and two-laser flash photolysis experiments246. The colored form 173 can be converted back to 172 by simple exposure to visible light (equation 65). 

The accurate determination of incident light intensity is of pivotal importance in any quantitative photochemical experiment. While various physical devices are available for making absolute intensity measurements,168 these devices can be difficult to calibrate and usually are rather expensive. A much simpler approach involves the use of a chemical actinometer. This type of system is based upon a photochemical reaction for which product quantum yields are reasonably insensitive to variations in reactant concentration, temperature, light intensity and excitation wavelength. Once the quantum yield is calibrated by an absolute method, a chemical actinometer becomes a rapid, inexpensive and highly accurate secondary standard for light intensity measurements. 

Table 4.8 Lamps, filters and actinometers used in the photochemical experiments...

The fractional light absorption can be measured in a separate experiment. Knowing and estimating the extent of decomposition, f , the incident intensity can be calculated in the units of einstein cm-3s-1 falling on the reaction cell. To avoid geometrical errors due to differences in absorptivity of the actinometer solution and the sample, the same cell is used for actinometry and for the reaction, under conditions of equal optical densities. There are a number of photochemical reactions which have been found suitable as actinometers. They are useful within their specific wavelength ranges. 

As the latter effect, which is comparable to the inner-filter effect during photolysis experiments in solution, does not always occur, radiation scattering problems are inherent to all solid-state photoreactions and are particularly relevant to the photostability testing of solid-state drugs, such as tablets, pills, or powders. Thus in this section, we will discuss three experimental approaches for the determination of solid-state photochemical quantum yields utilizing chemical actinometers. 

A classical photochemical research is that of Bunsen and H. E. Roscoe on the union of hydrogen and chlorine. They say that in Draper s tithonometer the pressure varied during the experiment and hence, since the amount of chlorine dissolved by the liquid varied, the composition of the gas was not constant. They used an apparatus which they call an actinometer in which the pressure remained constant. It consisted (Fig. 59) of a half-blackened flat... 

Apart from the role in mechanistic photochemistry, the scarce dependence on conditions continues to make this compound a convenient actinometer. A recent study has confirmed this fact, supporting its choice as a practical, photochemically sensitive, and thermally robust standard [72]. The study showed that the molar absorptivities of the aldehyde were only weakly dependent upon temperature and that the quantum yield was virtually independent of temperature as well as of the wavelength. This compound was used as an actinometer in field experiments on photochemistry in ice, with excellent results. The value of 0.41 was recommended... 

---