4 PHOTOCHEMICAL PRINCIPLES 



introduction of electron-donating substituents (such as methyl am- 

 monium groups as in, for example, the triphenyl methane dyes or 

 nitrogen groups in the conjugated chain as in the porphyrins, in the 

 bile colors, and in the cyanine dyes) would shift the maximum to 

 longer wavelengths. 



The introduction of sulfur into the conjugated system alters the 

 spectrum profoundly. Thus, replacing the oxygen atom in benzo- 

 phenone (absorption maximum at 255 m/x) by a sulfur atom results 

 in a blue substance, thiobenzophenone (absorption maximum at 620 

 m/j.) — a shift of 365 m^tt! Sulfur compounds exhibit unexpected spec- 

 tral properties owing to the easily polarizable nature of the -n- electrons 

 of the sulfur bonds (Rosenthal and Oster, 1954). We have found that 

 alkaline thiobenzophenone is readily bleached by red light (Oster and 

 Citarel, to be published). It is quite likely that the chromophoric ma- 

 terial of visual purple (absorption maximum at 500 m/x) is thio vitamin 

 A aldehyde (perhaps in the bound state) and is bleached by light to 

 give vitamin A aldehyde (absorption maximum at 387 m/i.) (compare 

 Wald and Brown, 1952, and Dartnall, 1957, Chap. 4, with Oster, 

 1958). 



Most dyes exhibit a shift to longer wavelengths when they are bound 

 to high polymeric substrates (for review, see Oster, 1955). For ex- 

 ample, a solution of malachite green in the free state appears blue. 

 When a small amount of polymer to which the dye will bind is added 

 to the solution, the absorption spectrum is shifted to longer wave- 

 lengths, and the solution appears green. The bound dye resists reduc- 

 tion by strong reducing agents which readily reduce the free dye 

 (Oster and Bellin, 1957). Those dyes, such as the triphenylmethanes 

 and stilbene derivatives, which are capable of internal rotation, ex- 

 hibit a much greater fluorescence when bound than when free (Oster 

 and Nishijima, 1956). The same enhancement of fluorescence is ob- 

 served if the viscosity of the medium is increased. Apparently planarity 

 of the dye molecule is a necessary condition for fluorescence. 



The profound changes in properties which a dye may undergo when 

 it is separated from its substrate indicate the problems that might be 

 encountered in attempts to isolate the pigments of photoperiodic sys- 

 tems. In fact, discrepancies between action spectra and absorption 

 spectra of isolated pigments of a photochemical system are not of 



