Resonance Raman studies of the structures and vibrational assignments of photochromic S‐alkyldithizones

Abstract

The resonance Raman spectra of S-rnethyldithizone (MeHDz), S-ethyldithizone (EtHDz) and S-isopropyldithizone (iPrHDz) have been measured in the solid state and in solution. It is shown that the red and yellow isomers of these S-alkyldithizones adopt in solution the trans-syn-s-trans and trans-unti-s-trans configurations, respectively, with respect to the N=N, C-N and C-N bonds of the formazan skeleton. It is also demonstrated that for EtHDz the red and yellow isomers can be isolated as different crystal forms. Vibrational assignments are given for both red and yellow isomers of MeHDz based on the frequency shifts of five kinds of isotopically substituted species. INI'RODUCTION S-Alkyldithizones (3-alkylthio-1,5-diphenylformazans)are known to exhibit colour changes in solution.'*2 The colour of a freshly prepared solution of Smethyldithizone (MeHDz) is permanganate pink. On keeping the solution in the dark or in diffused light at room temperature, the colour slowly changes to yellowbrown with a concomitant spectral change; the absorption peak originally at 545 nm decreases in intensity as a new peak appears at 415 nm (Fig. 1A). This process is greatly accelerated on illumination with green light, but is reversed with light of shorter wavelength.S-Isopropyldithizone (iPrHDz) also exhibits a colour change in solution. However, the change is opposite : the lcolour of a fresh solution is yellow ( A, , , = 410 nm), and it gradually turns brownish on standing with a new absorption maximum at 550 nm (Fig. 1B).Although it is generally accepted that these colour changes of S-alkyldithizones are due to thermal and r---1