PHOTOCHROMISM OF THE SYNTHETIC 4',7-DIHYDROXYFLAVYLIUM CHLORIDE

The synthetic compound 4',7-dihydroxyflavylium chloride shows an inter esting photochromic effect at moderately acidic pH values. Pale yellow solutions of this compound, equilibrated in the dark at pH 3.4, becom e bright yellow upon irradiation with 313-nm light. Switching off the light, gives back the initial pale yellow solution. Photochemical meth ods, H-1 NMR, and molecular orbital calculations were used to elucidat e this photochromic behavior. In solutions equilibrated in the dark (p H < 5.7), the existence of two main species in equilibrium is demonstr ated: the colored flavylium cation and the colorless trans-chalcone. F reshly prepared solutions of this compound at pH 6.2 reveal the existe nce of another colored form, the quinonoidal base, which reacts therma lly to yield trans-chalcone. An analogous reaction takes place in fres hly prepared solutions at pH 3.4: the flavylium cation is partially co nverted into trans-chalcone. The extent of such conversion is pH depen dent and nearly complete at pH 6.2. Irradiation of trans-chalcone at t he wavelength of 313 nm partially gives back the initial colored form, with concomitant formation of small amounts of photodegradation produ cts. The turnover of the photochromic reaction is greater at pH 3.4 th an at pH 6.2. The photochromism is explained on the basis of a trans-c is photoisomerization. In fact the relative stability of the two chalc one isomers is reversed in the excited state, as predicted from molecu lar orbital calculations.