PHOTOCHROMIC PROPERTIES OF 4'-AMINO-SUBSTITUTED 2-HYDROXYCHALCONES

2-Hydroxychalcones with 4'-amino substituents undergo facile photochem ical conversion into highly coloured flavylium ions on irradiation wit h 365-436 nm light in aqueous ethanoI solution in the pH region 4-6. T he coloured solutions are substantially stable against prolonged irrad iation with visible light. The absorption coefficients of the flavyliu m ions around 530-560 nm are strong (approximately 5 X 10(4) mol(-1) d m(3) cm(-1)) and well separated from those of the chalcones around 400 nm. Some of the derivatives feature good reversibility for repeated p hotochemical colouration and thermal reversion without significant fat igue in dilute solution, while others feature high thermal stability. Similar photochemical colouration occurs in many solid polymers, but i rreversible thermal bleaching takes place. The photochromic properties are discussed in terms of the frontier orbital theory.