Photochromism of spiro-naphthoxazines: Molar absorption coefficients and quantum efficiencies

A method is presented, which allows, with a minimum of assumptions, the determination of the molar absorption coefficients of the coloured form of photochromic compounds and their forward and reverse quantum efficiencies. The approach used combines a modified Fischer's method with an improved intensity variation technique. The efficacy and accuracy of the proposed method are demonstrated by measuring the photochromic parameters of several naphthoxazinespiroindolines. The forward quantum efficiencies were found to vary from 0.2 to 0.9 and the ratios of reverse to forward quantum efficiencies were shown to be uniformly low, in the range 0-0.07. The molar absorption coefficients of the coloured form at the peak maximum were in the range (3.1-5.1) × 10⁴ dm³ mol^-1 cm^-1.