Photochromic behavior of 2,2-spiro-adamantylidene-2H-naphtho[1,2-b]pyran: A new thermoreversible and photoreversible photochromic system

The photokinetic behavior of a photochromic compound, 2,2-spiro-adamantylid ene-2H-naphtho[1,2-b]pyran (Py), has been investigated by using monochromat ic irradiation in the UV and visible ranges. An unusually complex photochem istry occurs whereby two colored forms (o-quinone-allides) are produced, on e of which Is thermoreversible whereas the other is not thermoreversible bu t photoreversible, These are the result of ring opening of the pyran C-O bo nd to a short lived open-form intermediate which converts to the two differ ent colored forms by twisting around different C-C bonds. Their spectra and molar absorption coefficients were obtained in acetonitrile solution. The kinetic parameters of the thermal bleaching (rate constant and activation e nergy) and photobleaching (quantum yield) were measured, A plausible reacti on mechanism is proposed. Based on this mechanism, mathematical methods wer e devised which were capable of analyzing the system during its dynamic evo lution and evaluating the quantum yields of the color-forming photoreaction s.