A LOW-TEMPERATURE LUMINESCENCE STUDY OF THE COLORED MEROCYANINE FORM OF SUBSTITUTED 6-NITROBENZO-SPIROPYRANS

The emission spectra of colored merocyanine forms of substituted 6-nitrobenzospiropyrans (1-4) were studied at 77 K. The merocyanine form obtained by UV irradiation of a solution of spiropyrans at room temperature is a dynamic equilibrium mixture of stable trans forms. The merocyanine form obtained by UV irradiation of the same solution at 77 K is a short-lived intermediate. The assignment of this short-lived intermediate to the trans-cis-cis (TCC) form is based on MM2 calculations. This TCC intermediate was also observed during the visible light irradiation of the hydrochloric acid salt form of 1 (5) prepared at 197 K.