Photochromic reaction of 6-nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline]: time-resolved resonance Raman and absorption study

Time-resolved resonance Raman spectra and time-resolved absorption spectra of transient species of photochromic 6-nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] (6-nitroBIPS) generated by irradiation with UV light in cyclohexane have been measured. Three transients were observed to appear sequentially in the nanosecond-microsecond time region. The first transient (transient I) with the shortest lifetime is identified as the T₁ state of photomerocyanine having an open structure (spiro CO bond-cleaved). The second transient (transient II) is attributed to photomerocyanine produced from transient I, and the last transient to appear (transient III) having the longest lifetime is assigned to a dimeric species of photomerocyanine because of the quadratic dependence of its quantum yield on both the pump UV laser power and the concentration of the solution. It is concluded that the cleavage of the spiro CO bond occurs in the S₁ state of the parent species.