Ultrafast photo-dynamics of a reversible photochromic spiropyran

We report the first ultrafast ring closure kinetics of the merocyanine (MC) of a reverse photochromic spiropyran 1′,3′,3′-trimethyl-6,8-dinitrospiro[2H-1-benzopyran-2,2-indoli ne] (6,8-dinitro BIPS) and its 1′-(2-carboxyethyl) substituted derivative. The reaction is fast and essentially over in a few hundred picoseconds. The rate of MC-S₁ state decay evaluated from transient absorbance can be correlated directly with the MC-S₁ state lifetime as determined using stimulated and ordinary emission measurements. The recovery of the MC-S₀ state from MC-S₁ has a component with a lifetime of 60-78 ps in acetonitrile with a second component of 350-470 ps probably due to a metastable intermediate. We can further establish that the excited MC-S₁ state takes on the order of 500 fs to form from higher-energy states after photoexcitation with 390 nm light.