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-indoline] (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-S 1 state decay evaluated from transient absorbance can be correlated directly with the MC-S 1 state lifetime as determined using stimulated and ordinary emission measurements. The recovery of the MC-S 0 state from MC-S 1 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-S 1 state takes on the order of 500 fs to form from higher-energy states after photoexcitation with 390 nm light.