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Title
| - Opening a Spiropyran Ring by Way of an Exciplex Intermediate
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Abstract
| - A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzeneresidue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyrangroup has been condensed to the central phenylene ring so as to position a weak electron donor close tothe pyrene unit. Illumination of the pyrene-based chromophore leads to formation of a fluorescent exciplexin polar solvents but pyrene-like fluorescence is observed in nonpolar solvents. The exciplex has a lifetimeof a few nanoseconds and undergoes intersystem crossing to the pyrene-like triplet state with low efficiency.Attaching a 4-nitrobenzene group to the open end of the spiropyran unit creates a new route for decayof the exciplex whereby the triplet state of the spiropyran is formed. Nonradiative decay of this latterspecies results in ring opening to form the corresponding merocyanine species. Rate constants for thevarious steps have been obtained from time-resolved fluorescence spectroscopy carried out over a modesttemperature range. Under visible light illumination, the merocyanine form reverts to the original spiropyrangeometry so that the cycle is closed. Energy transfer from the pyrene chromophore to the merocyanineunit leads to an increased rate of ring closure and serves to push the steady-state composition in favor ofthe spiropyran form.
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