Stepwise Two-Photon-Gated Photochemical Reaction in Photochromic [2.2]Paracyclophane-Bridged Bis(imidazole dimer)

Stepwise
two-photon processes not only have great potential for
efficient light harvesting but also can provide valuable insights
into novel photochemical sciences. Here we have designed a [2.2]­paracyclophane-bridged
bis­(imidazole dimer), a molecule that is composed of two photochromic
units and absorbs two photons in a stepwise manner. The absorption
of the first photon leads to the formation of a short-lived biradical
species (half-life = 88 ms at 298 K), while the absorption of the
additional photon by the biradical species triggers a subsequent photochromic
reaction to afford a long-lived quinoid species. The short-lived biradical
species and the long-lived quinoid species display significantly different
absorption spectra and rates of the thermal back-reaction. The stepwise
two-photon excitation process in this photochromic system can be initiated
even by incoherent continuous-wave light irradiation, indicating that
this two-photon reaction is highly efficient. Our molecule based on
the bridged bis­(imidazole dimer) unit should be a good candidate for
multiphoton-gated optical materials.