Reaction amplification with a gain: Triplet exciton-mediated quantum chain using mixed crystals with a tailor-made triplet sensitizer.

Abstract

Photochemical valence bond isomerization of a crystalline Dewar benzene (<b>DB</b>) diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (<b>DB-AcPh-Pz</b>) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (Φ ≈ 450). By contrast, isomorphous crystals of the <b>DB</b> diacid monosalt of an ethylbenzene-linked piperazinium (<b>DB-EtPh-Pz</b>) lacking a triplet sensitizer showed a less impressive quantum yield of ca. Φ ≈ 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with <b>DB-AcPh-Pz</b> as a dilute triplet sensitizer guest in crystals of <b>DB-EtPh-Pz</b>. As expected from their high structural similarities, solid solutions were easily formed with the triplet sensitizer salt in the range of 0.1 to 10%. Experiments carried out under conditions where light is absorbed by the triplet sensitizer-linked <b>DB-AcPh-Pz</b> can be used to initiate a triplet state adiabatic reaction from <b>³DB-AcPh-Pz</b> to <b>³HB*-AcPh-Pz</b>, which can serve as a chain carrier and transfer energy to an unreacted <b>DB-EtPh-Pz</b> where exciton delocalization in the crystalline solid solution can help carry out an efficient energy transfer and enable a quantum chain employing the photoproduct as a triplet chain carrier. Excitation of mixed crystals with as little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield Φ ≈ 517.