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

Photochemical valence bond isomerization of a crystalline Dewar benzene (DB) diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (DB- AcPh-Pz) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (Phi = 450). By contrast, isomorphous crystals of the DB diacid monosalt of an ethylbenzene- linked piperazinium (DB- EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. Phi = 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with DB- AcPh-Pz as a dilute triplet sensitizer guest in crystals of DB- EtPh-Pz. 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 DB- AcPh-Pz can be used to initiate a triplet state adiabatic reaction from 3DB- AcPh-Pz to 3HB*- AcPh-Pz, which can serve as a chain carrier and transfer energy to an unreacted DB- EtPh-Pz 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 Phi = 517.