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Photochemical valence bond isomerization of a crystalline Dewar benzene 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 (F ≈ 450). By contrast, isomorphous crystals of the Dewar benzene diacid monosalt of an ethylbenzene-linked piperazinium (DB-EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. F ≈ 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 the 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 a little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield F ≈ 517. 

Quantum chain reactions are characterized by the formation of several photoproducts per photon absorbed (FQC > 1) and constitute a promising signal amplification mechanism. The triplet-sensitized isomerization of Dewar benzene is known to undergo quantum chain reactions characterized by an adiabatic valence-bond isomerization to the excited state of Hückel benzene, which is able to transfer its triplet energy to a new ground state Dewar benzene that reacts to continue the chain. Given that diffusion-mediated energy transfer is the chain-limiting event in solution, we demonstrate here that reactions in crystals are significantly more efficient by taking advantage of energy transfer by a presumed exciton delocalization mechanism. Using Dewar benzenes with covalently attached, high energy triplet sensitizers we have demonstrated the efficiency of the solid state by the amplification of a quantum yield of ca. FQC ≈ 76 in acetonitrile solution to as much as ca. FQC ≈ 100–120 in submicron size specimens prepared by the re-precipitation method, and up to ca. FQC ≈ 300 with microcrystalline powders suspended in water.