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Highly reactive arylalkylcarbenes generated in solution by photolysis of their aryldiazoalkane precursors tend to undergo competing inter- and intramolecular reactions to yield a complex mixture of products. Having previously shown the use of crystals to effectively control the reactivity of arylalkylcarbenes to afford high yields of a single product, it was of interest to investigate whether the crystalline environment could also enable spectroscopic detection of these intermediates en route to photoproduct. Using 1,2,2-triphenyldiazoethane (3) as a model substrate to probe the effect of alternative reaction trajectories that yield triphenylethylene (5) by competing 1,2-H shift or 1,2-Ph migration, we report selectivities consistent with reaction from a spin-equilibrated carbene 4 in solution, while reactions in crystals primarily afford alkene 5 via a lattice-controlled 1,2-H shift. Attempts to detect 1,2,2-triphenylethylidene 4 in crystals by nanosecond laser flash photolysis or by triplet-triplet fluorescence at 77 K were unsuccessful, indicating that arylalkylcarbenes possessing α-H substituents undergo facile 1,2-H shifts both in solution and in the solid state. However, related tert-butylphenylmethylene with no α-H substituents could be observed by triplet-triplet fluorescence at 77 K in glassy matrices. 

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.