Emission Spectroscopic Investigation of Triplet Diarylcarbene Generated in Molecular Sieve VPI-5

Abstract

We report an attempt to generate and characterize a triplet carbene, bis(2,4,6-trichlorophenyl)carbene ((3)1), in zeolites Y and L and in a molecular sieve VPI-5 in which a possible dimerization and reaction with the precursor of carbene are significantly retarded, thus making the triplet carbene longer lived than in solution at room temperature. The adsorption of a corresponding diazomethane (1-N(2)), the precursor of 1, was carefully examined by comparing the absorption spectrum after adsorption with that of 1-N(2) in n-pentane, which revealed that 1-N(2) was adsorbed with the diazo group intact only in VPI-5, while in other zeolites 1-N(2) was found to be decomposed upon adsorption. This difference in reactivity of the hosts was ascribed to the absence of Brønsted-acid sites in VPI-5. The photoirradiation of 1-N(2) in VPI-5 at 77 K was monitored by emission spectroscopy, which revealed that bis(2,4,6-trichlorophenyl)methyl radical (1-H) was produced as the only detectable species under these conditions. This is interpreted as indicating that nascent (3)1 may undergo efficient hydrogen abstraction as a result of multiple excitation by repeated refraction and reflection of the light in a light-scattering medium. In accord with this interpretation, the emission due to (3)1 was observed when irradiation was carried out on a translucent glassy sample prepared by submerging VPI-5 incorporating 1-N(2) in a refractive-index-matching fluid such as propylene glycol or glycerol. ESR signals ascribable to (3)1 were also observed under these conditions. Laser photolysis of 1-N(2) in VPI-5 at room temperature with fast detection of both emission and absorption showed that the bands due to 1-H were detected in the nanosecond time regime probably because of the extremely fast H abstraction by (3)1. However, a variable-temperature ESR study showed that the signals due to (3)1 survive up to 220 K in VPI-5 while the signals disappear at 120 K in 2-methyltetrahydrofuran, suggesting that triplet carbene is stabilized in VPI-5. Thus, a triplet carbene was generated and characterized in a zeolite for the first time and shown to be stabilized extrinsically. The present study also proposes a solution to the issues of acidic sites and multiple excitation often observed in zeolites.