Observation of Azo Chromophore Fluorescence and Phosphorescence Emissions from DBH by Applying Exclusively the Orbital Confinement Effect in Siliceous Zeolites Devoid of Charge-Balancing Cations

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Abstract

The molecular orbital (MO) confinement theory predicts that the HOMO and LUMO of an organic guest may become distorted when the dimensions of the host cavity approach closely those of the MO (Corma, A.; García, H.; Sastre, G.; Viruela, P. M. J. Phys. Chem. B 1997, 101, 4575-4582). Generally, quantum chemical calculations assume that the MO extends over all the space. However, the fact that a molecule is confined within a restricted cavity might impose certain limits to its MO. In agreement with this theory, we have observed for the first time the phosphorescence emission from 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) upon incorporation in zeolites devoid of charge-balancing cations, even at room temperature. This observation may be rationalized on the basis of two possible effects: (i) Decrease of the HOMO-LUMO gap induced by confinement of the azo np* chromophore favors ISC, and (ii) inhibition of radiationless deactivation pathway by the immobilization of DBH within the zeolite host favors long lifetimes.

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