Alkali Ion-Controlled Excited-State Ordering of Acetophenones Included in Zeolites: Emission, Solid-State NMR, and Computational Studies

Abstract

The nature of the lowest triplet excited state of acetophenones included in zeolites has been inferred through steady-state and time-resolved emission spectra. Acetophenone shows cation-dependent state switching. Within NaLiY and NaY zeolites, the emitting state is identified to have ππ* character, whereas in NaRbY and NaCsY, two emissions characteristic of nπ* and ππ* were observed. In contrast, 4‘-methoxyacetophenone does not show cation-dependent state switching; in all alkali cation-exchanged zeolites, the lowest triplet is identified to have ππ* character. The results are attributed to a specific cation−acetophenone interaction. Static, MAS, and CP-MAS spectra of 13C-enriched acetophenone included in MY zeolites confirm the presence of such an interaction. The data reveals that the extent of interaction, as reflected by the molecular mobility, depends on the cation. Small cations such as Li+ and Na+ interact strongly whereas large cations such as Rb+ and Cs+ interact weakly with acetophenone. Consistent with these trends, small cations are found to switch the lowest triplet to ππ* character, whereas the large cations leave the nπ* and ππ* triplet states of acetophenone close to each other. Computational studies provide strong support for these interpretations. B3LYP/6-31G* calculations were carried out on acetophenone and 4‘-methoxyacetophenone as well as their Li+ and Na+ complexes. Geometries with cations bound to the carbonyl, phenyl, and methoxy groups were examined. The most-stable structures involve a cation−carbonyl interaction, which stabilizes the n orbital and, in turn, destabilizes the nπ* triplet state. Excited-state energetics were quantified using TDDFT/6-31+G* calculations. Consistent with experimental observations, acetophenone and 4‘-methoxyacetophenone are predicted to have nπ* and ππ* as their lowest triplet states, respectively. Complexation with Li+ or Na+ is predicted to lead to a ππ* triplet as the lowest excited state for both compounds. The present study, combining steady-state and time-resolved emission spectra, solid state NMR, and computations, demonstrates the occurrence of cation-dependent state switching in acetophenones and offers an internally consistent explanation of the effect in terms of specific cation−carbonyl interaction.