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 13 C-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.The generality and simplicity of the orbital-based nomenclature for electronic transitions introduced by Kasha 1 allowed photochemists to classify and predict photoreactions on the basis of the orbital nature of the excited state. 2 Since the pioneering studies by Hammond on the photoreduction of benzophenone, it has become clear that in carbonyl compounds the nπ* state is more reactive than the ππ* state. 3 This has led photochemists to seek conditions under which a given carbonyl compound would possess nπ* as the lowest excited state. Since the early 1960s, it has been known that aryl-alkyl ketones and diaryl ketones may possess closely placed nπ* and ππ* triplet states. 4 The relative energies of the two states, each with its own distinc...

Controlling Photoreactions Through Noncovalent Interactions Within Zeolite Nanocages

“…Based on the results with Li + and Na + , the BA was rationalized to follow the trend Li + > Na + > K + > Rb + > Cs + . 20 Solid -state NMR results supported the computational predictions. 20 The three independent measurements, static, magic angle spectra (MAS), and cross -polarized magic angle spectra (CP -MAS), of 13 (2) Greater mobility of ACP molecules on silica, which lack alkali metal ion, than within NaY zeolites, which has at least four type II alkali metal ions in a supercage, speaks for the importance of alkali metal ion · · · ACP interaction.…”

“…20 Solid -state NMR results supported the computational predictions. 20 The three independent measurements, static, magic angle spectra (MAS), and cross -polarized magic angle spectra (CP -MAS), of 13 (2) Greater mobility of ACP molecules on silica, which lack alkali metal ion, than within NaY zeolites, which has at least four type II alkali metal ions in a supercage, speaks for the importance of alkali metal ion · · · ACP interaction. (3) From the comparison of the line width in MAS spectra of ACP in NaY zeolites of varying Si:Al ratio (line width at half height: 500, 390, 350, and 300 Hz in zeolites of Si:Al ratio 2.5, 6, 15, and 40), it is obvious that ACP molecules are more restricted within NaY of high alumina content (consequently more alkali ion).…”

“…10.6 ). 20 Binding affi nities (BAs) of Li + to ACP in these two modes in both the ground state (55.3 and 37.7 kcal mol − 1 ) and the triplet state (57.3 and 47.2 kcal mol − 1 ) are large. As would be expected, Li + binds more strongly (BA in ground state: 55.3 kcal mol − 1 ) than Na + ion (39.9 kcal mol − 1 ) to ACP.…”

“…10.9 ). 20 Based on the orbital energies, the lowest triplet state of free and Li + bound ACP is predicted to have n π * and π π * confi gurations. Time -dependent density functional theory (TDDFT) calculation besides correctly predicting the ordering in the case of unperturbed ACP and 4 -methoxyacetophenone (MACP), identifi ed the triplet energies within a few kilocalorie per mole ( < 2 kcal mol − 1 ) of experimental values.…”

“…20 It is known that at 77 K, ACP in nonpolar as well as moderately polar organic solvents shows a structured emission (phosphorescence) with a short lifetime (millisecond range) characteristic of n π * states, while MACP shows a broad emission with a long lifetime (seconds) characteristic of π π * triplet state. As illustrated in Figure 10.10 , ACP within NaY shows, independent of the time slice, broad phosphorescence, suggesting the emitting state to be π π * in character.…”