Intrazeolite Photochemistry. 12. Ship-in-a-Bottle Synthesis and Control of the Photophysical Properties of 9-(4-Methoxyphenyl)xanthenium Ion Imprisoned into Large-Pore Zeolites

Abstract

The 9-(4-methoxyphenyl)xanthylium ion (AnX + ), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 Å pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X + ) adsorbed within the zeolite by treatment of xanthydrol onto the H + form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using R-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX + . Entrapment within the zeolite framework restricts AnX + conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds), as well as the observation of room-temperature fluorescence in the nanosecond range.

Supramolecular Host‐Guest Systems in Zeolites Prepared by Ship‐in‐a‐Bottle Synthesis

“…9‐(4‐Methoxyphenyl)xanthylium has also been prepared by a ship‐in‐a‐bottle synthesis inside the cavities of zeolites Y and Beta by electrophilic attack of preformed xanthylium cation180 on electron‐rich anisole (Scheme ) 181. The resulting 9‐arylxanthylium remains entrapped inside the zeolite supercages, molecular modelling indicating that the conformational rotation of the anisyl substituent over the xanthylium cores is impeded by the encapsulation (Figure 7).…”

“… Molecular model of the 9‐(4‐methoxyphenyl)xanthylium cation encapsulated within an all ‐ silica zeolite Y supercage (taken from ref 181…”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…Once formed, the synthesized molecules are permanently entrapped within the zeolite supercage and can neither escape nor diffuse between cavities. This strategy was initially applied to the preparation of metallic complexes [47,48] but has been expanded in recent years to the synthesis of encapsulated metallophthalocyanines [49] and organic ions and molecules [50][51][52][53]. This topic has also been recently reviewed [12,34].…”

“…It has also been demonstrated that a number of organic cations, having a submillisecond lifetime in solution, become indefinitely persistent when incorporated inside the rigid framework of a zeolite [50][51][52]140]. In one example, the encapsulated dibenzotropylium cation was observed to persist within the zeolite nanochannel for longer than 10 years.…”

Photophysical Properties of Methyl Triazone Included in MCM‐41¶

“…In comparison with homogeneous solution, the channels, cages, and cavities within these porous materials exhibit microheterogeneity in polarity, acidity and electric field and restricted mobility, and this has been frequently invoked to explain the novel photochemistry and photophysics observed. Many investigations have been made into the photophysical properties of incorporated guests such as neutral molecules (4,5), organic cations (6–9) and more recently inorganic photosemicon‐ductors such as titanium dioxide (TiO 2 ) (10,11). Depending on the zeolite type and the nature of the guest, several intriguing phenomena have been observed including delayed fluorescence, room temperature phosphorescence and novel decay kinetics i.e.…”