Surface photochemistry: benzophenone as a probe for the study of silica and reversed-phase silica surfaces

Abstract

This work reports the use of benzophenone, a very well characterized probe, to study new hosts: two reversed-phase silicas. Laser-induced room temperature luminescence of argon purged solid powdered samples of benzophenone adsorbed onto the two different reversed-phase silicas, RP-18 and RP-8, revealed the existence of a low energy emission band in contrast with the benzophenone adsorbed on 60 Å pore silica, where only triplet benzophenone emits. This low energy emission band was identified as the fluorescence of the ketyl radical of benzophenone, which is formed as the result of a hydrogen atom abstraction reaction of the triplet excited benzophenone from the alkyl groups of the surface of the reversed silicas. Such emission does not exist for benzophenone adsorbed onto 60 Å pore silica. Room temperature phosphorescence was obtained in argon purged samples for all the surfaces under use. The decay times of the benzophenone emission vary greatly with the alkylation of the silica surface when compared with "normal" silica surface. A lifetime distributions analysis has shown that the shortest lifetimes for the benzophenone emission exist in the former case. Triplet-triplet absorption of benzophenone was detected in all cases and is the predominant absorption in the case of 60 Å pore silica, while benzophenone ketyl radical formation occurs in the case of the reversed silicas. Diffuse reflectance laser flash photolysis and gas chromatography-mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after laser irradiation, both at 266 nm or 355 nm. Product analysis and identification show that the degradation photoproducts are dependent on the excitation wavelength, the photochemistry being much more rich and complex in the 266 nm excitation case, where an a-cleavage reaction occurs. A detailed mechanistic analysis is proposed.

Achieving Thermally Activated Delayed Fluorescence from Benzophenone by Host–Guest Complexation

“…34,35 Note that it was reported that Bzp in triplet states could subtract H-atom from the reverse phased silica and form diphenyl ketyl radical with emission maxima around 575 nm. 36 However, the lack of observation of emission centered around 575 nm in Bzp:MSN indicates that this radical luminescence is negligible in our experiments.…”

“…The τ DF component occurring via the rISC process increases exponentially with temperature, which is the opposite trend to the typical TADF process. This kind of opposite trend of temperature on the PL emission was rarely observed previously in TADF emitters; if the RTP minor contribution also impacts the PL dynamics along with the TADF, this behavior is evident. − The environmentally induced phosphorescence properties of Bzp have been used effectively to understand solid mesoporous silica surfaces. , Note that it was reported that Bzp in triplet states could subtract H-atom from the reverse phased silica and form diphenyl ketyl radical with emission maxima around 575 nm . However, the lack of observation of emission centered around 575 nm in Bzp:MSN indicates that this radical luminescence is negligible in our experiments.…”

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

“…Some molecules can be protonated when they are encapsulated in SBMs. − The guest forms one or more cationic species depending on the available proton-acceptor groups and the acidity of the encapsulating material. − In these systems, intermolecular H-bonds may stabilize the encapsulated guest and increase its fluorescence lifetime.…”

“…Other molecules, such as ketones and benzophenone, have been used as probes to characterize new solid supports. − ,− Benzophenone (BZP, Scheme ) is widely used to investigate the acidic character of the environment. , As in the previous composites, BZP that is encapsulated in materials that contain Brønsted or Lewis acid sites [ZSM-5 and aluminophosphate (AlPO 4 ) microporous solid] exhibits protonation processes. , In BZP/AlPO 4 complexes, the luminescence spectra show two bands (440 and 470 nm) that are assigned to the H-bonded and protonated BZP species, respectively. The behavior was compared with that of BZP encapsulated in ZSM-5, a host that is less hydrophilic and polar than AlPO 4 . , When Na + cations are introduced into the ZSM-5 framework, Lewis acid sites are favored.…”

Hydroxylation of benzophenone with ammonium phosphomolybdate in the solid state via UV photoactivation

“…Otherwise if the photoexcited benzophenone ketyl radical receives a hydrogen radical, it gives rise to the product, diphenylcarbinol. 10 Initially the photoproducts from benzophenone were examined by UV visible spectroscopy. Benzophenone shows an absorption peak at 253 nm in ethanol before the reaction, and after the reaction a new peak appeared at l max 4 300 nm.…”