A Diffuse Reflectance Comparative Study of Benzil Inclusion within p-tert-Butylcalix[n]arenes (n = 4, 6, and 8) and Silicalite

Abstract

Diffuse reflectance and laser-induced techniques were used to access photochemical and photophysical processes of benzil in solid supports, namely p-tert-butylcalix[n]arenes with n = 4, 6, and 8. A comparative study was performed using these results and those obtained with another electronically inert support, silicalite, which is a hydrophobic zeolite. In the latter substrate, ground-state benzil has the two carbonyl groups in an s-trans planar conformation while in the calixarenes a distribution of conformers exists, largely dominated by skew conformations where the carbonyl groups are twisted one to the other. In all substrates, room-temperature phosphorescence was obtained in air-equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for benzil/p-tert-butylcalix[6]arene, showing that this host cavity well accommodates benzil, enhancing its room-temperature phosphorescence. p-tert-Butylcalix[6] and [8]arene molecules provide larger hydrophobic cavities than silicalite, and inclusion complexes are formed with these hosts and benzil as guest; p-tert-butylcalix[4]arene does not include benzil. This probe is deposited outside the calix[4] cavity, in the form of microcrystals. Triplet−triplet absorption of benzil was detected in all cases and is predominant in the silicalite channel inclusion case. Benzil ketyl radical formation occurs with inclusion in calix[6]arene and calix[8]arene. In the three cases, benzoyl radical was detected at long times (in the millisecond time scale). Product analysis and identification clearly show that the main detected degradation photoproducts in all substrates are benzoyl radical derivatives. Calix[6] and [8]arenes are able to supply hydrogen atoms that allow also another reaction, the reduction to benzoin through benzil ketyl radical formation.

Luminescence Lifetime Distributions Analysis in Heterogeneous Systems by the Use of Excel's Solver

“…22,23 What we have here is the calixarene protection from the oxygen existing in the air because of the inclusion complex formation and at the same time some pyrene also remained outside the calixarene cavities forming "pools" of pyrene microcrystals. A similar behavior was reported by us for benzophenone included within calix[n]arenes (n ) 4, 6, and 8) 8 and microcrystalline cellulose. 27 A spectral decomposition is presented both for pyrene/calix- [4]arene and pyrene microcrystals room-temperature luminescence emission (microsecond time scale) in parts a and b of Figure 4, where Voigt profiles were used.…”

“…29,33 From other surface photochemistry studies performed using benzophenone as a probe we know now that ground state absorption spectra (obtained with the use of an integrating sphere, measuring reflectances as a function of wavelength) are highly indicative of the surface environmental polarity. 8,27,32 In this report, we are showing, we believe for the first time in the literature, that a carefully study of the decay curve of benzophenone in a large time scale obtained with the use of a gated detector, followed by a lifetime distribution analysis with this new methodology can provide very accurate information relative to the existence of different adsorption sites in a specific solid powdered host. …”

“…18,19 The ability of calixarenes and calixarene derivatives to form inclusion complexes, accommodating guest molecules in their intramolecular cavities, greatly depends on the size and geometry of guest molecule but also on the host cavity. 7,8,11,18,19 Calixarenes can be extremely useful hosts for use in environmental chemistry studies due to their ability to selectively bind, separate, and sense neutral organic molecules (and ions) of contaminants, depending not only on the different sizes of its nanocavities but also on different substituents in the upper or lower rims. 19 The time-resolved fluorescence spectrum of the 1:50 mol: mol pyrene/calix [4]arene sample in the nanosecond time range (data not shown) is very simple, exhibiting monomer emission peaking at 396 nm and also excimer emission peaking at about 470 nm, as reported for pyrene on other solid powdered surfaces.…”

“…Good examples of this are photochemical studies of polycyclic aromatic hydrocarbons, ketones, and dyes, among others, on solid powdered substrates, such as hydrophobic or hydrophilic zeolites, 4 silicas with different porosities, 1a aluminas, and silica-aluminas. 5 Supramolecular studies of probes within cavities of cyclodextrins 6 or calixarenes 7,8 should also be mentioned, among other supramolecular studies.…”

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

“…3c when compared to Fig. 3b, where 266 nm radiation was used to excite BZP adsorbed on RP-8 (similar results were obtained for RP-18, but those data are not shown) is that the benzoyl radical transient absorption 11 is now well visible in the shorter wavelength range and at longer timescales. The identification could be made, again by comparison with previously published work, 11 and this means that the BZP photochemistry is wavelength dependent, i.e.…”