The thermolytic molecular precursor method was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA15 support. The resulting surface Si-OH/Ti-OH sites of the Ti-SBA15 catalysts were modified with a series of (N,N-dimethylamino)trialkylsilanes, Me(2)N-SiMe(2)(R) (where R = Me, (n)Bu, or (n)Oc). Compared with the unmodified catalysts, the surface-modified catalysts are more active in the oxidation of cyclohexene with H(2)O(2) and exhibit a significantly higher selectivity (up to 58%) for cyclohexene oxide formation (vs allylic oxidation products). In situ Fourier transform infrared (FTIR) and diffuse reflectance UV visible (DRUV-vis) spectroscopies were used to probe this phenomenon, and it was determined that active sites with capped titanol centers, (SiO(surface))(3)Ti(OSiR(3)), likely undergo Ti-OOH formation upon addition of H(2)O(2) in a manner analogous to that for active sites of the type (SiO(surface))(3)TiOH. On the basis of the observation of similar Ti-OOH intermediates for both species, the electron-withdrawing effects on the Ti(IV) active site, resulting from the surface modification, are likely responsible for the observed increase in selectivity.

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Singlet oxygen has been detected in single nerve cells by its weak 1270 nm phosphorescence (a1deltag --> X3sigmag-) upon irradiation of a photosensitizer incorporated in the cell. Thus, one can now consider the application of direct optical imaging techniques to mechanistic studies of singlet oxygen at the single-cell level.

The lowest excited electronic state of molecular oxygen, singlet molecular oxygen (a1Deltag), is an intermediate in many chemical and biological processes. Tools and methods have been developed to create singlet-oxygen-based optical images of heterogeneous samples that range from phase-separated polymers to biological cells. Such images provide unique insight into a variety of oxygen-dependent phenomena, including the photoinitiated death of cells.

Optical images of glassy, phase-separated polymer films have been generated using the 1270 nm phosphorescence of singlet molecular oxygen. Specifically, upon irradiation of immiscible blends of polystyrene and poly(ethylene-co-norbornene) that contain a singlet oxygen sensitizer, phase-separated droplets as small as ∼10 μm in diameter could be resolved using a microscope designed to detect singlet oxygen phosphorescence. This study demonstrates that it is possible to create singlet oxygen images of systems in which the sensitizer is not mobile (i.e., systems in which the effects of sensitizer bleaching cannot be rectified by diffusion of more dye into a given volume). The effect of singlet oxygen diffusion across the interfacial boundary between phase-separated domains of both liquid and polymer samples has also been examined. For singlet oxygen created in one phase, diffusion into a second phase in which the quantum yield of singlet oxygen phosphorescence is larger and oxygen is more soluble gives rise to a significant change in the intensity of the singlet oxygen signal at the interface. This effect can be pronounced when a long singlet oxygen lifetime facilitates singlet oxygen diffusion over large distances, but can be mitigated when interfacial tension yields a phase boundary with appreciable curvature. Boundary curvature in thin, phase-separated films of polystyrene/poly(ethylene-co-norbornene) is slight. Moreover, the singlet oxygen lifetime in these polymers is sufficiently short that, within its lifetime, singlet oxygen cannot diffuse over an appreciable distance. Under such conditions, singlet oxygen images of interfacial boundaries are sufficiently sharp as to make this optical technique useful for a range of fundamental studies.

Upon irradiation of a photosensitizer, singlet molecular oxygen has been detected via its aΔg → X3Σg - phosphorescence with a microscope. Using this tool, a singlet oxygen image with 2.5 μm resolution has been constructed of a phase-separated toluene/water mixture. For the a → X transition at ∼ 7850 cm-1 (∼ 1270 nm), a lateral resolution of 2.5 μm is close to the diffraction limit of ∼1.5−2.0 μm.

The time-resolved absorption spectrum of singlet molecular oxygen (a 1 ∆ g f b 1 Σ g + ) has been recorded in liquid D 2 O using a step-scan Fourier transform spectrometer. A molar absorption coefficient of 6 ( 2 M -1 cm -1 was determined for the peak of the absorption band at 5228 ( 6 cm -1 . In this report, issues pertinent to the detection of this signal are outlined. Specifically, for O 2 (a 1 ∆ g ) produced upon pulsed laser irradiation of a sensitizer, it is necessary to account for a laser-induced, temperature-dependent shift in a D 2 O absorption band that interferes with the weak a f b signal. Along with a f b absorption coefficients obtained previously in other solvents, the D 2 O data reported herein enable an appropriate test of a topical model for the perturbing effect of solvent on radiative transitions in oxygen.