PHOTOCHEMISTRY ON SURFACES: TRIPLET‐TRIPLET ENERGY TRANSFER ON MICROCRYSTALLINE CELLULOSE STUDIED BY DIFFUSE REFLECTANCE TRANSIENT ABSORPTION and EMISSION SPECTROSCOPY*

Abstract

Triplet‐triplet energy transfer has been studied between benzophenone and an oxazine dye (2,7‐bis(diethyl‐amino)‐phenazoxonium chloride) co‐adsorbed on the surface of microcrystalline cellulose. Ground state absorption and fluorescence measurements provide evidence for dimer formation of the oxazine dye when adsorbed on cellulose in contrast to the behaviour in ethanol solution where no dimerization is observed. The equilibrium constant for dimerization, which is found to be (1.0 × 0.1) × 106 mol−1 g (2560 × 250 dm3 mol−1) for oxazine alone on cellulose decreases in the presence of co‐adsorbed benzophenone. Fluorescence is detected from excited monomeric but not from excited dimeric oxazine. The absorption spectrum of the triplet state of oxazine adsorbed on cellulose was obtained and its extinction coefficient evaluated relative to that of triplet benzophenone which was used as a sensitizer. The lifetime of adsorbed triplet oxazine is 4.3 ms which is 300 times longer than that in acetonitrile solution. The efficiency of energy transfer from triplet benzophenone to oxazine on cellulose was studied using both time resolved sensitized absorption and phosphorescence intensity measurements as a function of oxazine concentration. Lifetime measurements show that the energy transfer process involves static quenching since the benzophenone lifetime is independent of oxazine loading at the surface. A mechanism is proposed to explain the results in which one oxazine molecule is suggested as being able to quench phosphorescence from a “pool” consisting of 2 to 3 benzophenone molecules.

Eosin Y Triplet State as a Probe of Spatial Heterogeneity in Microcrystalline Cellulose

“…Adsorbed species are immobilized in the rigid cellulose matrix by hydrogen bonding between the hydroxyl groups of the polymer chain and the guest molecule, reducing the molecular motion and increasing the adsorbate emission lifetime and quantum yield as reported in many cases. It was also reported that dyes adsorbed on cellulose are protected from quenching by molecular oxygen, as the mobility of this species is highly reduced in this medium, provided cellulose is well dried (20). In this way, interference from oxygen is easily avoided.…”

Photophysical Studies of a New Water Soluble Indocarbocyanine Dye Adsorbed onto Microcrystalline Cellulose and beta-Cyclodextrin

“…10 −3 mbar) at 60 °C for at least 24 h before sample preparation. The samples were prepared using the solvent evaporation method [30,31,32,33,34,35,36,40]. This method consists of the addition of a solution containing the indocarbocyanine in dried ethanol to the previously dried microcrystalline cellulose, followed by slow solvent evaporation from the slurry under stirring in a fume cupboard.…”

“…1 cm quartz cells were used for measurement of powdered samples. The remission function, F(R), of a probe adsorbed or included onto a solid powdered adsorbent and calculated with the use of the Kubelka-Munk equation for optically thick samples (F(R) = (1−R) 2 /2R ), after correction for the blank, is proportional to the host concentration [30,31,32,33,34,35,36,40]. Solution measurements were made using the same apparatus in the normal transmission mode.…”

“…One of our group’s interests is the photophysical processes of several probes [30,31,32,33,34,35,36,37,38,39,40,41,42], including cyanine dyes [30,31,32,40,43] adsorbed on various heterogeneous powdered supports, particularly onto microcrystalline cellulose. Probes included within cyclodextrin [40,41] and calixarene [42] cavities were also investigated.…”

“…When a solution of the probe in one of these swelling solvents is added to microcrystalline cellulose, the cellulose to cellulose hydrogen bonds are replaced by cellulose to solvent bonds and the matrix swells to a degree which depends on the solvent used for sample preparation [34]. Probes can then penetrate into sub-microscopic pores of the solid substrate and stay strongly entrapped within the cellulose polymeric chains, after solvent removal [30,31,32,33,34,35,36,43]. …”