Ultraviolet−Visible and Fourier Transform Infrared Diffuse Reflectance Studies of Benzophenone and Fluorenone Adsorbed onto Microcrystalline Cellulose

Abstract

Benzophenone and fluorenone, which have a nonrigid and a rigid structure, respectively, were used as probes to study the nature of the adsorption process onto microcrystalline cellulose. Diffuse reflectance techniques were used in the UV−vis and infrared regions. Luminescence studies revealed that whenever fluorenone or benzophenone are entrapped into the natural polymer chains and in close contact with the substrate, a strong quenching effect exists for both probe's luminescence at room temperature. For fluorenone, the fluorescence quantum yields (ΦF) determined were about 0.10 when dichloromethane, cyclohexane, and benzene (solvents which do not swell cellulose) were used for sample preparation, while for dioxane, acetone, ethanol, and methanol (solvents which efficiently swell cellulose) ΦF was approximately 0.01. These values are about 1 order of magnitude higher than those obtained in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (π,π*) fluorenone first excited singlet state. Complementary, infrared studies showed that the carbonyl group of benzophenone is affected by entrapment (when the solvents used induce the swelling of cellulose), whereas in fluorenone the same band is insensitive to the adsorption process, not allowing the differentiation between entrapped molecules and surface crystallites of this ketone. These observations implied that benzophenone is entrapped between the chains of the polymer forming hydrogen bonds between the carbonyl and the hydroxyl groups of the glycosidic chains, while the rigidity of fluorenone apparently restrains the ketone−substrate interactions to the aromatic rings. Through the modifications observed in the carbonyl stretching band of benzophenone, it was possible to establish a swelling effect scale for the solvents, which is compared with previous results.

Surface Photochemistry: Organic Molecules within Nanocavities of Calixarenes

“…A smaller band assigned to the C=C/C=O coupled stretching mode can be observed at about 1626 cm -1 in accordance with previous studies [54,55]. Benzophenone exhibits an extended conjugated system, resonance interactions being very important for the planar conformations of benzophenone in the microcrystals case.…”

“…The more important they are, the weaker the C=O energy becomes as it assumes more single bond character. Inclusion of the two ketones into the calixarenes cavities may promote deviations from planarity as explained previously [54,55], therefore decreasing resonance and increasing the energy of the C=O stretching mode due to the C=O double bond character enhancement (when compared with ketone microcrystals). Indeed, the carbonyl stretching band wavenumbers obtained from FTIR absorption spectra [9] show that this is the case, which is consistent with inclusion in all cases.…”

Semiconducting Polyfluorenes—Towards Reliable Structure-Property Relationships

“…From a comparison with literature results, this emission band is assigned to stem from keto defect sites, namely fluorenone building blocks, which have been found to emit exactly at this energetic position. [40] The observed solvatochromism of the low-energy emission band at ca. 2.3 eV supports the assignment to fluorenone building blocks and reflects the considerable permanent dipole moment of the >C=O moiety (see Fig.…”

Fluorenone defects in fluorene-based conjugated polymers

“…Even from copolymers containing only 0.005 mol% of orange-emitting co-monomers the orange emission is suffi ciently strong to produce an overall white emission (Tu et al , 2006). Since the green emission band from PDAFs closely resembles the solid state emission reported from fl uorenone 26 (Ilharco et al , 1997), it thus seemed quite plausible that it could arise from defects 25 and the slightly more red-shifted emission seen in related polymers could thus arise from similar defects, e.g. 27 for PIFs.…”