The quantum yields of singlet oxygen production and
lifetimes at the gas–solid interface in silica gel material
are determined. Different photosensitizers (PS) are encapsulated
in parallelepipedic xerogel monoliths (PS-SG). PS were chosen according
to their known photooxidation properties: 9,10-dicyanoanthracene
(DCA), 9,10-anthraquinone (ANT), and a benzophenone
derivative, 4-benzoyl benzoic acid (4BB). These experiments
are mainly based on time-resolved 1O2 phosphorescence detection,
and the obtained FD and tD values are compared with
those of a reference sensitizer for production, 1H-phenalen-1-
one (PN), included in the same xerogel. The trend between their
ability to oxidize organic pollutants in the gas phase and their efficiency
for production is investigated through photooxidation
experiments of a test pollutant dimethylsulfide (DMS). The
FD value is high for DCA-SG relative to the PN reference, whereas
it is slightly lower for 4BB-SG and for ANT-SG. FD is related to
the production of sulfoxide and sulfone as the main oxidation
products for DMS photosensitized oxidation. Additional mechanisms,
leading to C!S bond cleaveage, appear to mainly occur
for the less efficient singlet oxygen sensitizers 4BB-SG and ANTSG.
Vacuum-ultraviolet (VUV) irradiation (kexc: 172 ± 12 nm) of polystyrene films in the presence of oxygen
produced not only oxidatively functionalized surfaces, but generated also morphological changes.
Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization,
enhanced aggregation or printing, processes leading to morphological changes open new possibilities of
microstructurization. Series of experiments made under different experimental conditions brought
evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene
pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen,
ozone) produced in the gas phase between the VUV-radiation source and the substrate. However,
oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight
and eventually to mineralization of the organic substrate is initiated by electronic excitation of the
polymer leading to C–C-bond homolysis and to a complex oxidation manifold after trapping of the
C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation
could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after
washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily
functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the
polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the
fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of
the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred
radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects,
optical microscopy, friction-mode AFM) and disproportionation (fluorescence).
Manoj, N(American chemical society, August 7, 2006)
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Abstract:
A combined experimental and theoretical study of the absorption spectra of a group of closely related pyrylium
perchlorates 1-11 are presented. Minor changes in the position of the substituents lead to drastic changes in
the absorption spectra in this series of compounds. We have attempted to explain the observed changes using
the x,y-band notation developed by Balaban and co-workers. Absorption spectra of all compounds are compared
with results from time-dependent density functional theory (TDDFT) and Zerner’s intermediate neglect of
differential overlap (ZINDO/S) level calculations. Results of the calculations are in good agreement with
experimental observations and an interesting correlation between Balaban’s notations and the MO transitions
are obtained for simple derivatives. It is suggested that for more complex systems such as R- and â-naphthyl
substituted systems, the empirical method is not appropriate.