Structure absorption spectra correlation in a series of 2,6-dimethyl-4-arylpyrylium salts.

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 alpha- and beta-naphthyl substituted systems, the empirical method is not appropriate.

Intramolecular charge transfer processes in donor-acceptor substituted vinyltetrahydropyrenes.

Two novel donor-acceptor-substituted vinyltetrahydropyrene derivatives, 2-N,N-dimethylamino-7-(1-carbethoxyvinyl)-4,5,9,10-tetrahydropyrene, , and 2-N,N-dimethylamino-7-(1,1-dicyanovinyl)-4,5,9,10-tetrahydropyrene, , were synthesized and their photophysical properties investigated in solvents of different polarities. Our studies revealed the existence of intramolecular charge transfer excited states in these molecules. For both compounds the fluorescence maxima exhibited solvent polarity-dependent red shifts. These were quantitatively analysed by the Lippert-Mataga and Liptay equations to obtain the excited state dipole moments. Our results indicated that in the case of , emission takes place from a planar (1)CT state in all non-protic solvents. In the case of , the nature of the excited state depends on the solvent. A fast relaxation to a triplet state is proposed in cyclohexane. The emitting state in medium polar solvents is a planar (1)CT state. In highly polar solvents a twisted (1)CT state is invoked to explain the low fluorescence quantum yield. For both compounds CT nature of the emitting states were further confirmed by studies in acidic medium. The ground and excited state pK(a) values for and were determined using absorption and emission spectral changes observed in the presence of protic acids.

Metal-core--organic shell dendrimers as unimolecular micelles.

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm(2) for a first-generation NCD (Au-G1(CO(2)Me)) to 0.80/nm(2) for a fourth-generation NCD (Au-G4(CO(2)Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding water-soluble sodium salts. Aqueous solutions of (Au-Gn(CO(2)Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis.

Photoinduced electron transfer in hydrogen bonded donor--acceptor systems. Free energy and distance dependence studies and an analysis of the role of diffusion.

The free energy dependence of electron transfer in a few small-molecule donor--acceptor systems having hydrogen-bonding appendages was studied to evaluate the role of diffusion in masking the inverted region in bimolecular PET reactions. A small fraction of the probe molecules associate and this led to the simultaneous observation of unimolecular and diffusion-mediated quenching of the probe fluorescence. Free energy dependence studies showed that the unimolecular electron transfer obeys Marcus behavior and the diffusion-mediated electron transfer obeys Rehm--Weller behavior. The absence of an inverted region in bimolecular PET reactions is thus attributed to diffusion. The results of the free energy dependence studies suggest that distance dependence of electron transfer plays a role in masking the inverted region. To ascertain this aspect we have carried out a study of the distance dependence of electron transfer in the hydrogen-bonded donor--acceptor systems. For a system in the normal region an exponential rate decrease was observed. For a system in the inverted region it was observed that the rate depends very feebly on distance. Thus distance dependence studies did not confirm the prediction of enhanced rates at larger distances in the inverted region.

Phototransformations of C-Benzoylaziridines. Dipolarophilic Trapping of Photogenerated Azomethine Ylides(1).

The phototransformations of a few 2,3-diaroylaziridines and 2-aryl-3-aroylaziridines have been studied by steady-state photolysis and product analysis. The formation of various photoproducts could be substantiated by ring opening via C-C bond cleavage (leading to azomethine ylides), intramolecular hydrogen abstraction, and C-N bond cleavage. Isolation of stereospecific 3-pyrroline derivatives from the photoreaction of benzoylaziridines in the presence of DMAD confirms our previous results concerning the azomethine ylides as major transient intermediates, produced under laser pulse photoexcitation. Dimethyl 1-cyclohexyl-2-benzoylpyrrole-3,4-dicarboxylate (25), one of the photoadducts derived from the reaction of 1a and 1b with DMAD undergoes a novel and unusual photorearrangement to give dimethyl 2-(1-benzoylcyclohexyl)pyrrole-3,4-dicarboxylate (27), the structure of which was confirmed through X-ray crystallographic analysis.