G. Di Silvestro

Energy transfer in nanostructured oligothiophene inclusion compounds

Resonant energy transfers from terthiophene (T3) to quinquethiophene (T5) oligomers embedded in the nanochannels of perhydrotriphenylene (PHTP) crystals are studied by cw and femtosecond spectroscopy. The stringent geometry imposed by the host results in a very peculiar supramolecular organization of the guest molecules consisting of parallel linear arrays of chromophores. This provides a unique opportunity to make a quantitative analysis of the energy transfer phenomena and in particular to distinguish between heterotransfers and homotransfers. The experimental data fitted by a suitable theoretical analysis indicate that homotransfers become important in the long time dynamics and that the heterotransfer rate is satisfactorily described by the analytical solution obtained by assuming a continuous intermolecular spacing.

Excitonic effects in thiophene oligomers

Abstract Excitonic interactions in conjugated chains are studied by analysing the optical and vibrational properties of unsubstituted thiophene oligomers with different supramolecular organization. Single quasiplanar molecules, electronically decoupled in the solid state, have been obtained by inclusion of the thiophene oligomers in perhydrotriphenylene. Resonant Raman scattering shows that the strong spectral changes which occur upon molecular aggregation can be explained only by electronic excited state interactions, in rather good agreement with the prediction of the molecular exciton model.

Optical properties and photoexcitations of an electronically decoupled thiophene oligomer in the solid state

Abstract The suppression of the collective excitations in a novel form of quinquithienyl prevents charge photoproduction and evidences the spectroscopical properties of the isolated molecules in their planar form. The strong reduction of the photoluminescence quantum yield and the non-exponential decay of the photoluminescence in the aggregated molecules are accounted for in terms of exciton dynamics.

Synthesis of (+)-(3S,7S,10S)- and (+)-(3S,7S,10R)-3,7,10-trimethylboratrane. Amplification of enantiomeric purity in the reacti

Abstract The synthesis is described of the two enantiomerically pure isomers (+)-(3 S ,7 S ,10 S )- and (+)-(3 S ,7 S ,10 R )-3,7,10-trimethylboratr The structures were determined by 1 H- and 13 C-NMR spectroscopy. A method for increasing the enantiomeric purity by trimerization reactions of partially-resolved ( S ) propylene oxide is proposed. The reaction was studied from the kinetic viewpoint and interpreted according to a binomial probability scheme. The experimental findings point to a rapid growth in enantiomeric purity for the ( SSS ) trimer compared with the starting material, whilst no increase was found for the ( SSR ) trimer.

Derivatization and structural studies by spectroscopic methods of humic acids from Leonardite

Abstract Humic acids (HA) from Leonardite were methylated with CH 3 I/Ag 2 O in a methanol suspension (yield 261±23.6 g kg −1 ; N =4) and dimethylformamide (DMF) (yield 839±57.6 g kg −1 ; N =4) solution. Methylated humic acids are soluble in organic solvents and have been examined by IR-FT, 1 H and 13 C-NMR spectroscopy methods. The OCH 3 /COOCH 3 ratio was calculated by 13 C-NMR for HA methylated in methanol suspension and DMF solution obtaining values of 0.38 and 0.82, respectively. Methylated acids molecular weight distribution was measured by Size-Exclusion Chromatography (SEC). The product obtained in methanol suspension has a molecular weight ( M n ) of 2918 Da and a dispersion index ( D = M w / M n ) of 1.13. The methylated product prepared in DMF solution is characterized both by higher molecular weight, ( M n =3623 Da) and dispersion index ( D =2.10).

Stability and Reactivity of Inclusion Compounds

Abstract The general aspects of polymerization of diene monomers included in the crystal lattice of suitable hosts are discussed. Particular attention has been devoted to the behavior of adducts formed by perhydrotriphenylene. Study of the phase diagrams revealed a strict correlation between stability and reactivity in the solid state. Some recent results, concerning inclusion copolymerization and production of polymers with unusual steric structure (e.g., hemitactic polypropylene), are reported.

Microstructure of poly(methyl sorbate)

Abstract Anionic polymerization of methyl sorbate produces polymers of different microstructures according to the catalytic system employed. An examination of the i.r., 1H- and 13C-NMR spectra, for poly(methyl sorbate) and for the saturated polymer obtained by non-catalytic reduction of the double bond, together with comparison with low-molecular-weight model compounds, makes it possible to assign an erythro-structure to the polymer obtained in the presence of butyl lithium and a threo-structure to the polymer obtained in the presence of the catalytic system(butyl lithium + tetramethylurea). The steric purity of such polymers is rather low.

Configurational equilibria and 13C NMR spectra of tetracyclic saturated hydrocarbons

Abstract The stereoisomeric equilibrium composition over Pd/C at high temperature of a series of tetracyclic saturated hydrocarbons having general formula C18H30 was determined by gas chromatography and compared with that calculated on the basis of conformational analysis. Several stereoisomers were isolated. Their 13C NMR spectra agree very well with those calculated according to standard procedures. This method of structural determination was applied to perhydronaphthacene, perhydrobenzo(a)anthracene, 2- and 5-methylper-hydrobenzo (d,e)anthracene and 2,7-dimethylperhydropyrene.

Energy transfer in oligothiophene inclusion compounds

Energy transfer between terthiophene and quinquethiophene oligomers embedded in the nanochannels of perhydrotriphenylene crystals is investigated by fs-time-resolved photoluminescence spectroscopy. Excitonic effects in these compounds are suppressed due to the large intermolecular distances imposed by the host crystal. Very efficient transfers of the electronic excitations from donor (terthiophene) to acceptor (quinquethiophene) molecules are observed. A quantitative analysis of the experimental data show that energy migration mainly occurs via direct donor-acceptor single-step transfer.

PHASE DIAGRAMS OF PERHYDROTRIPHENYLENE AND AROMATIC COMPOUNDS II. Terthiophene and related thiophene oligomers

Terthiophene included in a perhydrotriphenylene (PHTP) adduct exhibits an UV spectrum different from that observed in the bulk or in solution. The X-ray diffraction pattern also differs significantly from that of pure PHTP. The UV spectrum is interpreted as due to an isolated molecule in a vacuum-like space, in agreement with a model developed from X-ray diffraction data. Knowledge of the phase diagram was essential for an interpretation of the optical properties and of some kinetic aspects of the formation of the inclusion compound phase. In this work, the phase diagrams of terthiophene (T3) are presented, and DSC evidence of the inclusion of quater- (T4), penta- (T5)and α,ω-dihexylquaterthiophene (DH4T) is discussed.