RESUMO
By applying a molecular orbital perturbation approach, we calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. It is found that the interchain bond-charge correlation has a strong influence on the relative probabilities for generating singlet and triplet excitons. Most importantly, application of our approach to a model system for poly-(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.
RESUMO
A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.
RESUMO
Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.
RESUMO
Monolayers of isophthalic acid derivatives at the liquid/solid interface have been studied with scanning tunneling microscopy (STM). We have investigated the dynamics related to the phenomenon of solvent co-deposition, which was previously observed by our research group when using octan-1-ol or undecan-1-ol as solvents for 5-alkoxy-isophthalic acid derivatives. This solvent co-deposition has now been visualized in real-time (two frames per second) for the first time. Dynamics of individual molecules were investigated in mixtures of semi-fluorinated molecules with video-STM. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to analyze the data obtained for the mixtures under investigation. Upon imaging the same region of a monolayer for a period of time we observed that non-fluorinated molecules progressively substitute the fluorinated molecules. These findings illustrate the metastable equilibrium that exists at the liquid/solid interface, between the physisorbed molecules and the supernatant solution.