Sergio O. Vásquez

A shell model for cross relaxation in elpasolite crystals: application to the 3P0 and 1G4 states of Cs2NaY1−xPrxCl6

Abstract A model for cross relaxation in high symmetry systems is developed. The positions of all acceptor ions and all multipole-multipole interactions may be included. The geometric factors are explicitly considered. A feature of the model is that it enables decay rates and the form of decay curves to be predicted a priori if the exponential decay curves of very dilute and stoichiometric materials are experimentally accessible. The model is applied to the title system and excellent agreement with experiment is achieved.

Endohedral terthiophene in zigzag carbon nanotubes: Density functional calculations

The inclusion and encapsulation of terthiophene T3 molecules inside zigzag single-walled carbon nanotubes CNT’s is addressed by density functional calculations. We consider the T3 molecule inside six semiconducting CNT’s with diameters ranging approximately from 8 to 13 A. Our results show that the T3 inclusion process is exothermic for CNT’s with diameters larger than 9 A. The highest energy gain is found to be of 2 eV, decreasing as the CNT diameter increases. This notable effect of stabilization is attributed to the positively charged CNT inner space, as induced by its curvature, which is able to accommodate the neutral T3 molecule. The band structure of the T3@CNT system shows that T3 preserves its electronic identity inside the CNT’s, superimposing their molecular orbitals onto the empty CNT band structure without hybridization. Our results predict that the electronic states added by the T3 molecules would give rise to optical effects and nonradiative relaxation from excited states.

A theoretical study of conformational aspects and energy transfer between terthiophene and quinquethiophene in perhydrotriphenylene inclusion compounds

A theoretical study of models with supramolecular architecture of co-inclusion compounds based on the host perhydrotriphenylene and guests terthiophene and quinquethiophene (PHTP:T3,T5) is carried out to elucidate in detail the conformational aspects of the oligomeric guest species in the PHTP matrix host. The factors that direct the geometry, location and separation of terthiophene and quinquethiophene within the channels of the PHTP host have been studied using semi-empirical and ab initio calculations. The movement of the guests inside the channel is subject to constraints preventing free rotations or axial displacements along the nanochannel. Optimal arrangement and the general trend of the relative order between T3 and T5 in the (PHTP:T3,T5) co-inclusion compound is obtained. Furthermore, excited state calculations allow the explanation of the spectral shifts of the included species in terms of the planarization of their geometries. An analysis of the energy transfer processes between the T3-T5 donor-acceptor pair based on the configurational details of the co-inclusion compound conclude that efficient transfer proceeds only through two different and perpendicular windows for the T3 --> T5 transfer. The results emphasize the importance for better understanding of the directional details of the energy transfer mechanisms in this kind of one-dimensional systems.

Statistical approach to the transient up-converted population in monodoped amorphous solids

Based on microscopic considerations, an analytic statistical model is presented for the study of the temporal variation of up-conversion luminescence produced by the interaction of excited state Ln3+ optical centers in monodoped amorphous materials. The resulting expressions are presented in terms of known mathematical functions, and they account for the detailed dependence on the systems experimental variables. The model allows a correct prediction of the shape of the up-conversion transient curves, and also accounts not only for the dependence of the up-conversion intensity on the square of the optical center concentration and of the external excitation intensity at relatively low pump power, but also allows the prediction of “saturation” effects with increasing values of these factors, a situation comparable to what happens with steady state luminescence. Finally, interpretations of the reproduced observables from a microscopic viewpoint are given.

Vibronic intensities in centrosymmetric lanthanide complex ions. I: A combined crystal field-ligand polarisation approach

SummaryA theoretical model to calculate the vibronic intensities induced by the odd vibrational modes in centrosymmetric lanthanide complexes is developed and applied to octahedral complex ions, LnX63−, such as occur in the hexachloroelpasolites Cs2NaLnCl6. Both the crystal field and the ligand polarisation contributions are evaluated using a standard set of symmetry coordinates. For the crystal field term a truncated expansion of the intermediates states is employed rather than the more conventional closure approximation. Special care is necessary to ensure that the phases of the contributions are correctly determined since the cross-term between the ligand polarisation and crystal field contributions is signed. General equations applicable to anyfn complex ion are derived and an example of their application to the PrCl63− ion is given The agreement with experiment is satisfactory.

Vibronic intensities in the electronic spectra of transition metal complex ions. Part X. Second order Herzberg-Teller contributions to the vibronic intensity of the 4A2g ↔ 4T2g transition in MnF2- 6

A method to estimate the contribution to the vibronic intensity in electronic transitions of inorganic complex ions due to second order terms in the Herzberg-Teller expansion of vibronic wavefunctions is developed and applied to the 4A2g ↔ 4T2g transition of the MnF2- 6 ion. Both crystal field and ligand polarization contributions are derived, the crystal field term being evaluated in the single intermediate p state approximation. Contributions to the intensity of the nine second order terms are evaluated for several force fields and compared with recent experimental data. Although there are some difficulties in this comparison, it provides some support both for the model and the attribution of the intensity mechanism.

Multiscale Approach to the Study of the Electronic Properties of Two Thiophene Curcuminoid Molecules.

We studied the electronic and conductance properties of two thiophene-curcuminoid molecules, 2-thphCCM (1) and 3-thphCCM (2), in which the only structural difference is the position of the sulfur atoms in the thiophene terminal groups. We used electrochemical techniques as well as UV/Vis absorption studies to obtain the values of the HOMO-LUMO band gap energies, showing that molecule 1 has lower values than 2. Theoretical calculations show the same trend. Self-assembled monolayers (SAMs) of these molecules were studied by using electrochemistry, showing that the interaction with gold reduces drastically the HOMO-LUMO gap in both molecules to almost the same value. Single-molecule conductance measurements show that molecule 2 has two different conductance values, whereas molecule 1 exhibits only one. Based on theoretical calculations, we conclude that the lowest conductance value, similar in both molecules, corresponds to a van der Waals interaction between the thiophene ring and the electrodes. The one order of magnitude higher conductance value for molecule 2 corresponds to a coordinate (dative covalent) interaction between the sulfur atoms and the gold electrodes.

Vibronic intensities in the electronic spectra of transition metal complex ions. Part IX. Experimental study of the relative vibronic intensities in the 2Eg → 4A2g transition of the MnF2- 6 ion in Cs2SiF6

The relative intensities of about 90 of the more prominent vibronic bands in the 2Eg → 4A2g luminescence spectrum of the MnF2- 6 ion in Cs2SiF6 have been measured at 80K. The reported intensities cover a range of more than 106. Corrected values of the important intensity ratios of the v 3, v 4, v 6 vibronic origins are given as 1:4·02:5·45. Most of the features can be assigned as progressions in the v 1, v 2 even parity vibrational modes based on the magnetic dipole allowed electronic origin and the v 3, v 4, v 6 odd parity vibronic origins, up to four members of the progressions being observed. Prominent features also include combinations of the three odd vibrational modes. Anharmonic effects are small. The intensity mechanisms of these features are briefly discussed. It is likely that second order and third order Herzberg-Teller vibronic coupling makes a significant contribution to the intensity mechanism. These data will be used in subsequent theoretical models of the intensity mechanism.

Vibronic intensities in the electronic spectra of transition metal complex ions VIII: Vibrational coordinates for octahedral ions and their application to the4A2g ↔2Eg transition of the MnF 6 2− ion

SummaryA reexamination of the vibrational coordinates appropriate in vibronic intensity calculations in octahedral coordination compounds is presented. We derive a complete set of symmetry coordinates that is orthonormal and transforms correctly under the group generators. The vibronic hamiltonian for the crystal field and ligand polarization contributions to the intensity are calculated in the basis of these coordinates. The crystal field term is evaluated both using a truncated basis set for the intermediate electronic states and using the closure approximation. These methods have been applied to the calculation of the vibronic intensity distribution for the4A2g ↔2Eg transition of the MnF62− ion and close agreement wtih experiment achieved.

Adsorption thermodynamic functions and the mobility of SO2 on aerosil

Abstract A study of the adsorption and thermodynamic functions of sulfur dioxide on Aerosil has been made, leading to some conclusions about the mobility of the SO 2 molecule. The differential and integral enthalpies and entropies of adsorption, together with a statistical mechanical interpretation of the system, are discussed. Particularly, the statistical mechanical calculation of the integral entropy of adsorption allows a comparison to be made between the experimental results for the SO 2 —Aerosil system and those obtained for adsorption in two limiting cases: an immobile or localized model that can be associated with chemisorption, and a mobile film model that can be related to physisorption. The results confirm a weak gas—solid interaction with a highly mobile adsorbed phase.