Electronic coupling responsible for energy transfer in columnar liquid crystals

Electronic Delocalization in Discotic Liquid Crystals: A Joint Experimental and Theoretical Study

Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2',3'-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper pi-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the pi-electrons along the discotic stacks.

Long-range resonance energy transfer in molecular systems

The current state of understanding of molecular resonance energy transfer (RET) and recent developments in the field are reviewed. The development of more general theoretical approaches has uncovered some new principles underlying RET processes. This review brings many of these important new concepts together into a generalization of Förster's original theory. The conclusions of studies investigating the various approximations in Förster theory are summarized. Areas of present and future activity are discussed. The review covers Förster theory for donor-acceptor pairs and electronic coupling for singlet-singlet, triplet-triplet, and superexchange-mediated energy transfer. This includes the transition density picture of Coulombic coupling as well as electronic coupling between molecular aggregates (excitons). Spectral overlaps and ensemble energy transfer rates in disordered aggregates, the role of dielectric properties of the medium, weak versus strong coupling, and new models for energy transfer in complex molecular assemblies are also described.

Comparison between J-aggregates in a self-assembled multilayer and polymer-bound J-aggregates in solution: a steady-state and time-resolved spectroscopic study

The self-assembly of a thiacarbocyanine dye on a charged substrate via the layer-by-layer alternate adsorption technique was studied in detail. The influence of the chemical structure of the oppositely charged polyelectrolyte (PEL) in the multilayer on J-aggregate formation was investigated by means of absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. J-Aggregates were formed yielding a bathochromic narrow absorption band. Fine tuning of the position of the absorption band is possible by incorporating other polyelectrolytes in the multilayer. The results obtained for the dye-polyelectrolyte multilayers were compared with measurements made on the aqueous dye-polyelectrolyte mixtures. The parameters obtained by fitting the fluorescence decays to several different models allowed us to suggest a mechanism for the non-radiative decay channel. The low fluorescence quantum yields and fast non-exponential fluorescence decays obtained for both the dye-polyelectrolyte multilayers and the aqueous dye polyelectrolyte mixtures were attributed to the trapping of a fast migrating exciton by a non-luminescent trap. The results related to J-aggregate formation, decay time and dimensionality of the aggregates were compared in solution and in a self-assembled multilayer.

Crystal and electronic structure of a fluorescent columnar liquid crystalline electron transport material

A very favorable LUMO energy for cathodic electron injection is found in the crystalline state of the mesogenic red fluorescent perylene derivative 1. It is one of the rare cases where the columnar crystal structure of such a compound could be established directly by X-ray investigation.