Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part I: A homologous series of 2,5-diheptyloxy substituted PV-oligomers

Pyrene fluorescence in 2,7-di(4-phenylethynyl)pyrene-bridged bis(alkenylruthenium) complexes

Complexes PyrDPE-RuCl and PyrDPE-Ruacac with a π-extended 2,7-di(4-phenylethynyl)pyrene linker undergo simultaneous one-electron oxidations of their {Ru}-styryl entities. The absence of an intervalence charge-transfer (IVCT) band at intermediate stages, where the mixed-valent, singly oxidized radical cation is present, and spin density confinement to the terminal styryl ruthenium site(s) are tokens of a lack of electronic coupling between the {Ru} entities across the π-conjugated linker. The close similarity of the linker-based π → π* bands in the complexes and the free ligand and their insensitivity towards oxidations at the terminal sites indicate that the central pyrenyl fluorophore is electronically decoupled from the electron-rich {Ru}-styryl termini. As a consequence, the complexes offer stable pyrene-based fluorescence emissions at 77 K, which are red-shifted from that of the linker.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Fluidic-directed assembly of aligned oligopeptides with π-conjugated cores

A microfluidic-based directed assembly strategy is employed to form highly aligned supramolecular structures. Formation of aligned synthetic oligopeptide nanostructures is accomplished using planar extensional flow, which induces alignment of underlying material suprastructures. Fluidic-directed assembly of supramolecular structures allows for unprecedented manipulation at the nano- and mesoscales, which has the potential to provide rapid and efficient control of functional material properties.

Electronic Excitation of Polyfluorenes: A Theoretical Study

We present systematic, theoretical investigations on structure-property correlations in polyfluorenes (PFs) derived mainly from the chain morphology, oligomer length, and chemical substitutent. Both the vertical absorptions and the vibrational contributions to electronic absorption and fluorescence spectra have been calculated. The effect of temperature on the nature of photoexcitations of PFs has been demonstrated. It is found that the vibronic (electronic and vibrational) structures of PFs are morphology-dependent. beta-phase oligofluorenes (beta-(FL)(n)) and ladder-type poly(p-phenylene) (LPPP) oligomers show a red shift compared to the spectra of alpha-(FL)(n). The asymmetry of the absorption and fluorescence spectra in alpha-(FL)(n) and the fluorenone (FLO) defect oligofluorenes alpha-(FL)(n)(-)(m)(FLO)(m) is significantly more pronounced than that in planarized beta-(FL)(n) and LPPP oligomers. By properly taking into account the anharmonic torsion potentials resulting from the strong electronic and nuclear coupling in the oligofluorenes, we have reasonably reproduced the experimentally observed spectroscopic features. The low-energy on-chain chemical defect sites such as FLO units act as charge-trapping sites for singlet excitations, are the predominantly lighting-emitting species, and thus alter the blue light-emitting properties of PFs whereas the blue-light-emitting properties of PFs are hardly influenced by the hole-transporting molecules. The optical properties of PFs have been predicted by the finite-size calculations. Energy gaps of PFs are estimated by extrapolations from excitation energies of oligofluorenes up to 21 FL units.

Energy-Transfer Efficiency in Stacked Oligo(p-phenylene vinylene)s: Pronounced Effects of Order

The supramolecular structure of two types of oligo(p-phenylene vinylene) (OPV) building blocks in dodecane solution is studied. Monofunctional chromophores (MOPV) form well-defined helical assemblies, whereas bifunctional molecules (BOPV) aggregate into so-called frustrated stacks, lacking any higher helical order. This difference in organization has a major influence on the transfer of excitation energy through the assemblies. Energy transfer to supramolecularly incorporated guests (MOPV with lower bandgap) is used to probe the intrinsic differences in exciton mobility in these two types of mixed aggregates. From the observed donor fluorescence quenching, it can be concluded that the helically ordered nature of the MOPV stacks facilitates the transfer of excitation energy, yielding evidence for higher exciton mobility in the well-ordered assemblies than in the frustrated stacks. Finally, the concept of energy transfer in supramolecular assemblies is extended to the solid state by the successful implementation in a light-emitting diode (LED).

Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.