Conformational characterization from modulated single molecule fluorescence intensity traces

Inhomogeneous Quenching as a Limit of the Correlation Between Fluorescence Polarization and Conformation of Single Molecules

The photophysical properties of conjugated polymers (CPs) largely depend on the interactions between the CP and its environment. We present a study of two polymers with identical conjugated backbones, bare and insulated, that showed different fluorescence excitation modulation depth histograms. However, the polarization differences are not related to differences in conformation, as commonly believed, but to the existence of "dark" chromophores in the bare polymer that are statically quenched. This results in inhomogeneous quenching of the polymer chain that breaks the correlation between excitation fluorescence polarization and conjugated polymer chain conformation.

Single-molecule spectroscopy for plastic electronics: materials analysis from the bottom-up

pi-conjugated polymers find a range of applications in electronic devices. These materials are generally highly disordered in terms of chain length and chain conformation, besides being influenced by a variety of chemical and physical defects. Although this characteristic can be of benefit in certain device applications, disorder severely complicates materials analysis. Accurate analytical techniques are, however, crucial to optimising synthetic procedures and assessing overall material purity. Fortunately, single-molecule spectroscopic techniques have emerged as an unlikely but uniquely powerful approach to unraveling intrinsic material properties from the bottom up. Building on the success of such techniques in the life sciences, single-molecule spectroscopy is finding increasing applicability in materials science, effectively enabling the dissection of the bulk down to the level of the individual molecular constituent. This article reviews recent progress in single molecule spectroscopy of conjugated polymers as used in organic electronics.

Solvent Mediated Intramolecular Photoinduced Electron Transfer in a Fluorene-Perylene Bisimide Derivative

Photoinduced electron transfer from fluorene to perylene bisimide has been studied for 2,7-bis(N-(1-hexylheptyl)-3,4:9,10-perylene-bisimide-N'-yl))-9,9-didodecylfluorene (PFP) in 11 different organic solvents. The intramolecular charge-separated state in PFP is almost isoenergetic with the locally excited state of the perylene bisimide. As a consequence of the small change in free energy for charge separation, the electron transfer rate strongly depends on subtle changes in the medium. The rate constant k(CS) for the electron transfer from fluorene to perylene bisimide moiety in the excited state varies over more than 2 orders of magnitude ( approximately 10(8)-10(10) s(-1)) with the solvent but does not show the familiar increase with polarity. The widely differing rate constants can be successfully explained by considering (1) the contribution of the polarization energy of the dipole moment in the transition state and by (2) the classical Marcus-Jortner model and assuming a spherical cavity for the charge-separated state. Using the first model, we show that lnk(CS) should vary linearly with Deltaf [Deltaf = (epsilon(r) - 1)/(2epsilon(r) + 1) - (n(2) - 1)/(2n(2) + 1), where epsilon(r) and n represent the static dielectric constant and the refractive index of the solvent, respectively], in accordance with experimental results. The second model, where the reorganization energy scales linearly with Deltaf, provides quantitative agreement with experimental rate constants within a factor of 2.

Flexibility of phenylene oligomers revealed by single molecule spectroscopy

The rigidity of a p-phenylene oligomer (p-terphenyl) has been investigated by single molecule confocal fluorescence microscopy. Two different rylene diimide dyes attached to the terminal positions of the oligomer allowed for wavelength selective excitation of the two chromophores. In combination with polarization modulation the spatial orientation of the transition dipoles of both end groups could be determined independently. We have analyzed 597 single molecules in two different polymer hosts, polymethylmethacrylate and Zeonex. On average we find a 22 degrees deviation from the linear gas phase geometry (T = 0 K), indicating a rather high flexibility of the p-phenylene oligomer independent of the matrix. To substantiate our experimental results, we have performed quantum chemical calculations at the density functional theory level for the molecular geometry and the electronic excitations. Our findings are in agreement with former experiments on the persistence length of poly(p-phenylenes).

The Chemistry of Organic Nanomaterials

The development of nanotechnology using organic materials is one of the most intellectually and commercially exciting stories of our times. Advances in synthetic chemistry and in methods for the investigation and manipulation of individual molecules and small ensembles of molecules have produced major advances in the field of organic nanomaterials. The new insights into the optical and electronic properties of molecules obtained by means of single-molecule spectroscopy and scanning probe microscopy have spurred chemists to conceive and make novel molecular and supramolecular designs. Methods have also been sought to exploit the properties of these materials in optoelectronic devices, and prototypes and models for new nanoscale devices have been demonstrated. This Review aims to show how the interaction between synthetic chemistry and spectroscopy has driven the field of organic nanomaterials forward towards the ultimate goal of new technology.

Synthesis and Energy-Transfer Properties of Hydrogen-Bonded Oligofluorenes

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.

Ladder-Type Pentaphenylenes and Their Polymers: Efficient Blue-Light Emitters and Electron-Accepting Materials via a Common Intermediate

A new route to ladder-type pentaphenylenes has been developed in which both good hole-accepting p-type and electron-accepting n-type materials can be prepared from a common intermediate. This key intermediate is a pentaphenylene diester 5 obtained in high yield by Suzuki coupling of 2 equiv of fluorene boronates with 2,5-dibromoterephthalate. Addition of aryllithium followed by ring closure with boron trifluoride produced a blue-emitting ladder-type pentaphenylene. Bromination followed by reductive polymerization with nickel(0) gave new high molecular mass polymers, which show efficient blue emission with a very small Stokes shift. These polymers bridge the gap in emission between polyfluorenes and fully ladder-type polyphenylenes. An alternative ring closure of the dibromopentaphenylene diester 14 with acid made a diketone that is a good electron-accepting material, as it displays a reversible two-electron reduction. The reduction onset potential of -0.875 V against Ag/Ag(+) corresponds to a lowest unoccupied molecular orbital (LUMO) energy level of 3.53 eV, comparable to the work function of magnesium, suggesting that this unit could be used to greatly increase the injection of electrons into polymers containing it in a light-emitting diode (LED) or solar cell. A red-emitting material was prepared by Suzuki coupling of the dibromopentaphenylene 10b with a perylene dye, thus offering the prospect of tuning the emission from pentaphenylene materials over the whole visible range by attachment of suitable dyes. Unoptimized single-layer organic LEDs that used 11b showed stable pure-blue emission with brightnesses of over 200 cd/m(2) at 7 V, with moderate efficiencies.

Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric molecules

We demonstrate measurements of the efficiency of competing Förster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energy-transfer pathways have to be considered in the analysis of single-molecule trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.