Temporal fluctuations in excimer-like interactions between pi-conjugated chromophores

Inter- or intramolecular coupling processes between chromophores such as excimer formation or H- and J-aggregation are crucial to describing the photophysics of closely packed films of conjugated polymers. Such coupling is highly distance dependent, and should be sensitive to both fluctuations in the spacing between chromophores as well as the actual position on the chromophore where the exciton localizes. Single-molecule spectroscopy reveals these intrinsic fluctuations in well-defined bi-chromophoric model systems of cofacial oligomers. Signatures of interchromophoric interactions in the excited state - spectral red-shifting and broadening, and a slowing of photoluminescence decay - correlate with each other but scatter strongly between single molecules, implying an extraordinary distribution in coupling strengths. Furthermore, these excimer-like spectral fingerprints vary with time, revealing intrinsic dynamics in the coupling strength within one single dimer molecule, which constitutes the starting point for describing a molecular solid. Such spectral sensitivity to sub-Angstrom molecular dynamics could prove complementary to conventional FRET-based molecular rulers

A Giant Molecular Spoked Wheel

The modular synthesis of a defined, rigid molecular spoked wheel structure with the sum formula C₁₈₇₈H₂₆₈₂ and a diameter of about 12 nm is described. The attached 96 dodecyl side chains provide the solubility of the 25 260 Da compound in common organic solvents. At the octanoic acid/highly oriented pyrolytic graphite interface, the molecules self-assemble to form an ordered 2D lattice, which is investigated by scanning tunneling microscopy, displaying their structure with submolecular resolution

Monodisperse Oligo(<i>p</i>‑phenylene–butadiynylene)s: GPC Conversion Factors and Self-Assembled Monolayers

Alkyl- and alkoxy-substituted <i>p</i>-phenylene–butadiynylene tetramers, obtained via a stepwise synthesis, are polymerized under Glaser coupling conditions. The fractionation of the oligomers by recycling gel permeation chromatography (recGPC) yields three sets of monodisperse oligo­(<i>p</i>-phenylene–butadiynylene)­s (OPBs) with degrees of polymerization <i>n</i> = 4, 8, 12, ..., 60. Subsequent GPC analysis of these oligomers provides three sets of size- and substituent-dependent conversion factors that quantify the overestimation of the oligomer molecular weight when a polystyrene (PS) calibrated GPC system is used for the data analysis. Scanning tunneling microscopy (STM) of representative oligomers at the solid/liquid interface visualizes the monodispersity, the length-dependent rigidity, and their assembling behavior

Time-Resolved Electron Paramagnetic Resonance and Theoretical Investigations of Metal-Free Room-Temperature Triplet Emitters

Utilization of triplets is important for preparing organic light-emitting diodes with high efficiency. Very recently, both electrophosphorescence and electrofluorescence could be observed at room temperature for thienyl-substituted phenazines without any heavy metals (Ratzke et al. J. Phys. Chem. Lett., 2016, 7, 4802). It was found that the phosphorescence efficiency depends on the orientation of fused thiophenes. In this work, the thienyl-substituted phenazines are investigated in more detail by time-resolved electron paramagnetic resonance (EPR) and quantum chemical calculations. Spin dynamics, zero-field splitting constants, and electron-spin structures of the excited triplet states for the metal-free room-temperature triplet emitters are correlated with phosphorescence efficiency. Complete active space self-consistent field (CASSCF) calculations clearly show that the electron spin density distributions of the first excited triplet states are strongly affected by the molecular geometry. For the phosphorescent molecules, the electron spins are localized on the phenazine unit, in which the sulfur atom of the fused thiophene points upward. The electron spins are delocalized onto the thiophene unit just by changing the orientation of the fused thiophenes from upward to downward, resulting in the suppression of phosphorescence. Time-resolved EPR measurements and time-dependent density functional theory (TD-DFT) calculations demonstrate that the electron spins delocalized onto the thiophene unit lead to the acceleration of nonradiative decays, in conjunction with the narrowing of the singlet–triplet energy gap

A Liquid-Crystalline Phenylene-Based Shape-Persistent Molecular Spoked Wheel

Molecular spoked wheels with an all-phenylene backbone and different alkoxy side chain substitution patterns were synthesized using a cobalt-catalyzed [2 + 2 + 2] cycloaddition and subsequent template-directed cyclization via Yamamoto coupling. The two-dimensional organization of the molecules at the solid/liquid interface was investigated by means of scanning tunneling microscopy, allowing imaging of the molecular structure with submolecular resolution. With the right proportion of the flexible alkyl corona to the rigid core, mesomorphic behavior of one compound could be observed over a wide temperature range

Temporal Switching of Homo-FRET Pathways in Single-Chromophore Dimer Models of π‑Conjugated Polymers

A set of π-conjugated oligomer dimers templated in molecular scaffolds is presented as a model system for studying the interactions between chromophores in conjugated polymers (CPs). Single-molecule spectroscopy was used to reveal energy transfer dynamics between two oligomers in either a parallel or oblique-angle geometry. In particular, the conformation of single molecules embedded in a host matrix was investigated via polarized excitation and emission fluorescence microscopy in combination with fluorescence correlation spectroscopy. While the intramolecular interchromophore conformation was found to have no impact on the fluorescence quantum yield, lifetime, or photon statistics (antibunching), the long-term nonequilibrium dynamics of energy transfer within these bichromophoric systems was accessible by studying the linear dichroism in emission at the single-molecule level, which revealed reversible switching of the emission between the two oligomers. In bulk polymer films, interchromophore coupling promotes the migration of excitation energy to quenching sites. Realizing the presence and dynamics of such interactions is crucial for understanding limitations on the quantum efficiency of larger CP materials

Synthesis and Arm Dissociation in Molecular Stars with a Spoked Wheel Core and Bottlebrush Arms

Unique star-like polymeric architectures composed of bottlebrush arms and a molecular spoked wheel (MSW) core were prepared by atom transfer radical polymerization (ATRP). A hexahydroxy-functionalized MSW (<b>MSW</b>_{<b>6‑OH</b>}) was synthesized and converted into a six-fold ATRP initiator (<b>MSW</b>_{<b>6‑Br</b>}). Linear chain arms were grafted from <b>MSW</b>_{<b>6‑Br</b>} and subsequently functionalized with ATRP moieties to form six-arm macroinitiators. Grafting of side chains from the macroinitiators yielded four different star-shaped bottlebrushes with varying lengths of arms and side chains, i.e., <b>(450</b><i><b>-<i>g</i>-</b></i><b>20)</b>_<b>6</b>, <b>(450</b><i><b>-<i>g</i>-</b></i><b>40)</b>_<b>6</b>, <b>(300</b><i><b>-<i>g</i>-</b></i><b>60)</b>_<b>6</b>, and <b>(300</b><i><b>-<i>g</i>-</b></i><b>150)</b>_<b>6</b>. Gel permeation chromatography analysis and molecular imaging by atomic force microscopy confirmed the formation of well-defined macromolecules with narrow molecular weight distributions. Upon adsorption to an aqueous substrate, the bottlebrush arms underwent prompt dissociation from the MSW core, followed by scission of covalent bonds in the bottlebrush backbones. The preferential cleavage of the arms is attributed to strong steric repulsion between bottlebrushes at the MSW branching center. Star-shaped macroinitiators may undergo aggregation which can be prevented by sonication

Coherent and Incoherent Interactions between Cofacial Π‑Conjugated Oligomer Dimers in Macrocycle Templates

The interactions between two π-conjugated oligomers templated in molecular scaffolds are revealed as a function of separation and orientation, providing models of intermolecular interactions in bulk organic semiconductor materials. For a variety of dimer geometries (acyclic and macrocyclic) of the same model oligomer, no change in fluorescence spectra, fluorescence dynamics, or low-temperature single-molecule emission characteristics is observed. A small red-shift and slowing of fluorescence in the most closely spaced macrocyclic dimer structure is thought to arise both due to an intramolecular solvatochromic shift as well as from weak intramolecular aggregate formation. No corresponding effect is observed in bulk films of the acyclic model oligomer, implying the absence of intermolecular aggregate or excimer formation due to random relative dipole orientations. The largest effect of intramolecular geometry of the model dimer structures is seen in transient fluorescence depolarization, where an open ring geometry leads to rapid depolarization, compared to the corresponding macrocycle, due to the presence of a range of molecular transition dipole moment orientations. Self-assembled monolayers of the molecules on HOPG investigated by scanning-tunneling microscopy further illustrate the conformational variability of the open dimers in contrast to the fixed conformation of the closed dimers

Flexible Phenanthracene Nanotubes for Explosive Detection

Phenanthracene nanotubes with arylene-ethynylene-butadiynylene rims and phenanthracene walls are synthesized in a modular bottom-up approach. One of the rims carries hexadecyloxy side chains, mediating the affinity to highly oriented pyrolytic graphite. Molecular dynamics simulations show that the nanotubes are much more flexible than their structural formulas suggest: In 12, the phenanthracene units act as hinges that flip the two macrocycles relative to each other to one of two possible sites, as quantum mechanical models suggest and scanning tunneling microscopy investigations prove. Unexpectedly, both theory and experiment show for 13 that the three phenanthracene hinges are deflected from the upright position, accompanied by a deformation of both macrocycles from their idealized sturdy macroporous geometry. This flexibility together with their affinity to carbon-rich substrates allows for an efficient host–guest chemistry at the solid/gas interface opening the potential for applications in single-walled carbon nanotube-based sensing, and the applicability to build new sensors for the detection of 2,4,6-trinitrotoluene via nitroaromatic markers is shown

Flexible Phenanthracene Nanotubes for Explosive Detection

Phenanthracene nanotubes with arylene-ethynylene-butadiynylene rims and phenanthracene walls are synthesized in a modular bottom-up approach. One of the rims carries hexadecyloxy side chains, mediating the affinity to highly oriented pyrolytic graphite. Molecular dynamics simulations show that the nanotubes are much more flexible than their structural formulas suggest: In 12, the phenanthracene units act as hinges that flip the two macrocycles relative to each other to one of two possible sites, as quantum mechanical models suggest and scanning tunneling microscopy investigations prove. Unexpectedly, both theory and experiment show for 13 that the three phenanthracene hinges are deflected from the upright position, accompanied by a deformation of both macrocycles from their idealized sturdy macroporous geometry. This flexibility together with their affinity to carbon-rich substrates allows for an efficient host–guest chemistry at the solid/gas interface opening the potential for applications in single-walled carbon nanotube-based sensing, and the applicability to build new sensors for the detection of 2,4,6-trinitrotoluene via nitroaromatic markers is shown