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Register a new userInteractions between pi-conjugated chromophores in a giant molecular spoked wheel
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We discuss the intriguing photophysics of a giant molecular spoked wheel of pi-conjugated arylenealkynylene chromophores on the single-molecule level. This molecular mesoscopic tructure, C1878H2682, shows fast switching between the 12 identical chromophores since the fluorescence is unpolarised but only one chromophore emits at a time.
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Conjugated polymers offer potential for many diverse applications but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations - excitons - span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not fully understood. For example, where is the exciton formed within a conjugated segment, is it always situated on the same repeat units? Here, we introduce structurally-rigid molecular spoked wheels, 6 nanometres in diameter, as a model of extended pi-conjugation. Single-molecule fluorescence reveals random exciton localisation, leading to temporally-varying emission polarisation. Initially, this random localisation arises after every photon absorption event because of temperature independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales following prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, however, emission polarisation can also switch without a spectral jump occurring, implying long-range homogeneity in local dielectric environment.
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.
The exciton relaxation dynamics of photoexcited electronic states in poly($p$-phenylenevinylene) (PPV) are theoretically investigated within a coarse-grained model, in which both the exciton and nuclear degrees of freedom are treated quantum mechanically. The Frenkel-Holstein Hamiltonian is used to describe the strong exciton-phonon coupling present in the system, while external damping of the internal nuclear degrees of freedom are accounted for by a Lindblad master equation. Numerically, the dynamics are computed using the time evolving block decimation (TEBD) and quantum jump trajectory techniques. The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: 1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. 2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. 3) Exciton density localization is driven by the external dissipation, arising from `wavefunction collapse occurring as a result of the system-environment interactions. Finally, we show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.
New charge transfer crystals of $pi$-conjugated, aromatic molecules (phenanthrene and picene) as donors were obtained by physical vapor transport. The melting behavior, optimization of crystal growth and the crystal structure is reported for charge transfer salts with (fluorinated) tetracyanoquinodimethane (TCNQ-F$_x$, x=0, 2, 4), which was used as acceptor material. The crystal structures were determined by single-crystal X-ray diffraction. Growth conditions for different vapor pressures in closed ampules were applied and the effect of these starting conditions for crystal size and quality is reported. The process of charge transfer was investigated by geometrical analysis of the crystal structure and by infrared spectroscopy on single crystals. With these three different acceptor strengths and the two sets of donor materials, it is possible to investigate the distribution of the charge transfer systematically. This helps to understand the charge transfer process in this class of materials with $pi$-conjugated donor molecules.
Anisotropic magnetoresistance (AMR), originating from spin-orbit coupling (SOC), is the sensitivity of the electrical resistance in magnetic systems to the direction of spin magnetization. Although this phenomenon has been experimentally reported for several nanoscale junctions, a clear understanding of the physical mechanism behind it is still elusive. Here we discuss a novel concept based on orbital symmetry considerations to attain a significant AMR of up to 95% for a broad class of $pi$-type molecular spin-valves. It is illustrated at the benzene-dithiolate molecule connected between two monoatomic nickel electrodes. We find that SOC opens, via spin-flip events at the ferromagnet-molecule interface, a new conduction channel, which is fully blocked by symmetry without SOC. Importantly, the interplay between main and new transport channels turns out to depend strongly on the magnetization direction in the nickel electrodes due to the tilting of molecular orbital. Moreover, due to multi-band quantum interference, appearing at the band edge of nickel electrodes, a transmission drop is observed just above the Fermi energy. Altogether, these effects lead to a significant AMR around the Fermi level, which even changes a sign. Our theoretical understanding, corroborated in terms of textit{ab initio} calculations and simplified analytical models, reveals the general principles for an efficient realization of AMR in molecule-based spintronic devices.
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