Johannes Gierschner

Multistimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H···N and C-H···O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.

A White-Light-Emitting Molecule: Frustrated Energy Transfer between Constituent Emitting Centers

White-light-emitting single molecules are promising materials for use in a new generation of displays and light sources because they offer the possibility of simple fabrication with perfect color reproducibility and stability. To realize white-light emission at the molecular scale, thereby eliminating the detrimental concentration- or environment-dependent energy transfer problem in conventional fluorescent or phosphorescent systems, energy transfer between a larger band-gap donor and a smaller band-gap acceptor must be fundamentally blocked. Here, we present the first example of a concentration-independent ultimate white-light-emitting molecule based on excited-state intramolecular proton transfer materials. Our molecule is composed of covalently linked blue- and orange-light-emitting moieties between which energy transfer is entirely frustrated, leading to the production of reproducible, stable white photo- and electroluminescence.

Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with n=1–4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S0↔S1 emission and excitation bands are mirror symmetrical with negligible 0–0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon–carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree–Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0–0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic ba...

Luminescent distyrylbenzenes: tailoring molecular structure and crystalline morphology

The last few years have seen a steady increase in small molecule based conjugated materials, which promise innovative (opto)electronic applications. This requires however a systematic understanding of structure–property relationships, which can only be achieved via libraries of structurally well-defined single crystalline materials based on systematically designed molecular structures. In this feature article, we are presenting structure–property relationships of functionalized distyrylbenzene (DSB), which is one of the most extensively investigated π-conjugated molecular materials. This will provide a general insight into the specific implications of intermolecular arrangements on their solid state optoelectronic properties, discussing H- vs. J-aggregation, herringbone vs. π-stacks, the occurrence of excimers, size effects, and polycrystallinity. The systematic insight into DSB functionalization will then suggest pathways towards targeted molecular design strategies, with special focus on the cyano-vinylene motif (DCS materials) which allows for highly fluorescence solid state samples due to synergetic packing effects promoted by its twist elasticity and secondary bonding interaction. Finally, recent advances in the application of DSB-/DCS-based materials are shortly reviewed.

Tailor-Made Highly Luminescent and Ambipolar Transporting Organic Mixed Stacked Charge-Transfer Crystals: An Isometric Donor–Acceptor Approach

We have rationally designed a densely packed 1:1 donor-acceptor (D-A) cocrystal system comprising two isometric distyrylbenzene- and dicyanodistyrylbenzene-based molecules, forming regular one-dimensional mixed stacks. The crystal exhibits strongly red-shifted, bright photoluminescence originating from an intermolecular charge-transfer state. The peculiar electronic situation gives rise to high and ambipolar p-/n-type field-effect mobility up to 6.7 × 10(-3) and 6.7 × 10(-2) cm(2) V(-1) s(-1), respectively, as observed in single-crystalline OFETs prepared via solvent vapor annealing process. The unique combination of favorable electric and optical properties arising from an appropriate design concept of isometric D-A cocrystal has been demonstrated as a promising candidate for next generation (opto-)electronic materials.

Solid-state optical properties of linear polyconjugated molecules: π-stack contra herringbone

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F(12)DSB) as well as the DSB:F(12)DSB cocrystals prefer cofacial pi-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant k(F). Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F(12)DSB and DSB:F(12)DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.

Unique Piezochromic Fluorescence Behavior of Dicyanodistyrylbenzene Based Donor–Acceptor–Donor Triad: Mechanically Controlled Photo-Induced Electron Transfer (eT) in Molecular Assemblies

A novel donor-acceptor-donor triad that exhibits fluorescence on-off switching with high contrast ratio (ca. 10(3) ) in response to a mechanical stimulus in the solid state is reported. This system provides a very unique example of high-contrast fluorescence switching that is driven by a mechanical-force-controlled photo-induced electron transfer (eT) in molecular assemblies.

Electronic deactivation in single chains, nano-aggregates and ultrathin films of conjugated oligomers

Abstract Fluorescence spectra, quantum yields Φ F , decay curves, and fluorescence anisotropies of oligothiophenes and oligophenylenevinylenes with varying chain lengths are determined in the dissolved state, in nano-aggregates and in ultrathin films. Φ F decreases in the series monomer > aggregate ≈ disordered film > oriented film by four powers of ten, whereas the average decay times remain unchanged. The orientational relaxation time is 35 ps in condensed oligothiophenes. The strong increase of Φ F with disorder is mainly attributed to the increasing density of luminescence centers.

Highly Fluorinated Benzobisbenzothiophenes

Expedient, facile syntheses of highly fluorinated benzobisbenzothiophenes (BBBT) are reported. Defined peripheral arrangements of sulfur and fluorine atoms lead to extensive crystalline networks of edge-to-edge S-F close contacts. The effects of various substitution patterns on self-assembly and electronic properties are described.

Excitonic versus electronic couplings in molecular assemblies: The importance of non-nearest neighbor interactions

We demonstrate that for a range of phenylene- and thiophene-based conjugated polymers of practical relevance for optoelectronic applications, exciton couplings in one-dimensional stacks deviate significantly from the nearest neighbor approximation. Instead, long-range interactions with non-nearest neighbors have to be included, which become increasingly important with growing oligomer size. While the exciton coupling vanishes for infinitely long ideal polymer chains and provides a sensitive measure of the actual conjugation length, the electronic coupling mediating charge transport shows rapid convergence with molecular size. Similar results have been obtained for very different molecular backbones, thus highlighting the general character of these findings.