Liangxuan Wang

Kurzzusammenfassung: Trotz intensiver Forschung an spitzenverstärkter Ramanspektroskopie (TERS) wurde die Winkelverteilung der Raman‐Streuung bisher nicht experimentell untersucht. Deshalb wird die Relevanz der Winkelverteilung für das gemessene TERS‐Signal selten diskutiert. In dieser Arbeit untersuchen wir die Winkelverteilung des spitzenverstärkten Ramansignals in der Fourier‐Ebene. Dafür verwenden wir ein Modellsystem aus flach liegenden Kobalt(II)hexadecaflouro‐Phthalocyanin Molekülen (CoPcF ₁₆ ), die auf einem glatten Goldfilm adsorbiert sind. Sowohl Vibrationen in der Molekülebene als auch solche senkrecht zur Ebene können beobachtet werden. Die Ramanmoden senkrecht zur Ebene bei 678 cm ⁻¹ und 740 cm ⁻¹ haben dabei andere winkelabhängige Intensitätsverteilungen als die Ramanmoden parallel zur Ebene bei 1309 cm ⁻¹ und 1373 cm ⁻¹ . Wir interpretieren das gemessene TERS‐Signal unter Berücksichtigung der vibronischen Moden, die mithilfe von Dichtefunktionaltheorie (DFT) berechnet wurden, und der direktionalen Ramanstreuung im Bereich (Lücke) zwischen Spitze und Goldfilm, die mithilfe der „Finite‐Difference Time‐Domain (FDTD)“ Methode simuliert wurde. Wir nehmen an, dass die TERS‐Lücke die Ramanschwingungsmoden unterschiedlich weiterleitet, was zu unterschiedlichen Winkelverteilungen in der resultierenden Intensität der Ramanstreuung führt. Unsere Ergebnisse zeigen, dass die TERS‐Detektionsgeometrie für das Verständnis der spektralen Eigenschaften nicht vernachlässigbar ist. Das betrifft beispielsweise die Variation relativer Peakintensitäten bei der Untersuchung molekularer Orientierungen oder die Verfolgung chemischer Reaktionen mithilfe der Ramanspektroskopie.

Despite intensive research in tip‐enhanced Raman spectroscopy (TERS), the angular distribution of Raman scattering in the TERS gap remains experimentally unreported leaving its relevance to the TERS signal formation to be seldomly discussed. Here, we investigate the angular distribution of the tip‐enhanced Raman signal in the Fourier plane using a model system composed of flat‐lying cobalt (II) hexadecafluoro‐phthalocyanine (CoPcF ₁₆ ) molecules physically adsorbed on a smooth gold surface. Both in‐plane and out‐of‐plane vibrational modes are observed, where the out‐of‐plane Raman modes at about 678 and 740 cm ⁻¹ have different angular intensity distributions than those of in‐plane Raman modes at 1309 and 1373 cm ⁻¹ . We interpret the angular spectrum of the TERS signal considering the molecular vibrational modes computed with density functional theory (DFT) for the free and gold‐deposited molecule, and the directed Raman scattering by the gap‐mode predicted by finite‐difference time‐domain (FDTD) simulations. We contend that the TERS gap directs the Raman vibrational modes differently, leading to distinct angularly distributed Raman scattering intensities. These findings emphasize the nonnegligible role of the TERS detection scheme in understanding spectral features, such as the relative peak intensity ratio variations for studying molecular orientations, or for monitoring chemical reactions.

Achieving high quantum yields for organic fluorophores in both solution and in the polycrystalline solid state is a significant yet challenging goal, as most fluorophores typically emit effectively in only one phase. In this study, we introduce 1,1’-(2,5-dimethoxyterephthaloyl)-bis(glutraimide) (TGlu), a novel ‘single benzene’-based organic fluorophore featuring an alkoxy donor and an N-acyl-glutarimide acceptor. TGlu achieves close-to-unity fluorescence quantum yields in both solution and in the polycrystalline state. Through systematic studies involving four additional structural analogues of TGlu, we unveiled that the acceptor group within the single benzene core plays a crucial steric and electronic role in achieving high fluorescence efficiency in both phases. Furthermore, we demonstrated the versatility of this new fluorophore in various applications. In solution, TGlu serves as a highly effective photocatalyst for photoinduced energy/electron transfer reversible addition−fragmentation chain-transfer polymerization while in its single-crystal form, TGlu exhibits highly efficient waveguiding properties.

A series of windmill‐shape heterocyclic molecules containing three N−B←N units, TBN and its derivatives, with quasi‐planar C ₃ symmetric backbone, are synthesized. The parent TBN exhibits a strongly allowed, doubly degenerate lowest excited state but suffers from very low fluorescence, due to very fast nonradiative decay rate through a conical intersection (CI) as revealed by femtosecond transient absorption spectroscopy and quantum‐chemical calculations. Introducing peripheral phenyl‐ or thienyl‐groups (Ph‐TBN or Th‐TBN) induces pronounced bathochromic shifts and enhances fluorescence, which is beneficial from inhibited nonradiative pathway by the increased energy barriers to access the CI at excited state. The understanding of this rather uncommon behaviour may open routes for the design of novel fluorescence materials.

A series of windmill‐shape heterocyclic molecules containing three N−B←N units, TBN and its derivatives, with quasi‐planar C ₃ symmetric backbone, are synthesized. The parent TBN exhibits a strongly allowed, doubly degenerate lowest excited state but suffers from very low fluorescence, due to very fast nonradiative decay rate through a conical intersection (CI) as revealed by femtosecond transient absorption spectroscopy and quantum‐chemical calculations. Introducing peripheral phenyl‐ or thienyl‐groups (Ph‐TBN or Th‐TBN) induces pronounced bathochromic shifts and enhances fluorescence, which is beneficial from inhibited nonradiative pathway by the increased energy barriers to access the CI at excited state. The understanding of this rather uncommon behaviour may open routes for the design of novel fluorescence materials.

The peripherical protons of the dye molecule hypericin can undergo structural interconversion (tautomerization) between different isomers separated by a low energy barrier with rates that depends sensitively on the interaction with local chemical environment defined by the nature of host material. We investigate the deuterium (D) isotope effect of hypericin tautomerism at the single‐molecule level to avoid ensemble averaging in different polymer matrices by a combined spectroscopic and computational approach. In the ′innocent′ PMMA matrix only intramolecular isotope effects on the internal conversion channel and tautomerization are observed; while PVA specifically interacts with the probe via H‐ and D‐bonding. This establishes a single molecular picture on intra‐ and intermolecular nano‐environment effects to control chromophore photophysics and ‐chemistry.

Current research on organic light emitters which utilize multiple resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials is gaining significant interest because of the materials’ ability to efficiently generate color‐pure blue emission. However, the underlying reasons for high color purity remain unclear. It is shown here that these emitters share a common electronic basis, which is deduced from resonance structure considerations following Clar's rule, and which is termed as “poly‐heteroaromatic omni‐delocalization” (PHOD). The simple and clear design rules derived from the PHOD concept allow extending the known chemical space by new structural motifs. Based on PHOD, a set of novel high‐efficiency color‐pure emitters with brilliant deep‐blue hue is specifically designed.

The complex photokinetics of donor–acceptor–donor triads with varying flexible spacer lengths ( n = 4–10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady‐state and transient fluorescence spectroscopy combined with computational studies. For the short spacer ( n = 4) in a liquid solution, dynamic charge‐transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli‐responsive high‐contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color ( n = 4). The work thus demonstrates how in‐depth analysis of complex photophysics can be put to practical use in materials science.