Ronald Siebert

Ruthenium(II) photosensitizers of tridentate click-derived cyclometalating ligands: a joint experimental and computational study.

A systematic series of heteroleptic bis(tridentate)ruthenium(II) complexes of click-derived 1,3-bis(1,2,3-triazol-4-yl)benzene N^C^N-coordinating ligands was synthesized, analyzed by single crystal X-ray diffraction, investigated photophysically and electrochemically, and studied by computational methods. The presented comprehensive characterization allows a more detailed understanding of the radiationless deactivation mechanisms. Furthermore, we provide a fully optimized synthesis and systematic variations towards redox-matched, broadly and intensely absorbing, cyclometalated ruthenium(II) complexes. Most of them show a weak room-temperature emission and a prolonged excited-state lifetime. They display a broad absorption up to 700 nm and high molar extinction coefficients up to 20 000 M(-1)cm(-1) of the metal-to-ligand charge transfer bands, resulting in a black color. Thus, the complexes reveal great potential for dye-sensitized solar-cell applications.

Dual Emission from Highly Conjugated 2,2′:6′:2″-Terpyridine Complexes—A Potential Route to White Emitters

Here, we present a new class of terpyridine complexes of the transition-metal ions, iron(II), ruthenium(II), and osmium(II), overcoming the poor emission properties typical for this class of polypyridyl complexes. These complexes show, besides an increased room-temperature emission quantum yield and a prolonged lifetime of the metal-to-ligand charge-transfer (MLCT) states, dual emission from two well-separated excited states of the same molecule. These experimental findings are attributed to a highly stabilized ligand chromophore, where photoinduced excited-state planarization causes an enhancement of electron delocalization. This planarization, in turn, reduces the potential energy of the S(1) state and minimizes electronic coupling to the MLCT state, which is prone to non-radiative deactivation via metal-centered excited states. Due to their dual emission the complexes presented here show emission covering the entire Vis spectral range upon excitation of the ππ* states in the near UV. Thus, by structurally tuning the electronic coupling of the ππ* and the MLCT states a new synthetic route toward white emitters, which can subsequently be incorporated into polymers, is opened.

Spectroscopic Investigation of the Ultrafast Photoinduced Dynamics in π‐Conjugated Terpyridines

Ultrafast light-induced processes in a series of pi-conjugated mono-, bis-, tris- and tetrakis(terpyridine) derivatives are investigated by femtosecond time-resolved spectroscopy. Non-exponential excited-state dynamics involving singlet-triplet intersystem crossing are observed which span from picoseconds to nanoseconds (see figure). Time-resolved spectroscopy is applied to investigate the ultrafast relaxation dynamics of several pi-conjugated mono-, bis-, tris- and tetrakis(terpyridine) derivatives. This particular series of structurally closely related systems was prepared applying efficient synthetic strategies and resembles key building blocks for a wide range of photoactive complexes, dendrimers and metallo-polymers with resulting potential applications, for example, in photovoltaics or as organic light-emitting diodes. Aiming for applications of supramolecular assemblies based on these recently presented terpyridine ligands a detailed knowledge of the light-induced processes of the ligands themselves represents a prerequisite. By applying femtosecond time-resolved absorption spectroscopy in concert with time-resolved fluorescence and Raman measurements, we detail the photophysical properties.

Light-induced dynamics in conjugated bis(terpyridine) ligands--a case study toward photoactive coordination polymers.

Terpyridine coordination compounds have recently revealed their potential toward optoelectronic applications, which are based on (i) their excellent charge-transfer properties or (ii) intriguing luminescence properties of the systems, depending on their design. This article features recent work in dissecting the photophysical properties of such materials by investigating the photoinduced processes in individual molecular fragments. The article starts considering the terpyridine ligands themselves before discussing the impact of metal coordination. After shedding light into the interplay of different electronic states in the terpyridine complexes, first results of a single-molecule fluorescence study are presented, which allow for correlation of the overall luminescence properties with the structure of the polymer.

Ultrafast plasmon dynamics and evanescent field distribution of reproducible surface-enhanced Raman-scattering substrates

AbstractSurface-enhanced Raman scattering (SERS) is a potent tool in bioanalytical science because the technique combines high sensitivity with molecular specificity. However, the widespread and routine use of SERS in quantitative biomedical diagnostics is limited by tight requirements on the reproducibility of the noble metal substrates used. To solve this problem, we recently introduced a novel approach to reproducible SERS substrates. In this contribution, we apply ultrafast time-resolved spectroscopy to investigate the photo-induced collective charge-carrier dynamics in such substrates, which represents the fundamental origin of the SERS mechanism. The ultrafast experiments are accompanied by scanning-near field optical microscopy and SERS experiments to correlate the appearance of plasmon dynamics with the resultant evanescent field distribution and the analytically relevant SERS enhancement. FigureUltrafast time-resolved differential absorption spectroscopy combined with scanning near-field optical microscopy (left) and atomic force microscopy (right) yields insight into the photoinduced charge-carrier dynamics in innovative reproducible SERS-substrates

Ruthenium(II)-bis(4'-(4-ethynylphenyl)-2,2':6', 2''-terpyridine) - A versatile synthon in supramolecular chemistry. Synthesis and characterization

AbstractA homoleptic ethynyl-substituted ruthenium(II)-bisterpyridine complex representing a versatile synthon in supramolecular chemistry was synthesized and analyzed by NMR spectroscopy, mass spectrometry and X-ray diffractometry. Furthermore, its photophysical properties were detailed by UV/Vis absorption, emission and resonance Raman spectroscopy. In order to place the results obtained in the context of the vast family of ruthenium coordination compounds, two structurally related complexes were investigated accordingly. These reference compounds bear either no or an increased chromophore in the 4′-position. The spectroscopic investigations reveal a systematic bathochromic shift of the absorption and emission maximum upon increasing chromophore size. This bathochromic shift of the steady state spectra occurs hand in hand with increasing resonance Raman intensities upon excitation of the metal-to-ligand charge-transfer transition. The latter feature is accompanied by an increased excitation delocalization over the chromophore in the 4′-position of the terpyridine. Thus, the results presented allow for a detailed investigation of the electronic effects of the ethynyl substituent on the metal-to-ligand charge-transfer states in the synthon for click reactions leading to coordination polymers.

Photo-induced processes in new materials for electro-optical applications

We present a spectroscopic study of the photoinduced excited-state processes in new materials for polymer solar cells and organic-light emitting diodes, which are based on extended conjugated terpyridine systems. Various spectroscopic tools were combined to investigate the photoinduced dynamics in such systems from the Franck-Condon point up to nanoseconds after light-absorption. The intriguing finding of dual luminescence from a novel class of terpyridine complexes are discussed.