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.

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.

The molecular mechanism of dual emission in terpyridine transition metal complexes--ultrafast investigations of photoinduced dynamics.

Temperature dependent luminescence experiments are combined with femtosecond time-resolved transient absorption spectroscopy to decipher the photoinduced excited-state relaxation pathway in mononuclear Fe, Ru and Os terpyridine complexes bearing a conjugated chromophore within the ligand framework. The herein presented complexes constitute a class of coordination compounds, which overcome the poor emission properties commonly observed for most terpyridine transition metal complexes. As reported earlier, the complexes reveal dual emission at room temperature stemming from ligand centered and metal-to-ligand charge-transfer states. The molecular mechanism of the room temperature dual luminescence is addressed experimentally in this contribution. The experimental results indicate an ultrafast branching reaction within the excited-state manifold upon photoexcitation of the ligand-centered S(1) state. This branching occurs from a "hot" excited state geometry close to the Franck-Condon point of absorption and within ∼100 fs, i.e. the temporal resolution of our experimental setup. The combination of ultrafast differential absorption experiments and temperature-dependent luminescence data allows not only to draw conclusions about the molecular mechanism underlying the observed dual emission but also to construct quantitative Jablonski diagrams and, thereby, to detail the excited-state topology determining the remarkable luminescence properties of the systems at hand.

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.

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

Surface-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.

Spectroscopic investigation of the ultrafast photoinduced dynamics in pi-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.