New organometallic materials such as two-dimensional metallacycles and three-dimensional metallacages are important for the development of novel optical, electronic, and energy related applications. In this article, the ultrafast dynamics of two different platinum-containing metallacycles have been investigated by femtosecond fluorescence upconversion and transient absorption. These measurements were carried out in an effort to probe the charge transfer dynamics and the rate of intersystem crossing in metallacycles of different geometries and dimensions. The processes of ultrafast intersystem crossing and charge transfer vary between the two different classes of metallacyclic systems studied. For rectangular anthracene-containing metallacycles, the electronic coupling between adjacent ligands was relatively weak, whereas for the triangular phenanthrene-containing structures, there was a clear interaction between the conjugated ligand and the metal complex center. The transient lifetimes increased with increasing conjugation in that case. The results show that differences in the dimensionality and structure of metallacycles result in different optical properties, which may be utilized in the design of nonlinear optical materials and potential new, longer-lived excited state materials for further electronic applications.

Four temperatures to characterize the gel transition in reversible associating polymers have been calculated in a novel mixed molecular dynamics/Monte Carlo model. (1) The temperature below which relaxation times no longer show Arrhenius dependence on temperature; (2) the Vogel-Fulcher temperature at which the structural relaxation time extrapolates to infinity; (3) the micelle formation temperature at which the number of reversible bonds sharply increases; and (4) a crossover temperature at which the viscosity exhibits a power law divergence as predicted by mode coupling theory. These specific temperatures are obtained from measurements of diffusivity, specific heat, and network topology.

Two-photon active green fluorescent protein -type chromophores were successfully synthesized following investigations directed toward a modified version of zFP538 chromophore, a structural analogue to the GFP-chromophore. A generalized approach for the chromophore synthesis via a well-studied cycloaddition reaction combining an iminoglycine methyl ester and a substituted benzaldehyde was developed allowing for flexibility in the incorporation of functional groups such as donorÀacceptor substituents and for additional groups to provide extended conjugation. Steady-state spectroscopy, fluorescence quantum yields, and time-resolved fluorescence lifetimes for synthesized chromophores were extensively investigated for the functionalized chromophores. Time-resolved fluorescence lifetimes were found to be biexponential generally with subpicosecond and picosecond components. The individual effects of substitution position of functional groups and relative bulk size were evaluated and found to be rather significant in changing the fluorescencedecay characteristics in the case of positioning, but ambiguous with respect to relative bulk. The GFP-type chromophores were found to possess modest to low two-photon absorption cross sections with the dimethylamino-substituted analogue possessing the largest value at nearly 40 GM. These molecules show promise as biological markers for application in the study of conformation changes and aggregation of amyloid peptides, known to play an important role in many neurodegenerative diseases.

Förster Resonance Energy Transfer (FRET) based measurements that calculate the stoichiometry of intermolecular interactions in living cells have recently been demonstrated, where the technique utilizes selective one-photon excitation of donor and acceptor fluorophores to isolate the pure FRET signal. Here, we present work towards extending this FRET stoichiometry method to employ two-photon excitation using a pulse-shaping methodology. In pulse-shaping, frequency-dependent phases are applied to a broadband femtosecond laser pulse to tailor the two-photon excitation conditions to preferentially excite donor and acceptor fluorophores. We have also generalized the existing stoichiometry theory to account for additional cross-talk terms that are non-vanishing under twophoton excitation conditions. Using the generalized theory we demonstrate two-photon FRET stoichiometry in live COS-7 cells expressing fluorescent proteins mAmetrine as the donor and tdTomato as the acceptor.

Ultrafast pump-probe spectroscopy and pulse-shaping techniques are providing new modes of contrast for the field of multiphoton microscopy. Endogenous species such as heme proteins show rich nonlinear spectroscopic signatures of excited state absorption, stimulated emission and ground-state bleaching. Commercially available octave-spanning Ti:sapphire oscillators offer new opportunities for imaging based on pump-probe contrast. Spatial light modulators take advantage of this large bandwidth, shaping pulses of light to selectively excite molecular structures with similar spectral properties. We present two-color pump-probe imaging of heme proteins solutions and red blood cells.