Isolated Neutral Organic Radical Unveiled Solvent-Radical Interaction in Highly Reducing Photocatalysis.

Diffusion-limited kinetics is a key mechanistic debate when consecutive photoelectron transfer (conPET) is discussed in photoredox catalysis. In-situ generated organic photoactive radicals can access catalytic systems as reducing as alkaline metals that can activate remarkably stable bonds. However, in many cases, the extremely short-lived transient nature of these doublet state open-shell species has led to debatable mechanistic studies, hindering adoption and development.

Advancing Organic Photoredox Catalysis: Mechanistic Insight through Time-Resolved Spectroscopy.

The rapid development of light-activated organic photoredox catalysts has led to the proliferation of powerful synthetic chemical strategies with industrial and pharmaceutical applications. Despite the advancement in synthetic approaches, a detailed understanding of the mechanisms governing these reactions has lagged. Time-resolved optical spectroscopy provides a method to track organic photoredox catalysis processes and reveal the energy pathways that drive reaction mechanisms.

Concentration-dependent aggregation of methylene blue acting as a photoredox catalyst.

Hydroxylation reactions are important in biological processes and synthetic schemes. Many challenging hydroxylation reactions have been achieved using photoredox catalytic methods. For the oxidative hydroxylation of arylboronic acids, methylene blue has been used successfully as a photoredox catalyst to produce phenyl groups.

Temperature-Dependent Spin-Driven Dimerization Determines the Ultrafast Dynamics of a Copper(II)-Bound Tripyrrindione Radical.

Radicals and other open-shell molecules play a central role in chemical transformations and redox chemistry. While radicals are often highly reactive, stable radical systems are desirable for a range of potential applications, ranging from materials chemistry and catalysis to spintronics and quantum information. Here we investigate the ultrafast properties of a stable radical system with temperature-dependent spin-tunable properties.

Time-resolved dynamics of stable open- and closed-shell neutral radical and oxidized tripyrrindione complexes.

Stable open- and closed-shell Pd(II) and Cu(II) complexes of hexaethyl tripyrrin-1,14-dione (TD1) produce triplet, doublet or singlet states depending on the metal center and the redox state of the ligand. Pd(II) and Cu(II) form neutral TD1 complexes featuring ligand-based radicals, thus resulting in doublet and triplet states, respectively. The reversible one-electron oxidation of the complexes removes an unpaired electron from the ligand, generating singlet and doublet states.

Ultrafast Dynamics of Tripyrrindiones in Solution Mediated by Hydrogen-Bonding Interactions.

The optical properties and ultrafast dynamics of hexaethyl tripyrrin-1,14-dione (HTD1) are tuned by hydrogen-bonding interactions between the solute and the solvent. In solvents with low hydrogen-bonding affinity, HTD1 preferentially forms hydrogen-bonded dimers, whereas in solvents that can either donate or accept hydrogen bonds HTD1 is present as a monomer. The distinction between dimer and monomer determines the dynamics of the system, with faster internal conversion observed in the dimer form.

Insights into the structural changes occurring upon photoconversion in the orange carotenoid protein from broadband two-dimensional electronic spectroscopy.

Carotenoids play an essential role in photoprotection, interacting with other pigments to safely dissipate excess absorbed energy as heat. In cyanobacteria, the short time scale photoprotective mechanisms involve the photoactive orange carotenoid protein (OCP), which binds a single carbonyl carotenoid. Blue-green light induces the photoswitching of OCP from its ground state form (OCPO) to a metastable photoproduct (OCPR).

Mapping the spatial overlap of excitons in a photosynthetic complex via coherent nonlinear frequency generation.

We experimentally demonstrate a nonlinear spectroscopic method that is sensitive to exciton-exciton interactions in a Frenkel exciton system. Spatial overlap of one-exciton wavefunctions leads to coupling between them, resulting in two-exciton eigenstates that have the character of many single-exciton pairs. The mixed character of the two-exciton wavefunctions gives rise to a four-wave-mixing nonlinear frequency generation signal.

Acoustic phonon strain induced mixing of the fine structure levels in colloidal CdSe quantum dots observed by a polarization grating technique.

Acoustic phonon modes in colloidal semiconductor nanocrystals are of significant interest due to their role in dephasing and as the main component of homogeneous line broadening. Despite their importance, these modes have proven elusive and have only recently been experimentally observed. This paper expands on results presented in our earlier paper [V.

Measurement of electron-electron interactions and correlations using two-dimensional electronic double-quantum coherence spectroscopy.

A two-dimensional (2D) optical coherent spectroscopy that correlates the double excited electronic states to constituent single excited states is described. The technique, termed two-dimensional double-quantum coherence spectroscopy (2D-DQCS), makes use of multiple, time-ordered ultrashort coherent optical pulses to create double and single quantum coherences over the time intervals between the pulses. The resulting 2D electronic spectra map out the energy correlation between the first excited state and two-photon-allowed double-quantum states.

CdSe nanoparticle elasticity and surface energy.

The acoustic phonon modes of colloidal CdSe nanoparticles in solution (293 K) are passively measured by a third order ultrafast heterodyne cross-polarized transient grating measurement. Using the observed size-dependence of the acoustic phonon frequency, the elastic properties of the nanoparticles are determined. The size-dependence of the elastic modulus is then used to ascertain information about the relative surface energies of the nanocrystals and suggests the extent and depth of surface reconstruction.

Nonlinear optical approach to multiexciton relaxation dynamics in quantum dots.

Unlike the majority of molecular systems quantum dots can accommodate multiple excitations, which is a particularly important attribute for potential lasing applications. We demonstrate in this work the concept of using nth order nonlinear spectroscopies in the transient grating configuration as a means of selectively exciting (n-1)/2 excitons in a semiconductor and probing the subsequent relaxation dynamics. We report a direct observation of multiparticle dynamics on ultrashort time scales through comparison of third and fifth order experiments for CdSe colloidal quantum dots.

Nanocrystal shape and the mechanism of exciton spin relaxation.

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals.

Dynamics within the exciton fine structure of colloidal CdSe quantum dots.

Evidence for an interaction between the quantum dot exciton fine structure states F = +/-1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping. An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported. By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system.