Photochemical Ring Slippage of Bis(pentafluorophenyl)titanocene: Reaction kinetics and matrix isolation of the primary photoproduct

Laser flash photolysis and matrix-isolation techniques were applied to elucidate the photochemistry of the orange complex bis(pentafluorophenyl)titanocene ( = bis(η⁵-cyclopentadienyl)bis(pentafluorophenyl)titanium- (IV), Cp₂Ti^IV{C₆F₅}₂, Cp = η⁵-C₅H₅, 1 ) which is used as a polymerization photo-initiator. The primary photo-reaction of 1 is the formation of a highly reactive blue isomer X with unit quantum yield. In carefully dried and degassed benzene solution, the photoisomer X rearranges to starting material 1 with a first-order reaction, k = 5 · 10³ s^?1 at room temperature. X is highly reactive towards H₂O, MeOH, acetone, MeCN, MeNO₂, butane-1,4-diyl diacrylate, 2,2,6,6-tetramethylpiperidine N-oxide, CO, O₂, and N₂; absolute bimolecular rate constants range from 10⁶ to 10⁹ M ^?1 · s^?1. The primary photoisomerization 1→X is tentatively ascribed to a cyclopentadienyl ring slippage from η⁵ to a lower hapticity, a process that opens up coordinative unsaturation.     

Imaging of tautomerism in a single molecule

Fluorescence imaging is used to visualize directly the transfer of two inner hydrogen atoms in single porphycene molecules. This reaction leads to a chemically equivalent but differently oriented structure and hence results in a rotation of the transition dipole moments. By probing single immobilized molecules with an azimuthally polarized laser beam in the focal spot of a confocal microscope we observe ring-like emission patterns, possible only for a chromophore with two nearly orthogonal transition dipole moments. Numerical simulations of the observed emission patterns yield a value of 72 degrees for the angle between the S0-S1 transition moments in the two tautomeric forms.     

Pathwise Convergence of a Rescaled Super-Brownian Catalyst Reactant Process

Consider the one-dimensional catalytic super-Brownian motion X (called the reactant) in the catalytic medium which is an autonomous classical super-Brownian motion. We characterize both in terms of a martingale problem and (in dimension one) as solution of a certain stochastic partial differential equation. The focus of this paper is for dimension one the analysis of the longtime behavior via a mass-time-space rescaling. When scaling time by a factor of K, space is scaled by K ^η and mass by K ^−η. We show that for every parameter value η ≥ 0 the rescaled processes converge as K→ ∞ in path space. While the catalyst’s limiting process exhibits a phase transition at η = 1, the reactant’s limit is always the same degenerate process.