Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization.

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor-acceptor chromophore two different -phenylene bridges in () and () (bpy = 5,5'-di--butyl-2,2'-bipyridine, CAT = 3--butylcatecholate, -Ph = -phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙, CAT˙ = semiquinone (SQ), and NN), charge-separated doublet S (S = chromophore excited spin singlet configuration) excited state. Following excitation, the S state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversion the T (T = chromophore excited spin triplet configuration) state.

Spectroscopic Signatures of Resonance Inhibition Reveal Differences in Donor-Bridge and Bridge-Acceptor Couplings.

The torsional dependence of the ground state magnetic exchange coupling () and the corresponding electronic coupling matrix element () for eight transition metal complexes possessing donor-acceptor (D-A) biradical ligands is presented. These biradical ligands are composed of an S = 1/2 metal semiquinone (SQ) donor and an S = 1/2 nitronylnitroxide (NN) acceptor, which are coupled to each other via -phenylene, methyl-substituted -phenylenes, or a bicyclo[2.2.

Isomerization of Linear to Hyperbranched Polymers: Two Isomeric Lactones Converge via Metastable Isostructural Polyesters to a Highly Branched Analogue.

We report here the Zn(II)-catalyzed convergence of two metastable and isostructural polyesters to an isomeric polymer having a hyperbranched architecture. Ring-opening transesterification polymerization (ROTEP) of 4-carbomethoxyvalerolactone () under Brønsted catalysis is known to give the linear polyester . We show here that this can be isomerized to the equilibrated (and highly branched) polyester .

Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts.

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LZn catalysts (L = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (P = 0.72-0.

Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone.

Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d by H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics.