3 research outputs found
Electron Transfer Assisted by Vibronic Coupling from Multiple Modes
Understanding
the effect of vibronic coupling on electron transfer
(ET) rates is a challenge common to a wide range of applications,
from electrochemical synthesis and catalysis to biochemical reactions
and solar energy conversion. The MarcusāJortnerāLevich
(MJL) theory offers a model of ET rates based on a simple analytic
expression with a few adjustable parameters. However, the MJL equation
in conjunction with density functional theory (DFT) has yet to be
established as a predictive first-principles methodology. A framework
is presented for calculating transfer rates modulated by molecular
vibrations, that circumvents the steep computational cost which has
previously necessitated approximations such as condensing the vibrational
manifold into a single empirical frequency. Our DFTāMJL approach
provides robust and accurate predictions of ET rates spanning over
4 orders of magnitude in the 10<sup>6</sup>ā10<sup>10</sup> s<sup>ā1</sup> range. We evaluate the full MJL equation with
a Monte Carlo sampling of the entire active space of thermally accessible
vibrational modes, while using no empirical parameters. The contribution
to the rate of individual modes is illustrated, providing insight
into the interplay between vibrational degrees of freedom and changes
in electronic state. The reported findings are valuable for understanding
ET rates modulated by multiple vibrational modes, relevant to a broad
range of systems within the chemical sciences
Preparation of Halogenated Fluorescent Diaminophenazine Building Blocks
A short, convenient,
and scalable protocol for the one-pot synthesis
of a series of fluorescent 7,8-dihalo-2,3-diaminophenazines is introduced.
The synthetic route is based on the oxidative condensation of 4,5-dihalo-1,2-diaminobenzenes
in aqueous conditions. The resulting diaminophenazines could be attractive
intermediates for the preparation of polyfunctional phenazines and
extended polyheteroacenes. We find that the undesired hydroxylation
byproducts, typically obtained in aqueous conditions, are completely
suppressed by addition of a stoichiometric amount of acetone during
the oxidation step allowing for selective formation of 7,8-dihalo-2,2-dimethyl-2,3-dihydro-1<i>H</i>-imidazoĀ[4,5-<i>b</i>]Āphenazine derivatives with
good to excellent yields. Under reductive conditions, the imidazolidine
ring can be hydrolyzed into the desired 7,8-dihalo-2,3-diaminophenazines.
Furthermore, we report a selective route under highly reducing conditions
to monohydrodeaminate the 2,3-diĀ(methylamino) phenazine derivatives,
which allows for further structural variations of these phenazine
building blocks. All of these derivatives are luminescent, with measured
fluorescence quantum-yields of up to 80% in ethanol for the more rigid
structures, highlighting the potential of such materials to provide
new fluorophores
Spectroscopic Analysis of a Biomimetic Model of Tyr<sub>Z</sub> Function in PSII
Using
natural photosynthesis as a model, bio-inspired constructs
for fuel generation from sunlight are being developed. Here we report
the synthesis and time-resolved spectroscopic analysis of a molecular
triad in which a porphyrin electron donor is covalently linked to
both a cyanoporphyrin electron acceptor and a benzimidazoleāphenol
model for the Tyr<sub>Z</sub>-D<sub>1</sub>His190 pair of PSII. A
dual-laser setup enabled us to record the ultrafast kinetics and long-living
species in a single experiment. From this data, the photophysical
relaxation pathways were elucidated for the triad and reference compounds.
For the triad, quenching of the cyanoporphyrin singlet excited state
lifetime was interpreted as photoinduced electron transfer from the
porphyrin to the excited cyanoporphyrin. In contrast to a previous
study of a related molecule, we were unable to observe subsequent
formation of a long-lived charge separated state involving the benzimidazoleāphenol
moiety. The lack of detection of a long-lived charge separated state
is attributed to a change in energetic landscape for charge separation/recombination
due to small differences in structure and solvation of the new triad