2 research outputs found
Photochemistry and Electron Transfer Kinetics in a Photocatalyst Model Assessed by Marcus Theory and Quantum Dynamics
The
present computational study aims at unraveling the competitive
photoinduced electron transfer (ET) kinetics in a supramolecular photocatalyst
model. Detailed understanding of the fundamental processes is essential
for the design of novel photocatalysts in the scope of solar energy
conversion that allows unidirectional ET from a light-harvesting photosensitizer
to the catalytically active site. Thus, the photophysics and the photochemistry
of the bimetallic complex <b>RuCo</b>, [(bpy)<sub>2</sub>Ru<sup>II</sup>(tpphz)ÂCo<sup>III</sup>(bpy)<sub>2</sub>]<sup>5+</sup>, where
excitation of the rutheniumÂ(II) moiety leads to an ET to the cobaltÂ(III),
were investigated by quantum chemical and quantum dynamical methods.
Time-dependent density functional theory (TDDFT) allowed us to determine
the bright singlet excitations as well as to identify the triplet
states involved in the photoexcited relaxation cascades associated
with charge-separation (CS) and charge-recombination (CR) processes.
Diabatic potential energy surfaces were constructed for selected pairs
of donor–acceptor states leading to CS and CR along linear
interpolated Cartesian coordinates to study the intramolecular ET
via Marcus theory, a semiempirical expression neglecting an explicit
description of the potential couplings and quantum dynamics (QD).
Both Marcus theory and QD predict very similar rate constants of 1.55
× 10<sup>12</sup> – 2.24 × 10<sup>13</sup> s<sup>–1</sup> and 1.21 × 10<sup>13</sup>–7.59 ×
10<sup>13</sup> s<sup>–1</sup> for CS processes, respectively.
ET rates obtained by the semiempirical expression are underestimated
by several orders of magnitude; thus, an explicit consideration of
electronic coupling is essential to describe intramolecular ET processes
in <b>RuCo</b>
Fate of Photoexcited Molecular Antennae - Intermolecular Energy Transfer versus Photodegradation Assessed by Quantum Dynamics
The
present computational study aims to unravel the competitive
photoinduced intermolecular energy transfer and electron transfer
phenomena in a light-harvesting antenna with potential applications
in dye-sensitized solar cells and photocatalysis. A series of three
thiazole dyes with hierarchically overlapping emission and absorption
spectra, embedded in a methacrylate-based polymer backbone, is employed
to absorb light over the entire visible region. Intermolecular energy
transfer in such antenna proceeds via energy transfer from dye-to-dye
and eventually to a photosensitizer. Initially, the ground and excited
state properties of the three push–pull-chromophores (e.g.,
with respect to their absorption and emission spectra as well as their
equilibrium structures) are thoroughly evaluated using state-of-the-art
multiconfigurational methods and computationally less demanding DFT
and TDDFT simulations. Subsequently, the potential energy landscape
for the three dyads, formed by the π-stacked dyes as occurring
in the polymer environment, is investigated along linear-interpolated
internal coordinates to elucidate the photoinduced dynamics associated
with intermolecular energy and electron transfer processes. While
energy transfer among the dyes is highly desired in such antenna,
electron transfer, or rather a light-induced redox chemistry, leading
to the degradation of the chromophores, is disadvantageous. We performed
quantum dynamical wavepacket calculations to investigate the excited
state dynamics following initial light-excitation. Our calculations
reveal for the two dyads with adjusted optical properties exclusively
efficient intermolecular energy transfer within 200 fs, while in the
case of the third dyad intermolecular electron transfer dynamics can
be observed. Thus, this computational study reveals that statistical
copolymerization of the individual dyes is disadvantageous with respect
to the energy transfer efficiency as well as regarding the photostability
of such antenna