16 research outputs found
Hole-transfer induced energy transfer in perylene diimide dyads with a donorâspacerâacceptor motif
We investigate the photoinduced dynamics of perylene diimide dyads based on a donorâspacerâacceptor motif with polyyne spacers of varying length by pumpâprobe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donorâspacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donorâspacer and the donorâspacerâacceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipoleâmultipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices
Circumcision-related tragedies seen in children at the Komfo Anokye Teaching Hospital, Kumasi, Ghana
Quantitative InâSitu NMR Illumination for Excitation and Kinetic Analysis of Molecular Motor Intermediates
Modern synthetic methods can be highly complex and proceed via multiple sequential processes. The intricate concepts can rarely be checked in all necessary detail due to the lack of suitable analysis methods. For photoinitiated reactions the starting point can be chosen to any desired precision by short or ultrafast light pulses. However, the optical analysis is blind to many aspects. Pairing the illumination of the reaction mixture with other online detection schemes proves extremely valuable. Herein we present a combination of variable temperature NMR spectroscopy with in-situ illumination and photon-counting to monitor the short-lived intermediates and gain a comprehensive and quantitative picture of the working of a hemithioindigo-based molecular motor. Application of the scheme is not limited to the field of molecular machines, we rather think that it will be of great use for any detailed identification and characterization of chemical intermediates in photochemistry, photoredox catalysis, photoswitching, or photoresponsive materials
Hole-transfer induced energy transfer in perylene diimide dyads with a donorâspacerâacceptor motif
We investigate the photoinduced dynamics of perylene diimide dyads based on a donorâspacerâacceptor motif with polyyne spacers of varying length by pumpâprobe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donorâspacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donorâspacer and the donorâspacerâacceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipoleâmultipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices
Excited-state dynamics of a molecular dyad with two orthogonally-oriented fluorophores
The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin-orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state
Untargeted Metabolomics Reveals Molecular Effects of Ketogenic Diet on Healthy and Tumor Xenograft Mouse Models
Complete Mechanism of Hemithioindigo Motor Rotation
Hemithioindigo-based molecular motors
are powered by nondamaging
visible light and provide very fast directional rotations at ambient
conditions. Their ground state energy profile has been probed in detail,
but the crucial excited state processes are completely unknown so
far. In addition, very fast processes in the ground state are also
still elusive to date and thus knowledge of the whole operational
mechanism remains to a large extent in the dark. In this work we elucidate
the complete light-driven rotation mechanism by a combination of multiscale
broadband transient absorption measurements covering a time scale
from fs to ms in conjunction with a high level theoretical description
of the excited state. In addition to a full description of the excited
state dynamics in the various time regimes, we also provide the first
experimental evidence for the elusive fourth intermediate ground state
of the original HTI motor. The fate of this intermediate also is followed
directly proving complete unidirectionality for both 180° rotation
steps. At the same time, we uncover the hitherto unknown involvement
of an unproductive triplet state pathway, which slightly diminishes
the quantum yield of the <i>E</i> to <i>Z</i> photoisomerization.
A rate model analysis shows that increasing the speed of motor rotation
is most effectively done by increasing the photoisomerization quantum
yields instead of barrier reduction for the thermal ratcheting steps.
Our findings are of crucial importance for improved future designs
of any light-driven molecular motor in general to yield better efficiencies
and applicability