5 research outputs found
Unraveling the Mechanism of Photoinduced Charge Transfer in CarotenoidâPorphyrinâC<sub>60</sub> Molecular Triad
Photoinduced
charge transfer (CT) plays a central role in biologically
significant systems and in applications that harvest solar energy.
We investigate the relationship of CT kinetics and conformation in
a molecular triad. The triad, consisting of carotenoid, porphyrin,
and fullerene is structurally flexible and able to acquire significantly
varied conformations under ambient conditions. With an integrated
approach of quantum calculations and molecular dynamics simulations,
we compute the rate of CT at two distinctive conformations. The linearly
extended conformation, in which the donor (carotenoid) and the acceptor
(fullerene) are separated by nearly 50 Ă
, enables charge separation
through a sequential CT process. A representative bent conformation
that is entropically dominant, however, attenuates the CT, although
the donor and the acceptor are spatially closer. Our computed rate
of CT at the linear conformation is in good agreement with measured
values. Our work provides unique fundamental understanding of the
photoinduced CT process in the molecular triad
Calculating High Energy Charge Transfer States Using Optimally Tuned Range-Separated Hybrid Functionals
Recently
developed optimally tuned range-separated hybrid (OT-RHS)
functionals within time-dependent density functional theory have been
shown to address existing limitations in calculating charge transfer
excited state energies. The RSH success in improving the calculation
of CT states stems from enforcing the correspondence of the frontier
molecular orbitals (FMOs) to physical properties, where the highest
occupied MO energy relates to the ionization potential and the lowest
unoccupied MO energy relates to the electron affinity. However, in
this work, we show that a less accurate description of CT states that
involves non-FMOs is afforded by the RSH approach. In order to achieve
a high quality description of such higher energy CT states, the parameter
tuning procedure, which lies at the foundation of the RSH approach,
needs to be generalized to consider the CT process. We demonstrate
the need for improved description of such CT states in donorâacceptor
systems, where the optimal tuning parameter is accounting for the
state itself
Quantitative Prediction of Optical Absorption in Molecular Solids from an Optimally Tuned Screened Range-Separated Hybrid Functional
We show that fundamental
gaps and optical spectra of molecular
solids can be predicted quantitatively and nonempirically within the
framework of time-dependent density functional theory (TDDFT) using
the recently developed optimally tuned screened range-separated hybrid
(OT-SRSH) functional approach. In this scheme, the electronic structure
of the gas-phase molecule is determined by optimal tuning of the range-separation
parameter in a range-separated hybrid functional. Screening and polarization
in the solid state are taken into account by adding long-range dielectric
screening to the functional form, with the modified functional used
to perform self-consistent periodic-boundary calculations for the
crystalline solid. We provide a comprehensive benchmark for the accuracy
of our approach by considering the X23 set of molecular solids and
comparing results obtained from TDDFT with those obtained from many-body
perturbation theory in the GW-BSE approximation. We additionally compare
results obtained from dielectric screening computed within the random-phase
approximation to those obtained from the computationally more efficient
many-body dispersion approach and find that this influences the fundamental
gap but has little effect on the optical spectra. Our approach is
therefore robust and can be used for studies of molecular solids that
are typically beyond the reach of computationally more intensive methods
Photoinduced Homolytic Bond Cleavage of the Central SiâC Bond in Porphyrin Macrocycles Is a Charge Polarization Driven Process
Photoinduced cleavage
of the bond between the central Si atom in
porphyrin macrocycles and the neighboring carbon atom of an axial
alkyl ligand is investigated by both experimental and computational
tools. Photolysis and electron paramagnetic resonance measurements
indicate that the SiâC bond cleavage of Siâphthalocyanine
occurs through a homolytic process. The homolytic process follows
a low-lying electronic excitation of about 1.8 eV that destabilizes
the carbide bond of similar bond dissociation energy. Using electronic
structure calculations, we provide insight into the nature of the
excited state and the resulting photocleavage mechanism. We explain
this process by finding that the electronic excited state is of a
charge transfer character from the axial ligand toward the macrocycle
in the reverse direction of the ground state polarization. We find
that the homolytic process yielding the radical intermediate is energetically
the most stable mechanistic route. Furthermore, we demonstrate using
our computational approach that changing the phthalocyanine to smaller
ring system enhances the homolytic photocleavage of the SiâC
bond by reducing the energetic barrier in the relevant excited states
Molecular Structure, Spectroscopy, and Photoinduced Kinetics in Trinuclear Cyanide Bridged Complex in Solution: A First-Principles Perspective
We
investigate the molecular structure of the solvated complex,
[(NC)<sub>6</sub>FeâPtÂ(NH<sub>3</sub>)<sub>4</sub>âFeÂ(CN)<sub>6</sub>]<sup>4â</sup>, and related dinuclear and mononuclear
model complexes using first-principles calculations. Mixed nuclear
complexes in both solution and crystal phases were widely studied
as models for charge transfer (CT) reactions using advanced spectroscopical
and electrochemical tools. In contrast to earlier interpretations,
we find that the most stable gas phase and solvated geometries are
substantially different from the crystal phase geometry, mainly due
to variance in the underlying oxidation numbers of the metal centers.
Specifically, in the crystal phase a PtÂ(IV) metal center resulting
from Fe â Pt backward electron transfers is stabilized by an
octahedral ligand field, whereas in the solution phase a PtÂ(II) metal
complex that prefers a square planar ligand field forms a CT salt
by bridging to the iron complexes through long-range electrostatic
interactions. The different geometry is shown to be consistent with
spectroscopical data and measured CT rates of the solvated complex.
Interestingly, we find that the experimentally indicated photoinduced
process in the solvated complex is of backward CT (Fe â Pt)