237 research outputs found
Solid-State Effects on the Optical Excitation of Push-Pull Molecular J-Aggregates by First-Principles Simulations
J-aggregates are a class of low-dimensional molecular crystals which display
enhanced interaction with light. These systems show interesting optical
properties as an intense and narrow red-shifted absorption peak (J-band) with
respect to the spectrum of the corresponding monomer. The need to theoretically
investigate optical excitations in J-aggregates is twofold: a thorough
first-principles description is still missing and a renewed interest is rising
recently in understanding the nature of the J-band, in particular regarding the
collective mechanisms involved in its formation. In this work, we investigate
the electronic and optical properties of a J-aggregate molecular crystal made
of ordered arrangements of organic push-pull chromophores. By using a time
dependent density functional theory approach, we assess the role of the
molecular packing in the enhancement and red shift of the J-band along with the
effects of confinement in the optical absorption, when moving from bulk to
low-dimensional crystal structures. We simulate the optical absorption of
different configurations (i.e., monomer, dimers, a polymer chain, and a
monolayer sheet) extracted from the bulk crystal. By analyzing the induced
charge density associated with the J-band, we conclude that it is a
longitudinal excitation, delocalized along parallel linear chains and that its
overall red shift results from competing coupling mechanisms, some giving red
shift and others giving blue shift, which derive from both coupling between
transition densities and renormalization of the single-particle energy levels.Comment: This is the published version of the work, distributed under the
terms of the ACS AuthorChoice licence
https://pubs.acs.org/page/policy/authorchoice_termsofuse.htm
Proposed alteration of images of molecular orbitals obtained using a scanning tunnelling microscope as a probe of electron correlation
Scanning tunneling spectroscopy (STS) allows to image single molecules
decoupled from the supporting substrate. The obtained images are routinely
interpreted as the square moduli of molecular orbitals, dressed by the
mean-field electron-electron interaction. Here we demonstrate that the effect
of electron correlation beyond mean field qualitatively alters the uncorrelated
STS images. Our evidence is based on the ab-initio many-body calculation of STS
images of planar molecules with metal centers. We find that many-body
correlations alter significantly the image spectral weight close to the metal
center of the molecules. This change is large enough to be accessed
experimentally, surviving to molecule-substrate interactions.Comment: 27 pages including Supplemental Information. To appear in Physical
Review Letter
A classical picture of subnanometer junctions: an atomistic Drude approach to nanoplasmonics
The description of optical properties of subnanometer junctions is
particularly challenging. Purely classical approaches fail, because the quantum
nature of electrons needs to be considered. Here we report on a novel classical
fully atomistic approach, {\omega}FQ, based on the Drude model for conduction
in metals, classical electrostatics and quantum tunneling. We show that
{\omega}FQ is able to reproduce the plasmonic behavior of complex metal
subnanometer junctions with quantitative fidelity to full ab initio
calculations. Besides the practical potentialities of our approach for large
scale nanoplasmonic simulations, we show that a classical approach, in which
the atomistic discretization of matter is properly accounted for, can
accurately describe the nanoplasmonics phenomena dominated by quantum effects.Comment: This article is licensed under a Creative Commons Attribution 3.0
Unported Licenc
Quantifying the Plasmonic Character of Optical Excitations in a Molecular J-Aggregate
The definition of plasmon at the microscopic scale is far from being
understood. Yet, it is very important to recognize plasmonic features in
optical excitations, as they can inspire new applications and trigger new
discoveries by analogy with the rich phenomenology of metal nanoparticle
plasmons. Recently, the concepts of plasmonicity index and the generalized
plasmonicity index (GPI) have been devised as computational tools to quantify
the plasmonic nature of optical excitations. The question may arise whether any
strong absorption band, possibly with some sort of collective character in its
microscopic origin, shares the status of plasmon. Here we demonstrate that this
is not always the case, by considering a well-known class of systems
represented by J-aggregates molecular crystals, characterized by the intense J
band of absorption. By means of first-principles simulations, based on a
many-body perturbation theory formalism, we investigate the optical properties
of a J-aggregate made of push-pull organic dyes. We show that the effect of
aggregation is to lower the GPI associated with the J-band with respect to the
isolated dye one, which corresponds to a nonplasmonic character of the
electronic excitations. In order to rationalize our finding, we then propose a
simplified one-dimensional theoretical model of the J-aggregate. A useful
microscopic picture of what discriminates a collective molecular crystal
excitation from a plasmon is eventually obtained.Comment: Published by ACS under ACS AuthorChoice licens
Interplay between Intra- and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates
In a first-principles study based on density functional theory and many-body
perturbation theory, we address the interplay between intra- and intermolecular
interactions in a J-aggregate formed by push-pull organic dyes by investigating
its electronic and optical properties. We find that the most intense excitation
dominating the spectral onset of the aggregate, i.e., the J-band, exhibits a
combination of intramolecular charge transfer, coming from the push-pull
character of the constituting dyes, and intermolecular charge transfer, due to
the dense molecular packing. We also show the presence of a pure intermolecular
charge-transfer excitation within the J-band, which is expected to play a
relevant role in the emission properties of the J-aggregate. Our results shed
light on the microscopic character of optical excitations of J-aggregates and
offer new perspectives to further understand the nature of collective
excitations in organic semiconductors.Comment: published under ACS Authorchoice licens
Predicting signatures of anisotropic resonance energy transfer in dye-functionalized nanoparticles
Resonance energy transfer (RET) is an inherently anisotropic process. Even
the simplest, well-known F\"orster theory, based on the transition
dipole-dipole coupling, implicitly incorporates the anisotropic character of
RET. In this theoretical work, we study possible signatures of the fundamental
anisotropic character of RET in hybrid nanomaterials composed of a
semiconductor nanoparticle (NP) decorated with molecular dyes. In particular,
by means of a realistic kinetic model, we show that the analysis of the dye
photoluminescence difference for orthogonal input polarizations reveals the
anisotropic character of the dye-NP RET which arises from the intrinsic
anisotropy of the NP lattice. In a prototypical core/shell wurtzite CdSe/ZnS NP
functionalized with cyanine dyes (Cy3B), this difference is predicted to be as
large as 75\% and it is strongly dependent in amplitude and sign on the dye-NP
distance. We account for all the possible RET processes within the system,
together with competing decay pathways in the separate segments. In addition,
we show that the anisotropic signature of RET is persistent up to a large
number of dyes per NP.Comment: 9 pages, 5 figures. Supplementary information available at
http://pubs.rsc.org/en/content/articlelanding/2016/ra/c6ra22433d/unauth#!divAbstrac
Enhanced light-harvesting of protein-pigment complexes assisted by a quantum dot antenna
We predict the enhanced light harvesting of a protein-pigment complex when
assembled to a quantum dot (QD) antenna. Our prototypical nanoassembly setup is
composed of a Fenna-Mattews-Olson system hosting 8 Bacteriochlorophyll (BChl) a
dyes, and a near-infrared emitting CdSeTe/ZnS alloy-core/shell
nanocrystal. BChl a has two wide windows of poor absorption in the green and
orange-red bands, precisely where most of the sunlight energy lies. The
selected QD is able to collect sunlight efficiently in a broader band and
funnel its energy by a (non-radiative) F\"orster resonance energy transfer
mechanism to the dyes embedded in the protein. By virtue of the coupling
between the QD and the dyes, the nanoassembly absorption is dramatically
improved in the poor absorption window of the BChl a.Comment: 5 pages, 3 figures, presented in the NANOFIM 2018 conference in
Mexico Cit
Visualizing electron correlation by means of ab-initio scanning tunneling spectroscopy images of single molecules
Scanning tunneling microscopy (STM) has been a fundamental tool to
characterize many-body effects in condensed matter systems, from extended
solids to quantum dots. STM of molecules decoupled from the supporting
conductive substrate has the potential to extend STM characterization of many
body effects to the molecular world as well. In this article, we describe a
many-body tunneling theory for molecules decoupled from the STM substrate, and
we report on the use of standard quantum chemical methods to calculate the
quantities necessary to provide the 'correlated' STM molecular image. The
developed approach has been applied to eighteen different molecules, to explore
the effects of their chemical nature and of their substituents, as well as to
verify the possible contribution by transition metal centers. Whereas the bulk
of calculations have been performed with CISD because of the computational
cost, some tests have been also performed with the more accurate CCSD method to
quantify the importance of the computational level on many-body STM images. We
have found that correlation induces a remarkable squeezing of the images, and
that correlated images are not derived from Hartree-Fock HOMO or LUMO alone,
but include contributions from other orbitals as well. Although correlation
effects are too small to be resolved by present STM experiments for the studied
molecules, our results provide hints for seeking out other species with larger,
and possibly experimentally detectable, correlation effects.Comment: Main text + Supplemental materia
Equation of Motion for the Solvent Polarization Apparent Charges in the Polarizable Continuum Model: Application to Time-Dependent CI
The dynamics of the electrons for a molecule in solution is coupled to the
dynamics of its polarizable environment, i.e., the solvent. To theoretically
investigate such electronic dynamics, we have recently developed equations of
motion (EOM) for the apparent solvent polarization charges that generate the
reaction field in the Polarizable Continuum Model (PCM) for solvation and we
have coupled them to a real-time time-dependent density functional theory (RT
TDDFT) description of the solute [Corni et al. J. Phys. Chem. A 119, 5405
(2014)]. Here we present an extension of the EOM-PCM approach to a
Time-Dependent Configuration Interaction (TD CI) description of the solute
dynamics, which is free from the qualitative artifacts of RT TDDFT in the
adiabatic approximation. As tests of the developed approach, we investigate the
solvent Debye relaxation after an electronic excitation of the solute obtained
either by a pulse of light or by assuming the idealized sudden promotion
to the excited state. Moreover, we present EOM for the Onsager solvation model
and we compare the results with PCM. The developed approach provides
qualitatively correct real-time evolutions and is promising as a general tool
to investigate the electron dynamics elicited by external electromagnetic
fields for molecules in solution.Comment: This is the final peer-reviewed manuscript accepted for publication
in The Journal of Chemical Physics. Copyright by AIP, the final published
version can be found at
http://scitation.aip.org/content/aip/journal/jcp/146/6/10.1063/1.497562
- âŠ