71 research outputs found
INDEX
Perylene diimides and related compounds
(naphthalene diimides,
anthracene diimides, etc.) are one of the most important classes of
organic dyes. Therefore, the prediction and the rationalization of
both their transition energies and the particular shape of their absorption
and emission spectra is essential to improve their design. Here, we
report the simulations of both adiabatic and vibronic signatures of
a series of perylene diimide derivatives with a state-of-the-art time-dependent
density functional theory (TD-DFT) approach. First, the 0â0
energies have been computed and compared to experimental data. In
a second stage, the determination of vibronic shapes has been performed
to shed light on the vibrational modes implied in the experimental
band topologies. Both anharmonicity and functionnal effects are also
discussed. It turns out that theory consistently reproduced 0â0
energies but does not always yield band shapes in perfect match with
experiment. In a last stage, new structures are designed, and it is
shown that a full push effect is more effective than a pushâpull
strategy for the present class of molecules
Diogen Laertije - Ćœivoti i miĆĄljenja istaknutih filozofa
The
vast majority of polyhedral assemblies prepared by combining
organic bent ligands and âphotophysically innocentâ
palladiumÂ(II) metal ions are nonemissive. We report here a simple
strategy to switch on the luminescence properties of a polyhedral
assembly by combining a thermally activated delayed fluorescence (TADF)
organic emitter based on a dipyridylcarbazole ligand scaffold with
Pd<sup>2+</sup> ions, giving rise to a luminescent Pd<sub>6</sub>L<sub>12</sub> molecular cube. The assembly is capable of encapsulating
within its cavity up to three molecules per cage of fluorescein, in
its neutral lactone form, and up to two molecules of Rose Bengal in
its dianionic quinoidal form. Photoinduced electron transfer (PeT)
between the photoactive cage and the encapsulated Fluorescein and
photoinduced energy transfer (PET) from the cage to encapsulated Rose
Bengal have been observed by steady-state and time-resolved emission
spectroscopy
Excited-State Dipole and Quadrupole Moments: TD-DFT versus CC2
The accuracies of
the excited-state dipole and quadrupole moments
obtained by TD-DFT are assessed by considering 16 different exchange-correlation
functionals and more than 30 medium and large molecules. Except for
excited-state presenting a significant charge-transfer character,
a relatively limited dependency on the nature of the functional is
found. It also turns out that while DFT ground-state dipole moments
tend to be too large, the reverse trend is obtained for their excited-state
counterparts, at least when hybrid functionals are used. Consequently,
the TD-DFT excess dipole moments are often too small, an error that
can be fortuitously corrected for charge-transfer transition by selecting
a pure or a hybrid functional containing a small share of exact exchange.
This error-cancelation phenomena explains the contradictory conclusions
obtained in previous investigations. Overall, the largest correlation
between CC2 and TD-DFT excess dipoles is obtained with M06-2X, but
at the price of a nearly systematic underestimation of this property
by ca. 1 D. For the excess quadrupole moments, the average errors
are of the order of 0.2â0.6 D·Ă
for the set of small
aromatic systems treated
What is the Key for Accurate Absorption and Emission Calculations, Energy or Geometry?
Using a hierarchy
of wave function methods, namely ADC(2), CC2,
CCSD, CCSDR(3), and CC3, we investigate the absorption and emission
energies in a set of 24 organic compounds. For all molecules, reference
values are determined at the CC3//CC3 or CCSDR(3)//CCSDR(3) levels
and the energetic and geometric effects are decomposed considering
all possible methodological combinations between the five considered
methods. For absorption, it is found that the errors are mainly energy-driven
for ADC(2), CC2, and CCSDR(3), but not for CCSD. There is also an
error compensation between the errors made on the geometries and transition
energies for the two former approaches. For emission, the total errors
are significantly larger than for absorption due to the significant
increase of the structural component of the error. Therefore, the
selection of a very refined method to compute the fluorescence energy
will not systematically provide high accuracy if the excited-state
geometry is not also optimized at a suitable level of theory. This
is further demonstrated using results obtained from TD-DFT and hybrid
TD-DFT/wave function protocols. We also found that, compared to full
CC3, only CCSDR(3) is able to deliver errors below the 0.1 eV threshold,
a statement holding for both absorption (mean absolute error of 0.033
eV) and emission (mean absolute error of 0.066 eV)
Structural and Optical Properties of Subporphyrinoids: A TD-DFT Study
Using <i>ab initio</i> approaches accounting for environmental effects,
we investigate the ground- and excited-state properties of four subporphyrinoids:
subporphyrin, subporphyrazine, tribenzosubporphyrin, and subphthalocyanine.
We first show that the selected level of theory, that is DFTÂ(PBE0),
is able to reproduce the structure and NMR spectra of all compounds.
The aromaticity of these four macrocyclic entities are next quantified
and it is showed that these bowl-shape induced molecules present very
strong aromatic characters. Next we analyze the spectral signatures
of all four compounds using an approach going beyond the vertical
approximation. The 0â0 energies are reproduced with a mean
absolute deviation smaller than 0.1 eV, and the very good agreement
obtained between experimental and theoretical band shapes allows us
to unravel the vibronic contributions responsible to the specific
band shapes of these subporphyrinoids. Finally, we investigate a large
series of substituted subporphyrins, demonstrate the quality of the
trends that are obtained with theory and design new compounds presenting
red-shifted optical bands
Tuning the Spectroscopic Properties of Ratiometric Fluorescent Metal Indicators: Experimental and Computational Studies on Mag-furaâ2 and Analogues
In
this joint theoretical and experimental work, we investigate
the properties of Mag-fura-2 and seven structurally related fluorescent
sensors designed for the ratiometric detection of Mg<sup>2+</sup> cations.
The synthesis of three new compounds is described, and the absorption
and emission spectra of all of the sensors in both their free and
metal-bound forms are reported. A time-dependent density functional
theory approach accounting for hydration effects using a hybrid implicit/explicit
model is employed to calculate the absorption and fluorescence emission
wavelengths, study the origins of the hypsochromic shift caused by
metal binding for all of the sensors in this family, and investigate
the auxochromic effects of various modifications of the âfuraâ
core. The metal-free forms of the sensors are shown to undergo a strong
intramolecular charge transfer upon light absorption, which is largely
suppressed by metal complexation, resulting in predominantly locally
excited states upon excitation of the metal complexes. Our computational
protocol might aid in the design of new generations of fluorescent
sensors with low-energy excitation and enhanced properties for ratiometric
imaging of metal cations in biological samples
Accurate Excited-State Geometries: A CASPT2 and Coupled-Cluster Reference Database for Small Molecules
We
present an investigation of the excited-state structural parameters
determined for a large set of small compounds with the dual goals
of defining reference values for further works and assessing the quality
of the geometries obtained with relatively cheap computational approaches.
In the first stage, we compare the excited-state geometries obtained
with ADC(2), CC2, CCSD, CCSDR(3), CC3, and CASPT2 and large atomic
basis sets. It is found that CASPT2 and CC3 results are generally
in very good agreement with one another (typical differences of ca.
3 Ă 10<sup>â3</sup> Ă
) when all electrons are correlated
and when the aug-cc-pVTZ atomic basis set is employed with both methods.
In a second stage, a statistical analysis reveals that, on the one
hand, the excited-state (ES) bond lengths are much more sensitive
to the selected level of theory than their ground-state (GS) counterparts
and, on the other hand, that CCSDR(3) is probably the most cost-effective
method delivering accurate structures. Indeed, CCSD tends to provide
too compact multiple bond lengths on an almost systematic basis, whereas
both CC2 and ADC(2) tend to exaggerate these bond distances, with
more erratic error patterns, especially for the latter method. The
deviations are particularly marked for the polarized CO and CN bonds,
as well as for the puckering angle in formaldehyde homologues. In
the last part of this contribution, we provide a series of CCSDR(3)
GS and ES geometries of medium-sized molecules to be used as references
in further investigations
Grafting Spiropyran Molecular Switches on TiO<sub>2</sub>: A First-Principles Study
To
explore the optoelectronic properties of spiropyran molecular
switches adsorbed onto TiO<sub>2</sub> anatase surfaces, we performed
a density functional theory (DFT)/time-dependent density functional
theory (TD-DFT) study considering the two isomeric forms of the photochromes
anchored by both their sides. A comparison between the features of
the hybrid and isolated systems is proposed to probe the adsorption
effects on both subsystems. This comparison considered, on the one
hand, the density of states and the alignment of the energy levels,
and, on the other hand, the UVâvisible spectra of these systems.
We show that several electronic and optical characteristics of the
hybrid systems are modulated by the open/closed state of the photochromes.
These properties are also modified by the localization of the anchor
group on the photochrome
How Adsorption Onto TiO<sub>2</sub> Modifies the Properties of Multiswitchable DTE Systems: Theoretical Insights
In
order to best employ multiphotochromes as complex molecular
gates, each isomer should ideally have a distinct optical profile
to be selectively addressable. In this ab initio DFT and TD-DFT study, we have modeled the electronic
and optical properties of a series of dithienylethene (DTE) dimers
grafted onto an anatase (101) surface. We seek to investigate how
grafting onto a TiO<sub>2</sub> surface modifies the energy levels
and UVâvisible spectra of the dimers and enhances the asymmetry
of the isomers. By extracting information from the density of states,
we have qualified the distinct degrees of interaction between the
substrate and each isomeric configuration as CO > CC > OC >
OO in
order of decreasing electronic coupling. We subsequently use this
information to interpret the UVâvis spectra computed for the
isomers. The results show that the grafted systems present new peaks
and shifted <i>S</i><sub>1</sub> energies compared with
the isolated photochrome, suggesting that adsorption onto a TiO<sub>2</sub> surface may induce an asymmetric character in the DTE dyad
Spectral Signatures of Perylene Diimide Derivatives: Insights From Theory
Perylene diimides and related compounds
(naphthalene diimides,
anthracene diimides, etc.) are one of the most important classes of
organic dyes. Therefore, the prediction and the rationalization of
both their transition energies and the particular shape of their absorption
and emission spectra is essential to improve their design. Here, we
report the simulations of both adiabatic and vibronic signatures of
a series of perylene diimide derivatives with a state-of-the-art time-dependent
density functional theory (TD-DFT) approach. First, the 0â0
energies have been computed and compared to experimental data. In
a second stage, the determination of vibronic shapes has been performed
to shed light on the vibrational modes implied in the experimental
band topologies. Both anharmonicity and functionnal effects are also
discussed. It turns out that theory consistently reproduced 0â0
energies but does not always yield band shapes in perfect match with
experiment. In a last stage, new structures are designed, and it is
shown that a full push effect is more effective than a pushâpull
strategy for the present class of molecules
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