1,720,966 research outputs found
Combining intra- and intermolecular charge-transfer: A new strategy towards molecular ferromagnets and multiferroics
No full textOrganic ferroelectric materials are currently a hot research topic, with mixed stack charge transfer crystals playing a prominent role with their large, electronic-in-origin polarization and the possibility to tune the transition temperature down to the quantum limit and/or to drive the ferroelectric transition via an optical stimulus. By contrast, and in spite of an impressive research effort, organic ferromagnets are rare and characterized by very low transition temperatures. Coexisting magnetic and electric orders in multiferroics offer the possibility to control magnetic (electric) properties by an applied electric (magnetic) field with impressive technological potential. Only few examples of multiferroics are known today, based on inorganics materials. Here we demonstrate that, by decorating mixed stack charge transfer crystals with organic radicals, a new family of robust molecular ferromagnets can be designed, stable up to ambient temperature, and with a clear tendency towards multiferroic behaviour
Dynamical disorder and Resonance Energy Transfer: a novel quantum-classical approach
Full text linkResonance Energy Transfer (RET), at the heart of photosynthesis, supports life on earth, but also guarantees the operation of several technological devices, like organic light-emitting diodes and solar cells. Medium properties and dynamics largely affect RET efficiency, but reliable models addressing how molecular electron-vibration motion and solvent dynamics jointly affect RET are still missing. Here we propose a novel quantum-classical approach to describe RET in a non-adiabatic molecular system embedded in a dynamic polar environment. The approach, validated against optical properties of a dye in solution, is then applied to a RET-pair, demonstrating that dynamic disorder, as induced by a liquid polar solvent, boosts RET efficiency
Shining Light on Inverted Singlet–Triplet Emitters
Full text link: The inversion of the lowest singlet and triplet excited states, observed in several triangle-shaped organic molecules containing conjugated carbon and nitrogen atoms, is an astonishing result that implies the breakdown of Hund's rule. The phenomenon attracted interest for its potential toward triplet harvesting in organic LEDs. On a more fundamental vein, the singlet-triplet (ST) inversion sheds new light on the role of electron correlations in the excited-state landscape of π-conjugated molecules. Relying on the celebrated Pariser-Parr-Pople model, the simplest model for correlated electrons in π-conjugated systems, we demonstrate that the ST inversion does not require triangle-shaped molecules nor any specific molecular symmetry. Indeed, the ST inversion does not require strictly non-overlapping HOMO and LUMO orbitals but rather a small gap and a small exchange integral between the frontier orbitals
Intermolecular Energy Transfer in Real Time
Full text linkResonance energy transfer (RET) is
a complex phenomenon where energy
is transferred between two nonequivalent molecules. In the Förster
picture, that applies to the weak coupling regime, RET occurs from
the energy donor molecule in the relaxed excited state toward the
acceptor, in an energy-conserving process. However, energy dissipation
is crucial for a more general picture of RET that also applies to
the strong coupling regime. Here we present a dynamical, nonadiabatic
model for RET also accounting for energy relaxation. We exploit the
essential state formalism to set up a model for the RET pair that
yields an accurate picture of the relevant physics, accounting for
just a few electronic states and a single coupled vibrational coordinate
per molecule. Molecular vibrations are treated in a nonadiabatic approach,
and energy dissipation is dealt within the Redfield formalism. The
approach is first validated on an isolated dye, demonstrating that
a very simple relaxation model, defined in terms of a single relaxation
parameter, properly describes the different regimes of energy dissipation
expected for a molecule, with a fast (fs time window) internal conversion
to the lowest excited state and a slow relaxation toward the ground
state (ns time window). The same approach is then applied to follow
the real time dynamics of a RET pair. In line with the Förster
model, in the weak coupling regime the internal conversion of the
donor molecule is completed before energy transfer takes place. Our
approach also applies to the strong coupling regime, where we observe
ultrafast energy transfer occurring well before the internal relaxation
of the energy donor is completed
Terahertz-pulse driven modulation of electronic spectra: Modeling electron-phonon coupling in charge-transfer crystals
No full textWe calculate the optical spectra of a charge-transfer crystal modulated by a terahertz pulse, accounting
for electron-vibration coupling. The model Hamiltonian is parametrized against first principle calculations
and adiabatic results are validated against a fully non-adiabatic calculation where relaxation phenomena are
introduced via the coupling of the quantum system to a dissipative bath of classic anharmonic oscillators. The
experiment is well reproduced by the proposed model with no need to introduce any ad hoc assumption on the
temporal dependence of model parameters, but just accounting for the quadratic dependence of the Hubbard U
on non-totally symmetric molecular coordinates
Optical spectra of molecular aggregates and crystals: testing approximation schemes
Full text linkThe interplay between exciton delocalization and molecular vibrations profoundly affects optical spectra of
molecular aggregates and crystals. The exciton motion occurs on a similar timescale as molecular vibrations,
leading to a complex and intrinsically non-adiabatic problem that has been handled over the years introducing
several approximation schemes. Here we discuss systems where intermolecular distances are large enough so
that only electrostatic intermolecular interactions enter into play and can be treated in the dipolar
approximation. Moreover, we only account for interactions between transition dipole moments, as relevant to
symmetric molecules, with negligible permanent (multi)polar moments in the ground and low-lying excited
states. Translational symmetry is fully exploited to obtain numerically exact solutions of the relevant
Hamiltonian for systems of comparatively large size. This offers a unique opportunity to assess the reliability of
different approximation schemes. The so-called Heitler–London approximation, only accounting for the effects
of intermolecular interactions among degenerate electronic states, leads to the celebrated exciton model,
widely adopted to describe optical spectra of molecular aggregates and crystals. We demonstrate that,
mainly due to a cancellation of errors, the exciton model approximates well the position of exciton bands
and reasonably well the bandshapes, but it fails to predict spectral intensities, leading to underestimated
intensities in J-aggregates and overestimated intensities in H-aggregates. This general result is validated
against an exact sum-rule. Finally, we address the validity of several approximation schemes adopted to
reduce the dimension of the vibrational basis
Shedding light on thermally-activated delayed fluorescence
No full textThermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications
in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF
is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet–triplet gaps,
sizable spin–orbit couplings, conformational flexibility and a balanced contribution of charge transfer
and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially
affected by the interaction of the TADF dye with its local environment. The concurrent optimization of
the dye and of the embedding medium is therefore of paramount importance to boost practical
applications of TADF. Towards this aim, refined theoretical and computational approaches must be
cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we
will address some of the most important issues in the field. Specifically, we will critically review
theoretical and computational approaches to TADF rates, highlighting the limits of widespread
approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the
major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices
Vibrational coherences in charge-transfer dyes: A non-adiabatic picture
No full textEssential-state models efficiently describe linear and nonlinear spectral properties of different families of charge-transfer chromophores. Here, the essential-state machinery is applied to the calculation of the early-stage dynamics after ultrafast (coherent) excitation of polar and quadrupolar chromophores. The fully non-adiabatic treatment of coupled electronic and vibrational motion allows for a reliable description of the dynamics of these intriguing systems. In particular, the proposed approach is reliable even when the adiabatic and harmonic approximations do not apply, such as for quadrupolar dyes that show a multistable, broken-symmetry excited state. Our approach quite naturally leads to a clear picture for a dynamical Jahn-Teller effect in these systems. The recovery of symmetry due to dynamical effects is however disrupted in polar solvents where a static symmetry lowering is observed. More generally, thermal disorder in polar solvents is responsible for dephasing phenomena, damping the coherent oscillations with particularly important effects in the case of polar dyes
Antiadiabatic View of Fast Environmental Effects on Optical Spectra
Full text linkAn antiadiabatic approach is proposed to model how the refractive index of the surrounding medium
affects optical spectra of molecular systems in condensed phases. The approach solves some of the issues
affecting current implementations of continuum solvation models and more generally of effective models
where a classical description is adopted for the molecular environment
- …
