Aminoalkyl-substituted heptamethine cyanine dyes are characterized by a large Stokes shift, an uncommon feature for cyanine molecules yet very promising for their application as fluorescent probes in ...bioimaging and as light harvesting antennas in biohybrid systems for solar energy conversion. The origin of this photophysical feature has not been adequately explored so far, and a combined experimental and theoretical work is herein provided to shed light on the role played by the central aminoalkyl substituent bound to the heptamethine cyanine backbone in defining the unusual properties of the dye. The spectra recorded in solvents of different polarities point to a marginal role of the medium in the definition of the Stokes shift, which conversely can be ascribed to the relaxation of the molecular geometry upon photoexcitation. This hypothesis is supported by an extensive theoretical investigation of the ground and excited states of the dye. TD-DFT results on the aminoalkyl-substituted dye and its unsubstituted precursor demonstrate a very similar cyanine-like structure for both molecules in the relaxed excited state. Conversely, in the ground state the amino substitution disrupts the conjugation in the polymethine chain, leading to a broken-symmetry, non-planar structure.
Aminoalkyl substituted heptamethine cyanines show a large Stokes shift, ascribed to the geometry relaxation upon photoexcitation that drives the molecule from a distorted ground state structure to a planar and cyanine-like excited state.
This tutorial provides a comprehensive description of the origin of chiroptical properties of supramolecular and plasmonic assemblies in the UV-visible region of the electromagnetic spectrum. The ...photophysical concepts essential for understanding chiroptical signatures are presented in the first section. Just as the oscillator strength (a positive quantity) is related to absorption, the rotational strength (either a positive or a negative quantity) defines the emergence of chiroptical signatures in molecular/plasmonic systems. In supramolecular systems, induced circular dichroism (ICD) originates through the off-resonance coupling of transition dipoles in chiral inclusion complexes, while exciton coupled circular dichroism (ECD) originates through the on-resonance exciton coupling of transition dipoles in chiral assemblies resulting in the formation of a bisignated CD signal. In bisignated ECD spectra, the sign of the couplet is determined not only by the handedness of chiral supramolecular assemblies, but also by the sign of the interaction energy between transition dipoles. Plasmonic chirality is briefly addressed in the last section, focusing on inherent chirality, induced chirality, and surface plasmon-coupled circular dichroism (SP-CD). The oscillator strength is of the order of 1 in molecular systems, while it becomes very large (10
4
-10
5
) in plasmonic systems due to the collective plasmonic excitations, resulting in intense CD signals, which can be exploited for the design of plasmonic metamaterial platforms for chiral sensing applications.
This tutorial provides a comprehensive description of the origin of chiroptical properties of supramolecular and plasmonic assemblies in the UV-visible region of the electromagnetic spectrum.
To investigate interchromophore interactions in azobenzene polymers, we have undertaken a thorough spectroscopic analysis of the azodye (S)‐3‐pivaloyloxy‐1‐(4′‐nitro‐4‐azobenzene)pyrrolidine by ...modeling the repeating unit of poly(S)‐3‐methacryloyloxy‐1‐(4′‐nitro‐4‐azobenzene)pyrrolidine) and its dimeric derivative whose synthesis is presented here. The analysis of the electronic and Raman spectra of the azodye in several solvents is based on a previously proposed model for polar chromophores in solution. Electronic and CD spectra of the dimeric unit are collected and analyzed within the framework of a new model. On the basis of the information collected from the spectroscopic analysis of the solvated dye, this model accounts for interchromophore interactions in the dimer. The large CD signal measured for the dimer (amounting to about a third of the signal measured for the polymer) suggests the presence of important chiral interactions in the dimeric unit, and is modeled in terms of a right‐handed relative orientation of the two chromophores.
To understand interchromophore interactions in azopolymers we have synthesized the dimeric unit in the picture and undertaken a thorough analysis of its spectroscopic behavior. The strong CD signal observed for the dimer in solution demonstrates the presence of important chiral interactions in the dimer. These results are interpreted on the basis of a new theoretical bottom‐up approach that describes the spectroscopic properties of interacting chromophores, based on information collected for the solvated isolated chromophores.
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We present a joint theoretical and experimental work aimed to understand the spectroscopic behavior of multipolar dyes of interest for nonlinear optics (NLO) applications. In particular, we focus on ...the occurrence of broken-symmetry states in quadrupolar organic dyes and their spectroscopic consequences. To gain a unified description, we have developed a model based on a few-state description of the charge-transfer processes characterizing the low-energy physics of these systems. The model takes into account the coupling between electrons and slow degrees of freedom, namely, molecular vibrations and polar solvation coordinates. We predict the occurrence of symmetry breaking in either the ground or first excited state. In this respect, quadrupolar chromophores are classified in three different classes, with distinctively different spectroscopic behavior. Cases of true and false symmetry breaking are discriminated and discussed by making resort to nonadiabatic calculations. The theoretical model is applied to three representative quadrupolar chromophores: their qualitatively different solvatochromic properties are connected to the presence or absence of broken-symmetry states and related to two-photon absorption (TPA) cross-sections. The proposed approach provides useful guidelines for the synthesis of dyes for TPA application and represents a general and unifying reference frame to understand energy-transfer processes in multipolar molecular systems, offering important clues to understand basic properties of materials of interest for NLO and energy-harvesting applications.
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Materials for molecular electronics and magnetism Biscarini, Fabio; Coronado, Eugenio; Painelli, Anna ...
Journal of materials chemistry. C, Materials for optical and electronic devices,
09/2021, Volume:
9, Issue:
33
Journal Article
Peer reviewed
Functional molecular materials exhibiting, in particular, electrical, magnetic or optical properties have been an active focus of research over the last 50 years. These materials have been a source ...of inspiration in two interrelated areas of research, namely, molecular electronics and molecular magnetism, with important implications in biomedical areas. This themed issue highlights recent progress and perspectives in these important areas of materials science. The issue covers topics from the chemical design and manipulation of novel molecular materials with unprecedented control over their properties, even at the single-molecule level, to exciting applications of these special molecular systems in emerging fields, including bioelectronics and molecular spintronics.
Fabio Biscarini, Eugenio Coronado, Anna Painelli and Masahiro Yamashita introduce the
Journal of Materials Chemistry C
themed issue on Materials for Molecular Electronics and Magnetism, in honour of Jaume Veciana and Concepció Rovira.
The photophysics of a structurally unique aza‐analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture was studied in detail. The combined ...experimental and computational investigation reveals that the lowest excited state has charge‐transfer character, in spite of the absence of any peripheral electron‐withdrawing groups. The exceptionally electron‐rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, which is qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states that are close in energy but have different polarization directions.
The emission solvatochromism of a centrosymmetric double helical π‐expanded pyrrolo3,2‐bpyrrole is related to symmetry breaking of the first excited state. Strong emission from the charge‐transfer state is interpreted as borrowing intensity from the close‐lying localized transitions, thus explaining the high and stable fluorescence quantum yield.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this contribution, we adopt an essential-state description for octupolar (AD3 or DA3) chromophores (where A is an electron-acceptor and D is an electron-donor) that also accounts for the coupling ...of electrons to molecular vibrations and for solvation effects. The first excited state of octupolar chromophores is always multistable and can therefore support symmetry breaking. In particular, symmetry is always broken in the relaxed excited state of octupolar dyes in polar solvents, with consequent localization of the excitation on one of the dipolar molecular branches. This rationalizes the common observation of strongly solvatochromic fluorescence spectra for octupolar chromophores. The model is validated through the comparison with experimental data. The essential-state model is also adopted to derive a perturbative expression for the electroabsorption spectrum: if compared with the formalism derived for dipolar molecules, a new term appears for octupolar chromophores, due to the field activation of an otherwise dark transition. The importance and implications of this term are discussed.
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We present the synthesis and spectroscopic characterization of a twisted push–pull biphenyl molecule undergoing photoinduced electron transfer. Steady‐state and transient absorption spectra suggest, ...in this rigid molecular structure, a subtle interplay between locally‐excited and charge‐transfer states, whose equilibrium and dynamics is only driven by solvation. A theoretical model is presented for the solvation dynamics and, with the support of quantum chemical calculations, we demonstrate the existence of two sets of states, having either local or charge‐transfer character, that only “communicate” thanks to solvation, which is the sole driving force for the charge‐separation process.
With the help of solvation: The photoinduced, solvent‐driven intramolecular electron transfer is followed in real time, demonstrating the existence of two mutually‐orthogonal sets of locally‐excited and charge‐transfer states.
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Upon decreasing temperature (T sub(c) ~ 81 K) tetrathiafulvalene-chloranil (TTF-CA) undergoes a discontinuous phase transition from a neutral regular stack to an ionic dimerized stack. The same ...system may undergo a phase transition upon photoexcitation. We discuss the early time (< 3 ps) dynamics following ultrafast (< 20 fs) excitation of TTF-CA crystals either near or far from the neutral-ionic transition (T ~ 90 and 260 K, respectively). Adopting a modified Hubbard model with linear coupling of electrons to a zone-center lattice (dimerization) phonon and to a molecular vibration, both assumed to be harmonic, we are able to quantitatively reproduce coherent oscillations recently reported for this system H. Uemura and H. Okamoto, Phys. Rev. Lett. 105, 258302 (2010) (http://dx.doi.org/10.1103/PhysRevLett.105.258302); T. Miyamoto, H. Uemura, and H. Okamoto, J. Phys. Soc. Jpn. 81, 073703 (2012) (http://dx.doi.org/10.1143/JPSJ.81.073703). We demonstrate that the microscopic model with parameters optimized for ground state properties of TTF-CA also applies to the coherent dynamics following photoexcitation. The modulation of the frequency of coherent molecular oscillations at the frequency of the lattice phonon emerges naturally as due to strong anharmonic coupling between the two harmonic vibrational degrees of freedom through coupling to delocalized electrons. Detailed comparison with experiment validates the model and sheds light on other aspects of photoinduced dynamics, including the almost instantaneous transfer of several electrons upon absorption of a single photon, as due to a cooperative effect of electronic correlations and molecular vibrations.
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The physics of aggregates of polar and polarizable donor-acceptor dyes is discussed, extending a previous model to account for the coupling of electronic and vibrational degrees of freedom. Fully ...exploiting translational symmetry, exact absorption and fluorescence spectra are calculated for aggregates with up to 6 molecules. A two-step procedure is presented: in the first step, a mean-field solution of the problem is proposed to define the excitonic basis
via
a rotation of the electronic basis. The rotation is also accompanied by a Lang-Firsov transformation of the vibrational basis. In the second step, the aggregate Hamiltonian, written on the exciton basis, is diagonalized towards exact results. The procedure leads to a reduction of the dimension of the problem, since, at least for weak coupling, only states with up to 3 excitons are needed to obtain reliable results. More interestingly, the mean-field solution represents the proper reference state to discuss excitonic and ultraexcitonic effects. The emerging picture demonstrates that the exciton model offers a reliable description of aggregates of polar and polarizable dyes in the weak coupling regime, while ultraexcitonic effects are important in the medium-strong coupling regimes, and particularly so for J-aggregates where ultraexcitonic effects show up most clearly with multistability and multiexciton generation.
Optical spectra of aggregates of polar dyes are discussed, fully accounting for polarizability of the dyes and for the coupling of electronic and vibrational degrees of freedom.
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