Collective Electronic Oscillator Research Articles

Signatures of Through‐Space Charge Transfer in Two‐Photon Absorption of Paracyclophane Derivatives

Third order polarizability, (γ) taken from the collective electronic oscillator (CEO) method was used to calculate the two‐photon absorption (TPA) of tetrastyryl‐[2,2]paracyclophane derivatives with different through‐space charge transfer configurations considering various donor and acceptor combinations at the terminal styryl groups. For the virtually same linear absorption, different TPA spectra were obtained. For controlling and fine‐tuning frequency and cross‐sections of TPA the through‐space charge transfer interactions can be used. The results are explained by the electronic density matrices corresponding to governing oscillators in one‐ and two‐photon absorption and the ground state. It is indicated that for the studied systems mainly the lowest four oscillators are responsible for the TPA cross‐sections rather than a simple effective three‐state model.

Nonadiabatic Excited-State Molecular Dynamics Modeling of Photoinduced Dynamics in Conjugated Molecules

Nonadiabatic dynamics generally defines the entire evolution of electronic excitations in optically active molecular materials. It is commonly associated with a number of fundamental and complex processes such as intraband relaxation, energy transfer, and light harvesting influenced by the spatial evolution of excitations and transformation of photoexcitation energy into electrical energy via charge separation (e.g., charge injection at interfaces). To treat ultrafast excited-state dynamics and exciton/charge transport we have developed a nonadiabatic excited-state molecular dynamics (NA-ESMD) framework incorporating quantum transitions. Our calculations rely on the use of the Collective Electronic Oscillator (CEO) package accounting for many-body effects and actual potential energy surfaces of the excited states combined with Tully's fewest switches algorithm for surface hopping for probing nonadiabatic processes. This method is applied to model the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene, PPV). Our analysis shows intricate details of photoinduced vibronic relaxation and identifies specific slow and fast nuclear motions that are strongly coupled to the electronic degrees of freedom, namely, torsion and bond length alternation, respectively. Nonadiabatic relaxation of the highly excited mA(g) state is predicted to occur on a femtosecond time scale at room temperature and on a picosecond time scale at low temperature.

Linear and third-order nonlinear optical properties for the heptamethine cyanine chromophore H-aggregates thin film

The thin film of a heptamethine cyanine chromophore HC was prepared by spin-coating technique. Its surface morphology and linear optical property were characterized by atomic force microscopy (AFM) and UV-visible absorption spectroscopy. The results show that HC molecules are arranged in a well-ordered H-aggregate type. The third-order nonlinear optical properties of the spin-coating film were also measured by degenerate four-wave mixing (DFWM) measurement. Enhanced third-order nonlinear susceptibility can be attributed to molecular aggregation effects, and the corresponding mechanism was dealt with by collective electronic oscillator (CEO) approach.

Electronic Excitations in a Ladder Type Fluoranthenopyracylene in its Neutral and Charged States: A Theoretical and Experimental Study

This work reports on theoretical and spectroelectrochemical studies of a thermally and chemically stable fluoranthenopyracylene ladder type molecule 7,10,17,20 tetra-p-dodecylphenyl(bisfluorantheno[8,9-a,c]) pyracylene. Optimal geometries were evaluated for neutral and positively/negatively charged states performing density functional theory (DFT) calculations. Absorption spectral line frequencies as well as oscillator strengths were calculated using the time-dependent DFT and collective electronic oscillator (CEO) methods. The calculated optical spectra are in good agreement with the experimental measurements in solution. The difference of charged states produced by reduction and/or oxidation versus the electronic neutral forms on the vertical excitation energies and CEO electronic density matrix distribution has been characterized and compared to the geometrical changes of the molecular fragments. The localization of excitation in the optical transition of the charged states shows that the excitation is less separated and shows larger electronic coherence in comparison to the neutral form.

Density Matrix Analysis, Simulation, and Measurements of Electronic Absorption and Fluorescence Spectra of Spirobifluorenes

The linear absorption and fluorescence spectra as well as the oscillator strengths of 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene (A), 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene (B), and 2,2',7,7'-tetrakis(9,9'-spirobifluoren-2-yl)-9,9'-spirobifluorene (C) are calculated on the basis of the collective electronic oscillator (CEO) approach of Mukamel et al. (see, e.g., Chem. Rev. 2002, 102, 3171). The graphical visualization and quantitative characterization of CEO modes allows one to extract the real-space distribution of electronic excitations of the molecules under study. Effects of the lengthening and branching of the oligophenylene segments have been analyzed. The influence of the lowest excited (S1) vs ground-state (S0) geometry changes on the CEO modes is investigated and related to the geometry changes of the molecular parts. The obtained theoretical results are in good agreement with experimental trends observed in absorption and fluorescence data.

Interchain interactions in charged diacetylenic oligomers carrying bulk substituents revisited

We are studying how the electronic properties of an aggregate, built with conjugated oligomers carrying bulk substituents, are affected by intermolecular interactions. In this paper we apply the CEO (Collective Electronic Oscillator) method, on the basis of the semiempirical INDO/S Hamiltonian, to compute the electronic density matrix modifications following the photon absorption in a doubly charged cluster of two units of a fully carbazolyl-substituted oligodiacetylene tetramer, taken as a model system. The picture that had emerged from our previous calculations based on the less sophisticated CIS (Configuration Interaction including Singles) approach is seen to be confirmed. Despite the large separation between the backbones, a through-space charge transfer occurs between the two oligomers due to the fact that the excess charge, contrary to what is generally believed, is not localized on the conjugated backbone, but is spread out over the carbazolyl moieties of the charged molecule. Consideration of this kind of interaction improves the theoretical results obtained for the isolated charged oligomer chain, and aids in better explaining some features of the experimental photoinduced spectra of the corresponding polymer.

Nonlinear Infrared and Optical Responses of a Holstein−Peirls−Hubbard Dimer

A simple model based on the Holstein-Peierls-Hubbard Hamiltonian has been used to calculate the nonlinear responses at infrared and optical frequencies. The model is applied to a molecular ion radical dimer in order to account for the contribution to the nonlinear responses arising from the intermolecular charge transfer excitations and from their coupling to both intramolecular and intermolecular phonons. A similar calculation has been performed on a model quadrupolar conjugated molecule characterized by intramolecular charge transfer excitations. The calculations are performed according to a collective electronic oscillator scheme by solving the Liouville equation for the bielectronic density matrix. Such a choice allows us to retain the ability, inherent in the Hubbard type models, to fully account for the on-site electron correlation effects. Calculated spectra are reported for one-photon and two-photon absorption (the latter in the form of the imaginary part of the Kerr susceptibility) and for third harmonic generation. Narrow resonances are observed in the infrared, related mostly to the coupling with intramolecular modes. The off-resonant contribution to the nonlinear susceptibility arising from the electron-phonon interactions appears to be marginal (in the order of 1%) in all cases.

Influence of Interchain Interactions on the Electronic Properties of Neutral and Charged Oligodiacetylenes Carrying Bulk Substituents

We discuss the effect of the interchain interactions on the electronic properties of a "dimer", named (CHD(4))(2), built from two molecules of a fully carbazolyl-substituted oligodiacetylene containing four repeating units. Each carbazolyl moiety is connected to its respective backbone through a methylene spacer, and the excitations of the neutral and doubly positively charged dimer are obtained using the CIS (configuration interaction including singles) and CEO (collective electronic oscillator) models. The separation distance between the backbones is fixed to a value that could possibly imply a very weak, if any, interchain interaction between the oligomers. In the charged dimer, where we have not been able to perform CEO calculations, it can be expected from previous results that the CIS method will behave satisfactorily. Contrary to the neutral case and surprisingly enough, the simulated photoinduced absorption (PA) spectra indicate a substituent-induced strong interaction between the oligomers, due to the very large amount of excess charge on the distorted oligomer which is spread over its carbazolyl moieties. Broadening of the lowest energy band due to the latter interaction improves the agreement between the theoretical and the experimental PA spectrum for the corresponding polymer. The through-space charge transfer directly connecting the charged with the neutral molecule could supply an efficient channel for charge transport in substituted polydiacetylenes.

Intermolecular forces and nonbonded interactions: Superoperator nonlinear time-dependent density-functional-theory response approach

Electrostatic and dispersive interactions of polarizable molecules are expressed in terms of generalized (nonretarded) charge-density response functions of the isolated molecules, which in turn are expanded using the collective electronic oscillator (CEO) eigenmodes of linearized time-dependent density-functional theory. Closed expressions for the intermolecular energy are derived to sixth order in charge fluctuation amplitudes.

Two-photon absorption in three-dimensional chromophores based on [2.2]-paracyclophane.

A series of alpha,omega-bis donor substituted oligophenylenevinylene dimers held together by the [2.2]paracyclophane core were synthesized to probe how the number of repeat units and through-space delocalization influence two-photon absorption cross sections. Specifically, the paracyclophane molecules are tetra(4,7,12,15)-(4'-dihexylaminostyryl)[2.2]paracyclophane (3R(D)), tetra(4,7,12,15)-(4' '-(4'-dihexylaminostyryl)styryl)[2.2]paracyclophane (5R(D)), and tetra(4,7,12,15)-(4' "-(4' '-(4'-dihexylaminostyryl)styryl)styryl)[2.2]paracyclophane (7R(D)). The compounds bis(1,4)-(4'-dihexylaminostyryl)benzene (3R) and bis(1,4)-(4' '-(4'-dihexylaminostyryl)styryl)benzene (5R) were also synthesized to reveal the properties of the "monomeric" counterparts. The two-photon absorption cross sections were determined by the two-photon induced fluorescence method using both femtosecond and nanosecond pulsed lasers as excitation sources. While there is a red shift in the linear absorption spectra when going from the "monomer" chromophore to the paracyclophane "dimer" (i.e., 3R --> 3R(D), 5R --> 5R(D)), there is no shift in the two-photon absorption maxima. A theoretical treatment of these trends and the dependence of transition dipole moments on molecular structure rely on calculations that interfaced time-dependent density functional theory (TDDFT) techniques with the collective electronic oscillator (CEO) program. These theoretical and experimental results indicate that intermolecular interactions can strongly affect B(u) states but weakly perturb A(g) states, due to the small dipole-dipole coupling between A(g) states on the chromophores in the dimer.

Theoretical Investigation of the Charge Injection Effects on the Electronic Properties of Substituted Oligodiacetylenes

We discuss the charge-induced modifications in the ground state geometry and in the electronic properties of two types of fully carbazolyl-substituted oligodiacetylenes, named CBDn and CHDn respectively, which are taken as models of the corresponding polymers. Quantum chemical calculations have been performed on the isolated oligomers carrying substituents, either directly linked to the backbone (CBDn) or connected to it through a methylene spacer (CHDn). The collective electronic oscillator (CEO) approach is used to compute the excitations in terms of the one-electron transition density matrixes (CEO-modes). The simulated photoinduced absorption spectra which are obtained show interesting features, and the allowed low-energy transitions compare favorably with experimental results where available. Two-dimensional plots in real space of the CEO-modes allow us to analyze the physical nature of the excitons in terms of the photoinduced electron/hole pairs motion. For charged oligomers, two different types of...

Efficient fluoride-selective fluorescent host: experiment and theory.

A new naphthalene derivative containing a urea group at the 1,8-position of naphthalene was synthesized and showed a unique absorption and fluorescence peak with fluoride ion. Calculations suggested that a new peak was attributed to the increased anionic character of urea nitrogen due to the strongly charged hydrogen bonding between fluoride and amide protons of the urea. The fluoride selectivity among halides (F(-), Cl(-), Br(-)) comes from the fact that the fluoride approaches much closer to the amide protons than other halides and resides in the cavity with fast dynamics. The nature of electronic transitions that were analyzed from the calculations by the collective electronic oscillator method also supports the anionic nature of the complex between host and fluoride.

Semiempirical calculations of the collective optical excitations in substituted oligodiacetylenes

In this paper, we present the results of a theoretical study of the electronic and optical excitations of isolated carbazolyl-substituted oligodiacetylenes using the collective electronic oscillator (CEO) method. Within this technique the excitations are directly connected to the corresponding one-electron transition density matrices, obtained as eigenmodes of the time dependent Hartree–Fock (TDHF) Liouville operator, which represent motions of electron/hole pairs. The two-dimensional plot of the transition density matrix (CEO mode) establishes the physical connection between the electronic spectra and the electronic rearrangement due to the photon absorption. Consequently this representation allows for an intuitive interpretation of the optical spectra in terms of the chemical structure, and gives a useful tool for the design of molecules showing the required physical properties.

Collective Electronic Oscillator Method: Application to Conjugated Organic Molecules

The collective electronic oscillator (CEO) method was developed by Mukamel and collaborators.[Phys. Rev. Lett. 1992, 69, 65; Science 1997, 277, 781] Recently Ⅰ have extended the CEO method to obtain the frequency dependent optical properties with all the contributing components. The brief introduction of the CEO fomalism and its recent applications to linear absorption and two-photon absorption (TPA) of conjugated organic molecules will be discussed. The size scaling of optical properties of polyenes and polyynes have studied by ab initio calculations, and this result is consistent with the coherence length of the time dependent densities to first (<TEX>$ρ^(1)$</TEX>) and second order (<TEX>$ρ^(2)$</TEX>) in the electric field obtained from the CEO method.

Collective optical excitations in one-dimensional porphyrin assemblies

The collective electronic oscillator (CEO) method allows to analyze the electronic excitations in molecular systems. Using this technique the excitations are directly connected to the corresponding one-electron transition density matrices, obtained as eigenmodes of the time dependent Hartree–Fock (TDHF) Liouville operator, which represent motions of electron–hole (e–h) pairs. Two-dimensional plots of the density matrices establish the connection between the electronic spectra and the electronic rearrangement due to the photon absorption. In this paper, we use the CEO procedure to study the nature of the electronic excitations on three series of one-dimensional porphyrin systems, showing through the analysis of the CEO eigenmodes how they can be described in terms of Frenkel or Wannier–Mott excitons.

Time-resolved x-ray Raman spectroscopy of photoexcited polydiacetylene oligomer: A simulation study

Off-resonant x-ray diffraction provides a novel real-space and real-time probe of electronic and vibrational dynamics in optically excited molecules. The entire manifold of valence electronic excitations may be monitored through the dependence of the x-ray Raman peaks on the scattering wave vector Δk and energy Δω. The electronic excitation energies and transition density matrices of a polydiacetylene oligomer, computed using the time-dependent Hartree–Fock collective electronic oscillator algorithm, are used to simulate the Raman signals and illustrate their information content.

Excited state molecular dynamics simulations of nonlinear push–pull chromophores

Julolidinemalononitrile, p-nitroaniline, and julolidinyl- n- N, N ′-diethylthiobarbituric acid are studied with ground and excited state molecular dynamics simulations in conjunction with the collective electronic oscillator formalism and Onsager’s cavity model. Ground and excited state geometries are calculated in the gas phase and four solvents. The results are interpreted in the context of a two-state valence bond model for charge-transfer transitions of conjugated organic molecules, and are compared to recent resonant Raman experimental results. The calculated geometries are qualitatively consistent with both the two-state model and experiment. In addition, calculated transition density matrices are presented to visualize the changes in charge distribution accompanying photoexcitation.

Control of Intrachromophore Excitonic Coherence in Electroluminescent Conjugated Dendrimers

We present coupled electronic oscillator calculations on conjugated dendrimers that can be used in organic light-emitting diodes. The results indicate that the nature of the excited state in these materials can be strongly influenced by electronic coherences. We use the collective electronic oscillator model approach to compare and contrast primary excitations of two families of dendrimers consisting of tris(distyrylbenzenyl) chromophores attached to either nitrogen or benzene in the 1, 3, and 5 positions. The choice of the central moiety of the dendrimer provides a direct control of the coherence between the conjugated segments, which form the emissive region of the hyperbranched molecular structure. For the case of the amine-centered dendrimer, we find that excitations are delocalized across the amine unit, whereas delocalization between distyrylbenzene units is effectively blocked by the use of a benzene core unit. The calculations provide an accurate description of the measured absorption data and an understanding of the fluorescence dynamics observed. In particular, they suggest a microscopic route to controlling the polarization anisotropy of dendrimers through choice of the central moiety. Additionally, we are able to develop a qualitative reasoning for the dependence of the solid-state emission on dendrimer generation using the results from the calculation. The level of intermolecular interactions is found to depend sensitively on the location of emissive dipoles in the dendrimer cores.

Optical and electronic properties of neutral and charged oligodiacetylene clustersPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

The collective electronic oscillator (CEO) approach recently proposed by Mukamel, which is based on an INDO/S Hamiltonian in the framework of the time-dependent Hartree–Fock approximation, is used to investigate the effect of the supramolecular architecture on the photogenerated excitations in different aggregates of a large-sized oligodiacetylene. The method gives the transition energies and the corresponding one-electron transition density matrices directly, which allows one to analyze, in terms of electron–hole motions in real space, the electronic redistribution upon photon absorption. Electronic excitations depend strongly on the arrangement of the oligomer molecules in the cluster, appreciable interchain effects being obtained even when the arrangement of the oligomer molecules corresponds to a minimum interchain interaction. Interchain interactions give rise to new transitions whose oscillator strengths depend on the nature of the aggregate (neutral or charged), and which can significantly modify the optical properties of the isolated molecular system.

Collective electronic oscillator/semiempirical calculations of static nonlinear polarizabilities in conjugated molecules

The collective electronic oscillator (CEO) approach based on the time-dependent Hartree–Fock approximation is combined with INDO/S, MNDO, AM1, and PM3 semiempirical Hamiltonians. This technique is applied to compute and analyze the static nonlinear polarizabilities of a series of donor/acceptor substituted oligomers. To mimic the experimental conditions, polarizabilities in substituted molecules are calculated for the isolated complex and in a dielectric medium, wherein the solvent contributions are incorporated using the self-consistent reaction field approach. The dielectric environment significantly increases second and third order static polarizabilities and considerably improves the agreement with experimental data. We find that calculated spectroscopic observables agree well with experimental values. We conclude that the CEO/semiempirical approach is an inexpensive and numerically efficient method of computing nonlinear molecular properties.