Large Oscillator Strength Research Articles

Optical Properties and Charge-Transfer Excitations in Edge-Functionalized All-Graphene Nanojunctions.

We investigate the optical properties of edge-functionalized graphene nanosystems, focusing on the formation of junctions and charge-transfer excitons. We consider a class of graphene structures that combine the main electronic features of graphene with the wide tunability of large polycyclic aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we show that, upon convenient choice of functional groups, low-energy excitations with remarkable charge-transfer character and large oscillator strength are obtained. These properties can be further modulated through an appropriate width variation, thus spanning a wide range in the low-energy region of the UV-vis spectra. Our results are relevant in view of designing all-graphene optoelectronic nanodevices, which take advantage of the versatility of molecular functionalization, together with the stability and the electronic properties of graphene nanostructures.

Photophysical properties and vibrational structure of ladder-type penta p-phenylene and carbazole derivatives based on SAC-CI calculations

The π-conjugated ladder-type molecules constitute an attractive field of organic photoactive materials. In this work, the photophysical properties of ladder-type penta-p-phenylene (LPP) and carbazole derivatives (bisindenocarbazole and diindolocarbazole) have been investigated theoretically using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The equilibrium geometries in the ground (S 0) and first excited (S 1) states were calculated to be planar, and the excitation is delocalized over the molecules. SAC-CI/DZP calculations have been applied to the absorption and emission spectra of these molecules. The absorption spectra were well reproduced in both peak positions and the shape of the absorption bands. The strong absorption is attributed to the highest occupied molecular orbital to the lowest unoccupied molecular orbital (H–L) transition; however, in carbazoles, the H–1→L transition is located below the H–L transition. The vibrational structure in the S 0–S 1 absorption band of LPP was analyzed by calculating the Franck–Condon (FC) factors based on the potential energy surfaces (PESs) along the normal coordinates that are relevant to the geometry change. The vibrational structure was well reproduced by the theoretical simulation. The C–C stretching mode dominantly contributes to the vibrational structure, while the breathing motion of the molecular frame does not influence the structure. The emission energies calculated by the SAC-CI method also agree well with the experimental values. The vibrational structure in the fluorescence band was also examined by the FC analysis; the theoretical spectrum is satisfactory for the two carbazoles, while the 0–0 transition is overestimated in LPP. In diindolocarbazole, the S 2 state has a large oscillator strength, while the S 1 state has a small oscillator strength.

Optical properties of wide‐bandgap monolithic pillar microcavities with different geometries

AbstractA customization of the optical properties of pillar microcavities on the desired applications is essential for their future use as quantum‐optical devices. Therefore, all‐epitaxial cavities with CdSe or InGaN quantum dots embedded in pillar structures with different geometries have been realized by focused‐ion‐beam etching. The high temperature stability of the emission and the large oscillator strength of quantum dots made of these material systems are combined with small mode volume pillar microcavities to utilize the light‐matter interaction. The quality factors of circularly shaped pillar microcavities have been measured and compared to theoretical calculations. As a possibility to achieve polarized light emission, asymmetrically shaped microcavities are presented. Examples of an elliptically shaped pillar as well as of photonic molecules are investigated with respect to their photoluminescence characteristics and polarization. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study of the low-lying electronic states of CCCF radical and its ions

The low-lying electronic states of the CCCF radical and its ionic states have been investigated systematically using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with aug-cc-pVTZ basis set. To investigated the Renner–Teller effect on the CCCF radical, Cs symmetry was used for CCCF. The CCCF has been found to have a X2A′ ground state with rotational constant B¯=4500.5MHz, which is in good agreement with the experimental values of 4555.8043MHz. The calculations of vertical excitation energies of CCCF at 2.893 and 4.180eV are attribute to the X2A′→32A″ and X2A′→52A′, respectively, which has larger oscillator strengths. The ionization potentials of CCCF are computed in order to provide a theoretical guidance to the photoelectron spectrum (PES) of the CCCF radical. The first adiabatic electron affinity (AEA) of CCCF is predicted to be 2.671eV. A comparison of the geometries and bonding among the CCCX (X=F, Cl, and Br) radicals presents the ground state X2A′ of CCCX can be described as allenic structures with the unpaired electron on the C3 atom, while the excited state 12A″ (12Π) of CCCX have the linear acetylenic structures with the unpaired electron on the C1 atom. The barriers to linearity decrease as follows: ΔE (CCCF)<ΔE (CCCCl)<ΔE (CCCBr).

Modeling the Vibrational Dynamics and Nonlinear Infrared Spectra of Coupled Amide I and II Modes in Peptides

The amide vibrational modes play an important role in energy transport and relaxation in polypeptides and proteins and provide us with spectral markers for structure and structural dynamics of these macromolecules. Here, we present a detailed model to describe the dynamic properties of the amide I and amide II modes and the resulting linear and nonlinear spectra. These two modes have large oscillator strengths, and their mutual coupling plays an important role in their relaxation. Using first-principles calculations of NMA-d(7) and a dipeptide in a fluctuating bath described by molecular dynamics simulations, we model the frequencies of the local vibrations as well as the coupling between them. Both the coherent couplings and the fluctuations induced by contact with their environment are taken into account. We apply the resulting model of interacting fluctuating oscillators to study the collective vibrations and the partially coherent transport of vibrational energy through a model α-helix. We find that the instantaneous vibrations are delocalized over a few (up to four) amide units, while the coherences in the helix survive for 0.5-1 ps, leading to coherent transport on a similar time scale.

Enhancing Long-Range Exciton Guiding in Molecular Nanowires by H-Aggregation Lifetime Engineering

Excitonic transitions in organic semiconductors are associated with large oscillator strength that limits the excited-state lifetime and can in turn impede long-range exciton migration. We present perylene-based emissive H-aggregate nanowires where the lowest energy state is only weakly coupled to the ground state, thus dramatically enhancing lifetime. Exciton migration occurs by thermally activated hopping, leading to luminescence quenching on topological wire defects. An atomic force microscope tip can introduce local topological quenchers by distorting the H-aggregate structure, demonstrating long-range exciton migration at room temperature and offering a potential route to writing fluorescent "nanobarcodes" and excitonic circuits.

Large quantum dots with small oscillator strength

We have measured the oscillator strength and quantum efficiency of excitons confined in large InGaAs quantum dots by recording the spontaneous emission decay rate while systematically varying the distance between the quantum dots and a semiconductor-air interface. The size of the quantum dots is measured by in-plane transmission electron microscopy and we find average in-plane diameters of 40 nm. We have calculated the oscillator strength of excitons of that size assuming a quantum-dot confinement given by a parabolic in-plane potential and a hard-wall vertical potential and predict a very large oscillator strength due to Coulomb effects. This is in stark contrast to the measured oscillator strength, which turns out to be so small that it can be described by excitons in the strong confinement regime. We attribute these findings to exciton localization in local potential minima arising from alloy intermixing inside the quantum dots.

Vibronic structure of the 3 s Rydberg state of the 2-methylallyl radical

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled 2-methylallyl (C 4H 7) radicals provides vibronic spectra of the 3 s and 3 p Rydberg states. Analysis of the vibronic structure following one-photon and two-photon excitation of rovibronically cold 2-methylallyl radicals and its isotopologues C 4H 4D 3 and C 4D 7 reveals transitions to more than 30 vibrational levels in the 3 s Rydberg state that are identified and reassigned on the basis of predictions from ab initio calculations and results from pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained with resonant multiphoton excitation via selected intermediate states. Depletion spectroscopy reveals transitions to short-lived 3 p Rydberg states that have a large oscillator strength.

Insights into the ultraviolet spectrum of liquid water from model calculations

With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.

GAS-PHASE ELECTRONIC SPECTRA OF POLYACETYLENE CATIONS: RELEVANCE OF HIGHER EXCITED STATES

Transitions to higher electronic states of polyacetylene cations (HC{sub 2n}H{sup +}, n = 4, 5, 6) have been measured in the gas phase at {approx}20 K. The absorption spectra were obtained using a resonant two-color, two-photon fragmentation technique in an ion trap, allowing a direct comparison between laboratory and astrophysical data. The purpose was to investigate the relevance of such transitions to astronomical observations because the general expectation is that the bands could be too broad due to fast intramolecular processes. It is shown that the origin bands are still narrow enough (1-10 cm{sup -1}) to be considered, especially as the higher-lying transitions often possess large oscillator strengths.

Molecular Modelling of Ground- and Excited-States Vibrations in Organic Conducting Devices: Hexakis(n-hexyloxy)triphenylene (HAT6) as Case Study

In order to gain insight into fundamental aspects of organic photocell materials, we have calculated ground and excited electronic-state structures and molecular vibrations for an isolated HAT6 molecule (hexakis(n-hexyloxy)triphenylene). Excited-state calculations are carried out using time-dependent density functional theory and frequencies are evaluated analytically using coupled perturbed Kohn–Sham equations. These model calculations have been validated against new infrared and ultraviolet data on HAT6 in solution. The main allowed valence excitation, having the largest oscillator strength, is chosen for the structural and vibrational investigations. Comparison with the ground-state vibrational dynamics reveals surprisingly large spectral differences. In addition, the alkoxy tails, which are usually considered to play only a structural role, are clearly involved in the molecular vibrations and the structural distortion of the excited electronic state compared with the ground state. The tails may play a more important role in charge separation, transport and excited-state relaxation than was previously thought. In this case, chemical modification of the tails would allow vibrational and related properties of organic photocell materials to be tailored.

Electronic Structures and Optical Properties of Indolocarbazole Isomers

Density functional theory(DFT) and the configuration interaction with single excitations(CIS) method were used to optimize the ground state and excited state structures of five indolocarbazole molecules using the 6-31G(d,p) basis set.Based on their geometric structures,the absorption and emission spectra were calculated using time-dependent DFT(TD-DFT) with the same basis set and employing the polarizable continuum medium model(PCM).There are obvious differences in the emission spectra of these isomers as isomer 5 has larger oscillator strength in its emission spectrum,but its transition energies are the lowest among the isomers.The emission peak value of isomer 4 is the highest and the oscillator strengths of isomer 2 are the weakest from 250 to 450 nm.This is because the structures change from ground state to the excited state and the molecular orbital(MO) energy levels of these molecules are different.We also evaluated the nonlinear optical response(first hyperpolarizability) of this series of molecules.The calculated polarizabilities are similar but the static first hyperpolarizabilities(β0) are different.The β0 of isomer 2 is the largest among the compounds investigated.

Strong exciton-photon coupling in inorganic-organic multiple quantum wells embedded low-Q microcavity

Optoelectronic-compatible heterostructures are fabricated from layered inorganic-organic multiple quantum wells (IO-MQW) of Cyclohexenyl ethyl ammonium lead iodide, (C(6)H(9)C(2)H(4)NH(3))(2)PbI(4) (CHPI). These hybrids possess strongly-resonant optical features, are thermally stable and compatible with hybrid photonics assembly. Room-temperature strong-coupling is observed when these hybrids are straightforwardly embedded in metal-air (M-A) and metal-metal (M-M) low-Q microcavities, due to the large oscillator strength of these IO-MQWs. The strength of the Rabi splitting is 130 meV for M-A and 160 meV for M-M cavities. These values are significantly higher than for J-aggregates in all-metal microcavities of similar length. These experimental results are in good agreement with transfer matrix simulations based on resonant excitons. Incorporating exciton-switching hybrids allows active control of the strong-coupling parameters by temperature, suggesting new device applications.

Symmetry breaking in cationic polymethine dyes, part 1: Ground state potential energy surfaces and solvent effects on electronic spectra of streptocyanines

AbstractCharge localization and dynamics in conjugated organic molecules, as well as their spectral signatures are of great importance for photonic and photovoltaic applications. Intramolecular charge delocalization in polymethine dyes occurs through π‐conjugated bridges and contributes to the appearance of low‐energy excited states that strongly influence their linear and nonlinear optical (NLO) properties. When the chain length in symmetrical cations exceeds the characteristic size of the soliton, the positive charge may localize at one of the terminal groups of the molecule and induce symmetry breaking of both the electron density distribution and molecular geometry. This charge localization is coupled with molecular vibrations and solvent effects. We investigated the mechanism of symmetry breaking in a series of cationic streptocyanines with different conjugated chain length and qualitatively predicted their electronic absorption spectra. This class of organic molecules is chosen as a model system to develop methodology which can subsequently be used to evaluate more complicated compounds for NLO applications. Our calculations show that the minimum number of vinylene groups in the conjugated chain necessary to break the symmetry of streptocyanines is eight in the gas phase and six in cyclohexane. We constructed the ground state potential energy surface (PES) in two dimensions using symmetry breaking and symmetry adapted coordinates. These were defined as the difference and the sum of the two central carbon–carbon bonds, respectively. This PES was found to have two equivalent minima for systems with symmetry breaking. The energy barrier between these two minima was estimated in the gas phase and in solution, which was taken into account by the polarizable continuum model. Charge localization in each minimum was found to be asymmetric. It is additionally stabilized by the solvent reaction field, which increases the energy barrier. The electronic absorption spectrum in solution is red shifted as compared to the gas phase. As the symmetry breaks, additional excited states with large oscillator strengths appear in the electronic spectrum. Geometry optimization and spectral predictions were also performed in a uniform external electric field in order to simulate nonequilibrium solvation effects due to the finite relaxation time of solvent molecules. Two asymmetric minima with different depths appear on the resulting PES. The lower minimum has charge localized at one of the two molecular terminal groups which is additionally stabilized by the solvent field, while the higher one has charge localized on another terminal group. This finding demonstrates the possibility that two forms with different charge distributions coexist in polar solvents. Our results suggest that nonequilibrium solvation may be a cause of absorption band broadening and splitting. This work is a first step in a larger study aimed at the analysis of the linear and nonlinear properties of long π‐conjugated systems of interest for NLO applications and plastic photovoltaics. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Selection rules for optical transitions in PbSe nanocrystal quantum dots: Drastic effect of structure inversion asymmetry

We resolve a paradox according to which optical transitions seemingly forbidden in PbSe nanocrystal quantum dots by the parity selection rule exhibit large oscillator strengths in optical absorption. The transitions become allowed if the center of a spherical nanocrystal is shifted away from an atomic site leading to the structure inversion asymmetry. This asymmetry is accounted for within a four-band envelope-function formalism. Transition strengths for various optical transitions are calculated.

Selection Rules for Optical Transitions in PbSe Nanocrystal Quantum Dots: Drastic Effect of Structure Inversion Asymmetry

We resolve a paradox according to which optical transitions seemingly forbidden in PbSe nanocrystal quantum dots by the parity selection rule exhibit large oscillator strengths in optical absorption. The transitions become allowed if the center of a spherical nanocrystal is shifted away from an atomic site leading to the structure inversion asymmetry. This asymmetry is accounted for within a four-band envelope-function formalism. Transition strengths for various optical transitions are calculated.

On the controversial nature of the 1 B1u and 2 B1u states of trans-stilbene: The n-electron valence state perturbation theory approach

The nature of two lowest-energy states of B(u) symmetry of trans-stilbene and the accurate calculation of their vertical excitation energy have been the subject of a controversy because time dependent density functional theory (TD-DFT) calculations, in agreement with experimental observations, have questioned the results obtained with multireference perturbation theory (MRPT) in the CASPT2 implementation. This paper aims to solve this controversy. By using a different version of MRPT, the n-electron valence state perturbation theory method, the description provided by TD-DFT is confirmed: the lowest B(u) singlet state has a HOMO-->LUMO nature and a large oscillator strength (HOMO refers to highest occupied molecular orbital and LUMO refers to lowest unoccupied molecular orbital), while the second B(u) singlet state has a mixed HOMO-1-->LUMO and HOMO-->LUMO+1 nature, has an oscillator strength almost vanishing, and is located at 0.6-0.7 eV higher than the first excited state. The computed vertical excitation energy to the first state (3.8-4.0 eV) is in good agreement with the experimental value.

Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature

Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2λ thick ZnO cavity and 9 pair SiO2/HfO2 DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.

Properties and prospects of blue–green emitting II–VI‐based monolithic microcavities

AbstractIn this paper we review basic optical properties of II–VI‐based monolithic microcavities with quantum wells or quantum dots embedded. The epitaxial growth of high‐reflectivity distributed Bragg reflectors for the blue–green spectral range is discussed since they are a key component for micropillar cavities. The pillar structured samples reveal three‐dimensionally confined optical modes with high quality factors and small mode volumes making them appropriate to investigate cavity quantum electrodynamic effects. The II–VI material system is very promising for the utilization of this effect for technological applications due to the large exciton oscillator strength and carrier confinement. The theoretical methods needed to describe light–matter interaction in such systems is reviewed. For the quantum well based cavities single‐mode stimulated emission at near room temperature is realized. Furthermore, the relatively large cavity polariton stability makes it possible to observe polariton condensation in this kind of samples. For the quantum dot based microcavities a strong enhancement of the emission intensity for quantum dots being in spectral resonance with the cavity modes is observed. To realize the necessary spectral matching a target fine tuning can be achieved by temperature variation or by post fabrication focused ion beam milling. A pronounced enhancement of the spontaneous emission rate of quantum dots coupled to the discrete optical modes of the cavities is demonstrated by the occurrence of the Purcell effect. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strong exciton‐photon coupling and its polarization dependence in a metal‐mirror microcavity with oriented PIC J‐aggregates

AbstractWe present a study of strong exciton‐photon coupling and its dependence on incident light polarization in a metal‐metal mirror microcavity containing PIC J‐aggregates. Rabi‐splitting energies between upper and lower polariton branches are estimated as 94 meV and 69 meV for s‐ and p‐polarized incident light, respectively. These large values are due to large oscillator strength of Frenkel excitons in the PIC J‐aggregates and strong confinement of light attributed to the metallic microcavity as well. As for the effective thickness of the active layer for s‐polarized light, a good agreement is obtained between Lfiteff = 201 nm deduced from the experimental data and Lcalceff = 207 nm calculated from the summation of the measured thickness of active layer with the estimated penetration depths into silver mirrors. We also discuss the difference in the polarization dependences of Rabi‐splitting energy, quantitatively. It is concluded that the polarization dependence is mainly due to an alignment of the J‐aggregates in the active layer and is not affected so much by anisotropy of the penetration depths into the silver mirrors. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)