Charge-transfer Exciton States Research Articles

Dynamics of accelerated excimer formation of coumarin dyes anchored on semiconductor films

We report the ultrafast dynamics of coumarin (7-hydroxycoumarin-3-carboxylic acid, OHCCA) anchored on ZrO2 and TiO2 films. Time-resolved fluorescence measurements reveal that OHCCA undergoes noticeable intramolecular structural relaxations on the film surface, yielding a large vibronic displacement on fluorescence spectra. OHCCA forms an excimer on the ZrO2 and TiO2 films with time constants of 940 and 370 fs for ZrO2, and TiO2 films, respectively, showing further red-shifted charge transfer (CT) emission in the TiO2 film. These suggest an enhanced electronic coupling between OHCCA and metal oxides and that stronger chemisorption in the TiO2 film can significantly hamper an interfacial charge injection via competitive formation of CT exciton state.

Mechanism of Ultrafast Triplet Exciton Formation in Single Cocrystals of π-Stacked Electron Donors and Acceptors.

Ultrafast triplet formation in donor-acceptor (D-A) systems typically occurs by spin-orbit charge-transfer intersystem crossing (SOCT-ISC), which requires a significant orbital angular momentum change and is thus usually observed when the adjacent π systems of D and A are orthogonal; however, the results presented here show that subnanosecond triplet formation occurs in a series of D-A cocrystals that form one-dimensional cofacial π stacks. Using ultrafast transient absorption microscopy, photoexcitation of D-A single cocrystals, where D is coronene (Cor) or pyrene (Pyr) and A is N,N-bis(3'-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (C5PDI) or naphthalene-1,4:5,8-tetracarboxydianhydride (NDA), results in formation of the charge transfer (CT) excitons Cor•+-C5PDI•-, Pyr•+-C5PDI•-, Cor•+-NDA•-, and Pyr•+-NDA•- in <300 fs, while triplet exciton formation occurs in τ = 125, 106, 484, and 958 ps, respectively. TDDFT calculations show that the SOCT-ISC rates correlate with charge delocalization in the CT exciton state. In addition, time-resolved EPR spectroscopy shows that Cor•+-C5PDI•- and Pyr•+-C5PDI•- recombine to form localized 3*C5PDI excitons with zero-field splittings of |D| = 1170 and 1250 MHz, respectively. In contrast, Cor•+-NDA•- and Pyr•+-NDA•- give triplet excitons in which |D| is only 1240 and 690 MHz, respectively, compared to that of NDA (2091 MHz), which is the lowest energy localized triplet exciton, indicating that the Cor-NDA and Pyr-NDA triplet excitons have significant CT character. These results show that charge delocalization in CT excitons impacts both ultrafast triplet formation as well as the CT character of the resultant triplet states.

Theory of the dark state of polyenes and carotenoids

A theory is developed to describe the singlet dark state (usually labeled S1 or 2Ag) of polyenes and carotenoids. The theory assumes that in principle this state is a linear combination of a singlet triplet-pair and an odd-parity charge-transfer exciton. Crucially, these components only couple when the triplet-pair occupies neighboring dimers, such that an electron transfer between the triplets creates a nearest-neighbor charge-transfer excitation. This local coupling stabilises the 2Ag state and induces a nearest neighbor attraction between the triplets. In addition, because of the electron-hole attraction in the exciton, the increased probability that the electron-hole pair occupies neighboring dimers enhances the triplet-triplet attraction: the triplet pair is `slaved' to the charge-transfer exciton. The theory also predicts that as the Coulomb interaction is increased, the 2Ag state evolves from a predominately odd-parity charge-transfer exciton state with a small component of triplet-pair character to a state predominately composed of a triplet-pair with some exciton character. Above a critical Coulomb interaction there is a decoupling of the triplet-pair and charge-transfer exciton subspaces, such that the 2Ag state becomes entirely composed of an unbound spin-correlated triplet pair. The predictions of this theory are qualitatively consistent with high-level density matrix renormalization group calculations of the Pariser-Parr-Pople (or extended Hubbard) model.

Two-photon excited fluorescence from ground-state charge-transfer cocrystals of acridine and 1,2,4,5-tetracyano-benzene

Near-infrared two-photon absorption (TPA) molecules designed based on intramolecular charge-transfer (CT) interactions have been widely investigated during the last few decades; however, studies on supramolecular TPA materials based on intermolecular CT interactions are quite rare. In this work, cocrystals of acridine and 1,2,4,5-tetracyano-benzene were successfully prepared by self-assembly. The cocrystals show yellow emission centered at ∼540 nm with a solid-state photoluminescence quantum yields (PLQY) of 22.48%. Two-photon excited fluorescence (TPEF) measurements show that the cocrystals display wide-window TPEF that is absent in their individual components. Optical absorption analysis indicate that the fluorescence is originated from excitation from the ground states to CT exciton states instead of exciton states of donor or acceptor molecules. This work may facilitate further two-component nonlinear optical material design and applications.

Ultrafast Charge Transfer and Relaxation at a Donor-Acceptor Interface.

The efficiency of charge separation in organic photovoltaic materials is crucially determined by the underlying dynamics of the charge transfer (CT) excitons and their dissociation into free electrons and holes. To unravel the main principles of the underlying mechanism on a molecular level, we construct a toy model of electronically coupled donors interacting with a manifold of CT exciton states. In particular, we set up a ladder of CT site energies to model the exciton dissociation. To mimic the complexity of the exciton dynamics at the donor-acceptor interface, the electronic CT manifold is designed to include two vibrational modes that are vibronically coupled to the excitons. We examine the impact of the electronic and vibrational coherences and the structure of the vibronic manifold on the transfer efficiency and charge recombination. Optimal configurations of the vibronic CT manifold are revealed. In particular, the rate of charge recombination can be minimized when the transient dynamics are carefully explored. Such a toy model can be used as a guide for the design of organic materials for efficient photovoltaic devices.

Full color emission of all-bromide inorganic perovskite nanocrystals

Inorganic perovskites have attracted intensive research attention. The perovskites ABX3 (X = Cl, Br, and I) can be used for blue, green, and red emission by adjusting the halogen X. However, it is very hard to avoid halogen exchanging when different halide perovskites are used together for optoelectronic devices. We synthesized the all-bromide inorganic perovskite nanocrystals with full color emission. The blue emission with a center wavelength of 405 nm originates from the exciton recombination through the charge-transfer exciton state in perovskite Cs4PbBr6. The green emission with a center wavelength of 520 nm results from band edge-exciton recombination in perovskite CsPbBr3. The red emission with a center wavelength of 606 nm comes from the trapped- and self-trapped-exciton recombination in perovskite CsPb2Br5. Our experimental results show that all-bromide inorganic perovskite nanocrystals have great potential for full color light emitting devices.

Interfacial Molecular Packing Determines Exciton Dynamics in Molecular Heterostructures: The Case of Pentacene-Perfluoropentacene.

The great majority of electronic and optoelectronic devices depend on interfaces between p-type and n-type semiconductors. Finding matching donor-acceptor systems in molecular semiconductors remains a challenging endeavor because structurally compatible molecules may not necessarily be suitable with respect to their optical and electronic properties, and the large exciton binding energy in these materials may favor bound electron-hole pairs rather than free carriers or charge transfer at an interface. Regardless, interfacial charge-transfer exciton states are commonly considered as an intermediate step to achieve exciton dissociation. The formation efficiency and decay dynamics of such states will strongly depend on the molecular makeup of the interface, especially the relative alignment of donor and acceptor molecules. Structurally well-defined pentacene-perfluoropentacene heterostructures of different molecular orientations are virtually ideal model systems to study the interrelation between molecular packing motifs at the interface and their electronic properties. Comparing the emission dynamics of the heterosystems and the corresponding unitary films enables accurate assignment of every observable emission signal in the heterosystems. These heterosystems feature two characteristic interface-specific luminescence channels at around 1.4 and 1.5 eV that are not observed in the unitary samples. Their emission strength strongly depends on the molecular alignment of the respective donor and acceptor molecules, emphasizing the importance of structural control for device construction.

Changes in optical properties of conjugated molecules and polymers upon adsorption onto graphene

ABSTRACTWe compared the changes in optical transmission spectrum induced by self-assembly on CVD graphene of (i) a model squarylium dye, namely n-butyl-squarylium dye (1,3-Bis[4-(dibutylamino)phenyl]-2,4-squaraine) and (ii) a conjugated polymer, poly(3-hexyl thiophene). We analyse the results based on Scanning Tunnelling Microscope images. The self-assembly of the zwitterionic dye is controlled by electrostatic interactions. A strong red-shift is observed and is explained in terms of electrostatic interactions with graphene. The self-assembly of polyalkylthiophenes presents a large red shift due to a strong alignment and planarization of the conjugated π-electron system and the formation of a new molecule-graphene charge-transfer exciton state.

Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice

We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.

Efficient and ultrafast formation of long-lived charge-transfer exciton state in atomically thin cadmium selenide/cadmium telluride type-II heteronanosheets.

Colloidal cadmium chalcogenide nanosheets with atomically precise thickness of a few atomic layers and size of 10-100 nm are two-dimensional (2D) quantum well materials with strong and precise quantum confinement in the thickness direction. Despite their many advantageous properties, excitons in these and other 2D metal chalcogenide materials are short-lived due to large radiative and nonradiative recombination rates, hindering their applications as light harvesting and charge separation/transport materials for solar energy conversion. We showed that these problems could be overcome in type-II CdSe/CdTe core/crown heteronanosheets (with CdTe crown laterally extending on the CdSe nanosheet core). Photoluminesence excitation measurement revealed that nearly all excitons generated in the CdSe and CdTe domains localized to the CdSe/CdTe interface to form long-lived charge transfer excitons (with electrons in the CdSe domain and hole in the CdTe domain). By ultrafast transient absorption spectroscopy, we showed that the efficient exciton localization efficiency could be attributed to ultrafast exciton localization (0.64 ± 0.07 ps), which was facilitated by large in-plane exciton mobility in these 2D materials and competed effectively with exiton trapping at the CdSe or CdTe domains. The spatial separation of electrons and holes across the CdSe/CdTe heterojunction effectively suppressed radiative and nonradiative recombination processes, leading to a long-lived charge transfer exciton state with a half-life of ∼ 41.7 ± 2.5 ns, ∼ 30 times longer than core-only CdSe nanosheets.

Theory of optical excitation spectra and depolarization dynamics in bilayerWS2from the viewpoint of excimers

We investigate the optical excitation spectra and the photoluminescence depolarization dynamics in bilayer WS$_2$. A different understanding of the optical excitation spectra in the recent photoluminescence experimentby Zhu {\em et al.} [arXiv:1403.6224] in bilayer WS$_2$ is proposed. In the experiment, four excitations (1.68, 1.93, 1.99 and 2.37 eV) are observed and identified to be indirect exciton for the $\Gamma$ valley, trion, A exciton and B exciton excitations, respectively, with the redshift for the A exciton energy measured to be 30$\sim$50 meV when the sample synthesized from monolayer to bilayer. According to our study, by considering there exist both the intra-layer and charge-transfer excitons in the bilayer WS$_2$, with inter-layer hopping of the hole, there exists excimer state composed by the superposition of the intra-layer and charge-transfer exciton states. Accordingly, we show that the four optical excitations in the bilayer WS$_2$ are the A charge-transfer exciton, ${\rm A}'$ excimer, ${\rm B}'$ excimer and B intra-layer exciton states, respectively, with the calculated resonance energies showing good agreement with the experiment. In our picture, the speculated indirect exciton, which involves a high-order phonon absorption/emission process, is not necessary. Furthermore, the binding energy for the excimer state is calculated to be 40 meV, providing reasonable explanation for the experimentally observed energy redshift of the A exciton. Based on the excimer states, we further derive the exchange interaction Hamiltonian. Then the photoluminescence depolarization dynamics due to the electron-hole exchange interaction is studied in the pump-probe setup by the kinetic spin Bloch equations. We find that ......

Charge-Transfer Excitons at Organic Semiconductor Surfaces and Interfaces

When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface.

Insights into Excitons Confined to Nanoscale Systems: Electron–Hole Interaction, Binding Energy, and Photodissociation

The characteristics of nanoscale excitons--the primary excited states of nanoscale systems like conjugated polymers, molecular aggregates, carbon nanotubes, and nanocrystalline quantum dots--are examined through exploration of model systems. On the basis of a valence bond-type model, an intuition is developed for understanding and comparing nanoscale systems. In particular, electron-hole interactions are examined in detail, showing how and why they affect spectroscopy and properties such as binding energy. The relationship between the bound exciton states and the nanoscale analogue of free carriers (charge-transfer exciton states) is developed. It is shown why the electron and hole act as independent particles in this manifold of states. The outlook for the field is discussed on the basis of the picture developed in the paper, with an emphasis on exciton binding and photodissociation.

Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators

We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.

Femtosecond dynamics of excited states ofCOadsorbed onMgO(001)‐(1×1)

We have investigated the time development of excited electronic states of $\mathrm{C}\mathrm{O}$ molecules adsorbed on the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface using a state-of-the-art first principles approach. Density-functional theory is used to calculate the ground state geometry of the clean surface and of the molecules adsorbed on the surface. Thereafter, the quasiparticle band structures of bulk MgO, of the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface, and of $\mathrm{C}\mathrm{O}$ adsorbed on the surface are calculated within the GW approximation. Taking the electron-hole interaction into consideration the electron-hole excitations and their optical spectrum are obtained from the solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function. The optical spectra of bulk MgO, the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface, and $\mathrm{C}\mathrm{O}$ adsorbed on the surface are calculated, yielding good agreement with available experimental data. Finally, based on the solution of the BSE for the adsorbate system $\mathrm{C}\mathrm{O}:\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$, the time propagation of molecular excitations is studied employing the time-dependent Schr\odinger equation. An initial $\mathrm{C}\mathrm{O}$ excitation exhibits a very fast decay due to its coupling to charge-transfer exciton states between the substrate and the adsorbate. The decay is characterized by a lifetime of about 1.6 fs, which is a factor of 5 faster than the decay of single-particle states.

Effects of compositional phase transitions on luminescence of Sr 1− xCa xTiO 3:Pr 3+

The mixed-compound of Sr 1− x Ca x TiO 3 has shown several compositional phase transformations. Photoluminescence and excitation spectra of the samples with different x and doped with 0.2% Pr 3+ were investigated. Changes in the emission spectra were observed in different phases. The blue emission at 491 nm from 3P 0 state was found quite strong in the tetragonal phase, and was thermally quenched in the orthorhombic phases. The intensity of the red luminescence from 1D 2 increases with increasing content of calcium. The strongest red emission is obtained from CaTiO 3:Pr 3+. The results are discussed based on the configuration coordinate model and interaction of Pr with the charge transfer exciton state of the Ti complex.

Interchain delocalization of an exciton in conjugated polymers

Optical excitations in conjugated polymers are considered primarily as intrachain one-dimensional excitons, while interchain coupling may be also important in the dynamical processes of excited states. Two aspects of interchain effects on excitons are discussed theoretically. The first one is the Frenkel-exciton type excitation transfer due to the Coulomb coupling. We examine how the excitation transfer rate depends on the chain length, the interchain distance, and the state. The other aspect is charge transfer due to the direct overlap of atomic orbitals. We demonstrate that the one-dimensional nature of the lowest exciton is not much altered by the interchain coupling. On the other hand, charge-transfer exciton states are located fairly close to the lowest intrachain exciton, so that they may be important in the dynamical processes of excited states, although they do not have large oscillator strength in the absorption spectrum.

Picosecond-spectroscopy of excited states based on nonlinear electron photoemission

A new general method was developed suitable for measuring the lifetime of excited states in the condensed phase. This method is based on a multistep photoemission process in which the decay of an excited state generated by one pulse is measured by the use of a delayed probe pulse. The effective lifetime of a charge-transfer exciton state in tetracene was measured to be [approximately]20 ps. 16 refs., 1 fig.

Charge transfer excitons in halogen-bridged mixed valence complexes

The charge transfer (CT) excitons in [Pt(en) 2][Pt(en) 2X 2](ClO 4) 4, (X=Br and Cl), single crystals have been studied by the electro-reflectance (ER) spectroscopy. By the Kramers-Kronig transformation, the electric field modulation spectra of imaginary part of the dielectric constant were obtained, in which a new absorption band is observed in the higher energy side of the optically allowed CT exciton. Assuming that the modulation mechanism is due to a mixing of parity-allowed and forbidden CT exciton states, modulation efficiency of those two excitons were calculated. It was found that there is a good consistence between the experimental and calculated results in both crystals.

Soliton-like charge-transfer exciton states in quasi-one-dimensional organic crystals

For charge-transfer excitons strongly interacting with vibrations of a donor-acceptor chain, soliton-like states are found to have an internal structure: at a certain ratio of polaronic shift and electron-hole relative motion energy, a transition from a symmetric to a dimerized soliton occurs accompanied by a crucial change of the lattice distortion pattern, soliton length, etc. This transition corresponds to spontaneous symmetry breaking within an extended state. The polarons on coupled chains are briefly discussed.