Local Charge-transfer State Research Articles

Room-Temperature Phosphorescence Materials Featuring Triplet Hybrid Local Charge Transfer Emission.

Room-temperature phosphorescence materials have found important applications in dissolved oxygen sensing, temperature monitoring, anticounterfeiting, etc., because of their prolonged phosphorescence lifetime. However, the known systems mainly utilize the triplet local excited state emission, which is generally less sensitive to microenvironment perturbation. In this work, we designed a series of 4-phenyl-1,8-naphthalimide (NMI) derivatives containing different numbers of carbazole (Cz) units (denoted as NMI-Cz, NMI-2Cz, and NMI-3Cz). Steady state and time-resolved spectroscopy studies determined that the compounds undergo intramolecular through-space charge transfer in solution, yielding a triplet hybrid local charge transfer state. Room-temperature phosphorescence emission was observed in compound-doped poly(methyl methacrylate) thin films upon ammonia treatment. Interestingly, emission from different films exhibited different persistence times. We believe a film-based, time-resolved luminescent ammonia sensor could be developed by making a device of the emissive films as fabricated.

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems.

Energy transfer across the donor-acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system.

Efficient near-infrared emission benefits from slowing down the internal conversion process.

Organic deep-red (DR) and near-infrared (NIR) emitters with high photoluminescence quantum yield (PLQY) are rare due to the strong non-radiative (knr) decay. Here, we report two DR/NIR emitters with high PLQY, TPANZPyPI and TPANZ3PI. Interestingly, the TPANZPyPI film exhibits 46.5% PLQY at 699 nm. Theoretical calculations indicate that TPANZPyPI can achieve this high PLQY in the near-infrared emission region due to its small S1 to S0 internal conversion (IC) rate. Meanwhile, research has found that, compared to TPANZ3PI, TPANZPyPI with a more rigid structure can effectively suppress the T2 to T1 IC process, which is conducive to higher exciton utilization efficiency (EUE). TPANZPyPI's non-doped OLED shows NIR emission with 4.6% @ 684 nm maximum external quantum efficiency (EQEmax). Its doped OLEDs radiate DR with an EQEmax of 6.9% @ 666 nm. These EQEs are among the highest values for hybridized local charge transfer state materials emitting more than 640 nm. This work demonstrates for the first time, based on a combination of theory and experiment, that increasing the molecular rigidity can inhibit the excited state IC process in addition to the S1 to S0 IC, realizing efficient electroluminescence.

Revisiting the Charge-Transfer States at Pentacene/C60 Interfaces with the GW/Bethe-Salpeter Equation Approach.

Molecular orientations and interfacial morphologies have critical effects on the electronic states of donor/acceptor interfaces and thus on the performance of organic photovoltaic devices. In this study, we explore the energy levels and charge-transfer states at the organic donor/acceptor interfaces on the basis of the fragment-based GW and Bethe–Salpeter equation approach. The face-on and edge-on orientations of pentacene/C60 bilayer heterojunctions have employed as model systems. GW+Bethe–Salpeter equation calculations were performed for the local interface structures in the face-on and edge-on bilayer heterojunctions, which contain approximately 2000 atoms. Calculated energy levels and charge-transfer state absorption spectra are in reasonable agreements with those obtained from experimental measurements. We found that the dependence of the energy levels on interfacial morphology is predominantly determined by the electrostatic contribution of polarization energy, while the effects of induction contribution in the edge-on interface are similar to those in the face-on. Moreover, the delocalized charge-transfer states contribute to the main absorption peak in the edge-on interface, while the face-on interface features relatively localized charge-transfer states in the main absorption peak. The impact of the interfacial morphologies on the polarization and charge delocalization effects is analyzed in detail.

Tailored Interface Energetics for Efficient Charge Separation in Metal Oxide-Polymer Solar Cells

Hybrid organic-inorganic heterointerfaces in solar cells suffer from inefficient charge separation yet the origin of performance limitations are widely unknown. In this work, we focus on the role of metal oxide-polymer interface energetics in a charge generation process. For this purpose, we present novel benzothiadiazole based thiophene oligomers that tailor the surface energetics of the inorganic acceptor TiO2 systematically. In a simple bilayer structure with the donor polymer poly(3-hexylthiophene) (P3HT), we are able to improve the charge generation process considerably. By means of an electronic characterization of solar cell devices in combination with ultrafast broadband transient absorption spectroscopy, we demonstrate that this remarkable improvement in performance originates from reduced recombination of localized charge transfer states. In this context, fundamental design rules for interlayers are revealed, which assist the charge separation at organic-inorganic interfaces. Beside acting as a physical spacer in between electrons and holes, interlayers should offer (1) a large energy offset to drive exciton dissociation, (2) a push-pull building block to reduce the Coulomb binding energy of charge transfer states and (3) an energy cascade to limit carrier back diffusion towards the interface.

Strategic tuning of excited-state properties of electroluminescent materials with enhanced hot exciton mixing.

Deep blue emitters with excellent stability, high quantum yield and multifunctionality are the major issues for full-color displays. In line with this, new multifunctional, thermally stable blue emitters viz., N-(4-(10-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)anthracen-9-yl)phenyl)-N-phenylbenzenamine (DPIAPPB) and 2-(10-(9H-carbazol-9-yl)anthracen-9-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CADPPI) with hybridized local charge transfer state (HLCT) and hot exciton properties have been synthesized. These molecules show high photoluminescence quantum yield (Φs/f): (DPIAPPB – 0.82/0.70 and CADPPI – 0.91/0.83). The CADPPI based device (EL – 467 nm) shows high efficiencies [ηc – 9.85 cd A−1; ηp – 10.84 lm W−1; ηex – 4.78% at 2.8 V; CIE (0.15, 0.10)] compared to the DPIAPPB device (EL − 472 nm) [ηc – 6.56 cd A−1; ηp – 6.16 lm W−1; ηex – 4.15% at 2.8 V with CIE (0.15, 0.12)]. The green device with CADPPI:Ir(ppy)3 exhibits a maximum L – 59 012 cd m−2; ηex – 16.8%; ηc – 37.3 cd A−1; ηp – 39.8 lm W−1 with CIE (0.30, 0.60) and the red device with CADPPI:Ir(MDQ)2(acac) shows a maximum L – 43 456 cd m−2; ηex – 21.9%; ηc – 36.0 cd A−1; ηp – 39.6 lm W−1 with CIE (0.64, 0.35).

Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells

Within this work, we emphasis on the importance of twisted intramolecular charge transfer (TICT) process in organic dyes based on triphenyl amine moiety to achieve high performance in dye-sensitized solar cells. Through the comparison between two recent made dyes, L1 and L1Fc, on different semiconductors (TiO2, and ZrO2), we could spectrally and dynamically detect for the first time the formation of TICT state for L1 on ZrO2 after localized charge transfer (LCT) state population, and an electron injection process from TICT state on TiO2. However, for the excited L1Fc dye, the ultrafast electron transfer from ferrocene (Fc) moiety to the L1 unit quenched the formation of TICT state in L1Fc on semiconductors, leading instead to an electron injection process from the LCT state. The electron injection from TICT state in L1 associated with structural rearrangements on TiO2 leads to slow recombination process and an efficiency improvement of about 325%, compared to solar cells based on L1Fc dye, in which TICT s...

Coordination-Driven Self-Assembly of Ruthenium Polypyridyl Nodes Resulting in Emergent Photophysical and Electrochemical Properties.

Ruthenium polypyridyl complexes are among the most studied molecular species for photochemical applications such as light-harvesting and photocatalysis, with [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) serving as an iconic example. We report the use of the [Ru(bpy)2]2+ fragment as a 90° acceptor tecton (M) in coordination-driven self-assembly to synthesize a M4L4 metallacycle (L = 4,4'-bipyridine) and a M6L4 truncated tetrahedral cage [L = 2,4,6-tris(4-pyridyl)-1,3,5-triazine]. The M6L4 cage possesses emergent properties attributed to its unique electronic structure, which results in increased visible-light absorption and an emission band that decays biexponentially with times of 3 and 790 ns. The presence of multiple ruthenium centers in the cage results in multiple RuIII/II reduction events, with a cathodic shift of the first reduction relative to that of [Ru(bpy)3]Cl2 (0.56 V vs 1.05 V). The ligand-centered reduction shifts anodically (-1.29 vs -1.64 V) versus the first bpy reduction observed in the parent [Ru(bpy)3]Cl2. The photophysical properties are explained by the existence of two localized charge-transfer states in the cage molecule: one that draws upon the bipyridine π* orbitals and the other upon the 2,4,6-tris(4-pyridyl)-1,3,5-triazine π* orbitals.

Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects.

We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

The lowest-energy charge-transfer state and its role in charge separation in organic photovoltaics.

Energy independent, yet higher than 90% internal quantum efficiency (IQE), has been observed in many organic photovoltaics (OPVs). However, its physical origin remains largely unknown and controversial. The hypothesis that the lowest charge-transfer (CT) state may be weakly bound at the interface has been proposed to rationalize the experimental observations. In this paper, we study the nature of the lowest-energy CT (CT1) state, and show conclusively that the CT1 state is localized in typical OPVs. The electronic couplings in the donor and acceptor are found to determine the localization of the CT1 state. We examine the geminate recombination of the CT1 state and estimate its lifetime from first principles. We identify the vibrational modes that contribute to the geminate recombination. Using material parameters determined from first principles and experiments, we carry out kinetic Monte Carlo simulations to examine the charge separation of the localized CT1 state. We find that the localized CT1 state can indeed yield efficient charge separation with IQE higher than 90%. Dynamic disorder and configuration entropy can provide the energetic and entropy driving force for charge separation. Charge separation efficiency depends more sensitively on the dimension and crystallinity of the acceptor parallel to the interface than that normal to the interface. Reorganization energy is found to be the most important material parameter for charge separation, and lowering the reorganization energy of the donor should be pursued in the materials design.

Resonant photoemission at the O1s threshold to characterize In2O3 single crystals

We report on spectroscopic investigations on In2O3 single crystals. We focus on the detailed analysis of the O1s resonance profile by resonant photoelectron spectroscopy. We analyze the electronic structure and assign the O2p to build the valence band and both, O2p and In5s5p states to contribute to the conduction band (CB). We determine the partial density of states for the valence and CBs and find a strong hybridization of O2p and In5s states. This is deduced from constant final state spectra on the O-KLL-Auger over the O K-edge and In M4,5-edge and a comparison to the corresponding X-ray absorption spectroscopy data. We also identify several types of defects. A broad band of oxygen derived defects is identified that extends throughout the gap. These are attributed to small polarons which cause an anti-resonance in the valence states around the O1s threshold. In addition, a separate Auger decay at resonance indicates the existence of localized charge transfer states which involves localized In5s5p states.

Control of Intrachain Charge Transfer in Model Systems for Block Copolymer Photovoltaic Materials

We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor-acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor-acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.

Ultrafast Intramolecular Charge Separation in a Donor–Acceptor Assembly Comprising Bis(η5-cyclopentadienyl)molybdenum Coordinated to an Ene-1,2-dithiolate-naphthalenetetracarboxylicdiimide Ligand

The first example of a Donor-spacer-Acceptor tryad, based upon a molybdenum-ene-1,2-dithiolate unit as the Donor and a naphthalene-diimide as the Acceptor, has been synthesized and its photophysical properties investigated. Synthesis required the preparation of a new pro-ligand containing a protected ene-1,2-dithiolate bound through a phenyl linkage to a naphthalenetetracarboxylicdiimide (NDI) group. Deprotection of this pro-ligand by base hydrolysis, followed by reaction with [Cp(2)MoCl(2)], produced the new dyad [Cp(2)Mo(SC(H)C(C(6)H(4)-NDI)S)] (2). Electrochemical studies showed that 2 can be reversibly oxidized to [2](+) and reduced to [2](-), [2](2-), and [2](3-). These studies, augmented by UV/vis, IR, and electron paramagnetic resonance (EPR) spectra of electrochemically generated [2](+) and [2](-), show that the highest occupied molecular orbital (HOMO) of 2 is ene-1,2-dithiolate-based and the lowest unoccupied molecular orbital (LUMO) is NDI-based; these conclusions are supported by density functional theory (DFT) calculations for the electronic ground state on a model of 2 which also showed that these two parts of the molecule are electronically distinct. The dynamics of the excited states of 2 in CH(2)Cl(2) solution were investigated by picosecond time-resolved IR spectroscopy following irradiation by a 400 nm ∼120 fs laser pulse. These investigations were complemented by an ultrafast transient absorption spectroscopic study from 420 to 760 nm of the nature of the excited states of 2 in CH(2)Cl(2) solution following irradiation by a 383 nm ∼120 fs laser pulse. These studies showed that irradiation of 2 at both 400 and 383 nm leads to the formation of the [(Cp)(2){Mo(dt)}(+)-Ph-{NDI}(-)] charge-separated state as a result of a cascade electron transfer initiated by the formation of an (1)NDI* excited state. (1)NDI* rapidly (ca. 0.2 ps) forms the local charge transfer state [Cp(2)Mo(dt)-{Ph}(+)-{NDI}(-)] which has a lifetime of about 1.7 ps and decays to produce the ground state and the charge-separated state [(Cp)(2){Mo(dt)}(+·)-Ph-{NDI}(-)]; the latter has an appreciable lifetime, about 15 ns in CH(2)Cl(2) at room temperature.

Resonant Photoemission at the O1s threshold to characterize β-Ga2O3 single crystals

We report on spectroscopic investigations on β-Ga2O3 single crystals. We focus on the detailed analysis of the O1s resonance profile by resonant photoelectron spectroscopy. We analyze the electronic structure and assign the O2p to build the valence band and both, O2p and Ga4s4p states to contribute to the conduction band. We determine the partial density of states for the valence and conduction bands and find a strong hybridization of O2p and Ga4s states. This is deduced from constant final state spectra on the O-KLL-Auger over the O K-edge and Ga L2,3-edge and a comparison to the corresponding X-ray absorption spectroscopy data. We also identify several types of defects. A broad band of oxygen derived defects is identified that extends throughout the gap. Small polarons are attributed to cause an anti-resonance in the valence states around the O1s threshold. In addition, a separate Auger decay at resonance indicates the existence of localized charge transfer states which involves localized Ga4sp states.

Condensed Phase Laser Induced Harpoon Reactions

Laser induced charge transfer reactions of halogens in rare gas solids and liquids provide a powerful means for the study of condensed phase dynamics. Many-body effects with respect to both electronic and nuclear coordinates, and cooperative interactions with radiation fields, are some of the studied phenomena that are highlighted in this article.The pertinence of these ionic reactions to chemistry in solids is demonstrated in photodissociation studies of molecular halogens in rare gas matrices. The coexistence of both delocalized and localized charge transfer states in solid xenon doped with atomic halogens is presented and dynamical consequences—charge separation, self-trapping and energy storage—are discussed. Static and dynamic solvent effects in liquid phase harpoon reactions are considered. The characterization of cooperative excitations— two-photon, two-electron transitions—in liquid solutions is presented.

Electronic spectroscopy of M(bpy) 2+3 (M = Fe, Ru, Os), Cr(bpy) 3+3 and related compounds

Absorption and luminescence spectroscopy of the title compounds have been studied in single crystals, PVA foils and alcohol solutions. Particular attention has been given to crystal sites with defined symmetry (D 3 and C 2 for Fe, Ru, Os). The effect of 4,4′ substitution in bpy has been studied for Fe, Ru, and OS complexes and significant intensity flow out of the ligand absorption bands into the metal to ligand charge transfer bands has been found for Ru and Os, consistent with back π-bonding. CD, CPL, MCD, MCPL and time resolved luminescence spectroscopy have been used to help make assignments. All of these data support a delocalized description of the luminescent states, except in fluid solutions where a viscosity dependent relaxation to a localized charge transfer state occurs. The first detailed investigation of the excited states of chromium (III) trisbipyridine in crystals is reported.