Molecular Emitters Research Articles

Theory of molecular emission power spectra. I. Macroscopic quantum electrodynamics formalism.

We study the emission power spectrum of a molecular emitter with multiple vibrational modes in the framework of macroscopic quantum electrodynamics. The theory we present is general for a molecular spontaneous emission spectrum in the presence of arbitrary inhomogeneous, dispersive, and absorbing media. Moreover, the theory shows that the molecular emission power spectra can be decomposed into the electromagnetic environment factor and lineshape function. In order to demonstrate the validity of the theory, we investigate the lineshape function in two limits. In the incoherent limit (single molecules in a vacuum), the lineshape function exactly corresponds to the Franck-Condon principle. In the coherent limit (single molecules strongly coupled with single polaritons or photons) together with the condition of high vibrational frequency, the lineshape function exhibits a Rabi splitting, the spacing of which is exactly the same as the magnitude of exciton-photon coupling estimated by our previous theory [S. Wang et al., J. Chem. Phys. 151, 014105 (2019)]. Finally, we explore the influence of exciton-photon and electron-phonon interactions on the lineshape function of a single molecule in a cavity. The theory shows that the vibronic structure of the lineshape function does not always disappear as the exciton-photon coupling increases, and it is related to the loss of a dielectric environment.

Triazine-based OLEDs with simplified structure and high efficiency by solution-processed procedure

Solution-processed Organic light-emitting diodes (OLEDs) have aroused serious attention because of its lower cost and easier fabrication. Herein, a novel triazine-based molecular emitter is reported for solution-processed OLEDs for the first time. The emitter, 4,4′,4′'-(1,3,5-triazine-2,4,6-triyl)tris(N,N-bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)aniline) denoted as Tri-2tCzPA-TRZ, shows excellent solubility in common organic solvents, facilitating solution-processed OLEDs. The resultant OLEDs doped with 1,3-di(9H-carbazol-9-yl)benzene (mCP) achieved a maximum current efficiency of 18.8 cd A−1 and maximum luminance of 12,580 cd m−2, which is higher than the maximum current efficiency of the most solution-processed OLEDs in simple structures.

Highly Efficient, Red Delayed Fluorescent Emitters with Exothermic Reverse Intersystem Crossing via Hot Excited Triplet States

Three donor–acceptor–donor molecular emitters have been designed by taking triphenylamine or N-phenylcarbazole as the donor and maleimide or phenyl maleimide as the acceptor, in which the highest occupied molecular orbital interaction between two donor units is maximized via the acceptor bridge. This is envisaged to enable both strong fluorescence radiation and fast exoergic reverse intersystem crossing via the second triplet state. Detailed photophysical characterization and theoretical calculations confirm that all the compounds have large oscillator strengths and short delayed fluorescence lifetimes of ∼0.2 μs. The fabricated organic light-emitting diodes (OLEDs) give red emission above 600 nm, luminance exceeding 6000 cd m–2, and external quantum efficiencies (EQEs) of over 6%. In particular, the best device shows an emission at 645 nm and a maximum EQE of 10.4%. Moreover, the EQEs remain above 3% at 1000 cd m–2 for all the emitters. This work provides an effective method to develop organic emitters for highly efficient OLEDs with low-efficiency roll-off.

Photo-induced electron transfer between a metal nanoparticle and a collection of molecular emitters

Hybrid polaritonic nanostructures combining molecular excitons and localized surface plasmons hold strong potential for promising applications in broad areas of nanophotonics. Here, using quantum mechanical calculations, we study the position- and configuration-dependent optical response modulations in a hybrid system composed of a plasmonic nanoparticle and a collection of molecular emitters. Our results underline the coupling between the plasmonic nanoparticle and molecular emitter states, confirmed from the transition density maps. Depending on the interparticle distance (d) and the number of coupled molecular emitters, we show that photo-induced electron transfer emerge. Our theoretical findings bear fundamental interest for the development of novel hybrid polaritonic nanostructures.

Thermally activated delayed fluorescent polymer- assisted morphological control on perfluorinated ionomer enriched surface and exciton harvesting for phosphorescent organic light-emitting devices

Poly(3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) is one of the most successful conductive polymers, usually used as hole injecting layer in organic light-emitting devices. Due to the enhanced conductivity and tunable work function (WF), the modification of PEDOT:PSS with some additives has been proved to be cost-effective. Perfluorinated ionomer (PFI) has been reported as an effective additive to improve the WF of PEDOT:PSS, increasing from 5.2 eV to 5.95 eV with a recipe of PEDOT(1):PSS(6):PFI(25). However, the strong hydrophobicity of PFI has limited the choice of the organic solvents used to fabricate the uniform emitting layer on the modified PEDOT:PSS. To address this issue, the polymers were introduced as the additives of the emitter to overcome the non-uniform morphology on the hydrophobic surface of PEDOT:PSS:PFI. A conventional fluorescent polymer poly(9-vinylcarbazole) (PVK) and a thermally activated delayed fluorescent polymer (4-(2-(9-(heptadecan-9-yl)-6-phenyl-9H-carbazol-3-yl)-9,9-dihexyl-7-phenylacridin-10(9H)-yl)phenyl) (phenyl)methanone (PABPC2.5) were compared in parallel to reveal the influence of the polymer additive of the emitters on the overall performances of the devices. The control device based on the PFI-modified PEDOT:PSS achieved a maximum current efficiency (CE) of 0.14 cd A−1, without any polymer additives in the emitting layer. In contrast, the CE of the devices with PVK and PABPC2.5 were dramatically elevated to 7.1 cd A-1 and 4.5 cd A−1 respectively, when a small molecular thermally activated delayed fluorescence compound was employed as the emitter. Enabled by an easily soluble phosphorescent emitter acetylacetonatobis(4-(4-tert-butylphenyl)-thieno[3,2-c]pyridinato-C2,N)iridium (PO-01-TB). The device with PABPC2.5 as the additive obtained a maximum CE of 42.0 cd A−1, which is significantly higher than 15.7 cd A−1 of the one with the additive PVK, attributed to the energy transfer in the emitting layer. The polymer additives not only overcome the issue of the hydrophobicity of the modified PEDOT:PSS but endow the exciton harvesting from the polymer to the small molecular emitters.

A 3D Polymeric Platform for Photonic Quantum Technologies

AbstractThe successful development of future photonic quantum technologies will much depend on the possibility of realizing robust and scalable nanophotonic devices. These should include quantum emitters like on‐demand single‐photon sources and non‐linear elements, provided their transition linewidth is broadened only by spontaneous emission. However, conventional strategies to on‐chip integration, based on lithographic processes in semiconductors, are typically detrimental to the coherence properties of the emitter. Moreover, such approaches are difficult to scale and bear limitations in terms of geometries. Here an alternative platform is discussed, based on molecules that preserve near‐Fourier‐limited fluorescence even when embedded in polymeric photonic structures. 3D patterns are achieved via direct laser writing around selected molecular emitters, with a fast, inexpensive, and scalable fabrication process. By using an integrated polymeric design, detected photon counts of about 2.4 Mcps from a single cold molecule are reported. The proposed technology will allow for competitive organic quantum devices, including integrated multi‐photon interferometers, arrays of indistinguishable single‐photon sources, and hybrid electro‐optical nanophotonic chips.

Cyclometalated Ir(III) complexes as tuneable multiband light sources for optical multisensor systems: Feasibility study

Development of novel analytical devices complying with modern requirements of simplicity and cost-effectiveness is an important task. Multisensor systems based on various types of chemical sensors are promising tools in this respect. Herein the development of a new approach to design of optical multisensor systems is reported. The idea is to use a combination of molecular emitters – cyclometalated Ir(III) compounds – as tuneable multiband light sources. Upon the 365 nm light irradiation these compounds emit in certain wavelength ranges, that can be tuned by changing of ligand environment of Ir(III). This gives an option to choose an appropriate set of emitters for particular analytical tasks, thus giving the way for construction of novel type of optical multisensor systems with tuneable emittance. The multiband light illuminates the analyzed sample in the specific range of wavelengths with consequent registration of sample absorption spectra. Chemometric processing of the resulted signals allows quantification of particular analytes in mixtures. The details of the system design and its’ performance validation in complex aqueous mixtures of metal ions (cobalt, nickel and copper) are described in this paper. It is shown that precise quantitative analysis of all three ions simultaneously is possible with root mean-square errors in prediction below 0.007 M in the range 0.01–0.1 M. We believe that this approach gives a wide variety of options for real-world applications.

Femtomolar Detection of Spermidine Using Au Decorated SiO2 Nanohybrid on Plasmon-Coupled Extended Cavity Nanointerface: A Smartphone-Based Fluorescence Dequenching Approach.

Coupling of photons with molecular emitters in different nanocavities have resulted in transformative plasmonic applications. The rapidly expanding field of surface plasmon-coupled emission (SPCE) has synergistically employed subwavelength optical properties of localized surface plasmon resonance (LSPR) supported by nanoparticles (NPs) and propagating surface plasmon polaritons assisted by metal thin films for diagnostic and point-of-care analysis. Gold nanoparticles (AuNPs) significantly quench the molecular emission from fluorescent molecules (at close distances <5 nm). More often, complex strategies are employed for providing a spacer layer around the AuNPs to avoid direct contact with fluorescent molecules, thereby preventing quenching. In this study we demonstrate a rapid and facile strategy with the use of Au-decorated SiO2 NPs (AuSil), a metal (Au)-dielectric (SiO2) hybrid material for dequenching the otherwise quenched fluorescence emission from radiating dipoles and to realize 88-fold enhancement using the SPCE platform. Different loading of AuNPs were studied to tailor fluorescence emission enhancements in spacer, cavity, and extended (ext.) cavity nanointerfaces. We also present femtomolar detection of spermidine using this nanohybrid in a highly desirable ext. cavity interface. This interface serves as an efficient coupling configuration with dual benefits of spacer and cavity architectures that has been widely explored hitherto. The multifold hot-spots rendered by the AuSil nanohybrids assist in augmented electromagnetic (EM)-field intensity that can be captured using a smartphone-based SPCE platform presenting excellent reliability and reproducibility in spermidine detection.

Plasmon enhanced second harmonic generation by periodic arrays of triangular nanoholes coupled to quantum emitters

Optical properties of periodic arrays of nanoholes of a triangular shape with experimentally realizable parameters are examined in both linear and nonlinear regimes. By utilizing a fully vectorial three-dimensional approach based on the nonlinear hydrodynamic Drude model describing metal coupled to Maxwell's equations and Bloch equations for molecular emitters, we analyze linear transmission, reflection, and nonlinear power spectra. Rigorous numerical calculations demonstrating second and third harmonic generation by the triangular hole arrays are performed. It is shown that both the Coulomb interaction of conduction electrons and the convective term contribute on equal footing to the nonlinear response of metal. It is demonstrated that the energy conversion efficiency in the second harmonic process is the highest when the system is pumped at the localized surface plasmon resonance. When molecular emitters are placed on a surface of the hole array line shapes, the second harmonic signal exhibits three peaks corresponding to second harmonics of the localized surface plasmon mode and upper and lower polaritonic states.

Preparation and Photophysical Properties of Bis(tridentate) Iridium(III) Emitters: Pincer Coordination of 2,6-Di(2-pyridyl)phenyl.

The way to prepare molecular emitters [5t + 4t'] of iridium(III) with a 5t ligand derived from the abstraction of the hydrogen atom at position 2 of the aryl group of 1,3-di(2-pyridyl)benzene (dpybH) is shown. In addition, the photophysical properties of the new emitters are compared with those of their counterparts resulting from the deprotonation of 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyMebH), at the same position, which are also synthesized. Treatment of 0.5 equiv of the dimer [Ir(μ-Cl)(η2-COE)2]2 (COE = cyclooctene) with 1.0 equiv of Hg(dpyb)Cl leads to the iridium(III) derivative IrCl2{κ3-N,C,N-(dpyb)}(η2-COE) (3), which reacts with 2-(1H-imidazol-2-yl)-6-phenylpyridine (HNImpyC6H5) and 2-(1H-benzimidazol-2-yl)-6-phenylpyridine (HNBzimpyC6H5) in the presence of Na2CO3 to give Ir{κ3-C,N,N-(NImpyC6H4)}{κ3-N,C,N-(dpyb)} (4) and Ir{κ3-C,N,N-(NBzimpyC6H4)}{κ3-N,C,N-(dpyb)} (5), respectively. Similar reactions of the Williams's dimer [IrCl(μ-Cl){κ3-N,C,N-(dpyMeb)}]2 with HNImpyC6H5 and HNBzimpyC6H5 in the presence of Na2CO3 afford the dimethylated counterparts Ir{κ3-C,N,N-(NImpyC6H4)}{κ3-N,C,N-(dpyMeb)} (6) and Ir{κ3-C,N,N-(NBzimpyC6H4)}{κ3-N,C,N-(dpyMeb)} (7), whereas 2-(6-phenylpyridine-2-yl)-1H-indole (HIndpyC6H5) initially gives IrH{κ2-N,N-(IndpyC6H5)}{κ3-N,C,N-(dpyMeb)} (8) and subsequently Ir{κ3-C,N,N-(IndpyC6H4)}{κ3-N,C,N-(dpyMeb)} (9). Complexes 4-7 are phosphorescent green emitters (λem 490-550 nm), whereas 9 is greenish yellow emissive (λem 547-624 nm). They display lifetimes in the range 0.5-9.7 μs and quantum yields in both doped poly(methyl)methacrylate films and in 2-methyltetrahydrofuran at room temperature depending upon the ligands: 0.5-0.7 for 6 and 7, about 0.4 for 4 and 5, and 0.3-0.2 for 9.

Realizing Efficient Single Organic Molecular White Light-Emitting Diodes from Conformational Isomerization of Quinazoline-Based Emitters.

Single pure organic molecular white light emitters (SPOMWLEs) are of significance as a new class of material for white lighting applications; however, few of them are able to emit white electroluminescence from organic light-emitting diodes. Herein, donor-π-acceptor conjugated emitters, 2PQ-PTZ and 4PQ-PTZ, were designed and synthesized as SPOMWLEs for white light emission considering the distinct advantages of their conformation isomers. The coexistence of conformational isomers in 2PQ-PTZ, which is the first experimental evidence of the coexisting quasi-axial and quasi-equatorial conformers, provides ideal flexibility to obtain white light emission from their simultaneous and well-separated fluorescence and thermally activated delayed fluorescence. With these remarkable properties, a 2PQ-PTZ-based white light-emitting diode (LED) with a CIE of (0.32, 0.34) and color rendering index (CRI) of 89 is demonstrated. Further, the white organic light-emitting diode (OLED) of 2PQ-PTZ exhibits a high external quantum efficiency (EQE) of 10.1%, which is the reported highest performance among SPOMWLE-based OLEDs.

Computational Studies of Molecular Materials for Unconventional Energy Conversion: The Challenge of Light Emission by Thermally Activated Delayed Fluorescence.

In this paper we describe the mechanism of light emission through thermally activated delayed fluorescence (TADF)—a process able to ideally achieve 100% quantum efficiencies upon fully harvesting the energy of triplet excitons, and thus minimizing the energy loss of common (i.e., fluorescence and phosphorescence) luminescence processes. If successful, this technology could be exploited for the manufacture of more efficient organic light-emitting diodes (OLEDs) made of only light elements for multiple daily applications, thus contributing to the rise of a sustainable electronic industry and energy savings worldwide. Computational and theoretical studies have fostered the design of these all-organic molecular emitters by disclosing helpful structure–property relationships and/or analyzing the physical origin of this mechanism. However, as the field advances further, some limitations have also appeared, particularly affecting TD-DFT calculations, which have prompted the use of a variety of methods at the molecular scale in recent years. Herein we try to provide a guide for beginners, after summarizing the current state-of-the-art of the most employed theoretical methods focusing on the singlet–triplet energy difference, with the additional aim of motivating complementary studies revealing the stronger and weaker aspects of computational modelling for this cutting-edge technology.

Influence of the Chemical Structure on Molecular Light Emission in Strongly Localized Plasmonic Fields

The coupling between a molecular emitter and an optical cavity is often addressed theoretically with the molecule regarded as a point dipole, thus lacking any information on chemical structure. Thi...

Rare-earth-free zinc aluminium borate white phosphors for LED lighting

A novel family of rare-earth-free white phosphors was synthesized by the polymeric precursor method, based on the trapping of molecular emitters in a zinc aluminium borate matrix, resulting in an intense and easily tuneable photoluminescence.

2D host–guest supramolecular chemistry for an on-monolayer graphene emitting platform

Electronic decoupling of molecular emitters from monolayer graphene allows luminescence of the hybrid platform, opening new perspectives for 2D materials-based nanophotonics.

Properties of quantum dots coupled to plasmons and optical cavities.

Quantum electrodynamics is rapidly finding a set of new applications in thresholdless lasing, photochemistry, and quantum entanglement due to the development of sophisticated patterning techniques to couple nanoscale photonic emitters with photonic and plasmonic cavities. Colloidal and epitaxial semiconductor nanocrystals or quantum dots (QDs) are promising candidates for emitters within these architectures but are dramatically less explored in this role than are molecular emitters. This perspective reviews the basic physics of emitter-cavity coupling in the weak-to-strong coupling regimes, describes common architectures for these systems, and lists possible applications (in particular, photochemistry), with a focus on the advantages and issues associated with using QDs as the emitters.

Charge Carrier Injection Electroluminescence with CO-Functionalized Tips on Single Molecular Emitters.

We investigate electroluminescence of single molecular emitters on NaCl on Ag(111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force, and light detection capabilities. The role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have an impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbitals with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.

Near-Infrared [Ir(N∧C)2(N∧N)]+ Emitters and Their Noncovalent Adducts with Human Serum Albumin: Synthesis and Photophysical and Computational Study

Near-infrared (NIR) molecular emitters based on transition-metal complexes have attracted growing attention due to their potential application for in vivo and in vitro bioimaging experiments. Their...

Triplet harvesting in the polaritonic regime: A variational polaron approach

We explore the electroluminescence efficiency for a quantum mechanical model of a large number of molecular emitters embedded in an optical microcavity. We characterize the circumstances under which a microcavity enhances harvesting of triplet excitons via reverse intersystem-crossing (R-ISC) into singlet populations that can emit light. For that end, we develop a time-local master equation in a variationally optimized frame, which allows for the exploration of the population dynamics of chemically relevant species in different regimes of emitter coupling to the condensed phase vibrational bath and to the microcavity photonic mode. For a vibrational bath that equilibrates faster than R-ISC (in emitters with weak singlet-triplet mixing), our results reveal that significant improvements in efficiencies with respect to the cavity-free counterpart can be obtained for strong coupling of the singlet exciton to a photonic mode, as long as the singlet to triplet exciton transition is within the inverted Marcus regime; under these circumstances, the activation energy barrier from the triplet to the lower polariton can be greatly reduced with respect to that from the triplet to the singlet exciton, thus overcoming the detrimental delocalization of the polariton states across a macroscopic number of molecules. On the other hand, for a vibrational bath that equilibrates slower than R-ISC (i.e., emitters with strong singlet-triplet mixing), we find that while enhancements in photoluminescence can be obtained via vibrational relaxation into polaritons, this only occurs for a small number of emitters coupled to the photon mode, with delocalization of the polaritons across many emitters eventually being detrimental to electroluminescence efficiency. These findings provide insight into the tunability of optoelectronic processes in molecular materials due to weak and strong light-matter coupling.

Anisotropic Fluorescence Emission and Photobleaching at the Surface of One-Dimensional Photonic Crystals Sustaining Bloch Surface Waves. I. Theory

Photonic crystal (PC) enhanced fluorescence has been proposed as a novel tool for early disease detection in a liquid biopsy format. However, photobleaching of the emitters has never been deeply investigated, although its cross section is expected to increase due to the large field intensity enhancement. Herein, we report on a comprehensive theoretical description of the stationary fluorescence emission of molecular emitters bound to the surface of a one-dimensional photonic crystal (1DPC) biosensor. The model considers coupling of the emission to the large local density of the states provided by the 1DPC, in particular to the Bloch surface waves, which can be characterized by different polarization states. The rotational diffusion equation in the presence of photobleaching was solved analytically by a Laplace spherical harmonics analytical approach. The results show that photobleaching can severely affect the fluorescence emission in terms of total intensity and polarization composition, suggesting that ...