Optical Transition Dipole Moment Research Articles

Highly-Directional, Highly-Efficient Solution-Processed Light-Emitting Diodes of All-Face-Down Oriented Colloidal Quantum Well Self-Assembly.

Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800cd m-2 with a long operational lifetime of 247h at 100cd m-2 as well as a stable saturated deep-red emission (651nm) with a low turn-on voltage of 1.7eV at a current density of 1mA cm-2 and a high J90 of 99.58mA cm-2 . These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.

Layer-dependent structure-optoelectronic property relationships for two-dimensional ruddlesden-popper phase (BA)2Csn−1PbnBr3n+1 perovskites

Recently, hybrid two-dimensional (2D) Ruddlesden-Popper (RP) phase layered halide perovskite structures of (BA)2Csn-1PbnBr3n+1 (BA is the butylammonium organic chain and n is the number of inorganic perovskite-like layers) have been experimental synthesized and indicated superior optical properties. Here, we reported an investigation of the above series of layered halide perovskites (n = 1, 2, 3 and ∞). Their structural, electronic and optical properties were calculated using first-principle approaches, including the bond angles, band structures, formation energy, carrier effective masses, density of states, differential charge densities, Bader charge analysis, optical absorption spectra and transition dipole moment. As a consequence of these findings, with the increase in the number of inorganic layers, perovskite materials showed lower bandgaps, improved light absorption and poorer stability. Hence, tuning the inorganic layer thickness of layered 2D RP phase perovskites is a suitable method to enhance their optoelectronic properties. This work provides a theoretical basis for layered 2D perovskite materials with large-scale and low-cost applications in the photoluminescent device light-emitting diodes.

Magnetophotoselection in the Investigation of Excitonically Coupled Chromophores: The Case of the Water-Soluble Chlorophyll Protein.

A magnetophotoselection (MPS) investigation of the photoexcited triplet state of chlorophyll a both in a frozen organic solvent and in a protein environment, provided by the water-soluble chlorophyll protein (WSCP) of Lepidium virginicum, is reported. The MPS experiment combines the photoselection achieved by exciting with linearly polarized light with the magnetic selection of electron paramagnetic resonance (EPR) spectroscopy, allowing the determination of the relative orientation of the optical transition dipole moment and the zero-field splitting tensor axes in both environments. We demonstrate the robustness of the proposed methodology for a quantitative description of the excitonic interactions among pigments. The orientation of the optical transition dipole moments determined by the EPR analysis in WSCP, identified as an appropriate model system, are in excellent agreement with those calculated in the point-dipole approximation. In addition, MPS provides information on the electronic properties of the triplet state, localized on a single chlorophyll a pigment of the protein cluster, in terms of orientation of the zero-field splitting tensor axes in the molecular frame.

Electronic dephasing in mixed quantum–classical molecular systems using the spin-boson model

Pure electronic dephasing is investigated using the spin-boson Hamiltonian in mixed quantum–classical environment. The spin-boson model used here is a composite system made up of a quantum subsystem, an electronic 2-level subsystem linearly coupled to harmonic vibrations, interacting with a classical bath. Experimental results for a multitude of molecular systems indicate that the zero-phonon line (ZPL) profile is determined by electronic dephasing, which is not accounted for in the multimode Brownian oscillator (MBO) model due to the unphysical contribution from the MBO bath modes to the ZPL profile. Mixed quantum–classical dynamics formalism of non-equilibrium systems is employed to assess the contribution of the bath modes to pure electronic dephasing by probing the ZPL profile when coupled to a classical bath in the mixed quantum–classical condensed systems. Pure electronic dephasing is discussed in the context of mixed quantum–classical dynamics formalism which starts with mixed quantum–classical Liouville equation in a mixed quantum–classical environment. It is noteworthy, however, that the fundamental difference between the fully quantum MBO model and the mixed quantum–classical Brownian oscillator, is that the zero-phonon line calculated by the former shows unphysical asymmetry on the low-energy side as it has not been observed in real systems, whereas the ZPL reported herein eliminates this asymmetry. A systematic approach using matrix mechanics is developed to treat this phenomenon. To this end, a closed-form expression of linear and nonlinear optical electronic transition dipole moment time correlation functions in a dissipative media are derived. Linear absorption spectra and 4-wave mixing signals at various temperatures showing a sound thermal broadening, temporal decay, and accurate pure dephasing further ratify the applicability and correctness of the mixed quantum–classical dynamics approach to spectroscopy and dynamics are computed.

Electronic dephasing of polyatomic molecules interacting with mixed quantum-classical media.

This paper offers an expedient, efficient, and unique treatment of multimode quantum subsystems (polyatomic molecules) interacting with a classical environment in which the time evolution of the coupling term is governed by the algebraic rules of statistical mechanics in mixed quantum-classical systems developed by Kapral and Nielsen [S. Nielsen, R. Kapral, and G. Ciccotti, J. Chem. Phys., 2001, 115, 5805]. This unique time evolution of the coupling term is neither quantal nor classical but rather something different that relies heavily on Wigner transform, thereby leading to non-Newtonian mechanics. As such, an argument is presented that the approach provided herein for treating polyatomic molecular systems in a mixed quantum-classical environment is new and different as opposed to the many other schemes of semiclassical dynamics that are normally employed to study such systems. The merits of expediency and efficiency of the herein mixed quantum-classical dynamics calculations emanate from avoiding using integrals for time evolutions, and, instead, employing matrix mechanics whereby LU decomposition and singular value decomposition (SVD) numerical techniques are utilized for diagonalization. An electronic 2-level subsystem interacting with a classical bath through the spin-boson model to render accurate pure electronic dephasing in multimode molecular systems by eliminating the unphysical asymmetry in the line shape of the zero-phonon line (ZPL) exhibited by other models is exploited. This work has a superior advantage over the single-mode spin-boson model, published previously, whereby a multitude of types of vibrational modes (slow, fast, or both) of the quantum subsystem may readily be handled using different spectral densities. The spin-boson model used here is a composite system made up of a quantum subsystem, i.e., a subsystem bilinearly coupled to a multidimensional harmonic oscillator (representing the intermediate quantum vibrational modes between the electronic subsystem and the bath), interacting with a classical bath, where the coupling term is governed by the mixed quantum-classical Liouville equation. A multidimensional coherent-state approach is employed to deal with the time evolution of the quantum subsystem. A closed-form expression of linear and nonlinear optical electronic transition dipole moment time correlation functions in mixed quantum-classical dissipative media is derived. Pure electronic dephasing is probed using the aforementioned approach. Linear absorption spectra and 4-wave mixing signals (e.g., photon echo and pump-probe) are calculated showing a reasonable thermal broadening, temporal decay, and accurate pure dephasing.

What Controls the Orientation of TADF Emitters?

Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.

Lattice Distortion in Mixed-Anion Lead Halide Perovskite Nanorods Leads to their High Fluorescence Anisotropy

Semiconductor nanorods (NRs) with strong fluorescence anisotropy are considered promising polarized light sources for liquid crystal displays. Here, we achieved a high fluorescence anisotropy value of 0.4 in the mixed-anion lead halide perovskite NRs, which constitutes a four-fold enhancement as compared to pristine CsPbBr3 NRs. This was realized by substituting some smaller bromine anions with larger iodine anions in the orthorhombic CsPbBr3 NRs via anion exchange. The resulting CsPbBr1.2I1.8 NRs kept a highly uniform length of ∼36 nm and width of ∼8.6 nm, and possessed a high emission quantum yield of 83%. On the basis of the DFT calculations, we attribute the improved polarized emission of the mixed-anion perovskite NRs to a moderate structural distortion of PbX6 octahedra units in the orthorhombic perovskite lattice, which results in a preferred direction of the optical transition dipole moment along the Pb–X–Pb axis. Our study provides a guidance for design and fabrication of perovskite NRs with high fluorescence anisotropy.

Azobenzene Sulphonic Dye Photoalignment as a Means to Fabricate Liquid Crystalline Conjugated Polymer Chain‐Orientation‐Based Optical Structures

AbstractThe use of a noncontact photoalignment method to fabricate in‐plane optical structures, defined by the local uniaxial ordering of liquid crystalline conjugated polymer chains, is reported. Molecular orientation is demonstrated for both green‐light‐emitting fluorene‐benzothiadiazole alternating copolymer F8BT and F8BT/red light emitting complex copolymer Red‐F binary blend films deposited on a well‐known azobenzene sulphonic dye photoalignment material SD1. Absorption anisotropy ratios of up to 9.7 are readily achieved for 150 nm thickness F8BT films. Spatial pattern definition, afforded by masking the UV polarized light exposure of the photoalignment layer, allows the fabrication of optical structures with a resolution down to the micron scale. The alignment process is further extended to enable the serial, independent orientation of films deposited on top of each other and to permit the molecular orientation to follow curvilinear patterns. In the former case, this allows F8BT bilayer structures to be fabricated that show even higher absorption anisotropy ratios, up to ≈12, close to the theoretical limit for the previously deduced ≈22° optical transition dipole moment angle relative to the chain axis.

Uniaxial transition dipole moments in semiconductor quantum rings caused by broken rotational symmetry

Semiconductor quantum rings are topological structures that support fascinating phenomena such as the Aharonov–Bohm effect and persistent current, which are of high relevance in the research of quantum information devices. The annular shape of quantum rings distinguishes them from other low-dimensional materials, and enables topologically induced properties such as geometry-dependent spin manipulation and emission. While optical transition dipole moments (TDMs) in zero to two-dimensional optical emitters have been well investigated, those in quantum rings remain obscure despite their utmost relevance to the quantum photonic applications of quantum rings. Here, we study the dimensionality and orientation of TDMs in CdSe quantum rings. In contrast to those in other two-dimensional optical emitters, we find that TDMs in CdSe quantum rings show a peculiar in-plane linear distribution. Our theoretical modeling reveals that this uniaxial TDM originates from broken rotational symmetry in the quantum ring geometries.

Spectrally Stable Defect Qubits with no Inversion Symmetry for Robust Spin-To-Photon Interface

Scalable spin-to-photon interfaces require quantum emitters with strong optical transition dipole moment and low coupling to phonons and stray electric fields. It is known that particularly for coupling to stray electric fields, these conditions can be simultaneously satisfied for emitters that show inversion symmetry. Here, we show that inversion symmetry is not a prerequisite criterion for a spectrally stable quantum emitter. We find that identical electron density in ground and excited states can eliminate the coupling to the stray electric fields. Further, a strong optical transition dipole moment is achieved in systems with altering sign of the ground and excited wavefunctions. We use density functional perturbation theory to investigate an optical center that lacks of inversion symmetry. Our results show that this system close to ideally satisfies the criteria for an ideal quantum emitter. Our study opens a novel rationale in seeking promising materials and point defects towards the realisation of robust spin-to-photon interfaces.

Observation of Bose-enhanced photoassociation products

We produce molecules using Bose-enhanced Raman photoassociation. We apply the stimulated Raman adiabatic passage (STIRAP) technique on a Bose-Einstein condensate (BEC) to produce more than ultracold molecules. This chemical reaction is only made possible because of the Bose enhancement of the optical transition dipole moment between the initial atomic state and an intermediate molecular state. We study the effect of Bose enhancement by measuring the transition Rabi frequency in a BEC and by comparing it with measurements for two atoms in sites of a Mott insulator. By breaking the dimers' bond and directly observing the separated atoms, we measure the molecular inelastic collision rate parameters. We discuss the possibility of applying Bose-enhanced STIRAP to convert a BEC of atoms into a BEC of molecules, and argue that the required efficiency for STIRAP is within experimental reach.

Suppression of transition dipole moments in radioactive atoms

Optical emission from radioactive atoms is investigated. It is shown that the radioactivity of the central nucleus results in the suppresion of the optical transition dipole moment in the corresponding atom. The situations are regarded when this suppression is significant and can be experimentally observed.

Angle-Dependent Photoluminescence Spectroscopy of Solution-Processed Organic Semiconducting Nanobelts

We report an anomalous anisotropy in photoluminescence (PL) from crystalline nanobelt of an organic small-molecule semiconductor, 6,13-dichloropentacene (DCP). Large-area well-aligned DCP nanobelt arrays are readily formed by self-assembly through solution method utilizing the strong anisotropic interactions between molecules. The absorption spectrum of the arrays suggests the formation of both intramolecular exciton and intermolecular exciton. However, the results of angle-dependent PL spectroscopy indicate that the PL arises only from the relaxation of intramolecular exciton, which has an optical transition dipole moment with an angle of 115° with the long-axis of the nanobelts. The angular dependence of PL signals follows a quartic rule (IPL(θ) ∝ cos4(θ – 115)) and agrees well with the optical selection rule of individual DCP molecules. The measured polarization ratio ρ from the individual nanobelts is on average 0.91 ± 0.02, superior to that of prior-art organic semiconductors. These results provide new insights into exciton behavior in 1D π–π stacking organic semiconductors and demonstrate DCP’s great potential in the photodetectors and optical switches for large-scale organic optoelectronics.

Morphology-dependent optical anisotropies in then-type polymer P(NDI2OD-T2)

Organic semiconductors tend to self-assemble into highly ordered and oriented morphologies with anisotropic optical properties. Studying these optical anisotropies provides insight into processing-dependent structural properties and informs the photonic design of organic photovoltaic and light-emitting devices. Here we measure the anisotropic optical properties of spin-cast films of the $n$-type polymer P(NDI2OD-T2) using momentum-resolved absorption and emission spectroscopies. We quantify differences in the optical anisotropies of films deposited with distinct face-on and edge-on morphologies. In particular, we infer a substantially larger out-of-plane tilt angle of the optical transition dipole moment in high temperature annealed, edge-on films. Measurements of spectral differences between in-plane and out-of-plane dipoles, further indicate regions of disordered polymers in low temperature annealed face-on films that are otherwise obscured in traditional x-ray and optical characterization techniques. The methods and analysis developed in this work provide a way to identify and quantify subtle optical and structural anisotropies in organic semiconductors that are important for understanding and designing highly efficient thin film devices.

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (ηEQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields ηEQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.

Combining Positive and Negative Dichroic Fluorophores for Advanced Light Management in Luminescent Solar Concentrators

This work describes the unexpected alignment of a common perylene bisimide fluorescent dye (Lumogen Red305) in a host liquid crystal matrix. The negative dichroic fluorophore orients with the primary absorption/emission dipole, corresponding to the physical long axis of the perylene bisimide core, perpendicular to the director of a host liquid crystal. A second absorption dipole, which lies perpendicular to the primary dipole and corresponding to the physical short axis of the perylene core, lies parallel to the host liquid crystal. Individual illumination of the two absorbing optical transition dipole moments of the Red305 dye results in a single linearly polarized emission. When Red305 is combined with a second positive dichroic Coumarin‐type fluorophore that aligns in the conventional manner in a liquid crystal host, that is, with an absorption dipole along the physical long axis of the fluorophore and the director of the host liquid crystal, advanced light management is possible, such as electrically switchable colors and directing emission light to different edges in a luminescent solar concentrator device.

Commentary: Assessing FRET using spectral techniques

The manuscript “Assessing FRET using spectral techniques” by Leavesley et al. (page 898 in this issue) addresses Förster resonance energy transfer (FRET) between organic molecules due to an interaction of optical transition dipole moments. Although occurring in nature for more than a billion of years, a theoretical description of this mechanism has only been given in 1948 1. Since that time energy transfer spectroscopy has been used to probe molecular distances and mechanisms in the nanometre range 2-4, profiting from the fact that the energy transfer rate depends on the sixth power of molecular distance (kET ∼ r−6) and is, therefore, a very sensitive parameter. FRET experiments gained considerable importance, when in the 1990s green fluorescent protein [GFP, naturally occurring in the jellyfish Aequorea Victoria, 5] and its mutants could be cloned and fused with almost any protein of a cell. So, nonradiative energy transfer from cyan fluorescent protein (CFP) to yellow fluorescent protein (YFP) or from GFP to red fluorescent protein was used to probe changes of molecular conformation, for example, upon binding of calcium 6, or interactions between adjacent molecules, for example, protein−protein interactions playing an important role in regulation of apoptosis 7 or in pathogenesis of M. Alzheimer and further neurodegenerative diseases 8, 9. Due to these experimental restrictions (and pronounced costs) of time-resolving FRET experiments, spectral techniques and algorithms upon stationary optical excitation still play an important role, as outlined in the manuscript by Leavesley et al. (page 898 in this issue). Here, the authors used various techniques and algorithms based on up to three filter sets (e.g., for measurement of FRET efficiency at variable donor concentration) as well as spectral unmixing. Using a probe, where CFP and YFP are linked by a cAMP binding protein, they measured FRET efficiency in cultivated cells as a function of cAMP concentration. Although a two-filter set approach was already able to describe FRET efficiency with rather low coefficients of variation, spectrally resolved experiments provided improved results. The authors concluded that hyperspectral confocal microscopy in combination with linear unmixing, cell segmentation, and quantitative image analysis was appropriate for FRET experiments in single cells. I would like to share this opinion, emphasizing that in an ideal case spectral and time-resolving FRET experiments might be combined. This holds in particular, if acceptor fluorescence is low 8, or if donor and acceptor fluorescence are spectrally indiscernible [Homo FRET, 11]. During the last years resolution in optical microscopy has been improved considerably. Starting from a value around 200 nm, as resulting from Abbé's diffraction theory, values of 10−30 nm were meanwhile obtained by super-localization microscopy based on single molecule detection 12, 13 or by stimulated emission depletion microscopy [STED, 14]. Resolution in FRET experiments still goes below these values and permits to measure molecular distances of 5 nm and less. In addition, high irradiance as needed for STED (and to some extent also for single molecule detection) and risking severe cell damage can be avoided in FRET experiments. Furthermore, FRET is not restricted to microscopy and not limited to adherent samples, but can be combined with any other method, for example, flow cytometry 15.

Exciton-polariton mediated light propagation in anisotropic waveguides

To analyze the exciton-polariton dispersion relation of highly anisotropic thiacyanine films and nanofibers, we formulated a plane-wave expansion method by which we could obtain the eigenfrequencies of polaritons as eigenvalues of a non-Hermitian and frequency-independent matrix. The group refractive index calculated from the slope of the dispersion curve agreed quite well with the Fabry-Perot interference patterns found in both the calculated and observed transmission spectra. We found that the dispersion relation of the anisotropic polariton was quite different from the isotropic case and depended strongly on the tilt angle of the optical transition dipole moment of the constituent molecules to the propagation direction. Material parameters such as the transverse and longitudinal exciton frequencies are also discussed.

Angle-resolved and internuclear-separation-resolved measurements of the ionization rate of theBstate of I2by strong laser fields

Angle-resolved and internuclear-separation-resolved measurements of the single ionization rate of neutral I${}_{2}$ have been obtained. By launching a wave packet in the $B{\phantom{\rule{0.16em}{0ex}}}^{3}\ensuremath{\Pi}{{}_{u}}^{+}$ state ($B$ state) of I${}_{2}$ with a 50-fs tunable pump pulse we can measure the ionization rate as a function of internuclear separation as the wave packet evolves in the $B$ state. Moreover, since the ground to $B$-state optical transition dipole moment is parallel to the internuclear axis, the $B$-state subpopulation of the I${}_{2}$ thermal ensemble will have a high degree of alignment, allowing for angular measurements. The $B$ state shows the well-known effect of enhanced ionization at a critical separation, $R$${}_{c}$, with an enhancement factor of 22 when the ionizing field is polarized along the internuclear axis and the enhanced ionization decreases when the angle between the field and the axis increases from 0 ${}^{\ensuremath{\circ}}$ to 90 ${}^{\ensuremath{\circ}}$, finally disappearing at 90 ${}^{\ensuremath{\circ}}$. These results on the enhanced ionization of the $B$ state of I${}_{2}$ give the most precise determination of $R$${}_{c}$ for any molecule and agree extremely well with the prediction from a simple model of electron localization.

Influence of mesoscopic structures on single molecule dynamics in thin smectic liquid crystal films

Mesoscopic structures in liquids have an impact on the diffusion dynamics of the constituting molecules. Smectic 8CB liquid crystals on silicon wafers show the formation of mesoscopic structures on the μm scale at a film thickness of 200 nm. Depending on the kind of substrate (thermally grown or native SiOx), we observed the formation of focal conic domains (FCDs) and a new type of terraced holes, respectively. Dynamics are described via single perylene diimide tracer molecule tracking of translational diffusion and in the case of FCDs by a combination of translation and rotation detected viafluorescence correlation spectroscopy. Tailoring perylene diimide molecules such that the optical transition dipole moment follows the liquid crystal director allows mapping out FCDs and investigating the dynamics within a single FCD.