Molecular Transition Dipole Moments Research Articles

Shape-Dependent Optical Waveguides and Low-Threshold Lasers from Polymorphic Two-Dimensional Organic Single Crystals.

Organic single crystals (OSCs) with uniform morphologies and highly ordered molecular aggregations are promising for high-performance optoelectronic devices, such as organic solid-state lasers (OSSLs), organic light-emitting transistors (OLETs), and organic light-emitting diodes (OLEDs). However, manipulating OSC morphologies and aggregation is challenging. In this study, we synthesized two-dimensional (2D) OSCs of 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) in hexagonal and parallelogram microplate (H-MP and P-MP) forms. Both types exhibit H-aggregation in the 2D plate plane but with different molecular transition dipole moment (TDM) orientations. This leads to different photon coupling modes with H-MP and P-MP microcavities. H-MPs enable isotropic 2D-waveguiding, forming whispering gallery mode (WGM) resonators, while P-MPs create unidirectional waveguiding, forming Fabry-Pérot mode (FP) resonators. These resonators can generate low-threshold laser emissions at 467 and 473 nm, respectively, and exhibit superior lasing stability with a half-life exceeding 2 h. Our BSBCz microplate OSCs are attractive candidates to combine controlled organic microcavities with photon transporting for realizing future integrated optoelectronic devices.

Observation of Coherent Symmetry-Breaking Vibration by Polarization-Dependent Femtosecond Spectroscopy.

Understanding photoinduced chemical reactions beyond the Born-Oppenheimer paradigm requires a comprehensive examination of vibronic interactions. Although femtosecond studies have unveiled the influence of vibrational modes strongly coupled to ultrafast intramolecular reactions in the excited state, they often lack direct observations of how vibrations modulate electronic properties due to the rapid disappearance of reactants. To address this gap, our research investigates the dynamics of photoexcited molecules that do not react. Specifically, we focus on the coherent librational motion of molecular transition dipole moments, discovering that the coherent libration primarily originates from symmetry-breaking components in vibronically excited vibrational modes. Symmetry breaking motion can significantly impact the excited-state dynamics of highly symmetric molecules, potentially leading to nonadiabatic transitions. In essence, the data analysis framework introduced in this study can be harnessed to uncover potential reactivity in photoexcited molecules, further enhancing our understanding of the mechanisms governing these reactions.

Identifying the Quantitative Relationship Between the Molecular Structure and the Horizontal Transition Dipole Orientation of TADF Emitters

AbstractRealizing the horizontal orientation of molecular transition dipole moment (TDM) can greatly improve the out‐coupling efficiency and the resultant external quantum efficiency (EQE) of organic light‐emitting diodes (OLEDs). Herein, key parameters governing the horizontal TDM have been continuously explored. However, quantitatively identifying the key parameters from the molecular structure viewpoint is rather challenging due to the complexity of the influencing parameters. Here, by training the machine learning (ML) models using the experimental results, the quantitative relationship between the molecular structure and the horizontal TDM ratio (ϴ) of thermally activated delayed fluorescent (TADF) emitters in the host‐guest films is identified. The molecular structure is represented by either quantum chemistry‐calculated structural descriptors or topological/physical/chemical molecular descriptors. Key descriptors are ranked and can be used for guiding molecular structure design. Moreover, the accuracy of ML models is double‐verified by comparing the predicted results with experimental ϴ values and the trend of experimental EQE based on a group of materials. Using compressed sensing technology, the low‐dimension material space is also visually constructed based on key descriptors, and the results are consistent with those of the ML models.

Enhanced performance of solution processed OLED devices based on PFO induced TADF emission layers

The internal quantum efficiency (IQE) of the OLED devices based on thermally activated delayed fluorescence (TADF) materials was thought to nearly 100 % by assuming the light outcoupling efficiency of 20–25 %. However, the external quantum efficiency (EQE) is still limited by the ηout. One crucial factor that affects the light output coupling efficiency is the molecular transition dipole moment of the emission molecule. In this study, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was chosen as an orientation inducer to enhance the horizontal orientation ratio of a TADF emitter, 2-Phenyl-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl] pyrimidine (DMAc-PPM), prepared using a solution method. With doping PFO in weight ratios ranging from 1 wt% to 9 wt%, the orientation factor of TADF emitter increased, which testified the inducing function of PFO. The TADF-OLED devices using PFO as an inducer exhibited an EQE increasing with the PFO doping weight ratio. The maximum EQE of 7.3 % was achieved at the doping weight ratio of 7 wt%, which increase about 1.7 times compared to the device without the PFO inducer. This observed variation of EQE differs from the trend observed in maximum luminance intensity. The increase of EQE can be attributed to the improvement of light output efficiency caused by the variation of orientation factor. The lower carrier mobility lead to the lower maximum luminance intensity. The synergistic effect of light output efficiency and carrier mobility enhanced the performance of OLED devices.

Collective States in Molecular Monolayers on 2D Materials.

Collective excited states form in organic two-dimensional layers through Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared with the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening, as well as spatial disorder that occurs in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space, we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with a peak position and integrated intensity similar to those for the perfectly ordered system. We measured the optical response of a monolayer of the perylene derivative MePTCDI on two-dimensional materials. On the wide-band-gap insulator hexagonal boron nitride, it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When the semimetal graphene is used as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multiwavelength Raman scattering, we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by nonradiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons and molecular vibrations. The excellent properties of organic monolayers make them promising candidates for components of soft-matter optoelectronic devices.

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks.

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Two‐Dimensional Organic Crystals Enabling Anisotropic Photon Transport and Dual‐Color Orthogonally Polarized Laser

AbstractAnisotropic transport of polarized photons in organic 2D crystals provides a great opportunity to construct integrated photonic circuits. However, such study remains a critical challenge limited by the precise orientation of molecular transition dipole moment (TDM) and modification of the interaction between photons and molecular TDM. Herein, a stilbene derivative with strong solid‐state photoluminescence is designed and synthesized. The excellent anisotropic polarization‐dependent waveguide occurs in its parallelogram 2D crystal and further enables 463‐ and 493‐nm lasing emissions along the short and long axes of the crystal, respectively. Polarization measurements show that the two lasers with the different wavelengths have mutually orthogonal linear polarizations. These results provide a new strategy for designing orthogonally polarized lasers in single‐component 2D crystals, which may inspire further development of organic photonic devices with controllable polarization properties.

Enhanced performance of solution-processed OLEDs by altering the molecular transition dipole moment orientation of emission layers

Transition dipole moment orientation is one of the crucial factors, which can enhance the performance of organic light-emitting diodes (OLEDs). In this work, the transition dipole moment orientation of the host-guest emission layers (EMLs) prepared by solution method annealed at different temperatures were systematically studied by analyzing the angle-dependent PL spectrum. When the EMLs of 2DPAc-MPM (20 wt%): DPEPO was annealed at the temperature of 115 °C, the photoluminescence quantum yield (PLQY) was enhanced to 78%±3.5%, and the vertical dipole ratio was reduced to 0.34. The lower vertical orientation of transition dipoles is conducted to improve the electron and hole mobility, which was testified by the hole and electron only devices. The lower vertical dipole ratio, higher PLQY and carrier mobility together enhanced the performance of solution processed OLEDs.

Enhanced Light–Matter Interaction and Polariton Relaxation by the Control of Molecular Orientation

AbstractExciton‐polaritons, in which the electronic state of an excited organic molecule and a photonic state are strongly coupled, can form a Bose–Einstein condensate (BEC) at room temperature. However, so far, the reported thresholds of organic polariton BECs under optical excitation are as high asPth = 11–500 μJ cm–2. One route toward lowering the condensation threshold is to increase the Rabi energy by aligning the molecular transition dipole moments. In this report, it is demonstrated that control of the orientation of a perylene‐based discotic dye, which is able to self‐organize in mesogenic columnar structures, can significantly enhance exciton–photon interaction and polariton relaxation rate in optical cavities. These results show the importance of the molecular orientation for strong light–matter interactions and provide a promising strategy toward the realization of an organic low threshold polariton BEC system and electrically driven organic polariton BEC.

Nanoscale Mapping of Morphology of Organic Thin Films.

We determine precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We used FIM microscopy to detect the orientation of molecular transition dipole moments from an extremely low density of luminescent dye molecules, which we call "morphology sensors". The orientation of the sensor molecules is driven by the local film structure and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. We demonstrate morphological mapping with a depth resolution to a few Ångstroms that is limited by the ability to determine thickness during deposition, along with an in-plane resolution limited by optical diffraction. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices.

Unveiling the Impact of Aggregation on Optical Anisotropy of Triazaacephenanthrylene Single Crystals. A Combined Quantum Crystallography and Conceptual Density Functional Theory Approach.

Triazaacephenanthrylene (TAAP) triclinic single crystals show substantial optical anisotropy of absorption and fluorescence. The maximum effect can be correlated with the direction perpendicular to the plane of chromophores connected in a head-to-tail manner via weak dispersive interactions. This phenomenon is uncommon as usually the existence of postulated π···π interactions between the molecules forming dimers or stacks cause quenching of fluorescence. Herein we present a comprehensive study of inter- and intramolecular interactions in the crystal of TAAP enriched with the investigation of aromaticity. Our results show that intramolecular interactions stabilize the overall conformation of the molecule whereas dispersive forces determine the aggregation between TAAP molecules. In fact, there is no conventional π···π interaction between the molecules in the dimer. Instead, we observed a close contact between the lone pair of the bridgehead N10B atom and π-deficient pyrazine ring from an adjacent molecule. Optical anisotropy in TAAP crystals was directly correlated with the alignment of the molecular transition dipole moments caused by specific molecular self-assembly.

Molecular aggregates of pyronin dyes with polyelectrolyte polystyrene sulfonate (PSS) in aqueous solution

Aggregation of the cationic dyes pyronin B (PyB) and pyronin Y (PyY) in aqueous solution containing anionic polyelectrolyte polystyrene sulfonate (PSS) has been studied by using steady-state and time-resolved fluorescence and UV–vis absorption spectroscopy techniques. Although H-aggregates of these pyronin dyes have been previously observed in colloidal suspension, this is the first report of their J-aggregates obtained as a function of dye concentration by using constant concentration of PSS in solution. The best dye concentrations for the J-aggregates were determined as 2.4 × 10−5 M and 1.2 × 10-4 M for PyB and PyY, respectively. Moreover, aggregation equilibrium constants (KD) and thermodynamic functions were determined as a function of temperature for H-type aggregation. By employing the exciton theory, angle (α) between the molecular transition dipole moments and the long aggregate axis, and the intermolecular distance (R) were also calculated for each dye aggregates. In addition, the fluorescence lifetime values were determined from fluorescence decay measurements for the monomer and aggregated species of pyronin compounds in polyelectrolyte aqueous solution.

Linear and nonlinear Herzberg−Teller vibronic coupling effects. II: Hole-burning and fluorescence line narrowing spectroscopy

Part I of this series showed that Herzberg-Teller vibronic coupling manifests itself in linear spectroscopy through the breakdown of the mirror image symmetry between the absorption and emission spectra, whereby a nuclear exponential function was used to represent the molecular electronic transition dipole moment. This part tests the applicability of the framework developed in Paper I to absorption-emission asymmetry in nonlinear signals such as hole-burned and fluorescence line narrowed spectra. Model calculations of nonlinear absorption and emission spectra showing asymmetry is provided. Hole-burning and fluorescence line narrowing signals are to exemplify nonlinear absorption and emission spectra so as to reveal the anticipated asymmetry in 4-wave mixing experiments, and probe the consequent zero-phonon hole and the phonon-sideband hole. These calculations prove to be efficient and fast, exhibiting remarkable numerical stability. The observation of richer phonon-side band (PSB) in linear spectra, phono-side band hole (PSBH) in Hole-burning and fluorescence line narrowing signals, and emergence of new quantum beats over longer time scale (despite the dissipative medium) under the non-Condon regime are linked to the long quantum coherence phenomenon and stimulated photon peak shift in photosynthetic systems.

Fast predictions of exciton diffusion length in organic materials

Simple computational protocol for the estimation of singlet exciton diffusion length in organic materials.

Spectroscopic properties and cold elastic collisions of alkaline-earth Mg + Mg+ system

In this theoretical work, we calculate molecular potential energy curves, spectroscopic constants and transition dipole moments for the ground and first excited electronic states (A2Πu) of alkaline-earth molecular ion We consider an ab initio multi-reference configuration interaction method using an aug-cc-pv5z Gaussian basis set and compare our results with those obtained by others. The comparison shows a very good agreement. In addition, we investigate the ion-atom elastic collisions for a wide range of energies including the isotopic effect. In close connections, we determine the scattering T-matrix elements for ground-state electronic potentials, which show divergent behavior at a particular energy, indicating the occurrence of scattering resonances. We find that the s-wave scattering length is positive for both ground-state potentials. We also study possible free-bound transitions and Franck–Condon overlap for our system.

Emergence of Chiroptical Properties in Molecular Assemblies of Phenyleneethynylenes: The Role of Quasi-degenerate Excitations.

Chiroptical properties of supramolecular assemblies originate through the asymmetric coupling of molecular transition dipole moments. Herein, we report a joint experimental and theoretical investigation to understand the influence of intermolecular interactions on chiroptical properties, particularly during the early stages of self-assembly. In this regard, phenyleneethynylene-based molecular systems appended with d- and l-isomers of phenylalanine have been synthesized with one as well as two electronic transitions in the spectral region of interest. When self-assembled, these molecules show distinctly different chiroptical properties with the right- and left-handed organizations, guided by the chirality of phenylalanines. The standard exciton approach explains the observation of a bisignated electronic circular dichroism signal in systems with a single transition but fails when applied to systems with two nearby transitions. Here, we present a generalized exciton approach that addresses the unusual chiroptical properties of systems with multiple transitions.

Tunable Light–Matter Hybridization in Open Organic Microcavities

Open microcavities represent a versatile cavity design that allows the external control of internal properties such as cavity thickness and mode detuning without changing the key parameters of the cavity itself, rendering them particularly interesting for light–matter interaction experiments. Here, we demonstrate the tunability of an open microcavity with an embedded active organic layer providing parallel alignment of molecular transition dipole moments as well as strong self-absorption inside the cavity. By decreasing the cavity thickness, we observe a transition from the weak coupling regime into the strong coupling regime evidenced by the onset of avoided crossing behavior between involved modes. This change of coupling mechanism is shown for 2D (planar) as well as 0D (hemispherical) cavities.

Principal molecular axis and transition dipole moment orientations in liquid crystal systems: an assessment based on studies of guest anthraquinone dyes in a nematic host.

An assessment of five different definitions of the principal molecular axis along which molecules align in a nematic liquid crystal system has been made by analysing fully atomistic molecular dynamics (MD) simulations of a set of anthraquinone dyes in the cyanobiphenyl-based nematic host mixture E7. Principal molecular axes of the dyes defined by minimum moment of inertia, minimum circumference, minimum area, maximum aspect ratio, and surface tensor models were tested, and the surface tensor model was found to give the best description. Analyses of MD simulations of E7 alone showed that the surface tensor model also gave a good description of the principal molecular axes of the host molecules, suggesting that this model may be applicable more generally. Calculated dichroic order parameters of the guest-host systems were obtained by combining the surface tensor analysis with fixed transition dipole moment (TDM) orientations from time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dyes, and the trend between the dyes generally matched the trend in the experimental values. Additional analyses of the guest-host simulations identified the range of conformers explored by the flexible chromophores within the dyes, and TD-DFT calculations on corresponding model structures showed that this flexibility has a significant effect on the TDM orientations within the molecular frames. Calculated dichroic order parameters that included the effects of this flexibility gave a significantly improved match with the experimental values for the more flexible dyes. Overall, the surface tensor model has been shown to provide a rationale for the experimental alignment trends that is based on molecular shape, and molecular flexibility within the chromophores has been shown to be significant for the guest-host systems: the computational approaches reported here may be used as a general aid in the predictive design of dyes with appropriate molecular shapes and flexibilities for guest-host applications.

Experimental and molecular dynamics studies of anthraquinone dyes in a nematic liquid-crystal host: a rationale for observed alignment trends.

Five anthraquinone dyes with bis(4-propylphenyl) substituents, connected via sulfide or amine linking groups at the 1,5-positions or directly at the 2,6-positions, have been studied as guests in the nematic liquid crystal host, E7. Polarized UV-visible absorption spectra of aligned samples were used to obtain experimental dichroic order parameters, which exhibit values in the range 0.51-0.74. Fully atomistic MD simulations of these guest-host systems were carried out, generally using default parameters but using new force constants derived here for the dyes containing flexible phenyl-sulfide and phenyl-amine linking groups. An analysis of the alignment of the dye molecules in these simulations provides calculated molecular order parameters, which are combined with calculated order parameters for the alignment of the transition dipole moments within the dyes, reported previously, to give calculated dichroic order parameters. The trend in the calculated dichroic order parameters between the dyes shows a good match with the trend in the experimental values, enabling the observed variation to be rationalised primarily by changes in the alignment of the calculated transition dipole moments within the dyes; the calculated molecular order parameters show a relatively small variation between the dyes. The results indicate that this computational approach may be used generally to rationalise trends in the alignment of guest molecules in liquid crystal hosts, suggesting that it may also be able to provide a predictive aid in the design of guest dyes.

Simulations of fluorescence enhancement and emission profile changes in porphyrin attached to gold-silica core–shell nanoparticles

Changes in the spectral and photophysical parameters of a porphyrin derivative caused by placing the molecule in the vicinity of a spherical gold or core–shell gold-silica nanoparticle have been predicted using Mie theory. The simulations allow the optimization of the system with regard to maximum brightness enhancement. The results demonstrate that, in addition to fluorescence quantum yield and decay time, the spectral emission profile may also be modified by an appropriate choice of the nanoparticle parameters, such as gold core radius and shell thickness. Both emission intensity and shape are also dependent on the relative orientation of the molecular transition dipole moment with respect to the nanoparticle surface.