The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

Effects of Deuterium Isotopes on Pt(II) Complexes and Their Impact on Organic NIR Emitters

Insight into effect of deuterium isotopes on organic near-IR (NIR) emitters was explored by the use of self-assembled Pt(II) complexes H-3-f and HPh-3-f, and their deuterated analogues D-3-f and DPh-3-f, respectively (Scheme 2). In vacuum deposited thin film, albeit having nearly identical emission spectral feature maximized at ~810 nm, H-3-f and D-3-f exhibit remarkable difference in photoluminescence quantum yield (PLQY) of 29 % and 50 %, respectively. Distinction in PLQY is also observed for HPh-3-f (800 nm, 50 %) and DPh-3-f (798 nm, 67 %). We then elucidated the theoretical differences in the impact on near-infrared (NIR) luminescence between Pt(II) complexes and organic small molecules upon deuteration. The results establish a general guideline for the deuteration on NIR emission efficiency. From a perspective of practical application, NIR OLEDs based on D-3-f and DPh-3-f emitters attain EQEmax of 15.5 % (radiance 31,287 mW Sr-1  m-2 ) and 16.6 % (radiance of 32,279 mW Sr-1  m-2 ) at 764 nm and 796 nm, respectively, both of which set new records for NIR OLEDs of >750 nm.

Minimizing non-radiative decay in molecular aggregates through control of excitonic coupling

The widely known "Energy Gap Law" (EGL) predicts a monotonically exponential increase in the non-radiative decay rate (knr) as the energy gap narrows, which hinders the development of near-infrared (NIR) emissive molecular materials. Recently, several experiments proposed that the exciton delocalization in molecular aggregates could counteract EGL to facilitate NIR emission. In this work, the nearly exact time-dependent density matrix renormalization group (TD-DMRG) method is developed to evaluate the non-radiative decay rate for exciton-phonon coupled molecular aggregates. Systematical numerical simulations show, by increasing the excitonic coupling, knr will first decrease, then reach a minimum, and finally start to increase to follow EGL, which is an overall result of two opposite effects of a smaller energy gap and a smaller effective electron-phonon coupling. This anomalous non-monotonic behavior is found robust in a number of models, including dimer, one-dimensional chain, and two-dimensional square lattice. The optimal excitonic coupling strength that gives the minimum knr is about half of the monomer reorganization energy and is also influenced by system size, dimensionality, and temperature.

Efficient Near-Infrared Luminescence of Self-Assembled Platinum(II) Complexes: From Fundamentals to Applications

ConspectusDesigning bright and efficient near-infrared (NIR) emitters has drawn much attention due to numerous applications ranging from biological imaging, medical therapy, optical communication, and night-vision devices. However, polyatomic organic and organometallic molecules with energy gaps close to the deep red and NIR regime are subject to dominant nonradiative internal conversion (IC) processes, which drastically reduces the emission intensity and exciton diffusion length of organic materials and hence hampers the optoelectronic performances. To suppress nonradiative IC rates, we suggested two complementary approaches to solve the issues: exciton delocalization and molecular deuteration. First, exciton delocalization efficiently suppresses the molecular reorganization energy through partitioning to all aggregated molecules. According to the IC theory together with the effect of exciton delocalization, the simulated nonradiative rates with the energy gap ΔE = 10⁴ cm^-1 decrease by around 10⁴ fold when the exciton delocalization length equals 5 (promoting vibronic frequency ω_l = 1500 cm^-1). Second, molecular deuterations reduce Franck-Condon vibrational overlaps and vibrational frequencies of promoting modes, which decreases IC rates by 1 order of magnitude in comparison to the rates of nondeuterated molecules under ΔE of 10⁴ cm^-1. Although deuteration of molecules has long been attempted to increase emission intensity, the results have been mixed. Here, we provide a robust derivation of the IC theory to demonstrate its validity, especially to emission in the NIR region.The concepts are experimentally verified by the strategic design and synthesis of a class of square-planar Pt(II) complexes, which form crystalline aggregates in vapor deposited thin films. The packing geometries are well characterized by the grazing angle X-ray diffraction (GIXD), showing domino-like packing arrangements with the short ππ separation of 3.4-3.7 Å. Upon photoexcitation, such closely packed assemblies exhibit intense NIR emission maximized in the 740-970 nm region through metal-metal-to-ligand charge transfer (MMLCT) transition with unprecedented photoluminescent quantum yield (PLQY) of 8-82%. To validate the existence of exciton delocalization, we applied time-resolved step-scan Fourier transform UV-vis spectroscopy to probe the exciton delocalization length of Pt(II) aggregates, which is 5-9 molecules (2.1-4.5 nm) assuming that excitons mainly delocalized along the direction of ππ stacking. According to the dependence of delocalization length vs simulated IC rates, we verify that the observed delocalization lengths contribute to the high NIR PLQY of the aggregated Pt(II) complexes. To probe the isotope effect, both partially and completely deuterated Pt(II) complexes were synthesized. For the case of the 970 nm Pt(II) emitter, the vapor deposited films of per-deuterated Pt(II) complexes exhibit the same emission peak as that of the nondeuterated one, whereas PLQY increases ∼50%. To put the fundamental studies into practice, organic light-emitting diodes (OLEDs) were fabricated with a variety of NIR Pt(II) complexes as the emitting layer, showing the outstanding external quantum efficiencies (EQEs) of 2-25% and the remarkable radiances 10-40 W sr^-1 m^-2 at 740-1002 nm. The prominent device performances not only successfully prove our designed concept but also reach a new milestone for highly efficient NIR OLED devices.This Account thus summarizes our approaches about how to boost the efficiency of the NIR emission of organic molecules from an in-depth fundamental basis, i.e., molecular design, photophysical characterization, and device fabrication. The concept of the exciton delocalization and molecular deuteration may also be applicable to a single molecular system to achieve efficient NIR radiance, which is worth further investigation in the future.

Periodic Fluorescence Variations of CdSe Quantum Dots Coupled to Aryleneethynylenes with Aggregation-Induced Emission

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anticorrelated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation-induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation-induced emission, can result in emerging optical properties.

Multiply “trapped” 3[trans-Pt(PR3)2(CCC6H4X)2]* conformers in rigid media

The triplet emission exhibits multiple well-defined decays depending on the θ₁ and θ₂ values and environment in the crystal sites.

Theoretical investigation on the crystal structures and electron transport properties of several nitrogen-rich pentacene derivatives

Exploring and synthesizing new simple n-channel organic semiconductor materials with large electron mobility and high air stability have remained a major challenge and hot issue in the field of organic electronics. In the current work, the electron transport properties of four novel nitrogen-rich pentacene derivatives (PBD1, PBD2, PBD3, and PBD4) with two cyano groups as potential n-channel OFET materials have been investigated at the molecular and crystal levels by means of density functional theory (DFT) calculations coupled with the prediction of crystal structures and the incoherent charge-hopping model. Calculations reveal that the studied compounds, which possess low-lying frontier molecular energy levels, large ionization potentials and electron affinities, are very stable exposed to air. Based on predicted crystal structures, the average electron mobility at room temperature (T = 300 K) for PBD1, PBD2, PBD3, and PBD4 is predicted to be as high as 0.950, 0.558, 0.518, and 1.052 cm²·V⁻¹·s⁻¹, which indicate that these four compounds are more than likely to be promising candidates as n-type OFET materials under favorable device conditions. However, this claim needs experimental verification. In addition, the angular-dependent simulation for electron mobility shows that the electron transport is remarkably anisotropic in these molecular crystals and the maximum μ(e) appears along the crystal axis direction since molecules along this direction exhibit the close face-to-face stacking arrangement with short interplanar distances (~3.6-4.0 Å), which induces large electronic couplings.

Room temperature triplet state spectroscopy of organic semiconductors

Organic light-emitting devices and solar cells are devices that create, manipulate, and convert excited states in organic semiconductors. It is crucial to characterize these excited states, or excitons, to optimize device performance in applications like displays and solar energy harvesting. This is complicated if the excited state is a triplet because the electronic transition is 'dark' with a vanishing oscillator strength. As a consequence, triplet state spectroscopy must usually be performed at cryogenic temperatures to reduce competition from non-radiative rates. Here, we control non-radiative rates by engineering a solid-state host matrix containing the target molecule, allowing the observation of phosphorescence at room temperature and alleviating constraints of cryogenic experiments. We test these techniques on a wide range of materials with functionalities spanning multi-exciton generation (singlet exciton fission), organic light emitting device host materials, and thermally activated delayed fluorescence type emitters. Control of non-radiative modes in the matrix surrounding a target molecule may also have broader applications in light-emitting and photovoltaic devices.

New di-ferrocenyl-ethynylpyridinyl triphenylphosphine copper halide complexes and related di-ferricenyl electro-crystallized materials

Three di-ferrocenyl-ethynylpyridinyl copper complexes have been synthesised and CV measurements made.