Saul T. E. Jones

High-performance light-emitting diodes based on carbene-metal-amides

Adding a twist for enhanced performance The efficiency of organic light-emitting diodes (OLEDs) is fundamentally governed by the ratio of emissive singlet to dark triplet excitons that are formed from spin-polarized electron and hole currents within the material. Typically, this has set an upper limit of 25% internal quantum efficiency for OLEDs. Di et al. manipulated the ratio of spin states through a modification of process chemistry. They introduced a rotation of the molecular structure, which inverted the spin-state energetics and enhanced OLED performance. Science, this issue p. 159 Spin-state inversion via intramolecular rotation can enhance the performance of solution-processed organic light-emitting diodes. Organic light-emitting diodes (OLEDs) promise highly efficient lighting and display technologies. We introduce a new class of linear donor-bridge-acceptor light-emitting molecules, which enable solution-processed OLEDs with near-100% internal quantum efficiency at high brightness. Key to this performance is their rapid and efficient utilization of triplet states. Using time-resolved spectroscopy, we establish that luminescence via triplets occurs within 350 nanoseconds at ambient temperature, after reverse intersystem crossing to singlets. We find that molecular geometries exist at which the singlet-triplet energy gap (exchange energy) is close to zero, so that rapid interconversion is possible. Calculations indicate that exchange energy is tuned by relative rotation of the donor and acceptor moieties about the bridge. Unlike other systems with low exchange energy, substantial oscillator strength is sustained at the singlet-triplet degeneracy point.

Luminescent Gold(III) Thiolates: Supramolecular Interactions Trigger and Control Switchable Photoemissions from Bimolecular Excited States

Abstract A new family of cyclometallated gold(III) thiolato complexes based on pyrazine‐centred pincer ligands has been prepared, (C^Npz^C)AuSR, where C^Npz^C=2,6‐bis(4‐ButC6H4)pyrazine dianion and R=Ph (1), C6H4 tBu‐4 (2), 2‐pyridyl (3), 1‐naphthyl (1‐Np, 4), 2‐Np (5), quinolinyl (Quin, 6), 4‐methylcoumarinyl (Coum, 7) and 1‐adamantyl (8). The complexes were isolated as yellow to red solids in high yields using mild synthetic conditions. The single‐crystal X‐ray structures revealed that the colour of the deep‐red solids is associated with the formation of a particular type of short (3.2–3.3 Å) intermolecular pyrazine⋅⋅⋅pyrazine π‐interactions. In some cases, yellow and red crystal polymorphs were formed; only the latter were emissive at room temperature. Combined NMR and UV/Vis techniques showed that the supramolecular π‐stacking interactions persist in solution and give rise to intense deep‐red photoluminescence. Monomeric molecules show vibronically structured green emissions at low temperature, assigned to ligand‐based 3IL(C^N^C) triplet emissions. By contrast, the unstructured red emissions correlate mainly with a 3LLCT(SR→{(C^Npz^C)2}) charge transfer transition from the thiolate ligand to the π⋅⋅⋅π dimerized pyrazine. Unusually, the π‐interactions can be influenced by sample treatment in solution, such that the emissions can switch reversibly from red to green. To our knowledge this is the first report of aggregation‐enhanced emission in gold(III) chemistry.

Star-shaped fluorene–BODIPY oligomers: versatile donor–acceptor systems for luminescent solar concentrators

Luminescent solar concentrators (LSCs) are waveguides doped with luminescent centers that can spectrally and spatially concentrate sunlight. They can reduce the cost of photovoltaic energy production and are attractive prospects for photobioreactors and building-integrated applications. Reabsorption, caused by non-zero overlap between the absorption and emission spectra of the light-emitting centers, often limits LSC efficiency. Donor–acceptor energy-transfer complexes are one method to mitigate reabsorption by shifting the emission away from the main absorption peak. Here we introduce versatile star-shaped donor–acceptor molecules based on a central BODIPY energy acceptor with oligofluorene donor side units. Varying the oligofluorene chain length alters the relative oscillator strengths of the donor and acceptor, changing the severity of reabsorption for a given donor density, but also changing the luminescence yield and emission spectrum. We performed comprehensive device measurements and Monte Carlo ray tracing simulations of LSCs containing three oligofluorene–BODIPY donor–acceptor systems with different oligofluorene chain lengths, and then extended the simulation to study hypothetical analogs with higher donor–acceptor ratios and different terminal acceptors. We found that the measured structures permit waveguide propagation lengths on a par with state-of-the-art nanocrystalline emitters, while the proposed structures are viable candidates for photobioreactor and energy production roles and should be synthesized.

Silicon phthalocyanines as dopant red emitters for efficient solution processed OLEDs

The optical characterisation and device functionality for a series of axially modified silicon phthalocyanines (SiPcs) as dopant red emitters in solution-processed and vapour-deposited organic light-emitting diodes (OLEDs) is presented. Control over the axial group composition enables bandgap maintenance around 1.8 eV and photoluminescence quantum yield values of up to 66% in solution. We confirm efficient energy transfer between a polyfluorene host matrix and a SiPc dopant under photoexcitation, and demonstrate solution-processed OLED devices with narrow electroluminescence spectra (20 nm full-width at half-maximum) close to 700 nm. Vapour-deposited OLEDs based on TCTA as host exhibit characteristic SiPc emission around 715 nm, but with greater spectral contamination from the host. An initial OLED optimisation exercise demonstrates external quantum efficiencies of up to 2.5% in solution-processed devices, indicating that such phthalocyanines may prove promising red dopant emitters for OLEDs with high colour purity.

Exploring the photophysics of carbene metal amides (Conference Presentation)

Light emission in organic semiconductors is governed by the spin of excitons formed upon electrical excitation. Conventionally, 25% of excitons form as emissive singlets and 75% form non-emissive triplets. Exceeding this limit for OLEDs requires designing new materials. Developments in molecular design have allowed utilization of triplet excitons through either direct phosphorescence (1) or secondary processes converting a triplet into a singlet via a spin flip, creating “delayed” fluorescence. (2) Thermally Activated Delayed Fluorescence (TADF) has provided guidelines for creating donor-acceptor molecules, but the effects governing spin dynamics are still being explored. Increasingly, there is consensus that intersystem crossing,(ISC) cannot be understood from a static picture of the molecules; a more dynamic approach is necessary. Carbene Metal Amide (CMA) emitters (3) provide an excellent example, displaying large spectral shifts due to conformational reorganisation and highly variable intersystem crossing rates. In solid films, they have produced solution processed green OLEDs with record efficiencies. Here we show, starting from the green CMA archetypes, we can alter the molecular design to probe the effects of steric hindrance, spin-orbit coupling, and dipole strength on the emission properties. Using fast time resolved cryogenic PL spectroscopy we demonstrate the impact of changing the metal bridge atom on ISC, and explore high molecular weight variants for flexible electronics. We demonstrate these emitters can be tuned across the visible spectrum whilst retaining similar photophysical properties, and achieve efficient OLED devices via both solution and vacuum processing. We discuss their structure property relationships for emission, explore a new set of high efficiency OLED dopants, and provide fundamental insight into their spin conversion mechanism. From these studies we derive the first set of design rules for this new class of organometallic TADF emitters. 1) Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Baldo et al. Appl. Phys. Lett. 1999 2) Highly efficient organic light-emitting diodes from delayed fluorescence. Uoyama et al. Nature 2012 3) High-performance light-emitting diodes based on carbene-metal-amides, Di et al. Science, 2017

Linear carbene metal amides as a new class of emitters for highly efficient solution-processed and vapor-deposited OLEDs (Conference Presentation)

Current materials leaders in OLED technology are largely based on phosphorescent iridium complexes and Thermally Activated Delayed Fluorescence (TADF) materials which emit by harvesting light from all excited states ensuring nearly 100% internal quantum efficiency (IQE). Although, high efficiency red, green and blue OLEDs were realized, very short operating stability remains a fundamental challenge for blue OLEDs. Here we present our materials design strategy. We have recently designed numerous linear coinage metal complexes with efficient photo- and electroluminescent properties.[1,2] Our materials are composed of the donor and acceptor ligands which are linked by a coinage metal atom. Linear geometry of coinage metal complexes enables rotational flexibility. Rotation about the metal-ligand bond allowed us to tune the energy gap between singlet and triplet excited states. When the gap is close to zero, facile intersystem crossing and reversed intersystem crossing are possible which enables efficient singlet and triplet excited state harvesting. Depending on the value of the energy gap we have designed various functional materials with phosphorescent or delayed fluorescence properties. As a proof of concept, we fabricated OLED devices with exceptionally high external quantum efficiencies (>28% EQE) in both solution-processed and vacuum-deposited OLEDs.[3] Power and current efficiency are comparable to or exceeding state-of-the-art phosphorescent OLEDs and quantum dot LEDs. Our materials possess short excited state lifetime (100-300 ns) for the delayed emission which is highly important for the fabrication of the long-lived OLEDs. [1] A.S. Romanov, D. Di, L. Yang, J. Fernandez-Cestau, C.R. Becker, C.E. James, B. Zhu, M. Linnolahti, D. Credgington, M. Bochmann, Chem. Commun., 52, 6379 (2016) [2] A.S. Romanov, C.R. Becker, C.E. James, D. Di, D. Credgington, M. Linnolahti, M. Bochmann, Chem. Eur. J., 23, 4625 (2017). [3] D. Di, A.S. Romanov, L. Yang, J.M. Richter, J.P.H. Rivett, S. Jones, T.H. Thomas, M.A. Jalebi, R.H. Friend, M. Linnolahti, M. Bochmann, D. Credgington, Science, 356, 159 (2017)

Research data supporting "Silicon phthalocyanines as dopant red emitters for efficient solution processed OLEDs".

Underlying UV-Vis, PL, EL and OLED current-voltage-luminance data for the samples discussed in the main article.

Research data supporting "High-performance light-emitting diodes based on carbene-metal-amides"

This dataset includes the experimental results of optical spectroscopy (cryogenic transient photoluminescence, transient absorption, ultrafast transient photoluminescence, transient electroluminescence, steady-state emission/absorption and Raman), OLED device characterization, electrochemistry, as well as data associated with quantum chemical (DFT) calculations.