Query: R H Friend

Candidate papers:

abstract: Organic light emitting diodes (OLEDs)1 5, quantum dot based LEDs6 10, perovskite based LEDs11 13 and micro LEDs14,15 have been championed to fabricate lightweight and flexible units for next generation displays and active lighting. Although there are already some high end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact. Here we demonstrate efficient action of radical based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres the highest reported value for deep red and infrared LEDs. For a standard closed shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin 1/2 doublet. If as expected on energetic grounds both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near unity internal quantum efficiency.Organic light emitting devices containing radical emitters can achieve an efficiency of 27 per cent at deep red and infrared wavelengths based on the excitation of spin doublets, rather than singlet or triplet states.
author_names: Xin Ai

E W Evans

Shengzhi Dong

Alexander J Gillett

Haoqing Guo

Yingxin Chen

Timothy J H Hele

Richard H Friend

Feng Li
title: Efficient radical based light emitting diodes with doublet emission
venue: Nature
year: 2018

abstract: Within the framework of the radical pair mechanism, magnetic fields may alter the rate and yields of chemical reactions involving spin correlated radical pairs as intermediates. Such effects have been studied in detail in a variety of chemical systems both experimentally and theoretically. In recent years, there has been growing interest in whether such magnetic field effects (MFEs) also occur in biological systems, a question driven most notably by the increasing body of evidence for the involvement of such effects in the magnetic compass sense of animals. The blue light photoreceptor cryptochrome is placed at the centre of this debate and photoexcitation of its bound flavin cofactor has indeed been shown to result in the formation of radical pairs. Here, we review studies of MFEs on free flavins in model systems as well as in blue light photoreceptor proteins and discuss the properties that are crucial in determining the magnetosensitivity of these systems.
author_names: E W Evans

Charlotte A Dodson

Kiminori Maeda

Till Biskup

Christopher J Wedge

Christiane R Timmel
title: Magnetic field effects in flavoproteins and related systems
venue: Interface Focus
year: 2013

abstract: With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor acceptor neutral radicals based on an electron poor perchlorotriphenylmethyl or tris(2,4,6 trichlorophenyl)methyl radical moiety combined with different electron rich groups. Experimental and quantum chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor acceptor radicals have a strong emission yield (up to 54% and high photostability, with estimated half lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light emitting diodes based on such a radical emitter show deep red/near infrared emission with a maximal external quantum efficiency of 5.3% Our results provide a simple molecular design strategy for stable, highly luminescent radicals with non Aufbau electronic structures. Organic radical molecules violating the Aufbau principle are shown to reach high luminescence quantum yield and photostability, with potential applications for deep red and near infrared light emitting sources.
author_names: Haoqing Guo

Qiming Peng

Xian-Kai Chen

Qinying Gu

Shengzhi Dong

E W Evans

Alexander J Gillett

Xin Ai

Ming Zhang

Dan Credgington

Veaceslav Coropceanu

Richard H Friend

Jean-Luc Bredas

Feng Li
title: High stability and luminescence efficiency in donor acceptor neutral radicals not following the Aufbau principle
venue: Nature Materials
year: 2019

abstract: Magnetic fields as weak as the Earth's can change the yields of radical pair reactions even though the energies involved are orders of magnitude smaller than the thermal energy, kBT, at room temperature. Proposed as the source of the light dependent magnetic compass in migratory birds, the radical pair mechanism is thought to operate in cryptochrome flavoproteins in the retina. Here we demonstrate that the primary magnetic field effect on flavin photoreactions can be amplified chemically by slow radical termination reactions under conditions of continuous photoexcitation. The nature and origin of the amplification are revealed by studies of the intermolecular flavin tryptophan and flavin ascorbic acid photocycles and the closely related intramolecular flavin tryptophan radical pair in cryptochrome. Amplification factors of up to 5.6 were observed for magnetic fields weaker than 1 mT. Substantial chemical amplification could have a significant impact on the viability of a cryptochrome based magnetic compass sensor.
author_names: Daniel R Kattnig

E W Evans

Victoire Dejean

Charlotte A Dodson

Mark Ian Wallace

Stuart R Mackenzie

Christiane R Timmel

P J Hore
title: Chemical amplification of magnetic field effects relevant to avian magnetoreception.
venue: Nature chemistry
year: 2016

abstract: Covalent organic frameworks (COFs) are an emerging class of highly tuneable crystalline, porous materials. Here we report the first COFs that change their electronic structure reversibly depending on the surrounding atmosphere. These COFs can act as solid state supramolecular solvatochromic sensors that show a strong colour change when exposed to humidity or solvent vapours, dependent on vapour concentration and solvent polarity. The excellent accessibility of the pores in vertically oriented films results in ultrafast response times below 200 ms, outperforming commercially available humidity sensors by more than an order of magnitude. Employing a solvatochromic COF film as a vapour sensitive light filter, we demonstrate a fast humidity sensor with full reversibility and stability over at least 4000 cycles. Considering their immense chemical diversity and modular design, COFs with fine tuned solvatochromic properties could broaden the range of possible applications for these materials in sensing and optoelectronics.Covalent organic frameworks (COFs) find increasing application as sensor material, but fast switching solvatochromism was not realized. Here the authors demonstrate that combination of electron rich and deficient building blocks leads to COFs which fast and reversibly change of their electronic structure depending on the surrounding atmosphere.
author_names: Laura Ascherl

E W Evans

Matthias Hennemann

Daniele Di Nuzzo

Alexander G Hufnagel

Michael Beetz

Richard H Friend

Timothy B Clark

Thomas Bein

Florian Auras
title: Solvatochromic covalent organic frameworks
venue: Nature Communications
year: 2018

abstract: Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (kBT) it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green's function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin selective recombination reactions, and the effects of the inter radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ~500 mT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes.
author_names: E W Evans

Daniel R Kattnig

Kevin B Henbest

P J Hore

Stuart R Mackenzie

Christiane R Timmel
title: Sub millitesla magnetic field effects on the recombination reaction of flavin and ascorbic acid radicals.
venue: The Journal of chemical physics
year: 2016

abstract: Electrically injected charge carriers in organic light emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for achieving high quantum efficiency. Dopants based on thermally activated delayed fluorescence (TADF) have emerged as promising candidates for addressing the spin statistics issue in OLEDs. In these materials, reverse singlet triplet intersystem crossing (rISC) becomes efficient, thereby activating luminescence pathways for weakly emissive triplet states. However, despite a growing consensus that torsional vibrations facilitate spin orbit coupling (SOC driven ISC in these molecules, there is a shortage of experimental evidence. We use transient electron spin resonance and theory to show unambiguously that SOC interactions drive spin conversion and that ISC is a dynamic process gated by conformational fluctuations for benchmark carbazolyl dicyanobenzene TADF emitters.
author_names: E W Evans

Yoann Olivier

Yuttapoom Puttisong

William K Myers

Timothy J H Hele

S Matthew Menke

Tudor H Thomas

Dan Credgington

David Beljonne

Richard H Friend

Neil C Greenham
title: Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules.
venue: The journal of physical chemistry letters
year: 2018

abstract: Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have not attracted much attention as functional components of the final material. We have developed a new series of dual pore perylene based COFs and demonstrated that their imine bonds can be protonated reversibly, causing significant protonation induced color shifts toward the near infrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapor sensors with a detection limit as low as 35 mg L 1 and a response range of at least 4 orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of nonaqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as function determining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their possible applications.
author_names: Laura Ascherl

E W Evans

Jeffrey Gorman

Sarah Orsborne

Derya Bessinger

Thomas Bein

Richard H Friend

Florian Auras
title: Perylene Based Covalent Organic Frameworks for Acid Vapor Sensing.
venue: Journal of the American Chemical Society
year: 2019

abstract: Efficient vacuum processed organic light emitting diodes are fabricated using a carbene metal amide material, CMA1. An electroluminescence (EL) external quantum efficiency of 23% is achieved in a host free emissive layer comprising pure CMA1. Furthermore external quantum efficiencies of up to 26.9% are achieved in host guest emissive layers. EL spectra are found to depend on both the emissive layer doping concentration and the choice of host material, enabling tuning of emission color from mid green (Commission Internationale de l'Eclairage co ordinates [0.24, 0.46] to sky blue [0.22 0.35] without changing dopant. This tuning is achieved without compromising luminescence efficiency >80% while maintaining a short radiative lifetime of triplets <1 ms)
author_names: Patrick J Conaghan

S Matthew Menke

Alexander S Romanov

Saul T E Jones

Andrew J Pearson

E W Evans

Manfred Bochmann

Neil C Greenham

Dan Credgington
title: Efficient Vacuum Processed Light Emitting Diodes Based on Carbene Metal Amides.
venue: Advanced materials
year: 2018

abstract: Cis(or trans) [RuCl2(CO)2(PPh3)2] react with two and one equivalents of AgBF4 to give the recently reported [Ru(CO)2(PPh3)2][BF4]2*CH2Cl2 (1) and novel [RuCl(CO)2(PPh3)2][BF4] 12 CH2Cl2 (2) respectively. Cis [RuCl2(CO)2(PPh3)2] also reacts with two equivalents of AgBF4 in the presence of CO to give [Ru(CO)3(PPh3)2][BF4]2 (3) Reactions of 1 and 2 with NaOMe and CO at 1 atm produce the carbomethoxy species [Ru(COOMe)2(CO)2(PPh3)2] (4) and [RuCl(COOMe)(CO)2(PPh3)2] (5) respectively. Complex 4 can also be formed from the reaction of 3 with NaOMe and CO. Alternatively, 4 is formed from cis [RuCl2(CO)2(PPh3)2] with NaOMe and CO at elevated pressure (10 atm) if these reactants are refluxed under 1 atm of CO, [Ru(CO)3(PPh3)2] is the product. The reaction of [RuCl(CO)3(PPh3)2][AlCl4] with NaOMe provides an alternative route to the preparation of 5, but the product is contaminated with [RuCl2(CO)2(PPh3)2] Compounds 1. 2, 4 and 5 have been characterised by IR, 1H NMR and analysis, whilst the formulation of 3 is proposed from spectroscopic data only. This account also examines the reactivity of [Ru(CO)2(PPh3)2][BF4]2 CH2Cl2 with NaBH4, conc. HCl, KI and, finally, MeCOONa in the presence of CO. The products of these reactions, namely cis [RuH2(CO)2(PPh3)2] cis [RuCl2(CO)2(PPh3)2] cis [RuI2(CO)2(PPh3)2] and [Ru(OOCMe)2(CO)2(PPh3)2] have been identified by comparison of their spectra with previous literature.
author_names: E W Evans

Mohammed B H Howlader

Mark T Atlay
title: Reactivity of cationic carbonyl/phosphine ruthenium(II) compounds
year: 1995