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Heavy metal free emitters for new-generation light sources

Periodic Reporting for period 1 - MEGA (Heavy metal free emitters for new-generation light sources)

Reporting period: 2019-01-01 to 2020-12-31

Organic heavy metal free fluorescent materials show exceptional potential for use in new-generation light sources, such as organic light-emitting devices (OLEDs) and organic lasers. It is anticipated that these new materials will enable organic electronic devices to be constructed with higher efficiency, simpler device structures, lower fabrication costs, and reduced environmental impact. Amongst the different types of materials currently being investigated, two show particular promise:

• Fluorescence materials exhibiting thermally activated delayed fluorescence (TADF) for use in OLEDs in displays and lighting devices.
• Fluorescent materials with low thresholds for amplified spontaneous emission (ASE) for use in organic lasers in spectroscopy and telecommunication.

However, in order to develop these materials for commercial industrial use, several challenges still remain to be overcome, including:

• Theories explaining TADF and ASE are still in their infancy.
• Organic material samples need to be extremely pure (>99.5%). Consequently, new synthesis routes need to be developed.
• TADF emitters for OLEDs have lifetimes that fall well short of industry requirements.
• Fluorescence emitters for lasers need high available optical gain, solution processability and narrow emission spectra with high efficiency.
• Properties of TADF and lasing materials are very sensitive to structural changes.

Thus, the overall goal of the MEGA project is to help develop organic heavy metal free fluorescent materials for commercial use by tackling these challenges. In order to develop the new materials, the following scientific and technical objectives will be targeted:

• Objective 1: Screen compounds with TADF or lasing properties by means of molecular modelling.
• Objective 2: Synthesise most promising compounds with TADF or lasing properties.
• Objective 3: Characterise most promising compounds with TADF or lasing properties.
• Objective 4: Test materials in device structures to meet industry requirement.
"The main achievements during Period 1 have been the following:

WP1 Theory and Screening

• 60+ new molecules grouped in thirteen (13) families of organic compounds for TADF emitters have been designed (Families 1 - 13).
• 30+ new molecules grouped in six (6) families of organic compounds as potential dyes for lasing applications have been designed (Families A - F).
• Calculations on the new molecules have been done to different levels and are on-going.
• A selection of compounds for TADF and lasing applications has been made.
• Progress has been made in scientific understanding of two important aspects of TADF: i) Replacement of ""static"" energy diagrams with ""dynamical"" energy diagrams and ii) TADF in boron nitrogen (BN) based compounds.

WP2 Synthesis

• Synthesis has started of molecules for the thirteen (13) families of organic compounds for TADF emitters (Families 1 - 13).
• Synthesis has started of molecules for the six (6) families of organic compounds as potential dyes for lasing applications (Families A - F).
• An optimized synthetic route has been developed for the preparation of donor-acceptor-donor type TADF emitters using readily available acceptor and donor moieties by efficient Buchwald-Hartwig cross-coupling reactions.
• It was discovered that, for the design and synthesis of stable boron containing emitters, the boron-oxygen bond should be exploited as it gives a higher thermal stability for the target emitters. Such boron-chelate acceptor moieties can be used in various TADF molecular systems as efficient acceptors.

WP3 Characterisation

• A variety of characterisation techniques have been performed on the organic molecules produced for TADF emitters (e.g. Families 1 - 4) and lasing applications (e.g. Families A - F).

WP4 Material Testing in Device Structures

• A protocol has been developed for characterising OLED and key device parameters.
• An OLED reference stack has been developed.
• Optical ellipsometry to be used to determine key optical constants for optical simulation of OLED stacks.

WP5 Project Co-ordination, Management and Dissemination

• All deliverables and milestones completed for Period 1.
• Kick-off and Year 1 meetings held.
• Midterm review held in Nov 2020 and successfully passed.
• 79.7 secondments implemented representing 21.9% of the overall total.
• 11 international journal papers published, which already exceeds the project’s overall target.
• 4 conference papers presented.
• Project video has received 420+ views.
• 7 tweets and 13 followers on project twitter account."
"During Period 1, substantial progress beyond the state of the art has been made in the fields of “theory” and “synthesis” of thermally activated delayed fluorescence (TADF) and amplified spontaneous emission (ASE) materials.

In the field of “theory”, progress has been made in understanding of two important aspects of TADF:

(i) Replacement of ""static"" energy diagrams with ""dynamical"" energy diagrams
This is due to joint contributions from different studies and is based on including the effect of the molecular vibrations in the energy diagrams. The difference with respect to the static diagrams (consisting of three well-distinguished energy levels) resides on two new aspects: (a) a continuum of triplet excited state energy levels is considered in the energy diagram of a given compound, and (b) due to the intra-molecular vibrations, the sense of energy-evolution through the continuum (up or down) is crucial for a better understanding when comparing several compounds.

(ii) TADF in boron nitrogen (BN) based compounds
This aspect concerns a recently discovered class of TADF compounds, in which donor- and acceptor atoms are intercalated in the same block instead of compounds consisting of separated donor- and acceptor blocks. This difference results in the crucial advantage of the BN compounds: both parameters mentioned above can be jointly improved in the BN compounds, as compared to the compounds designed as donor-acceptor intercalated blocks, for which improving one of the parameters can be only achieved at the expense of deteriorating the other parameter.

The new insights achieved by our studies (preliminary conclusion, not yet published) bring to light the reasons why the two parameters can be improved independently: while the energy parameter (EP) can be improved through space extension of the molecular pi-system, the intensity parameter (IP) is by design-concept destined to be large, given that the space-localizations of the electron and hole in the excited state are imbricated in the same block due to the resonance effect, in turn enhanced by the intercalated donor- and acceptor atoms.

In the field of “synthesis”, progress has been made in the following areas:

(i) An optimized synthetic route has been developed for the preparation of donor-acceptor-donor type TADF emitters using readily available acceptor and donor moieties by efficient Buchwald-Hartwig cross-coupling reactions.
(ii) It was discovered that, for the design and synthesis of stable boron containing emitters, the boron-oxygen bond should be exploited as it gives a higher thermal stability for the target emitters. Such boron-chelate acceptor moieties can be used in various TADF molecular systems as efficient acceptors.

By the end of the project, MEGA is expected to advance the economic potential of organic heavy metal free fluorescent materials."
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