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Designing Singlet Fission Materials Using Excited State Aromaticity

Periodic Reporting for period 1 - EXAM (Designing Singlet Fission Materials Using Excited State Aromaticity)

Reporting period: 2020-04-01 to 2022-03-31

Singlet exciton fission (SF) is a carrier multiplication process in organic semiconductors that generates two electron-hole pairs for one photon absorbed, affording quantum efficiencies up to 200%. Photovoltaic devices based on SF have received large attention recently for their potential in efficiency enhancement and to break the Shockley-Queisser limit on the efficiency of single-junction photovoltaics. The main aim of the action is to demonstrate highly stable, tuneable organic materials which undergo SF through the exploitation of the aromaticity of both the ground state and excited states and feasible design rules for these materials. The materials are expected to be promising candidates as SF functional layer for solar cells and other multiple exciton generation applications.
The main objective of the action is to demonstrate highly stable, tuneable organic materials which undergo singlet fission through the exploitation of the aromaticity of both the ground state and excited states and feasible design rules for these materials. The materials are expected to be promising candidates as SF functional layer for solar cells and other multiple exciton generation applications. The resulting concept represents better understanding and tailoring excited state properties of organic semiconductors, which can be expended to wide range of materials with particular excited state nature for even wider application prospect.
During this project the beneficiary has developed a series of organic conjugated materials for the application of singlet fission, and studied the structure-performance relationship behind these systems. The beneficiary also conducted deepened analysis on the Clibalackrot-type compounds, taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters. The theoretical results enable rational designs based on this scaffold. The resulting concept represents better understanding and tailoring excited state properties of organic semiconductors, which can be expended to wide range of materials with particular excited state nature for even wider application prospect.
A series of experiments to synthesize and purify the target Pechmann dyes compouds, which have been further characterized with NMR, HRMS and single crystal XRD. By selecting different end groups, the dyes can be adjusted with various bulky end groups to tuning their solubility or morphology features in solid states.
A series of fulvalene backbone based fuse ring dyes, Cibalacklot scaffold, was selected as the parent model. Comprehensive analysis based on computed aromaticity indices, including nucleus independent chemical shifts scans (NICS-XY) and NICS values at 1.7 Å (NICS(1.7)zz) plots of the anisotropy of the induced current density (AICD), the geometric harmonic oscillator model of aromaticity (HOMA), and the electronic aromatic fluctuation (FLU) and multicentre (MCI) indices, were performed. We extracted a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic, instead of Baird-aromatic, as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory.
A major delay has been caused by the COVID-19 pandemic since Mar 2020 and all through the period of the project, due to lockdowns and limited lab occupation after the reopening, hence the delay of getting experimental data and manuscript writing. Nevertheless, several papers related to the work in this project have been published in leading scientific journals, and several are currently under review or close to submission. The beneficiary couldn’t manage to attend any conference due to the COVID-19 pandemic. However part of the findings have also been presented by other members in the research group in other conferences. The beneficiary has also attended multiple online seminars/workshops and presented the findings in this project.
For commercialisation, the beneficiary has conducted close collaboration with Cambridge Photon Technology, a spin-off company of the university. The candidate has provided important technology advices to the company, which has helped them in securing several patents.
For public engagement, Cambridge University offers a wide range of public engagement and outreach opportunities which the applicant will participate in. The department is very conscious of its role in communicating the importance and excitement of contemporary chemistry and its cognate disciplines to young people and the general public. The applicant will participate in the Cambridge Science Festival (CSF) which is a two-week celebration of science specifically designed to bring cutting edge research to the general public. During the festival, department organized Chemistry Open Day (COD) is an ever-popular activity that open the door for scientists aged from five to 95 to try hands-on activities that explore the fascination of chemistry. In addition to this, communication and public engagement will be conducted where possible by articles in the popular press, radio/television/podcast programmes, etc. For this purpose the University has a dedicated press office to publicise research work at the University. The work of the host team has been cited in the popular press on several occasions. Although this was cancelled in 2020 and the work was not presented in 2021, the beneficiary will seek the chance to join this event in 2022.
We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Also the results elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.
The findings from this project have been part of some exciting collaboration works between the research group in Cambridge and those across Europe. The results are expected to provide further impact on a highly competitive research field in the future. The European scientific and wider community will benefit from this work since it has the potential to deliver an entirely new platform for organic chemistry and optoelectronics. In addition to the direct science and technological advantages of the project, Europe will benefit from the human capital of bringing a highly-trained researcher from outside Europe (China). In addition to the cultural exchange, this will enhance Europe’s scientific collaboration networks.
The candidate has closely worked with Cambridge Photon Technology, and the results have provided significant guidance in developing the photon-multiplying-films, a major targeted product of the company. The findings from this project are expected to provide further impact to the field of optoelectronics, as singlet fission materials are attracting more attentions due to their unique optoelectronic properties.
Hückel-aromatic in both ground state and excited state