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Modulation of Charge Transfer by N-Alkylation to Control Photoluminescence Energy and Quantum Yield

preprint
submitted on 20.04.2020, 20:04 and posted on 22.04.2020, 12:04 by Andrew T. Turley, Andrew Danos, Antonio Prlj, Andrew P. Monkman, Basile F.E. Curchod, Paul R. McGonigal, Marc K. Etherington
Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimetnal, e.g., mediating emission quenching. N-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. N-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine’s two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an N-alkylated analog of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This N-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

Funding

HyperOLED Horizon 2020 project no. 732013

EPSRC Doctoral Training Grant

History

Email Address of Submitting Author

marc.k.etherington@northumbria.ac.uk

Institution

Northumbria University, Ellison Place, Newcastle upon Tyne, NE1 8ST

Country

UK

ORCID For Submitting Author

0000-0003-2101-5757

Declaration of Conflict of Interest

The authors declare no conflict of interest

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