Modulation of charge transfer by N-alkylation to control photoluminescence energy and quantum yield†
Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimental, 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 analogue 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.