Theory and principles of luminescent organic radical materials for OLED and sensor applications

Introduction

Organic luminescent radicals (OLRs) are quite rare emitters demonstrating doublet-doublet fluorescence and having a number of advantages towards practical applications in organic light emitting diodes (OLEDs), ratiometric fluorescence sensors, and anti-counterfeiting labeling.

Doublet-Doublet Fluorescence

The doublet-doublet fluorescence by OLRs is a spin-allowed process similarly to the common singlet-singlet fluorescence. However, in contrast to closed-shell molecules for which “bright” excited singlet states are higher in energy than “dark” triplet states, the doublet excited states of OLRs are always lower in energy than the quartet states.

Theoretical Efficiency

That is why the theoretical limit for internal quantum efficiency of OLR-based OLEDs is expected to be 100%, and why the quartet excited states in general have not been considered for OLRs. However, recent experimental studies indicate that quartet states of OLR emitters indeed can be populated in OLEDs, thus reducing their efficiency.

Project Aim

In the current project, I aim to develop general theory and principles of OLRs in order to involve quartet states into the emission process and to boost the efficiency of OLEDs beyond state-of-the-art results.

Sensor Applications

Another challenge for this project is the design of OLRs for sensor applications. Most OLRs possess low-lying first excited doublet states that make them perfect anti-Kasha emitters, for which emission occurs from higher excited states. This is something that is rare for closed-shell systems but required for ratiometric fluorescence sensors.

Anti-Kasha Emission

In this project, I aim to extend the principles of anti-Kasha emission for OLRs to make a breakthrough in state-of-the-art ratiometric detection of radicals.

Anti-Counterfeiting Labelling

Finally, a special kind of sensing called two-step anti-counterfeiting labeling will be developed in this project based on photoresponsive aromatic carbonyls. These compounds are environmentally friendly and can easily generate stable OLRs upon UV irradiation, which I will utilize for innovative anti-counterfeiting applications.