Rational design of multi-functional thermally activated delayed fluorescence emitters for both sensor and OLED applications

Abstract

Combining two functional applications into one molecule presents great challenges towards molecular design and synthesis. In this work, we developed novel thermally activated delayed fluorescence (TADF) emitters, CzSO-1CO and CzSO-2CO, which not only exhibited a highly selective response to Fe(III) among 10 tested metals in aqueous solutions but also served as efficient hosts for blue phosphorescence organic light-emitting diodes (OLEDs). Due to the sufficient frontier orbital separation, both experimental testing and Gauss simulation demonstrate that the probes have small single/triplet splitting (ΔE_ST), which results in a long fluorescence lifetime of 1.5 μs. The delayed PL spectra demonstrate that the TADF emitter can go beyond the short-time traditional fluorescence and eliminate the strong background interference. Moreover, CzSO-2CO possesses high thermal stability with a T_g of 106 °C and stable fluorescence intensity under UV light irradiation over a wide range of pH values. This photo and thermal stability will ensure long-term utilization in practical applications. In view of its high triplet energy, the solution-processed blue OLED host with CzSO-2CO achieves a maximum current efficiency of 22.5 cd A⁻¹, almost two times that of CzSO-1CO, and a bright luminance of 7500 cd m⁻². As far as we know, this is the first report to develop TADF emitters for application in both ion detection and OLEDs, and the test results indicate that the proposed non-conjugated linking between the luminescence group and identifying moieties is a promising molecular design strategy to construct novel structures to achieve the TADF feature and sensor property in a single system.