# Enhanced third-order optical nonlinearity in a dipolar carbene-metal-amide material with two-photon excited delayed fluorescence

## TL;DR

A new carbene-metal-amide material shows strong two-photon absorption and delayed fluorescence, making it promising for advanced photonic technologies.

## Contribution

The first dipolar carbene-metal-amide material with enhanced two-photon absorption and thermally activated delayed fluorescence is reported.

## Key findings

- The material exhibits a two-photon absorption cross-section up to 105 GM.
- It shows a high radiative rate of 2.18·10⁶ s⁻¹ for thermally activated delayed fluorescence.
- The material maintains excellent photostability under femtosecond laser excitation.

## Abstract

Advanced photonic materials showing two-photon absorption (2PA) have been widely explored to develop three-dimensional imaging, micro and nanofabrication, all-optical switching, lithography on a nanoscale and many other enabling technologies. These all require nonlinear absorption chromophores with intrinsic 2PA cross-sections and long-term photo- and thermal stability. Here, we disclose the very first example of the dipolar carbene-metal-amide (CMA) material showing a enhanced 2PA cross-section up to 105 GM. Overall molecular design considerations such as extended π-conjugation (to increase polarizability), minimizing the singlet-triplet energy gap (ΔEST), and using heavy metal atoms are the first design principles to obtain bright one- and two-photon excited thermally activated delayed fluorescence (TADF) material, showing one of the highest radiative rate of 2.18·106 s-1 across CMA materials. Bright red CMA 2P-TADF material shows excellent photostability (LT50 = 3 h) to 20 mW femtosecond pulsed laser excitation at 1000 nm, encouraging further CMA exploration for future applications in advanced photonic technologies requiring third-order nonlinear optical properties.

The development of advanced photonic materials exhibiting two-photon absorption (2PA) relies on nonlinear absorption chromophores with enhanced 2PA cross-sections and long-term photo- and thermal stability. Here, the authors report a dipolar carbene-metal-amide (CMA) with a 2PA cross-section up to 105 GM that exhibits third-order nonlinear optical properties — red two-photon–excited thermally activated delayed fluorescence with radiative rates of up to 2.1·106 s–1 — while maintaining excellent photostability.