Enhanced Third-Order Optical Nonlinearity in a Dipolar Carbene-Metal-Amide Material with Two-Photon Excited Delayed Fluorescence

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 carbenemetal-amide (CMA) material showing a enhanced 2PA cross-section up to 105 GM. Overall molecular design considerations such as extended $\pi$-conjugation (to increase polarizability), minimizing the singlet-triplet energy gap ($\Delta$E ST ), 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$\bullet$10 6 s -1 across CMA materials. Bright red CMA 2P-TADF material shows excellent photostability (LT 50 = 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.