Nonalternant acenaphtho[1,2-k]fluoranthene diimide derivatives with outstanding optical-limiting performance

We report the synthesis and characterization of a series of nonalternant acenaphtho[1,2-k]fluoranthene diimide derivatives (L1-L4) featuring different aryl substituents and for the first time examine their reverse saturable absorption (RSA) properties and optical-limiting (OL) application potentials. The combined benefits of a large nonalternant π-framework and unique donor-acceptor type conformation make it possible to improve their chemical stability and intramolecular charge-transfer interaction, offering excellent feasibility of finely regulating their OL behavior based on the RSA mechanism. Z-scan experiments uncover that all compounds exhibit outstanding OL performance, far superior to most reported classical OL materials (e.g., C₆₀). By applying 532 nm nanosecond laser pulses, the thiophene-bearing L1 achieves the lowest OL onset threshold of 7 mJ cm^-2, the lowest OL threshold of 0.123 J cm^-2, and the largest nonlinear absorption coefficient of 941 cm GW^-1, all of which significantly outweigh those of benchmark C₆₀. The RSA mechanism of L1-L4 is well identified by time-resolved transient absorption spectroscopy. The stronger excited-state absorption and longer excited-state lifetimes of L1, L2, and L4 contribute to higher OL performance compared to L3. Our findings demonstrate the effectiveness of imide-functionalized nonalternant polycyclic aromatic hydrocarbons towards constructing high-performance OL materials.