Black aluminum molecular rings embedded in glass: ultralow-bandgap composites for broadband optical limiting

Developing nonlinear optical (NLO) materials from earth-abundant elements is key to sustainable photonics. We report aluminum molecular rings (Al₆) stabilized by catechol ligands-the smallest main-group molecular rings known. This versatile strategy enables precise structural control and systematic co-ligand tuning, establishing a robust platform for structure-property studies. Unlike conventional transparent aluminum-oxygen clusters (AlOCs), these catecholate Al₆ systems exhibit a striking black color, ultranarrow band gaps, and exceptional third-order NLO responses, unveiling a new frontier for aluminum-based hybrid materials. For practical implementation, the Al₆ rings were successfully incorporated into bulk silicate glass matrices via a sol-gel process while fully retaining their predefined NLO functionalities. The resulting composites exhibit strong nonlinear optical responses at both 532 nm and 1064 nm, indicating broad-band operational capability. Among these, AlOC-186, with the smallest dimension (1.8 nm), stands out by delivering the highest third-order NLO susceptibility Imχ⁽³⁾ under picosecond excitation at both wavelengths. This work highlights the potential of cost-effective, lightweight elements as competitive alternatives to conventional heavy-metal-based optical limiters and establishes a new avenue for developing high-performance customizable NLO devices.