Broadband Photoresponsive Bismuth Halide Hybrid Semiconductors Built with π-Stacked Photoactive Polycyclic Viologen.

Photoresponse ranges of commercially prevailing photoelectric semiconductors, typically Si and InGaAs, are far from fully covering the whole solar spectrum (∼295-2500 nm), resulting in insufficient solar energy conversion or narrow wave bands for photoelectric detection. Recent studies have shown that infinite π-aggregation of viologen radicals can provide semiconductors with a photoelectric response range covering the solar spectrum. However, controlled assembly of an infinite π-aggregate is still a great challenge in material design. Through directional self-assembly of electron-transfer photoactive polycyclic ligands, two crystalline inorganic-organic hybrid photochromic viologen-based bismuth halide semiconductors, ((Me)3pytpy)[BiCl6]·2H2O [1; (Me)3pytpy = N,N',N″-trimethyl-2,4,6-tris(4-pyridyl)pyridine] and ((Me)3pytpy)[Bi2Cl9]·H2O (2), have been synthesized. They represent the first series of pytpy-based photochromic compounds. After photoinduced coloration, the conductivities of both 1 and 2 increased. The radical products have electron absorption bands in the range of 200-1600 nm, exceeding that of Si. Both the conductivity and the photocurrent intensity of 2 are stronger than those of 1, due to better planarity, tighter π-stacking, and higher degrees of overlap of ((Me)3pytpy)3+ cations. This study not only provides a new design idea for synthesizing radical-based multispectral photoelectric semiconductors but also enriches the family of electron-transfer photochromic compounds.