There are obvious advantages to photoluminescent materials built from globally abundant, nontoxic base elements. With this in mind, our group has been developing the synthesis and structure-property relationships of bismuth-organic materials. We have previously shown that the electronic and structure directing properties of bismuth may be used to achieve long-lived organic phosphorescence, with emission derived from an outer coordination sphere emissive molecule such as 1,10-phenanthrolinium (Hphen).  Building upon that work, we more recently examined the effects of substituted phens (HPhen-R) on the emissive behavior of bismuth-organic materials. Five photoluminescent bismuth(III)-organic compounds were synthesized: two display solely phosphorescence, two exhibit fluorescence, and one shows both fluorescence and phosphorescence. The structural and electronic origins of these differences were examined experimentally using single crystal X-ray diffraction and computationally with TD-DFT. It was found that the phosphorescence originates from the triplet state (T<sub>1</sub>) of the substituted HPhen-R, with excitation involving the bismuth-organic moiety. Bismuth’s involvement induces spin-orbit coupling for triplet state population and additionally acts as a heavy metal attenuator to achieve X-ray luminescence (radioluminescence). It is proposed that the strong π-π interactions in the phosphorescent compounds promote the energy transfer from the bismuth-organic moiety; the fluorescent compounds lack any such π-π interactions, and thus show fluorescence and no radioluminescence. The effect of hydrogen bond strength on the luminescent lifetimes is also discussed.

Controlling phosphorescence and radioluminescence in bismuth-organic materials

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