Electronic Structure Studies and Photophysics of Luminescent Th(IV) Anilido and Imido Complexes

A series of thorium anilide compounds [ThNHAr^R(TriNOx)] (R = para-OCH₃ (1-Ar), para-H (1-Ar), para-Cl (1-Ar), para-CF₃ (1-Ar), TriNOx^3- = tris(2-tert-butylhydroxylaminato)benzylamine), and their corresponding imido compounds [Li(DME)][Th═NAr^R(TriNOx)] (2-Ar) as well as the alkyl congeners [ThNHAd(TriNOx)] (1-Ad) and [Li(DME)][Th═NAd(TriNOx)] (2-Ad), have been prepared. The para-substituents on the arylimido moiety were introduced for systematic variation of their electron-donating and withdrawing abilities, changes that were evident in measurements of the ¹³C{¹H} NMR chemical shifts of the ipso-C atom of the Ar^R moiety. Room temperature, solution-state luminescence of the four new thorium imido compounds, along with the previously reported [Li(THF)₂][Th═NAr^3,5-CF3(TriNOx)] (2-Ar) and [Li(THF)(Et₂O)][Ce═NAr^3,5-CF3(TriNOx)] (3-Ar) have been described. Among these complexes, 2-Ar demonstrated the most intense luminescence feature with excitation at 398 nm and emission at 453 nm. The luminescence measurements, together with a time-dependent density functional theory (TD-DFT) study, helped uncover an intra-ligand n → π* transition that was assigned as the origin of the bright blue luminescence; 3-Ar has an 1.2 eV redshift in excitation energy compared with its proligand. The weak luminescence of other derivatives (2-Ar and 3-Ar) was attributed to non-radiative decay from low-lying excited states originating from inter-ligand transitions (2-Ar) or ligand-to-metal charge transfer bands (3-Ar). Overall, the results expand the range of the thorium imido organometallic compounds and demonstrate that thorium(IV) complexes can support strong ligand luminescence. The results also demonstrate the utility of applying a Th(IV) center for tuning the n → π* luminescence energy and intensity of an associated imido moiety.