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

A series of thorium anilide compounds [ThNHAr R (TriNOx)] (R = para -OCH 3 ( 1-Ar OMe ), para -H ( 1-Ar H ), para -Cl ( 1-Ar Cl ), para -CF 3 ( 1-Ar 4-CF3 ), TriNOx 3- = tris (2- tert -butylhydroxylaminato)benzylamine), and their corresponding imido compounds [Li(DME)][Th═NAr R (TriNOx)] ( 2-Ar R ) 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 13 C{ 1 H} 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 3,5-CF3 ) and [Li(THF)(Et 2 O)][Ce═NAr 3,5-CF3 (TriNOx)] ( 3-Ar 3,5-CF3 ) have been described. Among these complexes, 2-Ar 3,5-CF3 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 3,5-CF3 has an 1.2 eV redshift in excitation energy compared with its proligand. The weak luminescence of other derivatives ( 2-Ar R and 3-Ar 3,5-CF3 ) was attributed to non-radiative decay from low-lying excited states originating from inter-ligand transitions ( 2-Ar R ) or ligand-to-metal charge transfer bands ( 3-Ar 3,5-CF3 ). 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.