Switchable ambient-temperature singlet-triplet dual emission in nonconjugated donor-acceptor triarylboron-Pt(II) complexes.

A triarylboron compound Si-BNPA (1) containing a BMes(2) acceptor and an N-(2'-pyridyl)-7-azaindolyl (NPA) donor linked by a tetrahedral silane group has been synthesised. This molecule displays unusual white singlet-triplet dual emission at 77 K, with an exceptionally long phosphorescent decay time (2.2 s). Fluoride titration experiments established that the singlet and triplet emission peaks are due to acceptor-based Mes-->B charge transfer and donor-based 3pi-->pi* transitions, respectively. This dual emission was found to be persistent and observable at ambient temperature in its Pt(II) complex [Pt(N,N-Si-BNPA)Ph2] (2a). Furthermore, 2a was found to undergo intramolecular "roll-over" C-H activation to produce the N,C-chelate complex [Pt(N,C-Si-BNPA)(SMe2)Ph] (2b). This compound also displays ambient temperature singlet-triplet dual emission, but with a much greater phosphorescent efficiency than 2a due to the formation of a more stable chelate ring. Addition of fluoride was found to have little impact on the phosphorescent emission of 2a, but resulted in a large enhancement of the phosphorescent emission intensity of 2b. To establish the impact of donor-acceptor geometry on this singlet-triplet dual emission, the properties of linearly conjugated donor-acceptor complexes [Pt(N,N-BNPA)Ph2] (3a) and [Pt(N,C-BNPA)Ph2] (3b) were also examined. Consistent with 2a and 2b, the N,C-chelate complex 3b has a much higher phosphorescent efficiency than the N,N-chelate 3a. Although 3a and 3b show bright and fluoride-switchable phosphorescence at ambient temperature, they are not dual emissive and show only metal-to-ligand chage-transfer-based phosphorescence. The non-conjugated donor-acceptor geometry and the overlap of the donor and acceptor singlet and triplet excitation bands in Si-BNPA and its Pt(II) complexes may thus be the key for achieving singlet-triplet dual emission on two separated chromophores in a single molecule.