Switchable Ambient-Temperature Singlet–Triplet Dual Emission in Nonconjugated Donor–Acceptor Triarylboron–Pt^II Complexes

Graphical Abstract

Double the fun! Singlet–triplet dual emission at ambient temperature has been achieved in compounds containing a triarylboron acceptor and an N-(2′-pyridyl)-7-azaindolyl donor group bridged by a tetrahedral Si linker (see figure). Pt^II chelation and chelate-mode switching from N,N to N,C have been found to greatly enhance phosphorescent emission. Furthermore, both singlet and triplet emission bands are responsive to fluoride ions.

Abstract

Double the fun! Singlet–triplet dual emission at ambient temperature has been achieved in compounds containing a triarylboron acceptor and an N-(2′-pyridyl)-7-azaindolyl donor group bridged by a tetrahedral Si linker (see figure). Pt^II chelation and chelate-mode switching from N,N to N,C have been found to greatly enhance phosphorescent emission. Furthermore, both singlet and triplet emission bands are responsive to fluoride ions.

A triarylboron compound Si-BNPA (1) containing a BMes₂ 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 ³π→π* 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] (2 a). Furthermore, 2 a was found to undergo intramolecular “roll-over” CH activation to produce the N,C-chelate complex [Pt(N,C-Si-BNPA)(SMe₂)Ph] (2 b). This compound also displays ambient temperature singlet–triplet dual emission, but with a much greater phosphorescent efficiency than 2 a due to the formation of a more stable chelate ring. Addition of fluoride was found to have little impact on the phosphorescent emission of 2 a, but resulted in a large enhancement of the phosphorescent emission intensity of 2 b. 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] (3 a) and [Pt(N,C-BNPA)Ph₂] (3 b) were also examined. Consistent with 2 a and 2 b, the N,C-chelate complex 3 b has a much higher phosphorescent efficiency than the N,N-chelate 3 a. Although 3 a and 3 b 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.