Cationic Zinc(II) Complexes with Carbazole-Type Counter-Anions: Intracomplex Donor/Acceptor Pairs Affording Exciplexes with Thermally Activated Delayed Fluorescence.

Two cationic zinc(II) complexes with carbazole-type counter-anions, namely, [Zn(tpy) 2 ] 2+ [CAZ- p -BF 3 - ] 2 (Zn- p ) and [Zn(tpy) 2 ] 2+ [CAZ- o -BF 3 - ] 2 (Zn- o ), have been designed and synthesized, where tpy is 2,2':6',2″-terpyridine, CAZ- p -BF 3 - is 4-((9 H -carbazol-9-yl)phenyl)trifluoroborate, and CAZ- o -BF 3 - is (2-(9 H -carbazol-9-yl)phenyl)trifluoroborate. The complex cation [Zn(tpy) 2 ] 2+ (as the acceptor) and the carbazole-type counter-anion CAZ- p -BF 3 - or CAZ- o -BF 3 - (as the donor) form an intracomplex donor/acceptor pair. Single-crystal structures reveal that compared to Zn- p , Zn- o exhibits a stronger π-π stacking interaction between the carbazole group (as the donor unit) of the counter-anion and the tpy ligand (as the acceptor unit) of [Zn(tpy) 2 ] 2+ because of the different anchoring position of the BF 3 - anion in the counter-anion. In a doped film, Zn- p and Zn- o afford an isolated exciplex formed between the carbazole group and the tpy ligand within the single complex, which gives green-yellow emission with a thermally activated delayed fluorescence (TADF) feature. In crystalline states, Zn- p and Zn- o afford exciplexes with blue emission centered at 468 nm and green-blue emission centered at 508 nm, respectively. The Zn- p crystalline sample shows a relatively large singlet-triplet energy gap (Δ E ST ) (0.33 eV) and no TADF, whereas the Zn- o crystalline sample exhibits a small Δ E ST (0.06 eV) and distinct TADF, with a reverse intersystem crossing rate at 3.3 × 10 5 s -1 . Zn- p and Zn- o both exhibit intriguing mechanochromic luminescence, with largely red-shifted (by over 70 nm) emission and modulated TADF properties upon mechanically grinding the crystalline samples. The work demonstrates that donor/acceptor pairs affording exciplexes can be formed within cationic metal complexes using counter-anions with donor nature, which opens a new avenue toward photo-active metal complexes with rich photophysical properties.