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

Abstract

Two cationic zinc(II) complexes with carbazole-type counter-anions, namely, [Zn(tpy)2]2+[CAZ-p-BF3–]2 (Zn-p) and [Zn(tpy)2]2+[CAZ-o-BF3–]2 (Zn-o), have been designed and synthesized, where tpy is 2,2′:6′,2″-terpyridine, CAZ-p-BF3– is 4-((9H-carbazol-9-yl)phenyl)trifluoroborate, and CAZ-o-BF3– is (2-(9H-carbazol-9-yl)phenyl)trifluoroborate. The complex cation [Zn(tpy)2]2+ (as the acceptor) and the carbazole-type counter-anion CAZ-p-BF3– or CAZ-o-BF3– (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 BF3– 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 (ΔEST) (0.33 eV) and no TADF, whereas the Zn-o crystalline sample exhibits a small ΔEST (0.06 eV) and distinct TADF, with a reverse intersystem crossing rate at 3.3 × 105 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