Switch of the Lowest Excited‐States of Terpyridylplatinum(II) Acetylide Complexes Bearing Amino or Azacrown Moieties by Proton and Cations

The investigation of the synthesis and unique photophysical properties of two terpyridylplatinum( II ) acetylide complexes [Pt(terpy)(C≡CC 6 H 4 ‐4‐R)] where terpy is 2,2′:6′,2′′‐terpyridyl and R = N(CH 3 ) 2 ( 1 ) or N ‐15‐monoazacrown‐5 ( 2 ) has been carried out. These complexes exhibit an absorption band in the region of 460−650 nm in neutral or basic solution that corresponds to a ligand‐to‐ligand charge transfer (LLCT) transition from the amino‐substituted acetylide to the terpyridyl acceptor. In acidic solutions the complexes show the lowest‐energy absorption band at 380−500 nm, which is assignable to a dπ(Pt)→π*(terpy) metal‐to‐ligand charge transfer (MLCT) transition. In acidic media the complexes are emissive with quantum yields of 1 × 10 −3 to 2 × 10 −3 , and lifetimes of 20−50 ns. Compound 2 may complex metal cations through its azacrown ether moiety. For Na + , Ba 2+ , Ca 2+ and Mg 2+ , a 1:1 binding mode with log K s of 2.52 to 4.18 was observed. As in the case of acidic solution, the cation‐bound complex also features the lowest‐lying MLCT excited state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)