Cyclometalated Pt(II) Complex Appending a Pyridyl Ligand with a Benzothienobenzothiophene (BTBT) Unit: Synthesis, Photophysical Properties, and an Unusual Shift of Centered Ligands in Excited States

A cyclometalated Pt(II) complex was synthesized by appending a pyridyl ligand containing a benzothienobenzothiophene (BTBT) unit, which is a well-known small organic molecule semiconductor. The target complex [Pt(ppy)(BTBT-py)Cl] (2) was characterized by H-1 NMR, steady-state UV-vis absorption, and photoluminescence spectroscopies in comparison with [Pt(ppy)(ppyH)Cl] (1) and BTBT-py (where ppy = 2-phenylpyridine and BTBT-py = 2-(3-pyridyl)-[1]benzothieno[3,2-b][1]benzothiophene). Complex 2 exhibited absorption bands derived from the ppy-centered pi-pi* transition and intramolecular charge transfer (ICT) from BTBT to pyridine moieties, indicating that 2 inherited the combined properties of the parent complex 1 and the parent ligand BTBT-py. Moreover, DFT calculations showed that 2 inherited HOMO and LUMO characteristics from BTBT-py as well as HOMO-1 and LUMO+1 characteristics from 1. While BTBT-py showed fluorescence from the (ICT)-I-1 state, the corresponding fluorescence was quenched in 2 because the Pt center, a heavy atom, enhanced the intersystem crossing. The ancillary ligand-centered (ICT)-I-3, rather than the ppy-centered (3)pi-pi*, state was successfully monitored in 2 using femtosecond transient absorption spectroscopy. Replacing the phenyl group with the BTBT unit in the cyclometalated Pt(II) complex restructured the electronic structure of the lowest triplet excited state, shifting the centered ligand from ppy to BTBT-py.