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 1H 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 π–π* 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 1ICT state, the corresponding fluorescence was quenched in 2 because the Pt center, a heavy atom, enhanced the intersystem
crossing. The ancillary ligand-centered 3ICT, rather than
the ppy-centered 3π–π*, 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.