Synthesis and Structural Characterization of a New Vapochromic Pt(II) Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) Ligand

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce
the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be
brightly luminescent in the solid state at room temperature and at 77 K, with no change in λemmax. The complex
exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red
(5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest
vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile
organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures
of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic
Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence
of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the
presence of non-interacting Pt···Pt separations with distances of 3.9092(9) and 4.5483(11) Å and a zigzag
arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt···Pt
separations of 3.2981(14) and 3.3427(14) Å, indicative of metallophilic interaction. The change in the emitting
state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8−d8
metallophilic interactions in the red form and its metal−metal-to-ligand charge transfer (MMLCT) excited state to a
more-localized Pt(dπ)-to-tpy(π*) metal-to-ligand charge transfer (MLCT) excited state in the green form.