Bis-alkynylphosphine Oxide Pt(II) Complexes: Aggregation-Induced Phosphorescence Enhancement and Mechanochromic Luminescence Properties

Four bis-alkynyl Pt(II) complexes [Pt(dtbpy)(C2–L–P(O)Ph2)2] with dtbpy = 4,4′-ditertbutyl-2,2′-bipyridine
and alkynylphosphine oxide ligands (L = no linker, Pt0; phenyl, Pt1; biphenyl, Pt2; naphthyl, Pt3) have been synthesized and fully characterized by spectroscopic
methods and single crystal XRD analysis. It has been found that the
nature of the π-conjugated linker is a key factor in fine-tuning
the emission energy of the complexes in solution and in achieving
the aggregation-induced phosphorescence enhancement (AIPE) effect
for complex Pt0 with the most compact linker. Phosphine
oxide fragment, which can be involved in weak intermolecular interactions,
promotes the existence of two solid forms with different luminescence
properties. These two forms can be switched from one to another upon
grinding, thus featuring distinct mechanochromic luminescence properties.
TDDFT calculations are consistent with the experimental results and
assign mixed 3MLCT and 3LL′CT solution
emission character and 3MMLCT and 3LL′CT
emission nature in supramolecular dimeric structures.