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Structure–Property Relationships and 1 O 2 Photosensitisation in Sterically Encumbered Diimine Pt II Acetylide Complexes
Author(s) -
Nolan Deanne,
Gil Belén,
Wang Longsheng,
Zhao Jianzhang,
Draper Sylvia M.
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201300759
Subject(s) - acetylide , steric effects , phosphorescence , diimine , chemistry , photochemistry , ligand (biochemistry) , excited state , porphyrin , crystallography , stereochemistry , fluorescence , organic chemistry , biochemistry , physics , receptor , quantum mechanics , nuclear physics , catalysis
A series of sterically encumbered [Pt( L )(σ‐acetylide) 2 ] complexes were prepared in which L , a dendritic polyaromatic diimine ligand, was held constant ( L =1‐(2,2′‐bipyrid‐6‐yl)‐2,3,4,5‐tetrakis(4‐ tert ‐butylphenyl)benzene) and the cis ethynyl co‐ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron‐withdrawing phenyl‐CF 3 substituents ( 4 ) with electron‐donating pyrenes ( 5 ) resulted in a red shift of both the lowest‐energy absorption (Δ E =3300 cm −1 , 61 nm) and emission bands (Δ E =1930 cm −1 , 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited‐state lifetimes, switched from being 3 MMLL′CT‐derived (mixed metal–ligand‐to‐ligand charge transfer) when phenyl/polyphenylene substituents ( 3 , 4 , 6 ) were present, to ligand‐centred 3 ππ* when the substituents were more conjugated aromatic platforms [pyrene ( 5 ) or hexa‐ peri‐ hexabenzocoronene ( 7 )]. The novel Pt II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited‐state lifetimes suggested they would be good candidates for use as singlet‐oxygen photosensitisers. Determined by in situ photooxidation of 1,5‐dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl‐ 5 as sensitiser ( k obs =39.3×10 −3 min −1 ) was over half that of the known 1 O 2 sensitiser tetraphenylporphyrin ( k obs =78.6×10 −3 min −1 ) under the same conditions. Measured 1 O 2 quantum yields of complexes 5 and 7 were half and one‐third, respectively, of that of TPP, and thus reveal an efficient triplet–triplet energy‐transfer process in both cases.