Structure–Property Relationships and ¹O₂ Photosensitisation in Sterically Encumbered Diimine Pt^II Acetylide Complexes

Abstract

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-CF3 substituents (4) with electron-donating pyrenes (5) resulted in a red shift of both the lowest-energy absorption (ΔE=3300 cm(-1) , 61 nm) and emission bands (ΔE=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 (kobs =39.3×10(-3) min(-1) ) was over half that of the known (1) O2 sensitiser tetraphenylporphyrin (kobs =78.6×10(-3) min(-1) ) under the same conditions. Measured (1) O2 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.

Luminescent Bis‐Cyclometalated Gold(III) Complexes with Alkynyl Ligands of Hexaphenylbenzene and Hexabenzocoronene Derivatives and Their Supramolecular Assembly

“…The absorption bands in 8 show slight redshifts relative to those in the free HBC–alkynyl ligand due to extension of π conjugation in the alkynyl ligand after the incorporation of the bis‐cyclometalated gold(III) moiety . The σ donation from the alkynyl group to the gold(III) center may also contribute to the redshifts observed in the HBC–alkynylgold(III) complex . The additional absorption band at λ =414 nm in the complex, which is absent in the free ligand, probably involves a metal‐perturbed π–π* IL transition of the bis‐cyclometalating ligand, since an absorption band at λ =412 nm has also been observed in 6 , 7 , and 9 , which contain the same bis‐cyclometalating ligand as that in 8 .…”

“…The additional absorption band at λ =414 nm in the complex, which is absent in the free ligand, probably involves a metal‐perturbed π–π* IL transition of the bis‐cyclometalating ligand, since an absorption band at λ =412 nm has also been observed in 6 , 7 , and 9 , which contain the same bis‐cyclometalating ligand as that in 8 . In related HBC–platinum(II) systems, the appearance of this additional band is attributed to the fact that the attachment of the metal complex moiety lowers the symmetry of the HBC–alkynyl ligand. A similar rationale may also be applied to the observation of the additional band in complex 8 .…”

“…On the other hand, the electronic absorption spectrum of complex 8,w hich possesses ap olycyclic aromatic HBC core in the alkynyl ligand, shows typical absorption features to those showni no ther HBC transition-metal complexes (a band at l = 448 nm;pband at l = 396 nm, and b band at l = 364 nm). [57][58][59][60]64] The absorption bands in 8 show slight redshifts relative to those in the free HBC-alkynyl ligand due to extension of p conjugation in the alkynyl ligand after the incorporation of the bis-cyclometalated gold(III) moiety. [59] The s donation from the alkynyl group to the gold(III) center may also contributet ot he redshifts observed in the HBC-alkynylgold(III) complex.…”

“…Despite recent reports on related HBC‐ and HPB‐containing isoelectronic platinum(II) complexes, which focused on photophysical property studies of the platinum(II) systems, corresponding supramolecular assembly studies of the platinum(II) complexes have not been made . Therefore, supramolecular assembly studies on HBC‐ and HPB‐containing gold(III) complexes would be an interesting area to explore.…”

“…Despite recent reports on relatedH BC-andH PB-containing isoelectronic platinum(II) complexes, which focused on photophysicalp roperty studies of the platinum(II) systems, corresponding supramolecular assembly studies of the platinum(II) complexes have not been made. [57][58][59][60][61] Therefore, supramolecular assembly studies on HBC-and HPB-containing gold(III) complexes would be an interesting area to explore. Herein, we report the synthesis;c haracterization;a nd photophysical, electrochemical, and supramolecular assembly studies of as eries of cyclometalated gold(III) complexes with various alkynyl-functionalized HPB and HBC ligands.…”