The synthesis, photophysical studies and applications of novel cyclometalated platinum(II) complexes

Abstract

A new class of tridentate cyclometalating ligands containing a Cphenyl, a Npyridyl and a Npyrazolyl donor moiety as well as a 1-pyrazolyl-NH that can be available for metal coordination and other chemical interactions have been developed. They were collectively known as the C,N,Npyrazolyl ligands. These new ligands were expected to generate a new series of cyclometalated late transition metal complexes, especially cycloplatinated complexes, which are the main focus of this research, with special spectroscopic and photophysical properties. First, a series of luminescent cyclometalated Pt(II) complexes, [Pt(L1)Cl], [Pt2(L1 -)2] and [Pt(L1)(PPh3)]ClO4 (where PPh3 = triphenylphosphine), containing the basic form of the new cyclometatating ligand system, HL1 (2-phenyl-6-(1H-pyrazol- 3-yl)-pyridine), were synthesized and characterized by X-Ray crystallography. [Pt(L1)Cl] displayed strong phosphorescence in acetonitrile solution at room temperature with emission max at 514 nm, which can be tentatively assigned to the triplet metal-to-ligand charge transfer (3MLCT) (d(L)) transition, with some triplet intraligand (3IL) ((L)*(L)) character. Deprotonation of [Pt(L1)Cl] in organic solvents yielded a neutral dimer [Pt2(L1 -)2], which showed relatively high room-temperature luminescent quantum yield of 0.59 in DMF (max = 509 nm). Substitution of the ancillary chloro- ligand in [Pt(L1)Cl] by PPh3 yielded [Pt(L1)(PPh3)]ClO4, which also possessed good room-temperature luminescent quantum yield of 0.52 in DMF (max = 504 nm) and a better solubility in water. [Pt(L1)(PPh3)]ClO4 was synthesized to demonstrate the pH dependence of luminescent properties of this class of C,N,Npyrazolyl cyclometalated Pt(II) system. Such a pH response was ascribable to the protonation / deprotonation of the 1-pyrazolyl-NH on the C,N,Npyrazolyl cyclometalating ligands. The pKa value of the 1-pyrazolyl-NH in [Pt(L1)(PPh3)]ClO4 was measured to be ca. 4.0 in 1:2 (v/v) aqueous DMF solutions. To probe the nature of the intramolecular d8 – d8 Pt – Pt interaction in these new cycloplatinated complexes, a diphosphine bridging ligand, bis(diphenylphosphino)methane (dppm), was employed to synthesize the bridged binuclear species, {[Pt(L1)]2(μ-dppm)}(ClO4)2. Emission max of {[Pt(L1)]2(μ- dppm)}(ClO4)2 in acetonitrile at 606 nm was typical of the analogous binuclear cycloplatinated systems and can be attributed to the triplet metal-metal-to-ligand charge-transfer (3MMLCT) (d*(Pt-Pt)(L)) transition. This is in line with the corresponding X-Ray crystal structure that the two [Pt(L1)]+ units were held in a parallel manner with close Pt – Pt separation of 3.193(2) Å, indicating d8 – d8 intramolecular Pt – Pt interaction. On the other hand, such an intramolecular Pt – Pt interaction was found to be cleavable following the deprotonation of the 1-pyrazolyl- NH on the C,N,Npyrazolyl cyclometalating ligand. X-Ray crystallography revealed that the two [Pt(L1 -)] units were not parallel to each other after deprotonation. Instead of intramolecular d8 – d8 interaction, the deprotonated species displayed a pair of intramolecular interactions between the two deprotonated pyrazolyl rings on the cyclometalating ligands and two phenyl rings of the μ-dppm. Distances between the interacting aromatic rings were 3.54 to 3.75 Å. Protonation of the ligand returned the complex back to the original state with the re-establishment of the intramolecular Pt – Pt interaction. The reversible switching between the “open” (no intramolecular Pt – Pt interaction) and “closed” (re-establishment of intramolecular Pt – Pt interaction) states of the binuclear molecule resembled the action of a pivot-hinge joint. Such a dynamic mechanical motion was observable in solution by the switching between the lowenergy 3MMLCT emission at 605 nm (“closed” status) and the monomeric 3MLCT emission at 507 nm (“open” status). To our knowledge, {[Pt(L1)]2(μ-dppm)}(ClO4)2 represents the first binuclear Pt(II) system with tunable intramolecular Pt – Pt interaction. To further understand the mechanism behind the action of such a molecular pivot-hinge joint, a commercially available diphosphine bridging ligand, bis(dicyclohexylphosphino)methane (dchpm), was used to replace dppm in the construction of a new binuclear C,N,Npyrazolyl cyclometalated Pt(II) complex, {[Pt(L1)]2(μ-dchpm)}(ClO4)2, which rendered interactions between the deprotonated pyrazolyl moiety and the bridging diphosphine ligand impossible. Detailed spectrofluorometric titrations and X-Ray crystallographic studies revealed that instead of the cleavage of the intramolecular Pt – Pt interaction, the two platinum centers of {[Pt(L1)]2(μ-dchpm)}(ClO4)2 got even closer (from 3.2388(8) to 3.1576(9)Å) upon deprotonation. This indicates that the ability to establish a pair of intramolecular interactions after deprotonation is important for the cleavage of the intramolecular Pt – Pt interaction and the subsequent “opening” mechanism of the molecular pivot-hinge fails to function in the absence of phenyl rings on the bridged diphosphine ligand which acts as the pivot. Besides their linear photophysical properties, two-photon absorption properties of the mononuclear and dppm bridged dinuclear C,N,Npyrazolyl cycloplatinated system were also investigated. Besides HL1, two new ligands, HL2 and HL3, possessing similar C,N,Npyrazolyl skeleton with more extended conjugation by the incorporation of additional phenyl rings (phenyl in HL2 and biphenyl in HL3) at the para- position of the central pyridine, were synthesized. Their corresponding chloro-, triphenylphosphine and bis(diphenylphosphino)methane (dppm) substituted cyclometalated Pt(II) complexes: [Pt(L1)Cl], [(Pt(L2)Cl], [Pt(L3)Cl], [Pt(L1)(PPh3)]ClO4, [Pt(L2)(PPh3)](ClO4), [Pt(L3)(PPh3)](ClO4), {[Pt(L1)]2(μ- dppm)}(ClO4)2, [Pt(L2)]2(μ-dppm)](ClO4)2 and [Pt(L3)]2(μ-dppm)](ClO4)2 were synthesized and their linear and two-photon photophysical properties were studied. Two-photon induced luminescence of these complexes were measured by nearinfrared (NIR) femto-second excitation at 720 nm. Their two-photon adsorption processes were confirmed by power dependence experiments and two-photon excitation spectrum (ex = 680 – 900 nm). [Pt(L1)Cl] demonstrated high two-photon absorption cross-section of ca. 20.8 GM (at concentration of 10-3 M in DMF, ex = 720 nm) with the emission profile highly resembling the linear one. In view of the interesting two-photon excitation properties of the C,N,Npyrazolyl cyclometalated Pt(II) system, their feasibility as two-photon luminescent molecular probes for in vitro imaging was explored. [Pt(L1)Cl] was found to actively accumulate in human cervical carcinoma (HeLa) cells in 1 – 2 minutes. The green luminescence from [Pt(L1)Cl] was observable under 400 nm and 730 nm laser excitation. To the best of our knowledge, this is the first example of a cyclometalated Pt(II) complex acting as a nonlinear two-photon induced emission dye for the labeling of living cells.