Luminescent Cycloplatinated Complexes Containing Poly(pyrazolyl)-borate and -methane Ligands

Abstract

Cycloplatinated neutral [Pt(C ∧ N){H 2 B(pz) 2 }] (1À3) [C ∧ N = benzoquinolate (bzq), 2-phenylpyridinate (ppy), and 2-phenylquinolate (pq)] and [Pt(pq){HB(pz) 3 }] 10 and cationic [Pt(C ∧ N){H 2 C(pz) 2 }] + (4À6) and [Pt(C ∧ N){HC(pz) 3 }] + (7À9)complexes were synthesized by the reaction of the corresponding precursors [Pt(C ∧ N)(μ-Cl)] 2 with the adequate poly-(pyrazolyl)-borate or -methane ligand. However, the reactions of [Pt(C ∧ N)(μ-Cl)] 2 (C ∧ N = bzq, ppy) with [HB(pz) 3 ] À evolve with BÀN bond cleavage, yielding the binuclear systems [Pt(C ∧ N)(μ-pz)] 2 as a mixture of cis and trans isomers. Complexes were characterized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structures of 1, 3, 6, 7À9, and [Pt(bzq)(μ-pz)] 2 were confirmed by X-ray single-crystal studies. The absorption, emission, and electrochemical properties of these complexes are mainly dominated by the nature of the cyclometalated ligand and the charge of the complex. On the basis of TD-DFT calculations (1, 7À9), the lowest-energy absorption for neutral 1 has been ascribed to a mixed 1 ILCT/ 1 MLCT transition, whereas for the cationic 7À9, it is mainly attributed to 1 ILCT combined with some CT to both ligands in 9 ( 1 MLCT/ML 0 CT 9) or to the HC(pz) 3 in 7 and 8 ( 1 ML 0 CT). These compounds are emissive in all media (except 4 and 10 in the solid state at 298 K). In the solid state at 298 K and at 77 K, these complexes display intense phosphorescence, which is typical of monomers. In deoxygenated CH 3 CN solutions at 298 K, phosphorescence is accompanied by higher-energy fluorescence in complexes 1, 4, and 8, which disappears at concentrated solutions and at 77 K. Complex 7 displays a special behavior, observing fluorescence and/or excimer fluorescence only at 298 K and excimeric emission (diluted glasses) and emission from aggregates in concentrated glasses. TD-DFT of the lowest-lying excited states responsible for the phosphorescence of 1 and 7À9 reveals a 3 ILCT origin with a mixed 3 MLCT character for 1 and, in the case of the cationic 7À9, a 3 ILCT transition mixed with 3 ML 0 CT (especially in 8) and with some 3 MLCT in 9. ' INTRODUCTIONCyclometalated square-planar platinum(II) complexes have attracted great interest in recent years due to their luminescent properties 1À12 with potential applications in optoelectronic devices, 2,3,7,10,11,13À25 photocatalysts, 26 and chemical or biochemical sensors. 27À31 Emission from these Pt(II) complexes is typically assigned to ligand-centered ( 3 LC) and/or metal-to-ligand charge-transfer ( 3 MLCT) states, although some degree of ligand-to-ligand 0 charge-transfer ( 3 LL 0 CT) states is also possible. Additionally, the square-planar nature of these complexes facilitates the formation of dimers or aggregates through noncovalent π 3 3 3 π and/or Pt 3 3 3 Pt interplanar stacking interactions, both in the solid state and in solution. 32À44 As a consequence, they exhibit rich photophysical behavior causing a marked red shift relative to the mononuclear emissio...

Photophysical and DFT studies on cycloplatinated complexes: modification in luminescence properties by expanding of π-conjugated systems

“…This is attributed to a higher electron delocalization (or larger π conjugated system) in the bzq ligand as compared to that in the ppy ligand. 22,26 Also, the emission of complex 1 in KBr pellet shows the same pattern with that obtained in powder form ( Figure S4). Upon cooling the temperature to 77 K in solid state, intensities of the emission bands are increased while the band shapes remain similar to those obtained at 298 K, with the same order in energy shifting as was mentioned before for the complexes 1 and 2.…”

“…45 This type of fluorescence/phosphorescence emission is rarely observed. 26,28,45,56 By excitation of the complex 1 at lower energies (λ ≥ 390 nm) only a structured emission band was observed,…”

“…[23][24][25][26][27] The best known types of luminescent complexes carrying a mono cycloplatinated(II) fragment have been reported in the literature based on C^N aromatic chelate derivatives such as 2-arylpyridine, in which the ligand includes a donor, C-bonded aryl group, and an acceptor, N-bonded pyridine ring, capabilities.…”

“…This energy gap can be effectively controlled by modifying the C ˄ N backbone at either phenyl or pyridine ring, such as incorporation of more polarizable atoms like sulfur in the ring system, introduction of electron withdrawing or donating substituent on the C ˄ N ligand, and extending size of the π-conjugation system in the C^N ligand. 29,30 Generally, expanding of the π-conjugated system causes reduction of the π-π* energy gap leading to red shift in emission energy, 22,26,27,31 although a blue shift behavior may be observed in some cases. 16 We have recently synthesized a series of Pt complexes containing different cyclometalated ligands [32][33][34] and investigated their chemistry in oxidative addition reactions, [35][36][37][38][39] ligand substitution reactions, 40,41 and metal-carbon bond protonolysis.…”

Attachment of Luminescent Neutral “Pt(pq)(C≡CtBu)” Units to Di and Tri N-Donor Connecting Ligands: Solution Behavior and Photophysical Properties

“…In the bpe-bridged binuclear compound 4a, the highly structured emission profile at low temperature, with peak maxima at 588, 648 and 702 nm, the observed vibronic spacing (close to that observed for the free ligand), and also the long lifetime (39.2 μs) are consistent with a predominantly bpe-centered 3 IL 3 ()* excited state. However, the emission profiles of 1a (pyz) and 2a (bpy) are similar (590 nm 298 K; 572, 610 77 K 1a, 574, 612 nm 77 K 2a) and compares to those seen for typical phenylquinolyl platinum complexes (i.e., [Pt(pq){H2B(pz)2}] em = 580, 610 nm) [58], what is consistent with emission from a 3 MLCT excited state likely mixed with alkynyl to pq charge transfer contribution ( 3 MLCT/ 3 L'LCT). For the remaining complexes (3a, 5a and 6a), the low temperature profiles are also similar to those of 1a and 2a, but the maxima are slightly red shifted in the 6a(tpac)  3a(bpa)  5a(bpac), pointing to some contribution of the central N-linker ligand.…”