Observation of low-energy metal-metal-to-ligand charge transfer absorption and emission: Electronic spectroscopy of cyclometalated platinum(II) complexes with isocyanide ligands

摘要

A series of luminescent mononuclear [(Cboolean ANDNboolean ANDN)Pt(Cequivalent toNR)]X (HCboolean ANDNboolean ANDN = 6-phenyl-2,2'-bipyridine; R = Bu-t (1), Bu-n (2), Pr-i (3), cyclohexyl. (Cy, 4), X = ClO4; R = 2,6-Me2C6H3 (Ar', 5), X = PF6) and [(Cboolean ANDNboolean ANDN)Pt(CO)]CF3SO3 (6(CF3SO3)) and binuclear [{(Cboolean ANDNboolean ANDN)Pt}(2)(mu-Cequivalent toN(CH2)(3)Nequivalent toC)](PF6)(2) (7(PF6)(2)) complexes were synthesized, and their spectroscopic and photophysical. properties have been investigated. The crystal lattices of 1(ClO4) and 5(PF6) reveal close [Cboolean ANDNboolean ANDN] pi-pi intermolecular contacts (3.4-3.6 Angstrom). Additional metal-metal interactions are evident in 5(PF6) with intermolecular Pt-Pt distances of 3.3831(9) Angstrom. At complex concentration greater than 5 x 10(-3) mol dm(-3), the UV-vis absorption spectrum of 2 displays a weak shoulder at 511 nm (epsilon = 120 dm(3) mol(-1) cm(-1)), which does not obey Beer's law. A metal-metal-to-ligand charge transfer (MMLCT) [dsigma*(Pt-Pt) --> pi*(Cboolean ANDNboolean AND)] absorption of a dimeric species via 2[Pt(Cboolean ANDNboolean ANDN)(Cequivalent toN(n)Bu)](+) reversible arrow [(Cboolean ANDNboolean ANDN)Pt(Cequivalent toN(n)Bu)](2)(2+) is proposed. The room-temperature emission spectrum of 4 for complex concentrations greater than or equal to 7 x 10(-3) mol dm(-3) shows a low-energy band at 710 nm originating from a (MMLCT)-M-3 excited state. A corresponding well-defined low-energy absorption at 500 nm in the excitation spectrum of 4 (lambda(em) 710 nm) is assigned to the (MMLCT)-M-1 transition. The 77 K emissions of complexes 1-6 in glassy solutions are sensitive to complex concentration. Upon increasing concentrations from 5 x 10(-5) to 2 x 10(-3) mol dm(-3) for 1-4, red emissions at 600-625 nm ascribed to pi-pi excimeric excited states develop at the expense of the vibronic (MLCT)-M-3 emissions at lambda(max) similar to502 nm. Complexes 2 and 4 show additional low-energy emissions at 739 and 710 nm, respectively, attributable to (MMLCT)-M-3 states. Concentrated 77 K glassy solutions of complexes 5-7 exhibit (MMLCT)-M-3 emissions (lambda(max) 711-744 nm). The broad structureless solid-state luminescence of 2(ClO4), 3(ClO4), 5(PF6), 6(CF3SO3), and 7(PF6)(2) at 298 K (lambda(max) 701-748 nm) and 77 K (lambda(max) 744-813 nm) are assigned to (MMLCT)-M-3 excited states arising from intermolecular stacking interactions in the solid state. The spectroscopic properties of the related [(Cboolean ANDNboolean ANDC)Pt(L)] (HCboolean ANDNboolean ANDCH = 2,6-diphenylpyridine; L = CO and Cequivalent toNAr') solids are compared, and no MMLCT emission is evident. [References: 63]