Synthesis, characterization, photophysics and electrophosphorescent applications of phosphorescent platinum cyclometalated complexes with 9-arylcarbazole moieties

Ultrafast Excited-State Dynamics of Photoluminescent Pt(II) Dimers Probed by a Coherent Vibrational Wavepacket

Intricate potential energy surfaces (PESs) of some transition metal complexes (TMCs) pose challenges in mapping out initial excited-state pathways that could influence photochemical outcomes. Ultrafast intersystem crossing (ISC) dynamics of four structurally related platinum(II) dimer complexes were examined by detecting their coherent vibrational wavepacket (CVWP) motions of Pt-Pt stretching mode in the metal-metal-to-ligand-charge-transfer excited states. Structurally dependent CVWP behaviors (frequency, dephasing time, and oscillation amplitudes) were captured by femtosecond transient absorption spectroscopy, analyzed by short-time Fourier transformation, and rationalized by quantum mechanical calculations, revealing dual ISC pathways. The results suggest that the ligands could fine-tune the PESs to influence the proximity of the conical intersections of the excited states with the Franck-Condon state and thus to control the branching ratio of the dual ISC pathways. This comparative study presents future opportunities in control excited-state trajectories of TMCs via ligand structures.

Resolving the ultrafast intersystem crossing in a bimetallic platinum complex

Bimetallic platinum complexes have interesting luminescent properties and feature long-lasting vibrational coherence and ultrafast intersystem crossing (ISC) after photoexcitation. Ultrafast triplet formation is driven by very strong spin-orbit coupling in these platinum (II) systems, where relativistic theoretical approaches beyond first-order perturbation theory are desirable. Using a fully variational relativistic theoretical method recently developed by the authors, we investigate the origins of ultrafast ISC in the [Pt(ppy) (μ-^tBu₂pz)]₂ complex (ppy = phenylpyridine, pz = pyrazolate). Spin-orbit coupling values, evaluated along a Born-Oppenheimer molecular dynamics trajectory, are used to propagate electronic populations in time. Using this technique, we estimate ultrafast ISC rates of 15-134 fs in this species for the possible ISC pathways into the three low-lying triplet states.

Polypyridyl ligands as a versatile platform for solid-state light-emitting devices

A comprehensive review of tuneable polypyridine complexes as the emissive components of OLED and LEC devices is presented, with a view to bridging the gap between molecular design and commercialization.

Achieving yellow emission by varying the donor/acceptor units in rod-shaped fluorenyl-alkynyl based π-conjugated oligomers and their binuclear gold(i) alkynyl complexes

Fluorenyl-alkynyl based π-conjugated rod-shaped oligomers bearing different central aromatic moieties and functionalizable di-alkynyl termini, such as H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH1), H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btz-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH2) and H-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btd-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-H (OH3) where Fl = 9,9-dioctylfluorene, Btz = N-hexylbenzotriazole, and Btd = benzothiadiazole, were successfully synthesized by a Pd(0) catalyzed Stille coupling protocol. Electron withdrawing benzothiadiazole and benzotriazole as strong to moderate acceptors and fluorene as the donor have been incorporated to adjust the Donor-Acceptor (D-A) strength for fine-tuning the bandgap (E_g) as well as the emission wavelength. The corresponding digold(i) σ-complexes (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM1), (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btz-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM2) and (PPh₃)Au-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Btd-[triple bond, length as m-dash]-Fl-[triple bond, length as m-dash]-Au(PPh₃) (OM3) have also been prepared by a reaction of Au(PPh₃)Cl and methanolic NaOMe in DCM with the corresponding alkynyl functionalized oligomers to take advantage of the heavy-atom effect on their emissive properties. The synthesized rod-shaped π-conjugated fluorene based oligomers and their binuclear Au(i) σ-complexes have been unambiguously characterized by various spectroscopic tools such as FTIR and multinuclear NMR as well as MALDI-TOF and CHN analyses. The absorption and emission spectral studies exhibited a progressive red shift with increasing the electron withdrawing character of the central aromatic unit. The rod-like oligomers having alkynyl termini and the corresponding digold(i) complexes are found to be blue, cyan and yellow emissive, demonstrating the fine-tuning of the emission wavelength. Most importantly, the fluorene based π-conjugated yellow light emitters OH3 and OM3 are successfully achieved by varying the donor/acceptor moiety to the fluorenyl-alkynyl backbone. The digold(i) diacetylide organometallic wires exhibit phosphorescence at 77 K in degassed CH₂Cl₂ due to the efficient intersystem crossing from the S₁ to the T₁ excited state as induced by heavy atoms.

Syntheses, photoluminescence and electroluminescence of two novel platinum(ii) complexes

Highly efficient green OLEDs based on two platinum complexes showed a peak current efficiency of 48.3 cd A⁻¹ with low efficiency roll-off; and even at a brightness of 1000 cd m⁻², a current efficiency of 47.0 cd A⁻¹ could still be obtained.

Platinum and Gold Complexes for OLEDs

Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d(6) electronic configuration and octahedral geometry, the coordination-unsaturated nature of d(8) transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d(8) platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d(8) metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

Luminescence properties of some monomeric and dimeric cycloplatinated(ii) complexes containing biphosphine ligands

The starting complexes [PtCl(C^N)(dmso)], in which C^N is either ppy = 2-phenylpyridinate, , or bhq = 7,8-benzo[h]quinolinate, , were prepared by a known method using the reaction of [PtCl2(dmso)2] with ppyH or bhqH, respectively, in toluene under reflux conditions. The reaction of [PtCl(C^N)(dmso)], or , with 1 equiv. of a number of biphosphine ligands, P^P, gave the cationic monomeric complexes [Pt(ppy)(P^P)]Cl, for which P^P is either 1,2-bis(diphenylphosphino)ethane (dppe), , 1,3-bis(diphenylphosphino)propane (dppp), , or bis(diphenylphosphino)methane (dppm), ; the bhq analogous complex [Pt(bhq)(dppe)]Cl, , was prepared similarly. However, the complex [Pt(ppy)(dfppe)]Cl, , in which dfppe is 1,2-bis(dipentafluorophenylphosphino)ethane, was prepared by the reaction of with excess amount of dfppe. When each of the starting complexes [PtCl(C^N)(dmso)], or , were reacted with 0.5 equiv. of any of the P^P ligands, the dimeric complexes [Pt2Cl2(ppy)2(μ-P^P)], , or [Pt2Cl2(bhq)2(μ-P^P)], , were formed. The complexes were fully characterized using multinuclear ((1)H and (31)P) NMR spectroscopy and elemental analysis. The structures of typical complexes , , , and were also confirmed by X-ray crystallography. The effect of ligands on the luminescent properties of the complexes was investigated and DFT calculations were performed to confirm the assignments.

Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices

Phosphorescent organic light-emitting devices (OLEDs) have attracted increased attention from both academic and industrial communities due to their potential practical application in high-resolution full-color displays and energy-saving solid-state lightings. The performance of phosphorescent OLEDs is mainly limited by the phosphorescent transition metal complexes (such as iridium(III), platinum(II), gold(III), ruthenium(II), copper(I) and osmium(II) complexes, etc.) which can play a crucial role in furnishing efficient energy transfer, balanced charge injection/transporting character and high quantum efficiency in the devices. It has been shown that functionalized main-group element (such as boron, silicon, nitrogen, phosphorus, oxygen, sulfur and fluorine, etc.) moieties can be incorporated into phosphorescent emitters and their host materials to tune their triplet energies, frontier molecular orbital energies, charge injection/transporting behavior, photophysical properties and thermal stability and hence bring about highly efficient phosphorescent OLEDs. So, in this review, the recent advances in the phosphorescent emitters and their host materials functionalized with various main-group moieties will be introduced from the point of view of their structure-property relationship. The main emphasis lies on the important role played by the main-group element groups in addressing the key issues of both phosphorescent emitters and their host materials to fulfill high-performance phosphorescent OLEDs.