Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs

Abstract

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near infrared regions of the spectrum, due to the transfer of electronic excited state energy into vibrations. We describe how this undesirable "energy gap law" can be side-stepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centres. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λ max = 610 nm, Φ = 0.85 in deoxygenated CH 2 Cl 2 at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53800 M-1 cm-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T 1 → S 0 phosphorescence process, allowing it to compete effectively with non-radiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode (OLED) prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

An efficient heterodinuclear Ir(iii)/Pt(ii) complex: synthesis, photophysics and application in light-emitting electrochemical cells

“…3 The measured PL quantum yield of 2 in degassed toluene at room temperature is very high at Φ PL = 85% with a decay time of only τ(300 K) = 640 ns. These values reveal an outstanding radiative rate (calculated as k r = Φ PL /τ) of k r =1.33 •10 6 s 1 that is an order of magnitude higher than of the fastest Pt(II) complexes 64,65 and notably higher than most other Ir(III) complexes. 54,63,66 This behavior is in line with the TD-DFT results, and shows the dominant role of the Ir(III) center in the properties of the T 1 state and the PL characteristics of complex 2.…”

“…Transition metal complexes that incorporate more than one metal centre can offer efficient phosphorescence as well as a broad range of design opportunities. [1][2][3][4][5][6][7][8] The emission and processing properties of such multinuclear complexes are tuneable by the molecular design, 4,[8][9][10][11][12][13] and, e.g., the employment of a rigid conjugated bridging ligand between the metal centers over a flexible nonconjugated counterpart will result in a red-shifted emission, but at the expense of lowered solubility. A highly symmetric complex will further be prone to crystallization in the solid state, whereas a nonsymmetric complex can allow for high solubility in common processing solvents and a facile solutionbased fabrication of uniform thin films fit for low-cost device applications.…”

Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design

“…This can be traced by measuring the emission decay time as a function of temperature. 6,34,[47][48][49] Measured at a cryogenic temperature T=1.7 K complex 5 shows emission with a long decay time of τ(1.7K) = 97.6 µs, assigned to the lowest sub-state I, I→S 0 . It is noted that the observed slight deviation of the decay curve from the mono-exponential profile at short time range is related to Spin-Lattice Relaxation (SLR) processes 50,51 , indicating a slow thermal relaxation of higher T 1 sub-states at T = 1.7 K (Figure 4(a)).…”

“…A further increase in temperature is followed by further shortening of the emission decay time due to the thermal population of substate III and activation of the III→S channel reaching the value of τ(120K) = 1.6 µs. The obtained temperature dependence of the emission decay time can be analyzed with equation 1, describing the thermal population of higher sub-states via Boltzmann type relation: 34,47,48 (1) ΔE(III−I) = 65 cm -1 . The average decay time, τ av , of the three sub-states calculated as 52,53 (2) = 3( III→S 0 relaxation channel is activated at temperatures above T = 30 K and with an increase of temperature up to T = 120 K, the emission decay time decreases down to τ(120K) = 1.0 µs.…”

Multinuclear PtII Complexes: Why Three is Better Than Two to Enhance Photophysical Properties

“…Square planarP t II complexes containingc onjugated coordinated ligandsa re particularly interesting building blocks for the creationo fs upramolecular nanostructures sincet heir flat geometry makes them prone to stack through noncovalent interaction, such as p-p stacking. [1][2][3][4][5][6][7][8][9][10] Furthermore, when Pt II complexes are close enough (distance below 3.5 ) [11][12][13][14] metallophilic interactions between Pt centers may be established and stable aggregates [15][16][17] can be observed that possess spectroscopic properties which can be dramatically different from those of the monomeric metal complex. [1][2][3][4][18][19][20][21][22][23] In fact, the establishmento fg round state intermolecular interactions between protruding d z 2 orbitals resultsi nt he formation of lower-lying molecular orbitals, thus an ew optical transition appears ascribed to metal-metal-to-ligand charget ransfer (MMLCT).…”