Efficient Near-Infrared Luminescence of Self-Assembled Platinum(II) Complexes: From Fundamentals to Applications

ConspectusDesigning bright and efficient near-infrared (NIR) emitters has drawn much attention due to numerous applications ranging from biological imaging, medical therapy, optical communication, and night-vision devices. However, polyatomic organic and organometallic molecules with energy gaps close to the deep red and NIR regime are subject to dominant nonradiative internal conversion (IC) processes, which drastically reduces the emission intensity and exciton diffusion length of organic materials and hence hampers the optoelectronic performances. To suppress nonradiative IC rates, we suggested two complementary approaches to solve the issues: exciton delocalization and molecular deuteration. First, exciton delocalization efficiently suppresses the molecular reorganization energy through partitioning to all aggregated molecules. According to the IC theory together with the effect of exciton delocalization, the simulated nonradiative rates with the energy gap ΔE = 10⁴ cm^-1 decrease by around 10⁴ fold when the exciton delocalization length equals 5 (promoting vibronic frequency ω_l = 1500 cm^-1). Second, molecular deuterations reduce Franck-Condon vibrational overlaps and vibrational frequencies of promoting modes, which decreases IC rates by 1 order of magnitude in comparison to the rates of nondeuterated molecules under ΔE of 10⁴ cm^-1. Although deuteration of molecules has long been attempted to increase emission intensity, the results have been mixed. Here, we provide a robust derivation of the IC theory to demonstrate its validity, especially to emission in the NIR region.The concepts are experimentally verified by the strategic design and synthesis of a class of square-planar Pt(II) complexes, which form crystalline aggregates in vapor deposited thin films. The packing geometries are well characterized by the grazing angle X-ray diffraction (GIXD), showing domino-like packing arrangements with the short ππ separation of 3.4-3.7 Å. Upon photoexcitation, such closely packed assemblies exhibit intense NIR emission maximized in the 740-970 nm region through metal-metal-to-ligand charge transfer (MMLCT) transition with unprecedented photoluminescent quantum yield (PLQY) of 8-82%. To validate the existence of exciton delocalization, we applied time-resolved step-scan Fourier transform UV-vis spectroscopy to probe the exciton delocalization length of Pt(II) aggregates, which is 5-9 molecules (2.1-4.5 nm) assuming that excitons mainly delocalized along the direction of ππ stacking. According to the dependence of delocalization length vs simulated IC rates, we verify that the observed delocalization lengths contribute to the high NIR PLQY of the aggregated Pt(II) complexes. To probe the isotope effect, both partially and completely deuterated Pt(II) complexes were synthesized. For the case of the 970 nm Pt(II) emitter, the vapor deposited films of per-deuterated Pt(II) complexes exhibit the same emission peak as that of the nondeuterated one, whereas PLQY increases ∼50%. To put the fundamental studies into practice, organic light-emitting diodes (OLEDs) were fabricated with a variety of NIR Pt(II) complexes as the emitting layer, showing the outstanding external quantum efficiencies (EQEs) of 2-25% and the remarkable radiances 10-40 W sr^-1 m^-2 at 740-1002 nm. The prominent device performances not only successfully prove our designed concept but also reach a new milestone for highly efficient NIR OLED devices.This Account thus summarizes our approaches about how to boost the efficiency of the NIR emission of organic molecules from an in-depth fundamental basis, i.e., molecular design, photophysical characterization, and device fabrication. The concept of the exciton delocalization and molecular deuteration may also be applicable to a single molecular system to achieve efficient NIR radiance, which is worth further investigation in the future.

Luminescence properties of [Ir(C^N)2(N^N)]+ complexes: relations between DFT computation results and emission band-shape analysis data

Luminescence properties of two series of [Ir(C^N)₂(N^N)]⁺ complexes bearing deprotonated 1-phenyl-1H-pyrazole or 1-(2,4-difluorophenyl)-1H-pyrazole as cyclometalating C^N ligands and different α-diimines (2,2′-bipyridine, 1,10-phenanthroline and their derivatives) as ancillary N^N ligands have been studied in acetonitrile solutions at room temperature and in 77 K methanol/ethanol (1 : 1) matrices. Ligand and temperature induced changes in the nature of the emissive ³*[Ir(C^N)₂(N^N)]⁺ species result in well-pronounced changes in their emission properties like emission wavelength, emission quantum yields and emission lifetimes. Depending on the nature of the coordinated C^N and N^N ligands and/or the measurement temperature, the investigated luminophores exhibit emissions arising from the intraligand transitions localized within the N^N ligand or from the metal-to-ligand charge-transfer transitions involving the Ir(C^N)₂⁺ and N^N moieties as confirmed by means of the DFT computations. The computed DFT energies of the excited ³*[Ir(C^N)₂(N^N)]⁺ states and outer/inner reorganization energies associated with the S₀ ← ³*[Ir(C^N)₂(N^N)]⁺ transitions remain in nice agreement with those available from the performed emission band-shape analyses. The observed agreement implies ordinary DFT computations at the B3LYP/LANL2DZ/6-31G(d,p) level of theory, even performed neglecting the spin–orbit phenomena, as enough accurate in the quantitative prediction of the most important parameters characterizing the investigated [Ir(C^N)₂(N^N)]⁺ luminophores.

For two series of [Ir(C^N)₂(N^N)]⁺ luminophores, the computed DFT quantities remain in nice agreement with those available from the emission band-shape analyses.

Heteroleptic Re(CO)2+ and Re(CO)3+ complexes with α-diimines: similarities and differences in their luminescence properties

The photophysical properties of two series of phosphorescent rhenium(i) complexes, [Re(CO)₂(N^N)(tpp)₂]⁺ and [Re(CO)₃(N^N)(tpp)]⁺ with carbon monoxide (CO), triphenylphosphine (tpp) and α-diimine (N^N) ligands have been investigated in deoxygenated acetonitrile solution at room temperature and in solid methanol/ethanol 1 : 1 matrices at 77 K. The complexes display moderate to strong phosphorescence which is related to the N^N ligand modulated metal-to-ligand charge-transfer S₀ ← ³*MLCT or intraligand S₀ ← ³*LC transitions. Luminescence properties of the investigated series have been found to be very similar but some intrinsic differences between them are clearly seen. Whereas the [Re(CO)₂(N^N)(tpp)₂]⁺ series shows MLCT emission in both temperature regimes studied, the [Re(CO)₃(N^N)(tpp)]⁺ series exhibits intrinsic changes in its emission character when the measurement temperature is lowered from 298 to 77 K. In both investigated series, their emission characteristics are strongly affected by the nature of coordinated α-diimine N^N ligands. The observed trends, changes in the radiative k_r and non-radiative k_nr deactivation rate constants, have been compared with those found for the previously investigated [Re(CO)₃(N^N)(Cl)], [Re(CO)₃(N^N)(CH₃CN)]⁺, and [Re(CO)₂(N^N)(dppv)]⁺ series (dppv = cis-1,2-bis(diphenylphosphino)-ethene). Similarities and differences in the spectroscopic and photophysical properties of five series of the Re(CO)₃⁺ and Re(CO)₂⁺ complexes have been analyzed in the view of results from DFT and TD-DFT computation and the emission band-shape analyses performed according to the Marcus–Jortner formalism.

We report results from comparative studies of luminescence properties of five series of α-diimine rhenium(i) complexes.

Heteroleptic [Os(Cl)(CO)(P^P)(pbi)] complexes bearing bidentate phosphine and 2-(2-pyridyl)benzimidazolate ligands: impact of isomerism on their luminescence properties

Two isomeric series of osmium(ii) complexes exhibit significant differences in their luminescence properties.

Heteroleptic [Os(H)(CO)(N∧N)(tpp)2]+and [Os(Cl)(CO)(N∧N)(tpp)2]+complexes – comparative studies of their luminescence properties

Two classes of luminescent osmium(ii) complexes – very similar (H ≈ Cl) and slightly different (H ≠ Cl).

The luminescence properties of the heteroleptic [Re(CO)3(N∩N)Cl] and [Re(CO)3(N∩N)(CH3CN)]+ complexes in view of the combined Marcus–Jortner and Mulliken–Hush formalism

Unified description of the radiative k_r and non-radiative k_nr rate constants characterizing the ³*MLCT → S₀ processes allows deeper insights into the luminescence properties of the heteroleptic [Re(CO)₃(α-diimine)(CH₃CN)]⁺ and [Re(CO)₃(α-diimine)Cl] chelates.