Construction and performance of OLED devices prepared from liquid-crystalline TADF materials

The device performance is reported for three compounds which show both thermally activated delayed fluorescence and liquid crystallinity, and use the donor 3,6-bis(3,4-didodecyloxyphenyl)carbazole. Two of the compounds, whose photophysics were reported previously, are based on a terephthalonitrile acceptor. A third and new compound is based on an isophthalonitrile acceptor and shows a more temperature-accessible mesophase and enhanced solution emission quantum yield. Two of the compounds show device external quantum efficiencies of between 2-3% and exhibit very small efficiency roll off. The responses are evaluated in terms of the specific nature of the materials

Unexpected, photochemically induced activation of the tetrabutylammonium cation by hexachloroplatinate(iv)

A dinuclear, butadiene-bridged complex, trans-μ2:η2,η2-1,3-butadiene-bis(trichloroplatinate(ii)) (1) was unexpectedly obtained on photolysis of acetone solutions of (NBu4)2[PtCl6]

Spin-Orbit Coupling and Phosphorescence Rate of Dinuclear Iridium(III) complexes

The dinuclear design of the Ir(III) complexes with chromophoric bridging ligand affords significant increase of the phosphorescence rate. Comparative analysis of dinuclear complexes with their mononuclear analogues showed that the enhancement of phosphorescence rate is brought by electronic coupling of the two coordination centers which results in doubled number of excited singlet states electronically suitable for direct spin-orbit coupling (SOC) with the emitting triplet state. The metal coordinated halide(s) in the structure of the complex can also work as SOC center(s) and thus further enhance the SOC efficiency of the emitting triplet state with singlet states. In the case of the Iridium coordinated Iodides the latter appear to be the major SOC centres influencing the rate of phosphorescence. The electronic character of the bridging ligand is an another tool to modulate the luminescence of Ir(III) complexes. The relatively high π-excessive ligand diminishes the metal and halide contribution to the lowest excited states, and, consequently the efficiency of SOC of emitting triplet state with singlets and the phosphorescence rate. This even may lead to a relatively slow intersystem crossing (ISC) from lowest excited singlet state to the triplet manifold manifested by the appearance of fluorescence, unusual for Ir(III) complexes. The π-deficient character of the chromophoric bridging ligand and relatively wide π-conjugated system can afford strong involvement of the metals to formation of the lowest excited states and relatively low energy of the latter which affords red emitting phosphors with unprecedented combination of quantum yield and rate of phosphorescence. An efficient design approach to materials showing red and fast phosphorescence is hetero-dinuclear structure combining Ir(III) and Pt(II) centers bridged by π-deficient chromophoric ligand. In this case the ligands π-conjugated system can be relatively small which affords strong involvement of the Ir(III) center to formation of the lowest excited singlet states and provide efficient SOC of the emitting triplet state with singlets. The Pt(II) center expands the chromophoric system of the ligand and allows to reach the red-optical range of emission, and yet serves as an SOC centre auxiliary to Ir(III). As a result, such a design affords very fast and intense red-phosphorescence unattainable via the conventional mononuclear molecular design

Ag(i) complex design affording intense phosphorescence with a landmark lifetime of over 100 milliseconds

A highly emissive Ag( I) complex comprising 2,9- dimethyl- 1,10- phenanthroline ( dmp) and bis[( 2- diphenylphosphino) phenyl] ether ( dpep) ligands was synthesized, characterized and investigated for its photophysical properties both experimentally and theoretically. The material exhibits intense phosphorescence from the triplet state of ligand centered ( 3LC) character featuring an unprecedented long lifetime of t = 110 ms and a quantum yield of FPL = 50%, as measured for a doped PMMA matrix under ambient conditions. This is an efficient yet exceptionally slow emission decay, breaking the previous record by several orders of magnitude. Such properties are attributed to two factors: ( i) the Ag( I) ion introduces weak spinorbit coupling and efficient population of the emitting triplet state; ( ii) the rigid molecular design combined with a matrix- based rigidity largely suppresses non- radiative relaxations

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

The design and detailed photophysical study of two novel Ir(III) complexes featuring mono- and dinuclear design are presented. Emission quantum yield and decay times in solution are Phi(PL) = 90% and tau(300 K) = 1.16 mu s for the mononuclear complex 5, and Phi(PL) = 95% and tau(300 K) = 0.44 mu s for the dinuclear complex 6. These data indicate an almost 3-fold increase in the phosphorescence rate for dinuclear complex 6 compared to 5. Zero-field splitting (ZFS) of the T-1 state also increases from ZFS = 65 cm(-1) for the mononuclear complex to ZFS = 205 cm(-1) for the dinuclear complex and is accompanied by a drastic shortening of the individual decay times of T-1 substates. With the help of TD-DFT calculations, we rationalize that the drastic changes in the T-1 state properties in the dinuclear complex originate from an increased number of excited states available for direct spin-orbit coupling (SOC) routes as a result of electronic coupling of Ir-Cl antibonding molecular orbitals of the two coordination sites

Cyclometalation Geometry of the Bridging Ligand as a Tuning Tool for Photophysics of Dinuclear Ir(III) Complexes

Bridging ligands play a crucial role in design of luminescent dinuclear metal complexes. Bis-cyclometalating ligands gave rise to a large family of highly efficient emitters. Herein, we investigate the effect of switching the cyclometalating function of the bridging (chromophoric) ligand on photophysical properties of dinuclear Ir(III) complexes. The new dinuclear Ir(III) complex (Ir-1), comprising a bridging chromophoric ligand with two terminal cyclometalating phenyl derivatives, conjugated to the central twice nitrogen-coordinating thiazolo[5,4-d]thiazole derivative, displays red phosphorescence of decent efficiency in CH2Cl2 solution at room temperature (Phi(PL) = 12%, tau = 1.5 mu s, and lambda = 635 nm). This is several times more efficient compared to the properties of the earlier reported dinuclear Ir(III) complex IrIr, with a bridging ligand comprising terminal nitrogen-coordinating pyridine derivatives and a central cyclometalating thieno[3,2-b]thiophene derivative, under the same conditions (Phi(PL) = 3.5%, tau = 2.9 mu s, and lambda = 714 nm). This "C/N swap" within the bridging ligand caused blue-shifted and improved efficiency of phosphorescence of Ir-1. The origin of this effect is the significantly reduced exchange interaction in state T-1 and, consequently, smaller Delta E(S-1 - T-1) energy gap. According to the density functional theory calculations, this comes from the more even (wider) distribution of the highest occupied molecular orbital within the bridging ligand and increased participation of the metal centers and halide atoms in the formation of states S-1 and T-1. Modulation of the substituent pattern on the bridging ligand in complex Ir-2, analogous to Ir-1, afforded selective tuning of the phosphorescence rate, whereas other properties of phosphorescence remained similar under the same conditions (Phi(PL) = 15%, tau = 3.1 mu s, and lambda = 632 nm)

Can Coumarins Break Kasha’s Rule?

Coumarin C-2 was reported (Signore et al., J. Am. Chem. Soc., 2010, 132, 1276 and Brancato et al., J. Phys. Chem. B, 2015, 119, 6144) to break Kasha's rule. However, the two lowest excited singlet states of C-2 are separated by less than 0.5 eV. To slow down the S-2 -> S-1 internal conversion and thus to enable the Kasha's rule-breaking S-2 fluorescence, a much larger energy separation seems to be necessary. Thus, the photophysical behavior reported for C-2 raised very basic questions concerning mechanisms of nonradiative transitions in organic molecules. Herein we reinvestigated luminescence of C-2 and found that thoroughly purified C-2 does not show any dual fluorescence in steady-state experiments, contrary to the previous findings. The higher-energy emission, previously erroneously assigned as S-2 -> S-0 fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1)

Halide-Enhanced Spin–Orbit Coupling and the Phosphorescence Rate in Ir(III) Complexes

The spin-forbidden nature of phosphorescence in Ir(III) complexes is relaxed by the metal-induced effect of spin-orbit coupling (SOC). A further increase of the phosphorescence rate could potentially be achieved by introducing additional centers capable of further enhancing the SOC effect, such as metal-coordinated halides. Herein, we present a dinuclear Ir(III) complex Ir2I2 that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of FPL(300 K) = 90% and a submicrosecond decay time of only tau(300 K) = 0.34 mu s, as measured under ambient temperature for the degassed toluene solution. These values correspond to a top value T-1 -> S-0 phosphorescence rate of kr = 2.65 x 10(6) s(-1). Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T1 ZFS(III-I) = 170 cm(-1) and unusually short individual decay times of T1 substates: t(I) = 6.4 mu s, tau(II) = 7.6 mu s, and t(III) = 0.05 mu s. This indicates a strong SOC of state T1 with singlet states. Theoretical investigations suggest that the SOC of state T1 with singlets is also contributed by halides. Strongly contributing to the higher occupied molecular orbitals of the complex (e.g., HOMO, HOMO - 1, and so forth), iodides work as important SOC centers that operate in tandem with metals. The examples of Ir2I2 and of earlier reported analogous complex Ir2Cl2 reveal that the metal-coordinated halides can enhance the SOC of state T-1 with singlets and, consequently, the phosphorescence rate. A comparative study of Ir2I2 and Ir2Cl2 shows that the share of halides in total contribution (halides plus metals) to the SOC of state T-1 with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T-1 state from ZFS(III-I) = 205 cm(-1) for Ir2Cl2 to T1 ZFS(III-I) = 170 cm(-1) for Ir2I2. This results in a more efficient thermal population of the fastest emitting T-1 substate III, thus further enhancing the roomtemperature phosphorescence rate

Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at Unprecedentedly Short Decay Time

The four new Ag(I) complexes Ag(phen)(P-2-nCB) (1), Ag(idmp)(P-2-nCB) (2), Ag(dmp)(P-2-nCB) (3), and Ag(dbp)(P-2-nCB) (4) with P-2-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S-1 -> S-0 transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S-1 and T-1 states. The oscillator strength of the S-1 -> S-0 transition f(S-1 S-0)- an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S-1 S-0) = 0.0258 for Ag(phen)(P-2-nCB) (1) to f(S-1 S-0) = 0.0536 for Ag(dbp)(P-2-nCB) (4), as calculated for the T-1 state optimized geometries. This parameter governs the radiative TADF decay time (tau(r)) at ambient temperature, found to be tau(r) = 5.6 mu s for Ag(phen)(P-2-nCB) (1) but only tau(r) = 1.4 mu s for Ag(dbp)(P-2-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (Phi(PL)) measured for powder samples at ambient temperature is boosted up from Phi(PL), = 36% for Ag(phen)(P-2-nCB) (1) to Phi(PL) = 100% for Ag(dbp)(P-2-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P-2-nCB) (1), Ag(idmp)(P-2-nCB) (2), and Ag(dmp)(P-2-nCB) (3) to Ag(dbp)(P-2-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S-1 state from the state T-1, Delta E(S-1-T-1). Experimentally it is determined for the complexes Ag(dmp)(P-2-nCB) (3) and Ag(dbp)(P-2-nCB) (4) to be of moderate size of Delta E(S-1-T-1) = 650 cm(-1)