P∩N Bridged Cu(I) Dimers Featuring Both TADF and Phosphorescence. From Overview towards Detailed Case Study of the Excited Singlet and Triplet States.

We present an overview over eight brightly luminescent Cu(I) dimers of the type Cu2X2(P∩N)3 with X = Cl, Br, I and P∩N = 2-diphenylphosphino-pyridine (Ph2Ppy), 2-diphenylphosphino-pyrimidine (Ph2Ppym), 1-diphenylphosphino-isoquinoline (Ph2Piqn) including three new crystal structures (Cu2Br2(Ph2Ppy)3 1-Br, Cu2I2(Ph2Ppym)3 2-I and Cu2I2(Ph2Piqn)3 3-I). However, we mainly focus on their photo-luminescence properties. All compounds exhibit combined thermally activated delayed fluorescence (TADF) and phosphorescence at ambient temperature. Emission color, decay time and quantum yield vary over large ranges. For deeper characterization, we select Cu2I2(Ph2Ppy)3, 1-I, showing a quantum yield of 81%. DFT and SOC-TDDFT calculations provide insight into the electronic structures of the singlet S1 and triplet T1 states. Both stem from metal+iodide-to-ligand charge transfer transitions. Evaluation of the emission decay dynamics, measured from 1.2 ≤ T ≤ 300 K, gives ∆E(S1-T1) = 380 cm-1 (47 meV), a transition rate of k(S1→S0) = 2.25 × 106 s-1 (445 ns), T1 zero-field splittings, transition rates from the triplet substates and spin-lattice relaxation times. We also discuss the interplay of S1-TADF and T1-phosphorescence. The combined emission paths shorten the overall decay time. For OLED applications, utilization of both singlet and triplet harvesting can be highly favorable for improvement of the device performance.

Design of Conformationally Distorted Donor-Acceptor Dyads Showing Efficient Thermally Activated Delayed Fluorescence.

A highly potent donor-acceptor biaryl thermally activated delayed fluorescence (TADF) dye is accessible by a concise two-step sequence employing two-fold Ullmann arylation and a sequentially Pd-catalyzed Masuda borylation-Suzuki arylation (MBSA). Photophysical investigations show efficient TADF at ambient temperature due to the sterical hindrance between the donor and acceptor moieties. The photoluminescence quantum yield amounts to ΦPL = 80% in toluene and 90% in PMMA arising from prompt and delayed fluorescence with decay times of 21 ns and 30 µs, respectively. From an Arrhenius plot, the energy gap Δ E(S1 - T1) between the lowest excited singlet S1 and triplet T1 state was determined to be 980 cm-1 (120 meV). A new procedure is proposed that allows us to estimate the intersystem crossing time to ∼102 ns.

Thermally Tunable Dual Emission of the d(8)-d(8) Dimer [Pt2(µ-P2O5(BF2)2)4](4).

High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu4N]4[Pt2(µ-P2O5(BF2)2)4], abbreviated Pt(pop-BF2), have been investigated over a wide temperature range, 1.3-310 K. We focus on the lowest excited states that result from dσ*pσ (5dz(2)-6pz) excitations, i.e., the singlet state S1 (of (1)A2u symmetry in D4h) and the lowest triplet T1, which splits into spin-orbit substates A1u((3)A2u) and Eu((3)A2u). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield of ∼ 100% over the investigated temperature range. Our investigation sheds light on this extraordinary dual emission behavior, which is unique for a heavy-atom transition metal compound. Direct ISC processes in Pt(pop-BF2) are largely forbidden due to spin-, symmetry-, and Franck-Condon overlap-restrictions and, therefore, the ISC time is as long as 29 ns for T < 100 K. With temperature increase, two different thermally activated pathways, albeit still relatively slow, are promoted by spin-vibronic and vibronic mechanisms, respectively. Thus, distinct temperature dependence of the ISC processes results and, as a consequence, also of the fluorescence/phosphorescence intensity ratio. The phosphorescence lifetime also is temperature-dependent, reflecting the relative population of the triplet T1 substates Eu and A1u. The highly resolved phosphorescence shows a ∼ 220 cm(-1) red shift below 10 K, attributable to zero-field splitting of 40 cm(-1) plus a promoting vibration of 180 cm(-1).

Highly efficient luminescence of Cu(I) compounds: thermally activated delayed fluorescence combined with short-lived phosphorescence.

Luminescent materials showing thermally activated delayed fluorescence (TADF) have gained high attractiveness as emitters in organic light emitting diodes (OLEDs) and other photonic applications. Nevertheless, even utilization of TADF can be further improved, introducing a novel concept. This is demonstrated by a new class of brightly luminescent low-cost Cu(I) compounds, for which the emission stems from both the lowest excited triplet T1 and singlet S1 state. At T = 300 K, these materials exhibit quantum yields of more than ΦPL = 90% at short emission decay times. About 80% of the emission intensity stems from the singlet due to TADF, but importantly, an additional 20% is contributed by the lower lying triplet state according to effective spin-orbit coupling (SOC). SOC induces also a relatively large zero-field splitting of the triplet being unusual for Cu(I) complexes. Thus, the overall emission decay time is distinctly reduced. Combined use of both decay paths opens novel photonic applications, in particular, for OLEDs.

Brightly luminescent Pt(II) pincer complexes with a sterically demanding carboranyl-phenylpyridine ligand: a new material class for diverse optoelectronic applications.

A series of three Pt(II) complexes with a doubly cyclometalating terdentate ligand L1, L1H2 = 3,6-bis(p-anizolyl)-2-carboranyl-pyridine, and diethyl sulfide (1), triphenylphosphine (2), and t-butylisonitrile (3) as ancillary ligands were synthesized. X-ray diffraction studies of 1 and 2 show a coordination of the L1 ligand in a C-N-C mode in which the bulky and rigid o-carborane fragment is cyclometalated via a C atom. Importantly, no close intermolecular Pt-Pt contacts occur with this ligand type. The new Pt(II) pincer complexes display very high luminescence quantum yields at decay times of several tens of µs even in solution under ambient conditions. On the basis of the low-temperature (T = 1.3 K) emission decay behavior, the emission is assigned to a ligand centered triplet excited state (3)LC with small (1,3)MLCT admixtures. Because the phosphorescence is effectively quenched by molecular oxygen, optical sensors operating in a wide range of oxygen pressure can be developed. Owing to the very high luminescence quantum yields, the new materials might also become attractive as emitter materials for diverse optoelectronic applications.

Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P

A series of highly luminescent dinuclear copper(I) complexes has been synthesized in good yields using a modular ligand system of easily accessible diphenylphosphinopyridine-type P^N ligands. Characterization of these complexes via X-ray crystallographic studies and elemental analysis revealed a dinuclear complex structure with a butterfly-shaped metal-halide core. The complexes feature emission covering the visible spectrum from blue to red together with high quantum yields up to 96%. Density functional theory calculations show that the HOMO consists mainly of orbitals of both the metal core and the bridging halides, while the LUMO resides dominantly on the heterocyclic part of the P^N ligands. Therefore, modification of the heterocyclic moiety of the bridging ligand allows for systematic tuning of the luminescence wavelength. By increasing the aromatic system of the N-heterocycle or through functionalization of the pyridyl moiety, complexes with emission maxima from 481 to 713 nm are obtained. For a representative compound, it is shown that the ambient-temperature emission can be assigned as a thermally activated delayed fluorescence, featuring an attractively short emission decay time of only 6.5 µs at ϕPL = 0.8. It is proposed to apply these compounds for singlet harvesting in OLEDs.

The triplet state of fac-Ir(ppy)3.

The emitting triplet state of fac-Ir(ppy)(3) (fac-tris(2-phenylpyridine)iridium) is studied for the first time on the basis of highly resolved optical spectra in the range of the electronic 0-0 transitions. For the compound dissolved in CH(2)Cl(2) and cooled to cryogenic temperatures, three 0-0 transitions corresponding to the triplet substates I, II, and III are identified. They lie at 19,693 cm(-1) (507.79 nm, I → 0), 19,712 cm(-1) (507.31 nm, II → 0), and 19,863 cm(-1) (503.45 nm, III → 0). From the large total zero-field splitting (ZFS) of 170 cm(-1), the assignment of the emitting triplet term as a (3)MLCT state (metal-to-ligand charge transfer state) is substantiated, and it is seen that spin-orbit couplings to higher lying (1,3)MLCT states are very effective. Moreover, the studies provide emission decay times for the three individual substates of τ(I) = 116 µs, τ(II) = 6.4 µs, and τ(III) = 200 ns. Further, group-theoretical considerations and investigations under application of high magnetic fields up to B = 12 T allow us to conclude that all three substates are nondegenerate and that the symmetry of the complex in the CH(2)Cl(2) matrix cage is lower than C(3). It follows that the triplet parent term is of (3)A character. Studies of the emission decay time and photoluminescence quantum yield, Φ(PL), of Ir(ppy)(3) in poly(methylmethacrylate) (PMMA) in the temperature range of 1.5 ≤ T ≤ 370 K reveal average and individual radiative and nonradiative decay rates and quantum yields of the substates. In the range 80 ≤ T ≤ 370 K, Φ(PL) is as high as almost 100%. The quantum yield Φ(PL) drops to ∼88% when cooled to T = 1.5 K. The investigations show further that the emission properties of Ir(ppy)(3) depend distinctly on the complex's environment or the matrix cage according to distinct changes of spin-orbit coupling effectiveness. These issues also have consequences for optimizations of the material's properties if applied as an organic light-emitting diode (OLED) emitter.

The lowest excited state of brightly emitting gold(I) triphosphine complexes.

The strongly luminescent neutral gold(I) triphosphine complexes [Au(dipnc)(PPh(3))] and [Au(dppnc)(PPh(3))] with dipnc = 7,8-bis(diisopropylphosphino)-nido-carborane ([(PiPr(2))(2)B(9)H(10)C(2))](-)) and dppnc = 7,8-bis(diphenylphosphino)-nido-carborane ([(PPh(2))(2)B(9)H(10)C(2)](-)) are studied in a wide range of temperatures of 1.5

Triplet state properties of the OLED emitter Ir(btp)2(acac): characterization by site-selective spectroscopy and application of high magnetic fields.

The well-known red emitting complex Ir(btp)2(acac) (bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix. The studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B=10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I-->0 at 16268 cm-1), the three substates I, II, and III of the T1 triplet state are separated by DeltaEII-I=2.9 cm-1 and DeltaEIII-I=25.0 cm-1, respectively. DeltaEIII-I represents the total zero-field splitting (ZFS). The individual decay times of these substates are tauI=150 micros, tauII=58 micros, and tauIII=2 micros, respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 micros at T=1.5 K, while the thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces Zeeman mixing of the substates of T1, resulting in an increased splitting between the two lower lying substates from 2.9 cm-1 at zero field to, for example, 6.8 cm-1 at B=10 T. Further, the decay time of the B-field perturbed lowest substate IB decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the three investigated states belong to the same triplet parent term of one single site. Other sites show a similar behavior, though the values of ZFS vary between 15 and 27 cm-1. Since the amount of ZFS reflects the extent of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T1 is a 3LC (ligand centered) state with significant admixtures of 1,3MLCT (metal-to-ligand charge transfer) character. Interestingly, the results show that the MLCT perturbation is different for the various sites. An empirical correlation between the amount of ZFS and the compound's potential for its use as emitter material in an OLED is presented. As a rule of thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10 cm-1.