Rafał Czerwieniec

Blue-light emission of Cu(I) complexes and singlet harvesting.

Strongly luminescent neutral copper(I) complexes of the type Cu(pop)(NN), with pop = bis(2-(diphenylphosphanyl)phenyl)ether and NN = bis(pyrazol-1-yl)borohydrate (pz(2)BH(2)), tetrakis(pyrazol-1-yl)borate (pz(4)B), or bis(pyrazol-1-yl)-biphenyl-borate (pz(2)Bph(2)), are readily accessible in reactions of Cu(acetonitrile)(4)(+) with equimolar amounts of the pop and NN ligands at ambient temperature. All products were characterized by means of single crystal X-ray diffractometry. The compounds exhibit very strong blue/white luminescence with emission quantum yields of up to 90%. Investigations of spectroscopic properties and the emission decay behavior in the temperature range between 1.6 K and ambient temperature allow us to assign the emitting electronic states. Below 100 K, the emission decay times are in the order of many hundreds of microseconds. Therefore, it is concluded that the emission stems from the lowest triplet state. This state is assigned to a metal-to-ligand charge-transfer state (3MLCT) involving Cu-3dand pop-π* orbitals. With temperature increase, the emission decay time is drastically reduced, e.g. to 13 μs [corrected] (Cu(pop)-(pz(2)Bph(2))), at ambient temperature. At this temperature, the complexes exhibit high emission quantum yields, as neat material or doped into poly(methyl methacrylate) (PMMA). This behavior is assigned to an efficient thermal population of a singlet state (being classified as (1)MLCT), which lies only 800 to 1300 cm(-1) above the triplet state, depending on the individual complex. Thus, the resulting emission at ambient temperature largely represents a fluorescence. For applications in OLEDs and LEECs, for example, this type of thermally activated delayed fluorescence (TADF) creates a new mechanism that allows to harvest both singlet and triplet excitons (excitations) in the lowest singlet state. This effect of singlet harvesting leads to drastically higher radiative rates than obtainable for emissions from triplet states of Cu(I) complexes.

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.

Diversity of Copper(I) Complexes Showing Thermally Activated Delayed Fluorescence: Basic Photophysical Analysis

A comparison of three copper(I) compounds [1, Cu(dppb)(pz2Bph2); 2, Cu(pop)(pz2Bph2); 3, Cu(dmp)(phanephos)(+)] that show pronounced thermally activated delayed fluorescence (TADF) at ambient temperature demonstrates a wide diversity of emission behavior. In this study, we focus on compound 1. A computational density functional theory (DFT)/time-dependent DFT approach allows us to predict detailed photophysical properties, while experimental emission studies over a wide temperature range down to T = 1.5 K lead to better insight into the electronic structures even with respect to spin-orbit coupling efficiencies, radiative rates, and zero-field splitting of the triplet state. All three compounds, with emission quantum yields higher than ϕPL = 70%, are potentially well suited as emitters for organic light-emitting diodes (OLEDs) based on the singlet-harvesting mechanism. Interestingly, compound 1 is by far the most attractive one because of a very small energy separation between the lowest excited singlet S1 and triplet T1 state of ΔE(S1-T1) = 370 cm(-1) (46 meV). Such a small value has not been reported so far. It is responsible for the very short decay time of τ(TADF, 300 K) = 3.3 μs. Hence, if focused on the requirements of a short TADF decay time for reduction of the saturation effects in OLEDs, copper(I) complexes are well comparable or even slightly better than the best purely organic TADF emitters.

Synthesis of Cyclometallated Platinum Complexes with Substituted Thienylpyridines and Detailed Characterization of Their Luminescence Properties

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (B = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of (3)pi pi* character, which experience only weak spin-orbit couplings to higher lying singlet states.

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 <or= T <or= 300 K. Analysis of the emission decay dynamics provides detailed information about the lowest triplet state. In particular, the magnitude of zero-field splitting of the emitting state is determined to be 47 and 29 cm(-1) for [Au(dipnc)(PPh(3))] and [Au(dppnc)(PPh(3))], respectively. The emission decay times of the individual triplet substates lie in the range of 4 to 130 mus. The observed photophysical properties suggest that the molecular orbitals involved in the lowest electronic transitions exhibit, beside gold orbitals, considerable contributions from nonmetallic ligand orbitals. OLED or sensor application of these complexes is suggested.

ReI(CO)3+ complexes with N∩O− bidentate ligands

Simple reactions between Re(CO)5Cl and 2-benzoxazol-2-ylphenol, 2-benzothiazol-2-ylphenol or 2-(1-methyl-1H- benzoimidazol-2-yl)phenol (performed in boiling toluene) lead to dimeric complexes of general formula Re2(CO)6(N∩O−)2. The obtained dimeric species are quite stable in non-coordinating solvents, but in coordinating media (e.g., pyridine – pyr) they undergo disscociative solvolysis to form monomeric derivatives of general formula Re(CO)3(N∩O−)pyr. Molecular structures of both the dimeric and monomeric forms have been confirmed by means of X-ray measurements, electron impact mass spectrometry and IR spectroscopic investigations. UV-VIS absorption and emission properties of the newly synthesized Re(CO)3+ complexes have been investigated and briefly compared with those previously reported for their 8-oxyquinolinato analogues.

Singlet harvesting with brightly emitting Cu(I) and metal-free organic compounds

In an electroluminescent excitation, singlet and triplet excitons are generated. In this contribution it is proposed to harvest all excitons in an efficiently emitting singlet state by use of molecules which exhibit distinct thermally activated delayed fluorescence at T = 300K. Highly attractive examples, comprising Cu(I) complexes and the metal-free acridine orange, are presented and discussed with respect to their photophysical properties.

Radiative electron transfer in planar donor–acceptor quinoxaline derivatives

Abstract Synthetic and spectral studies have been performed for a family of electron donor–acceptor (D-A) quinoxaline derivatives possessing an aromatic amine as an electron donor. A photophysical behaviour of the compounds with an internal degree of freedom for internal D-A rotation and their rigid analogues with a fixed planar conformation appears to be very similar. Electronic transition dipole moments related to the charge-transfer (CT) absorption and fluorescence are determined by both the direct interactions between the 1 CT and ground states and by the contributions from the locally excited configurations. The radiative properties of the D-A systems under study can be explained in terms of the simple model which assumes that the electronic coupling elements are mainly determined by the interactions between the atoms forming the A-D bond.

TADF for singlet harvesting: next generation OLED materials based on brightly green and blue emitting Cu(I) and Ag(I) compounds

Detailed photophysical studies are presented for Cu2Cl2(dppb)2 and Ag2Cl2(dppb)2. Both compounds show very effective thermally activated delayed fluorescence (TADF) at ambient temperature with an emission quantum yield of the Ag(I) complex of ΦPL(300 K) = 93 %. This emission is blue shifted by 65 nm (2500 cm-1) with respect to the emission of the Cu(I) complex, demonstrating a valuable strategy for engineering blue light emitters. Potentially, these materials are well suited for taking advantage of the singlet harvesting effect in an OLED device. Moreover, both compounds do not show effects of concentration quenching at high emitter concentration, a property which might be attractive for reducing the efficiency roll-off at higher current densities. Investigations down to T = 1.6 K show that spin-orbit coupling (SOC) is particularly weak. This is displayed in the very long emission decay times of the triplet states (T1 states) of metal-toligand charge transfer (3MLCT) character, amounting to τ(Ag2Cl2(dppb)2) = 1.1 ms and τ(Cu2Cl2(dppb)2) = 2.2 ms. According to the TADF mechanism, which leads to the additional decay channel at ambient temperature via the S1 state (of 1MLCT character), an increase of the radiative rate by a factor of 70 and almost 500 for Ag2Cl2(dppb)2 and Cu2Cl2(dppb)2, respectively, is induced. This results in radiative rates at ambient temperature of kr = 6.2 ∙ 104 s-1 (τr = 16 μs, Ag(I) complex) and 11.7 ∙ 104 s-1 (τr = 8.5 μs, Cu(I) complex). Simple approaches are presented that allow us to understand the weakness of SOC on the basis of results from DFT and TD-DFT calculations. Investigations of the emission decay properties down to T = 1.6 K further support the conclusions with respect to the SOC strength.

Temperature-dependent phosphorescence spectra of Pd- and Pt-porphins and their applications

In this paper the temperature dependence of radiative deactivation of Pd- and Pt-porphin triplet states in Shpolskii matrices has been studied in the temperature range of 4.2-210 K. Dramatic changes in the phosphorescence spectra, with increasing temperature, were observed. It has been proposed that this is due to the inclusion of thermally activated states of the studied molecules in the processes of radiative deactivation of triplet states. For Pd-porphin the T1 → S0 emission is merely observed at liquid helium temperature. An increase in temperature leads to the activation of several radiative Ti → S0 transitions, so additional bands appear in the phosphorescence spectrum. In the case of Pt-porphin, temperature activation takes place as early as at liquid helium temperature. The degeneracy lifting of lowest T1,2 states due to crystal field splitting was observed for both Pd- and Pt-porphins. The splitting value δ = ΔE(T2 − T1) has been measured in different matrices and the dependencies on both the metal ion in the given matrix and the matrix type for the given metal ion, have been revealed. The possibility of designing the luminescent molecular thermometers for cryogenic temperatures based on the high temperature sensitivity of the Pd- and Pt-porphin phosphorescence in n-alkane matrices, has been discussed. The workability of the dependence of Pd- and Pt-porphin phosphorescent properties on temperature in the design of the molecular thermometer family for the temperature range from a few K up to temperatures close to the melting point of n-alkane matrix (150-200 K) is demonstrated.