Excited State Emission Research Articles

Synthesis and characterization of acrylate cyanide bridged dimeric fac-Rhenium(I) complex: Photophysical, selective CO2 adsorption and theoretical studies

Reaction of ethyl 2-cyano-3-(2-hydroxy-5-methyl-3-(p-tolylimino)methyl)phenyl)acrylate) (HL) based on 2, 6-diformyl-4-methylphenol with [Re(CO)5Cl] in toluene afforded a product of composition [Re2(CO)6(L)2]. X-ray structure of the representative complex [Re2(CO)6(L)2] was determined to confirm the molecular species unequivocally. The molecular structure of the rhenium complex exhibited distorted octahedral geometry around the Re(I) center, consistent with the d6 configuration. The complex exhibited excellent photoluminescence behavior in acetonitrile solution. To gain a deeper insight into the ground and excited-state geometries, absorption, and emission properties of the binuclear Re(I) core, complex was further scrutinized by DFT and TDDFT methods. In addition, the synthesized binuclear Re(I) complex was used as a porous material for the selective adsorption of CO2 gas at low temperature (273 K), as well as at ambient condition (298 K). To the best of our knowledge, this compound is the first Re(I)-based binuclear complex which efficiently adsorbs CO2 selectively.

Structure and Function of Tryptophan-Tyrosine Dyads in Biomimetic β Hairpins.

Tyrosine–tryptophan (YW) dyads are ubiquitous structural motifs in enzymes and play roles in proton-coupled electron transfer (PCET) and, possibly, protection from oxidative stress. Here, we describe the function of YW dyads in de novo designed 18-mer, β hairpins. In Peptide M, a YW dyad is formed between W14 and Y5. A UV hypochromic effect and an excitonic Cotton signal are observed, in addition to singlet, excited state (W*) and fluorescence emission spectral shifts. In a second Peptide, Peptide MW, a Y5–W13 dyad is formed diagonally across the strand and distorts the backbone. On a picosecond timescale, the W* excited-state decay kinetics are similar in all peptides but are accelerated relative to amino acids in solution. In Peptide MW, the W* spectrum is consistent with increased conformational flexibility. In Peptide M and MW, the electron paramagnetic resonance spectra obtained after UV photolysis are characteristic of tyrosine and tryptophan radicals at 160 K. Notably, at pH 9, the radical photolysis yield is decreased in Peptide M and MW, compared to that in a tyrosine and tryptophan mixture. This protective effect is not observed at pH 11 and is not observed in peptides containing a tryptophan–histidine dyad or tryptophan alone. The YW dyad protective effect is attributed to an increase in the radical recombination rate. This increase in rate can be facilitated by hydrogen-bonding interactions, which lower the barrier for the PCET reaction at pH 9. These results suggest that the YW dyad structural motif promotes radical quenching under conditions of reactive oxygen stress.

InAs/InP quantum dots stacking: Impact of spacer layer on optical properties

The optical properties of a vertical multilayer stack of semiconductor self-assembled quantum dots (QDs) depend on the coupling status. We present an improved modeling method for analysis of QD stacks, consisting of several separate steps in the elastic strain analysis, to reproduce the realistic unidirectional strain accumulation along the QD growth direction from lower and upper dots. Based on this modeling method, by varying the spacer thickness, we systematically study a large variety of QD stack structures, for better understanding the influence of strain and electronic coupling mechanisms. A “quasi continuum band” or highly degenerate excited state (ES) is found in the closely multi-stacked QDs, probably accounting for the occurrence of distinct blueshifting of the photoluminescence peak wavelength for the decreasing spacer thickness. The enhanced ES emission from this type of stacking QD modeled by using rate equations may pave a way for designing potential high-performance laser devices operating on ES.

Synthesis and comparative studies of photophysical and electrochemical properties of three different types of new heteroleptic 5-arylazo-8-hydroxyquinoline complexes of rhodium including trans → cis isomerism studies

Refluxing [Rh(2-phpy)2(Cl)]2 separately with the equivalent amount of 5-phenylazo-8-hydroxyquinoline (Hq1), 5-(4-fluorophenylazo)-8-hydroxyquinoline (Hq2) and 5-(4-nitrophenylazo)-8-hydroxyquinoline (Hq3) in toluene in aerobic condition afforded mononuclear orange coloured compounds having general formula [Rh(2-phpy)2(q)] in excellent yields. The complexes were substantiated by IR, 1H NMR, ESI-mass, EPR, UV–Vis and emission spectra, cyclic voltammetry, single-crystal X-ray structure determinations and density functional theory (DFT) calculations. Synthesis and comparison of their photophysical and electrochemical properties of a new branch of rhodium(III) complexes of 8-hydroxyarylazo analogues are presented which are never studied before. The ground and excited-state geometries, absorption, and emission properties of the complexes were examined by DFT and TDDFT methods. The excitations and emissions are investigated by the natural transition orbital (NTO) analysis. The emission occurred via 3MLCT or 3ILCT transition by theoretical analysis of triplet state DFT calculations. The cis-trans isomerism studies for the azo complexes have been done by experimental as well as theoretical analysis.

A lanthanide complex for selective sensing of Fe(III) ions

The lanthanide complex [Eu(Tpy–NH2)2Cl2]Cl (1), where Tpy–NH2 is 4-(paminophenyl)-2,2:6,2”-terpyridine, was synthesized. Its photophysical properties, e.g., Fourier Transform Infrared (FTIR) spectra, UV–visible absorption spectra, and luminescence properties, in the presence of various metal ions were studied. Tpy-NH2 acts as chromophore, or light-harvesting photo sensitizing antenna moiety, for Eu3+ luminescence. The bright luminescence in 1 suggests that favourable photo-induced energy transfer occurs from the Tpy-NH2 antenna to the Eu3+ to fill the long-lived emissive excited states of Eu3+ f–f transitions. The luminescence spectral profile of complex 1 showed a characteristic sharp emission band maximum for 5D0 →7F2 f–f transitions at 614 nm originating from Eu3+ ions. The luminescence studies revealed that this material possesses a highly selective fluorescence quenching response to Fe3+ ions in liquid suspension. The dose-dependent fluorescent quenching showed good linearity, and a Fe3+ detection limit of 0.655 μM was calculated for complex 1. This material can potentially be used as a fluorosensor for sensing Fe3+ ions; its luminescence quenching response can be explained in terms of a competitive absorption mechanism.

Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments.

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

Fast creation of a three-atom singlet state with a dissipative mechanism and Rydberg blockade

We present a protocol to prepare rapidly a stationary three-atom singlet state with high fidelity, which takes advantage of the dissipative factor originating from the spontaneous emission of short-lived excited states and Rydberg blockade. The short lifetime of the excited states is an active ingredient in the state preparation, and the numerical simulation results of the master equation show that the fidelity of the created state can achieve 0.98 when the evolution time of the system is as short as 300 $\ensuremath{\mu}\mathrm{s}$ under certain reasonable conditions. Moreover, the protocol is only weakly dependent on the Rydberg interaction strength and needs no state initialization, optical cavity, feedback controls, and fine controlling of the system parameters, which can greatly reduce the experimental difficulty.

Tailoring the Emission of Fluorinated Bipyridine-Chelated Iridium Complexes.

New functionalized tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III) ((dfpypy)3Irs) complexes, including small molecules and their dendrimer embedded analogoues, were synthesized and characterized. It is demonstrated that both the fac-(dfpypy)3Ir-based polyphenylene dendrimers and (triisopropylsilyl)ethynyl (TIPSE)-substituted (dfpypy)3Ir complexes induce large bathochromic shifts (∼50 nm) of emission bands compared with fac-(dfpypy)3Ir. This is due to the pronounced 3π–π* character of emissive excited states and the extended conjugation. A further remarkable feature is the small bathochromic shift of the emissions of fac-tris(2-phenylpyridine)iridium (fac-(ppy)3Ir)-based polyphenylene dendrimers when compared to those of the iridium (Ir) complex core. Obviously, the triplet metal-to-ligand charge transfer makes emission less sensitive to extended conjugation than the 3π–π* transition. This finding suggests new concepts for designing blue phosphorescent dendrimer emitters. Both the dendrimers and the TIPSE-substituted (dfpypy)3Ir complexes represent new green and the trimethylsilyl-functionalized (dfpypy)3Ir new blue phosphorescent emitters. Incorporation of TIPSE moieties into the ligands of iridium complex gives rise to enhanced phosphorescence.

Quantum-Chemical Insights into the Phosphorescence Efficiencies of Blue-Emitting Platinum Complexes with Phenylene-Bridged Pincer Ligands.

Blue phosphorescent platinum complexes with phenylene-bridged pincer ligands, [Pt(dmib)Cl] [1; dmib = m-bis(methylimidazolyl)benzene], [Pt(mizb)Cl] [2; mizb = bis( N-methylimidazolium)benzene], and [Pt(dpzb)Cl] [3; dpzb = m-bis(3,5-dimethylpyrazolyl)benzene], have been investigated theoretically to rationalize the marked differences of their phosphorescence efficiencies. On the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, the geometrical and electronic structures, absorption and emission properties, and radiative and nonradiative processes are analyzed in detail. The emission from the emissive lowest triplet state (T1) originates from a mixture of metal-to-ligand charge-transfer (3MLCT) and intraligand charge-transfer (3ILCT) states. The calculated radiative decay rate constants of T1 of the complexes are comparable and in the same order of magnitude with the experimental measurements. Therefore, the potential energy profiles for the deactivation processes from T1 via temperature-independent and -dependent pathways are explored to reveal the effect of nonradiative decay on phosphorescence. The calculated results indicate that the very weak emission of 3 could be ascribed to the deactivation process via the metal-centered (3MC) state, which can be readily accessible via a spontaneous process from the T1 state. This work provides more in-depth insight into the nature of the emissive excited state, shielding light on a better understanding of the excited-state behavior of phosphorescent platinum complexes.

Exciton States in Monolayer MoSe2 and MoTe2 Probed by Upconversion Spectroscopy

Transitions metal dichalcogenides (TMDs) are direct semiconductors in the atomic monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20 % for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in MoSe$_2$ and MoTe$_2$ monolayers have so far been elusive due to their low oscillator strength and strong inhomogeneous broadening. Here we show that encapsulation in hexagonal boron nitride results in emission line width of the A:1$s$ exciton below 1.5 meV and 3 meV in our MoSe$_2$ and MoTe$_2$ monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1$s$ transition and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this non-linear optical process. At the origin of upconversion our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band thus generating high energy excitons with small wave-vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation and resonant Raman scattering confirming their origin as excited excitonic states in monolayer thin semiconductors.

Metal-Organic Frameworks with Organogold(III) Complexes for Photocatalytic Amine Oxidation with Enhanced Efficiency and Selectivity.

Luminescent organogold(III) complex AuIII with highly emissive triplet excited state was encapsulated in two metal-organic frameworks (MOFs) with different pore sizes and structures (MOF1 and ZJU-28). Compared with the AuIII complex in solution, the resultant composites AuIII @MOF1 and AuIII @ZJU-28 exhibit enhanced emission intensity, lifetime, and quantum yield. Under irradiation, AuIII @MOFs are efficient, selective, and recyclable catalysts for light-induced aerobic C-N bond formation. When used as a heterogeneous catalyst for oxidizing secondary amines to the corresponding imines, AuIII @ZJU-28 achieved high TONs of 876-1548, which are about 2.8-3.5 times higher than that of the homogenous AuIII complex. In addition, different selectivities in oxidizing mixed substrates is realized by means of different host MOFs, and thus encapsulating the AuIII complex in an appropriate MOF allowed the desired product to be obtained. Inherent shortcomings of homogeneous catalysts in cyclic use are also overcome by using composite catalysts, and high conversion of the AuIII @ZJU-28 catalyst was still observed after ten cycles.

Luminescence of Atomic Barium in Rare Gas Matrices: A Two-Dimensional Excitation/Emission Spectroscopy Study.

A detailed characterization is made of the distinct sites occupied by atomic barium isolated in the three rare gas hosts Ar, Kr, and Xe in excitation scans extracted from the recorded total 6s6p 1P1 → (6s)21S0 fluorescence. Extensive use has been made of two-dimensional excitation/emission (2D-EE) spectroscopy to achieve a comprehensive characterization for the wide variety of sites present in the Ba/RG matrix systems. The 2D-EE technique has proved to be a very powerful method to probe the effects of strong intersite reabsorption when extensive spectral overlap occurs between emission and resonance 6s6p 1P1 ← (6s)21S0 absorption of barium atoms occupying multiple sites. Two-dimensional excitation/emission scans have also been used in this study to monitor the effects of sample annealing and thereby identify the thermally stable sites of isolation. Sites of the same type occupied by atomic barium in the three host solids are identified in resolved excitation spectra and are associated on the basis of the observed matrix shift versus host polarizability. Following site associations, the photophysical properties of each matrix site were characterized revealing that the Stokes shift was greatest in the blue site, smallest for the violet site, and intermediate for the green site. The emission temperature dependences and excited state lifetimes were recorded, indicating that measured radiative lifetimes of 4-5 ns were in good agreement with the gas phase value of 8.4 ns when corrected for the effective field of the solids. The only exception to this was the blue site in Ba/Xe, where a nonradiative quenching channel exists even at 9.8 K that competes effectively with the nanosecond fluorescence. An unusual, asymmetric 2 + 1 excitation band has been recorded for atomic barium in the three rare gas hosts in addition to the threefold split, Jahn-Teller bands typically observed for P ← S absorptions of matrix-isolated metal atoms. Possible assignments of the sites responsible for these band shapes are made on the basis of recent spectral simulations obtained from molecular dynamics calculations on the Ba/Xe system.

First-principles study of efficient phenothiazine-based D–π–A organic sensitizers with various spacers for DSSCs

Design of highly efficient sensitizers is one of the most significant issues for dye-sensitized solar cells (DSSCs). In this work, we designed and screened a series of phenothiazine-derivative dyes with donor–π spacer–acceptor (D–π–A) architecture based on a structural configuration with a phenothiazine group as the donor (D) part, a thiophene as the linker (π), and a 2-cyanoacrylic acid moiety (A) as the electron-acceptor segment, comparing the effects of different π-spacer groups between the electron donor and acceptor. The effects of different electron-deficient linker units on the absorption/emission spectra and photovoltaic properties were investigated by a combination of density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches. Based on the ground- and excited-state geometries, the absorption and emission spectra were calculated using the TD-DFT method. Different exchange–correlation functionals were first applied to establish a proper methodology for calculating the excited-state energy of the reference dye, known as TC104. The theoretical results calculated using the TD-CAM-B3LYP/B3LYP method with 6-311++G(d,p)/6-31G(d,p) basis set were found to agree well with the absorption and emission wavelengths for TC104 found experimentally. Overall, dye molecules SP1–SP4 (especially SP2 and SP4) were found to be promising sensitizers for highly efficient organic DSSCs.

Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin.

This study revealed S2 fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin(1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broad-band fluorescence upconversion technique. This is the first S2 fluorescence from chromophores with twisted Möbius topology, and the observation of S2 fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand the S2 fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the SB state to the SQ state, which occurs prior to relaxation to the optically dark, lowest electronic state (SD). Therefore, the SQ state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the SB state to the S0 state. In deprotonated [32]heptaphyrin, the internal conversion from the SB state to the SQ state proceeds with a slower time constant of 150 fsfor owing to its rigid structure, helping the observation of its S2 fluorescence.

Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction

We have previously developed photocatalytic CO2 reduction systems using graphitic carbon nitride (g-C3N4) and a Ru(II) mononuclear complex (e.g., trans(Cl)–[RuII{4,4′-(H2PO3)2bpy}2(CO)2Cl2] bpy = 2,2′-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C3N4 and electron-transfer process to RuP, here we examined the photophysical properties of g-C3N4 as well as mesoporous g-C3N4 (mpg-C3N4) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C3N4 (as well as mpg-C3N4) has at least three emissive excited states with different lifetimes (g-C3N4; 1.3 ± 0.4, 3.9 ± 0.9, and 15 ± 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though phot...

Unveiling the Dual Emission Photo-Deactivation Mechanism of a Neutral Heteroleptic Iridium (III) Complex.

In this article, the photo-deactivation mechanism of dual emission of a neutral iridium (III) complex is explored by using density function theory (DFT) and time-dependent density function theory (TD-DFT) calculations. To explore the phosphorescence quantum yield of the iridium (III) complex, the radiative decay constant of each emission excited state was computed by TD-DFT calculations, including spin-orbit coupling (SOC). In these calculations, factors such as the transition dipole moments, energy gaps, and SOC elements between the emission states and singlet excited states are taken into account in the evaluation of the radiative decay constants. Additionally, the non-radiative decay is revealed by considering the temperature-independent and the temperature-dependent non-radiative processes. The computational results indicate that the order of the two emission excited states can exert a significant effect on the phosphorescence quantum yield, which is beneficial for understanding the properties of photo-deactivation of phosphorescent emitters.

Solvent effect on multiple emission and ultrafast dynamics of higher excited states

We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the visible region: S2 → S0, and the other at NIR region: S1 → S0. We show that exact band-positions, intensities, and fluorescence-decay-timescales (τ) depend strongly on viscosity and polarity of solvents. Interestingly, though the faster component of τ increased for IR144 with increasing viscosity and chain-length of alcohols, the reverse was seen for IR140, indicating the possible formation of ion-pair of IR140 with alcohols.

A "Flexible" Rigid Rod, trans-Pt(PMe3)2(C≡CC6H4CN)2 (L1), to Form 2D [{Cu2(μ2-X)2}2(μ4-L1)] n Polymers (X = Br, I) Exhibiting the Largest Bathochromic Emissions.

The trans-Pt(PMe3)2(C≡CC6H4CN)2 organometallic ligand L1, which is prepared from 4-ethynylbenzonitrile and cis-Pt(PMe3)2Cl2, binds CuX salts to form two strongly luminescent two-dimensional coordination polymers (CPs) [{Cu2(μ2-X)2}2(μ4-L1)] n (X = I, CP1; X = Br, CP2). The emission quantum yields, Φe ≈ 30% at 298 K, are the largest ones for all CPs built upon the trans-Pt(PMe3)2(C≡CC6H4X)2 motifs (X = SMe, CN). X-ray crystallography reveals that, to accommodate these layered CPs, L1 must undergo major distortions of the C≡C-C angles (∼159°) and significant rotations about the Pt-CC bonds, so that the dihedral angles made by the two aromatic planes is 90° in a quasi-identical manner for both CPs. Together, these two features represent the largest distortion for trans-Pt(PMe3)2(C≡CC6H4X)2 complexes among all of the CPs built upon this type of ligand (2 of 16 entries). Concurrently, CP1 and CP2 also exhibit the most red-shifted emissions (λmax = 650 and 640 nm, respectively) known for this type of chromophore at room temperature. The {Cu2(μ2-X)2} rhomboids adopt the trans- (X = I, common) and cis-geometries (X = Br, extremely rare) making them "isomers" if excluding the fact that the halides are different. Density functional theory (DFT) and time-dependent DFT suggest that the triplet emissive excited state is metal/halide-to-ligand charge transfer in both cases despite this difference in rhomboid geometry.

A new molecular switch based on a symmetrical dinuclear complex of two tricarbonylrhenium(I) moieties bridged by 4,4″-azobis-(2,2′-bipyridine)

A new symmetrical dinuclear complex of two tricarbonylrhenium(I) moieties, of formula [(CH3CN)(CO)3Re(4,4″-azobpy) Re(CO)3(CH3CN)](PF6)2, with 4,4″-azobpy = 4,4″-azobis-(2,2′-bipyridine), has been synthesized and characterized by spectroscopic, electrochemical, spectroelectrochemical, photophysical and computational techniques. The bridging azo group in the bipyridyl ring decreases the emission quantum yield of the 3MLCT lowest-lying excited state respect to similar Re(I) complexes and introduces a new emissive excited state with a longer lifetime, due to increased electronic delocalization in the bridging ligand. When reducing the azo group in CH3CN/H2O mixtures with sodium dithionite, the emission is enhanced by an order of magnitude. Therefore, this complex can be used as a “molecular switch” with electron and proton additions. Besides, changes in the absorption spectrum on addition of l-Cysteine can be applied for sensing aminoacids with reducing thiol groups. The electronic structures calculated by DFT methods agree reasonably well with experimental results.

Tunable Multicolor Phosphorescence of Crystalline Polymeric Complex Salts with Metallophilic Backbones.

A total of 35 [Au(NHC)2 ][MX2 ] (NHC=N-heterocyclic carbene; M=Au or Cu; X=halide, cyanide or arylacetylide) complex salts were synthesized by co-precipitation of [Au(NHC)2 ]+ cations and [MX2 ]- anions. These salts contain crystallographically determined polymeric Au⋅⋅⋅Au or Au⋅⋅⋅Cu interactions and are highly phosphorescent with quantum yields up to unity and emission color tunable in the entire visible regions. The nature of the emissive excited states is generally assigned to ligand (anion)-to-ligand (cation) charge-transfer transitions assisted by d10 ⋅⋅⋅d10 metallophilicity. The emission properties can be further tuned by controlled triple-component co-crystallization or by epitaxial growth. Correct recipes for white light-emitting phosphors with quantum yields higher than 70 % have been achieved by screening the combinatorial pool.