How the Quantum Efficiency of a Highly Emissive Binuclear Copper

Article pubs.acs.org/Langmuir

How the Quantum Efficiency of a Highly Emissive Binuclear Copper Complex Is Enhanced by Changing the Processing Solvent Daniel Volz,† Martin Nieger,‡ Jana Friedrichs,§ Thomas Baumann,*,§ and Stefan Bras̈ e*,† †

Institut für Organische Chemie, KIT, Karlsruhe, Germany Department of Chemistry, University of Helsinki, PO Box 55 (A.I. Virtasen aukio 1), 00014 Helsinki, Finland § cynora GmbH, Eggenstein-Leopoldshafen, Germany ‡

S Supporting Information *

ABSTRACT: Polymorphism is often linked to the choice of processing solvents. Packing effects or the preference of one certain conformer as possible causes of this phenomenon are strongly dependent on solvents and especially on their polarity. Even in amorphous solids, the microstructure can be controlled by the choice of solvents. Polymorphs or amorphous solids featuring different packing densities can exhibit different properties in terms of stability or optical effects. The influence of these effects on a binuclear, strongly luminescent copper(I) complex was investigated. Many possible applications for luminescent, amorphous coordination compounds, such as organic light-emitting diodes, sensors, and organic lasers, rely on photophysical properties like quantum efficiency to be repeatable. The effect of processing solvents in this context is often underestimated, but very relevant for utilization in device manufacturing and should therefore be understood more deeply. In this work, theoretical derivations, DFT calculations, X-ray-diffraction, photoluminescence spectroscopy, and the timedependent single-photon-counting-technique (TDSPC) were used to understand this phenomenon more deeply. The influence of five different solvents on Cu2I2(MePyrPHOS)3 was probed. This resulted in a modulation of the photoluminescence quantum yield ϕ between 0.5 and 0.9 in amorphous solid state. A new polymorph of the material with slightly reduced values for ϕ has been identified. The reduced efficiency could be correlated with a higher porosity and a reduced packing density. Dense packing reduces nonradiative decay by geometrical fixation and thus increases the quantum efficiency. The existence of similar effects on aluminum and iridium compounds has been confirmed by application of different processing solvents on Alq3 and Ir(ppy)3. These results show that a tuning of the efficiency of a emissive metal complexes by choosing a proper processing solvent is possible. If highly efficient materials for practical applications are desired, an evaluation of multiple solvents has to be considered.

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thermochromism and solvatochromism14 or a π−π-stackinginduced enhancement of photoluminescence15 have been observed for copper complexes. Due to the π−π-stacking of diimine ligands in [Cu(diimine)(POP)]BF4 in the solid state,16 the photoluminescence quantum efficiency (PLQY) was enhanced up to 6 times in comparison to a sample which was obtained by simply evaporating the dichloromethane solution to dryness. In contrast, the enhanced form was processed from hexane or petrol ether. A cationic [Cu(Ph2Pqn)2]+ complex has been crystallized in two distortion isomers by using different solvents.17 Another work demonstrated how four polymorphic forms of isomeric Au−Cuclusters [CuAu(PPh2py)3]2+ were obtained using different solvents,18 showing PLQYs ranging from 0.001 to 0.250: If the structure of the complex is flexible enough, subtle changes of the amount and position of incorporated solvents and the relative positions of cations and anions can have a profound influence on the emission properties. An additional, fifth cluster

INTRODUCTION Copper is certainly one of the most versatile central atoms used in coordination chemistry: A plethora of reactions are efficiently catalyzed by Cu(I) or Cu(II). Complexes or materials containing copper ions often show tremendous extinction when used in dyes. Recently, the search for new luminescent coordination compounds has led to the development of ubiquitous photoluminescent copper(I)complexes.1−5 Their employment as emitters in organic light-emitting diodes (OLEDs),6,7 light-emitting electrochemical cells,8 oxygen sensors,9 and even luminescent markers in biochemistry10,11 promises new applications for these materials. Due to the favorable d10-electron-configuration found for Cu(I), many common quenching mechanisms involving empty, metalcentered d-orbitals are prohibited.5 In most of these examples, the copper moieties are directly participating in the luminescence because metal-to-ligand-charge transfer (MLCT) emission occurs. Recently, Hartmut Yersin showed a thermally induced delayed fluorescence phenomenon for copper complexes featuring a small energy gap between the S1 and T1 states.12,13 Beyond that, other fascinating photophysical phenomena like © 2013 American Chemical Society

Received: October 5, 2012 Revised: February 2, 2013 Published: February 2, 2013 3034

dx.doi.org/10.1021/la3039522 | Langmuir 2013, 29, 3034−3044

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Scheme 1. By Changing the Solvent, Another Polymorph of Cu2I2(MePyrPHOS)3 Can Be Obtaineda

a

While the emission spectra are quite similar, changes concerning the photoluminescence quantum efficiency (PLQY) were found.

Table 1. Photophysical Properties of Samples 1−5a sample

processing solvent

λmax [nm]

λ1/2, exc [nm]

PLQY ϕ

CIE xb

CIE yb

precipitation time

1 2 3 4 5

diethyl ether pentane hexane methanol dichloromethane

515 520 520 512 546

425 430 432 413 396

0.79 0.70 0.66 0.89 0.52

0.285 0.299 0.300 0.276 0.397

0.507 0.518 0.520 0.502 0.536

15 min seconds seconds 3h c

λmax: emission (peak maximum, excitation at 350 nm, ±1 nm). λ1/2, exc: excitation (half-height wavelength, emission peak was observed ±1 nm). PLQY: all values were obtained from powder samples, ±0.02. bCIE x, CIE y: position in the CIE 1931 XYZ color space, where x + y + z = 1.31 c Samples were obtained by evaporation of the solvent rather than precipitation. a

[Au3Cu2(PPh2py)5]5+ exists.19 The emission color of these five compounds ranges from blue to red. On the other hand, a decrease of PLQY after the preparation of thin films of Cu-complexes is a well-known phenomenon. For example, the PLQY of Cu(POP)(Bpz4)12 decreases from 0.90 to 0.30, while the emission maximum shifts toward lower energies, indicating a competing loss of energy of the excited complexes before radiative relaxation to the ground state is possible. Relations between the chemical environment of a chromophore and its properties are of course not limited to copper compounds. Especially for organic luminescent molecules, an entanglement effect of side chains in matrix materials is wellknown:20−23 Depending on the substance, the processing conditions, and the solvent used, packing is more or less dense. Polymorphs of organic crystals bearing different luminescent behavior have been synthesized by using different solvents.24 Accordingly, processing solvents are often not only inert media, but also an important factor influencing the properties of the material. For transition-metal complexes compounds featuring MLCT emission like Ir(4,6-dFppy)2pic,25 a grave influence of solid matrix materials like frozen CH2Cl2 or THF on lifetime and emission behavior has been shown. Incorporation of luminescent ruthenium-complexes in zeolite-networks as an example for a very rigid environment resulted in a slight blue shift, accompanied by a suppression of nonradiative decay rates.26 For ruthenium and osmium complexes, a blue shift and luminescence enhancement was found upon cooling solutions of the complexes to the melting point of the solvent,27 embedding them in PMMA matrices28 or oxalate networks.29 By single-pulse time-dependent synchrotron experiments, an interesting effect for the two different molecules of [Cu(phen)(PPh3)2]BF4 in crystalline samples has been found:30 Due to lattice restriction effects, it was possible to discriminate both nonequivalent complex molecules.

In this study, a luminescence enhancement phenomenon observed for a dinuclear copper(I) complex is reported. Depending on the processing solvent used for sample preparation (samples 1 to 5), different photoluminescence quantum yields ranging from 0.52 to 0.89 could be obtained for the same compound. This observation is rationalized by comparison of two different polymorphs of the compound (forms a and b; see Scheme 1), which were obtained by using solvents of different polarity. By the means of PL spectroscopy and TDSPC measurements, we identified the stiffness of the obtained solids as a key parameter for the quantum efficiency. More rigid environments efficiently suppress the nonradiative decay of the excited states of these compounds, thus enhancing the quantum efficiency of PyrPHOS-complexes.

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EXPERIMENTAL METHODS

Basic properties of the five different samples are given in Table 1. Diethyl ether, pentane, hexane, methanol, and dichloromethane were used as processing solvents in order to cover a wide range of solvent polarity. Pentane and hexane as solvents featuring very similar properties were chosen. The differences between samples processed from these two solvents were only marginal. Our sample preparation protocol, as well as the conditions used for the preparation of single crystals, are collected in the Supporting Information, including details such as solvent volumes, temperatures, and so on. Further details regarding the synthesis, chemical, and spectroscopical features of the amorphous and crystalline samples are also given in the SI. Photophysical Measurements. Absorption spectra were measured on a Thermo Scientific (Evolution 201) UV−visible spectrophotometer in acetonitrile. Emission and excitation spectra in the solid state were measured with an Horiba Scientific FluoroMax-4 spectrofluorometer using a JX monochromator and a R928P PMT detector. Fluorescence lifetime measurements were recorded and detected on the same system using the TCSPC method with the FM2013 accessory and a TDSPC hub from Horiba Yvon Jobin. For this, a NanoLED 370 was used as excitation source (λ = 370 nm, 1.5 ns pulse). Decay curves were analyzed with the software DAS-6 and DataStation provided by Horiba Yvon Jobin. The quality of the fit was 3035


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determined by the Chi-Square-method by Pearson.32 The quality factor χ2 should be as close to unity as possible. The definition for the factor can be written as follows: i

χ2 =

∑ k=1

(ei − oi)2 ei

with ei = value proposed by the fit and oi = actual value. For the determination of PLQY ϕ, an absolute PL quantum yield measurement system from Hamamatsu Photonics was used. The system consisted of a photonic multichannel analyzer PMA-12, a model C99200−02G calibrated integrating sphere, and a monochromatic light source L9799−02 (150 W Xe- and Hg−Xe-lamps). Data analysis was done with the PLQY measurement software U6039−05, provided by Hamamatsu Photonics. DFT Calculations. DFT calculations were performed using BP8633,34 and B3LYP35,36 functionals and the def2-SV(P)37,38 basis set. For numerical integration, the m4 grid was employed. In the BP86 calculations, the resolution-of-identity approximation39−41 was used. The initial structures were obtained from single-crystal X-ray diffraction data (sample 1, previous work;42 sample 2, this work). All calculations were done using the Turbomole program package (version 6.4)43 X-ray Diffraction. The single-crystal X-ray diffraction study of form b was carried out on a Bruker-Nonius Kappa-CCD diffractometer at 123(2) K using Mo Kα radiation (λ = 0.710 73 Å). X-ray data for form a have been published and discussed elsewhere42 and are included for comparison purposes. Direct methods (SHELXS97)44 were used for structure solution, and refinement was carried out using SHELXL-9744 (full-matrix least-squares on F2). Hydrogen atoms were localized by difference electron density determination and refined using a riding model. A semiempirical absorption correction was applied. In the structure are two voids with disordered solvent molecules, which could not be refined reasonably. Therefore, the program SQUEEZE (module in the program PLATON45) was used. The “squeezed out” solvent molecules were addressed as disordered CH2Cl2 (over a center of symmetry, respectively). With the voids taken into account (ignoring the solvent), the porosity is 10.0%45 and the packing index is 64.0.46 Pale yellow crystals, C54H48Cu2I2N3P3 − CH2Cl2, M = 1297.67, crystal size 0.24 × 0.12 × 0.06 mm3, triclinic, space group P-1 (no. 2): a = 12.408(1) Å, b = 13.275(1) Å, c = 16.157(1) Å, α = 96.34(1)°, β = 102.32(1)°, γ = 94.16(1)°, V = 2571.4(4) Å3, Z = 2, ρ(calc) = 1.676 Mg m−3, F(000) = 1288, μ = 2.266 mm−1, 9021 reflections (2θmax = 50°), 9012 unique [Rint = 0.000, due to the use of SQUEEZE], 580 parameters, R1 (I > 2σ(I)) = 0.054, wR2 (all data) = 0.124, S = 1.03, largest diff. peak and hole 1.428 (near I1)/ −0.725 e Å−3. Crystallographic data (excluding structure factors; see Figure 1) for the structures reported in this work have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-894321 (form b) [CCDC-838848 (form a)].42 Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: int. code +(1223)336−033; e-mail: [email protected]).

Figure 1. Molecular structure of form b of Cu2I2MePyrPHOS3 (displacement parameters are drawn at 50% probability level).

molecules of CuI. The complexes are neutral and the Cu2I2 moiety forms a butterfly-shaped metal−halide core without any metal−metal or iodide−iodide interactions. The coordination of the equal MePyrPHOS ligands to the two copper ions occurs in two different ways. One of the three ligands coordinates in a chelating fashion through the nitrogen and the phosphorus atom to both copper ions. The remaining two ligands act as monodentate ligands and coordinate only with their phosphor atoms to one copper ion, respectively. For both monodentate MePyrPHOS ligands, nitrogen atoms are free of coordination. Even though both forms a and b feature these structural motifs, a major difference exists: The monodentate MePyrPHOS coordinating CuP is twisted, as demonstrated in Scheme 1. Form a can be transformed into form b by turning said ligand by 120° so that the methyl-group lies above the Cu2I2-butterfly. In a first approximation, the molecular differences between the two conformers are negligible, as the bond lengths and angles for both forms determined from the single crystals are very comparable, as shown in Table 2. Geometry optimizations of both conformers using the BP86 density functional confirm the Table 2. Structural Comparison of Polymorphs Based on Single-Crystal X-ray Diffraction (XRD) and DFT Calculationsa form a XRD

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Cu−Cu CuN−P CuP−Pbridge CuN−I

RESULTS AND DISCUSSION The molecular structure and basic photophysical properties of one of the two conformers of Cu2I2MePyrPHOS3 (form a) has been determined before.42 During the present study, we identified an alternative conformer (form b, see Figure 1) and investigated the effect of the processing solvents on decay lifetime, PL, and PLQY. Comparison of the Two Polymorphs. Form a was crystallized from diethyl ether, while crystals of form b were obtained from pentane (Figure 1).The structural analysis of the two polymorphs revealed a uniform complex structure composed of three molecules of MePyrPHOS and two

I−CuN−I Cu−I−Cu

form b DFT

Distances (Å) 2.757(1) 2.65 2.253(1) 2.30 2.246(1) 2.29 2.682(1) 2.70 2.688(1) 2.74 Angles (deg) 106.74(1) 113.3 62.34(1) 58.4 61.85(1) 58.3

XRD 2.753(1) 2.243(2) 2.247(2) 2.642(1) 2.709(1) 106.39(3) 62.26(3) 61.96(3)

DFT 2.68 2.29 2.30 2.71 2.76 111.3 59.4 58.7

a

Selected distances and angles are given. CuN: copper coordinated by iodide, phosphine and nitrogen. CuP: copper coordinated by iodide, monodentate phosphine, and bridging phosphine. Values for each iodide atom are given. 3036


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Figure 2. Frontier orbitals of the conformer forms a and b are calculated using the B3LYP functional. In both forms, the highest occupied molecular orbital (HOMO, left) is located mainly on the Cu2I2 core, while the lowest unoccupied molecular orbital (LUMO, right) is located on the pyridine moiety of the bidentate MePyrPHOS ligand.

fact that the influence of the rotated ligand on the Cu2I2 core is very small. However, there are some differences between both observed forms. As a result from the slightly different shapes of the two conformers, both porosity45 and packing indices I46 are different. Form a, which has been obtained from diethyl ether, shows a porosity of 5.8% and the packing index I is 64.8%. For form b, the packing is less close (I = 64.0%) and the porosity is much higher (10.0%) when omitting the solvent contribution. It is already known that Cu(I) complexes readily incorporate solvents in their crystals47−49 and are very sensitive toward stoichiometry.6,47,50−52 Probably as a result of the higher porosity, we found two molecules of dichloromethane incorporated in the unit cell of form b, while form a incorporated no solvent. Validity of the Crystalline Forms as Models for Amorphous Samples. We believe that both forms a and b can be regarded as structural model systems for the powder samples obtained from diethyl ether (sample 1) and pentane (sample 2), providing a possible explanation for the different PLQY-values found in the amorphous state. By using different solvents, we proved that the formation of different conformers is possible. Forms a and b and the powder samples 1−5 incorporated different amounts of dichloromethane. Because of this, powder samples 1/2 and crystalline forms a/b are not

exactly identical by means of their chemical composition and will not be treated as equal substances here. Nevertheless, we were able to derive useful conclusions from the structural data of form a and b. Since the basic geometry of the Cu 2 I 2 MePyrPHOS 3 molecules in solid state is nearly identical for both forms, the PL spectra should be very similar. However, PLQY is sensitive toward the rigidity of the solid matrix and should therefore depend on packing density. Theoretically, even more conformers and mixtures of two or more conformers are possible in the amorphous state. With two nonbridging ligands and three different conformer positions, up to nine conformers are theoretically possible. So far, two of them were structurally characterized, but it is likely that, in an amorphous powder sample (like samples 1−5), mixtures of two or more conformers are present. The comparison of the PLQY of the forms a and b is admittedly problematic: Both crystalline samples contained a different amount of solvent (form a: no solvent, form b: 2 molecules per elemental cell), so the rigidity is not comparable. Because of this, we used the amorphous samples 1−5 for the further discussions of PLQY ϕ and lifetimes τ. All amorphous powder samples showed the same chemical properties and molecular structure and contained the same amount of 3037


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calculations using the B3LYP functional.35,36 According to these calculations, form a has a dipole moment of 7.3 D, while form b has a considerably lower dipole moment of 4.3 D. Apparently, a correlation between solvent polarity and the preferred conformer exists: From diethyl ether, the polar form a was crystallized, while the unpolar solvent pentane favors the formation of form b. It is important to point out that the solubility of both forms a and b in ether and pentane is quite comparable. To confirm this, we prepared saturated solutions of both forms in either solvent and compared the amount of dissolved material by UV−vis-spectroscopy. (Spectra are given in the SI.) This can be interpreted as a similar lattice energy for forms a and b. From these considerations, our X-ray crystallography results, and our DFT calculations, we derived a simple mechanism to rationalize the phenomenon (Scheme 2). Since the solubility of

dichloromethane (1/3 per Cu2I2MePyrPHOS3) as proven by elemental analysis and FT-IR spectroscopy. It shall be emphasized that it is highly unlikely to obtain crystalline material by the processing protocol we used to prepare samples 1−5. This has been proven in other works before.15,53 DFT Calculations of the S(0) and T(1) Geometry. To get a better understanding of the photophysical properties of the copper compound, we performed single-point DFT calculations of both conformers using the coordinates obtained from X-ray analyses. The most striking structural difference between the two forms is a rotation of one of the monodentante ligands around a Cu−P-bond. Minor changes between a and b, like the exact relative positions of the phenyl rings, are more subtle, but add up and should not be neglected. This makes an estimation of the barrier height quite elaborate. In addition, effects of the environment would have to be taken into account to be able to compare the theoretical value to our experimental data. A reliable computation of the barrier height is therefore beyond the scope of this work. The frontier orbitals of forms a and b show very similar features (see Figure 2): the highest occupied molecular orbital (HOMO) is mainly located on the copperiodide cluster, while the lowest unoccupied molecular orbital (LUMO) is located on the bidentate MePyrPHOS ligand with most of the electron density on the pyridine moiety. The HOMO−LUMO excitation thus has pronounced charge transfer character from the copper-iodide core to the MePyrPHOS ligand. TDDFT calculations with the BP86 functional show that the contribution of the HOMO−LUMO excitation to the lowest excited singlet state are >99% at the optimized ground-state structure. These results indicate that the green emission of both conformers results from a metal/halide-to-ligand charge transfer (MXLCT) state. Since the frontier orbitals are only weakly influenced by the structural changes between forms a and b, the emission wavelength remains similar.

Scheme 2. Proposed Mechanism for the Formation of the Two Different Conformers a and ba

a

From the polar diethyl ether, we isolated the more polar form a, while crystallization with the nonpolar pentane gave form b. This suggests equilibrium between at least these two conformer forms in solution, which can be manipulated by the polarity of the processing solvent.

both forms is comparable, we assumed that the precipitation rates kprecip,a and kprecip,b of solid forms a and b, which includes nucleation and growth of particles, are also comparable. If this is valid, equilibria between all possible conformer forms of Cu 2I2(MePyrPHOS)3 in solution are the main factors governing the formation of one of the two forms. The existence of dynamic equilibria in solution has been shown for many copper(I)-phosphine complexes before.54−58 A similar situation seems to apply for Cu2I2(MePyrPHOS)3 as well; despite the inequality of the three MePyrPHOS ligands in the complex, only onevery broadsignal has been found in the 31 P NMR spectrum in CDCl3 (the spectrum is given in the SI). The polarity of the processing solvent should influence these equilibria: polar solvents favor the formation of polar molecules by lowering the standard enthalpy of formation. In amorphous samples, the case is expected to be much more complicated: Precipitation of the amorphous solids is usually quite fast, which should give a mixture of different conformers, depending on the solvents and the processing time, whose influence we discuss later. This is consistent with our results for powder samples 1−4. For sample 5, we used a modified processing procedure. Rather than by precipitation, the sample was obtained by solvent evaporation. This resulted in a sample showing a significant red-shift of the emission maximum and a drastic decrease of PLQY. Photoluminescence Spectroscopy, PLQY Measurements, and Emission Decay. First, we compared the

Figure 3. Calculated structures (BP86) of the central fragment of a in the ground state (left) and the lowest triplet state (right).

Using the BP86 functional,33,34 structure optimizations in the ground state and the lowest triplet state were performed. Upon relaxation in the triplet state, the Cu−Cu distance shortens significantly from 2.65 Å to 2.55 Å (form a) and from 2.68 Å to 2.55 Å (form b), respectively. The most striking structural change is the reduction of the I−Cu−Cu−I dihedral angle by more than 15° (form a, 145° to 129°; form b, 143° to 126°). The structural changes in both conformers are very similar. This agrees well with the assumption that the orientation of the monodentate ligand scarcely influences neither the ground state nor the MXLCT-state, leading to similar photophysical properties. Mechanistic Considerations for the Formation of the Two Conformers. The dipole moment for the two structures analyzed by X-ray diffraction has been determined with DFT3038


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evaporated amorphous solids.15 In our case, a similar effect is expected to be responsible for the large differences between the precipitated samples 1−4 and the evaporated sample 5. According to the DFT calculations, the bright emission of samples 1−5 is a result of a metal/halide-to-ligand charge transfer transition (MXLCT). The two nonbridging MePyrPHOS-ligands are not participating in this transition. We found very broad emission and excitation bands (Figure 6). For all

photoluminescence properties of the crystalline forms a and b and amorphous samples 1 and 2 (Figure 4). The spectra of

Figure 4. PL spectra of Cu2I2(MePyrPHOS)3 in amorphous and crystalline state. Samples were processed from diethyl ether and pentane. Excitation wavelength λex = 350 nm. Emission maxima were determined by using a Gaussian fit curve between 500 and 540 nm.

form a/sample 1 are almost superimposable, indicating only minor differences of the crystalline and the amorphous sample. Spectra are, however, slightly shifted for form b/sample 2. The broad, ill-structured emission bands are indicative of chargetransfer emission, which was confirmed by our DFT calculations. This suggests that the two crystalline polymorphs may be sufficient enough for qualitative interpretations, but the overall situation seems to be more complex in an amorphous state. As stated above, the crystalline and amorphous samples have to be considered different substances. Photoluminescence spectra of all samples 1−5 are given in Figure 5. The data and the values for PLQY are summarized in

Figure 6. Excitation spectra of Cu2I2(MePyrPHOS)3 processed from various solvents. For each sample, the individual emission maxima were monitored.

spectra, excitation was monitored at the individual emission maxima. We noted a shift toward shorter wavelengths between samples 1−4 and sample 5. All curves were complex, showing one peak and three shoulders between 300 and 450 nm. This shows that the involved states lie closely together. The Stokes shift for all five samples is greater than 150 nm, which is typical for charge-transfer emission and an indicator for a rather high structural distortion between ground and excited states. Excitation spectra of the crystalline forms a and b are similar to those of the amorphous samples and are given in the SI. Emission lifetime data are given in Table 3. Full decay curves are given in the SI. The data have been fitted using a biexponential function: ⎛ t ⎞ ⎛ t⎞ I(t ) = a1 exp⎜ − ⎟ + a 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

The quality of the fitting as represented by the quality factor χ2 was good. For each sample, a shorter lifetime τ1, ranging from 0.4 to 0.6 μs, and a longer lifetime τ2, ranging from 2.4 to 3.3 μs, could be found. There are several possible explanations for this effect; the situation could be similar to measurements for [Cu(phen)(PPh3)2]BF4 published by the Coppens group,30 where different structural environments of disordered molecules in the same crystalline sample led to different lifetimes. Su et al. explained a similar effect for amorphous [Cu(phen)(pop)]BF4 as solid-state emission from two luminescent states with different lifetimes.15 Assuming that a similar situation applies for the amorphous samples 1−5, one can analyze the contribution of the two lifetimes to the overall process: If no other states are contributing to the emission, the population ratios of these states equal a1 and a2.59,60 Population ratios deviate around 0.50. However, the contribution to luminescence is proportional to aiτi; therefore, the emission yield for an individual decay is formulated as aiτi Ri = ∑i aiτi

Figure 5. Photoluminescence spectra of Cu2I2(MePyrPHOS)3. All samples were powders. Excitation wavelength λex = 350 nm. Emission maxima were determined by using a Gaussian fit curve between 500 and 540 nm.

Table 1. Obviously, samples 1−4 show a clear trend in the PLQY: The higher the dipole moment of the processing solvent, the higher the ϕ. Due to the alternative processing protocol, sample 5 does not match this series. There is a red-shift of λmax and the decrease of ϕ is considerably high, though the dipole moment of dichloromethane lies between methanol and diethyl ether. The solvent effect must be overlain by another effect resulting from the different processing method. A plausible explanation is given in the literature: Zhang, Li, and Su found a similar effect for a copper complex and explained it with a different degree of order in the amorphous solids between precipitated and 3039


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Table 3. Emission Lifetime Dataa fitting results (biexponential fit) sample sample sample sample sample a

1 2 3 4 5

τ1 [μs]

a1

± ± ± ± ±

0.58 0.44 0.57 0.52 0.45

0.39 0.49 0.42 0.41 0.58

0.06 0.12 0.05 0.05 0.10

average lifetimes

τ2 [μs]

a2

χ2

⟨τ⟩1/e [μs]

⟨τ⟩a [μs]

⟨τ⟩f [μs]

± ± ± ± ±

0.42 0.56 0.43 0.48 0.55

1.00 0.99 1.01 0.99 1.00

1.1 1.7 1.2 1.2 1.8

1.4 2.0 1.4 1.4 2.1

2.3 2.7 2.5 2.0 3.0

2.68 3.11 2.82 2.41 3.33

0.25 0.35 0.17 0.15 0.22

Spectra were measured at 550 nm with an excitation wavelength of 370 nm. ⟨τ⟩av = ±0.40 μs.

With similar pre-exponential values ai, emission from longlived states dominates the emission behavior of Cu2I2(MePyrPHOS)3. If there are no quenching factors (impurities, trace amounts of solvent) present,59 the PLQY ϕ can be generally formulated as a function of the decay lifetime τ as

Table 4. Comparison of Averaged Radiative and Nonradiative Decay Rates kr and knr for Samples 1−5, Sorted by Decreasing PLQY and Correlation with the Precipitation Timea PLQY ϕ kr [106 s−1]

ϕ = k rτ

sample sample sample sample sample

with kr the radiative decay rate. Additionally, with the relation: τ −1 = k r + k nr

the nonradiative decay rate knr can be calculated. This set of equations is valid for a monoexponential emission decay, e.g., for fluorescence of organic molecules.61 According to the rule of Kasha and Vavilov,62 ϕ is independent of the observed wavelength in those cases and the same applies for kr and τ. Unfortunately, τ is not properly defined for biexponential decay profiles. There are several methods of extracting averaged, wavelength-dependent lifetimes ⟨τ⟩, which are useful for a qualitative estimation of averaged, wavelength-dependent radiative and nonradiative decay rates.61 First, ⟨τ⟩1/e is defined as the time after which the intensity I(t) has dropped to 1/e of the initial intensity I(t = 0). Second, the amplitude average lifetime is defined by

⟨τ ⟩a =

0.89 0.79 0.70 0.66 0.52

0.64 0.56 0.35 0.47 0.25

kr/ knr

precipitation time

0.07 0.14 0.15 0.24 0.22

9.0 4.0 2.3 2.0 1.1

3h 15 min seconds seconds b

PLQY = ±0.02. Samples were obtained by evaporation of the solvent rather than precipitation.

a

b

increased ϕ-values is an effective suppression of nonradiative decay processes, accompanied by a generally increasing radiative rate kr. Again, sample 5 does not fit properly in the series, suggesting that structural distortions are considerably higher. This effect should result not only from the solvents alone, but also from the alternative processing methods used for preparation of sample 5. Given that kr and knr have been calculated for samples showing biexponential emission decay, the absolute values are only approximate and should be interpreted with care. Interestingly, there is a correlation between the precipitation time and the PLQY-values: While precipitation occurs almost immediately from apolar solvents like pentane or hexane (sample 2 and 3), even after complete addition of the reaction mixture into the processing solvent, no precipitate is formed when polar solvents are used. For sample 1, the precipitation of the product is finished after approximately 15 min, while sample 4 takes up to 3 h. Table 4 reveals that there is a clear trend: the longer the precipitation time, the higher the quantum efficiency and the higher the ratio kr/knr. Again, sample 5 cannot be correlated with precipitation time because of the evaporation technique used for sample preparation. For the other samples, apparently, by slower precipitation, a higher degree of packing can be achieved, thus enhancing the quantum efficiency. A correlation between parameters determining the rigidity of a solid, like particle size, porosity, and packing density, and the method of sample preparation is trivial, but the drastic effect on the reproducibility of PLQY-values is mostly neglected in the literature concerning the synthesis and characterization of new emissive compounds. Relevance of Our Findings for Other Luminescent Metal−Organic Compounds. To validate whether our findings are relevant for other emissive metal−organic compounds, we performed a series of brief experiments with fac-tris-(2-phenylpyridine)iridium, Irppy3, and fac-tris-(8hydroxyquinolato)aluminum, Alq3, both yellow- to greenemitting metal complexes like Cu2I2MePyrPHOS3 (Figure 7

∑ aiτi i

and last, the intensity average lifetime can be written as

⟨τ ⟩f =

4 1 2 3 5

knr [106 s−1]

∑i aiτi 2 ∑i aiτi

For a monoexponential decay, all these lifetimes are equivalent with τ, while there are massive aberrations between the different ⟨τ⟩-values for biexponential decays. In this work, we used the amplitude average lifetime to calculate decay rates, as proposed by O’Connor and co-workers.61 Values for all three average lifetimes are given in Table 3. Regardless of the method, lifetimes ranging between 1 and 3 μs were obtained. In Table 4, the averaged values for kr and knr are given, which were calculated from ϕ and ⟨τ⟩a according to the formulas stated above. In general, for high values of ϕ, kr tends to be high, while knr is lower, respectively. A quantitative comparison between the values for the averaged kr and knr is not trivial because of the wavelength dependency mentioned above, but the ratio between kr and knr is a valuable qualitative indicator for the existence of efficient nonradiative decay processes. As stated in Table 4, the ratio between kr and knr decreases with ϕ from 9.0 to 1.1. For instance, the product obtained from methanol has a dominant radiative decay rate, while nonradiative and radiative decay is in the same order of magnitude for that of the sample processed from dichloromethane. The reason for the 3040


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Figure 7. Molecular structures of the PyrPHOS-complex, Irppy3, and Alq3.

Table 5. Influence of the Processing Conditions on Powder Samples of Irppy3 and Alq3a Irppy3 Irppy3 Irppy3 Irppy3 Irppy3 Alq3 Alq3 Alq3 Alq3

PLQY ϕ

λmax [nm]

CIE x

CIE y

0.17 0.05 0.09 0.09 0.01 0.36 0.36 0.29 0.31

540 535 538 535 537 514 500 517 515

0.356 0.318 0.371 0.312 0.365 0.286 0.253 0.309 0.308

0.611 0.620 0.572 0.628 0.583 0.511 0.465 0.528 0.535

sample preparation sublimated see sample see sample see sample see sample sublimated see sample see sample see sample

1 3 4 5 1 3 5

precipitation time seconds 15 min 15 min b

− 30 min 15 min b

Excitation wavelength: 350 nm. All samples are neat powders. PLQY = ±0.02; λmax = ±2 nm. Because of the small differences between samples 2 and 3 for the copper complex, only hexane was used. As a result of the good solubility of Alq3 in methanol, we were not able to prepare a powder sample from this solvent. bSamples were obtained by evaporation of the solvent rather than precipitation. a

sample with a PLQY comparable to the sublimated sample, but a considerable blue shift to 500 nm. A clear trend that explains the correlation between solvent properties, ϕ and λmax, for all three substances could not be found. This is not unexpected, considering the chemical differences between those substances. However, a relation between the time between the addition of the dissolved complex to the processing solvent and the formation of amorphous powder seems to exist for Alq3 and Irppy3, too. In general, a slower precipitation led to samples with a higher quantum yield. This is in good agreement with observations that were made for the PyrPHOS-complex (see Table 1). The fact that solvent effects seem to be much stronger for Irppy3 and Cu2I2MePyrPHOS3 than for Alq3 can be explained with the longer emission decay time: the longer the lifetime of the excited states, the greater the chance for nonradiative relaxation, which depends strongly on the environment.

and Table 5). These two substances were chosen because of their high practical relevance in the field of organic lightemitting diodes.63−66Additionally, extensive work has been done to probe their photophysical behavior.13,22,67−69 While the phosphorescent Irppy3 has a decay lifetime of 2.1 μs,68 quite similar to Cu2I2MePyrPHOS3, Alq3 is a fluorescence emitter with an emission decay lifetime of 2 ns.69 Both substances feature a much more rigid structure than the PyrPHOScomplex, thus ruling out effects from solvent-dependent equilibria between different conformers. The isomerization from the fac- to the mer-isomers is not expected at ambient temperature. Because of triplet-annihilation of excited states, the PLQY of Irppy3 in solid state is reduced considerably compared to diluted thin films in matrix materials like PMMA, where quantum yields close to 1.00 have been reported.67 Because pentane and hexane gave samples with almost identical properties, we omitted the pentane samples in this series. Additionally, we measured ϕ for sublimated samples of Alq3 and Irppy3, which were obtained from commercial sources. The results are summarized in Table 5. We found an influence of the processing solvents on the PLQY-values for both Irppy3 and Alq3. For Irppy3, ϕ changed within one order of magnitude, from 0.01 to 0.17, while λmax showed only small deviations, similar to the PyrPHOScomplex. Compared to the sublimated sample, all solutionprocessed samples showed reduced quantum efficiency. Evaporation of the solvent (see sample 5 of the PyrPHOS complex) led to a sample featuring a much smaller ϕ-value of only 0.01. No red-shift was found. For Alq3, values for ϕ were between 0.29 and 0.36, which is, considering the sensitivity of this measurement, a very small but nevertheless clearly detectable fluctuation. Surprisingly, processing from diethyl ether (compare sample 1 of the PyrPHOS-complexes) led to a

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CONCLUSION How Processing Solvents Influence the PLQY of Cu2I2(MePyrPHOS)3. The structural and photophysical analysis can be used to explain the enhancement of the quantum efficiency of Cu2I2MePyrPHOS3 by certain processing solvents. We showed that the PyrPHOS-complex has a rather high excited-state lifetime by TDSPC, while DFT calculations of the lowest triple state suggested a structural distortion of this state compared to that of the ground state. We also analyzed the emission lifetime, which is in the order of microseconds. This isin terms of molecular dynamicsa rather long time and even is in the order of magnitude for protein-unfolding processes.70 Our comparison of the PyrPHOS complexes with a long-lived Ir-complex and a very short-lived Al-complex suggests that the processing solvent especially affects the long-lived compounds. Especially long-lived excited states tend 3041


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containing luminescent three-coordinate copper(I) complexes. J. Am. Chem. Soc. 2011, 133, 10348−10351. (3) Ogura, T.; Fernando, Q. Mass Spectrometry and Structures of Copper(1) Carboxylates in the Vapor Phase. Inorg. Chem. 1973, 12, 2611−2615. (4) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. Systematic investigation of the metalstructure-photophysics relationship of emissive d10-complexes of group 11 elements: the prospect of application in organic light emitting devices. J. Am. Chem. Soc. 2011, 133, 12085−12099. (5) Armaroli, N.; Accorsi, G.; Cardinali, F.; Lostorti, A. Photochemistry and Photophysics of Coordination Compounds I. Top. Curr. Chem. 2007, 280, 69−115. (6) Liu, Z.; Qayyum, M. F.; Wu, C.; Whited, M. T.; Djurovich, P. I.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Thompson, M. E. A codeposition route to CuI-pyridine coordination complexes for organic light-emitting diodes. J. Am. Chem. Soc. 2011, 133, 3700−3703. (7) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. E-type delayed fluorescence of a phosphine-supported Cu2(μ-NAr2)2 diamond core: harvesting singlet and triplet excitons in OLEDs. J. Am. Chem. Soc. 2010, 132, 9499− 9508. (8) Costa, R. D.; Tordera, D.; Ortí, E.; Bolink, H. J.; Schönle, J.; Graber, S.; Housecroft, C. E.; Constable, E. C.; Zampese, J. A. Copper(I) complexes for sustainable light-emitting electrochemical cells. J. Mater. Chem. 2011, 16108−16118. (9) Liu, X.; Sun, W.; Zou, L.; Xie, Z.; Li, X.; Lu, C.; Wang, L.; Cheng, Y. Neutral cuprous complexes as ratiometric oxygen gas sensors. Dalton Trans. 2012, 41, 1312−1319. (10) Beltramini, M.; Dimuro, P.; Rocco, G. P.; Salvato, B. Luminescence Properties of the Dinuclear Copper Complex in the Active Site of Hemocyanins. Arch. Biochem. Biophys. 1994, 313, 318− 327. (11) McMillin, D. R.; Hudson, B. P.; Liu, F.; Sou, J.; Berger, D. J.; Meadows, K. A. Luminescence Probes of DNA-Binding Interactions Involving Copper Complexes. In Photosensitive MetalOrganic Systems, Kutal, C.; Serpone, N., Eds.; American Chemical Society: Washington, DC, 1993; pp 211−231. (12) Czerwieniec, R.; Yu, J.; Yersin, H. Blue-light emission of Cu(I) complexes and singlet harvesting. Inorg. Chem. 2011, 50, 8293−8301. (13) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs. Coord. Chem. Rev. 2011, 255, 2622−2652. (14) Dias, H. V. R.; Diyabalanage, H. V. K.; Rawashdeh-Omary, M. A.; Franzman, M. A.; Omary, M. A. Bright phosphorescence of a trinuclear copper(I) complex: luminescence thermochromism, solvatochromism, and concentration luminochromism. J. Am. Chem. Soc. 2003, 125, 12072−12073. (15) Zhang, L.; Li, B.; Su, Z. Phosphorescence enhancement triggered by pi stacking in solid-state [Cu(N-N)(P-P)]BF4 complexes. Langmuir 2009, 25, 2068−2074. (16) Kuang, S.-M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.; Walton, R. a Synthesis and structural characterization of Cu(I) and Ni(II) complexes that contain the bis[2-(diphenylphosphino)phenyl]ether ligand. Novel emission properties for the Cu(I) species. Inorg. Chem. 2002, 41, 3313−3322. (17) Suzuki, T.; Yamaguchi, H.; Hashimoto, A.; Nozaki, K.; Doi, M.; Inazumi, N.; Ikeda, N.; Kawata, S.; Kojima, M.; Takagi, H. D. Orange and yellow crystals of copper(I) complexes bearing 8(diphenylphosphino)quinoline: a pair of distortion isomers of an intrinsic tetrahedral complex. Inorg. Chem. 2011, 50, 3981−3987. (18) Koshevoy, I. O.; Chang, Y.-C.; Karttunen, A. J.; Haukka, M.; Pakkanen, T.; Chou, P.-T. Modulation of metallophilic bonds: solventinduced isomerization and luminescence vapochromism of a polymorphic Au-Cu cluster. J. Am. Chem. Soc. 2012, 134, 6564−6567. (19) Chen, K.; Strasser, C. E.; Schmitt, J. C.; Shearer, J.; Catalano, V. J. Modulation of luminescence by subtle anion-cation and anion-π

to be influenced by the effects of a rigid matrix. All substances analyzed in this study were neat materials, so the molecules of the materials are only surrounded by the material itself in the solid state. The rigidity of the material and therefore the matrix is ruled by morphology and packing. For these effects, the processing time and the conditions (precipitation, evaporation of the solvent, sublimation) is a vital factor. Apparently, by slower formation of the solid, a higher degree of packing can be achieved, thus enhancing the quantum efficiency. The conclusion that the matrix should be as rigid as possible if high PLQY values are desired, should, however, not be drawn in general until experimental data for more fluorescent and/or phosphorescent molecules are available. Two final conclusions can be drawn: (1) When comparing photophysical data of different substances, it is important to analyze the substance history in terms of processing solvents, especially when materials from different sources are analyzed. (2) From a synthetic chemist’s point of view, high reaction yields are desirable, which often rules the choice of processing solvents. This solvent does not necessarily lead to the material with a maximized quantum yield, which is preferred for most applications of luminescent materials like sensors, LEECs, and OLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of compounds and their chemical properties. Sample preparation. 31P NMR of MePyrPHOS and Cu2I2(MePyrPHOS)3 in solution. IR spectra for samples 1− 5. Pictures of samples 1−5 under UV irradiation (366 nm). Comparison of the emission spectra between crystalline and amorphous samples. Luminescence decay spectra for samples 1−5. Emission spectra for all Irppy3 and Alq3 samples. Crystallographic data in CIF (CCDC-894321 (form b)). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Dr. Thomas Baumann: Tel +49 721 608 29006; E-mail [email protected]. Prof. Dr. Stefan Bräse: Fax +49 721 608 48581; Tel +49 721 608 42903; E-mail [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Verband der Chemischen Industrie (VCI) and the Deutsche Forschungsgemeinschaft (DFG) via support for the transregional collaborative research centre SFB/ TRR 88 “3MET” are acknowledged.

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REFERENCES

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How the Quantum Efficiency of a Highly Emissive Binuclear Copper

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