Effect of Conjugation Pathway in Metal-free Room-temperature Dual

Letter pubs.acs.org/JPCL

Effect of Conjugation Pathway in Metal-Free Room-Temperature Dual Singlet−Triplet Emitters for Organic Light-Emitting Diodes Wolfram Ratzke,†,# Lisa Schmitt,‡,# Hideto Matsuoka,§,# Christoph Bannwarth,∥,# Marius Retegan,⊥ Sebastian Bange,† Philippe Klemm,† Frank Neese,*,⊥ Stefan Grimme,*,∥ Olav Schiemann,*,§ John M. Lupton,*,† and Sigurd Höger*,‡ †

Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstr. 31, 93040 Regensburg, Germany Kekulé-Institut für Organische Chemie und Biochemie, University of Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany § Institute für Physikalische und Theoretische Chemie, University of Bonn, Wegelerstr. 12, 53115 Bonn, Germany ∥ Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstr. 4, 53115 Bonn, Germany ⊥ Max-Planck-Institut für Chemische Energiekonversion, Stiftstr. 34−36, 45470 Mülheim an der Ruhr, Germany ‡

S Supporting Information *

ABSTRACT: Metal-free dual singlet−triplet organic light-emitting diode (OLED) emitters can provide direct insight into spin statistics, spin correlations and spin relaxation phenomena, through a comparison of fluorescence to phosphorescence intensity. Remarkably, such materials can also function at room temperature, exhibiting phosphorescence lifetimes of several milliseconds. Using electroluminescence, quantum chemistry, and electron paramagnetic resonance spectroscopy, we investigate the effect of the conjugation pathway on radiative and nonradiative relaxation of the triplet state in phenazine-based compounds and demonstrate that the contribution of the phenazine nπ* excited state is crucial to enabling phosphorescence.

N

complexes, raising the phosphorescence rate by 2−4 orders of magnitude. It has been widely exploited in the context of the technologically most crucial spin-dependent recombination process, in OLED electrophosphorescence.20,25 More recently, decreasing ΔE has received significant attention in the form of thermally activated delayed fluorescence (TADF) emitters.26−30 In contrast, the role of both the spatial distribution of the molecular orbitals and the nonradiative decay has so far remained rather obscure: there are only very few examples of metal-free phosphorescent OLED materials in the literature.4,23 This state of affairs is surprising given the substantial body of literature on the nature of phosphorescent transitions in a range of hydrocarbon materials.3−17 Nonbonding (n-) orbitals from lone electron pairs, for example, in nitrogen-containing heterocycles such as pyridines15 or pyrimidines,31 can undergo substantial coupling to π-electron orbitals as described by ElSayed’s rule.32 The spatial part of the SOC operator can be understood as a rotation acting on the molecular orbitals. In the case of spatially orthogonal n- and π-orbitals, this rotation leads to strong one-center contributions to the SOC elements. Such low-Z phosphorescent emitters are technologically of interest for two reasons: given sufficiently slow nonradiative relaxation,

ongeminate recombination of radical pairs as occurs in many electron-transfer processes is governed by transition rates that depend sensitively on the magnetic field.1 Reaction products of positive and negative charges form in either the singlet or triplet excited-state manifold of a molecule. A versatile tool to study such reactions in the solid state is given by the organic light-emitting diode (OLED),2 in which nongeminate pairs are formed following electrical injection. Spin statistics imply a maximum OLED efficiency of 25% as a result of the spin singlet-state pair recombination. It has long been known that triplet excited states, the primary (75%) reaction product in an OLED, can couple radiatively to the molecular ground state by phosphorescence, although in many cases such emission is extremely weak, particularly in pure hydrocarbons.3−19 There are two principal routes to controlling the yield of phosphorescence in molecular emitters: raising radiative rates20,21 or suppressing nonradiative decay.22,23 Fundamentally, phosphorescence is caused through perturbation of the pure spin states by spin−orbit coupling (SOC). For atoms, this perturbation induces singlet−triplet mixing at a rate proportional to Z2, r−6, and ΔE−2 with Z the atomic number, r the distance between the electron and atomic center, and ΔE the energy difference between coupled states.24 Although quantitatively different for more general systems (i.e., molecules), the strong dependence on Z and on distance is a general feature of SOC and thus of phosphorescence. Increasing Z has been the usual approach in organometallic © 2016 American Chemical Society

Received: August 23, 2016 Accepted: October 27, 2016 Published: October 27, 2016 4802

DOI: 10.1021/acs.jpclett.6b01907 J. Phys. Chem. Lett. 2016, 7, 4802−4808

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The Journal of Physical Chemistry Letters

Figure 1. Electroluminescence (EL) spectra and spectrally integrated transient EL after device switch-off for six different phenazine-based thiophene isomers, embedded in an OLED matrix (PVK blend or CBP) at 3% by weight concentration, measured at room temperature. For some compounds, residual host matrix emission (thin black line) is observed in EL. For compounds 1−5, phosphorescence is observed for only the “β”-isomer (1b−5b, sulfur in the upper position, marked red) in the form of a second emission peak (labeled T) and an afterglow with a single-exponential decay of lifetime τ, and not in the “α”-isomers (1a−5a, sulfur in the lower position, marked blue). For compound 6, no lateral conjugation exists and the position of the sulfur atom in the isomer does not affect phosphorescence significantly. This material shows the clearest dual emission with the longest phosphorescence lifetime. The dominant singlet (S) and triplet (T) peaks are 1a, 606 nm; 1b, 544 and 697 nm; 2a, 759 nm; 2b, 674 and 745 nm; 3a, 582 nm; 3b, 525 and 697 nm; 4a, 614 nm; 4b, 629 and 765 nm; 5a, 651 nm; 5b, 605 and 787 nm; 6a, 487 and 630 nm; 6b, 478 and 650 nm.

without the need for any heavy metal high-Z substituents.23 We now investigate the influence of π-conjugation pathway within the emitter structure, which controls both the radiative and nonradiative triplet decay channel. The electroluminescence (EL) results are compared to quantum chemical calculations and to measurements of spin density using electron paramagnetic resonance (EPR) spectroscopy. The compounds under investigation are shown in Figure 1. Two series of dithieno[2,3-a:3′,2′-c]phenazine constitutional isomers were investigated in which the sulfur atom of the thiophene unit is either in the upper position (red curves), thus prohibiting a direct conjugation between the two central thiophene units due to their β,β′-connection, or in the lower position (blue curves) so that the central thiophene units are conjugated through their α,α′-connection, as discussed in more

they can potentially enable high internal quantum efficiencies without having to resort to expensive rare earth metal compounds because both singlets and triplets contribute to emission, albeit at the expense of radiative rate and thus maximum OLED brightness. More importantly, dual singlet− triplet emitters report directly on spin statistics, thereby offering a window to spin correlations and spin relaxation channels, which in turn can be highly sensitive to magnetic fields.33−39 In addition, dual emitters provide direct spectroscopic access to the interconversion processes of singlet fission40,41 and triplet− triplet annihilation.18 Recently, by using the promising nπ* transition in Nheterocyclic aromatics,42,43 we described thienyl-substituted phenazines as the basic building block of dual emitters and succeeded, for the first time, in observing room-temperature electrophosphorescence and electrofluorescence in parallel 4803

DOI: 10.1021/acs.jpclett.6b01907 J. Phys. Chem. Lett. 2016, 7, 4802−4808

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afterglow in 1a is dominated by delayed fluorescence due to thermal detrapping of triplet exciplexes of the PVK-matrix (see Figure S17),47 preventing any detection of weak phosphorescence, in contrast to 1b, where radiative triplet emission is clearly visible through the single-exponential luminescence decay. Evidently, displacing the conjugation path in 1a from the phenazine moiety to the thiophene unit dramatically reduces the phosphorescence yield compared to that of 1b. The simplest way to tune emission color is by changing the electronic structure of the aromatic groups of the donor− acceptor system by substitution of the protons with inductive or mesomeric units. Attaching cyano groups to the phenazine in compound 2b shifts the singlet by 130 nm to the red with respect to 1b but has a smaller effect on the triplet (leading to a red shift of 48 nm). In 2a, the singlet shifts to the red by 85 nm with respect to 2b and the triplet remains undetectable. The conjugation of the system can be extended further both on the head of the phenazine unit and on the thiophene tail. Adding phenylenes to the phenazine in structure 3b does not affect conjugation because their steric demand enforces orthogonal conformation with respect to the condensed hydrocarbon; thus, the spectral position of the singlet remains conserved when compared to 1b. Again, for the isomer 3a, no triplet emission is detectable. Condensation of the phenylenes in compound 4b increases conjugation, thus shifting both singlet and triplet peaks to the red when compared to 1b. The position of the singlet peak is the same for 4a; however, again no triplet peak is visible in the α-isomer. Further extension of the conjugation of the thiophene unit is realized in 5. This extension of conjugation reduces the phosphorescence lifetime but otherwise has little effect on the EL spectra. A small gap opens between the singlet spectra of the two isomers, and once again the α-isomer 5a shows no discernible triplet afterglow. The clearest phosphorescence intensities, independent of isomer, are found for compounds 6a and 6b, with only minimal substitution of the thienophenazines. Further photophysical and device characteristics can be found in the Supporting Information. The origin of the phosphorescence can be rationalized on the basis of quantum chemical calculations using a densityfunctional theory multireference configuration interaction (DFT/MRCI) method followed by a mean-field approximated SOC configuration interaction calculation (SOC-CI).48,49 The combined method is referred to as SOC-DFT/MRCI. This approach allows computation of radiative decay rates from the

detail in Figure 2.23,44,45 Details of the synthesis and characterization are given in the Supporting Information.

Figure 2. Position of the sulfur atom controls the dominant conjugation path and the localization of spin density in the phenazine-based dual emitters. (a, b) The conjugation path from the phenazine to the two bithiophene units is impeded in 1a, where conjugation extends over the quaterthiophenes, whereas the phenazine is fully conjugated with the bithiophene in 1b. (c, d) Spin density computed for the excited triplet state at the UB3LYP/def2-SVP level. In 1a the spin density predominantly covers the thiophenes, whereas in 1b it is localized on the phenazine moiety. Electrophosphorescence is observed for only 1b.

Figure 1 summarizes steady-state EL spectra and spectrally integrated EL decay transients recorded following device switch-off for the 12 compounds.46 The emission peaks are attributed to either singlets (S) or triplets (T). The emissive species are easy to differentiate because there is virtually no phosphorescence under steady-state optical excitation of the material when diluted in an oxygen-saturated solvent at ambient conditions (see the Supporting Information for steady-state photoluminescence spectra).23 We compare the effect of conjugation on the EL characteristics of the different compounds. The conjugation of the central thiophene units in 1a leads to a red-shift of 62 nm of the singlet emission compared to 1b. At the same time, the triplet emission is no longer detectable in the steady-state EL spectrum. The

Table 1. Triplet State Parameters from Different Experimental and Computational Methods method

ELa (RT)

ELa (4 K)

SOC-DFT/MRCIb

SOC-DFT/MRCIb

parameter

τT (ms)

τT (ms)

τT (s)

|⟨T1|HSOC|S0⟩| (cm )

1a 1b 2a 2b 6a 6b

− 6.0 − 4.3 23.0 38.5

− 10.1 − 7.2 25.3 47.1

0.77 2.8 1.4 4.7 0.56 5.3

2

6.1 8.4 7.9 3.6 1.0 3.0

× × × × × ×

10−4 10−8 10−4 10−5 10−3 10−4

−2

EPRc (10 K)

EPRc (10 K)

τT (ms)

D-parameter (cm−1)

2.7 18.3 0.8 8.1 >5.5 >7.0

0.053 0.068 0.038 0.049 0.072 0.069

a

Measured triplet lifetimes obtained by spectrally integrated transient EL at either room temperature or at 4 K. Electrophosphorescence for 1a and 2a was too weak to be observed. bThe triplet lifetime and the squared T1 → S0 spin−orbit coupling (SOC) matrix elements are computed in the mean-field approximation from the pure DFT/MRCI spin states. All values given were obtained at the T1 (ms = 1) minimum geometry, which was optimized at the UPBE0-D3(BJ)/def2-TZVP level of theory. cTriplet lifetime measured by EPR at a temperature of 10 K. The dipolar contribution to the zero-field splitting (D-parameter) of the triplet excited state is extracted from fitting to the EPR spectra and provides a measure of spatial delocalization. 4804

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ments and the fact that triplet emission is observed from both isomers. Whereas weak phosphorescence may be masked by strong fluorescence in EL experiments, EPR can give insight into the relaxation processes of triplet excited states for both emissive as well as nonemissive excitations. Time-resolved EPR (TR-EPR) measurements were performed in frozen toluene solutions at 10 K for the isomer pairs 1, 2, and 6.51,52 At this low temperature, optically excited triplet states could be detected for all compounds, even for those that are nonphosphorescent. From the spin-polarized dipolar Pake pattern the zero-field splitting (ZFS) constants could be extracted. Simulations of the EPR spectra enabled us to determine the dipolar contribution, D, to the ZFS precisely, providing a direct measure of the delocalization of the triplet wave function.51,52 The results are summarized in Table 1. The triplet lifetimes determined by TR-EPR at 10 K correlate well with the OLED phosphorescence lifetimes. For 6, the spin relaxation was too fast relative to the triplet lifetime, and only a lower limit of the lifetime was determined. The nonphosphorescent molecules generally show EPR triplet lifetimes ten times shorter than those of the phosphorescent isomers. This difference in excited-state lifetime implies an order-ofmagnitude increase in nonradiative decay rate for 1a and 2a compared to 1b and 2b, supporting the trends of increased nonradiative decay rates in the α over the β isomers identified in the quantum chemical calculations. In all cases, static or dynamic distortions of the molecule can give rise to sufficient vibrational overlap between states to allow SOC to induce changes in spin multiplicity as well as nonradiative decay to the singlet ground state at a crossing point of the molecular potential surfaces; as a consequence, ISC occurs.7 In addition, the sulfur atom of the thiophene offers a large SOC constant for accelerated ISC.53 Considering the thiophene moiety on its own, this enhanced ISC leads to two competing effects, phosphorescence and nonradiative triplet decay. As shown above, the origin of the phosphorescence can be assigned to the nπ* state located on the nitrogen atoms. It is therefore more likely that, in the case of the nonphosphorescent compounds, the thiophene moieties promote nonradiative triplet deactivation more strongly than phosphorescence. By investigating the localization of the spin density of the excited triplet state, we can identify possible regions within the molecules where SOC can occur. Comparison of each pair of isomers of 1 and 2 reveals that the nonphosphorescent molecules have smaller D-values, implying more delocalized spin density distributions. Figure 2 shows the conjugation path and the calculated spin density in the isomers of 1. The calculations were performed using the unrestricted B3LYP hybrid density functional (see the Supporting Information for details)54 and confirm the above TR-EPR results in that the spin density in 1a is further spread out in space than it is for 1b.44 Because of the interruption of the conjugated thiophene backbone for the βisomer, the spin density is localized more on the phenazine than on the thiophene moiety. Since the absence of spin density on the outer (nonfused) thiophene moieties for the β-isomers correlates with the appearance of phosphorescence, we deduce that these thiophene rings are responsible for the nonradiative deactivation of the triplet excited state. We conclude that two distinct effects control phosphorescence in these low-Z compounds. Mixing of the triplet excited state with the higher-lying nπ* singlet state ensures sufficient SOC and fundamentally enables a radiative decay of

triplet states to the ground state. In the calculations, the hexyl groups in compounds 1 and 2 were replaced by methyl groups, while for 6 they were replaced by hydrogen atoms. The computed lifetimes are given in Table 1. It can be seen that based on the radiative decay rates, which differ at most by 1 order of magnitude, the triplet lifetimes for the phosphorescent β-isomers are longer than those for the non-phosphorescent αisomers. In principle, SOC may lead to phosphorescence in two different ways: the singlet ground state can couple with triplet excited states. In this way, spin- and electric-dipole allowed excitations in the triplet manifold, from T1 to various triplet states that mix with S0, may contribute. Alternatively, one of the ms components of the T1 state couples with the excited singlet states, mixing in contributions from electric-dipole allowed excitations in the singlet manifold. Because of the molecular symmetry, the singlet nπ*-state can couple with the T1 (ms = 0) ππ*-state, as discussed in detail in the Supporting Information. Furthermore, this singlet nπ*-state has a nonvanishing oscillator strength of roughly fosc(nπ*) ∼ 0.004−0.005 for transitions to the ground state S0 (see Table S2 for compoundspecific values). To investigate the relevance of this nπ*-state for the isomer pairs 1 and 6, additional calculations were performed in which both the singlet and triplet nπ*-states were explicitly excluded in the DFT/MRCI and SOC-CI treatments (see computational details below). In this case, the radiative transition rates decrease by 3−5 orders of magnitude, suggesting that no phosphorescence would be observed in the experiment (see Table S1). Including only one of the nπ*-states, either the singlet or the triplet, revealed that it is the singlet nπ*-state that determines the magnitude of the radiative triplet transition rate. It can therefore be concluded that it is almost exclusively the mixing of the singlet nπ*-state with the T1 (ms = 0) ππ*-state which results in phosphorescence. The calculated radiative triplet lifetimes (Table 1, column 3) are up to 3 orders of magnitude longer than the measured electrophosphorescence lifetimes. Furthermore, the trends observed in experiment within pairs of isomers cannot be rationalized from the calculated radiative transition rates. Consequently, nonradiative decay processes must strongly impact the observed triplet lifetimes and are likely responsible for the isomeric differences. Given that the conditions (i.e., host matrix, temperature, etc.) were the same for a set of isomers, one can assume that the observed differences between isomers are an inherent property of the compounds themselves, and result predominantly from the different intersystem crossing (ISC) rates from the T1 to the S0 state. In the Franck−Condon approximation, the ISC rate is directly proportional to the squared T1 → S0 SOC element evaluated for the relaxed geometry of the T1 state.50 The SOC elements between the pure spin states are computed from DFT/MRCI employing the same mean-field approximation used in the SOC-CI step, as summarized in Table 1, column 4. For the α- and β-isomers of compounds 1 and 2, the squared SOC elements |⟨T1|HSOC|S0⟩|2 differ by a factor of approximately 104 and 10, respectively. These differences reflect, at least qualitatively, the experimental trends. The lack of phosphorescence in 1a and 2a is thus likely a consequence of the significantly faster ISC in the α-isomers compared to that in the β-isomers. Conversely, the weak ISC to the singlet ground state suppresses nonradiative decay and enhances phosphorescence for the β-isomers. In contrast, for 6, the order of magnitude of the squared SOC matrix elements is found to be similar for both isomers. This calculation is again in qualitative agreement with the lifetimes from the EL measure4805

DOI: 10.1021/acs.jpclett.6b01907 J. Phys. Chem. Lett. 2016, 7, 4802−4808

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The geometries of the α- and β-isomers of compounds 1, 2, and 6 in the T1 state were optimized at the unrestricted PBE0D3(BJ)/def2-TZVP level of theory.57−62 The Turbomole suite of programs is used for this purpose and all electronic groundstate calculations.63,64 The resolution-of-the-identity (RI-J) approximation to accelerate the computation of Coulomb integrals together with matching basis sets was used throughout.65,66 All structures turned out to be planar and were verified as energetic minima by computing harmonic frequencies at the same level of theory. Except for 1b and 2b, where a slight in-plane distortion is observed at this level of theory (retaining the Cs symmetry of the structure), the optimizations yield C2v symmetric geometries. On these geometries, DFT/MRCI48 single-point calculations (exploiting the Cs symmetry) based on a restricted BHLYP/ TZVP reference61,67−69 were performed. Three spin-pure excited singlet and triplet states were computed within each irreducible representation A′ and A″. All configurations up to 0.8 Hartree above the reference configurations are considered in these DFT/MRCI calculations. The reference space is iteratively increased until all dominant contributions of the computed states are included (see Table S1). The obtained pure spin states are then coupled in a SOC configuration interaction calculation,49 and the four lowest eigenstates are calculated using the SPOCK.CI program.70 These four states correspond to the singlet ground state as well as the three ms components of T1. Radiative transition rates can then be obtained as the sum of the individual transition rates from the second, third, and fourth state to the lowest one (the S0 state). The T1 → S0 SOC matrix elements of the T1 geometry are computed with the SPOCK.CI program using the pure spin states obtained from DFT/MRCI. Time-resolved continuous wave and pulsed EPR measurements at 9.7 GHz (X-band) were performed on a Bruker E580 spectrometer. The sample temperature inside the EPR cavity was controlled by a helium flow cryostat (Oxford Instruments model CF935) and a temperature controller (Oxford Instruments model ITC500). An optical parametric oscillator (Continuum Surelite OPO Plus) pumped by a Q-switched Nd:YAG laser (Continuum SL-II 10) was used for optical excitation. The molecules were dissolved in toluene at a concentration of 1 mM. The solutions were transferred into 4 mm outer diameter quartz EPR tubes. The molecules were excited at the peak wavelengths of their absorption spectra in the visible region. Delay after laser flash (DAF) measurements based on pulsed EPR were carried out to monitor the dynamics of the triplet states. In DAF measurements, the laser flash was followed by a two-pulse spin−echo sequence at a range of delay times. The decay kinetics were observed by monitoring the echo intensities as a function of the delay time.

the triplet. The calculated radiative rates, however, are insufficient to explain the strong isomer effect on phosphorescence yield in EL. This difference is dominated by the nonradiative decay channel. The SOC of the triplet excited state to the singlet ground state controls the probability of spin flips which are necessary for nonradiative relaxation. This probability is directly impacted by the accessibility of the thiophene moiety by the excited-state wave function. For the molecular structures studied here, the small isomeric modification associated with switching the position of two sulfur atoms strongly alters the effective conjugation path and localizes the spin density preferentially either in the proximity of the phenazine unit or the thiophene groups. Quantumchemical calculations show that the first case is associated with a decrease of the SOC between the triplet and singlet ground state. This reduction in SOC by localization on the phenazine suppresses nonradiative decay and thereby allows the observation of radiative triplet decay at room temperature. The thiophene units are not necessary for generating phosphorescence but merely shift absorption and emission wavelengths of the fundamental phenazine transition to the red, because of the effectively increased conjugation. This red shift is useful because the spectral overlap with the background EL of the matrix molecules is reduced. In this way, we obtain emitters which can be used as dopants in most common organic semiconductor matrices. Further enhancements of phosphorescence quantum yield are anticipated by increasing molecular rigidity of the thiophene moieties55 and by raising the energy of the vibrational modes dominant in internal conversion, such as by deuteration.56

■

EXPERIMENTAL METHODS The phenazine derivatives were obtained by condensation of the corresponding diamines and diketones. Details of the synthesis of the compounds used are given in the Supporting Information. OLEDs were fabricated on precleaned glass substrates covered by 100 nm of a structured indium tin oxide layer. The substrates were successively cleaned in an ultrasonic cleaner with acetone, 2% Hellmanex solution, and ultrapure water, and they were UV cleaned and ozonized before spin coating with a hole-injecting layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), resulting in a layer thickness of ∼80 nm. For the emissive layer, two different host matrices were used. The PVK-matrix is a blend of PVK:PBD:TPD in a ratio of 65:25:10 by weight. Alternatively, a CBP matrix was used.46 The emitter was added with 3% by weight to the corresponding matrix. The compounds were dissolved in chlorobenzene and spin coated in a nitrogen glovebox, yielding layers of ∼30 nm thickness. The top contact was provided by a thermally deposited electrode comprising 10 nm of calcium and 250 nm of aluminum. Devices were encapsulated in the glovebox with Apiezon N grease and a glass plate to prevent electrode degradation by atmospheric oxygen and water during the short sample transfer times. EL was measured in a coldfinger liquid helium cryostat under a vacuum of