Light-Induced Structural Change in Iridium Complexes Studied by

May 8, 2014 - A. Batagin-Neto†, A. P. Assis‡, J. F. Lima§, C. J. Magon§, L. Yan∥, M. Shao∥, B. Hu∥, and C. F. O. Graeff*⊥. † Campus Ex...
1 downloads 4 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCA

Light-Induced Structural Change in Iridium Complexes Studied by Electron Spin Resonance A. Batagin-Neto,† A. P. Assis,‡ J. F. Lima,§ C. J. Magon,§ L. Yan,∥ M. Shao,∥ B. Hu,∥ and C. F. O. Graeff*,⊥ †

Campus Experimental de Itapeva, UNESP - Univ Estadual Paulista, Rua Geraldo Alckmin 519, Itapeva, São Paulo 18409-010, Brazil POSMAT − Programa de Pós-Graduaçaõ em Ciência e Tecnologia de Materiais, UNESP − Univ Estadual Paulista, Bauru, São Paulo 17033-360, Brazil § Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, São Paulo 05508-070, Brazil ∥ The University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ DF-FC, UNESP, Univ Estadual Paulista, Avenida Eng. Luiz Edmundo Carrijo Coube, 14-01, Bauru, São Paulo, 17033-360, Brazil ‡

S Supporting Information *

ABSTRACT: Iridium-based compounds are materials of great interest in the production of highly efficient organic light emitting diodes and several other applications. However, these organometallic compounds present relative low stability due to photodegradation processes still not well understood. In this work we investigated paramagnetic states induced by UV photoexcitation on iridium(III) bis[(4,6-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) and iridium(III)-tris(2-phenylpyridine) (Ir(ppy)3) complexes dispersed in different polymeric matrices by electron spin resonance (ESR). Photogenerated charged states with relatively strong hyperfine interactions were observed and attributed to matrix/complex charge-transfer processes. Measurements of the signal amplitude decay after photoexcitation interruption were performed as a function of temperature. The photoinduced centers are thermally activated with energy barrier between 0.3 and 0.6 eV. Electronic structure calculations suggest that the signals observed by ESR are associated with metastable negatively charged Ir complexes distorted structures.

1. INTRODUCTION Iridium complexes are highly efficient photoluminescent materials largely employed as dopants in active layers of organic light emitting diodes (OLEDs),1−4 oxygen sensing,5−8 and catalytic applications.9 The high spin−orbital coupling in these complexes results in quite unique optoelectronic properties, such as triplet emission and quantum efficiencies close to 100%. For the production of electronic devices, Ir-based dyes are commonly dispersed in polymeric hosts. These dispersions frequently confer improved properties to the resulting systems, as high triplet/singlet excitons harvesting in OLEDs4,10 or mechanical stability in oxygen sensors.6,11 In particular, dispersions containing low spin−orbit-coupled polymeric hosts have also attracted great attention due to significant changes induced in the optoelectronic properties of these systems by external magnetic fields.12,13 © 2014 American Chemical Society

Despite these promising technological applications, the use of metal complexes is still limited due to stability problems. In fact, distinct mechanisms have been proposed to explain the degradation of Ir compounds;14 however, the processes involved are not understood in detail. Recently the existence of intrinsic chemical degradation routes of Ir-based phosphorescent dyes was imputed to the complexes’ chemical dissociation during OLED operation. Based on mass spectrometry experiments, Moraes et al. and Sivasubramaniam et al. proposed that ligand’s decomplexation generates charged subproducts in the system, which subsequently react with host segments producing nonradiative species.14−16 Structural changes of Ir complexes have also been associated with nonradiative transitions involving triplet excited states of Received: October 14, 2013 Published: May 8, 2014 3717

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

these compounds.17 It has been proposed that internal conversions from photogenerated metal-to-ligand charge-transfer states (1MLCT/3MLCT) to metal-centered triplet states (3MC) could be responsible for Ir−N bond dissociation and ligand rotation resulting in five coordinated distorted trigonal bipyramidal (TBP) structures. However, although there is some evidence of such nonradiative transitions, until now there has been no direct experimental measurements able to prove the existence of such distorted structures,18,19 as well as their role in the complex degradation. In order to better evaluate degradation and/or charge transfer (CT) processes that can take place in organometallicbased systems, in this work we present electron spin resonance (ESR) studies of photogenerated paramagnetic states on pristine and polymer dispersed Ir complexes, namely, iridium(III)-tris(2-phenylpyridine) (Ir(ppy)3) and iridium(III) bis[(4,6-fluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic). ESR signals are found and attributed to photoinduced structural defects on Ir-complexes. Electronic structure calculations performed on distinct paramagnetic species suggest that the signals can be associated with negatively charged distorted structures, indicating that TBP species can in fact be formed and stabilized by CT processes in the system.

range of 278 and 363 K. An UV LED (370 nm, 1 mW) was used for photoexcitation. Manganese markers were used for gfactor calibration. Some measurements were made on a Bruker Elexsys E580 spectrometer operating at X-band. In this spectrometer temperature was controlled by a continuous flow of liquid helium using an Oxford cryogenic system. For electronic structure studies, different isomers of the Ircomplexes were considered: four isomers for FIrpic (FIrpic(iso1−4)) and two for Ir(ppy)3 (Ir(ppy)3(fac/mer)).15,19 The ground-state geometries were optimized by DFT approach with Becke’s LYP (B3LYP) exchange-correlation functional. Quasi-relativistic pseudopotentials20,21 and double-ξ quality basis set (LANL2DZ) were employed for Ir atom description, while the 6-31G basis set was adopted for lighter atoms. Optimization studies were performed with the aid of GAMESS computational package.22 ESR parameters were obtained in a DFT approach by using two different hybrid functionals, B3LYP and PBE0 (Perdew−Burke−Erzerhoff GGA hybrid functional). Relativistic effects were taken into account by employing the scalar-relativistic ZORA (zero-order regular approximation) Hamiltonian.23 Ir atoms were described by segmented all-electron relativistically contracted (SARC) polarized triple-ξ-valence basis set (TZVPPP), and the lighter atoms were described by the SARC-TZVP basis set.24 The calculations were performed using the ORCA computational package.25

2. METHODOLOGY Poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly[9,9-di(2-ethylhexyl)-fluorenyl-2,7-diyl] (PFO) polymers were purchased from Sigma-Aldrich and employed as matrices. The dyes iridium(III) bis(2-(4,6-difluorephenyl)pyridinatoN,C2) (FIrpic) and tris[2-phenylpyridine] iridium (Ir(ppy)3) were purchased from American Dye Source, Inc. and used as received. Figure 1 shows the structure of the compounds.

3. RESULTS Figure 2 shows the LESR signals obtained for the complexes FIrpic and Ir(ppy)3 in pure films and dispersed into the polymeric matrices polyfluorene (PFO), polystyrene (PS), and poly(methyl-methacrylate) (PMMA). The spectra were normalized by the masses of the Ir complexes present in the dispersions. All measurements were made at the same experimental conditions at room temperature. As can be seen, the signal is composed of four transition lines for FIrpic+PFO, FIrpic/Ir(ppy)3+PS, FIrpic/Ir(ppy)3+PMMA systems, and FIrpic (pristine). On the other hand, no signal was obtained for Ir(ppy)3+PFO and Ir(ppy)3 (pristine). In general, the signals were more intense in the PS and PMMA matrix. One relevant feature of the spectra is the difference in amplitude between the transitions. This asymmetry was observed even at high temperature (363 K) and can in principle be ascribed to different factors, such as the presence of equivalent hydrogen and/or nitrogen hyperfine interactions,26 g and A tensors with distinct principal axes,27 or Ir isotopes with different quadrupole and hyperfine ratios (Q/A).28 A small distortion in the line shape of the first two transitions in the PMMA matrix suggests the presence of additional defects in this matrix. Table 1 summarizes relevant parameters obtained from LESR spectra for the different systems. For all cases, the hyperfine constants (A) were estimated by considering the average difference between adjacent transition peaks. The resonant field, employed in g-factors estimation, was obtained by evaluating the position of the spectrum at its center (region between the second and third transitions) and then corrected by its relative position regarding to Mn markers. Note that both line shape and ESR parameters are quite similar, independent of Ir-complex and matrix, which suggests that the centers experience very similar chemical environments. The signals shown in Figure 1 were obtained under UV irradiation, presenting a significant decrease after illumination

Figure 1. Structures of Ir complexes and polymeric matrices.

Different compositions were considered: pure FIrpic, FIrpic +PS (4:3 w/w), FIrpic+PMMA (6:50 w/w), FIrpic+PFO (4:3 w/w), pure Ir(ppy) 3 , Ir(ppy) 3 +PS (3:25 w/w), Ir(ppy)3+PMMA (6:50 w/w), and Ir(ppy)3+PFO (1:100 w/w). All samples were prepared by dissolving the complex and the polymer matrix in THF. After solvent evaporation, the films obtained were placed in the quartz tubes with 4.5 mm in diameter for ESR analysis. Light-induced electron spin resonance (LESR) measurements were performed using an X band spectrometer MiniScope MS300 from Magnettech in the temperature 3718

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

Figure 2. LESR signals obtained for the complexes FIrpic (a) and Ir(ppy)3 (b), pristine and in different hosts.

FIrpic+PS is presented. As can be seen, the decay time dependence can be fitted by

Table 1. Experimental ESR Parameters system

g-factor

A (mT)

FIrpic FIrpic+PS FIrpic+PMMA FIrpic+PFO Ir(ppy)3 Ir(ppy)3+PS Ir(ppy)3+PMMA Ir(ppy)3+PFO

2.0067 ± 0.0005 2.0068 ± 0.0005 2.0068 ± 0.0002 2.0066 ± 0.0003 No signal 2.0070 ± 0.0004 2.0067 ± 0.0003 no signal

1.31 1.19 1.19 1.19 No signal 1.18 1.19 no signal

1/τ1 = k·e−Ea / T

ln(1/τ1) = ln(k) − Ea(1/T )

(1)

where k represents a pre-exponential term, and Ea the activation energy associated with the signal decay.29 Table 2 summarizes the average activation energy obtained for each system following eq 1. Table 2. LESR Decay Time Activation Energy for Different Ir Complex Systems system

interruption. Figure 3a illustrates the signal amplitude dependence with elapsed time after photoexcitation interruption for the system FIrpic+PS in different temperatures (peak-to-peak difference between the maximum and minimum of the central transition). The curves can be well adjusted by a second-order exponential decay with two components: a fast-temperature independent decay, τ0, and a slow-temperature dependent decay, τ1(T). A similar behavior was observed for all systems (see Supporting Information). This decay suggests that the LESR signal is associated with photogenerated metastable paramagnetic states, which are influenced by the polymer matrix. The decay time constant is temperature dependent; in Figure 3b the dependence of ln[1/ τ1] with the reciprocal of the temperature (1/T) for the system

FIrpic+PS FIrpic+PMMA Ir(ppy)3+PS Ir(ppy)3+PMMA

Ea (eV) 0.36 0.37 0.5 0.5

± ± ± ±

0.02 0.08 0.1 0.2

In general, larger activation energies were obtained for Ir(ppy)3-based systems, suggesting the presence of more stable paramagnetic states in this complex. Quite similar activation energies were obtained for PS and PMMA matrices, indicating that these hosts play a similar role on the paramagnetic centers’ stability. Figure 4 presents the comparison of LESR spectra acquired at 77 K and room temperature for the complexes Ir(ppy)3 and

Figure 3. (a) Signal amplitude decay as a function of the elapsed time from the photoexcitation interruption. (b) Linear fit related to eq 1 3719

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

Figure 4. LESR spectra measured at 77 K and room temperature for the complexes Ir(ppy)3 and FIrpic in PS.

environments, where different relaxation times and saturation dynamics are expected. Considering that paramagnetic centers are located on the Ircomplex ligands, the similarity between the spectra of Ir(ppy)3 and FIrppy indicate that the unpaired spins are located on similar ligands, for instance, the phenylpyridine (ppy) and difluorophenylpyridine (2Fppy) for Ir(ppy)3 and FIrpic, respectively. In this same context, another relevant feature that can bring valuable information about the mechanism responsible for the paramagnetic center formation is the difference in the response of pristine FIrpic and Ir(ppy)3 films. Despite both complexes presenting similar structures, just the FIrpic films present detectable ESR signals after irradiation. Considering the structure of both Ir complexes (Figure 1), this fact could in principle be associated with the higher asymmetry of FIrpic compared to Ir(ppy)3. Indeed this feature could result in a stronger molecular electric dipole on this compound that could facilitate the dissociation of photogenerated excitons. Since this asymmetry is not observed in Ir(ppy)3, it could hinder the occurrence of such process in pure films of this complex. However, the presence of intense signals in Ir(ppy)3+PS and Ir(ppy)3+PMMA indicates that the complex−matrix interactions facilitate the paramagnetic center formation, possibly by charge transfer followed by charge trapping. In fact, it is difficult to underline the exact mechanism associated with the paramagnetic center formation in our samples, or even to define whether it is associated with CT processes, degradation routes, or additional reactions involving matrix segments. Regarding the first possibility, it is known that Ir complexes often present lower oxidation than reduction potential34 with respect to a common reference electrode. In this sense, one expects cationic species to be more easily formed in comparison to anionic counterparts. As a matter of fact, from energy level analysis, the formation of positively charged Ir species is a plausible process after UV excitation, since the difference between ELUMO of the complexes and the hosts (ΔELUMO) is lower than ΔEHOMO (see Figure 5). In this case, the dissociation of photo excited excitons results in an electron transfer from the complex to the matrix, generating cationic Ir structures in the system. On the other hand, the formation of negatively charged Ir structures is not extensively discussed in the literature, although these structures are also considered to be stable if they are formed.34

FIrpic in PS (see Supporting Information for PMMA-based systems, including spectra acquired at 40 K). Low-temperature spectra present stronger asymmetry between the central and peripheral transitions and a lower line width with respect to those presented in Figure 2. Note that no additional changes in line shape are observed, suggesting that the main features of the obtained spectra cannot be just associated with anisotropic effects as discussed previously.

4. DISCUSSION The spectra presented in Figure 2 evidence the formation of paramagnetic species in the systems after UV irradiation. The existence of similar line shapes and ESR parameters for all the systems under study, including pure FIrpic film, suggests that the LESR signal is mainly associated with paramagnetic species generated in the complexes. In this way, the polymer matrix plays a secondary role, being probably responsible for stabilizing the photogenerated species. The spectra presented in this work, however, is quite different from those commonly reported for Ir complexes.30−33 In general, ESR spectra found in the literature have broad line shapes and strong hyperfine interactions, which are related to paramagnetic centers close to the Ir atoms. Thus, our results show that the LESR-induced paramagnetic spins experience weaker interactions with Ir, suggesting that they should be delocalized on the complex ligands instead of being concentrated on the Ir atom. Indeed this assumption is compatible with lower hyperfine coupling constant and g-values closer to the free electron, as observed. In this context, in principle, the presence of four transitions in the spectra can be ascribed to hyperfine interactions with Ir atom (with a weak coupling) or with equivalent hydrogen (I = 1/2) and/or nitrogen (I = 1) atoms, or even to the superposition of the ESR response of two or more centers located on distinct chemical environments, for instance, one on the complex and another on the polymeric matrix. This latter hypothesis actually seems to be less plausible in the present case since LESR signals were observed in pure FIrpic films, discrediting the relevance of host signals. In addition, no significant changes in line shape could be observed by varying the temperature and microwave power, in contradiction to the expected behavior of spins systems located in distinct 3720

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

• The cationic species FIrpic+ and Ir(ppy)3+ provide large values of g-factor and hyperfine coupling (g: 2.31959− 3.38852; Aiso > 2.5 mT), mainly due to the high spin density on the Ir atom. • The anionic species FIrpic− and Ir(ppy)3− have g-factors very close to the experimental values (g: 2.00425− 2.01180); however, they have weaker hyperfine interactions than that obtained from experiments (Aiso < 0.5 mT). • The subproduct Ir(ppy)2• and the distorted structure Ir(ppy)3-Nrot+fac have the same characteristics of cationic structure Ir(ppy)3+ (g: 2.14307−4.02210), due to the high spin density on the Ir atom. • The subproduct ppy• has a small g-factor value (g: 2.00234−2.00236); however, it cannot reproduce the hyperfine interaction, even considering N and/or H hyperfine couplings. • Finally, among all structures studied, the most suitable set of parameters were obtained for Ir(ppy)3-Nrot−fac species, which are summarized in Table 3 and Figure 6.

Figure 5. Energy levels of the materials under study. Dotted lines represent triplet level energies.35−38

In addition to CT processes, degradation mechanisms such as light-induced ligand decomplexation of Ir compounds,14,16 ligand rotation,17,19 and interaction/incorporation of reactive oxygen species in the system6,8 should also be considered in order to explain the generation of ESR stable species. The ligand decomplexation has been proposed as a degradation route of Ir-complexes based OLEDs. It has been suggested that lateral ligands like ppy (or picolinate−pic) could dissociate from the Ir central atom of Ir(ppy)3 (or FIrpic) in a reversible or irreversible way, generating the radical species Ir(ppy)2• (Ir(Fppy)2•) and ppy• (pic•). These radical species could be the origin of the LESR signals. In the same context, it has been proposed that distorted structures, coming from Ir−N bond cleavage and ligand rotation, were also formed after photoexcitation. These structural rearrangements would be induced by triplet transitions from metal-to-ligand charge transfer (3MLCT) states to metal-centered (3MC) states,19 generating metastable distorted structures in the system. The presence of such distorted structures, with distinct frontier levels and electroaffinity, could induce quite uncommon CT processes in the system, which could generate paramagnetic states. Finally, since our experiments were conducted with samples exposed to the air, another relevant possibility is oxygeninduced signals. It is known that reactive oxygen species (ROS) are generated by the interaction of diatomic oxygen with triplet excited Ir compounds. Such reactive species could follow subsequent reactions, possibly generating radicals or traps in the systems.5 In order to better understand the real nature of the observed paramagnetic centers, as well as the processes involved, electronic structure calculations were performed considering distinct complexes’ structures related to the above presented hypothesis: cationic and anionic ground state optimized species: FIrpic−, FIrpic+, Ir(ppy)3− and Ir(ppy)3+; optimized and nonoptimized subproducts coming from ligand decomplexation: Ir(ppy)2• and ppy•; and negatively and positively charged distorted species obtained by pyridine ring rotation: Ir(ppy)3-Nrot+ and Ir(ppy)3-Nrot−. Distorted structures were evaluated just for Irppy3, since it would be difficult to determine which ligand suffers rotation in FIrpic. In the following, we present a summary of the results obtained. Details regarding the calculations, as well as results for all structures, are available in the Supporting Information.

Table 3. g-Factors and Ir Hyperfine Couplings Obtained for Negatively Charged Distorted Structures species

functional

g-value (giso)

Ir hyperfine (Aiso − mT)

Ir(ppy)3-Nrot−fac (A)

B3LYP PBE0 B3LYP PBE0

2.043 68 2.043 29 2.018 96 2.020 93

1.269 1.481 0.852 1.038

Ir(ppy)3-Nrot−fac (B)

As can be seen, the results suggest that the hyperfine interaction with the iridium atom is responsible for the quartet structure of the LESR signal. Note that, despite good hyperfine matching with the experimental observations, we are not able to reproduce the g-factors. Such deviations could be in principle ascribed to the influence of the matrix that was not considered in the calculations. Nonetheless, it is important to emphasize that DFT calculations give us a qualitative idea regarding the nature of the observed LESR signal, indicating that photogenerated paramagnetic centers are not compatible with simple charged species or degradation byproducts. Figure 7 illustrates the structure and spin density of two optimized of Ir(ppy)3-Nrot− species, considered in the calculations; for comparison with the charged structures see the Supporting Information. As can be seen, in the distorted conformations, the unpaired spin is mainly delocalized on the ligands, presenting a smaller interaction with the central Ir atom, which ensures reduced spin−orbit coupling in these species in comparison to the cationic structures. In addition, the presence of a rotated ring promotes higher spin localization on the other two ligands, in comparison to that expected for undistorted anionic structures. These features culminate in intermediary hyperfine couplings and g-factors close to the free electron in these structures that are compatible with our data. In fact, the formation of five-coordinate distorted trigonal bipyramidal (TBP) structures, similar to those presented in Figure 7, has been associated with nonradiative transitions of Ir complexes. DFT calculations performed by Treboux et al. and Sajoto et al. have shown that TBP structures are related to the lowest triplet state of these complexes, which is associated with a metal-centered triplet state (3MC).17,19,39 Following a similar mechanism, already proposed for Ru-based compounds, it has 3721

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

Figure 6. Hyperfine constants for hydrogen, nitrogen, and iridium atoms obtained from electronic structure calculations from the species Ir(ppy)3rot−fac (A) and Ir(ppy)3-rot−fac (B).

negatively charged species in the system. Nevertheless, it is important to keep in mind that traditional CT processes must be considered with some restriction in the present case, since they are based on ground-state Ir complexes structures. In fact, additional and specific CT processes, quite different from those expected for ground-state conformations, can take place in TBP structures. In order to test the hypothesis of LESR spectra being associated with negatively charged Ir-species, complementary studies were performed by adding an acceptor compound (tetracyanobenzene, TCNB) into FIrpic+PS samples. Since we propose that LESR spectra comes from electron transfer, from the hosts to Ir complexes, the presence of electron acceptors could hinder the formation of paramagnetic centers, promoting changes in the spectra. Figure 8 presents a comparison between both FIrpic+PS and FIrpic+TCNB+PS spectra, acquired at the same experimental conditions. As can be seen, small differences in the line shapes are induced by the presence of TCNB molecule. The changes can be attributed to the superposition of Ir complexes signals and

Figure 7. Spin density on Ir(ppy)3-rot−fac (A) and Ir(ppy)3-rot−fac (B) optimized structures.

been suggested that internal conversions involving 3MLCT and 3 MC states, as well as additional intermediary states, could take place in Ir complexes, generating such distorted structures in the system. Nevertheless, due to the regeneration of the ground-state conformation after the decay, until now there is no direct experimental evidence of these structures.19 The association of such five-coordinated species to the observed spectra is compatible to ESR signals of some cobalt complexes. Indeed, quite similar spectra have been already reported for five-coordinate Co species, with octet lines instead of a quartet. In these cases, the main features of the spectra have been attributed to an axial g-factor and 59Co hyperfine matrices with different parallel axes.27 Additionally, a high spin delocalization was observed on the Co-complex that is also compatible with the spin density presented in Figure 7. Despite our results suggesting that distorted anionic structures can be responsible for the observed signals, indicating that TBP-like structures formed during irradiation can act as electron traps in the polymer hosts, the origin of the negative charge is not well understood. As already stated, the reduction of Ir complexes is a less plausible process, so it is very difficult to explain the presence of

Figure 8. Comparison between LESR spectra of FIrpic in PS and TCNB+PS matrices. 3722

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

after excitation interruption, which can be recovered after additional UV excitation. From the fitting, the curves can be decomposed in two components with distinct decay constants: (i) a temperatureindependent component with τ1 ∼ 5−7 s, and (ii) a temperature-dependent component with τ2 = τ2(T), which can be attributed to several distinct processes. Since our results suggest that the observed paramagnetic centers are associated with anionic distorted structures, it is reasonable to suppose that the signal decay involves CT and structural reorganization processes. Indeed, the large values of τ in the order of seconds and its temperature dependence reinforces that the ESR signal quenching can be associated with structural relaxation processes. In this context, the Ea values (Table 2) should be considered as the energy required for conformational relaxation from TBP structures to ground-state (GS), plus some small additional relaxation induced by the CT process. The activation energy associated with transitions from ground-state (GS) to 3MC-TBP states (EGS‑TBP) has been estimated to be higher than 0.5 eV for Ir complexes with high quantum efficiency (η > 0.93), which is the case of FIrpic and Ir(ppy)3.19 Considering the enthalpy difference between ground state (GS) and TBP structures (ΔH), the energy required for conformational regeneration is supposed to be ETBP‑GS = EGS‑TBP − ΔH, resulting in activation energies around 0.3−0.5 eV,17 and thus compatible with our result (see Figure 9). In this context, the lower activation barrier observed for

additional charged species. The TCNB-based sample also shows less intense ESR signals in relation to FIrpic+PS, around a factor of 10. This quenching effect suggests that the presence of an acceptor molecule defines a competing route for the matrix oxidation, reinforcing our hypothesis that Ir complexes acts as electron traps in the original systems without TCNB. Indeed, it is quite difficult to outline the exact mechanism associated with CT processes occurring between polymer hosts and Ir complexes. In this context, the simplest route that could be proposed is the direct transfer of intrinsic charges, present on the polymers, to the distorted complexes. Actually, PMMA and PS polymers are known to charge easily by contact with others polymers or metals, commonly presenting some residual negative charges (called “crypto-electrons”) that can induce uncommon redox reactions, like Ru-complexes reduction.40 However, in our case, preliminary LESR studies performed on pure matrices, evidence that no significant charges are present on the pristine polymers, which discredits the hypothesis of simple matrix-to-complex electron-transfer processes. Besides also being related to TBP-like structures formation, the strong dependence of the LESR signal amplitude with UV illumination can suggest the relevance of traps states in the hosts for the generation of charged species. As a matter of fact, the existence of trap states coming from charge-induced relaxation processes has already been reported for PMMA and PS matrices.41,42 These states have strong influence in the acceptor/donor properties of these materials and have been associated with intra- and intermolecular relaxation of polymers segments after charge injection. In this sense, specific interactions between polymer segments and the organometallic compounds, in addition to the presence of “crypto-electrons”, could be responsible for such traps in the hosts. In this same context, oxygen or hydrated oxygen species sorbed on the polymer surface, or even coming from ROS incorporation in the polymer hosts,5 could also be responsible for trap states in the system (see Figure 5).43,44 Nevertheless, more studies are still necessary in order to better evaluate all these hypotheses. It is important to note that additionally to the presence of negatively charged species, our results also suggest the relevance of structural distortions in the Ir compounds coming from nonradiative triplet state evolution. In this sense, the absence of expressive signals in PFO matrix reinforces the relevance of long-lived triplet states for the ESR signal observation. As a matter of fact, given the alignment of Ircompounds and PFO triplet states (see Figure 5), guest-to-host energy transfer processes are expected to occur in PFO-based systems.35,45 Such excited state diffusions have already been observed in transient photoluminescence experiments of Ir complexes in different hosts and are also expected to occur in our experiment.45 Following this mechanism the photogenerated 3MLCT states (coming from 1MLCT to 3MLCT transitions) would diffuse from the complex to the host, hindering the formation of TBP-like structures and thus the observation of a LESR signal. This hypothesis is indeed corroborated by the observation of weak signals in FIrpic+PFO. Given the larger dissimilarity between the guest and host triplet levels, lower resonant triplet interaction is expected between them,46 in such way that 3 MLCT/3MC-TBP transitions can occur in a more efficient way. Finally, another relevant factor regarding the presence of metastable structures is the thermally activated signal decay. As shown in Figure 3, the LESR signals present amplitude decay

Figure 9. Diagram representing the relation between ETBP‑GS, EGS‑TBP, and ΔH and their relation with Ea values presented in Table 2.

FIrpic-based systems (Table 2) can be explained by the fact that asymmetric complexes presents lower EGS‑TBP barriers than symmetric ones.17 Lastly, it is also important to note that the reversibility associated with the amplitude of the LESR signal indicates that the structures responsible for the ESR signal cannot be directly associated with the complexes degradation, which reinforces our interpretation. 3723

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

Complexes: Principles and Biomedical Applications. Coord. Chem. Rev. 2011, 255, 2542−2554. (9) Ju, X.; Liang, Y.; Jia, P.; Li, W.; Yu, W. Synthesis of Oxindoles via Visible Light Photoredox Catalysis. Org. Biomol. Chem. 2012, 10, 498− 501. (10) Rothe, C.; King, S.; Monkman, A. Systematic Study of the Dynamics of Triplet Exciton Transfer Between Conjugated Host Polymers and Phosphorescent Iridium (III) Guest Emitters. Phys. Rev. B 2006, 73, 245208. (11) DeRosa, M. C.; Crutchley, R. J. Photosensitized Singlet Oxygen and Its Applications. Coord. Chem. Rev. 2002, 233−234, 351−371. (12) Hu, B.; Yan, L.; Shao, M. Magnetic-Field Effects in Organic Semiconducting Materials and Devices. Adv. Mater. 2009, 21, 1500− 1516. (13) Yan, L.; Shao, M.; Graeff, C. F. O.; Hummelgen, I.; Ma, D.; Hu, B. Changing Inter-molecular Spin-orbital Coupling for Generating Magnetic Field Effects in Phosphorescent Organic Semiconductors. Appl. Phys. Lett. 2012, 100, 013301. (14) Moraes, I. R. de; Scholz, S.; Lüssem, B.; Leo, K. Analysis of Chemical Degradation Mechanism Within Sky Blue Phosphorescent Organic Light Emitting Diodes by Laser-desorption/ionization Timeof-flight Mass Spectrometry. Org. Electron. 2011, 12, 341−347. (15) Sivasubramaniam, V.; Brodkorb, F.; Hanning, S.; Loebl, H. P.; van Elsbergen, V.; Boerner, H.; Scherf, U.; Kreyenschmidt, M. Fluorine Cleavage of the Light Blue Heteroleptic Triplet Emitter FIrpic. J. Fluorine Chem. 2009, 130, 640−649. (16) De Moraes, I. R.; Scholz, S.; Lüssem, B.; Leo, K. Role of Oxygen-bonds in the Degradation Process of Phosphorescent Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 99, 053302. (17) Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E. Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes. J. Am. Chem. Soc. 2009, 131, 9813−9822. (18) Kobayashi, T.; Ide, N.; Matsusue, N.; Naito, H. Temperature Dependence of Photoluminescence Lifetime and Quantum Efficiency in Neat fac-Ir(ppy)3 Thin Films. Jpn. J. Appl. Phys. 2005, 44, 1966− 1969. (19) Wagenknecht, P. S.; Ford, P. C. Metal Centered Ligand Field Excited States: Their Roles in the Design and Performance of Transition Metal Based Photochemical Molecular Devices. Coord. Chem. Rev. 2011, 255, 591−616. (20) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (21) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310. (22) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; et al. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (23) Van Wüllen, C. Molecular Density Functional Calculations in the Regular Relativistic Approximation: Method, Application to Coinage Metal Diatomics, Hydrides, Fluorides and Chlorides, and Comparison with First-Order Relativistic Calculations. J. Chem. Phys. 1998, 109, 392−399. (24) Pantazis, D. A.; Chen, X.-Y.; Landis, C. R.; Neese, F. AllElectron Scalar Relativistic Basis Sets for Third-Row Transition Metal Atoms. J. Chem. Theory Comput. 2008, 4, 908−919. (25) Neese, F. The ORCA Program System. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73−78. (26) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. (27) Carpenter, G. B.; Clark, G. S.; Rieger, A. L.; Rieger, P. H.; Sweigart, D. A. Dithiolenes Revisited: An Electron Spin Resonance Study of Some Five-co-ordinate Cobalt Complexes and the Crystal Structures of [Co{S2C2(CF3)2}2{P(OPh)3] and [Co{S2C2(CF3)}2(PPh3)]. J. Chem. Soc., Dalton Trans. 1994, 20, 2903− 2910.

5. CONCLUSION Paramagnetic centers induced by UV photoexcitation on iridium(III)bis[(4,6-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) and iridium(III)fac-tris(2-phenylpyridine) (Ir(ppy)3) complexes were evaluated by electron spin resonance. The results suggest that paramagnetic centers are generated on the Ir complexes after UV irradiation. By electronic structure calculations they are associated with negatively charged trigonal bipyramidal species coming from nonradiative evolution of triplet states and stabilized by charge transfer processes involving the polymer matrix. Furthermore, our results indicate that once these species are formed, they can act as electron traps in the system, and thus influence the electronic and transport properties of these materials.

■

ASSOCIATED CONTENT

S Supporting Information *

Amplitude signal decay, spectra at 40 K, 77 K, and room temperature, and electronic structure calculation details. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: graeff@fc.unesp.br. Phone: +55 (14) 3103-6084, r: 7662. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to thank the Brazilian agencies CAPES, FAPESP (Proc. 2012/03116-7), and CNPq for financial support. This research was also supported by resources supplied by the Center for Scientific Computing (NCC/GridUNESP) of the São Paulo State University (UNESP).

■

REFERENCES

(1) Seo, J. H.; Kim, Y. K.; Ha, Y. Efficient Blue-Green Organic LightEmitting Diodes Based on Heteroleptic Tris-Cyclometalated Iridium(III) Complexes. Thin Solid Films 2009, 517, 1807−1810. (2) Endo, A.; Suzuki, K.; Yoshihara, T.; Tobita, S.; Yahiro, M.; Adachi, C. Measurement of Photoluminescence Efficiency of Ir(III) Phenylpyridine Derivatives in Solution and Solid-State Films. Chem. Phys. Lett. 2008, 460, 155−157. (3) Rausch, A. F.; Thompson, M. E.; Yersin, H. Blue Light Emitting Ir(III) Compounds for OLEDs - New Insights into Ancillary Ligand Effects on the Emitting Triplet State. J. Phys. Chem. A 2009, 113, 5927−5932. (4) Rausch, A. F.; Thompson, M. E.; Yersin, H. Matrix Effects on the Triplet State of the OLED Emitter Ir(4,6-dFppy)2(pic) (FIrpic): Investigations by High-Resolution Optical Spectroscopy. Inorg. Chem. 2009, 48, 1928−1937. (5) Djurovich, P. I.; Murphy, D.; Thompson, M. E.; Hernandez, B.; Gao, R.; Hunt, P. L.; Selke, M. Cyclometalated Iridium and Platinum Complexes as Singlet Oxygen Photosensitizers: Quantum Yields, Quenching Rates and Correlation with Electronic Structures. Dalton Trans. 2007, 3763−3770. (6) Amao, Y. Probes and Polymers for Optical Sensing of Oxygen. Microchim. Acta 2003, 143, 1−12. (7) Amao, Y.; Ishikawa, Y.; Okura, I. Green Luminescent iridium(III) Complex Immobilized in Fluoropolymer Film as Optical OxygenSensing Material. Anal. Chim. Acta 2001, 445, 177−182. (8) Ruggi, A.; van Leeuwen, F. W. B.; Velders, A. H. Interaction of Dioxygen with the Electronic Excited State of Ir(III) and Ru(II) 3724

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725

The Journal of Physical Chemistry A

Article

(28) Raizman, A.; Suss, J. T.; Low, W. Quadrupole Interaction and Static Jahn−Teller Effect in the EPR Spectra of Ir2+ in MgO and CaO. Phys. Rev. B 1977, 15, 5184−5196. (29) Shigemori, K.; Mino, H.; Kawamori, A. pH and Temperature Dependence of Tyrosine Z̈ Decay Kinetics in Tris-Treated PSII Particles Studied by Time-Resolved EPR. Plant Cell Physiol. 1997, 38, 1007−1011. (30) Hetterscheid, D. G. H.; Klop, M.; Kicken, R. J. N. A. M.; Smits, J. M. M.; Reijerse, E. J.; de Bruin, B. Hydrogen-Atom Transfer in Open-Shell Organometallic Chemistry: The Reactivity of RhII(cod) and IrII(cod) Radicals. Chem.Eur. J. 2007, 13, 3386−3405. (31) Hetterscheid, D. G. H.; Kaiser, J.; Reijerse, E.; Peters, T. P. J.; Thewissen, S.; Blok, A. N. J.; Smits, J. M. M.; de Gelder, R.; de Bruin, B. IrII (Ethene): Metal or Carbon Radical? J. Am. Chem. Soc. 2005, 127, 1895−1905. (32) Connelly, N. G.; Emslie, D. J. H.; Klangsinsirikul, P.; Rieger, P. H. Analysis of Electron Paramagnetic Resonance Spectra with Very Large Quadrupole Couplings. J. Phys. Chem. A 2002, 106, 12214− 12220. (33) Takaoka, A.; Peters, J. C. A Homologous Series of Cobalt, Rhodium, and Iridium Metalloradicals. Inorg. Chem. 2012, 51, 16−18. (34) Kapturkiewicz, A.; Angulo, G. Extremely Efficient Electrochemiluminescence Systems Based on Tris(2-phenylpyridine)iridium(III). Dalton Trans. 2003, 20, 3907−3913. (35) Niu, Q.; Zhang, Y.; Wang, Y.; Wang, X.; He, M. High-Efficiency Conjugated-Polymer-Hosted Blue Phosphorescent Light-Emitting Diodes. Chin. Sci. Bull. 2012, 57, 3639−3643. (36) Hofbeck, T.; Yersin, H. The Triplet State of fac-Ir(ppy)3. Inorg. Chem. 2010, 49, 9290−9299. (37) Burkhart, R. D.; Burrows, J. A. J.; Haggquist, G. W. Triplet Photophysics Of Polystyrene with and without Selected Terminating Substituents. In SPIE Proceedings; Menzel, E. R., Ed.; SPIE: Bellingham, WA, 1989; Vol. 1054, pp 130−137. (38) Graves, W. E. Temperature Dependence of Phosphorescence Characteristics of Aromatic Hydrocarbons in Poly(methylmethacrylate). J. Chem. Phys. 1972, 56, 1309−1314. (39) Treboux, G.; Mizukami, J.; Yabe, M.; Nakamura, S. Blue Phosphorescent Iridium(III) Complex. A Reaction Path on the Triplet Potential Energy Surface. Chem. Lett. 2007, 36, 1344−1345. (40) Liu, C.; Bard, A. J. Chemical Redox Reactions Induced by Cryptoelectrons on a PMMA Surface. J. Am. Chem. Soc. 2009, 131, 6397−6401. (41) Bixby, T. J.; Cordones, A. A.; Leone, S. R. CdSe/ZnS Quantum Dot Intermittency in N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′biphenyl)-4,4′-diamine (TPD). Chem. Phys. Lett. 2012, 521, 7−11. (42) Duke, C. B.; Fabish, T. J. Charge-Induced Relaxation in Polymers. Phys. Rev. Lett. 1976, 37, 1075−1078. (43) Nicolai, H. T.; Kuik, M.; Wetzelaer, G. a. H.; de Boer, B.; Campbell, C.; Risko, C.; Brédas, J. L.; Blom, P. W. M. Unification of Trap-limited Electron Transport in Semiconducting Polymers. Nat. Mater. 2012, 11, 882−887. (44) Zhuo, J.-M.; Zhao, L.-H.; Png, R.-Q.; Wong, L.-Y.; Chia, P.-J.; Tang, J.-C.; Sivaramakrishnan, S.; Zhou, M.; Ou, E. C.-W.; Chua, S.-J.; et al. Direct Spectroscopic Evidence for a Photodoping Mechanism in Polythiophene and Poly(bithiophene-alt-thienothiophene) Organic Semiconductor Thin Films Involving Oxygen and Sorbed Moisture. Adv. Mater. 2009, 21, 4747−4752. (45) Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. 100% Phosphorescence Quantum Efficiency of Ir(III) Complexes in Organic Semiconductor Films. Appl. Phys. Lett. 2005, 86, 071104. (46) Rothe, C.; King, S.; Monkman, A. Long-Range Resonantly Enhanced Triplet Formation in Luminescent Polymers Doped with Iridium Complexes. Nat. Mater. 2006, 5, 463−466.

3725

dx.doi.org/10.1021/jp503831p | J. Phys. Chem. A 2014, 118, 3717−3725