Article pubs.acs.org/JPCC
Dual Luminescence, Interligand Decay, and Nonradiative Electronic Relaxation of Cyclometalated Iridium Complexes in Solution E. Pomarico,†,‡ M. Silatani,† F. Messina,†,§ O. Braem,† A. Cannizzo,†,∥ E. Barranoff,⊥ J. H. Klein,#,∇ C. Lambert,# and M. Chergui*,† Laboratoire de Spectroscopie Ultrarapide (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC, FSB, CH-1015 Lausanne, Switzerland ⊥ School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom # Institut für Organische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany †
S Supporting Information *
ABSTRACT: Femtosecond broadband photoluminescence studies are presented for Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), Ir(ppy)2(bpy)(PF6) (Ir3), Ir(ppz)3 (Ir4), and Ir(ppz)2dipy (Ir5) (where ppy = 2-phenylpyridine, pic = picolinate, bpy = 2,2′-bipyridine, ppz = 1-phenylpyrazole, and dipy = 5-phenyldipyrrinato) in solution. Upon 400-nm excitation of Ir1−Ir3, we observed a prompt population of the lowest MLCT states. The higher states decay on an ultrafast time scale (1 ns, attributed to population equilibration between the multiplet components of the 3MLCT state, intramolecular vibrational cooling, and radiative recombination through phosphorescence, respectively. In a later study,40 on the heteroleptic iridium complex bis[2(9,9-dibutylfluorenyl)-1-isoquinoline(acetylacetonate)] iridium(III), denoted as Ir(dbfliq)2acac, the same group investigated the internal conversion (IC) between three singlet MLCT states and found it to be 1 >1 >1 >1 >1 >1
>1 >1 >1 >1 >1 >1
80 fs (78%) and >300 ps (22%). The latter is probably associated with the 1.6-ns time constant determined by nanosecond-resolved measurements.
manifold and a slight blue shift of the DAS associated with the first two time constants, because higher-energy MLCT states were transiently populated and contributed to the early-time luminescence.41 In the case of Ir5, both the GF and SVD were optimal using a biexponential function, with time constants of ∼110 fs and ∼3.5 ps. The spectrum associated with the short-time component was found to have a maximum in the blue part of the spectrum (at 520 nm) and to show an asymmetric profile (Figure S5). The spectrum associated with the longer time constant appeared broader quite symmetric and was centered at about 580 nm. Finally, regarding Ir4, the multiexponential fit (convoluted with a Gaussian instrument response function) of its kinetic trace (Figure S4) yields a rise time of 140 ± 50 fs (−1), followed by a biexponential decay with time constants of 530 ±
3. DISCUSSION We first discuss complexes Ir1−Ir3, as the evolution of their luminescence shows a number of common trends. As already mentioned, the time-zero emission in Figure 4 is centered at 520−530 nm, with a shoulder on the blue wing at ∼500 nm and a red wing extending out to 700 nm. Evidently, there is more than one emissive component in these spectra, as reflected in Figures 7 and S6. In Ir1, the lowest 3MLCT state absorbs at 485 nm (20620 cm−1), and it is responsible for the steady-state phosphorescence in the 520−540-nm region, with a Stokes shift of ∼2100 cm−1. As already discussed in ref 41 where the same 16464
DOI: 10.1021/acs.jpcc.6b04896 J. Phys. Chem. C 2016, 120, 16459−16469
Article
The Journal of Physical Chemistry C
case of Ir3, this luminescence must be due to an upper state involving the ppy ligands. In refs 42, 44, and 45, the authors report a phosphorescence band of Ir1 at 1.5 K and upon excitation at 375 nm with a maximum at ∼529 nm and a shoulder at 555 nm. With an increase in temperature to 30 K, the maximum increases in intensity and shifts to shorter wavelengths of about 514 nm, whereas the shoulder on the red side barely moves. The authors attributed the emission maximum at 1.5 K to substate I, which is vibronically induced by Herzberg−Teller (HT) coupling, because the transition is strongly forbidden. This implies that the main peak (from v′ = 0 to v″ = 1) is red-shifted with respect to the electronic origin (0−0), but this was not borne out by our measurements. With increasing temperature, substates II and III become thermally populated, and their transition to the ground state is significantly more allowed than that of substate I. Thus, the corresponding emission is dominated by electronic 0−0 transitions and Franck−Condon- (FC-) active modes. Because the vibrational energies of HT- and FC-active modes are usually different, an emission band with a blue-shifted maximum grows in near 514 nm. Although this scenario is not discussed here, the temperature behavior of the shoulder in ref 45 suggests that it might correspond to another emitting state that is none of the three substates. The observed shift over long times that we report in the present work (Figure 5) cannot be rationalized in terms of the population of the three substates: Because the systems are at RT, all three substates would be promptly populated, and it is unphysical to imagine substate I being populated at early times and then the higher substates being populated at later times. Rather, our results can be rationalized by invoking a mechanism of dual luminescence, with the red component dominating at times on the order of hundreds of picoseconds whereas the bluer component, stemming from a lower-energy conformation of the potential surface, would be populated over infinite times. For this to occur, the upper state would need to be characterized by a double-minimum potential surface, as schematically depicted in Figure 8. The higher-energy minimum would emit in the red range because of the curvature of the ground-state potential surface at the equilibrium configuration of the upper state, whereas the lower-energy minimum would emit in the blue range, as it samples a deeper region of the ground-state surface. Dual emission mechanisms based on double-well excited states or the availability of other triplet states closer in energy to the lowest-lying 3MLCT have already been proposed to explain the RT emission of various Ru-54−56 and Ir-based complexes.57−60 Recently, Kleinschmidt et al. theoretically predicted that different minima can be found for the lowest-energy triplet state of Ir1, corresponding to different structural configurations.61 Our results seem to confirm these predictions. As already mentioned for Ir1 and Ir2, the emission involves the ppy moiety. The fact that it decays to another minimum over extended times (hundreds of picoseconds to infinite times) implies that the relaxation from the red-emitting minimum to the blue-emitting minimum is slow and, therefore, that the barrier from one configuration to the other is somewhat higher than kT. We now turn to the case of Ir3. Given the similarity of the early-time spectra of Ir1−Ir3 (Figure 3), we conclude that the ppy moiety is involved in the early-time emission of Ir3. However, at later times, the bpy moiety is responsible for the steady-state emission, which is populated on a picosecond time scale (Figure 3). Thus, energy relaxation seems to cause a ppy-
emission band was reported for 266-nm excitation, using the same Stokes shift, the 500-nm shoulder (reflected as DAS1 in Figures 7 and S6) most likely represents emission from the state absorbing at 460 nm (21700 cm−1), which is characterized by strong singlet−triplet mixing. By analogy, the same applies to Ir2, whereas for Ir3, an absorption in this region is not clearly visible but cannot be ruled out. Thus, a higher state transiently emits in these complexes, as already reported from short-lived higher-lying excited states of Re35 and Os38 complexes. The ensuing emission quickly shifts red in all three complexes and stabilizes at ∼550 nm in Ir1 and Ir2 and at ∼600 nm in Ir3, as also reflected in the DAS2 traces for Ir1 and Ir2 and the DAS3 trace for Ir3 (Figures S5 and S6). The maximum of DAS2 corresponds to the maximum of the t = 0 emission in Figure 4, and it is very close to the maximum of the steady-state emission for complexes Ir1 and Ir2. Therefore, this emission is clearly due to the promptly populated lowest vibrationally hot 3MLCT state. Unlike that from Ir1, emission from Ir2 and Ir3 undergoes a strong reduction (almost 50%) in the first 200 fs with no apparent rise in the red part of the spectrum (Figure 3). The corresponding DAS1 traces (Figures 7 and S6) are broader and red-shifted with respect to that of Ir1 and have a much larger amplitude (about 70%) with respect to DAS2 and DAS3. This strong loss of emission intensity in Ir2 and Ir3 can be explained by the fact that, upon cascading from higher- to lower-energy triplet states, the singlet character of these states decreases dramatically. This is suggested by the steep intensity change of the absorption spectrum at about 440−470 nm for these two complexes (Figure 4). As a consequence, a strong decrease of the radiative rate occurs. In previous studies, single-wavelength ultrafast luminescence studies attributed the early dynamics of Ir1, upon excitation into the 1MLCT manifold, to population equilibration between the substates of the lowest-energy 3MLCT state44,45 with a time constant of about 200 fs.34 Yersin and co-workers investigated the steady-state PL excitation (PLE) and PL spectra of Ir1 doped into CH2Cl2, poly(methyl methacrylate), and tetrahydrofuran at temperatures of 1.5 and 370 K.42,44,45 They clearly identified three substates (I, II, III) of the lowest triplet manifold, with a maximum splitting of 170 cm−1. The transition of the lowest substate (I) to the ground state has the most forbidden character with a lifetime of 116 μs at 4.2 K, whereas that of the next (II) has a lifetime of 6.4 μs, and that of the highest (III) has the most allowed character with a lifetime of 200 ns. We rule out the possibility that the 200-fs time scale might be due to equilibration among substates I−III,34 because the zero-field energy splitting between these substates is less than kT (1-ns component, which excludes emission from an intermediate state, and their evolutions look identical in CHCl3 and DMSO, a weakly polar and a strongly polar solvent, respectively, suggesting that they reflect vibrational relaxation. The long-lived luminescence (>1 ns, Figure 3) is represented by DAS3 in Figures 7 and S6, and it should be due to the steady-state emission for both Ir1 and Ir2. However, as noted above, the fact that it lies to the red of the steady-state emission (Figure 5) is quite remarkable. As no such shift occurs in the 16465
DOI: 10.1021/acs.jpcc.6b04896 J. Phys. Chem. C 2016, 120, 16459−16469
Article
The Journal of Physical Chemistry C
radiative rate of the transition. As shown in Figure S3, the Ir4 phosphorescence signal has a lifetime of 1.6 ns in RT DMSO, which is about 2 orders of magnitude shorter than that of Ir1 (170 ns), whereas at low temperatures, its lifetime becomes on the order of tens of microseconds.7 Therefore, the strong quenching at RT can be attributed to the thermal activation of a nonradiative channel to a lower metal-centered state, to the ground state, or to a thermally activated chemical reaction, once the steady-state 3MLCT state has formed. On this point, according to refs 7 and 12, the calculated energy barrier from the ground state to the nonradiative state for Ir4 has a value comparable to the Ir−N bond strength between the Ir ion and a ppz ligand. Therefore, at RT, thermal energy would be sufficient to cause the rupture of this bond because of the high energy content of the excited complex. A rotation of the ppz ligand would then stabilize the complex into a trigonal bipyramidal geometry, leading to a five-coordinate species with a triplet metal-centered (3MC) character.7 However, this would imply that putting more energy into the system would favor the reactive path and, therefore, lead to a shortened lifetime, even at low temperatures, because the laser would heat the sample. In ref 7, the excitation wavelengths are all below 400 nm, which gives quite a large excess of energy, yet the low-temperature measurements do yield long lifetimes. We therefore conclude that the shortened Ir4 phosphorescence lifetime is due to the activation of nonradiative channels to the ground state at RT that do not involve a chemical transformation, contrary to the hypothesis of ref 7. It is interesting to note that, in that work, the temperature dependence was investigated in tetrahydrofuran crystals, which have a melting point of 137 K, whereas the decay times change dramatically in the 130−140 K range. This might be a coincidence, but it suggests that the rigidity of the matrix plays an important role, either because of differential shifts of the vibronic levels of the triplet and ground states, which might open relaxation channels, or more likely, because of the release of steric hindrances on the lowest excited triplet state that open a nonradiative channel to the ground state or to lower metal-centered states. Such relaxation channels have been known for some time for ruthenium complexes,62−64 whereas deactivation to the ground state has been proposed for copper diimine complexes in solution.65 In the case of Ir5, the time-zero signal peaks at about 540 nm (18500 cm−1), with a Stokes shift of approximately 2300 cm−1 with respect to the maximum of the 1LC absorption band at 480 nm. As the absorption of the 3LC state is at lower energy (660 nm, Figure S2), the t = 0 luminescence signal is clearly due to fluorescence from the 1LC state. In addition, the spectral profile of the observed early-time luminescence is very similar to the steady-state fluorescence of boron dipyrrin complexes,26 where ISC into the 3LC state is inefficient. The analysis retrieved two time constants, with DAS1 being almost coincident with the t = 0 fluorescence (Figure S5). The decay time constant (∼110 fs) can be attributed to the depopulation of the 1LC state through ISC into the manifold of 3 LC states. In Figure 5, the red-most emission band is centered at about 580−600 nm. This is far from the steady-state phosphorescence (Figure 1), which implies that it is due to higher-lying triplet states that transiently emit with a lifetime of 3.5 ps. In the red part of the detected region (>660 nm), we did not observe a rise of the phosphorescent signal, because of the limit of our detection system (see Supporting Information) and the low radiative rate of the lowest triplet state of Ir5, which is over an order of magnitude lower than that of Ir1.7,27
Figure 8. Schematic representation of the mechanism of dual luminescence from the lowest-energy excited state (ES) to the ground state (GS) of complexes Ir1 and Ir2, with the low-energy emission dominating at early times on the order of hundreds of picoseconds and stemming from the upper minimum, whereas the higher-energy emission stems from a lower-energy conformation that is populated at infinite times. The emission energy is determined by the curvature of the GS surface with respect to the ES minima.
to-bpy decay. King et al. proposed that the large Stokes shift in Ir3 in liquid solutions occurs because of a large distortion of the bpy ligand in the corresponding 3MLCT excited state, induced by solvent relaxation.23 However, this explanation is not needed because the bpy-related emission band is at its characteristic energies, as observed in various bpy-containing TM complexes.52 The time scale for the ppy-to-bpy decay is on the order of 1.5 ps (Table 1), which might include some vibrational relaxation, as has been seen for Ir1 and Ir2. In summary, for Ir1−Ir3, the ultrafast response upon 400-nm excitation is characterized by the prompt population of lowerlying excited states, including the lowest 3MLCT state. The higher-energy emissions decay on a very short time scale (
