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Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes Coordinated with Perylenebisimide and Its π‑Radical Anion Ligands Wenbo Yang,†,¶ Brennan Ashwood,‡,¶ Jianzhang Zhao,*,† Wei Ji,† Daniel Escudero,*,§ Denis Jacquemin,*,§,∥ and Carlos E. Crespo-Hernández*,‡ †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian 116024, People’s Republic of China ‡ Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, Cleveland, Ohio 44106, United States § CEISAM UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France ∥ Institut Universitaire de France, 1, rue Descartes, 75005 Paris Cedex 5, France S Supporting Information *

ABSTRACT: Cyclometalated Ir(III) complexes showing strong absorption of visible light and a long-lived triplet state are of particular interest as photocatalysts and photosensitizers. Steady-state and femtosecond transient absorption spectroscopy, complemented with TD-DFT computations, are used to study the ultrafast intersystem crossing (ISC) dynamics of two perylenebisimide (PBI)-containing cyclometalated Ir(III) complexes (Ir−PBI and Ir−NPBI). The S1 → Tn ISC dynamics of these complexes is ultrafast, similar to the conventional Ir(III) complexes bearing a 1MLCT → 3MLCT transition. Much longer triplet decay lifetimes are observed in Ir−PBI (τT = 11.4 μs) and Ir−NPBI (τT = 7.9 μs) compared to conventional Ir(III) complexes (τT < 5 μs). This can be rationalized by the character of the lowest-energy triplet excited state (T1). In the case of the PBI-containing complexes, the T1 is of intraligand character (3IL), and the T1 → S0 spin−orbital coupling (SOC) is 3 cm−1. This is in contrast with conventional Ir(III) complexes, where the 3MLCT → S0 has SOCs of 490 cm−1. In the presence of a typical sacrificial electron donor, triethanolamine (TEOA), the coordinated PBI ligands are transformed into their radical form under an inert atmosphere, either with or without photoexcitation. We found that the triplet-state population is decreased with formation of the radical. The doublet excited state has a lifetime in the range of 4−7 ps.

1. INTRODUCTION Cyclometalated Ir(III) complexes have attracted much attention due to their applications in photocatalysis,1 such as hydrogen (H2) production,2−4 photoredox catalytic organic reactions, photodynamic therapy, electroluminescence,5−15 luminescent bioimaging, molecular sensors,16−20 luminescent oxygen sensing,21−24 triplet−triplet annihilation upconversion,25−29 as well as fundamental photochemical studies.30,31 To improve their properties, it is crucial to understand the underlying photophysics of the complexes. For instance, most foreseen applications require strong absorption of visible light, the population of the triplet manifold in high yield, and a longlived lowest-energy triplet state. However, conventional Ir(III) complexes weakly absorb in the visible region and exhibit rather short triplet-state lifetimes.32 Recently, a series of cyclometalated Ir(III) complexes showing strong absorption in the visible domain and a longlived triplet state were reported.25−28,33−42 Although the photophysical properties of the conventional cyclometalated © 2017 American Chemical Society

Ir(III) complexes, such as Ir(ppy)3 (ppy = 2-phenylpyridine), have been well-documented,43 the cyclometalated Ir(III) complexes with visible light-harvesting ligands are less studied. For instance, the excited-state dynamics and intersystem crossing (ISC) kinetics of Ir(III) complexes containing visible light-harvesting ligands, which populate an intraligand lowestenergy triplet state (3IL) rather than a traditional 3MLCT state, have rarely been investigated.39 It is known that ligand-based, that is, 1IL → 3IL, and ligand-to-ligand charge transfer, that is, 1 LLCT → 3LLCT, ISC can occur with a smaller heavy atom effect than the conventional 1MLCT → 3MLCT ISC.44 Conventional Ir(III) complexes show ultrafast ISC, for instance, both Ir(piq)3 (piq = 1-phenylisoquinoline) and Ir(ppy)3 show ISC within 70 fs.45,46 Therefore, it is interesting Received: July 10, 2017 Revised: August 25, 2017 Published: August 29, 2017 21184

DOI: 10.1021/acs.jpcc.7b06714 J. Phys. Chem. C 2017, 121, 21184−21198

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The Journal of Physical Chemistry C Scheme 1. Structures of Ir−PBI and Ir−NPBI and Corresponding PBI and NPBI Ligands

in the Ir(III) complexes in the presence of an electron donor. This is also inspired by our previous study of the steady-state and long-lived triplet-state photophysical properties of the Ir(III) complexes.37 The complexes show strong absorption of visible light and a long-lived triplet state (τT = 11.4 μs for Ir− PBI, τT = 7.9 μs for Ir−NPBI), which are the signatures of ligand-based photophysics. Here with femtosecond transient absorption spectroscopy, we show that the ISC occurs within hundreds of femtoseconds. Moreover, we report the formation of the PBI ligands in their radical form upon addition of a typical sacrificial electron donor, triethanolamine (TEOA). The formation of the radical ligand exerts a significant influence on the photophysical properties of the complexes.

to study the kinetics of the ISC processes in Ir(III) complexes containing visible light-absorbing ligands. Heavy atoms significantly affect the triplet-state lifetimes of perylenebisimide (PBI) derivatives. For instance, when the ethynyl PBI is coordinated to a Pt(II) center, a very short triplet-state lifetime of 246 ns is obtained.47,48 Previously, we attached the PBI moiety to C60, which is a spin converter, thus without any heavy atom effect, and observed an exceptionally long lived triplet state for the PBI moiety (105 μs).38 On the other hand, the ISC lifetime of the PBI acetylide ligand coordinated to Pt(II) center was determined as 1.2 ps.39 A cyclometalated Ir(III) complex with a dangled PBI moiety on the coordinated ligand was synthesized, but its ISC kinetics was not measured.49 To our knowledge, the impact of the heavy atom effect on the ISC kinetics of the PBI moiety in cyclometalated Ir(III) complexes has not been reported yet. Moreover, some typical visible light-harvesting ligands, such as PBI, have high electron-accepting ability; hence, the properties of the complexes in the presence of electron donor need to be elucidated. Such a system is very common for photocatalysis, in which a sacrificial electron donor is often added. It is known that PBI can be transformed into a radical anion in the presence of reductants. For instance, PBI radicals were prepared by various methods, for example, by dangling a phenoxide radical on the PBI core,50 by obtaining a zwitterionic structure,51 or by electrochemical39,52,53 or chemical54,55 reductions, but the PBI chromophores coordinated to a transition metal atom, such as Ir(III), have not been converted into the radical form previously. Inspired by the aforementioned challenges, we study in the present work the ISC process of two PBI-containing Ir(III) complexes (Scheme 1) and the formation of radical PBI ligands

2. RESULTS AND DISCUSSION 2.1. Steady-State UV−Vis Absorption Spectra. Previously, we reported the steady-state and long-lived excitedstate properties of the Ir−PBI and Ir−NPBI complexes in acetonitrile (ACN).37,38 They both show strong absorption of visible light and population of a long-lived triplet excited state. As stated above, these features differ from those of most Ir(III) complexes, which show weak absorption in the visible range and a relatively short-lived triplet state (usually 1) due to the large energy gap between the S1 and T1 states (ΔES1/T1 = 0.82 eV)37,66−68 as well as the fact that the T3 and T2 states are nearly isoenergetic with the S1 state according to TD-DFT calculations (discussed

Figure 5. Transient absorption spectra of Ir−NPBI in ACN upon excitation at (a,b) 383, (c,d) 660, and (e−g) 725 nm during the initial 3 ns time window. A portion of the spectral window is omitted at each excitation wavelength due to Rayleigh scattering from the pump beam at each excitation wavelength. 21188


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The Journal of Physical Chemistry C

Figure 6. Representative excited-state decay traces of Ir−NPBI in ACN following excitation at (a) 383, (b) 660, and (c) 725 nm. Traces following 383 and 660 nm excitation were globally fit across the entire spectral probe window to a two-component sequential kinetic model containing an offset, while a two-component sequential model was used to fit the kinetics following 725 nm excitation. Decay-associated spectra of Ir−NPBI from global analysis are shown for (d) 383, (e) 660, and (f) 725 nm excitation. The residual spectrum (blue) in panels (e) and (f) were magnified by 5 and 10, respectively. The transition from the red to green spectra in (f) is assigned to the triplet−triplet self-quenching from an aggregate complex.

state dynamics (Figure 6c). Initial excitation generates absorption bands centered at 363, 475, and 530 nm and negative-amplitudes bands at 412 and >650 nm. The latter bands are assigned to ground-state depopulation (Figure 1a). Within 1 ps (τ1), the band at 475 nm slightly decays and the band at 530 nm narrows (Figure 5e). This is followed by a decay of the 363, 475, and 530 nm bands with apparent ground-state repopulation and blue shifting of the band at 412 to 404 nm (Figure 5f) over tens of picoseconds (τ2). The negative-amplitude signal at around 650 nm rises to form an absorption band, indicating that absorption from an excitedstate species overlaps with the ground-state depopulation signal. Then, the entire transient spectrum decays with a lifetime of 200 ps (τ3, Figure 5g). Only a very small yield of the long-lived transient species can be detected in the transient absorption spectra after a 1.2 ns time delay, contrary to the cases when excitation was performed at 383 and 660 nm. However, magnification of the residual decay-associated spectrum following 725 nm excitation reveals that in fact a very small amount of the long-lived species is present (Figure 6f). The excited-state relaxation dynamics of Ir−NPBI are dependent on the excitation energy. On the basis of the relative offsets of the transient absorption spectra (Figure 6d− f), the yield for population of the long-lived (>3 ns) excitedstate species increases with excitation energy. The offset spectrum is identical to that previously assigned to the T1 state of Ir−NPBI using nanosecond transient absorption spectroscopy, suggesting that this offset corresponds to the T1 state spectrum.37 Measurements following 383 and 660 nm excitation were made using approximately the same groundstate absorbance at the excitation wavelength and the same excitation intensity; thus, the relative amplitudes of the offset spectrum indicate that the triplet manifold is being populated in greater yield following 383 nm excitation. The significantly

Table 2. Kinetic Lifetimes Obtained from Global Analysis of Ir−NPBI Transient Absorption Spectra conditions 725 660 383 383

nm, nm, nm, nm,

ACN, 0.08 mM ACN, 0.01 mM ACN, 0.01 mM MeOH, 0.01 mM

τ1 (ps)

τ2 (ps)

± ± ± ±

33 ± 1b − − −

0.90 0.90 0.90 0.90

0.05a 0.06a 0.04a 0.04a

τ3 (ps) 203 190 209 77

± ± ± ±

4c 3c 4c 3c

τ1 is assigned to intramolecular vibrational relaxation following all excitation wavelengths. bτ2 following 725 nm excitation is tentatively assigned to triplet self-quenching and/or triplet−triplet annihilation. c τ3 following 725 nm excitation is assigned to charge recombination to repopulate the ground state in both solvents. a

assigned to ground-state depopulation (Figure 1a). Within 1 ps (τ1), the absorption band red shifts to 530 nm (Figure 5a). Then, the bands decay significantly over hundreds of picoseconds (τ2), while the 660 nm maximum blue shifts (Figure 5b). Somewhat different spectral changes are observed following excitation at 660 nm. An initial rise (Figure 5c) forms maxima at 362, 403, 478, and 528 nm in the transient absorption spectra. The negative-amplitude band at 400 nm is assigned to ground-state depopulation, in agreement with the steady-state absorption spectrum (Figure 1a). Within the next 1 ps (τ1), the maximum at 478 nm slightly decays and the band at 528 nm red shifts (Figure 5c). This is followed by decay across the entire probe window (τ2), resulting in a long-lived transient species (Figure 5d), which is identical to, but lower in amplitude than, the transient species observed following excitation at 383 nm. The spectral changes of Ir−NPBI in ACN (Figure 5e−g) are considerably different following excitation at 725 nm than those at excitation wavelengths of 383 and 660 nm and require a three-component sequential model to globally fit the excited21189


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Figure 7. (a,b) Transient absorption spectra of the Ir−PBI radical ([Ir−PBI]−•) following 725 nm excitation in N2-saturated ACN. (c) Representative kinetic decay traces of the Ir−PBI radical ([Ir−PBI]−•) fit to a two-component sequential model. (d) Decay-associated spectra extracted from global analysis of the excited-state kinetics using a two-component sequential model.

To the best of our knowledge, this is the first report on the ISC dynamics of Ir(III) complexes that show strong absorption of visible light and populate a 3IL state. Our results show that despite the bulky ligands and T1 state being of 3IL character, both Ir−PBI and Ir−NPBI may undergo ultrafast ISC to populate the triplet manifold. Ir−PBI undergoes ISC slower than that observed for some conventional Ir(III) complexes containing small ligands. For instance, Ir(piq)3 shows ISC with 70 fs (piq = 1-phenylisoquinoline).45 Previously, some cyclometalated Ir(III) complexes containing a Bodipy chromophore were reported, but the ISC kinetics were not studied.27,33,34,71 However, ISC in the present Ir(III) complexes, which populate the 3IL state, is still much faster than that in some metal-free PBI derivatives, which have ISC lifetimes of 30 ps,66 or those in PBI cofacial dimers, for which the ISC takes more than 1 ns.72 Ir−PBI undergoes ISC on time scales comparable to the 0.88 ps lifetime previously reported for thionated PBI derivatives.73 In binuclear trans-bis(phosphine) Pt(II) bisacetylide complexes, with the heavy atom effect of the Pt(II) ion, the ISC of the coordinated PBI takes 1.2 ps.39 2.3. Ultrafast Decay in Ir−PBI Complexes with Radical PBI Ligands: Femtosecond Transient Absorption Spectroscopy. It was reported that the doublet excited state of the PBI−phenoxide radical is short-lived (1.9−11 ps), but the radical is not localized within the π network of the PBI ligand.50 A longer lifetime of 145 ± 15 ps was determined for the D1 state of a PBI π radical prepared by electrochemical reduction.52 Here, we report on the excited-state dynamics of Ir−PBI and Ir−NPBI π radicals in ACN using femtosecond transient absorption spectroscopy. The π radical of Ir−PBI ([Ir−PBI]−•) was selectively excited using an excitation wavelength of 725 nm (Figure 7a,b). Following initial excitation, excited-state absorption bands centered at 456 and 614 nm and ground-state depopulation bands at 405, 573, and >690 nm are observed. The entire excited-state decay process can be globally fit satisfactorily to a two-component sequential kinetic model (Figure 7d and Table 3). The first lifetime (τ1) corresponds to a decay and blue shift

smaller triplet absorption upon excitation at 725 nm is due to triplet self-quenching and/or triplet−triplet annihilation, which is proposed to result from aggregation due to the 8-fold greater concentration of Ir−NPBI used for this measurement. According to TD-DFT vertical energies,37 excitation with 383 nm radiation populates a high-lying singlet state (S11 or S17 with oscillator strengths of 0.38 or 0.15, respectively), while 660 and 725 nm excitations are predicted to populate the S1 state of Ir−NPBI that has an oscillator strength of 0.45. As recently shown for nonmetal PBI derivatives,65 excitation to higherenergy singlet states may provide access to additional efficient ISC pathways, which may explain the higher triplet yield upon 383 nm excitation. With the above discussion, it is now possible to assign the kinetic lifetimes extracted from the transient absorption spectra of Ir−NPBI. ISC and charge separation are proposed to be competitive pathways that occur within our time resolution of ∼200 fs. Then, following each excitation wavelength, we assign τ1 to intramolecular vibrational relaxation in the triplet manifold. The lifetime for this at each excitation wavelength is within experimental error; therefore, a common lifetime is reported (Table 2). τ3 in Table 2 corresponds to a decay of excited-state absorption bands and ground-state repopulation across the probe window within 200 ps, which we assign to charge recombination. Previous TD-DFT calculations for Ir− NPBI showed that the S1 state, as well as higher-energy singlet states, have significant intramolecular charge transfer (ILCT) character in the Franck−Condon region,37 suggesting that charge separation may compete with ISC to the triplet manifold on an ultrafast time scale, as proposed above. The assignment of charge recombination is further supported by the solvent dependence of τ3 following 383 nm excitation in which the process occurs nearly three times faster in MeOH than ACN (Table 2). Charge-separated species are expected to become destabilized in less polar solvents, leading to a faster recombination time. τ2 in Table 2 is assigned to either triplet self-quenching and/or triplet−triplet annihilation upon 725 nm excitation, as explained above. 21190


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wavelength; therefore, the excited-state dynamics of [Ir− NPBI]−• + Ir−NPBI were monitored back-to-back with just Ir−NPBI (Figures 5e−g and 8a−c). In both cases, the full excited-state dynamics were globally fit to a three-component sequential model (Figure 8e and Table 3). Bands at 473 and 570 nm are formed within the cross-correlation of the pump and probe beams. This is followed by the decay of each excitedstate absorption band within ∼150 fs (τ1) to populate an excited-state band similar to Ir−NPBI (see Figure S12a−f in the Supporting Information for direct comparison). The following 6 ps lifetime (τ2) shows an identical spectral evolution to τ2 observed in Ir−NPBI following 725 nm excitation. Finally, the remaining excited-state population decays within 200 ps (τ3), which can be assigned to charge recombination in Ir−NPBI (discussed in section 2.2). The T1 state is not detected when studying the mixture of Ir−NPBI and [Ir−NPBI]−•, and this is due to the smaller concentration of Ir−NPBI in solution. Just as for [Ir−PBI]−•, excitation of [Ir−NPBI]−• with 725 nm radiation is expected to populate the high-lying D2 state (Table 5). Therefore, τ1 may be assigned to the excited-state decay of [Ir−NPBI]−• through ultrafast charge recombination or IC to repopulate the ground state. While the spectral evolution suggests that identical processes occur during τ2 for [Ir−NPBI]−• + Ir−NPBI and Ir−NPBI, the significant difference in the lifetime requires further explanation. Similar to [Ir−PBI]−•, we suggest that [Ir− NPBI]−• exhibits a biexponential decay in which the second process dominates τ2. The fact that this time constant obtained is similar to τ2 for [Ir−PBI]−• and that 725 nm excitation is expected to populate the D2 state further supports that [Ir− PBI]−• and [Ir−NPBI]−• follow similar relaxation dynamics. 2.4. Effect of Formation of a Radical Anion Ligand on the Triplet-State Property of the Ir(III) Complexes. The

Table 3. Lifetimes Obtained from Global Analysis of [Ir− NPBI]−• and [Ir−NPBI]−• + Ir−NPBI in 0.08 mM ACN Following 725 nm Excitation compound

τ1 (fs)a

τ2 (ps)b

τ3 (ps)c

[Ir−PBI]−• [Ir−NPBI]−• + Ir−NPBI

280 ± 50 150 ± 50

3.7 ± 0.6 6.9 ± 0.6

n/a 205 ± 8

a c

Assigned to IC from Dn to D0. bAssigned to IC from D1 to D0. Assigned to charge recombination in Ir−NPBI.

of the excited-state absorption bands and partial repopulation of the ground state within hundreds of femtoseconds. This is followed by a few picosecond uniform decay of the excited-state absorption signal (τ2). The biexponential decay of the excited state found in [Ir− PBI]−• has previously been observed for radicals in large π systems.74 Some of these π radicals decayed with a subpicosecond and picosecond component, as observed for [Ir− PBI]−• herein. However, the authors provided no discussion regarding the origin of this ultrafast decay or biexponential behavior. Vertical excitation energies of the excited doublet states for [Ir−PBI]−• at the TD-DFT level of theory (Table 5) indicate that excitation at 725 nm leads to population of the second excited D2 state. Therefore, it is possible that IC to the ground state occurs quickly from a D2 state (τ1), while the remaining population decays from the lower-lying D1 state within τ2. The vertical excitation energies also predict that both D1 and D2 states have majority charge-transfer character, suggesting that charge recombination could play an important role in the biexponential decay to the ground state. The ultrafast dynamics of a mixture of Ir−NPBI and [Ir− NPBI]−• were also studied following 725 nm excitation. Both the radical species and neutral compound absorb at this

Figure 8. Transient absorption spectra for [Ir−NPBI]−• + Ir−NPBI (a−c) as well as representative kinetic traces (d) and decay-associated spectra (e) following 725 nm excitation in ACN. Sharp positive signals from 575 to 650 nm in (a) and (b) result from stimulated anti-Raman emissions from the solvent and are observed within the cross-correlation of the pump and probe beams. Representative traces were globally fit to a three-lifetime sequential kinetic model. 21191


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S13a−d). We confirmed that the transient signal observed in the nanosecond transient absorption spectra is due to the triplet state by studying the quenched decay in aerated solution (see the Supporting Information, Figure S14a−d). 2.5. Theoretical Computations of the Ir−PBI Complexes: Rationalization of the (i) Ultrafast ISC, (ii) Emissive Properties, and (iii) Doublet Excited States. Computational studies have been proved successful to unveil the photodeactivation dynamics of Ir(III) complexes.6 Herein, we focus on the Ir−PBI complex as a representative example of the PBI-containing complexes. For comparative purposes, we also study the related Ir(ppy)2(bpy) complex, which exhibits conventional cyclometalated Ir(III) complex photophysics. Table 4 reports the vertical energies of the lowest-lying singlet and triplet excited states for Ir−PBI calculated with the TDDFT method. The orbitals involved in these excited states are plotted in Figure 10a. S2 and S4 are the two low-lying bright states (which are of mixed 1IL/LLCT character), and they are responsible for the experimental absorption band in the 500− 600 nm domain (see Figure 1a). First we aim to rationalize the very different triplet decay lifetimes for Ir−PBI and for its related Ir(ppy)2(bpy) complex. One of the main factors controlling the triplet excited-state lifetimes is the magnitude of the SOC between the lowest triplet excited state (T1) and the ground state. We assume here that these complexes obey Kasha’s rule such that the well of the T1 will be always populated after ISC and IC processes.76 For Ir(ppy)2(bpy), it is well described from both experimental77 and computational78 viewpoints that the lowest-lying singlet and triplet excited states are mainly of MLCT character. Herein, we have optimized the geometry of T1 for both complexes and computed the T1 → S0 SOC values at these geometries (see Computational Details). For Ir(ppy)2(bpy), the T1 → S0 SOC amounts to 490 cm−1, hence clearly indicating a T1 state with pronounced 3MLCT character. In comparison, the T1 → S0 SOC of Ir(piq)(ppy)2 (piq = 1-phenylisoquinoline) was previously reported as 391 cm−1.79 In contrast, for Ir−PBI, the T1 → S0 SOC is 3 cm−1, indicating that the T1 has reduced metal contribution. Indeed, the spin density distribution at the T1 geometry is in accordance with a 3IL state located on the PBI ligand (see Figure 10b). Therefore, ISC to the ground state is significantly less favorable for Ir−PBI, and consequently, much longer triplet excited-states lifetimes are observed. We next rationalize why Ir−PBI undergoes ultrafast ISC to the triplet manifold even though ISC to the ground state occurs

changes of the triplet state of the Ir(III) complexes upon formation of the radical ligands was studied (Figure 9a−d).

Figure 9. Nanosecond transient absorption spectra of the compounds (a) Ir−PBI and (c) Ir−PBI/TEOA (1:10) and decay curves of (b) Ir−PBI and (d) Ir−PBI/TEOA (1:10) at 540 nm. The radical was produced by irradiation of a 532 nm laser. λex = 532 nm (pulsed laser); [Ir−PBI] = 1.0 × 10−5 M; in deaerated ACN; 20 °C.

First, the nanosecond transient absorption spectra of Ir−PBI were recorded and are similar to those previously observed.37,38 Bleaching bands at 384 and 544 nm were observed, which are due to the ground-state depletion. An excited-state absorption band in the region of 400−700 nm was observed. The tripletstate lifetime was determined as 11.4 μs, which is different from that of the previous report (τT = 22.3 μs); this is not unexpected because the triplet-state lifetime is dependent on the excitation power of the laser.75 Upon addition of 10 equiv of TEOA and photoirradiation to transform the PBI ligands to their radical form, the difference absorption spectra are identical to the untreated Ir−PBI spectra (Figure 9c). However, for the triplet state quenched by a radical, the lifetime decreased slightly from 11.4 to 9.5 μs. A similar study was performed for Ir−NPBI. No new transient band was observed (see the Supporting Information, Figure

Table 4. Lowest-Lying Vertical (at S0 Geometry) Singlet and Triplet Electronic Transition Energies (eV/nm) and Oscillator Strengths (in Parentheses) of Ir−PBI at the TD-B3LYP/6-31G(d) Level of Theory and Photophysically Relevant Spin−Orbit Matrix Elements (SOCMEs in cm−1)a statesb Ir−PBI

S1 S2 S3 S4 T1 T2 T3 T4 T5

energy ( fc) 1.91/650 (0.000) 2.01/617 (0.413) 2.46/504 (0.0134) 2.48/500 (0.296) 1.17/1057 (0.000) 1.89/656 (0.000) 1.99/623 (0.000) 2.42/513 (0.000) 2.45/505 (0.000)

characterd MLCT/LLCT(H−1 → IL/LLCT(H → L) 1 MLCT/LLCT(H−1 → 1 IL/LLCT(H → L+1) 3 IL/LLCT (H → L) 3 MLCT/LLCT(H−1 → 3 IL/LLCT(H → L+1) 3 MLCT/IL(H−5 → L) 1 MLCT/LLCT(H−1 → 1

SOCMEs Tn/Sme (x-; y-; z-components) L)

1

L+1)

L)

L+1)

− − − − T1/S0: (−1.2; −0.2; 2.6) T2/S2: (−66.2; −64.4; 37.3) − T4/S4: (−21.7; −32.2; 26.3) −

a

See also the orbitals in Figure 10. bOnly the selected low-lying excited states are presented. cOscillator strengths. dOnly the main configurations are presented. eValues obtained at the QR-TD-DFT/6-31G(d) level of theory. 21192


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Figure 10. (a) Kohn−Sham (B3LYP/6-31G(d)) orbitals involved in the lowest-lying excited states of Ir−PBI. (b) Spin density distribution (B3LYP/6-31G(d)) at the optimized geometry of T1 of Ir−PBI. Alkyl chains are simplified as methyl groups.

Figure 11. Spin distribution of (a) [Ir−PBI]−• and (b) [Ir−NPBI]−• at the optimized doublet-state geometries. Alkyl chains are simplified as methyl groups.

Table 5. Lowest-Lying Vertical (at D0 Geometry) Doublet Electronic Transition Energies (eV/nm), Oscillator Strengths (in Parentheses), and Square of the Spin Angular Momentum of [Ir−NPBI]−• at the TD-UB3LYP/LANL2DZ Level of Theorya −•

[Ir−NPBI]

statesb

energy ( fc)

characterd

S2e

D1 D2 D4 D9

0.93/1330 (0.1946) 1.37/904 (0.1002) 1.82/681 (0.4877) 2.25/550 (0.1695)

LLCT(H α → L α) LLCT/ILCT(H β → L β) ILCT(H α → L+4 α) LLCT(H−1 α → L α)

0.819 0.783 0.944 0.834

a

See also the orbitals in Figure S15. bOnly the selected low-lying excited states are presented. cOscillator strengths. dOnly the main configurations are presented. eThe square of the spin angular momentum.

on microsecond time scales. The ISC rate between the n-singlet (Sn) and m-triplet (Tm) excited states (kISC) with the Fermi Golden approximation obeys kISC =

2π ⟨Sn|H̑ SO|Tm⟩2 × [FCWD] ℏ

comparing the relative energy levels, several ISC channels appear, for example, the S2 → T2 and the S4 → T4 ISC channels. Some of these states possess non-negligible participation of the Ir(III) center. These ISCs are characterized by SOCs of up to 66 and 32 cm−1. Having in mind these values, we can easily rationalize why Ir−PBI still shows ultrafast ISC decay, which is in agreement with the experimental observations. Population of these higher-lying triplet states is followed by IC to the lowest-energy triplet state. Not surprisingly, for the Ir(ppy)2(bpy) complex, the SOCs values for several Tn → S0 ISC decays amount to up to 400−500 cm−1. Finally, we assessed if the near-IR absorption of the complexes with the PBI radical ligands can be modeled computationally. Toward this end, we computed vertical energies for the D0 → Dn transitions at the TD-DFT level of theory. Indeed, the bands are greatly shifted to the near-IR. Hence, the low-lying doublet states appear at 1639 nm (D1) and 873 nm (D4). The spin density surfaces of the Ir(III) complexes with radical ligands at the D0 state (ground state)

(1)

where the bracket term stands for their associated SOCs and FCWD accounts for the Franck−Condon weighted density of states. Computing all parameters in eq 1 becomes rapidly prohibitive for large compounds. Instead, we can qualitatively estimate the efficiency of ISC processes. It is well described that mainly two factors, that is, substantial SOC values and small Tm−Sn energy gaps, govern the efficiency of the ISC processes.80 We therefore determined theoretical estimates of SOCs between the manifold of higher-lying singlet and triplet excited states for both Ir−PBI and Ir(ppy)2(bpy) (see details in section 3.4). For Ir−PBI, upon irradiation with the longer wavelength, we can directly and indirectly populate a manifold of singlet excited states (S1−4; see Table 4). These states interact with the manifold of triplets (see T1−5 in Table 4). By 21193


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study the excited-state dynamics of Ir−PBI, Ir−NPBI, [Ir− PBI]−•, and [Ir−NPBI]−•. The laser and spectrometer setup is described in significant detail in previous publications.81,82 In brief, a Ti:sapphire oscillator (Coherent Vitesse) seeds a chirped pulse regenerative amplifier (Coherent Libra-HE). The amplifier generates 4 mJ, 100 fs pulses at 800 nm with a 1 kHz repetition rate, which are used to pump an optical parametric amplifier (OPA, TOPAS, Quantronix/Light Conversion). A small fraction of the pump beam is split off prior to the OPA and focused through a constantly translating 2 mm CaF2 crystal to generate a broad-band probe pulse (320−710 nm). The OPA was tuned to excitation wavelengths of 383 and 530 nm for Ir−PBI, 383, 660, and 725 nm for Ir−NPBI, and 725 nm for [Ir−PBI]−• and [Ir−NPBI]−•. Excitation pulses were set to an intensity of 1 μJ at the sample using a neutral density optical filter in order to minimize cross-phase modulation effects as well as sample degradation.83 Stimulated Raman emission bands from the solvent were observed that obscured data analysis during the cross-correlation of the pump and probe beams. For this reason, the time zero in the femtosecond transient absorption spectra reported in this work was defined at the end of the pump−probe cross-correlation. The setup employs a mechanical delay stage to adjust the time that it takes the probe pulse to reach the sample up to a maximum delay of 3 ns. The broad-band probe pulses are split into two fractions. Both beams pass through the sample (2 mm path length), but only one probes the dynamics, while the other serves to correct for changes in the white light continuum throughout the experiment. The pulses are focused into optical fibers leading to CMOS detection units. Generation of the reference signal comes via a chopper wheel, which blocks every other pump pulse, providing an alternating sequence of spectra with and without sample excitation and thus providing ΔA transient absorption data. During the experiments, solutions were continuously stirred using a Teflon stir bar. Samples of Ir− PBI and Ir−NPBI were prepared at a concentration of 0.01 mM in ACN when exciting at 383, 530, and 660 nm and in methanol at 383 nm excitation. Concentrations of ∼0.08 mM were used for experiments performed with 725 nm excitation. Degradation of Ir−PBI and Ir−NPBI was monitored as the change in absorbance at 545 and 660 nm, respectively. Samples were replaced with fresh solutions before the change in absorbance reached 5%. Homemade software (Labview, National Instruments, Inc.) was used for data collection. Prior to analysis, the data was corrected for group velocity dispersion of the probe pulse. Approximately 50 kinetic traces were extracted from the multidimensional transient absorption data for global and target analysis with Igor Pro 6.36.84 The instrument response function was held at 200 fs in all cases, as determined from the twophoton coherent signal from neat methanol (IRF of 200 ± 50 fs).85 3.4. Computational Details. All calculations are based on DFT. The geometries of the singlet ground state (S0), doublet (D0), and the lowest triplet excited state (T1) were optimized for complexes Ir−PBI and Ir(ppy)2(bpy) using the B3LYP86,87 functional in combination with the 6-31G(d) atomic basis set for all light atoms. Relativistic effects were included for the Ir atom by using the ECP-60-mwb Stuttgart/Dresden pseudopotential.88 The nature of the stationary points was confirmed by computing the Hessian at the same level of theory. Gas-phase TD-B3LYP vertical singlet and triplet excitation energies were determined on the S0 geometry, while the doublet excited states

were also studied (Figure 11a,b). It was found that the spin density surfaces are mainly localized on the PBI ligands, but delocalization to the bpy moiety was also found. Likewise to the Ir-PBI complex, we also studied the absorption of [Ir−NPBI]−•, the main absorption bands are predicted to be 1330, 904, 681, 550 nm, etc. (Table 5). These values are in good agreement with the experimental results of 900, 680, and 500 nm (Figure 2c). By examination of the frontier molecular orbitals of the open-shell compound, the transitions can be assigned as NPBI moiety-localized transition, such as the 680 nm bands (H α → L+4 α), 900 nm bands (H β → L β), or as intraligand charge-transfer transition at 550 nm (H−1 α → L α) (Figure S15 and Table 5). The result for [Ir− PBI]−• is given in Figure S16 and Table S1).

3. EXPERIMENTAL SECTION 3.1. General Methods. All of the chemicals used in synthesis are analytically pure and were used as received. The preparation and molecular structural characterization of the compounds were reported previously.37 NMR spectra were recorded by an OXFORD NMR 400 MHz spectrometer with CDCl3 as the solvent and tetramethylsilane (TMS) as the standard at 0.00 ppm. HRMS was accomplished with a matrixassisted laser desorption/ionization (MALDI) micro-MALDI TOF mass spectrometer (Waters, U.K.). Absorption spectra were recorded on HP 8453 spectrophotometer (Agilent, U.S.). Compound PBI. 1H NMR (400 MHz, CDCl3): δ 10.18 (d, 1H, J = 8.1 Hz), 9.00 (s, 1H), 8.84−8.66 (m, 9H), 8.17−8.09 (m, 2H), 7.58 (s, 1H), 4.20−4.15 (m, 4H), 2.00−1.97 (m, 2H), 1.45−1.36 (m, 16H), 0.98−0.90 (m, 12H). MALDI-TOFHRMS ([C52H48N4O4 + H]+): calcd m/z = 793.3754, found m/z = 793.3753. Compound NPBI. 1H NMR (400 MHz, CDCl3): δ 9.54 (s, 1H), 8.91 (s, 1H), 8.48−8.18 (m, 7H), 7.70−7.44 (m, 3H), 6.70 (s, 1H), 4.14−3.62 (m, 7H), 1.67−1.64 (m, 4H), 1.41− 1.26 (m, 18H), 0.98−0.87 (m, 15H). MALDI-TOF-HRMS ([C56H57N5O4 + H]+): calcd m/z = 864.4489, found m/z = 864.4465. Compound Ir−PBI. 1H NMR (400 MHz, CDCl3): δ 9.82 (d, 2H, J = 37.3 Hz), 9.40 (s, 1H), 8.59−8.47 (m, 4H), 8.33− 7.90 (m, 9H), 7.73−7.69 (m, 3H), 7.54−7.45 (m, 3H), 7.08− 6.81 (m, 6H), 6.44 (s, 1H), 6.30 (s, 1H), 4.19−4.03 (m, 4H), 2.07−1.87 (m, 2H), 1.47−1.35 (m, 16H), 1.00−0.90 (m, 12H). MALDI-TOF-HRMS ([C 74 H 64 N 6 O 4 Ir] + ): calcd m/z = 1293.4618, found m/z = 1293.4628. Compound Ir−NPBI. 1H NMR (400 MHz, CDCl3): δ 9.85 (s, 2H), 9.33 (d, 1H, J = 6.7 Hz), 8.84 (d, 1H, J = 6.9 Hz), 8.68−8.57 (m, 2H), 8.30−8.10 (m, 4H), 7.99−7.92 (m, 6H), 7.78−7.69 (m, 3H), 7.56−7.46 (m, 2H), 7.08−6.90 (m, 6H), 6.38−6.30 (m, 2H), 4.18−4.09 (m, 4H), 3.54−3.50 (m, 1H), 1.84−1.81 (m, 2H), 1.59−1.53 (m, 2H), 1.42−1.32 (m, 20H), 0.97−0.90 (m, 15H). MALDI-TOF-HRMS ([C78H73N7O4Ir]+): calcd m/z = 1364.5353, found m/z = 1364.5345. 3.2. Nanosecond Transient Absorption. The nanosecond transient absorption spectroscopy was studied with a LP920 laser flash photolysis spectrometer (Edinburgh Instruments, Livingston, U.K.). The samples in laser flash photolysis experiments were purged with N2 for 15 min before measurements. The signal was digitized with a Tektronix TDS 3012B oscilloscope. 3.3. Femtosecond Transient Absorption. Femtosecond broad-band transient absorption spectroscopy was employed to 21194



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were obtained on the D0 optimized geometry. T1 → S0 and Tm → Sn SOCs were computed using the linear and quadratic response TD-DFT approaches,89,90 respectively, as implemented in the Dalton program.91 The T1 → S0 SOCs were computed at the T1 optimized geometries, while the Tm → Sn SOCs were calculated at the S0 optimized geometry. For the SOC calculations, the B3LYP functional in combination with the 6-31G and raf-r atomic basis set was used for light atoms and Ir, respectively. Scalar relativistic effects were included with the Douglas−Kroll−Hess second-order (DKH2) Hamiltonian.92,93 The SOC operator applied in all of our calculations makes use of the effective single-electron approximation.

ACKNOWLEDGMENTS We thank the NSFC (21473020, 21673031, 21273028, 21421005, and 21603021), Program for Changjiang Scholars and Innovative Research Team in University [IRT_13R06], and Dalian University of Technology (DUT16TD25, DUT15ZD224, DUT2016TB12) for financial support. D.E. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-Curie Grant Agreement No 700961. D.J. and D.E. are indebted to the GDRI-RFCCT for financial support in the framework of the Zhiwu program (TTA-UP project). Calculations were performed using the resources of the GENCI-CINES/IDRIS, those of the Centre de Calcul Intensif des Pays de Loire, as well as those of the local Troy cluster acquired thanks to Région des Pays de la Loire. B.A. and C.E.C.-H acknowledge funding from the NSF CAREER Program (Grants CHE-1255084 and CHE-1539808).

4. CONCLUSIONS In summary, with steady-state and femtosecond/nanosecond transient absorption spectroscopies, we demonstrated that two cyclometalated Ir(III) complexes with a PBI ligand undergo ISC within hundreds of femtoseconds, while exhibiting strong absorption of visible light and a long-lived triplet state. This result is interesting because it was well accepted that the intraligand state (3IL) has a weaker heavy atom effect than the conventional MLCT state. In accordance to our calculations, the SOCs are sufficient for the Sn → Tn transition, yet the T1 → S0 transition is much slower (SOC = 3 cm−1). Moreover, we found that the PBI ligand is transformed to the radical anion form in the presence of a typical sacrificial electron donor (TEOA), which reduces the triplet-state yield of the complexes. The doublet excited states of the complexes were investigated with DFT/TD-DFT computations, and the near-IR absorption of the Ir(III) complexes with the PBI ligand in the radical anion forms was rationalized. The decay of the D1 state of the radical anion of the coordinated PBI ligand is ultrafast (4−7 ps). Our results are useful for understanding the fundamental photophysical properties of the Ir(III) complexes and for their application in photocatalysis and photosensitizing.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06714. Molecular structure characterization and additional photophysical spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +8641184986236 (J. Z.). *E-mail: [email protected] (D.E.). *E-mail: [email protected]. Telephone: +33251125564 (D.J.). *E-mail: [email protected] (C.E.C.-H.). ORCID

Jianzhang Zhao: 0000-0002-5405-6398 Wei Ji: 0000-0001-6391-9768 Denis Jacquemin: 0000-0002-4217-0708 Carlos E. Crespo-Hernández: 0000-0002-3594-0890 Author Contributions ¶

W.Y. and B.A. contributed equally to this work.

Notes

The authors declare no competing financial interest. 21195


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