Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes

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Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes Coordinated with Perylenebisimide and Its #-Radical Anions Ligands Wenbo Yang, Brennan Ashwood, Jianzhang Zhao, Wei Ji, Daniel Escudero, Denis Jacquemin, and Carlos E. Crespo-Hernández J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06714 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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The Journal of Physical Chemistry

Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes Coordinated with Perylenebisimide and Its π-Radical Anion Ligands Wenbo Yang,a¶ Brennan Ashwood,b¶ Jianzhang Zhao,a* Wei Ji,a Daniel Escudero,c* Denis Jacqueminc,d* Carlos E. Crespo-Hernándezb* a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian 116024, P. R. China Telephone: +8641184986236 E-mail: [email protected] (J. Z.) b

Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, Cleveland, Ohio 44106, United States E-mail: [email protected] (C.C.)

c

CEISAM UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France E-mail: [email protected]. (D.E.) d

Institut Universitaire de France, 1, rue Descartes, 75005 Paris Cedex 5, France Telephone: +33251125564 E-mail: [email protected] (D.J.)

¶ These authors contribute equally to this work.

ACS Paragon Plus Environment

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Abstract: Cyclometalated Ir(III) complexes showing strong absorption of visible light and longlived 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 1

MLCT→3MLCT transitions. 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 spinorbital 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 inert atmosphere, either with or without photoexcitation. We found that the triplet state population is decreased with formation of the radical. The doublet excited states 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 for 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


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strong absorption of visible light, population of the triplet manifold in high yield, and long-lived 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 long-lived triplet state were reported.25−28,33−42 Although the photophysical properties of the conventional cyclometalated Ir(III) complexes, such as Ir(ppy)3 (ppy = 2phenylpyridine), 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 lowest-energy triplet state (3IL), rather than a traditional 3MLCT state, have rarely been investigated.39 It is known that ligand-based, i.e., 1IL→3IL and ligand-to-ligand charge transfer, i.e., 1LLCT→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, Ir(piq)3 (piq = 1-phenylisoquinoline) shows ISC within 70 fs.45 Similarly, Ir(ppy)3 undergoes ISC in 70 ± 10 fs.46 Therefore, it is interesting 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 In contrast, 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


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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 have 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 needs 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 radical anion in the presence of reductants. For instance, PBI radicals were prepared by various methods, e.g., by dangling a phenoxide radical on the PBI core,50 by obtaining of 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 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 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 long-lived triplet state (τT = 11.4 μs for Ir−PBI, τT = 7.9 μs for Ir−NPBI), which are the signatures of a 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.

2. RESULTS AND DISCUSSION


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2.1. Steady State UV−vis Absorption Spectra. Previously, we reported the steady-state and long-lived excited-state 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 < 5 μs). The PBI and NPBI ligands were used as reference in the study of the photophysical properties (Scheme 1). The preparation and molecular structural characterization were reported previously.37 Scheme 1. Structures of Ir−PBI, Ir−NPBI and Corresponding PBI, NPBI Ligands


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The steady state UV−vis absorption spectra of the complexes and the ligands were recorded (Figure 1a,b). Without amino substituent, the PBI ligand shows a more structured absorption band compared to the NPBI ligand, which presents a structureless and red-shifted absorption. These spectral features pertain to a large extent in the Ir(III) complexes, indicating that the perturbation of the π-conjugation system of the PBI/NPBI ligands is limited upon coordination to the Ir(III) center. In contrast, with a less bulky ligand, the cyclometalation exerts a significant effect on the UV−vis absorption features of the ligands.6−8,30,49,56,57 0.6

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400

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Figure 1. UV−vis absorption spectra: (a) Ir−PBI, Ir−NPBI and (b) PBI, NPBI. 20°C. CH2Cl2 was used for PBI and ACN was used for the other compounds. PBI is a well-known electron acceptor,58−62 and it forms a radical anion in the presence of electron donors.54,55,63 Previously, a phenoxide radical was attached to the PBI core, but the spin density was barely located on the PBI core, that is, it was not a π-radical localized on the PBI moiety.50 Consequently, while the π-radical PBI was observed, the formation of the π-radical with a coordinated PBI ligand was not reached. In order to study the formation of the radical anion with a coordinated PBI ligand, we monitored the UV−vis absorption changes of the complexes in the presence of a typical electron


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donor triethanolamine (TEOA, see Figure 2a). For Ir−PBI, the presence of TEOA induces the formation of new absorption bands in the 600 −1000 nm region. These new bands are signatures of the created radical anion of PBI,39,52−55 and are assigned to the D0→Dn transitions (see the

without irradiation in air upon irradiation in N2

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Figure 2. (a) UV−vis absorption spectra of Ir−PBI in the presence of TEOA in aerated solution and after irradiation by 541 nm Xe lamp for 15 minutes in deaerated solution (N2 atmosphere). (b) The reversibility of the formation of [Ir−PBI]−• monitored by the absorbance at 710 nm. [Ir−PBI] = 1.0 × 10−5 M, [TEOA] = 1.0 × 10−4 M. (c) UV−vis absorption spectra of Ir−NPBI in the presence of TEOA in aerated solution and in deaerated solution (N2 atmosphere). (d) The reversibility of the formation of [Ir−NPBI]−• monitored by the absorbance at 900 nm. [Ir−NPBI] = 1.0 × 10−5 M, [TEOA] = 1.0 × 10−3 M. ‘In air’ and ‘in N2’ in the figures are abbreviation for ‘under air atmosphere’ and ‘under N2 atmosphere’, respectively. 20 °C.


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computed TD-DFT values below) of the PBI radical anions.54,55,64 The absorption bands at 384 nm and 541 nm show decreased intensity upon formation of the radical. This allowed us to determine that the transition yield from Ir−PBI to its radical form is 40 %, using the molar extinction coefficient of the radical (13700 M−1 cm−1 at 710 nm). The absorption features are different from those of non-substituted PBI π-radical prepared by electrochemical reduction, for which the absorption bands at 700 nm and 800 nm are well-separated.54,55,64 The absorption of this π-radical of PBI moiety is also drastically different from the PBI dangled with phenoxide radicals (not PBI π-radicals).50 For Ir−PBI, we found that no radical anions were formed without photoexcitation. Moreover, after exposure to air, the radical disappeared, probably due to the oxidation of the radical by O2. By purging the solution with N2, to remove O2, the radical can be formed again by photoexcitation. The reversibility of the process is illustrated in Figure 2b. Interestingly, for Ir−NPBI, no photoexcitation was required to obtain the radical anion. In the presence of the sacrificial electron donor, TEOA, the radical anion was formed without photoexcitation, as indicated by the appearance of a near IR absorption band at 900 nm accompanied by the decrease of the absorption bands at 402 nm and 668 nm. The transition yield for Ir−NPBI to its radical form was determined to be 27 %. The molar extinction coefficient at 900 nm was estimated as 23700 M−1 cm−1. Upon exposure to air, the radical anion disappeared. Upon removal of the O2 by purging with N2, the radical anion was formed again without the need of photoexcitation. Similarly to Ir−PBI, the reversibility of the process slightly decreases after a few cycles. The change of the uncoordinated ligands under similar conditions was studied as well. In that case, no radical anion formation was observed (see Supporting Information, Figure S9a,b). This


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result indicates that coordination to Ir(III) ion is essential to form radical anions of the PBI ligands. Study of the effect of radical ligands, may be formed in situ in the presence of reductants, on the photophysical property of the complexes is important because the application of the transition metal complexes in areas such as photocatalysis, photovoltaics, photodynamic therapy and photon upconversion is dependent on the photophysical properties of the complexes. 2.2. Ultrafast Intersystem Crossing in Ir−PBI and Ir−NPBI Complexes: Femtosecond Transient Absorption Spectroscopy. The femtosecond transient absorption spectra of Ir−PBI in ACN were monitored upon excitation at 530 nm (Figure 3a,c,e) or 383 nm (Figure 3b,d,f) wavelengths in order to monitor the excited-state dynamics following population of low- and high-lying singlet states, respectively. Similar results were obtained following 383 nm excitation in methanol (MeOH, see Figure S10a,c in the Supporting Information). An initial rise in the transient absorption spectra with maxima at 425 and 700 nm is observed (Figure 3a,b) independent of the excitation wavelength, while the 700 nm band is much broader following 383 nm excitation. A negative-amplitude band centered at 530 and 540 nm is formed simultaneously (Figure 3b), and is masked by Rayleigh scattering from the pump beam when excitation is performed at 530 nm (Figure 3a). The negative-amplitude band is assigned to ground-state depopulation, which is in agreement with the steady-state absorption spectrum (Figure 1a). Over the next ~3 ps, the band at 425 nm slightly decays and shifts to higher energy. In addition, a shoulder is formed at 480 nm, and the band centered at 700 nm begins to decay. An isosbestic point is observed at 445 nm. The decay and rise of the 700 and 490 nm bands, respectively, continue for ~15 ps. Following 530 nm excitation, a small decay of the band with maximum at 490 nm is apparent over hundreds of picoseconds, which is not observed upon excitation at 383 nm. We assign this apparent decay of the transient signal to an artifact caused by the Rayleigh scattering from the excitation beam. Firstly, the decrease in transient signal is not observed


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anywhere in the transient spectra upon 530 nm, including the ground-state depopulation signal shown in Figure 4a at the probe wavelength of 370 nm. Secondly, the decay is also not observed in the transient spectra when excitation is performed at 383 nm, while the dynamics of the ground-state depopulation signals at both excitation wavelengths closely resemble one another within experimental errors (compare kinetic traces at 370 and 540 nm probe wavelengths in Figure 4a and 4b, respectively). λex= 383 nm

λex= 530 nm 6

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-4 400

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Figure 3. Transient absorption spectra of Ir−PBI upon (a, c, e) 530 nm and (b, d, f) 383 nm excitation in ACN at sub-picosecond and picosecond time-delays. Wavelengths from 510 to 550 nm are omitted in a, c, and e due to Rayleigh scattering from the pump beam. The transient absorption kinetics of Ir−PBI were globally fitted across the entire spectral probe window using a three- or a two-component sequential model containing a constant offset


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for excitation at 530 or 383 nm, respectively (Figure 4a,b). Global and target analyses were used to extract the corresponding decay-associated spectra. The lifetimes obtained from these fits are provided in Table 1. τ1 corresponds to the simultaneous rise and decay of the 490 and 700 nm bands, respectively, which is assigned to ISC from the singlet to triplet manifold. A recent study

Figure 4. Representative kinetic traces of Ir−PBI upon (a) 530 nm and (b) 383 nm excitation in ACN. Traces obtained with an excitation wavelength of 530 and 383 nm were globally fit to sequential kinetic models, respectively, generating decay-associated spectra shown in (c) upon 530 nm excitation and in (d) upon 383 nm excitation. Within experimental uncertainties, the excited-state dynamics of Ir−PBI are independent of whether excitation is performed at 530 or 383 nm.


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Table 1. Kinetic Lifetimes Obtained from Global Analysis of Ir−PBI Transient Absorption Spectra

a

Conditions

τ1 (ps)

τ2 (ps)

530 nm, ACN

0.94 ± 0.04a

5.85 ± 0.10b

383 nm, ACN

0.94 ± 0.04a

5.85 ± 0.30b

383 nm, MeOH

1.23 ± 0.04a

7.90 ± 0.50b

τ1 is assigned to intersystem crossing from the singlet to triplet manifold. bτ2 is assigned to a

combination of internal conversion from a triplet receiver state (Tn) to the T1 state and vibrational cooling dynamics in the T1 state. Within experimental uncertainties, the global lifetimes are independent of whether excitation is performed at 530 or 383 nm. showed that non-metal PBI derivatives exhibit similar spectral features as in Figure 3c,d, and these features were also assigned to ISC from the singlet to triplet manifold.65 ISC is proposed to occur through a higher energy triplet state (Tn, n>1) due to the large energy gap between the S1 and T1 state (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 below, Table 4). ISC occurs slightly slower in MeOH than ACN (Table 1) following 383 nm excitation, and this is expected to result from the shifting of singlet/triplet energy gaps in different solvent environments. Following τ1, the 490 nm band continues to grow, while that at 700 nm narrows and decays, suggesting that the T1 state is completely populated on this time scale (τ2). τ2 becomes slightly longer in going from ACN to MeOH (Table 1), which is suggestive of intermolecular vibrational relaxation. Global analysis reveals that the lifetime for the rise of the 490 nm band is wavelength-independent, while the decay of the 700 nm band becomes faster at longer wavelengths. The later observation, in combination with the solvent-dependence of τ2, suggests that the narrowing of the 700 nm band corresponds to intermolecular vibrational


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relaxation.69,70 However, the rise of the 490 nm band is assigned to population of the T1 state via internal conversion from a higher energy triplet state (T3 or T2 based on TD-DFT calculations in Table 4). Therefore, τ2 encompasses both internal conversion and intermolecular vibrational relaxation within the triplet manifold. The offset observed in Figures 3 and 4 then corresponds to the long-lived T1 state.

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Figure 5. Transient absorption spectra of Ir−NPBI in ACN upon excitation at (a, b) 383 nm, (c, d) 660 nm, and (e−g) 725 nm during the initial 3 ns time window. Sharp negative bands from 400 to 450 nm following 383 nm excitation and positive bands from 550 to 600 nm following 660 and 725 nm excitation result from stimulated Raman and anti-Raman emission, respectively, from the solvent that occurs during the cross correlation of the pump and probe beams. A portion of the spectral window is omitted at each excitation wavelength due to Rayleigh scattering from the pump beam at each excitation wavelengths.


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Transient absorption experiments were also performed for Ir−NPBI in ACN using excitation wavelengths of 383, 660, and 725 nm to excite Ir−NPBI to different energy levels (Figure 5a−g). Similar measurements in MeOH are shown in Figure S10b,d exciting at 383 nm. A twocomponent sequential model containing a constant offset was used to globally fit the kinetic data following 383 and 660 nm excitation (Figure 6a,b), while a three-component sequential model was necessary when excitation was done at 725 nm (Figure 6c). The global lifetimes at each excitation wavelength are provided in Table 2. Upon excitation at 383 nm, a broad band centered at 510 nm and negative-amplitude bands at 403 and 662 nm are observed in ACN (Figure 5a). The negative-amplitude signals are assigned to ground-state depopulation (Figure 1a). Within 1 ps (τ1), the absorption band redshifts 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 redshifts (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, 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 excited-state dynamics (Figure 6c). Initial excitation generates absorption bands centered at 363, 475, and 530 nm and negative-amplitudes


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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-sate repopulation and blue shifting of the bands at 412 to 404 nm (Figure 5f) over tens of picoseconds (τ2). The negative-amplitude signal around 650 nm rises to form an absorption band, indicating that absorption from an excited-state species overlaps with the ground-state depopulation signal.

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Wavelength (nm)

Wavelength (nm)

Wavelength (nm)

Figure 6. Representative excited-state decay traces of Ir−NPBI in ACN following excitation at (a) 383 nm, (b) 660 nm, and (c) 725 nm. Traces following 383 and 660 nm excitation were globally fit across the entire spectral probe window to a two-lifetime sequential kinetic model containing an offset while a three-lifetime 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 nm, (e) 660 nm, and (f) 725 nm excitation. The residual spectrum (blue) in panels e and f were magnified by 5 and 10, respectively.


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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 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 excitation energy. Based on the relative offsets of the transient absorption spectra (Figure 6d, e, f), the yield for population of the long-lived (> 3 ns) excited-state 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 ground-state 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 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 eightfold greater concentration of Ir−NPBI used for this measurement. According to TD-DFT vertical energies,37 excitation with 383 nm radiation populates a highlying singlet state (S11 or S17 with oscillator strengths of 0.38 or 0.15, respectively), while 660 and 725 nm excitation are predicted to populate the S1 state of Ir−NPBI that has an oscillator strength of 0.45. As recently shown for non-metal PBI derivatives,65 excitation to higher energy singlet states may provide access to additional efficient ISC pathways, which may explain the higher relative triplet yield upon 383 nm excitation.


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

a

Conditions

τ1 (ps)

τ2 (ps)

τ3 (ps)

725 nm, ACN, 0.08 mM

0.90 ± 0.05a

33 ± 1b

203 ± 4c

660 nm, ACN, 0.01 mM

0.90 ± 0.06a

--

190 ± 3c

383 nm, ACN, 0.01 mM

0.90 ± 0.04a

--

209 ± 4c

383 nm, MeOH, 0.01 mM

0.90 ± 0.04a

--

77 ± 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. cτ3 following 725 nm excitation are assigned to charge recombination to repopulate the ground-state in both solvents.

With the above discussion, it is now possible to assign the kinetic lifetimes extracted from the transient absorption spectra of Ir−NPBI. Intersystem crossing 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, so 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 show 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 intersystem crossing to the triplet manifold in 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


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solvents, leading to faster recombination time. τ2 in Table 2 is assigned to either triplet selfquenching and/or triplet-triplet annihilation upon 725 nm excitation, as explained above. 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 even with bulky ligands and therefore a 3IL state, both Ir−PBI and Ir−NPBI may undergo ultrafast intersystem crossing to populate the triplet manifold. Ir−PBI undergoes ISC slower than observed for some conventional Ir(III) complexes containing small ligands. For instance, Ir(piq)3 shows ISC with 70 fs (piq = 1phenylisoquinoline).45 Previously, some cyclometalated Ir(III) complexes containing 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 in some metalfree PBI derivatives, which have ISC lifetimes of 30 ps,66 or in PBI co-facial dimers, for which the ISC takes more than 1 ns.72 Ir−PBI undergoes ISC on timescales 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 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.


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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-lifetime sequential model. (d) Decay-associated spectra extracted from global analysis of the excited-state kinetics using a two-lifetime sequential model. 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). The first lifetime (τ1) corresponds to a decay and blueshift 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).


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The bi-exponential 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 bi-exponential 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 internal conversion 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 internal conversion 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 wavelength, so 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). 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 excited-state absorption band within a ~150 fs (τ1) to populate an excited-state band similar to Ir−NPBI (See Figure S12a−f in 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


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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

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 antiRaman 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. may be assigned to the excited-state decay of [Ir−NPBI]−• through ultrafast charge recombination or internal conversion to re-populate the ground-state. While the spectral


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

τ1 (fs)a

τ2 (ps)b

[Ir−PBI]−•

280 ± 50

3.7 ± 0.6

n. a.

[Ir−NPBI]−• + Ir−NPBI

150 ± 50

6.9 ± 0.6

205 ± 8

Compound

τ3 (ps)c

a

Assigned to internal conversion from Dn to D0. b Assigned to internal conversion from D1 to D0. c Assigned to charge recombination in Ir−NPBI. 2.4. Effect on Formation Radical Anion Ligand on the Triplet State Property of the Ir(III) Complexes. The changes of the triplet state of the Ir(III) complexes upon formation of the radical ligands was studied (Figure 9a−d). First the nanosecond transient absorption spectra of Ir−PBI were recorded and are similar to those previously observed.37,38 Bleaching bands at 384 nm and 544 nm were observed, which are due to the ground state depletion. Excited state absorption band in the region 400−700 nm was observed. The triplet state lifetime was determined as 11.4 μs, which is different from 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 TEOA and photoirradiation to transform the PBI ligands to its radical form, the difference absorption spectra are identical to the untreated Ir−PBI spectra


22

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(Figure 9c). However, the triplet state quenched by radical the lifetime decreased slightly from 11.4 μs to 9.5 μs. A similar study was performed for Ir−NPBI. No new transient band was observed (see Supporting Information, Figure S13a−d). We confirmed that the transient signal observed in the nanosecond transient absorption spectra is due to triplet state, by studying the quenched decay in aerated solution (see Supporting Information, Figure S14a−d).

0.08

0.04

a

0.04

c

0.00

Δ O.D.

Δ O.D.

0.00 -0.04

-0.04

-0.08 Time (μs) 0 16 32

-0.12

Time (μs) 4 20 36

8

12 24 40

0 28 56

-0.08

28

7 35 63

14 42 70

21 49

400 500 600 700 Wavelength / nm

400 500 600 700 Wavelength / nm

0.00

Δ O.D.

0.00

-0.05

Δ O.D.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.05 τT = 11.4 μs

-0.10

τT = 9.5 μs

b

-0.15 0

50

100 150 Time / μs

200

d

-0.10 0

50

100 150 Time / μs

200

Figure 9. Nanosecond transient absorption spectra of the compounds (a) Ir−PBI and (c) Ir−PBI/TEOA (1:10), decay curves of (b) Ir−PBI and (d) Ir−PBI/TEOA (1:10) at 540 nm. Radical was produced by irradiation of 532 nm laser. λex = 532 nm (pulsed laser); [Ir−PBI] = 1.0 × 10−5 M; in deaerated ACN. 20 °C. 2.5. Theoretical Computations on the Ir−PBI Complexes: Rationalization of the i) Ultrafast ISC, ii) the Emissive Properties and iii) the Doublet Excited States.


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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 TD-DFT 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 internal conversion (IC) processes.76 For Ir(ppy)2(bpy), it is well described from both experimental,77 and computational,78 viewpoints, that the lowestlying 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 indicate 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.


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We next rationalize why Ir−PBI undergoes ultrafast ISC to the triplet manifold even though ISC to the ground state occurs 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 S n H SO Tm =

2

× [ FCWD ]

(1)

where the bracket term stands for their associated SOCs and FCWD accounts for the FranckCondon weighted density of states. Computing all parameters on 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, i.e., 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 4.4). 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 comparing the relative energy levels, several ISC channels appear, e.g., the S2→T2 and the S4→T4 ISC channels. Some of these states possess non-negligible participation of the Ir(III) center. These ISC 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 internal conversion to the lowest-energy triplet state. Not surprisingly, for the Ir(ppy)2(bpy) complex the SOCs values for several Tn→S0 ISC decay amount up to 400−500 cm−1.


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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/631G(d) Level of Theory and Photophysically Relevant Spin-Orbit Matrix Elements (SOCMEs in cm−1). See Also the Orbitals in Figure 10.

Ir−PBI

Statesa

Energy (fb)

Characterc

SOCMEs Tn/Smd (x-; y-; z-components)

S1

1.91 / 650 (0.000)

1

MLCT/LLCT (H−1→L)

−

S2

2.01 / 617 (0.413)

1

IL/LLCT (H→L)

−

S3

2.46 / 504 (0.0134)

1

MLCT/LLCT (H−1→L+1)

−

S4

2.48 / 500 (0.296)

1

IL/LLCT (H→L+1)

−

T1

1.17 / 1057 (0.000)

3

IL/LLCT (H→L)

T1/S0: (−1.2; −0.2; 2.6)

T2

1.89 / 656 (0.000)

3

MLCT/LLCT (H−1→L)

T2/S2: (−66.2; −64.4; 37.3)

T3

1.99 / 623 (0.000)

3

IL/LLCT (H→L+1)

−

T4

2.42 / 513 (0.000)

3

MLCT/IL (H−5→L)

T4/S4: (−21.7; −32.2; 26.3)

T5

2.45 /505 (0.000)

1

MLCT/LLCT (H−1→L+1)

−

a

Only the selected low−lying excited states are presented. b Oscillator strengths. c Only the main configurations are presented. d Values obtained at the QR-TD-DFT/6-31G(d) level of theory. Finally, we assessed if the near IR absorption of the complexes with the PBI radical ligands can be modeled computationally. Towards 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 1649 nm (D1) and 873 nm (D4). The spin density surfaces of the Ir(III) complexes with radical ligands at D0 state (ground state) were also


26

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studied (Figure 11a, b). It was found that the spin density surfaces are main localized on the PBI ligands, but delocalization to the bpy moiety was also found.

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 group to reduce computation time.

a)

b)

Figure 11. Isosurface of spin density of (a) [Ir−PBI]−• and (b) [Ir−NPBI]−•. Calculated at the optimized doublet-state geometries at UB3LYP/GENECP/LANL2DZ level with Gaussian 09W. Alkyl chains are simplified as methyl group to reduce computation time.


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We also studied the absorption of the Ir(III) complexes with radical ligands. For [Ir−NPBI]−•, the main absorption bands are predicted as 1330 nm, 904 nm, 681 nm and 550 nm, etc. (Table 5). These values are in good agreement with the experimental results of 900 nm, 680 nm and 500 nm (Figure 2c). By examination of the frontier molecular orbitals of the open-shell compound, the transitions can be attributed to either the NPBI moiety-localized transition, such as the 680 nm bands (H α → L+4 α) 900 nm bands (H β → L β), or an intraligand charge transfer transition at 550 nm (H−1 α → L α) (Figure S15 and Table 5). The result for [Ir−PBI]−• was given in Figure S16 and Table S1).

Table 5. Lowest-Lying Vertical (at D0 Geometry) Doublet Electronic Transition Energies (eV / nm) Oscillator Strengths (in Parentheses) and Square of Spin Angular Momentum of [Ir−NPBI]−• at the TD-UB3LYP/LANL2DZ Level of Theory. See Also the Orbitals in Figure S15 States a [Ir−NPBI]−•

Energy (f b)

Character c

S2 d

D1

0.93 / 1330 (0.1946)

LLCT (H α→L α)

0.819

D2

1.37 / 904 (0.1002)

LLCT/ILCT (H β→L β)

0.783

D4

1.82 / 681 (0.4877)

ILCT (H α→L+4 α)

0.944

D9

2.25 / 550 (0.1695)

LLCT (H−1 α→L α)

0.834

a

Only the selected low−lying excited states are presented. b Oscillator strengths. c Only the main configurations are presented. d The square of the spin angular momentum.

3. EXPERIMENTAL SECTION 3.1 General Methods. All the chemicals used in synthesis are analytical pure and were used as received. The preparation and molecular structural characterization of the compounds were reported previously.37 NMR spectra were recorded by OXFORD NMR 400 MHz spectrometer


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with CDCl3 as solvent and tetramethylsilane (TMS) as standard at 0.00 ppm. HRMS was accomplished with matrix-assisted 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−TOF−HRMS ([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 ([C74H64N6O4Ir]+): 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.


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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 and 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 broadband transient absorption spectroscopy was employed to study the excited-state dynamics of Ir−PBI, Ir−NPBI, [Ir−PBI]−•, [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 broadband 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 it takes the probe pulse to reach the sample up to a maximum delay of 3 ns. The broadband 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


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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 concentrations of 0.01 mM in ACN when exciting at 383, 530, and 660 nm and 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%. A 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 two-photon coherent signal from neat methanol (IRF of 200 ± 50 fs).85 3.4 Computational Details. All calculations are based on density functional theory (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, whilst the doublet excited states were obtained on the D0 optimized geometry.


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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 whilst the Tm→Sn SOCs were calculated at the S0 optimized geometry. For the SOC calculations the B3LYP functional in combination to the 631G and raf-r atomic basis set were 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 our calculations makes use of the effective single-electron approximation.

4. CONCLUSIONS In summary, with steady state and femtosecond/nanosecond transient absorption spectroscopies, we demonstrated that two cyclometalated Ir(III) complexes with a perylenebisimide (PBI) ligand undergo intersystem crossing 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 the intraligand state (3IL) is with weaker heavy atom effect than the conventional MLCT state. With DFT/TDDFT computation the spin-orbital coupling (SOC) is sufficient for the S1→T1 transition, yet the T1→S0 transition is retarded (SOC = 3 cm−1), vs. the 490 cm−1 for the T1→S0 transition of Ir(ppy)3. Moreover, we found that PBI ligand is transformed to the radical anion form in the presence of a typical sacrificial electron donor (triethanolamine, TEOA), which reduces triplet state yield of the complexes. The doublet excited states of the complexes were investigated with DFT/TDDFT computations, and the near IR absorption of the Ir(III) complexes with the PBI ligand in the radical anion forms were 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 of the fundamental photophysical


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properties of the Ir(III) complexes and for the application of the similar complexes in photocatalysis and photosensitizing.

■ ACKNOWLEDGMENT 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. DE thanks funding from the European Union’s Horizon 2020 research and innovation programme under the 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 GENCICINES/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 CHE1539808).

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Molecular structure characterization and additional photophysical spectra (PDF)

■ AUTHOR INFORMATION Notes The authors declare no competing financial interest.

■ REFERENCES


33


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(1) Reithmeier, R. O.; Meister, S.; Rieger, B.; Siebel, A.; Tschurl, M.; Heiz, U.; Herdtweck, E. Mono- and Bimetallic Ir(III) Based Catalysts for the Homogeneous Photocatalytic Reduction of CO2 under Visible Light Irradiation. New Insights into Catalyst Deactivation. Dalton Trans. 2014, 43, 13259−13269. (2) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. Discovery and High-Throughput Screening of Heteroleptic Iridium Complexes for Photoinduced Hydrogen Production. J. Am. Chem. Soc. 2005, 127, 7502−7510. (3) Metz, S.; Bernhard, S. Robust Photocatalytic Water Reduction with Cyclometalated Ir(III) 4-Vinyl-2,2[Prime or Minute]-Bipyridine Complexes. Chem. Commun. 2010, 46, 7551−7553. (4) Yuan, Y.-J.; Zhang, J.-Y.; Yu, Z.-T.; Feng, J.-Y.; Luo, W.-J.; Ye, J.-H.; Zou, Z.-G. Impact of Ligand Modification on Hydrogen Photogeneration and Light-Harvesting Applications Using Cyclometalated Iridium Complexes. Inorg. Chem. 2012, 51, 4123−4133. (5) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Highly Phosphorescent Bis-Cyclometalated

Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light

Emitting Diodes. J. Am. Chem. Soc. 2001, 123, 4304−4312. (6) Escudero, D.; Jacquemin, D. Computational Insights into the Photodeactivation Dynamics of Phosphors for OLEDs: A Perspective. Dalton Trans. 2015, 44, 8346−8355.


34

Page 35 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Escudero, D. Quantitative Prediction of Photoluminescence Quantum Yields of Phosphors from First Principles. Chem. Sci. 2016, 7, 1262−1267. (8) Chi, Y.; Chou, P.-T. Transition-Metal Phosphors with Cyclometalating Ligands: Fundamentals and Applications. Chem. Soc. Rev. 2010, 39, 638−655. (9) Liu, Z. W.; Guan, M.; Bian, Z. Q.; Nie, D. B.; Gong, Z. L.; Li, Z. B.; Huang, C. H. Red Phosphorescent Iridium Complex Containing Carbazole-Functionalized Β-Diketonate for Highly Efficient Nondoped Organic Light-Emitting Diodes. Adv. Funct. Mater. 2006, 16, 1441−1448. (10) Williams, E. L.; Li, J.; Jabbour, G. E. Organic Light-Emitting Diodes Having Exclusive Near-Infrared Electrophosphorescence. Appl. Phys. Lett. 2006, 89, 083506. (11) Yang, C. L.; Zhang, X. W.; You, H.; Zhu, L. Y.; Chen, L. Q.; Zhu, L. N.; Tao, Y. T.; Ma, D. G.; Shuai, Z. G.; Qin, J. G. Tuning the Energy Level and Photophysical and Electroluminescent Properties of Heavy Metal Complexes by Controlling the Ligation of the Metal with the Carbon of the Carbazole Unit. Adv. Funct. Mater. 2007, 17, 651−661. (12) Zhou, G.-J.; Wang, Q.; Wong, W.-Y.; Ma, D.; Wang, L.; Lin, Z. A Versatile Color Tuning Strategy for Iridium(III) and Platinum(II) Electrophosphors by Shifting the ChargeTransfer States with an Electron-Deficient Core. J. Mater. Chem. 2009, 19, 1872−1883. (13) Kim, T. H.; Lee, H. K.; Park, O. O.; Chin, B. D.; Lee, S. H.; Kim, J. K. White-LightEmitting Diodes Based on Iridium Complexes via Efficient Energy Transfer from a Conjugated Polymer. Adv. Funct. Mater. 2006, 16, 611−617.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 47

(14) Liu, Y.; Ye, K.; Fan, Y.; Song, W.; Wang, Y.; Hou, Z. Amidinate-Ligated Iridium(III) Bis(2-Pyridyl)Phenyl

Complex

as

an

Excellent

Phosphorescent

Material

for

Electroluminescence Devices. Chem. Commun. 2009, 3699−3701. (15) Chou, P.-T.; Chi, Y. Phosphorescent Dyes for Organic Light-Emitting Diodes. Chem.

−Eur. J. 2007, 13, 380−395. (16) Han, Y.; You, Y.; Lee, Y.-M.; Nam, W. Double Action: Toward Phosphorescence Ratiometric Sensing of Chromium Ion. Adv. Mater. 2012, 24, 2748−2754. (17) Zhao, Q.; Li, F.; Huang, C. Phosphorescent Chemosensors Based on Heavy-Metal Complexes. Chem. Soc. Rev. 2010, 39, 3007−3030. (18) Fernández-Moreira, V.; Thorp-Greenwood, F. L.; Coogan, M. P. Application of d6 Transition Metal Complexes in Fluorescence Cell Imaging. Chem. Commun. 2010, 46, 186−202. (19) You, Y.; Nam, W. Photofunctional Triplet Excited States of Cyclometalated Ir(III) Complexes: Beyond Electroluminescence. Chem. Soc. Rev. 2012, 41, 7061−7084. (20) Lo, K. K.-W.; Chung, C.-K.; Lee, T. K.-M.; Lui, L.-H.; Tsang, K. H.-K.; Zhu, N. New Luminescent Cyclometalated Iridium(III) Diimine Complexes as Biological Labeling Reagents. Inorg. Chem. 2003, 42, 6886−6897. (21) Borisov, S. M.; Klimant, I. Ultrabright Oxygen Optodes Based on Cyclometalated Iridium(III) Coumarin Complexes. Anal. Chem. 2007, 79, 7501−7509.


36

Page 37 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Fischer, L. H.; Stich, M. I. J.; Wolfbeis, O. S.; Tian, N.; Holder, E.; Schäferling, M. Redand Green-Emitting Iridium(III) Complexes for a Dual Barometric and Temperature-Sensitive Paint. Chem.−Eur. J. 2009, 15, 10857−10863. (23) Feng, Y.; Cheng, J.; Zhou, L.; Zhou, X.; Xiang, H. Ratiometric Optical Oxygen Sensing: A Review in Respect of Material Design. Analyst 2012, 137, 4885−4901. (24) Mak, C. S. K.; Pentlehner, D.; Stich, M.; Wolfbeis, O. S.; Chan, W. K.; Yersin, H. Exceptional Oxygen Sensing Capabilities and Triplet State Properties of Ir(ppy-NPh2)3. Chem. Mater. 2009, 21, 2173−2175. (25) Sun, J.; Wu, W.; Guo, H.; Zhao, J. Visible-Light Harvesting with Cyclometalated Iridium(III) Complexes Having Long-Lived 3IL Excited States and Their Application in Triplet– Triplet-Annihilation Based Upconversion. Eur. J. Inorg. Chem. 2011, 2011, 3165−3173. (26) Ma, L.; Guo, S.; Sun, J.; Zhang, C.; Zhao, J.; Guo, H. Green Light-Excitable Naphthalenediimide Acetylide-Containing Cyclometalated Ir(III) Complex with Long-Lived Triplet Excited States as Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversion. Dalton Trans. 2013, 42, 6478−6488. (27) Sun, J.; Zhong, F.; Yi, X.; Zhao, J. Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet–Triplet Annihilation Upconversion. Inorg. Chem. 2013, 52, 6299−6310.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

(28) Ma, L.; Guo, H.; Li, Q.; Guo, S.; Zhao, J. Visible Light-Harvesting Cyclometalated Ir(III) Complexes as Triplet Photosensitizers for Triplet-Triplet Annihilation Based Upconversion. Dalton Trans. 2012, 41, 10680−10689. (29) Peng, J.; Jiang, X.; Guo, X.; Zhao, D.; Ma, Y. Sensitizer Design for Efficient TripletTriplet

Annihilation

Upconversion:

Annihilator-Appended

Tris-Cyclometalated

Ir(III)

Complexes. Chem. Commun. 2014, 50, 7828−7830. (30) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Photochemistry and Photophysics of Coordination Compounds: Iridium. Top. Curr. Chem. 2007, 281, 143−203. (31) Langdon-Jones, E. E.; Hallett, A. J.; Routledge, J. D.; Crole, D. A.; Ward, B. D.; Platts, J. A.; Pope, S. J. A. Using Substituted Cyclometalated Quinoxaline Ligands to Finely Tune the Luminescence Properties of Iridium(III) Complexes. Inorg. Chem. 2013, 52, 448−456. (32) Dragonetti, C.; Falciola, L.; Mussini, P.; Righetto, S.; Roberto, D.; Ugo, R.; Valore, A.; De Angelis, F.; Fantacci, S.; Sgamellotti, A., et al. The Role of Substituents on Functionalized 1,10-Phenanthroline in Controlling the Emission Properties of Cationic Iridium(III) Complexes of Interest for Electroluminescent Devices. Inorg. Chem. 2007, 46, 8533−8547. (33) Rachford, A. A.; Ziessel, R.; Bura, T.; Retailleau, P.; Castellano, F. N. Boron Dipyrromethene (Bodipy) Phosphorescence Revealed in [Ir(ppy)2(bpy−C≡C−Bodipy)]+. Inorg. Chem. 2010, 49, 3730−3736. (34) Hanson, K.; Tamayo, A.; Diev, V. V.; Whited, M. T.; Djurovich, P. I.; Thompson, M. E. Efficient Dipyrrin-Centered Phosphorescence at Room Temperature from Bis-Cyclometalated Iridium(III) Dipyrrinato Complexes. Inorg. Chem. 2010, 49, 6077−6084.


38

Page 39 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Bronner, C.; Baudron, S. A.; Hosseini, M. W.; Strassert, C. A.; Guenet, A.; De Cola, L. Dipyrrin Based Luminescent Cyclometallated Palladium and Platinum Complexes. Dalton Trans. 2010, 39, 180−184. (36) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.; Eisenberg, R.

Sensitizing

the

Sensitizer:

The

Synthesis

and

Photophysical

Study

of

Bodipy−Pt(II)(Diimine)(Dithiolate) Conjugates. J. Am. Chem. Soc. 2011, 133, 350−364. (37) Sun, J.; Zhong, F.; Zhao, J. Observation of the Long-Lived Triplet Excited State of Perylenebisimide (PBI) in C^N Cyclometalated Ir(III) Complexes and Application in Photocatalytic Oxidation. Dalton Trans. 2013, 42, 9595−9605. (38) Liu, Y.; Zhao, J. Visible Light-Harvesting Perylenebisimide-Fullerene (C60) Dyads with Bidirectional "Ping-Pong" Energy Transfer as Triplet Photosensitizers for Photooxidation of 1,5Dihydroxynaphthalene. Chem. Commun. 2012, 48, 3751−3753. (39) Llewellyn, B. A.; Slater, A. G.; Goretzki, G.; Easun, T. L.; Sun, X.-Z.; Davies, E. S.; Argent, S. P.; Lewis, W.; Beeby, A.; George, M. W., et al. Photophysics and Electrochemistry of a Platinum-Acetylide Disubstituted Perylenediimide. Dalton Trans. 2014, 43, 85−94. (40) Schulze, M.; Steffen, A.; Würthner, F. Near-IR Phosphorescent Ruthenium(II) and Iridium(III) Perylene Bisimide Metal Complexes. Angew. Chem. Int. Ed. 2015, 54, 1570−1573. (41) McCusker, C. E.; Hablot, D.; Ziessel, R.; Castellano, F. N. Metal Coordination Induced πExtension and Triplet State Production in Diketopyrrolopyrrole Chromophores. Inorg. Chem. 2012, 51, 7957−7959.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 47

(42) Geist, F.; Jackel, A.; Winter, R. F. Ligand Based Dual Fluorescence and Phosphorescence Emission from Bodipy Platinum Complexes and Its Application to Ratiometric Singlet Oxygen Detection. Inorg. Chem. 2015, 54, 10946−10957. (43) Denisov, S. A.; Cudré, Y.; Verwilst, P.; Jonusauskas, G.; Marín-Suárez, M.; FernándezSánchez, J. F.; Baranoff, E.; McClenaghan, N. D. Direct Observation of Reversible Electronic Energy Transfer Involving an Iridium Center. Inorg. Chem. 2014, 53, 2677−2682. (44) Kozlov, D. V.; Tyson, D. S.; Goze, C.; Ziessel, R.; Castellano, F. N. Room Temperature Phosphorescence from Ruthenium(II) Complexes Bearing Conjugated Pyrenylethynylene Subunits. Inorg. Chem. 2004, 43, 6083−6092. (45) Hedley, G. J.; Ruseckas, A.; Samuel, I. D. W. Ultrafast Intersystem Crossing in a Red Phosphorescent Iridium Complex. J. Phys. Chem. A 2009, 113, 2−4. (46) Messina, F.; Pomarico, E.; Silatani, M.; Baranoff, E.; Chergui, M. Ligand-Centred Fluorescence and Electronic Relaxation Cascade at Vibrational Time Scales in Transition-Metal Complexes. J. Phys. Chem. Lett. 2015, 6, 4475−4480. (47) Rachford, A. A.; Goeb, S.; Castellano, F. N. Accessing the Triplet Excited State in Perylenediimides. J. Am. Chem. Soc. 2008, 130, 2766−2767. (48) Prusakova, V.; McCusker, C. E.; Castellano, F. N. Ligand-Localized Triplet-State Photophysics in a Platinum(II) Terpyridyl Perylenediimideacetylide. Inorg. Chem. 2012, 51, 8589−8598.


40

Page 41 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(49) Costa, R. D.; Céspedes-Guirao, F. J.; Bolink, H. J.; Fernández-Lázaro, F.; Sastre-Santos, Á.; Ortí, E.; Gierschner, J. A Deep-Red-Emitting Perylenediimide−Iridium-Complex Dyad: Following the Photophysical Deactivation Pathways. J. Phys. Chem. C 2009, 113, 19292−19297. (50) Schmidt, D.; Son, M.; Lim, J. M.; Lin, M.-J.; Krummenacher, I.; Braunschweig, H.; Kim, D.; Würthner, F. Perylene Bisimide Radicals and Biradicals: Synthesis and Molecular Properties. Angew. Chem. Int. Ed. 2015, 54, 13980−13984. (51) Schmidt, D.; Bialas, D.; Würthner, F. Ambient Stable Zwitterionic Perylene BisimideCentered Radical. Angew. Chem. Int. Ed. 2015, 54, 3611−3614. (52) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R. Excited Doublet States of Electrochemically Generated Aromatic Imide and Diimide Radical Anions. J. Phys. Chem. A 2000, 104, 6545−6551. (53) Roznyatovskiy, V. V.; Gardner, D. M.; Eaton, S. W.; Wasielewski, M. R. Radical Anions of Trifluoromethylated Perylene and Naphthalene Imide and Diimide Electron Acceptors. Org. Lett. 2014, 16, 696−699. (54) Goodson, F. S.; Panda, D. K.; Ray, S.; Mitra, A.; Guha, S.; Saha, S. Tunable Electronic Interactions between Anions and Perylenediimide. Org. Biomol. Chem. 2013, 11, 4797−4803. (55) Jiao, Y.; Liu, K.; Wang, G.; Wang, Y.; Zhang, X. Supramolecular Free Radicals: NearInfrared Organic Materials with Enhanced Photothermal Conversion. Chem. Sci. 2015, 6, 3975−3980.


41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 47

(56) Wu, W.; Wu, W.; Ji, S.; Guo, H.; Zhao, J. Accessing the Long-Lived Emissive 3IL Triplet Excited States of Coumarin Fluorophores by Direct Cyclometallation and Its Application for Oxygen Sensing and Upconversion. Dalton Trans. 2011, 40, 5953−5963. (57) Costa, R. D.; Céspedes-Guirao, F. J.; Ortí, E.; Bolink, H. J.; Gierschner, J.; FernándezLázaro, F.; Sastre-Santos, Á. Efficient Deep-Red Light-Emitting Electrochemical Cells Based on a Perylenediimide-Iridium-Complex Dyad. Chem. Commun. 2009, 3886−3888. (58) Jiménez, Á. J.; Sekita, M.; Caballero, E.; Marcos, M. L.; Rodríguez-Morgade, M. S.; Guldi, D. M.; Torres, T. Assembling a Phthalocyanine and Perylenediimide Donor–Acceptor Hybrid through a Platinum(II) Diacetylide Linker. Chem. −Eur. J. 2013, 19, 14506−14514. (59) Suzuki, S.; Kozaki, M.; Nozaki, K.; Okada, K. Recent Progress in Controlling Photophysical Processes of Donor–Acceptor Arrays Involving Perylene Diimides and BoronDipyrromethenes. J. Photoch. Photobio. C 2011, 12, 269−292. (60) Wu, H.; Wang, H.; Xue, L.; Shi, Y.; Li, X. Hindered Intramolecular Electron Transfer in Room-Temperature Ionic Liquid. J. Phys. Chem. B 2010, 114, 14420−14425. (61) Supur, M.; Sung, Y. M.; Kim, D.; Fukuzumi, S. Enhancement of Photodriven Charge Separation

by

Conformational

and

Intermolecular

Adaptations

of

an

Anthracene–

Perylenediimide–Anthracene Triad to an Aqueous Environment. J. Phys. Chem. C 2013, 117, 12438−12445. (62) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Ratner, M. A.; Wasielewski, M. R. Solvent Control of Spin-Dependent Charge Recombination Mechanisms within Donor−Conjugated Bridge−Acceptor Molecules. J. Am. Chem. Soc. 2004, 126, 9510−9511.


42

Page 43 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(63) Montilla, F.; Esquembre, R.; Gómez, R.; Blanco, R.; Segura, J. L. Spectroelectrochemical Study of Electron and Energy Transfer in Poly(Fluorene-Alt-Phenylene) with Perylenediimide Pendant Groups. J. Phys. Chem. C 2008, 112, 16668−16674. (64) Marcon, R. O.; Brochsztain, S. Aggregation of 3,4,9,10-Perylenediimide Radical Anions and Dianions Generated by Reduction with Dithionite in Aqueous Solutions. J. Phys. Chem. A 2009, 113, 1747−1752. (65) Yang, W.; Zhao, J.; Sonn, C.; Escudero, D.; Karatay, A.; Yaglioglu, H. G.; Küçüköz, B.; Hayvali, M.; Li, C.; Jacquemin, D. Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives. J. Phys. Chem. C 2016, 120, 10162−10175. (66) Yu, Z.; Wu, Y.; Peng, Q.; Sun, C.; Chen, J.; Yao, J.; Fu, H. Accessing the Triplet State in Heavy-Atom-Free Perylene Diimides. Chem. −Eur. J. 2016, 22, 4717−4722. (67) Gerbich, T.; Schmitt, H.-C.; Fischer, I.; Mitrić, R.; Petersen, J. Dynamics of Isolated 1,8Naphthalimide and N-Methyl-1,8-Naphthalimide: An Experimental and Computational Study. J. Phys. Chem. A 2016, 120, 2089−2095. (68) Yushchenko, O.; Licari, G.; Mosquera-Vazquez, S.; Sakai, N.; Matile, S.; Vauthey, E. Ultrafast Intersystem-Crossing Dynamics and Breakdown of the Kasha–Vavilov’s Rule of Naphthalenediimides. J. Phys. Chem. Lett. 2015, 6, 2096−2100. (69) Berera, R.; van Grondelle, R.; Kennis, J. T. M. Ultrafast Transient Absorption Spectroscopy: Principles and Application to Photosynthetic Systems. Photosynth. Res. 2009, 101, 105−118.


43


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

(70) Reichardt, C.; Sainuddin, T.; Wächtler, M.; Monro, S.; Kupfer, S.; Guthmuller, J.; Gräfe, S.; McFarland, S.; Dietzek, B. Influence of Protonation State on the Excited State Dynamics of a Photobiologically Active Ru(II) Dyad. J. Phys. Chem. A 2016, 120, 6379−6388. (71) Majumdar, P.; Yuan, X.; Li, S.; Le Guennic, B.; Ma, J.; Zhang, C.; Jacquemin, D.; Zhao, J. Cyclometalated

Ir(III)

Complexes

with

Styryl-Bodipy

Ligands

Showing

Near

IR

Absorption/Emission: Preparation, Study of Photophysical Properties and Application as Photodynamic/Luminescence Imaging Materials. J. Mater. Chem. B 2014, 2, 2838−2854. (72) Veldman, D.; Chopin, S. M. A.; Meskers, S. C. J.; Groeneveld, M. M.; Williams, R. M.; Janssen, R. A. J. Triplet Formation Involving a Polar Transition State in a Well-Defined Intramolecular Perylenediimide Dimeric Aggregate. J. Phys. Chem. A 2008, 112, 5846−5857. (73) Tilley, A. J.; Pensack, R. D.; Lee, T. S.; Djukic, B.; Scholes, G. D.; Seferos, D. S. Ultrafast Triplet Formation in Thionated Perylene Diimides. J. Phys. Chem. C 2014, 118, 9996−10004. (74) Hu, P.; Lee, S.; Herng, T. S.; Aratani, N.; Gonçalves, T. P.; Qi, Q.; Shi, X.; Yamada, H.; Huang, K.-W.; Ding, J., et al. Toward Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity. J. Am. Chem. Soc. 2016, 138, 1065−1077. (75) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. The Triplet Excited State of Bodipy: Formation, Modulation and Application. Chem. Soc. Rev. 2015, 44, 8904−8939. (76) Kasha, M. Characterization of Electronic Transitions in Complex Molecules. Discuss. Faraday Soc. 1950, 9, 14−19.


44

Page 45 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(77) Wu, S.-H.; Ling, J.-W.; Lai, S.-H.; Huang, M.-J.; Cheng, C. H.; Chen, I. C. Dynamics of the Excited States of [Ir(ppy)2bpy]+ with Triple Phosphorescence. J. Phys. Chem. A 2010, 114, 10339−10344. (78) Bokarev, S. I.; Bokareva, O. S.; Kühn, O. Electronic Excitation Spectrum of the Photosensitizer [Ir(ppy)2(bpy)]+. J. Chem. Phys. 2012, 136, 214305. (79) Li, X.; Minaev, B.; Ågren, H.; Tian, H. Density Functional Theory Study of Photophysical Properties of Iridium(III) Complexes with Phenylisoquinoline and Phenylpyridine Ligands. J. Phys. Chem. C 2011, 115, 20724−20731. (80) Marian, C. M. Spin–Orbit Coupling and Intersystem Crossing in Molecules. WIREs Comput. Mol. Sci. 2012, 2, 187−203. (81) Reichardt, C.; Vogt, R. A.; Crespo-Hernández, C. E. On the Origin of Ultrafast Nonradiative Transitions in Nitro-Polycyclic Aromatic Hydrocarbons: Excited-State Dynamics in 1-Nitronaphthalene. J. Chem. Phys. 2009, 131, 224518. (82) Reichardt, C.; Wen, C.; Vogt, R. A.; Crespo-Hernández, C. E. Role of Intersystem Crossing in the Fluorescence Quenching of 2-Aminopurine 2[Prime or Minute]-Deoxyriboside in Solution. Photochem. Photobiol. Sci. 2013, 12, 1341−1350. (83) Vogt, R. A.; Gray, T. G.; Crespo-Hernández, C. E. Subpicosecond Intersystem Crossing in Mono- and Di(Organophosphine)Gold(I) Naphthalene Derivatives in Solution. J. Am. Chem. Soc. 2012, 134, 14808−14817. (84) Capellos, C.; Bielski, B. H. Kinetic Systems: Mathematical Description of Chemical Kinetics in Solution, Wiley Interscience, New York, 1972.


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

(85) Rasmusson, M.; Tarnovsky, A. N.; Åkesson, E.; Sundström, V. On the Use of TwoPhoton Absorption for Determination of Femtosecond Pump–Probe Cross-Correlation Functions. Chem. Phys. Lett. 2001, 335, 201−208. (86) Becke, A. D. Density‐Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (87) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (88) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-Adjustedab Initio Pseudopotentials for the Second and Third Row Transition Elements. Theor. Chim. Acta. 1990, 77, 123−141. (89) Tunell, I.; Rinkevicius, Z.; Vahtras, O.; Sałek, P.; Helgaker, T.; Ågren, H. Density Functional Theory of Nonlinear Triplet Response Properties with Applications to Phosphorescence. J. Chem. Phys. 2003, 119, 11024−11034. (90) Ågren, H.; Vahtras, O.; Minaev, B. Response Theory and Calculations of Spin-Orbit Coupling Phenomena in Molecules. Adv. Quantum. Chem. 1996, 27, 71−162. (91) Helgaker, T.; Jensen, H. J. A.; Jorgensen, P.; Olsen, J.; Ruud, K.; Ågren, H.; Andersen, T.; Bak, K. L.; Bakken, V.; Christiansen, O. Dalton, A Molecular Electronic Structure Program, Release 1.2, 2012. (92) Douglas, M.; Kroll, N. M. Quantum Electrodynamical Corrections to the Fine Structure of Helium. Ann. Phys. 1974, 82, 89−155.


46

Page 47 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(93) Hess, B. A. Relativistic Electronic-Structure Calculations Employing a Two-Component No-Pair Formalism with External-Field Projection Operators. Phys. Rev. A 1986, 33, 3742−3748.

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Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes

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