Ultrafast Dynamics of the Metal-to-Ligand Charge Transfer Excited

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A: Spectroscopy, Photochemistry, and Excited States

Ultrafast Dynamics of the MLCT Excited States of Ir(III) Proteo and Deutero Dihydrides Chelsea Marie Taliaferro, Evgeny O. Danilov, and Felix N. Castellano J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02266 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Ultrafast Dynamics of the MLCT Excited States of Ir(III) Proteo and Deutero Dihydrides Chelsea M. Taliaferro, Evgeny O. Danilov, and Felix N. Castellano* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States ABSTRACT For decades, transition metal hydrides have been at the forefront of numerous photocatalytic reactions leveraging either photoacid or photohydride generation. Of upmost importance is the nature of the M-H bond itself, which is typically the major site of photochemical reactivity, particularly in Ir(III) hydrides featuring metal-to-ligand charge transfer (MLCT) excited states. As a departure point for understanding the fundamental spectroscopy and photophysics of the MLCT excited states of Ir(III) diimine hydrides, cis-[Ir(bpy)2H2]+ (bpy = 2,2’-bipyridine) and its deuterated analog cis-[Ir(bpy)2D2]+ were prepared and investigated. The robust nature of these molecules enabled detailed solution-based photophysical studies using ultrafast transient absorption and infrared spectroscopy, executed without the generation of permanent photoproducts. Static FT-IR and Raman spectra (λex = 785 nm) of these two molecules revealed weak but measurable Ir-H and Ir-D stretching vibrations centered at 2120 cm-1 and 1510 cm-1, respectively. Short-lived (τ = 25 ps) MLCT excited states were observed for both cis[Ir(bpy)2H2]+ and cis-[Ir(bpy)2D2]+ following femtosecond pulsed laser excitation at 480 nm in

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visible and near-IR transient absorption experiments. A similar time constant was measured for the in-phase and out-of-phase Ir-H stretching modes of the triplet excited state between 1900 and 2200 cm-1 using transient IR spectroscopy. The Ir-D stretching modes in the MLCT excited state were masked by bpy-localized vibrations rendering quantitative evaluation of these modes difficult. The time-resolved infrared data were consistent with DFT-calculated mid-IR difference spectra in both of these molecules, yielding quantitative matches to the measured IR difference spectra. The information presented here provides valuable insight for understanding the primary photophysical events and transient absorption and IR spectroscopic signatures likely to be encountered throughout metal hydride photochemistry.

Introduction Despite their name, metal “hydrides” can be either hydridic or acidic, implying the nature of the metal hydride bond significantly affects its behavior in different solvent media. A popular example is the intensively investigated [Cp*Ir(bpy)H]+ (Cp* = pentamethylcyclopentadiene and bpy = 2,2’-bipyridine) which varies between hydridic and acidic character dependent upon solvent environment.1–6 Although there has been extensive research performed on the photochemical properties and product analysis of these Ir(III) hydrides, the photophysics and dynamics of these species and their highly reactive intermediates relevant to the Ir-H bond cleavage and/or formation remain largely unexplored. One example to the contrary was the observation of light-induced proton transfer from [Cp*Ir(bpy)H]+ to methanol, ultimately recombining on the millisecond scale, reforming the hydride species.4 More recently, Miller and coworkers probed excited state self-quenching reactions of this same molecule in CH3CN, suggesting that H2 generation directly resulted from a bimolecular reaction between an energized


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Ir(III) chromophore and ground state [Cp*Ir(bpy)H]+ species.7 While supra-nanosecond timescales are valuable for observing the formation of photoproducts from such reactions, ultrafast time-scales are necessarily required to study excited state kinetics related to the initial Ir-H bondbreaking and/or bond-forming chemistry. However, the presence of highly reactive species formed in such photoreactions makes it difficult to study the pure photophysics and the primary photoactivation events of the isolated compounds. Taking a step back from the spectroscopic observation of photochemical processes commonly observed in species such as [Cp*Ir(bpy)H]+, non-photoreactive Ir-H containing compounds become valuable for comprehending the spectroscopic signatures inherent to these Ir(III) hydrides. Transient absorption spectroscopy can often times be nondescript and does not permit clear observation of bond breaking/formation following photoexcitation if the products do not absorb light in a distinctive manner. Alternatively, vibrational spectroscopy can serve as a most useful method for investigating transition metal hydrides as the M-H bond and changes therein can be directly monitored in a localized and characteristic manner. M-H bonds are easily identifiable as they typically lie outside of the highly congested fingerprint region and are usually fairly broad (△νfwhm = 10-30 cm-1).8 Their intensities can vary, but they are almost always weaker than those characteristic of M-CO or M-CN bonds, which can also lie within the M-H window.1,8 Deuterium labeling provides another method of identifying terminal metal-hydride bonds. Upon replacing the hydride with a more massive deuterium atom, the frequency of the bond vibration drastically shifts to lower energy as a result of the increased reduced mass. Therefore, isotopic labeling is not only a useful characterization tool but can unequivocally identify the path of hydride or proton transfer photoprocesses in reactive transition metal hydrides. Additionally, this ability to directly monitor the M-H/D bond formation/breaking


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renders time-resolved infrared (TRIR) spectroscopy most valuable in photophysical studies of transition metal hydride complexes. Typically, TRIR experiments of transition metal complexes take advantage of strongly allowed absorptions from C≡O or C≡N bonds.9–17 IR bands due to MH bond stretching vibrations possess very low molar absorptivity for relevant excited state features to be nominally observed.1 However, improved sensitivity in these experimental apparatus’ now appear promising for observing such weaker but uniquely characteristic molecular vibrations. Combining the benefits of photostable Ir(III) hydrides with modern ultrafast transient IR (TRIR) techniques, M-H stretching modes following visible pumping of the MLCT excited state were observed for the first time. The work presented here represents a key step towards understanding the underlying photophysics of Ir(III) polypyridyl hydrides and the nature of the M-H bond(s) in the MLCT excited state. Moving forward, we seek to apply these results to more reactive metal hydrides where the underlying photophysics are difficult to evaluate when short-lived intermediates and other photoproducts become abundant.

Experimental Section General All commercially available reagents were used as purchased, without further purification. 1H and 13

C NMR spectra were collected on a 400 MHz Varian Innova Spectrometer, and the resulting

spectra were processed with the MestReNova 10.0.2 software package (Figure S1 and Figure S2). Electrospray ionization (ESI) mass spectra were measured at the Michigan State University Mass Spectrometry Core, East Lansing, MI (Figures S3 and Figure S4). Solid-state ATR-FTIR was performed using a Bruker Alpha ATR-FTIR and OPUS Spectroscopy Software (v. 7.2). Solid-state Raman spectra were collected at NCSU’s Analytical Instrumentation Facility (AIF) using a Horiba XploRA PLUS Confocal Raman Microscope. The precursors, [Irbpy2Cl2]PF6 and


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[Irbpy2CF3SO3]CF3SO3 were synthesized according to literature procedures, and structural characterization data matches previously reported values.18–22 cis-[Irbpy2H2]PF6. This molecule was synthesized using a slight modification of previously reported procedures.18,19 [Irbpy2CF3SO3]CF3SO3 (150 mg) was added to 4:1 H2O:EtOH (20 mL). NaBH4 (800 mg) was added, resulting in a dark green mixture that was refluxed for 1 hour under nitrogen. The final yellow-orange solution was then cooled to room temperature and aqueous KPF6 was added, producing a yellow-orange solid that was washed with water and dried. The solid was then dissolved in minimal acetone and recrystallized with excess diethyl ether. 67% yield. IR 𝜈max (cm-1): 2125, 2093 (Ir-H); 1H NMR (DMSO-d6, 400 MHz): δ 9.36 (d, 2H), 8.70 (t, 4H), 8.20 (t, 2H), 8.11 (t, 2H), 7.77 (d, 2H), 7.59 (t, 2H), 7.49 (t, 2H), -17.92 (s, 2H); 13C NMR (DMSO-d6, 100 MHz) δ 158.22, 156.86, 155.85, 147.74, 138.11, 136.78, 128.54, 127.43, 124.27, 123.71; ESI-MS m/z: calcd for 507.1166; found 507.1161. cis-[Irbpy2D2]PF6 Synthesized in the same manner as [Irbpy2H2]PF6; replaced the solvent environment with deuterated analogs, D2O and EtOD, and the reducing agent with NaBD4. 60% yield. IR 𝜈max (cm-1): 1527, 1505 (Ir-D); 1H NMR (DMSO-d6, 400 MHz): δ 9.36 (d, 2H), 8.70 (t, 4H), 8.20 (t, 2H), 8.11 (t, 2H), 7.77 (d, 2H), 7.59 (t, 2H), 7.49 (t, 2H); 13C NMR (DMSO-d6, 100 MHz) δ 158.22, 156.87, 155.87, 147.77, 138.14, 136.81, 128.57, 127.47, 124.30, 123.74; ESIMS m/z: calcd for 509.1287; found 509.1282. Ultrafast UV-VIS Transient Absorption (TA) Spectroscopy Sub-picosecond transient absorption (TA) measurements were performed at the NCSU Imaging and Kinetic Spectroscopy (IMAKS) Laboratory in the Department of Chemistry and used a Ti:sapphire laser system described previously.23 Briefly, a portion of the output from a 1 kHz Ti:sapphire Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into


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the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent OPerA Solo) to generate the 480 nm pump pulse used in these experiments, while the probe beam was delayed in a 6.6 ns optical delay stage. The probe beam was focused into a CaF2 crystal to generate white light continuum (WLC) spanning 340−750 nm or into the NIR with a proprietary crystal. The two beams were focused and spatially and temporally overlapped into a spot on the sample, with the relative polarizations of the pump and probe beams set at the magic angle. All solvents were fresh and spectrophotometric grade, and the ground-state absorption spectra were taken before and after each experiment using an Agilent 8453 UV−visible spectrophotometer to ensure there was no sample decomposition. Samples were prepared in 2 mm path length quartz cuvettes. The transient spectra and kinetics were obtained using a commercially available transient absorption spectrometer (Helios, Ultrafast Systems), averaging at least three scans and using 2 s of averaging at every given delay. Transient kinetics were evaluated using Igor Pro 7. Nanosecond UV-VIS Transient Absorption (TA) Spectroscopy Nanosecond transient absorption measurements were collected with a LP920 laser flash photolysis system (Edinburgh Instruments). Briefly, a tunable Vibrant 355 Nd:YAG/OPO system (OPOTEK) was used for pulsed laser excitation. To collect transient difference spectra in the visible portion of the spectrum, an iStar ICCD camera (Andor Technology), controlled by the LP900 software program (Edinburgh Instruments), was used. Samples were prepared in a glovebox prior to measurement in 1 cm path length quartz optical cells, prepared to have optical densities around 0.4 at the excitation wavelength (420 nm, ∼ 2.2 mJ/pulse). Ground state UV− vis absorbance measurements were taken before and after experiments to ensure sample stability. Transient absorption decay kinetics were fit and analyzed with IGOR Pro.


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Ultrafast Mid-IR Transient Absorption (TRIR) Spectroscopy Sub-picosecond mid-infrared time-resolved measurements were performed using an in-house built pump-probe transient absorption spectrometer. The output from a 1 kHz Ti:Sapphire Coherent Libra regenerative amplifier (4 mJ, 100 fs (fwhm) at 800 nm) was split into the pump and probe beams. The pump beam was directed into an optical parametric amplifier (Coherent OPerA Solo, UV-VIS) to generate tunable excitation (480 nm), delayed in a 3.3 ns optical delay stage, and then focused into the sample. An optical chopper synchronized with the laser output at 500 Hz was placed in the pump beam for delta OD calculation, and a wavelength-appropriate half-wave plate rotated polarization to ensure the excitation at the magic angle. The probe beam was directed into another optical parametric amplifier (Coherent OPerA Solo, DFG) to generate the Mid IR probe. After entering a N2-purged sample compartment, the probe beam was split 50/50 into probe and reference beams which were both focused into the sample sealed in a demountable flow cell with round 25 mm Diam. x 1 mm BaF2 windows and a variable spacer (0.95 mm) but only the probe was overlapped with the pump. Both probe and reference were re-focused onto the entrance slit of a Horiba Scientific iHR320 imaging spectrometer. The signal was collected using a 64x2 dual array MCT liquid N2–cooled detector (FPAS integrator and electronics from Infrared Systems Development Corporation). The experiment was controlled by in-house built LabVIEW software. Typically, 1600 laser pulses were averaged. To aid in sample stability, scanning was stopped shortly after signals returned to the baseline, though one full 3.3 ns scan was performed during setup to confirm no further features evolved after this point. The sample solutions were prepared to have FTIR absorbance values of ca. 0.3 at the metal hydride stretch. The ground state IR absorption spectra were taken


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before and after each experiment using a Bruker Vertex 80V FTIR spectrophotometer operating with OPUS v.7.2 software to ensure there was no sample decomposition observable in the midIR fingerprint regions. DFT Calculations The electronic structure calculations utilized in this study were performed using the Gaussian 0924 software package and the computation resources of the North Carolina State University High Performance Computing Center. Geometry optimizations were performed at the B3LYPD3/6-31G*/LANL2DZ level of theory.25–32 The polarizable continuum model (PCM) was used to simulate the acetonitrile solvent environment for all calculations.33 Frequency calculations were performed on all optimized structures and no imaginary frequencies were obtained. The representative Gaussian input file is available in section S14.

Results and Discussion +

+

N N

N H

N

H

N

Ir N N

1

D Ir D N

2

Figure 1. Molecular structures of cis-[Ir(bpy)2H2]PF6 (1) and cis-[Ir(bpy)2D2]PF6 (2). Synthesis and Characterization The proteo and deutero containing Ir(III) bipyridyl complexes 1 and 2 (Figure 1) were synthesized using a modified 3-step synthesis originally reported by Meyer and coworkers.18,19 Starting with commercially available 2,2’-bipyridine and IrCl3•xH2O, the dichloride,


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[Ir(bpy)2Cl2]PF6 was prepared in high yield before replacing the chlorides with triflate groups using CF3SO3H.18,19,34,22 The reducing agents NaBH4 and NaBD4 in either CH3OH or CH3OD, respectively, were then used to synthesize 1 and 2, in moderate yields.18,19 Each product was purified through multiple recrystallizations. The final synthesized molecules were structurally characterized by 1H and 13C NMR spectroscopy, solid-state FT-IR and Raman spectroscopy, and ESI-MS.

FT-IR Spectroscopy and Solid-State Raman Spectroscopy

Normalized Absorbance

(a)

2200 2100 2000 -1 Wavenumber (cm )

1600 1500 1400 -1 Wavenumber (cm )

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

(b)

Normalized Intensity

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


2200 2100 2000 -1 Raman Shift (cm )

1550 1500 1450 -1 Raman Shift (cm )

2200

2000

1800

1600

1400

1200

-1

Raman Shift (cm )

Figure 2. (a) Solid-state FT-IR (ATR) and (b) Raman spectra (λex = 785 nm) of 1 (red) and 2 (blue). Insets depict enlarged Ir-H and Ir-D bands.


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Figure 2 presents the solid-state FT-IR (ATR) and Raman spectra (λex = 785 nm) measured for 1 and 2. Deuterium labeling resulted in red-shifting the metal hydride stretching frequencies between 1 and 2, making these techniques facile indicators of successful isotope substitution. The magnitude of the isotopically induced frequency shift can be approximated using Equations 1 and 2, where µ is the reduced mass calculated from respective masses of the two bonded atoms (Ir and H or D; ma and mb, respectively) and ν is the vibrational frequency value (in cm-1) calculated from the force constant (k), µ, and the speed of light c. Using Eq. 2 with the measured Ir-H stretching frequency (2120 cm-1) and µIr-H (~1), k can be readily calculated and then substituted into the same equation with µIr-D (~2) to estimate the Ir-D frequency as 1500 cm-1, which is in quantitative agreement with experimental FT-IR and Raman data presented in Figure 2. ! !

µμ = ! !!!! !

ν=

!

!

!

!"!

!

(1) (2)

Upon inspection, the relevant IR and Raman spectra reveal symmetric and anti-symmetric H-MH stretches are only separated by a few wavenumbers. The higher frequency vibration has been assigned to the symmetric stretch and the anti-symmetric stretch to the lower frequency vibration using DFT calculations as described below. The FTIR results are useful indicators for the relevant spectroscopic windows to be used in transient IR studies where the Ir-H/D vibrations are probed. In particular, the lack of bpy-and solvent-based vibrations near the characteristic Ir-H stretches near 2100 cm-1 will be shown to be paramount in evaluating the excited state features of 1. Unfortunately, the Ir-D stretches were shifted into a highly congested spectral region inhabited by many bpy-localized vibrations; a


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situation that was detrimental for quantitatively studying the Ir-D modes of 2 in the MLCT excited state. DFT calculations were also used to assign the structuring of the Ir-D stretch in the FTIR data in Figure 2b as mixed bpy and Ir-D vibrations.

Ultrafast Transient Absorption (TA) Spectroscopy

Figure 3. Sub-picosecond transient absorption difference spectra of 1 measured in acetonitrile following excitation by 480 nm laser pulses (0.436 µJ/pulse, 100 fs fwhm). The NIR difference spectra are presented in the inset measured under identical experimental conditions. The laser scatter surrounding the excitation wavelength at 480 nm was removed for clarity. The MLCT excited states of both complexes 1 and 2 were measured on sub-nanosecond timescales using transient absorption difference spectra and single wavelength kinetics following excitation at 480 nm. Four positive features were evident at 370, 430, 500, and the tail end of a broad feature starting below 900 nm, as can be seen in Figures 3 and S5. Absorption features at 370 and 500 nm are common markers used to confirm the formation of bpy•- which results from transient MLCT one-electron oxidation of the metal center and one-electron reduction of bpy.35–


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The NIR difference spectra also agree with established spectra of other bpy-containing MLCT

complexes where a broad absorption feature appears between 600-1000 nm.37,38 The entire absorption profile of the excited state follows the identical decay kinetics as a function of monitoring wavelength, suggesting all the transient features observed originate from the same lowest energy MLCT excited state. The MLCT excited state in 1 is short lived, as presented in Figure S6, with a single exponential lifetime of 25 ps for all transient features and no long-lived excited states were persistent in supra-nanosecond transient absorption spectroscopy experiments. TA data for 2 revealed similar spectral features and excited state lifetime (Figures S5 and S6), suggesting no obvious deuterium isotope effect on MLCT excited state decay. The short lifetime and resulting photostability of these complexes verified their utility as non-reactive species ideal for photophysical investigations of Ir(III) metal hydrides. Photostability was confirmed by UV-Vis absorption spectroscopy performed before and after all TA experiments confirming no changes to their ground state absorption spectra, highlighting the robustness of these molecules for photophysical investigations. The analogous compound cis-[Irbpy2Cl2]+ was also briefly studied using ultrafast and nanosecond TA. Although the short-lived features seen in Figure S7 are similar to those of 1 and 2, the lifetime is about halved at about 12 ps. Strikingly, there is a long-lived excited state not seen in the TA spectra of 1 or 2. This long-lived state, also belonging to bpy•- is visible using nanosecond TA in Figure S8.35–37 These stark differences between the dichloride and 1 and 2 suggest replacing the chlorides with strongly σ-donating hydrides separates the very small energy gap believed to exist between the π- π* and d- π* states in the dichloride.20,21,39–45 Ultrafast Transient IR (TRIR) Spectroscopy


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Figure 4. Experimental TRIR difference spectrum of 1 measured in acetonitrile following excitation at 480 nm (5 µJ/pulse, 100 fs fwhm). The inset depicts the DFT calculated IR difference spectrum of 1 over the same Ir-H spectral window. The M-H stretching modes observed by TRIR spectroscopy in the MLCT excited state of 1 following 480 nm excitation are presented in Figure 4. The high photostability of this molecule allows for high excitation powers (5 µJ/pulse) to be used without use of a flow cell to reconcile the low extinction coefficients characteristic of the Ir-H vibrations. Bleaching signals were observed for both the proteo and deutero compounds at 2128 and 1523 cm-1, respectively (Figure 4 and Figure 5b). However, these observed bleaches were broad, structureless, and do not resolve the symmetric and anti-symmetric H-M-H stretches. No other vibrational modes were observed (or calculated to be present) across the 2000-2200 cm-1 energy window allowing for clear fingerprint observation of the stretching modes of 1 in the MLCT excited state. This proteo molecule exhibited two distinct transient absorption features, one that is better resolved at higher energy and another that is less resolved and somewhat broad at lower energy. The higher energy stretch is similar in intensity and shape to the bleaching feature and was expected as a result of


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MLCT oxidation of the Ir(III) center to Ir(IV), which should transiently increase the strength of the Ir-H bonds in the excited state. The window could not be moved to higher energy to observe more of this feature as the solvent absorbs appreciably past this point. The significantly broader lower energy feature was unexpected and as the presented spectra are the result of background subtraction, is not simply a result of experimental artifacts. One possible explanation for the appearance of this second feature is the larger energy separation between symmetric and antisymmetric stretching modes of H-Ir-H modes in the triplet excited state, as revealed through the electronic structure calculations described below. The extreme broadness of this feature is also unusual, and when the window is shifted to lower energy as in Figure S9, it appears to end just below 1900 cm-1. To test for possible solvent interactions, this experiment was repeated in DCM, though 1 proved photochemically unstable in this solvent. No features were observed when 2 was examined at this window.

Figure 5. Ultrafast TRIR difference spectra of (a) 1 and (b) 2 in acetonitrile-d3 following 480 nm excitation. The Ir-D stretches appear at much lower energy due to the increase in reduced mass. Unfortunately, these stretching modes are moved to the center of the spectroscopic window occupied by multiple bpy vibrations, which can be seen in the FTIR spectra presented in Figure


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2. All TRIR studies performed in the 1400-1650 cm-1 window had to be measured in CD3CN which doesn’t absorb as strongly as CH3CN in this region of the spectrum. The TRIR difference spectra revealed multiple transient features at high and low energy when compared to the bleach -1

at 1523 cm , making it difficult to identify any pure Ir-D stretching mode shift resulting from MLCT excited state formation. When observing the relevant shifts for 1 in this lower energy window, the same positive features were still present confirming they do not belong to any Ir-D related stretch (Figure 5). The locations of bpy vibrations present for 1 and 2 agree with previous transient infrared experiments performed on [Ru(bpy)3]+.46–48 Lifetimes of the features within the 1430-1650 cm-1 window were measured to fall between 25 and 30 ps, suggesting all transient features originate from the identical excited state, confirming there was no observed isotope effect on the MLCT excited state lifetimes (Figure S11). Although infrared spectroelectrochemistry would be a useful technique here to further evaluate these molecules, enabling confirmation of all TRIR features, 1 unfortunately irreversibly oxidizes at the Ir(III) center on electrochemical time scales,18,19 precluding any possible bulk electrolysis characterization. DFT Calculated IR Difference Spectra Calculated IR difference spectra were generated for comparison to the experimental TRIR data. All electronic structure and spectral simulation calculations were performed using the Gaussian09 software package. Geometries of both Ir(III) complexes were optimized at the B3LYP-D3/6-31G*/LANL2DZ level of theory. The polarizable continuum model was used to simulate the acetonitrile solvent environment used in all TRIR experiments. [Ir(dmbpy)2Cl2]+ (dmbpy = 4,4′-dimethyl-2,2′-bipyridine) was used as the benchmark for geometry optimizations as there are no available crystal structures for 1 or 2.49 Calculated TRIR spectra were produced


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by subtracting the optimized ground state’s structure’s normalized IR data from that of the lowest triplet excited state; both are shown in Figure S12. Upon inspection, the experimentally determined TRIR frequencies for 1 as presented in Figure 4 and Figure 5 agree well with calculated difference spectra shown in Figure 4 (inset) and Figure S13. According to the simulated difference spectrum, the symmetric and anti-symmetric stretches are essentially split into two different energy regions in the MLCT state of 1. This most likely explains the appearance of the two energetically well-separated transient features in the experimental TRIR data occurring at both low and high energy with respect to the vibrations in the ground state of this molecule. Based on the electronic structure calculations, the higher energy feature (2156 cm-1) is therefore assigned to the symmetric stretch while the lower energy feature (2086 cm-1) is attributed to the anti-symmetric stretch in the MLCT excited state. The calculated IR difference spectrum of 2 also agrees with experimental TRIR results in that there is good overlap between the observed vibrations in the 1430-1650 cm-1 window of 1 and 2, except for the ground state bleach occurring near 1500 cm-1 that is only present in 2.

Conclusions The photophysical characterization of cis-[Ir(bpy)2H2]+ and cis-[Ir(bpy)2D2]+ has been presented in order to better understand the fundamental spectroscopy of the MLCT excited state of Ir(III) diimine hydrides. These compounds proved to be particularly robust, without producing permanent photoproducts, rendering them ideal for fundamental photophysical studies of the IrH bond vibrational dynamics. Short-lived (τ = 25 ps) MLCT excited states were observed in both compounds using ultrafast transient UV-vis and mid-IR absorption spectroscopy. TRIR results revealed two Ir-H stretching modes surrounding the ground state bleach of the dihydride at 2130 cm-1. These features have been assigned to symmetric and anti-symmetric stretching modes of


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the H-Ir-H fragment in the MLCT excited state using computational methods. Similar observations could not be made for the deutero compound, where the Ir-D stretching modes in the excited state were masked by bpy-localized vibrations. The transient absorption and IR spectroscopic features presented here provide insight into the photophysical behavior of Ir(III) diimine hydrides and can be utilized in future investigations involving photocatalytic hydrides and their highly reactive intermediates, particularly in relation to the production of solar fuels. ASSOCIATED CONTENT Supporting Information Available: Additional experimental and calculated TA and TRIR spectra and their associated kinetic fits for 1 and 2, their 1H and 13C NMR and ESI-MS spectra, ultrafast and nanosecond TA of cis-[Irbpy2Cl2]+, and the Gaussian input file for 1 are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Felix N. Castellano. Email: [email protected] ACKNOWLEDGMENT This work was supported by the National Science Foundation (CHE-1465068). The solid state Raman spectra were acquired at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (Award Number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).


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Ultrafast Dynamics of the Metal-to-Ligand Charge Transfer Excited

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