Photochemical Excitation, Transient Spectroscopy and Luminescence Spectroscopy of [NiH( tBu P=N=P)] (12)

Based on the simplified mechanistic picture presented in Chapter 2.4.1, the excited state evolution of hydride 12 was investigated to extend the mechanistic understanding of the transformation of 12 to 16. As shown in Figure 94, 12 shows strong absorption in the near UV spectrum, indicating ligand centered or charge transfer transitions. Interested in the nature of the electronic excitation responsible for the observed photochemical behavior of 12, TD-DFT analysis of 12 was performed.²⁵ Figure 92a presents the overlap of the experimental and computational electronic spectra of 12, showing a good agreement of the fine structure with a blue shift of approximately 0.4 eV which is commonly observed in TD-DFT.^[317]

Figure 94: (a) Experimental UV-vis spectrum of 12 in THF and electronic transitions predicted by TD-DFT (ZORA-b3LYP/def2-TZVPP (Cosmo)). (b) Difference electron density of the transition at  = 304 nm (blue color denotes

increase and red color denotes a decrease in electron density; the hydride ligand is not shown).

The strongest absorption in the region responsible for the photochemical reactivity of 12 is present at

 = 334 nm in THF solvent. Theory predicts this transition to be a MLCT type transition which can be described as excitation from the metal centered d_z2 to a pincer ligand centered * orbital (Figure 94b).

Formal dehydrogenation of the pincer ligand therefore is crucial for by introducing low lying ligand centered unpopulated orbitals.

25 Computational analysis was performed by Dr. Markus Finger.

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While optical excitation proceeds within t = 10^-15 s, the subsequent excited state evolution takes place on a picosecond timescale and can therefore be monitored using transient UV-pump spectroscopy which has a time resolution of 10^-13 s. UV-pump-UV-vis- and IR-probe spectroscopy can be performed using a translation stage to adjust the time delay between the pump and probe pulse and is utilized to investigate 12 and isotopologue 12-D in THF solution under argon atmosphere.²⁶

Excitation of a THF solution of 12 at exc. = 385 nm results in immediate broad absorption in the whole visible spectrum (Figure 95a). Within 1 ps, the transient undergoes a decrease of the absorption above  = 500 nm. The transient absorption further gets lower in intensity on the picosecond timescale and a persistent absorption centered at  = 450 nm is obtained. The time trace of the spectral behavior at  = 550 nm and

 = 480 nm is used to extract the time constants  by a global biexponential fit giving 1 = 0.9±0.5 ps,

2 = 13±1 ps and 3 >> 1 ns. While the first time constant1 is attributed to fast internal conversion and vibrational cooling from the initial populated Frank-Condon-state, the second time constant 2 is consistent with internal conversion and vibrational cooling into the electronic ground state of 12. Time constant 3

describes the conversion of a persistent species which exceed the timescale of the experiment (1 ns).

Two possibilities arise for the assignment of the persistent absorption which is visible in the TR-UV-vis spectrum: staying on the singlet hypersurface, a lifetime in the nanosecond timescale of a metal complex most likely means formation of a photoproduct PP since fluorescence and thermal relaxation processes in most cases rule out long-lived excited state of the same multiplicity as the ground state.^[316] Assuming intersystem crossing (ISC), population of a triplet state is possible as well, resulting in slow radiative (phosphorescence) and non-radiative (ISC) relaxation processes. While efficient ISC is rarely observed in 3d metal complexes due to a lack of spin-orbit coupling, Scholes and Doyle have recently shown that a series of [NiX(o-tolyl)(dtbbpy)] (X = Cl, Br, I, dtbbpy = 4,4’-di-tert-butyl-2,2’-bipyridyl) complexes undergoes formation of long-lived ³MLCT states after ISC from ¹MLCT and ¹LLCT states on the picosecond timescale.^[318] As an alternative to spin orbit coupling, close energetic proximity of excited singlet and triplet states giving rise to strong multiconfigurational interactions is discusses as the origin of the unusual fast ISC process.

26 Transient spectroscopy was performed by Dr. Jennifer Ahrens from the group of Prof. Dirk Schwarzer, Max Planck Institute for biophysical chemistry.

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Figure 95: Transient UV-vis spectra generated by exc. = 385 nm excitation of a 6 mM solution of (a) 12 and (b) 12-D in THF for selected pump-probe delays. The stationary UV-vis spectrum of 12 in THF solution is shown on top,

while the time traces with global triexponential fits are shown as inlets.

Transient UV-vis-probe spectroscopy of isotopologues 12-D gives similar results as 12 (Figure 95b). The time constants  show minor variation with 1 = 1.1±0.2 ps, 2 = 10±1 ps and 3 >> 1 ns. The determined time constants allow for the calculation of rates with = 1/k and therefore kinetic isotope effects (KIEs) giving rise to KIEt1 = 0.8 and KIEt2 = 1.2 for the two processes, respectively (see Chapter 2.4.3). These values are best interpreted as negligible given the error of the experiment. In general, care has to be taken upon interpretation of the kinetic isotope effect determined from transient spectroscopy: Photochemical excitation proceeds vertically, as stated by the Franck-Condon principle, therefore resulting in population of vibrational excited states, resulting in a lower difference in activation energy between both isotopologues which accordingly results in a lower KIE, experimentally.^[319] Additionally, the determined quantum yield

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of Φ410 = 9% for conversion of 12 to 16 suggests that repopulation of the vibrational and electronic ground state after photoexcitation might contribute more strongly to the determined lifetime than the actual formation of the persistent species PS.

To obtain structural information on the species observed in the transient UV-vis spectrum, transient IR spectra of 12 and 12-D were recorded. The spectral region containing the Ni-H stretch can be accessed using THF solvent. To observe the C-C double bond stretch and Ni-D stretch resonating at lower energy, measurement in THF-d8 is required. The combination of these two solvents allows for the measurement in the region of ῦ =1250–3500 cm^-1 in which no additional transient resonances can be observed aside from the signals shown in Figure 96.

Figure 96: Transient IR spectra generated by exc. = 400 nm excitation of (a) a 11 mM solution of 12 in THF and (b) a 3 mM solution of 12 in THF-d8 for selected pump-probe delays. The stationary IR spectrum of 12 in THF/ THF-d8

solution are shown on top, while the time traces with biexponential fits are shown as inlets.

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Starting with the Ni-H stretch shown in Figure 96a, an immediate bleach of the ground state vibration at ῦ = 1850 cm^-1 is observed. The spectral evolution shows gradual relaxation to the ground state, which results in complete repopulation after ca. 80 ps. The absence of any positive signals in the region between 1775 cm^-1 and 1900 cm^-1 shows that photochemical excitation has a strong influence in the Ni-H stretching vibration.

The repopulation of the ground state indicates that the persistent species monitored in the TR-UV-vis spectrum shows either the exact same IR signature in the Ni-H stretch region as parent 12 or, most likely, cannot be observed due to lower spectral resolution and a change in excitation wavelength from

exc. = 385 nm in the TR-UV-vis (TR: time resolved) to  exc. = 400 nm in the TR-IR. The biexponential fit of the time trace at the signal peak gives time constants which are in good agreement with the TR-UV-vis measurement. Monitoring the C-C double bond stretch also shows an immediate bleach at ῦ = 1518 cm^-1 which repopulates within ca. 80 ps confirming the absence of the persistent species in TR-IR (Figure 96b).

However, a positive red shifted signal can be observed, indicating population of π* orbitals in the excited states, which is in agreement with a d_z2-π*-MLCT predicted by theory. Time traces at ῦ = 1518 cm^-1 and ῦ = 1505 cm^-1 show similar spectral evolution, giving biexponentially fitted time constants 1 = 1.0 ps and

2 = 12 ps, and globally fitted time constants 1 = 1.3±0.2 ps and 2 = 12±0.5 ps. While the Ni-H and C-C bond bleach show no underlying peak structure, the positive resonance at ῦ = 1507 cm^-1 undergoes dynamic behavior which results in a shift of the maximum to ῦ = 1513 cm^-1 after 7.8 ps. This can be interpreted as two overlapping resonances which decay with different time constants.

Similar to the bleach of the Ni-H stretch in TR-IR of 12, transient spectra of deuterated 12-D show a bleach of the Ni-D fermi doublet at ῦ = 1328 cm^-1 and ῦ = 1340 cm^-1 (Figure 97a, see Chapter 1.4.3 for detailed description of the IR spectrum of 12-D). Repopulation with 1 = 10±2 ps fitted globally over both resonances is in good agreement with the value extracted from the Ni-H vibration (the initial very fast process is not considered in the global fit). Comparing the region at ῦ = 1470–1540 cm^-1 however shows a difference between both isotopologues (Figure 97b). While the shape and temporal evolution of the bleach is mostly identical, it is shifted to ῦ = 1512 cm^-1 for 12-D. Since both spectra are measured in 3 mM solution, the magnitude of the bleach is nearly identical with approximately 2 mOD. Upon comparing the positive signals of 12 and 12-D a huge difference in the intensity can be observed with 12* showing approximately threefold signal strength. Importantly, two maxima at ῦ = 1505 cm^-1 and ῦ = 1507 cm^-1 are observed in the excited state 12-D*. In contrast to 12 an overall increase in intensity of the positive signal is observed until t = 4.8 ps. Over this time, the maximum shifts from ῦ = 1505 cm^-1 to ῦ = 1507 cm^-1, therefore resembling the behavior observed for 12. Additionally, measurement at short pump-probe delay (t < 1 ps) reveals vibrationally hot C-C stretching vibrations for 12-D*.

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The difference in intensity between the resonance monitored for 12 and 12-D is best interpreted as an overlap of the C-C double bond and Ni-H stretching vibration in the transient spectrum. The transient Ni-H stretch is therefore visible with ῦ = 1507 cm^-1 in the initial spectral evolution of 12, whereas 12-D shows only the transient C-C double bond. Importantly, the huge difference in ῦ(Ni-H) for the electronic ground state and the transient spectrum indicates population of an electronically excited state rather than a vibrationally excited molecule in the electronic ground state. The timescale of conversion of this electronically excited state 2 = 12±0.5 ps suggests population of the S1 state. While transition metal hydride complexes have been examined by transient infrared spectroscopy, no excited state hydride stretching vibrations are reported in the literature.^[247]

Figure 97: Transient IR spectra generated by exc. = 400 nm excitation of (a) a 11 mM solution of 12-D in THF-d8

and (b) a 3 mM solution of 12-D in THF-d8 for selected pump-probe delays. The stationary IR spectrum of 12-D in THF-d8 solution are shown on top, while the time traces with biexponential fits are shown as inlets.

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UV-pump-UV-vis-pump spectroscopy reveals the formation of a persistent species PS with a life time allowing for bimolecular reactivity. Since this species is not observed in the UV-pump-IR-probe experiment, it is not possible to state if this species is produced from S1 or higher electronically excited states Sn. However, TR-IR spectroscopy gives structural information on the excited state species populated after photoexcitation of 12. Initially, weakening of the pincer C-C double bond is observed which is in agreement with population of * orbitals in the Franck-Condon state as predicted by theory. Internal conversion (IC) results in population of the S1 state which shows severe lowering of the Ni-H bond based on a red shift of the Ni-H stretching vibration by Δῦ = 343 cm^-1 compared to the ground state. Such a strong influence on the Ni-H bond order is in agreement with population of *Ni-H orbitals, and therefore population of a metal centered excited state with occupation of d_x2-y² (Figure 98). Summing up, the excited state evolution of 12* results in population of state similar to the product obtained by excitation of a d-d transition.

However, this state is obtained by a MLCT transition which shows a much greater extinction coefficient than symmetry forbidden metal centered transitions.

Figure 98: Simplified excited state evolution of 12.

While kinetic analysis of the reaction of 12 to 16 allow for the formulation of a simple mechanistic model, transient spectroscopy and computational analysis give further insight in the initial photophysical events. In transient UV-vis spectroscopy a persistent absorption at  = 450 nm is observed and assigned to the persistent species PS proposed based on kinetic analysis (Chapter 2.4.1), which is either a long-lived triplet state or a photoproduct.

To determine whether population of triplet excited states plays a role in the photochemistry of 12, luminescence measurement of a benzene solution of 12 was performed. The long life time of triplet states usually results in efficient phosphorescence, even though Scholes and Doyle do not report luminescence for their nickel complexes.^[318] Fluorescence on the contrary competes with fast internal conversion, therefore lowering the luminescence quantum yield.^[152] While most data on the conversion 12 to 16 is obtained in THF solvent, polar solvents may result in excited state quenching due to exciplex formation, therefore rendering benzene more suitable for luminescence measurement.^[320]

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Figure 99: Fluorescence spectra of (black) benzene and (red) a 1∙10^-4M solution of 12 in benzene.

The emission spectrum of a 1∙10^-4M solution of 12 in benzene shows a sharp feature at  = 386 nm close to the excitation wavelength of exc. = 345 nm (Figure 99). The energetic difference ῦ = 3079 cm^-1 between excitation wavelength and observed emission suggests Raman scattering at the solvent C-H bond which is confirmed by blank measurement. Since no significant emission of an excited state can be observed by fluorescence spectroscopy, population of triplet states by photoexcitation of 12 is excluded. Accordingly, the long-lived species proposed by kinetic anlysis and observed in transient spectroscopy is assigned as a ground-state photoproduct PP.