Ligand Rigidity Steers the Selectivity and Efficiency of the Photosubstitution Reaction of Strained Ruthenium Polypyridyl Complexes

While photosubstitution reactions in metal complexes are usually thought of as dissociative processes poorly dependent on the environment, they are, in fact, very sensitive to solvent effects. Therefore, it is crucial to explicitly consider solvent molecules in theoretical models of these reactions. Here, we experimentally and computationally investigated the selectivity of the photosubstitution of diimine chelates in a series of sterically strained ruthenium(II) polypyridyl complexes in water and acetonitrile. The complexes differ essentially by the rigidity of the chelates, which strongly influenced the observed selectivity of the photosubstitution. As the ratio between the different photoproducts was also influenced by the solvent, we developed a full density functional theory modeling of the reaction mechanism that included explicit solvent molecules. Three reaction pathways leading to photodissociation were identified on the triplet hypersurface, each characterized by either one or two energy barriers. Photodissociation in water was promoted by a proton transfer in the triplet state, which was facilitated by the dissociated pyridine ring acting as a pendent base. We show that the temperature variation of the photosubstitution quantum yield is an excellent tool to compare theory with experiments. An unusual phenomenon was observed for one of the compounds in acetonitrile, for which an increase in temperature led to a surprising decrease in the photosubstitution reaction rate. We interpret this experimental observation based on complete mapping of the triplet hypersurface of this complex, revealing thermal deactivation to the singlet ground state through intersystem crossing.


Experimental Supporting Information General synthesis
Dry DMF was obtained by storing a freshly opened bottle of DMF over 3 Å molecular sieves under N2 atmosphere. Dry acetonitrile was collected from a Pure Solve MD5 dry solvent dispenser (Demaco). All other reagents and solvents, were purchased from Sigma-Aldrich, and used as received. Ruthenium complex synthesis was conducted in oxygen-free atmosphere using standard Schlenk-line techniques in the absence of light. Flash column chromatography was performed on silica gel (Screening devices B.V.) with a particle size of 40-64 µm and a pore size of 60 Å. TLC analysis was conducted on TLC aluminium foils with silica gel matrix (Supelco, silica gel 60, art. nr. 56524) with detection by UV-absorption (254 nm). cis-[Ru(bpy)2(CH3CN)2](CF3SO3)2 was prepared following a literature method. 1 All 1 H NMR spectra were recorded on a Bruker AV-300 or AV-400 spectrometer. Chemical shifts (δ) are indicated in ppm relative to TMS or the solvent peak. Mass spectra were recorded by using a MSQ Plus Spectrometer fitted with a Dionex automatic sample injection system. High resolution mass spectra were recorded by direct injection (2 µl of 1 µM solution in MeOH or acetonitrile and 0.1% formic acid) in a mass spectrometer (Thermo Finnigan LTQ Orbitrap) equipped with an electrospray (250 °C) with resolution R = 60,000 at m/z = 400 (mass range m/z = 150-2000) and dioctylphthalate (m/z = 391.28428) as a lock mass. The high-resolution mass spectrometer was calibrated prior to measurements with a calibration mixture (Thermo Finnigan).

Single crystal X-ray crystallography of [2a](PF6)2
Single crystals were obtained by slow vapour diffusion of 2-propanol (~ 3 mL) into a solution of [2a](PF6)2 (1.0 mg, 1.1 μmol) in MeOH (1.0 mL), which was filtered through a 0.2 μm cellulose acetate syringe filter prior to the start of the crystallization. Red, needle-shaped crystals of X-ray quality were obtained after ten days. The structures crystallizes in the C2/c space group, unlike the crystal structure of this compound published earlier by Yoshikawa et al. (space group P21/c). 6 All reflection intensities were measured at 110(2) K using a SuperNova diffractometer (equipped with Atlas detector) with Mo Kα radiation ( = 0.71073 Å) under the program CrysAlisPro (Version 1.171.36.32, Agilent Technologies, 2013). The same program was used to refine the cell dimensions and for data reduction. The structure was solved with the program SHELXS-2014/7 (Sheldrick, 2015) and was refined on F 2 with SHELXL-2014/7 (Sheldrick, 2015). Numerical absorption correction based on Gaussian integration over a multifaceted crystal model was applied using CrysAlisPro. The temperature of the data collection was controlled using the system Cryojet (Oxford Instruments). The H atoms were placed at calculated positions using the instructions AFIX 43 or AFIX 137 with isotropic displacement parameters having values 1.2 or 1.5 Ueq of the attached C atoms. The structure is partly disordered.
The two PF6 − counter ions are disordered over two orientations, and the occupancy factors of the major components of the disorder refine to 0.875(3) and 0.793 (6).

Singlet oxygen generation and phosphorescence quantum yield of [1a-2b](PF6)2
The quantum yields of singlet oxygen generation and phosphorescence were determined in a custom-built setup described below (Scheme S1). 500 μL of sample, consisting of the compound in deuterated methanol, was added to a 104F-OS semi-micro fluorescence cuvette from Hellma Analytics, and placed in the temperature-controlled cuvette holder. The sample was allowed to equilibrate at 298 K for 5 minutes. Emission spectroscopy was performed with a 450 nm fibre-coupled laser (LRD-0450, Laserglow), which was set to 50 mW at the cuvette (4 mm beam diameter; 0.4 W•cm −2 ) at a 90° angle with respect to the spectrometer. The excitation power was measured using a S310C thermal sensor connected to a PM100USB power meter (Thorlabs). The emission spectra were recorded using two separate spectrometers for the UV-Vis and NIR emission, i.e. from 300 nm to 1000 nm for the phosphorescence of the complex (Avantes 2048L StarLine spectrometer) and from 1000 nm to 1700 nm for the phosphorescence of singlet oxygen ( 1 g) around 1275 nm (Avantes NIR256-1.7TEC spectrometer, detector set to −12 °C). The infrared emission spectrum was acquired within 9 seconds, after which the laser was turned off directly. Similarly, the visible emission spectrum was acquired within 2 seconds. UV-Vis absorption spectra before and after emission spectroscopy were measured using an Avalight-DHc halogen-deuterium lamp (Avantes) as light source (turned off during emission spectroscopy) and the before mentioned UV-Vis spectrometer as detector, both connected to the cuvette holder at a 180° angle. All spectra were recorded using Avasoft 8.5 software from Avantes and further processed using Microsoft Office Excel 2010 and Origin Pro 9.1 software.
The quantum yields of phosphorescence and singlet oxygen production was calculated using the relative method with [Ru(bpy)3]Cl2 as the standard (  = 0.73, 7,8  P = 0.015 in CD3OD), according to Equation S2. The phosphorescence quantum yield of [Ru(bpy)3Cl2] in CD3OD was determined by absolute methods (see below).
where  is the quantum yield, A 450 is the absorbance at 450 nm (always kept below 0.1 for a 4 mm path length), E is the integrated emission peak of singlet oxygen at 1270 nm or the integrated phosphorescence emission peak between 520 and 950 nm, and sam and std denote the sample and standard, respectively.

Absolute phosphorescence quantum yield of [Ru(bpy)3Cl2]
The phosphorescence quantum yield (P) of [Ru(bpy)3Cl2] in deuterated methanol in air at room temperature (293 ± 2 K) was determined by absolute methods using an integrating sphere-based setup, to serve as a standard for relative phosphorescence quantum yield measurements. P was found to be 0.015 ± 0.002. In order to validate the method used, P was also determined in water, where we obtained a P in air of 0.044 ± 0.005, which compares well to the literature value (0.040 ± 0.002). 9 Experimental setup: The integrating sphere setup used for determining the phosphorescence quantum yield is depicted in Scheme S2. The excitation source was a fibre-coupled CW 450nm diode laser (LRD-0450, Laserglow), coupled into a 200-μm multimode optical fibre, leading to a collimating lens (F220SMA-B, Thorlabs). After collimation, the light passed a mechanical iris to produce a 4-mm diameter beam (vide infra) with 10 mW optical power (Pexc = 80 mW•cm −2 ). The excitation power was measured using a S310C thermal sensor connected to a PM100USB power meter (Thorlabs). A PTFE-coated AvaSphere-30-IRRAD integrating sphere (30 mm diameter, reflectance > 98%), fitted with three ports (entry, exit and sample port), and an AvaSpec-ULS2048L StarLine CCD spectrometer were obtained from Avantes. The integrating sphere and spectrometer were calibrated together using an AvalightHALCALISP30 NIST traceable calibration lamp from Avantes (9.5% relative uncertainty versus NIST standard), so that the observed intensities are expressed as a photon flux (mol photons•s −1 •nm −1 ). The filter holder was fabricated by our own mechanical department, and held a NE520A (OD = 2) absorptive neutral density filter (Thorlabs) or a 500nm longpass filter (FEL0500, Thorlabs, T520-1000 nm = 92.1 ± 0.9%). An Avalight-DHc (Avantes) deuterium-halogen lamp was used as a white-light source for the determination of the transmission functions of the filters used. The spectra were recorded with Avasoft 8.5 software from Avantes, and further processed with Microsoft Office Excel 2010 and Origin Pro 9.1 software.
Experimental procedure: A measurement tube, made of a quartz EPRtube bottom (± 7 cm length) fused to a NS14 glass connector (± 2 cm length), was filled with a solution of [Ru(bpy)3Cl2] (50 µL, 50 µM in CD3OD), and closed with a septum under air. A second tube was filled with CD3OD (50 µL) and served as the blank. The tube precisely fitted into a hole made in the integrating sphere, and was suspended in the centre of the sphere, in the middle of the excitation laser beam. The laser diode was allowed to warm up for 10 minutes prior to the experiment to guarantee a stable optical power output. The measurements were always performed in the same order, i.e. (1) absorption measurement of the blank, (2) absorption measurement of the sample, and (3) emission measurement of the sample. In this way, the neutral density filter is not moved between the measurement of the blank and sample, ensuring equal attenuation of the non-absorbed excitation light for both spectra. Equally, the sample is not moved between the measurement of its absorption and emission. For the absorption measurements, the neutral density filter was placed in the filter, and replaced by the 500-nm longpass filter for the emission measurement.

Quantum yield calculation method: The phosphorescence quantum yield (P) is defined by Equation S3
: Here, qpem is the emission photon flux (in photons•s −1 ) integrated over the spectral range 1 to 2 (520-950 nm), qpabs is the photon flux absorbed by the ruthenium complex (in photons•s −1 ), and IP() is the spectral luminescence intensity (in photons•s −1 •nm −1 ). qpabs is determined by subtracting the spectral light intensity of the excitation source that has passed through the sample (Iexcsample, in photons•s −1 •nm −1 ) from the spectral light intensity of the excitation source that has passed through the blank sample (Iexcblank, in photons•s −1 •nm −1 ), and by integrating over the excitation wavelength range 3 to 4 (400-500 nm).
The spectrometer and the integrating sphere were calibrated so that the observed intensities are directly proportional to the photon flux, i.e. IP()  [mol photons·s −1 ·nm −1 ] Therefore, integrating these values over the relevant wavelength regions directly provided the flux of photons that arrived at the spectrometer.
Because the intensity of the emitted light is relatively low compared to that of the exciting laser source the absorption and emission of the sample cannot be measured at the same time. In other words, the laser light saturates the spectrometer, which prevents the emission to be measured simultaneously. To circumvent this problem, the absorption was measured using a neutral density filter with known transmittance (typically Fattn ≈ 0.0062, i.e. ~ 99.4% attenuation). This filter was placed between the integrating sphere and the spectrometer to measure the absorbed photon flux. The data was corrected for the attenuation by the neutral density filter (Fattn()) at each wavelength. For the measurement of the emission, this filter was replaced by a longpass filter (> 500 nm) to remove the excitation light. Additionally, the intensity of the emission measured was corrected for the minimal absorbance of this light by the shortpass filter used. This was performed by dividing the luminescence intensity at each wavelength by the transmission curve T() of the longpass filter at this wavelength. The accordingly corrected equation for UC is Equation S4:

Mulliken spin (Ru)
------  Table S12. The relative energy to the ground state of the starting complex (kJ/mol), the bond distances (Å), the dihedral angle of the straining ligand () and the Mulliken spin on ruthenium of the triplet states found in the photodissociation reaction pathways of complex 1 [1a] 2+ in water.  Table S14. The relative energy to the ground state of the starting complex (kJ/mol), the bond distances (Å), the dihedral angle of the straining ligand () and the Mulliken spin on ruthenium of the triplet states found in the photodissociation reaction pathways of complex 1 [1a] 2+ in acetonitrile.

Reaction Pathways of First Solvent Molecule
Complex [1a] 2+ -H2O -Pathway 1 Figure S16. Plot of the linear transits and the obtained mechanistic states for the singlet ground state (red) and for the triplet excited state (blue) for pathway 1 of complex [1a] 2+ in H2O. The Ru-O1 distance is plotted on the x-axis, the Ru-N2 distance on the y-axis, and the energy relative to the ground state 1 [1a] 2+ is plotted on the z-axis.              Figure S26. Plot of the linear transits and the obtained mechanistic states for the singlet ground state (red) and for the triplet excited state (blue) for pathway 1 of complex [1a] 2+ in CH3CN. The Ru-N7 distance is plotted on the x-axis, the Ru-N4 distance on the y-axis, and the energy relative to the ground state 1 [1a] 2+ is plotted on the z-axis.