Visible-Light Actinometry and Intermittent Illumination as Convenient Tools to Study Ru(bpy)3Cl2 Mediated Photoredox Transformations

Photoredox catalysis provides many green opportunities for radical-mediated synthetic transformations. However, the determination of the underlying mechanisms has been challenging due to lack of quantitative methods that can be easily implemented in synthetic labs, where this research tends to be centered. We report here on the development, characterization and calibration of a novel actinometer based on the photocatalyst tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)3Cl2). By using the same molecule as the photocatalyst and the actinometer, we eliminate problems associated with matching sample spectral distribution, lamp-sample spectral overlap and other problems intrinsic to doing quantitative photochemistry in a laboratory that has little expertise in this area. In order to validate our actinometer system in determining the quantum yield of a Ru(bpy)3Cl2 photosensitized reaction, we test the Ru(bpy)3Cl2 catalyzed oxidation of benzhydrol to benzophenone as a model chain reaction. We also revive the rotating sector method by updating the technique for modern LED technologies and demonstrate how intermittent illumination on the timescale of milliseconds to seconds can help probe a chain reaction, using the benzhydrol to benzophenone oxidation to validate the technique. We envision these methods to have great implications in the field of photoredox catalysis, providing researchers with valuable research tools.


A. General Information
Tris(bipyridine)ruthenium(II) chloride (Ru(bpy) 3 Cl 2 ) was purchased from Fisher Scientific and used as received. Diphenylanthracene (DPA), benzhydrol, 4-cyanopyridine N-oxide, and trimethyloxonium tetrafluoroborate were purchased from Sigma Aldrich and used as received. Flash column chromatography was performed using 230-400 mesh silica gel. All 1 H and 13 C NMR were recorded on a Bruker AVANCE 400 spectrometer. Chemical shifts (δ) are reported in ppm from the solvent resonance as the internal standard (CDCl 3 : δ 7.26 ppm).

D. Preparation of potassium ferrioxalate
The solid potassium ferrioxalate was prepared as described by Hatchard, C.G.; Parker, C.A. Proc. Roy. Soc. (London) 1956, A235, 518-536. Briefly, 1.5 M potassium oxalate and 1.5 M ferric chloride were prepared in Milli-Q H 2 O. The two solutions were then combined under stirring in a 3:1 ratio (potassium oxalate:ferric chloride). After letting the solution stir for 2 hours the resulting precipitate was filtered off and recrystallized three times from warm water. The solid was then allowed to dry under vacuum and was stored in the dark. In order to determine the effective intensity of the 460 nm LED fitted with a 440 nm notch filter (FWHM 10 nm) we utilized the ferrioxalate actinometer, which has been described in detail in the online methods section.
From the previously described experiment we have found the following results after 1 minute of irradiation:  From this we can determine the concentration of ferrous ions (Fe 2+ ) produced during the the 1 minute irradiation as follows: Since we know the volume of the sample to be 3.5 mL we can find the total number of moles Fe 2+ .

G. Power dependence of Ru(bpy) 3 Cl 2 based actinometer
To be useful with a variety of different illumination sources the actinometer should respond linearly to increasing powers of visible light illumination. For this reason we have examined the Ru(bpy) 3 Cl 2 based actinometer using the 460 nm LED at different irradiances. Through modulation of the systems current we have been able to adjust the power of the LED. Powers were measured using a Luzchem SPR-4001 spectroradiometer. I. Sensitization of 1,9-diphenylanthracene by 3 Ru(bpy) 3 Cl 2 Through the use of LFP one can demonstrate the sensitization of DPA by triplet 3 Ru(bpy) 3 Cl 2 even though the triplet-triplet absorption of DPA (λ max 440 nm) and ground-state absorption of the Ru(bpy) 3 Cl 2 overlap. This is due to the fact that 3 DPA has a relatively large ε and a τ that is considerably longer than that of 3 Ru(bpy) 3 Cl 2 , making it possible to observe the growth and decay of the signal corresponding to 3 DPA upon 460 nm laser (10 mJ per pulse) excitation of a dearated solution of Ru(bpy) 3 Cl 2 and DPA in CH 3 CN. As can be seen in Figure S6, in the absence of DPA (black trace) one observes only bleaching of Ru(bpy) 3 Cl 2 at 440 nm. However, upon addition of DPA (blue trace) there is a clear growth and decay of a signal that we attribute to 3 DPA. Figure S6: LFP traces obtained upon 460 nm (10mJ per pulse) excitation of a dearated solution of Ru(bpy) 3 Cl 2 (black) and Ru(bpy) 3 Cl 2 + 1,9-diphenylanthrancene (blue) in CH 3 CN while monitoring at 440 nm.

J. End user instructions (SOP)
1) Due to the visible light nature of the Ru(bpy) 3 Cl 2 actinometer, it is important that experiments be performed in a dark room as to avoid conversion due to ambient lighting. 2) Ensure that the geometry of irradiation of both the actinometer and reaction of interest is consistent. I. Utilize a sample holder which allows for reproducible sample placement II.
Secure both the sample holder and light source to ensure fixed distance III.
Utilize the same reaction vessel or perfectly matched pairs for all measurements 3) As the analysis of the actinometer is done spectrophotometrically, it may be advantageous to perform the irradiation in a cuvette. However, this is not necessary, as in many cases the samples will require dilution prior to analysis. 4) Prepare a solution of Ru(bpy) 3 Cl 2 (same concentration as that used in the reaction of interest) and DPA (0.10 mM) in acetonitrile. As Ru(bpy) 3 Cl 2 is both the photocatalyst and actinometer, this is a simple way to match lamp-sample spectral overlap.

Important:
The accuracy of the actinometer relies on this concentration match. 5) Record a UV-Vis spectra of the Ru(bpy) 3 Cl 2 and DPA solution. This will be your "time zero" spectrum. 6) As the absorption of Ru(bpy) 3 Cl 2 and DPA at 372 nm is additive, depending on the concentration of Ru(bpy) 3 Cl 2 it may be necessary to dilute the sample for analysis to ensure that the absorption is within the linear response region of the spectrophotometer (see the specifications of your spectrophotometer). Most spectrometers do not perform well at absorbances above 2.5. 7) Note the absorbance at 372 nm (A INITIAL ). 8) Irradiate the sample for a set period of time. 9) Record a new UV-Vis Spectrum of the irradiated sample and note the absorbance at 372 nm (A FINAL ). 10) As Ru(bpy) 3 Cl 2 should not photobleach on the timescale of the irradiation, there will be a residual absorption at 372 nm, even when all DPA has been consumed. For this reason, A FINAL will be larger than the absorbance found at 372 nm in the absence of DPA. 11) Knowing the volume of the irradiated sample and that the molar extinction coefficient (ε) for DPA is 11,100 M -1 cm -1 one can determine the number of moles of DPA consumed during the irradiation using equation (S.1).

# of moles DPA consumed =
Where A INITIAL and A FINAL are the absorbance of the solution at 372 nm before and after irradiation, respectively; ε !"# nm is the extinction coefficient of DPA at 372 nm in acetonitrile, is the path length of the cuvette, and V is the volume of the sample for which the absorption was measured. If a dilution was made before the measurement this must be taken into account by applying standard volumetric dilution factors.
• Since the quantum yield (Φ) for the actinometer is known to be 0.019, ζ we can then use the number of moles DPA consumed to determine the number of moles of photons absorbed by our sample per unit time ( where Φ is the quantum yield of DPA consumption, and t is the irradiation time in seconds. • To determine the Φ for the reaction of interest you must then irradiate the sample of interest using the same geometry employed for the actinometer. Once irradiation is complete you must then determine the number of moles of product formed or substrate consumed per unit time using your favourite analytical technique. From this one can apply equation (S.3) to determine the Φ of their reaction.  In some cases it may be necessary to perform the actinometer and the reaction of interest on different time scales and then correct for the time of exposure.

K. History and theory behind the rotating sector method
The use of intermittent illumination in photo-initiated reactions is in no way a new idea, having been initially suggested in 1929 by Briers, Chapman and Walters. 1 However, it was not until 1946 when Burnett and Melville expanded on the theory of intermittent illumination and applied it to the direct photo-polymerization of vinyl acetate that it became evident one could use such intermittent illumination to characterize chain reactions, in particular the average lifetime of a chain sequence. 2 The technique has since become known as the "rotating sector" method as at the time the simplest route to intermittent illumination was to place a rotating sectored disk in the path of the light source. Using this set-up, one could control the light/dark ratio by controlling the size and/or number of sectors in the disk as well as the duration of the light-on and light-off periods through its speed of rotation. 2 Over the next few decades the theory and the instrumentation behind the technique expanded, as it was a relatively simple technique for obtaining kinetic data for photochemically initiated chain reactions exhibiting bimolecular chain termination. The technique eventually fell out of favour as it was demonstrated that electron spin resonance (ESR) spectroscopy could measure the rates of bimolecular radical reactions involved in both terminating and non-terminating chain events, some of which were only previously accessible through the use of intermittent illumination. 3 Although the idea of utilizing a rotating sectored disk to intermittently irradiate a sample seems simple in principle, in practice its implementation is met with many difficulties the least of which is the procurement of a functioning rotating sector apparatus. Other challenges in its operation include its large physical footprint, the requirement of two different sectored disks (one for short pulses and one for long pulses), 4 and poor control of the speed of rotation. High frequency of exposure also becomes difficult as fast rotating disks present safety risks.
In a typical rotating sector experiment the light is interrupted in such a way that the period of irradiation (t on ) is followed by an off time (t off ) that is equal to or longer than that of t on . When the flashing rate is slow, t on is much longer than the radical lifetime and because of this the radical concentration quickly increases to the value reached during steady-state irradiation [R] SS . However, since the period over which the radicals decay is short in comparison to the dark period (t off ), during the slow flashing cycle the radical concentration drops off to essentially zero whenever the light is off. The average concentration of radicals [R] Avg in the system throughout the experiment during slow flashing would thus be 1/2[R] SS if the period of light on and off were equal. Such a situation is depicted in Figure S8a. On the other hand if the flashing rate is increased to the point where t on is significantly shorter than τ s the situation is much different. Under these conditions the radicals initiated during a single t on period will continue to grow through several successive on-off cycles until they eventually level off. Since the radical concentrations does not reach the extreme high and low values it does when the flashing rate is slow, the [R] Avg of the system approaches the steady-state concentration [R] SS when the illumination is uninterrupted ( Figure S8b). From the plots in Figure S11, it becomes evident that as long as the intensity of irradiation and total irradiation time are kept constant, one should see a difference in the average rate of reaction for the fast and slow flashing experiments with the change in rate becoming apparent as the t off time reaches that of the radical chain lifetime (τ s ).
One of the simplest ways to observe the change in reaction rate when moving from fast to slow flashing is to plot the percent conversion of the reaction versus log(t on ) of the flashing cycle. The first point of inflection in this plot corresponds to the average chain lifetime (τ s ). Although it may also be interesting to compare the conversions obtained under intermittent illumination with those obtained using steady-state illumination, it is not a requirement in the determination of τ s . In doing so it is also important to account for the fact that intermittently illuminated samples are only irradiated for one-half or onethird of the experiment depending on the on/off ratio used and therefore this should also be accounted for in the constant illumination of samples. It may also be important to monitor sample temperature as constant illumination may result in temperature increases, which may influence the observed rate of reaction.  Figure S9: Photographs of the irradiation apparatus used for both steady-state and intermittent illuminations. A) power supply, B) pulse generator, C) Oscilloscope, D) 460 nm LED, E) sample holder, F) magnetic stir plate, G) photodiode, H) removable notch filter, and I) constant current driver.

M. Synthesis of 4-cyano-N-methoxypyridinium tetrafluoroborate
An oven-dried 100 mL round bottom flask was charged with 4-cyanopyridine N-oxide (6 mmol, 720 mg) and trimethyloxonium tetrafluoroborate (11.37 mmol, 1.68 g). The contents were dissolved in 30 mL of dry DCM, and the reaction mixture was purged with argon for 15 minutes and stirred overnight. The reaction was quenched with 15 mL of MeOH, and the solvent was removed under reduced pressure. The crude solid was recrystallized in MeOH to afford 4-cyano-N-methoxypyridium tetrafluoroborate as a white crystalline solid (910 mg, 68% isolated yield).

N. Thermodynamics and kinetics of the Ru(bpy) 3 Cl 2 catalyzed oxidation of benzhydrol
If we calculate the Gibbs free energy for electron transfer between * Ru(bpy) 3 Cl 2 and 4cyano-N-methoxypyridinium tetrafluoroborate, we find that the initial electron transfer between * Ru(bpy) 3 Cl 2 [E ox (Ru 3+ / * Ru 2+ ) = -0.81 V vs. SCE] and the pyridinium salt [E red salt -0.50 V vs. SCE] is exothermic by 0.31 eV. While this indicates that the initial excited state electron transfer is thermodynamically possible, it does not confirm that it is also kinetically favorable. Through the use of LFP, we have also determined that 4cyano-N-methoxypyridinium tetrafluoroborate quenches * Ru(bpy) 3 Cl 2 at a rate of 6.9 x 10 8 M -1 s -1 (See Figure S10). Although we would expect the electron transfer to be diffusion controlled since it is exothermic by more than 0.2 eV, the potential of the pyridium salts is only an estimate based on its corresponding N-alkyl analogue. We have also determined that the lifetime of Ru(bpy) 3 Cl 2 remains unchanged in the presence of benzhydrol. With this information in hand, the percentage of 3 Ru(bpy) 3 Cl 2 quenched by the pyridinium salt under initial reaction conditions can be calculated using the following equation: From this we calculate that 89.7.% of Ru(bpy) 3 Cl 2 triplets are quenched by the pyridinium salt under initial reaction conditions. When we combine this with the fact that the efficiency of intersystem crossing in Ru(bpy) 3 Cl 2 nearly unity (η ISC ≈ 1), we find that the initiation efficiency of the reaction is approxamitely 0.90.

O. Further examination of the quantum yield for the Ru(bpy) 3 Cl 2 mediated oxidation of benzhydrol
We have explored the dependency of the quantum yield on the power of irradiance, as well as the concentration of benzhydrol and 4-cyano-N-methoxypyridinium tetrafluoroborate. As the intensity of the absorbed light increases we have found that the quantum yield of the reaction decreases, and that there is a relationship between the square root of the light intensity and the quantum yield of the chain (See Figure S11). Although such a result would be unexpected in a direct photolysis experiment under photosensitized conditions this is anticipated. It is also seen that the quantum yield is independent of benzhydrol concentration but dependent on the concentration of the pyridinium tetrafluoroborate. Unexpectedly in the presence of 60 mM 4-cyano-Nmethoxypyridinium tetrafluoroborate the conversion over 2 minutes is reduced to 43.2%. One would expect an increase in the concentration of the pyridinium salt to result in a decrease in termination events and therefore an increase the yield. A possible explanation for this observation is that the BF 4 anion of the pyridinium salt is dynamically quenching or displacing the Clcounter-ion of the Ru(bpy) 3 2+ resulting in a drastically reduced excited state lifetime, which would ultimately decrease the initiation efficiency (see Figure S12).