Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs Matt Gray Chemistry 7350 December 11, 2017

Outline Description of the problem Related work in the field Prior work from the Meyer group Results from this article Interpretation and applications Conclusion, future work, and critiques

e- sources for CO2 reduction CO2 is an abundant, cheap carbon source Carbon must be reduced to produce renewable solar fuels Halide oxidation can be initiated by a single electron transfer Br2 is a liquid halide source with lower toxicity than Cl2 https://www.chem.tamu.edu/rgroup/marcetta/chem462/lectures/GoldsteinParkCO2.pdf

Field Background Variety of approaches for selective halide oxidation 1 2 3 4 1. Sykora, M.; Kincaid, J. R. Nature 1997, 387, 162−164.; 2. J. Am. Chem. Soc. 2013, 135, 862−869.; 3. Adams, R. E.; Schmehl, R. H. Langmuir 2016, 32, 8598−8607.; 4. J. Chem. Rev. 2015, 115, 13006−13049.

Meyer Group Background 2 1 3 Focused on ionic donor-acceptor interactions, since Coulombic attraction brings in ions, but releases dihalide; freeing active site for more turnovers. 1. J. Dalton Trans. 2011, 40, 3830−3838.; 2. Inorg. Chem. 2015, 54, 4512− 4519.; 3. J. Am. Chem. Soc. 2016, 138, 16815−16826.

Goal and Hypothesis Goal: Determine the excited state electron transfer oxidation reactions as a function of [Br-] Hypothesized that adding bromide would tend to quench PL (an indicator of photo-oxidation potential of a system) [Ru(deeb)(bpy)2][(PF6)2]; deeb = 4,4’-diethylester-2,2’-bipyridine bpz = 2,2’-bipyrazine

Bromide Titration Equivalences of bromide were added to the system through titration of a tetrabutylammonium bromide salt Up to two additions led to changes in the photo-physical properties First Br- eq. was stoichiometric (466 nm isosbestic point); second has high K (estimated by UV-vis change at 430 nm, isosbestic point); additional Br- didn’t change PL properties

Jablonski Diagram S0 T1 S2 S1 hν ISC IC kNR kR

UV-vis Comparison Two MLCT peaks seen (425 nm, 450 nm) for [Ru(deeb)(bpz)2]2+ system Consecutive ion interactions with the complex were tracked through the formation of distinct isosbestic points, along with intensity shifts of MLCT bands Absorption change for first ion pair increased linearly with additional Br-. Second ion addition was in equilibrium.

SS-photoluminescence 460 nm excitation used; at an isosbestic wavelength Φ = 3.3% in absence of Br– ; dropped linearly to zero during the first bromide titration; PLmax red-shifted The second equivalence increased the PL (Φ = 0.7%) Excitation done at isosbestic point so total number of photons absorbed remained unchanged throughout the titration. PL red-shift was assigned to a small equilibrium concentration of the second ion pair, however, this was not explained well at all.

TR-photoluminescence Pulsed excitation at 445 nm; near the isosbestic points Lifetime of 900 ns seen during first bromide titration; overall intensity decreased Second titration led to biexponential kinetics (τ = 65 ns) Both data sets were fit to the PLI equation N2 pumped dye laser used. Not mechanism specific, so no further conclusions were drawn from this study.

Transient Absorption 532 nm pulsed excitation; causes bleach in visible range Two isosbestic points monitored at 400 nm and 550 nm 405 nm (Br2·-) and 500 nm (Ru2+) were monitored Br2·- formation was Br– dependent; this was used to elucidate k5 (Br2·- formation rate) K5 is a secondary rate, since two species come together to form it

Photo-physical Properties

Conclusions/Future Work Br2·- is a promising electron source for solar fuels applications; can be formed from visible light Unusual PL behavior during Br- titration (quenching and re-formation) is a result of the unique electronics of the ‘deeb’ ligand Increasing absorption onset into red/IR and testing excited bromide ion pair as CO2 reductants Assuming this is a Dexter electron transfer process; given the proximity of the species.

Critiques Attempted to explain odd PL behavior using solvent ε values, while only using CH2Cl2 to test this claim Self-citation as many references in discussion section Some qualitative comparisons to acetone were done based on literature values, but no evidence of in-house testing was shown in the paper.s

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Determination of Properties in Table 2. Φ = efficiency of a primary process = number of molecules undergoing a process/total photons τ = 1/k

SS-PL visible