Bromide Photo-oxidation Sensitized to Visible Light in Consecutive

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs Matt Gray Chemistry 7350 December 11, 2017 Outline o Description of the problem o Related work in the field o Prior work from the Meyer group

o Results from this article o Interpretation and applications o Conclusion, future work, and critiques 2 e- sources for CO2 reduction o CO2 is an abundant, cheap carbon source o Carbon must be reduced to produce renewable

solar fuels o Halide oxidation can be initiated by a single electron transfer o Br2 is a liquid halide source with lower toxicity than Cl2 3

Field Background o Variety of approaches for selective halide oxidation 1 3 1. Sykora, M.; Kincaid, J. R. Nature 1997, 387, 162164.; 2. J. Am. Chem. Soc. 2013, 135, 862869.; 3. Adams, R. E.; 4 Schmehl, R. H. Langmuir 2016, 32, 85988607.; 4. J. Chem. Rev. 2015, 115, 1300613049.

2 4 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, 38303838.; 2. Inorg. Chem. 2015, 54, 4512 4519.; 3. J. Am. Chem. Soc. 2016, 138, 1681516826. 5 6 Goal and Hypothesis

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

a system) bpz = 2,2-bipyrazine 7 Bromide Titration o Equivalences of bromide were added to the system through titration of a tetrabutylammonium bromide salt o Up to two additions led to changes in the photophysical properties

8 Jablonski Diagram S2 IC S1 ISC T1

h kR kNR S0 9

UV-vis Comparison o Two MLCT peaks seen (425 nm, 450 nm) for [Ru(deeb)(bpz)2]2+ system o Consecutive ion interactions with the complex were tracked through the formation of distinct isosbestic points, along with intensity shifts of MLCT bands 10 SS-photoluminescence

o 460 nm excitation used; at an isosbestic wavelength o = 3.3% in absence of Br ; dropped linearly to zero during the first bromide titration; PLmax red-shifted o The second equivalence increased the PL ( = 0.7%) 11 TR-photoluminescence o Pulsed excitation at 445 nm; near the isosbestic points o

Lifetime of 900 ns seen during first bromide titration; overall intensity decreased o Second titration led to biexponential kinetics ( = 65 ns) o Both data sets were fit to the PLI equation 12 Transient Absorption o 532 nm pulsed excitation; causes bleach in visible range

o Two isosbestic points monitored at 400 nm and 550 nm o 405 nm (Br2-) and 500 nm (Ru2+) were monitored o Br2- formation was Br dependent; this was used to elucidate k5 (Br2- formation rate) 13 Photo-physical Properties

14 Conclusions/Future Work o Br2- is a promising electron source for solar fuels applications; can be formed from visible light o Unusual PL behavior during Br- titration (quenching and re-formation) is a result of the unique electronics of the deeb ligand o Increasing absorption onset into red/IR and testing excited bromide ion pair as CO2

reductants 15 Critiques o Attempted to explain odd PL behavior using solvent values, while only using CH2Cl2 to test this

claim o Self-citation as many references in discussion section 16 Thank you for your time! Any questions?

17 Determination of Properties in Table 2. o = efficiency of a primary process = number of molecules undergoing a process/total photons o = 1/k 18

SS-PL visible 19

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