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

2. 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

3. 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

4. Field Background Variety of approaches for selective halide oxidation1 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.

5. Meyer Group Background2 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.

7. 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

8. 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

9. Jablonski Diagram S0 T1 S2 S1 hν
ISC IC
kNR kR

10. 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.

11. 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.

12. 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.

13. 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

14. Photo-physical Properties

15. 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.

16. 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

17. Thank you for your time!
Any questions?

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

19. SS-PL visible