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J. Phys. Chem. Lett. 2012, 3, 1363-1368
Eric H. Hill†‡, Subhadip Goswami§,
Deborah G. Evans‡, Kirk S. Schanze*§, and
David G. Whitten*†
†Center for Biomedical Engineering, Department of Chemical and
Nuclear Engineering, University of New Mexico, Albuquerque, New
Mexico 87131-1341
‡The Nanoscience and Microsystems Program and Department of
Chemistry and Chemical Biology, University of New Mexico,
Albuquerque, New Mexico 87131-1341
§Department of Chemistry, University of Florida, Gainesville, Florida
32611-7200
Photochemistry of a Model Cationic p-
Phenylene Ethynylene in Water

2. Background
• These compounds exhibit strong antimicrobial activity in the light and
dark, and are useful as biosensors
– Long-term applications require photostability
• First relevant photochemical study by T. Roberts and T. Wooldridge
(1973) and Wan, Culshaw, and Yates (1982) with phenylacetylenes
– Suggested nucleophilic attack of structured water on or the protonation of a
triple bond could take place depending on the substituent
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J. Phys. Chem. Lett. 2012, 3, 1363-1368

3. Photophysical Data
1
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J. Phys. Chem. Lett. 2012, 3, 1363-1368
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
250 300 350 400 450 500
Absorbance
and
Fluorescence
(arb.
units) Wavelength (nm)
Absorbance and Fluorescence of 1
Abs
Flu

4. The photolysis is followed by
absorbance spectroscopy
Figure 2. UV-visible absorption spectra of 20 µM compound 1 in water as a function of photolysis for a)
air-saturated solution and b) argon-degassed solution. Arrows are indicative of change with increasing
time. Photolysis effected by using broad-band near-UV light (300 – 400 nm).
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J. Phys. Chem. Lett. 2012, 3, 1363-1368

5. Quantum Yields of Starting Material
Disappearance
Solvent (- ) Argon (- ) Air
H2O 0.000377 0.000242
D2O 0.000075 0.000112
Isotope effect on pathways involving protonation was observed
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J. Phys. Chem. Lett. 2012, 3, 1363-1368

6. Photoproducts and Analysis of
Isoptope-labeled Products
Compound m (g/mol) z m/z Resulting from Labeled atoms added
1 510.324 2 255.162 Starting mat’l Unlabeled – starting material
2 528.334 2 264.167 1 + H2O 2 D, 1 18O
3 546.345 2 273.172 1 + 2 H2O 4 D, 1-2 18O
*7 542.313 2 271.157 1 + O2 1 18O
*8 574.303 2 287.152 1 + 2 O2
1-2 18O
4 410.212 1 410.212 (1 – 6) + OH 1 D
5 428.223 1 428.223 (2 – 6) + OH 4 D, 1-2 18O
*9 442.202 1 442.202 (7 – 6) + OH 1 D, 1-3 18O
6 118.123 1 118.123 1 D (15%), 1 18O
Table 2. List of products and their change upon incorporation of isotopes
* Exchange of 16O by 18O occurs when the compound is dissolved in 10% 18O-labeled water for 12 hours.
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7. Product Summary
Primary photoproducts of photolysis
Consecutive and Hybrid photoproducts of photolysis
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J. Phys. Chem. Lett. 2012, 3, 1363-1368

8. Proprietary and Confidential
American Chemical Society
8
Conclusions
The main pathways of degradation were:
• Photoprotonation of the triple bond followed by
addition of water
• Addition of oxygen across the triple-bond
• Cleavage of the quaternary ammonium side-
chains.
The finding of consecutive and hybrid products
shows that the primary products are also
photoreactive
Acknowledgments
• Ken Sherrell and the UNM Mass Spectrometry
Facility
• Dr. Anand Parthasarathy for assistance with
quantum yield determinations
• US Defense Threat Reduction Agency for
funding through Grants W911NF07-1-0079 and
HDTRA1-08-1-0053.
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J. Phys. Chem. Lett. 2012, 3, 1363-168