Reports: UNI450467-UNI4: Photophysical Characterization of Free-base Bis(arylethynyl)porphyrin Diacids and Aggregates

Kathryn E. Splan, Macalester College

The narrow absorption profiles exhibited by porphyrins limit their use in many photophysical applications. Arylethynylporphyrins that feature expanded conjugation of the porphyrin macrocycle hold great potential as chromophores that exhibit enhanced light-harvesting properties in the red region of the spectrum. However, to date, use of arylethynylporphyrins in optical applications has centered primarily on metalloporphyrin derivatives. Our project extends aspects of free-base porphyrin chemistry to include arylethynyl porphyrin derivatives.

Results from Year 1.

The first year of the funding period focused on the fundamental characterization of free-base arylethynyl photophysics in both the neutral and diacid form. To delineate the impact of the ethynyl linkers on free-base arylethynyl porphyrin photophysics, we completed the synthesis of arylethynyl porphyrins 1-5 and the trans-disubstituted H2TPP derivatives lacking arylethynyl functionalities and conducted a comprehensive photophysical study of the compounds. This study comprises the first photophysical investigation of free-base arylethynyl porphyrins and was published in The Journal of Physical Chemistry A.

Results from Year 2.

Efforts during year 2 have centered upon the synthesis of the water soluble porphyrins 6-11. Porphyrins bearing sulfonate and carboxylate functionalities have been shown to aggregate into well-defined structures under acidic conditions, most often forming J-aggregate structures. We predict that J-aggregates formed from porphyrins 6 and 9 will exhibit more dramatic spectral shifts relative to the tetraphenylsulfonate and tetraphenylcarboxylate derivatives owing to the elongation of the transition dipole moment along one axis of the molecule. Similarly, aggregates grown from the di-substituted porphyrins 7, 8, 10, and 11 are likely to exhibit aggregates where both the size and shape of the aggregates may differ significantly from those formed from symmetric tetraphenylporphyrin derivatives, owing to the asymmetric nature of the molecule.

Porphyrins 6-8 where synthesized by Kaarin Evens (Macalester '14) during the summer of 2012 and preliminary aggregation studies were performed. We did not observe spectra that were indicative of J-aggregate formation for 6-8 when measured under the same conditions that promote aggregation in TCPP. Efforts are now focused on altering both pH and ionic strength in attempt to generate stable aggregate structures from these porphyrins. Synthetic efforts are underway toward porphyrins 9-11.

Impact on Faculty and Students

Support from the Petroleum Research Fund has been highly beneficial for both the PI and for Macalester undergraduate students involved in the project. The grant has provided support for summer salary and travel to conferences, supplies for the project, and funding for instrumentation that would not have been available through other sources. Support for the project has also allowed Peter Goldberg (supported internally by Macalester prior to PRF funding) to continue his research and bring the project to the point of publication. Peter presented his results at the National American Chemical Society meeting in March 2011, and he is now enrolled as a first year graduate student at the University of Michigan Department of Chemistry. The grant also provided a summer stipend to Kaarin Evens, who is currently continuing her work during the academic year. Moreover, Kaarin also benefits from exposure to the nature of scientific research early in her education, and her experience this past summer has reinforced her motivation to pursue a research career. The project has allowed Kaarin to gain in-depth lab experience in organic synthesis and characterization as well as electronic absorption characterization before she encounters these subjects in the classroom.