Toward Advanced Exciton Charge Separation with Metal-Organic Framework Films

Joseph Hupp^a, Omar Farha^a, Hassan Beyzavi^a, Monica So^a, Benjamin Klahr^a

^aDepartment of Chemistry, Northwestern University, United States, Sheridan Road, 2145, Evanston, United States

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Poster, Benjamin Klahr, 040
Publication date: 16th April 2014

Metal-organic frameworks (MOFs) have been shown to possess interesting properties that may prove beneficial for efficient solar energy conversion schemes. For example, light harvesting MOF crystals have been prepared which exhibit ultrafast energy migration.¹ This fast energy migration is hypothesized to be facilitated by the spatial separation of chromophores and the anisotropy of the crystals. To take advantage of the anisotropy of MOFs in solar energy conversion, MOF films must be grown in a controlled and oriented fashion on a conductive substrate. Perhaps the best way to for to deposit films in an oriented fashion is by the layer-by-layer method (LBL), where individual components of the MOF are introduced to the substrate one at a time. This self-limiting method has been show to produce uniform films for which the orientation can be controlled through selective functionalization of the substrate.²
In this work, simple porphyrin based MOF films were deposited on conductive substrates using LBL in order to analyze their properties relevant to solar energy conversion. Thickness and absorption measurements allowed us to deduce that the porphyrins are oriented perpendicular to the substrate in 2D sheets. These sheets are separated by diazabicyclooctane molecules, the presence of which is indicated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis. Comparing absorption and electrochemical characterization allowed us to determine that all porphyrins were reversibly accessible electrochemically. Initial photoelectrochemical results show that a redox active species in solution can reduce the photooxidized porphyrin. This work represents a promising first step of employing precisely controlled and oriented MOFs in solar energy conversion schemes.
References:
1. Son, H.-J.; Jin, S.; Patwardhan, S.; Wezenberg, S. J.; Jeong, N. C.; So, M.; Wilmer, C. E.; Sarjeant, A. a.; Schatz, G. C.; Snurr, R. Q.; Farha, O. K.; Wiederrecht, G. P.; Hupp, J. T., J. Am. Chem. Soc., 2013, 135, 862.
2. Zacher, D.; Yusenko, K.; Bétard, A.; Henke, S.; Molon, M.; Ladnorg, T.; Shekhah, O.; Schüpbach, B.; de los Arcos, T.; Krasnopolski, M.; Meilikhov, M.; Winter, J.; Terfort, A.; Wöll, C.; Fischer, R. a., Chem. Eur. J., 2011, 17, 1448.

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