title: Spectroscopic determination of proton position in the proton-coupled electron transfer pathways of donor-acceptor supramolecule assemblies.
abstract: A homologous set of porphyrin derivatives possessing an isocyclic five-membered ring appended with an amidinium functionality has been used to examine proton-coupled electron transfer (PCET) through well-characterized amidine-carboxylic acid interfaces. Conjugation between the porphyrin chromophore and the amidinium interface can be altered by selective reduction of the isocyclic ring of an amidinium-purpurin to produce an amidinium-chlorin. The highly conjugated amidinium-purpurin displays large spectral shifts in the visible region upon alteration of the amidinium/amidine protonation state; no such change is observed for the chlorin homologue. Analysis of the UV-vis absorption and emission profiles of the amidinium-purpurin upon deprotonation allows for the measurement of the porphyrinic-amidinium acidity constant for the ground state (pKa = 9.55 +/- 0.1 in CH3CN) and excited state (pKa)= 10.40 +/- 0.1 in CH3CN). The absorption spectrum of the purpurin also provides a convenient handle for determining the protonation state of assembled interfaces. In this way, the purpurin macrocycle provides a general tool for PCET studies because it can be used to determine the location of a proton within PCET interfaces formed from carboxylic acid electron acceptors including dinitrobenzenes (DNBs) and naphthalenediimide (NI), which have been used extensively in previous PCET studies. An amidine-carboxylic acid interface is observed for electron-rich acceptors, whereas the ionized amidinium-carboxylate interface is observed for electron-poor acceptors. The PCET kinetics for purpurin/chlorin associated to NI are consistent with an amidine-carboxylic acid interface, which is also verified spectrally.
authors.affiliation: Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA.
authors.forename: Joel
authors.initials: J
authors.lastname: Rosenthal
authors.forename: Justin M
authors.initials: JM
authors.lastname: Hodgkiss
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Young
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Nocera
mesh_terms: D000465:Algorithms; D000577:Amides / Q000737:chemistry; D000880:Anthraquinones / Q000138:chemical synthesis / Q000737:chemistry*; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry; D009570:Nitriles / Q000737:chemistry; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D011522:Protons; D013057:Spectrum Analysis; D015032:Zinc / Q000737:chemistry
chemical_list: D000577:Amides; D000880:Anthraquinones; D009570:Nitriles; D011166:Porphyrins; D011522:Protons; C006969:chlorin; D009532:Nickel; D015032:Zinc; C511975:purpurin anthraquinone