Unlike numerous known examples of exciplexes (products of charge formation reactions), we reported recently that cationic exciplexes (products of charge shift reactions) can be formed with N-methylisoquinolinium as an excited acceptor and alkyl benzene donors. We have now synthesized five intramolecular analogues (isoquinolinium linked by a trimethylene tether to alkyl benzenes) that proved to be well suited to demonstrating that emissive exciplexes can be formed in water from purely organic components. Three conformers (anti, gauche, and folded) leading to electron transfer were identified using a combination of absorption spectroscopy, fluorometry, and time-correlated single photon counting. The hydrophobicity of the donor moiety was found to enhance the formation of the folded conformer, which leads directly to exciplex formation. Electronic coupling matrix elements between ground, charge-transfer, and locally excited states were determined from correlations between radiative rate constants and average emission frequencies. The charge transfer (CT) character of the exciplexes (88–97%) was calculated from the electronic coupling. In spite of such a high CT character in a highly polar solvent, exciplex fluorescence quantum yields up to 0.03 and lifetimes up to 17 ns were observed.
Nanosecond transient absorption spectroscopy was used to generate ethoxyl radicals and demonstrate that they react with 2,6-lutidine and 4-phenylpyridine to give the corresponding N-hydropyridinyl radicalsproducts of a novel hydrogen atom transfer from the alkoxyl radical to the nitrogen atom of the substituted pyridines. Nanosecond kinetics show that both reactions are rapid (k ∼ 107 M–1 s–1) in acetonitrile at room temperature. Rate constants measured for reaction of the ethoxyl vs. d 5-ethoxyl radical with 2,6-lutidine and 4-phenylpyridine show that both reactions exhibit primary H/D kinetic isotope effects for the hydrogen (deuterium) atom transfer reactions.
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