| Abstract
| - Metal-to-ligand charge-transfer (MLCT) photolyses (λ ≥ 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), [Cu(bpod)2]PF6 (2), and [Cu(bpod)2](NO3)2 (3) yieldBergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)2(CH3COO)2compound (4) are photochemically inert under the same conditions. In the case of 4, sensitizedphotochemical generation of the lowest energy 3π−π* state, which is localized on the enediyne unit, leadsto production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemicalstudies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between Epvalues of −1.75 and −1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E1/2 = +0.15 and +0.38 V, respectively. Combined withthe observed Cu(I)-to-pyridine(π*) MLCT and pyridine(π*)-to-Cu(II) ligand-to-metal charge transfer (LMCT)absorption centered near ∼315 nm, the results suggest a mechanism for photo-Bergman cyclization thatis derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediatesproduced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5‘-phosphate and 3‘-phosphoglycolate termini that are derived from H-atom abstraction from the 4‘-position of the deoxyribosering rather than redox-induced base oxidation.
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