Metal-to-ligand charge-transfer (MLCT) photolyses (
395 nm) of copper complexes of
cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod,
1), [Cu(bpod)
2]PF
6 (
2), and [Cu(bpod)
2](NO
3)
2 (
3) yieldBergman cyclization of the bound ligands. In contrast, the uncomplexed ligand
1 and Zn(bpod)
2(CH
3COO)
2compound (
4) are photochemically inert under the same conditions. In the case of
4, sensitizedphotochemical generation of the lowest energy
3-
* 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(
*) MLCT and pyridine(
*)-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.