A possible explanation for the very efficient DNA photocleavage
of 3 is the additional involvement of the 3MLCT
excited state.With E00B2.1 eV and E1/2([Ru]2+/+])=0.46 V
vs. NHE, an excited state reduction potential, Ered*, of
B1.64 V vs. NHE can be estimated for 3, making it a strong
oxidizing agent. Using +1.29 V vs. NHE as the oxidation
potential of guanine in water at pH = 7,21 a favorable driving
force of 0.35 V for the generation of G+ from the 3MLCT
state of 3 is calculated. Guanine oxidation by the excited states
of Ru(II) complexes has been previously shown to result in
DNA cleavage and adduct formation.3a,22 Although DNA
photocleavage from guanine oxidation was reported for
[Ru(hat)3]2+ (hat = 1,4,5,8,9,12-hexaazatriphenylene), a
complex with similar excited state reduction potential as that
of 3, 80% DNA damage required 3 min irradiation at
436 nm.22b As shown in Fig. 4a, irradiation of 3 for only 30 s
results in nearly quantitative DNA cleavage.
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