A chromophore-quencher complex incorporating a photoredox-active ligand
Inorganic Chemistry , Volume 33 - Issue 9 p. 1899- 1906
A photoredox-active ligand (CL), the chromophore-quencher complexes Cu(CL)(CF3SO3)2 (1) and Cu(CL)(H2O)2(CF3SO3)2(2), where CL = 4,7-di-n-butoxy-1,3-bis((3′-pyridylmethyl)imino)isoindoline, and the reference complexes Cu(L)(H2O)(CF3SO3)2(3), where L = 1,3-bis((3′-pyridylmethyl)imino)isoindoline, and Zn(CL)(H2O)2(ClO4)2 (4) have been synthesized and characterized. The ligand CL is subject to a protonation equilibrium with ground state pKa = 8.99 ± 0.06 and an approximate excited-state pKa = 10.5 ± 0.5. In 4:1 acetonitrile:water, the excited ligand CL* at pH > 13.5 has λem = 419 nm (ϕem = 0.19), while the excited protonated ligand CLH+* at pH < 7 has λem = 486 nm (ϕem = 0.86). In hexane the lifetime of the fluorescent excited state of CL was determined to be 2.00 ± 0.07 ns. In “dry” acetonitrile the emission came from both CL* and CLH+* but only one decay was resolved with π = 2.64 ± 0.03 ns. Picosecond transient absorption spectroscopy and emission decay studies of CL in 4:1 acetonitrile: water solution showed an absorption feature (λmax = 625 nm and τ = 9.6 ± 0.6 ns) which is assigned to CLH+* in an extensive hydrogen-bonding solvation network. Oxidative quenching of CL* by methyl viologen dication, p-benzoquinone, or Co(bpy)3 3+ in acetonitrile generated a common transient absorbance centered at 480 nm (ε ≈ 6000 M-1 cm-1) which is assigned to the cation radical CL+. Picosecond absorption spectroscopy of the zinc reference complex 4 showed an absorption feature (λmax = 625 nm) similar to that seen for CL in 4:1 acetonitrile: water and assigned to CLH+*. Excitation of 4 results in emission with λem = 486 nm (ϕem = 0.92) and τ = 9.3 ± 0.3 ns. While the chromophore-quencher complexes 1 and 2 show the same emission band as that seen for 4, the quantum yields and lifetimes of this emission are much reduced (ϕem = 0.03, τ= 3.14 ± 0.04 ns and ϕem = 0.22, τ = 4.2 ± 0.1 ns, respectively). Transient absorption studies of 1 and 2 both display oxidative quenching of the chromophore by Cu(II) as evidenced by the generation of cation radical CL+ absorbance. The charge separated state for both complexes is formed in less than 25 ps and decays with a lifetime of approximately 300 ps. The difference in forward and backward electron-transfer rates is attributed to Marcus inverted region behavior.