A CHROMOPHORE QUENCHER COMPLEX INCORPORATING A PHOTOREDOX-ACTIVE LIGAND

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 equilibr ium with ground state pK(a) = 8.99 +/- 0.06 and an approximate excited -state pK(a) = 10.5 +/- 0.5. In 4:1 acetonitrile:water, the excited li gand CL at pH > 13.5 has lambda(em) = 419 nm (phi(em) = 0.19), while the excited protonated ligand CLH+ at pH <7 has lambda(em) = 486 nm ( phi(em) = 0.86). In hexane the lifetime of the fluorescent excited sta te of CL was determined to be 2.00 +/- 0.07 ns. In ''dry'' acetonitril e the emission came from both CL and CLH+* but only one decay was res olved with tau = 2.64 +/- 0.03 ns. Picosecond transient absorption spe ctroscopy and emission decay studies of CL in 4:1 acetonitrile:water s olution showed an absorption feature (lambda(max) = 625 nm and tau = 9 .6 +/- 0.6 ns) which is assigned to CLH+ in an extensive hydrogen-bon ding 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 (epsilon almost-equal-t o 6000 M-1 cm-1) which is assigned to the cation radical CL+. Picoseco nd absorption spectroscopy of the zinc reference complex 4 showed an a bsorption feature (lambda(max) = 625 nm) similar to that seen for CL i n 4:1 acetonitrile: water and assigned to CLH+. Excitation of 4 resul ts in emission with lambda(em) = 486 nm (phi(em) = 0.92) and tau = 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 lifetim es of this emission are much reduced (phi(em) = 0.03, tau = 3.14 +/- 0 .04 ns and phi(em) = 0.22, tau = 4.2 +/- 0.1 ns, respectively). Transi ent absorption studies of 1 and 2 both display oxidative quenching of the chromophore by Cu(II) as evidenced by the generation of cation rad ical 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.