Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics

Abstract

In the interest of expanding the inventory of available long lifetime, photochemically robust, and strongly reducing Cu(I) MLCT sensitizers, we present detailed structural, photophysical, and electrochemical characterization of [Cu(dipp)₂]⁺, dipp = 2,9-diisopropyl-1,10-phenanthroline, and its sterically encumbered tetramethyl analogue [Cu(diptmp)₂]⁺, diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline. The achiral isopropyl substituents enable similar steric bulk effects to the previously investigated sec-butyl substituents while eliminating the complex NMR structural analyses associated with the presence of two chiral centers in the latter. The photophysical properties of [Cu(diptmp)₂]⁺ are impressive, possessing a 2.3 μs lifetime in deaerated CH₂Cl₂ and a photoluminescence quantum yield of 4.7%, which were slightly attenuated in coordinating tetrahydrofuran (THF) solutions. Nanosecond transient absorption spectroscopy results matched the transient photoluminescence kinetics enabling complete characterization of MLCT excited-state decay in these molecules. The calculated excited-state potential for the Cu²⁺/Cu⁺* couple (E = -1.74 V vs Fc^+/0) indicated that [Cu(diptmp)₂]⁺* is a strong photoreductant potentially useful for myriad applications. Ultrafast transient absorption measurements performed in THF solutions are also reported, yielding the relative time scales for both the pseudo-Jahn-Teller distortion (0.4-0.8 ps in [Cu(dipp)₂]⁺ and 0.12-0.5 ps in [Cu(diptmp)₂]⁺) and singlet-triplet intersystem crossing (6.4-10.1 ps for [Cu(dipp)₂]⁺ and 3.5-5.4 ps for [Cu(diptmp)₂]⁺) within these molecules. The disparity in the time scales of pseudo-Jahn-Teller distortion and intersystem crossing between two complexes with different anticipated excited-state geometries suggests that strongly impeded structural distortion in the MLCT excited state (i.e., [Cu(diptmp)₂]⁺) enables more rapid surface crossings in the initial deactivation dynamics.