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.

Peter Dylan Crapps

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