Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines

This
work presents a series of Cu(I) heteroleptic 1,10-phenanthroline
chromophores featuring enhanced UVA and visible-light-harvesting properties
manifested through vectorial control of the copper-to-phenanthroline
charge-transfer transitions. The molecules were prepared using the
HETPHEN strategy, wherein a sterically congested 2,9-dimesityl-1,10-phenanthrolne
(mesPhen) ligand was paired with a second phenanthroline ligand incorporating
extended π-systems in their 4,7-positions. The combination of
electrochemistry, static and time-resolved electronic spectroscopy,
77 K photoluminescence spectra, and time-dependent density functional
theory calculations corroborated all of the experimental findings.
The model chromophore, [Cu(mesPhen)(phen)]+ (1), lacking 4,7-substitutions preferentially reduces the mesPhen ligand
in the lowest energy metal-to-ligand charge-transfer (MLCT) excited
state. The remaining cuprous phenanthrolines (2–4) preferentially reduce their π-conjugated ligands
in the low-lying MLCT excited state. The absorption cross sections
of 2–4 were enhanced (εMLCTmax = 7430–9980 M–1 cm–1) and significantly broadened across the UVA and visible regions
of the spectrum compared to 1 (εMLCTmax = 6494 M–1 cm–1). The excited-state
decay mechanism mirrored those of long-lived homoleptic Cu(I) phenanthrolines,
yielding three distinguishable time constants in ultrafast transient
absorption experiments. These represent pseudo-Jahn–Teller
distortion (τ1), singlet–triplet intersystem
crossing (τ2), and the relaxed MLCT excited-state
lifetime (τ3). Effective light-harvesting from Cu(I)-based
chromophores can now be rationalized within the HETPHEN strategy while
achieving directionality in their respective MLCT transitions, valuable
for integration into more complex donor–acceptor architectures
and longer-lived photosensitizers.