The
electronic and structural properties of ten heteroleptic [Cu(NN)(PP)]+ complexes have been investigated. NN indicates 1,10-phenanthroline
(phen) or 4,7-diphenyl-1,10-phenanthroline (Bphen); each of these
ligands is combined with five PP bis-phosphine chelators, i.e., bis(diphenylphosphino)methane
(dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane
(dppp), 1,2-bis(diphenylphosphino)benzene (dppb), and bis[(2-diphenylphosphino)phenyl]
ether (POP). All complexes are mononuclear, apart from those based
on dppm, which are dinuclear. Experimental dataalso taken
from the literature and including electrochemical properties, X-ray
crystal structures, UV–vis absorption spectra in CH2Cl2, luminescence spectra and lifetimes in solution, in PMMA,
and as powdershave been rationalized with the support of density
functional theory calculations. Temperature dependent studies (78–358
K) have been performed for selected complexes to assess thermally
activated delayed fluorescence. The main findings are (i) dependence
of the ground-state geometry on the crystallization conditions, with
the same complex often yielding different crystal structures; (ii)
simple model compounds with imposed C
2v
symmetry ([Cu(phen)(PX3)2]+; X = H or CH3) are capable of modeling structural
parameters as a function of the P–Cu–P bite angle, which
plays a key role in dictating the overall structure of [Cu(NN)(PP)]+ complexes; (iii) as the P–Cu–P angle increases,
the energy of the metal-to-ligand charge transfer absorption bands
linearly increases; (iv) the former correlation does not hold for
emission spectra, which are red-shifted for the weaker luminophores;
(v) the larger the number of intramolecular π-interactions within
the complex in the ground state, the higher the luminescence quantum
yield, underpinning a geometry locking effect that limits the structural
flattening of the excited state. This work provides a general framework
to rationalize the structure–property relationships of [Cu(NN)(PP)]+, a class of compounds of increasing relevance for electroluminescent
devices, photoredox catalysis, and solar-to-fuels conversion, which
so far have been investigated in an unsystematic fashion, eluding
a comprehensive understanding.