Photochemistry and Photophysics of Coordination Compounds

92 N. Armaroli et al.
Fig. 22 Ligand 12, 2-(3,5-di-tert-butyl-4-methoxyphenyl)-9-(2,4,6-trimethylphenyl)-1,10phenanthroline
(Fig. 22). This finding gives further support to the notion that steric factors
are by far more important than electronic factors.
The room temperature lifetime of [Cu(12)2] + in oxygen free solution (285
ns) is scarcely affected under air-equilibrated conditions (266 ns) and is remarkably
high also in CH3OH (182 ns), a solvent where most [Cu(NN)2] +
do not show any luminescence. This rather unusual solvent insensitivity of
the lifetime is observed also for [Cu(5)2] + (Fig. 14) suggesting that tert-butyl
groups are very effective in preventing the formation of the pentacoordinated
exciplex at room temperature [74]. On the other hand, in striking contrast
with [Cu(12)2] + , [Cu(5)2] + is virtually non-luminescent in 77 K rigid matrix,
highlighting once again the subtle factors affecting the molecular structure
and, accordingly, the emission performance. Probably, in [Cu(12)2] + ,thekey
structural features causing good luminescence performance are the methyl
residues on the 2 and 6 position of a phenyl substituent (Fig. 22).
Solid state measurements confirm the strong dependence of the excited
state lifetimes of [Cu(NN)2] + complexes on geometric distortions. In particular
it has been found that there is a good linear correlation between the
ξCD parameter (see Sect. 2.1, Eq. 1) and the measured lifetime both at room
temperature and at 17 K: the smaller the distortion from the ideal pseudotetrahedral
geometry (high ξCD parameter), the longer the lifetime [56].
2.5
Photoinduced Processes in Multicomponent Systems Based on
[Cu(NN)2] + Complexes
Templated synthesis of Cu(I)-bisphenanthrolines has prompted the design
and construction of sophisticated multicomponent architectures comprising
one or more [Cu(NN)2] + centers in tandem with other chromophores, typ-