Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 89
wards exciplex quenching. It has been hypothesized that emission stems from
the pentacoordinated exciplex itself that deactivates to a pentacoordinated
groundstatespecieswhicheventuallylosesthe“fifth”nucleophilicligandto
regenerate the initial ground state pseudotetrahedral complex [81]. In recent
years it has also been evidenced that some [Cu(NN)2] + complexes exhibit
a prompt luminescence signal with a lifetime of the order of 13–16 ps, which
is attributed to deactivation of 1 MLCT [88], whereas the long-lived component
(above 50 ns) would be phosphorescence from 3 MLCT, which borrows
intensity from upper lying singlet levels [89].
The shortest and longest values reported to date (oxygen-free CH2Cl2
solution, longer-lived component) are 80 [68] and 930 ns [71] and refer to homoleptic
[Cu(NN)2] + complexes of the two phenanthroline ligands depicted
in Fig. 19.
Fig. 19 Ligand 9 (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) and 10 (2,9-di-n-butyl-
3,4,7,8-tetramethyl-1,10-phenanthroline). Lifetime values of their [Cu(NN)2] + complexes
differ by more than one order of magnitude
The value for [Cu(9)2] + is very similar to that of [Cu(1)2] + under the
same conditions (80 ns), suggesting that electronic delocalization of the ligands
[73], very strong for 9, is less important than steric factors in determining
the lifetimes value. Notably, substitution on the 3 and 8 position
with a simple methyl residue in 10 is particularly effective to limit exciplex
quenching and yields a lifetime of almost 1 µs. In general, the large majority
of [Cu(NN)2] + homoleptic complexes exhibit excited state lifetimes in the
range 80–350 ns in oxygen-free solution [15]. A longer lifetime (730 ns, Φem
= 0.01, oxygen-free CH2Cl2) has been found with the suitably designed heteroleptic
complex [Cu(1)(11)] + [90], Fig. 20, in which excited state distortion
is strongly limited by the cumbersome tert-butyl substituent. Interestingly,
steric contraints make the formation of the homoleptic analogue [Cu(11)2] +
very difficult.
The picture describing the luminescent excited states of Cu(I)-bisphenanthrolinesisnotstraightforwardalthoughParkeretal.,inlightoftheob-