Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Rhodium 219
properties of the very weak emission measured from room-temperature solutions
of Rh(phen)3 3+ (bandshape, lifetimes) are consistent with a small
amount of MC excited state being in thermal equilibrium with the lowest LC
state [34]. In related 2,2 ′ -bipyridine complexes, changes in LC-MC energy gap
and emission spectral profile can be induced by methyl substitution in the
3,3 ′ positions, as a consequence of changes in the degree of planarity of the
ligand [37].
The interplay of LC and MC triplets in the photophysics of this type of
complexes has been investigated in detail by studying the series of mixedligand
complexes cis-Rh(phen)2XY n+ (X=Y=CN,n =1;X=Y=NH3, n =3;
X=NH3, Y=Cl,n = 2), where the energy of the MC states is controlled by
the ligand field strength of the X, Y ancillary ligands [38]. While at room temperature
all the complexes are very weakly emissive, at 77 K, the di-cyano and
di-amino complexes give the typical, structured LC emission, whereas the
amino-chloro complex exhibits a broad emission of MC type (Fig. 1a). This
can be readily explained on the basis of the energy diagram of Fig. 1b, where
the relative energies of the LC triplet (appreciably constant for the three complexes)
and of the lowest MC state (dependent on the ligand field strength of
the ancillary ligands) are schematically depicted. For the amino-chloro complex
the MC state is the lowest excited state of the system. For the di-amino
case the situation is similar to that of the Rh(phen)3 3+ complex, with LC as
the lowest excited state but with MC sufficiently close in energy to provide
an efficient thermally activated decay path for LC (actually, in an appropriate
temperature regime the two states are in equilibrium). In the di-cyano
complex, the MC state is sufficiently high in energy that the LC state has
a substantial lifetime (1.2 µs) even in room-temperature solution [38]. The
actual energy gap between the LC and MC states (2000 cm –1 for the di-cyano
Fig. 1 a 77 K emission spectra of cis-Rh(phen)2XY n+ complexes with different X, Y ancillary
ligands. b Rationalization of the emission properties in terms of relative energies of
ligand-centered (LC) and metal-centered (MC) excited states (adapted from [38])