Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Rhodium 221
bite angle, two tpy ligands provide a lower ligand field strength compared
to three phen (or bpy) ligands [46]. Accordingly, whereas Rh(bpy) 3+
3 is a LC
emitter, Rh(tpy) 3+ and related compounds only show at 77 Kbroadstructure-
2
less emissions of MC type [47].
In heteroleptic bis-imine Rh(III) complexes, multiple emissions originating
from LC states localized on different ligands can be present at
77 K. For example, in complexes of type [Rh(bpy)n(phen)3–n] 3+ , the LC
emissions localized on bpy and phen are spectrally very similar but can
be distinguished on the basis of their different lifetimes in time-resolved
experiments [30, 48]. Clear indication that the excitation is trapped at either
a bipyridyl or a phenanthroline ligand in the phosphorescent triplet state
can also be obtained from phosphorescence microwave double resonance
(PMDR) experiments [49]. The heteroleptic complex Rh(phi)2(phen) 3+ (5)
has been particularly developed as a DNA photocleavage agent (see Sect. 4).
The complexes absorb strongly in UV region with a significant broad absorption
in the visible range, which tails to ca. 500 nm [50]. The spectrum
is dominated by overlapping of the ligand centered (LC) transitions of the
component ligands with the phi centered bands at lower energy. These bands
are strongly pH dependent with shifts to the blue upon increasing the pH.
At 77 K the complex exhibits ligand centered (LC) dual emission from both
phi and phen ligands. At room temperature no emission can be detected
whereas a long-lived excited state (τ ≈ 200 ns in polar solvent) has been
observed by transient absorption. On the basis of several experimental evidences
this excited state, lying in energy at ca. 2 eV above the ground state,
is assigned by the authors to be intraligand charge transfer in nature (ILCT).
Quenching experiments with organic electron donors clearly indicate that the
ILCT triplet state is a strong oxidizing agent with E1/2( ∗Rh3+ /Rh2+ )=2.0 V
vs. NHE [50]. Multiple LC emissions have also been suggested to occur in
heteroleptic complexes containing bipyridine or phenanthroline and pyridyl
triazole ligands [51].
While for the vast majority of Rh(III) polypyridine complexes the photophysics
and photochemistry are dominated by LC and MC states, in a few