Photochemistry and Photophysics of Coordination Compounds

126 S. Campagna et al.
With increasing temperature, the emission lifetime (Fig. 8) and quantum
yield decrease [1, 4, 6, 8, 32, 61–79]. This behavior may be accounted for by
a stepwise term and two Arrhenius terms [1–3]:
B
1/τ =k0 +
1+exp[C(1/T –1/TB)] + A1 exp(– ∆E1/RT)
+ A2 exp(– ∆E2/RT). (1)
The value of the various parameters is somewhat dependent on the nature
of the solvent. In propionitrile–butyronitrile (4 : 5 v/v) the values are as
follows [70, 71]: k0 = 2 × 10 5 s –1 ; B = 2.1 × 10 5 s –1 ; A1 = 5.6 × 10 5 s –1 ; ∆E1 =
90 cm –1 ; A2 = 1.3 × 10 14 s –1 ; ∆E2 = 3960 cm –1 .Includedink0 are the radiative
k0(r) and nonradiative k0(nr) rate constants at 84 K. The stepwise term B is
due to the melting of the matrix (100–150 K) and corresponds to the coming
into play of vibrations capable of facilitating radiationless deactivation [8, 71].
Inthesametemperaturerangearedshiftof∼ 1000 cm –1 is observed in the
maximum of the emission band, and it is mainly attributed to reorganization
of solvent molecules around the excited state in fluid solution before
emission takes place [8, 71]. The Arrhenius term with A1 = 5.6 × 10 5 s –1 and
∆E1 = 90 cm –1 is thought to correspond to the thermal equilibration with
a level lying at slightly higher energy and having the same electronic nature
(so it would be a fourth MLCT state [6], considering the lowest-lying MLCT
state is made of three sublevels as described before). The second Arrhenius
term corresponds to a thermally activated surface crossing to an upper-lying
3 MC level which undergoes fast deactivation. Identification of this higher
level as a 3 MC state is based upon the observed photosubstitution behavior
at elevated temperatures [61], consistent with established photoreactivity
patterns for d 6 metal complexes [17, 52].
Experiments carried out with [Ru(bpy)3] 2+ and [Ru(bpy-d8)3] 2+ in H2O
and D2O [61, 80, 81] indicate that k0(nr) is sensitive to deuteration, as expected
for a weak-coupled radiationless process [6, 82–84]. By contrast, A2
is insensitive to deuteration, supporting a strong-coupled (surface crossing)
deactivation pathway, which may be related to the observed photosensitivity.
It should be noted that the decrease in lifetime on melting has also been explained
on the basis of the energy gap law because of the corresponding red
shift in the emission band [6].
Finally, it should be noted that at 77 K the emission spectrum of
[Ru(bpy)3] 2+ , as well as that of most Ru(II) polypyridine complexes, exhibits
a vibrational structure (see Fig. 7). This structure is assigned to the vibrational
progression, and its energy spacing is about 1300 cm –1 ,equivalentto
the C – NandC– C stretching energy of the aromatic rings, thus indicating
that such stretchings are the dominant accepting modes for deactivation of
the 3 MLCT state.