Photodeactivation Channels of Transition Metal Complexes …
279
addition, the interplay between two extrinsic factors, i.e., temperature and environment, was concomitantly assessed in this study.
Two different anti-Kasha emissions are possible: those arising between thermally equilibrated ES (case A in Fig. 7a) and those resulting from non-thermally
equilibrated ESs (case B in Fig. 7a). The thermally-equilibrated case is characterized by: (i) rather small ES n − ES 1 energetic differences, and thus thermal
equilibrium between the ES is at play; and (ii) absence of excitation wavelengthdependent emissive properties. Conversely, a non-thermally equilibrated scenario is
typified by: (i) large ES n − ES 1 energy gaps, so that photon emission from ES n
is faster than the sluggish ES n → ES 1 IC or ISC processes, and (ii) excitation
wavelength-dependent emissive properties. In Fig. 7b are depicted the two heteroleptic [Ru(H)(CO)(NˆN)(tpp) 2 ]
+ complexes (tpp triphenylphosphine) under study
in [99]. In the case of 1, [Ru(H)(CO)(dmphen)(tpp) 2 ]
+ (dmphen 5,6-dimethyl-1,
10-phenanthroline), temperature-dependent emissive properties were experimentally
observed [100]. More in details, an emission switch from solely
3 MLCT emission
at 298 K to dual photoluminescence at 77 K was measured. This
3 MLCT state
and the higher-lying
3 LC state are responsible for the two emissive bands at 77 K.
Conversely, for 2, [Ru(H)(CO)(bpy)(tpp) 2 ]
+ , only
3 MLCT emission was observed
regardless of the temperature. Two possible origins of the anti-Kasha emissions in 1
arise, namely: (i) different temperature-dependent solvent relaxation effects for the
3 MLCT and
3 LC states or (ii) a kinetically controlled scenario of photoluminescence.
The solvent hypothesis relies on the fact that both (i) solute geometry relaxation in
the ES and (ii) ES density-dependent relaxation of the solvent polarization occur at
298 K before the actual photon emission takes place. Conversely, at 77 K, the glassy
conformation of the media hinders the solvent molecules to reorganize themselves to
adapt to the solute charge distribution in the electronic ES. Therefore, if the
3 MLCT
and
3 LC excitations involve different density rearrangements, it could be the case
that the
3 MLCT state is the lowest lying ES (Kasha state) at 298 K but not at 77 K.
The second hypothesis (kinetically controlled scenario of photoluminescence) relies
on the fact that thermal equilibrium between the involved ES could be possible at
298 K, but not at 77 K.
The two above hypotheses were examined in [99]. In a first step, the geometries
of the relevant stationary points of 1, i.e., the two
3 ES minima and the TS connecting
both minima, were optimized with UB3LYP. In Fig. 8a, the schematic Jablonski
diagram is shown while Fig. 8b contains the spin density distribution at the
3 MLCT
and
3 LC minima. The
3 MLCT state is the lowest adiabatic emissive state, i.e., the
Kasha state, being 4.0 kcal/mol below the
3 LC state. In addition, there is a TS
connecting both minima, which is found 1.8 kcal/mol above the
3 LC minimum.
The α P-Ru-P bite angle at the
3 LC minimum (169°) is much larger than at the
3 MLCT
minimum (139°). Therefore, due to the steric hindrance between the two −PPh 3 units
at the
3 MLCT minimum, one of the phenyl rings is forced to twist at this geometry
(see Fig. 8b). The imaginary mode (31i cm
−1 ) of the TS connecting both minima
mainly involves the torsion of this phenyl ring. The photodeactivation channel of 1
can be summarized as follows. Photoexcitation to the singlet manifold (S n ) leads to
the population of the triplet manifold (T m ) with almost unity yield, as very efficient
279
addition, the interplay between two extrinsic factors, i.e., temperature and environment, was concomitantly assessed in this study.
Two different anti-Kasha emissions are possible: those arising between thermally equilibrated ES (case A in Fig. 7a) and those resulting from non-thermally
equilibrated ESs (case B in Fig. 7a). The thermally-equilibrated case is characterized by: (i) rather small ES n − ES 1 energetic differences, and thus thermal
equilibrium between the ES is at play; and (ii) absence of excitation wavelengthdependent emissive properties. Conversely, a non-thermally equilibrated scenario is
typified by: (i) large ES n − ES 1 energy gaps, so that photon emission from ES n
is faster than the sluggish ES n → ES 1 IC or ISC processes, and (ii) excitation
wavelength-dependent emissive properties. In Fig. 7b are depicted the two heteroleptic [Ru(H)(CO)(NˆN)(tpp) 2 ]
+ complexes (tpp triphenylphosphine) under study
in [99]. In the case of 1, [Ru(H)(CO)(dmphen)(tpp) 2 ]
+ (dmphen 5,6-dimethyl-1,
10-phenanthroline), temperature-dependent emissive properties were experimentally
observed [100]. More in details, an emission switch from solely
3 MLCT emission
at 298 K to dual photoluminescence at 77 K was measured. This
3 MLCT state
and the higher-lying
3 LC state are responsible for the two emissive bands at 77 K.
Conversely, for 2, [Ru(H)(CO)(bpy)(tpp) 2 ]
+ , only
3 MLCT emission was observed
regardless of the temperature. Two possible origins of the anti-Kasha emissions in 1
arise, namely: (i) different temperature-dependent solvent relaxation effects for the
3 MLCT and
3 LC states or (ii) a kinetically controlled scenario of photoluminescence.
The solvent hypothesis relies on the fact that both (i) solute geometry relaxation in
the ES and (ii) ES density-dependent relaxation of the solvent polarization occur at
298 K before the actual photon emission takes place. Conversely, at 77 K, the glassy
conformation of the media hinders the solvent molecules to reorganize themselves to
adapt to the solute charge distribution in the electronic ES. Therefore, if the
3 MLCT
and
3 LC excitations involve different density rearrangements, it could be the case
that the
3 MLCT state is the lowest lying ES (Kasha state) at 298 K but not at 77 K.
The second hypothesis (kinetically controlled scenario of photoluminescence) relies
on the fact that thermal equilibrium between the involved ES could be possible at
298 K, but not at 77 K.
The two above hypotheses were examined in [99]. In a first step, the geometries
of the relevant stationary points of 1, i.e., the two
3 ES minima and the TS connecting
both minima, were optimized with UB3LYP. In Fig. 8a, the schematic Jablonski
diagram is shown while Fig. 8b contains the spin density distribution at the
3 MLCT
and
3 LC minima. The
3 MLCT state is the lowest adiabatic emissive state, i.e., the
Kasha state, being 4.0 kcal/mol below the
3 LC state. In addition, there is a TS
connecting both minima, which is found 1.8 kcal/mol above the
3 LC minimum.
The α P-Ru-P bite angle at the
3 LC minimum (169°) is much larger than at the
3 MLCT
minimum (139°). Therefore, due to the steric hindrance between the two −PPh 3 units
at the
3 MLCT minimum, one of the phenyl rings is forced to twist at this geometry
(see Fig. 8b). The imaginary mode (31i cm
−1 ) of the TS connecting both minima
mainly involves the torsion of this phenyl ring. The photodeactivation channel of 1
can be summarized as follows. Photoexcitation to the singlet manifold (S n ) leads to
the population of the triplet manifold (T m ) with almost unity yield, as very efficient