Photodeactivation Channels of Transition Metal Complexes …
261
will showcase several anti-Kasha scenarios for TMCs (see, e.g., Sects. 3.2 and 3.3).
Next, starting from the thermally equilibrated S 1 state, fluorescence can then occur if
the emission of a photon is competitive to other radiationless processes depopulating
this state, such as S 1 → S 0 IC and/or S 1 → T n ISC processes (note that the k f values
typically range between 10
11 and 10
8 s
−1 ). In the case that ISC is fast enough to
compete with fluorescence and IC, triplet ESs will be populated with a certain yield.
Importantly, for many TMCs, due to: (i) their high density of near-degenerate ES of
different spin symmetry and (ii) the large size of the SOCs between these states; it
often leads to an important grade of admixture between these pure states, and thus
resulting in mixed spin–orbit states. This situation makes often meaningless not only
(i) the classical nomenclature of “pure” (e.g., singlet, triplet, or any other multiplicity) ESs, (ii) but also the fundamental difference between an IC process and an ISC
process. Moreover, in these systems, SOCs might not always be the main driving
force for ISC, but instead it could be controlled by vibronic effects [9]. Let us assume
now that the “pure” lowest triplet ES is finally predominantly populated. Once the
well of T 1 is populated, long-lived phosphorescence with a rate constant k r typically
ranging between 10
6 and 10
−2 s
−1 will occur from this thermally equilibrated ES if
it is fast enough to compete with T 1 → S 0 nonradiative decay. An important feature
of triplet states is that they consist of three different spin sublevels (T x , T y , and T z ),
in accordance with the three different M s quantum numbers (+1, 0, and −1), and that
result in the three different components of their associated rates (see Fig. 1) [10]. To
complete this plethora of interconnected relaxation processes, the manifold of singlet
and triplet ES may additionally be depopulated through many other photophysical
and/or photochemical processes. Among others, we highlight: (i) reverse ISC (rISC)
processes leading to the repopulation of the S 1 state, and which are responsible of
the thermally activated delayed fluorescence (TADF) phenomenon that has recently
found tremendous interest for OLED applications [11], (ii) photochemical reactions
leading to photoproducts, and (iii) intermolecular photoinduced energy and electron
transfer processes occurring due to the interaction of the molecular system with its
environment. Having in mind the above discussion, one can easily recognize how
complex and intricate the ES dynamics is, being the final outcomes of the photochemical reaction controlled by a subtle interplay of electronic and geometrical
rearrangements that take place during the ES deactivation dynamics [12]. While
intrinsic factors of the molecular systems, such as (i) the spin and character of the
ES involved in the process and (ii) the energetic alignment and effective couplings
between these states, do play a protagonist role in determining the preferred deactivation channels, other factors of extrinsic nature, such as temperature, pressure,
excitation wavelength, and environmental effects, can often strongly modify the outcome of the photochemical processes. To illustrate this complexity, computational
works covering the early-time photophysics but also the long-lived ES decay regime
are highlighted in this book chapter (see selected examples in Sect. 3).
Approaching the study of the ES dynamics of TMCs from an experimental
viewpoint requires techniques that can provide sensitivity to the electronic states,
their spin and the vibrational dynamics. Obtaining a simultaneous description of all
these variables within a single technique is challenging, and thus, a combination
261
will showcase several anti-Kasha scenarios for TMCs (see, e.g., Sects. 3.2 and 3.3).
Next, starting from the thermally equilibrated S 1 state, fluorescence can then occur if
the emission of a photon is competitive to other radiationless processes depopulating
this state, such as S 1 → S 0 IC and/or S 1 → T n ISC processes (note that the k f values
typically range between 10
11 and 10
8 s
−1 ). In the case that ISC is fast enough to
compete with fluorescence and IC, triplet ESs will be populated with a certain yield.
Importantly, for many TMCs, due to: (i) their high density of near-degenerate ES of
different spin symmetry and (ii) the large size of the SOCs between these states; it
often leads to an important grade of admixture between these pure states, and thus
resulting in mixed spin–orbit states. This situation makes often meaningless not only
(i) the classical nomenclature of “pure” (e.g., singlet, triplet, or any other multiplicity) ESs, (ii) but also the fundamental difference between an IC process and an ISC
process. Moreover, in these systems, SOCs might not always be the main driving
force for ISC, but instead it could be controlled by vibronic effects [9]. Let us assume
now that the “pure” lowest triplet ES is finally predominantly populated. Once the
well of T 1 is populated, long-lived phosphorescence with a rate constant k r typically
ranging between 10
6 and 10
−2 s
−1 will occur from this thermally equilibrated ES if
it is fast enough to compete with T 1 → S 0 nonradiative decay. An important feature
of triplet states is that they consist of three different spin sublevels (T x , T y , and T z ),
in accordance with the three different M s quantum numbers (+1, 0, and −1), and that
result in the three different components of their associated rates (see Fig. 1) [10]. To
complete this plethora of interconnected relaxation processes, the manifold of singlet
and triplet ES may additionally be depopulated through many other photophysical
and/or photochemical processes. Among others, we highlight: (i) reverse ISC (rISC)
processes leading to the repopulation of the S 1 state, and which are responsible of
the thermally activated delayed fluorescence (TADF) phenomenon that has recently
found tremendous interest for OLED applications [11], (ii) photochemical reactions
leading to photoproducts, and (iii) intermolecular photoinduced energy and electron
transfer processes occurring due to the interaction of the molecular system with its
environment. Having in mind the above discussion, one can easily recognize how
complex and intricate the ES dynamics is, being the final outcomes of the photochemical reaction controlled by a subtle interplay of electronic and geometrical
rearrangements that take place during the ES deactivation dynamics [12]. While
intrinsic factors of the molecular systems, such as (i) the spin and character of the
ES involved in the process and (ii) the energetic alignment and effective couplings
between these states, do play a protagonist role in determining the preferred deactivation channels, other factors of extrinsic nature, such as temperature, pressure,
excitation wavelength, and environmental effects, can often strongly modify the outcome of the photochemical processes. To illustrate this complexity, computational
works covering the early-time photophysics but also the long-lived ES decay regime
are highlighted in this book chapter (see selected examples in Sect. 3).
Approaching the study of the ES dynamics of TMCs from an experimental
viewpoint requires techniques that can provide sensitivity to the electronic states,
their spin and the vibrational dynamics. Obtaining a simultaneous description of all
these variables within a single technique is challenging, and thus, a combination