260
D. Escudero
1 General Overview
In a molecular system, the chemistry in the electronically excited states (ES) tends
to significantly differ from that of the ground state (GS). First, as the photon absorption conducts the molecular system to a high-energy level, the ESs are generally
short-lived and significantly more reactive than the GS. The molecular system in
their ESs will tend to dissipate this excess of energy in a radiative or radiationless
manner. Second, the GS and the ES’s molecular properties and thereto their chemical
reactivity strongly differ. GS molecular properties can easily be analyzed in terms
of basic concepts of bonding, anti-bonding, and/or non-bonding interactions, and
furthermore, numerous (i) structure–property relationships and (ii) rules to predict
chemical reactivity are available for the GS. In contrast, ES molecular properties and
ES reactivity are hardly predictable. This complex behavior is even more dramatic
in the case of transition metal complexes (TMCs), which compared with organic
molecules, possess (i) a much higher density of ESs in the near-UV/visible spectral
regions, (ii) easily populated triplet ESs due to larger spin–orbit couplings (SOCs),
and (ii) a larger variety of ESs of different character [1, 2]. Among these types of
ESs, I highlight: (i) metal-centered (MC) states, i.e., fully localized on the transition
metal; (ii) ligand-centered (LC) states, i.e., localized on a single ligand; (iii) ligandto-metal charge transfer (LMCT) states, where the ligand acts as a donor and the
transition metal as an acceptor; (iv) metal-to-ligand charge transfer (MLCT) states;
and (v) ligand-to-ligand charge transfer (LLCT) states [3, 4]. In some cases, the ES
character is readily recognizable from a visual inspection of the involved orbitals.
However, it is often found that there is a certain degree of admixture between different ES, leading to, e.g., mixed MLCT/LC ES. To alleviate these issues and avoid
subjective assignments of the ES characters, several quantitative analysis tools have
been developed in recent years [5–7].
Let us now recall the possible pathways of a given photochemical reaction. These
processes are schematically presented in the Jablonski diagram depicted in Fig. 1.
Such a textbook representation is chosen herein for several reasons: first because it
is generally valid for small organic chromophores and second because it is presumably well known by most of the readers. Note that the photodeactivation pathways
highlighted in Fig. 1 and their corresponding timescales might not always apply
for TMCs. Upon photoexcitation, which is a very fast process taking place on a
timescale of ca. 10
−15 s, and thus the nuclei remain fixed during this process (i.e.,
Franck–Condon (FC) approximation), the manifold of singlet ESs (S m ) is populated
vertically. According to the Kasha rule [8], which it is generally assumed to always
hold true, radiationless deactivation would occur next through internal conversion
(IC) and vibrational relaxation processes, which ultimately lead to the population of
the lowest vibrational level of the S 1 state. The latter processes are generally very
fast; i.e., the k IC rate constant is usually higher than 10
12 s
−1 , so that other possible
deactivation processes (such as S m → T n intersystem crossing (ISC) processes and/or
anti-Kasha S m → T 0 radiative deactivation) are often not competitive to IC. This is
the reason that Kasha rule holds true for many molecular systems. In this chapter, we
D. Escudero
1 General Overview
In a molecular system, the chemistry in the electronically excited states (ES) tends
to significantly differ from that of the ground state (GS). First, as the photon absorption conducts the molecular system to a high-energy level, the ESs are generally
short-lived and significantly more reactive than the GS. The molecular system in
their ESs will tend to dissipate this excess of energy in a radiative or radiationless
manner. Second, the GS and the ES’s molecular properties and thereto their chemical
reactivity strongly differ. GS molecular properties can easily be analyzed in terms
of basic concepts of bonding, anti-bonding, and/or non-bonding interactions, and
furthermore, numerous (i) structure–property relationships and (ii) rules to predict
chemical reactivity are available for the GS. In contrast, ES molecular properties and
ES reactivity are hardly predictable. This complex behavior is even more dramatic
in the case of transition metal complexes (TMCs), which compared with organic
molecules, possess (i) a much higher density of ESs in the near-UV/visible spectral
regions, (ii) easily populated triplet ESs due to larger spin–orbit couplings (SOCs),
and (ii) a larger variety of ESs of different character [1, 2]. Among these types of
ESs, I highlight: (i) metal-centered (MC) states, i.e., fully localized on the transition
metal; (ii) ligand-centered (LC) states, i.e., localized on a single ligand; (iii) ligandto-metal charge transfer (LMCT) states, where the ligand acts as a donor and the
transition metal as an acceptor; (iv) metal-to-ligand charge transfer (MLCT) states;
and (v) ligand-to-ligand charge transfer (LLCT) states [3, 4]. In some cases, the ES
character is readily recognizable from a visual inspection of the involved orbitals.
However, it is often found that there is a certain degree of admixture between different ES, leading to, e.g., mixed MLCT/LC ES. To alleviate these issues and avoid
subjective assignments of the ES characters, several quantitative analysis tools have
been developed in recent years [5–7].
Let us now recall the possible pathways of a given photochemical reaction. These
processes are schematically presented in the Jablonski diagram depicted in Fig. 1.
Such a textbook representation is chosen herein for several reasons: first because it
is generally valid for small organic chromophores and second because it is presumably well known by most of the readers. Note that the photodeactivation pathways
highlighted in Fig. 1 and their corresponding timescales might not always apply
for TMCs. Upon photoexcitation, which is a very fast process taking place on a
timescale of ca. 10
−15 s, and thus the nuclei remain fixed during this process (i.e.,
Franck–Condon (FC) approximation), the manifold of singlet ESs (S m ) is populated
vertically. According to the Kasha rule [8], which it is generally assumed to always
hold true, radiationless deactivation would occur next through internal conversion
(IC) and vibrational relaxation processes, which ultimately lead to the population of
the lowest vibrational level of the S 1 state. The latter processes are generally very
fast; i.e., the k IC rate constant is usually higher than 10
12 s
−1 , so that other possible
deactivation processes (such as S m → T n intersystem crossing (ISC) processes and/or
anti-Kasha S m → T 0 radiative deactivation) are often not competitive to IC. This is
the reason that Kasha rule holds true for many molecular systems. In this chapter, we