266
D. Escudero
One important aspect when modeling ES is that the impact of the environment is
often larger in the ES than in the GS, and thus, the outcome of the photochemical
processes can strongly vary from the gas phase to the condensed phase and/or in the
presence of complex environments. This poses significant challenges from a modeling viewpoint, as the explicit quantum chemical consideration of the environment can
become easily prohibitive for the ES. Thus, the most popular approaches to mimic
the environment are (i) hybrid techniques such as quantum mechanical/molecular
mechanics (QM/MM), which provide an atomistic description of the environment
at a reasonable cost [41], and (ii) polarizable continuum models (such as the PCM
approach [42]), where the solvent is modeled as a polarizable continuum rather than
with its atomistic description. PCM models are commonly used to mimic solvation
effects and provide the best compromise between accuracy and computational cost.
Problems of the PCM approach may arise (i) in the presence of protic solvents,
where specific solvent–solute hydrogen bonding interactions may arise and (ii) for
excitations involving large density rearrangements. A first shell of explicit solvent
molecules in combination with the bulk PCM effects may alleviate the first problem,
as exemplarily shown for a Fe(II) complex [43], while the latter problem appears
in linear response approaches and requires the use of advanced state-specific (SS)
PCM models [44, 45]. The SS-PCM approaches are best suited to model emission
processes. This is so because photon emission is often a “slow” process, involving
not only the solute geometry relaxation in the ES but also the reorganization of the
solvent molecules to adapt to the solute charge distribution in the ES (an example
of the interplay between temperature and solvation effects is presented in Sect. 3.3).
Alternatively, QM/MM methods can be used straightforwardly in systems where one
can clearly define the partitioning between the photoactive part and the environment,
such as protein embedding, modeling caging, and solvent effects, as well as in the
case of molecular crystals. However, it is not always possible to split the level of accuracy of the different parts of the system, since for instance, in photosynthetic systems
many photoactive units are spread within the protein, and therefore, the whole system
would require to be described quantum mechanically. Other related problems may
arise from the appearance of excimers and exciplexes due to aggregation in the condensed phase. These situations require the inclusion of several photoactive units in
the QM part. An alternative to study large multichromophoric systems is to use exciton models [46]. Aiming at an accurate recovery of the environmental effects for the
ES in a QM/MM framework, increased efforts in the community have been recently
devoted to the development of polarizable QM/MM schemes [47], where both QM
and MM parts polarize each other. Finally, as a prominent alternative for QM/MM,
subsystem and embedding quantum chemical schemes have found increased interest
in recent years [48].
In this chapter, in Sect. 2 an overview of the state-of-the-art theoretical/
computational methods to unveil photodeactivation channels in TMCs is presented,
while selected recent computational works are highlighted in Sect. 3. These examples were selected to illustrate the progress achieved in deciphering photodeactivation channels of TMCs and which importantly cover the ultrafast ES decay
D. Escudero
One important aspect when modeling ES is that the impact of the environment is
often larger in the ES than in the GS, and thus, the outcome of the photochemical
processes can strongly vary from the gas phase to the condensed phase and/or in the
presence of complex environments. This poses significant challenges from a modeling viewpoint, as the explicit quantum chemical consideration of the environment can
become easily prohibitive for the ES. Thus, the most popular approaches to mimic
the environment are (i) hybrid techniques such as quantum mechanical/molecular
mechanics (QM/MM), which provide an atomistic description of the environment
at a reasonable cost [41], and (ii) polarizable continuum models (such as the PCM
approach [42]), where the solvent is modeled as a polarizable continuum rather than
with its atomistic description. PCM models are commonly used to mimic solvation
effects and provide the best compromise between accuracy and computational cost.
Problems of the PCM approach may arise (i) in the presence of protic solvents,
where specific solvent–solute hydrogen bonding interactions may arise and (ii) for
excitations involving large density rearrangements. A first shell of explicit solvent
molecules in combination with the bulk PCM effects may alleviate the first problem,
as exemplarily shown for a Fe(II) complex [43], while the latter problem appears
in linear response approaches and requires the use of advanced state-specific (SS)
PCM models [44, 45]. The SS-PCM approaches are best suited to model emission
processes. This is so because photon emission is often a “slow” process, involving
not only the solute geometry relaxation in the ES but also the reorganization of the
solvent molecules to adapt to the solute charge distribution in the ES (an example
of the interplay between temperature and solvation effects is presented in Sect. 3.3).
Alternatively, QM/MM methods can be used straightforwardly in systems where one
can clearly define the partitioning between the photoactive part and the environment,
such as protein embedding, modeling caging, and solvent effects, as well as in the
case of molecular crystals. However, it is not always possible to split the level of accuracy of the different parts of the system, since for instance, in photosynthetic systems
many photoactive units are spread within the protein, and therefore, the whole system
would require to be described quantum mechanically. Other related problems may
arise from the appearance of excimers and exciplexes due to aggregation in the condensed phase. These situations require the inclusion of several photoactive units in
the QM part. An alternative to study large multichromophoric systems is to use exciton models [46]. Aiming at an accurate recovery of the environmental effects for the
ES in a QM/MM framework, increased efforts in the community have been recently
devoted to the development of polarizable QM/MM schemes [47], where both QM
and MM parts polarize each other. Finally, as a prominent alternative for QM/MM,
subsystem and embedding quantum chemical schemes have found increased interest
in recent years [48].
In this chapter, in Sect. 2 an overview of the state-of-the-art theoretical/
computational methods to unveil photodeactivation channels in TMCs is presented,
while selected recent computational works are highlighted in Sect. 3. These examples were selected to illustrate the progress achieved in deciphering photodeactivation channels of TMCs and which importantly cover the ultrafast ES decay