Photodeactivation Channels of Transition Metal Complexes …
263
used to track the ES decay dynamics of [IrBr 6 ]
2− [26]. Finally, these techniques can
often be used to analyze the effect of external stimulus on the photochemical properties, such as temperature, pressure, environmental effects (importantly aggregation
effects), and excitation wavelength. For instance, the effect of temperature on the
ISC timescales and yields in [Pt 2 (P 2 O 5 H 2 ) 4 ]
4− was deeply analyzed in [19]. In this
study, temperature was found to play a very important role in controlling the ISC
processes and thereto in determining the fluorescence/phosphorescence ratio of this
binuclear complex at a given temperature. Other recent examples highlighting the
effect of extrinsic factors on TMCs photochemistry are given in Sect. 3.3.
All the time-resolved techniques described above require very expensive and intricate experimental setups that prevent their use in a systematic manner. This is why
theoretical and computational investigations have become increasingly popular in the
field, as they offer an access to the ES properties and decay dynamics at a reasonable
cost. In this regard, since the pioneer work on TMCs photochemistry of Daniel and
coworkers in the 1990s [27, 28], the boost in the theoretical activity during recent
years would have not been possible without the developments done on: (i) quantum
chemical methods for the ES, (ii) algorithms to locate the optimal geometries of the
relevant stationary points on the ES potential energy surfaces (PESs), (iii) ES decay
rate theories, and (iv) ES reaction dynamics methods. In analogy with the experimental techniques, there is not a single state-of-the-art methodology able to capture
all the inherent complexities of a photochemical reaction, so that often a combination of them is required. Thus, depending on the problematic to be solved one has
to focus on attaining an accurate description of the ESs (e.g., modeling absorption
and emission processes), or on obtaining accurate ES PESs (e.g., modeling photoreactivity), or conversely on choosing an adequate ES reaction dynamic method
to model the early-time photophysics of TMCs. The absorption spectra of TMCs
and the associated ES properties (i.e., ES dipoles, densities, and charges) at the FC
point can now be routinely modeled [3], but still not with same accuracy as the ESs
of organic systems (see Sect. 2.1). Conversely, accessing emission properties and
vibronic effects remains more difficult and costly. Still, for some quantum chemical
methods for the ES, such as time-dependent density functional theory (TD-DFT)
[29], both analytic first (gradient) and second (Hessian) derivatives are already available, so that ES geometry optimizations and calculations of the ES vibrational levels
are possible at a reduced computational cost. This has facilitated the calculation of a
large panel of emissive (long-lived) properties in molecular systems containing up to
ca. 100 atoms [30], including fluorescence spectra, k f decay rates, and vibrationally
resolved emission band shapes. Additionally, one can compute additional magnitudes, such as the adiabatic energy differences and the nonadiabatic couplings and
SOCs between the electronic states of interest, which can be used in combination
with ES decay rate formalisms (mainly Fermi-Golden rule-based) for the calculation
of nonradiative decays rate, i.e., k IC and k ISC . Note that since Fermi-Golden expressions are based on perturbation theory, their application is strictly valid only when
the size of the coupling is significantly smaller than the adiabatic energy gap between
the final and initial states, so that these expressions should cautiously be used when
dealing with close-lying ESs. The latter calculations, along with the calculation of
263
used to track the ES decay dynamics of [IrBr 6 ]
2− [26]. Finally, these techniques can
often be used to analyze the effect of external stimulus on the photochemical properties, such as temperature, pressure, environmental effects (importantly aggregation
effects), and excitation wavelength. For instance, the effect of temperature on the
ISC timescales and yields in [Pt 2 (P 2 O 5 H 2 ) 4 ]
4− was deeply analyzed in [19]. In this
study, temperature was found to play a very important role in controlling the ISC
processes and thereto in determining the fluorescence/phosphorescence ratio of this
binuclear complex at a given temperature. Other recent examples highlighting the
effect of extrinsic factors on TMCs photochemistry are given in Sect. 3.3.
All the time-resolved techniques described above require very expensive and intricate experimental setups that prevent their use in a systematic manner. This is why
theoretical and computational investigations have become increasingly popular in the
field, as they offer an access to the ES properties and decay dynamics at a reasonable
cost. In this regard, since the pioneer work on TMCs photochemistry of Daniel and
coworkers in the 1990s [27, 28], the boost in the theoretical activity during recent
years would have not been possible without the developments done on: (i) quantum
chemical methods for the ES, (ii) algorithms to locate the optimal geometries of the
relevant stationary points on the ES potential energy surfaces (PESs), (iii) ES decay
rate theories, and (iv) ES reaction dynamics methods. In analogy with the experimental techniques, there is not a single state-of-the-art methodology able to capture
all the inherent complexities of a photochemical reaction, so that often a combination of them is required. Thus, depending on the problematic to be solved one has
to focus on attaining an accurate description of the ESs (e.g., modeling absorption
and emission processes), or on obtaining accurate ES PESs (e.g., modeling photoreactivity), or conversely on choosing an adequate ES reaction dynamic method
to model the early-time photophysics of TMCs. The absorption spectra of TMCs
and the associated ES properties (i.e., ES dipoles, densities, and charges) at the FC
point can now be routinely modeled [3], but still not with same accuracy as the ESs
of organic systems (see Sect. 2.1). Conversely, accessing emission properties and
vibronic effects remains more difficult and costly. Still, for some quantum chemical
methods for the ES, such as time-dependent density functional theory (TD-DFT)
[29], both analytic first (gradient) and second (Hessian) derivatives are already available, so that ES geometry optimizations and calculations of the ES vibrational levels
are possible at a reduced computational cost. This has facilitated the calculation of a
large panel of emissive (long-lived) properties in molecular systems containing up to
ca. 100 atoms [30], including fluorescence spectra, k f decay rates, and vibrationally
resolved emission band shapes. Additionally, one can compute additional magnitudes, such as the adiabatic energy differences and the nonadiabatic couplings and
SOCs between the electronic states of interest, which can be used in combination
with ES decay rate formalisms (mainly Fermi-Golden rule-based) for the calculation
of nonradiative decays rate, i.e., k IC and k ISC . Note that since Fermi-Golden expressions are based on perturbation theory, their application is strictly valid only when
the size of the coupling is significantly smaller than the adiabatic energy gap between
the final and initial states, so that these expressions should cautiously be used when
dealing with close-lying ESs. The latter calculations, along with the calculation of