MC
Metal-centered
MCQDPT
Multiconfiguration quasi-degenerate perturbation theory
MCTDH
Multiconfiguration time-dependent Hartree
MLCT
Metal-to-ligand-charge-transfer
MS-CASPT2 Multi-state CASPT2
PBE0
Perdew–Burke–Ernzerhof
PCM
Polarized continuum model
PES
Potential energy surfaces
PW91
Perdew–Wang 1991
RASSI
Restricted active space state interaction
SOC
Spin-orbit coupling
XANES
X-Ray absorption near-edge structure
ZFS
Zero field splitting
ZORA
Zeroth order regular approximation
1 Introduction
Triplet electronic excited states play a central role in the spectroscopy, photochemistry, and photophysics of transition metal complexes. They perturb the fine structure of absorption spectra and are responsible for long-lived emission via low-lying
metal-to-ligand-charge-transfer (MLCT) states over a wide range of energy
domains [1–12]. They can quench emission by triggering electron transfer processes via charge-separated (CS) states [13–15] or by inducing competitive dissociation via metal-centered (MC) states [16–20]. Triplet sigma-bond charge transfer
states induce metal–alkyl bond homolysis [21, 22] sigma-bond sigma-bond excited
states are precursors of metal–metal bond homolysis [23] whereas intra-ligand
(IL) localized triplet states conduct isomerization pathways under visible irradiation [24–26].
The kinetics of intersystem crossing (ISC) processes entirely control the population of the low-lying triplet states after UV/visible absorption and strongly
influence the branching ratio between radiative and non-radiative decays. The
development of time-resolved spectroscopy, within femtosecond (fs)/picosecond
(ps) time scales, has opened the route to new experimental investigations in the field
of first-, second-, and third-row transition metal complexes photophysics
supporting evidence of ultra-fast ISC [27–32]. In order to understand the role of
the high spin states and to interpret these experimental findings, quantum chemistry
needs powerful methods able to describe correctly the excited states properties:
(1) electronic and geometrical structures; (2) transition energies; (3) spin-orbit
interaction between states of different multiplicities; and (4) multiplet spin-orbit
splitting. The electronic structure calculations should also figure out the shape of
the potential energy surfaces (PES) underlying the non-adiabatic excited states
dynamics. A direct correlation between the experimental data and the outcome of
Absorption Spectroscopy, Emissive Properties, and Ultrafast Intersystem. . .
379
Metal-centered
MCQDPT
Multiconfiguration quasi-degenerate perturbation theory
MCTDH
Multiconfiguration time-dependent Hartree
MLCT
Metal-to-ligand-charge-transfer
MS-CASPT2 Multi-state CASPT2
PBE0
Perdew–Burke–Ernzerhof
PCM
Polarized continuum model
PES
Potential energy surfaces
PW91
Perdew–Wang 1991
RASSI
Restricted active space state interaction
SOC
Spin-orbit coupling
XANES
X-Ray absorption near-edge structure
ZFS
Zero field splitting
ZORA
Zeroth order regular approximation
1 Introduction
Triplet electronic excited states play a central role in the spectroscopy, photochemistry, and photophysics of transition metal complexes. They perturb the fine structure of absorption spectra and are responsible for long-lived emission via low-lying
metal-to-ligand-charge-transfer (MLCT) states over a wide range of energy
domains [1–12]. They can quench emission by triggering electron transfer processes via charge-separated (CS) states [13–15] or by inducing competitive dissociation via metal-centered (MC) states [16–20]. Triplet sigma-bond charge transfer
states induce metal–alkyl bond homolysis [21, 22] sigma-bond sigma-bond excited
states are precursors of metal–metal bond homolysis [23] whereas intra-ligand
(IL) localized triplet states conduct isomerization pathways under visible irradiation [24–26].
The kinetics of intersystem crossing (ISC) processes entirely control the population of the low-lying triplet states after UV/visible absorption and strongly
influence the branching ratio between radiative and non-radiative decays. The
development of time-resolved spectroscopy, within femtosecond (fs)/picosecond
(ps) time scales, has opened the route to new experimental investigations in the field
of first-, second-, and third-row transition metal complexes photophysics
supporting evidence of ultra-fast ISC [27–32]. In order to understand the role of
the high spin states and to interpret these experimental findings, quantum chemistry
needs powerful methods able to describe correctly the excited states properties:
(1) electronic and geometrical structures; (2) transition energies; (3) spin-orbit
interaction between states of different multiplicities; and (4) multiplet spin-orbit
splitting. The electronic structure calculations should also figure out the shape of
the potential energy surfaces (PES) underlying the non-adiabatic excited states
dynamics. A direct correlation between the experimental data and the outcome of
Absorption Spectroscopy, Emissive Properties, and Ultrafast Intersystem. . .
379