17 research outputs found
Probing the Fate of Lowest-Energy Near-Infrared Metal-Centered Electronic Excited States: CuCl<sub>4</sub><sup>2ā</sup> and IrBr<sub>6</sub><sup>2ā</sup>
Ultrafast
transient absorption spectroscopy is used to investigate
the radiationless relaxation dynamics of CuCl<sub>4</sub><sup>2ā</sup> and IrBr<sub>6</sub><sup>2ā</sup> complexes directly promoted
into their lowest-energy excited metal-centered states upon near-infrared
femtosecond excitation at 2000 nm. Both the excited CuCl<sub>4</sub><sup>2ā</sup> <sup>2</sup><i>E</i> and IrBr<sub>6</sub><sup>2ā</sup> <sup>2</sup><i>U</i><sub><i>g</i></sub>ā²(<i>T</i><sub>2<i>g</i></sub>) states undergo internal conversion to the ground electronic
states, yet with significantly different lifetimes (55 fs and 360
ps, respectively) despite the fact that the <sup>2</sup><i>E</i> and <sup>2</sup><i>U</i><sub><i>g</i></sub>ā²(<i>T</i><sub>2<i>g</i></sub>) states are separated by
the same energy gap (ā¼5000 cm<sup>ā1</sup>) from the
respective ground state. This difference likely arises from the predominance
of the JahnāTeller effect in a Cu<sup>2+</sup> ion and the
spināorbit coupling effect in an Ir<sup>4+</sup> ion. The approach
documented in this work may be used for elucidating the role of low-energy
metal-centered states in relaxation cascades of a number of coordination
compounds, allowing for design of efficient light-triggered metal
complexes
5āAzido-2-aminopyridine, a New Nitrene/Nitrenium Ion Photoaffinity Labeling Agent That Exhibits Reversible Intersystem Crossing between Singlet and Triplet Nitrenes
The
photochemistry of a new photoaffinity labeling (PAL) agent,
5-azido-2-(<i>N</i>,<i>N</i>-diethylamino)Āpyridine,
was studied in aprotic and protic solvents using femtosecond-to-microsecond
transient absorption and product analysis, in conjunction with <i>ab initio</i> multiconfigurational and multireference quantum
chemical calculations. The excited singlet S<sub>1</sub> state is
spectroscopically dark, whereas photoexcitation to higher-lying singlet
excited S<sub>2</sub> and S<sub>3</sub> states drives the photochemical
reaction toward a barrierless ultrafast relaxation path via two conical
intersections to S<sub>1</sub>, where N<sub>2</sub> elimination leads
to the formation of the closed-shell singlet nitrene. The singlet
nitrene undergoes intersystem crossing (ISC) to the triplet nitrene
in aprotic and protic solvents as well as protonation to form the
nitrenium ion. The ISC rate constants in aprotic solvents increase
with solvent polarity, displaying a ādirectā gap effect,
whereas an āinverseā gap effect is observed in protic
solvents. Transient absorption actinometry experiments suggest that
a solvent-dependent fraction from 20% to 50% of nitrenium ions is
generated on a time scale of a few tens of picoseconds. The closed-shell
singlet and triplet nitrene are separated by a small energy gap in
protic solvents. As a result, the unreactive triplet state nitrene
undergoes delayed, thermally activated reverse ISC to reform the reactive
closed-shell singlet nitrene, which subsequently protonates, forming
the remaining fraction of nitrenium ions. The product studies demonstrate
that the resulting nitrenium ion stabilized by the electron-donating
4-amino group yields the final cross-linked product with high, almost
quantitative efficiency. The enhanced PAL function of this new azide
with respect to the widely applied 4-amino-3-nitrophenyl azide is
discussed
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Photochemistry of Monochloro Complexes of Copper(II) in Methanol Probed by Ultrafast Transient Absorption Spectroscopy
Ultrafast transient absorption spectra in the deep to near UV range (212ā384 nm) were measured for the [Cu<sup>II</sup>(MeOH)<sub>5</sub>Cl]<sup>+</sup> complexes in methanol following 255-nm excitation of the complex into the ligand-to-metal charge-transfer excited state. The electronically excited complex undergoes sub-200 fs radiationless decay, predominantly via back electron transfer, to the hot electronic ground state followed by fast vibrational relaxation on a 0.4ā4 ps time scale. A minor photochemical channel is CuāCl bond dissociation, leading to the reduction of copper(II) to copper(I) and the formation of MeOHĀ·Cl charge-transfer complexes. The depletion of ground-state [Cu<sup>II</sup>(MeOH)<sub>5</sub>Cl]<sup>+</sup> perturbs the equilibrium between several forms of copper(II) complexes present in solution. Complete re-equilibration between [Cu<sup>II</sup>(MeOH)<sub>5</sub>Cl]<sup>+</sup> and [Cu<sup>II</sup>(MeOH)<sub>4</sub>Cl<sub>2</sub>] is established on a 10ā500 ps time scale, slower than methanol diffusion, suggesting that the involved ligand exchange mechanism is dissociative
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources