14 research outputs found
Laser-induced fluorescence spectroscopy of the Ga-N[subscript 2] cluster
The first spectroscopic observation of the GaāN[subscript 2] van der Waals cluster is reported. This was formed by laser ablation of a GaAs or GaP target followed by pulsed supersonic expansion in pure nitrogen. Laser-induced fluorescence spectra have revealed two strong band systems above 30000 cm[superscript -1]. The first, which has an onset at 33468 cm[superscript -1], is composed of eight observable members of a progression in the van der Waals stretching vibration. This has been assigned to the 2Īā[X with combining tilde]2Ī [subscript 3/2] transition correlating with the Ga 4s[superscript 2]4D[superscript 1] [superscript 2]D ā 4s[superscript 2]4p[superscript 1] [superscript 2]P[subscript 3/2] transition. A BirgeāSponer extrapolation gives a lower limit of 1270 cm[superscript -1] for the van der Waals binding energy in the excited state. At higher wavenumbers another prominent vibrational progression is observed which is attributed to a spin-forbidden transition correlating with the 4s[superscript 1]4p[superscript 2] [superscript 4]P ā 4s[superscript 2]4p[superscript 1] [superscript 2]P atomic asymptote, the cluster excited state most likely having [superscript 4]Ī£[superscript -] symmetry. The excited state undergoes relatively slow spināorbit-induced predissociation onto the repulsive [B with combining tilde][superscript 2]Ī£[superscript +] potential surface followed by rapid emission from the Ga 4s[superscript 2]5s[superscript 1] [superscript 2]S dissociation product
Ultrafast Vibrational Spectroscopic Studies on the Photoionization of the Ī±āTocopherol Analogue Trolox C
The
initial events after photoexcitation and photoionization of
Ī±-tocopherol (vitamin E) and the analogue Trolox C have been
studied by femtosecond stimulated Raman spectroscopy, transient absorption
spectroscopy and time-resolved infrared spectroscopy. Using these
techniques it was possible to follow the formation and decay of the
excited state, neutral and radical cation radicals and the hydrated
electron that are produced under the various conditions examined.
Ī±-Tocopherol and Trolox C in methanol solution appear to undergo
efficient homolytic dissociation of the phenolic āOH bond to
directly produce the tocopheroxyl radical. In contrast, Trolox C photochemistry
in neutral aqueous solutions involves intermediate formation of a
radical cation and the hydrated electron which undergo geminate recombination
within 100 ps in competition with deprotonation of the radical cation.
The results are discussed in relation to recently proposed mechanisms
for the reaction of Ī±-tocopherol with peroxyl radicals, which
represents the best understood biological activity of this vitamin
Ultrafast Wiggling and Jiggling: Ir<sub>2</sub>(1,8-diisocyanomenthane)<sub>4</sub><sup>2+</sup>
Binuclear complexes of d<sup>8</sup> metals (Pt<sup>II</sup>, Ir<sup>I</sup>, Rh<sup>I</sup>,) exhibit
diverse photonic behavior, including
dual emission from relatively long-lived singlet and triplet excited
states, as well as photochemical energy, electron, and atom transfer.
Time-resolved optical spectroscopic and X-ray studies have revealed
the behavior of the dimetallic core, confirming that MāM bonding
is strengthened upon dĻ* ā pĻ excitation. We report
the bridging ligand dynamics of Ir<sub>2</sub>(1,8-diisocyanomenthane)<sub>4</sub><sup>2+</sup> (IrĀ(dimen)), investigated by fsāns time-resolved
IR spectroscopy (TRIR) in the region of Cī¼N stretching vibrations,
Ī½Ā(Cī¼N), 2000ā2300 cm<sup>ā1</sup>. The
Ī½Ā(Cī¼N) IR band of the singlet and triplet dĻ*pĻ
excited states is shifted by ā22 and ā16 cm<sup>ā1</sup> relative to the ground state due to delocalization of the pĻ
LUMO over the bridging ligands. Ultrafast relaxation dynamics of the <sup>1</sup>dĻ*pĻ state depend on the initially excited FranckāCondon
molecular geometry, whereby the same relaxed singlet excited state
is populated by two different pathways depending on the starting point
at the excited-state potential energy surface. Exciting the long/eclipsed
isomer triggers two-stage structural relaxation: 0.5 ps large-scale
IrāIr contraction and 5 ps IrāIr contraction/intramolecular
rotation. Exciting the short/twisted isomer induces a ā¼5 ps
bond shortening combined with vibrational cooling. Intersystem crossing
(70 ps) follows, populating a <sup>3</sup>dĻ*pĻ state
that lives for hundreds of nanoseconds. During the first 2 ps, the
Ī½Ā(Cī¼N) IR bandwidth oscillates with the frequency of
the Ī½Ā(IrāIr) wave packet, ca. 80 cm<sup>ā1</sup>, indicating that the dephasing time of the high-frequency (16 fs)<sup>ā1</sup> Cī¼N stretch responds to much slower (ā¼400
fs)<sup>ā1</sup> IrāIr coherent oscillations. We conclude
that the bonding and dynamics of bridging di-isocyanide ligands are
coupled to the dynamics of the metalāmetal unit and that the
coherent IrāIr motion induced by ultrafast excitation drives
vibrational dephasing processes over the entire binuclear cation
Vibrational Excitation of Both Products of the Reaction of CN Radicals with Acetone in Solution
Transient electronic and vibrational
absorption spectroscopy unravel
the mechanisms and dynamics of bimolecular reactions of CN radicals
with acetone in deuterated chloroform solutions. The CN radicals are
produced by ultrafast ultraviolet photolysis of dissolved ICN. Two
reactive forms of CN radicals are distinguished by their electronic
absorption bands: āfreeā (uncomplexed) CN radicals,
and āsolvatedā CN radicals that are complexed with solvent
molecules. The lifetimes of the free CN radicals are limited to a
few picoseconds following their photolytic production because of geminate
recombination to ICN and INC, complexation with CDCl<sub>3</sub> molecules,
and reaction with acetone. The acetone reaction occurs with a rate
coefficient of (8.0 Ā± 0.5) Ć 10<sup>10</sup> M<sup>ā1</sup> s<sup>ā1</sup> and transient vibrational spectra in the Cī»N
and Cī»O stretching regions reveal that <i>both</i> the nascent HCN and 2-oxopropyl (CH<sub>3</sub>CĀ(O)ĀCH<sub>2</sub>) radical products are vibrationally excited. The rate coefficient
for the reaction of solvated CN with acetone is 40 times slower than
for free CN, with a rate coefficient of (2.0 Ā± 0.9) Ć 10<sup>9</sup> M<sup>ā1</sup> s<sup>ā1</sup> obtained from
the rise in the HCN product <i>v</i><sub>1</sub>(Cī»N
stretch) IR absorption band. Evidence is also presented for CN complexes
with acetone that are more strongly bound than the CNāCDCl<sub>3</sub> complexes because of CN interactions with the carbonyl group.
The rates of reactions of these more strongly associated radicals
are slower still
Photophysics of Threaded sp-Carbon Chains: The Polyyne is a Sink for Singlet and Triplet Excitation
We
have used single-crystal X-ray diffraction and time-resolved
UVāNIRāIR absorption spectroscopy to gain insights into
the structures and excited-state dynamics of a rotaxane consisting
of a hexayne chain threaded through a phenanthroline macrocycle and
a family of related compounds, including the rheniumĀ(I) chlorocarbonyl
complex of this rotaxane. The hexayne unit in the rhenium-rotaxane
is severely nonlinear; it is bent into an arc with an angle of 155.6(1)Ā°
between the terminal C1 and C12 atoms and the centroid of the central
CāC bond, with the most acute distortion at the point where
the polyyne chain pushes against the ReĀ(CO)<sub>3</sub>Cl unit. There
are strong through-space excited-state interactions between the components
of the rotaxanes. In the metal-free rotaxane, there is rapid singlet
excitation energy transfer (EET) from the macrocycle to the hexayne
(Ļ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet
EET, from the macrocycle complex <sup>3</sup>MLCT state to the hexayne
(Ļ = 1.5 ns). This study revealed detailed information on the
short-lived higher excited state of the hexayne (lifetime ā¼1
ps) and on structural reorganization and cooling of hot polyyne chains,
following internal conversion (over ā¼5 ps). Comparison of the
observed IR bands of the excited states of the hexayne with results
from time-dependent density functional calculations (TD DFT) shows
that these excited states have high cumulenic character (low bond
length alternation) around the central region of the chain. These
findings shed light on the complex interactions between the components
of this supramolecular rotaxane and are important for the development
of materials for the emerging molecular and nanoscale electronics
Tracking a PaternoĢāBuĢchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies
A detailed mechanistic investigation
of the early stages of the
PaternoĢāBuĢchi reaction following 267 nm excitation
of benzaldehyde in cyclohexene has been completed using ultrafast,
broadband transient UVāvisible and IR absorption spectroscopies.
Absorption due to electronically excited triplet state benzaldehyde
decays on a 80 ps time scale via reaction with cyclohexene. The growth
and subsequent decay of the biradical intermediate produced following
CāO bond formation is followed by transient vibrational spectroscopy.
The biradical decays by ring closure to an oxetane or by dissociating,
reforming the ground state reactants. Detailed kinetic analysis allowed
derivation of quantum yields and rate constants for these competing
biradical decay processes, Ļ<sub>oxetane</sub> = 0.53, Ļ<sub>diss</sub> = 0.47, <i>k</i><sub>oxetane</sub> = 0.27 Ā±
0.09 ns<sup>ā1</sup> and <i>k</i><sub>diss</sub> =
0.24 Ā± 0.09 ns<sup>ā1</sup>. This study provides a striking
illustration of the ways in which contemporary ultrafast transient
absorption spectroscopy methods can be used to dissect the mechanism
and kinetics of a classic photoreaction
Excited State Structure and Dynamics of the Neutral and Anionic Flavin Radical Revealed by Ultrafast Transient Mid-IR to Visible Spectroscopy
Neutral and anionic flavin radicals are involved in numerous
photochemical
processes and play an essential part in forming the signaling state
of various photoactive flavoproteins such as cryptochromes and BLUF
domain proteins. A stable neutral radical flavin has been prepared
for study in aqueous solution, and both neutral and anion radical
states have been stabilized in the proteins flavodoxin and glucose
oxidase. Ultrafast transient absorption measurements were performed
in the visible and mid-infrared region in order to characterize the
excited state dynamics and the excited and ground state vibrational
spectra and to probe the effect of the protein matrix on them. These
data are compared with the results of density functional theory calculations.
Excited state decay dynamics were found to be a strong function of
the protein matrix. The ultrafast electron transfer quenching mechanism
of the excited flavin moiety in glucose oxidase is characterized by
vibrational spectroscopy. Such data will be critical in the ongoing
analysis of the photocycle of photoactive flavoproteins
2D-IR Spectroscopy Shows that Optimized DNA Minor Groove Binding of Hoechst33258 Follows an Induced Fit Model
The
induced fit binding model describes a conformational change
occurring when a small molecule binds to its biomacromolecular target.
The result is enhanced noncovalent interactions between the ligand
and biomolecule. Induced fit is well-established for small moleculeāprotein
interactions, but its relevance to small moleculeāDNA binding
is less clear. We investigate the molecular determinants of Hoechst33258
binding to its preferred A-tract sequence relative to a suboptimal
alternating A-T sequence. Results from two-dimensional infrared spectroscopy,
which is sensitive to H-bonding and molecular structure changes, show
that Hoechst33258 binding results in loss of the minor groove spine
of hydration in both sequences, but an additional perturbation of
the base propeller twists occurs in the A-tract binding region. This
induced fit maximizes favorable ligandāDNA enthalpic contributions
in the optimal binding case and demonstrates that controlling the
molecular details that induce subtle changes in DNA structure may
hold the key to designing next-generation DNA-binding molecules
Photofragmentation Dynamics in Solution Probed by Transient IR Absorption Spectroscopy: ĻĻ*-Mediated Bond Cleavage in <i>p</i>āMethylthiophenol and <i>p</i>āMethylthioanisole
The 267 nm photodissociation dynamics of <i>p</i>-methylthiophenol
(<i>p-</i>MePhSH) and <i>p</i>-methylthioanisole
(<i>p</i>-MePhSMe) dissolved in CD<sub>3</sub>CN have been
probed by subpicosecond time-resolved broadband infrared spectroscopy.
Prompt (Ļ < 1 ps) SāH bond fission in <i>p</i>-MePhSH is confirmed by monitoring the time-evolution of the parent
(S<sub>0</sub>) bleach and the transient absorption of the <i>p</i>-MePhS products. Vibrational relaxation of the latter occurs
on a ā¼8.5 ps time scale, and ā¼40% of the total radical
population undergoes geminate recombination over a ā¼150 ps
time scale, yielding (mainly) the <i>p</i>-MePhSHĀ(S<sub>0</sub>) parent. SāMe bond fission following photoexcitation
to the S<sub>1</sub> state of <i>p</i>-MePhSMe occurs over
a much longer timescale, with a rate that is very dependent on the
degree of vibrational excitation within S<sub>1</sub>. The various
findings are compared and contrasted with results from complementary
gas-phase photofragmentation studies of both molecules, which are
shown to provide a valuable starting point for describing the solution-phase
dynamics
Efficient Quenching of TGA-Capped CdTe Quantum Dot Emission by a Surface-Coordinated Europium(III) Cyclen Complex
Extremely efficient quenching of
the excited state of aqueous CdTe quantum dots (QDs) by photoinduced
electron transfer to a europium cyclen complex is facilitated by surface
coordination to the thioglycolic acid capping ligand. The quenching
dynamics are elucidated using steady-state emission and picosecond
transient absorption