Laser-induced fluorescence spectroscopy of the Ga-N[subscript 2] cluster

The first spectroscopic observation of the Ga–N[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 2Δ–[X with combining tilde]2Π[subscript 3/2] transition correlating with the Ga 4s[superscript 2]4D[superscript 1] [superscript 2]D ← 4s[superscript 2]4p[superscript 1] [superscript 2]P[subscript 3/2] transition. A Birge–Sponer 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 ← 4s[superscript 2]4p[superscript 1] [superscript 2]P atomic asymptote, the cluster excited state most likely having [superscript 4]Σ[superscript -] symmetry. The excited state undergoes relatively slow spin–orbit-induced predissociation onto the repulsive [B with combining tilde][superscript 2]Σ[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 α‑Tocopherol Analogue Trolox C

The initial events after photoexcitation and photoionization of α-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. α-Tocopherol and Trolox C in methanol solution appear to undergo efficient homolytic dissociation of the phenolic −OH 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 α-tocopherol with peroxyl radicals, which represents the best understood biological activity of this vitamin

Ultrafast Wiggling and Jiggling: Ir₂(1,8-diisocyanomenthane)₄²⁺

Binuclear complexes of d⁸ metals (Pt^II, Ir^I, Rh^I,) 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 M–M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir₂(1,8-diisocyanomenthane)₄²⁺ (Ir­(dimen)), investigated by fs–ns time-resolved IR spectroscopy (TRIR) in the region of CN stretching vibrations, ν­(CN), 2000–2300 cm^–1. The ν­(CN) IR band of the singlet and triplet dσ*pσ excited states is shifted by −22 and −16 cm^–1 relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ¹dσ*pσ state depend on the initially excited Franck–Condon 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 Ir–Ir contraction and 5 ps Ir–Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ³dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν­(CN) IR bandwidth oscillates with the frequency of the ν­(Ir–Ir) wave packet, ca. 80 cm^–1, indicating that the dephasing time of the high-frequency (16 fs)⁻¹ CN stretch responds to much slower (∼400 fs)⁻¹ Ir–Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal–metal unit and that the coherent Ir–Ir 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: “free” (uncomplexed) CN radicals, and “solvated” 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 CDCl₃ molecules, and reaction with acetone. The acetone reaction occurs with a rate coefficient of (8.0 ± 0.5) × 10¹⁰ M^–1 s^–1 and transient vibrational spectra in the CN and CO stretching regions reveal that <i>both</i> the nascent HCN and 2-oxopropyl (CH₃C­(O)­CH₂) 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 ± 0.9) × 10⁹ M^–1 s^–1 obtained from the rise in the HCN product <i>v</i>₁(CN stretch) IR absorption band. Evidence is also presented for CN complexes with acetone that are more strongly bound than the CN–CDCl₃ 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 UV–NIR–IR 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 rhenium­(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)° between the terminal C1 and C12 atoms and the centroid of the central C–C bond, with the most acute distortion at the point where the polyyne chain pushes against the Re­(CO)₃Cl 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 (τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet EET, from the macrocycle complex ³MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on the short-lived higher excited state of the hexayne (lifetime ∼1 ps) and on structural reorganization and cooling of hot polyyne chains, following internal conversion (over ∼5 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 Paternò–Büchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies

A detailed mechanistic investigation of the early stages of the Paternò–Büchi reaction following 267 nm excitation of benzaldehyde in cyclohexene has been completed using ultrafast, broadband transient UV–visible 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 C–O 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, ϕ_oxetane = 0.53, ϕ_diss = 0.47, <i>k</i>_oxetane = 0.27 ± 0.09 ns^–1 and <i>k</i>_diss = 0.24 ± 0.09 ns^–1. 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 molecule–protein interactions, but its relevance to small molecule–DNA 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 ligand–DNA 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: πσ*-Mediated Bond Cleavage in <i>p</i>‑Methylthiophenol and <i>p</i>‑Methylthioanisole

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 CD₃CN have been probed by subpicosecond time-resolved broadband infrared spectroscopy. Prompt (τ < 1 ps) S–H bond fission in <i>p</i>-MePhSH is confirmed by monitoring the time-evolution of the parent (S₀) bleach and the transient absorption of the <i>p</i>-MePhS products. Vibrational relaxation of the latter occurs on a ∼8.5 ps time scale, and ∼40% of the total radical population undergoes geminate recombination over a ∼150 ps time scale, yielding (mainly) the <i>p</i>-MePhSH­(S₀) parent. S–Me bond fission following photoexcitation to the S₁ 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 S₁. 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