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

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

Ultrafast Infrared Spectral Fingerprints of Vitamin B₁₂ and Related Cobalamins

Vitamin B₁₂ (cyanocobalamin, CNCbl) and its derivatives are structurally complex and functionally diverse biomolecules. The excited state and radical pair reaction dynamics that follow their photoexcitation have been previously studied in detail using UV–visible techniques. Similar time-resolved infrared (TRIR) data are limited, however. Herein we present TRIR difference spectra in the 1300–1700 cm^–1 region between 2 ps and 2 ns for adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), CNCbl, and hydroxocobalamin (OHCbl). The spectral profiles of all four cobalamins are complex, with broad similarities that suggest the vibrational excited states are related, but with a number of identifiable variations. The majority of the signals from AdoCbl and MeCbl decay with kinetics similar to those reported in the literature from UV–visible studies. However, there are regions of rapid (<10 ps) vibrational relaxation (peak shifts to higher frequencies from 1551, 1442, and 1337 cm^–1) that are more pronounced in AdoCbl than in MeCbl. The AdoCbl data also exhibit more substantial changes in the amide I region and a number of more gradual peak shifts elsewhere (e.g., from 1549 to 1563 cm^–1), which are not apparent in the MeCbl data. We attribute these differences to interactions between the bulky adenosyl and the corrin ring after photoexcitation and during radical pair recombination, respectively. Although spectrally similar to the initial excited state, the long-lived metal-to-ligand charge transfer state of MeCbl is clearly resolved in the kinetic analysis. The excited states of CNCbl and OHCbl relax to the ground state within 40 ps with few significant peak shifts, suggesting little or no homolysis of the bond between the Co and the upper axial ligand. Difference spectra from density functional theory calculations (where spectra from simplified cobalamins with an upper axial methyl were subtracted from those without) show qualitative agreement with the experimental data. They imply the excited state intermediates in the TRIR difference spectra resemble the dissociated states vibrationally (the cobalamin with the upper axial ligand missing) relative to the ground state with a methyl in this position. They also indicate that most of the TRIR signals arise from vibrations involving some degree of motion in the corrin ring. Such coupling of motions throughout the ring makes specific peak assignments neither trivial nor always meaningful, suggesting our data should be regarded as IR spectral fingerprints

Ultrafast Infrared Spectral Fingerprints of Vitamin B₁₂ and Related Cobalamins

Vitamin B₁₂ (cyanocobalamin, CNCbl) and its derivatives are structurally complex and functionally diverse biomolecules. The excited state and radical pair reaction dynamics that follow their photoexcitation have been previously studied in detail using UV–visible techniques. Similar time-resolved infrared (TRIR) data are limited, however. Herein we present TRIR difference spectra in the 1300–1700 cm^–1 region between 2 ps and 2 ns for adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), CNCbl, and hydroxocobalamin (OHCbl). The spectral profiles of all four cobalamins are complex, with broad similarities that suggest the vibrational excited states are related, but with a number of identifiable variations. The majority of the signals from AdoCbl and MeCbl decay with kinetics similar to those reported in the literature from UV–visible studies. However, there are regions of rapid (<10 ps) vibrational relaxation (peak shifts to higher frequencies from 1551, 1442, and 1337 cm^–1) that are more pronounced in AdoCbl than in MeCbl. The AdoCbl data also exhibit more substantial changes in the amide I region and a number of more gradual peak shifts elsewhere (e.g., from 1549 to 1563 cm^–1), which are not apparent in the MeCbl data. We attribute these differences to interactions between the bulky adenosyl and the corrin ring after photoexcitation and during radical pair recombination, respectively. Although spectrally similar to the initial excited state, the long-lived metal-to-ligand charge transfer state of MeCbl is clearly resolved in the kinetic analysis. The excited states of CNCbl and OHCbl relax to the ground state within 40 ps with few significant peak shifts, suggesting little or no homolysis of the bond between the Co and the upper axial ligand. Difference spectra from density functional theory calculations (where spectra from simplified cobalamins with an upper axial methyl were subtracted from those without) show qualitative agreement with the experimental data. They imply the excited state intermediates in the TRIR difference spectra resemble the dissociated states vibrationally (the cobalamin with the upper axial ligand missing) relative to the ground state with a methyl in this position. They also indicate that most of the TRIR signals arise from vibrations involving some degree of motion in the corrin ring. Such coupling of motions throughout the ring makes specific peak assignments neither trivial nor always meaningful, suggesting our data should be regarded as IR spectral fingerprints

Photophysics of Singlet and Triplet Intraligand Excited States in [ReCl(CO)₃(1-(2-pyridyl)-imidazo[1,5-α]pyridine)] Complexes

Excited-state characters and dynamics of [ReCl­(CO)₃(3-R-1-(2-pyridyl)-imidazo­[1,5-α]­pyridine)] complexes (abbreviated <b>ReGV-R</b>, R = CH₃, Ph, PhBu^<i>t</i>, PhCF₃, PhNO₂, PhNMe₂) were investigated by pico- and nanosecond time-resolved infrared spectroscopy (TRIR) and excited-state DFT and TD-DFT calculations. Near UV excitation populates the lowest singlet state S₁ that undergoes picosecond intersystem crossing (ISC) to the lowest triplet T₁. Both states are initially formed hot and relax with ∼20 ps lifetime. TRIR together with quantum chemical calculations reveal that S₁ is predominantly a ππ* state localized at the 1-(2-pyridyl)-imidazo­[1,5-α]­pyridine (= impy) ligand core, with impy → PhNO₂ and PhNMe₂ → impy intraligand charge-transfer contributions in the case of <b>ReGV-PhNO</b>_<b>2</b> and <b>ReGV-PhNMe</b>_<b>2</b>, respectively. T₁ is predominantly ππ*­(impy) in all cases. It follows that excited singlet and corresponding triplet states have to some extent different characters and structures even if originating nominally from the same preponderant one-electron excitations. ISC occurs with a solvent-independent (CH₂Cl₂, MeCN) 20–30 ps lifetime, except for <b>ReGV-PhNMe</b>_<b>2</b> (10 ps in CH₂Cl₂, 100 ps in MeCN). ISC is 200–300 times slower than in analogous complexes with low-lying MLCT states. This difference is interpreted in terms of spin–orbit interaction and characters of orbitals involved in one-electron excitations that give rise to S₁ and T₁ states. <b>ReGV-R</b> present a unique case of octahedral heavy-metal complexes where the S₁ lifetime is long enough to allow for separate spectroscopic characterization of singlet and triplet excited states. This study provides an insight into dynamics and intersystem crossing pathways of low-lying singlet and triplet excited states localized at bidentate ligands bound directly to a heavy metal atom. Rather long ¹IL lifetimes indicate the possibility of photonic applications of singlet excited states

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

Dinitrogen Release from Arylpentazole: A Picosecond Time-Resolved Infrared, Spectroelectrochemical, and DFT Computational Study

<i>p</i>-(Dimethylamino)­phenyl pentazole, DMAP-N₅ (DMAP = Me₂N–C₆H₄), was characterized by picosecond transient infrared spectroscopy and infrared spectroelectrochemistry. Femtosecond laser excitation at 310 or 330 nm produces the DMAP-N₅ (S₁) excited state, part of which returns to the ground state (τ = 82 ± 4 ps), while DMAP-N and DMAP-N₃ (S₀) are generated as double and single N₂-loss photoproducts with η ≈ 0.14. The lifetime of DMAP-N₅ (S₁) is temperature and solvent dependent. [DMAP-N₃]⁺ is produced from DMAP-N₅ in a quasireversible, one-electron oxidation process (<i>E</i>_1/2 = +0.67 V). Control experiments with DMAP-N₃ support the findings. DFT B3LYP/6-311G** calculations were used to identify DMAP-N₅ (S₁), DMAP-N₃⁺, and DMAP-N in the infrared spectra. Both DMAP-N₅ (S₁) and [DMAP-N₅]⁺ have a weakened N₅ ring structure

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

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^<i>x</i>)Pt(pap^<i>y</i>)] (Q = Substituted <i>o</i>‑Quinone or <i>o</i>‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (<b>1</b>; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-<i>tert</i>-butyl-<i>N</i>-phenyl-<i>o</i>-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([<b>1</b>]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [<b>1</b>]⁺. Simulation of the cyclic voltammograms of <b>1</b> recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([<b>2</b>]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex <b>1b</b> (one of the positional isomers of <b>1</b>) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (<b>3</b>; ^O,OQ = 3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex <b>1b</b> is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of <b>3</b>. Density functional theory calculations on [<b>1</b>]^<i>n</i>+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of <b>1</b> with electron donors and propose a mechanism for this process

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^<i>x</i>)Pt(pap^<i>y</i>)] (Q = Substituted <i>o</i>‑Quinone or <i>o</i>‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (<b>1</b>; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-<i>tert</i>-butyl-<i>N</i>-phenyl-<i>o</i>-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([<b>1</b>]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [<b>1</b>]⁺. Simulation of the cyclic voltammograms of <b>1</b> recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([<b>2</b>]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex <b>1b</b> (one of the positional isomers of <b>1</b>) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (<b>3</b>; ^O,OQ = 3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex <b>1b</b> is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of <b>3</b>. Density functional theory calculations on [<b>1</b>]^<i>n</i>+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of <b>1</b> with electron donors and propose a mechanism for this process