Investigating the Photodissociation Dynamics of CF2BrCF2I in CCl4 through Femtosecond Time-Resolved Infrared Spectroscopy.

The photodissociation dynamics of CF2BrCF2I in CCl4 at 280 ± 2 K were investigated by probing the C-F stretching mode from 300 fs to 10 µs after excitation at 267 nm using time-resolved infrared spectroscopy. The excitation led to the dissociation of I or Br atoms within 300 fs, producing the CF2BrCF2 or CF2ICF2 radicals, respectively. All nascent CF2ICF2 underwent further dissociation of I, producing CF2CF2 with a time constant of 56 ± 5 ns. All nascent g-CF2BrCF2 isomerized into the more stable a-CF2BrCF2 with a time constant of 47 ± 5 ps. Furthermore, a-CF2BrCF2 underwent a bimolecular reaction with either itself (producing CF2BrCF2Br and CF2CF2) or Br in the CCl4 solution (producing CF2BrCF2Br) at a diffusion-limited rate. The secondary dissociation of Br from a-CF2BrCF2 was significantly slow to compete with the bimolecular reactions. Overall, approximately half of the excited CF2BrCF2I at 267 nm produced CF2BrCF2Br, whereas the other half produced CF2CF2. The excess energies in the nascent radicals were thermalized much faster than the secondary dissociation of I from CF2ICF2 and the observed bimolecular reactions, implying that the secondary reactions proceeded under thermal conditions. This study further demonstrates that structure-sensitive time-resolved infrared spectroscopy can be used to study various reaction dynamics in solution in real time.

Modulations of a Metal-Ligand Interaction and Photophysical Behaviors by Hückel-Möbius Aromatic Switching.

In organometallic complexes containing π-conjugated macrocyclic chelate ligands, conformational change significantly affects metal-ligand electronic interactions, hence tuning properties of the complexes. In this regard, we investigated the metal-ligand interactions in hexaphyrin mono-Pd(II) complexes Pd[28]M and Pd[26]H, which exhibit a redox-induced switching of Hückel-Möbius aromaticity and subsequent molecular conformation, and their effect on the electronic structure and photophysical behaviors. In Möbius aromatic Pd[28]M, the weak metal-ligand interaction leads to the π electronic structure of the hexaphyrin ligand remaining almost intact, which undergoes efficient intersystem crossing (ISC) assisted by the heavy-atom effect of the Pd metal. In Hückel aromatic Pd[26]H, the significant metal-ligand interaction results in ligand-to-metal charge-transfer (LMCT) in the excited-state dynamics. These contrasting metal-ligand electronic interactions have been revealed by time-resolved electronic and vibrational spectroscopies and time-dependent DFT calculations. This work indicates that the conspicuous modulation of metal-ligand interaction by Hückel-Möbius aromaticity switching is an appealing approach to manipulate molecular properties of metal complexes, further enabling the fine-tuning of metal-ligand interactions and the novel design of functional organometallic materials.

Photoacid-induced aqueous acid-base reactions probed by femtosecond infrared spectroscopy.

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N3-) in D2O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*-), and the conjugate base of DA (A-)) were probed in the spectral region of 1400-1670 cm-1 in the time range of 1 ps-1 µs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400 nm; thus, reactions related to both DA* and A- could be probed. Furthermore, changes in the populations of N3- and DN3 were monitored using the absorption bands at 2042 and 2133 cm-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04 ns. The remainder underwent an acid-base reaction to produce A*-, which relaxed to A- with a time constant of 3.9 ± 0.3 ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D2O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1 M) were diffusion controlled, they could be described using a simple rate equation.

Photoexcitation dynamics of bromodiphenyl ethers in acetonitrile-d3 studied by femtosecond time-resolved infrared spectroscopy.

The efficient decomposition of polybrominated diphenyl ethers (PBDEs), onetime prevalent flame retardants, is central to the reduction of their harmful effects on human health. PBDE photodecomposition is a promising method, but its mechanism and products are not well understood. The photoexcitation dynamics of 3- and 4-bromodiphenyl ethers (BDE-2 and BDE-3) in CD3CN were studied from 0.3 ps to 10 µs using time-resolved infrared spectroscopy. An excitation at 267 nm dissociated the Br atom from BDE-2 and BDE-3 within 0.3 ps and 14 ± 3 ps, respectively, producing a radical compound (R) and a Br atom. About 85% of R formed an intermediate (IM) that weakly interacted with the Br atom and the surrounding CD3CN solvent in 7-12 ps. The remaining R separated from the dissociated Br and underwent slow geminate rebinding (GR) with Br within 35 to 54 ns. The IM competitively engaged in GR with the interacting Br in 40-60 ps or formed CD3CN-bound radical compounds (RS) in 100-130 ps. The RS further degraded via either the dissociation of CD3-producing a cyano-bound diphenyl ether (DE) in 150 or 550 ns-or the deuterium abstraction of CD3CN in 180 or 430 ns-producing a deuterated DE. Overall, 33 ± 3 (22 ± 3)% of the photoexcited BDE-2 (BDE-3) decomposed in CD3CN under 267 nm excitation. Efficient binding of the CD3CN solvent to R deterred the yield-diminishing GR and slowed the rate of product formation. The observed photoexcitation dynamics of BDE suggest methods for the efficient decomposition of PBDE.

meso-Oxoisocorroles: Tunable Antiaromaticity by Metalation and Coordination of Lewis Acids as Well as Aromaticity Reversal in the Triplet Excited State.

The corrole derivative meso-oxoisocorrole has been theoretically predicted to be antiaromatic, despite its formally cross conjugated electronic system. In this study, this prediction has been experimentally proven by the facile preparation of meso-oxoisocorrole via the oxidation of a meso free corrole with MnO2 and its comprehensive characterization using NMR, UV/vis absorption, FT-IR, and transient-absorption spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Furthermore, the free base meso-oxoisocorrole was metalated by treatment with Ni(acac)2, PdCl2(PhCN)2, and Zn(OAc)2 to give the corresponding metal complexes. These complexes are more strongly antiaromatic, and their degree of paratropicity depends on their planarity. Thus, fine tuning of their antiaromaticity was achieved with concomitant modulation of their HOMO-LUMO gaps. In the presence of tris(pentafluorophenyl)borane, their antiaromaticity is significantly enhanced due to the elongation of the C═O bond, which promotes the polarized C+-O- resonance state. Furthermore, a distinct frequency shift of the C═O vibrational mode in the triplet state was observed in the time-resolved IR spectra in accordance with the Baird rule, which indicates aromaticity reversal in the excited state.

Dynamics of photodissociation of nitric oxide from S-nitrosylated cysteine and N-acetylated cysteine derivatives in water.

Cysteine and N-acetylated cysteine derivatives are ubiquitous in biological systems; they have thiol groups that bind NO to form S-nitrosothiols (RSNOs) such as S-nitrosocysteine (CySNO), S-nitroso-N-acetylcysteine (NacSNO), and S-nitroso-N-acetylpenicillamine (NapSNO). Although they have been utilised as thermally or catalytically decomposing NO donors, their photochemical applications are yet to be fully explored owing to the lack of photodissociation dynamics. To this end, the photoexcitation dynamics of these RSNOs in water at 330 nm were investigated using femtosecond time-resolved infrared (TRIR) spectroscopy over a broad time range encompassing the entire reaction, which includes the primary reaction, secondary reactions of the reaction intermediates, and product formation. We discovered that the acetate and amide groups in these RSNOs have strong vibrational bands sensitive to the bondage of NO and the electronic state of the compound, which facilitates the identification of reaction intermediates involved in photoexcitation. The simplest thiol available with the acetate group-thioglycolic acid-was nitrosylated; it produced S-nitrosothioglycolic acid (TgSNO) and was comparatively investigated. Transient absorption bands in the TRIR spectra of the RSNOs were assigned using quantum chemical calculations. Photoexcited cysteine-related RSNOs either decompose into RS and NO within 0.3 ps after excitation at 330 nm with a primary quantum yield (Φ1) of 0.46-1 or relax into an electronically excited intermediate state lying at 42 ± 3 kcal mol-1 above the ground state, which relaxes into the ground state with a time constant of 460-520 ps. A majority (62-80%) of the RS radical geminately rebinds with NO at a time constant of 3-7 ps. The remaining RS reacts with the neighbouring RSNO, which produces additional NO and RSSR with a (nearly) diffusion-limited rate constant that doubles the amount of NO produced; further, it remarkably extends the time window for the dissociated NO to react with the target compound. The final fraction of NO produced from these RSNOs at 330 nm was 0.32-0.58, and it depends on the geminate rebinding yield and Φ1. The detailed dynamics of the photoexcited RSNO can be utilised in the quantitative application of these RSNOs in practical use and in the synthesis of more efficient photoactivated NO precursors.

Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy.

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The SQ states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T1 states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.

Complete photodissociation dynamics of CF2I2 in solution.

Photodissociation dynamics of CF2I2 in cyclohexane were evaluated by probing the C-F stretching mode over a wide time range after ultraviolet excitation using femtosecond infrared spectroscopy. After the ultrafast (<0.2 ps) state-selective photodissociation of CF2I2 as in the gas phase (267 nm excitation led to exclusive three-body dissociation (CF2 + I + I), 350 nm to exclusive two-body dissociation (CF2I + I), and 310 nm to a mixture of three- and two-body dissociations), various secondary reactions were observed. Once produced, some nascent CF2 radicals immediately formed a complex with the departing I atom (ICF2), which produced either CF2I or CF2 radicals. The produced CF2I geminately recombined with the I atom, whereas the CF2 radical reacted bimolecularly to produce C2F4 with a diffusion-limited rate constant of 8.1 × 109 M-1 s-1. Some nascent CF2I radicals were produced with sufficient excess energy to further dissociate into CF2 and I, or immediately reacted with the dissociated I atom to form the I2-CF2 isomer that rapidly dissociated into CF2 and I2. Other nascent CF2I radicals geminately recombined with the I atom with various time constants. Thus, the nascent photoproducts, CF2 and CF2I take various reaction paths: complex formation, secondary dissociation, isomer formation, and fast and slow germinate rebindings. The ensuing reaction path of the nascent photoproduct is dictated by its internal energy as well as solvent environment, which leads to different interactions between the photoproduct and solvent. Measurement over a broad time range with a structure-sensitive probe could reveal the fate of all the reaction intermediates, which allows evaluation of the complete reaction dynamics in solution.

Insilico direct folding of thrombin-binding aptamer G-quadruplex at all-atom level.

The reversible folding of the thrombin-binding DNA aptamer G-quadruplexes (GQs) (TBA-15) starting from fully unfolded states was demonstrated using a prolonged time scale (10-12 µs) parallel tempering metadynamics (PTMetaD) simulation method in conjunction with a modified version of the AMBER bsc1 force field. For unbiased descriptions of the folding free energy landscape of TBA-15, this force field was minimally modified. From this direct folding simulation using the modified bsc1 force field, reasonably converged free energy landscapes were obtained in K+-rich aqueous solution (150 mM), providing detailed atomistic pictures of GQ folding mechanisms for TBA-15. This study found that the TBA folding occurred via multiple folding pathways with two major free energy barriers of 13 and 15 kcal/mol in the presence of several intermediate states of G-triplex variants. The early formation of these intermediates was associated with a single K+ ion capturing. Interestingly, these intermediate states appear to undergo facile transitions among themselves through relatively small energy barriers.

Photodissociation of CF2ICF2I in solid para-hydrogen: infrared spectra of anti- and gauche-˙C2F4I radicals.

The photolysis of 1,2-diiodotetrafluoroethane (CF2ICF2I) has served as a prototypical system in ultrafast reaction dynamics. Even though the intermediates, anti- and gauche-iodotetrafluoroethyl (˙C2F4I) radicals, have been characterized with electron diffraction and X-ray diffraction, their infrared spectra are unreported. We report the formation and infrared identification of these radical intermediates upon ultraviolet photodissociation of CF2ICF2I in solid para-hydrogen (p-H2) at 3.3 K. Lines at 1364.9/1358.5, 1283.2, 1177.1, 1162.2, 1126.8, 837.3, 658.0, 574.2, and 555.2 cm-1 are assigned to anti-˙C2F4I, and lines at 1325.9, 1259.7, 1143.4, 1063.4, 921.0, and 765.3 cm-1 to gauche-˙C2F4I. A secondary photodissociation leading to C2F4 was also observed. The assignments were derived according to behavior on secondary photolysis, comparison of the vibrational wavenumbers and the IR intensities of the observed lines with values predicted with the B3PW91/aug-cc-pVTZ-pp method. This spectral identification provides valuable information for future direct spectral probes of these important intermediates.

Symmetry-breaking charge transfer in the excited state of directly linked push-pull porphyrin arrays.

Herein, we revealed a symmetry-breaking charge transfer (SBCT) process in the excited state of a directly linked push-pull porphyrin dyad (AD) and triad (ADA) via spectroscopic measurements including steady-state absorption and fluorescence, time-resolved fluorescence (TRF), femtosecond transient absorption (fs-TA), and time-resolved infrared (TRIR) measurements. Unprecedented broad fluorescence spectra were observed for porphyrin arrays in polar solvents; these were attributed to the existence of a charge transfer state as evidenced by the TRF measurements. TA measurements also revealed emerging features of a CT state for AD and ADA in polar solvents. These dynamics were also confirmed via TRIR measurements, which provided further information on the solvation and structural relaxation processes of the SBCT process. This is the first observation of an SBCT process in porphyrin arrays, providing fundamental understanding of the strongly coupled porphyrin arrays. Thus, the results of this study reveal the potential of the porphyrin arrays in relevant applications requiring SBCT.

Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst.

We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light-responsive photocatalysts for the photoconversion of CO2 into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO2 photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO2 photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO2 photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compared with AuNPs.

Rebinding dynamics of NO to microperoxidase-8 probed by time-resolved vibrational spectroscopy.

Femtosecond vibrational spectroscopy was used to probe the rebinding kinetics of NO to microperoxidase-8 (Mp), an ideal model system for the active site of ligand-binding heme proteins, including myoglobin and hemoglobin, after the photodeligation of MpNO in glycerol/water (G/W) solutions at 294 K. The geminate rebinding (GR) of NO to Mp in viscous solutions was highly efficient and ultrafast and negligibly dependent on the solution viscosity, which was adjusted by changing the glycerol content from 65% to 90% by volume in G/W mixtures. The kinetics of the GR of NO to Mp in viscous solutions was well represented by an exponential function with a time constant of ca. 11 ps. Although the kinetic traces of the GR of NO to Mp in solutions with three different viscosities (18, 81, and 252 cP) almost overlap, they show a slight difference early in the decay process. The kinetic traces were also described by the diffusion-controlled reaction theory with a Coulomb potential. Since the ligand is deligated in a neutral form, an ionic pair of NO(-) and Mp(+) may be produced before forming the Mp-NO bond by an electron transfer from Mp to NO as the deligated NO is sufficiently near to the Fe atom of Mp. The strong reactivity between NO and ferrous heme may arise from the Coulomb interaction between the reacting pair, which is consistent with the harpooning mechanism for NO binding to heme.

A very rapid electronic relaxation process in a highly conjugated Zn(II)porphyrin-[26]hexaphyrin-Zn(II)porphyrin hybrid tape.

The excited-state energy relaxation processes of a Zn(II)porphyrin­[26]hexaphyrin­Zn(II)porphyrin triply linked hybrid tape, FZn, have been investigated by femtosecond transient absorption spectroscopy (TA), using a directly meso­meso linked hybrid trimer, HZn, as a reference compound. FZn has a very small S1­S0 energy gap through the expansion of π-conjugation and the absorption band at 1897 nm corresponds to its lowest singlet excited-state as a consequence of enhanced transition dipole moment that lies parallel to the long molecular axis. In TA measurements, we observe an energy transfer process (0.4 ps) from the Zn(II)porphyrin moiety to the [26]hexaphyrin core in HZn. In contrast to HZn, a biexponential decay with the time constants of 0.25 and 6.5 ps was observed in the decay profile of FZn. The detailed analysis of excitation wavelength, temperature and solvent dependent TA in FZn revealed that the electronic relaxation process (0.25 ps) from S1 to S0 is faster than the vibrational relaxation processes (5.9 ps) in the excited and ground states due to a very small S1­S0 energy gap through the expansion of π-conjugation. Accordingly, we demonstrate that electronic deactivation overtakes vibrational relaxation processes in a highly conjugated FZn.

A Description of Vibrational Modes in Hexaphyrins: Understanding the Aromaticity Reversal in the Lowest Triplet State.

Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of [26]- and [28]hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of [26]bis(rhodium) hexaphyrin (R26H), those of [28]bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.

Identification of an emitting molecular species by time-resolved fluorescence applied to the excited state dynamics of pigment yellow 101.

Time-resolved fluorescence (TRF) with a resolution higher than the periods of vibrations may provide the vibrational spectrum of an emitting species by directly recording the vibrational wave packet motions in time. We applied high-resolution TRF to investigate the excited-state dynamics of pigment yellow 101 (P.Y.101). The TRF spectra of P.Y.101 in dichloromethane showed that upon photoexcitation of the enol isomer, dynamics occur in the S1 state to form a product in two time constants at 30 and 140 fs. TRF signals were modulated due to the vibrational wave packet motions in the excited states, which provided the vibrational spectra of the emitting species. Depending on the emission wavelength, two different vibrational spectra were evident. With the help of theoretical calculations, the two spectra were assigned to the enol and keto isomers of P.Y.101 in the S1 state, leading to the conclusion that P.Y.101 undergoes ultrafast excited-state intramolecular proton transfer (ESIPT) with a quantum yield close to 1. Visible-pump infrared-probe transient absorption spectra were recorded to corroborate this conclusion.

Effect of Electron-donating Group on NO Photolysis of {RuNO}6 Ruthenium Nitrosyl Complexes with N2 O2 Lgands Bearing π-Extended Rings.

In this study, we introduced the electron-donating group (-OH) to the aromatic rings of Ru(salophen)(NO)Cl (0) (salophenH2 =N,N'-(1,2-phenylene)bis(salicylideneimine)) to investigate the influence of the substitution on NO photolysis and NO-releasing dynamics. Three derivative complexes, Ru((o-OH)2 -salophen)(NO)Cl (1), Ru((m-OH)2 -salophen)(NO)Cl (2), and Ru((p-OH)2 -salophen)(NO)Cl (3) were developed and their NO photolysis was monitored by using UV/Vis, EPR, NMR, and IR spectroscopies under white room light. Spectroscopic results indicated that the complexes were diamagnetic Ru(II)-NO+ species which were converted to low-spin Ru(III) species (d5 , S=1/2) and released NO radicals by photons. The conversion was also confirmed by determining the single-crystal structure of the photoproduct of 1. The photochemical quantum yields (ΦNO s) of the photolysis were determined to be 0>1, 2, 3 at both the visible and UV excitations. Femtosecond (fs) time-resolved mid-IR spectroscopy was employed for studying NO-releasing dynamics. The geminate rebinding (GR) rates of the photoreleased NO to the photolyzed complexes were estimated to be 0≃1, 2, 3. DFT and TDDFT computations found that the introduction of the hydroxyl groups elevated the ligand π-bonding orbitals (π (salophen)), resulting in decrease of the HOMO-LUMO gaps in 1-3. The theoretical calculations suggested that the Ru-NNO bond dissociations of the complexes were mostly initiated by the ligand-to-ligand charge transfer (LLCT) of π(salophen)→π*(Ru-NO) with both the visible and UV excitations and the decreasing ΦNO s could be explained by the changes of the electronic structures in which the photoactivable bands of 1-3 have relatively less contribution of transitions related with Ru-NO bond than those of 0.

Geminate rebinding dynamics of nitric oxide to ferric hemoglobin in D2O solution.

Femtosecond mid-infrared (mid-IR) spectroscopy was used to probe geminate rebinding (GR) dynamics of photo-released nitric oxide (NO) to ferric hemoglobin (Hb(III)) in D2O solution at room temperature. Time-resolved vibrational spectra exhibit two overlapping NO bands for NO-bound Hb(III) (Hb(III)NO), a major band at 1925 cm(-1) (89%) and a minor one at 1905 cm(-1) (11%), suggesting that Hb(III)NO has at least two conformational substates. Both bands decay nonexponentially, each with a different time scale, and the decays are described by a stretched exponential function; the major band's decay is described by 0.96 exp(-t/40 ps)(0.86) + 0.04 and the minor band's decay is described by exp(-t/85 ps)(0.75). These decays arise mainly from the GR of the photo-released NO to Hb(III), indicating that the bound state's conformer influences the NO binding. In particular, the His64 residue, known to have inward conformation in the major band and outward conformation in the minor band, plays a significant role in controlling the binding of NO to Hb(III). The GR of NO to ferric Hb is slower than that to ferrous Hb, which shows fast and efficient GR due to the high reactivity of NO to the heme Fe(ii). The slower GR of NO to Hb(III) may be caused by the lower reactivity of NO to the heme Fe(iii).

Dynamics of Irreversible NO Release from Photoexcited Molsidomine.

Molsidomine (SIN-10), an orally administered NO-delivery drug for vasodilation, cannot be used to alleviate hypertensive crisis because it releases NO at a slow rate. SIN-10 may be used to treat sudden cardiac abnormalities if the rapid and immediate release of NO is achieved via photoactivation. The photodissociation dynamics associated with the NO release process from SIN-10 in CHCl3 was investigated using time-resolved infrared spectroscopy. Approximately 41% of photoexcited SIN-10 at 360 nm decomposed into CO2, CH2CH3 radical, and the remaining radical fragment [SIN-1A(-H)] with a time constant of 43 ps. All SIN-1A(-H) released NO spontaneously with a time constant of 68 ns, becoming N-morpholino-aminoacetonitrile, resulting in 41% for the quantum yield of immediate NO release from SIN-10. The results obtained can be used to realize the quantitative control of the NO administration at a specific time, and SIN-10 can be potentially used to address the phenomenon of hypertensive crisis.

Rotational Isomerization of Carbon-Carbon Single Bonds in Ethyl Radical Derivatives in a Room-Temperature Solution.

The rotational isomerization of 1,2-disubstituted ethyl radical derivatives, reaction intermediates often found in the reaction of 1,2-disubstituted ethane derivatives, has never been measured because of their short lifetime and ultrafast rotation. However, the rotational time constant is critical for understanding the detailed reaction mechanism involving these radicals, which determine the stereoisomers of compounds produced via the intermediates. Using time-resolved infrared spectroscopy, we found that the CF2BrCF2 radical in a CCl4 solution rotationally isomerizes with a time constant of 47 ± 5 ps at 280 ± 2 K. From this value and the rotational barrier heights of related compounds, CH3CH2 and CH3CH2CHCH3 radicals in CCl4 were estimated to rotationally isomerize within 1 ps at 298 K, considerably faster than ethane and n-butane, which rotationally isomerize with time constants of 1.8 and 81 ps, respectively. The time constant for the rotational isomerization was similar to that calculated using transition state theory with a transmission coefficient of 0.75.