Dynamics of NO Release and Linkage Isomer Formation from S-Nitroso-Mercaptoethanol in Aqueous Solutions: Insights from Femtosecond Infrared Spectroscopy.

Understanding the photodynamics of S-nitroso-thiol (RSNO), an effective NO transporter in biological systems, is essential for its photochemical applications. S-nitroso-mercaptoethanol (MceSNO), a simple water-soluble RSNO, facilitates high-level quantum calculations. We investigated the photoexcitation dynamics of MceSNO in an aqueous solution, focusing on NO dissociation, recombination, and linkage isomerization using quantum calculations and femtosecond infrared spectroscopy. Upon excitation at 320 nm, MceSNO rapidly dissociates into NO and MceS radicals. Approximately 31 ± 3% of MceS reacts with unexcited MceSNO molecules, forming MceSSMce and releasing additional NO. The remaining MceS undergoes geminate recombination with NO, forming either MceSNO (41 ± 4%) or MceSON (28 ± 3%), the latter being a sulfur-ON linkage isomer observed for the first time in a room-temperature solution. MceSON isomerizes back to MceSNO in 470 ± 30 ps. The formation mechanism of MceSON was verified through a potential energy surface constructed at the CASPT2D(16,11)/cc-pVTZ level. The isomerization barrier was determined to be 3.3 ± 1.2 kcal/mol in water.

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.

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.

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.

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.

Visible-light NO photolysis of ruthenium nitrosyl complexes with N2O2 ligands bearing π-extended rings and their photorelease dynamics.

NO photorelease and its dynamics for two {RuNO}6 complexes, Ru(salophen)(NO)Cl (1) and Ru(naphophen)(NO)Cl (2), with salen-type ligands bearing π-extended systems (salophenH2 = N,N'-(1,2-phenylene)-bis(salicylideneimine) and naphophenH2 = N,N'-1,2-phenylene-bis(2-hydroxy-1-naphthylmethyleneimine)) were investigated. NO photolysis was performed under white room light and monitored by UV/Vis, EPR, and NMR spectroscopies. NO photolysis was also performed under 459 and 489 nm irradiation for 1 and 2, respectively. The photochemical quantum yields of the NO photolysis (ΦNO) of both 1 and 2 were determined to be 9% at the irradiation wavelengths. The structural and spectroscopic characteristics of the complexes before and after the photolysis confirmed the conversion of diamagnetic Ru(II)(L)(Cl)-NO+ to paramagnetic S = ½ Ru(III)(L)(Cl)-solvent by photons (L = salophen2- and naphophen2-). The photoreleased NO radicals were detected by spin-trapping EPR. DFT and TDDFT calculations found that the photoactive bands are configured as mostly the ligand-to-ligand charge transfer (LLCT) of π(L) → π*(Ru-NO), suggesting that the NO photorelease was initiated by the LLCT. Dynamics of NO photorelease from the complexes in DMSO under 320 nm excitation were investigated by femtosecond (fs) time-resolved mid-IR spectroscopy. The primary photorelease of NO occurred for less than 0.32 ps after the excitation. The rate constants (k-1) of the geminate rebinding of NO to the photolyzed 1 and 2 were determined to be (15 ps)-1 and (13 ps)-1, respectively. The photochemical quantum yields of NO photolysis (ΦNO, λ = 320 nm) were estimated to be no higher than 14% for 1 and 11% for 2, based on the analysis of the fs time-resolved IR data. The results of fs time-resolved IR spectroscopy and theoretical calculations provided some insight into the overall kinetic reaction pathway, localized electron pathway or resonance pathway, of the NO photolysis of 1 and 2. Overall, our study found that the investigated {RuNO}6 complexes, 1 and 2, with planar N2O2 ligands bearing π-extended rings effectively released NO under visible light.

Midwavelength Infrared Colloidal Nanowire Laser.

Realizing bright colloidal infrared emitters in the midwavelength infrared (or mid-IR), which can be used for low-power IR light-emitting diodes (LEDs), sensors, and deep-tissue imaging, has been a challenge for the last few decades. Here, we present colloidal tellurium nanowires with strong emission intensity at room temperature and even lasing at 3.6 µm (ω) under cryotemperature. Furthermore, the second-harmonic field at 1.8 µm (2ω) and the third-harmonic field at 1.2 µm (3ω) are successfully generated thanks to the intrinsic property of the tellurium nanowire. These unique optical features have never been reported for colloidal tellurium nanocrystals. With the colloidal midwavelength infrared (MWIR) Te nanowire laser, we demonstrate its potential in biomedical applications. MWIR lasing has been clearly observed from nanowires embedded in a human neuroblastoma cell, which could further realize deep-tissue imaging and thermotherapy in the near future.

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.

Photodissociation Dynamics of Nitric Oxide from N-Acetylcysteine- or N-Acetylpenicillamine-bound Roussin's Red Ester.

The photochemical release of nitric oxide (NO) from a NO precursor is advantageous in terms of spatial, temporal, and dosage control of NO delivery to target sites. To realize full control of the quantitative NO administration from photoactivated NO precursors, it is necessary to have detailed dynamical information on the photodissociation of NO from NO precursors. We synthesized two new water-soluble Roussin's red esters (RREs), [Fe2(µ-N-acetylcysteine)2(NO)4] and [Fe2(µ-N-acetylpenicillamine)2(NO)4], which have five times longer lifetime than the well-known [Fe2(µ-cysteine)2(NO)4]. The photodissociation dynamics of NO from these RREs in water were investigated over a broad time range from 0.3 ps to 10 µs after excitation at 310 and 400 nm using femtosecond time-resolved infrared (IR) spectroscopy. When these RREs are excited, they either release one NO, producing a radical species deficient in one NO (R), [Fe2(µ-RS)2(NO)3], or relax into the ground state without photodeligation via an electronically excited intermediate state (M). R appears immediately after photoexcitation, suggesting that one NO is photodissociated faster than 0.3 ps. A certain fraction of R undergoes geminate recombination (GR) with NO with a time constant of 7-9 ps, while the remaining R competitively binds to the solvent. Solvent-bound R eventually bimolecularly recombines with NO with a rate constant of (1.3-1.6) × 108 M-1 s-1. For a given RRE molecule, the fractional yield of M (0.62-0.76) depends on the excitation wavelength (λex); however, the relaxation time of M (6 ± 1 ns) is independent of λex. Although the primary quantum yield of NO photodissociation (Φ1) was found to be 0.24-0.38, the final yield of NO suitable for other reactions (Φ2) was reduced to 0.14-0.29 due to the picosecond GR of the dissociated NO with R. Detailed photoexcitation dynamics of RRE can be utilized in the quantitative control of NO administration at a specific site and time, promoting pin-point usage of NO in chemistry and biology. We demonstrate that femtosecond IR spectroscopy combined with quantum chemical calculations is a powerful method for obtaining detailed dynamic information on photoactivated NO precursors such as Φ1 and Φ2, the GR yield, and secondary reactions of the nascent photoproducts, which are essential information for the design of efficient photoactivated NO precursors and their quantitative utilization.

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.