Influence of Curvature on the Physical Properties and Reactivity of Triplet Corannulene Nitrene.

Although nitrene chemistry is promising for the light-induced modification of organic compounds, the reactivity of large polycyclic aromatic compounds and the effects of their curvature remain unexplored. Irradiation of azidocorannulene () in methanol/acetonitrile followed by HCl addition produced diastereomers and . Azirine is apparently trapped by methanol to form diastereomeric acetal derivatives that are hydrolyzed with HCl to yield and '.

α-Cleavage of 2,2-Diazido-2,3-dihydroinden-1-one in Solution and Cryogenic Matrices.

The Norrish type I (α-cleavage) reaction is an excellent photochemical method for radical-pair formation in solution. However, in cryogenic matrices, the starting material typically re-forms before the radical pair diffuses apart. This study focused on N extrusion from an azido alkyl radical to prevent radical-pair recombination.

Unraveling the Solid-State Photoreactivity of Carbonylbis(4,1-Phenylene)dicarbonazidate with Laser Flash Photolysis.

Solid-state photoreactions are generally controlled by the rigid and ordered nature of crystals. Herein, the solution and solid-state photoreactivities of carbonylbis(4,1-phenylene)dicarbonazidate () were investigated to elucidate the solid-state reaction mechanism. Irradiation of in methanol yielded primarily the corresponding amine, whereas irradiation in the solid state gave a mixture of photoproducts.

Light-Mediated Synthesis of 2-(4-Methoxyphenyl)-1-pyrroline via Intramolecular Reductive Cyclization of a Triplet Alkylnitrene.

Irradiation of -methoxyazidobutyrophenone () in methanol yielded 2-(4-methoxyphenyl)-1-pyrroline () and several other photoproducts. However, in the presence of tris(trimethylsilyl)silane (TTMSS), is formed selectively. Transient absorption and ESR spectroscopy verify that the irradiation of forms triplet alkylnitrene through intramolecular energy transfer from the triplet ketone (T).

Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the -Keto Tautomer.

Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety () were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that - exist as both enol and keto tautomers, with the enol form being predominant.

Formation of Stilbene Azo-Dimer by Direct Irradiation of p-Azidostilbene.

Triplet arylnitrenes may provide direct access to aryl azo-dimers, which have broad commercial applicability. Herein, the photolysis of p-azidostilbene (1) in argon-saturated methanol yielded stilbene azo-dimer (2) through the dimerization of triplet p-nitrenostilbene ( 1N). The formation of 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ  ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices.

Photolysis of 3-Azido-3-phenyl-3H-isobenzofuran-1-one at Ambient and Cryogenic Temperatures.

Although alkyl azides are known to typically form imines under direct irradiation, the product formation mechanism remains ambiguous as some alkyl azides also yield the corresponding triplet alkylnitrenes at cryogenic temperatures. The photoreactivity of 3-azido-3-phenyl-3H-isobenzofuran-1-one (1) was investigated in solution and in cryogenic matrices. Irradiation (λ = 254 nm) of azide 1 in acetonitrile yielded a mixture of imines 2 and 3.

From Incident Light to Persistent and Regenerable Radicals of Urea-Assembled Benzophenone Frameworks: A Structural Investigation.

Herein we probe the effects of crystalline structure and geometry on benzophenone photophysics, self-quenching, and the regenerable formation of persistent triplet radical pairs at room temperature. Radical pairs are not observed in solution but appear via an emergent pathway within the solid-state assembly. Single crystal X-ray diffraction (SC-XRD) of two sets of constitutional isomers, benzophenone -urea macrocycles, and methylene urea-tethered dibenzophenones are compared.

Visible Light Absorption and Long-Lived Excited States in Dinuclear Silver(I) Complexes with Redox-Active Ligands.

Well-defined dinuclear silver(I) complexes have been targeted for applications in catalysis and materials chemistry, and the effect of close silver-silver interactions on electronic structure remains an area of active inquiry. In this study, we describe the synthesis, structure, and photophysical properties of dimeric silver complexes featuring a redox-active naphthyridine diimine ligand. Unusually for silver(I), these complexes display absorption features in the visible region due to metal-metal to ligand charge transfer (MMLCT) transitions, which arise from the combination of close silver-silver interactions and low-lying ligand π* orbitals.

Cracking under Internal Pressure: Photodynamic Behavior of Vinyl Azide Crystals through N Release.

When exposed to UV light, single crystals of the vinyl azides 3-azido-1-phenylpropenone (), 3-azido-1-(4-methoxyphenyl)propenone (), and 3-azido-1-(4-chlorophenyl)propenone () exhibit dramatic mechanical effects by cracking or bending with the release of N. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals.

Photoinduced α-Cleavage of 2-Azido-2-phenyl-1,3-indandione at Cryogenic Temperatures.

To expand the utility of α-cleavage at cryogenic temperatures, we investigated the photoreactivity of 2-azido-2-phenyl-1,3-indandione (). EPR spectroscopy revealed that irradiating in 2-methyltetrahydrofuran (mTHF) matrices forms alkylnitrene , which has zero-field splitting parameters (/c = 1.5837 cm; /c = 0.

Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-phenyl)-4-oxo-but-2-enoate Ester Derivatives.

To clarify the cis-trans isomerization mechanism of simple alkenes on the triplet excited state surface, the photochemistry of acyclic and cyclic vinyl ketones with a -methoxyacetophenone moiety as a built-in triplet sensitizer ( and , respectively) was compared. When irradiated, ketone produces its cis-isomer, whereas ketone does not yield any photoproducts. Laser flash photolysis of ketone yields a transient spectrum with λ ∼ 400 nm (τ ∼ 125 ns).

Photolysis of 5-Azido-3-Phenylisoxazole at Cryogenic Temperature: Formation and Direct Detection of a Nitrosoalkene.

To enhance the versatility of organic azides in organic synthesis, a better understanding of their photochemistry is required. Herein, the photoreactivity of azidoisoxazole was characterized in cryogenic matrices with IR and UV-Vis absorption spectroscopy. The irradiation (λ = 254 nm) of azidoisoxazole in an argon matrix at 13 K and in glassy 2-methyltetrahydrofuran (mTHF) at 77 K yielded nitrosoalkene .

Formation and Reactivity of Triplet Vinylnitrenes as a Function of Ring Size.

The photoreactivity of cyclic vinyl azides (3-azido-2-methyl-cyclopenten-1-one) and (3-azido-2-methyl-2-cyclohexen-1-one), which have five- and six-membered rings, respectively, was characterized at cryogenic temperature with electron paramagnetic resonance (EPR), IR, and UV spectroscopy. EPR spectroscopy revealed that irradiating (λ > 250 nm) vinyl azides and in 2-methyltetrahydrofuran at 10 K resulted in the corresponding triplet vinylnitrenes (/ = 0.611 cm and / = 0.

Steric Demand and Rate-determining Step for Photoenolization of Di-ortho-substituted Acetophenone Derivatives.

Laser flash photolysis of ketone 1 in argon-saturated methanol yields triplet biradical 1BR (τ = 63 ns) that intersystem crosses to form photoenols Z-1P (λ = 350 nm, τ ~ 10 μs) and E-1P (λ = 350 nm, τ > 6 ms). The activation barrier for Z-1P re-forming ketone 1 through a 1,5-H shift was determined as 7.7 ± 0.

Laser flash photolysis of nanocrystalline α-azido-p-methoxy-acetophenone.

Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast, solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (λ ∼ 400 nm), which is short-lived (τ = 833 ns) as it intersystem crosses to form imine 2.

Using Molecular Architecture to Control the Reactivity of a Triplet Vinylnitrene.

Photolysis of 3-azido-1-indenone (1) with a light-emitting diode (LED, λ = 405 nm) or mercury arc lamp (Pyrex) resulted in the formation of heterodimer 3 in excellent yield, through dimerization of triplet vinylnitrene 2. At ambient temperature, vinylnitrene 2 (λ at 340 and 480 nm) was detected directly with laser flash photolysis of vinyl azide 1. The vinylnitrene intermediate was also characterized directly with IR and ESR spectroscopy in cryogenic matrices.

Photoenolization of o-Methylvalerophenone Ester Derivative.

Photolysis of ester 1 in argon-saturated methanol and acetonitrile does not produce any product, whereas irradiation of 1 in oxygen-saturated methanol yields peroxide 2. Laser flash photolysis studies demonstrate that 1 undergoes intramolecular H atom abstraction to form biradical 3 (λmax ~340 nm), which intersystem crosses to form photoenols Z-4 and E-4 (λmax ~380 nm). Photoenols 4 decay by regenerating ester 1.

Direct detection of a triplet vinylnitrene, 1,4-naphthoquinone-2-ylnitrene, in solution and cryogenic matrices.

The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene (3)2, which was detected directly by IR spectroscopy. Vinylnitrene (3)2 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene (3)2, which is stable up to at least 100 K.

Infrared matrix isolation study of the thermal and photochemical reactions of ozone with trimethylgallium.

The thermal and photochemical reactions of (CH3)3Ga and O3 have been explored using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental data using twin jet deposition and theoretical calculations demonstrate the formation of multiple product species after deposition, annealing to 35 K, and UV irradiation of the matrices. The products were identified as (CH3)2GaOCH3, (CH3)2GaCH2OH, (CH3)(CH3O)Ga(OCH3), (CH3)2GaCHO, and (CH3)Ga(OCH3)(CH2OH).

Phototautomerization on the singlet and triplet surface in o-hydroxyacetophenone derivatives in polar solvents.

Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (λmax 430 nm, τ ≈ 3 μs) and 2b (λmax 400 nm, τ ≈ 1 μs), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (λmax 400 nm, τ ≈ 10 μs), whereas 2b forms both Z-3b and E-3b (λmax 350 nm, τ ≈ 5 and 72 μs). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b.

Triplet sensitized photolysis of a vinyl azide: direct detection of a triplet vinyl azide and nitrene.

Photolysis of vinylazide 1, which has a built-in acetophenone triplet sensitizer, in argon-saturated toluene results in azirine 2, whereas irradiation in oxygen-saturated toluene yields cyanide derivatives 3 and 4. Laser flash photolysis of azide 1 in argon-saturated acetonitrile shows formation of vinylnitrene 1c, which has a λmax at ∼300 nm and a lifetime of ∼1 ms. Vinylnitrene 1c is formed with a rate constant of 4.

Trans-cis isomerization of vinylketones through triplet 1,2-biradicals.

The irradiation of trans-vinylketones 1a-c yields the corresponding cis isomers 2a-c. Laser flash photolysis of 1a and 1b with 308 and 355 nm lasers results in their triplet ketones (T1K of 1), which rearrange to form triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation with a 266 nm laser produces their cis-isomers through singlet reactivity. Time-resolved IR spectroscopy of 1a with 266 nm irradiation confirmed that 2a is formed within the laser pulse.