EPR spectroscopy of multicomponent,multispin molecular system obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon

Complex multicomponent, multispin molecular system, consisting of a septet trinitrene, two quintet dinitrenes, and three triplet mononitrenes, was obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon. To identify these paramagnetic products, electron paramagnetic resonance spectroscopy in combination with line-shape spectral simulations and density functional theory calculations was used. The products of the photolysis was found to be triplet 2,4-diazido-3-cyano-5-fluoropyridyl-6-nitrene (D_T = 1.000 cm⁻¹, E_T = 0), triplet 2,4-diazido-3-cyano-5-fluoropyridyl-2-nitrene (D_T = 1.043 cm⁻¹, E_T = 0), triplet 2,6-diazido-3-cyano-5-fluoropyridyl-4-nitrene (D_T = 1.128 cm⁻¹, E_T = 0 cm⁻¹), quintet 4-azido-3-cyano-5-fluoropyridyl-2,6-dinitrene (D_Q = 0.211 cm⁻¹, E_Q = 0.0532 cm⁻¹), quintet 2-azido-3-cyano-5-fluoropyridyl-4,6-dinitrene (D_Q = 0.208 cm⁻¹, E_Q = 0.0386 cm⁻¹), and septet 3-cyano-5-fluoropyridyl-2,4,6-trinitrene (D_S = −0.1017 cm⁻¹, E_S = −0.0042 cm⁻¹) in a 38:4:7:22:14:4 ratio, respectively.     

Magnetic anisotropy parameters of matrix-isolated septet 1,3,5-trinitreno-2,4,6-trichlorobenzene

An ESR spectrum of high-symmetry septet 1,3,5-trinitreno-2,4,6-trichlorobenzene generated under photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in solid argon at 15 K was recorded. Computer simulation revealed that the spectrum corresponds to the septet spin state with the fine structure parameters D _S = ?0.0957±0.0006 cm^?1 and E _S = 0±0.0004 cm^?1. These values of the magnetic anisotropy parameters D _S and E _S are in good agreement with the results of UDFT calculations. The spin-spin (D _SS) and spin-orbit (D _SO) coupling parameters of septet molecules with D _3h symmetry are negative and mutually enhance the magnetic anisotropy of these molecules. The contribution of the spin-orbit coupling to the magnetic anisotropy of 1,3,5-trinitreno-2,4,6-trichlorobenzene is higher than 11% due to the presence of three chlorine atoms in the molecule. This suggests the possibility of further strengthening the magnetic properties of septet 1,3,5-trinitrenobenzenes by introducing bromine and iodine atoms into positions 2, 4, and 6 of their benzene rings.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

Persistent Organic High‐Spin Trinitrenes

The septet ground state trinitrenes 1,3,5‐trichloro‐2,4,6‐trinitrenobenzene and 1,3,5‐tribromo‐2,4,6‐trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H₂, O₂, and H₂O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.     

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 (³ 1N ). The formation of ³ 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed ³ 1N (λ_max ~ 370 nm, 2.85 × 10⁷ s⁻¹). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s⁻¹) was assigned to a triplet excited state (T) of 1 . Density functional theory calculations yielded three configurations for T of 1 , with the unpaired electrons on the azido (T_A) or stilbene moiety (T_Tw, twisted and T_Fl, flat). The transient was assigned to T_Tw based on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol⁻¹ for 1 N ; thus, intersystem crossing of ¹ 1N to ³ 1N is unlikely at ambient temperature, supporting the formation of ³ 1N from T of 1 . Thus, sustainable synthetic methods for aryl azo-dimers can be developed using the visible-light irradiation of aryl azides to form triplet arylnitrenes.     

Sensitized photo-redox reaction: VII. The Synthesis,FAB mass spectrum,stationary absorption,emission, transient absorption spectra,redox potential of dihydroxy-tin (IV)-mesoporphyrin dimethyl ester and its sensitized photo-reduction of methyl viologen

In DMSO/water (4:1), photolysis of the dihydroxy-Sn (IV)-mesoporphyrin dimethyl ester (SnP)/methyl viologen (MV²⁺)/ethylene diamine tetraacetic acid (EDTA) ternary system produces methyl viologen cation radical with a quantum yield of 0.67, much higher than that of systems with other metal complexes of mesoporphyrin dimethyl ester. Neither EDTA nor MV²⁺ quenches the stationary fluorescence of SnP, implying that the reaction does not take place at the singlet state. With flash photolysis we obtain the T-T absorption spectrum of SnP (λ_max 440 nm). By following the decay of this absorption, the triplet life time of SnP is estimated to be 41 μs. The life time is related to the concentration of either MV²⁺ or EDTA. Good linear relationships are obtained by plotting τ₀τ vs. the concentration of MV²⁺ or EDTA (Stern-Volmer plot), from which we determine the quenching constants: _kq(MV²⁺) =5.5 × 10⁷ mol^?1, s^?1; k_q (EDTA) =2.7 × 10⁷ mol^?1, s^?1. The data suggests that upon photolysis of the above ternary system, both oxidative quenching and reductive quenching of the triplet state of the sensitizer are occurring. From the measured phosphorescence spectrum (λ_max 704 nm) and the ground state redox, potentials (E^red_1/2?-0.84V, E^ox_1/2?+1.43 V, vs. Ag/AgCl, KCl (sat.)), we obtain the redox potential of triplet SnP to be E(P⁺/P*_T)?-0.33 V, E(P*_T+/P^?)?+0.92 V. Matching this data with the redox potential of MV²⁺ and EDTA, we establish the fact that during the photolysis of the SnP/MV²⁺/EDTA ternary system, both oxidative and reductive quenching are thermodynamically favorable processes. This is also the reason why the SnP sensitized reaction is much more efficient relative to other mesoporphyrin derivatives.     

没食子酸的激光光解研究

The transient species of gallic acid(GA)have been studied by 266 nm nanosecond laser flash photolysis inaqueous solution and acetonitrile.The intermediate with absorption at 320 nm was identified as excited triplet state(~3GA~*),the decay rates of which were obtained in aqueous solution and acetonitrile respectively.Energy transferfrom ~3GA~* to β-carotene was observed and the energy transfer rate constant k_(ent)was determined to be 2.2×10~9mol~(-1)·L·s~(-1).GA underwent photoionization during photolysis and the quantum yield of photoionization was de-termined to be 0.12 at room temperature with KI as a reference.     

Anomalous rotational state distribution for the O2 photofragment in the UV photodissociation of ozone

The coherent anti-Stokes Raman scattering (CARS) spectrum of O₂(¹Δ_g) formed in the photodissociation of ozone at 266 nm is presented. An anomalous propensity for even (J = 2,4,6, …) retational states is observed. This propensity effect is discussed in terms of symmetry restrictions and dynamical bias in the photolysis.     

Photosensitization mechanisms in photopolymer coating film containing photoinitiators sensitized by aminochalcone‐type dye for computer‐to‐photopolymer plate

Photosensitization mechanisms in photopolymer coating film containing an aminochalcone‐type dye sensitizer and a radical generating reagent, sensitizer dyes, (E)‐3‐(9‐julolidinyl)‐1‐phenyl‐2‐propen‐1‐one (A), (E)‐2‐(9‐julolidinyl)‐methylene‐1‐indanone (B), 9‐benzoyl‐2,3,6,7‐tetrahydro‐1H,5H‐benzo[i,j]‐furano‐[3,2‐g]quinolizine (C), 4‐(dimethylamino) chalcone (D) and a radical‐generating reagent, 2,4,6‐tris (trichloromethyl)‐1,3,5‐triazine (TCT), were investigated by laser flash photolysis using a total reflection cell. Weak fluorescence and strong broad triplet absorption were detected. The fluorescence was statically quenched by TCT at quenching distances (R_f) of 15, 14, 20 and 14 Å for A, B, C and D as well as the triplet initial absorption, at quenching distances (R_t) of 16, 16, 16 and 14 for A, B, C and D, similar to the fluorescence quenching distances. The triplet decay time of the dyes was inefficiently quenched by TCT with the rate constants (k _q) of 1.9, 3.1, 0.7 and 1.0×10⁵ mol⁻¹/dm³/s for A, B, C and D. The sensitivity of photopolymers containing a sensitizer dye and a TCT was obtained at an excitation of 488 nm corresponding to the emission peaks of argon ion laser of 1.1, 0.2, 0.54 and 9.1 mJ cm² for A, B, C and D. The results indicated that the static sensitization process from the fluorescent singlet excited state of the dyes to the ground state of TCT was predominant, and the high sensitivity for A and B was caused by the high absorbance at 488 nm and that for C by the high fluorescent quenching distance. Copyright © 1999 John Wiley & Sons, Ltd.     

Photochemical generation of triplet—triplet nitrene pairs in aromatic diazide crystals

The molecular and crystal structures of 4-amino-2,6-diazido-3,5-dichloropyridine and 6-amino-2,4-diazido-1,3,5-triazine, as well as the paramagnetic photolysis products of their crystals at 77 K, were studied using X-ray diffraction analysis and ESR spectroscopy. Triplet nitrenes generated during the photolysis of diazidopyridine form triplet—triplet nitrene pairs, whose ESR spectrum corresponds to the quintet spin state. The high-spin state (S = 2) results from the exchange interaction between two triplet molecules with the zero-field splitting parameters |D| = 1.0280 cm⁻¹ and |E| = 0.0038 cm⁻¹ and the γ angle between two C—N nitrene bonds equal to 133°. This angle is close to an angle of 136.2° between the C-N bonds of two adjacent molecules in the crystal structure. No formation of the triplet—triplet nitrene pairs is observed during the photolysis of crystalline diazidotriazine, whose molecules lie in the parallel planes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 513–520, March, 2008.     

Conjugated Biradical Intermediates: Spectroscopic,Kinetic, and Trapping Studies of 2,2-Dimethyl-1,3-perinaphthadiyl

The biradical 2,2-dimethyl-1,3-perinaphthadiyl ( a ) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2 , and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low-temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (³ a ) and rather long-lived (400 μs) at room temperature. When formed in its lowest singlet state (¹ a ), the biradical is too short-lived to undergo intersystem crossing to ³ a or bimolecular reactions. Thus, ³ a is formed exclusively from the excited triplet state of the precursor compounds, ³ 1 * or ³ 3 *. The monomolecular decay of ³ a is retarded by the spin barrier; ³ a initiates the polymerization of acrylonitrile and is trapped by ³O₂.     

Electronic structure and spin bonds in the [(NH3)5Cr-O-Cr(NH3)5]4+ complex

The electronic structure of the lowest singlet, triplet, quintet, and septet states of the binuclear complex [Cr(NH₃)₅]₂O⁴⁺ are calculated by the semiempirical MC SCF method in the INDO approximation. Schemes of spin coupling of Cr-O-Cr in these states are analyzed. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 3, pp. 457–464, May–June, 1997.     

Laser flash photolysis studies on radical formation by H-atom abstraction of triplet vitamin K3 and by H-atom transfer via the triplet exciplex between triplet methyldihydroxynaphthalene and benzophenone

Nanosecond laser photolysis techniques were incorporated to obtain (1) the absorption spectra and coefficients of triplet vitamin K₃ (2-methyl-1,4-naphthoquinone, MNQ) and its ketyl radical (2-methylnaphthosemiquinone, 2MNQH^*) in acetonitrile (ACN) as well as to reveal (2) the mechanisms for hydrogen atom abstraction of triplet MNQ (³MNQ^*) from phenol which proceeded in a diffusion process with an efficiency of unity. On the other hand, the hydroxymethylnaphthoxy radical was produced with the benzophenone ketyl radical (BPK) by the hydrogen atom transfer from triplet 2-methyl-1,4-dihydroxynaphthalene (MDHNp) sensitized by triplet benzophenone to benzophenone (BP) via the triplet exciplex. The question to be addressed was, which was produced in the MDHNp-BP system, the 2-methyl or 3-methylnaphthosemiquinone radical? Comparing the absorption spectrum and coefficient of the radical produced via the triplet exciplex with those of the 2MNQH^* obtained by H-atom abstraction of ³MNQ^*, the radical formed with BPK was revealed to be 2MNQH^*. The reasons for the preferable formation of 2MNQH^* are discussed for H-atom abstraction as well as the transfer reactions.     

Triplet states of humic acids studied by laser flash photolysis using different excitation wavelengths

The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (T_HFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of T_HFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of T_HFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of T_HFA quenching by molecular oxygen are equal to (7—8)·10⁸ L mol^–1 s^–1.     

PHOTOLYSIS MECHANISM OF AQUEOUS TYROSINE AND TYROSYL PEPTIDES*

Abstract— Laser flash photolysis at 265 nm has been employed for measuring the initial hydrated electron (e^-_aq) and p-alanylphenoxyl radical (Tyr) in aqueous Tyr, small Tyr peptides and R Nase A. The results indicate that monophotonic photolysis not involving the fluorescent or triplet states is the principal initial process. Equivalent yields of e and Tyr were found in all cases except Tyr, where the Tyr yield was 60% higher than e^-_aq attributed to splitting of the phenolic bond. Computer analysis of e^-_aq and Tyr decays for Tyr indicates the importance of electron-radical recombination in competition with electron scavenging and bimolecular radical-radical reactions. Evidence for intramolecular electron migration has been obtained in cystinyl-bis-Tyr.     

Photochemical synthesis and properties of quintet pyridyl-2,6-dinitrenes

The structures and properties of quintet pyridyl-2,6-dinitrenes formed by low-temperature photolysis of 2,6-diazidopyridines were studied by ESR and quantum chemical (B3LYP/6-31G^*) methods. The synthesized dinitrenes represent a new type of quintet tetraradicals for which the ratio of zero-field splitting parameters is | E _q|/|D _q| = 1/3. This ratio is characteristic of quintet molecules in which the vector angle between two biradical centers is close to 114.5°. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1536–1540, September, 2006.     

Methylene radical reaction with cis-butene-2

A study of the pressure dependence of the C₅ products from the reaction of cis-butene-2 and methylene is reported. Methylene was produced by the photolysis of diazomethane with 4358 Å light at 23° or 56°, and by photolysis of ketene with 3200 Å radiation at 23° or 100°. The change with increasing pressure of the relative amounts of the characteristically “triplet products” (trans-1,2-dimethylcyclopropane, trans-pentene-2 (TP2), and 3-methylbutene-1 (3MB1)) and “singlet products” (cis-1,2-dimethylcyclopropane (CDMC) and cis-pentene-2 (CP2)) are discussed. The behavior is reminiscent of that found in ³CH₂-cis-butene-2 systems and can be interpreted in terms of the rapid rate of rearrangement of an initial triplet diradical product component, due to ³CH₂, relative to the slower rate and readier collisional stabilization of an initial vibrationally-excited dimethyl cyclopropane product component, due to ¹CH₂. Relative rates of reactions of ¹CH₂ with allylic CH:vinyl CH:C?C in the neat liquid were, for diazomethane, 1:1.1:7.2 and, for ketene, 1:1.2:6.7.     

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment

Low‐temperature UV‐photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X‐band EPR spectrum of triplet MesP and the derived zero‐field splitting parameter D =4.116 cm⁻¹ differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations.     

High-spin organic molecules with dominant spin-orbit contribution and unprecedentedly large magnetic anisotropy

High-spin organic molecules with dominant spin-orbit contribution to magnetic anisotropy are reported. Quintet 4-azido-3,5-dibromopyridyl-2,6-dinitrene (Q-1), quintet 2-azido-3,5-dibromopyridyl-4,6-dinitrene (Q-2), and septet 3,5-dibromopyridyl-2,4,6-trinitrene (S-1) were generated in solid argon matrices by ultraviolet irradiation of 2,4,6-triazido-3,5-dibromopyridine. The zero-field splitting (ZFS) parameters, derived from electron spin resonance spectra, show unprecedentedly large magnitudes of the parameters D: ∣D(Q1)∣ = 0.289, ∣D(Q2)∣ = 0.373, and ∣D(S1)∣ = 0.297 cm(-1). The experimental ZFS parameters were successfully reproduced by density functional theory calculations, confirming that magnetic anisotropy of high-spin organic molecules can considerably be enhanced by the "heavy atom effect." In bromine-containing high-spin nitrenes, the spin-orbit term is dominant and governs both the magnitude and the sign of magnetic anisotropy. The largest negative value of D among septet trinitrenes is predicted for 1,3,5-trinitrenobenzene bearing three heavy atoms (Br) in positions 2, 4, and 6 of the benzene ring.     

Triplet excited states of nitronaphthalenes. III. 1,4-dinitronaphthalene

Nanosecond flash photolysis of 1,4-dinitronaphthalene (1,4-DNO₂N) in aerated and deaerated solvents shows a transient species with absorption maximum at 545 nm. The maximum of the transient absorption is independent of solvent polarity and its lifetime seems to be a function of the hydrogen donor efficiency of the solvent. The transient absorption is attributed to the lowest excited triplet state of 1,4-DNO₂N. The reactivity of this state for hydrogen abstraction from tributyl tin hydride (Bu₃SnH), K_q = 3.8 × 10⁸M^?1 sec, is almost equal to that of nitrobezene triplet state which has been characterized as an n → π* state. Based on spectroscopic and kinetic evidence obtained in the present work, the triplet state of 1,4-DNO₂N behaves as an n → π* state in nonpolar solvents, while in polar solvents the state is predominantly n → π* with a small amount of intramolecular charge transfer character.