Triplet ground state laser flash photolysis

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Nanosecond laser flash photolysis showed that pyrrole-2-carboxyaldehyde does not exhibit fluorescence emission, but undergoes intersystem crossing to the triplet state with an efficiency of 0.80 in benzene <2003PPS418>. 3,5-Dimethyl-2-(2 -pyridyl)pyrrole forms weak 1 1 H-bonded complexes with methanol and /r-ry-butanol in the ground state, while 3,5-di-fet7-butyl-2-(2 -pyridyl)pyrrole forms both 1 1 and 1 2 complexes with the same alcohols, but no excited state proton transfer appears to occur in such complexes <2004MI3948>. 

The mechanistic details of this reaction have been examined by several workers. Absorption of a photon produces the enamine excited singlet state EA, which undergoes inter system crossing to the excited triplet state EA. Cyclization leads to an excited zwitterion ZW, which relaxes to the zwitterionic ground state (Scheme 8) . A suprafacial hydride shift produces the observed product. The presence of the zwitterionic intermediate has been detected by laser flash photolysis . In studies on the related sulfide ring closure, the zwitterion was trapped by a dipolarophile ... 

On the basis of product studies, it is clear that irradiation of the naphthyl azides leads to loss of nitrogen with the likely consequent formation of nitrenes. Just as for phenyl azide, the initially formed singlet nitrenes may intersystem cross to the triplet and then dimerize to azo compounds. Clearly in the case of 2-naphthyl azide, but not 1-naphthyl azide, a closed-shell ground-state intermediate that can be trapped with diethylamine can be generated. The intermediate was formulated as the azirine on the basis of product studies [57]. Low temperature absorption spectroscopy and time-resolved laser flash photolysis experiments to be described later support the formation of azirines and provide an explanation for the different reactivity observed between the 1- and 2-substituted azides. 

The brominated diaryldiazomethane (7) was found to be stable enough to survive Suzuki coupling, thus allowing the synthesis of mono-, bis- and tris(diazo) compounds, e.g. (8). The photoproducts of these diazo compounds were characterized by EPR and SQUID measurements, which indicated that triplet, quintet and septet ground-state species were generated from the mono-, bis- and tris(diazo) precursors, respectively. Temperature-dependent EPR and UV-visible measurements and also laser flash photolysis studies showed that all three species are stable up to 160 K, and have half-lives of a few seconds in solution at room temperature. 

The effed of thienyl groups on the photoisomerization and rotamerism of symmetric and asymmetric stUbenes has been studied [73]. Stationary and pulsed fluorimetric techniques, laser flash photolysis, and conventional photochemical methods and theoretical calculations were used for investigating photochemistry of flve symmetric (bis-substituted) and asymmetric (mono-substituted) analogues of -stilbene, where one or both side aryls are 2 -thienyl or 3 -thienyl groups. It was shown that the presence of one or two thienyl groups and their positional isomerism afled the spectral behavior, the relaxation properties (radiative/reactive competition), the photoisomerization mechanism (singlet/triplet), and the ground-state rotamerism. 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

The effect of substituents placed para to the nitrene center was investigated in a series of matrix isolation, laser flash photolysis, and product studies. Table 44.1 Ksts a number of rate constants (k,sc for intersystem crossing to the triplet ground-state and for rearrangement to the didehydroazepine). For comparison, the kinetic parameters of several other singlet aryl nitrenes are also given. 

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