Excited state Flash photolysis

Electron density calculations are less successful in accounting tor the reactions of benzenes with substituents such as methoxy, and there is strong evidence with these for a different pathway that involves ejection of an electron to form a radical cation (3.7) this is in keeping with the greatly enhanced electron-donor properties of an excited state. Flash photolysis studies support therormation of radical cations for methoxybenzenes on irradiation, and solvated electrons have also been detected in scavenging experiments. Subsequent attack by the nucleophile on the radical cation can then be rationalized by calculations based on this species rather than on the excited state. 

Free radicals or excited states Flash photolysis (Flashlamp (Laser... 

In fact, it is well known that Cr(lll) polypyridyl complexes are reduced to the labile Cr(ll) species upon irradiation with 351 nm laser, in agreement with their strong oxidant character in the excited state. Flash-photolysis experiments showed transient decay constants in the ps and sub-ps timescale that could be quenched by both, oxidants ([Ru (NH3)6]) and reductants (F, triethanolamine and alcohols) [39]. A very complex photophysic and photochemical behavior were described. 

Primary photochemical processes also may yield vibrationally excited products. Flash photolysis of cyanogen, cyanogen bromide and cyanogen iodide in each case yields a product whose absorption spectrum identifies it as a vibrationally hot CN radical with up to 6 quanta of excitation. VibrationaUy excited nitric oxide in the ground electronic state (detected by the A absorption system) is... 

CT term is lowest in energy followed by a LF term lying about 200 cm above. Orbital schemes are proposed for both compounds.The temperature dependence of the luminescence of the hexanuclear Mo(II) chloride cluster [Mo Cl, ] has been investigated over the rauige 1.4-300 K and analysed in terms of the emission from several Boltzmeuin populated triplet sublevels. The lowest triplet state was identified as t, and is due to the tjg to t y orbital excitation.A flash photolysis study of the quenching of [Mo Cl, ] by [IrCl ] shows it to occur by a one electron transfer process from the cluster medium euid pressure effects have been noted.An investigation of the... 

Porter G and Topp M R 1968 Nanosecond flash photolysis and the absorption spectra of excited singlet states Nature 220 1228-9... 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

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... 

This brings us to U(VI) as an electron acceptor in MMCT transitions. A few examples were mentioned above already. Krol et al. have shown and used the U(V)-U(VI) MMCT transition in oxygen-deficient uranates [78]. It is situated in the near-infrared. This transition plays an important role in the quenching of the luminescence of uranates. The luminescence of U(VI)02 in solution is quenched by Mn(II) and it has been shown by flash photolysis that this is due to an excited MMCT state Mn(III)U(V) [79]. 

In the flash photolysis technique a large population of ground state molecules are raised to an excited singlet state by the initial photolysis flash. In a time r (singlet lifetime) after the photolysis flash a certain proportion of... 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

For the investigation of triplet state properties a laser flash photolysis apparatus was used. The excitation source was a Lambda Physik 1 M 50A nitrogen laser which furnished pulses of 3.5 ns half-width and 2 mJ energy. The fluorescence decay times were measured with the phase fluorimeter developed by Hauser et al. (11). 

Ikegami M, Arai T (2000) Laser flash photolysis study on hydrogen atom transfer of 2-(2-hydn>xyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole in the triplet excited state. Chem Lett 9 996-997... 

As with solution experiments, flash photolysis in the gas phase has produced evidence for the existence of intermediates but no information about their structure. In principle gas phase IR spectra can provide much more information, although the small rotational B value of gaseous carbonyls and low lying vibrational excited states preclude the observation of rotational fine structure. As described in Section II, time-resolved IR experiments in the gas phase do not suffer from problems of solvent absorption, but they do require very fast detection systems. This requirement arises because gas-kinetic reactions in the gas phase are usually one... 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

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