Excited state Stem-Volmer plot

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

A different explanation was identified in the quenching properties of these heterocycles. Thiophene and mono methyl derivatives are efficient quenchers of triplet benzophenone. The Stem-Volmer plot showed a linear relationship [104, 105]. On the contrary, 2,5-dimethylthiophene (a compound able to give the cycloaddition reaction) is not a good quencher of benzophene [106]. N-Benzoylpyrrole also does not act as a quencher of the triplet benzophenone [106]. On the contrary, pyrrole and selenophene are quenchers of the excited benzophenone [106]. In this case, the Stern-Volmer plot is not linear. This situation is commonly encountered when the quencher employed quenches two excited states. It seems reasonable that pyrrole acts as quencher of both triplet benzophenone and the exciplex between triplet benzophenone and pyrrole [106]. 

Stark-Einstein law, 4 Stem-Volmer plot, 34 slilbene, absorption spectrum, 1 3 cis-trans isomerization, 42 cvclization, 97 excited state energies, 17 styrenes, addition reactions, 58... 

The vibrational energy levels of the B rio electronic state of I2 were studied by absorption spectroscopy in Exp. 39. In the present experiment, selected vibrational-rotational levels of this state will be populated using a pulsed laser. The fluorescence decay of these levels will be measured to determine the lifetime of excited iodine and to see the effect of fluorescence quenching caused by collisions with unexcited I2 molecules and with other molecules. In addition to giving experience with fast lifetime measurements, the experiment will illustrate a Stem-Volmer plot and the determination of quenching cross-sections for iodine. Student results for different quenching molecules will be pooled and the dependence of the cross sections on the molecular properties of the collision parmers will be compared with predictions of two simple models. 

In fact, quenching effects can be evaluated and linearized through classic Stem-Volmer plots. Rate constants responsible for dechlorination, decay of triplets, and quenching can be estimated according to a proposed mechanism. A Stern-Volmer analysis of photochemical kinetics postulates that a reaction mechanism involves a competition between unimolecular decay of pollutant in the excited state, D, and a bimolecular quenching reaction involving D and the quencher, Q (Turro N.J.. 1978). The kinetics are modeled with the steady-state approximation, where the excited intermediate is assumed to exist at a steady-state concentration ... 

Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both yobs and fcobs- The typical Stem-Volmer plot is no longer represented by Equation (30.7a), but rather by Equation (30.7b), where obs is defined by Equation (30.6b), fcq is the bimolecular quenching rate constant, ko is the probe s mean excited-state unimolecular decay rate constant, fcobs is the mean observed decay rate constant, yo is the distribution parameter of the Gaussian for the unimolecular decay, and yobs is the distribution parameter for the observed unimolecular decay rate. 

Electron-transfer quenching of the photoexcited Ru complex by has been investigated in a polysiloxane film containing dispersed Ru(bpy)3 and MV, [86] (see Section 13.2.3.4). The Stem-Volmer plots for the relative emission quantum yield (equal to /(//) and the relative lifetime of the excited state (Tq/t) against concentration showed that the electron-transfer... 

From the Stem-Volmer plots, Matsuzaki and Nagakura found the collision-free lifetime to become shorter in the presence of a magnetic field and they explained the reduction in fluorescence lifetime and the decrease in fluorescence intensity by an enhancement of ISC (due to the perturbation of the excited singlet state by triplet states). The influence of the magnetic field on the emission spectrum, which consists of a banded structure and a continuum, has not been examined and hence the magnetic effect on the continuum emission is unknown. 

The photophysics of the quenching of the excited states of trans-[Cr(en)2(NCS)F] ion, by [Cr(CN)6] or [Cr(ox)3] ions, has been investigated using Stem-Volmer plots. Two quartet states are shown to be involved in addition to the doublet state.The concentration dependence of the quenching of excited states of [CvLsY ions (L = bipy or phen) has also been investigated by laser flash photolysis. 

Berger et al. (1973) reported that process (I) forming benzene and CO is important in photolyses at 276 and 284 nm. Their data for quantum yields of benzaldehyde loss were near equal to those for benzene and carbon monoxide formation. At low pressure, was very near unity. However, even at these short wavelengths, quenching of the excited states was significant, and quenching is expected to be important for photolyses in air at atmospheric pressure. See figure IX-L-5. Near linearity in the Stem-Volmer plots of versus pressure of benzaldehyde can be seen for the experiments at 276... 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

As with other quantitative photochemical studies, it is important to design Stem-Volmer experiments carefully the quencher should be chosen to ensure that it interacts only with the excited state that is of interest, the extent of reaction should be small enough to ensure that substrate depletion does not affect the intensity of light absorbed, and the concentration of quencher should not be so small that it is significantly depleted by its sensitized reaction. Stern-Volmer plots may turn out to be non-linear, for example because the quencher interacts with more than one excited state on the reaction pathway, or because two different excited states lead to the same chemical product, and such results are of value in unravelling the mechanism. 

---