Benzene excimer

In a complementary study an interest in the mechanistic pathways which are operative in micellar media led to a characterization of the triplet pentachloro-benzene excimer. Pentachlorobenzene (1) was irradiated in 0.100 M CTAB (hexadecyltriethylammoniumbromide) aqueous solution at 254 nm. The major dechlorinated products were the tetrachlorobenzenes (2—4) with an accompanying trace of trichlorobenzenes. Bromotetrachlorobenzenes were observed as byproducts [19]. Building on our results in homogeneous solution, the mechanism for photohydrodehalogenation is represented in Scheme 3. Since... 

The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et al., 1972 Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). 

The transient absorption spectrum obtained from the pulse radiolysis of CMS solutions in benzene as shown in Fig. 2 (b) is very similar to the absorption spectrum obtained in a solid resist film as shown in Fig. 2 (a). However, the absorption in solution around 500 nm is mainly due to the complex of benzene with chlorine atoms whereas in the solid film, it is due to the complex of the phenyl ring of CMS with Cl. The absorption due to Pj is observed in both solid CMS film and CMS solutions in benzene. The absorption around 320 nm may be due to the biradical of benzene. The very short lived species with absorption around 500 nm is mainly due to the benzene excimer with a small contribution from the CMS excimer. Benzene solutions of CMS have proven to be a very good model system for reactions occurring in solid films of CMS. 

Figure 2. Inverse of transient absorbance vs. deiay time reiation of benzene excimer, excited at 266nm and observed at 505nm in neat liquid state. High excitation intensity for closed circles and lo w for open circles.

The intrinsic weakness of benzene excimer emission derives from its extreme dipole forbiddeness (7). Its weakness relative to monomer emission under dilute solution conditions additionally results from the diffusion limit upon excimer formation rates, along with thermally induced excimer dissociation. These latter considerations are largely irrelevant to polymers, vide infra. 

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. 

Excimer laser-assisted destruction of organic molecules is used to treat vapor-phase benzene, toluene, ethylbenzene, and xylenes (BTEX) produced from air stripping of contaminated groundwater. One apphcation of this technology has been to enhance the performance of high-temperature incineration of organic contaminants. 

At higher concentrations in solution, the photodimerization of tS has been studied by means of picosecond electronic absorption spectroscopy. The 5i state of tS in benzene at 22°C is quenched with a diffusion-controlled rate constant of 2.03 X lO M s to give a new reactive intermediate exhibiting an absorption maximum at 480 nm. This new species decays unimolecularly with a rate constant of (2.40 0.37) X 10 s. It has tentatively been assigned to either the excimer or a biradicaloid species located at the pericyclic minimum. 

In the case of benzene (point group D6h) excimer states arise from configuration interaction of the 8-fold degenerate CR states and exciton states of both 1La and 1Lb origin. Owing to the very large separation of 1La and 1Lb states in this molecule, the lowest exciton state is of 1Lb character and contributes to the lowest excimer state after configuration interaction.68... 

Simple MO considerations show that the dimer cation of an aromatic hydrocarbon M, with one less antibonding electron than the ground excimer configuration, should be stable with respect to its constituents M+ + M this is confirmed by esr129 and optical absorption8 studies of y-irradiated solutions of benzene, naphthalene, and anthracene in low temperature glasses. 

Direct evidence for photoassociation is limited to those systems that exhibit either excimer (or exciplex) luminescence or excimer absorption which has recently been observed in benzene following pulse radiolysis.141... 

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

In cyclohexane mainly excimer emission (cf. Figure 4). dIn benzene. 

Values of ke = 2.5 x 1010, 1.4 x 1010, and 1.2 x 1010 M-1s-1 are calculated using eqs. 7-9, respectively, and data from Table 1. These values are similar to the calculated rate of diffusion in benzene solution (2 x 1C)10 M ls-1), thus indicating that excimer formation (eq. 5) is essentially an irreversible process. 

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

The curved Stern-Volmer plot for the HH isomer suggests that it arises from both an excited singlet and an excited triplet state. This suggests that about 3, 1.5, and 3% respectively of the HH isomer is singlet derived in each solvent. The 360-nm irradiation of MeTND and benzophenone was explored in a solution of benzene, dibromomethane, and 1-bromobutane. The very efficient quenching of the HH isomer formation by COT implies that the majority of this isomer is formed from a triplet-state precursor. The unquenchable part of the HH isomer appears to be derived from singlet-state precursors, possibly singlet excimers. 

The protonated dendrimer exhibits a far more intense excimer band than corresponding Frechet dendrons without a cyclam core. One possible reason for this behaviour is that excimer formation is facilitated by folding of the - flexible - benzyl ether framework. Whereas a change in emission intensity is observed in the course of protonation of the cyclam dendrimer, a reference substance containing no cyclam and benzene moieties (Fig. 5.17) shows a linear increase... 

In 1977, Scharf and Mattay [123] found that benzene undergoes ortho as well as meta photocycloaddition with 2,2-dimethyl-1,3-dioxole and, subsequently, Leismann et al. [179,180] reported that they had observed exciplex fluorescence from solutions in acetonitrile of benzene with 2,2-dimethyl-l,3-dioxole, 2-methyl-l,3-dioxole, 1,3-dioxole, 1,4-dioxene, and (Z)-2,2,7,7-tetram-ethyl-3,6-dioxa-2,7-disilaoct-4-ene. The wavelength of maximum emission was around 390 nm. In cyclohexane, no exciplex emission could be detected. No obvious correlation could be found among the ionization potentials of the alkenes, the Stern-Volmer constants of quenching of benzene fluorescence, and the fluorescence emission energies of the exciplexes. Therefore, the observed exciplexes were characterized as weak exciplexes with dipole-dipole rather than charge-transfer stabilization. Such exciplexes have been designated as mixed excimers by Weller [181],... 

Finally, Goto et al.284 carried out a comparative fluorescence study on thin films of benzene-, biphenyl-, naphthalene-, and anthracene-bridged PMOs, thereby introducing new PMOs with 2,6-naphthalene (30) and 9,10-anthracene (31) organic bridges. The fluorescence spectra of the PMO films were significantly red-shifted compared with those of their precursor solutions, suggesting excimer formation. 

An important aspect of the photophysics of the Pt(diimine)(dithiolate) photochemistry that has received increasing attention is the ability of the excited-state complexes to undergo self-quenching. Initial work by Connick and Gray (111) showed that the lifetime of the complex Pt(bpy)(bdt) (bdt = benzene-1,2-dithiolate, 31) decreased with increasing solution concentration. The bimolecular self-quenching rate constant, calculated from a Stem-Volmer quenching analysis, was found to be 9.5 x 109 A/-1 s-1 in acetonitrile and 4 x 109 M 1 s 1 in chloroform. However, no evidence of excimer formation... 

A plot of the total quantum yield versus the concentration of pentachloro-benzene provides a very nice linear plot (r = 0.995) (Fig. 2), suggesting that (kT + k,d) > k2[l]. Thus, by extrapolation of [1] to zero, one can calculate the quantum yield independent of triplet excimer (siaglet + triplet)- Subtraction of the quantum yields for direct fission from singlet and triplet states ( si gIe, + triplet) from ToM provides the expression for the dependence of the remainder ( ) upon concentration (Eq. 7). A plot of the reciprocal (l/cx) versus the reciprocal of the concentration of the substrate is linear (r = 0.950), which is consistent with the mechanism and is illustrated in Fig. 3 [2]. 

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