Naphthalene singlet

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

The product e N G (t) is the decay function for the monomer naphthalene singlet state where kj is the unperturbed naphthalene lifetime and Cjj(t) is a general, non-exponential function that describes the naphthalene fluorescence decay In the presence of X mole fraction of mergy accepter. Using our multlexponentlal... 

Calculate the concentration required to quench 99% of singlet excited naphthalene, given that the lifetime of the naphthalene singlet state is 100 ns in the absence of dioxygen and A. /V-di methyl aniline and that the rate constant of diffusion /cdill is 5 x 109m 1 s [0.2m]... 

The light-induced reactions of fluoro-, chloro-, and bromo-naphthalenes have been studied in the nucleophilic solvents methanol and diethylamine.83 The fluoro-derivative yields substituted products from the naphthalene singlet state, but the other halogenonaphthalenes react from the Tx state to give mainly the radical products naphthalene and binaphthyl, although some substitution is observed. Attempts to quench the reactions led to anomalous behaviour in that the quantum yields were actually increased in the presence of several potential triplet quenchers.84 85 Aryl iodides have been reported to react rapidly with potassium dialkyl phosphates in liquid ammonia when irradiated with 350 nm wavelength light.86 Dialkyl arylphosphonates are formed in 87—96% yield... 

Oxidative repair is not a unique feature of our Rh(III) complexes. We also demonstrated efficient long-range repair using a covalently tethered naphthalene diimide intercalator (li /0 1.9 V vs NHE) [151]. An intercalated ethidium derivative was ineffective at dimer repair, consistent with the fact that the reduction potential of Et is significantly below the potential of the dimer. Thymine dimer repair by a series of anthraquinone derivatives was also evaluated [151]. Despite the fact that the excited triplets are of sufficient potential to oxidize the thymine dimer ( 3 -/0 1.9 V vs NHE), the anthraquinone derivatives were unable to effect repair [152]. We attribute the lack of repair by these anthraquinone derivatives to their particularly short-lived singlet states anthraquinone derivatives that do not rapidly interconvert to the excited triplet state are indeed effective at thymine dimer repair [151]. These observations suggest that interaction of the dimer with the singlet state may be essential for repair. 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

They observed a constant quantum yield of fluorescence (Or = 0.3) for all members of the series independent of whether the anthracene moiety absorbed and emitted the energy or the naphthalene moiety absorbed the energy and transferred it to the anthracene moiety. Thus at these short distances singlet energy transfer is 100% efficient. 

Figure 6.11. Electronic transitions to the first excited singlet (s) and lowest triplet (0 states from the ground states (g) of benzophenone (B) and naphthalene (N) moieties in compounds (4), n = 1-3. Possible radiative transitions are represented by straight arrows, radiationless transitions by wavy arrows.(80> Reprinted by permission of the American Chemical Society.

A singlet naphthalene or a singlet exciplex is thought to be the reactive species in this reaction since the quantum yield of cycloaddition parallels the quenching of naphthalene fluorescence by acrylonitrile. 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

The benzo-derivative 16 is accessible through 1,4-dimethylnaphthalene, which on singlet oxygenation leads to the thermally labile naphthalene-1,4-endoperoxide. This endoperoxide expels singlet oxygen at ca. 10 °C, but diimide reduction below 0 °C in MeOH affords the stable dihydro derivative 16 (Eq. 11). 

Almost all the reported compounds have been characterized with the help of various nuclear magnetic resonance (NMR) techniques. For previous studies of the compounds, refer to CHEC-II(1996) <1996CHEC-II(8)713>.The H NMR spectrum (300MHz) of 2,3,7-trirnethyl-3a,9a-dihydro-1,8-dithiaMa,5,9-triazacyclopenta[3]naphthalene-4,6-dione 47 <2000JHC1161> showed the presence of one quartet at 8 4.23 corresponding to the CH. Another broad singlet corresponds to the presence of the N-H proton. 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Molecular structure can have a profound effect on the position in the spectrum where fluorescence occurs, as well as on its intensity. It can be shown by quantum mechanics that the more extended a conjugated system is, the smaller will be the separation in energy between the ground state and the lowest excited singlet state. This is evident in the fact that benzene, naphthalene, and anthracene, having one, two, and three rings, fluoresce maximally at 262 nm, 320 nm, and 379 nm, respectively. 

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