Phosphorescence naphthalene

Spectroscopic evidence has also been adduced for the ability of the anthracene T2 to transfer energy to other substances.239 Selective excitation of guest anthracene in a host dibenzofuran crystal also containing napthalene-d8 as a guest resulted in naphthalene phosphorescence. As shown in Figure 8, the path of the energy is S0 -> Sj excitation of anthracene, intersystem crossing... 

Triplet energy migration can be probed by addition of free quenchers to the polymer, and observing which kinetics, Stern-Volmer or Perrin (see Section II above), pertain. In the case of the quenching of naphthalene phosphorescence or delayed fluorescence in a variety of co-polymers by the dloleflns plperylene and cycloocta-1,3-diene at low temperature where material diffusion... 

Let us illustrate this chemistry with an example. Benzophenone is a well-known photosensitizer, which will phosphoresce at low temperature (77 K) after excitation in the range 360-370 run. This phosphorescence of benzophenone is quenched by adding a polyaromatic hydrocarbon (PAH) such as naphthalene phosphorescence is then observed from this species even though it has no absorption band around 360-370 nm. Such observations clearly describe the activation at wavelengths otherwise transparent for a given medium. This sequence of processes can be described as follows ... 

The excitation of benzophenone in solid solution at 77 K with light of wavelength 366 nm emits phosphorescence. When naphthalene is added to this solid solution, the benzophenone phosphorescene is replaced by naphthalene phosphorescence even naphthalene does not absorb photons from light of wavelength 366 nm. This quenching process takes place by formation of exciplex between excited benzophenone and ground-state naphthalene followed by triplet-triplet energy transfer and emission of phosphorescence from the triplet naphthalene [15]. 

Quenching poly(l-vinylnaphthalene) and 1-ethyl-naphthalene phosphorescence at 77 K as a function of piperylene molarity. From Cozzens and Fox (1). 

The nonequivalence of the rates of photosensitized reactions in heterogeneous nanophases of glassy polymers is proved in experiments with naphthalene phosphorescence decay. For example, it is shown [13] that in aerated PMMA films fluorescence of singlet-excited naphthalene molecules N can be decayed by tinuvin P, but this does not affect the rate of naphthalene dissociation. The latter is consumed in the process, the rate of which is not defined by the concentration of particles responsible for fluorescence. Under such conditions, according to definition by the authors [13], primary chemical acts are inevitable. However, in the absence of oxygen tinuvin P slows the photochemical process down in accordance with a decrease of singlet-excited naphthalene molecule concentration. 

The kj and terms were evaluated from Equation 2 and found to be roughly comparable to the values for naphthalene adsorbed on highly activated alumina (11,30). Generally, the smaller the and values, the more strongly the compound interacts with the surface (29,30). In the future, Equation 2 should be a useful means of comparing various surfaces for RTF in solid-surface phosphorescence analytical work. 

Some evidence for the process (AQ) - (AQ) when QsA, or for photoassociation of the triplet A + A - (AA)(, has been provided by Hoytink and co-workers,<1B8) who reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene. 

Unless otherwise specified, both qy and x refer to the same process, indicated by f for fluorescence and p for phosphorescence. For naphthalene, qy is for fluorescence and T is for phosphorescence. bNote heavy atom effect in phosphorescence. 

The biphenyl, naphthalene, pyrene, and triphenylene adducts display intense room-temperature phosphorescence.237 238 These observations indicate the occurrence of a mercury heavy atom effect, which promotes... 

Excitation of benzophenone in solid solution at 77K with light of wavelength 366nm produces phosphorescence. As naphthalene is added, the... 

Phosphorescence emission occurs from the triplet naphthalene 3Ci0Hs —> CioHg + hv... 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

Fig. 27. Semilogarithmic plot of the nonradiative triplet rate constant against (E— o)/> for the normal and deuterated hydrocarbons listed in Ref. t)). The broken line, derived from phosphorescence spectra, is taken from Ref. t). The slopes of the two solid lines differ by a factor 1.35. (O.Ci-jjH, E = 4000 cm l 0 Ci fl Z>u, =5500 cm t). The following totally deuterated hydrocarbons are included benzene, triphenylene, acenaphtene, naphthalene, phenanthrene, chrysene, biphenyl, p-terphenyl, pyrene, 1,2-benzanthracene, anthracene (in the order of increasing /S). (From Siebrand and Williams, Ref. l)...

Spectroscopic data of nitroaromatics have been reviewed D in addition, several papers on luminescence of nitroaromatic compounds have appeared recently. The phosphorescence polarization of several aromatic nitro compounds has been studied and recent triplet-triplet absorption data on 1- and 2-nitro-naphthalene have become available ). 

Naphthalene deriv. Typei) Phosphorescence (77 K) Ethanol 1 in Photosubstitution ... 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

Fig. 9. Decay of luminescence with time. Ordinate In (luminescence intensity) one division = 0.25. Abscissa time one division = 0.0003 sec. for curves (a) and (6) 0.0005 sec. for curve (c) 1.0 sec. for curve (d) and 0.1 sec. for curve (e). (a) and (6) Delayed fluorescence of pyrene monomer and dimer in ethanol at +23°C. (c) Delayed fluorescence of naphthalene in ethanol at —23°C. (d) Triplet-singlet phosphorescence of 10-W phenanthrene in EPA at 77°K. (e) Delayed fluorescence of 10-lAf phenanthrene in EPA at 77°K.

Substituents have considerable influence on emission characteristics of aromatic compounds. Heavy atom substituents tend to reduce the fluorescence quantum yield 4>f in favour of phosphorescence emission f. In halogen series the effect increases in the order F < Cl < Br < I. In Table 5.1 are recorded experimental data for halogen substituted Naphthalenes. 

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