Triplet state of naphthalene

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

Figure 6.3 Energy level diagram for the triplet state of naphthalene (D = 0.1003 cm-1, E= -0.0137 cm-1, g = 2.003). Solid lines correspond to orientation of the magnetic field along the z-axis, dashed lines for orientation along the x-axis. Arrows show the allowed transitions for 9.50 GHz microwave radiation.

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

The n-7i triplet of quinoxaline has been observed in a single crystal in durene, and the fine structure was found to be practically identical to that of the triplet state of naphthalene.254... 

By contrast, in the pulse radiolysis of naphthalene and xenon mixtures in the gaseous phase, the triplet state of naphthalene was produced almost instantaneously (< 0.5 /isec)109. This was attributed to energy transfer by direct impact of sub-excitation electrons. 

If two strongly associated aromatic molecules undergo charge-transfer to the D+A- state, one would expect some coupling between the electrons to give a triplet level, unless, subsequently, the ions separated. Thus if the complex existed as D+A-, a major doublet would be expected, comparable with that found for the Ams = 1 transitions for the triplet state of naphthalene (Hutchison and Mangum, 1958). 

Wagner and Sakamoto reported the first example of the intramolecular (2 + 2) photocycloaddition of simple alkenes to the excited triplet state of naphthalene [290], Cyclizations of 266 and 267 are entirely triplet-state reactions The formation of cycloadducts is sensitized by Michler s ketone and is quenched by conju-... 

FIGURE 3. Fine structure of the triplet state of naphthalene as a function of the external field Hp. Heavy lines H0 parallel to y-axis light lines H0 parallel to z-axis dotted arrows the aM = 2 transition. 

FIGURE 12. ESR spectrum of the triplet state of naphthalene showing the fine structure and the hyperfine splittings. From Varian Associates EPR at Work Series, No. 26. 

Polydimethylsiloxane in itself is very stable to near-UV light. Commercial products, however, become yellow and evolve gases on exposure to UV irradiation. This phenomenon seems to be due to photosensitized decomposition of polymers. In connection with this, Siegel et al. (53) carried out ESR studies on photosensitized decomposition of polydimethylsiloxane. When this polymer containing naphthalene was irradiated at -196° C with light of wavelengths longer than 240 nm, there were observed ESR spectra due to free radicals produced by decomposition of polymers and due to the photo-excited triplet state of naphthalene molecules. The ESR spectrum due to the free radicals observed immediately after irradiation is shown in Fig. 25a. Fig. 25b... 

Siegel and Judeikis [124] have shown that the photodegradation of polydimethylsiloxane is sensitized by naphthalene. A stepwise biphotonic absorption process, involving the first triplet state of naphthalene as an intermediate, followed by energy transfer, has been proposed. It seems that the decomposition of the polymer occurs via Si—CH3 bond rupture and abstraction of hydrogen atoms by the methyl radicals to produce —Si—CHj radicals. Breaking of the — Si—CH3 bond has also been proposed as the most probable reaction in the vacuum ultraviolet photolysis of polydimethylsiloxane at 123.6 and 147 nm [125]. 

For solutions of naphthalene in cyclohexane the yield of triplet states of naphthalene, the intensity of monomer and excimer fluorescence, and the yield of singlet state naphthalene have been measured as a function of naphthalene concentration by pulse radiolysis methods. The influence of a number of electron scavengers on these yields has been determined... 

Besides fluorescence spectroscopy, time-resolved spectroscopy can rely on the measurement of excited (singlet or triplet) state absorption. Similarly to ground-state absorption, the spectral and absorbance properties may be altered by CyD complexation and yield information about the behavior of the complex in the excited state in addition, the time dependence (formation and decay) of the excited state absorption yields information about the kinetics and dynamics of the system. This is illustrated by the behavior of the lowest triplet state of naphthalene as measured by nanosecond spectroscopy using a Q-switched Nd YAG laser at 266 nm for excitation [21]. The triplet-triplet absorption spectra were measured in neat solvents (water and ethanol) and in the presence of a- and -CyD (Fig. 10.3.3). The spectra in ethanol and H2O had the same absorption maximum, but the transition was considerably weaker and broadened in H2O. Both CyDs induced a red shift, and a-CyD additionally narrowed the main band considerably. Fig. 10.3.4 shows the effect of a-CD concentration on the time evolution of the triplet-triplet absorption at 416 nm in the microsecond range. Triplet decay was caused by O2 quenching a detailed kinetic analysis of the time dependence yielded two main components which could be assigned to the free guest and the 1 2 complex, in full... 

In their pioneering work on the lowest triplet state of naphthalene in a durene host crystal, Hutchison and Mangum (1958, 1961) found that the degeneracy of the three magnetic sublevels is lifted even in the absence of an external field and attributed this effect to a magnetic dipole-dipole interaction between the unpaired spins. This was the first measurement of the (microscopic) magnetic properties of a triplet state and marks the starting... 

Spin Hamiltonian Parameters of the Lowest Triplet States of Naphthalene and Its 1-Halo Derivatives (IXN) in Durene... 

In their EPR experiments on the lowest triplet state of naphthalene at liquid helium temperatures, Schwoerer and Wolf (1966) observed that one... 

Stemlicht H, McConnell HM (1960) Effect of deuterium substitution on the lifetime of the phosphorescent triplet state of naphthalene. J Chem Phys 33 302-303... 

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