Naphthalene triplet, spectrum

FIGURE 13. ESR spectrum of randomly oriented photoexcited naphthalene triplets showing the aM = 1 transitions. Taken from reference (283). 

A typical 9.4 GHz-ESR triplet spectrum of such a host-guest system is shown in Eig. 7.3. The host crystal was a perdeuterated naphthalene crystal, (N-dg), with 2%... 

The appearance of the sms = 2 transition as a single line at approximately half field of the centre of the Am = 1 powder spectrum can often be taken as evidence that the observed species is really a triplet state molecule. The intensity increases with increasing D as (D/ gfisB). As seen in Fig. 3.24 its amplitude is quite strong for the naphthalene triplet at X-band because of the small anisotropy of this feature, see Exercise E3.18. 

Fig. 3.24 Spectrum of the naphthalene triplet state computed with D = 1,074 G, E = -164 G, [C.A. Hutchison, B.W. Mangum, J. Chem. Phys. 34, 908 (1961)]. A diagonalization of the spin Hamiltonien matrix. B second order perturbation treatment of the spin Hamiltonian. Spectra are adapted from http //www.esr-spectsim-softw.fr/programs with permission from Dr. C. Chachaty...

These authors observed phosphorescence from 4,4-diphenylcyclohexadie-none at 77°K (0-0 band at 68.8 kcal per mole) and pointed out that the similarity between the phosphorescence spectrum of the dienone and that of benzophenone indicates that the lowest triplet state of the former compound is n — IT. This evidence suggests that a triplet state of the dienone may be involved in the unsensitized rearrangement but is not sufficient to justify Zimmerman and Swenton s conclusion that such a state is demanded. Interestingly, naphthalene (triplet 0-0 band at 61 kcal per mole) sensitizes rather than quenches Reaction (110). Such sensitization has also been observed by Caspar and Hammond (unpublished results) in a similar system and can be best interpreted as involving singlet excitation transfer from naphthalene to the dienone. 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

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. 

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. 22 B. Exeitation spectrum at room temperature showing the intensity of delayed fluorescence of a naphthalene crystal as a function of the wavelength of the exciting light. The ordinate is proportional to the square of the singlet-triplet absorption coefficient. (From Avakian and Abramson, Ref.52))...

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm"1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198... 

The observed quenching effects suggest that the excitation energy of the reactive triplet may be close to the 71 kcal Zimmerman estimated from the phosphorescence spectrum of 35. On the other hand, lumi-santonin, also an enone, must have a relatively low-energy reactive triplet, since its rearrangement is sensitized by Michler s ketone.401 From a Stern-Volmer plot of the effect of naphthalene on the quantum yield for photorearrangement of 35, kjkr was measured to equal 7000.416 Thus if naphthalene quenches triplet 35 at the diffusion-con-... 

The well-known photorearrangement of urtAo-nitrobenzaldehyde to orfAo-nitrosobenzoic acid458 may well involve hydrogen abstraction as the initial photoprocess. Nitrobenzene is photoreduced in reactive solvents to iV-phenylhydroxylamine,459 and the intermolecular hydrogen abstraction apparently proceeds from the triplet state, since, when perfluoronaphthalene is present in the sample, the ESR spectrum of the PhN02H radical is replaced by the typical triplet ESR spectrum of the naphthalene.460... 

The polarized phosphorescence spectra of 1,5-naphthyridine and its d6 isomer in durene and in durene-d14 mixed crystals have been obtained at 4°K. The lowest singlet state is at 27123 and 27200 cm-1, whereas the corresponding triplet state is at 23215 and 23288 cm 1 for the proto and deutero compounds, respectively. The phosphorescence lifetime in durene crystals is 0.23 sec.140 The polarized spectra (4°K) of 1,5-naphthyridine and its d6 isomer in naphthalene have also been examined. There is a difference between the spectra in naphthalene and in durene attributable to a decrease in the strong vibronic coupling that is considered to exist between the n- n state and the higher n -> n states.141 TheEPR spectrum of 1,6-naphthyridine in its lowest triplet state has been observed for solid solutions in single crystals of durene. The nuclear hyperfine structure allows an estimate of 0.14 to be made for the spin density on the nitrogen atom in the 1-position.142... 

This MCPA calculation has been carried out for the triplet naphthalene mixed crystal, with a square cell of nine sites (3 x 3). The density-of-states distribution for cA = 30% is shown in Fig. 4.14. One first notices that the smoothed shape disagrees generally with the numerical-simulation spectrum. The finest structure, at the maximum of the spectrum, corresponds to a configuration with a guest molecule A completely surrounded by host... 

Figure 16 shows the absorption spectrum obtained by additive-free polyethylene [67], At ambient temperature the absorption observed on nanosecond time-scale increased continuously from 500 to 200 nm without showing any maximum. The absorption in UV is similar to that obtained by y-irradiation. Considering the results obtained by liquid alkanes, the absorption seems to be comprised of several different free radicals. At 95 K additional absorption due to the trapped electron was observed at wavelengths longer than 600 nm the band was observable even at ambient temperature in the picosecond time-domain [96]. The electron decays presumably by the hole-electron recombination. The decay of the trapped electron was independent of the presence of carbon tetrachloride, suggesting that the additives reacted with a mobile electron but not with the trapped electron. On adding naphthalene, the radiation-induced spectrum showed the bands due to the first excited triplet state and the radical... 

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. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

One method for studying triplets was devised by Hutchinson and Mangum (1961) who obtained the spectrum of naphthalene in its triplet state by irradiating an oriented solid solution of naphthalene in a single crystal of durene. 

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