Benzene, absorption spectrum fluorescence

Figure 4.2 Absorption spectrum (continuous line) and fluorescence spectrum (dashed line) of anthracene in benzene...

Absorption maxima of the open-ring form in benzene, THF, and acetonitrile were observed in the wavelengths ranging from 335 to 340 nm. Although the maximum showed a small hypsochromic shift in hexane, the solvent shift in the absorption spectrum was rather small. On the other hand, the fluorescence spectra showed remarkable Stokes shifts depending on the solvent polarity. The maximum at 488 nm in hexane shifted to 560 nm in THF. At the same time, the intensity decreased. The fluorescence intensity in acetonitrile was <1% of the intensity in hexane. The results indicate that the excited state of the open-ring form has a polar structure with a large dipole moment. 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

Figure 5.10. Absorption and fluorescence spectrum of perylene in benzene (by permission from Lakowicz, 1983).

Although the valence ji-ji excitation spectra of benzene derivatives have been extensively studied over the past 65 years both experimentally and theoretically, much less is known about that of phenol, apart from its lowest excited state. In general, absorption and fluorescence spectroscopy of a benzene ring can be used to detect its presence in a larger compound and to probe its environment. While the relative constancy of the valence jt-n excitation spectrum allows a qualitative identification of spectral bands by a correspondence with those in free benzene, detailed quantitative differences could indicate the nature of substituents, ligands or medium. Key information on substituted benzene includes the excitation energies, transition moments and their direction, and electrostatic... 

An observation of the triplet state of anthracene in benzene (27) is of special interest in that, in addition to the absorption spectrum of the triplet, a delayed fluorescence emission from the singlet at 4300 A. was recorded. The intensity of fluorescence, which was approximately proportional to the square of the triplet concentration at all times, was attributed to triplet-triplet quenching ... 

Monoclinic plates from ale. Sublimes. When pure, colorless with violet fluoresence when impure (due to tetracene, naphthacene), yellow with green fluorescence. Strongly triboluminescent and triboelectric. dj 1.25. mp 218. bp,M 342. Absorption spectrum Clar, Ber. 65, 506(1932). Less soluble than the isomeric phenanthrene. Insol in water one gram dissolves in 67 ml abs alcohol, 70 ml methanol, 62 ml benzene, 85 ml chloroform, 200 ml ether, 31 ml carbon disulfide, 86 ml carbon tetrachloride, 125 ml toluene. Anthracene darkens in sunlight. According to Downs, U.S. pat. 1,303.639 (1919), when solns of crude anthracene in coal tar naphtha are exposed to ultraviolet irradiation, the anthracene is precipitated as dianthracene (para-anthracene) which is reconverted to anthracene by sublimation. 

Orthorhombic needles from benzene + petr ether, mp 155. Absorption spectrum Martin, Ann. Combustibles Lit. 12, 967 (1937). Sol in most organic solvents without fluorescence. Any fluorescence present is due to anthrano). Tendency to change to anthraquinone. Equilibrium in abs ale 89% anthrone ] 1% anthrano]. 

Pale yellow needles from alcohol or xylene, mp 170°. Absorption spectrum Clar, Ber. 65, 846 (1932). Solubility at 20° = 0.52 g in 100 g glacial acetic acid 1.61 g in 100 g benzene 2.05 g in 100 g chlorobenzene. Solution in HjS04 is orange with green fluorescence. 

Long orange needles with 1 HjO from dil alcohol, anhydr at 100. Anhydr red needles from abs alcohol or by sublimation around 150 in high vacuum (less than 2 mm Hg). mp 257. Absorption spectrum Meek, J. Chem. Soc. Ill, 969 (1917) Ezaby, ibid. (B) 1970, 1293. More sol in boiling water than alizarin (yellow color with yellowish hue). Freely sol in alcohol (red), in ether (intensely yellow with fluorescence). Soluble in benzene, toluene, xylene (dark yellow), in boiling alum soln (red). 

Orange crystals from acetic acid, mp 200-203 (Org. Syn.. Orange plates from ether. Deep red needles from alcohol, benzene, toluene, xylene, mp 196. Sublimes in high vacuum. Absorption spectrum Meek, Watson, J. Chem. Soc. 1 9,544 (1916) Meek, Ibid 111,969 (1917). K at 18" = 3.1 x 10-,°. Moderately sol in alcohol with red color. Sol in ether with brown color and yellow fluorescence. Sol with violet color in aq alkalies and in ammonia. Black precipitate with CO,. One gram dissolves in about 13 g of boiling glacial acetic acid. 

One can see even in the low-resolution spectrum of Fig. 2 that the 2600 A absorption spectrum of benzene is unusually discrete for a molecule of such size. Further examination at high resolution reveals many regions in the spectrum where vibronic bands are not seriously overlapped by others. This sets the stage for obtaining a unique set of resonance fluorescence spectra. By pumping one of these bands with tuned monochromatic light, molecules can be prepared in the specific vibronic level reached... 

In the case of the dicyanodiphenyltriafulvene (VI), the dipole moment of the compound in the ground state is 7.9D (dioxane solution, 30°C) [14]. Application of the Lippert-Mataga approach, with the assumption of a cavity radius of 4 A, leads to a value of ID for the dipole moment in the excited state. This is of course a very crude estimation. Too much reliance should not be placed on the quantitative meaning of this result [31]. In any event, the blue shift in the absorption spectrum and the red shift in the fluorescence spectrum on going to more polar solvents or from cyclohexane to benzene, support the substantive decline of the dipole moment in the transition from the ground state to the first excited state. 

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