14.2 Absorption Spectra of Alkenes and Aromatics

• spectral data for linear polyenes • spectral data for linear, fused aromatics • spectral data for non-linear, fused aromatics • spectral data for linear polyphenyls • quite often the absorption spectrum of a new compound can be estimated by comparison to known analogs

14.2 : 1/8 Linear Polyenes

CH CH n -1 -1 n name λmax (nm) εmax (M cm ) 1 ethylene 163 ? 2 butadiene 217 21,000 3 hexatriene 268 35,000 4 octatetraene 304 ? 5 decapentaene 328 120,000

• as the number of double bonds increases, the long wavelength absorption shifts to higher values (called a red-shift) • the molar absorptivity increases as the molecular orbital size increases • to anticipate the spectrum, use the number of conjugated double

bonds, i.e. CH2=CH-CH2-CH=CH2 has a spectrum closer to ethylene than butadiene.

14.2 : 2/8 Linear Fused Aromatics (1)

-1 -1 structure name λmax (nm) εmax (M cm )

benzene 255 220

naphthalene 315 320

anthracene 357 10,000

tetracene 471 10,000

• as the number of fused rings increases, the long wavelength absorption shifts to higher values • the long wavelength transition is forbidden in benzene and naphthalene, but allowed in anthracene and tetracene • to anticipate the spectrum use the number of conjugated double bonds, i.e. diphenylmethane has a spectrum that resembles toluene 14.2 : 3/8 Linear Fused Aromatics (2)

14.2 : 4/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951. Non-Linear Fused Aromatics (1)

-1 -1 structure name 0-0 band (nm) ε0-0 (M cm ) 3,4-benzo 370 170 phenanthrene

chrysene 360 800

pyrene 370 120

perylene 437 37,000

• the 0-0 band appears at lower wavelengths than would be predicted by the number of fused rings (379 for anthracene and 479 for tetracene) • the first three have band positions similar to anthracene and molar absorptivities similar to naphthalene • perylene has properties between anthracene and tetracene

14.2 : 5/8 Non-Linear Fused Aromatics (2)

14.2 : 6/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951. Linear Polyphenyls (1)

-1 -1 n name 0-0 band (nm) λmax(nm) εmax (M cm ) 1 benzene 264 255 220 2 biphenyl 288 248 1,600 3 p-terphenyl 320 276 3,300 4 p-quaterphenyl 340 294 4,000

• as the number of conjugated rings increases, the 0-0 band shifts to higher wavelengths • the increase in wavelength is not as fast as the polyenes or linear aromatics because of the bond between the rings is twisted • the spectrum is featureless because thermally induced oscillation about the twist angle adds width to the vibronic bands • the molar absorptivity increases because the number of double bonds is increasing

14.2 : 7/8 Linear Polyphenyls (2)

14.2 : 8/8 R. A. Freidel and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, Wiley, New York, 1951.