Polyenes linear absorption

V-UV Application First Excited State of Linear Polyenes. The first electronic absorption band of perfect linear aromatic polyenes (CH)X, or perfect polyacetylene shifts to the red (to lower energies) as the molecule becomes longer, and the bond length alternation (BLA) would be zero. This was discussed as the free-electron molecular orbital theory (FEMO) in Section 3.3. If this particle-in-a-box analysis were correct, then as x > oo, the energy-level difference between ground and first excited state would go to zero. This does not happen, however first, because BLA V 0, next, because these linear polyenes do not remain linear, but are distorted from planarity and linearity for x > 6. 

Elizabe. L. Yeo. L. Harris. K.D.M. Sankar. G. Thomas. J.M. Conformational properties of guest molecules in constrained solid-state environments Bromine K-edge x-ray absorption spectroscopy of 2-bromoalkane/urea inclusion compounds. Chem. Mater. 1998, 10. 1220. Shang, Q. Y., Dou, X. Hudson, B.S. Off-axis orientation of the electronic transition moment for a linear conjugated polyene. Nature 1991, 352. 703. 

To understand the effects of additional states beyond the two-level model in centrosymmetric molecular materials, it is helpful to analyze symmetry conditions that dictate allowable transitions between states. Because the ground state (also called the lAg state) is of Ag symmetry, the lowest optically allowed one-photon transition is to the IBu state (of opposite ssmimetry), which is the lowest lying one-photon excited state in the linear absorption spectrum. The IBu state is not necessarily the lowest lying excited state, however. One fundamental difference between centrosymmetric polyenes and cyanines is the location of another excited state, the lowest lying two-photon state or 2Ag state. Studies show that the 2Ag state lies energetically below the linear absorption peak (IBu state) for polyenes, but above the peak for cyanines. Electron correlations that lower the... 

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

Ultraviolet/visible absorption, fluorescence, infrared and Raman spectroscopies are useful for studying structures (configuration, conformation, symmetry etc.) of electronically ground and excited states of linear polyenes, which have attracted much attention of... 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

A two-photon allowed absorption of poly(dihexylsilane) has been observed at 0.9 eV above the one-photon excitation63 -65, in contrast to the two-photon absorption of linear polyenes, which is about 0.5 eV below the intense one-photon absorption. Absorption spectra and the dynamics of other high-lying excited states of the polysilane have been investigated by means of femtosecond time-resolved excited-state absorption spectroscopy66. 

The vacuum UV absorption spectrum of 2-vinylfuran has been recorded and interpreted. The electronically excited molecule was found to behave like a linear polyene rather than like furan <2004JCP10972>. 

Essential features of the electronic spectra of linear polyenes have already been mentioned in Chapter 1. The HOMO LUMO transition is shifted to longer wavelengths as the number of conjugated double bonds increases, and this is easy to rationalize in the FEMO model. However, the next transition that is to be expected from the simple MO model is only allowed if the molecule does not have a center of symmetry that is, if it is not in the all-trans configuration. (Cf. Example 1.7.) The absorption spectrum of /S-caro-... 

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