Linear optical spectrum, polyenes

The principal feature in the optical spectrum of linear polyenes is the strongly allowed absorption that lies in the near ultraviolet for the shortest polyene butadiene and shifts to... 

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... 

We mentioned in Section III.A that one of the unique features of radical ion optical spectroscopy is that it allows one to measure excited-state energies of a molecule at two different geometries, namely that of the neutral species (If in PE spectra) and that of the relaxed radical cation (Xmax of the EA bands). In many cases this feature is of little relevance because either the geometry changes upon ionization are too small to lead to noticeable effects (e.g. in aromatic hydrocarbons), or because such effects are obscured, due to the invisibility of the states in one or other of the two experiments (i.e. strong cr-ionizations in the PE spectrum) or because of the near-cancellation of opposing effects (as in the case of linear conjugated polyene radical cations). 

Change in the dipole moment and polarizability upon electronic excitation was determined for various linear polyenes by adopting electric field-induced changes in the optical absorption spectrum [70]. Polyenes studied (Exhibit 2) were diphenylbutadiene (DPB 7), diphenylhexatriene (DPH 8), diphenyloctate-traene (DPO 9), diphenyldecapentaene (DPD 10), and all-tranj-retinal (3). Exhibit 2 describes the experimental values determined. It is proposed that (based on the results obtained) the excited state dipole moments determined on these polyene systems have a role to play in the mechanism of trans-cis photoisomerization. 

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