Excitation energies linear conjugated polyenes

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). 

The results of the particle in a one-dimensional box problem can be used to describe the delocalized n electrons in (linear) conjugated polyenes. Such an approximation is called the free-electron model. Take the butadiene molecule CH2=CH-CH=CH2 as an example. The four n electrons of this system would fill up the [Pg.16]

Excitation Energies of Linear Conjugated Polyene Radical Cations... 

Butadiene. - The ab initio calculation of the linear polarizability and second hyperpolarizability tensors for f/ww-butadiene has attracted a great deal of attention. The molecule is the smallest stable structure showing -electron delocalisation and is the first member of the fully conjugated polyene series. Extended conjugated structures are associated with low excitation energies and large response functions. 

The convergence of the linear polyene Sq to S2 excitation energy to a finite limit as the length of the conjugated chain increases is strong evidence that there is bond length alternation in the long chains. This is supported by the limited number of direct structure determinations that have been published. 

Energies of the lowest singlet and triplet excited states of linear polyenes containing 2 to 20 conjugated double bonds were recently calculated (94). These values are considerably lower than the known experimental values, given in this table—e.g., calc, of 3-carotene 11,5 kcal/mole (48 kj/mole). 

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