Polyenes excited states

Retinal as Visual Pigment Model Spectroscopy and Physical Chemistry. As in previous years, several theoretical, spectroscopic, and photochemical studies of retinal (136) and related compounds, especially Schiffs bases, have been reported,and in many cases the main aim was to obtain information relevant to the functioning of rhodopsin and related visual pigments. Particularly valuable are surveys of the year s literature on the photochemistry of polyenes, excited states of biomolecules,and recent developments in the molecular biology of vision. 

Since Schulten and Karplus showed that the experimentally observed ordering of polyene excited states reflected the importance of electron correlation in these molecules [2], the idea that explicit consideration of electron-electron coulomb interaction is essential for a correct description of polyene electronic structure has been reinforced by a number of theoretical studies [4-8]. Unfortunately, as theory more closely agrees with experiment, it also becomes more complex and computationally intensive. This is the reason that we have... 

Woodward and Hoffmann have first disclosed that the thermal (4M+2)-cyclization (and also the photochemical (4M)-cyclization) takes place via Type I process, and the photochemical (4m+2)-cyclization (and also the thermal (4m)-cyclization) via Type II process 51>. They called the former (Type I) process "disrotatory", while the latter (Type II) process was referred to as "conrotatory". They attributed this difference in selectivity to the symmetry of HO and SO MO in the ground-state and excited-state polyene molecules, respectively (Fig. 7.33). The former is symmetric with respect to the middle of the chain, and the latter antisymmetric, so that the intramolecular overlapping of the end regions having the same sign might lead to the Type I and Type II interactions, respectively. 

Salares VR, Young NM, Carey PR, and Bernstein HJ. 1977. Excited-state (exciton) interactions in polyene aggregates—Resonance Raman and absorption spectroscopic evidence. Journal of Raman Spectroscopy 6(6) 282-288. 

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

Similarly to shorter polyenes, calculations of the excited states of longer polyenes have shown that the lengths of the double bonds increase upon excitation while those of the single bonds decrease75-78. However, these changes are not equally distributed along the chain. Instead, they tend to localize in the central region of the molecule and are more pronounced in the 2 kg state, for which calculations indicate a reversal of the bond alternation pattern. 

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... 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

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. 

III. EXCITED STATES OF POLYENE RADICAL CATIONS BY OTHER METHODS A. Introduction... 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

Finally, before turning to a brief review of methodological and theoretical aspects, we mention that one of the distinguishing features of planar conjugated polyene radical cations (cf Section II.D) is that their EA spectra reveal a breakdown of the single-configuration picture for ionic excited states which had been used so successfully in interpreting their PE spectra (cf. Section II). 

In ab initio methods (which, by definiton, should not contain empirical parameters), the dynamic correlation energy must be recovered by a true extension of the (single configuration or small Cl) model. This can be done by using a very large basis of configurations, but there are more economical methods based on many-body perturbation theory which allow one to circumvent the expensive (and often impracticable) large variational Cl calculation. Due to their importance in calculations of polyene radical ion excited states, these will be briefly described in Section 4. 

Figure 29 raises the question of how the energies of these two excited states evolve as one goes to longer polyene chains, in analogy to those found in polyacetylenes which become conductive upon oxidative doping (= ionization) or photoexcitation. 

Because the extension of the polaron in polyene radical cations is finite (10-20 double bonds depending on the type of calculation), its electronic structure is independent of the number of double bonds attached to either side of it, so that the two lines in Figure 29 must bend at some point to meet the abscissa horizontally, as indicated by the dashed curves. Apparently, the point of inflection has not been reached for n = 15, but it is of interest that the curve for the first excited state could well extrapolate to 0.35 eV, which happens to be where the absorption of a polaron in polyacetylene has been observed300. If this is true, a second, more intense absorption band should occur between 0.5 and 0.7 eV, but the corresponding experiments have not yet been carried out. 

The realization of the polaronic nature of polyene radical cations leads naturally to the question, to what extent the pronounced relaxation of polyenes upon ionization affects their excited-state energies. Such changes can be assessed by comparing the ionization energy differences I) —I] obtained from PE spectra with the positions of the band maxima in the radical cation s EA spectra which measure the same quantities at the radical cation... 

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). 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

---