Conjugated polyenes electronic spectra

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

In the ground electronic state of (16.9), the highest occupied and lowest vacant tt MOs have j = jMc 5 c + respectively HMO theory predicts the longest wavelength band of the electronic absorption spectrum of a conjugated polyene to occur at... 

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-... 

Figure 28 shows the UV—visible spectra of monomer 72. poly-76, and copolymers 77—80. The monomer spectrum was taken in CHCI3. and the polymer spectra were taken in spin-coated films on a quartz substrate. Compound 75 shows the characteristic broad band at 540 nm due to the tt—jt electronic transition of the conjugated cyclic polyene backbone. Copolymers 77—82 containing a chromophore show two maximum absorption values around 390 and 550 nm due to a pendant chromophore and conjugated cyclic polyene backbone, respectively. 

Simple aldehydes and ketones show only weak absorption in the ultraviolet region of the spectrum owing to an n to tt electronic transition of the carbonyl group. If, however, the carbonyl group is conjugated with one or more carbon-carbon double bonds, intense absorption (g = 8,000 - 20,000 M cm ) occurs as a result of a 77 to tt transition as with polyenes, the position of absorption is shifted... 

Our goal is to model quantitatively 7r-electronic contributions to both vibrational and electronic spectra. The general e-ph analysis introduced in Section II combines the microscopic AM formalism [18,19] with the spectroscopic ECC model [22]. The reference force field F for PA provides an experimental identiHcation of delocalization effects. Transferable e-ph coupling constants are presented in Section III for polyenes and isotopes of trans- and a s-PA. The polymer force field in internal coordinates directly shows greater delocalization in t-PA, while coupling to C—C—C bends illustrates V(/ ) participation and different coupling constants a(/ a) and a(Jis) in Eq. (3) support an exponential r(/ ). NLO spectra of PDA crystals and films are presented in Section IV, with multiphoton resonances related to excited states of PPP models and vibronic contributions included in the Condon approximation. Linear and electroabsorption (EA) spectra of PDA crystals provide an experimental separation of vibrational and electronic contributions, and the full tt-tt spectrum is needed to model EA. We turn in Section V to correlated descriptions of electronic excitations, with particular attention to theoretical and experimental evidence for one- and two-photon thresholds of centrosymmetric backbones. The final section comments on parameters for conjugated polymers, extensions, and open questions. 

We emphasize that 7(a>) and 7 (w) in Eq. (33) contain information about the ground and 1 potential surfaces sketched in Fig. 6.10. The force constants in Eq. (10) require all electrons and are formidable calculations for polyenes [21]. The F-dependent coefficients in Eq. (33), on the other hand, are due to virtual tt-tt excitations. Similarly, the vibronics of two-photon excitations appear in Eq. (34), and the induced intensity depends on the TT-TT spectrum. These EA expressions hold for an isolated molecule or polymer. An isotropic distribution of conjugated backbones in films also gives r(cu) terms due to internal fields [106] or site disorder, and such profiles have been reported in PA [107] and PDA [108] films. Since EA of extended states in semiconductors [109,110] also goes as T (o) and disorder is poorly understood, films are more difficult to model. In crystals. Stark shifts scale as 7 (a>) and induced moments as 7(a>) or TPA. 

---