Polyenes fluorescence spectra

There is a one-to-one correspondence between the vibrational frequencies that are prominent in resonance enhanced Raman spectra and the frequencies that form progressions in vibrationally resolved fluorescence spectra. As is the case for the resonance enhanced Raman spectra the vibrational development of linear polyene fluorescence spectra are dominated by two modes loosely referred to as the C-C and C=C stretch frequencies at approximately 1200 cnr and 1600 cm"l, respectively. Frequencies of these two modes for unsubstituted, a,co-dimethyl substituted and a,co-diphenyl substituted polyenes [33-42] are summarized in Table 2. 

Fluorescence spectra from a polystyrene film photolyzed in vacuum are shown in Figure 2. Similar but less intense spectra were observed in films irradiated in air. The Product I responsible for this spectrum was partially extractable from the film with methanol the fluorescence spectrum of the extract is shown also in Figure 2. Comparison of these spectra with those of a wide variety of reasonable model compounds suggests that Product I is related to 1,3-diphenyl-l,3-butadiene since the spectral match with 1,4-diphenyl-l,3-butadiene, shown in Figure 2, is quite close. Product I spectra were obtained also from the residual films after extraction, indicating that the diene moiety may form part of a photolyzed chain as well as exist as a short-chain fragment. Fluorescence spectra that could be related to higher polyenes were not detected in the vacuum exposures. In air exposures, however, the prompt emission spectra from films did exhibit a weak shoulder superimposed on the... 

Most current interest in u.v. spectra is concerned with retinal and its cis-trans isomers. Studies at 77 K include theoretical calculations, fluorescence, and the effect of protonation on the corresponding Schiff base. The singlet-triplet absorption spectrum of all-trans-retinol has been measured. Calculations on the absorption-emission spectra of rhodopsin suggest that there is little dependence on the angular twist in the polyene chain. Pulse radiolysis has made it... 

As a typical example of polyene spectroscopy, absorption and fluorescence spectra of frons,frans-l,3,5,7-octatetraene in hexane at 23 °C are shown in Figure 3. An absorption band with several peaks is observed in Figure 3a. It should be noted that the positions of electronic absorption bands strongly depend on solvents . This absorption band is dipole allowed, because the molar absorption coefficient of this band is very large (Table 1). This band is attributed to the transition from the 1 kg ground state to the 1 feu excited state (tt-tt transition). Although the absorption peaks are due to vibrational transitions, a precise vibrational analysis cannot be made because of the broad band widths. The position of the observed emission spectrum (Figure 3b) shows a considerable red shift in... 

Many linear polyenes exhibit anomalous fluorescence behaviour in the sense that the fluorescence rate constant kf calculated from the absorption spectrum (Equation 2.11) is much smaller than that determined by lifetime and quantum yield measurements (Equation 3.33). In 1972, Hudson and Kohler reported high-resolution absorption and emission spectra of all-trart.v-l, 8-diphenylocta-l, 3,5,7-tetraene at low temperature, which proved that the lowest excited singlet state Si was not that reached by the strongly allowed 7t,Jt transition (/ 1.5) that is predicted by MO theory and observed at 410nm.3" Rather, very weak (f 0.06), structured absorption that had been hidden under the tail of the jt,jt -absorption in solution spectra was detected at slightly longer wavelengths. 

Petek H, Bell AJ, Choi YS, Yoshihara K, Tounge BA and Christensen RL (1995) One- and two-photon fluorescence excitation spectra of the 2 Ag states of linear tetraenes in free jet expansions. J Chem Phys 102 4726 739 Ramasesha S and Zoos ZG (1984) Correlated states in linear polyenes, radicals, and ions Exact PPP transition moments and spin densities. J Chem Phys 80 3278-3287 Raubach RA and Guzzo AV (1973) Singlet-triplet absorption spectrum of uH-trans-retinal. J Phys Chem 75 983-984 Ricci M, Bradforth SE, Jimenez R and Fleming G (1996) Internal conversion and energy transfer dynamics of spheroidene in solution and in the LH-1 and LH-2 light-harvesting complexes. Chem Phys Lett 259 381-390... 

The presence of certain metal ions, notably Fe and generates an additional colour, characterized by strong absorption in the red region (500-800 nm) of the spectrum [1667]. Fluorescence measurements confirm that the species responsible for the colours are rapidly dissociated in tetrahydrofuran (THF), leaving polyene sequences, which are closely related to those formed in the absence of metal ions. The presence of HCl, which is also a primary product, is necessary for the efficient generation of the colour. 

---