Stilbene Excited States

The observation of triplet sensitized cycloaddition reactions of 10 but not t-1 with vinyl ethers reflects the requirement of a planar stilbene excited state for effective interaction with ground state electron-rich or electron-poor alkenes. While triplet sensitized reactions of other cyclic stilbene analogues (e.g., 5-9) have not been reported, it appears quite likely that they should occur. 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57], Table 4-1 collects the INDO/SCl-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

Regarding the emission properties, AM I/Cl calculations, performed on a cluster containing three stilbene molecules separated by 4 A, show that the main lattice deformations take place on the central unit in the lowest excited state. It is therefore reasonable to assume that the wavefunction of the relaxed electron-hole pair extends at most over three interacting chains. The results further demonstrate that the weak coupling calculated between the ground state and the lowest excited state evolves in a way veiy similar to that reported for cofacial dimers. 

Figure 3.13. Resonance Raman spectra of Sj excited state trans-stilbene in decane at delay times indicated. The pump wavelength was 292.9 nm and the probe wavelength was 585.8nm. The vertical dashed lines illustrated the substantial spectral evolution of the 1565 cm compared to the 1239cm band. (Reprinted with permission from reference [56]. Copyright (1993) American Chemical Society.)...

Example Singlet Excited State of Trawi-Stilbene... 

Electronic excited states are often reactive intermediates in many photochemical reactions. In a number of cases, the excited state may undergo energy relaxation. The photoisomerization reaction of fra i-stilbene provides a well-studied... 

It is interesting to note that while the electrocyclic reaction shown in Eq. (9.104) has been developed into a very useful synthetic reaction, not all stilbene-type systems cyclize. For the reaction to occur, the sum of the free valence indices (2 F ) for the first excited state at atoms between which the new bond is formed must be greater than unityu83,184) ... 

Kleinschmidt J, Rentsch S, Tottleben W, Wilhelmi B (1974) Measurement of strong nonlinear absorption in stilbene-chloroform solutions, explained by the superposition of two-photon absorption and one-photon absorption from the excited state. Chem Phys Lett 24 133-135... 

Rettig W, Strehmel B, Majenz W (1993) The excited states of stilbene and stilbenoid donor-acceptor dye systems. A theoretical study. Chem Phys 173(3) 525-537... 

Shinohara Y, Arai T (2008) Effect of methoxy substituents on the excited state properties of stilbene. Bull Chem Soc Jpn 81(11) 1500—1504... 

Many authors may object to our usage of the word intermediate for the spectroscopic excited state at the original geometry, such as trans-stilbene triplet, and there may even be some opposition to the usage of this term for the non-spectroscopic excited state, such as twisted stilbene triplet, and a tendency to reserve the term intermediate only for those species which have a minimum in the So hypersurface. 

Very few examples of singlet n-n dimerizations are reported, the reactions of 2-butenes in Eqs. 36 and 37 being examples with observable stereochemistry. The reactions are stereospecific in agreement with theory. The unsensitized reactions of styrene 115> and some stilbene derivatives 116> may also proceed via excited states, Eq. 47 and... 

Frederick JH, Fujiwara Y, Penn JH, Yoshihara K, Petek H (1991) Models for stilbene photoisomerisation experimental and theoretical studies of the excited-states dynamics of 1,2-diphenylcycloalkenes. J Phys Chem 95 2845... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

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