Trans Franck-Condon state

The assignment of the observed fast kinetic rate to trans-cis isomerization is strongly supported by our experimental data, which show that the time constant is related to the viscosity by approximately The Forster and Hoffman modeP was developed originally to explain the Q= relationship where Q is the fluorescence quantum yield and 17 is the viscosity of the medium for triphenylmethane dyes. In addition, it was predicted that the fluorescence lifetime, r, should follow a similar relationship t = c if According to this model, absorption of light produces a vertically excited Franck-Condon state with the phenyl rings still at a ground state equilibrium... 

Excitation of the investigated stilbene molecule from its ground state H to the Franck-Condon state occurs in a few femtoseconds. As a result, only fast electronic polarization techniques can follow a drastic change of the charge distribution around the zwitterionic exited FC state. The latter has been proved particularly by the excitation energy dependence on the solvent refractive index [23]. The first step after excitation to the Franck-Condon state of the trans-stilbene configuration is vibrational relaxation followed by solvent-solute relaxation that leads to a rapid population of the H state from which fluorescence occurs. These relaxation processes result in a Stokes shift (A ). 

The time of excitation to the Franck-Condon state is approximately 1 fs (10 s). Thus, these techniques will freeze out any equilibrium that involves the actual movement of nuclei, such as a cis-trans interconversion. Electron exchange can, however, be faster than a vibration. These very fast electron transfer processes can be probed by exposing a sample to laser radiation with a pulse shorter than 10 s, i.e., the realms of femtosecond (10 s) and attosecond (10 s) spectroscopy. ... 

Figure 7.5 (a) Franck-Condon n->n excitation of ethylene and subsequent relaxation (b) trans-cis isomerization in the excited state. 

Oxford University Press, Oxford Zechmeister L (1962) Cis-Trans Isomeric Carotenoids, Vitamins A and Arylpolyenes. Academic Press, New York Zerbetto F and Zgierski MZ (1990) The missing fluorescence of s-tmns butadiene. J Chem Phys 93 1235-1245 Zerbetto F and Zgierski MZ (1994) Franck-Condon modeling of the structure of the Sfl-iS2 transition of trans,trans, cis,trans-, and cis.ds-octatetraene. J Chem Phys 93 1842-1851 Ziegler LD and Hudson BS (1983) Resonance Raman scattering of ethylene. Evidence for a twisted geometry in the V state. J Chem Phys 79 1197-1202... 

The vertical electronic spectrum of acetylene shows many similarities with that of the isoelectronic molecule HCN. Both are linear molecules in their electronic ground states, but their lowest-valence excited states are bent. Photoexcitation of acetylene in the near-UV (190-240 nm) induces a electron promotion, resulting in population of the planar, trans bent A A state ( S in the linear limit) [312], The A A —transition is dominated by long progressions involving v 3 (the excited state trans bending mode), as would be expected on Franck-Condon grounds. The A—X... 

In some cases the rate constant for formation of the cis isomer k ) is greater than for formation of the trans isomer (Ir,). Because the cis isomer is higher in energy than the trans, there may be better overlap of the vibrational component of the wave function of the cis ground state with the vibrational component of the excited state than is the case with the trans isomer. Thus, the Franck-Condon term for the twisted singlet to cis reaction could be more favorable than for the trans. 

The available electronic-structure data indicate the existence of a conical intersection of the optically allowed l S(7r7r ) state with the dark 2 Ag state in the vicinity of the Franck-Condon zone of the Sq tran-... 

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