Fluorescence Franck—Condon state

Figure 2.30. SFo, Franck-Condon state Sp, fluorescence state. Reproduced with permission from Ref. 65.

In contrast, if the medium is too viscous to allow solvent molecules to reorganize, emission arises from a state close to the Franck-Condon state (FC) (as in the case of a nonpolar medium) and no shift of the fluorescence spectrum will be observed (F in Figure 7.2). 

Slow reorganization dynamics in the adsorbate/adsorbent complex after excitation of the Franck-Condon state. This could explain the spectral redshift of fluorescence with time (Figure 8.8, upper right, and Figure 8.9). In liquid solution, the excited state equilibrates on the picosecond timescale, but is has been shown(23) that this process can slow down on surfaces to 10-50 nsec. 

The most likely electronic transition will occur without changes in the positions of the nuclei (e.g., little change in the distance between atoms) in the molecular entity and its environment. Such a state is known as a Franck-Condon state, and the transition is referred to as a vertical transition. In such transitions, the intensity of the vibronic transition is proportional to the square of the overlap interval between the vibrational wavefunctions of the two states. See Fluorescence Jablonski Diagram Comm, on Photochem. (1988) Pure and Appl. Chem. 60, 1055. 

On the ultrafast conversions from excited FC (Franck-Condon) state to FI (Fluorescence) state of chromophores in PNS of photoactive proteins PYP, Rh and FP... 

The enrichment of the concentration of the polar solvent component in the cage and, therefore, the relative amount of the red shift of the fluorescence band is a function of viscosity, since the diffusion-controlled reaction time must be smaller than the excited-state lifetime. This lifetime limitation of the red shift is even more severe if the higher value of the excited-state dipole moment is not a property of the initial Franck-Condon state but of the final state of an adiabatic reaction. Nevertheless, the additional red shift has been observed for the fluorescence of TICT biradical excited states due to their nanosecond lifetime together with a quenching effect of the total fluorescence since the A to 50 transition is weak (symmetry forbidden) (Fig. 2.25). 

The above described model sequences have been studied both as oligomers [7,8,11-13,19] and as polymers [9,11,20]. An increase in the size of the helix is known to reinforce its stability, as revealed by their melting curves [18] and attested by X-ray diffraction measurements in solution [21]. Therefore, in this chapter we focus on the polymeric duplexes poly(dGdC).poly(dGdC) [= 1000 base-pairs], poly(dAdT).poly(dAdT) [= 200-400 base-pairs] and poly(dA).poly(dT) [= 2000 base-pairs] studied by us. First we discuss the absorption spectra, which reflect the properties of Franck-Condon states, in connection with theoretical studies. Then we turn to fluorescence properties fluorescence intensity decays (hereafter called simply fluorescence decays ), fluorescence anisotropy decays and time-resolved fluorescence spectra. We... 

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... 

A study of the low-temperature (77 °K) emission and absorption spectra of naphthalene43 has established the nature of the comparatively weak absorption band on the long wavelength side of its ultra-violet spectrum the lowest singlet-singlet band in the crystal has been shown to be at 322 Low-temperature fluorescence study of some molecular and ionic hydroxynaph-thalenes in rigid solvents44 shows that emission occurs from the Franck-Condon state at a temperature of 77 °K, whereas at room temperature, normal fluorescence is observed. 

Figure 4 A diagram of the four-level model of a dye molecule in solution is shown. The equilibrated ground state, 1, is surrounded by a solvent cage. Upon transition to the Franck-Condon excited state, 2, the nuclear configuration of the dye and the solvent cage remains stationary. Resonance fluorescence from the Franck-Condon state is shown. Equilibration in the excited state of the dye molecule and the surround solvent is obtained in level 3. The major portion of fluorescence takes place between levels 3 and 4, although emission also occurs continuously from the intermediate levels (Reproduced by permission from Chem. Phys. Letters, 1975, 32, 476)...

The analysis of the solvatochromic effects on molecular absorption and emission (fluorescence and phosphorescence) spectra is further complicated by the variation of time scales for the solvent relaxation after the spectral excitation of the solute molecule. The spectral transition is a very fast process that takes place within approximately 10 s. Thus, during this short period of time the atomic nuclei do not practically move. The excited state reached by the respective vertical transition is often called the Franck-Condon state (Figure 11.1.4). 

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 ). 

Stabilization of the relaxed excited state as compared to the Franck-Condon state. Thus, as a rule, the solvatochromic shifts in the absorption and fluorescence spectra are not equal. 

The primary process of rhodopsin is summarized in Figure 125.6. Rhodopsin goes to the Franck-Condon state (FC state) by photon absorption. Then, the Franck-Condon state moves rapidly to a fluorescent state (FI state) within 25 fs. The fluorescent state then goes to the ground state intermediate photor-hodopsin, within about 200 fs. 

---