Stokes shifted fluorescence energy

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Typical absorption and fluorescence spectra are shown in Figure 11.3. Since energy is lost in the activated state (S ) before fluorescence, the emission maximum always occurs at a lower wavenumber than the absorption maximum. The difference, which is termed the Stokes shift, can be calculated approximately from the absorption spectrum using the Pestemer rule [17,18]. This rule states that the Stokes shift is 2.5 times the half-bandwidth at the absorption maximum. 

Fig. 1 Jablonski diagram of energy level for describing processes absorption, fluorescence and phosphorescence in complex molecules where kf and /c arc the radiative and nonradiative rates of fluorescence, respectively, kj and kTnr are the radiative and nonradiative rates of phosphorescence, respectively, k-lsc is the interconversion rate, and kmt is the rate of intermolecular processes Av denotes the Stokes shift of fluorescence...

The experimental peak energies of both fluorescence and absorption are in excellent agreement - Stokes shift in eV, 0.80 experimental and 0.83 (TD-DFT performed by package TURBOMOLE) (Ahlrichs et at. TURBOMOLE version 5.6 University of Karlsruhe Karlsruhe, Germany) with the theoretical values for compound 20 (Table 2) <2005PCB6004>. 

A detailed study of the electronic structure and optical properties was published for the spiro derivative of f-Bu-PBD, Spiro-PBD (40) [108]. The vibronic structure of the lowest energy absorption band is well resolved, in solution as well as in the amorphous him. The 0-0 transition is at 351 nm (3.53 eV), the 0-1 and 0-2 vibronic bands that have a higher oscillator strength, are at 336 nm (3.69 eV) and 318 nm (3.90 eV), respectively. The fluorescence spectrum of this compound is symmetrical to the absorption spectrum with a Stokes shift of 43 nm. 

Figure 2.5. Energy level diagram (top) and spectra (bottom) illustrating the two-state model of relaxation. The energy of the absorbed quantum is Av , and the energies of the emitted quanta are hvfl (unrelaxed) and hvF (relaxed). The fluorescence spectrum of the unrelaxed state (solid curve) is shifted relative to the absorption spectrum (dotted curve) due to the Stokes shift. The emission intensity from the unrelaxed state decreases and that from the relaxed state (dashed curve) increases as a result of relaxation.

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