Excited-state lifetimes transitions

A major advantage of fluorescence as a sensing property stems from the sensitivity to the precise local environment of the intensity, i.e., quantum yield (excited state lifetime (xf), and peak wavelength (Xmax). In particular, it is the local electric field strength and direction that determine whether the fluorescence will be red or blue shifted and whether an electron acceptor will or will not quench the fluorescence. An equivalent statement, but more practical, is that these quantities depend primarily on the change in average electrostatic potential (volts) experienced by the electrons during an electronic transition (See Appendix for a brief tutorial on electric fields and potentials as pertains to electrochromism). The reason this is more practical is that even at the molecular scale, the instantaneous electric... 

It would be desirable to obtain symmetry information on V2H. All efforts to generate optical spectra of ground-to-bound excited state hole transitions have failed so far. Because of the low concentration of V2H, we attempted to use PTIS. This technique works well if the bound excited state lifetimes are long so that a phonon can interact with the bound hole... 

The point is now to estimate the maximum number of photons that can be detected from a burst. The maximum rate at which a molecule can emit is roughly the reciprocal of the excited-state lifetime. Therefore, the maximum number of photons emitted in a burst is approximately equal to the transit time divided by the excited-state lifetime. For a transit time of 1 ms and a lifetime of 1 ns, the maximum number is 106. However, photobleaching limits this number to about 105 photons for the most stable fluorescent molecules. The detection efficiency of specially designed optical systems with high numerical aperture being about 1%, we cannot expect to detect more than 1000 photons per burst. The background can be minimized by careful dean-up of the solvent and by using small excitation volumes ( 1 pL in hydrodynamically focused sample streams, 1 fL in confocal exdtation and detection with one- and two-photon excitation, and even smaller volumes with near-field excitation). 

In order to build up dendrimers crqrable of exhibiting redox activity and light-induced functions, appropriate building blocks have to be used. In the last 20 years, extensive investigations carried out on the photochemical and electrochemical properties of transition metal compounds have shown that Ru(II) and Os(ll) complexes of aromatic M-heterocycles (Figure 1), e.g., Ru(bpy)j and Os(bpy)j (bpy = 2,2 -bipyridine), exhibit a unique combination of chemical stability, redox properties, excited state reactivity, luminescence, and excited state lifetime. Furthermore all these properties can be tuned within rather broad ranges by... 

Values of +0.4 and +0.5 are theoretically expected for r0 and pm respectively, if the absorption and emission transition moments are in the same direction, as is often the case with excitation at the longest-wavelength absorption maximum. However, due to rapid internal rotation of the emission transition moments immediately after excitation, the experimentally determined values of rQ and p0 are always lower than the maximal values. Thus, the highest value ever observed for rQ is+ 0.35. In the common case where the fluorophore undergoes rotational motion during the excited-state lifetime, values of r or p closer to zero are observed depending on the extent of depolarization, and in the case of complete depolarization these parameters assume values of zero. The dependence of the anisotropy on rotational motion is described by eq 9[55]... 

Excitation spectra have been of considerable use recently in studying both hydration numbers (by lifetime measurements) and inner-sphere complexation by anions (by observing appearance of the characteristic frequencies for e.g. the Eu3+ 5D0-+ 7F0 transition for the different possible species). Thus using a pulsed dye laser source, it was possible to demonstrate the occurrence of inner sphere complexes of Eu3+ with SCN, CI or NO3 in aqueous solution, the K values being 5.96 2, 0.13 0.01 and 1.41 0.2 respectively. The CIO4 ion did not coordinate. Excited state lifetimes suggest the nitrate species is [Eu(N03)(HzO)6,s o.4]2+ the technique here is to compare the lifetimes of the HzO and the corresponding D20 species, where the vibrational deactivation pathway is virtually inoperative.219 The reduction in lifetime is proportional to the number of water molecules complexed.217 218... 

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). 

As a result of ion-phonon interaction, the population of the excited state decreases via nonradiative transition from the excited state to a lower electronic state. The energy difference between the two electronic states is converted into phonon energy. This process of population relaxation is characterized by a relaxation time, xj, which depends on the energy gap between the two electronic states, the frequencies of vibration modes, and temperature (Miyakawa and Dexter, 1970 Riseberg and Moos, 1968). At room temperature, the excited state lifetime is dominated by the nonradiative relaxation except in a few cases such as the 5Do level of Eu3+ and 6P7/2 level of Gd3+ for which the energy gap is much larger than the highest phonon frequency of the lattice vibrations. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

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