Excited-state lifetime determination

The excited state lifetimes determined from the na-AIMD simulations are generally in good agreement with experimental data. In addition, the na-AIMD simulations provide detailed insights into the dynamical mechanism of radiationless decay. The time evolution of the nonadiabatic transition probability could be correlated with certain vibrational motions. In this way, the simulations yield the driving modes of internal conversion. 

Excited State Lifetime Determination with I tosecond Laser Pulses... 

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... 

Excited-state lifetimes can be measured directly by monitoring the decay of luminescence, but impurities present affect both the lifetime and the luminescence spectrum. Also, because low temperatures are necessary for phosphorescence studies, the excited-state properties determined may differ from those at room temperature. 

The ability of fluorescence to provide temporal information is of major importance. Great progress has been made since the first determination of an excited-state lifetime by Gaviola in 1926 using a phase fluorometer. A time resolution of a few tens of picosecond can easily be achieved in both pulse and phase fluorometries by using high repetition rate picosecond lasers and microchannel plate photo-... 

Dr can be determined by time-resolved fluorescence polarization measurements, either by pulse fluorometry from the recorded decays of the polarized components I l and 11, or by phase fluorometry from the variations in the phase shift between J and I as a function of frequency (see Chapter 6). If the excited-state lifetime is unique and determined separately, steady-state anisotropy measurements allow us to determine Dr from the following equation, which results from Eqs (5.10) and (5.41) ... 

Dynamic quenching of fluorescence is described in Section 4.2.2. This translational diffusion process is viscosity-dependent and is thus expected to provide information on the fluidity of a microenvironment, but it must occur in a time-scale comparable to the excited-state lifetime of the fluorophore (experimental time window). When transient effects are negligible, the rate constant kq for quenching can be easily determined by measuring the fluorescence intensity or lifetime as a function of the quencher concentration the results can be analyzed using the Stern-Volmer relation ... 

The dependence of knr on the value or concentration of a [Parameter], in the vicinity of the excited sensor, determines both the luminescence intensity and the excited state lifetime of the sensor. 

The value of fEdetermines all other variables in the equations above. In turn, fE is determined by the temporal resolution of interest of the system studied. To resolve an average excited state lifetime t, the required data sampling rate, in frequency domain techniques is at least an order ofmagnitude slower than it is in the time domain as stated by the following relation (when Np > 32 and Nw= 1) ... 

One can expect that the analysis of continuous distributions of electronic excited-state lifetimes will not only provide a higher level of description of fluorescence decay kinetics in proteins but also will allow the physical mechanisms determining the interactions of fluorophores with their environment in protein molecules to be elucidated. Two physical causes for such distributions of lifetimes may be considered ... 

Pairwise EET rates cannot be directly measured in antenna systems. The closest approach to direct determination is offered on the one hand by time resolved picosecond and sub-picosecond absorption and fluorescence measurements and on the other hand by hole burning spectroscopies. Time resolved techniques do not detect transfer between isoenergetic sites. A somewhat more indirect approach to determining pairwise rates is that of analysing excited state lifetime data in terms of a particular antenna and an EET model. 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

A kinetic technique for determining a fluorophore s excited state lifetime by using a light source whose intensity is modulated sinusoidally at a certain frequency, such that the intensity of the fluorescence emission likewise varies sinusoidally but with an added delay from the finite relaxation constant for fluorescence decay. The period of the sinusoidal modulation is chosen to be in the neighborhood of the magnitude of the fluorescence lifetime. 

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]... 

For the determination of the excited-state lifetime, r, energy-transfer measurements in steady-state experiments, florescence decay measurements, or transient photon absorption measurements were applied. 

The rather short lifetime (a few nanoseconds) of the triplet excited carbene makes extensive studies of intermolecular processes difficult. However, the excited-state lifetime (60 ns) of triplet dimesitylcarbene (19c) is exceptionally large, probably because of decreased efficiency of both intermolecular and intramolecular deactivation pathways. Intermolecular rate constants for the reaction with CCI4, O2 and 1,4-cyclohexadiene have been determined. 

Secondly, the emission of a particular complex species is characteristic of that species and can be used to identify the species present. Particularly is this so if excited state lifetimes are measured, as these vary dramatically depending on the number of OH groups coordinated to the Eu3+ or Tb3+ ion. This is because multiple excitation of the OH stretching mode provides an alternative deexcitation route. Measurement of lifetimes thus can be used to determine the number of coordinated water molecules.218... 

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