Quantum efficiency of fluorescence

Quantitative aspects. The total fluorescence intensity, F, is given by the equation F = Ia(f> where Ia is the intensity of light absorption and 4>f the quantum efficiency of fluorescence. Since 70 = 7a + 7t where 70 = intensity of incident light and 7t = intensity of transmitted light, then... 

The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist. 

Griseofulvin exhibits both fluorescence and luminescence. A report by Neely et al., (7) gives corrected fluorescence excitation (max. 295 nm) and emission (max. 420 nm) spectra, values for quantum efficiency of fluorescence (0.108) calculated fluorescence lifetime (0.663 nsec) and phosphorescence decay time (0.11 sec.). The fluorescence excitation and emission spectra are given in Figure 7. 

The third rule of practical value concerns the intensity of fluorescence obtained and its variation with frequency of exciting light. The rate of emission of fluorescence is by definition equal to the rate of light absorption, measured in quanta, multiplied by the quantum efficiency of fluorescence, i.e.,... 

In dianions of xanthene dyes although halosubstituents decrease the quantum efficiency of fluorescence, phosphorescence efficiency is not increased proportionately. The phosphorescence lifetime decreases with fa. It is suggested that in these dyes, besides enhancement of Sx intersystem crossing rates, S nonradiative transition is promoted by heavy atom... 

For aromatic hydrocarbons, the quantum efficiencies of fluorescence and phosphorescence in low temperature glasses (Appendix H) such as EPA (ether isopentane ethyl alcohol in the ratio 2 2 5) add up to unity. This suggests that direct nonradiative decay from Si —> S0 is of very low probability. All the nonradiative paths to the ground state are coupled via the triplet state. The sequence of transfers is ... 

Melhuish, W. H. (1961) Quantum efficiencies of fluorescence of organic substances effect of solvent and concentration of the fluorescent solute, J. Phys. Chem. 65, 229-235. 

Using the charge-balance factory y, the efficiency of production for singlet excitons Tjr, and the quantum efficiency of fluorescence < />/, the internal quantum efficiency... 

In most molecules, the quantum efficiency of fluorescence decreases with increasing temperature because the increased frequency of collision at elevated temperatures increases the probability of collisional relaxation. A decrease in solvent viscosity leads to the same result. 

The microscopic behaviour between the ions in dilute systems results from multipolar interaction. On the other hand in the rate equations which are used for measurement of macroscopic data such as quantum efficiencies of fluorescence, the multipole questions are absent. The macroscopic treatment of energy transfer was performed recently independently by Fong and Diestler (5) and Grant ( >) who conclude that the concentration... 

Po is the radiant power of the exciting radiation note that the fluorescence intensity is directly proportional to Pq t is the quantum efficiency of fluorescence,... 

For most practical purposes the oscillator strengths and the connected radiative transition probabiUties obtained from the absorption spectra correspond to some average values due to the total number of sites. This is also true for the nonradiative transition probabilities, which are obtained from the measured average lifetimes and average quantum efficiencies of fluorescence. 

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