Quantum yield 450 INDEX

The fluorescent components are denoted by I (intensity) followed by a capitalized subscript (D, A or s, for respectively Donors, Acceptors, or Donor/ Acceptor FRET pairs) to indicate the particular population of molecules responsible for emission of/and a lower-case superscript (d or, s) that indicates the detection channel (or filter cube). For example, / denotes the intensity of the donors as detected in the donor channel and reads as Intensity of donors in the donor channel, etc. Similarly, properties of molecules (number of molecules, N quantum yield, Q) are specified with capitalized subscript and properties of channels (laser intensity, gain, g) are specified with lowercase superscript. Factors that depend on both molecular species and on detection channel (excitation efficiency, s fraction of the emission spectrum detected in a channel, F) are indexed with both. Note that for all factorized symbols it is assumed that we work in the linear (excitation-fluorescence) regime with negligible donor or acceptor saturation or triplet states. In case such conditions are not met, the FRET estimation will not be correct. See Chap. 12 (FRET calculator) for more details. 

Thus, E is defined as the product of the energy transfer rate constant, ku and the fluorescence lifetime, xDA, of the donor experiencing quenching by the acceptor. The other quantities in Eq. (12.1) are the DA separation, rDA the DA overlap integral, / the refractive index of the transfer medium, n the orientation factor, k2 the normalized (to unit area) donor emission spectrum, (2) the acceptor extinction coefficient, eA(k) and the unperturbed donor quantum yield, QD. 

We now focus our attention on the presence of the unperturbed donor quantum yield, Qd, in the definition of R60 [Eq. (12.1)]. We have pointed out previously [1, 2] that xd appears both in the numerator and denominator of kt and, therefore, cancels out. In fact, xo is absent from the more fundamental expression representing the essence of the Forster relationship, namely the ratio of the rate of energy transfer, kt, to the radiative rate constant, kf [Eq. (12.3)]. Thus, this quantity can be expressed in the form of a simplified Forster constant we denote as rc. We propose that ro is better suited to FRET measurements based on acceptor ( donor) properties in that it avoids the arbitrary introduction into the definition of Ra of a quantity (i />) that can vary from one position to another in an unknown and indeterminate manner (for example due to changes in refractive index, [3]), and thereby bypasses the requirement for an estimation of E [Eq. (12.1)]. 

Attention should be paid to possible problems in the measurement of fluorescence quantum yields (some of which are discussed Section 6.1.5) inner filter effects, possible wavelength effects on Op, refractive index corrections, polarization effects, temperature effects, impurity effects, photochemical instability and Raman scattering. 

For vacuum sublimed thin films, Grabuzov et al. [138] reported a photoluminescence quantum efficiency of 32 2%. In the same paper, data on the absorption coefficient at the maximum, a = (4.4 0.1) x 104 cm 1, and the refractive index at 633 nm (n = 1.73 0.05) can be found. Other reported values for the photoluminescence quantum efficiency that can be found in the literature are 30 5% [124] and 25 5% [139]. Naito et al. [109] reported a quantum yield of 5% in the amorphous film compared to 35% in the crystalline state. The fluorescence lifetime is reported to be biexponential with x = 3.4 and 8.4 ns, which is much shorter than in the crystal (17.0 ns). In the amorphous state, the larger free volume allows more vibrations and rotations to take place, which favors nonradiative decay. 

When heat is produced in the sample after the photolytic flash, the refractive index of the liquid changes and the probe beam is deflected. The intensity of this probe beam measured by a photomultiplier tube placed behind the pinhole decreases as the temperature of the irradiated volume increases (then its density and its refractive index decrease). The total optical signal change is a measurement of all the heat produced in the sample, i.e. the sum of non-radiative transitions, chemical reactions and solvation energies. Luminescence does not contribute to this signal (nor does scattered light) and for this reason thermal lensing can be used to determine luminescence quantum yields. 

The Battelle group found that photoisomerization of various indigo derivatives could be used to record holograms which had low (<0.2%) scattering efficiencies (36). Their refractive-index data yield nQ = 2.9 x 10 cm for an N,N -dibenzoylindigo derivative. These systems were reversible. The quantum yields of isomerization were much lower in poly(methyl methacrylate) or polystyrene than in fluid solution, in accord with arguments presented earlier. Concentration quenching was also observed in solution. 

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