Fluorescence quantum yield ratio

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

For systems in which the triplet state decays quickly and radiationlessly to ground, the excimer-to-monomer fluorescence quantum yield ratio is given by... 

A phosphorescence to fluorescence quantum yield ratio higher than one. 

Figure 4. Graphs of fluorescence quantum yield and phosphorescence quantum yield ( ) versus log of the ratio of millimoles of dissolved sodium acetate to millimoles of p-aminobenzoic acid anion. (Reproduced from reference 12. Copyright 1988 American Chemical Society.)...

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

In order to go further into the experimental check we constructed Arrhenius plots of the fluorescence quantum yield of BMPC in a few solvents (methanol, ethanol, propanol, hexanol and methylene chloride), all of which showed good linearity. The activation energies and A/kp ratios, calculated from the slopes and intercepts of those plots, are collected in Table 1. The smooth increase of both parameters in the alcohol series is mainly associated with the increase of solvent viscosity. On the other hand, decrease of the solvent dielectric constant from 32.7 (methanol) to 8.9 (dichloromethane) causes a small but significant increase of the activation energy also, this increase is probably somewhat compensated by the decrease of the viscous-flow... 

The ratio of the number of photons emitted by Si to the number of photons absorbed by S0 is known as the fluorescence quantum yield,... 

Now, the rate of fluorescence emission is Jf = kf [Si], and since the fluorescence quantum yield was shown in Section 3.5 to be equal to the ratio of the rate of fluorescence to the total rate of deactivation, the fluorescence quantum yields in the presence and absence of a quencher, Q( )f and < )f respectively, are ... 

In other words, the fluorescence quantum yield is the ratio of the number of emitted photons (over the whole duration of the decay) to the number of absorbed photons. According to Eq. (3.4), the ratio of the d-pulse response tp(f) and the number of absorbed photons is given by... 

R0. Calculation of the integral of leads to the following expression for the ratio of the fluorescence quantum yields in the presence and absence of energy transfer ... 

The cationic form has a higher fluorescence quantum yield than the neutral form, which allows the excitation ratio (490/435 nm) measurement to be extended down to pH 2. 

Such an enhancement of the fluorescence quantum yield can be explained in terms of the relative locations of the singlet n-n and n-n states. In the absence of cation the lowest excited states has n-n character, which results in an efficient intersystem crossing to the triplet state and consequently a low fluorescence quantum yield. In the presence of cation, which strongly interacts with the lone pair of the carbonyl group, the n-n state is likely to be shifted to higher energy so that the lowest excited state becomes n-n. An outstanding selectivity of Na+ versus K+ was found the ratio of the stability constants is 1300 in a mixture of ethanol and water (60 40 v/v). 

S-2, in which the spacer between the two boronic acids is flexible, has the additional capability of forming excimers. The 1 1 binding of a saccharide leads to an increase in the monomer fluorescence intensity. This increase has two origins the decrease in excimer formation, and the increase in fluorescence quantum yield resulting from suppression of the PET process. The 1 1 complex is formed at low saccharide concentrations, but increasing the concentration leads to the formation of the 1 2 complex, as revealed by the increase in the ratio of the intensities of the excimer band to the monomer band. The selectivity of S-2 was found to be similar to that of S-l. 

A quantity that in many cases can easily be measured, even in a heterogeneous system, is the ratio between these two fluorescence quantum yields. We therefore write... 

This equation shows that the ratio between the acceptor and donor fluorescence quantum yields is directly proportional to the energy-transfer rate constant kET. We have shown that this leads to the following linear relation between the fluorescence intensity of the acceptor 70x and that of the donor 7py, and the occupation probability pQx of the acceptor [3, 77] ... 

If there is no emission shift, but only variation in the fluorescence quantum yield on protonation, one follows the fluorescence intensity at one emission wavelength when the acid and the base forms of the probe are excited. All the advantages of the ratio method described with two emission wavelengths also apply to this fluorescence excitation method. 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

In the absence of photodimerization (kg = 0) the ratio of excimer/molec-ular fluorescence quantum yields is given by (Eqs. 1-3)... 

The quantum yields and decay rates of the intermolecular excimer of naphthalene and its derivatives are given in Table 8. The solvent ethanol water 95 5 v/v is one of the few solvents in which the fluorescence of these compounds has been completely characterized. Examination of the values of kD and QM for other solvents shows that 95 % EtOH does not belong in the same class as the hydrocarbon solvents, or even anhydrous ethahol. In the latter solvents, kD/kM falls between 0.8 for 1,6-dimethylnaphthalene and 1.4 for naphthalene. Although the quantity k /k has been measured only once for a naphthyl compound in a hydrocarbon solvent (see Table 5), the values 0.3 and 0.4 seem appropriate for 1,6-dimethylnaphthalene and naphthalene, respectively, in hydrocarbon solvents. Since QD/QM = (kpD/kpM) -s-(kD/kM), we obtain QD/QM = 0.4 for 1,6-dimethylnaphthalene and 0.3 for naphthalene. The intrinsic quantum yield ratio as determined in 95 % EtOH solvent is about seven... 

Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E rotamer A, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state [L]/[A] equilibrium ratio, gives rise to locally excited A, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A into E is difficult to detect because it proceeds at 298 K... 

Figure 5.11 shows the behavior of the ratio of the dual fluorescence quantum yields in polar n-butyl chloride in a large temperature range. It can be seen that for both DMA compounds, Tm is situated at around 200 K, whereas for both DMPYR compounds, it lies above 330 K. With respect to temperature, the behavior for ester and nitrile with a similar amino substituent is comparable, but the increased importance of the TICT band for the esters reflects as an upward shifting of the In A/ B curves without horizontal displacement. 

Fig. 5. 11. Temperature dependence of the ratio of the dual-fluorescence quantum yields for the nitriles DMABN and DMPYRBN (left) and the esters DMABEE and DMPYRBEE (right) in n-butyl chloride.33...

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