Dual fluorescence quantum yield

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...

The emission spectra of 10-CPT in water-methanol mixtures exhibits dual fluorescence (Fig. 1 left). The appearance of the low energy emission band at 570 nm for 10-CPT in a neat methanol solution indicates an efficient PTTS process. The large fluorescence quantum yield and similarity of the emission at neutral and basic solutions is evidence of the excited anion (RO ) formation, in contrast to 6HQ, for which double PTTS leads to the tautomer [2], With the increase of water content in the mixtures, we observed a substantial decrease in the fluorescence intensity of the nondissociated form of 10-CPT at 430 nm and a concomitant increase of RO " intensity at 570 nm. This is a well-known effect in hydroxyaromatic compounds [4] and is attributed to the increase of the protolytic photodissociation rate with increasing water concentration. 

The temperature dependence of the dual fluorescence of DMABN as shown in Fig. 2.1 displays several characteristic features. The short wavelength B band at around 350 nm shows an intensity minimum at a certain temperature Tm around 200 K, whereas the quantum yield of the long wavelength A band slightly but monotonically increases with temperature. Thus, above Tm, the total fluorescence quantum yield increases with temperature, a feature which is rather uncommon in photophysics. 

The fluorescence quantum yields of pyrene-1-carboxaldehyde in water and methanol are 0.98 and 0.07/ an effect attributed to solvent effects on 7c,n and n,n states. Cycloaddition reactions of 1-naphthonitrile to 1,2-dimethyl-cyclopentene are attributed to both and Lj, states.It is pointed out that although dual fluorescence is known, this is the first example of divergent reaction from two nearly isoenergetic singlet states. An analysis of the u.v. spectra of some acyl pyridines, including a theoretical examination of the molecular geometry, and excited states of bipyrimidine compounds have also been made. Photo tautomerism and the fluorescence of the cation of 4-amino-pyrazole[3,4-iflpyrimidine, an analogue of adenine, has been published by Wierzchowski et Intramolecular heteroexcimer formation in... 

Another TICT molecule, ethyl p-(A,A -diethylamino)benzoate (DEAEB), was used to probe solute-solvent interactions in supercritical ethane, CO2, and fluoroform (3,50,56). Unlike DMABN and DMAEB, DEAEB forms a TICT state even in nonpolar solvents (Figure 13) (50), resulting in dual fluorescence emissions. Because of the excited-state thermodynamic equilibrium, the relative intensities (or fluorescence quantum yields) of the LE-state (xle) and TICT-state ( Ticr) emissions may be correlated with the enthalpy (AH) and entropy (A5) differences between the two excited states ... 

Azulene is the best-known exception to Kasha s rule and serves as a model for other nonaltemant aromatic compounds, which also exhibit anomalous fluorescence from their second excited singlet states. This anomalous anission of the Sj state was first observed unambiguously by Beer and Longuet-Higgins [2] and has been confirmed many times in more recent studies [15,16]. The second excited singlet state of azulene has a lifetime of ca. 1 to 2 ns in both the gas phase and in solution, and exhibits dual emission, decaying radiatively to S, with a minute quantum yield (< 10" ) [17,18] and to So with a quantum yield most recently determined to be 0.041 (in ethanol at room temperature) [16]. Earlier studies placed the Sj - Sq fluorescence quantum yield near 0.03 [19,20]. Small et al also recently measured the quantum yield of azulene s Sj-Sq nonradialive decay using a completely independent... 

Often all three approaches ((1) to (3)) are simultaneously active, e.g., in the dual fluorescence of the classical HCT compounds like DMABN where both the energy of the long-wavelength emission (factor (1)) as well as the ratio between the two bands (factors (2) and (3) above) varies. The simultaneous presence of two bands allows very accurate ratio measuring, and small changes in this ratio can be detected more accurately as if only changes in absolute quantum yields have to be observed. These probes with dual fluorescence will be called type-3. ... 

Fig. 2.18. Low-temperaure slopes of the ratio of the quantum yields of dual fluorescence for diethylaminobenzonitrile in two polar solvents, propionitrile and capronitrile, of different E .87...

Chlorophyll a fluorescence was measured with a dual-modulated fluorometer (Photosystem Instruments, Trtilek et al. 1997). Minimum (Fa) and maximum fluorescence (Fm) was recorded after dark adaptation (5 min at 4°C, sufficient to attain stabilization of the fluorescence signal for all light regimes). The maximum quantum yield of photosynthesis FvIFm was calculated as (Fm-F0)/Fm (Krause and Weis 1991). 

Based on the interplay between the complexing ability of crown-ethers or cry tands and the dual fluorescence (monomer/excimer) of aromatic rings (especially anthracene), new supramolecular receptors were designed and synthesized. Modulation of the fluorescrace (wavelengA, quantum yield, excimer/monomer ratio) of different photoactive systems is observed as a specific response to the complexation of metal (Na ", K" ", Ag" ", Tl", Rb" ") or molecular Ql3N -(CH2)n- NH3) cations. These properties could be used for light frequency conversion (anthraceno-coronand), detection and estimation of cations at low concentration (craterophanes, tonnelet) and evaluation of the solvent polarity (semaphorene). 

The fluorescence of A33 in methanol is dual but the quantum yield is low ( p 0.04) compared with that of 9,10-dipropyl anthracene, the reference compound ( p = 0.76). By complexation with an excess of metal catirms such as K ( p = 0.30) or protonation with an excess of CF COOH ( = 0.76), a large fluorescence intensity increase is observed with the disappearance of the exciplex emission (Scheme 2) it is consistent with die fact that the nitrogen lone pairs interact no more with the aromatic ring but with the metal cations or the protons (8a). It is thus possible to use this fluorescence enhancement to evaluate cations as shown in Fig. 7 for K in CH3OH. Other systems closely related to these cryptands designed and prepared by Czamik (20a) and De Silva (2(m) are described in this book. 

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