Analytical Chem istry - DePauw University

616 Analytical Chemistry 2.0quinineOHOFigure 10.49 Tonic water, which containsquinine, is fluorescent when placedunder a UV lamp. Source: Splarka (commons.wikipedia.org).NNweak fluorescence. Most unsubstituted, nonheterocyclic aromatic compoundshave favorable fluorescence quantum yields, although substitutionson the aromatic ring can significantly effect F f . For example, the presenceof an electron-withdrawing group, such as –NO 2 , decreases F f , while addingan electron-donating group, such as –OH, increases F f . Fluorescencealso increases for aromatic ring systems and for aromatic molecules withrigid planar structures. Figure 10.49 shows the fluorescence of quinineunder a UV lamp.A molecule’s fluorescent quantum yield is also influenced by externalvariables, such as temperature and solvent. Increasing the temperature generallydecreases F f because more frequent collisions between the moleculeand the solvent increases external conversion. A decrease in the solvent’sviscosity decreases F f for similar reasons. For an analyte with acidic or basicfunctional groups, a change in pH may change the analyte’s structure andits fluorescent properties.As shown in Figure 10.48, fluorescence may return the molecule to anyof several vibrational energy levels in the ground electronic state. Fluorescence,therefore, occurs over a range of wavelengths. Because the change inenergy for fluorescent emission is generally less than that for absorption, amolecule’s fluorescence spectrum is shifted to higher wavelengths than itsabsorption spectrum.Re l a x a t i o n b y Ph o s p h o r e s c e n c eA molecule in a triplet electronic excited state’s lowest vibrational energylevel normally relaxes to the ground state by an intersystem crossing to a singletstate or by an external conversion. Phosphorescence occurs when themolecule relaxes by emitting a photon. As shown in Figure 10.48, phosphorescenceoccurs over a range of wavelengths, all of which are at lower energiesthan the molecule’s absorption band. The intensity of phosphorescence,I p , is given by an equation similar to equation 10.28 for fluorescenceI = 2. 303kΦεbCP = kP ′pp 0 010.29where F p is the phosphorescent quantum yield.Phosphorescence is most favorable for molecules with n p* transitions,which have a higher probability for an intersystem crossing thanp p* transitions. For example, phosphorescence is observed with aromaticmolecules containing carbonyl groups or heteroatoms. Aromaticcompounds containing halide atoms also have a higher efficiency for phosphorescence.In general, an increase in phosphorescence corresponds to adecrease in fluorescence.Because the average lifetime for phosphorescence is very long, rangingfrom 10 –4 –10 4 s, the phosphorescent quantum yield is usually quite small.An improvement in F p is realized by decreasing the efficiency of externalconversion. This may be accomplished in several ways, including loweringthe temperature, using a more viscous solvent, depositing the sample on a