Fluorescent Species

As shown in Figure 27-1, fluorescence is one of several mechanisrns by which a molecule returns to the ground state after it has been excited by absorption of radiation. All absorbing molecules have the potential to fluoresce, but most compounds do not because their structure provides radiationless pathways for relaxation to occur at a greater rate than fluorescence emission. The quantum yield of molecular fluorescence is simply the ratio of the number of molecules that fluoresce to the total number of excited molecules, or the ratio of photons emitted to photons absorbed. Highly fluorescent rnolecules, such as fluorescein, have quantum efficiencies that approach unity under some conditions. Nonfluorescent species have efficiencies that are essentially zero.

Fluorescence and Structure

Compounds containing aromatic rings give the most intense and most useful molecular fluorescence emission. While certain aliphatic and alicyclic carbonyl compounds as well as highly conjugate double- bonded structures also fluoresce. there are very few of these compared with the nurnber of fluorescent cornpounds containing arornatic systerns. Most un substituted aromatic hydrocarbons fluoresce in solution, with the quanturn efficiency increasing with the nurnber of rings and their degree of condensation. The simplest heterocyclics. such as pyridine, furan, thiophene. and pyrrole, do not exhibit rnolecular fluorescence (Figure 27-3), but fused-ring structures containing these rings often do (Figure 27-4). Substitution on an aromatic ring causes shifts in the wavelength of absorption maxima and corresponding changes in the fluorescence peaks. In addition, substitution frequently affects the fluorescence efficiency. These effects are demonstrated by the data in Table 27-1. The Effect of Structural Rigidity

Experiments show that fluorescence is particularly favored in rigid rnolecules. For example, under similar measurement conditions, the quantum efficiency of fluorene is nearly 1.0, while that of biphenyl is about 0.2 (Figure 27-5). The difference in behavior is a result of the increased rigidity provided by the bridging rnethylene group in fluorene. This rigidity lowers the rate of nonradiative relaxation to the point where relaxation by fluorescence has time to occur. There are many similar examples of this type of behavior. In addition, enhanced ernission frequently results when fluorescing dyes are adsorbed on a solid surface; here again, the added rigidity provided by the solid may account for the observed effect. The influence of rigidity also explains the increase in fluorescence of certain organic chelating agents when they are cornplexed with a metal ion. For example, the fluorescence intensity of 8-hydroxyquinoline is rnuch less than that of the zinc complex (Figure ).

Temperature and Solvent Effects

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