Fluorescence and Structure

The most intense fluorescence is found in compounds containing aromatic group with low-energy π→π∗ transitions. A few aliphatic, alicyclic carbonyl, and highly conjugated double-bond structures also exhibit fluorescence as well. Most unsubstituted aromatic hydrocarbons fluoresce in solution too. The quantum efficiency increases as the number of rings and the degree of condensation increases. Simple heterocycles such as the structures listed below do not exhibit fluorescence.

Pyridine Pyrrole Furan Thiophene

With nitrogen heterocyclics, the lowest energy transitions is involved in n→π∗ system that rapidly converts to the triplet state and prevents fluorescence. Although simple heterocyclics do not fluoresce, fused-ring structures do. For instance, a fusion of a benzene ring to a hetercyclic structure results in an increase in molar absorptivity of the absorption band. The lifetime of the excited state in fused structure and fluorescence is observed. Examples of fluorescent compounds is shown below.

quinoline

Benzene ring substitution causes a shift in the absorption maxima of the wavelength and changes in fluorescence emission. The table below is used to demonstrate and visually show that as benzene is substituted with increasing methyl addition, the relative intensity of fluorescence increases.

Compound	Structure	Wavelength of Fluorescence (nm)	Relative intensity of Fluorescence
Benzene		270-310
Toluene		270-320
Propyl Benzene		270-320

Table 2. Relative intensity of fluorescence comparison with alkane substituted benzenes.

The relative intensity of fluorescence increases as oxygenated species increases in substitution. The values for such increase is demonstrated in the table below.

Compound	Structure	Wavelength of Fluorescence (nm)	Relative intensity of Fluorescence
Phenol		285-365
Phenolate ion		310-400
Anisole		285-345

Table 3. Relative intensity of fluorescence comparison with benzene with oxygenated substituted benzene

Influence of a halogen substitution decreases fluorescence as the molar mass of the halogen increases. This is an example of the “heavy atom effect” which suggest that the probability of intersystem crossing increases as the size of the molecule increases. As demonstrated in the table below, as the molar mass of the substituted compound increases, the relative intensity of the fluorescence decreases.

Compound	Structure	Wavelength of Fluorescence (nm)	Relative intensity of Fluorescence
Fluorobenzene		270-320
Chlorobenzene		275-345
Bromobenzene		290-380

Table 4. Relative intensity fluorescence comparison with halogen substituted compounds

In heavy atom substitution such as nitro derivatives or heavy halogen substitution such as iodobenzene, the compounds are subject to predissociation. These compounds have bonds that easily rupture that can then absorb excitation energy and go through internal conversion. Therefore, the relative intensity of fluorescence and fluorescent wavelength is not observed and this is demonstrated in the table below.

Compound	Structure	Wavelength of Fluorescence (nm)	Relative intensity of Fluorescence
Iodobenzene		None
Anilinium ion		None
Nitrobenzene		None

Table 5. Relative fluorescent intensities of iodobenzene and nitro derivative compounds

Carboxylic acid or carbonyl group on aromatic ring generally inhibits fluorescence since the energy of the transition is less than transition. Therefore, the fluorescence yield from transition is low.