Page 42 - Instrumental Analysis - Pharm D Clinical- 07-PA403

Spectrofluorimetry

1- Temperature
Increasing temperature leads to increased molecular motion and collisions →
more non-radiative relaxation → F decreases.

2- Solvent effects
• Solvent polarity: polar solvents enhance fluorescence.
• Solvent viscosity: fluorescence increases as solvent viscosity increases

because energy loss by molecular collision decreases.
• pH: changing pH may alter the structure of the molecule and affect its

quantum efficiency. Compounds with acidic and basic groups show different
F intensity and wavelength for ionized and unionized forms according to pH
changes but no general rule. F may increase or decrease.

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3- Structure of the compound (structural considerations)
• The majority of molecules that give fluorescence have structures made up

of aromatic rings, fused aromatic ring structures or conjugated double bond
structures.
• As the degree of conjugation increases → the number of electrons involved
in the  orbitals increases → absorption becomes more intense → ε and Φ
both increase.

Benzene Naphthalene Anthracene

Fluorescence increases

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