Fluorescence

Fluorescence is an example of a radiative photophysical process. It occurs when the return of a singlet electronic excited state to the singlet electronic ground state is accompanied by the emission of a photon. A typical time scale for the lifetime of the excited state is on the order of 10 ns.

Excitation and Emission Spectra

Two distinguishable types of spectra are associated with fluorescence. The fluorometer contains two monochromators. M1, located between the light source and the sample, controls the spectral distribution of light impinging on the sample. The other, M2, is located between the sample and the detector. Source, sample, and detector form a right angle.

• Excitation spectrum: obtained by scanning with M1, holding M2 constant with a setting that allows light emitted by the sample to pass through to the detector. We learn the spectral distribution of the light that produces the excited state responsible for fluorescence. The excitation spectrum for a dilute solution of benzene in cyclohexane, G Fig. 1.5, is similar to its absorption spectrum, showing a band at about 250 nm, with vibrational fine structure arising from the vibrational spacings in S₁.

• Emission spectrum: obtained by scanning with M2, holding M1 constant at a setting that produces the excited electronic state responsible for fluorescence. We learn the spectral distribution of the light emitted by fluorescence. For benzene in cyclohexane, with excitation at 254 nm, the emission band is centered at about 280 nm, with vibrational fine structure determined by the vibrations in S₀, G Fig. 1.5. The energetic separation of the excitation and emission spectra is called the Stoke's shift.

Samples studied as dilute solutions (usually with an absorbance no larger than about 0.1 at the wavelength of excitation) are often contained in 1 x 1 cm cuvettes, with all four sides polished. More concentrated samples, and polymer films, are often studied using front-face illumination. In this technique, the excitating light beam is not normal to the sample, but instead impinges on it at a smaller angle, which is often 45^o. Then the emission from the surface of the sample is monitored by a detector set at an angle of 135^o (90^o from the incident beam).

Quantum Yield and Quenching

The quantum yield for fluorescence, Q, is defined as the number of photons emitted as fluorescence, divided by the number of excited states that were produced in the excitation. Q = 0 if the molecule is non-fluorescent. For fluorescent molecules, the value of Q often is a sensitive function of the environment, and this attribute of fluorescence is often exploited in experiments on polymers. Processes that compete with ordinary fluorescence, such as excimer formation and nonradiative singlet energy transfer, produce a decrease in the value of Q for the normal fluoresence of the chromophore.

Substances which reduce the value of Q are termed quenchers. Atmospheric oxygen is a common quencher of fluorescence. For example, a dilute (0.001 M, in terms of monomer units) solution of atactic polystyrene in dichloroethane at room temperature has a fluorescence that is, G Fig. 7.19:

• Strongest if the sample is purged with N₂
• Weaker if the sample is aerated
• Still weaker if the sample is purged with O₂.

Additional information can be obtained from fluorescence depolarization, where the incident exciting beam is polarized.

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