Q8(a). Fluorescence and phosphorescence

Introduction. Both are photoluminescence: light emitted by a molecule after it has absorbed light. They differ in the spin of the emitting state. Fluorescence comes from the lowest excited singlet S₁; phosphorescence from the lowest triplet T₁, reached by intersystem crossing.

1. Origin: fluorescence S₁ → S₀; phosphorescence T₁ → S₀.
2. Spin: fluorescence keeps the spin (Δ S = 0), spin-allowed; phosphorescence reverses a spin, spin-forbidden, made weakly allowed only by spin–orbit coupling.
3. Lifetime: fluorescence about 10⁻⁹–10⁻⁷ s; it stops at once when the light is switched off. Phosphorescence about 10⁻⁴ s to many seconds: an afterglow.

Figure: Jablonski diagram: A absorption, VR vibrational relaxation, F fluorescence (S₁→ S₀), ISC intersystem crossing, P phosphorescence (T₁→ S₀).

1. Wavelength: both are longer than the absorbed light (Stokes shift); phosphorescence is at longer wavelength than fluorescence because T₁ lies below S₁ (Hund's rule).
2. Conditions: fluorescence is seen in fluid solution at room temperature. The long-lived triplet is quenched by O₂ and by collisions, so phosphorescence is usually seen in rigid media (glasses at 77 K, solids); heavy atoms (Br, I) increase ISC and favour phosphorescence.
3. Examples: fluorescence: fluorescein (green in alkali), quinine sulfate in dilute H₂SO₄ (blue), anthracene, chlorophyll (red). Phosphorescence: benzophenone and biacetyl in frozen glasses, copper-activated ZnS in luminous dials, strontium aluminate glow-in-the-dark paints.

Conclusion. Fluorescence: singlet–singlet, spin-allowed, nanosecond lifetime, stops with the light (fluorescein, quinine). Phosphorescence: triplet–singlet, spin-forbidden, long lifetime with afterglow, longer wavelength, needs rigid media (benzophenone at 77 K, ZnS:Cu).

Application. Fluorescence is used in spectrofluorimetric analysis (quinine in tonic water, riboflavin), fluorescent brighteners and microscopy; phosphorescence in luminous paints, oxygen sensors and OLED emitters.