Q.39 Which of the following statements are CORRECT?
(A) Fluorescence has a much longer decay period than that of phosphorescence
(B) Radiative transition from T1 to S 0 is phosphorescence
(C) Radiative transition from S 1 to S 0 is fluorescence
(D) Enhancing the life time of the excited state is quenching
Phosphorescence exhibits a longer decay period than fluorescence due to spin-forbidden transitions. Correct statements are (B) and (C).
Option Analysis
A: Incorrect. Fluorescence decay occurs rapidly (nanoseconds), while phosphorescence lasts milliseconds to seconds because of intersystem crossing to the triplet state.
B: Correct. Phosphorescence involves radiative decay from the triplet state (T1) to ground state (S0), a spin-forbidden process enabling delayed emission.
C: Correct. Fluorescence is the spin-allowed radiative transition from singlet excited state (S1) to ground state (S0).
D: Incorrect. Quenching shortens the excited state lifetime by promoting non-radiative decay; enhancing lifetime opposes quenching.
Fluorescence and phosphorescence represent key photoluminescent processes distinguished by decay period, electronic transitions, and quenching mechanisms. These concepts are essential for CSIR NET Life Sciences exams, covering radiative transitions from S1 to S0 (fluorescence) and T1 to S0 (phosphorescence).
Key Differences
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Fluorescence Decay: Fast (10^-9 to 10^-7 s) spin-allowed S1 → S0 emission.
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Phosphorescence Decay: Slower (10^-3 s to minutes) due to T1 → S0 spin-forbidden transition after intersystem crossing.
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Quenching Impact: Reduces excited state lifetime via non-radiative paths, opposite of lifetime enhancement.
Jablonski Diagram Insights
In Jablonski diagrams, absorption populates S1 or higher singlets. Fluorescence emits promptly from S1 to S0. Phosphorescence follows triplet formation via intersystem crossing, with longer decay from T1 to S0.
Exam Relevance
For competitive exams, note: Fluorescence has shorter decay than phosphorescence; quenching deactivates excited states.
