Q.3 Which one of the following is not true according to Kasha’s rule about fluorescence?
- Bathochromic shift is observed in emission relative to absorption
- Hypsochromic shift is observed in absorption relative to emission
- Bathochromic shift is observed in absorption relative to emission
- None of these
Kasha’s rule states that fluorescence emission occurs from the lowest excited singlet state (S1), leading to a bathochromic (red) shift in emission relative to absorption due to vibrational relaxation. Option C is not true, as absorption occurs at higher energy (shorter wavelength) than emission.
Introduction
Kasha’s rule fluorescence bathochromic shift explains why emission spectra show longer wavelengths than absorption in molecules. This principle, key for molecular biology and spectroscopy exams, involves non-radiative relaxation to the lowest excited state before photon emission.
Kasha’s Rule Basics
Kasha’s rule states that fluorescence (or phosphorescence) happens only from the lowest excited state of a given multiplicity, typically S1 for singlets. Molecules absorb light to higher states (S2 or Sn) but relax vibrationally to S1’s ground vibrational level before emitting. Emission wavelength stays constant regardless of excitation wavelength, causing a Stokes shift.
Spectral Shifts Defined
Bathochromic shift moves λmax to longer (redder) wavelengths; hypsochromic shift moves it to shorter (bluer) ones. In fluorescence, absorption (S0 to S1/Sn) is at higher energy (shorter λ); emission (S1 to S0) follows relaxation, so emission λ > absorption λ—a bathochromic shift for emission.
Option Analysis
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A: Bathochromic shift in emission relative to absorption—True. Emission from relaxed S1 occurs at lower energy (longer λ) than initial absorption.
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B: Hypsochromic shift in absorption relative to emission—True. Absorption λ is shorter (higher energy) than emission λ, so absorption appears hypsochromic vs emission.
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C: Bathochromic shift in absorption relative to emission—False. Absorption is at shorter λ than emission; no bathochromic (longer λ) shift occurs in absorption relative to emission.
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D: None of these—Incorrect, as C is false.
Option Statement True/False per Kasha’s Rule Reason A Bathochromic shift emission vs absorption True Emission λ > absorption λ post-relaxation B Hypsochromic shift absorption vs emission True Absorption λ < emission λ C Bathochromic shift absorption vs emission False Absorption at higher energy D None False C violates rule Exceptions and Applications
Rare exceptions occur with large energy gaps between states, allowing emission from higher levels. In plant sciences or biotech, this aids fluorophore design in microscopy/PCR probes.
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