26. After absorbing light, chlorophyll molecules in green plants exist in singlet and triplet states. Following are certain statement on singlet and triplet states of chlorophyll molecules:
A. Singlet state is short lived compared to triplet state.
B. Singlet state is long lived compared to triplet state.
C. Singlet state contains electrons with anti-parallel. Spins while triplet state has electrons with parallel spins.
D. Singlet state contains electrons with parallel spins while triplet state has electrons with anti-parallel spins.
Which one of the following combinations is correct?
(1) A and B                                (2) B and C
(3) A and C                               (4) B and D

The correct answer is (3) A and C:
A. Singlet state is short-lived compared to triplet state.
C. Singlet state contains electrons with anti-parallel spins, while triplet state has electrons with parallel spins.

These are the true physical properties of singlet and triplet excited states in chlorophyll and other molecules after light absorption.​

Singlet and Triplet States of Chlorophyll: Properties Explained

Introduction

Key phrase: chlorophyll singlet triplet state electron spin lifetime photochemistry

After absorbing light, chlorophyll molecules in photosynthetic systems may exist in singlet and triplet excited states. These states differ by electron spin configuration and their lifetimes—two factors that are fundamental to understanding photochemistry, fluorescence, and biological photoprotection.​

Explanation of Each Statement

• A. Singlet state is short lived compared to triplet state.

  • Correct. The singlet excited state decays very rapidly (nanoseconds), while the triplet state can persist much longer (microseconds to milliseconds) because its relaxation back to the ground state is quantum mechanically forbidden, making it “long lived”.​

• B. Singlet state is long lived compared to triplet state.

  • Incorrect. Triplet states are longer lived than singlet states because transitions are spin forbidden.​

• C. Singlet state contains electrons with anti-parallel spins while triplet state has electrons with parallel spins.

  • Correct. In a singlet state, two electrons have opposite (“anti-parallel”) spins, resulting in a net spin of zero. In a triplet state, their spins are parallel (net spin one).​

• D. Singlet state contains electrons with parallel spins while triplet state has electrons with anti-parallel spins.

  • Incorrect. This reverses the definitions for singlet and triplet states.​

Key Facts: Singlet vs Triplet Chlorophyll States

• Singlet state: short-lived, anti-parallel/spin-paired electrons.

• Triplet state: long-lived, parallel/spin-unpaired electrons.​

• Triplet states are more prone to generating reactive oxygen species (ROS) and require carotenoid-mediated quenching for plant protection.​