Chlorophyll States After Light Absorption

When chlorophyll molecules in green plants absorb light energy, their electrons are excited to higher energy levels. These excited states can exist in different forms based on the spin of the electrons. The primary excited state initially formed is usually the singlet state, which can then undergo transitions to other states, including the triplet state.

Understanding Singlet and Triplet States

The difference between singlet and triplet states lies in the spin alignment of the excited electron relative to the ground state electron (or the remaining electron in the orbital). The multiplicity of a state is given by the formula \(2S + 1\), where \(S\) is the total spin angular momentum, which is the sum of the individual electron spins.

• In the singlet state, the spins of the two electrons are anti-parallel (opposite directions). If the spins are \(+1/2\) and \(-1/2\), the total spin \(S = +1/2 + (-1/2) = 0\). The multiplicity is \(2(0) + 1 = 1\), hence 'singlet'. Electrons have anti-parallel spins.
• In the triplet state, the spins of the two unpaired electrons are parallel (same direction). If the spins are \(+1/2\) and \(+1/2\), the total spin \(S = +1/2 + (+1/2) = 1\). The multiplicity is \(2(1) + 1 = 3\), hence 'triplet'. Electrons have parallel spins.

Lifetime of Chlorophyll States

The lifetime of an excited state refers to how long the molecule stays in that state before returning to the ground state or undergoing a transition to another state. The transitions from excited states back to the ground state or to other states have different probabilities and pathways.

• Singlet state lifetime: The singlet excited state (\(S_1\)) is typically very short-lived, usually on the order of nanoseconds (\(10^{-9}\) s). It can quickly return to the ground state (\(S_0\)) through processes like fluorescence, internal conversion, or intersystem crossing to the triplet state.
• Triplet state lifetime: The triplet excited state (\(T_1\)) is generally much longer-lived than the singlet state, often on the order of microseconds or even milliseconds (\(10^{-6}\) to \(10^{-3}\) s). This is because transitions from the triplet state to the singlet ground state are spin-forbidden (require a change in spin multiplicity), making the decay process slower.

Analyzing the Statements

Let's evaluate the given statements based on our understanding:

• A. Singlet state is short lived compared to triplet state. This statement is correct. Singlet states decay much faster than triplet states.
• B. Singlet state is long lived compared to triplet state. This statement is incorrect, as it is the opposite of statement A.
• C. Singlet state contains electrons with anti-parallel spins while triplet state has electrons with parallel spins. This statement is correct based on the definition of singlet (\(S=0\), anti-parallel spins) and triplet (\(S=1\), parallel spins) states.
• D. Singlet state contains electrons with parallel spins while triplet state has electrons with anti-parallel spins. This statement is incorrect, as it is the opposite of statement C.

Based on the analysis, statements A and C are correct.