Red colour of oxyhemoglobin is due to

GUJARAT SET 2018

Red colour of oxyhemoglobin is mainly due to :

• (A) d-d transition
• (B) Metal to ligand charge transfer transition
• (C) Interligand π-π* transition
• (D) Ligand to metal charge transfer transition

Correct Answer: (C) Interligand π-π* transition

The intense red color of blood (oxyhemoglobin) arises from the heme group, which is a highly conjugated porphyrin ring.

• The Transition: Light absorption occurs due to electronic transitions between the π and π* molecular orbitals of the porphyrin ring (the ligand).
• Soret Band: This transition causes strong absorption in the blue-green region. The remaining light reflected/transmitted is seen as bright red.
• Why not d-d? Iron d-d transitions are "Laporte forbidden" and generally too weak to produce such intense coloration.

Transition Comparison

Transition Type	Intensity	Example/Origin
d-d Transition	Weak	Hydrated Cu2+ (Pale blue)
Charge Transfer	Very Strong	KMnO4 (Deep Purple)
π-π* Transition	Intense	Oxyhemoglobin (Red)