"Beautiful..."
Kana gazed at the test tube. Blue solution.
"Copper ion complex," Milia explained. "Water molecules are coordinated."
"Coordinated?"
"A state where molecules or anions surround a metal ion."
Rei added. "Copper ions are blue. But have you thought about why?"
Kana shook her head. "I thought it just has color..."
"Color arises from light absorption," Milia said.
"Absorption?"
"When white light hits, specific wavelengths are absorbed. The rest reflects and reaches our eyes."
Rei drew a diagram. "Copper complex absorbs red light. That's why it looks blue."
Kana wrote in her notebook. "Absorption and color are complementary?"
"Exactly. Why specific wavelengths are absorbed is explained by ligand field theory."
Milia showed her tablet. "Transition metal ions have d-orbitals."
"D-orbitals?"
"A type of orbital where electrons reside. There are five."
Rei continued. "In an isolated metal ion, the five d-orbitals have equal energy."
"Equal?"
"Called degeneracy. But when ligands approach, this symmetry breaks."
Kana was confused. "Breaks?"
"Because ligand electrons repel. Energy of some d-orbitals increases."
Milia drew a diagram. "In octahedral complexes, the five d-orbitals split into two groups."
"eg orbitals and t2g orbitals," Rei added.
"Why do they split?"
"Different distances from ligands. eg orbitals point toward ligands. Stronger repulsion."
Kana began to understand. "So higher energy?"
"Yes. This splitting width is called crystal field splitting energy."
Milia continued. "d-electrons want to be in the lower energy t2g orbitals."
"But?"
"By absorbing light, they can be excited to eg orbitals."
Rei emphasized. "That light energy must match the splitting width."
Kana's eyes sparkled. "That's why specific wavelengths!"
"Correct. In copper complex, the splitting width corresponds to red light energy."
Milia took out another test tube. "This is green. Nickel complex."
"Different splitting width?"
"Different metal ions mean different numbers of d-electrons. Ligands also have effects."
Rei explained. "Strong ligands create large splitting. Weak ligands, small splitting."
"Ligand strength?"
"There's an order called the spectrochemical series. CN minus is strong. I minus is weak."
Kana was moved. "Everything connects..."
Milia continued. "Hemoglobin's red color is also the same principle."
"What?"
"Iron complex. Iron ion coordinated in porphyrin ring."
Rei added. "When oxygen binds, the ligand field changes. Color changes too."
"That's why arterial and venous blood have different colors!" Kana exclaimed.
"Yes. Life phenomena also rest on coordination chemistry."
Kana looked at the test tube again. "This blue color, the cry of d-orbitals?"
Milia laughed. "Poetic, but in a sense correct."
"Electrons rise to excited states and return immediately. That repetition," Rei said.
"Do they emit light when returning?"
"In most cases, released as heat. Fluorescence is a different story."
Kana asked. "Then why are plants green?"
"Chlorophyll. Magnesium complex," Milia answered.
"Absorbs red and blue, reflects green," Rei added.
"Choosing optimal wavelengths for photosynthesis?"
"Probably result of evolution."
Kana murmured. "Color is the voice of molecules."
"Beautiful expression," Milia acknowledged.
Rei said quietly. "Quantum mechanics explains color. Invisible structure of d-orbital splitting creates visible color."
"The invisible determines the visible."
"That's the joy of chemistry."
The three gazed at the blue complex. Now they understand why it absorbs light.