Crystal Engineering on Organic Pigments

H
N
O
H
N
O
Isolated molecule
Weak conjugation
of benzene rings
=> yellow
Colours of quinacridone
O
N
H
O
N
H
H
N
O
H
N
O
O
N
H
O
N
H
<strong>Crystal</strong>
Reasons for the colour change:
(1) Enhanced conjugation
=> HOMO-LUMO distance smaller
=> Absorption changes from violet to green-yellow
=> Colour changes from yellow to red-violet
(2) Interaction of transition dipole moments
(Exciton coupling)
N
O
H
N
O
H
H
O
N
H
O
N
[Calculation by P. Erk]