S
R
2
3
4
5
2 - Head
5 - Tail
a
b
S
R
HH
HT
TT
c
S
R
S
R
S
R
S
S
R
R
HT-HT
TT-HT
S
S
R
R
S
R
S
S
R
S
R
R
HH-TT
S
S
S
R
R
R
S
S
S
R
R
HH-TH
R
Grignard Reagent
Br
Br
S
R
Br
BrMg
S
R
MgBr
Br
Fig. 2 (a) Numbering of the carbon atoms on the thiophene ring, indicating the head and tail
positions on the ring and the isomers obtained on reaction with a Grignard reagent. (b) Structures
of possible couplings in the dimers of 3-alkylthiophene rings. (c) Structures of the four possible
regioisomeric triads of 3-alkylthiophene
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Steric Repulsions
HT coupling
a
b
Fig. 3 Structures showing (a) the solid-state packing efficiency of HT-coupled P3HT and (b)
steric repulsions in regioirregular P3HT that disrupt the solid-state packing
6
P. Sista and C.K. Luscombe