Chemistry Reference
In-Depth Information
should be approximately 3 kcal/mole greater than that of the acceptor [
113
,
114
]. This type of
sensitization is believed to be diffusion controlled [
115
].
Trecker [
116
] lists the steps of the sensitized dimerization reaction as follows:
S
Excitation: S
ð
sensitizer
Þþhn !
Energy loss: S
3
!
S
Sensitization: S
3
þ
D
!
S
D
D
þ
D
Dimerization:
S
þ
D
D
!
S
D
!
Dimer:
D
Þ
dimer(s
The quantum yield expression for this scheme of dimerization is shown as follows [
115
]:
1
=F ¼
1
=fa þ k
3
=fak
4
ðDÞ
where
F
represents the quantum yield of dimer formation,
f
is the efficiency of sensitizer intersystem
crossing,
D
is the initial concentration of the reactive groups, and
k
's are the specific rate constants for
the reactions shown in the above scheme.
10.5.1.1 Polymers with Pendant Cinnamoyl Functional Groups
Minsk et al. [
117
] may have been the first to synthesize a photocross-linkable polymer, namely poly
(vinyl cinnamate). The photochemistry of this compound is similar to the photo-cyclization of
cinnamic acid that is discussed bellow in this section. It is interesting that the reaction of cyclization
of cinnamic acid can take place even in the solid crystalline stage. This illustrates that the reaction
requires very little molecular motion. Similar reactions occur in polymeric materials that are
functionalized with cinnamate groups. The photocross-linking of poly(vinyl cinnamate) is illustrated
below:
n
2
n
O
O
O
UV light
O
O
O
n
Much earlier, well before Minsk, in 1895, Bertram and Kursten [
118
] recognized that solid
cinnamic acid undergoes a chemical change when exposed to light. Following this, Ruber [
119
]
established that the change is a dimerization of the acid to form a cyclobutane derivative. This
dimerization results in formation of truxillic and truxinic acids:
2
O
2
O
H
hν
O
