Chemistry Reference
In-Depth Information
NO
2
UV light
N
NO
2
visible light
N
R
O
R
When the irradiation is interrupted and the membrane is irradiated with visible light, the potential
returns to its original value, because the ring closes back to the spiran structure [
125
].
The addition of electron donors, like dialkyl aniline, to Kapton polyimide film
O
O
N
N
O
n
O
O
results in an enhancement of photocurrent by as much as five orders of magnitude, compared with the
virgin material [
258
]. Freulich explains the mechanism of enhancement as a result of radiation
absorption by the charge-transfer complex formed between the added electron donor and the imide
portion of the polymer backbone. Excitations are followed by rapid and complete electron transfer
from the donor to pyromelitimide to yield the radical anion of the polymer and the radical cation of
the donor [
258
]. These species undergo rapid back-electron transfer. In other words, the dialkyl
aniline donates one of the unpaired electrons in a typical photo-reduction reaction to the carbonyl
group. The reaction is reversible and the photo-conduction is by a mechanism of the ion radical
returning to the ground state.
Studies of the thermochromic, solvatochromic, and photoconductive properties of 3-, 4-, 6-, and 9-
poly[(butoxycarbonylinethyl urethane)-diacetylenes] result from changes from rod (red or blue) to
coil (yellow) conformations of the polymer backbone [
259
]. Photo-excitations of the solutions of
these polymers in the rod state result in a large transient photo-conductivity, while only very small
conductivity signals are observed in the coil state. The thermochromic shift that occurs in going from
the rod state at room temperature to the coil state at 65
C is accompanied by a decrease in the photo-
conductivity. The large conductivity signal in the rod state is attributed to the formation of mobile
charge carriers possibly via interchain charge transfer within aggregates. The decay of the photo-
conductivity is nonexponential and extends to microseconds.
Wong et al. [
260
] reported that they prepared a soluble rigid-rod organometallic polymer containing
electron-donating and electron-withdrawing
trans
-[-Pt-(tibutylphosphine)
2
-acetylene-R-acetylene-]
n
(where R
bithiazol-ediyl) groups. The polymer was formed by cuprous iodide-catalyzed dehydroha-
logenation reaction. The electron-donating and electron-withdrawing properties of the thiazole ring
confer solubility to the polymer. This polyacetylene is luminescent with a singlet emission peak at
539 nm and photo-conducting. The glass transition temperature of the polymer is 215
and it shows
relatively good thermal stability. The
ΒΌ
-
conjugation of the ligands extends into and through the metal
core and the absorption peaks show a significant red-shift of 17-26 nm compared to the bithienyl
counterparts due to the presence of the electron-withdrawing imine nitrogen atoms [
260
].
Kimura et al. [
259
] reported applying organic photochromic compounds to photochemical
switching of metal-ion complexation and ionic conduction by combining photochromism with
metal-ion binding property of crown ether derivatives. They synthesized vinyl polymers,
incorporating a crowned spirobenzopyran moiety at the side chain:
p
