Page 104 - Photoreactive Organic Thin Films

3. PHOTO-ORIENTATION BY PHOTOISOMERIZATION

COS
S A

Equation 3.27 dictates that AA 4SOnm /AA 360nm is constant regardless of the
irradiation dose, a result confirmed by the data in Table 3.1. It is noteworthy
60 450 360 B 350
that the parameters F 2(cos wj ^ ) and p£( >-» < >, whkh characterize
the symmetry of the 360 and 450 nm transitions in the cis isomer and the
reorientation of the UV transition of the azobenzene chromophore during
the trans— »cis photoisomerization, respectively, can be determined when
450
60
P 2 (cos o>B ~* ) is measured at the steady state of photo-orientation using
Equation 3.22. This type of experiment will be discussed eventually for
spiropyran and diarylethene chromophores in films of PMMA. Next, I
compare reorientation observations after cis— Hrans thermal isomerization of
azobenzene to the theoretical developments in Section 3.2.3.2.

3.4.2 Reorientation within the cis-)trans Thermal Isomerization
The process of reorientation during cis— »trans thermal isomerization can be
seen at the value of Q 2 in Equation 3.11, which shows that the cis anisotropy
does not contribute to the trans anisotropy if the trans isomer loses total
memory of the orientation in the cis isomer (Q 2 - 0). It is informative to note
that in the realistic physical case — i.e., the case of the azobenzene molecule
chemically attached to a polymer, where the cis and trans diffusion rates are
negligible in comparison to the cis— »trans isomerization rate — the relaxation
B A
of the cis and trans anisotropy, AA and AA , can be written respectively in
the form:
.
(l - exp(-^)) (3.28)
= exp(- it) and j , 1 + Q 2
B
where AAo' is the anisotropy at the moment the irradiation is stopped.
Figure 3.10 shows that trans-azobenzene anisotropy increases during
cis-»trans thermal isomerization in the P 2)io azo-polyglutamate. Thus, it can be
concluded that the azobenzene molecule has retained memory of its orienta-
tion when returning from the cis to the trans form {Q 2 & 0 in Equation 3.28).
A similar behavior (not shown) was found for an azobenzene self-assembled
28
monolayer. The value of Q 2 can be estimated by comparing reorientation
measurements to Equation 3.28. In the next section, I discuss the photo-
orientation of push-pull azo dyes.

3.5 PHOTO-ORIENTATION OF AZO DYES: SPECTRALLY OVERLAPPING ISOMERS

This section describes how coupled photo-orientation and photo-orientation
can be quantified in spectrally overlapping isomers. Four azo-polyurathanes
(Azo-PURs), PUR-1, PUR-2, PUR-3, and PUR-4 (see Figure 3.11) were
photo-oriented by polarized 488 nm blue light from an Argon-ion laser, and
real-time dichroism analysis at the irradiation wavelength was utilized to