Page 110 - Photoreactive Organic Thin Films

3. PHOTO-ORIENTATION BY PHOTOISOMERIZATION gO.

rnerization reactions. The isosbestic points shifted slightly when film samples
were irradiated with green versus UV lights. No spectral change was observed
during the B-^A thermal back reaction of the DE chromophore over 24
hours, meaning that the closed-ring form (the B isomer) of the DE
chromophore is stable at room temperature for this time period. The colored
photomerocyanine isomer (B isomer) of the SP chromophore persists for
several minutes at room temperature. A biexponential fading (B—>A thermal
recovery) of this form at room temperature was observed with rate constants
1
l
(k) of k t = 0.00125 s' and k 2= 0.00009 s~ and weighting factors (amplitudes)
0.5157 for kj and 0.4017 for k 2 in reasonable agreement with previous
38 39
results. ' Although the polymer free volume, the free-volume distribution,
or both, could influence the fading-rate constants, in the absence of
aggregation, the biexponential behavior is usually attributed to the dual form
of the photomerocyanine (i.e., the quinonic and zwitterionic forms). The
assessment of the photo- and thermal isomerization features is necessary for
photo-orientation studies.

3.6.2 Spectral Features of Photo-orientation
Figure 3.14 shows the dichroic spectra observed in films of DE/PMMA and
SP/PMMA. The insets in Figure 3.15A are expanded views of both the UV
and the visible absorptions of the DE chromophore. These spectra were
2
obtained 30 s after polarized UV irradiation (irradiation dose: 78 mj/cm ).
It is clear that Abs// and Absx are different; in other words, the irradiated
samples show anisotropic absorbance upon polarized UV irradiation.
Identical spectra were recorded for Abs// and Abs ± before UV irradiation,
demonstrating that the samples were in-plane isotropic at that time.
It is particularly remarkable in Figure 3.15 that Abs// is higher than Abs x
in the visible band, where the absorption of the A form is negligible and only
the B form exhibits an appreciable absorption. This result is confirmed by
real-time dichroism experiments (vide infra). Figure 3.16 shows the
absorbance of linearly polarized probe light (at 360 and 520 nm for DE, and
570 nm for SP) at various angles, H*, between the polarizations of the probe
2
and UV lights. The UV irradiation dose was 401 mj/cm . Sinusoidal behavior
is clearly shown, which demonstrates the nonpolar orientational distribution
of the isomers' transitions in both the UV and visible bands. For DE,
perpendicularly oriented transitions are clearly shown for the UV and visible
bands. The small drift (smearing-out of the modulation) in the absorption
data is due to rotational diffusion of the chromophores. Photo-orientation
also occurs during B—»A photoisomerization for both DE and SP in PMMA
(not shown), and the spectral features of photo-orientation are similar to
those observed after A—»B photo-orientation.

3.6.3 Photo-Orientation Dynamics and Transitions Symmetry
Real-time dichroism experiments were used to investigate the dynamics of
photo-orientation of SP and DE in films of PMMA. The samples were
irradiated with linearly polarized light while in situ transmittance