Page 112 - Photoreactive Organic Thin Films

3. PHOTO-ORIENTATION BY PHOTOISOMERIZATION

360nm);» (X = 520nm) • (X = 570 ran)

0 100 200
Angle, ¥ (degrees), between Pump and Probe Polarizations
FIGURE 3, 1 6 Dependence of the absorbance of PMMA films containing (left) diarylethene and
(right) spiropyran on the angle, *F, between UV irradiation and probe beam polarizations. The
normalized absorbance is defined as (Abs - Abs min)/(Abs - , where Abs min and Abs,^ are the
maximum and minimum absorbances, respectively. The // and J_ signs stand for the directions parallel
and perpendicular to the UV polarization, respectively. The markers are experimental data points, and
2
the full and dashed lines are cos *P theoretical fits. The analysis wavelengths are indicated. Note the
apparent it/2 angle-shift between the UV and visible orientational distributions of the diarylethene
chromophore. After reference 29, redrawn by permission.

were separated by a Wollaston prism and detected separately. The probe
beam was the 633 nm red and 532 nm green light for SP and DE,
respectively, so that photo-orientation processes of the isomer B were probed
independently from those of the isomer A. Absf t and Absf were calculated
from the amount of absorbed light, and the isotropic absorbance Ag, the
anisotropy AA B, and the order parameter S B were deduced. S B was calculated
at the steady state of photo-orientation. Photo-orientation of SP in PMMA is
discussed first.
Figure 3.17 shows the time evolution of A B and AA B of SP/PMMA during
and after linearly polarized UV irradiation for different irradiation power

l-» 0.18X10"°; (0.144)
8
2-»0.37xlO" ; (0.147)
!
3-»0.73xlO~ ; (0.150)
8
4-* I.28xl0~ ;(0.147)
8
5-» 1.82XIO" ; (0.160)

150 0 50
Time t/minutes
FIGURE 3.17 Real-time evolution of the isotropic absorbance (left) and the anisotropy (right) of
SP in PMMA upon linearly polarized UV irradiation for several irradiation intensities.The numbers from
2
I to 5 indicate the value of the irradiation intensity in units of Einstein.s~'.cm~ with the corresponding
sample absorbance (value in parentheses) at the irradiation wavelength (365 nm). The moments of
turning the irradiation light on and off are indicated. After reference 26, redrawn by permission.