Page 108 - Photoreactive Organic Thin Films

3, PHOTO-ORIENTATION BY PHOTOISOMERIZATION gj

were done by recording the transmitted light of a probe propagating
perpendicular to the sample and polarized at the magic angle, - 54.7 degrees,
from the vertically polarized pump, so as to eliminate anisotropy contribu-
tions to absorption changes. The extent of cis concentration, i.e., the a value,
found at 488 nm for PUR-1 and PUR-3 are 0.158 and 0.077, respectively.
The extinction coefficients are also given in Table 3.2.
For all Azo-PURs, the quantum yields of the forth, i.e., trans-^cis, are
small compared to those of the back, i.e., cis~>trans, isomerization—a feature
that shows that the azo-chromophore is often in the trans form during
trans«-»cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every
10(30 photons absorbed, and once in the cis, it isomerizes back to the trans
for about 2 absorbed photons. In addition, the rate of cis—>trans thermal
1
isomerization is quite high: 0.45 s" . Q~l shows that upon isomerization, the
azo-chromophore rotates in a manner that maximizes molecular nonpolar
orientation during isomerization; in other words, it maximizes the second-
order Legendre polynomial, i.e., the second moment, of the distribution of
the isomeric reorientation. Q~l also shows that the chromophore retains full
memory of its orientation before isomerization and does not shake
indiscriminately before it relaxes; otherwise, it would be Q~Q. The fact that
the azo-chromophore moves, i.e., rotates, and retains full orientational
memory after isomerization dictates that it reorients only by a well-defined,
discrete angle upon isomerization. Next, I discuss photo-orientation processes
in chromophores that isornerize by cyclization, a process that differs from the
isomeric shape change of azobenzene derivatives.

3.6 PHOTO-ORIENTATION OF PHOTOCHROMIC SPIROPYRANS AND
DiAKfLETHENES

Most optically induced reorientation studies have focused on azo-dye-con-
taining materials; studies of the light-induced orientation of chromophores
other than azobenzene derivatives and azobenzene-type molecules have rarely
been reported. It will be shown that it is possible to individualize photo-
oriented isomers of diarylethene and spiropyran derivatives in thin films of
PMMA. Such photochromic chromophores have been extensively studied,
32 34
not only from a photochemical point of view, "" but also for use in near-
35 37
field and three-dimensional optical data storage, "" and for photo-assisted
38
poling and hyperpolarizability switching. Photoisomerization of diaryiethenes
and spiropyrans is different from that of azobenzene derivatives. The photo-
isomerization of the former two leads to reversible ring closing (photo-
cyclization) and opening, and their apparent photo-orientation changes sign
for the UV versus the visible photochemical transition bands, a feature that is
due to the perpendicular UV and visible transitions in the B isomer. The
photo-orientation processes of both isomers of each of the diarylethene
and spiropyran chromophores studied will be separated by using polarized
LTV-vis and real time dichroism spectroscopies and by taking advantage of the
natural spectral differences exhibited by the photoisomers in the UV-vis
region.