Page 152 - Photoreactive Organic Thin Films

4. PHOTOISOHERIZATION AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER | 3 j

demonstrates chromophore photo-orientation. When irradiation is turned
off at time t = 10 minutes, the observed relaxation indicates that cis—>trans
thermal isomerization, which is completed after a few seconds in PUR-1 and
PUR-2, and takes several minutes to more than an hour in PUR-3 and PUR-4
(vide infra), converts cis to trans isomers, and the remnant anisotropy
demonstrates that the trans molecules are oriented after isomerization,
Photo-orientation by photoisomerization occurs through a polarization-
sensitive photoexcitation, i.e., photoselection. Two competing limiting
cases of photoselection are worth discussing. If the chroniophores are
photoisomerized only through photoselection and are not rotated, a large cis
population is anisotropically generated, and a hole is burned into the trans
isomer's orientational distribution (orientational hole burning, OHB, cosine
square probability of photoexcitation). In this case, both Abs (/ and Abs L
change in the same direction with (Abs// - Abs Q) = + 3 (Abs L - Abs^). Ab$ 0 is
the sample's absorbance before irradiation. Pure photo-reorientation occurs
when only the trans isomer is rotated by a discrete angle for each absorbed
photon, a feature that implies high reorientation rates for high-irradiation
intensities. Pure photo-reorientation can involve the cis isomer, but only when
it returns immediately to the trans isomer; therefore, the concentration of cis
isomers is negligible during pure photo-orientation, and the chromophore is
in the trans form most of the time during cis4~»trans isomerization cycling.
Pure photo-reorientation is theoretically characterized by high anisotropy
values for high-irradiation intensities and by a dynamic behavior in which
Abs//- and Abs ± evolve in opposite directions starting from the moment when
polarized light impinges the sample with (Abs// - Abs 0) = - 2 (Abs± - Abs Q),
The factors + 3 and - 2 originate from the orientational averaging of the
chroniophores' polarizability after isomerization and the orientation by
photoselection, respectively. Upon polarized irradiation, both OHB and pure
photo-reorientation decrease Abs//, whereas pure photo-reorientation
increases Abs ± and OHB decreases it in a competing manner. The trends of
Figures 4.18 and 4.19 can be explained by the competitive scheme of OHB
versus pure photo-reorientation. Upon polarized irradiation, Abs f/ decreases
in all four Azo-PURs, and Abs ± increases for PUR-1 and PUR-2 and decreases
for PUR-3 and PUR-4. OHB is dominant in PUR-3 and PUR-4 because of a
long-living cis isomer; in addition, the increase, after some time, of Abs± in
PUR-3 is indicative of molecular reorientation following OHB.
Near-pure photo-orientation of PUR-1 by polarized irradiation is shown
in Figure 4.16. Indeed, when the irradiating light is turned on, Abs L starts
nearly immediately exceeding the absorbance before irradiation, i.e., Abs 0. At
this same time, Abs// and Abs L change in opposite directions, and the higher
the pump intensity the faster and the larger the increase of Abs L - Abs 0 and
of the anisotropy. The near-pure photo-orientation dynamics observed for
PUR-1 fits the very first model developed for photo-orientation by photo-
isomerization, which assumed that the chromophore is constantly in the trans
state, or in other words, returns immediately to the trans state upon excitation,
and rotates during the excitation cycle by a discrete angle. For all Azo-PURs,
the quantum yields of the forth isomerization (trans—»cis,) are small compared
to those of the back (cis—Hrans) isomerization (cf. Chapter 3); in addition,