Polar Order in PDF1M Azobenzene Polymer Films
J. Phys. Chem. B, Vol. 107, No. 36, 2003 9739
TABLE 1: Linear Optical Constants in a PDR1M Thin
Film after Poling, Determined from a Treatment of the
Absorption Spectraa
film thickness: 60 nm
1064 nm
(fundamental
wave)
532 nm
(harmonic wave)
ne ) n| ) 1.969 no ) n ) 1.668 ne ) n| ) 2.175 no ) n ) 1.838
Re ) R| ) 0 Ro ) R )0 Re ) R| ) Ro ) R )
112 800 cm-1 28 700 cm-1
a The absorption coefficient is defined by Ri ) 4πki/λ.
p-polarized absorption spectra under oblique incidences (see
Figure 3) cannot be assigned to a new peak polarized along the
poling field. Indeed, no new band appears around 400 nm in
the variations of the anisotropic extinction coefficients presented
in Figure 2. In fact, this “apparent band” arising from an optical
effect is mainly due to the very strong anisotropic dispersion
of the refractive indices in this spectral range. This point
confirms that the anisotropy in the extinction coefficients
obtained through the procedure described in the previous section
Figure 5. Calculated refractive index spectra, n(λ), of a pDR1M thin
film (60 nm) before poling (full line) and after poling, in the plane (no,
ko: b) and out of the plane (ne, ke: 9) of the film. Mean refractive
index ( n ) (2no + ne)/3) spectrum (dotted line) after poling is also
reported for a better comparison. In the near-infrared range, refractive
indices extrapolated using Sellmeier’s law are also reported.
absorption, the intensity of the p-p polarized SHG responses
is several orders of magnitude stronger than that of a quartz
plate. The ratio of the maxima of the p-p versus the s-p fringes
is known to be dependent on the strength of the applied poling
field. It is worth noting that this ratio, nearly equal to 19.0 here,
is by far larger than the limit (∼9.0) of a low poling field
condition.24 Nevertheless, a careful analysis of the patterns
must be undertaken because they are probably perturbed by
resonance enhancement effects and by the high number den-
sity of chromophores. In particular, it has been evidenced that
large discrepancies in the dij values may appear when the
linear dichroism effects are neglected.17 Furthermore, we have
observed that the SHG responses are stable for a long period
of time in all poled samples, particularly in those thicker than
∼150 nm.
2
is really an indication of the degree of orientation, cos θ , of
the molecular polar groups.
Currently, the axial ordering of the chromophores in poled
films has been estimated using Φ as the order parameter5,6 and,
therefore, this crude approach leads to an overestimation of the
ordering, as demonstrated hereafter. Differently, from the aniso-
tropic extinction coefficients it is possible to define precisely
an axial order parameter by using the relation P2 ) (ke -
ko)/(ke + 2ko). This order parameter, P2 , is now adequately
defined because it is calculated from only the anisotropic
extinction coefficients of the poled film; its value P2 ) +0.43
is significantly lower than the suggested previous one, Φ )
+0.54. However, an apparent discrepancy is evidenced when
looking carefully at the mean extinction coefficient k ) (ke +
2ko)/3 after poling, as shown in Figure 2. The mean extinction
coefficient exhibits a different shape and is markedly less intense
than the initial one before poling, particularly in the high-energy
part of the spectrum. Similarly, the mean refractive index n
) (ne + 2no)/3 of the poled film in the near-infrared part of the
spectrum is also found smaller than the initial one before poling
(see Figure 5). All these differences in the optical constants are
thus due to perturbations in the molecular interactions, which
are induced under poling. Looking now at the anisotropic
extinction coefficients, the out-of-plane ke(νj) spectrum clearly
exhibits an intense contribution around ∼525 nm. In this respect,
it is well-known in donor-acceptor azobenzene systems that a
net stabilization of the first electronic excited states with
increasing polarity is associated with a bathochromic shift of
the π-π* absorption band; indeed, a photoexcitation leads to
the formation of delocalized excited forms (π-π* transitions),
which can be converted into stable charge-transfer (CT) forms
(zwitterionic structures), particularly when the surrounding is
strongly polar.25,26 The bathochromic effect observed for the
out-of-plane and in-plane extinction coefficient spectra is thus
consistent with a stabilization of the charge-transfer state and
with an increase in the polarity of the medium; obviously, this
effect is predominant for the out-of-plane band, because it
coincides with the uniaxial electrical constraint.
Discussion
Analysis of the Macroscopic Susceptibilities: Chro-
mophore Orientation. We have already pointed out that the
polarized absorption spectra under normal and oblique inci-
dences as well as the SHG Maker fringes patterns do reflect
efficient poling effects to a large extent. They are all consistent
with a strong structuration in the film, which tends to align the
polar chromophores along the applied field direction. In a first
crude approximation, one may describe the axial orientation of
the chromophores by the order parameter Φ ) 1 - A/A0,5 where
A0 and A are respectively the absorbance maxima for the unpoled
and poled samples at normal incidence. Using this relation, we
obtain Φ ) +0.54. This order parameter definition holds valid
under the assumption that, first, there is no degradation of the
chromophores during the poling process and, second, the
molecular ordering does not modify the overall characteristics
of the band. In fact, it is rather difficult to proove experimentally
that the first condition is fulfilled. The second assumption is
obviously in contradiction with the large changes that could
occur during and after an efficient poling process.
After poling, a very strong anisotropy in the extinction
coefficients does appear, a result consistent with a new structural
organization of the chromophores in the oriented polymer
system. This fact is also nicely illustrated in Figure 5, which
shows the variations of the refractive indices in the visible; the
variations were extrapolated in the infrared range, far from any
charge-transfer resonance, where the birefringence is estimated
as large as ∼0.21 near 1.064 µm. It is thus clearly demonstated
that the “narrow band” observed around 400 nm in the
A strong breaking of symmetry in the film induced by the
poling process, as evidenced in the LO responses, has also been
evidenced in the NLO quadratic responses, confirming that the
final molecular organization in the poled films is markedly polar.
Using the values of LO constants above determined (see Table
1), the p-p and s-p polarized Maker fringe patterns recorded
at 1064 nm were analyzed; in thin films (∼60 nm thickness)