16. Polarization Spectroscopy of Ordered Samples
A slight perturbation of the molecular structure and symmetry, e.g., from a
solvent, and even more from a substituent that destroys the symmetry, may result
in an excited state that is a mixture of two close-lying states. This happens
regularly for the L
b
and L
a
states of conjugated molecules. If one of the two
unperturbed transitions, say the L
a
transition, is much stronger than the L
b
transition, it may dominate the transition moment for a resulting transition, and
the transition moment will turn from the y-axis to a position closer to the z-axis.
Because of the large difference in the original intensities, even a small dose of L
a
mixed into L
b
will rotate the transition moment significantly.
A remarkable example of this solvent effect has been observed for pyrene,
aligned in stretched polyethylene [2, 28]. The low symmetry interaction with the
polymer solvent is able to mix a small amount of L
a
into L
b
. The result is that
the transition moment for the 0-0 transition of L
b
is located almost in the middle
between the y- and z-axes, and not along y as expected according to
the molecular symmetry. This is demonstrated by the observed K-value for the
0-0 transition that is neither 0.33, nor 0.58, but between the two values, actually
corresponding to a transition moment angle of around 37° with the y-axis.
It may be added that the complicated picture of vibronic peaks between the L
b
0-0 transition and L
a
are quite dominated by z-polarized intensity, gained
through vibronic mixing of the two states through non-totally symmetric
vibrations of B
3g
symmetry. It is said that the L
b
transition “borrows” or “steals”
intensity from L
a
. This is a common phenomenon in such spectra, and LD
spectroscopy is particularly well suited to study these processes. However, the
twisting of a transition moment for the (L
b
) 0-0 transition due to solvent effects
is very rare. The best known other case of this kind is the appearance of the 0-0
band of the symmetry forbidden first singlet-singlet transition (to B
2u
) in benzene
in various solvents (it is not seen in the gas phase).
For the overall analysis of the pyrene UV spectrum, the TEM procedure is
required because of the extensive spectral overlap between two or more
transitions. The TEM treatment results in the expected K-values for the other 0-0
peaks, as indicated above. The case of symmetrical S-electron systems in
spectral regions with only in-plane polarized S-S* transitions makes a very
detailed analysis possible, as discussed above.
With only intensity from transitions polarized along the y- and z-axes, the two
observed spectra
()
U
E and ()
V
E
provide sufficient information for a
mathematical construction of A
y ()
and A
z ()
, the sums of transitions polarized
along molecular axes y and z, respectively. Equations (16.3) and (16.4) provide
the results that are shown in the bottom of Figure 16.8.
Q
Q
Q
Q
710