Page 124 - Photoreactive Organic Thin Films

3, PHOTO-ORIENTATION BY PHOTOISOMERIZATION

1
/"" / \ / "2 "O /" *
A = (X L/ (£jj — ff^.) \JJD.D)
where A A = e A. C and A are the sample absorbances before and during
irradiation, respectively. Note that the sample is all-trans prior to irradiation.
Substituting 3B.5 into 3B.6 yields the following equation, the slope of which
is given by Equation 3.1.
A
A = 100010(1 - l(r °')<£; B(£ B - e A}.t (3B.7)
For the demonstration of Equation 3.2, we set n - 2 in Equation 3B.3 and
2
solve for oA 2B with t « t in mind. This gives:

28 2 A AB 2B 2
5 ' 5
The total anisotropy, AA = AA A + AA B, is derived by summing the aniso-
tropies due to the orientation of both isomers, i.e., AA A)B = 3 C e A$
P 2{cos w AjB)A 2A)2B, and by using Equation 3B.8. AA reads:
A
A
B
AA = f 1000/ 0'(1 - 10~ o)<^ B{P -* P 2(cos a) B)e B - P 2(cos <*) A)s A}.t (3B.9)
5
When isomeric reorientation is assumed (vide infra,} and irradiation is
performed at the irradiation wavelength, the slope of AA resumes to:
A
p(AA) = |lOOO/ 0'd - IO- O)^ B (Q% - *A) (3B.10)

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