Page 40 - Photoreactive Organic Thin Films

PHQTOISOMERJZATiON OF AZOBENZENES | f

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the distortion energy to be 56 kj mol" for the E-form and 77 kj mol" for the
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Z-form. Pragst used triplet sensitizers created in the recombination reaction
of electrochemically produced radicals and reported 170 ± 10 kj moH.
1.3.1.1.8 Electronic State Calculations
Electronic state calculations for azobenzene in early papers suffered from
the inability of older methods to take into account the mixing of (n,xc*)
and (n,n*) states. New calculations using ab-initio methods are successful,
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even in mastering donor/acceptor substituted azobenzenes. A survey of calcu-
lations in connection with the isomerization mechanism will be given in
Section 1.6.

1.3.1.1.9 Conclusion
The spectroscopy of azobenzene-type azo compounds is characterized by
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the large energy gap between the low-lying (n,n*) state and the next higher
'(Tt:,^*) state. A certain floppiness of the molecular shape leads to state inter-
actions, which, together with the ability of the molecules to isomerize, leads
to fast, radiationless deactivation of azobenzene and its derivatives. This photo-
stability is an asset for practical use. Aggregation rigidifies the molecular
structure, as clearly demonstrated by the increase of vibrational structure of
the bands. It also lowers the (TC,JE*) energy states with longer (head-to-tail) or
shorter (card-packed arrangements) lifetimes. An investigation of the photo-
stability of aggregated molecules compared to nonaggregated or dispersed
molecules is needed to assess possible disadvantages of the aggregates.
1,3.1.2 IR and Raman Spectroscopy
The relevant vibrations for this review are the N=N and C-N (Ph-N)
stretching vibrations and, perhaps, torsional vibrations around the C-N
bond. The E-azobenzene molecule has a center of inversion, and therefore the
N=N vibration is infrared-inactive, but Raman-active, and has been found to
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be at 1442 cm" . By IR spectroscopy, Kiibler et al. located the symmetric
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C-N stretching vibration at 1223 cm" in E- and at 866 cm"" in Z-azobenzene.
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The N=N vibration in Z-azobenzene is at 1511 cm"" (in KBr pellets). These
numbers are confirmed by newer work; Biswas and Umapathy report 1439
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and 1142 cm' for the N=N and C-N vibrations (in CC1 4), and Fujino and
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Tahara found nearly identical results (1440 cm" and 1142 cm" ). A thorough
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vibrational analysis of the E-isomer is given by Amstrong et al. The vibra-
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tions in the (n,n*) excited state are very similar: 1428 cm" and 1130 cm" .
1.3.I.3 Picosecond and Femtosecond Spectroscopy
The short pulse duration combined with the high photon density of ps-
and fs-lasers have provided the means to study the properties of the excited
states by emission and transient absorption measurements. Fluorescence of
the lowest and higher excited states of azobenzene can be detected, but most
work is being directed toward the dynamics of isomerization. Because
questions about the isomerization mechanism are prominent in this field, this
work will be discussed in Section 1.6: The Isomerization Mechanism.