Page 39 - Photoreactive Organic Thin Films

18 HERMANN RAU

Some azobenzenes that are locked against rotation by bulky substituents
in all four ortho positions may show fluorescence when frozen rigidly at
77 K: 2,2',4,4',6,6'-hexaisopropyl; 2,2'-dimethyl-4,4'6,6'-tetra-tert-butyl
62
azobenzene belong to this series. Azobenzenophanes 7 to 13 do not emit,
even at 77 K; this is the expectation for card-packed dimers.
Some reports on fluorescence occurring in, for instance, porous materials
63 64
such as Nafion or aluminophosphates, do not refer to azobenzene but to
protonated azobenzene, which is classified as a pseudostilbene (see Section
1.5). Emission from nonprotonated, isolated azobenzene-type molecules is
still very rare. Aggregated systems, however, seem more prone to show fluo-
56
rescence emission. Shinornura and Kunitake have detected fluorescence
bands with a maximum of near 600 nm in bilayer systems built from the
monomers of 15. They have shown that the ability to emit is tied.to the type
of aggregation: Head-to-tail aggregates emit relatively strongly, with quantum
3
yields of up to <j> = 10~ and lifetimes below 2 ns. Their prototype of card-
packed dimers does not emit at all. This is expected because of the low transi-
tion probability at the lower band edge, which favors radlationless
deactivation, probably via the Si state {see Figure 1.7).
65
Tsuda et al. found the same fluorescence at X^* = 600 nm in a giant
vesicle in a card-packed azobenzene arrangement. The noisy appearance of
their fluorescence trace (if not due to the technique of cqnfocal laser
microscopy), however, suggests a very low emission intensity. Both
56 65
Shinornura and Kunitake and Tsuda et al. report time-dependent orienta-
tion phenomena on Z~E isomerization in the supramolecular arrangement,
which is reflected in the fluorescence intensity. So the former general state-
3
ment that azobenzene-type azo molecules do not emit needs to be modified,
On the other hand, azobenzene can quench the fluorescence of other
molecules. This has been investigated in molecules containing both a fluoresc-
66
ing and an azobenzene unit. It was found that the E-forni is about 3 times, 67
68
or even up to 13 times, as effective a quencher as the Z-form. The influence
of the environment on such bichromophoric molecules was studied by
Eisenbach et al. 69
o-hydroxyazobenzene and other o- or p-hydroxy substituted azo com-
pounds show emission at low temperature. Although this seems to be an
unexpected n 4- TI* fluorescence, in reality it is not the fluorescence of an azo
compound but that of the tautomeric hydrazone form. 70

I.3.I.L7 The Triplet State
Triplet state data for azobenzene-type azo compounds are very limited.
1
Direct absorption of a 0.51 mol I" solution in C 7H 15J in 5 cm cells has not
47
been detectable. Neither has phosphorescence been detected. The energy of
triplet states has been located only by "chemical spectroscopy," i.e., the
quenching of other molecules' triplet states by azobenzene. Ronayette
71 72
et al, ' found two relevant triplet states at about 196 and 180 kj mol" 1
1
(E-azobenzene) and about 192 and 142 kj mol" (Z-azobenzene). Monti et
1
73
al. located triplet states at 146 (E-azobenzene) and 121 kj mol"" (Z-azoben-
zene). From their kinetic results, they inferred that the azobenzene acceptor
should be twisted (phantom triplet) when accepting the energy and calculated