Page 35 - Photoreactive Organic Thin Films

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fore to separate the data and mechanism sections, and as a result some redun-
dancy will be encountered.

.3.1 Spectroscopic Properties

1.3.1.1 UV/VIS Spectroscopy
The absorption spectra of E- and Z-azobenzene in EtOH solution were
presented in Figure LIB. In these spectra, a weak, low-energy band that is
well separated from the intense, higher-energy bands is identified. This consti-
1
tutes the azobenzene type. The lowest-lying excited state is of the '{n,Jt*)
type; there is a large energy gap between this lowest (n,7i*) state and the next
higher state that is of the (7t,7C*) type.

1,3./././ E-Azohenzene
In the spectrum of E-azobenzene, the unstructured low intensity and low
energy n -> n* band is identified in the 400 to 500 nrn region (n-hexane: K mAX
1 43
= 449 nm, £ max = 405 1 moH cm" ). This band corresponds to an n. —> n*
excitation and is forbidden under the symmetry C2h of E-azobenzene.
Compared to n —» n* transitions in other molecules that of azobenzene is
very intense. This may be due to nonplanar distortion and vibrational cou-
44
pling. The n -» n* intensity is stolen from the relatively far-off first n —> n*
45 46
band, as shown by their common direction of polarization. ' e of the (n,JC*}
3
state decreases by about 20% when the solution is frozen at 77K , which may
be interpreted as better planarity at low temperature. The n —» n* band of
the E-compound is continuous; a case with vibrational features has never
been found. This band extends to 620 nm, where e becomes smaller than
1
3
1
1
5'10~ 1 mol" cm" , which gives a state energy of ca. 17 500 cm" (2.1 eV,
1 47
205 kj mol- ).
The energy gap between the low-lying (n,it*) and the next(jc,7C*) state is
1
about 9000 cm"" if the band origin of the continuous n —> it* band is taken,
as usual, at 10% intensity—perhaps even greater. The intense n —> n* band is
at 1
^-max = 316 nm (fimax = 23,000 1 moHcm" ), and a second n —> TC* band
1 43
appears at 229 nm (n-hexane: e max = 14,400 1 moHcm" ) . In the short wave-
length region, the spectrum of E-azobenzene is very similar to that of stilbene,
1
but red shifted by some 2000 cm" . The n -» 71* bands show weakly
expressed vibrational structures at room temperature, much less strongly
expressed than in stilbene. But when solutions are cooled to 77 K in a rigid
solvent, a well-defined structure with vibrational spacings of 1200 to 1400
1
3 46
cm"" appears on both bands. ' Less expressed vibrational structure can be
induced by very viscous environments. 3
1.3.1.1.2 Z-Azobenzene
In the spectrum of Z-azobenzene, the n —> xc* transition is allowed
under the symmetry of C 2v. The n —» n* band maximum is 440 nm with
43
1
1
e = 1250 1 mol"" cm" , much higher than that of E-azobenzene. This is rele-
vant to the E-Z isomerization reaction. This band is also continuous, as in all
nonrigid Z-azo compounds.