Page 48 - Photoreactive Organic Thin Films

I. PHOTOISOMERIZATIQNOFAZOBENZENES 27

128
measured directly. Extrapolation is possible, however (Figure 1.12). So for
130
4-dimethylamino-azobenzene flash excitation, a rotating-shutter tech-
131
nique 128 or low-temperature techniques have been applied. Another tech-
nique is based on the intensity-dependent photostationary state of the
reaction; this technique extrapolates to infinite light intensity, where the ther-
mal reaction becomes unimportant. 23
1.4.2.2.1 Quantum Yields
as we as
The method of variation of the irradiation intensity gives <J>E_>Z ^
<J>Z_»E« For 4-dimethyl-aminoazobenzene in n-hexane irradiation at 366 nm or
405 nm, both of which are wavelengths of the n -> n* band, the results are
7 23 134
<J>E-*z = O-l and <|> Z_» E = 0.25. Albini et al., who calculated the extent of Z
—> E isomerization by an extrapolation of this conversion back to zero time
of irradiation, found somewhat higher <|>E-»Z values for 4-diethylarnino-
azobenzene in de-aerated cyclohexane solution (Table 1.3).
The quantum yield at 436 nm is significantly higher than it is at shorter
wavelength irradiation (Table 1.3) which is like in azobenzene-type com-
pounds. Indeed, in hydrocarbon solution, the absorption spectrum shows the
azobenzene-type feature of a long wavelength n -» n* band (Figure 1.11).
The photostability of aminoazobenzene-type azo compounds in hydro-
carbon solution is high, at least for irradiation in the visible and near UV
(A,>313nm) and is comparable to the azobenzene-type compounds. For
short wavelength (254 nm) irradiation, photoreduction is detected, but with
quantum yields of less than 1%. 134

.5 AZOAROMATICS OFTHE PSEUDO-STILBENETYPE

.5.1 Spectroscopic Properties
Azoaromatics of the pseudo-stilbene type are characterized by the feature that
is of the (n,n*) the lowest-excited singlet state type (Figure 1.13). Compared
l
to the azobenzene type, the ^(nji*) and (n,n*) states are rearranged. This
reordering can happen by lowering the energy of the (rc,rc*) state (e.g., by
donor/acceptor substituents) and/or by increasing the energy of the (n,xc*)
135 136
state (e.g., by asymmetric ' protonation or by complexing the n-electrons
of the azo group).

TABLE 1.3 Quantum Yields for E —> Z Isomerization of 4-
Diethylaminoazobenzene (4-DEAMAB) and 4-Diethylamino-4'-
methoxyazobenzene (4-DEAM-4'-MOAB) in De-arerated Cyclohexane (Adapted
from reference 134.)

Irradiation wavelength 254 nm 313 nm 366 nm 436 nm
4-DEAMAB 0.23 0.25 0.21 0.72
4-DEAM-4'-MOAB 0.34 0.31 0.27 0.84