Page 26 - Photoreactive Organic Thin Films

I. PHOTOISOMERIZATIONOFAZOBENZENES 5

the azo group, the kinetics of the E-Z isomerization, and the use of kinetic
evaluation methods. Thereafter, Section 1.3 covers the spectroscopic and iso-
merization data of the azobenzene type azo compounds; Section 1.4 includes
those of the aminoazobenzene type, and Section 1.5 presents those of the
pseudostilbene type azo compounds. In Section 1.6,1 discuss the mechanistic
aspects of the E-Z isomerization, and I make concluding remarks in Section 1.7.
Few publications on the spectroscopic and isomerization properties of
simple azo compounds have appeared in the last 15 years, as compared to the
decades before. There is, however, one exception: Ultrashort time-resolved
spectroscopy of azobenzene and its relatives has opened new access to the
dynamics following pico- and femtosecond excitation. The results are most
relevant for the mechanisms of the photophysical and photochemical pro-
cesses, which in azoaromatic compounds primarily are isomerizations. There is,
however, a host of newer investigations into the isomerization of azobenzene
and its family that are directed to applications in photoswitchable systems
and devices. Some of them are relevant for the understanding of the parent
molecules and therefore are included in this chapter.

1.2 THE AZO GROUP
1.2.1 Spectroscopic Properties
The azo group is planar and observed in the E- and Z-configurations
(Fig. 1.1 A). It is characterized by the ethenic it-electron system which has
antisymmetric wave functions relative to the molecular plane and by the
unique n-electron system. The n-orbitals centered at the two adjacent nitro-
gen atoms are symmetric in relation to the molecular plane. At a distance of
8
123 pm, they interact strongly and split into an n+ and n_ molecular orbital
with a large energy separation (photoelectron spectra give 3.3 eV ~ 25000 cm" 1
9
in azomethane 2 ). The n- and ic-systems are orthogonal for symmetry reasons.
The states built of these orbitals determine the spectroscopic and photo-
chemical behavior. The features of the azo group are best represented in the
spectra of the aliphatic azo compounds diazene H-N=N-H, azomethane
10 11
CH 3-N=N-CH 3 or 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, 3). In
azomethane, a floppy E-azo compound, the forbidden n —» n* band in the
1
1
region of 350 to 400 nm is very weak (e ~ 10 1 mol" cm" ) and continuous.
11 12
In DBH and the homologous DBO (diazanorbornene, 4), ' rigid Z-azo
1 1
compounds, the n —> n* band is weakly allowed (e ~ 300 1 mol" cm" ), struc-
tured sharply in the gas phase and weakly in solution. A large energy gap of
1
ca. 20,000 cm" separates the weak n —> n* bands from the intense allowed
n -» TI* bands around 215 nm in E-azomethane as well as in Z-DBO (here
also sharply structured in the gas phase). 12
/CH 3
A

CH,