Photoisomerization of Azobenzenes

Hermann Rau    Institut für Chemie, Universität Hohenheim, D-70593 Stuttgart, Germany

1.1 INTRODUCTION

Most of the phenomena described in this monograph on photoreactive organic thin films are based on the isomerization of units deliberately built into molecules, molecular assemblies, or polymers. Most especially, the spectroscopic and isomerization behavior of these units determines the switching and triggering properties of the photoreactive systems and devices. Information storage and nonlinear optical properties, as well as photo-control of equilibria and of polymer, membrane, and other properties are exploited in applications.

The majority of the systems outlined in this monograph contain the azobenzene moiety as the photoswitchable unit. Therefore, the first chapter of this monograph deals with the properties of this simple molecule and its simple derivatives. Applications for various purposes are covered in some of the following chapters.

Azobenzene 1 (Figure 1.1A) is particularly useful for these applications for the following reasons:

1. The azobenzene unit is chemically stable at moderate temperatures and against UV/VIS radiation;

2. On E-Z (trans-cis) conversion,1 it changes its absorption spectrum considerably (Figure 1.1B);

3. On E-Z conversion, it changes its molecular shape, reducing the distance between the p-positions from 1.0 to 0.59 nm (Figure 1.1A) and increasing the dipole moment from 0 to ca. 3 Debye;2

4. Donor/acceptor substituted azobenzenes, which have large second- and third-order nonlinear optical properties, show a fast thermal Z-E (cis-trans) conversion.

Items 2 to 4 warrant this chapter on photo-induced and thermal E-Z isomerization of the parent molecules, which are incorporated in numerous macromolecular and supramolecular assemblies as the photo-active element.

Azo compounds are systematically addressed as “diazenes.” This has to be borne in mind when one conducts a literature research. The azo (diazene) group is isosteric with the ethene group; stilbene and azobenzene have many related features, but they also possess relevant different properties that make azobenzenes superior for use as photo-switches.

To understand the photoresponsive properties of azobenzene and its molecular family, it is necessary to discuss their spectroscopy and the mechanistic options of isomerization. A review of the spectroscopic properties of azo compounds appeared in 1973.3 The isomerization properties of azobenzene were reviewed for several periods. Wyman4 covered the literature up to 1954, Ross and Blanc5 up to 1969, and Rau6,7 up to 1988. The present standalone review is restricted to the spectroscopy and isomerization of simple aromatic azo compounds. It is meant to serve as a basis for the detailed treatments of complex photoresponsive systems in the following chapters of this monograph.

The spectroscopic and photochemical, especially isomerization, features of the azoaromatics warrant separation into three classes according to the relative energy of the lowest lying 1(n,π*) and 1(π,π*) states: the azobenzene type, the aminoazobenzene type, and the pseudo-stilbene type.6 This determines the structure of this chapter. Section 1.2 provides basic information on 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 isomerization 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, I 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 processes, 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. 1.1A). It is characterized by the ethenic π-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 nitrogen atoms are symmetric in relation to the molecular plane. At a distance of 123 pm,8 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 in azomethane 29). The n- and π-systems are orthogonal for symmetry reasons.

The states built of these orbitals determine the spectroscopic and photochemical behavior. The features of the azo group are best represented in the spectra of the aliphatic azo compounds diazene H-N=N-H, azomethane 3-N=N-CH310 or 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, 3).11 In azomethane, a floppy E-azo compound, the forbidden n → π* band in the region of 350 to 400 nm is very weak (ε ≈ 10 1 mol−1 cm−1) and continuous. In DBH and the homologous DBO (diazanorbornene, 4),11,12 rigid Z-azo compounds, the n → π* band is weakly allowed (ε ≈ 300 1 mol−1 cm−1), structured sharply in the gas phase and weakly in solution. A large energy gap of ca. 20,000 cm−1 separates the weak n → π* bands from the intense allowed π → π* bands around 215 nm in E-azomethane as well as in Z-DBO (here also sharply structured in the gas phase).12

Although there are some “reluctant”13 aliphatic azo compounds, in general these molecules are photochemically not very stable.14 Thus, they are not used in the systems covered in this book and will not be reviewed in this contribution.

In aromatic azo compounds, the π system is extended. X-ray data2,15 for E-azobenzene give the N=N distance as 124.7 pm, not much different from that in azomethane, and the C-N distance as 142.8 pm. The NNC angle is 114.10, somewhat off the sp2 hybridization angle, and the molecule is planar (>CNNC = 1800). The corresponding values for Z-azobenzene are: N=N 125.3 pm, C-N 144.9 pm, >NNC 121.90, >CNNC 1720, and the twist angle of the phenyl rings is 53.30. This is in agreement with earlier work.16 Electron diffraction data17 for E-azobenzene do not differ more than 2 pm from the X-ray results, but they indicate a small twist angle >C-N of 300.

The extension of the conjugation system increases the photostability of the molecules and lowers the excitation energies compared to those of the aliphatic compounds: The n → π* absorption of aromatic azo compounds occurs in the visible region; they are colored (Figure 1.1B). The (n,π*) energy is moderately lowered by 5000 to 7000 cm−1, and the (π,π*) state energy strongly by about 20,000 cm−1; thus the band gap is reduced to about 10,000 cm−1 in azobenzene. This energy gap is very sensitive to substitution, which influences the spectroscopic and photochemical features of different azoaro-matics....