Photoreactive Organic Thin Films (eBook)

Wolfgang Knoll, Zouheir Sekkat (Herausgeber)

eBook Download: EPUB | PDF

2002 | 1. Auflage
559 Seiten
Elsevier Science (Verlag)
978-0-08-047997-2 (ISBN)

Infos

Wolfgang Knoll is a former Directory of Polymer research at the Max Planck Institute. He is extremely well know for his research in this area. Zouheir Sekkat was a Postdoctoral researcher at Max Planck working under Professor Knoll. With Knoll's involvement, we can be confident that the best people in this field will be contributing to the reference.

Inhalt

CONTENTS 6
CONTRIBUTORS 16
PREFACE 20
I: PHOTOISOMERIZATION AND PHOTO-ORIENTATION OF AZOBENZENES 22
1 Photoisomerization of Benzenes 24
1.1 Introduction 24
1.2 The Azo Group 26
1.3 Azoaromatics of the Azobenzene Type 34
1.4 Azoaromatics of the Aminoazobenzene Type 46
1.5 Azoaromatics of the Pseudo-Stilbene Type 48
1.6 The Isomerization Mechanism 52
1.7 Concluding Remarks 59
2 Ultrafast Dyamics in the Excited States of Azo Compounds 70
2.1 Introduction 71
2.2 Experimental Section 73
2.3 Results and Discussion 73
3 Photo-Orientation by Photoisomerization 84
3.1 Introduction 85
3.2 Photoisomerization of Azobenzenes 86
3.3 Photo-Orientation by Photoisomerization 89
3.4 Photo-Orientation of Azobenzenes: Individualizable Isomers 100
3.5 Photo-Orientation of Azo Dyes: Spectrally Overlapping Isomers 104
3.6 Photo-Orientation of Photochromic Spiropyrans and Diarylethenes 108
3.7 Conclusion 117
APPENDIX 3A: Quantum Yields Determination 119
APPENDIX 3B: Demonstration of Equations 3.12 through 3.15 123
II: PHOTOISOMERIZATIQN IN ORGANIC THIN FILMS 126
III: PHOTOCHEMISTRY AND ORGANIC NONLINEAR OPTICS 290
8 Photoisomerization Effects in Organic Nonlinear Optics: Photo-Assisted Poling and Depoling and Polarizability Switching 292
8.1 Introduction 293
8.2 Photo-Assisted Poling 293
8.3 Photo-Induced Depoling 299
8.4 Polarizability Switching by Photoisomerization 301
8.5 Conclusion 304
APPENDIX 8A: From Molecular to Macroscopic Nonlinear Optical Properties 305
9 Photoisomerization in Polymer Films in the Presence of Electrostatic and Optical Fields 310
9.1 Introduction 310
9.2 Photoisomerization and Nonlinear Polarizability 310
9.3 Alignment of Isomers in Polymers with Electric Fields 314
9.4 Second Harmonic In-situ Investigation of Photoisomerization 318
9.5 Conclusion 324
10 Photoassisted Poling and Photoswitching of NLO Properties of Spiropyrans and Other Photochromic Molecules in Polymers and Crystals 326
10.1 Introduction 327
10.2 Molecular Second-Order Nonlinear Optical Polarizabilities of Photochromic Molecules 328
10.3 Photoassisted Poling of Photochromes Other Than Azo Derivatives in Polymers 336
10.4 Photoswitching of NLO Properties in Organized Systems and Materials 342
10.5 Conclusion 347
11 All Optical Poling in Polymers and Applications 352
11.1 Standard Poling Techniques 353
11.2 All Optical Poling 355
12 Photoinduced Third-Order Nonlinear Optical Phenomena in Azo-Dye Polymers 386
12.1 Introduction 387
12.2. Third Harmonic Generation 388
12.3 Electric Field Induced Second Harmonic Generation 402
12.4 Degenerate Four-Wave Mixing 409
12.5 Prospective and Conclusions 413
IV: OPTICAL MANIPULATION AND MEMORY 418
13 Photoinduced Motions in Azobenzene-Based Polymers 420
13.1 Introduction 421
13.2 Photoinduced Motions 421
13.3 Possible Photonic Devices 441
13.4 Conclusions 444
14 Surface-Relief Gratings on Azobenzene-Containing Films 450
14.1 Introduction 451
14.2 Processes of SRG Formation 453
14.3 Theoretical Models 467
14.4 Factors Influencing the Formation of SRGs 475
14.5 Open Questions and Challenges for the Near Future 493
14.6 Possible Applications 494
14.7 Final Remarks 502
15 Dynamic Photocontrols of Molecular Organization and Motion of Materials by Two-Dimensionally Arranged Azobenzene Assemblies 509
15.1 Introduction 509
15.2 Photocontrol of Liquid Motion by Azobenzene Monolayers 511
15.3 Photocontrol of Polymer Chain Organizations 517
15.4 Photoinduced Motions and Mass Migrations 522
15.5 Concluding Remarks 530
16 3D Data Storage and Near-Field Recording 535
16.1 Introduction 536
16.2 Bit-Oriented 3D Memory 536
16.3 Photochromic Materials for 3D Optical Memory 538
16.4 Recording and Readout Optics 545
16.5 Near-Field Recording 555
16.6 Concluding Remarks 559
17 Synthesis and Applications of Amorphous Diarylethenes 563
17.1 Quasi-Stable Amorphous Diarylethenes 564
17.2 Thermally Stable Amorphous Diarylethenes 565
17.3 Optical Properties of Amorphous Diarylethenes 567
17.4 Charge Transport in Amorphous Diarylethene Films 571
17.5 Summary 573
INDEX 575
A 575
B 576
C 577
D 577
E 577
F 578
G 578
H 578
I 578
J 578
K 578
L 578
M 578
N 578
O 579
P 579
Q 581
R 581
S 581
T 582
U 582
V 582
W 582
Z 582

Textauszug

1

Photoisomerization of Azobenzenes

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

1.1 INTRODUCTION

1.2 THE AZO GROUP

1.2.1 Spectroscopic Properties

1.2.2 Isomerization

1.3 AZOAROMATICS OF THE AZOBENZENE TYPE

1.3.1 Spectroscopic Properties

1.3.2 Isomerization of Molecules of the Azobenzene Type

1.4 AZOAROMATICS OF THE AMINOAZOBENZENE TYPE

1.4.1 Spectroscopic Properties

1.4.2 Isomerization of Molecules of the Aminoazobenzene Type

1.5 AZOAROMATICS OF THE PSEUDO-STILBENE TYPE

1.5.1 Spectroscopic Properties

1.5.2 Isomerization of Molecules of the Pseudo-Stilbene Type

1.6 THE ISOMERIZATION MECHANISM

1.6.1 Azobenzene-Type Molecules

1.6.2 Pseudo-Stilbene–Type Molecules

1.7 CONCLUDING REMARKS

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:

Figure 1.1 (A) The E/Z isomerization system of azobenzene. (B) Absorption spectra of E- and Z-azobenzene in EtOH solution.

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....

Produkt-Details

• Erscheint lt. Verlag: 7.12.2002
• Co-Autor: Aleksandra Apostoluk, Elena Ishow, O.N. Oliveira Jr., Eugenii Katz, Tsuyoshi Kawai, Satoshi Kawata, Yoshimasa Kawata, Andre Knoesen, Takayoshi Kobayashi, Mikhail V. Kozlovsky, Jayant Kumar, Lev M. Blinov, L. Li, Henning Menzel, Keitaro Nakatani, Almeria Natansohn, Jean-Michel Nunzi, H. Rau, Paul Rochon, Takashi Saito, Takahiro Seki, Andrew N. Shipway, Victor M. Churikov, S.K. Tripathy, Itamar Willner, Jacques A. Delaire, Celine Fiorini, W. Haase, Chia-Chen Hsu, Kunihiro Ichimura, Masahiro Irie
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Organische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Technische Chemie
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Optik
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Technik ► Maschinenbau
• Themenwelt: Technik ► Umwelttechnik / Biotechnologie
• Themenwelt: Wirtschaft
• ISBN-10: 0-08-047997-9 / 0080479979
• ISBN-13: 978-0-08-047997-2 / 9780080479972