Photochromism: Molecules and Systems

Heinz Dürr, Henri Bouas-Laurent

• Publisher: Elsevier Science
• Year: 2003
• Print ISBN: 9780444513229
• eBook ISBN: 9780080538839
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Photochromism is simply defined as the light induced reversible change of colour. The field has developed rapidly during the past decade as a result of attempts to improve the established materials and to discover new devices for applications.As photochromism bridges molecular, supramolecular and solid state chemistry, as well as organic, inorganic and physical chemistry, such a treatment requires a multidisciplinary approach and a broad presentation. The first edition (1990) provided an enormous amount of new concepts and data, such as the presentation of main families based on the pericyclic reaction mechanism, the review of new families, some bimolecular photocycloadditions and some promising systems. This new edition provides an efficient entry into this flourishing field, with the core content retained from the original work to provide a basic introduction into the different subjects.*Second edition of a work first published in 1990, now revised due to constant development of research. *Including updated lists of references (1989-2001), offering immediate access to recent developments.*Providing great basic interest and high application potential bringing scientists together from chemistry, physics and engineering.

Photochromism Based on Pericyclic Reactions: Electrocyclization Reactions

Chapter 5

4n Systems Based on 1,3-Electrocyclization

C. Schulz

H. Dürr

1 INTRODUCTION

The photochromic behavior of many classes of compounds is based on electrocyclizations. In this chapter, photochromic systems with four electrons which undergo 1,3-electrocyclic reactions are dealt with. 1,5-Electrocyclizations involving six electrons are described elsewhere.

In scheme 1, a survey of electrocyclic processes in 4n-systems (n = 1,2,…) is shown.

Scheme 1 Electrocyclic Processes in 4n-Systems (n = 1,2,…)

Important photochromic heterocycles and open ring forms isoelectronic with the cyclopropyl and allyl anion, respectively, are given in scheme 2.

Scheme 2 Photochromism based on 1,3-electrocyclization (4 electrons)

The cyclobutene-butadiene interconversion, where a wealth of derivatives has been studied both theoretically and experimentally, has not given rise to important photochromic systems. This reaction is therefore not treated in this book. The reader is referred to earlier literature (ref. 1). Similar arguments hold for the cyclooctatriene-octatetraene interconversion which is excluded from this book as well.

2 STEREOCHEMISTRY IN 1,3-ELECTROCYCLIC REACTIONS

An outstanding feature of electrocyclic reactions is their stereospecificity. The orbital symmetry conservation principle in the form of the Woodward-Hoffmann rules (ref. 2) predicts the following behavior for symmetrical hydrocarbons: When the number of interacting electrons in the cyclic array is 4n (n = 1,2,.) the thermal electrocyclic reactions proceed via conrotatory pathways, the electrocyclic reactions which occur as primary photochemical processes via disrotatory pathways.

The rules may be applied to slightly disturbed systems. The question which arises concerns the application of the Woodward-Hoffmann rules to unsymmetrical substances including both highly substituted hydrocarbons as well as compounds containing heteroatoms. A paper by Snyder (ref. 3) describes the CNDO-calculated electrocyclization of the open ring form 1, isoelectronic with the allyl anion, to the corresponding three membered ring 2.

The results of these calculations show that heteroatom substitution of 1, 2. leads to three types of ring closures:

1. Thermal conrotation is allowed:

2. Thermal conrotation and disrotation are allowed:

3. Conrotation and disrotation are thermally forbidden but photochemically allowed:

3 OXIRANES

3.1 Monocyclic Aryloxiranes

The literature on oxiranes has been reviewed (refs. 4–6). It has been shown that room temperature photolysis of aryloxiranes (incorporating 3) causes cycloelimination to give arylcarbenes and carbonyl compounds.

Photolysis (77 K) in rigid glasses produces highly colored intermediates (ref. 7), which are stable at low temperatures, but are bleached by warming to 25 °C or upon irradiation with visible light. Color formation is attributed to C-C-bond cleavage with formation of carbonyl ylides 7.

Thap DoMinh, Trozzolo and Griffin (ref. 8) investigated the photochromism of cis- and trans-stilbene oxide. In the present review these compounds are treated as typical examples of other photochromic monocyclic oxiranes.

Irradiation of trans-stilbene oxide in ethanol glass at 77 K produces an orange material (λ_max = 490 nm) along with small amounts of benzaldehyde, phenylmethylene and desoxybenzoin. Irradiation of the cis isomer gives similar products, but the colored intermediate is a deep red compound (λ_max = 510 nm). At 140 K, both colors disappear (the bleaching occurring somewhat faster in the trans than in the cis isomer) giving benzaldehyde and phenylmethylene. The amount of fragmentation products formed by this photolysis-warm-up-procedure is estimated to be 20–25 times more than the amount originally produced by photolysis. Irradiation in the visible causes rapid fading and regenerates the original oxirane together with small amounts of fragmentation products.

The process may be envisaged as an electrocyclic reaction which interconverts oxiranes and open chain carbonyl ylides by a conrotatory or disrotatory mode. Three possible ylides may be derived from stilbene oxides: The order of stability is 9 > 8 >> 10. From the relative stabilities and absorption spectra it is possible to assign cis-oxoylide 9 to the more stable red shifted intermediate from cis-stilbene o...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface (1990 Edition)
• Foreword (2003 Revised Edition)
• Preface (2003 Revised Edition)
• See Literature Survey Update (1989 – 2001)
• ORGANIC PHOTOCHROMISM (IUPAC Technical Report)
• General Introduction
• Photophysical, Photochemical and Photokinetic Properties of Photochromic Systems
• Photochromism Based on “E-Z” Isomerhation of Double Bonds
• Photochromism Based on Pericyclic Reactions: Electrocyclization Reactions
• Photochromism Based on Pericyclic Reactions: Cycloaddition Reactions
• Photochromism Based on Tautomerism (Hydrogen Transfer)
• Photochromism Based of Dissociation Processes
• Photochromism in Biological Systems
• Environmental Effects on Organic Photochromic Systems
• The Use of Silver Salts for Photochromic Glasses
• Applications
• New Developments Highly Promising for Applications
• Glossary of Terms
• Appendix of Literature Updates
• Subject Index