Table Of ContentChemistry and Applications
of Leuco Dyes
TOPICS IN APPLIED CHEMISTRY
Series Editors: Alan R. Katritzky, FRS
Kenan Professor of Chemistry
University of Florida, Gainesville, Florida
Gebran J. Sabongi
Laboratory Manager, Encapsulation Technology Center
3M Company, St. Paul, Minnesota
Current volumes in the series:
CHEMISTRY AND APPLICATIONS OF LEUCO DYES
Edited by Ramaiah Muthyala
FROM CHEMICAL TOPOLOGY TO THREE-DIMENSIONAL
GEOMETRY
Edited by Alexandru T. Balaban
INFRARED ABSORBING DYES
Edited by Masaru Matsuoka
LEAD-BASED PAINT HANDBOOK
Jan W. Gooch
ORGANOFLUORINE CHEMISTRY
Principles and Commercial Applications
Edited by R. E. Banks, B. E. Smart, and J. C. Tatlow
PHOSPHATE FIBERS
Edward J. Griffith
POLY(ETHYLENE GLYCOL) CHEMISTRY
Biotechnical and Biomedical Applications
Edited by J. Milton Harris
RADIATION CURING
Science and Technology
Edited by S. Peter Pappas
RESORCINOL
Its Uses and Derivatives
Hans Dressler
TARGET SITES FOR HERBICIDE ACTION
Edited by Ralph C. Kirkwood
A Continuation Order Pian is available for this series. A continuation order will bring delivery of each new
volume immediately upon publication. Volumes are billed only upon actual shipment. For further informa-
tion please contact the publisher.
Chemistry and Applications
of Leuco Dyes
Edited by
Ramaiah Muthyala
3M Company
St. Paul, Minnesota
KLUWERACADEMICPUBLISHERS
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Contributors
Daniel S. Daniel, Clinical Chemistry Research and Development, Johnson
& Johnson Clinical Diagnostics, Rochester, New York 14650-21 13
Ian J. Fletcher, Research and Development Department, Chemicals Divi-
sion, Ciba-Geigy AG, CH-4002 Basel, Switzerland; present address: Con-
sumer Care Division, Ciba Speciality Chemicals, D79639 Grenzach-Wyhlen,
Germany
Yoshihiro Hatano, Research and Development, Yamamoto Chemicals, Inc.,
Yao, Osaka 581, Japan
Xiangfu Lan, Clariant Corporation, Charlotte, North Carolina 28269
Masaru Matsuoka, Department of Applied Materials Science, Osaka Pre-
fecture University, Sakai, Osaka 593, Japan; present address: Faculty of
Home Economics, Kyoto Women’s University, Imakumano, Higashiyama-
ku, Kyoto 605, Japan
Ramaiah Muthyala, 3M Company, St. Paul, Minnesota 55144
Hiroyuki Nakazumi, Department of Applied Materials Science, Osaka
Prefecture University, Sakai, Osaka 593, Japan
Tran Van Thien, Minnesota 3M Research Ltd., Pinnacles, Harlow, Essex
CM19 5AE, England
Rudolf Zink, Research and Development Department, Chemicals Division,
Ciba-Geigy AG, CH-4002 Basel, Switzerland; present address: Consumer
Care Division, Ciba Speciality Chemicals, D79639 Grenzach-Wyhlen,
Germany
V
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Foreword
Dye chemistry has seen a major resurgence of interest in recent years, as
evidenced by increased conference activity and the publication of many new
books in the field. This can be attributed to a virtual explosion of interest
in dyes for speciality and high-technology applications (the functional dyes),
and it is in these areas that the most exciting developments in dye chemistry
and dye applications are occurring. For example, one of the consequences
of the new information technology age is an unprecedented demand for
“hard copy,” be it written text (e.g., laser-, ink-jet, or thermally printed), or
photographic images (analogue or digitally generated). All of these processes
use dyes and pigments in one form or another, colorants which command
a very high unit price, and which often have very sophisticated structures.
In this general area of imaging and copying, leuco dyes have a major part
to play, and this brings us to the subject matter of this book.
The term leuco, meaning white, comes from the Greek, and was
originally applied to the reduced form of vat dyes, e.g. indigoids or quinones,
which were often (but by no means always) colorless. The alkali soluble
leuco-vat dyes were, of course, extremely important in textile dyeing, as they
provided the only means by which the highly insoluble parent vat dyes
could be applied permanently to the fabric. The applications of leuco-dyes
are now far more diverse than this, and the term is now applied to describe
the (reversibly) reduced form of any class of dye. The term is also sometimes
applied to the colorless form of a dye which may be produced by a
nonreductive process, as for example, in the case of intramolecular cycli-
zation, reactions induced by pH change, heat, or light. The important
feature of all these reactions is color change, and it is principally this
phenomenon that is exploited in so many new high-technology applications.
vii
viii Foreword
Given the diversity and the technical importance of leuco-dyes, it is
surprising that until now no text book dedicated to this field has been
published. The editor, Dr. Ramaiah Muthyala, a noted research chemist of
long standing in the area of photoreprography, is to be commended for his
perspicacity in appreciating this gap in the scientific literature, and for his
skill in assembling a range of experts, including himself, to write chapters
based on their own research experience, so producing a comprehensive and
balanced book that will certainly be a classical text in the literature of dye
chemistry.
The first chapter of the book deals with leuco-spiropyrans and related
spiro compounds, which constitute one of the classes of leuco compounds
not of the redox type. Such materials are photochromic, and are of major
technical importance. The author, Hiroyuki Nakazumi of the Department
of Applied Chemistry at the University of Osaka Prefecture, is well known
for his researches in functional dye chemistry, particularly photochromic
materials, and he provides a very useful update of the field, covering
mechanisms, synthesis, spectra and applications, together with a useful
section on approaches to near-infrared absorbing photochromic dyes.
Another noted and prolific Japanese author in the field of functional
dyes, Masaru Matsuoka of the Laboratory of Materials Science, Kyoto
Women’s University, has written the second chapter, dealing with leuco-
quinone dyes. These are the traditional redox leuco systems employed for so
many years in the vat dyeing industry, and this chapter is an invaluable
review of the chemistry of these systems, drawing on recent mechanistic and
structural investigations. Applications considered include not only textile
dyeing, but also other more specialized areas, such as hair dyeing, color
formers, and photoimaging materials.
The third chapter deals with leuco derivatives of the oxazine, thiazine,
and phenazine dyes, and is written by Tran Van Thien, who has had many
years experience in photoimaging at the 3M Research Center, Harlow. His
industrial experience in leuco dyes has enabled him to produce a collation
of material impossible to find elsewhere in a single review, and again recipes
for the synthesis of representative examples abound. Consideration is also
given to the numerous applications of these molecules in thermal and
photo-imaging systems.
Chapter 4 is concerned with a technically important group of leuco
compounds which like the spiropyrans are not formed by reduction of the
parent dye, but by formation of a spiro structure from the dye in such a way
that the newly created sp³ center destroys the conjugation, and hence, the
color of the chromophore. These are the phthalides (spirolactones) and the
position of equilibrium is determined by pH rather than a redox process.
Such materials are used mainly as color formers in pressure-sensitive
Foreword ix
copying paper, and in thermal recording paper. The two industrial authors,
Ian J. Fletcher and Rudolf Zink, have long and distinguished careers in this
field with the Chemicals Division of Ciba-Geigy, Grenzach, Germany, and
they bring to bear on this chapter their intimate knowledge and wide
experience of these materials.
The chemistry of these phthalides has much in common with that of
the compounds reviewed in the next chapter, i.e., the leuco-triarylmethanes,
and in fact, the parent dyes of the two classes share the same basic
chromophoric system. However, the latter are true redox systems, rather
than pH indicators, and consequently have a different range of technical
applications. The situation is complicated further in that the triarylmethane
cationic dyes can also bleach at high pH, giving a hydroxide addition
product which is better described as a “carbinol base” rather than a “leuco”
dye. Other nucleophiles (e.g., amines and cyanide ion) can add similarly to
give colorless products. The authors have succeeded in covering all these
complications thoroughly and logically, again giving due attention to
technical applications and synthetic procedures. The editor has co-authored
this chapter with Xiangfu Lan, who has had considerable academic and
industrial experience in the field of heterocyclic chemistry and dye synthesis,
and is currently a Senior Research Chemist with Clariant Corporation,
United States.
Chapter 6 is rather more specialized than the others, in that it deals
with a structurally narrow group of compounds, namely the leuco-fluorans.
These are spiro-lactones and are dibenzofluoran analogues of the phthalides
reviewed in Chapter 4. It might be argued that these compounds could have
been subsumed into Chapter 4, but in practice the major technical import-
ance of these materials in pressure-sensitive and thermal recording paper,
and the very extensive patent literature that has built up around them, fully
justifies their having a chapter to themselves. Yoshihiro Hatano is one of the
world‘s leading experts in fluoran chemistry, and having had more than
thirty years experience in this field with Yamamoto Chemicals Inc., Osaka,
there could be no one more appropriate to author this chapter.
The final chapter deals with a unique class of leuco-compounds which
turn the original concept of redox leuco dyes on its head. Tetrazolium salts
are (largely) colorless and can justifiably be called leuco compounds, and yet
on reduction they give intensely colored formazan dyes. Whereas most of
the color-change systems dealt with in earlier chapters find greatest techni-
cal use in reprography, tetrazolium salts are more important in biochemical
analysis. Nevertheless, it is clear from this chapter that the applications of
tetrazolium salts are probably more diverse than those of any other class of
leuco-compounds. It is fitting that the author, Daniel S. Daniel, is an expert
in the biochemical applications of leuco-dyes, having had more than twenty
