Photochemistry volume 41

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Photochemistry
Volume 41


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A Specialist Periodical Report

Photochemistry
Volume 41
A Review of the Literature Published between
January 2011 and December 2012
Editors
Angelo Albini, University of Pavia, Pavia, Italy
Elisa Fasani, University of Pavia, Pavia, Italy

Authors
Bruce A. Armitage, Carnegie Mellon University, USA
Gonzalo Cosa, McGill University, Canada

Telma Costa, University of Coimbra, Portugal
Catherine S. de Castro, University of Coimbra, Portugal
Maria Letizia Di Pietro, Universita` degli Studi di Messina, Italy
Daniele Dondi, University of Pavia, Italy
Rui Fausto, University of Coimbra, Portugal
Aurore Fraix, University of Catania, Italy
Andrea Go´mez-Zavaglia, University of Coimbra, Portugal
K. Kalyanasundaram, Swiss Federal Inst. of Technology (EPFL), Switzerland
Noufal Kandoth, University of Catania, Italy
Katerina Krumova, McGill University, Canada
Anto´nio L. Mac¸anita, Technical University of Lisbon, Portugal
Andrea Maldotti, Universita` degli Studi di Ferrara, Italy
Daniele Merli, University of Pavia, Italy
Francesco Nastasi, Universita` degli Studi di Messina, Italy
Fausto Puntoriero, Universita` degli Studi di Messina, Italy
J. Se´rgio Seixas de Melo, University of Coimbra, Portugal
Salvatore Sortino, University of Catania, Italy
Xinjing Tang, Peking University, China
Emanuela Trovato, Universita` degli Studi di Messina, Italy
Alberto Zeffiro, University of Pavia, Italy


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ISBN: 978-1-84973-580-3
ISSN: 0556-3860
DOI: 10.1039/9781849737722
A catalogue record for this book is available from the British Library
& The Royal Society of Chemistry 2013
All rights reserved
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For further information see our web site at www.rsc.org


Preface

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Vol. 41 takes up again the biennial cycle, with the regular consideration of
the main aspects of photochemistry every other year. Thus, organic and
theoretical aspects have been reviewed in Vol. 40 for the years 2010 and
2011, and the physical and inorganic aspects along with solar energy conversion are reviewed in the present volume for the years 2011 and 2012. The
reviews are preceded by a general introduction and review of 2012 and
followed by a series of highlights. The last part has become an established
feature of the series. The variety of the topics, the diversity of the language
used are really impressive. Photochemistry, just as chemistry in general and
perhaps to a higher degree, is more and more becoming the science that pick
up problems from other sciences, from physics to biology and engineering
and figures out, and then actually prepare materials and devices able to
perform the required function. This may be exciting, but makes more and
more difficult to give an image of what photochemistry is. As H. J. Kuhn
commented several years ago when presenting some previous volumes of
this series, ‘‘the reports should not compete with Chemical Abstract. . .in
giving just names, references and very short abstracts but should instead
complement these approved media by transmitting the essence of the year’s
scientific progress’’ (H. J. Kuhn, EPA Bull. 1988, 34, 91–92). We do remain
of the mind that it is worthwhile to present side by side a representative (?)
selection of such different aspects because there is still a unitary photochemical basic science and practitioners of different aspects may gain
something from a common discussion.
Prof. Elisa Fasani takes the job of co-editor from this volume. We thank
the staff of Specialist Periodical Reports at RSC and our coworkers at the
Photochemical Unit at the University of Pavia for their help.
Angelo Albini and Elisa Fasani

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The Royal Society of Chemistry 2013


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CONTENTS

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Cover
In 1913 Albert Einstein pointed
out that the ‘‘equivalence law’’
he had demonstrated does
not require the quantum
hypothesis.

Preface
Angelo Albini and Elisa Fasani

v

Periodical Reports: Physical, Inorganic Aspects and
Solar Energy Conversion
Introduction and review of the year 2012
Angelo Albini
1 Introduction
2 Review of the year 2012
References

3
3
4
9

Light induced reactions in cryogenic matrices (highlights 2011–2012)
Rui Fausto and Andrea Go´mez-Zavaglia
1 Introduction
2 Light induced conformational isomerizations in
cryomatrices
3 Tautomerizations and other structural isomerizations
4 Fragmentation reactions, unstable intermediates and
formation of complexes or weakly bound species
5 Noble gas chemistry
Acknowledgements
References

12
12
14
27

32
52
54
54

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Photophysics of fluorescently labeled oligomers and polymers
J. Se´rgio Seixas de Melo, Telma Costa, Catherine S. de Castro and
Anto´nio L. Mac¸anita
1 General view of polymer systems: fields, applications and
techniques
2 Polymers in solution: characteristics
3 Fluorescent probes
4 Photophysics of pyrene
5 Dynamics of excimer formation in oligomers
and polymers
6 Models for kinetics of excimer formation
7 Thermodynamics of excimer formation
8 Inclusion complexes (structures and stoichiometry)

Abbreviations
Acknowledgements
References

Photochemical and photocatalytic properties of
transition-metal compounds
Andrea Maldotti
1 Introduction
2 Tungsten
3 Manganese
4 Rhenium
5 Iron
6 Ruthenium
7 Osmium
8 Cobalt
9 Rhodium
10 Iridium
11 Nickel
12 Palladium
13 Platinum
14 Copper
15 Others
References

Photophysics of transition metal complexes
Francesco Nastasi, Maria Letizia Di Pietro, Emanuela Trovato and
Fausto Puntoriero
1 Introduction
2 Ruthenium and osmium
3 Rhenium

4 Iridium
viii | Photochemistry, 2013, 41, vii–x

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59
61
65
68
75
86
105
112
116
119
119

127

127
127
128
129
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5 Platinum and gold
6 Copper
7 Lanthanides
8 Miscellanea
9 Abbreviations
References

163
166
167
170

173
174

Photochemical applications of solar energy: photocatalysis
and photodecomposition of water
K. Kalyanasundaram
1 Introduction and scope
2 Photocatalysis
3 Photodecomposition of water
4 Concluding remarks
References

182

182
183
224
250
250

Highlights in Photochemistry

Enlightening the Americas: A History of the Inter-American
Photochemical Society (1975–2013)

269

Bruce A. Armitage
1 Birth of a Society
2 Newsletters

3 Elections
4 Efforts to promote and disseminate
photochemistry research
5 The I-APS winter meeting
6 Society awards
7 International collaboration
8 I-APS in 2013
Acknowledgements

272
274
275
276
277

Fluorogenic probes for imaging reactive oxygen species

279

269
271
271
271

Katerina Krumova and Gonzalo Cosa
1 Introduction
2 Reactive oxygen species
3 Fluorogenic probes
4 Conclusions
References

279
280
282
297
298

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Nitric oxide photoreleasing nanoconstructs with
multiple photofunctionalities
Aurore Fraix, Noufal Kandoth and Salvatore Sortino
1 Introduction
2 NO photodonors with fluorescence imaging modalities
3 NO photodonors with multiple phototherapeutic modalities
4 Closing remarks
Acknowledgements
References

302

302
303
308

315
315
316

Photochemical biology of caged nucleic acids
Xinjing Tang
1 Introduction
2 Caging groups and their photochemistry
3 Photochemical applications in biological studies
4 Summary
References

319

Photochemistry of the prebiotic atmosphere
Daniele Dondi, Daniele Merli and Alberto Zeffiro
1 Introduction
2 Composition of the early Earth atmosphere
3 Titan
References

342

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320
326
336
336

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Introduction and review of the year 2012
Angelo Albini

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DOI: 10.1039/9781849737722-00003

After a short introduction on the format adopted for this series of reports, a few
representative findings on photochemistry and its applications published in 2012 are

discussed.

1

Introduction

The present volume, no. 41 in the series ‘‘Photochemistry’’ of the Specialist
Reports published by the Royal Society of Chemistry follows the format
adopted from volume 39 on, that is a review of each of the main aspects of
photochemistry is presented every other year, in order to offer more
room for discussion of the development occurring. Thus, reviews on
physico-chemical aspects (photochemistry in matrix and dynamic aspects
in polymers) are presented here, along with inorganic photochemistry
(physical, chemical and catalytic aspects) and photochemical energy
conversion. These are referred to the 2011–2012 period. Computational and
organic photochemistry will be reviewed in next volume with reference to
the period 2012–2013.
The second part of the volume, and a fixed feature of the series, contains a
number of highlights, containing personal accounts of various application
of photochemistry. In this case, the history of the Inter-American Photochemistry Association is presented, following that of Asian-Oceanian and
European societies published in the previous volume. The other accounts
mainly deal with applications in the biological field, with such timely issues
as fluorogenic probes for reactive oxygen species, nitric oxide photoreleasing nanoconstructs and the photobiology of caged nucleic acids. A further
highlight is devoted to the photochemistry of prebiotic atmosphere.
2012 has marked the centennial of the celebrated presentation by
Giacomo Ciamician in New York on the photochemistry of the future that
made a large impression at the time and indeed makes a pleasant and
inspiring reading a century afterwards.1 Among the anniversaries of the
year, that of Kurt Schaffner, who turned 80. He has been Professor in
Geneva and then Director of the Max Plank Institute of Radiation

Chemistry (now of Bioorganic Chemistry) in Mu¨lheim-Ruhr, Germany,
from 1976 on. His choice to initiate a new and experimentally very difficult
theme, the structure and function of the plant photoreceptor phytochrome
and the highly important results obtained have been highlighted.2 A tribute3
followed by over 30 papers have been published in his honour by former
collaborators and scientific guests in Mu¨lheim. A tribute has been dedicated
to Francesco Lenci, a leading photobiologist in Pisa, Italy, on occasion of

Dipartimento di Chimica, Universita` di Pavia, viale Taramelli 12, 27100, Pavia, Italy.
Fax: 39 0382987323; Tel: 39 0382987316; E-mail:

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4

his 70 birthday. He is well known for his studies on motile responses of
microorganism to light stimuli.
Lamentably, this year has seen also the death of two of the protagonists
of modern photochemistry, H. E. Zimmerman and N. J. Turro. The contribution of both scientists to the development of this discipline is very well
known. An issue of JOC has been dedicated to Howard Zimmerman and
contains, after a biographic sketch,5 54 contributions by former co-workers
and colleagues, including one of which himself is a co-author.6 Nick Turro
is perhaps best commemorated by the account of his life, ‘‘Skating on the

edge of the paradigm’’, he gave himself some years ago.7
2

Review of the year 2012

Activity in photochemistry seems to proceed at no slacking pace. CAS lists
12860 reports on photochemistry in 2012, of which a remarkable 2467
patents and 698 reviews.
An important addition to the literature are books and in this sense
the main event of 2012 is the publication in two volumes of the third edition
of the CRC Handbook of Organic Photochemistry and Photobiology.8
With respect to the second edition (2003), new editors have taken charge
both of the chemical part (A Griesbeck and M. Olgemo¨ller), and of the
biological part (F. Ghetti). The handbook has grown to 1694 pages
and joins chapters devoted to a particular reaction (e.g. di-p-methane
rearrangement) or classes of intermediates (radical photochemistry) with
other illustrating technical aspects (e.g. excilamp photochemistry, solar
photochemistry). On the other hand, Paola Ceroni has edited an introduction to photochemical techniques, both steady state and time-resolved
spectroscopy and their use for the study of supramolecular spectroscopy
and nanostructures.9
As for research reports, one cannot but remark once again the trend
towards application that characterizes photochemistry. If chemistry in
general is the science that makes, the one that actually carries on what other
disciplines dream of, photochemistry is giving a more and more important
contribution to this end.
Reviewing the most important advancements published in the two
previous years in JACS,10 one of the editors has indeed remarked the
spectacular increased impact that photochemistry and photophysics have
on other, as well as in all modern technologies. Among the most important
disciplines he has mentioned the manipulation and design of multiphotonic

processes (non-resonant, multiphotonic excitations, two- or three-photons
reactions, generation of second and third harmonic able to excite different
entities in a linear resonant manner, formation of multiple excited states
from a single, high-energy, molecular excitation, etc.), molecular and
supramolecular systems able to perform complex functions, solid-state
photochemical phenomena (from synthesis to the transformation of a
photon’s energy into mechanical work), the application to sustainable
chemistry, photochemistry with visible light, plasmonics (also from the
generation of nanostructures to the release of DNA from a plasmonic
nanostructure).
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It is probably appropriate to present the small (and obviously personal)
choice of examples for the review of the year according to the application,
rather than to the reaction itself.
As mentioned in last year report, the work on solar energy conversion
seems to be again in a state of grace. A historical account of the chemical
approach to the artificial harnessing of solar energy has been published.11
An introductive, large scope review has compared the preparative techniques for efficient photocatalysts with reference to the best exploitation of
sun light.12 For this aim, iron(III)oxide in the form of hematite is, in many
respects, an attractive material for the photocatalytic production of molecular oxygen from water. Extensive research on this material has indeed
shown favourable features, but also limitations.13 The thermo- and
photochemical conversion of water and carbon dioxide into fuels has been
confronted by a theoretical analysis in general thermodynamic terms.14
Solar thermal conversion is more mature than the photochemical method,

and dramatic improvements are required in photo- and electrocatalytic
technologies.15 Imitation of nature by assembling complex artificial systems
that mimic chlorophyll photosynthesis have been reviewed. These contain
light harvesting units such as porphyrins assembled e.g. on protected metal
nanoparticles connected by non covalent bonding to acceptors.16
The construction of an ‘artificial leaf’ closely mimicking the functions of
green organisms and built by using earth-abundant elements has been
reported. This appears to be a simple, stand-alone device that provides a
means for an inexpensive and highly distributed solar-to-fuels system.17
Since the engineering and manufacturing of the system are rather
inexpensive, this may become a viable way for providing energy to the part
world lacking fossil resources. The social significance of the developing
nanotechnology-based artificial photosynthesis has been stressed also in
other papers, provided that the challenges involved in such key areas as light
capture, photochemical conversion, and energy storage are overcome.18
Another approach involves ‘biofuels’, which is the manipulation of
organisms for the production of fuels (or chemicals in general). Examples
are diverting the natural flow of photosynthetic electron transport in green
microalgae toward sustained generation of hydrogen gas or toward the
generation of volatile hydrocarbons from carbon dioxide and water. Long
chain hydrocarbons have been obtained from green microalgae in the place
of normal products. Also improving the solar-to-biomass energy conversion
efficiency of photosynthesis is an important goal and it has been demonstrated that a theoretical maximum of about 10% can be reached from the
current-best of 2–3%.19
Among the abundant literature on semiconductor photocatalysis, a
review on past, present and future outlook should be mentioned,20 along
with a discussion on the application for heterogeneous light conversion with
reference to charge transport characteristics, radical chemistry organic
degradation mechanisms, as well as the engineering aspect.21 Probably the
most investigated field involves pushing activity to lower wavelengths and a

comprehensive review has been devoted to titanium dioxide materials that
are activated by visible-light22 and another one to the chemical and physical
consequences of band bending at surfaces and interfaces.23
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With respect to extending the activity well into the visible, the most notable
advancement has been recently obtained by the use of fluorinated titania. The
methods of arriving at such materials, whether via post-synthesis or in-situ
fluorination, have been studied in such a way as to control the chemical
nature of fluorine incorporated. Surface fluorination induces adsorption and
thus the photocatalytic redox properties of the photocatalysts. On the other
hand, if fluorine is incorporated in the lattice, sometimes associated with
other codopants, new localized electronic structures and surface defects are
formed. This gives rise to the exceptional visible-light photoactivity of these
materials.24 Further review concern opposite aspects, new solid state aspects
of the photocatalyst on one hand, with the introduction of nanoachitectures
for varied applications,25 the role that organic molecules may have as
photocatalysts in water depollution.26 Organic molecules generally are not
as robust as inorganic oxides, although on the other one this limitation can be
at least partially circumvented by adsorption on a solid support, but are certainly more versatile with respect to light absorption and chemical behaviour.
Macromolecules are a field where photochemistry is finding an ever
increasing role, suggesting a large potential. As for the initiation, the choice
of a photochemical method allows a better control at a lower temperature
and further often minimizes chain transfer and depolymerization.27
Photoresponsive chromophores have been incorporated giving access to

interesting materials. As an example, the characteristic reactivity of the
o-nitrobenzyl ester group has been exploited for the photochemical control
of electrostatics and for making conjugated materials with photocleavable
pendants.28 A further step forward, one arrives at materials that depolymerise photochemically (Self Immolative Polymers), a quite appealing issue,
that is macromolecules bearing appropriate moieties at chain ends that
when activated undergo spontaneous head-to-tail depolymerizaton, giving
an amplified response output.29,30
In biology, photolabile polymers find important application for
biopatterning, as well as hydrogels for tissue engineering and many others.31
A further direction is nanomaterials. Varied photochemical methods, based
on monolayers of thiols, silanes and phosphonic acids, and thin films of
nanoparticles and polymers, have been developed for use on metal and
oxide surfaces. In this way, metal nanowires, nanostructured polymers and
nanopatterned oligonucleotides and proteins have been fabricated.32
Synthetic organic photochemistry sees a renaissance of electron transfer
processes. These give the possibility of bringing to reaction pairs of non
matched reagents, both electron-rich or both electron-poor, thanks to the
Umpolung caused in this way. Many of the new reactions reported involve
the use of a photocatalyst absorbing in the visible and are actually
conveniently carried out by using either solar radiation or inexpensive
‘fluorescent’ household lamps. Both transition-metal complexes (mainly Ru
and Ir complexes) and organic dyes are used to this purpose.33,34 The
mechanism of the overall process is at times somewhat involved, but the
target ‘visible light’ makes such processes appealing from the viewpoint of
cost, safety, availability, and environmental friendliness.33,35
Metal complexes that have long be used for inorganic application for
water splitting, photovoltaic cells and energy storage, found their way in
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organic chemistry. As it is well known, Ru(bipy) absorbs a significant part
of the visible (lmax 452 nm) and efficient intersystem crossing leads to the
lowest MTLCT triplet state that is a long lived species convenient for
bimolecular interactions. the idea has found application both for the
reductive side (left, the complex is oxidized, applied e.g. to the reduction of
diazonium salts) and for the oxidative side (right, the complex is reduced)
have found application, e.g. the oxidation of amines, followed by
deprotonation to a-amino radicals and further oxidation to such versatile
intermediates as iminium cation (see Scheme 1).36,37 The possibility of tuning
the redox properties of the metal complexes (by changing the metal, e.g. Ir(II)
in the place of Ru(II), or by changing the ligand) makes the system appealing.
Another interesting example is the arylation of the carbazole anion or Cu
complexes by aromatic halides, an analogue of the Ullmann reaction that,
contrary to the thermal version, occurs at room temperature or below
(see Scheme 2).38 Aromatic and benzylnitrile, but not aliphatic nitriles, are
smoothy reduced to the corresponding amines by visible light irradiation in
the presence of samarium iodide.39 Recent evidence that the principle of
photochemical switching can be applied also to activating a catalyst by
exposing the key moiety has been highlighted.40 Among the many
other reactions reported, an interesting case is the rearrangement of some
naturally occurring secolabdane diterpenoids with a b-homoallyl-a,b-unsaturated ketone structure. Irradiation led to cyclopropyl derivatives,
likewise found in nature, in accordance with the postulated radical
mechanism (see Scheme 3).41
Photons are by their nature the reagents of nanodimensions and this
aspect receives increasing attention. Molecular switches are often based on a
photochemical reaction. A basic question here is where differentiation

begins. Thus, in a seemingly symmetric system such as MQn (nZ2), where
1

2+

[Ru(bipy)3 ]

3

–0.87V

Ru(bipy)3

3+

+1.26V

2+

[Ru(bipy)3 ]

hν

+0.78V

Ru(bipy)3

+

–1.35V

Ru(bipy)3

2+

Scheme 1

N-Cu(PAr 3) 2
10%
Ph-X +

+

Li-N

Ph-N
hν

Scheme 2

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O

O
O

R

O

R'

R'
O
O

R

hν

O
O

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Scheme 3

M is a cromophore and Q a quencher, electron transfer may involve either
of the two Q moieties, and the same holds for a MM system in which M may
act both as an acceptor and as a donor. These symmetry breaking processes
and the condition for their operation have been discussed.42 On the practical
side, the fabrication of such systems is no simple task.
As an example, having available multichromophoric systems would be a
big step forward with respect to a monochromophoric one. Thus, an
asymmetric triad could in principle store a byte, rather than a bit, of data.
However, the behaviour of such systems is largely affected by the reciprocal
influence of the chromophores. Experimental efforts and theoretical
predictions on these devices have been discussed.43

The conversion of electronic energy into motion in molecular motors
presently finds ‘‘speed limits’’. The improvements suggested by a better
understanding of the dynamics involved by ultrafast luminescence
measurements have been reviewed.44 On the other hand, working at the
single molecule level enables to reach a better knowledge of chemical
reactions, in particular in catalysis. Thus, the use of high-resolution imaging
techniques with suitable fluorogenic probes has revealed the location of
the catalytically active sites and their relation to heterogeneities on the
catalyst surface of the solid catalyst and how the reactivity fluctuate.45
Exploring the photoinduced nuclear dynamics at the solid-liquid
interface should bridge the gap between surface electrochemistry and
photochemistry.46
Although a large part of photobiologic research has relatively little
chemical character, in some cases the interaction with photochemistry is
strong and it is certainly productive that the two disciplines are presented
side-by-side, as in the three major journals in the field and in the CRC
Handbook. Applications are quite varied, from photochemical labelling47
for determining the presence and the role of a molecule, to the intervention
in some biologic mechanism. As an example, a photoswitchable ligand may
be introduced into ion channel structures for exploring the diverse roles
of neurotransmitters and receptors in the brain,48,49 or to control the biological function of small molecules, oligonucleotides, and proteins involved
as parts of natural or artificial gene circuits in living cells.50 Light can be
precisely regulated in timing, location, intensity, and wavelength and thus is
the ideal mean for such investigations.
The relation between drugs and photochemistry encompasses a variety of
aspects. One is using photochemistry for pharmaceutical application, a
relatively simple example being the use of photocatalysis for disinfection.51
This seems to be a credible alternative to chlorination, with a reduced
production of harmful byproducts. The largest pharmaceutical activity is
photodynamic therapy (PDT), the dye-photosensitized destruction of

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cancer cells. Research in the field is active, although bedside application
remains limited at present. This requires that important drawbacks are
eliminated, such as obtaining a precise targeting of the photosensitizer,
based on the difference or different patterns of expression between cancerous and normal cells, avoiding general distribution to the whole body that
would otherwise lead to generalized photosensitization.
Strategies towards this end have been reviewed,53 as well as the
application to wound healing,54 where the appropriate choice of the
photosensitizer coupled with the use of lasers has led to positive results.
While PDT uses dyes and thus visible light, activation by UV-A light is
characteristic of different methods, such as that based on psoralens
(PUVA). The use of short wavelength irradiation increases the chance of
cancer development and this has fostered the research of alternative systems
endowed with a better selectivity and/or lower radiation doses. The possibility of exploiting thionucleoside-mediated DNA photosensitization in this
role has been explored.55
Another photochemical application is based on photoactivatable drugs or
on the precise delivery of drugs. Several photoresponsive nanocarriers
have been developed, based on different photoinduced effect, viz. either
molecular reactions, such as isomerization and oxidation; changes in a
polymer structure, such as fragmentation of the backbone or depolymerization; changes in the hydrophobicity; photothermal effect that control
surface absorption.56 A method that is fast developing is photochemical
internalisation. This is based on light-activated release of biologically active

compounds retained within endosomes or lysosomes. In this way, compounds that do not pass freely through the phospholipid membrane,
such as macromolecules are internalized via endocytosis. The method has
been considered to have progressed up to the threshold of clinical
application.57
The reverse side of the coin is the unwanted photochemistry of drugs that
may lead to serious phototoxic effect. The photochemical safety of new
drugs has thus to be clearly established. A number of screening tests
has been proposed in addition to those previously considered.58 At any rate,
UV light might interact with DNA and cause cancer, particular when
exogenous chromophores are present. The understanding of the biological
consequences is thus important, e.g. for devising appropriate protective
systems. An in-depth investigation has been carried out by using one of the
best known photoactive compounds, benzophenone, and a chiral derivative,
ketoprofen, and determining the damage to DNA and oligonucleotides. The
work confirmed the participation of singlet oxygen, energy transfer to
thymine bases resulting in cyclobutane formation competing with oxetane
generation, electron transfer with guanine.59
References
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Photochemistry, 2013, 41, 1–11 | 11


Light induced reactions in cryogenic
matrices (highlights 2011–2012)w
Rui Fausto* and Andrea Go´mez-Zavaglia

Published on 31 October 2013 on | doi:10.1039/9781849737722-00012

DOI: 10.1039/9781849737722-00012

In this chapter, relevant studies published in 2011 and 2012 and focusing on
the subject of light induced reactions in cryogenic matrices are reviewed.
These studies range from conformational isomerizations to complex (bondbreaking)/(bond-forming) processes induced either by ultraviolet-visible or infrared
light, and illustrate recent applications of the matrix isolation technique in these
domains. Photochemical processes in which noble gas atoms participate directly,
leading to formation of covalently bound noble gas containing molecules, are also
addressed briefly.

1

Introduction

In volumes 37, 38 and 39 of this series,1–3 we provided extensive reviews on
the literature dealing with light induced reactions in cryogenic matrices that
were published during the period July 2004 – December 2010. The present

chapter highlights relevant reports on the same subject appearing in the
specialized literature during 2011 and 2012.
As in our previous publications,1–3 we will focus on studies dealing with
organic compounds. Section 2 centers on conformational isomerization
processes, induced either by ultraviolet-visible (UV/visible) (2.1) or infrared
(IR) (2.2) light, while section 3 is dedicated to reports on photoinduced
tautomerizations and structural isomerizations in general. Fragmentation
photoreactions, which in general imply formation of unstable intermediates
are covered in section 4, which is divided in several subsections dealing
with specific topics within this general subject. This section addresses also
briefly the subject of photofragmentation processes leading to formation of
complexes or weakly bound species, The last section (section 5) refers to
photochemical processes where the matrix noble gas atoms participate
directly and lead to formation of covalently bound noble gas containing
molecules.
For those that are less familiarized with the matrix isolation
fundamentals and technical descriptions, the seminal books by Meyer,4
Andrews and Moskovits,5 Barnes et al.,6 Dunkin7 and Fausto,8 are
recommended as introductory literature to this chapter. Several more
specific reviews on matrix isolation and its application to the study of ligh
induced processes have been published in the last 5 years,9–16 including
Department of Chemistry, University of Coimbra, P-3004-535, Coimbra, Portugal.
Fax:+351 239 27703; Tel: +351 239 852063. E-mail:
w
Copyright and Licenses Note: The following figures were based on or copied from the original
articles, cited in the corresponding captions, with permission of their publishers: Figures 22, 25
and 31, John Wiley and Sons; Figures 1–3, 7, 12, 14–16, 19–21, 27, 29, 32, 33, 36 and 37,
Elsevier; Figures 6, 8, 9, 11, 13, 23, 24, 26, 30, 38, 39 and 41, American Chemical Society,
Figures 4, 5 and 17, Royal Society of Chemistry; Figure 34, Springer; Figures 10, 28 and 40,
American Institute of Physics; Figure 35, NRC Research Press.

12 | Photochemistry, 2013, 41, 12–58

c

The Royal Society of Chemistry 2013


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special issues of the Annual Reports on the Progress of Chemistry (the fifth
report in that series dedicated to matrix isolation, after those published in
1985, 1991, 1997 and 2001),15 Low Temperature Physics (special issue edited
by Arakawa and Ra¨sa¨nen and dedicated to Elena Savchenko on the
occasion of her 70th birthday)16 and the Journal of Molecular Structure.17
The last publication, edited by Fausto, Lapinski and Reva, provides a
general up to date impression of the current developments of the matrix
isolation technique and its applications in the study of light induced
processes. This includes the occurrence of photoinduced conformational
transformations described for 1-propanol (Wassermann, Suhm, Roubin
and Coussan), oxalic acid monoamide (Maier, Endres and Reisenauer),
b-alanine (Stepanian, Ivanov, Smyrnova and Adamowicz) and glycine
(Bazso, Magyarfalvi and Tarczay); halogen and hydrogen atom detachment
and transfer processes are reported for adenine (Iizumi, Ninomiya, Sekine
and Nakata), o-chlorobenzaldehyde (Tanaka, Fujiwara, Ogawa and Nishikiori), chloro-derivatives of resorcinol (Nagaya, Iizumi, Sekine and
Nakata), iso-tribromomethanes (George, Kalume and Reid), halogenated
phenol (Nanbu, Sekine and Nakata), ethyltrioxo-rhenium (Morris, Greene,
Green and Downs), maleic hydrazide (Reva, Almeida, Lapinski and

Fausto) and isocytosine (Ivanov, Stepanian and Adamowicz). Other types
of rearrangements and isomerizations are represented by the works on the
tetrazole-saccharyl conjugate (Ismael, Borba, Duarte, Giuliano, Go´mezZavaglia and Cristiano) and six-atomic [2C,2N,2S] isomeric structures
(Voros, Bazso, Tarczay and Pasinszki). In addition, investigations on the
structure and light induced reactions of complexes between nitrous acid and
methanethiol (Grzechnik and Mielke) and between formic acid and xenon
(Cao, Melavuori, Lundell, Ra¨sa¨nen and Khriachtchev) are also reported.
Noble gas cryochemistry is represented by the study of Turowski,
Gronowski, Guillemin and Kolos on molecules containing covalentlybound xenon atoms prepared by photolysis of cyanodiacetylene in
xenon matrices. Finally, the uses of low temperature matrices to trap
mass-selected protonated pyrene and coronene cations, as well as products
of 3-azidopropionitrile pyrolysis are represented by the contributions of
Garkusha, Fulara and Maier, and Pinto, Dias, Levita, Rodrigues, Barros,
Dyke and Costa, respectively.
The 3rd edition of the ‘‘Handbook of organic photochemistry and photobiology’’, by Griesbeck, Oelgemo¨ller and Ghetti, appeared in 2012 and
included a chapter dedicated to cryogenic matrix photochemistry, authored
by Bucher,18 which gives emphasys to the photoproduction and characterization of short lived species (radicals, diradicals and oligoradicals,
carbenes, nitrenes) as well as to the photochemistry of matrix-isolated
heterocycles. The book ‘‘Physics and chemistry at low temperatures’’,19
edited by Khriachtchev, was also launched during 2012. The book covers
several relevant topics involving low temperature chemistry and physics,
from fundamental investigations of weakly coupled systems to complicated
processes taking place in the outer solar system. Several chapters of
the book deal with photoinduced processes, including conformational
isomerizations (Andrews; Fausto, Khriachtchev and Hamm), production of
unstable species (Jacox; Andrews; Khmelenko, Lee and Vasiliev),
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photodynamics (Apkarian and Pettersson; Tiellens and Allamandola)
and photolysis of water ice (Johnson). The book includes also chapters
dedicated to theoretical modeling of trapped species (Nemukhin and
Grigorenko), noble gas chemistry (Grochala, Khriachtchev and Ra¨sa¨nen),
cryo-spectroscopy of biological molecules (Gerber and Sebek), high
resolution single molecule spectroscopy in condensed matter (Orrit and
Moerner), spectroscopy of surface species (Tsyganenko), cryogenic
solutions (Herrebout and van der Veken) and spectroscopy in helium
droplets (Kuyanov-Prozument, Skvortsov, Slipchenki, Sartakov and
Vilesov) and in solid parahydrogen (Fajardo). Recent reviews on light
induced noble gas chemistry in cryogenic matrices have also been reported
by Nemukhin et al.,20 Khriachtchev, Ra¨sa¨nen and Gerber21 and Gerber,
Tsivion, Khriachtchev and Ra¨sa¨nen,22 the last addressing the intrinsic
lifetimes and kinetic stability in different media of noble gas hydrides.
Particularly sound studies on the general subject of light induced reactions in cryogenic matrices reported during the period 2011–2012 must be
here mentioned: (i) the investigation by Krupa et al.,23 where an interesting
example of a conformer-selective photoreaction was described for isoeugenol, (ii) the production of a crystalline variety of formic acid based
on the higher energy conformer of the molecule (initially obtained by
vibrational excitation of the ground conformational state) by Hakala
et al.,24 (iii) the first unequivocal experimental detection of the five lowest
energy isomeric forms of cytosine, by Lapinski et al.,25 (iv) the elegant
studies of the triplet-sensitized photoreactivity of a geminal diazidoalkane
and of isoxazole, by Ranaweera et al.26 and Nunes et al.,27 which provided
detailed information on the photochemistry of nitrenes, (v) the probable
first direct experimental evidence of the photo-induced dissociationassociation (PIDA) mechanism of tautomerization of non-hydrogen
bonded molecules, ilustrated by Iizumi et al.28 on adenine photoreactions in
low temperature argon matrices, and (vi) the first preparation of a noble gas

hydride (HXeBr) in a molecular solid (CO2) by Tsuge et al.29
2

Light induced conformational isomerizations in cryomatrices

The study of high energy conformers, not accessible by other techniques,
produced in a cryogenic matrix upon in situ irradiation either in the
UV/visible or infrared ranges has experienced great progress during the
period covered by this review. The use of selective vibrational excitation of
the OH stretching overtone or combination modes involving the OH
stretching coordinate of low energy conformers, in the near-IR range, to
produce new conformers of high energy can nowadays be considered a wellestablished technique. In the last years, it has been applied with great
success to an increasing number of relevant molecular systems, including
monomeric species and associates. Excitation of other vibrations has been
less effectively used. On the other hand, the detailed understanding of the
mechanisms of energy relaxation which are on the basis of the possibility of
promoting conformational changes upon excitation of high frequency
vibrational modes is still an opened question. Dynamic studies on matrixisolated HONO have demonstrated that the excitation of the OH stretching
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mode in this molecule leads to cis-trans conformational isomerization with a
quantum yield close to 100%,30 which appears to be a clear demonstration
that the energy initially introduced in the molecule through vibrational
excitation is not thermalized or randomized on the timescale of the isomerization, and that the reaction is not statistical. Otherwise, the initially
pumped OH stretching vibration shall relax preferentially into a subset of

relatively few reactive states which are closely resonant to the former. These
reactive states are mostly combination modes involving bending modes and
the torsional reaction coordinate in high quantum states. Thus, they can
tunnel through the isomerization barrier efficiently, while other, nonreactive states act as energy storage devices. The time-dependent results
obtained for the prototype HONO molecule,30 can then be considered as a
gateway for exploration of the details of the processes of vibrational energy
relaxation in cryogenic matrices, a subject that will certainly receive great
development in the near future.
The use of UV/visible light to induce conformational changes in matrixisolated molecules is an intrinsecally less attractive technique, since in this
case conformational isomerizations occur most of times simultaneously
with other photochemical processes (e.g., fragmentation, stuctural isomerization, tautomerization) and, almost invariably, it results in the attainement of photostationary equilibria, thus preventing any efficient
conformer selection. Nevertheless, during the period covered by this review
a few studies using this strategy have also been reported, as described below.
As it could be expected, most of these studies focused on aromatic
molecules.
2.1 Conformational isomerizations induced by UV/visible light
Stepanian et al. investigated the simplest b-amino acid, b-alanine, isolated
in argon matrices and subjected to broadband UV irradiation provided by a
standard deuterium lamp.31 The analysis of the obtained IR spectra confirmed the presence of five b-alanine conformers in the matrices. Two of
these conformers (I, II in Fig. 1), have an N–HÁ Á ÁO intramolecular H-bond,
while the remaining observed conformers have one O–HÁ Á ÁN hydrogen
bond (V in Fig. 1), or no hydrogen bonds (IV and VII). The relative
populations of the five conformers trapped in the matrices were found to fit
well those determined using the relative Gibbs free energies calculated at the
CCSD(T)/CBS level of theory at the sublimation temperature (420 K).
According to theoretical calculations, all the non-observed conformers
represented in Fig. 1 are separated from the experimentally observed lower
energy forms by low energy barriers, thus relaxing to these latter forms
during matrix deposition. Both UV irradiation and matrix annealing
allowed discrimination of the spectral bands of the different conformers.

In particular, under UV irradiation the band intensities of conformers I and
II were found to decrease while those of conformers IV and V increase,
showing that UV excitation disrupts the intramolecular N–HÁ Á ÁO hydrogen
bond quite efficientely and, simultaneously, promotes internal rotation
about different bonds, including C–O and C–N. Though the authors
had presented a quite complete characterization of b-alanine ground
state potential energy surface, no excited states calculations were
Photochemistry, 2013, 41, 12–58 | 15