Photochemistry volume 38

Specialist Periodical Reports

Editor Angelo Albini

Photochemistry
Volume 38


Photochemistry

Volume 38



A Specialist Periodical Report

Photochemistry
Volume 38
A Review of the Literature Published between
July 2007 and December 2009
Editor
Angelo Albini, University of Pavia, Pavia, Italy
Authors
Francesco Barigelletti, Istituto ISOF-CNR, Italy
Adalbert Braig, Performance Chemicals Research, Switzerland
M. Consuelo Jime´nez, Universidad Polite´cnica de Valencia, Spain
Telma Costa, University of Coimbra, Portugal
Kurt Dietliker, Performance Chemicals Research, Switzerland
Daniele Dondi, University of Pavia, Italy
Rui Fausto, University of Coimbra, Portugal
Andrea Go´mez-Zavaglia, University of Coimbra, Portugal

Sylvie Lacombe, Universite´ de Pau et Pays de l’Adour, France
Andrea Maldotti, Universita´ degli Studi di Ferrara, Italy
Daniele Merli, University of Pavia, Italy
Miguel A. Miranda, Universidad Polite´cnica de Valencia, Spain
Kazuhiko Mizuno, Osaka Prefecture University, Japan
Gloria Olaso-Gonza´lez, Universitat de Vale`ncia, Spain
Thierry Pigot, Universite´ de Pau et Pays de l’Adour, France
Joa˜o Pina, University of Coimbra, Portugal
Luca Pretali, University of Pavia, Italy
Andrea Ricci, Performance Chemicals Research, Switzerland
Daniel Roca-Sanjua´n, Universitat de Vale`ncia, Spain
J. Se´rgio Seixas de Melo, University of Coimbra, Portugal
Luis Serrano-Andre´s, Universitat de Vale`ncia, Spain
Takashi Tsuno, Nihon University, Japan


If you buy this title on standing order, you will be given FREE access
to the chapters online. Please contact with proof of
purchase to arrange access to be set up.
Thank you.

ISBN 978-1-84755-054-5
ISSN 0556-3860
DOI: 10.1039/9781849730860
A catalogue record for this book is available from the British Library
& The Royal Society of Chemistry 2011
All rights reserved
Apart from fair dealing for the purposes of research or private study for
non-commercial purposes, or for private study, criticism or review, as
permitted under the Copyright, Designs and Patents Act, 1988 and the

Copyright and Related Rights Regulations 2003, this publication may not be
reproduced, stored or transmitted, in any form or by any means, without the
prior permission in writing of The Royal Society of Chemistry, or in the case
of reproduction in accordance with the terms of the licences issued by the
Copyright Licensing Agency in the UK, or in accordance with the terms of the
licences issued by the appropriate Reproduction Rights Organization outside
the UK. Enquiries concerning reproduction outside the terms stated here
should be sent to The Royal Society of Chemistry at the address printed on this
page.
Published by The Royal Society of Chemistry,
Thomas Graham House, Science Park, Milton Road,
Cambridge CB4 0WF, UK
Registered Charity Number 207890
For further information see our web site at www.rsc.org


Preface
Angelo Albinia
DOI: 10.1039/9781849730860-FP005

This is Volume 38 in the series Specialist Periodical Reports on Photochemistry and completes the updating process by reviewing the 212 years
period from July 2007 to December 2009. From 2010 on it is hoped that the
yearly periodicity will be restated and every effort will be given for reducing
the gap between the publication date of the original research and the review
in this series. On the other hand, a rapid publication is no more the only
issue. New papers are nowadays available on the PC screen of everybody,
whether at work or at home, sometimes several months in advance to the
actual publication date and retrieving a desired information is easy and
rapid via any of the available search engines.
Thus, the newest aspects of each field will continue to find a mention in

this series, but this will be based on a in depth discussion of a limited choice
of papers, rather than on a comprehensive presentation of the activity in the
field. The review part will be accompanied by a second part containing
highlights on specific aspects. This is expected to become as important as the
former one in this series – and has already expanded in this volume.
The plan of the reports remains the same, articulated in three sections,
devoted respectively to the physical and theoretical, to the organic and to
the inorganic aspects. In the first section a review on theoretical aspects has
been inserted (by Prof. Serrano-Andre`s) and in the last one the very extensive material on inorganic photochemistry has been subdivided between
prof. Maldotti and Prof. Barigelletti.
The highlights section has doubled, growing fom 2 to 4 contributions.
These mainly concern applied and industrial aspects, viz. new materials as
photosensitizers, prebiotic photochemistry and a field characterized by a
rapid grow, such has photolithography. A further highlight concerns
coatings in the automobile industry and may be followed by further contributions evidencing where one may look for photochemistry in the industry. This is probably not where photochemistry practitioners think it
should, but this is the state of affairs and there goes most of the money
invested in photochemistry. We are happy that well known experts from the
industry found the time for contributing.
As always, comments are highly welcome, we need to know whether this
work has to be continued – and how. The impression of the present reporter
is that having available a broad-scope information in a single book may
help in discovering connections with areas outside the everyday field of
activity, which is surely a plus. But it is the readers’ opinion that matters.

a

Organic Chemistry Department, Pavia University, viale Taramelli, 10, 27100, Pavia, Italy

Photochemistry, 2011, 38, v–v | v


c

The Royal Society of Chemistry 2011



CONTENTS
Cover
An energy level diagram
overlaid on the sun.
Background image reporduced
by permission of NASA.

v

Preface
Angelo Albini

Reports
Review of the period July 2007–December 2009
Angelo Albini
1 A bit of history
2 Photochemical literature: the present state
3 Review
References

1
1
1
3

8

Physical and theoretical aspects
Recent trends in computational photochemistry
Luis Serrano-Andre´s, Daniel Roca-Sanjua´n and
Gloria Olaso-Gonza´lez
1 Introduction
2 Theoretical methods and concepts for excited states

10

10
12

Photochemistry, 2011, 38, vii–xi | vii

c

The Royal Society of Chemistry 2011


3 Recent studies on photophysics and photochemistry
4 Photochemistry: nonadiabatic processes and reactivity
5 Conclusions
References

16
19
28
29

Light induced reactions in cryogenic matrices

37

Rui Fausto and Andrea Go´mez-Zavaglia
1 Introduction
2 UV/visible-induced reactions in cryomatrices
3 IR-induced reactions in cryomatrices
References

37
38
60
63

Dynamics and photophysics of oligomers and polymers
Joa˜o Pina, Telma Costa and J. Se´rgio Seixas de Melo
1 Organic conjugated polymers and oligomers
2 Dynamics of polymers hydrophobically modified with
fluorescent probes
3 Conclusions
References

67
67
90
105
105

Organic aspects
Alkenes, alkynes, dienes,
Takashi Tsuno
1 Photochemistry
2 Photochemistry
3 Photochemistry
4 Photochemistry
5 Photooxidation
References

polyenes

110

of
of
of
of

110
131
132
133
134
135

alkenes
polyenes
alkynes
enynes

Oxygen-containing functions
M. Consuelo Jime´nez and Miguel A. Miranda
1 Introduction
2 Norrish Type I reactions
3 Hydrogen abstraction
viii | Photochemistry, 2011, 38, vii–xi

143
143
143
145


4 Paterno`-Bu¨chi photocycloadditions
5 Photoreactions of enones and quinones
6 Photodecarbonylation
7 Photodecarboxylation
8 Photo-fries and photo-claisen rearrangements
9 Photocleavage of cyclic ethers
References

150
152
158
158
160
161
162

Photochemistry of aromatic compounds
Kazuhiko Mizuno
1 Introduction
2 Isomerization reactions
3 Addition and cycloaddition reactions
4 Substitution reactions
5 Intramolecular cyclization reactions
6 Inter- and intra-molecular dimerization reactions
7 Lateral-nuclear rearrangements
References

168

Functions containing a heteroatom different from oxygen
Angelo Albini and Elisa Fasani
1 Nitrogen containing functions
2 Functions containing different heteroatoms
References

210

168
168
173
180
183
189
202
203

210
226
230

Inorganic aspects and solar energy conversion
Photophysics of transition metal complexes
Francesco Barigelletti
1 Introduction
2 Ruthenium
3 Osmium
4 Rhenium
5 Iridium
6 Rhodium
7 Platinum
8 Palladium
9 Copper
10 Gold

234
234
234
240
241
243
247
248
250
251
252

Photochemistry, 2011, 38, vii–xi | ix


11 Chromium
12 Lanthanides
Abbreviations
Abbreviations for Ligands
Schematic formula
References

252
253
253
254
254
255

Photochemical and photocatalytic properties of transition-metal
compounds
Andrea Maldotti
1 Introduction
2 Titanium, niobium, chromium, molybdenum,
tungsten
3 Manganese, rhenium
4 Iron
5 Ruthenium
6 Osmium
7 Cobalt, rhodium, iridium
8 Nickel, Palladium, Platinum
9 Copper

10 Silver, gold
11 Zinc
Abbreviations
References

275

275
276
279
281
284
291
292
295
296
297
298
300
300

Highlights
New materials for sensitized photo-oxygenation
Sylvie Lacombe and Thierry Pigot
1 Introduction: the mechanisms of sensitized
photo-oxygenation reactions
2 Why designing new materials for photo-oxidation
3 Supported photosensitizers as convenient materials
for sensitized photooxygenationtion: different
approaches

4 Conclusion
References

x | Photochemistry, 2011, 38, vii–xi

307
307
311
311

324
325


Prebiotic photochemistry
Daniele Dondi, Daniele Merli and Luca Pretali
1 Introduction: why prebiotic photochemistry?
2 Hystorical background
3 Synthesis of nucleobases
4 Synthesis of sugars
5 Synthesis of Amino Acids
References

330

Industrial applications of photochemistry: automotive coatings
and beyond

344

Kurt Dietliker, Adalbert Braig and Andrea Ricci
1 Introduction
2 Stabilizaiton of automotive coatings
3 UV Curing in automotive applications
4 Some future applications of photochemistry in the
automotive industry
5 Conclusions
References

330
331
333
336
339
341

344
344
349
364
366
366

369

Trends in Photolithography Materials
Will Conley and Cesar Garza
1 Formation of the relief image
2 Formation of the relief image in chemically amplified resists
3 ArF materials, immersion lithography and extension of

ArF
References

369
371
373
385

Photochemistry, 2011, 38, vii–xi | xi



Review of the period July 2007–December
2009
Angelo Albinia
DOI: 10.1039/9781849730860-00001

1

A bit of history

Anniversaries have come up in these years. The present reporter has remarked that a century has elapsed since photochemistry came of age. The
chemical effects that light produced had of course been known since the
beginning of chemistry itself and the interest had much grown in the 19th
Century due to the development of photography. However, photochemical
experiments had remained sparse and conclusive evidence about the exact
nature of that effect had been very limited until the beginning of the following century, when things changed mainly thanks to the contribution by
Ciamician and Silber and by Paterno` in Italy and by Stobbe in Germany.
All of the three groups published their view of the state of the art in 1909
and recognized the great advancement that had taken place. It is indeed

remarkable that most of the key reactions of (unsaturated) carbonyl derivatives, nitro compounds and alkenes, oxygenations reactions and photochromism were then discovered and rationalized in a way that has resisted
time.1
In the hundred years since, photochemistry has been first neglected, then
has taken a considerable time in rediscovering what had been in meantime
forgotten. When this happened, however, (in the 1950s) the understanding
of molecular structure and bonding had much grown and the new ‘molecular’ photochemistry, as indicated in the title of Turro’s book2 became an
essential part of ‘mechanistic’ chemistry in research and in university
courses. The first volume of the present Royal Chemical Society series
edited by D. Bryce-Smith was printed 40 years ago at the high mark of this
process and represented photochemistry as a consistent and articulated
theory, growing at a lively pace in different (applicative) directions.
After four decades, what most impresses an observer is how far the applications of photochemistry has become detached from the core of the discipline. Indeed, photochemistry has pervaded fields so far one from another
that they are not only independent one from another, but are even forgetful
that there is a single core discipline.

2

Photochemical literature: the present state

Examining the photochemical literature in the 212 years period considered,
one first of all notices that this discipline has an important role and certainly
advances at no slackened pace, with regard to both research papers and
patents. The yearly number of photochemical papers is since some time
essentially unchanged. A more detailed consideration evidences some
a

Organic Chemistry Department, Pavia University, viale Taramelli, 10, 27100, Pavia, Italy

Photochemistry, 2011, 38, 1–9 | 1


c

The Royal Society of Chemistry 2011


characteristics of the photochemical literature that had been highlighted in
the Introduction to the previous volume.
Thus, if one takes into consideration the journals that have most often
hosted photochemical papers, as an example referring to year 2009, and lists
the journals according to the number of papers on this subject published
that year, one find that 32 journals contained about 35% of the total
number of the papers of that year. The type of journal is an indication of the
audience that photochemistry practitioners think to address. What comes
out is that the percentage of papers is distributed according to the key topic
of the journal as follows.
– General chemistry, 6.6% of the total (JACS, the single journal most
often chosen makes 3.2 %, the others are Chem. Commun., Angew. Chem.,
Chem. Eur. J., Proc. Natl. Acad. Sci.)
– Physical chemistry, 7% (J. Phis. Chem. A, B, C, Phys. Chem. Chem.
Phys., Chem. Phys. Lett.)
– Organic chemistry 2.6% (J. Org. Chem, Org. Lett., Org. Biom. Chem.)
– Inorganic chemistry 2.1% (Inorg. Chem., Dalton Trans.)
– Materials and surfaces 5.1% (J. Hazard. Mat., J. Mater. Chem.,
Langmuir, J. Coll. Inter. Sci.)
– Environment 3.7% (Env. Sci. Technol., Atm. Chem., Atm. Environ.,
Chemosph.)
Further topics among the most used specialist journals containing photochemical papers are catalysis (Appl. Cat. B), applied physics (Proc. SPIEE,
Opt. Express), polymer science (J. App. Pol. Sci.), biochemistry (Biochem.).
As remarked in the introduction to Vol. 37 in this series, a noticeable fact
is the relatively small amount of papers published in journals specifically

devoted to photochemistry. The three journals in the field (J. Photochem.
Photobiol., Photochem. Photobiol., Photochem. Photobiol Sci.) make together 3.5% of the total (see Fig. 1), a half of the papers in the general
chemistry category and a much smaller number than in other fields.
In the opinion of the present reporter, this fact does not necessarily imply
a negative connotation. It simply indicates that photochemistry is important
in many fields and plays a role in each of them that is felt more important
than that in photochemistry itself. In particular, remarkable is the high

Applied Physics

Other

Photochemistry

Environmental

General

Materials
Inorganic

Organic

Physical

Fig. 1 Distribution of photochemical papers in chemistry journals according to the discipline
(referred to the 32 most used journals).

2 | Photochemistry, 2011, 38, 1–9

number of papers

12

9

6

3

0
1955

1970

1985

2000

year
Fig. 2 Percentage of photochemical papers in JACS.

fraction of papers in general chemistry journals, the largest part of them
appearing as fast communication in prestigious journals, an indication of
the recognized position that this discipline has maintained. The use of devoted journals is much more extensive in other chemical disciplines, e.g. in
electrochemistry, but this has little to do with the importance and the role
that each discipline has.
The determining fact is that dedicated journals are available so that any
scientist can refer to them for good science, if a further portion of good

science is found elsewhere, no problem. In this sense, if a concern must be
expressed, this is rather that photochemistry, while remaining in the first
line, has lost some position with respect to other advancing fields. As an
example, if one considers JACS, inevitably the reference journal, the papers
in photochemistry certainly remain a high fractions of the articles published, but clearly the highest point has been reached two-three decades ago
and such levels are no more to be reached (see Fig. 2). This corresponds to
the feeling one has when browsing other chemistry journals or attending
meetings.
As to where photochemistry is done, there are a considerable number of
laboratories where photochemistry is the main businness. In 2009 the most
prolific author has been Prof. Shunichi Fukuzumi from the University of
Osake, but there are many other scientists following with a slightly lower
production, almost equally distributed between Japan, USA, Europe and
China. Fortunately, there is also an important production from laboratories where photochemistry is only one of the research theme and, importantly, patents maintain a large share in the photochemical literature.
3

Review

Some years have elapsed from the last publication of a textbook in
photochemistry and in 2009 we had the much wellcome opportunity of
having two in a few months. One of these is the new edition of what
indoubtely has been the reference text for over 40 years, Turro’s book now
titled ‘Modern Molecular Photochemistry of Organic Molecules’, with
Photochemistry, 2011, 38, 1–9 | 3


V. Ramamurthy and J. C. Scaiano as co-Authors,3 grown to over 1000
pages, but maintaining the same, quite captivating approach due to the
origin from courses and lectures (the first part of the text, exluding the
chapters on the chromophore photochemistry, is separately available).4

True to its title, the second one, Photochemistry of Organic Compounds.
From Principles to Practice by Kla`n and Wirz5 presents a substantial course
of photochemistry (5 chapters) followed by a long and very informative
chapter (250 pages) on the chemistry of excited states, presented by chromophore. The discussion is enlivened by the frequent introduction of ‘Case
Studies’ and ‘Special Topics’ that greatly help both in understanding the
mechanism involved and in appreciating the application in diverse fields.
Another important event is the publication of the two volumes Photochemistry and Photophysics of Coordination Compounds, edited by
V. Balzani and S. Campagna (part of the Top. Curr. Chem. Series, two
volumes, 273 þ 627 pages).6 This gives a complete account of the really
varied photochemistry of the complexes of block d and block f ions.
As mentioned, research reports in the field continue to appear at a steady
pace. Here, the reporter avows that he is unable to distinguish the main lines
of the development among many thousand papers. Browsing through the
literature causes panic, first of all because of the rapid advancement of
experimental techniques and computational methods that allow to arrive at
an in-depth understanding of the mechanism in cases that were not even
taken into consideration only a few years ago.
The advancement of computational chemistry is particularly apparent in
photochemistry. ‘Old’ problems have been confronted in a new way. Thus, a
multiconfiguration complete active space self-consistent field (CASSCF)
method has been applied to the determination of intermediates involved in
radiationless processes for acetophenone and derivatives.7
An excellent agreement has been obtained between experimental and
computed coulombic coupling matrix elements for donor-spacer-acceptor
systems, which consist of a boron dipyrromethane donor and acceptor in
various stages of protonation. Noteworthy, this correlation holds, despite
the fact that the validity of Fo¨rster theory applied to intramolecular electronic energy transfer (ET) over short (e.g. 20 A˚) distances is disputed.8
New approaches are used for fundamental processes such as proton
transfer (e.g. from the dimethylaniline radical cation to benzophenone
radical anion), where a new theory suggest that the transition state occurs

within the solvent coordinate, not the proton transfer coordinate, and
proton transfer may occur either adiabatically or nonadiabatically.9 Quite
interesting a computational study rationalized the mechanism of intramolecular oxo-hydroxy phototautomerism in pyridones and analogues that
has been obtained by IR irradiation in matrix. The tautomerism involves
ps* states that are repulsive toward the stretching of N-H or O-H bond.10
How one of the key photoreactions, C=C isomerization, is confronted
computationally and experimentally can be appreciated e.g. in a study on
fumaric amide.11
On the other hand, it has been shown that orbital-energy correlation
diagrams (by using an artificially high-spin ROHF method) and stateenergy correlation diagrams (by using a state-averaged CASSCF method)
4 | Photochemistry, 2011, 38, 1–9


can be computed ab initio, as shown for the electrocyclic ring opening of
cyclobutene and the addition between photoexcited oxygen and nitrogen.12
The computational approach extends to materials, as shown e.g. for the
application of a hybrid molecular dynamics–Monte Carlo technique to
simulate laser ablation in poly(methyl methacrylate).13
Photochemistry remains one of the best techniques for the generation of
intermediates under controlled conditions. A typical example is that of
benzylic carbanions that are smoothly generated by photolysis of the corresponding phenylpropionates. Under these conditions the lifetime of the
carbanion is a remarkable 200 ns in water and up to several minutes in a
rigorously anhydrous solvent (see Scheme 1).14
New intermediates are often attainable in matrix. Among the many noticeable advancements is the synthesis of the first molecule containing two
noble gas atoms, HXeOXeH, by UV photolysis of water in solid xenon and
subsequent annealing of the matrix at 40-45 K. This may be considered the
first step towards the preparation of linear (Xe-O)n chains (and thus contribute to the debate on the ‘missing xenon’ question).15 Apropos matrices,
it has been noticed that irradiation may give different results in different
matrices. This depends not on the chemical composition but on the different
rigidity of the organic glass at the temperature of the experiment.16

Inorganic photochemistry is enjoying a period of hefty development.
Metal carbonyls, as an example, are a favourite topic due both to the efficient and varied photochemistry these undergo and to the complex vibrational spectroscopy that allows a prompt identification. Two-dimensional
infrared (2DIR) spectroscopy is an excellent tool for testing the accuracy of
ab initio quantum chemical calculations.17 Another interesting topics is the
chirality conservation, as observed e.g. in the photochemical mer-fac
geometrical isomerization of Tris(1-phenylpyrazolato,N,C2 0 )iridium(III),
a complex pertaining to a class of highly fluorescent complexes used in
organic LEDs (OLEDs).18
The photochemistry of metal complexes and that of materials finds ample
application in solar energy conversion. A noticeable progress is taking place
with regard to water oxidation to dioxygen. This is a key feature in fundamental processes, such as both water splitting into hydrogen and oxygen
2H2 O þ 4hn ! O2 þ 2H2
and reduction of CO2 to methanol or hydrocarbons
2H2 O þ CO2 þ 8hn ! 2O2 þ CH4 :
This function is carried out by a cubic manganese oxide moiety in PhotoSystem II (see below) which continues to be studied and modeled.19,20

O

COO

O

hν

+ CO2

Scheme 1

Photochemistry, 2011, 38, 1–9 | 5

AcO

O

AcO

hν
TIPSO

O

TIPSO

O
-75°

O

O

O

H

HO
HO2C

H

O
O

O
HO

Scheme 2

Boc

Boc
N

Boc
N

O

COOCH3

N
H

O

N

O

OH

OTMS

OTMS
hν

OCH3

H
N
H

O

H
N
H

O

Scheme 3

Organic molecules are no less valuable substrates for photochemical reaction and have been exploited as key steps in complex synthetic sequences
(e.g. 2 þ 2 cycloaddition in the total synthesis of tetracyclic sesquiterpene
( Æ )-Punctaporonin C (see Scheme 2)21 as well as in the enantioselective
total synthesis of the Melodinus alkaloid ( þ )-Meloscine (see part of the
synthesis in Scheme 3).22
Further examples of solid-state asymmetric photochemical studies using
the ionic chiral auxiliary approach have been reported.23 Contrary to previous conclusions, a recent study on the dimerization of 2-anthracenecarboxylic acid derivatives demonstrated that remarkable regio-, diastereo-,
and even enantioselectivities can be induced by liquid crystals in a photochemical reaction. Indeed, the selectivity is conserved also upon changing

the shape of the molecule, much more than in the solid state.24
Although external factors such as crystal packaging may be determining
in many cases, intramolecular interactions may dominate in other ones.
Thus, although dibenzylketones usually fragment, the bis-(thienylmethyl)
ketone S,S-dioxide in Scheme 4 undergoes cycloaddition both in solution
and in the solid state (see Scheme 4).25
An interesting case of different products depending on the impinging flux
has been observed in the photochemistry of 2-ethylindandione.
In this case the use of a 312 nm lamp leads to Norrish Type II fragmentation or Yang cyclization via the triplet, whereas irradiation by
6 | Photochemistry, 2011, 38, 1–9


O

O

CO
hν
S
O

hν

S
O

100%

S

S
O

O

O

S

S

O

Scheme 4

O

O

.

H

O

.
O
Norrish
tipe I

O
H

T1

S1

O

O

HO
OH

.

.
Yang
cyclization

H
Norrish
tipe II

O

H

O

O
O

OH

H
H
H
O

O
Scheme 5

a 355 nm laser causes Norrish Type I fragmentation via the singlet.
Apparently, in the latter case T1 is re-excited and reforms S1 before it reacts
(see Scheme 5).26
The peculiar mildness of photochemical reactions is advantageous in
many cases, as exemplified for the generation of carbenes from diazo
compounds or diazirines. An example is the addition of a carbene unto a
fullerene. Encapsulation of a metal ions in the cavity of the molecule imparts a reactivity not present in the metal free fullerene, e.g. by favoring
addition onto the positions around fused pentagons (see Scheme 6).27
Likewise, the photolabilization of a ligand is an excellent method for the
synthesis of 2-azetidinone incorporating carbene chromium units that come
useful for the synthesis of peptides containing penicillin or cephalosporin
moieties.28
Photochemistry, 2011, 38, 1–9 | 7


hν
La2@C72

+
n
N

N
La2@C72

n = 1,2

Scheme 6

F

N
F
F

Ti

N

.

hν

+ (Me3Si)3SiH

F
F

F

N
polymer
M

F
N
+ (Me3Si)3Si

.

+
Ph2I

(Me3Si)3Si

+

+

Ph

.

+ PhI

F
(Me3Si)3SiH

PhH
Scheme 7

New photoinitiators with considerable advantages continue to be introduced. Silyl derivatives are often used for cationic polyymerization, as in the
case in Scheme 7, based on the generation of silyl radicals. The oxygen
consumption and the oxidation ability of these species makes the titanyl/
tris-(trimethylsilyl)silane/diphenyliodonium system quite effective under
aerated conditions (see Scheme 7).29
Photomedicinal applications continue to appear in search for selectivity.
In an interesting case, suitable species have been developed that are able to
recognize a molecular target that is present only in a specific bacterium and
that can be activated by light only after such an interaction.30 This should
allow to use photodynamic therapy more broadly for general infections
rather than, as it is presently, only for a highly localized infections.
References
1 A. Albini and V. Dichiarante, Photochem. Photobiol. Sci., 2009, 8, 248.
2 N. J. Turro, Molecular Photochemistry, 1965, Benjamin, New York.
8 | Photochemistry, 2011, 38, 1–9


3 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Modern Molecular Photochemistry of Organic Molecules, 2009, University Science Books, Sausalito,
Calif.
4 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Principles of Molecular
Photochemistry: An Introduction, 2009, University Science Books, Sausalito
Calif.
5 P. J. Kla`n and J. Wirz, Photochemistry of Organic Compounds. From Principles
to Practice, 2009, Wiley, Weinheim.
6 V. Balzani and S. Campagna, Photochemistry of Coordination Compounds,
Vol I & II, Top. Curr. Chem. 280, 281, Springer, Berlin, 2007
7 W. H. Fang, Acc. Chem. Res., 2008, 41, 452.

8 R. Ziessel, M. A.H. Alamiry, K. J. Elliott and A. Harriman, Angew. Chem. Int.
Ed. Engl., 2009, 30, 2772.
9 K. S. Peters, Acc. Chem. Res., 2009, 42, 89.
10 B. Chmura, M. F. Rode, A. L. Sobolewski, L. Lapinski and M. J. Nowak, J.
Phys. Chem. A, 2009, 112, 13655.
11 P. Altoe`, N. Haraszkiewicz, F. G. Gatti, P. G. Wiering, C. Frochot, A. M.
Brouwer, G. Balkowski, D. Shaw, S. Woutersen, W. J. Buma, F. Zerbetto, G.
Orlandi, D. A. Leigh and M. Garavelli, J. Am. Chem. Soc., 2009, 131, 104.
12 H. Shi, D. C. Roettger and A. L.L. East, J. Comput. Chem., 2008, 29, 883.
13 P. F. Conforti, M. Prasad and B. J. Garrison, Acc. Chem. Res., 2008, 41, 915.
14 G. Cosa, M. Lukeman and J. C. Scaiano, Acc. Chem. Res., 2009, 42, 599.
15 L. Khriachtchev, K. Isokoski, A. Cohen, M. Ra¨sa¨nen and R. B. Gerber, J. Am.
Chem. Soc., 2008, 130, 6114.
16 Y. P. Zhao, L. Y. Yang, C. J. Simmons and R. H. Liu, Chem. Asian J., 2009, 4,
754.
17 C. R. Baiz, P. L. McRobbie, J. M. Anna, E. Geva and K. J. Kubarych, Acc.
Chem. Res., 2009, 42, 1395.
18 K. Tsuchiya, E. Ito, S. Yagai, A. Kitamura and T. Karatsu, Eur. J. Inorg.
Chem., 2009, 2104.
19 J. J. Concepcion, J. W. Jurss, M. K. Brennaman, P. G. Hoertz, A. O.T.
Patrocinio, N. Y. Murakami Iha, J. L. Templeton and T. J. Meyer, Acc. Chem.
Res., 2009, 42, 1954.
20 G. C. Dismukes, R. Brimblecombe, G. A.N. Felton, R. S. Pryadun, J. E.
Sheats, L. Spiccia and G. F. Swiegers, Acc. Chem. Res., 2009, 42, 1935.
21 M. Fleck and T. Bach, Angew. Chem. Int. Ed. Engl., 2008, 47, 6189.
22 P. Selig and T. Bach, Angew. Chem. Int. Ed. Engl., 2008, 47, 5082.
23 C. Yang and W. Xia, Chem. As. J., 2009, 4, 1774.
24 Y. Ishida, Y. Kai, S. Kato, A. Misawa, S. Amano, Y. Matsuoka and K. Saigo,
Angew. Chem. Int. Ed. Engl., 2008, 47, 8241.
25 M. J. E. Resendiz, J. Taing, S. I. Khan and M. A. Garcia-Garibay, J. Org.

Chem., 2008, 73, 638.
26 C. Roscini, D. M. E. Davies, M. Berry, A. J. Orr-Ewing and K. I. BookerMilburn, Angew. Chem. Int. Ed. Engl., 2008, 47, 2283.
27 X. Lu, H. Nikawa, T. Tsuchiya, Y. Maeda, M. O. Ishitsuka, T. Akasaka, M.
Toki, H. Sawa, Z. Slanina, N. Mizorogi and S. Nagase, Angew. Chem. Int. Ed.
Engl., 2008, 47, 8642.
28 M. A. Sierra, M. Rodrı´ guez-Ferna´ndez, L. Casarrubios, M. Go´mez-Gallego
and M. J. Manchen˜o, Eur. J. Org. Chem., 2009, 2998.
29 M. A. Tehfe, J. Laleve´e, X. Allonas and J. P. Fouassier, Macromolecules, 2009,
42, 8669.
30 X. Zheng, U. W. Sallum, S. Verma, H. Athar, C. L. Evans and T. Hasan,
Angew. Chem. Int. Ed. Engl., 2009, 48, 2148.
Photochemistry, 2011, 38, 1–9 | 9


Recent trends in computational
photochemistry
Luis Serrano-Andre´s,a Daniel Roca-Sanjua´na and Gloria OlasoGonza´leza
DOI: 10.1039/9781849730860-00010

Recent advances in theoretical photophysics and photochemistry derive from the
improved capabilities of ab initio quantum-chemical methods to deal with different
types of excited states phenomena in molecules of increasing size and complexity.
Whereas the widespread use of time-dependent density functional (TD-DFT) based
techniques for the excited state have extended the study of absorption and emission
processes to large molecular systems and coupled-cluster (CC) methods have increased the accuracy of spectroscopic studies in medium-size compounds, multiconfigurational ab initio approaches such as CASPT2 and MRCI are now able to
cope accurately with all types of photochemical processes in medium to relatively
large systems, including nonadiabatic processes involving conical intersections, i.e.,
energy degeneracies, between potential energy hypersurfaces (PEHs), which are out
of reach for the other single reference approaches. The coupling of accurate electronic structure calculations based on PEHs with reaction dynamic procedures is
starting to make available the theoretical determination of both static and dynamic,

time-dependent and statistical, photoinduced properties in systems of different type
and complexity. Examples of the studied processes and the most commonly used
approaches are given below.

1

Introduction

Theoretical photochemistry has been always hampered by the inherent
difficulties related to the resolution of the Schro¨dinger equation to get energy solutions of the electronic Hamiltonian higher than the lowest one.1
Except for the quite successful behavior of parametrized semiempirical
methods in the 60s to estimate transition energies of organic molecules,
excited state quantum chemistry was often several steps behind the most
accurate computation of the molecular ground state, in which many more
chemical properties could be also determined because the used approaches
were simpler and faster, and the required computational tools were easier to
implement.2 The path for ab initio methods for excited states to increase
their accuracy with respect to semiempirical methods was not without
problems.2,3 The development of the Configuration Interaction techniques
in the 70s4,5 opened the field for the precise characterization of the excited
states of small polyatomic systems. It was soon realized that correlation
energy effects were more important for excited than for ground states, and
that a proper way to deal with the problem was to use a multiconfigurational reference wave function. The 80s witnessed the development
of the Complete Active Space Self-Consistent Field (CASSCF) approach,6
which was able to provide a much better description of the reference state,
soon complemented with the Multireference Configuration Interaction
a

Instituto de Ciencia Molecular, Universitat de Vale`ncia, Valencia, Spain

10 | Photochemistry, 2011, 38, 10–36

c

The Royal Society of Chemistry 2011


(MRCI) method,7 ready to add most of the remaining correlation effects.
Still, the drawbacks of the Configuration Interaction technology regarding
their excessive cost and poor scalability limited the dimension and accuracy
in the description of excited states to a precision often not lower than 1.0 eV
for medium-size molecules. A number of methods, such as the propagator
approaches, was developed and used, but lacked accuracy, while they were
plagued with problems. The most important breakthrough came with the
development of a multireference second-order perturbation theory approach, the CASPT2 method,8 in the beginning of the 90s, that brought the
level of accuracy to 0.3-0.1 eV for molecules up to the size of free base
porphin (30 atoms).9 The availability of the multiconfigurational methodologies in photochemistry was more important than merely increasing the
accuracy level. It opened the door for the determination of potential energy
hypersurface (PEH) degenerations, the so-called conical intersections (CI),
which are regions of the PEH space in which two or more states interact
nonadiabatically providing the conditions for a rapid transfer of the energy
from one state to the other. Modern photochemistry is fully based on the CI
concept.1,10,11 So far, only multiconfigurational approaches can describe
properly the dimensionality of the CI space, therefore theoretical photochemistry relies on them. Other methods based on a single reference are,
however, available to calculate excited states in some cases. The family of
coupled-cluster approaches for excited states12 are the most accurate procedures at hand (0.2-0.1 eV) to compute excited states of medium-size
molecules, but only in those cases in which the ground state is clearly defined as a single reference, basically near the equilibrium geometries. Far
from there, for instance in degeneracy situations, dissociations, diexcited
states or open-shell cases, the accuracy drops, in some cases dramatically up
to errors close to several eVs. More important than that, as mentioned, and

despite being used in some studies, single-reference approaches cannot and
must not be applied to describe conical intersections. Another popular
group of methods are the Time-Dependent Density Functional Theory
(TD-DFT) approaches for excited states.13,14 Their immediate advantage is
that they can be used in larger systems with a sort of black-box behavior
and in many cases a reasonable overall degree of predictability. They are
plagued with problems too. First, they can be considered a new version of
the parametrized semiempirical approaches that require one set of different
parameters and corrections for each of the electronic structure cases and
effects, and, additionally, they are particularly poor in all situations were the
single-reference methods are bad too (CIs among them), plus many other
cases: charge transfer situations, biradicals, anions, extended p systems, etc,
lacking in general the required accuracy.15 Some more balanced and precise
approaches have been also developed from them, like for instance the DFT/
MRCI procedure,16 but so far its use is not extensive.17 New ab initio
techniques bring on the other hand promising perspectives, such as the
Completely Renormalized Coupled-Cluster (CR-CC) approach,18 able to
deal with cases with near degeneracy effects, or the Density Matrix
Renormalization Group (DMRG)19 and the Restricted Active Space
Multiconfigurational Second-Order Perturbation (RASPT2)20 procedures,
both expanding the active space concept. Combined with the recent efficient
Photochemistry, 2011, 38, 10–36 | 11


implementations of the revisited Density Fitting (DF) or Resolution of the
Identity (RI), or the Cholesky Decomposition (CD) techniques21 to reduce
the cost of handling two-electron integrals and enlarging the size of the
molecular systems under study, ab initio approaches are making a step
forward in their applicability to deal with larger and more difficult
problems.

The complexity of the approaches also limits the use of computational
tools earlier developed for ground states, such as geometry optimizers, wave
function analyzers, or models to simulate the environment such as the QM/
MM procedures. Apart from that, and in order to fully describe photoinduced processes, it is first required not only to determine wide regions of
the PEH spaces for states of different multiplicity, including minima,
transition states, conical intersections and reaction paths, but also to take
into account transition and interaction properties between different states or
PEHs, such as transition dipole moments (TDM) or nonadiabatic, spinorbit, and vibronic couplings for instance. Once that described, timedependent and statistical properties such as state lifetimes, photochemical
rates or quantum yields require the resolution of time-dependent reaction
dynamics equations. Coupling both static and dynamic steps at the proper
level of accuracy it is still out of reach except for very small molecular
systems. Progress in that direction is, however, on their way. It is not surprising that the type and number of processes and properties to be studied,
and the molecular sizes available for computational chemists of the excited
state is somewhat more restricted than for ground states.
In the present contribution we include a brief recollection of some of the
most important concepts used in theoretical spectroscopy, photophysics,
and photochemistry, and a number of examples recently addressed in the
computational chemistry of the excited state, together with a comment on
the most widely used approaches, discussing their strengths and flaws. The
goal is not to make an exhaustive account of the methods and contributions,
but a critical overview of recent trends. Many more problems have been
published and a large number of methods exists which are less employed for
different reasons. We have tried to capture the most significant cases. In the
next section a collection of examples in spectroscopy and photochemistry
will be used to illustrate the panorama of the field in the last three-four
years.
2

Theoretical methods and concepts for excited states

Last years have witnessed developments which have pushed ahead the field
of modern photophysics and photochemistry, such as for instance the improved molecular beams and femtosecond laser techniques that permit
time-resolved studies of photoinduced phenomena also on single molecules,
and, undoubtedly, the extension in size and accuracy of the applicability of
theoretical, quantum-chemical methods to study the molecular electronic
excited states. Indeed, now it is possible to get information about reaction
intermediates at very short times from femtochemical techniques, and, more
than ever, the participation of quantum chemistry to interpret such findings
has become crucial. A constructive interplay between theory and
12 | Photochemistry, 2011, 38, 10–36