Photochemistry vol 39 a review of the literature published between

Specialist Periodical Reports Edited by Angelo Albini Photochemistry Volume 39 Photochemistry Volume 39 A Specialist Periodical Report Photochemistry Volume 39 A Review of the Literature Published between May 2010 and April 2011 Editor Angelo Albini, University of Pavia, Pavia, Italy Authors Catia Clementi, Dipartimento di Chimica, Perugia, Italy Telma Costa, University of Coimbra, Portugal Rui Fausto, University of Coimbra, Portugal Gianna Favaro, Dipartimento di Chimica, Perugia, Italy Andrea Go´mez-Zavaglia, University of Coimbra, Portugal Elizabeth J Harbron, College of William and Mary, Williamsburg, USA Linda J Johnston, National Research Council of Canada, Ottawa, Canada Kuppuswamy Kalyanasundaram, Laboratory of Photonics and Interfaces, Lausanne, Switzerland Andrea Maldotti, Universita` degli Studi di Ferrara, Italy Costanza Miliani, Dipartimento di Chimica, Perugia, Italy Hiroaki Misawa, Hokkaido University, Japan Joa˜o Pina, University of Coimbra, Portugal Fausto Puntoriero, Universita` degli Studi di Messina, Italy Aldo Romani, Dipartimento di Chimica, Perugia, Italy J Se´rgio Seixas de Melo, University of Coimbra, Portugal Kosei Ueno, Hokkaido University, Japan If you buy this title on standing order, you will be given FREE access to the chapters online Please contact sales@rsc.org with proof of purchase to arrange access to be set up Thank you ISBN: 978-1-84973-165-2 ISSN: 0556-3860 DOI: 10.1039/9781849732826 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: 10.1039/9781849732826-FP005 This is Volume 39 of the Series ‘‘Photochemistry’’ of the Specialist Periodical Reports published by the Royal Society of Chemistry, the first volume of which was printed in 1970 and reviewed the photochemical literature from July 1968 to June 1969 The Editor of Volume 1, Derek Bryce Smith, remarked that in preparing their contributions, ‘‘the authors have adopted as a guiding principle the need to present as unified a treatment as possible; and although various Chapters stress, say, the more ‘physical’ or ‘organic’ aspects of the subject, in reflection of the emphasis in the original papers, the aim has been to produce a work which as a whole will be of value to both physical and organic chemists, and even, with the inclusion of a section on Inorganic Photochemistry, to inorganic and organometallic chemists’’ (D Bryce Smith, Photochemistry, Vol 1, p iii, Chem Soc., London, 1970) Forty years later it does not appear possible to maintain the same format This is due in part to a change in the way scientific literature is currently searched and used As Ian Dunkin, the Editor of Volumes 34–36, remarked ‘‘it has been apparent in recent years that we are experiencing a profound change in the way in which information of all types is disseminated In particular, more-or-less universally available on-line searching for scientific papers and data has greatly speeded up the task of searching the literature for material relevant to any area of interest So much so, that the inevitable time delay inherent in the production of review serials, such as the Specialist Periodical Reports, now seems excessive’’ (Dunkin, Photochemistry, Vol 36, p 1, R Chem Soc., Cambridge, 2007) Furthermore, and perhaps even more importantly, photochemistry has expanded into so many diverse paths that presenting ‘‘a unified treatment’’ seems not to be a viable choice, let alone that this should be done in a few months after the original publications Photochemistry practitioners live in different communities that often have very little contact This very fact, however, suggests that publishing a yearly volune on photochemistry may make sense The idea is making available in a single volume a large collection of results from the literature, organized in a such a way that, even if a unitary treatment can not be offered, readers from different chemical disciplines may have some indication of the role that the peculiar physical and chemical properties of electronically excited states play in each case, and of the existence (and usefullness) of a basic core of notions of photochemistry Therefore, the last volumes aim, though not necessarily with success, to offer rather a crytical than a comprehensive coverage Taking a further step in this direction, from this volume on the coverage of is alternate, with the limitation each year to a half of the subjects chosen a Organic Chemistry Department, Pavia University, viale Taramelli, 10, 27100, Pavia, Italy Photochemistry, 2011, 39, v–vi | v  c The Royal Society of Chemistry 2011 for review (that on the other hand certainly not exhaust the applications of photochemistry) This change should give a better opportunity for a crytical treament of each topic This year, the reader will find inorganic photochemistry, solar energy conversion, transient and matrix spectroscopy, next year organic and theoretical In addition to these chapters, in every volume a short but general overview of (some of) the advancements in photochemistry and a number of ‘‘highlights’’ will be presented The last contributions are high-level account of special topics, and those treated this year concern strong photon-molecule coupling field and its effect on the efficiency on the efficiency of photochemical reactions, nanoscale spatial resolution in optical microscopy, photochromism in nanoparticles, as well as the relation between photochemistry and art masterpieces It is hoped that such state-of-the art specific contributions are of some general interest and that their publication along with general coverage chapter may be useful for the reader, the suggestions of whom are eagerly waited for and will be attentively considered A sincere thank goes to the Authors of the individual chapters, both those that took this job when the series started again from Vol 37 and the new entries (F Punturiero and K Kalyanasundaram) for their invaluable contribution, as well as to Bruce Gilbert, chair of the SPR series, for his encouragement and to Merlin Fox from RSC for his continuous support Professor Derek Bryce-Smith passed away in June 2011 A well-known scientist and first chairman of the European Photochemistry Association (EPA), he established the series of volumes on Photochemistry published by RSC in the Specialist Periodical Reports, with the first volume published in 1970 At the time, photochemistry was a rapidly-growing field and there was a clear requirement for a critical assessment of the literature that would help the many new groups that entered the field and those that were already involved The way he devised was assembling a team with a varied expertise and a shared target As he stated in the Introduction, ‘‘The authors have attempted to achieve both a report and a critical commentary, as far as possible; but the aim to have an edited manuscript at the printers only two and a half months after the end of the period under review has inevitably restricted the desirable element of commentary The name of the author principally responsible for compilation is given for each Chapter, although the authors accept Cabinet responsibility for the work as a whole’’ The series, which he served as the sole Editor up to Volume 18 and, in association with Professor Andrew Gilbert, up to Volume 25, was highly successful and the ‘‘red books’’ became a familiar sight in every photochemical laboratory and one of the main instruments for the advancement of this discipline vi | Photochemistry, 2011, 39, v–vi CONTENTS Cover An energy level diagram overlaid on the sun Background image reproduced by permission of NASA Preface Angelo Albini v Light induced reactions in cryogenic matrices Rui Fausto and Andrea Go´mez-Zavaglia Introduction UV-visible-induced reactions in cryomatrices IR-induced reactions in cryomatrices Acknowledgements References Excited state dynamics in p-conjugated polymers Joa˜o Pina, Hugh D Burrows and J Se´rgio Seixas de Melo Decay mechanisms in organic conjugated polymers and oligomers: energy transfer and on chain exciton migration Triplet state in p-conjugated polymers PhosLEDS and white light systems Photovoltaics Stability and degradation Water soluble conjugated polymers Conclusions Acknowledgments References 24 27 27 30 30 37 48 50 51 53 58 58 58 Photochemistry, 2011, 39, vii–x | vii  c The Royal Society of Chemistry 2011 Photophysics of transition metal complexes 65 Fausto Puntoriero Introduction Ruthenium and osmium Iridium Platinum Rhenium Interaction of transition metal complexes with DNA Lanthanides Copper Miscellanea References 65 66 71 75 76 78 79 80 82 82 88 Photochemical and photocatalytic properties of transition-metal compounds Andrea Maldotti Introduction Vanadium, chromium, molybdenum and tungsten Manganese Iron Ruthenium Osmium Cobalt, rhodium and iridium Platinum and palladium Copper and silver 10 Others References 88 88 90 91 94 102 102 104 105 107 107 Photochemical energy conversion 112 Kuppuswamy Kalyanasundaram Photodecomposition of water to molecular H2 and O2 Dye sensitized solar cells (DSC) Photoanodes Photosensitizers for DSC Electrolytes for DSC References 112 121 137 154 177 Fluorescence imaging on the nanoscale: bioimaging using near-field scanning optical microscopy Linda J Johnston Introduction Near-field approaches to overcome the diffraction limit Cellular imaging with aperture-based NSOM viii | Photochemistry, 2011, 39, vii–ix 191 191 192 196 Antennas and apertureless Hybrid NSOM methods Conclusions and outlook References 202 205 207 208 Photochromic nanoparticles 211 Elizabeth J Harbron Introduction All-organic photochromic nanoparticles Inorganic nanoparticles with organic photochromes Conclusions References 211 213 223 225 225 Strong photon–molecule coupling fields for chemical reactions 228 Kosei Ueno and Hiroaki Misawa Introduction Localized surface plasmon resonance and its enhancement of electromagnetic fields Preparation of metallic nanoparticles and their plasmonic spectral properties Two-photon photoluminescence from closely spaced gold nanostructures based on near-field enhancement effects Plasmonic enhancement effects proved by surface-enhanced Raman scattering spectroscopy (SERS) Plasmon-assisted two-photon polymerization Two-photon photochemical reactions by an incoherent light source Plasmon-assisted photocurrent generation by visible and near-infrared light irradiation Conclusion Acknowledgments References Photochemistry and cultural heritage What is the impact of light on works of art? Aldo Romani, Catia Clementi, Costanza Miliani and Gianna Favaro Introduction Photochemical tools Photophysical investigations: knowledge and diagnostics Photochemistry: photodegradation Control of efficiency of cleaning/restoring Summary and conclusions References 228 230 231 235 239 242 246 248 251 252 252 256 256 257 260 270 278 279 280 Photochemistry, 2011, 39, vii–x | ix (a) Counts 800 400 12 t / ns (b) 1000 Counts 800 600 400 200 12 16 t / ns Fig 10 Luminescence decays of organic colourants present in The Book of Kells (a) Blue areas (lexc=635 nm, lem=700 nm – red dots) compared with a natural indigo standard sample spread on paper by Arabic gum (lexc=635 nm, lem=700 nm) The two decay profiles are identical (b) Purple areas (lexc=455 nm, lem=640 nm – light grey dots) compared with an ancient parchment coloured with orcein (lexc=455 nm, lem=640 nm – grey dots) The pulsed source shape is in black concerning the metal ion involved, the ligands present in the complex and the media surrounding the dye Photochemistry: photodegradation Often, ancient, as well as modern paintings, little resemble today the way they looked when they were first completed by the artist An emblematic example are van Gogh s famous Sunflowers paintings, where the chrome yellow pigment was a bright colour, but it faded with time, turning to the opaque brown-yellow that we observe today Going back in time, coloured mural paintings survive at Pompeii, but colours are often not well preserved Due to exposure to sun and air they discoloured, leaving the paintings bare of their original vivacity and making the classical world much more pale than it probably was Dye and pigment ageing processes noticeably depend on environmental and conservation conditions, such as the spectral composition and intensity 270 | Photochemistry, 2011, 39, 256–283 of illumination, temperature, humidity and atmospheric pollutants.70 Photochemical degradation is one of the most important factors of ageing The lightfastness properties (that is the degree to which a dye resists fading due to light exposure) is today assessable using standardised methods for different classes of dyes in solution and in different substrates.71–77 Artworks exposed to atmospheric pollutants and solar or artificial irradiation undergo natural ageing Oxidant pollutants (NOX) and hydrocarbons (HC) are often present in the atmosphere because they are released from factories and vehicles After absorbing sunlight they undergo photochemical changes to secondary materials such as ozone, which are the main cause of the occurrence of photochemical degradation Several studies about protection from oxidation have been reported.78–80 However, it was also demonstrated that a low-oxygen atmosphere, does not necessarily slow fading in all dyes but may, in some instances, induce or accelerate unwanted colour changes and fading of some dyes in the presence of light Therefore, it is generally advisable to store polychromatic objects in the dark.81 4.1 Photochemical studies in solution and on laboratory model samples Because of the foregoing, photochemical investigations that deal with cultural heritage objects have been mainly addressed to study the effects produced by ageing on solutions and on laboratory samples to understand which are the variations that might occur with time in artworks To reproduce in laboratory the ageing conditions under which artworks might be subjected indoors in a museum, two operative modes were followed: natural ageing and accelerated ageing.48,57 Concerning natural ageing, the laboratory samples were exposed to light–dark daily cycles for a period of the order of one year in a specially home-made ageing chamber where indirect solar light passed through a Pyrex-glass covering and the average temperature ranged between 16 and 25 1C in winter and summer, respectively For artificial ageing, the samples were irradiated with a lamp simulating daylight (Xenon lamp) through a silica-fused window under controlled temperature (25 Æ 1C) and atmosphere (air or nitrogen) for some tenths of hours Changes occurring in solution or in laboratory samples were periodically recorded in terms of variations of absorption and luminescence spectra and colourimetric indices Such studies of the chromatic modifications of aged samples gave interesting information about the degradation processes, but the mechanisms involved are often complex, and progresses are being made in disclosing them Several organic dyes (anthraquinoids, flavonoids, etc.) contain groups which are able to give inter- and/or intra-molecular hydrogen bonding Their spectral behaviour depends on the stability of the bonding in the ground and excited states Moreover, upon electronic excitation, intramolecular proton transfer may occur in the excited state (ESIPT) As also outlined above, this excited state process has consequences on the features of both absorption and emission and also influences the interactions with the environment Among these interactions those leading either to degradation or protection of the dyed materials are of particular interest Photochemistry, 2011, 39, 256–283 | 271 As examples of laboratory investigations and results that they can provide, we report the cases of the photochemical and thermal degradation of saffron82 and carminic acid48 in solution and on painted surfaces and of madder in solution and dyed threads.57 4.1.1 Photodegradation of saffron Saffron, a naturally occurring colourant obtained from the dried stigmas of the flower of Crocus sativus L., was one of the most frequently used yellow dye in manuscripts83 and, less extensively, in textile dyeing.84 The basic components of saffron, which are responsible for its strong yellow-red colour, are cis- and trans-crocins, a family of water-soluble carotenoids, Fig 11.85,86 Their relative amounts markedly depend on the source from where saffron has been extracted.87 Six components (trans and cis crocins) were separated from natural saffron by HPLC and were characterised by their absorption and emission spectra Photochemical and thermal processes CH3 CH3 OR'' O O CH3 OR' CH3 CH3 CH3 All-trans O OR' 13-cis CH3 CH3 OR'' O HOH2C O HO HO O HO HO HOH2C O OH CH2 O HO HO OH X Y HO Crocins: R’ = R’’ = H – crocetin R’ = R’’ = Y - crocetin di(β-D-gentiobiosyl) ester R’ = X, R’’= Y - crocetin (β-D-glucosyl)-(β-D-gentiobiosyl) ester R’ = Y, R’’= H - crocetin (β-D-gentiobiosyl) ester R’ = X, R’’= H - crocetin (β-D-glucosyl) ester Fig 11 Structures of the main components of natural saffron 272 | Photochemistry, 2011, 39, 256–283 Emission intensity / a.u 400 500 600 700 800 λ / nm Fig 12 Fluorescence spectra (lexc=254 nm) of saffron painted surfaces normalized on the paper emission (439 nm): freshly prepared (1); stored for 26 months in the dark at room temperature (2) and exposed for 26 months to daylight (3) which degrade the dye were investigated on the individual components The effect of light promoted the trans - cis isomerization, while the thermal effect detached the glycosyl moieties Such information obtained from solutions was exploited to interpret results from aged laboratory samples prepared by spreading the saffron dye using Arabic gum as binder onto a paper support (water-painting technique) An example of the effect of ageing on fluorescence emission is illustrated in Fig 12 The painted papers were naturally aged, keeping them at ambient temperature, in the dark and exposed to sunlight over a period of 26 months Fluorescence spectra of a freshly prepared painted paper exhibits a band at 439 nm which is due to the paper and a band at 546 nm belonging to the colourant (spectrum 1) The latter faded in the sample kept in the dark (spectrum 2) and disappeared in that exposed to sunlight (spectrum 3) The extent of degradation is strongly influenced by the environment and is favoured by a closely packed molecular arrangement For these reasons the degradation of this colourant on water-paintings is very significant 4.1.2 Photodegradation of Carminic acid and lake Carminic acid is a b-C-glycopyranosyl derivative of anthraquinone which is extracted from a tropical American insect (cochineal) Its lake is an aluminium two-ligand complex, Fig 13 This compound has been studied extensively due to its practical application as colourant (e.g., in paints, foods, cloths).88–92 It behaves like an acidichromic molecule since its aqueous solutions change colour as the pH changes Emission is less sensitive to changes in the pH than absorption, since the fluorescence spectra of the differently protonated forms widely overlap.48 Accelerated photo-ageing of carminic acid and lake was investigated in solution, in the presence and absence of a binder, Arabic gum, commonly used in water-colour painting Natural ageing was followed for several months on water-colour painted paper The results indicate that both carminic acid and lake bleach upon light exposure The binder prevents lake Photochemistry, 2011, 39, 256–283 | 273 HOCH2 OH O HO HO O CH3 COOH O OH OH HO O O Al HOCH2 HO O HO O O OH HO O COOH OH HO O CH3 Fig 13 Structure of carminic lake (a) Absorbance 0.4 0.2 0.0 (b) 0.4 0.2 0.0 300 400 500 λ / nm 600 Fig 14 Photodegradation of carminic lake in DMSO upon light exposure for 60 hours in the absence (a) and presence (b) of Arabic gum from fading, but destabilises the carminic acid (Fig 14) The behaviour of painted papers upon ageing reflects the situation found in solution The effect of the binder on the degradation of carminic acid and carminic lake is different: the durability of carminic acid decreases, while the lake is efficiently stabilised This explains why lakes have been widely used in painting The reflectance spectra taken on water–painted papers (Fig l5) after 18 months natural ageing exhibit a red shift of the inflection point, calculated from the first derivative (see inserts of Fig 15), which is smaller for the lake than for the acid (17 vs 43 nm), confirming that the aluminium complex is the most stable colourant, as indicated by the study in solution The information about the spectral properties and their pH dependence has shed light on the spectral changes occurring in the presence of the binder The absorption spectrum of the dye, before and after irradiation, depends on its environment Thus, colour can be a diagnostic test of the medium used by the artist 274 | Photochemistry, 2011, 39, 256–283 (a) d(A')/d λ 1.2 A' 0.9 0.6 595 nm 400 600 638 nm 800 λ / nm 0.3 0.0 200 300 400 500 600 700 800 900 1000 λ / nm (b) d(A')/d λ 1.2 A' 0.9 618nm 601 nm 400 0.6 600 λ / nm 800 0.3 0.0 200 300 400 500 600 700 800 900 λ / nm Fig 15 Effect of 18 months natural ageing on the reflectance spectrum of water-painted paper with carminic acid (a) and carminic lake (b) A’=log 1/R (R is the reflectance signal) 4.1.3 Photodegradation of Madder dyestuffs Madder has been the object of several studies carried out in solution as well as in dyed textiles.2,17,45–47,55,57,58,69 Wool samples dyed with Madder and its main components, alizarin and purpurin, were also subjected to natural ageing in both the presence and absence of alum mordant and degradation was followed by reflectance absorption, fluorimetry and colourimetry, and by chromatography.57 Here we report results obtained by colourimetry (Fig 16) Measurements of colourimetric indices showed that lightness (L*) and redness (a*) mostly contributed to the total colour changes (DE), whereas variations in yellowness (b*) are negligible, Fig 16a In Fig 16b the DE indices, recorded during year of natural ageing of alizarin, purpurin and madder on mordanted (/m) and unmordanted wool, are compared Alizarin dyed samples were less affected by ageing and less sensitive to the presence of mordant than purpurin and madder dyed samples, which fastest underwent degradation but were also somewhat stabilized by alum mordant Colourimetric measurements are in agreement with results obtained by following photodegradation using other techniques Spectrophotometric measurements reveal a decrease of the dye absorbance in the visible, the Photochemistry, 2011, 39, 256–283 | 275 (a) colourimetric indices 10 ΔE ΔL * * Δb -5 * Δa 2000 4000 6000 8000 10000 time / h 15 (b) alizarin alizarin /m purpurin purpurin /m madder madder /m 10 ΔE 0 2000 4000 6000 8000 10000 time / h Fig 16 (a): Changes in colourimetric indices monitored on mordanted wool dyed with purpurin during year natural ageing, (b): Changes of colourimetric indices (DE) recorded during year natural ageing for alizarin, purpurin and madder on mordanted (/m) and unmordanted wool bleaching is more marked for purpurin and madder than for alizarin and is more pronounced in the absence of alum mordant Fluorimetric results show that degradation leads to an intensity decrease and blue shift of the dye fluorescence maximum Fluorimetry also evidences the degradation of wool, undetectable with other methods The photoproducts of wool proteins were recently identified.93 Results from chromatography were in line with those obtained by optical techniques 4.2 Photochemical investigations on real artworks Preliminary experiments carried out at laboratory level are of primary importance for photochemical investigations on real artworks In fact, what has to be considered on a real work is its actual status which is consequent to its history and in particular, but very important, to light exposure To face up to this situation, what can be done is a reconstruction of previous history, based on the present status This cannot be an easy task, especially 276 | Photochemistry, 2011, 39, 256–283 for the most ancient works for which the conservation conditions can be unknown for long periods In other words, photochemistry on originals is studied when it has already followed its natural course 4.2.1 Light induced alterations in inorganic pigments In spite of the fact that inorganic pigments are generally considered by far more stable than organic dyes, some alteration processes of both natural and synthetic pigments are proved to be activated by sunlight Studies of micro-samples from degraded paintings by microscopic X-ray absorption near-edge spectroscopy (m-XANES) experiments allowed to demonstrate that in the case of mercury sulphide (cinnabar), cadmium sulphide (cadmium yellow) and lead chromate (chrome yellow), the sunlight induces redox processes which are responsible for remarkable changes of the original colours.94 The blackening of mercury sulphide is a well know process occurring on mural paintings as well as on easel paintings with disfiguring effects, but not yet fully understood.95 The degraded areas originally painted with cinnabar resulted always enriched in chlorine that has been proved to catalyze the photochemical redox of HgS into Hg(0) and S(0); alternatively, a two step process was proposed, involving first the formation of calomel (HgCl)2 and then a light-induced disproportion of calomel into Hg and HgCl2.94 Cadmium sulphide is a synthetic pigment very popular among 19th to 20th century painters (for example, Monet, Ensor, Van Gogh, Picasso) due to its high covering power and bright yellow colour In many paintings cadmium sulphide showed the tendency to fading of originally bright yellow colour with the formation of small white-coloured globules on the top of the original paint surface Van der Snickt and co-workers demonstrated that sulphur, originally present in sulphidic form (S2), is oxidized to the sulphate form (S6 ỵ ) producing the white cadmium sulphate through a process which is initially activated by light.96 Diversely, another yellow pigment very appreciated by Impressionist painters, lead chromate (PbCrO4), showed the tendency to undergo a lightactivated reduction bringing the formation of various Cr(III) alteration compounds such as hydrated oxides, acetates or oxalates, all exhibiting a green colour.97 As a consequence, on several paintings by artists of the end of the 19th century and the beginning of the 20th Century, a browning of the original yellow areas, painted with the chrome yellow pigment, was observed The most famous of these are the various Sunflowers paintings Vincent van Gogh made during his life.98 4.2.2 Light induced alterations in organic dyes Following preliminary investigations carried out in solution and laboratory samples, degradation products can be today recognized in original artworks and help to reveal which was the colourant used at the time Photodegradation was clearly detected in studying Renaissance tapestries conserved at the Vatican Museum The obverse side of the tapestries gave weak fluorescence signals, due to photo-deterioration of the dyes, whereas, on the reverse, where the colours appeared well conserved, the emission signals were more intense, see Fig Photochemistry, 2011, 39, 256–283 | 277 Recent photochemical studies on degradation of indigo in solution demonstrated that isatin, the major photoproduct of indigo, is formed with very low quantum yield (r 10À6) in water and somewhat higher in the presence of oxygen based radicals and reducing species.99 This result was compared with those obtained by analysing blues in ancient Andean textiles, kept for millennia protected from light in a dry ambient in South Peru Isatin, found at low concentrations in these textiles, was assigned to degradation of indigo Nevertheless, considering the long time elapsed, indigo demonstrates to be a rather stable molecule in certain environments Isatin is thermodynamically more stable than indigo,100 this confirms that when protected from light in a dry ambient, indigo is fairly stable The unexpected stability of this organic molecule was related to the central double bond which enables a very fast non-radiative relaxation to the ground state On the other side, the central double bond is the most reactive bond of indigo.101 These results can be generalized: photodegradation quantum yields as found in water, below 10 À 6, could be representative for indigo in many works of art, and the values ranging from 10 À to 10 À could be representative of environments where oxygen based radicals or reducing species are present Control of efficiency of cleaning/restoring In-situ optics based techniques can give precious help during restoration or cleaning procedures An example is here reported where both fluorimetry and colourimetry were applied.102 The fresco Madonna delle Grazie e Santi (Fig 17) by Pietro Vannucci (Il Perugino), conserved in the S Agnese Fig 17 Madonna delle Grazie e Santi fresco: selected measurement points for the colourimetric determinations are indicated 278 | Photochemistry, 2011, 39, 256–283 Table Variations of colourimetric indices determined after cleaning of the fresco sample colour DL* Da* Db* DE* Nun’s veil Nun’s veil Virgin’s veil Virgin’s cheek white white Blue pink 12.45 14.2 1.83 8.5 À 0.36 À 1.31 0.05 2.45 À 4.85 À 3.23 À 1.0 2.86 13.36 14.64 2.09 9.31 Monastery in Perugia, during a recent restoration, was in depth investigated using various in-situ and micro-destructive techniques.103 UV-visible fluorescence and colourimetry were applied in order to obtain information about the possible presence of luminescent organic materials and to control the efficiency of cleaning procedures carried out by restorers Fluorescence signals and colourimetric indices were collected before and after cleaning The absence of luminescence in different coloured points of the fresco allowed to exclude that organic colourants had been used, whereas the presence of a weak fluorescence in some white areas, which disappeared after cleaning, indicated that organic materials were superimposed in different times Measurement points selected for colourimetric analyses are indicated in Fig 17 Some example of the variations in colourimetric indices occurred upon cleaning are reported in Table After cleaning, lightness increases (DL* W 0) and yellowness decreases (Db*o0) Redness-greenness changes are negligible (Da* E 0) Increase in brightness highlights the efficiency of the cleaning procedure, while decrease in yellowness is likely due to removing of dust and some patina from a tempera additional touch Repeated from time to time, such measurements allow to keep the fresco conditions under continuous control Summary and conclusions In this review recent results are reported on photophysical and photochemical investigations aimed to an in depth knowledge of artworks but also to provide curators and restorers with scientific supports for their correct conservation and restoration In the huge amount of materials that could be investigated as artwork components and the variety of techniques to be applied for investigating, this review is bounded to coloured materials studied by using UV-visible spectroscopic techniques, that is those techniques which exploit the electromagnetic radiation range traditionally belonging to photochemistry The importance of laboratory experiments in solution and on mock-up samples is highlighted in both the photophysical characterization and photochemical investigation on real cases The original works, for which investigations carried out are here described as examples, spam over time from 200 BC (ancient Andean textiles), through Middle Ages (parchments) and Renaissance (tapestries at the Vatican Museum), to modern (impressionism) and contemporary (Burri) art The results obtained have shown that photochemical techniques are suitable to be set-up for in-situ investigations and often have the potential to Photochemistry, 2011, 39, 256–283 | 279 recognize the chemical identity of colourants in different environments, sometimes coupled to (and/or supported by) other analytical techniques Photochemical effects of light on artworks are more difficult to investigate; they can only be observed a posteriori and are interpreted based on virtual laboratory experiments, for which 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Canada, Ottawa, Canada Kuppuswamy Kalyanasundaram, Laboratory of Photonics and Interfaces, Lausanne, Switzerland Andrea Maldotti, Universita` degli Studi di Ferrara, Italy Costanza Miliani, Dipartimento... Chimica, Perugia, Italy Andrea Go´mez-Zavaglia, University of Coimbra, Portugal Elizabeth J Harbron, College of William and Mary, Williamsburg, USA Linda J Johnston, National Research Council of Canada,.. .Photochemistry Volume 39 A Specialist Periodical Report Photochemistry Volume 39 A Review of the Literature Published between May 2010 and April 2011 Editor Angelo Albini, University of Pavia,