Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP001 Photochemistry Volume 41 Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP001 View Online View Online Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP001 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 View Online 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 Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP001 Thank you 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 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 Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP005 DOI: 10.1039/9781849737722-FP005 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 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 Photochemistry, 2013, 41, v–v | v  c The Royal Society of Chemistry 2013 Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP005 View Online CONTENTS Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP007 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 Introduction Review of the year 2012 References 3 Light induced reactions in cryogenic matrices (highlights 2011–2012) Rui Fausto and Andrea Go´mez-Zavaglia Introduction Light induced conformational isomerizations in cryomatrices Tautomerizations and other structural isomerizations Fragmentation reactions, unstable intermediates and formation of complexes or weakly bound species Noble gas chemistry Acknowledgements References 12 12 14 27 32 52 54 54 Photochemistry, 2013, 41, vii–x | vii  c The Royal Society of Chemistry 2013 Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP007 View Online 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 General view of polymer systems: fields, applications and techniques Polymers in solution: characteristics Fluorescent probes Photophysics of pyrene Dynamics of excimer formation in oligomers and polymers Models for kinetics of excimer formation Thermodynamics of excimer formation Inclusion complexes (structures and stoichiometry) Abbreviations Acknowledgements References Photochemical and photocatalytic properties of transition-metal compounds Andrea Maldotti Introduction Tungsten Manganese Rhenium Iron Ruthenium Osmium Cobalt 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 Introduction Ruthenium and osmium Rhenium Iridium viii | Photochemistry, 2013, 41, vii–x 59 59 61 65 68 75 86 105 112 116 119 119 127 127 127 128 129 130 132 138 139 140 142 143 145 146 147 149 150 156 156 157 160 161 View Online Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP007 Platinum and gold Copper Lanthanides Miscellanea Abbreviations References 163 166 167 170 173 174 Photochemical applications of solar energy: photocatalysis and photodecomposition of water K Kalyanasundaram Introduction and scope Photocatalysis Photodecomposition of water 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 Birth of a Society Newsletters Elections Efforts to promote and disseminate photochemistry research The I-APS winter meeting Society awards International collaboration 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 Introduction Reactive oxygen species Fluorogenic probes Conclusions References 279 280 282 297 298 Photochemistry, 2013, 41, vii–x | ix Downloaded Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-FP007 View Online Nitric oxide photoreleasing nanoconstructs with multiple photofunctionalities Aurore Fraix, Noufal Kandoth and Salvatore Sortino Introduction NO photodonors with fluorescence imaging modalities NO photodonors with multiple phototherapeutic modalities Closing remarks Acknowledgements References 302 302 303 308 315 315 316 Photochemical biology of caged nucleic acids Xinjing Tang Introduction Caging groups and their photochemistry Photochemical applications in biological studies Summary References 319 Photochemistry of the prebiotic atmosphere Daniele Dondi, Daniele Merli and Alberto Zeffiro Introduction Composition of the early Earth atmosphere Titan References 342 x | Photochemistry, 2013, 41, vii–x 319 320 326 336 336 342 345 351 356 View Online to be reminded, however, that almost certainly such primitive atmosphere contained traces of molecular oxygen In fact, oxygen can be produced by photodissociation of water 2H2 O ! O2 ỵ 2H2 Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 and of carbon dioxide CO2 ! CO ỵ O The last reaction has been calculated to occur at a B1 Â 10À8 g cmÀ2 yrÀ1 rate under present conditions,45 which is virtually negligible with respect to the present rate of biological fixation In primordial times, this reaction should have been significant, but due to the presence of reduced iron (II) in primeval sea water, it seems likely that the oxygen concentration at steady state would remain sufficiently low to neglect its presence when treating the photochemistry of prebiotic atmosphere The low concentrations of oxygen in the early atmosphere is further supported by the presence of uraninite (UO2) and galena (PbS) deposits from 2–2.8 billion year ago, since both of these species would be easily oxidized to UO3 and PbSO4, respectively, in presence of a large amount of oxygen.45 Although other scientists are of the view that the amount of O2 in the prebiological paleoatmosphere was sufficient for the production of ozone,46 a limiting value of 10À3 – 10À4 bar of oxygen partial pressure is currently accepted.47 Carbon dioxide, nitrogen and water vapor appear to have been the main constituents of primitive atmosphere However, the assumption that the key roles the early Earth atmosphere chemistry was played only by these (low reactive) chemical species seems to be too simplistic In fact, when considering the present atmosphere, it is apparent that it is far from the thermodynamic equilibrium with respect to oceans, rocks and, obviously, to biota It is not unreasonable to suppose that even during Hadean period a considerable amount of chemical potential was present (i.e methane, carbon monoxide, hydrogen etc.) In fact, a large chemical potential is a necessary condition for the birth and development of organisms Different energy sources (Sun irradiation, lightning, volcanoes) might have contributed to this energy harvesting by the production of a steady-state concentration of reactive species In particular, solar irradiation seems to be the more likely energy source due to its low entropy Importantly, light absorption is dependent on the presence of suitable chromophores, and even low amounts of such species could absorb a large amount of energy Therefore, early photochemistry could be driven by compounds present in low concentration The following discussion is mainly focused on the photochemistry of carbon dioxide and nitrogen Interest on this topic has greatly increased recently and only a limited selection of the studies published will be presented On the other hand, the choice will not be limited to strictu sensu prebiotic studies, but reports on the photochemistry of carbon dioxide and nitrogen relevant for prebiotic chemistry in general sense will be likewise considered 346 | Photochemistry, 2013, 41, 342–359 Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 View Online 2.1 Photochemistry of carbon dioxide The photodissociation reaction by direct absorption of light in the vacuumUV region seems to be a side mechanism for the production of carbon monoxide and oxygen that has a role only in the outer part of the atmosphere On 1938, Groth and Suess claimed to have detected glyoxal and formaldehyde from the irradiation of water and CO2 in the gas phase.48 In the past, several Authors used aqueous solutions of metal salts for CO2 photoreduction The first report is possibly the one by Bach49 that in 1893 claimed the formation of formaldehyde from a CO2 – saturated solution of uranyl acetate upon UV-irradiation Unfortunately, the presence of formaldehyde was only indirectly inferred from the formation of various colored uranium oxides Uranium salts as photosensitizers for carbon dioxide reduction to formaldehyde were used up to 1930 with controversial results.50 Colloidal iron hydroxides were likewise used and claimed to act as CO2 photoreducing agents Actually, later investigations demonstrated that iron (II) hydroxide decomposes spontaneously in aqueous environment to give hydrogen (the true reductant of carbon dioxide) and magnetite.51 More interesting is the reaction of CO2 on the surface of minerals that act as photocatalysts Despite the current interest for the CO2 photoreactions, only a few examples in literature could be applied to the prebiotic environment.52 Indeed, well known semiconductor photocatalysts such as titanium dioxide and metal sulfides could have been contributed to carbon dioxide fixation In theory, by using a photocatalyst having a band gap sufficiently large for the one-electron reduction of CO2 to its radical anion, CO2dÀ (E1= À1.90 V), the photoreduction of carbon dioxide could be viable However, the process is particularly unfavorable due to the thermodynamic stability of the CO2 molecule and furthermore the transient radical anion is unstable and undergoes either disproportionation to carbonate and CO or dimerization to oxalate.53,54 Despite this situation, proton coupled multiple-electron, stepwise reduction of CO2 is more favorable than single electron reduction, as thermodynamically more stable molecules are produced More than 30 years ago, Inoue et al reported55 the formation of formaldehyde and methanol after the irradiation of aqueous suspensions of different semiconductors under a continuous flow of carbon dioxide Among the semiconductors tested in that experiment, titanium and zinc oxides, and to a lesser extent cadmium sulfide and tungsten (VI) oxide, might be of some prebiotic relevance Silicon carbide was also found to exhibit a photocatalytic reductive property with respect to CO2, but any role in prebiotic chemistry is unlikely, since its presence on Earth as a mineral is quite rare, although this compound was detected in interstellar medium and in meteorites (included that entitled to Murchison, a meteorite well-known to prebiotic aficionados) Titanium dioxide can promote carbon dioxide photoreduction to methane when irradiated at 350 nm; a sacrificial hole scavenger like 2-propanol was shown to enhance the yield.56 Metal sulfides are well-known photocatalysts that are able to promote carbon dioxide photoreduction, particularly in the presence of sacrificial donors such as soluble sulfide ions Care has to be exerted in the experiments, because the method of preparation of such photocatalysts affects the Photochemistry, 2013, 41, 342–359 | 347 View Online Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 overall efficiency of the process The presence of such materials in prebiotic media might be related to the emission of hydrothermal vents.57–59 Among them, colloidal zinc sulfide can have an initial quantum efficiency of 10% for the production of formate from CO2 at pH 6.3.59 Moreover, zinc sulfide was shown to be stable with respect to carbonation ZnS ỵ H2 O ỵ CO2 ! ZnCO3 ỵ H2 S when the partial pressure ratio P(H2S) : P(CO2) is larger59 than 1.3 Â 10À12 Current best estimates suggest the limits 10À5oP(H2S) : P(CO2) o 10À3 for this ratio on prebiotic Earth.60 A quantum efficiency as high as 80% was reported for the photocatalytic reduction of CO2 to formic acid by ZnS colloids61 if sulfite was used as sacrificial donor Photochemical CO2 fixation in C-2 and C-3 compounds may also have played a role in prebiotic synthesis, as demonstrated by the irradiation of MnS in the presence of HS- ions (HCO3À was used instead of CO2)62 leading to formate as the initial photoproduct (quantum efficiency of 4.2% at pH=7.5) In addition to formate, longer chain carbon products were also detected C-2 - C-4 compounds were likewise photoproduced by irradiating aqueous suspensions of CdS and ZnS in the presence of tetramethylammonium chloride.63,64 Carbon dioxide has a key role in the formation of intermediates in important processes of the first prebiotic metabolic cycles, such as the reductive tricarboxylic acid (rTCA) cycle59 that is required for the development of more advanced self-replicating ‘living’ system Most of the rTCA photoreductive steps involving CO2 could actually be carried out by using ZnS and sulfides (indicated by bold arrows in Fig 1),65,66 although some of HOOC OH COOH COOH Citrate HOOC COOH COOH cis - Aconitate OH O S HOOC CoA COOH COOH H +, e- Isocitrate Acetyl CoA CO O O HOOC 50% (0.36%) Pyruvate COOH Oxalosuccinate COOH COOH H +, e-, CO CO O Oxaloacetate COOH HOOC O H +, e– HOOC HOOC COOH Oxoglutarate 70% (3.9%) Oxoglutarate is a minor product 2.5% H +, e -, CO2 COOH Malate 100% (1.9%) OH COOH HOOC Fumarate HOOC COOH Succinate H +, e– number in parentheses represents quantum yield Fig Reductive reverse tricarboxylic acid cycle 348 | Photochemistry, 2013, 41, 342–359 Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 View Online these reactions occur with a low yield A pyruvate to oxalosuccinate shortcut was detected by using similar reaction conditions (marked by a white arrow).67 Different reaction conditions are instead required for the isocitrate production (grey arrow) The reaction was obtained by irradiation of CdS in the presence of the enzyme isocitrate dehydrogenase and of methyl viologen as electron mediator (grey arrow),68 although this reaction conditions hardly fits in the prebiotic frame The prebiotic carbon dioxide fixation could more reasonably be related to reduction of hydrogen sulfide rather than of water through the following idealized reactions H2 O ỵ CO2 ! CH2 O ỵ O2 photosynthesis H2 S ỵ CO2 ! CH2 O ỵ S prebiotic carbon dioxide fixation Actually, the latter chemical equation requires only 78 kcal with respect to 118 kcal for the former one.69 Another possibility is the use of ferrous ions via the idealized reaction: CO2 ỵ 4FeO þ H2 O ! ‘‘CH2 O’’ þ 2Fe2 O3 This process has been found to operate in purple bacteria70 and has been suggested for rationalizing the formation of banded-iron sedimentary rocks.71 Despite this possibility, however, no examples of prebiotic interest have been reported Present time heterotrophic microbes obtain usable energy from the back reaction with ferric iron of the same process.72,73 2.2 Photochemistry of nitrogen N2 is the most inert diatomic molecule, a characteristic due to the large energy difference between filled and vacant molecular orbitals Nitrogen has a negative value of electron affinity (À1.8 eV) and a high bond dissociation energy (BDE) of 225 kcal/mol However, the latter property does not, by itself, explain the inertness of dinitrogen In fact the triple bond of acetylene has similar BDE (230 kcal/mol) and carbon monoxide BDE an even higher one (256 kcal/mol), although these molecules undergo several chemical reactions Dissociation of the first of the three bonds of dinitrogen requires about 130 kcal/mol74; that corresponds to more than half of the total triple bond energy As a comparison, the splitting of the first bond in the isoelectronic molecule acetylene is only 53 kcal/mol This difference is explained by the repulsion of the two unshared electron pairs and the electron pair of the remaining p-bond For this reason hydrogenation of acetylene is strongly exothermic whereas the corresponding nitrogen reaction N2 ỵ H2 ! N2 H2 is endothermic for 51 and 56 kcal/mol for trans- and cis-diimide, respectively.75 Photochemistry, 2013, 41, 342–359 | 349 View Online Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 The strength of the bonds of nitrogen is further reflected in the values of the redox potentials.76 N ỵ e ỵ H ỵ ! N H N2 ỵ 2e ỵ 2Hỵ ! N2 H2 N2 ỵ 4e ỵ 4Hỵ ! N2 H4 N2 ỵ 6e ỵ 6Hỵ ! 2NH3  E  E  E  E ¼ À3:2 V ¼ À1:1 V ¼ À0:36 V ẳ ỵ0:55 V As it is apparent from the reduction potentials, the second and third bonds are, on the contrary, very weak Furthermore, the reaction leading to hydrazine is endothermic by 20.7 kcal/mol in the gas phase and only by 8.2 kcal/mol in aqueous solution In the case of ammonia, exothermic values of 22 kcal/mol and 38 kcal/mol, respectively, were found for reduction in gas phase and in aqueous solution The high reduction potentials of the first and second reactions also clarify why the intermediate products are unstable and tend to revert to nitrogen and hydrogen In present time, the natural process of nitrogen fixation is carried out by some bacteria and occurs at a rate of 1.7 Â 108 tons per year.77 The process is made possible by the enzyme nitrogenase, that has a Fe/Mo/S cofactor.78 A mild abiotic reduction of nitrogen to ammonia can be achieved by using again H2S as reactant and freshly precipitated iron (II) sulfide as mediator The reaction takes place at atmospheric nitrogen pressure and temperatures of the order of 70–80 1C.79 The driving forces of the overall reaction are the oxidation of iron (II) sulfide to iron disulfide (pyrite) and the formation of hydrogen from H2S.79 FeSsị ỵ H2 Saqị ! FeS2sị ỵ H2gị DG ẳ 38:6 kJ=mol at pH ẳ 6:5 The overall reaction thus proceeds via the following equation N2 ðgÞ þ 3FeSðsÞ þ 3H2 SðaqÞ ! 3FeS2ðsÞ þ 2NH3ðgÞ The reaction yield is in the order of mmol of NH3 for each mol of FeS Other Authors reported similar results.80 If metallic iron or magnetite was used in the place of iron sulfide for the fixation of nitrogen harsher conditions were required, similar to those occurring in the Earth’s crust (800 1C and pressure of 0.1–0.4 GPa).81 The reported results rely on the formation of molecular hydrogen from iron (II) and water or hydrogen sulfide; thus could be classified as masked variants of the Haber-Bosch process In 1977 Schrauzer et al reported the photoreduction of nitrogen by irradiating titanium dioxide in the presence of water vapor, a reaction occurring with the formation of ammonia and traces of hydrazine.82 It was observed that iron doping of titanium dioxide enhanced the photocatalytic activity These results are however difficult to reproduce and have been questioned by some Authors.83 More recently, Kish et al reported the effectiveness of nitrogen photoreduction by nanostructured iron titanate 350 | Photochemistry, 2013, 41, 342–359 View Online CO2 N2 Plasma, Shock heating H 2O CO + NO CO hν hν CO Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 HO● NO ●CHO CO Liquid water dark reactions HNO N 2O H 2O (l) H + NO Fig Summary of possible reaction pathways for the prebiotic fixation of dinitrogen films and offered a tentative rationalization for the low reproducibility of earlier studies.84 Nitrogen can also be fixed by thermal reaction followed by a series of photochemical reactions in the gas phase and in liquid water These convert the products of strong heating (due to lightning or plasma generated by meteors, or to volcanism) into nitrite and nitrate.85–87 The first step is due to high temperature reactions that form NO and CO from N2 and CO2 Subsequently, NO was irradiated in gas phase in the presence of CO and of water, and N2O was the main product On the other hand, irradiation in presence of carbon dioxide (in the gas phase) rather led to NO2 formation (Fig 2).88 The series of reaction occurring could be described as follows: a in the presence of water CO ỵ H2 O ỵ hv ! HOd ỵ dCHO dCHO ỵ NO ! CO ỵ HNO 2HNO ! H2 N2 O2 ! N2 O ỵ H2 O b in the absence of water NO ỵ CO2 ỵ hn ! NO2 ỵ CO Another chemical pathway for NO fixation is the dark reaction with FeS to produce ammonia.89 Titan Titan, the largest moon of Saturn and second largest in the solar system, was discovered by Christian Huygens in 1655 Titan is the only object other than Earth for which clear evidence of lakes and oceans has been found.90 Another unique aspect is that the thickness of its atmosphere is almost ten times of that of Earth This factor generates a density of the atmosphere Photochemistry, 2013, 41, 342–359 | 351 View Online Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 larger than the Earth’s one (1.225 g/mm ) (Table 1) A further characteristics related to the high density of the atmosphere is its opaqueness, so that most of the incident light is reflected.91 Due to these characteristics, Titan has been the object of several studies; among them, the Cassini-Huygens mission made a mapping of all the most important chemical species on Titan at different altitudes, from 450 to 1600 km The atmosphere could be subdivided in thermosphere (0–50 km), stratosphere (50–250 km), mesosphere (250–500 km), thermosphere (500–1500 km) (Fig 3) Titan is characterized by a massively reducing atmosphere, containing carbon, nitrogen and oxygen, both in the form of small molecules such as CH4, CO, N2, and in the form of more complex molecules such as polymers.92 The mean atmospheric composition is dominated by nitrogen (98%), a feature that makes it the nitrogen-richest atmosphere in the Solar System aside from the Earth; the remaining 2% is composed mainly of methane (1.8%) In Table the major constituents of Titan’s stratosphere are reported Table Selected properties of Titan (Ref 91 and references therein) Surface radius 2575 km Surface gravity Distance from Saturn Average density Distance from Saturn Distance from Sun Orbital period Period around the Sun Solar flux Surface pressure Temperature at the surface Tropopause temperature (42 km) Stratosphere temperature (200 km) Exobase temperature (1600 km) 135 cm/s2 1.226 Â 106 km 1.881 g/cm3 1.226 Â 106 km 9.546 AU 15.95 Earth days 30 years 1.1% Earth 1496 Ỉ 20 mbar 94 K 71.4 K 170 K 186 Ỉ 20 K Km 50 Earth 280 K argon nitrogen oxygen Km 500 Titan 160K nitrogen methane thermosphere mesosphere 240 K 30 Orange haze ozone 160K stratosphere 200 210 K 70K 10 95K 290 K Fig 50 Methane, ethane troposphere Atmospheric temperature dependence at different altitudes for Earth and Titan 352 | Photochemistry, 2013, 41, 342–359 View Online Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 Table Stratospheric composition of Titan (Ref 91 and references therein) Constituents Weight fraction N2 CH4 H2 C2H6 C2H4 C2H2 C4H2 C3H8 HCN CO2 H2O 0.98 0.018 0.002 1.30 Â 10À5 9.00 Â 10À8 2.20 Â 10À6 1.40 Â 10À9 7.00 Â 10À7 1.60 Â 10À7 1.40 Â 10À8 1.00 Â 10À9 The polymerization of the organic constituents generates an orange haze, which due to its density falls on the surface of the moon The similarity with the Earth’s atmosphere lay in the fact that chemical reactions are driven by impinging UV radiation from the Sun and in the likelihood of some properties of Titan with respect to those of the primordial Earth Among these, the role of atmospheric and geological processes, such as erosion and precipitation However, there are at least two major differences between Titan and the prebiotic Earth, viz the lower temperature (94 K) and the lower exposure to solar UV radiation (see Fig 3).93 It is noteworthy that even at these low temperatures several chemical reactions take place Actually, photochemistry is the only viable path, although the photon flux on Titan is lower than on Earth due to its distance from the Sun.94 Furthermore, although Titan has no magnetic field, is subjected to electron bombardment due to Saturn magnetosphere The low temperature on Titan is obviously not compatible with the presence of liquid water on the surface, where rather oceans of ethane are present In view of these characteristics, Titan can be considered a small scale laboratory to study the photochemical evolution of the prebiotic Earth The main part of the incident photons with wavelength o155 nm is absorbed by N2 and CH4 in the upper atmosphere, while wavelengths around 200 nm are absorbed in the stratosphere by acetylene C2H2 (160olo200) and diacetylene C4H2 (lW200 nm).95 In a simulation of Titan atmospheric conditions, Khare and Sagan demonstrated in 198696 the presence of 16 amino acids in the obtained tholin (a brownish-red mixture formed in many prebiotic experiments), after treatment with N hydrochloric acid The simulation started from a gas mixture N2/CH4 in a 1/9 ratio at the pressure of mbar submitted to continuous spark discharge A large quantity of urea, aspartic acid, glycine and alanine was detected Several interesting aspects concerning photochemical reactions occurring in Titan atmosphere have been reported in literature97 and some of them will be discussed below in some detail The photodissociations of N2 and CH4 in the upper atmosphere and in the magnetosphere generate the key intermediates involved in the main Photochemistry, 2013, 41, 342–359 | 353 Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 View Online reactions occurring The most abundant products are acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propane (C3H8), propyne (CH3C2H), diacetylene (C4H2) and hydrogen cyanide (HCN) Many models have been proposed to justify the formation of such a large variety of products, and some mechanisms have been partially confirmed in the laboratory All of these models found their experimental counterpart in the direct observations of Titan from the Cassini space mission in 2006.98 One of the most reliable kinetic theoretical model proposed to justify the presence of more complex organics starting from nitrogen, methane, and water was elaborated by Yung in 1984.99 Accordingly to this model, excited N2 molecules present in the thermosphere (generated either by direct light absorption or by electron impact) react with methane forming hydrocyanic acid, methyl radicals and hydrogen The production of CO and CO2 in the upper atmosphere is due to the photodissociation of meteoric water by UV solar radiation (lo155 nm) and its reaction with CH3 radicals arising from the photodissociation of methane95: H2 O ỵ hv ! Hd ỵ dOH dCH3 ỵ dOH ! CO ỵ 2H2 dOH ỵ CO ! Hd þ CO2 In the mesosphere, the photoreduction of methane leads to the formation of acetylene, ethylene and propyne, accompanied by the formation of hydrogen atoms and molecules In the lower atmosphere, photochemistry is characterized by the photolysis of acetylene and HCN at wavelengths o200 nm C2 H2 ỵ hv ! HCRCd ỵ Hd HCN ỵ hv ! Hd ỵ dCN In order to explain how these unsaturated compounds are not immediately photolyzed, the following regeneration reactions with methane (present in a large concentration) were considered: HCRCd ỵ CH4 ! C2 H2 ỵ dCH3 dCN ỵ CH4 ! HCN ỵ dCH3 The fact that highly reactive hydrogen atoms generated from the above photolysis reactions did not undergo addition onto triple bond was explained by the following deactivation scheme (where M is a third body): C4 H2 ỵ Hd ỵ M ! C4 H3 ỵ M C4 H3 ỵ Hd ! C4 H2 ỵ H2 The abundance of ethane (estimated to be in the order of 13 ppm is due to the dimerization of the CH3 radicals; these last originates mainly from the photodissociation of methane.100 dCH3 ỵ dCH3 ! C2 H6 354 | Photochemistry, 2013, 41, 342–359 View Online The presence of propargyl radical C3H3 is justified by the reaction of acetylene with carbene (likewise resulting from the photodissociation of methane) Published on 31 October 2013 on http://pubs.rsc.org | doi:10.1039/9781849737722-00342 CH4 !: CH2 ỵ H2 : CH2 ỵ C2 H2 ! H2 Cd CRCH ỵ Hd In the atmosphere and in the haze, benzene has been detected by Cassini mission with UVIS (ultraviolet imaging spectrometer) at altitudes ranging from 450 km to 1600 km.101 On this basis, Vuitton concluded that the presence of benzene in the stratosphere is due to the cyclization of the unstable propargyl (and vinyl) radicals.101,102 H2C C CH + H2C C CH The below reported reactions conducing to cyanoacetylene and acrylonitrile take place in the upper atmosphere (W250 km), where the photolysis of HCN generates cyanide radicals CN that react respectively with acetylene and ethylene: dCN þ C2 H2 ! HCRC À CN þ Hd dCN þ C2 H4 ! H2 C ¼ CH À CN þ Hd The presence of these species where first predicted by theoretical models and finally confirmed by Cassini-Huygens space mission, as was the dependence from altitude of cyanide radicals Acrylonitrile could be also produced by reaction of vinyl radicals (stemming from ethylene photolysis) and hydrogen cyanide This reaction is interesting because is an important chemical sink for HCN100 H2 C ẳ CHd ỵ HCN ! H2 C ẳ CH CN ỵ Hd As for acrylonitrile, this easily photolyzes and gives molecular hydrogen and cyanoacetylene.100 H C H2C hν CN HC C CN + H2 3.1 The origin of the polymers detected in the orange haze There are three main polymer classes in the Titan orange haze, viz highcarbon content polymers, high-nitrogen content polymers and mixed C-N polymers Concerning C-rich polymers, acetylene undergoes photolysis and produces acetylene radicals These intermediates in turn participate into chain elongation processes reacting with acetylene (and its oligomers) and producing polyynes.100 H-CRCịn -H ỵ HCRCd ! 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