Faraday Discuss. 1995,100,C3-C7 The Labile Molecule since 1947 George Porter Imperial College London We are here to celebrate our favourite science of Physical Chemistry and it is right that the celebration should be here in the Royal Institution the House of Michael Faraday where he made all his discoveries where he lectured in this theatre for over 40 years and where he lived most of his life as the Resident professor in the rooms on the floor above. I count it as the greatest privilege of my life to have been for 20 years one of Faraday’s successors as Resident professor working in his laboratories living in his rooms and lecturing in his theatre. It was in this theatre on 30th June 1903 that the Faraday Society held its founda- tion meeting for the purpose of ‘promoting the study of electrochemistry electrometal- lurgy chemical physics metallography and kindred subjects’.It was a society of physical chemists named after one who never described himself as such or even as a physicist-a species unrecognised in his time-but who liked to be called a chemist or better still a natural philosopher. The first General Discussion of the new society was held in 1907 and there were about one hundred of them before they began to be numbered as separate publications with No. 1 in 1948 and the 100th in the new series held this month. The Faraday Discussions have been the seed beds from which Physical Chemistry has developed during this century. Many of the early discussions had obvious practical applications and the early presidents were pioneers in the industrial applications of physical chem- istry Sir Joseph Swann whose electric lamp preceded Edison’s by 20 years was the first President; Lord Kelvin whose researches made possible the laying of the first trans- atlantic cable was second and Sir William Perkin who founded the synthetic dye industry and was present in this theatre in 1906 for the golden jubilee of his discovery of Mauvine was the third.Unfortunately I was myself unable to be present at that first meeting in 1903 but I was present when the society celebrated its fiftieth anniversary in this same room in 1953. The discussions of the Faraday Society over the next decade or two were to me where everything in physical chemistry happened where the very latest advances were published and discussed and where one met like-minded scientists from this country and overseas who became lifelong friends.To me there was no society like it it was part of my life. To give a balanced impersonal; account of the many themes of the Faraday Dis- cussions which have taken place during the 48 years that I have been a member even if it were possible would be exceedingly dull. Instead I offer a very personal account of the one area which has concerned me most and how for me it has been inseparable from the Faraday Discussions of the time. I am sure that many of those here could tell similar stories about the importance of Faraday Discussions to their own work. I have called this talk ‘The Labile Molecule since 1947’ because it combines the title and date of the first Faraday Discussion that I attended in Oxford 48 years ago.It was only the second in the series of published discussions whose centenary we celebrate. The pattern of the Faraday Discussions is unique; the papers are written in full and distrib- c3 C4 The Labile Molecule since 1947 uted before the meeting; the presentation lasts only five minutes and is followed by questions and discussion all of which are published. I presented my first paper on work done with Professor R. G. W. Norrish at that 1947 meeting. There was nothing remarkable about the paper but it was important to me and its presentation was to a large Oxford audience including Hinshelwood Steacie Melville Coulson M. G. Evans and other names that I ranked at least equal to the old testament prophets.And there were younger prophets in waiting like Longuet-Higgins Rex Richards and Fred Dainton. It was a daunting occasion for a second year research student from Cambridge. I rather expected to be eaten for breakfast along with the Oxford marmalade. But it was a friendly society and everybody was very kind. Our paper was about the detection and measurement of free radicals in gases by a flow technique developed in 1929 by F. Paneth who used the removal of metal mirrors as a measure of the free radicals present after thermal decompositions. It was an unspecific and difficult method for photochemical reactions even with the help of a large army searchlight as the light source but it was the most direct method available.In the whole 400 pages of this discussion there was no mention of a direct observ- ation of any free radical except for some emission spectra in flames and electrical dis- charges and indeed some were still sceptical about their very existence under normal conditions. Sir Harry Melville had no doubt about their existence but was sanguine about our chances of direct observation. In his opening address he said. The direct physical methods of measurement simply cannot reach these magnitudes far less make accurate measurements in a limited period of time for example lop3sec. This was a challenge that I heard with glee because I had that summer decided to try a new line of work which if successful would do just this. I discarded my searchlight for a flash arrangement with a large capacitor bank by courtesy of my friends in the Royal Navy.The total cost was very small or even negative since the Navy were kind enough to say that there was five shillings due to me for every crate that I returned. The idea which I called flash photolysis was very simple in principle though less so in practice. A flash of light was used to produce chemical change in a substance and a second flash of light then recorded spectroscopically the products that were present a short time afterwards. By repeating the process with increasing time intervals between the flashes a movie of the very fast chemistry of free radicals and other intermediates was recorded. The first paper (published with Professor R.G. W. Norrish) on the use of high- energy flashes in photochemistry appeared in Nature in 1949 and the second paper on the double flash pulse and probe-the flash photolysis technique-was submitted to the Royal Society in the same year (in some haste since I was to be married two days later). I read this paper the following January at an afternoon meeting of the Royal Society in Burlington House with the President the redoubtable Sir Robert Robinson in the chair. He was one of those organic chemists who had grave doubts about the existence of these labile molecules but he was gracious enough to comment after my paper ‘there does now seem to be some experimental evidence for free radicals’. The time was now opportune to attend my second discussion of the Faraday Society held in Cambridge in 1950 on ‘Spectroscopy and Molecular Structure’ where I was able to describe in more detail the first absorption spectra of several free radicals obtained by the new method.The most interesting free radical was C10 obtained by flash photolysis of a mixture of gaseous chlorine and oxygen. The audience included Gerhard Herzberg and James Franck as well as Michael Kasha; truly a contempory of mine since we were born on the same day but owing to the transatlantic time difference neither of us has been able to claim priority. To me Michael’s paper was the most memorable of this meeting. It dealt with types of molecu- lar transition that were little known singlet-triplet and n-n* transitions. It heralded the George Porter c5 coming of a new era in photochemistry but received little attention at the time least of all from photochemists.My third Faraday Discussion held in Toronto in 1953 was attended by 45 members of the society from Europe including R. P. Bell E. J. Bowen Charles Goodeve Alfred Egerton R. G. W. Norrish Harry Melville F. S. Dainton Peter Gray and John Polanyi along with E. W. R. Steacie H. S. Taylor George Kistiakowsky Linus Pauling and the Society’s secretaries F. C. Tomkins and Beatrice Kornitzer. Many of us were shipmates on the Empress of Britain for the crossing. It was the first post-war overseas meeting of the Faraday Society and a quite memorable one for all of us. The subject of this Canada meeting was ‘The Reactivity of Free Radicals’.It seemed as if the Discussion subjects were being chosen especially for me because having inter- preted the spectra of some free radicals I had now turned to the original purpose the use of these spectra to measure reactivity. The chemical reactions of the free radical ClO made a very interesting story but were of little practical importance as I had to admit to visitors to the laboratory in Cam- bridge who might become benefactors and who quite reasonably were interested in possible applications. It was 30 years later that the information gained in those days became of very practical importance. It turned out that chlorine in the stratosphere formed by photochemical decomposition of the chlorofluorocarbons used as aerosols and refrigerants were responsible for the decomposition of ozone in the stratosphere and the recently discovered ozone hole over Antarctica.It was gratifying that very shortly afterwards C10 was shown from stratospheric flights over Antarctica to be a key intermediate in this photochemical cycle of destruction. The Faraday stories run and run and the 100th Faraday discussion held this month is on ‘Atmospheric Chemistry ’. Within a few years free-radical studies were everywhere dozens of free-radical absorption spectra had been described and a new free-radical society was about to be created. We were particularly interested in the organic and particularly the aromatic free radicals and we recorded for the first time the spectra of benzyl anilino phenoxyl and phenyl and many of their derivatives.It fell to Herzberg to use flash photolysis and ultra-violet spectroscopy to discover the spectra of methylene and methyl which had been two of our original targents. About this time in 1954 Irwin Norman and I described a complementary technique for recording free-radical spectra by photolysis of low-temperature solid solutions. We now turned to the flash photolysis study of another very important class of short lived intermediates the electronically excited states of molecules. Flash photolysis was still limited by the duration of the shortest flashes available to times of a few micro- seconds whilst typical allowed electronic transitions take place in much shorter times in the nanosecond range. However the triplet states involve much slower forbidden tran- sitions and their absorption spectra had just been detected in low-temperature solids by Donald McClure.All seemed set to detect these excited species in gases and in fluid solvents at normal temperatures although it was not at all certain that their lifetimes under these conditions would be long enough for our methods. We were fortunate; the lifetimes were ten or a hundred times longer than our flashes and a whole new chemistry became available to us the chemistry of the triplet state which as soon became appar- ent was as important to a photochemist as free radical chemistry. These transient triplet-state molecules in solution formed the subject of a paper with Maurice Windsor in my fourth Faraday Discussion held in Birmingham in 1954 on the subject of ‘The Study of Fast Reactions’.Our paper was on ‘The Triplet State in Fluid Solvents’ and it described the detection and kinetic study of about a dozen aromatic molecules as well as along with Robert Livingston chlorophyll in their triplet states at room temperature where their lifetimes were very conveniently for us a few hundred microseconds. C6 The Labile Molecule since 1947 R. G. W. Norrish and I read a paper at this same discussion on the flash photolysis technique itself. This was one of the first of many discussions on techniques for fast reaction chemistry. Other methods described included the shock tube flow (and stopped flow) methods and the temperature jump and relaxation techniques described by Eigen in his paper ‘Study of Ionic Reactions in Solutions in Times as Short as seconds.” Notice that although ‘microsecond’ was now in the chemical vocabulary the ‘nanosecond’ had still to arrive.We were all getting excited about fast reactions and were already talking about very fast reactions. There was a story going around at this meeting that Manfred Eigen sought the advice of an Oxford man as to what in the English language one could call reactions that were faster than very fast. Without any hesitation Ronnie Bell replied ‘Damn fast reactions Manfred and if they get faster than that the English language will not fail you you can call them. ‘Damn fast reactions indeed!’ For chemists biologists and physical chemists this meeting began ‘the race against time’.We awaited shorter pulses in the nanosecond region to enable us to study those very labile molecules the excited singlet states of molecules. The laser which was to trans- form the technique of flash photolysis and to make it the fastest of all these fast methods appeared in 1960. It took a few years to get hold of a laser and apply it to photochem- istry but this was done by Jeff Steinfeld in Shefield by 1965 using pulses ofa few nanoseconds. In 1966 I moved from Sheffield University to the Royal Institution and a student Micheal Topp who moved with me brought the precious laser and set up our first nanosecond flash apparatus here. We had to solve several further problems before we could do nanosecond flash photolysis. We had to produce a nanosecond white light pulse and to synchronise the two pulses within a few nanoseconds delay.We were then able to detect the singlet state absorption spectra of many molecules. As new laser pulse techniques became available especially the mode-locked colliding-pulse-mode laser developed by Shank picosecond and then femtosecond flash photolysis became possible. The steps in this race against time are shown in comparison with the cosmic time scale in Fig. 1. In the femtosecond region of physical chemistry there are two areas which are advancing particularly rapidly at this moment. The first the fastest step in the course of a molecular reaction is the crossing of the transition state. This was the subject of the Faraday Discussion of 1991 on ‘Structure and Dynamics of Reactive Transition States’ at which speakers included John the second Polanyi in this field his fellow Laureate Dudley Hershbach and two pioneers of femtosecond transition state dynamics Dick Zare and Ahmed Zewail.The progress through the transition state of a dissociating NaI molecule which becomes trapped in the resonant excited state for an average of 10 vibrations was followed by Zewail using fluoresence of the product Na* and the off-resonance fluorescence of NaL*. The second rapidly developing area of femtosecond molecule dynamics is photosyn- thesis. At this point I should remind you that in 1951 the Faraday Society broadened its terms of reference to read ‘To promote the study of sciences lying between chemistry physics and biology.’ This was a wise move though it is a matter for regret to me that there have been relatively very few Faraday Discussions in these interdisciplinary fields probably because of the immense growth in the number of societies and their associated journals in the biophysico-chemical areas.Today many of the most exciting develop- ments are in the biophysical sciences. Particularly active at the present time are studies of the primary processes of photo- synthesis. These begin with the transfer of the absorbed solar excitation energy between pigment molecules in the leaf membrane followed by the transfer of electrons and protons across the membrane It turns out that both of these processes occur in picose- conds or less and it was fortunate that at the time that these systems were being dis- covered femtosecond flash photolysis was becoming available.George Porter c7 SECONDS 1ol8 Age of Earth 1015 First Man lo 12 Pyramids 10 9 10 103 1 nse FLASH PHOTOLYSIS 10 -3 OND Fig. 1 Timescale of flash photolysis compared with the cosmic scale The most recent studies of these reactions concern the reaction centre of photo-system 2 of the green leaf which is the simplest unit that still carries out both energy and electron transfer. The structure of its associated light-harvesting unit (LHC 2) is now known from the electron diffraction work of Kuhlbrandt. The structure of the reaction centre (containing six chlorophylls and two pheophytins) has not yet been fully deter-mined but much can be deduced by comparison with the structures of the reaction centres of the photosynthetic bacteria determined by Huber Mlchel and Deisenhoffer which have many close homologies with photosystem 2.The processes of energy and electron transfer in the reaction centre occur in times that lie in the femtosecond and picosecond regions respectively. The theory of energy transfer was the subject of the Spiers Memorial lecture given by Theodore Forster at the Faraday Discussion of 1959 on ‘Energy Transfer with Special Reference to Biological Systems’. Theories of electron transfer have frequently been discussed notably by Rudy Marcus in his R. A. Robinson Memorial lecture at the Faraday Discussion of 1982 on ‘Electron and Proton Transfer’. The rates predicted for these transfers are about 100 femtoseconds and a few picoseconds respectively which agree very well with those mea-sured directly by flash photolysis.There is no part of the natural world which does not come under the scrutiny of physical chemists from time to time the electrical magnetic and optical properties of matter and its internal motions rotations vibrations and electronic transitions; the structure of molecules new ones like buckminsterfullerene and the eternal complex and beautiful ones of nature; chemical changes in strange places like the stratosphere and outer space in the hottest flames and in solids near to the absolute zero. And chosen as the subject of this talk the changes in these structures that occur both in the laboratory and in nature in times so short that we are near the point where chemists biologists and most physicists must accept that they have reached the end of the timescale and the limits of certainty. Furaday Discussion 100 Celebration Paper; Presented 24th April 1995