Early Times of Photochemistry

The perception of light is one of the fundamental experiences of living beings and its power to reveal the world has always been recognized. In Genesis, creation of matter is immediately followed by creation of light that allows to detect the created world through its forms and colors  

And God saw the Light, that it was good and God divided the Light from the darkness. 

Similar tales are present in the oldest texts of other cultures. The deepest impressions are expressed as an overwhelming flood of light, as Dante mentions for the vision of Paradise  

I think the keenness of the living ray Which I endured would have bewildered me. 

If but mine eyes had been averted from it (Paradise XXXffl 76-78) 

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Perhaps best known of Perrin s work is his spirited defense of kinetic theory and physical atomism entitled Les atomes (1913), in which he made use of his own work on Brownian motion, in combination with studies of cathode rays and x-rays, ionization, radioactivity, radiation, and quantum theory.72 About the time of the 1911 Solvay physics conference, Perrin shifted from Brownian motion to work on thin films, fluorescence, and photochemistry, partly to test the early quantum theory and especially to study individual atom-based fluctuations. 

It is a pleasure to introduce and commend this volume about photochemistry by my one-time student, my colleague, and friend, Paul Suppan. We shared for many years, first at the University of Sheffield and then at the Royal Institution in London, our fascination with chemical reactions induced by light—electronic flash lamps in the early days, lasers later. This account of photochemistry continues the enthusiasm and talent for clear exposition that Dr Suppan has always shown. It is a notable addition to the literature of photochemistry and will be welcomed by students and research workers alike. 

The need for photophysical studies in this area is indicated by the diversity of the photochemistry observed. Flash photolysis experiments in the early time domain may identify precursors to later transient species, as well as provide general information on aechanisas. 

In an alternative approach, exact (numerical) time-dependent quantum wave-packet methods have been employed since the early eighties of the last century to explore the d3mamics of ob-initio-haseA models of conical intersections, see Refs. 6-8 for reviews. It has been found by these calculations that the fundamental dissipative processes of population and phase relaxation at femtosecond time scales are clearly expressed already in fewmode systems, if a directly accessible conical intersection of the PE surfaces is involved. The results strongly support the idea that conical intersections provide the microscopic mechanism for ultrafast relaxation processes in polyatomic molecules. " More recently, these calculations have been extended to describe photodissociation and photoisomerization processes associated with conical intersections. The latter are particularly relevant for our understanding of the elementary mechanisms of photochemistry. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

Since the early 1970s, coordination chemistry and photochemistry have combined to allow development of a wide range of responsive metal complexes. These allow non-invasive monitoring of metal ion concentrations. Time-resolved measurements are particularly powerful, since they allow detection of very small amounts of substrate and have optimal signal-to-noise ratios. Nonetheless, much remains to be done using the tools which these early studies have provided, particularly with reference to the development of effective sensor systems for a range of ions by time-resolved techniques. 

It has been suggested that P BChl (where BChl is one of the two monomeric or "accessory BChls that are not part of P) is a transient state prior to P "I (14,16,19), although the evidence supporting this view has been criticized (23, 24) Recent subpicosecond studies find no evidence for P "BChl (8,9) These new results do not preclude some involvement of a monomeric BChl in the early photochemistry, only that P BChl apparently is not a kinetically resolved transient state Perhaps P itself contains some charge-transfer character between its component BChls, or between P and one or both of the monomeric BChls (8,9,25-27) One of the two monomeric BChls apparently can be removed by treatment of the reaction center with sodium borohydride (28) and subsequent chromatography, with no impairment of the primary electron transfer reactions (29) Thus, at present it appears that P I is the first resolved radical-pair state, and it forms with a time constant of about 4 ps in Rps sphaeroides ... 

Several naphthynes have already been investigated in the early 1970s. Griitzma-cher and Lohmann ° generated some derivatives by pyrolysis of different precursors and measured the ionisation potentials of the pyrolysis products. At about the same time, Lohmann investigated the photochemistry of the isomeric naphthalenedicar-boxylic anhydrides 61 and 62 using LFP and found that dimerization of the 2,3-naphthyne (57) is significantly faster than that of 4. The 1,2-isomer 50, on the other hand, hardly dimerizes at all. 

One of the enticing consequences of the excited state dynamics of base pairs is the possible role this property may have played in chemistry on the early earth. Prior to the existence of living organisms photosynthesis would have been absent. Consequently there would have been no free oxygen in the atmosphere and no ozone layer would have existed. The earth s surface would have been exposed to deeper (more energetic) UV irradiation than is the case today. Therefore UV photochemistry is part of the set of rules that may have governed the chemistry that could take place at that time. 

As hopefully will be evident by now much work has been carried out on the structure and photochemistry particularly over the latter half of the last century, up to the present day, with ever more sophisticated methods being used, from early spectroscopic detection using various matrices, to pure gas and liquid-phase work, to modern time-resolved spectroscopy and state-of-the-art theoretical studies. Here we concentrate on the spectroscopic work that has revived interest in these paradigm systems over the last few years. This will then link up with theoretical studies, including our own continuing work, in the final section. 

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