Photochemistry — development and achievements Jozef Šima Institute of Inorganic Chemistry, Technology and Materials, FCHPT STU, Radlinského 9, 812 37 Bratislava, Slovakia [email protected] Abstract: Photochemistry has been subjected to the enormous development within the last two centuries. This development has been driven mainly by two key factors: inherent scientific thirst for knowledge and worldwide foodstuff and energy needs. Within the development of photochemistry, mutual conditionality of global needs, progress in theory, improving of existing and birth of qualitatively new experimental techniques can be identified. Photochemistry has found its application in various fields of our life, development and protection of the nature. Keywords: applications, energy needs, experimental techniques, history, photochemistry Photochemistry — definition Photochemistry — stages of its development Photochemistry is defined as the branch of che- mistry concerned with the chemical effects of ultra- Human beings have known the impacts of light on violet, visible, or infrared radiation (Braslavsky, various aspects of life from the beginning of their 2007). Photochemistry cannot be separated either existence. This knowledge has been based on expe- from photophysics investigating processes occur- rience. In the field of therapy, sunlight was used in ring without changes in chemical identity of the the treatment of skin diseases over many centuries involved compounds, or from ground-state chemis- and the method was named heliotherapy. More try not involving formation and deactivation of than 3,500 years ago, ancient Egyptian and Indian electronically excited states. Taking these facts into healers used the ingestion of plant extracts or seeds account, a broader definition of photochemisty can in addition to sunlight for treating leucoderma. be formulated as follows: photochemistry is the Natural desire not only to notice light-induced branch of chemistry dealing with causes and courses phenomena but also to understand them has led to of chemical deactivation processes of particles from gradual formation of photochemistry, photophy- their electronically excited states, usually with the sics, and photobiology as scientific disciplines. participation of ultraviolet, visible or near-infrared In an introduction to photochemistry, brief infor- radiation (Šima, 2015). Anyway, an inherent feature mation of photography cannot be missing. The both of photochemistry and photophysics is the existence of photography has roots in the fact that involvement of electronically excited state(s). some substances are visibly altered by exposure to The term photochemistry is composed of parts photo, light. The coining of the word photography is attrib- meaning light, and chemistry. Along with this term it uted to Sir John Herschel (1839) and it is composed seems to be worth mentioning that the term photon from the Greek phōtós (meaning light) and graphê was coined neither by Planck nor Einstein who intro- (meaning drawing or writing). Following previ- duced the concept of light quantum (das Lichtquant ous experiments, Nicéphore Niépce was the first as in the original German language). Instead, it was managing to fix an image that was captured with a used by Lewis in his paper The conservation of Photons camera after several hours of exposition in 1826 or published in Nature (Lewis, 1926). In his paper Lewis 1827. It was made on a polished sheet of pewter and presented the following hypothesis “we are dealing here the light-sensitive substance was a thin coating of with a new type of atom, an identifiable entity, uncreatable bitumen. and indestructible, which acts as the carrier of radiant energy As a pioneering stage of photochemistry, the pur- and, after absorption, persists as an essential constituent of poseful investigation of chemical processes induced the absorbing atom until it is later sent out… I therefore take by sunlight may be declared. When introducing this the liberty of proposing for this hypothetical new atom, which stage, the name Giacomo Ciamician cannot be omit- is not light but plays an essential part in every process of ted. Due to the variation of the intensity and wave- radiation, the name photon”. It is a classical scientific length of solar radiation this stage was of empirical paradox that his theory and explanation about the nature with missing quantitative aspects. The main light failed but the word photon has survived. goals of the research consisted in pre paring new 84 Acta Chimica Slovaca, Vol. 10, No. 2, 2017, pp. 84—90, DOI: 10.1515/acs-2017-0015 organic compounds through the impact of sunlight electronic deactivation and vibrational relaxation) on investigated systems (Ciamician, 1912). and chemical reactions (formation and reactions of The invention of discharge and fluorescent intermediates). It should be pointed out that cur- tubes and filters, as well as spectral methods rent techniques allow scientists also to determine allowed specifying and determining radiation the structure of electronically excited molecules intensity and wavelength. This was a condition to and nanoparticles (Coppens et al., 2002; Coppens follow wavelength-dependence of photochemical et al., 2014). It should be pointed out that outcomes processes. Photochemical experiments were still of of attosecond (photo)physics penetrate to chemistry steady-state nature using continuous irradiation, providing valuable information on the electron the main parameter obtained being the quantum dynamics connected to chemical processes (Nisoli yield of final product(s) of photoreactions. Along et al., 2017; Kumpulainen et al., 2017). with searching for final reaction products, atten- tion was paid to the mechanism of photochemical Down of photochemistry as a science processes identifying intermediates, e.g. radicals in photoredox processes through their ground-state Any field of human activity to be called scientific reactions and spectroscopic techniques. This stage must be covered by consistent terminology, laws continued as the dominant mode of photochemical or principles, and hypotheses and scientific aims. investigation till the 1960s. As for terminology of photochemistry, IUPAC The advent of fast flash techniques working publishes Glossary of Terms in Photochemistry, the from microsecond to femtosecond timescale has latest one in 2007 (Braslavsky, 2007). The glossary open a qualitatively new chapter in investigating forms terminological and matter-of-fact basis of photochemical and photophysical behaviour of photochemistry and photophysics. chemical species. Flash photolysis was developed Photochemistry is based on just two photochemical in 1949 and Eigen, Norrish and Porter won the laws dealing more with the absorption of radiation 1967 Nobel Prize in Chemistry “for their studies (photophysical phenomenon) than with its photo- of extremely fast chemical reactions, effected by chemical consequences. The first one is called disturbing the equilibrium by means of very short according to its inventors the Grotthuss-Draper pulses of energy”. In the field of chemistry, this law (for chemists Theodor Grotthuss 1817 and John stage started with Porter’s introduction of micro- W. Draper 1841) and states that only radiation that is second flash photolysis allowing him to monitor absorbed by a molecule can produce a photochemi- reaction intermediates (van Houten, 2002). Over cal change in that molecule. four decades the instrumentation has evolved up The development of the quantitative aspects of to the femtosecond timescale — nine orders of photochemistry began with the enunciation of the magnitude faster. The concept of (ultra)fast flash quantum theory by Max Planck in 1900. The second techniques is in principle simple — to distort the law of photochemistry (Stark, 1908 and Einstein, system at equilibrium using a high-energy flash of 1912) states that for each photon of light absorbed by (usually monochromatized) radiation and detect a chemical system, only one molecule is activated for how fast the system restores to the equilibrium. It its subsequent reaction. This photochemical equiva- should be noted that at the time being also atto- lence law was definitely derived by Einstein during second (subfemtosecond) technique is at disposal, his development of the quantum (photon) theory of however, it is exploited mainly to follow physical light. It should be noted that irradiation of a system processes (Schultz and Vrakking, 2013). We thus by very intensive lasers can caused that one mole- have femtochemistry (no chemical process is faster) cule absorbs simultaneously two (in general even and attophysics. In this connection it is worth men- more) photons. Such a non-linear optical processes tioning that the first papers describing femtosecond were predicted originally in 1931 (Goeppert-Meyer, experiments originated not from a university but 1931), observed thirty years later (Keiser, 1961) and from AT&T Bell Laboratories (Shank and Ippen, are of practical importance. 1974) and orientation to solve tasks of practical im- The Bunsen-Roscoe Law of Reciprocity (1862) stat- portance can be demonstrated, e.g. by investigation ing that a photochemical effect is directly propor- of thermal energy dissipation after excitation of tional to the total energy dose, irrespective of the silicon (111) surface by 80-fs optical pulses (Shank, time required to deliver the dose, is not included 1986). The field of academic research focused on into fundamental photochemical laws. It