Photochemistry and Photophysics E-Book

Vincenzo Balzani

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 985

Veröffentlichungsjahr: 2024

Beschreibung

Connects principles, processes, and experimental techniques with current research in the continuously expanding field of photochemistry and photophysics

Photochemistry and Photophysics covers a wide spectrum of concepts in photochemistry and photophysics, introducing principles, processes, and experimental techniques, with a wealth of examples of current applications and research spanning natural photosynthesis, photomedicine, photochromism, luminescent sensors, energy conversion and storage, and sustainability issues.

In this Second Edition, several chapters have been revised considerably and others have been almost entirely rewritten. A number of schemes and figures have been added, and the reference list at the end of each chapter has been extended and updated.

Clearly structured, the first part of the text discusses the formation, properties, and reactivity of excited states of inorganic and organic molecules and supramolecular species, and the second part focuses on photochemical and photophysical processes in natural and artificial systems. Readers will learn how photochemical and photophysical processes can be exploited for novel, unusual, and unexpected applications.

Written by world-renowned experts in the field, Photochemistry and Photophysics includes information on:

• Formation, electronic structure, properties, chemical reactivity, and radiative and nonradiative decay of electronically excited states
• Fundamental concepts and theoretical approaches concerning energy transfer and electron transfer
• Peculiar light absorption/emission spectra and the photochemical properties of the various families of organic molecules and metal complexes
• Equipment, techniques, procedures, and reference data concerning photochemical and photophysical experiments, including warnings to avoid mistakes and misinterpretations
• Relationships between photochemical, photophysical, and electrochemical properties of molecules that enable interconversion between light and chemical energy

With an appropriate mix of introductory, intermediate, and advanced content, this is an ideal textbook resource for related undergraduate and postgraduate courses. The text is also valuable for scientists already active in photochemical and photophysical research who will find helpful suggestions to undertake novel scientific projects.

Leseprobe

Photochemistry and Photophysics

Concepts, Research, Applications

Second Edition

Vincenzo Balzani, Paola Ceroni, and Alberto Juris

Authors

Prof. Vincenzo BalzaniUniversity of BolognaDepartment of Chemistry Giacomo Ciamicianvia Selmi 2BolognaIT, 40126

Prof. Paola CeroniUniversity of BolognaDepartment of Chemistry Giacomo CiamicianVia Selmi 2BolognaIT, 40126

Prof. Alberto JurisUniversity of BolognaDepartment of Chemistry Giacomo CiamicianVia Selmi 2BolognaIT, 40126

Cover: © agsandrew/Shutterstock

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Print ISBN: 978-3-527-35276-0

ePDF ISBN: 978-3-527-84407-4

ePub ISBN: 978-3-527-84408-1

To Carla, Carlo, and Teresa

From the Preface to the First Edition

And God said: “Let there be light”;And there was light.And God saw that the light was good.(Genesis 1, 3-4)

Photochemistry and photophysics are natural phenomena as old as the world. Our life depends on photosynthesis, a natural photochemical and photophysical process. We get information about the surrounding space by photochemical and photophysical processes that occur in our eyes.

Currently, photochemistry and photophysics represent a modern branch of science, at the interface between light and matter and at the crossroads of several disciplines including chemistry, physics, materials science, ecology, biology, and medicine. In our day life, we are surrounded by products obtained with the aid of photochemistry and photophysics and by devices that exploit photochemical and photophysical processes to perform useful functions in various places, from industries to hospitals.

We are moving toward a future in which energy and information will be the dominant features of civilization. We will be forced to exploit sunlight as our ultimate energy source, converting it into useful energy forms by photochemical and photophysical processes. We will continue to miniaturize devices for information and communication technology down to the molecular level and we will use, more and more, light signals to transfer, store, and retrieve information.

The number of researchers working in the area of light-matter interaction is increasing, but several of them did (and still do) not receive an appropriate training. Light is often used in chemical laboratories as a silver bullet reactant to obtain products unavailable by thermal activation. In general, however, researchers lack the basis to fully understand how photochemical and photophysical processes can be exploited for novel, unusual, and unexpected applications in fields of energy conversion, information technology, nanotechnology, and medicine.

Preface to the Second Edition

In the past 10 years, several textbooks and reference books on photochemistry have been published. However, most of them essentially focus on the photoreactions of organic molecules. In some textbooks, the fundamental bases of excited-state properties are confined in a few pages; in others, theoretical aspects are presented in too much detail, including boring and unnecessary mathematical treatments. Most of the available books ignore, or barely mention, the photochemical and photophysical properties of metal complexes, a class of molecules that is attracting increasing theoretical and applicative interest. No textbook emphasizes the most recent trends in photochemistry and photophysics, such as information processing by reading, writing, and erasing molecules with light signals, the capability of powering and controlling molecular machines by light, the conversion of sunlight into electrical energy by inorganic and organic solar cells, the recent developments in the field of light-emitting diodes, and the first achievements along the road toward artificial photosynthesis.

For all these reasons, we felt there was the need for a book capable of (i) presenting a clear picture of the concepts required to understand the excited-state properties of the most important types of molecules, (ii) showing recent applications concerning photochemistry and photophysics, and (iii) opening the eyes of young researchers toward forefront developments or even futuristic visions of the light–matter interaction. The usefulness of the first edition of this book was testified by the prompt publication of a Chinese edition in 2015. In the following years, the frontiers of photochemistry and photophysics continued to expand with the development of new molecules, new materials, and new processes. There is no doubt that photochemistry and photophysics will play an increasingly important role in the development of science and technology.

Although the organization of this second edition is essentially the same as that of the first edition, several chapters have been revised considerably, others have been almost entirely rewritten, a number of schemes and figures have been added, and the reference list at the end of each chapter has been extended and updated.

We believe that this book, which originates from our long experience in teaching photochemistry and photophysics at the University of Bologna, can be a basic text for graduate and postgraduate courses because of its balanced content. We feel that it can also be useful for scientists who desire to enter photochemistry and photophysics research even if they did not have a chance, during their university training, to get the fundamental bases of this field. Scientist already active in photochemical and photophysical research can find suggestions to undertake novel scientific adventures.

Chapters 1–4 of this book deal with fundamental concepts concerning the nature of light, the principles that govern its interaction with matter, and the formation, electronic structure, properties, chemical reactivity, and radiative and nonradiative decay of excited states. Each concept is illustrated making reference to important classes of molecules. The notion that an excited state is a new chemical species with its own chemical and physical properties compared with the ground state is underlined, leading to the conclusion that photochemistry is a new dimension of chemistry.

Chapter 5 extends the above concepts from molecules to supramolecular (multicomponent) systems where a fundamental role is played by structural organization and component interactions.

Chapter 6 illustrates the fundamental concepts and the theoretical approaches concerning the two most important photochemical and photophysical processes, namely, energy transfer and electron transfer.

Chapters 7 and 8 deal with the photochemical and photophysical properties of organic molecules and metal complexes, respectively. The peculiar light absorption/emission spectra and the photochemical properties of the various families of organic molecules are illustrated by detailed discussions of several examples. For metal complexes, the discussion of the relationship between structure and photochemical and photophysical properties is underlined, with particular emphasis on the nature of the metal(s) involved, the outstanding luminescence properties of some classes of these compounds, and the relationships between luminescence and electrochemical properties.

Chapter 9 offers a detailed presentation of equipment, techniques, procedures, and reference data concerning photochemical and photophysical experiments, including warnings to avoid mistakes and misinterpretations.

Chapter 10 describes the relationships between photochemical, photophysical, and electrochemical properties of molecules that can be exploited for the interconversion between light and chemical energy.

Chapter 11 deals with the mechanisms of homogeneous and heterogeneous photocatalytic processes based on electron and hydrogen transfer reactions, including two-photon-driven photoredox catalysis and applications of photocatalysis for environmental protection.

Chapter 12 concerns the hot topic of light-powered molecular devices and machines. The concepts of exploiting the interaction between molecules and light to read, write, and erase information are illustrated, together with their application in the field of molecular logics. Various molecular devices (e.g., wires, switches, extension cables, pumps, and light-harvesting antennas) based on energy transfer, photoinduced electron transfer, or photoisomerization processes are described and important examples of light-powered molecular machines (e.g., linear and rotary motors) are discussed.

Chapter 13 illustrates in detail the reactions taking place in the natural photosynthetic processes of bacteria and green plants and describes the first achievements along the road toward photochemical water splitting by photocatalytic semiconductor nanoparticles and photoelectrochemical cells.

Chapter 14 illustrates the relationships between light and life, starting from vision and including damages caused by exposure to UV light, benefits deriving from light-based therapeutic processes, fluorescent sensors and their applications, and a brief description of bioluminescence processes.

Chapter 15 deals with applications of photochemistry and photophysics, covering various topics: photochromic compounds, luminescent sensors (including their use in fields as diverse as wind tunnel, thermometers, measuring blood analytes, detecting explosives, and warfare chemical agents), optical brighteners, atmospheric photochemistry, solar cells (PV, OSC, DSSC), electrochemiluminescent materials (LED, OLED, LEC), numerous applications concerning the interaction between polymers and light (e.g., photodegradation, photostabilization, photolitography, and stereolitography), and the photochemical syntheses of industrial products.

After having presented the fundamental concepts of photochemistry and photophysics and described the most important natural and artificial photochemical and photophysical processes, in Chapter 16, we offer the reader the opportunity to make acquaintance with forefront research through the discussion of selected topics taken from recent literature. The choice of the examples has been based not only on their intrinsic interest, but especially on their educational capacity to illustrate connections among fundamental photochemical and photophysical concepts.

In several chapters, additional information on specific topics is presented in boxes interlaced with the text. An important feature of the book is the abundance of illustrations that are essential for an easier understanding of the concepts discussed. References have been updated up to December 2023.

Before closing, we express our feeling concerning science, society, and Earth, the place on which we live. Planet Earth is a very special spaceship that cannot land or dock anywhere for being refueled or repaired. We can only rely on the limited resources available on the spaceship and the energy coming from the Sun. We are concerned about the increasing consumption of natural resources [1], the climate change [2], the energy crisis [3], and the degradation of the environment [4–6], which is accompanied by an increased social disparity. As Pope Francis warns [7–9], we are faced with a complex crisis that is both social and environmental. Strategies for a solution demand an integrated approach to combating poverty and protecting nature.

If we want to continue living on Earth, we must achieve the goals of ecological and social sustainability by implementing three transitions: from fossil fuels to renewable energies, from a linear to a circular economy, and from consumerism to sobriety [10], but we also need to create new resources. In principle, this is possible by exploiting the only abundant, inexhaustible, and well-distributed resource on which we can rely: solar energy. Starting from seawater, the fundamental components of our atmosphere, and mineral resources, by means of sunshine, we need to “fabricate” fuels, electricity, pure water, polymers, food, and other things we need [11].

Until now, humankind has taken from spaceship Earth enormous amounts of resources [12]. Hopefully, future generations will pay back Earth with a capital produced from human intelligence. Photochemistry and photophysics can help. Indeed, science can greatly benefit humankind, but science and technology alone will not take us where we need to go: a fair, open, responsible, friendly, united, and peaceful society. Responsible scientists, while creating, with the greatest moral care, new science and technology, should also play an important role as authoritative, informed, and concerned citizens of Earth [13]. They should teach their students not only to make science but also to distinguish what is worth making with science. As pointed out by Albert Einstein, “Concern for man himself and his fate must always constitute the chief objective of all technological endeavors … never forget this in the midst of your diagrams and equations.” We need scientists watching that science and technology are used for peace, not for war; for alleviating poverty, not for maintaining privileges; for reducing, not for increasing the gap between developed and underdeveloped countries; for protecting, not for destroying our planet that, beyond any foreseeable development of science, will remain the only place where mankind can live. Science, but also consciousness, responsibility, compassion, and care must be the roots of a new knowledge-based society.

References

1

Richardson, K., Steffen, W., Lucht, W., Bendtsen, J., Cornell, S. E., Donges, J. F., Drüke, M., Fetzer, I., Bala, G., von Bloh, W., et al. (2023) Earth beyond six of nine planetary boundaries.

Sci. Adv.

,

9

, eadh2458.

2

Masson-Delmotte, V., Zhai, P., Pirani, S., Connors, C., Péan, S., Berger, N., Caud, Y., Chen, L., et al. (2023) Summary for policymakers in

Climate Change 2021 – The Physical Science Basis

, Cambridge University Press, pp. 3–32.

3

Armaroli, N. and Balzani, V. (2010)

Energy for a Sustainable World: From the Oil Age to a Sun-Powered Future

, Wiley-VCH, Weinheim.

4

Wilson, E. O. (2006)

The Creation: An Appeal to Save Life on Earth

, Norton, New York.

5

Brown, L. R. (2011)

World on the Edge: How to Prevent Environmental and Economic Collapse

, Norton, New York.

6

Ehrlich, P. R. and Ehrlich, A. H. (2013) Can a collapse of global civilization be avoided?

Proc. R. Soc. B Biol. Sci.

,

280

, 20122845.

7

Pope Francis (2015)

On Care for Our Common Home, Laudato si’

,

https://www.vatican.va/content/francesco/en/encyclicals/documents/papa-francesco_20150524_enciclica-laudato-si.html

(3 October 2023).

8

Pope Francis (2020)

Fratelli Tutti

,

https://www.vatican.va/content/francesco/en/encyclicals/documents/papa-francesco_20201003_enciclica-fratelli-tutti.html

(1 December 2023).

9

Pope Francis (2023)

Apostolic Exhortation Laudate Deum of The Holy Father Francis To All People of Good Will on the Climate Crisis

,

https://press.vatican.va/content/salastampa/it/bollettino/pubblico/2023/10/04/0692/01509.html#inglese

(10 October 2023).

10

Balzani, V. (2019) Saving the planet and the human society: renewable energy, circular economy, sobriety.

Substantia

,

3

, 9–15.

11

Jacobson, M. Z. (2023)

No Miracles Needed: How Today’s Technology Can Save Our Climate and Clean Our Air

, Cambridge University Press, Cambridge.

12

Krausmann, F., Erb, K.-H., Gingrich, S., Haberl, H., Bondeau, A., Gaube, V., Lauk, C., Plutzar, C., Searchinger, T. D. (2013) Global human appropriation of net primary production doubled in the 20th century.

Proc. Natl. Acad. Sci.

,

110

, 10324–10329.

13

Balzani, V., Credi, A., Venturi, M. (2008) The role of science in our time in

Molecular Devices and Machines: Concepts and Perspectives for the Nanoworld

, Wiley-VCH, Weinheim.

Acknowledgments

In 1675, Isaac Newton in a letter to Hooke wrote: If I have seen further, it is by standing on the shoulders of Giants. This aphorism can be applied to any scientific paper and especially to any scientific book. Therefore, first of all we thank the thousands of authors whose papers have allowed us to gain a deeper understanding of the topics we have tried to illustrate and discuss in this book. We also thank a number of colleagues encountered at international meetings and in other occasions for enlightening discussions that have contributed to better focus the basic role played by photochemistry and photophysics in modern science and technology.

We are profoundly grateful to all the members of the photochemistry research group of the Giacomo Ciamician Department of Chemistry of the University of Bologna for several years of friendly research activity. In particular, we thank Alberto Credi, Giacomo Bergamini, and Serena Silvi for many helpful discussions. We also thank Nicola Armaroli and other colleagues of the Istituto ISOF/CNR of Bologna, Mirco Natali, Stefano Caramori and the late Franco Scandola (University of Ferrara), Sebastiano Campagna (University of Messina), Maurizio Fagnoni (University of Pavia), Nick Serpone (Concordia University, Montreal), Felix Castellano (North Carolina State University), Fred Brouwer and René Williams (University of Amsterdam), Gerald Meyer (University of North Carolina), and A. Prasanna de Silva (University of Belfast) who suggested improvements at various levels. Last but not least, we are grateful to the students who, over the years, have attended the photochemistry and photophysical courses in our university. They have greatly contributed to clarify our ideas and improve our teaching with their clever questions and punctual observations.

Bologna, January 2024         

Vincenzo Balzani, Paola Ceroni, Alberto Juris

List of Abbreviations

acac

acetylacetonate

AFM

atomic force microscopy

AIE

aggregation-induced emission

AIQ

aggregation-induced quenching

AO

atomic orbital

AQY

apparent quantum yield

biq

2,2′-biquinoline

BO

Born–Oppenheimer

BODIPY

boron-dipyrromethenes

bpi

1,3-bis(2′-pyridylimino)-isoindoline

bpy

2,2′-bipyridine

bpym

2,2′-bipyrimidine

bpz

2,2′-bipyrazine

btz

3,3′-dimethyl-1,1′-bis(

p

-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)

CAAC

cyclic alkyl(amino)carbene

CB

conduction band

CCD

charge-coupled device

CD

circular dichroism

CFC

chlorofluorocarbon

CISS

chirality-induced spin selectivity

ConPeT

consecutive photoinduced electron transfer

CPL

circularly polarized luminescence

CT

charge transfer

CTTS

charge-transfer-to-solvent

ddpd

N

,

N

′-dimethyl-

N

,

N

′-dipyridine-2-ylpyridine-2,6-diamine

DFT

density functional theory

DHP

dihydrophenanthrene

DLS

dynamic light scattering

4,4′-dm-bpy

4,4′-dimethyl-2,2′-bipyridine

4,4′-dph-bpy

4,4′-diphenyl-2,2′-bipyridine

DMF

N

,

N

-dimethylformamide

dbp

2,9-di-

n

-butyl-1,10-phenanthroline

dmp

2,9-dimethyl-1,10-phenanthroline

dpc

3,6-di-

tert

-butyl-1,8-di(pyridine-2-yl)-carbazolato

dpp

2,9-diphenyl-1,10-phenanthroline

DSPEC

dye-sensitized photoelectrosynthesis cell

DSSCs

dye-sensitized solar cells

EC

electrolyzer cell

ECL

electrochemiluminescence

en

ethylenediamine

FCS

fluorescence correlation spectroscopy

FRET

Förster-resonance energy transfer or fluorescence resonance energy transfer

GFP

green fluorescent protein

gly

glycine

HAT

hydrogen atom transfer

HCFC

hydrochlorofluorocarbon

HOMO

higher occupied molecular orbital

i-biq

3,3′-biisoquinoline

ic

internal conversion

ICT

interligand charge transfer

IPCE

incident photon-to-current efficiency

isc

intersystem crossing

ITO

indium tin oxide

LAS

light absorption sensitizer

LC

ligand-centered

LCAOs

linear combinations of atomic orbitals

LD

linear dichroism

LEC

light-emitting electrochemical cell

LED

light-emitting diode

LES

light emission sensitizer

LMCT

ligand-to-metal charge transfer

LMT

luminescent molecular thermometer

LUMO

lowest unoccupied molecular orbital

MC

metal-centered

MCP

microchannel plate

MLCT

metal-to-ligand charge transfer

MO

molecular orbital

NHE

normal hydrogen electrode

NIR

near-infrared

NLC

nonlinear crystal

NMI

naphthalimide

OEC

oxygen-evolving complex

OEP

octaethylporphyrin

OLED

organic light-emitting diode

OPA

optical parametric amplifier

OSCs

organic solar cells

PACT

photoactivated chemotherapy

PC

photocatalyst, photocatalytic

PCET

proton-coupled electron transfer

PDI

perylenediimide

PDT

photodynamic therapy

PEC

photoelectrochemical cell

PeT

photoinduced electron transfer

PES

potential energy surface

phen

1,10-phenanthroline

phq

−

2-phenylquinolyl

phtmeimb

phenyl[tris(3-methylimidazol-1-ylidene)]borate ion

PM

photomultiplier

POM

polyoxometalate

POP

bis[2-(diphenylphosphino)phenyl] ether

ppy

−

2-phenylpyridyl

pq

2-(2-pyridyl)-quinoline

PS

photosensitizer

PSP

pressure-sensitive paint

PTFE

polytetrafluoroethylene

PV

photovoltaic

py

pyridine

pz

pyrazine

QD

quantum dot

RC

reaction center

RP-isc

radical-pair intersystem crossing

SCE

saturated calomel electrode

SCO

spin crossover

sep

1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane

SET

single electron transfer

SMS

single-molecule spectroscopy

SOCT-isc

spin-orbit charge-transfer intersystem crossing

STH

solar-to-hydrogen

TADF

thermally activated delayed fluorescence

TCNP

tetracyanoporphyrin

thpy

−

2-(2′-thienyl)pyridyl

TICT

twisted intramolecular charge transfer

T.M.

transition moment

TPP

tetrakis(phenyl)porphyrin

tpy

2,2′ : 6′,2″-terpyridine

UV

ultraviolet

VB

valence band

vr

vibrational relaxation

XANES

X-ray absorption near-edge structure

YAG

yttrium aluminum garnet

1Introduction

1.1 Photochemistry and Photophysics in Science and Technology

Photochemical and photophysical processes have been intimately related to the development of humans and their environment even before their appearance on the planet. Ever since the first morning of creation, life has not been merely a chemical process, but one in which light from the Sun played a significant role: thus, photochemistry. In the first instance, simple photochemical reactions caused by the Sun’s rays generated organic molecules from the constituents of the primitive atmosphere on the Earth. Subsequently, a sophisticated series of photochemical and photophysical processes, now referred to as photosynthesis, made it possible for simple cells to become autotrophic, provided the necessities of life, stored solar energy in the form of fossil fuels, and still supply us with practically all our food.

From the point of view of living matter, however, photochemistry is more than the means of using the energy of light. It is also a means of sensing the environment (vision), an indicator of the time of day and the season, a source of damage to cellular constituents, and a mechanism for repairing some cellular damages. Photochemistry is also heavily involved in processes that determine the composition of matter in the interstellar space and in the formation of atmospheric pollutants. Of course, photophysical processes also occur in nature. Suffice it to say that the world would not be colored if sunlight were completely absorbed or completely reflected by the objects that surround us, and we would not be able to enjoy fireflies or other beautiful scenes without bioluminescence.

Each of these natural processes provides a sufficient reason for a scientific interest in photochemistry and photophysics. However, photochemistry and photophysics are also important from an artificial viewpoint. Their impact on the chemical, physical, biological, and medical sciences and technologies, including nanotechnology, is being felt increasing in a spectacular manner. Photochemical methods are used for producing polymeric printing plates and printed circuits, for UV curing of surface coatings and printing inks, and for laboratory and commercial synthesis of high-value chemical compounds. Photochemical and photophysical concepts are at the basis of important applications such as protection of dyes and plastics (and also human skin) from the damaging effect of sunlight; wastewater cleaning; design of fluorescent compounds for various sensing applications (wind galleries, security, optical brighteners, pollutant detectors, display devices, molecular switches and logic gates, biological markers, and cellular properties and functions); creation of photochromic materials used in sunglasses, fashion clothes, and optical memories; and development of laser devices and light-powered molecular machines. Other interesting fields concern photomedicine, multiphotonic processes, solar-powered green synthesis, molecular photovoltaics, and solar energy conversion by water photodissociation. These and other topics are dealt with in the subsequent chapters of this book.

1.2 Historical Notes

Artificial photochemical reactions have been observed as long as chemistry has been studied. Most of the earlier observations, however, were accidental and remained unexplained. The first investigation was made in 1777 by the Swedish chemist Carl W. Scheele, who observed that violet light was the most effective in darkening silver chloride. But it was only in 1817 that Theodor von Grotthuss established that only the light absorbed is effective in producing photochemical change. This first general principle of photochemistry passed unnoticed until 1841, when it was restated by John W. Draper and, as a consequence, is now termed the Grotthuss–Draper law.

Photochemistry emerged from its empirical stage when modern physics established that light is radiated in discrete quanta, called photons, with an energy proportional to the frequency of the light and that absorption corresponds to the capture of a photon by an atom or a molecule. With this concept in mind, Johannes Stark and Albert Einstein between 1908 and 1913 independently formulated the photoequivalence law that essentially states that there should be a 1 : 1 equivalence between the number of molecules decomposed and the number of photons absorbed. Experiments, however, showed that usually this 1 : 1 ratio is not observed, indicating that the Stark–Einstein law is not sufficient to characterize a photochemical process and that absorption of a photon can be followed by other processes. A distinction was thus introduced between the light-initiated reaction (photochemical primary process) and any subsequent chemical reactions (photochemical secondary processes). In some cases, such secondary reactions can proceed by a chain mechanism, which explains why one photon can decompose a great number of molecules.

An obvious reason why in photofragmentation reactions the number of decomposed molecules is smaller than the number of photons absorbed could be efficient recombination of primary products. It was soon realized, however, that even for other types of photoreactions (e.g., photoisomerization), the number of reacted molecules can be much less than the number of absorbed photons. It was thus clear that absorption of a photon is a necessary, but not a sufficient condition to cause a photoreaction and that light energy can be used by a molecule for other purposes. It was found, indeed, that in some cases photoexcitation does not cause any reaction, but leads to emission of light, i.e., a photophysical process, and that in other cases neither a chemical change nor light emission is observed.

An important limitation to the development of photochemistry until the second decade of the twentieth century was the unavailability of adequate light sources and analytical techniques. In fact, the only light source used by the early pioneers, like Georges Lemoine in Paris [1] and Giacomo Ciamician [2] in Bologna, was the Sun (see Box 13.1).