66836 - Molecular and Supramolecular Photochemistry

Learning outcomes

At the end of the course, the student knows the theoretical bases of photophysics and photochemistry. In particular, the students is able to: - analyze intra- and intermolecular processes involving excited states; - understand the working principle and use of the simplest photochemical techniques; - evidence the possible applications of photochemistry in the industrial field, for energy conversion, and luminescent sensors; - use the simplest photophysical and photochemical techniques both in steady-state and time-resolved regime, usually employed to characterize molecular and supramolecular systems.

Course contents

Prof. Paola Ceroni

Introduction : what is photochemistry; history of photochemistry with particular reference to Giacomo Ciamician. Excited states as new chemical species: different energy, lifetime, geometry, dipole moment, redox and acid/base properties, reactivity. Deactivation processes of electronic excited states: rate constants, efficiencies, quantum yields. Lifetimes of an electronic excited state: definition and relation to deactivation rate constants.

State diagram of atoms and molecules : orbitals, electronic configurations and excited states of atoms (e.g., oxygen) and diatomic and polyatomic molecules (e.g., water, formaldehyde, coordination compounds). Outlines of symmetry group theory.

Radiative and radiationless processes : molecular wavefunctions and Born-Oppenheimer approximation. Probability and selection rules for radiative transition of absorption, spontaneous and stimulated emission, and for radiationless transitions. Frank-Condon principle, Jablonski diagram: approximation and information that can be obtained from it. Correlation between absorption/emission spectra and the corresponding Jablonski diagram (e.g., benzophenone, naphthalene). Potential energy curves and surfaces for ground and electronic excited states of simple molecules.

Bimolecular quenching processes involving electronically excited states: Stern-Volmer equation, excimers and exciplexes. Catalyzed deactivation, photoinduced energy- and electron-transfer. Examples of quenching experiments by measurements of excited state lifetimes and emission quantum yields: static and dynamic quenching.

Electronic energy transfer : Coulombic and exchange mechanisms; spin selection rules and distance dependence. Applications of sensitization and quenching processes.

Photoinduced electron transfer : redox potential of electronic excited states. Examples of photocatalysis, conversion of light into chemical energy and vice versa (chemiluminescence and electrochemiluminescence).

Laser : principles of lasers based on 3 or 4 levels. Examples of the most commonly used lasers. Applications in photochemistry, medicine, material science, memories.

Equipment for photochemical reactions : irradiation sources (incandescent or arc lamps, lasers), interference or cut-off filters. Chemical actinometry: principles, and experimental aspects.

Spectrofluorimetry : block diagrams of the instrumentation, main components. Emission and excitation spectra, correction curves for the instrumental response. Lifetimes in the m s-s time scales by the sampling technique for luminescent excited states.

Time-resolved measurements of absorption and emission spectra by laser excitation: conventional laser flash photolysis and “pump-probe” technique. Block diagram, main components, excited state lifetimes in the ps-ns time range.

Single molecule fluorescence : confocal and wide-field microscopy: principles and applications.

Overview of photochemistry applications (a) in the biological and medical field (vision, protection against biological damages (sunscreens), photodynamic therapy); (b) to ecological issues (photosmog, photodegradation of pollutants); (c) in the industrial field  (photochromic materials, optical stabilizers and brighteners, LED, photovoltaic cells).

Supramolecular chemistry : definition of a supramolecular system, nature of the interactions between the component units.

Supramolecular photochemistry : quenching processes, Marcus and superexchange theories for electron transfer, energy transfer processes.

Examples of supramolecular devices driven by light.

Supramolecular wires for photoinduced electron and energy transfer: systems based on metal complexes, organic compounds, and DNA. Molecular switches operated by a photochemical mechanism. Plug-socket systems and elongation cables.

Natural and artificial photosynthesis : electron transfer process in the natural reaction center. Antenna systems for collecting light energy: antennae in natural photosynthesis; artificial antenna systems based on porphyrin, cyclodextrin, dendrimers, polymers, zeolites. Artificial photosynthesis (splitting of water by solar energy).

Photoactive dendrimers and their applications: photoswitchable boxes, systems for changing the frequency of light, sensors with signal amplification.

Molecular memories and logic gates based on supramolecular systems that respond to light inputs: YES, NOT, AND, OR, NOR, NAND, XOR, XNOR logic gates, more complex function. Neural-type systems.

Molecular machines operated by light: basic principles, energy inputs and signals. Why is light the best energy input? Natural molecular machines: basic features. Light powered hybrid systems. Artificial molecular machines. Rotary and linear movements. A nanomotor powered by visible light: sacrificial, kinetically assisted and intramolecular mechanisms.

Prof. Marco Montalti

The course content can be split in two parts; introductive and theorethical the former, practical and based on the direct use of photochemical instrumentations and technics the latter. Firts part: Fast survey on electronic absorption spectroscopy (technics and instrumentation) - Relation among absorption bands and electronic transitions - Emission spectroscopy - technics and instrumentation - emission and excitation spectra - interferences (light and raman scattering, harmonics) - corrected spectra - emission lifetimes determination (instrumentation)- Relation among emission bands and electronic transitions - Irradiation technics (lamps, filters, actinometers, etc.) - sensititation and quenching - transient absorption spectroscopy (technics and instrumentation). Second part: Spectroscopic characteritation of some compounds by practical determination of their absorption and emission spectra, emission lifetimes, photochemical reactivity, rection and emission quantum yields, actinometers. Moreover systems exhibiting sensitisation and quenching will be analized. Then practical experience of transient absorption spectroscopy will end the course.

Readings/Bibliography

Prof. Paola Ceroni

Lecture notes on the teacher web site .

- P. Klán, J. Wirz, Photochemistry of Organic Compounds, Wiley 2009.

- V. Balzani, V. Scandola: Supramolecular Photochemistry, Horwood, Chichester, 1991.

- V. Balzani, A. Credi, M. Venturi: : Molecular Devices and Machines. Concepts and Perspectives for the Nanoworld, 2° edizione, Wiley-VCH, 2008

Prof. Marco Montalti

"Chemistry and light", Paul Suppan, Cambridge, The Royal Society of Chemistry, 1994. Copies of the transparencies used in the class

Teaching methods

Prof. Paola Ceroni: lectures and discussions.

Prof. Marco Montalti: class lessons: theoretical lessons will be devoted to the presentation of the various technics usefull to study the photochemical and photophycal behaviour of compounds. The laboratory experiences will be held by groups of max 3 students with the aim of reaching the manual ability and the necessary knowledge to perform a photochemical study.

Assessment methods

Oral examination with 3 questions at least. Possibility for the student to briefly present an application of photochemistry with slides and discussion during the course.

Teaching tools

Power Point slides and blackboard.

spectrophotometers, spectrofluorometers, lasers, single photon counting and transient spectroscopy instrumentations for aboratory experiences.

Office hours

See the website of Paola Ceroni

See the website of Marco Montalti