Course "Photophysics of Organic Molecules"

Led by Dr. Oleg P. Dimitriev (Lashkaryov Institute of Semiconducor Physics, NAS of Ukraine), the course "Photophysics of Organic Molecules" will take place on September 8, 9, 10, 11, 12, and 15, from 10:00 to 11:30 in the CiQUS seminar room.

Course Description in Brief

Subject of the “Photophysics of organic molecules” deals with the issues where the processes of electronic excitation in organic materials and related consequences of the excitation take place. This includes electronic photoexcitation and formation of excitons, exciton dynamics, energy transfer, energy conversion, light emission, excitation quenching, photoinduced generation of charge carriers, etc. Understanding of the above fundamental processes are important in the field of material science, organic electronics and spectroscopy, chemistry of the related compounds, where exciton and charge carriers generation, recombination and transfer are important issues, used in related devices like organic solar cells and organic light-emitting diodes. In addition to fundamental processes listed above, the course provides an overview of experimental methods used for studies of photophysical properties of the materials, including steady-state and time-resolved techniques, and description of operational principles of the related devices. Therefore, the above course represents an essential addition to knowledge necessary for successful work in science and application of organic-based devices where photoexcitation plays key roles.

Course Goals

The major goal of the course is to provide and extend the knowledge of students who deals with organic chemistry, photochemistry and photophysics, and who are interested in a deeper understanding of photophysical processes in organic molecules and their application in the related fields. The course provides understanding of the following issues:

• the origin of color in organic and inorganic world;

• the rules for allowed electronic transitions;

• the nature of exciton and the difference between exciton and atomic excitation;

• the factors influencing molecule’s bandgap;

• the effects of different types of disorder on excitation and light emission behavior of molecules;

• the routes and regimes of excitation energy transfer, including the factors that control the different regimes of exciton energy transfer in multichromophoric systems;

• the mechanisms of energy upconversion, down-conversion, and intersystem crossing.

• the mechanisms and routes of excitation energy fading;

• the practical knowledge of the main photophysical techniques.

Specifically, the course is divided into two parts. The purpose of the first part is to give students basic knowledge of photophysical processes, disclosing the basic routes of molecular excitations, radiative and non-radiative decays, energy transfer and energy conversion. This include explanation of such issues as basic structural elements responsible for light absorption and emission, the difference between exciton and localized excitation, break of symmetry rules for electronic transitions, the difference between incoherent and coherent energy transfer, why infrared dyes have a low quantum yield of photoluminescence, and many other issues.

The second part of the course is more focused on experimental studies, practical application of photophysical phenomena and on advanced analysis of how conformational changes, disorder, and complexity of multichromophoric systems influence photophysical behavior. This includes description of basic experimental techniques, both steady-state and time-resolved, underlying issues affecting collection and interpretation of the results, contribution of different types of molecular disorder to experimental results, variety and complexity of excitons, factors and molecular building blocks influencing bandgap engineering, principles of operation of organic solar cells and light-emitting diodes of the different generations.

Basic course content

• Lecture 1: Generation of color in organic and inorganic world. Natural pigments. Perception of the human eye to colors.
• Lecture 2: Jablonski diagram. Allowed electronic transitions.
• Lecture 3: Exciton concept. Major exciton characteristics: binding energies, exciton radius and delocalization length, coherence length, exciton dynamics, etc.
• Lecture 4: Excitation energy transfer. Factors controlling different regimes of exciton energy transfer in multichromophoric systems. Exciton diffusion and coherent energy transfer. The preferred pathways and control of exciton energy transfer.
• Lecture 5: Energy upconversion and down-conversion. Intersystem crossing.
• Lecture 6: Radiative, non-radiative decay and dissociation of excitons. Energy gap law. Kasha-Vavilov rule. Hot luminescence.

Advanced course content

• Lecture 7: Experimental methods in photophysics.
• Lecture 8: Excitonic zoo: singlet and triplet, coherent and incoherent, localized and delocalized, hot and cold, bright and dark, charge-transfer and vibronic.
• Lecture 9: Effects of structural, energetic and dynamic disorder on excitation and light emission behavior of molecules. Kasha and non-Kasha behavior of electronic transitions in molecular aggregates.
• Lecture 10: Factors influencing molecule’s bandgap. Polyenes versus cyanines. Cyanine limit. Bandgap engineering.
• Lecture 11: Photoinduced driving forces and related effects: photoinduced mechanical motion, photo-driven molecular machines, photoinduced molecular assembling, supramolecular photochemistry, optical trapping, excimers.
• Lecture 12: Application of organic molecules in OLEDs and photovoltaics.

Course literature

• V. May, O. Kühn, Charge and energy transfer dynamics in molecular systems. John Wiley & Sons (2023).
• J. R. Lakowicz, Principles of Fluorescence Spectroscopy. Springer, New York (2006).
• M. Kasha, Characterization of Electronic Transitions in Complex Molecules. Discuss. Faraday Soc. 1950, 9, 14–19.
• N. J. Hestand, F. C. Spano, Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials. Acc. Chem. Res. 2017, 50, 2, 341–350.
• D. L. Andrews, Molecular Photophysics and Spectroscopy, Morgan & Claypool Publishers, 2014, 978-1-627-05288-7
• O. P. Dimitriev, Dynamics of excitons in conjugated molecules and organic semiconductor systems. Chem. Rev. 2022, 122, 8487-8593.
• S. J. Finkelmeyer, M. Presselt. Tuning Optical Properties of Organic Thin Films through Intermolecular Interactions–Fundamentals, Advances and Strategies. Chem. Europ. J. 2025, 31, e202403500.
• J. M. Dos Santos, et al. The golden age of thermally activated delayed fluorescence materials: design and exploitation. Chem. Rev. 2024, 124, 13736-14110.