Simon Pope Lecture 4

Luminescent Properties of Organic Aromatic Molecules

Introduction

• Continuation of discussion on luminescence.

• Focus on fluorescent characteristics of organic aromatic molecules, particularly the role of substituents.

• Mention of applications in fluorescence bioimaging.

Excimer Formation

• Stacking interactions in solution can lead to excimer formation (excited state dimers).

• Unique fluorescence spectrum signatures arise from excimer formation.

Influence of Substituents

Electron Donating Groups

• Examples: alcohol, ether, and amine substituents which have at least one lone pair of electrons.

• Lone pairs can push into the pi system, affecting fluorescence properties.

Effects on Spectra

• Presence of substituents increases molar absorption.

• Absorption and fluorescence emission spectra shift due to increased conjugation.

• Example: Phenol's lone pair enables resonance structures that extend the pi system.

  • Results in a shift in both absorption wavelength and emission spectrum.

  • Emission is dominated by pi to pi* transition.

Electronic Transitions and Acidity

• Introduction of substituents may lead to n to pi* transitions as well.

• Combination of electron donating and withdrawing groups can result in intramolecular charge transfer.

• Acidity of molecules changes in excited states compared to ground states.

  • Example: Phenol (pKa = 10.6) is more acidic in the excited state due to electron density redistribution affecting O-H bond strength.

  • Conjugate base stability affects acidity; increased conjugation leads to improved stability.

Importance in Chemical Reactions

• Excited state dynamics like proton transfer are crucial in processes such as photosynthesis.

• Understanding substituent effects is essential for elucidating excited state properties.

Carbonyl Groups in Luminescence

• Benzophenone as a model for studying electronic transitions.

  • Absorption can occur via pi to pi* and n to pi* transitions.

Energy Level Diagrams

• Diagram illustrating singlet excited states and transitions (internal conversion, fluorescence, phosphorescence).

• Spin allowed transitions lead to fluorescence, whereas spin forbidden transitions lead to phosphorescence.

• Conditions of solvent polarity affect excited state ordering, which influences fluorescence intensity.

  • Non-polar solvents lead to weak fluorescence, while polar solvents enhance fluorescence.

Heterocyclic Compounds

• Solvent effects also apply to compounds with heteroatoms (e.g., pyridine, quinoline).

• Fluorescence behavior linked to solvent polarity and hydrogen bonding.

Analytical Applications of Fluorescence

• Fluorescence as a diagnostic tool in biological system analysis.

• Common biological fluorophores include amino acids like tryptophan and tyrosine.

  • Key factors: UV absorption and emission in the nanoseconds range.

Green Fluorescent Protein (GFP)

• Discovery of GFP incorporates a spontaneous chromophore formation.

  • Emits at 520-530 nm.

  • Fluorescence microscopy uses GFP for detailed cellular imaging.

Structural Insights into GFP

• GFP's beta sheet polypeptide folds to form a chromophore via nucleophilic attack and bond formation.

• Intramolecular charge transfer gives rise to the fluorescence observed.

Transition to Lanthanides and Inorganic Phosphors

• Introduction of the F block of the periodic table focusing on lanthanides.

• Lanthanides have unique luminescence properties due to electronic transitions in 4f orbitals.

Characteristics of Lanthanides

• Lanthanides have long luminescence lifetimes due to forbidden transitions.

• Their electronic transitions are significantly impacted by their coordination environment.

• Applications in technology: bioassays, protein labeling, cell imaging, security labeling.

Conclusion

• The relationship between substituents, excited states, and solvent interactions is critical for understanding luminescence in various systems.