Simon Pope Lecture 4

Luminescent Properties of Organic Aromatic Molecules

Continuation of discussion on luminescence.
Focus on fluorescent characteristics of organic aromatic molecules, particularly the role of substituents.
Mention of applications in fluorescence bioimaging.

Stacking interactions in solution can lead to excimer formation (excited state dimers).
Unique fluorescence spectrum signatures arise from excimer formation.

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.

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.

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.

Excited state dynamics like proton transfer are crucial in processes such as photosynthesis.
Understanding substituent effects is essential for elucidating excited state properties.

Benzophenone as a model for studying electronic transitions.
- Absorption can occur via pi to pi* and n to pi* transitions.

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.

Solvent effects also apply to compounds with heteroatoms (e.g., pyridine, quinoline).
Fluorescence behavior linked to solvent polarity and hydrogen bonding.

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.

Discovery of GFP incorporates a spontaneous chromophore formation.
- Emits at 520-530 nm.
- Fluorescence microscopy uses GFP for detailed cellular imaging.

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.

Introduction of the F block of the periodic table focusing on lanthanides.
Lanthanides have unique luminescence properties due to electronic transitions in 4f orbitals.

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.

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