This is a lecture at the MSc level for chemistry students that are interested in molecular photochemistry. From the nature of the excimer to the orbitals and configuration interaction to interaction of electronic transitions and vibrational transitions.
Learning objectives:

The nature of the excimer is an excitonic interaction with only a slight charge transfer interaction.
The exciton is delocalized over the two pyrene units.

The transitions in pyrene are not charaterized by a simply HOMO LUMO transition. The description of the excited state is more complex. We have to use configuration interaction.

A molecule in its excited state is like a new species, a new entity, with special (sometimes unexpected) properties that are not present in its ground state.

Photochemistry has a direct link with quantum chemistry.

Transitions that are (symmetry) forbidden can still be observed because molecular vibrations will slightly break the forbidden character.

Intermolecular (or interchromophoric) interactions in the excited state can lead to electronic interactions enabling transfer processes.

Two YouTube video's are used in this work:
Chemiscope catches chemistry in the act - Science Nation, National Science Foundation
   • Chemiscope catches chemistry in the a...  

Vibrational Coupling, Berkeley Lab:
   • Vibrational Coupling  

Relevant papers on Pyrene:
Forbidden transition of pyrene and the nature of the excimer:

"Experimental and Theoretical Studies of the Photophysical Properties of 2- and 2,7-Functionalized Pyrene Derivatives"
https://doi.org/10.1021/ja2006862


"Pyrene Molecular Orbital Shuffle—Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals"
https://doi.org/10.1002/chem.201702594


The Nature of Excimer Formation in Crystalline Pyrene Nanoparticles
https://doi.org/10.1021/acs.jpcc.8b03963


I have published Eight papers featuring pyrene, including a JACS (2005) paper.
(See my Google Scholar list, do "Find", pyre).


Background literature on pyrene and its excimer:
Nanosecond Time-Resolved Fluorescence Spectroscopy in the Physical Chemistry Laboratory: Formation of the Pyrene Excimer in Solution
https://pubs.acs.org/doi/abs/10.1021/...

N. J. Turro , V. Ramamurthy and J. C. Scaiano , Principles of Molecular Photochemistry: An Introduction , University Science Books, 2009
https://www.uscibooks.com/turro2.htm

Review on "Pyrene-Based Materials for Organic Electronics", with loads of references,
(also on some photochemistry):
https://doi.org/10.1021/cr100428a


More information on Pyrene as an emissive polarity probe:
https://doi.org/10.1021/j100012a014



publish: 10-01-2022
Pyrene 2 28 12 2021