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Luminescent Molecular Thermometers
The Featured Molecules this month come from the paper by Uchiyama, Prasanna de Silva, and Iwai exploring the many ways that photophysical properties can be used as temperature probes. They introduce a variety of molecule types, many of them now in our molecule collection. Excited states play a central role in this paper and it provides an opportunity to introduce students to some excited state properties. Students in introductory chemistry frequently see the effect of electron promotion on bond strengths in diatomic molecules, usually in the context of the molecular orbital model. A good starting point for excited state properties of larger molecules is ethene, where the first excited state is nonplanar, with HCCH torsion angles of 90°. In addition, there is significant lengthening of the CC bond. Structure files for the ground and first excited singlet state of ethene are included below.
The computation of excited state geometries of molecules of the size described in the paper is very problematic, but some insight may be gained for a molecule of moderate symmetry such as 9-methylanthracene (structure 1 on page 721). Figures 1 and 2 below show the HOMO and LUMO orbitals of 9-methylanthracene resulting from an ab initio 3-21G* calculation. Figures 3 and 4 show the two singly occupied molecular orbitals of the first excited singlet state, from a similar calculation including configuration interaction. The similarity between the orbitals—the HOMO of the ground state with the lowest of the singly occupied orbitals of the excited state, and the LUMO of the ground state with the higher of the singly occupied orbitals of the excited state—is quite apparent. There is minimal excited state distortion, rather, the system is behaving much as a particle in a box and the difference in energy between the two states arises from the redistribution of electron density in a box of essentially the same size.
Viewing Requirements
In addition to the static images, two fully manipulable versions
(Jmol, MDLChime)
of these molecules appear below.
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Chime (registration required)
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