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Aromatic Hydrocarbon Anions: Structural Overview

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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
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Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain — the strain caused by the deviation of bond angles from the ideal 109.5° tetrahedral value for sp3 hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer’s theory that...

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
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Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure.

Miriam Peña-Álvarez¹, Samuele Fanetti², Naomi Falsini³

¹School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, UK. mpenaal@ed.ac.uk.

Molecules (Basel, Switzerland)

|September 29, 2019

Summary

We investigated the optical properties of phenylene chains (LPP) and cyclic systems (CPP), including hydrogenated variants (H4[n]CPP). Compression revealed intramolecular π-π interactions in H4[n]CPP, uniquely increasing fluorescence lifetime.

Keywords:

absorption fluorescence linear and cyclic paraphenylene pressure vibrational spectroscopy

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Area of Science:

Materials Science
Organic Chemistry
Physical Chemistry

Background:

The n-paraphenylene family consists of phenylene units linked by C-C bonds.
Optical properties of these systems are sensitive to structural arrangements and external stimuli.
Understanding structure-property relationships is crucial for developing new organic materials.

Purpose of the Study:

To compare the pressure-dependent optical properties of linear phenylene chains (LPP) and cyclic phenylene systems (CPP).
To investigate the influence of hydrogenation and cyclic structures on optical responses under pressure.
To elucidate the mechanisms behind unique optical behavior in hydrogenated cyclic systems.

Main Methods:

Synthesis and characterization of [6]LPP, [12]- and [6]CPP, and H4[6]CPP.
High-pressure studies (up to 25 GPa) using Raman and infrared spectroscopies.
Absorption and fluorescence spectroscopy (one- and two-photon excitation) under varying pressure conditions.

Main Results:

Observed distinct optical property responses to pressure across different phenylene architectures.
Revealed unprecedented pressure-dependent crystallographic data for H4[n]CPP.
Identified intramolecular π-π interactions in H4[n]CPP upon compression, correlating with optical changes.

Conclusions:

Intramolecular π-π interactions in H4[n]CPP under pressure are responsible for their unique optical properties.
Fluorescence lifetime in H4[n]CPP increases significantly with applied pressure.
The study provides insights into the pressure-induced modifications of π-conjugated systems.