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Disubstituted Cyclohexanes: cis-trans Isomerism

Disubstituted Cyclohexanes: cis-trans Isomerism

Depending upon the different spatial orientation of the substituents, the disubstituted cycloalkanes exhibit two types of stereoisomers. The cis isomers have the substituents on the same side of the ring, whereas the trans isomers have the substituents on the opposite sides. These stereoisomers exhibit different physical properties and cannot be interconverted without breaking the carbon-carbon bonds.
In cyclohexane, the substituents can occupy different positions generating distinct isomers....

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n +...

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...

Aromatic Hydrocarbon Anions: Structural Overview

Aromatic Hydrocarbon Anions: Structural Overview

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.
Due to the absence of continuous...

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

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The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.

Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

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.
Removing one hydrogen from the intervening CH2 group...

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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Published on: September 18, 2016

Enhanced intersystem crossing in core-twisted aromatics.

Kalaivanan Nagarajan¹, Ajith R Mallia¹, Keerthi Muraleedharan¹

¹School of Chemistry , Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM) , CET Campus, Sreekaryam , Thiruvananthapuram , Kerala 695 016 , India .

Chemical Science

|July 12, 2017

Summary

This summary is machine-generated.

Twisted aromatic molecules 1c and 2d exhibit efficient intersystem crossing due to their nonplanar geometry, leading to enhanced triplet excited states. This makes them promising for organic electronic applications like solar cells.

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Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

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Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

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Area of Science:

Organic Chemistry
Materials Science
Photochemistry

Background:

Efficient intersystem crossing (ISC) is crucial for developing advanced organic electronic materials.
Achieving high triplet state yields in organic molecules often requires specific structural modifications.
Core-twisted aromatics offer a unique platform for exploring photophysical properties.

Purpose of the Study:

To design and synthesize novel twisted aromatic compounds (1c and 2d) for efficient intersystem crossing.
To investigate the relationship between molecular twist and the yield of triplet excited states.
To evaluate the potential of these materials in organic electronics.

Main Methods:

Bottom-up synthesis and X-ray single crystal structure determination of twisted aromatics.
Femtosecond and nanosecond transient absorption spectroscopy to monitor intersystem crossing rates and yields.
Comparison of photophysical properties between twisted and planar aromatic analogs.

Main Results:

Systematically twisted aromatics 1c and 2d were synthesized, revealing nonplanar geometries due to steric congestion.
Increased molecular twist led to enhanced rates and yields of intersystem crossing.
Triplet quantum yields (ΦT) of 10 ± 1% for 1c and 30 ± 2% for 2d were observed, significantly higher than the planar analog 3c (<1%).
Twist-induced spin-orbit coupling via out-of-plane vibrations was identified as the mechanism for triplet state formation.

Conclusions:

Nonplanar geometry in core-twisted aromatics effectively promotes efficient intersystem crossing.
The degree of molecular twist directly correlates with the enhancement in triplet excited state formation.
Imide-functionalized core-twisted aromatics are promising candidates for organic electronics, including solar cells, due to their synthetic accessibility, solubility, and photophysical properties.