photophysical and photochemical properties of aromatic compounds pdf

Photophysical And Photochemical Properties Of Aromatic Compounds Pdf

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Studies on the gas-phase two-photon photochemistry of aromatic compounds performed in the past 25 years were summarized. Data on the kinetics and mechanisms of primary two-photon photoexcitation, photodissociation, and photoionization of aromatic molecules were systematized, and the nature of electronically excited states responsible for these processes was determined. Data on the reactivity of transient species and on the kinetics of elementary steps of laser UV photolysis bimolecular charge transfer and heavy particle transfer reactions and termolecular association and cluster ion formation reactions were presented.

Two-Photon Photochemical Reactions of Aromatic Compounds in the Gas Phase

In chemistry , aromaticity is a property of cyclic ring-shaped , planar flat structures with pi bonds in resonance those containing delocalized electrons that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds —they might be cyclic , but only aromatic rings have enhanced stability.

Since the most common aromatic compounds are derivatives of benzene an aromatic hydrocarbon common in petroleum and its distillates , the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist.

An aromatic functional group or other substituent is called an aryl group. The earliest use of the term aromatic was in an article by August Wilhelm Hofmann in Aromaticity as a chemical property bears no general relationship with the olfactory properties of such compounds how they smell , although in , before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants , such as terpenes , had chemical properties that we recognize today are similar to unsaturated petroleum hydrocarbons like benzene.

In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability.

The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another see Theory below. Rather, the molecule exhibits bond lengths in between those of single and double bonds.

The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization. As it is a standard for resonance diagrams, the use of a double-headed arrow indicates that two structures are not distinct entities but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid average of these structures.

Benzene is a regular hexagon —it is planar and all six carbon—carbon bonds have the same length , which is intermediate between that of a single and that of a double bond. In a cyclic molecule with three alternating double bonds, cyclohexatriene, the bond length of the single bond would be 1. However, in a molecule of benzene, the length of each of the bonds is 1.

This model more correctly represents the location of electron density within the aromatic ring. The following diagram shows the positions of these p-orbitals:. Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalized.

This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The first known use of the word "aromatic" as a chemical term—namely, to apply to compounds that contain the phenyl group—occurs in an article by August Wilhelm Hofmann in Also, many of the most odoriferous organic substances known are terpenes , which are not aromatic in the chemical sense.

But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation than many aliphatic compounds , and Hofmann may not have been making a distinction between the two categories. Many of the earliest-known examples of aromatic compounds, such as benzene and toluene, have distinctive pleasant smells.

This property led to the term "aromatic" for this class of compounds, and hence the term "aromaticity" for the eventually discovered electronic property. In the 19th century chemists found it puzzling that benzene could be so unreactive toward addition reactions, given its presumed high degree of unsaturation. The hexagonal structure explains why only one isomer of benzene exists and why disubstituted compounds have three isomers.

Between and , J. Thomson , the discoverer of the electron, proposed three equivalent electrons between each pair of carbon atoms in benzene. An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson , who was apparently the first in [9] to coin the term aromatic sextet as a group of six electrons that resists disruption.

In fact, this concept can be traced further back, via Ernest Crocker in , [10] to Henry Edward Armstrong , who in wrote "the [six] centric affinities act within a cycle … benzene may be represented by a double ring … and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition". Here, Armstrong is describing at least four modern concepts.

Second, he is describing electrophilic aromatic substitution , proceeding third through a Wheland intermediate , in which fourth the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle , thus anticipating Erich Clar 's notation. It is argued that he also anticipated the nature of wave mechanics , since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring much as the distribution of the electric charge in a body is altered by bringing it near to another body.

He was the first to separate the bonding electrons into sigma and pi electrons. Aromaticity of an arbitrary aromatic compound can be measured quantitatively by the nucleus-independent chemical shift NICS computational method [12] and aromaticity percentage [13] methods.

An aromatic or aryl ring contains a set of covalently bound atoms with specific characteristics:. In furan , the oxygen atom is sp 2 hybridized. Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules.

A molecule that can be aromatic will tend to change toward aromaticity, and the added stability changes the chemistry of the molecule. Aromatic compounds undergo electrophilic aromatic substitution and nucleophilic aromatic substitution reactions, but not electrophilic addition reactions as happens with carbon—carbon double bonds.

This is an important way of detecting aromaticity. By the same mechanism, the signals of protons located near the ring axis are shifted upfield. Aromatic molecules are also able to interact with each other in an "edge-to-face" orientation: The slight positive charge of the substituents on the ring atoms of one molecule are attracted to the slight negative charge of the aromatic system on another molecule.

Molecules that could be antiaromatic will tend to change from this electronic or conformation, thereby becoming non-aromatic. Recent studies have determined that cyclobutadiene adopts an asymmetric, rectangular configuration in which single and double bonds indeed alternate, with no resonance; the single bonds are markedly longer than the double bonds, reducing unfavorable p-orbital overlap. This reduction of symmetry lifts the degeneracy of the two formerly non-bonding molecular orbitals, which by Hund's rule forces the two unpaired electrons into a new, weakly bonding orbital and also creates a weakly antibonding orbital.

Hence, cyclobutadiene is non-aromatic; the strain of the asymmetric configuration outweighs the anti-aromatic destabilization that would afflict the symmetric, square configuration. The stability trends of the compounds described here are found to be reversed in the lowest lying triplet and singlet excited states T 1 and S 1 , according to Baird's rule. Aromatic compounds play key roles in the biochemistry of all living things. The four aromatic amino acids histidine , phenylalanine , tryptophan , and tyrosine each serve as one of the 20 basic building-blocks of proteins.

Further, all 5 nucleotides adenine , thymine , cytosine , guanine , and uracil that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines. Chlorophyll also has a similar aromatic system. Aromatic compounds are important in industry. Key aromatic hydrocarbons of commercial interest are benzene , toluene , ortho -xylene and para -xylene.

About 35 million tonnes are produced worldwide every year. They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including styrene , phenol , aniline , polyester and nylon. In heterocyclic aromatics heteroaromatics , one or more of the atoms in the aromatic ring is of an element other than carbon. This can lessen the ring's aromaticity, and thus as in the case of furan increase its reactivity.

Other examples include pyridine , pyrazine , pyrrole , imidazole , pyrazole , oxazole , thiophene , and their benzannulated analogs benzimidazole , for example. Additionally, the nitrogen atom is also sp 2 -hybridized and has one electron in a p-orbital, which adds up to 6 p-electrons, thus making pyridine aromatic. Pyrrole and imidazole are both five membered aromatic rings that contain heteroatoms. Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms see also simple aromatic rings.

Examples are naphthalene , anthracene , and phenanthrene. In fused aromatics, not all carbon—carbon bonds are necessarily equivalent, as the electrons are not delocalized over the entire molecule. The aromaticity of these molecules can be explained using their orbital picture. Like benzene and other monocyclic aromatic molecules, polycyclics have a cyclic conjugated pi system with p-orbital overlap above and below the plane of the ring. Many chemical compounds are aromatic rings with other functional groups attached.

Aromatic molecules need not be neutral molecules. For example, the cyclopentadienyl anion and the cycloheptatrienylium cation are both considered to be aromatic ions, and the azulene molecule can be approximated as a combination of both. In order to convert the atom from sp 3 to sp 2 , a carbocation , carbanion , or carbon radical must be formed.

Like neutral aromatic compounds, these compounds are stable and form easily. The cyclopentadienyl anion is formed very easily and thus 1,3-cyclopentadiene is a very acidic hydrocarbon with a p K a of Aromaticity also occurs in compounds that are not carbocyclic or heterocyclic; inorganic six-membered-ring compounds analogous to benzene have been synthesized. For example, borazine is a six-membered ring composed of alternating boron and nitrogen atoms, each with one hydrogen attached.

Homoaromaticity is the state of systems where conjugation is interrupted by a single sp 3 hybridized carbon atom. Y-aromaticity is used to describe a Y-shaped, planar flat molecule with resonance bonds. The concept was developed to explain the extraordinary stability and high basicity of the guanidinium cation.

The concept is controversial and some authors emphasize different effects. It is often invoked in cluster chemistry and is closely related to Wade's Rule. Spherical aromaticity is aromaticity that occurs in fullerenes. In , Andreas Hirsch and coworkers in Erlangen , Germany , formulated a rule to determine when a fullerene would be aromatic. This follows from the fact that an aromatic fullerene must have full icosahedral or other appropriate symmetry, so the molecular orbitals must be entirely filled.

From Wikipedia, the free encyclopedia. Phenomenon providing chemical stability in resonating hybrids of cyclic organic compounds. For meanings related to odor, see aroma compound. Main article: Aromatic compound. This section needs expansion. You can help by adding to it. April This scientific article needs additional citations to secondary or tertiary sources such as review articles, monographs, or textbooks.

Please add such references to provide context and establish the relevance of any primary research articles cited. Unsourced or poorly sourced material may be challenged and removed.

Aromaticity

China E-mail: bhkenlo cityu. Although the interactions of phosphorescent transition metal complexes with biological molecules such as DNA and proteins have been extensively investigated, there is a rapidly emerging area of research that focuses specifically on the cellular uptake of phosphorescent transition metal complexes and their applications as cellular reagents such as biosensors for ions and small molecules, and bioimaging reagents for organelles and tissues. This interest stems from the rich phosphorescence properties, and in some cases the controllable photochemical reactivity of transition metal complexes, which complement fluorescent organic dyes and luminescent lanthanide chelates. In this review article, we briefly introduce the fundamental photophysical properties of phosphorescent transition metal complexes and explain the reasons why the complexes are attractive candidates as cellular reagents. Very recent applications of these complexes in biosensing and imaging in an intracellular environment using different strategies are also summarized. Additionally, phosphorescent transition metal complexes with potential therapeutic applications are highlighted in this article. If you are not the author of this article and you wish to reproduce material from it in a third party non-RSC publication you must formally request permission using Copyright Clearance Center.

These metrics are regularly updated to reflect usage leading up to the last few days. Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts. The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric. Find more information on the Altmetric Attention Score and how the score is calculated. Cite this: J.

Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. In , Baird published rules describing aromaticity and antiaromaticity in the lowest triplet excited states of annulenes. Aromaticity is a key concept in organic chemistry, and can be used to explain a range of properties and processes. Forty years ago, Colin Baird published rules, developed using perturbation molecular orbital theory, that apply to aromaticity and antiaromaticity in the lowest triplet excited state 1.

Aromaticity

Various single-ring aromatic compounds in water sources are of great concern due to its hazardous impact on the environment and human health. The fluorescence excitation-emission matrix EEMs spectrophotometry is a useful method to identify organic pollutants in water. This study provides a detailed insight into the fluorescence properties of the 14 selected toxic single-ring aromatic compounds by experimental and theoretical analysis.

Two-Photon Photochemical Reactions of Aromatic Compounds in the Gas Phase

Exciting excited-state aromaticity

Acetylchrysenes, as the key precursors to ChBs, were photochemically prepared from the corresponding acetylphenyl naphthylethenes by means of a modified photocyclization method. The absorption and emission spectra of the BF 2 DKs were obtained in chloroform and acetonitrile, and the quantum yields and lifetimes of the fluorescence were determined. Excimeric fluorescence from PyB was absent even in highly concentrated solution.

In chemistry , aromaticity is a property of cyclic ring-shaped , planar flat structures with pi bonds in resonance those containing delocalized electrons that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds —they might be cyclic , but only aromatic rings have enhanced stability. Since the most common aromatic compounds are derivatives of benzene an aromatic hydrocarbon common in petroleum and its distillates , the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined.


Download PDF. Photophysical and Photochemical Properties of Aromatic Compounds is the first book to collect and classify all available quantitative data on the.


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Photophysical and Photochemical Properties of Aromatic Compounds. Jacob Malkin. Photophysical and Photochemical Properties of Aromatic Compounds is the first book to collect and classify all available quantitative data on the photochemistry and luminescence of aromatic compounds. Compounds are classified by both spectral-luminescent e. In addition, all of the quantum yields available have been collected. The variety of photochemical reactions of aromatics is examined based on eight types of elementary monomolecular and bimolecular photochemical processes.

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Two-Photon Photochemical Reactions of Aromatic Compounds in the Gas Phase
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1 Comments

  1. Arthur M.

    Photophysical and Photochemical Properties of Aromatic Compounds is the first book to collect and classify all available quantitative data on the photochemistry.

    19.04.2021 at 11:12 Reply

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