An Appreciation for Dmitri Mendeleev

Every so often in history, there comes a great mind that contributes a great amount of knowledge to the scientific community and the world, so much so that their names become impossible not to mention when speaking of a concept or invention created by them. Names such as Isaac Newton, Galileo Galilei, Albert Einstein, Niels Bohr and Thomas Edison come to mind. 200 years ago, another man was added to the list for his preparation of a periodic table of the elements, which is now a crucial foundation for chemistry, his name was Dmitri Mendeleev.

On February 17^th, 1869, on what is dubbed as the birth of the modern periodic table, Mendeleev completed his chart containing all 63 known elements in order of increasing atomic weight.¹ Thanks to his creative genius, chemists now have a single document that contains enough information that explains almost all the properties of any element ever known to humankind.

From early childhood to death, Mendeleev dedicated his life to chemistry and science. Never did he turn his back to science. Even when he was not on the verge of great discoveries, Mendeleev was using his knowledge and expertise to further Russian industry and economy through scientific means. The Periodic Table of the Elements, though, was by far his greatest contribution to chemistry; however he could not achieve this without the help from previous chemists and alchemists. Even with the previous contributions made to such a remarkable achievement, Mendeleev is considered the father of the modern Periodic Table.

Chapter 02: Early Attempts at Periodic Table

Prior to Mendeleev, there were many attempts to organize the existing elements into a working order that would accurately portray their properties. Scientists like Johann Wolfgang Doberiener, Alexandre Beguyer de Chancourtois and John Newlands all found similarities between a few elements, and came close to organizing the elements accurately, but all failed.

Dobereiner developed the Law of Triads in 1829, which stated that the middle element in a triad (three consecutive elements in a column) had an atomic weight that was the average of the other two elements.² Dobereiner’s work encouraged another chemist, Peter Kremers of Cologne, Germany, to study the law of triads. Kremers suggested some elements could belong to two triads, if the second triad was perpendicular to the first.³

In 1862, de Chancourtois published all the known elements, and displayed them in a helical graph wrapped around a cylinder, where elements with similar properties would be found on the same vertical line. His work was largely ignored until Mendeleev published his table. ⁴

John Newlands, an English chemist, arranged the 62 known elements in order of increasing atomic weights, and noticed that after every 8 elements, similar properties reoccurred. In 1863, Newlands proposed his Law of Octaves, which stated that “elements exhibit similar behaviours to the eighth element following it in the table”. ⁵

All these chemists had the right ideas but were not complete in their ideas. Only in 1869, did Dmitri Mendeleev get it right, when he proposed arranging the elements by atomic weights and properties.

Growing Up & Educational Accomplishments

Dmitri Ivanovich Mendeleev was born and raised in Tobolsk, Russia, as the youngest child in a family with 17 children. Dmitri’s father, Ivan Pavlovich Mendeleev, worked as the Director of the Tobolsk High School, where Dmitri himself attended and graduated from in 1850. His mother, Mariya Dmitrievna Mendeleev operated a glass factory owned by his family, during his childhood.⁶

Mendeleev was a very well educated person, who had a tremendous amount of interest in chemistry, an interest that was nurtured by his mother. Throughout his childhood, Mendeleev’s mother offered him special favours. From the beginning, his mother began saving money, little by little, so that he could one day be able to go to university. She also allowed him to go with her to work and observe the chemist who worked at the factory, and learn about glass making from him. This special attention by his mother, combined with his early influences towards chemistry only increased his love for learning and science.

Throughout his educational career, Mendeleev received many awards and had many achievements, including receiving the gold medal for natural sciences in 1855 from the University of St. Petersburg’s Physics and Mathematical Department.

Mendeleev not only studied and researched chemistry, he also wrote books about it. Among his more notable books (and also his first) was Organic Chemisrty, published in 1861, when he was only 27. Another famous book of Mendeleev’s was Fundamentals of Chemistry, published in 1868, which explained his basis for the periodic law.⁷ His books eventually gained enough recognition to become standard chemistry textbooks in Russian schools, and even to this day, many of the ideas and concepts discussed by Mendeleev in these books continue to find a place in Russian schools.⁸

Between 1859 and 1861, Mendeleev worked with R. W. Bunsen at Heidelberg University. Bunsen later went on to invent the Bunsen burner.⁹

Mendeleev was a very well revered chemist around the world. During his lifetime, he received more than 130 honorary degrees and titles from Russian and foreign academies, learned societies and educational establishments. Some of his more prestigious honorary degrees were received from the University of Oxford and the University of Cambridge (in the United Kingdom) in 1904. In 1905, Mendeleev received among the highest awards possible to a chemist; the Copley Award, given to him by the Royal Society, in London, England.¹⁰ The one award, which would have cemented him amongst the greatest chemists and scientists of all-time, was the Nobel Prize, which he never won, but came within one vote of winning in 1906.¹¹

Creation of Modern Periodic Table

While teaching as a Professor of Chemistry at the University of St. Petersburg between 1857 and 1890, Mendeleev felt the need to bring the same kind of order to inorganic chemistry that organic chemistry was gaining through the Theory of Molecular Structure.¹² Like many other chemists, Mendeleev believed the best way to organize the existing elements was to place them in increasing order according to their atomic weight.¹³ Mendeleev based his periodic table on the ‘four aspects of matter’, which were isomorphism, specific volumes of similar compounds or elements, composition of compound salts, and relations among atomic weights.¹⁴ Naturally, the easiest of the ‘four aspects of matter’ on which to base his table on were the relations among atomic weight between elements.

Once Mendeleev completed his table, he learned that the elements did not have to be artificially grouped according to familiar properties, because they naturally formed their own groups. Mendeleev left some blank spaces on his original periodic table, because he believed that there were elements that belonged in the empty spaces that had not been discovered yet.¹⁵ Mendeleev, however, did accurately predict the density, radii, combining ratios with oxygen as well as some other properties of some unknown elements because of the properties of the elements surrounding those blank spaces. His predictions were later confirmed by the discovery of these elements.¹⁶ His three most accurate predictions turned out to be those of germanium, gallium, and scandium, which had almost the exact quantitative properties predicted by Mendeleev.

The official discovery date of the periodic table is dated as February 17^th, 1869. Mendeleev did not even call the table ‘The Periodic Table’, but rather, he named it ‘practical experience with a system of elements based on their atomic weight and chemical similarity’.¹⁷ When presenting his concept for his version of the Periodic Table to the Russian Chemical Society, Mendeleev took some bold points to his colleagues, like, “the magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body”.¹⁸ It was statements like these that his colleagues at first did not accept. Once his predictions came true, though, more and more chemists and scientists began to accept his model of organized elements.

How Periodic Table Became So Successful

Even with the changes that Chemistry has gone through over the course of the past 100 years, i.e., Quantum Theory, the basic structure and concept of the periodic table has remained the same and it is safe to say that the periodic table will continue to remain relatively the same over the next 100 years.

Prior to Mendeleev, chemistry and alchemy went hundreds, if not thousands of years with the knowledge of only 63 elements, but when the periodic table was discovered, the world was enlightened to another 55 elements over the course of the next 150 years.

The Periodic Table, from its inception into the scientific world, has become a vital tool for chemists and budding-chemists alike. Without it, students learning chemistry would be forced to learn all 118 elements and all their relevant properties. Memorizing the periodic table and all its properties would become a course all on its own. However, thanks to the genius of Dmitri Mendeleev, that is not necessary, anyone who wants to know almost anything about any element can simply look to the periodic table for the desired information.

Winning over the scientific community wasn’t easy for Mendeleev when he first introduced his proposal for an organization of the existing elements, but when he was proved right in his predictions of gallium, germanium and scandium, not only did he cause the scientific community to reconsider their scepticism, but he discovered something that would be the basis of future study for many years to come.

Its Modifications since Mendeleev

Not every scientist is totally correct in any radical new concept introduced. There will always be others who come along and study a previous discovery, only to realize that there was either some sort of mistake, or something overlooked, or there may just be a better way of doing something. This was no different for Mendeleev. In 1894, William Ramsay of University College London, discovered argon (Ar), and over the next few years, he also discovered Helium (He), Krypton (Kr), Neon (Ne) and Xenon (Xe).¹⁹ Ramsay had essentially discovered what are now known as the noble gases. They had received the name “noble” because they seemed to stand out from almost all the other elements, rarely, if ever, interacting with any of them. It was for this reason that they became difficult for chemists to add to the periodic table. Not only had they not been predicted by Mendeleev, but they didn’t even have a column in which to go. Only after 6 years of research and study, did chemists and physicists finally find a place for them on the periodic table, when they created a whole new column for them at the end. They were introduced between the halogens and the alkali metals.²⁰

Another controversy surrounding the periodic table was when Dutch Physicist Anton van den Broek suggested that the key to correctly ordering the periodic table was not according to their atomic weight, but according to their nuclear charge.²¹ Another physicist, Henry Moseley, put the theory of van den Broek into play by photographing the x-ray spectrum of 12 elements. He discovered that the frequencies of features called K-lines in the spectrum of each element were directly proportional to the square of the integers representing the position of each successive element in the table. As a result of his discovery, he was able to determine that “there is in the atom a fundamental quantity, which increases by regular steps as we pass from eon element to the next.” This became known as atomic number in 1920 by Ernest Rutherford, and is now known as the number of protons in the nucleus. Moseley’s work further allowed for chemists to predict how many elements were yet to be discovered and what their properties might be.²²

The last major changes made to the periodic table occurred around 1940, when chemist, Glenn Seaborg discovered the element, plutonium. This led him to the discovery of more trans-uranium elements (elements 94 through 102) and forced him to reconfigure the table to include the lanthanide and actinide series.²³

Seaborg’s modifications were among the last major changes to the periodic table. Since then, it has remained relatively the same.

Description of Current Periodic Table

The periodic table as it looks today, contains 118 elements, and is organized into the different periods and groups, and was essentially created by Dmitri Mendeleev. It is also ordered in accordance to atomic numbers, where Hydrogen is the first element because of its one proton in the nucleus, and Ununoctium as the last element with 118 protons in the nucleus.

The periodic table also contains almost all quantitative properties of the elements, including such important ones as atomic number, mass number, boiling point, first ionization potential, electro negativity, oxidation states, electron configuration and much more.

Other Important Contributions by Mendeleev

Dmitri Mendeleev was not a one dimensional man. He was not only interested in applying his knowledge to the vast, wonderful world of chemistry; he also showed much interest and knowledge in other subjects, including writing, physics, agriculture, aeronautics, chemical technology, economics, meteorology, and national education.²⁴

Among Mendeleev’s contributions to other subjects, his most notable ones outside of chemistry came in physics. Mendeleev is much responsible for the theory behind the solvation of ions. He also indicated that there is the existence of an ‘absolute boiling point’, later named critical temperature. Mendeleev is also responsible for the equation of state for one mole of an ideal gas.²⁵

In 1890, Mendeleev invented new smokeless gunpowder called ‘pyrocollodium’ and in 1892, he began manufacturing the product.²⁶

His scientific investigations were closely linked to the economic development of Russia. As a prominent public figure at the time, he constantly spoke out for the industrial development and economic independence of Russia.²⁷

In 1865, Mendeleev managed a farm where he used his chemical knowledge to increase his crop yield, something which he then tried to apply to the Russian agriculture industry to help with their food production.

In 1867, Mendeleev was sent to Paris for the Paris Exposition. While in Paris, he studied the French chemical industry. The knowledge he gained from their industry allowed him to apply similar knowledge to Russia’s industry, and eventually led to an overall improvement in their soda industry.²⁸

Mendeleev had so many different influences on Russian society, and on the world, that it is almost impossible to track them all.

Concluding Thoughts

After living in St. Petersburg at the turn of the century, Mendeleev caught a bad case of pneumonia and never recovered. He died on January 07^th, 1907.²⁹

Mendeleev truly was a man of science. His contributions in science reached far and wide. Without a doubt, Dmitri Mendeleev is one of the greatest, if not the greatest contributor to the world of Chemistry. Without his accomplishments, the world would be a very different place. It is in part to him, that any technology exists today. Up to 1869, there were only 63 known elements, and after his discovery of organizing the known elements by increasing atomic weight, we had reached 118 by 1940. With his discovery of the Periodic Table, Mendeleev truly has cemented himself as one of the greatest scientists ever, putting him in an exclusive list, which would include Albert Einstein, Thomas Edison, Niels Bohr, Isaac Newton and Galileo Galilei.

Markush Structures

When you're sitting there with your chemistry book open and are staring at the examples of reactions, ever wonder why they use that generic form for the molecule? You know, the one that gives you an R-group instead of something specific. Well, those are called Markush structures; they have quite a history, dating back to the 1920s.

First, a definition for reference: A Markush structure is a structure that denotes a substance or substituent, agent, reactant, or other material that is described as being from a group consisting of certain specified materials. Specified structures can be an element, a chemical structure, a functional group, a class of chemical structures (e.g., alkyls, aryls, etc), or a class of functional groups (e.g., esters, alcohols, carboxylic acids, etc).

In 1924, Eugene A. Markush filed a patent for the preparation of a powder dye with its base compound pyrazolone, which could be used to dye wool and silk in an acid bath. The structure groups he specified in his now famous patent claim, however, were generalized and used to make claim to many variations of the pyralozone structure. He designed his patent claim in such a way so he could avoid having to file an individual patent for every variation he made.

As a result for filing such a patent the United States Patent and Trademark Office challenged it for not being specific enough. For being too vague of a structure. After much appeals and court cases by different parties, the issue had gone to court in 1935, 1938, 1947, 1970. Finally, in 1978, in the case of Horst Hornisch vs. the United States Patent and Trademark Office in the U.S. Court of Customs and Patent Appeals, the original rejection was reversed and Markush structures became patentable.

Markush claims are now legal and patentable.

A Markush Claim is a form of claim that allows claiming of members of a finite group by means of a phrase like “...a member of the class consisting of...” followed by a list of members of the group linked by the word “and”. The members of a Markush group must have at least one property that is mainly responsible for their membership in the group.

The claiming of a Markush Group means reciting “selected from the group consisting of...” This statement creates a limited type of generic claim. To be permitted, the members of the designated group must have at least one property in common which is primarily responsible for their function in the claimed relationsihp.

An example is if we have:

The values of filing Markush structures in patents is that a number of different compounds can be desribed in a single claim. There is no need to file a patent for every compound that you want to lay claim to. Similar compounds are covered under a single patent claim.

An example of how to determine the number of possible structures you can lay claim to is as follows:

Suppose we have 4 compounds:

We want to make the same structure from the Markush structure, so we lay claim to ___ where, based on the 4 above structures, Rn represent:

R-Group	Group	# of Groups
R1	H, F, Cl	3
R2	H, Cl	2
R3	H, Cl, F	3
R4	H	1
R5	H	1
R6	H, F	2

We multiply all the types of compounds: 3x2x3x1x1x2 = 36.

We therefore have 36 different compounds we have laid claim to. The 4 original compounds, and 32 different combinations consisting of the listed groups. All can be filed as patents (assuming all laid claim to are neither naturally occuring or already patented).

So the next time you see a molecule with an R-group attached, you will know that that compound is a Markush structure, and that a lot of patent claims, and legal court battles have occurred to make that little group important contributor to chemical and pharmaceutical discovery.

PPV (Polyphenylene Vinylene) - Is This the LED Technology of Tomorrow?

Light-emission, resultant from the excitation of electrons in a conjugated polymer, is typically referred to as electroluminescence (1). Discovered first by the Cambridge Research Group, and led by physicist Richard H. Friend at the University of Cambridge; the group was initially trying to create an organic semiconductor out of the polymer, Poly (para-phenylene vinylene) (PPV) (2).1 PPV was already understood to have electrical properties, not unlike semiconductors, because of its conjugated structure (2). It was also understood, at the time, to have a strong material resilient enough to use as a base for thin-filmed devices (2). What the Cambridge Research Group, nor anyone else, had foreseen was, when a small voltage was applied across a purified version of the polymeric material, a bright light was emitted (2). Ultimately, the group went on to demonstrate the practicality of the active emissive properties of PPV in polymeric light-emitting diodes (5), and even started Cambridge Display Technology (CDT), a group that licenses the use of polymeric light-emitting diodes (PLED) to companies around the world (9).
Since the discovery of PPV as an electroluminescent polymer, physicists, chemists, and material scientists, have come together to formulate a wide array of light-emitting polymers (1). Recent research has led to enough enhancements in efficiency and reproducibility for the field to allow for commercialization of LED-based products (1). Largely due to their easy processing, there is an effort in the display industry to create cheap, flexible displays with full color, capable of being used in computer monitors and televisions, among other things (5). CDT, Richard H. Friend’s company, has stated that they intend to enter the marketplace for cathode-ray tubes, light-emitting diodes, fluorescent lights, and Liquid Crystal Display (LCD) technology. As a luminescent technology, it is expected to surpass LCD as the standard form of displayed luminescence. Until now, however, Organic Light-Emitting Diode (OLED) technology, based on polymerized PPV, has emerged as a common replacement for LCD screens in smaller items, like MP3 players.
The structure of PPV and the components that make up its monomer are conducive to its electroluminating abilities. The monomer consists of two compounds, a phenylene ring and a vinyl ethylene. The phenylene ring, a di-substituted benzene ring, is bonded in the para-position (C1 and C4), to the vinyl ethylene (or vinylene) and the adjacent monomer. The vinylene molecule, on the other hand, is bonded to the phenylene ring in the monomer, and sandwiched between two phenylene rings when the monomers are polymerized (7).
The structure of the overall polymer, poly (p-phenylene vinylene), is such that when the monomers are adjoined, the vinylene is connected to its neighbors in a staggered conformation at an angle of 123˚ (7). This affects the planarity of the polymer. The deviation in planarity can affect the excitation of π-electrons in the conjugated regions of the monomer. Extended π-conjugation is required for the best excitation of electrons, but the non-planarity of the polymer diminishes conjugation and lowers spectral emission (7).
The angles between the two compounds are caused by steric hindrance between the vinylene compound and the hydrogens on the phenylene ring (7). The intermolecular steric repulsion forces the non-planer conformation of the polymer to form (7).
Despite the non-planarity caused by the angles, PPV has morphology such that high levels of crystallinity are capable of formation (7).
The synthesis of PPV is important for its influence on electroluminescent properties. The goal becomes to attain as pure a polymer as possible. Increased levels of impurity can alter the luminescent properties of the polymer, and shift the spectrum of emitted visible light away from that emitted by the pure compound. Synthesis methods that cause the highest yield of pure polymer thus become an important factor in formulating electroluminescent polymers. It is also important for the likelihood of adding derivatives to alter the properties of the polymer.
There are several methods for the synthesis of PPV. The first attempt made at synthesizing PPV was a direct chemical polymerization (9). This yielded insoluble crystals that limited the range of applications of the polymer (9). As of 2003, the most popular method for the synthesis of PPV was a base-induced anionic polymerization of sulfonium salts, like p-xylene-bis-(diethylsulfonium chloride) (9). In this method, the polymer is procured by adjoining the monomers via thermal elimination, to remove the diethyl sulfide moieties at the ends of the compound, and the HCl by-product (9). 2
After the formation of the monomer, derivatives can be grafted on the positions ortho- to the 1,4-disubstitutions on the phenylene to alter the electrochemical properties of the polymer. These grafts, depending on what the deposited derivative compound is, can change properties of the lead compound, like voltage intake before degradation, intensity of luminescence, lifetime, and spectral emission, among others (4).
It appears to be difficult to synthesize large amounts of the monomer first, and then polymerize them in a large solution. This is likely because of the conjugation that exists in the vinylene precursor. It would be highly reactive, and end up causing the polymer to not be a homologous chain, but instead a copolymer consisting of separate vinylene and phenylene monomers. It would also prove much more difficult to form an alternating polymer, required for the most efficient and favorable emission. Rather, the compound would likely form a random (or statistical) polymer, resulting in less predictable and less controlled results. The current synthetic methods appear to favor methods that require the elimination of easily removable side moieties and leave units of the required para-phenylene vinylene monomer intact. This promotes more controllable intramolecular reactivity of the units, and thus, more predictable results.
The current method is chemical synthesis via an elimination of diethyl sulfide moiety from p-xylene-bis-(diethylsulfonium chloride). This is among the easiest methods of synthesis, and hence, the most popular. Synthetically, this process is preferred because it forms the lower energy trans-conformations of the chain. It also forms under relatively low thermal conditions of 246ºc for 6 hours (9). Perhaps the most favorable reason for its usage though, lies in the simple fact that this precursor to PPV can be easily handled in a solution (2). According to Richard J. Artley, a consultant with the Generics Group, an investor in Cambridge Display Technologies, it can be “shipped around the world like paint, put it on a substrate, then just heat it up to 200 degrees Celsius.” (2). Because of the ability to put PPV on a surface like one would paint, it is capable of being put on a wide range of surface-types, and as will be explained later, make it a versatile substance when it comes to application.
Compared to electrochemical synthetic measures, where the synthesis and elimination of side moieties are dependent on oxidation-reduction methods, the formation of PPV via chemical synthesis contains higher thermal stability (9). Thermal stability becomes an important property for the application of PPV to electronics, since the conduction that causes luminescence generates heat (9). A higher thermal stability will result in a greater range of colors as well as an ability to heat to higher temperatures without risking degradation, allowing for a longer lifetime for the electronic it is being used in.
From the earliest discoveries of PPV as an electroluminescent polymer, it had been understood that this polymer, and other polymers like it, could be placed between a transparent cathode and anode, and used to transmit light of various color (1). These early applications though, proved to be inefficient at converting injected charges into usable light (1). Naturally, developments continued and derivatives of PPV were made, like PPV/SiO2. New electroluminescent polymers were also synthesized, like poly (9,9dioctylfluorene), and have now led to enough stability and efficiency to allow for broader commercialization of PLED-based products (1).
In its applications, the polymer film is placed between a transparent anode and cathode. The anode is usually made out of indium tin oxide. The cathode, on the other hand, is usually made out of Calcium or Barium (1). The principle involved in the actual luminescence is the sandwiching of the polymer film between the diode, and upon injection of a charge, causing electrons from the Highest Occupied Molecular Orbital (HOMO) to become excited and jump to the Lowest Unoccupied Molecular Orbital (LUMO) (7). The HOMO and LUMO are separated by an energy gap (7). The energy required to cause an electron to jump from one molecular orbital level to the other falls within the range of the visible energy spectrum (7). This means upon excitation it will emit wavelengths somewhere between 400-700nm, allowing for visible light to be produced (7). This is the essence of how electroluminescence works in PLEDs.
Unmodified, pure PPV emits a greenish-yellow light upon injection of charge (2). It was thought that, until a “good” blue was formed by the alteration of PPV, only two-color displays could be created. However, after much intensive experimentation, the discoverer of electroluminescence, Richard H. Friend, and his fellow Cambridge colleague, Andrew B. Holmes, created derivatives of PPV capable of spanning the entire visible spectrum. PPV was also found to be capable of shining up to 10x brighter than the average television screen (1). With brighter luminescence, and the ability to give off any wavelength in the visible spectrum, PLED technology is quickly becoming a viable replacement for other, less efficient technologies, like LCD and Plasma. PLED applications are fast including other areas too, like emergency lights, and signs, since they can shine brighter and can give off more than two colors (1).
The key to widespread commercialization though, is the increase in light-output-to-current-input (efficiency) of the technology. Currently, the goal is to increase the efficiency of electroluminescent polymers as much as possible. So far, four major steps in device operation from which improvement can occur have been looked at in detail, and are considered to be key areas for optimal efficiency; charge injection from the electrodes into the polymer, charge transport, charge recombination, and decay of the excited state (1).
Improvement in efficiency can come from simple precautions, like lowering exposure of PPV to oxygen, especially near the cathode, since photooxidation reactions can cause the reduction of efficiency by up to ~50% (3). The cathode, being made of a compound, like calcium (Ca2+), can be grafted on to a polar locale on the polymer, and work similarly, in principle, to deposition of Ca2+ in PPV. During grafting or deposition, Ca2+ works by diffusing into the PPV and becoming reversibly encapsulated as a single ion. A higher concentration of oxygen near the cathode and near the polymer works to decrease the efficiency of the product by decreasing the ion diffusion coefficient. The reduction in value of this coefficient, by the presence of oxygen, decreases the distance between the potential minima and increases the average binding energy between PPV and Ca2+ (3). Grafting calcium onto the polymer thus decreases the ability of oxygen to interject itself between the two atoms via oxidation reaction. This increases the ability of PPV, or whatever other electroluminescent polymer is between the diodes, to emit light at, or near, its optimal efficiency (3).
While PPV is not the most efficient electroluminescent polymer in its pure form, it is worth noting that electroluminescence of organic polymers were initially expected to reach an efficiency of 10%, according to reports from the 1990s (2). Inorganic polymers have already been concluded to have an efficiency of 1% (2). This is indicative of how important the structure and conductivity of the polymer is in relation to the electroluminescence (2). Perhaps this conductive potential is the very reason why Richard H. Friend and the Cambridge Research Group were using PPV as their test subject in their studies on semiconducting properties or organic compounds.
Derivatives of compounds are another way of increasing the efficiency of electroluminescent polymers. Different derivative groups alter the properties of the polymer and increase efficiency to much higher levels than pure PPV. While it would take too long to go through all the derivatives or even many derivatives and what makes them so efficient, it should just be noted quickly, as a point of comparison, that derivatives like Poly (2,3-dibutoxy-1,4-phenylene vinylene) and 2,5-disilylated PPV have efficiencies of 40% and 60%. This is a significant difference, compared to pure PPV, and indicative of just how well they conduct charge through the interdiode polymeric film and how easily they excite their electrons from the HOMO to the LUMO (4).
Changing the way the world will watch television in the next decade, electroluminescence has currently had its largest effects on screen technology. OLED and PLED are allowing for thinner, brighter, more flexible screens to be made for televisions, digital cameras, cellular phones, computer monitors, MP3 players and billboards, among other things (6). The thin polymeric film allows for brighter displays with the same array of colors as current LCD-based technologies, but with efficiency anywhere from 5x-10x greater (8). This means that they use less electricity, and give off better quality picture. This is important, especially if put in a larger societal context, where energy consumption is only increasing on an annual basis. The employment of electroluminescent technologies will make energy consumption by monitors and televisions lower. This same technology will also allow for longer battery-life in cellular phones, MP3 players and digital cameras that employ display screens (2).
By 1993, the lifetime of PPV had already surpassed 1000 hours, and was expected to surpass 100,000 hours (11 years and 5 months) soon thereafter (2). Similar materials were measured to be stable for up to 10,000 hours (2). Current data on the lifetime of PPV or its derivatives, or its possible replacements were unavailable; however, if these were the numbers for 1993, then it is reasonable to assume that these goals have been surpassed in the 15 years since the predictions were made.
The future of electroluminescent polymer technology lies in the usage of these thin films in PLEDs for screen technology. This appears to be, not only its most practical application, but also its most commercially viable. Because of the flexibility of the material, the future entails monitors and television that are capable of being rolled up like carpets or projection screens, as well as portable battlefield displays for real-time updates, and plastic laminates that could replace lighting fixtures (6).
Looking further into the future, the possibilities are endless for where these developments can lead to. So long as the electrodes are made to be transparent, PPV, or any other electroluminescent polymer can be used to create transparent windows that can be doubled as screens with the flick of a switch (6). Alterations to the windows can also allow them to function as touch screens. This could bring to reality the science fiction concept of a transparent touch-screen, like the one seen in the movie Minority Report. These same screens could very well end up having better color and resolution than the monitors and screens presently in use.
As of right now, most companies are still focusing on the usage of this technology on smaller screens (6). Once they are able to put together a model capable of transferring the same resolution and color display to larger screens, it should not be long before LCD, cathode-ray tubes, and other present technologies become phased out.
It is quite apparent that the future of screen technology and all aspects of television-viewing and computer-using will be dependent on the simple notion of polymeric electroluminescence. It is a marvel that a long chain of a single monomer can be causal in the development of entirely new technologies, and lead to a sub-branch of chemistry that had previously been untouched and largely unforeseen. The fact that these developments are in conjunction with the on-going developments in nanotechnology indicate that there is a great deal of potential in compounds like PPV for widespread commercial application. Just the examples of television and the impact that would have on everyday life would appear to be enough to designate the discovery of electroluminescence a grand scientific discovery. In 1993, when lifetimes of PPV and its electroluminescence was much less than it is today, and its emission of color in the visible spectrum was severely limited, the estimate of commercial profitability was projected at $30 billion (2). With the more recent advances that have lead to it being a viable replacement for current technologies, it is near unfathomable as to just how commercially successful electroluminescent polymers might become.

A Little Info on Lead Arsenate

Currently, in your local hardware store, or even in your local Wal-Mart or K-Mart, there are many products under the term “pesticide”. Among those products available are fungicides, herbicides, and insecticides. All of these products, at one time or another, in North America, have contained raw materials within their chemical makeup that has been hazardous to human health and/or environmentally unfriendly.

Lead Arsenate (AsHO₄Pb) is an inorganic chemical compound composed of the elements Arsenic (Ar), Hydrogen (H), Oxygen (O) and Lead (Pb). Lead Arsenate can be used as a raw material for many different kinds of pesticides, including herbicide, insecticide and as growth regulator. Typically lead arsenate is used as an insecticide.

(Compound structure of Lead Arsenate)

Its first introduction as an insecticide was in 1908, when the toxic product was created by Cal Spray, a branch company of the chemical conglomerate, Chevron Chemical Company. It was used to kill coddling moths in the Pajaro Valley apple orchards. Typically, throughout its commercial use, lead arsenate has been used to kill bugs and regulate growth in fruit and vegetable orchards where massive amounts of commercial crops are grown. It has also been used to kill off bugs and critters from tobacco plantations.

Lead arsenate is no longer used in many parts of North America, including many states in the USA, some of which have had the substance banned since the 1950’s for fears of its hazardous effects to humans.

While lead arsenate does eliminate bugs and rodents from fields, it also has a lot of negative effects, both to the environment and to humans. This pesticide is composed in such a way that the lead and arsenic bind tightly to the soil surface, where they may remain for decades, without any real possibility of removal. The longer a particular region remains an orchard using this pesticide, the more concentrated the lead arsenate becomes within the soil, and this can create serious health effects for children who play in that soil. Typically, in regions where this substance has been used, have reported higher rates of cancer among both the elderly and the young.

When regions that were once orchards and fields that used this compound are transformed into subdivisions and parks, the effects of the lead arsenate remain. This may result in great amounts of health hazards to children, as they become exposed to such contaminants. The substance is also carcinogen, and therefore may create high cancer rates in regions it was once used.

As a result of its environmental and human health effects, North American, as well as foreign governments finally banned the use of this product as a pesticide. After the 1950’s, however, governments were introduced to another product that could act as an insecticide in a similar fashion to that of lead arsenate, this was DDT (dichlorodiphenyltrichloroethane). DDT proved just as, if not even more effective, until it was realized that DDT’s were an even greater hazard to the ozone layer. It turned out that the chlorine in the DDT would rise up to the atmosphere and break up O3 (ozone) bonds, leading to the eventual hole in the ozone layer.

So far, no pesticides have been quite as effective as lead arsenate, yet still environmentally friendly and humanly safe. The only one that came close was DDT, but that has been banned since the 1990’s. There are though, many alternatives to lead arsenate insecticides currently available on the open market, including acephate, carbofuran, dimethoate, endosulfan and methyl parathion, as well as many others. The aforementioned, though, have not been banned in North America and Europe, the way DDT and lead arsenate have.

A Quick Overview on the Evolution of the Atom

Throughout history, there have been many scientists who have contributed to previous theories which have resulted in revolutionizing the way humankind see the world. An example would be how Isaac Newton contributed to Galileo’s already existent theories on forces and gravity. Because of the modifications made by Newton to Galileo’s already existent theory, the way people understood movement was revolutionized, and ever since, the world has never been the same. Another such series of great contributions can be seen in the evolution of the structure of the atom. Just like forces, the theories surrounding the atom evolved mainly in the BCE era by Greek philosophers, and was took up by European and North American chemists in the 18^th, 19^th, and 20^th, centuries.

Perhaps the first contributor to what had been dubbed ‘Atomism’ was the Greek Philosopher, Leucippus. His very existence has been questioned, but if he did exist, he did during the 5^th Century BCE. Leucippus claimed that besides empty space there was body, which was occupied by what Parmenides described as ‘real’, but these real could not be infinitely divided because of their smallness.¹

Democritus (460-370 BCE), who was a disciple of Leucippus, and also expanded on his atomic theory by confirming that objects could not be divided into infinity. Democritus described atoms to be originally similar, impenetrable, and have a density equal to their volume. Democritus based many of the properties of atoms on the Law of Necessity, and claimed that the properties of all the things in the world, including humans were the result of ‘the endless multiplicity of falling atoms’. Democritus also explained that all atomic motion was the result of previous atomic collisions, plus the inertia of atoms.²

Epicurus (341-271 BCE), one of the major philosophers of the Hellenistic period, taught his students that the basic constituents of the world are composed of atoms, and atoms are uncuttable bits of matter flying through empty space and could be of any size. He argued against his critics that we see that there are bodies in motion, and that nothing comes into existence from what does not exist. Epicurus also believed that, because of the Principle for Sufficient Reason, the universe always existed, and therefore, atoms always existed. To account for Democritus’ theory of collisions, Epicurus stated that when falling, atoms may ‘swerve’, thus causing collisions.³

Lucretius (99-55 BCE) only really made one contribution to the existent atomic theory, and that was a correction of Epicurus’ statement that atoms could be of any size, he argued that if they were to be of any size, then some would be visible to the naked eye and some very large in size.⁴

For a long time, there was no real progress concerning atoms until 1805 when John Dalton introduced his atomic theory. He stated that the atom differed from element to element and was in the shape of a sphere, similar to a billiard ball, and that the smallest piece of matter was an atom, which was indivisible. This theory was deemed successful because it satisfied the Law of Conservation of Mass, the Law of Definite Composition and the Law of Multiple Proportions.⁵

Michael Faraday’s experiments with Electrolysis and electricity as a whole suggested that electric charges were components of matter, and as a result of this discovery by Faraday, J.J. Thomson was able to enhance the existent atomic theory. J.J. Thomson, using cathode rays, discovered the electron in an atom, leading to his own modification of the atomic theory. Thomson created a model of the atom suggesting it was a positively charged sphere containing negatively charged electrons scattered about like raisons in a bun.⁶

Ernest Rutherford, a student of J.J. Thomson, devised an experiment, in 1911, with some of his own students to test the Thomson Theory of Atomic Structure. Rutherford used radium as a source of alpha particles and fired them at a thin sheet of gold. Rutherford predicted, based on the Thomson model, that the alpha particles would barely deflect, if at all. When they did shoot the alpha particles, some went through, and others bounced around. This led Rutherford to the conclusion that the positive charge had to be in small volume compared to the atom itself, and that it was in the centre of the atom, in a nucleus, and had the negatively charged electrons circle it in the surroundings.⁷

In 1932, James Chadwick was bombarding elements with alpha particles to calculate the masses of the nuclei. The results he was expecting was that the sum of the masses of the protons and the masses of the nuclei would be equal, since it was predicted that the nucleus was composed of positively charged electrons. When he saw his actual results, they did not agree with the prediction, but the atoms were still neutral. This discovery led Chadwick to believe that the nucleus contained neutral charges, which he dubbed ‘neutrons’. Now, isotopes could be accurately explained as an atom containing a different number of protons and neutrons.⁸

When Niels Bohr began studying the atom in 1913, he discovered that Rutherford’s model of the atom was incorrect, structurally. He determined that the if Rutherford’s explanation were to be true, then, according to the classical laws of physics, the atom would collapse, since as the electron circles the nucleus, it would accelerate, causing it to lose energy, and eventually spiral into the nucleus. This does not happen in real life though, and as a result, Bohr concluded that the atom does not follow the classical laws of physics, but rather, has its own set of laws. Bohr stated that the electrons were actually orbiting the nucleus without losing any energy because they were orbiting in energy levels. According to Bohr’s theory, electrons were able to move to higher energy levels, but only when they acquired more energy, otherwise they would remain in their ground state.⁹

In 1923, French Chemist, Louis De Broglie suggested that if a wave can act like a particle, as is the case with light, then maybe a particle can act like a wave. Later, Erwin Schrödinger and others used this theory to explain the activity of the electron inside the atom. Basing his work on Quantum Mechanics, Schrödinger proposed that electrons can only have certain quantized energies because of the requirement for only whole numbers of wavelengths for the electron wave, thus confirming that the electrons (a particle) act like a wave in an atom.¹⁰

It is easy to see how these different philosophers and chemists and physicists have contributed to such an important aspect of science. Every time one of them has contributed their idea, it has changed the way people may view the world around them. It was through these scientists, that we are now able to portray the atom as accurately as modern technology has allowed us to.