Tuning Color Through Substitution

Transcription

1 1 Tuning Color Through Substitution Introduction In this experiment, the effect of substituents on the absorbance spectra of molecules will be related to the structure of the molecular orbitals involved in the transition. Both the nature and the position of the substituent on the ring will be investigated and related to theoretical predictions based on the structure of the frontier molecular orbitals of the molecule. Molecular Orbitals Molecular Orbital Theory (MO theory) can be used to relate the formation and nature of chemical bonds to properties of molecules. This experiment will expand on the discussion of structure-property relationships by considering changes in optical properties within a family of similar compounds upon structural modifications. The molecular orbital approaches are used to (1) rationalize these changes and (2) guide the structural design to obtain compounds with desired absorption characteristics. As you have read through the text associated with this course, you have learned about the use of various theoretical tools chemists use to understand electrons, atoms and chemical bonding. In the particle-in-a-box experiment, the structure of the molecule was shown to affect the absorbance spectrum of the molecule in a predictable way. In another experiment, the valence electron-pair repulsion theory, VSEPR, was used to approximate the geometry of multi-atom molecules, and the molecular orbitals were calculated. One of the challenges associated with the Schrödinger equation is the difficulty in solving the equation for multi-atom systems. However, one can use a simplified approach to approximate energies of the electrons in a multi-atom molecule. By mathematically combining wavefunctions of simpler systems, it is possible to approximate the description of a more complex molecule. This technique is designated as the LCAO, Linear Combination of Atomic Orbitals, method and involves constructing molecular orbitals (MOs) for the molecule using the atomic orbitals of the individual atoms. Your textbook provides the details of the mathematics and theory behind this method. As the atomic orbitals combine to form molecular orbitals, the general rules of filling these molecular orbitals with electrons are as follows: (1) electrons occupy orbitals of the lowest energy in the ground state; (2) no more than two electrons can be placed in a single molecular orbital; and (3) two electrons in a single orbital must have anti-parallel spins (i.e., be paired ). In addition, the total number of molecular orbitals must be equal to the total number of atomic orbitals used in the MO construction and the total number of electrons distributed among the molecular orbitals must equal the total number of electrons from all of the bonding atoms.

2 In compounds, electrons can be excited from one molecular orbital to another of higher energy by using electromagnetic radiation (usually in the UV-visible range). This process can be used to evaluate the energies of molecular orbitals by means of spectrophotometric techniques, as was done with the Particle-in-a-Box model. Typically, the lowest energy transitions involve what are termed Frontier Molecular Orbitals. The highest energy molecular orbital filled with (an) electron(s) is called the Highest Occupied Molecular Orbital (HOMO). The lowest energy vacant molecular orbital (i.e., containing no electrons) is called the Lowest Unoccupied Molecular Orbital (LUMO). The difference in energy between the HOMO and the LUMO can sometimes be determined using UV-visible spectroscopy. Compounds of Interest 2 The two images above are ball and stick models of benzene (C6H6, left) and naphthalene (C10H8, right). Both of these molecules are highly conjugated, as were the molecules in the Particle-in-a-Box lab. These particular species belong to a special class of compounds known as aromatic. A cyclic planar molecule is aromatic if its conjugated π-system has 4n+2 electrons, where n is an integer (n = 1, 2, 3, ). For example, n = 1 for benzene (6 π electrons) and n = 2 for naphthalene (10 π electrons). The term aromatic has historic roots and most of the time has nothing to do with the actual odor of the compound. To be even more specific, benzene and naphthalene are members of the benzenoid aromatic hydrocarbon family because they are both based on the sixmembered ring of benzene. While benzenoid aromatic compounds are the most abundant, non-benzenoid aromatic molecules are known as well. The two images below are ball and stick models of naphthalene (left) and azulene (right). Although both of these molecules have the same molecular formula, C10H8, they have very different physical and chemical properties. Naphthalene is composed of two fused 6-membered rings and has three chemically different kinds of carbon atoms. Azulene is an unusual aromatic hydrocarbon that comprises an edge sharing combination of five- and seven- membered sp 2 -carbon rings. Due to the difference in arrangement of the carbon atoms, these structural isomers exhibit different spectroscopic behavior upon substitution. In particular, you will analyze and rationalize

3 the impact of hydrogen atom substitution in azulene by various functional groups on the color of the molecule. 3 Many branches of science use the electronic nature of molecules to design novel materials for various applications. For instance, advances in the design and fabrication of flat screen televisions and computer monitors have exposed the importance of developing molecules and/or materials that have specific and controllable optical properties (e.g., color) to improve the viewing experience of consumers. One such technology employs liquid crystals as the means for communicating a color scheme to viewers. The ability to accurately predict the color of a particular compound is a significant challenge. Regiospecific (e.g., controlled) substitution in a molecule may also be a difficult task that many synthetic chemists perform every day. To a first approximation, the C-H substitution at a benzenoid aromatic ring (e.g. benzene, naphthalene, anthracene) alters the energies of both the HOMO and the LUMO of the molecule in the same way (i.e. the energies of the Frontier MOs shift in the same direction by the same magnitude). Thus, such a structural modification will hardly affect the energy of the HOMO LUMO transition in the electronic spectrum of a benzenoid compound regardless of the nature of the substituent. Typically chemists create colored materials by working with highly conjugated polar organic molecules (dyes) or metal-containing compounds. The energy separation between the HOMO and LUMO of transition metal-based molecules often falls in the visible region of the electromagnetic spectrum, as was seen in the Introduction to Spectroscopy lab. Because of its deep color and the relatively simple structure, the azulenic framework has been attracting chemists attention as a potential optical marker that could be incorporated into larger molecular ensembles. In addition, azulene is planar (flat) and polar (the dipole moment of azulene is 1 Debye), which makes it attractive for liquid crystal design. In this two-week experiment, you will investigate Molecular Orbital Theory (MO theory) and use this theory to rationalize and predict certain structure-property relationships. In this experiment, you will calculate the contour plots and the energies of the HOMO and LUMO orbitals for a variety of structurally similar molecules and experimentally acquire their absorbance spectra. The effect of the structure of the HOMO and LUMO orbitals on the shift in absorption energy upon substitution will be determined. Additionally, you will determine the feasibility of using substituted

4 azulenes to control the color observed and absorbed. You will also relate the type of substituent and its position on the azulenic scaffold to the impact on color by analyzing changes in the energy of the HOMO LUMO electronic transition upon substitution. Pre-lab Familiarize yourself with the structures of the naphthalene and azulene molecules involved in this experiment. Note the differences between the different types of molecules studied. Read the paper referenced below by Lui et al. in J. Chem. Ed. and be prepared to answer questions about this paper and use it as a reference to understand your data. Pre-lab Assignment: Please answer the following questions in your lab notebook. This assignment is due at the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and submitted to your TA. The only molecules you will need to look up safety information for this lab are ethanol, naphthalene, azulene, and anthracene. For Week 1: 1) Define the term isomer. What are structural isomers? 2) Draw the naphthalene and azulene molecules and number the carbon atoms according to convention. 3) What is meant by the terms electron donating group (EDG) and electron withdrawing group (EWG) with respect to functional groups? 4) What are major differences between the HOMO and LUMO orbitals of naphthalene and azulene (Hint: Use Figure 2 in the Liu et al. article to help with this question). For Week 2: 5) Predict what effect the different functional groups (see pre-lab question 3 from Week 1 above for types) will have. 6) Following Figure 3 from Liu et al., draw the schemes of the frontier orbitals of azulene before and after adding an electron withdrawing or electron donating group to azulene at both the even and odd positions of azulene. There should be a total of four sketches (i.e., EWG on an even, EWG on an odd, EDG on an even, EDG on an odd). 7) Consider the case of an EDG on an odd position and an EWG on an even position of azulene. Using the figures from question six, predict the shift in the absorbance spectrum of these substitutions from the unsubstituted azulenes (i.e., either red or blue shifted). 4

5 5 Procedure Safety: Goggles must be worn at all times. Do not empty any of these cuvettes without explicit permission of your TA!! Do not pour any solvents down the drain. Week 1 Theoretical Models of Compounds and Absorbance Spectra Using the programs ChemDraw and Chem3D, create drawings of benzene, naphthalene, anthracene, and azulene. Under the Calculations tab in Chem3D, choose MM2 then Minimize Energy. Once the calculation is finished, under the Surfaces tab, select Choose Surface and then Molecular Orbital (Hückel Calculation). Under the Surfaces tab, highlight Select Molecular Orbital. A drop down window will open that allows you to choose the different molecular orbitals. The last number in brackets is the estimated energy in electron volts for the given molecular orbital. For each of the molecules above, record the energies of the HOMO and LUMO orbitals and determine the difference in these energies. Save pictures of the minimized structures for each of the molecules. Also look at the electron density distributions in the Frontier Molecular Orbitals. In your notebook, be sure to draw representations of the HOMO and LUMO electron density distributions for later reference. Make particular note of any differences in electron distribution in the HOMO and LUMO orbitals of these molecules. NOTE: The Chem3D program uses the semi-empirical Extended Hückel Method to calculate the energies of MOs and may not be particularly accurate in predicting these energies, especially for certain classes of compounds (e.g., non-benzenoid). However, this theoretical tool provides a reasonable, qualitative (or even semi-quantitative) description of the electron density distributions within the MOs and, in some cases, the energies of the MOs. Similar calculations can be carried out for the substituted azulene and naphthalene molecules studied experimentally below. Simply draw the substituted molecules starting with the structures for the unsubstituted molecules and reminimize the energy. Do this for both electron withdrawing and electron donating groups on different positions on the rings. Record the differences in the HOMO and LUMO energies compared to what is observed for unsubstituted compounds. You will compare these predictions with what you observe experimentally for the substituted molecules. Experimental Absorbance Spectra Use the Ocean Optics spectrophotometer to record the absorbance spectra of the unsubstituted model compounds. Record the spectrum down to the lowest possible wavelength. Consult you TA for the choice of solvent for your blank spectrum. You should record spectra of naphthalene, anthracene and azulene. Benzene will not be used since it does not absorb in the region of the Ocean Optics Spectrometer.

6 Effect of Substituents on Absorbance Spectra Pre-made solutions of several substituted azulenes are provided in glass vials in the room. Transfer these solutions to the cuvettes provided and record the absorbance spectrum. Once you have recorded your spectrum, transfer the solution back to the glass vial. Do not discard this solution!! You will collect the absorbance spectra of several substituted azulene molecules (you should already have collected the spectra of the unsubstituted molecules in the previous part of the experiment). Your TA will provide you with a list of the molecules to be investigated. Make careful note of the molecular structure for each molecule. Be sure to record the wavelength and absorbance values for all the provided solutions as well as the color of the solution. Record the spectra for each of the monosubstituted molecules and determine the direction of the shift, either blue or red shift, in the absorbance spectra relative to the unsubstituted molecules. Be careful to note the wavelength of the lowest energy (highest wavelength) absorbance and not necessarily the most intense absorption peak in the spectrum. Record and save each spectrum so that you can refer to the spectrum later when you analyze your data. Week 2 Predict the direction of the shift for the absorbance spectra for each of the polysubstituted molecules provided by your TA and then acquire their absorbance spectra. Compile all of your absorbance spectra and make a detailed comparison between your predictions and the observed experimental data. Relate these shifts to the structures of the frontier orbitals involved in the transition. Discuss the theoretical calculations for the absorbance spectra with those observed experimentally. Discuss the differences in the HOMO and LUMO orbitals for the different unsubstituted molecules and relate the structure of these orbitals to the effect of substituents on different positions in the structure. Report There will only be one report required for this experiment and it will be due in lab the week after completion of the two-week experiment. In your report you should discuss your predictions for the shifts in absorbance spectra and any deviations from your predictions. Also, your report should discuss the areas outlined in the rubric. References Oxtoby, Gillis, and Butler, Principles of Modern Chemistry, 8th Edition, Cengage Learning, Boston MA (2015), Chapter 6. Liu, Robert S.H. Colorful azulene and its equally colorful derivatives. Journal of Chemical Education, 2002, 79(2),