Absorption spectrum of a conjugated dye


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Absorption spectrum of a conjugated dye

  1. 1. Running Head: ABSORPTION SPECTRUM OF A CONJUGATED DYE 1 Absorption Spectrum of a Conjugated Dye Name Institution
  2. 2. Absorption Spectrum of a Conjugated Dye 2 Absorption Spectrum of a Conjugated DyeObjectives This experiment main objective is to probe the quantized nature of molecular electronicstates. This probe will be done by spectroscopy. The homologous series of molecules is studiedand the electronic energy levels variation of molecules will be examined during this experiment.Also, theoretical molecules will be used in studying the way electronic absorption energy of amolecule alternates with size. Lastly, skills of comparing theoretical values obtained from simplemodels with more complicated and robust models will be leant.Introduction The interpretation of spectroscopic transitions requires quantum mechanics. Thisexperiment will employ the use of quantum mechanics in modeling electronic transitional energyof a molecule between its ground state and its first excited state. Colored compounds such ascyanine and polymethine when excited results in absorption which occurs in visible region ofspectrum. Absorption spectrum of several dyes will be obtained in this experiment and thewavelength of the maximum absorption used in determining the energy difference betweenexcited state and ground state. The experimental results will then be compared with theoreticalresults.Background The absorption band which is in the visible region of a spectrum corresponding to thechange from molecular state to excited electronic state is 170kj/mole – 300 Kj / mole above the
  3. 3. Absorption Spectrum of a Conjugated Dye 3ground state. Dyes that absorb in the visible spectrum have weakly bound or delocalizedelectrons (free radicals or  electrons) in conjugated systems.Polymethine dyes’ electronic transitions involve the electrons along the polymethine chain. Thischain is conjugated; that is, it contains a string of alternating double and single carbonbonds. The number of bonds in this string is connoted by the nomenclature, P(#carbon-carbonbonds) Since wavelength of these bands depend on the spacing of the electronic energy levels,one must know the transition associated with any given absorption. The simple free-electronmodel (Kuhn) is accepted as the most precise model for explaining the energy of the absorptionmaxima, max. The free electron model assumes that  electrons are free to move unfetteredalong a conjugated carbon system. There is a correlation between the length of the conjugatedsystem and max. One of the objectives of this laboratory exercise is the elucidation of thisrelationship. The absorbance wavelength is a population average of the absorbance of both structuresaccording to the Boltzmann distribution equation. The conjugated chain is defined as the shortestchain from nitrogen to nitrogen and has a length L. Since the accepted value of a C=C isknown, L can be determined for each structure using equation.The quantum mechanical solution for the energy level of this model is 2 2 h n En  2 8 mL(1)
  4. 4. Absorption Spectrum of a Conjugated Dye 4Where m is the mass of the electron and h is the Planck constant. The ground state of a moleculewith N  electrons will have N/2 lowest levels filled. The electron transition is from HOMO toLUMO where n1 = N/2 and n2 = N/2+1. Thus the energy of transition is related to HOMO andLUMO by equation (2).(2) h 2 E  (n 2  n1 ) 2 2 2 8 mL The Particle in a Box model can be applied to conjugated systems, such as a hexatrienemolecule. For hexatriene, there are carbon six carbons in the conjugated system and there are sixpi electrons; 2 per double bond. Observing the Pauli Exclusion Principle, one can distribute theelectrons in the energy levels starting from the lowest as per the Aufbau Principle (See below). n= 4 S1  S0 n= 3 U (x) ap p rox n= 2 n= 1 xAbove graphic from Hope College “Absorption Spectra of Conjugated Polyenes”It can be seen in hexatriene that the S1 ¬ S0 transition relates to n=4 ¬ n=3 change of the particlein the Box model. The wave functions and energy for this model are 1  22 nx n    sin  (3) a  a And the energy of discrete level En
  5. 5. Absorption Spectrum of a Conjugated Dye 5 (4) E =hSo 2 8 mc L   h N 1(5)In case the amount of carbon atoms that are in the chain = p, Then the number of pi electrons inthe system is N = p+3 (remember 2 carbon atoms = double bond, 2 electrons per bond and Lis the length of the conjugated chain plus one bond length. L = (p+3) l (where l is the bondlength between the atoms in the chain. (Remember: a conjugated bond length is an averagebetween a single and a double bond. A) l =1.39 A = .139 nm (p 3) 2  ( nm )  63 . 7 p4(6) If there polarizable groups at eh end of the chain the conjugation of the group lengthens thechain. This lengthening is the quantity by. If there are no groups attached to the nitrogen then = 0. Rangesbetween 0 and 1 and is specific for each constituent.ExperimentalThe computer was turned on and allowed to completely boot up. The spectrometer was turned onand when the amber light turned green, the spectrometer program was opened on the computerand the both the lamps were turned on. The spectrum range window was then filled in such away to display the spectral range from 360 nm to 900nm.Using 2.5ml graduated pipette, 1.00mLs of the stock solution of the dyes was dispensed to 10Mlvolumetric flask and diluted to mark with the methanol. The visible spectrum of the solution was
  6. 6. Absorption Spectrum of a Conjugated Dye 6then taken using the plastic disposable cuvettes. Aliquots of the solution were diluted similar tothe above manner until an absorbance reading of one was obtained forming the workingconcentration.8 ml 0f the sample was then diluted to 10 ml. The other dyes were also made in the samemanner. The spectra of all the dyes were taken and recorded as spectra overlays.Results