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Absorption Spectra of Conjugated Dyes Objective To obtain absorption spectra of three related conjugated dycs and uso the data model the system with the quantum mechanical particle ina box problem Introduction: basic problem in quantun mechanics is that of the "particle ina box In thisproblem particle mass Isconstrainedto move one dimension along flength The particle has constant otential thergy, and isconfined to a box because the potential chergy infinite beyond the linc. According quantum mechanic the encrgy the particli must be quantized meaning that can only have spedific values andis equal to n2h2 (1) where the quantized chergy, nis the quantum number (which isa positive intege= and h Planck constant- These energies definc cheroy levels the partide lithe particle moves from one energy leve another the change chergy, AE s the difference energics between states with different values ofn other words AE 8mL² 8mL2 (2) Ifthe energy leveloris defined as nen+ the cquation an be simplified to AE +1) (3) 8ml2 The energy change, AE, between to states can be observed bectroscopically Since wavelength the bsorbance maximum Absort anco spectroscopy apply the particleina box model to systems which meet the constraints of that model In other words AE (4) This equation can thenbe solved for the wavelength SmcL² 2 (5) h(2n +1) Inthis exteriment. the particle ina hox model will bo applied to serics of conjugated (and coloFful) dycs- Conjugatedsystemshave serics alternating single double bonds electron that are part This conjugates system are free move anywhere nlong conjugated system. Figure shows three tugated dycs thatwillb studiedinthis Looking figuro you can sce that conjugated system exists between the two nitrogen atorns dve there are three pairso electrons the conjugated system (the non bonding pair on the hitrogen andthe two bonding pairs between the two nitrogen atoms Dyc has four pairs of electronsand dyc 3 has five pairs. 02.00 CH, can O C,H, C,H, e Ondo C,H, Figure The 1.1" 2,2° Toapply the particle in to these conjugate dyes voll will measure absorption maxima using spectroscopy determing the length of the conjugated (L). then calculate a bond leneth for the conjugated system- The absorbance maxima represent the transitions between the highest occupied molecular bital (HOMO) tothe lowest unoccupied molecular orbital(t LUMO). Because each energy level c hold electrons can determine quantum number HQM06 taking the totalnumber electrons the system and dividing by For example, dyc has three pair electrons the conjugated system so the HOMO Is n=3 Using the absorbance maxima and value of for each dye molecule. you will determine valuco rashon bond length- Theoretically, carbon carbon dlength should be equal that of benzene (b=0) nm). This means that should be equal tob times the number of bonds (b) between the two nitrogen atoms plus) factor accolnt for the fact that electrons can spillover past the nitrogen atoms other words =bls tlext Defining asb 2(p -1), where Isthe number oft electron pates, equation 5can berewritten (6) h(2p+1 You willuse this equation to determine your theoretical carbon carbon bond length. Experimental: Obtain 10+Maglicolls olutions of cach dve from Volle instructor. Add 0-30 dyc. add 2.70 mL wate* and mix well. (This will dilute the dyes factor 10). Using lvv/Vis tometer obtain spectrum between 400 and 800 nm. Record the wavelength of maximum absorbance for cach dye (Ifthe absorbance greater that 08 further dilute the dvc by factor of and recorda new spectru Safety Dyc molecules can be carcinogenic Wear gloves when working with the dyes and dispose of any contan mated material including pipette tips and paper towels used to clean up spills) as haza+dous waste Data Analysis: 1. Use equation 5to leterminça value of for cach of Voll dyc molecules 2. Plot the number each molecule and the equation determine value of both andlest from your data- 3 Prepare plot Using Igor Pro. fit qullation the plot (Yoll will need to defing Vour own equation Igor Pro- You will be solving for lext the resto the values includingla (=0 139 nm are known Watch your units Discussion: Comment the following 1. Why the benzene carbon carbon bond length used the theoretica value model? 2. Compare the valucsofl obtained from item 2 in the Data Analysis the bond length of benzene and discuss. 3 Compare the values of lest obtained from items and in the Data Analysis- The parameter counts for the fact that the nitt ogen toms not the hard walls particleina box, with infinite potential energy. Insteadt electrons can "spillover' pasta length ofa boxcalculated lasa multiple of carbon- carbon bond lengths. How does your value Fless compare with that of Docs the value make sense 5 Agng increases absorption wavelength decrease but as L increases wavelength should increase Which of these two effectsis dominant the system studied (and why?) 6- From the absorption spectra the dycs comment on why the dyc solutions are colors you observed References Halpern, AM. McBane C.C. Experimental Physical Chemistry Third Edition New York W.H Freeman d Company 2006.

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