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Twisted Intramolecular Charge Transfer in a Donor Acceptor System
Jensen, Liesl
Pham, Anna
Overby, Jasper
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Denver, CO
Metropolitan State University of Denver
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Collected for Auraria Institutional Repository by the Self-Submittal tool. Submitted by Matthew Mariner.
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Faculty mentor: Megan Lazorsky
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Major: Biochemistry

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3D PRINTED SEC CELLS 3D Printing Optimization SYNTHESIS: DONOR ACCEPTOR COMPLEX Jensen, L., Overbey, J., Pham, A., Lazorski M. Metropolitan State University of Denver Department of Chemistry and Biochemistry, Denver, CO Twisted Intramolecular Charge Transfer in a Donor Acceptor System ACKNOWLEDGEMENTS The Lazorski Research Group would like to thank Dr . Andrew Bonham, Dr . Mike West, Riley Gest, James Laughlin, and the MSU Denver Chemistry and Biochemistry Department faculty for their assistance and support with this project . Another thank you to the Applied Learning Center for providing funding through the Undergraduate Student Research Grant . References 1. Hirsch, T .; Port, H .; Wolf, H . C . , J . Phys Chem B . , 1997 , 101 , 4525 4535 . 2. Sabnis, R . W .; Guerrero, D . J .; Brewer, T .; Spencer, J . , US 20010021481 A 1 , September 13 , 2001 . 3. Neelakandan, P . P .; Hariharan, M .; Ramaiah, D . , Org . Lett , 2005 , 7 , 5767 5768 . BACKGROUND Novel Widely Reproducible Cell Design Since commercial SEC cells are limited to use with UV vis, IR, and Raman spectroscopy and often expensive, a prototype 3 D printable SEC cell was designed . The intent of this approach is to provide an open source method to the scientific community for producing SEC cells applicable to a variety of spectroscopic instrumentation . The end goal is to produce SEC cells with chemically resistant, inexpensive materials, tailored for inert atmosphere, and made compatible with optically transparent electrode architectures . ABSTRACT Cheaper, more accessible solar energy conversion materials and instrumentation could help improve solar energy technologies, the focus of the Lazorski lab research . Donor Acceptor (D A) systems can be fabricated cheaply to absorb and solar energy in a charge transfer (CT) state via electron transfer . The CT state energy can then be used for solar energy conversion applications . Thus, the first project in our lab evaluates electron transfer rates in D A systems forced to adopt specific geometric arrangements, i . e . Twisted Intramolecular Charge Transfer (TICT) . The synthesis and characterization of two D A systems is ongoing : an control, and a test D A . Thus, SEC is used to measure the Ultraviolet visible (UV vis) spectra of the oxidized/reduced D A systems to produce a model of their CT state spectra . The SEC model will be compared to Transient Absorption (TA) spectroscopy data to evaluate the effect of the geometric twist on the CT state lifetime . Our first project feeds into the second, which utilizes 3 D printing technology for fabrication of spectroelectrochemistry (SEC) sample cells . This project aims to provide affordable, accessible, reproducible, and chemically compatible instrumentation for evaluation of solar energy conversion materials, such as D As . Our synthesized D A systems serve as a proof of concept for our 3 D printed SEC cell designs, which have been fabricated and optimized on a MakerBot 3 D Replicator 2 X printer . While the current focus is SEC designs, a secondary goal is to provide a generalized technique enabling 3 D printing technology to be more applicable in the chemistry laboratory . CHARACTERIZATION: DONOR ACCEPTOR COMPLEX Nuclear Magnetic Resonance Spectroscopy Ultraviolet visible Spectroscopy and Spectroelectrochemistry CONCLUSIONS AND FUTURE WORK BMA : One focus of the research was the purification of BMA . The 1 H NMR ( 300 MHz, CDCl 3 ) spectra of the reaction product mixture, an An standard, and purified product mixture are compared in Figure 6 . This data suggests successful synthesis of the BMA starting material . An was found to be preferentially soluble in hot ethyl acetate ( EtOAC ) over BMA . Thus, hot filtration of the crude BMA solids was performed to remove excess An . This technique allowed for purification of BMA from the product mixture . Figure 9 . Comparison of the UV vis spectra of anthracene and V (methyl viologen) standards as well as the SEC spectrum of V to demonstrate spectral shifting upon reduction of V . * * Figure 6 . Comparison of 1 H NMR spectra of an An standard, the reaction product mixture to confirm the synthesis and the purified product mixture after EtOAc Wash UV vis spectra of the An and V were measured and compared to the SEC spectrum of the reduced V in Figure 9 . These data demonstrate how UV vis absorbances can shift upon oxidation/ reduction of the D/A . Once the cyclic system is ready for analysis, SEC will be performed resulting in a different absorption spectrum than the ground state species . The SEC spectrum of the oxidized/reduced D A dyad will then serve as a model of the photoinduced electron transfer state that would be expected in the transient absorption spectrum . BMA V 2 : The 1 H NMR spectra of the BMA V 2 product, BMA starting material, and a 4 , 4 bpy standard, are compared in Figure 7 . While all these spectra had to be performed in different solvents due to solubility, the spectral changes are consistent with our expectations for the formation of BMA V 2 . The two doublet peaks ( 4 H each) from the 4 , 4 bpy rings split into four doublets ( 2 H each x 2 due to disubstitution ) and shift downfield, consistent with deshielding of the ring protons by the positively charged nitrogen and a change in symmetry . The splitting of the two doublets from the BMA starting material is constant, consistent with unchanged shielding . While downfield shifting of all protons is observed, small shifts in the peaks due to the ring protons could be mostly due to differences in solvent environment . However, the large, downfield shift of the singlet (c . a . 2 ppm ) is likely due to deshielding of the methylene protons as a result of their proximity to the positively charged nitrogen on the 4 , 4 ring . Taken together these results suggest a successful BMA V 2 synthesis . Figure 10 . The UV vis SEC cell insert prototype machined from PTFE (Left/Center) . The insert contained within a typical UV vis cuvette (Right) A UV vis SEC cell insert was designed, and a prototype was machined from PTFE as shown in Figure 10 . The 3 D printing parameters have been varied in order to optimize the printing procedure as shown in Table 1 . Table 1 . 3 D Printed UV vis SEC Insert Optimization Trials . The optimization of the electrochemical cells began with cells printed in a more vertical fashion with distortion around the over hangs and exterior ridging due to low infill and high layer thickness . After decreasing the layer thickness from 0 . 2 mm to 0 . 1 mm there was a significant chance in the exterior ridging . To eliminate overhang distortion a HIPS support structure was added, because HIPS dissolves in limonene . To reduce dissolve times further structural modifications were made . And the solubility was tested in organic solvents . Internal cavity for RE Notch for CE Window/Slot for OTTLE (WE) and light path Window insert seals in solution Light path a a b b c c c c b b b b c c a a Print Observations Modifications Materials Exterior ridging, Overhang distortion First prints ABS Exterior ridging is gone. Limonene dissolve time is too long Increased infill 15% to 20% Added supports High Impact poly styrene (HIPS). ABS HIPS Wider on the side pressed against the build plate. Increased HIPS surface area to reduce dissolve time. ABS HIPS Limonene dissolve time was reduced Rotated gap 90 degrees to further reduce HIPS used ABS HIPS Figure 2 . Literature synthesis of 9 , 10 bis(bromomethyl)anthracene (BMA), hemicycle (BMA V 2 ), and the non twisted cyclic D A complex . Modified from Ref 2 and 3 . Figure 14 . Literature synthesis of tetramethyl 4 , 4 bipyridine and the twisted cyclic D A complex . Future goal is to synthesize this molecule and observe how geometry affects backwards electron transfer rate . Figure 1 . The simplified mechanism of photoinduced electron transfer in a D A dyad . The rates of forward and backward electron transfer are indicated as k CS and k CR , respectively . Reproduced from Ref 1 . Figure 3 . Reaction mixture from a synthesis trial of BMA V 2 Figure 5 . Non Twisted reaction mix at the end of reflux . Figure 4 . Non Twisted r eaction mix before reflux . Figure 7 . Comparison of 1 H NMR spectra of BMA V 2 , BMA reaction mixture and 4 , 4 Bipyridine Standard Non Twisted D A Complex : The insolubility of the full cycle in 1 H NMR solvents has made NMR characterization difficult . However, the combination of TLC evidence and comparison of our 1 H NMR spectra to the results of Neelakandan et al . , 2 suggest successful synthesis . In Figure 8 , TLC plates of the reaction mixture at start of reaction and the product mixture after reaction completion are compared . The plates are dipped in basic sodium hydrosulfite solution, which reduces the positively charged nitrogen species resulting in the purple coloration . The BMA V 2 has two reducible and the full cycle has four reducible . Thus, the full cycle is expected to Figure 8 . (Left to Right) Full Cycle (A before reaction & D after reaction) ; BMA V 2 & BMA (purified product) turn a darker purple color and have a smaller R f value than the BMA V 2. Over the course of the reaction, the purple coloration of the reaction mixture spot became much darker and the R f decreased, suggesting that the Full Cycle has been synthesized. Figure 11. ABS in ACN Figure 12. ABS in DMSO Figure 13. ABS and HIPS in ACN Synthesis reactions seem to produce BMA V2 and the non twisted cyclic D A complex, but further work needs to be done on purification. Different solvents are being considered for filtration to obtain the purest form of BMA V2 as possible. More synthesis trials of the non twisted D A complex will be completed. Additionally, library research is being done to determine what molecules may change the backward rate of transfer when attached as an R group to the twisted D A complex (Figure 11). Currently, two are being considered: 2, 2', 6, 6' tetraphenyl 4, bipyridine and 2 (3 bromophenyl ) 2', 6, 6' triphenyl bipyridine. Regarding the 3 D printing project, issues still need to be addressed such as poor chemical resistance of ABS to organic solvents and the build plate expansion causing the cell to break the cuvette when inserted. One possible solution includes switching the HIPS and ABS; i.e. using an organic solvent to dissolve the ABS away and using a HIPS cell. Other possibilities include coating the cell with a polymer coating such as Teflon, polyether ether ketone (PEEK), and fluorinated silica. Additionally, the cells need to be redesigned for use without a cuvette. d e f g BMA V 2 : Five synthesis trials for BMA V 2 have been performed following procedure . To a warm three neck round bottomed flask, 1 . 747 g of 4 , 4 bipyridine and 5 . 00 mL dry acetonitrile were added and melted together . In a separate container, BMA product from a previous synthesis reaction was dissolved in 32 . 00 mL of dry acetonitrile then added dropwise to the 4 , 4 bipyridine ( 4 , 4 bpy) solution . The reaction mixture was refluxed for 4 . 5 hours . Thin layer chromatography (TLC) plates were run comparing the hemicycle product to pure BMA solids and bpy every hour during the reaction . The final hemicycle product was filtered with c . a . 5 . 00 mL dry acetonitrile . The solids were dissolved in c . a . 30 . 00 mL deionized water and washed with c . a . 5 . 00 mL dichloromethane . All solids and filtrate solution were kept for analysis using TLC plates and 1 H NMR . Non twisted D A Complex : This synthesis has been performed following procedure . 3 The following were added to a three necked flask that was brought to reflux for 24 hours : 0 . 333 g of BMA V 2 , 0 . 099 g of BMA, and 34 . 00 mL of dry Optima acetonitrile . TLC plates were taken every hour comparing the reaction mixture to BMA V 2 and BMA . The reaction mixture was washed with ~ 10 . 00 mL acetonitrile and the precipitate was dissolved in water and washed with dichloromethane . TLC plates were run to identify the fraction containing the product . Thus, the precipitate was kept but filtrates were discarded because it did not contain the cyclic complex . SEC Spectroelectrochemistry combines two techniques : spectroscopy and electrochemistry to detect changes in the spectral response with respect to oxidation state . SEC can accurately model electron transfer intermediates and products of photoinduced reactions in artificial photosynthetic systems . As proof of concept, SEC is implemented in this project to model the spectral response of the photoinduced electron transfer products of a D A dyad : the oxidized anthracene (An) donor and the reduced 4 , 4 bipyridinium (V) acceptor .