Electronic Structures and UV/Visible Absorption Spectra of Organic Compunds

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Quantum chemistry ab initio and semi-empirical calculations

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Electronic Structures and UV/Visible Absorption Spectra of Organic Compunds

  1. 1. Electronic Structures and UV/Visible Absorption Spectra of Organic Compounds Fa-Gung Fan Xerox 2/5/2010 Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  2. 2. • Building upon the previous experience on quantum chemistry ab inito and semi-empirical calculations [1], here I would like to demonstrate our ability to model and predict the absorption spectra of solar cell materials. • I computed the electronic structures and the UV/visible spectra of several organic compounds. The results are shown in this set of slides. • Among the compounds studied, the NKX dyes and the several electron donor-acceptor structures are of high interest to solar cell development. [1] In modeling CNT field emission and electron-molecule collision cross section. Slides 44-51 and 66-68 of 2/6/2007 ME&IL Seminar presented some of the earlier calculations. Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  3. 3. Compounds Studied Coumarin derivative dyes: NKX-2311 (s-trans) NKX-2586 (s-trans) Flavone Grey – C White – H Red – O Blue – N Yellow - S Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  4. 4. TPA-BTD donor-acceptor system Pentacene-fullerene donor-acceptor system Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  5. 5. I first calculated the ground state electronic structures of these compounds, next examined their MOs and the LUMO-HOMO energy gaps, and finally computed the UV/visible absorption spectra. Note: The LUMO-HOMO gap can only be a very crude approximation to the actual energy gap. I think the LUMO-HOMO gap overestimates the actual excitation energy. Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  6. 6. Modeling Methods Used General procedure for ab initio calculation [2]: 1. Molecular geometry optimization using DFT method with B3LYP XC functional on a relatively small basis set. e.g. 3-21G, 6-31G. 2. Energy/MO calculation by the DFT method on a larger basis set. e.g., 6-31(d), 6-31(d,p), … 6-311+G(d,p) 3. Excited state calculation by TD-DFT method, taking into account the solvent effect (by PCM model), on the basis set used for the energy/MO calculation. 4. UV/vis spectrum evaluation by Gaussian convolution. Procedure for semi-empirical calculation: 1. Molecular geometry optimization using PM3 semi-empirical method. 2. Energy/MO calculation by PM3 method. 3. Excited state calculation by semi-empirical ZINDO/S model and CI method with solvent effect included (by SCRF Onsager model). 4. UV/vis spectrum evaluation by Gaussian convolution. [2] Some in the field do not regard Density Functional Theory (DFT) an ab initio method, while many other researchers in the same field refer the method ab inito. Here, we use the latter point of view. Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  7. 7. For the results presented in the following, the calculations were based on the ab initio method except for the last case (pentacene-fullerene), which was based on the semi-empirical method. Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  8. 8. NKX-2311 Dye HOMO LUMO LUMO - HOMO = 2.8 eV UV/vis Spectrum of NKX-2311 Dye in Ethanol Absorbance, L/(mol-cm) 1.2E+05 1.0E+05 8.0E+04 6.0E+04 4.0E+04 2.0E+04 0.0E+00 300 350 400 450 500 550 600 650 700 750 800 wavelength (nm) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  9. 9. NKX-2586 Dye HOMO LUMO LUMO - HOMO = 2.6 eV UV/vis Spectrum of NKX-2586 Dye in Ethanol 1.4E+05 Absorbance, L/(mol-cm) 1.2E+05 1.0E+05 8.0E+04 6.0E+04 4.0E+04 2.0E+04 0.0E+00 300 350 400 450 500 550 600 650 700 750 800 wavelength (nm) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  10. 10. Flavone HOMO LUMO LUMO - HOMO = 4.5 eV UV/vis Spectrum of Flavone in Methanol Absorbance, L/(mol-cm) 60000 50000 40000 30000 20000 10000 0 200 250 300 350 400 wavelength (nm) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  11. 11. TPA-BTD donor-acceptor HOMO LUMO - HOMO = 1.86 eV LUMO Notice the intramolecular charge transfer UV/Vis Spectrum of TPA-BTD in Dichloromethane (CH2Cl2) 1.00E+05 Absorbance, L/(mol-cm) 8.00E+04 6.00E+04 4.00E+04 2.00E+04 0.00E+00 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  12. 12. Pentacene-fullerene donor-acceptor HOMO PM3 semi-empirical LUMO LUMO - HOMO = 6.25 eV Notice the intramolecular charge transfer UV/vis Spectrum of Pentacene-Fullerene HOMO (This spectrum is pentacene-fullerence in gas phase, not in solution.) LUMO HOMO LUMO Absorbance, L/(mol-cm) 900 800 700 600 500 400 300 200 100 0 300 350 400 450 500 550 600 650 700 750 800 850 900 wavelength (nm) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  13. 13. Summary Notes • I compared the spectra of NKX-2311, NKX-2586, Flavone, and TPA-BTD with published calculations of others (below), and found the agreements were quite good. Zhang et al., J. Photochemistry and Photobiology A: Chemistry, 194, pp.167-172 (2008) Seijas et al., 10th International Electronic Conference on Synthetic Organic Chemistry, 2006 http://www.usc.es/congresos/ecsoc/10/CC/g012/index.htm He et al., Thin Solid Film, 516, pp. 5935-5940 (2008) • The model can be used in selecting and fine-tuning materials for solar cells to maximize sunlight absorption. Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)
  14. 14. • We can examine the frontier orbitals for better pictures of the transitions. The following shows those of NKX-2311. HOMO-3 HOMO-2 HOMO-1 HOMO LUMO LUMO+1 LUMO+2 LUMO+3 (-0.2716 Ha) (-0.2453 Ha) (-0.2371 Ha) (-0.1967 Ha) (-0.0931 Ha) (-0.0469 Ha) (-0.0138 Ha) (0.0046 Ha) Fa-Gung Fan For discussion with Prof. G. Ahmadi of Clarkson University (2/16/2010)

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