Quantum chemistry ab initio and semi-empirical calculations

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. • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. • 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)