Electronic Structures and UV/Visible Absorption Spectra of Organic Compounds

Fa-Gung Fan For discussions with Prof. Damian Allis of Syracuse University and Prof. Goodarz Ahmadi of Clarkson University (2/16/2010)

Electronic Structures and UV/Visible Absorption

Spectra of Organic Compounds

Fa-Gung Fan2/5/2010


[1] In modeling CNT field emission and electron-mol ecule collision cross section. Slides 44-51 and 66-68 of 2/6/2007 ME&IL Seminar pr esented some of the earlier calculations.

• Building upon the previous experience on quantum chemistry ab inito and semi-empirical calculations [1], here I would like to demonstrate our ability to mod el 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.


Compounds StudiedCoumarin derivative dyes:

Flavone

Grey – CWhite – HRed – OBlue – NYellow - S

NKX-2311 (s-trans) NKX-2586 (s-trans)


TPA-BTD donor-acceptor system

Pentacene-fullerene donor-acceptor system


Note: The LUMO-HOMO gap can only be a very crude approximation to the actual energy gap. I think the LUMO-HOMO gap o verestimates the actual excitation energy.

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.


Modeling Methods UsedGeneral 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 Functio nal Theory (DFT) an ab initiomethod, while many other researchers in the same fi eld refer the method ab inito. Here, we use the latter point of view.


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.


NKX-2311 Dye

LUMO - HOMO = 2.8 eV

UV/vis Spectrum of NKX-2311 Dye in Ethanol

0.0E+00

2.0E+04

4.0E+04

6.0E+04

8.0E+04

1.0E+05

1.2E+05

300 350 400 450 500 550 600 650 700 750 800

wavelength (nm)

Abs

orba

nce,

L/(m

ol-c

m)

HOMO LUMO


NKX-2586 Dye


UV/vis Spectrum of NKX-2586 Dye in Ethanol

0.0E+00

2.0E+04

4.0E+04

6.0E+04

8.0E+04

1.0E+05

1.2E+05

1.4E+05

300 350 400 450 500 550 600 650 700 750 800

wavelength (nm)

Abs

orba

nce,

L/(

mol

-cm

)

HOMO LUMO


Flavone


UV/vis Spectrum of Flavone in Methanol

0

10000

20000

30000

40000

50000

60000

200 250 300 350 400

wavelength (nm)

Abs

orba

nce,

L/(m

ol-c

m)

HOMO LUMO


TPA-BTD donor -acceptorLUMO - HOMO = 1.86 eV

Notice the intramolecularcharge transfer

UV/Vis Spectrum of TPA-BTD in Dichloromethane (CH2C l2)

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

200 250 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Abs

orba

nce,

L/(

mol

-cm

)

HOMO LUMO


Pentacene -fullerene donor -acceptorPM3 semi-empirical


HOMO LUMO

HOMO LUMOHOMO LUMO

Notice the intramolecularcharge transfer

UV/vis Spectrum of Pentacene-Fullerene

0100200300400500600700800900

300 350 400 450 500 550 600 650 700 750 800 850 900

wavelength (nm)

Abs

orba

nce,

L/(

mol

-cm

)

(This spectrum is pentacene-fullerence in gas phase, not in solution.)


• 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.htmHe 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.

Summary Notes


• We can examine the frontier orbitals for better pictures of the transitions. The following shows those of NKX-2311.

HOMO-3(-0.2716 Ha)

HOMO-2(-0.2453 Ha)

HOMO-1(-0.2371 Ha)

HOMO(-0.1967 Ha)

LUMO(-0.0931 Ha)

LUMO+1(-0.0469 Ha)

LUMO+2(-0.0138 Ha)

LUMO+3(0.0046 Ha)

Technology

Electronic Structures and UV/Visible Absorption Spectra of Organic Compounds