Study the Luminescence of Phosphors and Doped Materials

• Polymer system structures are optimized using the PM3 method within Hyperchem software.
• INDO/1 parameterized for spectroscopy is used to calculate the many absorption spectra using ZINDO software.

• In most cases the emission spectrum is a mirror image of the absorption spectrum shifted to lower energy.
• ZINDO is a well parameterized to accurately calculate the absorption spectra but does not calculate emission spectra.
• In the past we have been using the experimental Stoke's shift but this does not allow the computational effort to be independent.
• The most desirable technique would be a blend of speed and accuracy due to the many structures we whish to investigate.

(a) Rough Estimate for Stoke's Shift

• Reoptimization energies of molecules in the first excited state are usually calculated with expensive Configuration interaction calculations.
• Because most of the S₀ to S₁ transitions are HOMO to LUMO transitons, we can get a rough estimate of the reoptimization energy for a single configuration calculation, where one electron is excited from the HOMO to the LUMO while keeping the total spin the same (singlet state).

a) General Method

• There are two ways to calculate an estimate for the luminescence peak.
• Both rely on calculating the geometry of the single configuration representing S₁.
• After the geometry is calculated the Stoke's shift can be calculated by either.
• Calculation of the energy used to changed the molecular structure
• Because the energy between S₀ and S₁ changes we can use ZINDO to calculate the energy difference between these states. (This is technically absorption but it does give a method of how far apart the energy states are at the new geometry).
• PPP-OR11 and PPP-R10 will be used as test cases.

b) PPP-OR11 This model contains a total of 142 atoms. The model is a reproduction of the system shown below with one repeat unit (n=1). Excitation spectra are calculated using models with four repeat units (n=4). i) Calculations Calculated with ZINDO method. This method calculates the energy difference between the S0 and S1 states. Very good agreement for this rough approximation. Reoptimization method calculates almost identical emission peak.

c) PPP-R10 This model contains a total of 126 atoms. The model is a reproduction of the system below with one repeat unit (n=1). Excitation spectra are calculated using models with four repeat units (n=4). i) Calculations Calculated with ZINDO method. This method calculates the energy difference between the S0 and S1 states. Again very good agreement for this rough approximation.

d) UV-Vis Absorbance and Emission of 2-Styrylthiophene

	Predicted Values	Published Values*
Solvent	Vacuum	MeOH	BuCN	AcCN	Aq.	n He
λABS(nm)	359	321	324	321	306	321
λEM(nm)	421	375	370	380	360	354

*Note: The experimental values were determined in solvent and exhibit a shift from predicted values in vacuum.

e) Dependence of S₀ to S₁ Transition on Structure and Angle

Relative total energy and S0 to S1 transition energy as a function of a fixed dihedral angle for the unsubstituted TV-PPV structure.

Relative total energy and S0 to S1 transition energy as a function of a fixed dihedral angle for the substituted TV-PPV structure.

• We have used the single configuration of one electron moving from the HOMO to the LUMO to estimate the geometry of the molecule in the first excited state.
• With this geometry we can calculate the reoptimization energy directly or we can calculate the energy difference between the S₀ and S₁ states.
• In the cases of PPP-R10 and PPP-OR11 both yield surprisingly accurate estimates for emission.
• Comparisons to the literature for functionalized oligomers shows good agreement between the predicted and measured values.