Hi, I tried to find the emission energies of a molecule. I optimized the S1 with gaussian (wB97XD/6-31+G(d,p)) and did also TDDFT singlepoint calculations with orca on this structure. I found very low oscillator strengths for the S1 emission with several methods and basis sets. Has anyone an idea why this happens or if this is common. (In the experiment you can clearly see a peak). Can it help to optimize the S1 with solvent(PCM, SMD, ...)?
Thank you
The discrepancy between the experimental peak and the low oscillator strength obtained from the theoretical calculations for the S1 emission energy could be due to a number of factors. Some possibilities include:
- Inadequate basis set: The basis set used in the calculations (6-31+G(d,p)) may not be large enough to accurately describe the electronic structure of the molecule, leading to errors in the calculated oscillator strength. Using a larger basis set or a more accurate method such as CCSD(T) or CCSD-F12 may help to improve the accuracy of the calculations.
- Inadequate treatment of electron correlation: The wB97XD functional used in the calculations is a hybrid functional that includes a portion of exact exchange, but it may not fully account for electron correlation effects in the molecule, leading to errors in the calculated oscillator strength. Using a more advanced method such as MRCI or CASSCF may help to improve the accuracy of the calculations.
- Inadequate treatment of solvent effects: The calculations were performed in vacuum, neglecting the presence of solvent, but the experimental measurement were performed in solution. The solvent can affect the electronic structure of the molecule, leading to errors in the calculated oscillator strength. Including a solvent model such as PCM, SMD, or COSMO can help to account for the effects of the solvent on the electronic structure of the molecule, and may help to improve the agreement between the theoretical and experimental results.
- Other electronic states: The S1 state is not the only excited state contributing to the peak, other electronic states such as S0-S1, S1-S2, etc. may also contribute to the peak and hence the oscillator strength of S1 alone might not be enough to explain the peak. Additionally, the presence of vibronic transitions also can affect the emission peak, and it is important to take these into account in the calculations.
- Experimental factors: The experimental conditions (temperature, pressure, solvent, etc.) may not be the same as the theoretical calculations and this can also affect the agreement between the experimental and theoretical results.
Thank you very much Ponniah Vajeeston
I also noticed that other singlet states do have much larger osc. strengths when I compute TDDFT on the S1 geometry:
emission:
WL f_osc
494.5 0.006362 S1
309.2 0.10241494 S2
296.2 0.00750443 S3
285.8 0.58413039 S4
281.3 0.39436335 S5
I do not know if this has any physical meaning. Btw. the singlepoints were calculated with wB97XD3/def2-SVP, CPCM THF, with tuning: w = 0.13
In comparison the absorption values (ground state geometry):
WL f_osc
284.1 1.14182716 S1
258.8 0.10342428 S2
255.3 0.0938676 S3
251.4 0.05463666 S4
246.9 0.01040448 S5
Here the higher states did not contribute much
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