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brunosamp

Member

Registered: 2023-04-21

Posts: 18

Fluorescence or absorption output for electron-hole for analysis?

I am trying to analyse the electron-hole population for a pure graphene quantum dot (no functional group) and a graphene quantum dot with hydroxyls. I used M062X/6-31+G(d,p), and the hirshfeld method to calculate the electron-hole overlaps. To achieve this i used the output of the absorption calculation, which i used the following input, for both structures
#p td=(nstates=100, singlet) m062x/6-31+G(d,p) gfinput pop=FULL IOP(9/40=4) density=current

The result of the overlaps between electron and hole was higher for the pure graphene. And the transition dipole moments were:

Transition dipole moment in X/Y/Z:  -0.001971  -2.531006  -0.000003 a.u.  (Pure graphene)
Transition dipole moment in X/Y/Z:   0.090665   0.026479   0.361728 a.u  (Graphene with OH group)

But the experimentation shows that functionalized graphene has a higher fluorescence intensity. I have two questions:

1) I should've used the output of the fluorescence for this analysis?
2) The negative transition dipole moment of graphene means that the graphene with the OH graphene would have higher fluorescence intensity?

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,633

Website

Re: Fluorescence or absorption output for electron-hole for analysis?

1 Which state did you choose for the hole-electron analysis? If S1 was selected for both cases, sometimes you need to consider the possibility of anti-Kasha's rule. Also note that the analysis should be performed at minimum structure of emission state (S1 state if Kasha's rule works), therefore don't forget to optimize excited state geometry first.
Note that to compare theoretical calculation result with experimental observation, many factors should be taken into account. For example, if decay of excited state to ground state via internal conversion is fast, even if transition dipole moment has a large magnitude (and thus large oscillator strength), the fluorescence intensity must still be very weak.
BTW: Adding density=current is not helpful in this case but significantly increase computational cost.

2 Emission strength is positively proportional to square of norm of transition dipole moment (please check equation of evaluating oscillator strength). So, sign of transition dipole moment is unimportant.

brunosamp

Member

Registered: 2023-04-21

Posts: 18

Re: Fluorescence or absorption output for electron-hole for analysis?

Yes, i chose the first excited state. I was using the fchk and output file from a absorption calculation, which i don't think it was correct for the analysis, right? I am now optimizing the excited geometry to do this analysis properly.
Just one more question, if i have a anti-kasha result, should i just choose another excited state?

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,633

Website

Re: Fluorescence or absorption output for electron-hole for analysis?

Yes, you should optimize excited state first.

If you believe the fluorescence emission comes from higher excited state than S1 (e.g. S2), you should optimize the corresponding excited state and perform the analysis again.