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Re^9: How to check for acceptance of Kasha's rule from TDDFT data...?

Ilya Ioffe

Dear Siddheshwar,

First, you need to decide which excited states are responsible for fluorescence. If the system was initially excited to S1 then the answer is more or less unambiguous. If it was excited to some higher excited state, make an educated guess whether the Kasha's rule holds. The lower is the gap between Sn and S1, the higher is the probability that it does, i.e. that Sn rapidly decays into S1 and one observes fluorescence from S1. In fact, there are theoretical approaches to estimate the spontaneous transition rates between the states but that would be much more complex and we'd better skip that.
As for the emission wavelength, for fluorescence from Sn you need, indeed, to subtract the DFT energy of S0 from the TDDFT energy of Sn. In fact, the result is already printed in the output, see "TDDFT EXCITATION ENERGIES".The HOMO-LUMO gap can be an inappropriate approximation for the S0-S1 transition and you've done TDDFT anyway. Note that TDDFT energies are not ideal either. You are advised to learn about the limitations of TDDFT from the appropriate sources, this discussion board is not a very suitable medium for that.

Best regards

On Sat Sep 27 '14 8:36am, Siddheshwar Chopra wrote
>Dear Ilya,
>I think I have finally started understanding now. I must really thank you
>for helping me.
>So now I understand that the excited states mentioned in the o/p are the same... Sn states.. like we get in abdorption calcs.. The only difference in emission calc. is that we start with optimized S1 geometry here.
>Now could you help me how to analyse the emission spectra from the EXCITED STATES information? As Pavlo sir mentioned that it is not mandatory for kasha's rule to hold good, please explain how do we discuss with respect to these excited states.
>Also how to get the value of emission wavelength? Is it just the difference between homo, lumo values? or I need to subtract the energy values of relaxed Sn and S0?

>Kind Regards,
>On Fri Sep 26 '14 8:36pm, Ilya Ioffe wrote
>>Dear Siddheshwar,

>>In TDDFT, the reference is always the ground state, irrespective of the state you optimize. In other words, TDDFT makes possible to minimize energy of the excited state with respect to geometric variations but electronically you still optimize the ground state by means of DFT and then compute the excitations from it in the linear response approximation. Accordingly, the output contains data for the transitions between the ground electronic state, S0, and the excited electronic states, S1 to S_NSTATE (same osc. str. for absorption and stimulated emission). There are no vibrational levels, that's a completely different task. For absorption spectra optimize S0, for fluorescence from Sn optimize Sn.
>>On Fri Sep 26 '14 4:49pm, Siddheshwar Chopra wrote
>>>Dear Ilya,
>>>Thats really nice of you to explain in such simple words. Ilya I understand the point 1 you said about kasha's rule. I also understood that osc. strs. are reported between S0 and excited states.
>>>My doubt is that:: Say I have the optimized geometry of S1 now.. Now Iwant the emission spectra. So in this case, the osc. strs. etc. printed in the output file will be for S1 and excited states(S0 vibr. levels) OR will be for S0 and excited states (S1 vibr. levels)??
>>>I hope you got my confusion well...

>>>Kind Regards,
>>>On Fri Sep 26 '14 2:40pm, Ilya Ioffe wrote
>>>>Dear Siddheshwar,

>>>>1) Kasha's rule is of kinetic nature. It suggests that radiationless relaxation from Sn to S1 (with further radiative decay to S0) is normally of higher probability than fluorescence directly from Sn. But nothing requires S1 to have higher oscillator strength.

>>>>2) ISTATE=x means that you are requesting properties for the excited state no. x and/or that you are optimizing it. The excitation energies and oscillator strengths are always reported between S0 and each of the excited states requested by NSTATE, not between the excited states (you would need a multiconfiguration method rather than TDDFT to compute that). If you'd like to calculate fluorescence energy from a particular relaxed Sx, optimize this very Sx.

>>>>Best regards,

>>>>On Thu Sep 25 '14 2:18pm, Siddheshwar Chopra wrote
>>>>>Dear Pavlo,

>>>>>Please have a look at the energies for 3 excited states from absorption and emission runs:

>>>>> EXCITED STATE 1 ENERGY= -1355.7217994424 S = 0.0 SPACE SYM = A

>>>>> EXCITED STATE 2 ENERGY= -1355.6780193460 S = 0.0 SPACE SYM = A

>>>>> EXCITED STATE 3 ENERGY= -1355.6756729663 S = 0.0 SPACE SYM = A

>>>>> EXCITED STATE 1 ENERGY= -1355.7080939866 S = 0.0 SPACE SYM = A

>>>>> EXCITED STATE 2 ENERGY= -1355.6840777343 S = 0.0 SPACE SYM = A

>>>>> EXCITED STATE 3 ENERGY= -1355.6782292662 S = 0.0 SPACE SYM = A

>>>>>then what are these excited states? Ideally speaking in this run, S1 is the ground state. The excited state 1 has the lowest energy (-1355.7217994424). Then it should be the lowest vibrational level of S0.
>>>>>Please help me in this analysis.

>>>>>Kind Regards,

>>>>>On Thu Sep 25 '14 6:47am, Siddheshwar Chopra wrote
>>>>>>Dear Pavlo,
>>>>>>That is some real relief... Actually the same under study is a doped one. Sir please read my last message and answer them. The picture of Emission process is unclear to me, with regard to the firefly calculations. The ISSTATE=1 is ENSURING that we are finding a transitions from S1 only..Is it correct?
>>>>>>So now SPECIFICALLY about the emission calculations::: What are these EXCITED STATE 1, 2 and so on (depending on NSTATE vaue)? I mean are these the S0 vibrational levels now (because now we are studying de-excitation)? If yes, then how exactly do I find the emission wavelength? Is the analysis similar to the absorption spectra analysis then? I mean using osc. str. etc.? Please guide me.
>>>>>>Also if Kasha's rule holds good, then do I get a high osc. str. at 1st excited state itself in the o/p file?


>>>>>>On Thu Sep 25 '14 1:42am, Pavlo Solntsev wrote
>>>>>>>Kasha's rule doesn't always work. For example, if you have redox active group near chromophore.

>>>>>>>Try to figure out absorbtion--emission situation for a porphyrin.

>>>>>>>On Wed Sep 24 '14 9:11pm, Siddheshwar Chopra wrote
>>>>>>>>Dear users,
>>>>>>>>Somebody please help me in understanding this concept. I need to know that by setting istate =1 in emission spectra calculation, I am actually setting or calculating the de-excitation from the S1 state. Am I correct? If yes, then the TDDFT o/p gives me a set of transitions possible for every excited state (set by variable nstate).What are these nstates now? Do they hold any significance in S1-->S0 transition? Do we mean that these are the vibrational levels of S0 level? Please explain this in the light of kasha's rule.
>>>>>>>>I am worried because I am unable to point out the emission wavelength. Please help me Alex Sir and Pavlo Sir. Please help me in analysing the emission calculations.

>>>>>>>>Kind Regards,
>>>>>>>>n Tue Sep 23 '14 12:21pm, Siddheshwar Chopra wrote
>>>>>>>>>Dear Users,
>>>>>>>>>Please have a look at the attached file which contains a table describing the contributions/transitions from first four states only for both the absorption and emission calculations. For absorption, I understand that the final state can be any of the excited states which has highest osc. strength. Am I right? But my doubt is regarding kasha's rule... If you see the emission data, the first excited state has a very low oscillator strength of 0.004 (HOMO-->LUMO(97 at 2678.9 nm. However the higher states have a better contribution than this state. So please tell me how to analyse this properly. To my knowledge emission wavelength is around 600-730nm.
>>>>>>>>>I would be grateful for any help.

>>>>>>>>>Kind Regards,

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