I am trying to explain a fluorescence quenching experiment which seems to be static quenching (the fluorescent molecule and the quencher forms a ground state complex which is does not glow). How can I give prove to this mechanism via quantum chemistry ?
For a fluorescent molecule, the emission λ can be calculated by TDDFT, and then be compared with the experiment. For my case, I need to check out whether the quenching was due to the formation of the complex. But can one tell from any calculation that a molecule or complex is NOT fluorescent ? Any computational evidence to prove the complex should be not luminescent ?
This could result from a potential energy surface crossing or avoided crossing. There are many important details when considering exploring these routes computationally. For example, DFT is very questionable and may not be a suitable method and a multireference method will likely be required as at least as a qualitative check. There are many caveats and details to be thought of when doing these calculations. See as a starting point this paper:
Role of conical intersections in molecular spectroscopy and photoinduced chemical dynamics.
Domcke W1, Yarkony DR., Annu Rev Phys Chem. 2012;63:325-52. doi: 10.1146/annurev-physchem-032210-103522.
Miss./Mrs Dao,
There is not possible to obtain absence of emission process using quantum chemistry, because of this method, nevertheless that it is combination of methods, provides you information about energetics of any molecular system (organic, inorganic, metal-organic) associated with energy levels and probability for transition between those levels. Those probabilities can be associated with excitation or emission (in your case), particularly highlighting the optical spectroscopic methods here. This energy profile is strongly specific and depends on molecular structures (respectively, molecular factors) and environmental factors. In other words the energy probability distribution exists. It determines the energy profile of the molecule. How you shall study/establish this electronic structure/profile/distribution, depends of the experimental method used. If you do not receive any TDDFT data means that you do not account for all phenomena associated with emission in metal-organics (or inorganics most probably).
Please pay attention to an attachment showing EAs and Fs of a simple examples of {[ZnII(OH)]+} complex and isolated Zn2+ ion, obtained by TDDFT.
Good idea, but impracticable. Try to find evidence of ground-state complex formation in the absorption spectra.
«Collisional quenching only affects the excited states of the fluorophores, and thus no changes in the absorption spectra are expected. In contrast, ground-state complex formation will frequently result in perturbation of the absorption spectrum of the fluorophore.» (J.R. Lakowicz. Principles of Fluorescence Spectroscopy.)
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