Vitaliy A Tolkachev; Vladimir S Kalinov

doi:10.19080/JOJMS.2024.09.555754

Research Article

Pure Electronic Transitions of Non-Optically Excited Luminescence

Vitaliy A Tolkachev and Vladimir S Kalinov*

Center Photonics of Atomic and Molecular Structures of the Institute of Physics, National Academy of Sciences of Belarus, Belarus

Submitted:October 23, 2024;Published: November 20, 2024

*Corresponding author: Vladimir S Kalinov, Center Photonics of Atomic and Molecular Structures of the Institute of Physics, National Academy of Sciences of Belarus, Nezavisimosty prospect 68, Minsk, Belarus

How to cite this article: Vitaliy A Tolkachev, Vladimir S Kalinov. Pure Electronic Transitions of Non-Optically Excited Luminescence. JOJ Material Sci. 2024; 9(1): 555753.DOI: 10.19080/JOJMS.2024.09.555754

Abstract

A method to obtain pure electronic transition (PET) value from the non-optically excited luminescence vibronic (electron-nuclear) spectra is described. This method is based on the hypothesis that microreversibility of optical transitions of elementary vibronic state and the started states of transition are in thermal equilibrium, and hence the emission spectrum follows the quantum fluctuation theorem (QFT), which connects Stokes and anti-Stokes emission regions. The solution of QFT relationship gives the PET frequency at the experimental condition. The method was tested on optical vibronic absorption and emission spectra of molecular structures, beginning with polyatomic molecules in vapor state and up to condensed complicatedly organized molecular structures. Here the tests show that PET of non-optically excited emission is detectible if luminophor ensemble is homogeneous and in thermal equilibrium. Examples to obtain PET for electro-, bio-, chemi-, mechano-, crystallo-, solvo- and lyoluminescence are given. The method indicates non-homogeneity of luminophore ensembles.

Keywords:Pure electronic transition; Electroluminescence; Chemiluminescence; Bioluminescence; Mechanoluminescence; Lyoluminescence; Luminophor homogeneity

Introduction

The attempts to determine the frequency of pure electronic transition (PET) from diffuse vibronic spectra have a long history. The first successive attempts were made in the middle of the last century. By the model of full mirror symmetry of absorption and emission spectra (cross-section spectra), relations have been obtained, which now are presented as a form of quantum fluctuation theorem (QFT) equation [1,2]. Soon after [3] it was proposed to use QFT to obtain the electronic transition value. But the model had a defect: electron structure of finished (after relaxation) and started states never coincide. So, the model had an indefinite level of approximation and was not accepted for application. But in quantum mechanics the transition between elementary states under external perturbation is reversible (‘microscopic reversibility’). The problem of microscopic reversibility in quantum system has been studied for a long time [4,5]. But there are no papers on microscopic reversibility for systems defined by diffuse vibronic spectra. So, the hypothesis of microscopic reversibility for vibronic transition was proposed too. The hypothesis resulted in obtaining QFT for vibronic transition probability (cross-section) and based the method to obtain pure electronic transition (PET). The method was previously tested on molecular structures in vapor [6,7], solutions [7,8], quantum dots, molecular and doped crystals [9,10] as well as complex molecular systems and polarization-selected [10-12] chromophore ensembles. Non-optically excited luminescence has a specific nature of excitation, and hence the method testing based on the considered microreversibility hypothesis for these systems is actual. The excited states of non-optically excited luminescence arise from a queue of complicated physical-chemical processes. Their nature is not vivid and needs to be tested with that method.

Theory

As it was stated before, the form of QFT for luminescence with the assumption of approximate mirror symmetry model was obtained in [1-3]. With microreversibility base it was obtained only in [6]. As luminescence intensity I(ν) and cross-section(σν)(σ(ν)=I(ν)/ν⁴)for frequency ν(here and further below in 1/cv units) and ννΔν=|v₀-v|, so for ν₀ as PET frequency, T as temperature and Boltzmann constant k QFT relation for luminescence has the form:

As it is seen from (3), non-homogeneous luminophor does not show a clear extremum, but the sum of overlapped extremes region. That behavior manifests non-homogeneity. The partial extremum intensities are proportional to partial luminescence input. In previous papers on non-optically excited luminescence [13,14] it was shown that electro-, chemi- and bioluminescence exhibit PET only through homogeneous luminophor ensembles. It was found that for an emitting luminophor ensemble the PET of optically and non-optically excited luminescence can coincide. This was observed for a number of substances and materials.

Results and Discussion

Figure 1A shows the PET of electroluminescence, optically excited luminescence and absorption of rigid poly(p-terphenylenvinilen) (P3/5V) film [15] and is found by relation (2). All emission and absorption PET coincide, what means the uniformity and identity of corresponding chromophors. In Fig 1B electroluminescence and optically excited luminescence spectra of MEH-PPV:PbS composite [16] are compared. For the comparison the electroluminescence spectrum of nanocrystals PbS is given. Their pure electronic transitions differ as well as the spectra, but pure nanocrystals electroluminescence shows two extrema: main relative to electroluminescence of pure nanocrystals and feeble second which coincide with composite ones. Similar manifestation of the method is seen in chemiluminescence and its appearance in biology [13]. Figure 2 gives an example of the behaviour [17]. For the spectra at pH 6.1 and 6.8 spectrum coincides. As the spectra in the region of are the same, one can subtract from spectra an additional component by bringing the regions into coincidence. The obtained by this way normalized additional component (5) is highlighted with red. So the considered approach brings two forms of luminophor in all area of pH only: acidic and alkaline. With that method the same result is obtained for the luminophor of Photinus piralis firefly [13].

The example of triboluminescence spectra of zinc sulfide base doped by manganese material (ZnS:Mn) is shown in Figure 3. The triboluminescence spectrum is compared to the UV-excited spectra, electron (20keV) and proton (3MeV) beams luminescence [18]. For all the spectra the frequency of pure electronic transition is the same (ν₀ = 18200 ± 200eV, error of numeration). So, the chromophore of the material is highly homogeneous. The “noise” of triboluminescence ϕ − function spectrum is a result of the luminescence spectrum “fluctuations”.

Figure 4 gives an example of crystalloluminescence spectra (XTL spectra) of methyl-n-buthyl ketone and N,Ndimethilacetamid mixture and spectrum of UV-excited by 320nm light phosphorescence for comparison [19,20]. Here the spectra and the pure electronic transitions are all different. The crystalloluminescent luminophore is non-excitable by UV-light probably because of its dynamic nature. It is short-living in the chain of dynamical transformation of structures where excited luminescent states are produced. So, it is detected by the emission only.

In Figure 5 the solvoluminescence of metal-organic crystals of cyclic trigold complex previously irradiated by UV-light is considered. The emission arises at the contact with acetone. In spite of the “noise”, it is seen that PET extremum is shifted to higher energies as usual maximum of spectrum. There are no other spectra to coincide with the solvoluminescence, so, it would be reasonable to suppose that dynamic luminophor behavior is the same as it was previously for XTL-luminescence. The mechanoluminescence of the composite based on SrAl2O3:Eu is effective for observation of stress distribution. The mechanoluminescence spectra of that material, photoluminescence and electroluminescence are shown in Figure 6. All extremums of PET are displaced to higher energy in relation to luminescence extremums. The electroluminescent and photoluminescent luminophores coincide, but nonoptically excited mechanoluminescent and electroluminescent luminophores differ.

Power supply

Figure 7 manifests piezoluminescent spectra of ZnS:Mn, with adapter PMN-PT. Though the spectra of piezo- and photoexited luminescence almost coincide, the -function spectra differ, showing, besides of the main one, an intensive transition of shortwave photoluminophores. The example of non-homogeneous non-optically excited luminophor luminescence is demonstrated in Figure 8. The luminescence of a flexible composite film based on combination of copper doped zinc sulfide with polydimethylsiloxane (7:3) was registered. The average size of ZnS particles was. In Figure 8A the mechano- and electroluminescence spectra are given. As it is seen, both of the spectra indicate full luminophor non-homogeneity. Even durable dynamic load does not change luminescence behavior, as it is seen from Figure 8B.

Conclusion

All presented data result in conclusion that emission mechanism of non-optically excited luminescence is identical to optically excited one. It is the same with any photo-induced process like photoconductivity. The method is mostly important as it gives PET at rigorous, limited by experiment conditions, even for dynamic short-living luminophores. It supports the microreversibility of transitions in complex molecular systems as it is shown for quantum systems by quantum-mechanical consideration at least in neighborhood of pure electronic transition.