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Features of migration of triplet excitons in organic nanostructures on the porous surface



Triplet-triplet annihilation in the cluster of aromatic molecules adsorbed in a pores of anodized aluminium is investigated. Production of porous oxide aluminium was carried out by the anodic oxidation method of the polished aluminium plates in solutions of sulfuric acid. Adsorption of 1,2 – benzanthracene molecules is realized from a ethanol solution. Measurements of spectral and kinetic characteristics are made with apparatus for the influence of magnetic field study with registration in the photon-counting regime. The pulsed N2 – laser (λgen = 337 nm, Е = 1 mcJ, τ = 10 ns) was used as the photoexcitation source. Registration of decay kinetics of delayed emission is performed after 2 μs after excitation of the sample. The value of magnetic field effect is estimated by the relative change in the intensity of delayed fluorescence in the magnetic field and the absence of field. At photoexcitation of samples the delayed luminescence with a spectrum characteristic for annihilation delayed fluorescence (ADF) of 1,2 – benzanthracene in the condensed matters was observed. Total kinetics of decay of emission is nonexponential. The analysis of decay kinetics has shown, that at the initial moments of time after excitation (t < 0,1 ms) the decay curves are well described by the power function of the form IDF ~ t-n, and on distant times (t > 0,1 ms) exponential decay is observed. The analysis of experimental data on the basis of is formal-kinetic model has shown, that in the chosen interval of temperatures the initial part of experimental kinetic curves well approximatedby power function with index n = 0,3  0,5. Modelling of a long-term part of kinetics of decay was carried out on basis of percolation model. The complicated form of a kinetic curve indicates that in a pores of aluminium there are oxide the 1,2 – benzanthracene clusters with various structure. In the range of time from 0 to 0,1 ms the kind of curve of ADF decay is defined by migration of triplet excitations in clusters with crystal structure. On later times kinetic is formed as a result of triplet annihilation in percolation clusters with higher local heterogeneity. Time dependence of ADF intensity of 1,2 – benzanthracene in porous aluminium oxide from temperature is studied. Tem-perature dependence has two maxima. The first maximum is on Т = 120 K, and the second on Т = 230 K. The first maxi-mum appears weaker with inctrease of delay time of measurement and at mes = 250 μs practically disappears. Occurrence of two maxima in temperature dependence of 1,2 – benzanthracene emission indicates about non-uniform broadering of luminophore T-levels in clusters of both types. The influence of external magnetic field on annihilation of the ADF intensity of 1,2 – benzanthracene at the various registration times at T = 100 K is investigated. The received dependence is characteristic for crystals of aromatic molecules. In the range of small fields from 0 to 0,1 Т the positive magnetic effect is observed. The maximum value of a positive effect equal 7 % for mes = 2 μs. with the course of time the effect size decreases and for mes = 1 ms equal 4 %. After В = 0,1 Т there is a change of a sign on magnetic effect. The maximum value of negative magnetic effect is reached at В = 0,5 Т and equal 27 %. Time dependence of negative magnetic effect at Т = 100 K it is not observed. Magnetic field influence on triplet annihilation at Т = 120 K and Т = 230 K, at which maximum quantum yield of ADF is observed, has shown that the observable positive magnetic effect with increase of registration time starts to fall down.
May 29, 2020

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