The competence of the reversible exciplex formation and parallel quenching of excitation (by electron or energy transfer) was considered using a non-Markovian pi-forms approach, identical to integral encounter theory (IET). General equations accounting for the reversible quenching and exciplex formation are derived in the contact approximation. Their general solution was obtained and adopted to the most common case when the ground state particles are in great excess. Particular cases of only photoionization or just exciplex formation separately studied earlier by means of IET are reproduced. In the case of the irreversible excitation quenching, the theory allows specifying the yields of the fluorescence and exciplex luminescence, as well as the long time kinetics of excitation and exciplex decays, in the absence of quenching. The theory distinguishes between the alternative regimes of (a) fast equilibration between excitations and exciplexes followed by their decay with a common average rate and (b) the fastest and deep excitation decay followed by the weaker and slower delayed fluorescence, backed by exciplex dissociation.

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We use two water based synthetic approaches to LaF 3 :Nd 3 + nanoparticles (NPs), hydrothermal microwave treatment (HTMW) and co-precipitation (CO) technique, with different temperature of the reaction mixture to study the correlation between the degree of crystallinity of LaF 3 :Nd 3 + NPs and their fluorescence properties. We showed that the fluorescence spectra and quenching kinetics can be a powerful tool to reveal the crystal lattice defects, namely agglomerations of the dopant ions (Nd 3 + ) and the OH À positioned in the volume of the doped NPs. We found that reduced number of such crystal lattice defects as Nd 3 + pairs and clusters and the OH groups in the volume of the HTMW NPs leads to much higher fluorescence brightness than for CO NPs. The higher temperature of reaction mixture during HTMW synthesis results in better crystallinity and much higher fluorescence brightness of the produced NPs. But we believe that these results could be applied more generally to the development of synthetic strategies for bright fluorescent NPs. In sum, it's not the condition of NPs surface, but the degree of their crystallinity should be a major concern while choosing the synthetic path, as it generally predetermines their fluorescent properties.

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