The pathways of electronic excitation back energy transfer processes (BET) in novel Eu3+ heterocyclic 1,3-diketonates bearing a perfluorinated moiety

Abstract

A series of novel brightly emitting Eu3+ coordination compounds with 1,3-diketones bearing a thiophene moiety and a linear CnF2n+1 chain (n = 1–8) were synthesized and fully characterized. The dependence of back energy transfer (BET) processes on the fluorinated chain length of the ligand environment was investigated in detail. The rates of radiative, non-radiative, and BET processes were estimated by a complete set of spectroscopic methods, including temperature stimulated luminescence measurements. It was demonstrated that fluorination provided a remarkable increase in the photoluminescence quantum yield up to 58% along with an increase in sensitization efficiency from 45 to 86%. It was explained by the reduction of non-radiative relaxation on molecular oscillators. The probability of the BET process has a trend to decrease from 500 to 100 s−1 upon an extension of the diketone fluorinated chain. Moreover, the back energy transfer process occurs from the resonant level of Eu3+ ion to ligand triplet state T1 through activation energy barrier ΔEA between potential energy surfaces of T1 and 5D0 states. It was found that extension of the fluorinated chain in ligand alter the activation energy and frequency factor of BET. The influence of the both parameters on BET was studied. The results obtained give a fundamental basis for further rational design of highly efficient luminescent compounds of Eu3+ ions.