Estimation of the fluorescence lifetime for optically inaccessible exciplexes in nonpolar solutions under ionizing irradiation

Abstract

X-irradiation of nonpolar solutions likely provides a possibility to create exciplexes for any donor-acceptor pair that would energetically and sterically allow this. Thorough study and characterization of X-radiation generated exciplexes usually cannot be carried out with conventional methods because of the complex and non-exponential formation and decay dynamics of these species. In this paper, we present a simple and universal experimental approach for the estimation of fluorescence lifetimes (τF) of X-radiation generated exciplexes. The suggested procedure is based on the comparison of quenching of the exciplex emission band and the emission band from a standard luminophore with a known excited state lifetime by dissolved oxygen. Using this approach we report the τF values for two systems with optically inaccessible exciplexes, diphenylacetylene-N,N-dimethylaniline (DMA) and p-terphenyl-DMA, and for two typical exciplex forming systems, naphthalene-DMA and anthracene-DMA. All the found τF values for the X-radiation generated exciplexes lie in the range of 50-70 ns. The accuracy of this approach was checked by time-resolved measurements under X- or near-UV irradiation for those pairs, whose properties make this feasible. The proposed method gives a possibility to avoid a complex numerical evaluation of the non-exponential kinetics of recombination luminescence, and can be used to estimate the characteristic τF values for luminophores and excited complexes formed under X-radiation.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

“…Lifetimes of recombination exciplexes for all diphenylacetylenes of this work with DMA in DDC were estimated using a universal CW-based approach previously suggested for radiation-generated exciplexes . Thus obtained lifetimes are given in Table and amount to ∼60 ns for all three acceptor-substituted diphenylacetylenes 1CF 3 , 2CF 3 , 3CF 3 ; ∼45 ns for the parent DPA , 1OMe , and 2OMe ; and ∼35 ns for 3OMe .…”

“…(A) Normalized X-ray generated luminescence spectra for the mixtures of 6 × 10 –3 M of the corresponding diphenylacetylene and 1 × 10 –2 M DMA in DDC. (B) Normalized emission spectra of the corresponding diphenylacetylene/ DMA exciplex obtained from emission spectra of the same sample solution of 6 × 10 –3 M of the corresponding diphenylacetylene and 1 × 10 –2 M DMA in DDC recorded in the presence and absence of dissolved oxygen (see the text and ref for details).…”

“…Integral quenching was evaluated by integrating the long wavelength side of the total emission band corresponding to the exciplex emission without contribution from the intrinsic luminophore emission, as indicated in Figure A and Figures SI3.3–SI3.9, and taking the ratio of the two integrals . For most spectra with well-separated exciplex and intrinsic emission bands this was done directly on experimental spectra, like spectra (1) and (2) from Figure A.…”

“…134 Integral quenching was evaluated by integrating the long wavelength side of the total emission band corresponding to the exciplex emission without contribution from the intrinsic luminophore emission, as indicated in Figure 8A and Figures SI3.3−SI3.9, and taking the ratio of the two integrals. 126 For most spectra with well-separated exciplex and intrinsic emission bands this was done directly on experimental spectra, like spectra (1) and ( 2) from Figure 8A. For the mixtures of 1OMe or 3OMe with DMA, where the two emission bands substantially overlap (Figures SI3.6 and SI3.8 of the SI), the spectra were decomposed into the sum of spectra of the individual emission components using the approach of refs 70 and 131.…”

“…Figure 6B shows the extracted exciplex emission spectra, obtained by taking pairs of luminescence spectra from the same sample with removed dissolved oxygen and equilibrated with atmosphere, normalizing them by the maximum of the short wavelength band, and subtracting from each other. This procedure takes advantage of the much longer emission lifetime, and thus more efficient quenching, of exciplexes in comparison with intrinsic emission producing the short wavelength band and was suggested by a reviewer to ref 126. More details of it will be given below when discussing exciplex lifetime measurements.…”