Photophysical, optical and luminescent characteristics of heterocyclic-substituted coumarins and their application in OLED-devices

The development of organic electronics stimulates the search for new materials. The priority task is to find compounds with high brightness, efficiency and stability of luminescence. Coumarin derivatives are considered as promising candidates for solving this problem. This paper presents the results of a study of organic light-emitting diodes in the emission layer of which a number of coumarin dyes with pronounced donor-acceptor properties are used. The aim of the study was to identify the influence of the structure of synthesized molecules on the photophysical characteristics as well as on the emission efficiency of LEDs based on them. A series of organic compounds of the coumarin series has been synthesized: (E)-3-(3-(anthracene-9-yl)acryloyl)coumarin (compound 1), 4-hydroxy-3-(5-(4-methoxyphenyl)-1-(p-tolyl)-4,5-dihydro-1H-pyrazol-3-yl)coumarin (compound 2), 3-(1-acetyl-5-(4-methoxyphenyl)-4,5-dihydro-1H-pyrazol-3-yl)-4-hydroxycoumarin (compound 3), ethyl 7-(diethylamino)Coumarin-3-carboxylate (compound 4) as well as the well-known laser dye Coumarin 6 (3-(benzo[d]thiazole-2-yl)-7-(diethylamino)coumarin) used as a reference compound. The LEDs were produced by vacuum thermal spraying and spin-coating. The fluorescence and electroluminescence spectra were studied using an Ocean Optics Maya 200 PRO spectrometer. A photomultiplier was used to obtain the luminescence decay curves, PicoQuant PMA-C 192-N-M. Spectral data (absorption, photoluminescence) as well as time-resolved measurements (fluorescence attenuation time) indicate the key role of donor-acceptor interactions as well as spatial effects in the formation of electronic transitions. The current-voltage characteristics confirmed the presence of conduction modes limited by spatial charge and conduction limited by carrier capture processes. The study of the voltage-brightness characteristics showed that compound 2 demonstrates brightness comparable to the reference compound Coumarin 6, which makes it the most promising for further optimization of organic light-emitting diodes. In addition, it is shown that compound 4 in the device provides white emission with chromaticity coordinates close to daylight, which makes it potentially possible for practical use in lighting systems. The data obtained confirm the influence of donor-acceptor interactions on the properties of coumarins. The degree of conjugation of donor and acceptor fragments is directly determined by spectral shifts in the absorption and fluorescence spectra. The high brightness of compound 2-based diodes, comparable to the Cou standard, is due to its efficient donor-acceptor system which optimizes intramolecular charge transfer and increases the likelihood of radiative transitions (long lifetime of the excited state 3.5 ns). On the contrary, the acetyl group in compound 3 disrupts conjugation, leading to low brightness and short fluorescence lifetime (1.7 ns) due to nonradiative relaxation. The ability of compound 4 to provide white radiation in diodes (correlated color temperature 6410 K, close to daylight) is related to the contribution of the electron transport layer to the emission spectrum.