Engineering fused coumarin dyes: a molecular level understanding of aggregation quenching and tuning electroluminescence via alkyl chain substitution

Abstract

Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their application fields. Combining long range excimer/exciplex emissions with a compound emission have been used to get white light. In this work, a series of new coumarin derivatives having a planar structure have been synthesized and characterized. The effects of systematic variation in alkyl chain functionalization providing morphological variations that permit interesting solid state emitting properties have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting diode) device applications. Carbon chains containing 0-16 carbon atoms have been studied in order to conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also helpful in establishing a template to control the unproductive interchromophore electronic couplings.Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement with experimental data. Electroluminescence of Device 2 with a turn on voltage (V on ) around 5-6 V having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl substitution of 14-carbons (compound 17) in Device 7 (V on ¼ 7 V) exhibited green emission. Thus a strategy helpful to tune the electroluminescence has been discussed.

Packing directed beneficial role of 3-D rigid alicyclic arms on the templated molecular aggregation problem

“…It is interesting to note that the HOMO-LUMO energy band gap of 2.83 eV showed a shi of 0.05 eV from the previously reported values of 2.88 eV for alkyl chain substituted coumarins. 43 Thus the present approach is useful to eradicate greater shis in optical band gap. These studies also show that one could expect improved emission properties of planar chromophores without noticeable modication in the precursor luminophore.…”

“…7a), and peaks at 515 and 554 nm originated from 1. The long wavelength emission occurring at 610 nm that tails up to 750 nm is the exciplex emission 43 formed at the interface of a-NPD/ 1. It could be noted that bathochromically shied EL is a combination of PL of 1 and the exciplex emission forming at the a-NPD/1 interface.…”

“…We reported earlier that a pristine layer of coumarin derivatives failed to show electroluminescence, rather an emission due to exciplex formed at the interface of the a-NPD/coumarin derivative layer was observed at very high driving voltages. 43 Thus to improve charge transport, 2,3,5,6-tetrauoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) was incorporated in Device 1 just before the a-NPD layer. Device 1 showed a turn on voltage (V on ) of 9 V having structured EL ranging between 450 nm-750 nm (Fig.…”

“…We recently addressed the ACQ problem via a substitution approach on a coumarin derivative and concluded that simple alkyl chains were effective in preventing the close packed arrangement of luminophores 43 to some extent. Since the key idea is to increase the dimensionality of planar molecules through structure engineering, one of the possible ways to achieve this is simply to incorporate three dimensional (3-D) alicyclic ring substituents into the molecular scaffold.…”

“…43,52 Structural modications to coumarin rings have been explored by other research groups to improve solid state emission properties.…”