Tunable emission in aggregated T-Shaped 2H-Benzo[d][1,2,3]triazoles with waveguide behaviour
Graphical abstract
Introduction
Organic molecules that exhibit intense and tunable emission have received a great deal of attention and play an important role in next-generation of electronic applications due to their processability, flexibility, ultrathin aspect and large area. Among these organic nanomaterials, one-dimensional (1D) nanostructures have proven to be effective building blocks for miniaturized devices. In this respect, it is worth highlighting tunable colour displays, like Organic Light-Emitting Diodes (OLEDs) [1], Organic Field-Effect Transistors (OFETs) [2], chemical sensors [3] and optical and optoelectronic devices such as lasers [4] and optical waveguides [5].
1D Organic nano- and micro- structures have emerged as effective flexible media to generate or propagate and manipulate light efficiently on the sub-wavelength scale and these are considered to be fundamental elements and interconnectors in optical circuits. Within this framework, organic molecular crystals can serve as active optical waveguides [6], [7] since higher crystallinity improves the photon-crystal lattice interactions and the charge-transport mobility. A number of articles have been published to date regarding the use of organic compounds as optical waveguides. In many cases these compounds emit light from 400 to 600 nm. However, examples of organic waveguide materials with a red emission above 600 nm are still rare [8].
The modification or alteration of chemical structures is a common approach to tune the solid-state luminescence properties of organic materials. Most 1D organic waveguides are fabricated from small molecules with a π-conjugated structure due to their capacity to delocalize (move) electrons [9]. This property, along with the ready availability, facile synthesis and high purity, makes these compounds attractive components for device applications.
The absorption in these π-conjugated systems can be modulated by the construction of low band gap organic materials in which electron donor (D) and electron acceptor (A) moieties are incorporated into the molecular building blocks. This design facilitates intramolecular charge transfer (ICT) [10]. As a consequence, the band gap levels and other related optical properties can be readily tuned through the modification of donors and acceptors. Compared to the conventional D-A systems, other donor-acceptor-donor (D-A-D) types of chromophore will facilitate stronger ICT and lower the band gap energy further [11]. Moreover, the modification of moieties and substituents in conjugated structures not only provides strong ICT but also modulates the self-assembly behaviour, facilitates crystal packing and contributes to their luminescence or the chromogenic phenomenon.
The introduction of heterocycles or heteroatoms into π-conjugated systems is also a useful approach for the construction of ICT compounds [12]. In this regard, benzotriazole is a moderate electron acceptor moiety for which it is assumed that the polarizable imine unit is responsible for its electronic character [13]. The introduction of different substituents on the nitrogen in the 2-position offers the possibility of modulating the electron acceptor properties [14].
We are actively working on the synthesis of 4H-1,2,4-triazole and 2H-benzo[d][1,2,3]triazole derivatives that form aggregates which are capable of acting as tunable optical waveguides [15], [16], [17]. The latter series has a T-shaped geometry with spatial overlap between the HOMO and the LUMO, which is necessary for the large transition dipole moments that give rise to intense absorptions.
In this context, and as part of our ongoing research, we describe here the synthesis of T-shaped 2H-benzo[d][1,2,3]triazole D-A-D derivatives, the design of which was based on a computational study aimed at modulating the photoluminescent properties and self-assembly behaviour based on the peripheral donor groups. The properties of these aggregates as organic waveguides with tunable colour emission were studied.
Section snippets
Results and discussion
Our initial goal was to develop a new series of T-shaped D-π-A-π-D benzotriazole derivatives with ICT character. The target compounds contain two electron-donor arms with different features and the benzotriazole unit as the electron-acceptor core. This design allows the efficient tuning of the HOMO-LUMO gap by changing the donor and acceptor groups [18].
Structural modifications in molecular aggregates have a marked influence on their fluorescence properties such as the emission wavelength [19].
Conclusions
T-shaped 2H-benzo[d][1,2,3]triazole derivatives with Internal Charge Transfer (ICT) behaviour have been studied by DFT/TD calculations. Several compounds were then synthesized using a sustainable methodology, i.e., microwave irradiation as the energy source, a marked decrease in the amount of solvent and the use of a reusable catalyst. The introduction of different donor substituents in the horizontal arm of these compounds allowed the HOMO-LUMO gap to be modulated in order to tune the emission
General
Reagents were used as purchased. All air-sensitive reactions were carried out under an argon atmosphere. Flash chromatography was performed using silica gel (Merck, Kieselgel 60, 230–240 mesh or Scharlau 60, 230–240 mesh). Analytical thin layer chromatography (TLC) was performed using aluminium-coated Merck Kieselgel 60 F254 plates. NMR spectra were recorded on a Varian Unity 500 (1H: 500 MHz; 13C: 125 MHz) spectrometer at 298 K using deuterated solvents an internally referenced against the
Acknowledgements
Financial support from the MINECO of Spain (project CTQ2014-53600-R) and CTQ2014-52331R), UCLM (GI20163531) and Gobierno de Aragón (Research Group E39) is gratefully acknowledged. I. Torres is indebted to MEC for an FPU studentship. Moreover, technical support from the High Performance Computing Service of the University of Castilla-La Mancha is gratefully acknowledged.
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