Balancing conjugation and charge transfer in benzimidazole-decorated benzothiadiazoles to achieve green/orange HLCT emission with an external quantum efficiency of 9.1%

Abstract

The molecular engineering of the multifunctional benzothiadiazole nucleus offers a classical way to design new fluorophores with excellent properties and opens a promising pathway to harvesting non-radiative triplet excitons via upper-state reverse intersystem crossing. Here, a series of new benzothiadiazole derivatives with a benzimidazole chromophore judiciously substituted in linear and lateral positions are designed and synthesized. A simple positional functionalization and a change of the linker from a phenyl (AB1) to a thiophene unit (AB2) resulted in emissions spanning the gamut from green to orange-red. The structure–property relationship of these molecules was established comprehensively by investigating their theoretical, photophysical, thermal, electrochemical, and electroluminescence properties. The linear-substituted derivative showed a bathochromic shift in absorption (∼53 nm) when compared to the lateral-substituted derivatives, attributed to the extended conjugation induced by systematic orientation. Similarly, the thiophene-linked derivative (AB2) exhibited a 95 nm longer wavelength emission than the phenyl-linked derivative (AB1), which is attributed to the extended conjugation supported by the planar framework of the thiophene unit. The substantial Stokes shift of the AB1 and AB2 molecules indicates the significant contribution of the charge-transfer (CT) component in the excited state, which is also confirmed by the positive solvatochromic behavior of the fluorescence spectra. The excited-state dipole moment, orientation polarizability, and time-resolved fluorescence studies demonstrate the hybridization of local (LE) and CT states (HLCT) in the excited state, with a moderate quantum yield of 70–78%. The excellent thermal stability of all the molecules at high decomposition temperatures reveals the presence of a rigid structure in the solid state. The multi-layered, solution-processed organic light-emitting diodes fabricated with the AB1 and AB2 dyes display maximum external quantum efficiencies (EQEs) of 8.9% and 9.1%, with green and orange-red emissions associated with the CIE coordinates of (0.21, 0.41) and (0.55, 0.44), respectively. The observed EQE is significantly higher than the theoretical limit of fluorescent molecules (5%), boosted by the simultaneous presence of the HLCT excited state in the molecules and the possibility of higher-order triplet to singlet energy transfer due to the small energy splitting between the states.