Experimental and Theoretical Insights into Light Confinement within 2D Waveguides of Alkylphenyl Benzothiadiazole Crystals

Abstract

In this work, an expermental and theoretical investigation was carried out to explore the key factors influencing light transmission mechanisms in organic crystalline materials. Three benzothiadiazole (BTD) derivatives with slight structural modifications were selected due to their strong self-assembly capabilities, which enabled control over molecular packing and waveguide behavior. High-quality single crystals were obtained via the slow diffusion method, revealing distinct waveguiding properties: B1 and B2 function as two-dimensional (2D) waveguides, whereas B3 behaves as a one-dimensional (1D) waveguide. These differences arise from variations in crystal packing and directional S–N interactions, which consistently guide growth along the [100] direction. In B3, the dominance of S–N interactions solely along this axis promotes fiber formation, while B1 and B2 benefit from a more favorable dipole-field alignment in both longitudinal and transverse directions.Optical loss coefficients (OLC) indicate that B1 and B2 perform better as optical waveguides, exhibiting significantly lower losses (2–8.7 × 10⁻³ dB μm⁻¹) compared to B3 (1.07 × 10⁻² dB μm⁻¹). These differences are attributed to variations in exciton energy, as determined by the CIS|CNDOL/1CS method applied to simulated aggregates for both molecules and crystals, which imply more localized Frenkel excitons with a greater ability to generate polaritons and, therefore, a better capability to transmit light. They are also attributed to disparities in microchannel size and density within the crystal structures. The insights gained provide a comprehensive understanding of the key factors that influence light transmission.