Observation of Ultranarrow Band Red Photoluminescence from Pure Organic Self‐Assembled T2T Micro‐Rods: A Route to High‐Resolution Light Sources

Ultranarrow emission linewidths and high spectral purity are essential for next-generation displays and advanced optoelectronic/photonic applications. A red photoluminescence (PL) peak with a full width at half-maximum (FWHM) of 3.2 nm at 625 nm is reported from pure organic π-conjugated 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) self-assembled micro-rods (SAMRs). The sharp PL emission intensifies under prolonged exposure and increased laser power, indicating a photo-brightening (PB) effect. T2T SAMRs are fabricated via thermal annealing of reprecipitated T2T, which facilitates the molecular-scale reorganization of T2T molecules into ordered domains, thereby promoting high-quality π-conjugated crystalline structures. Structural and spectroscopic analyses-including Raman spectroscopy, grazing-incidence wide-angle X-ray scattering, and density functional theory calculations-reveal that the narrow 625 nm PL originates from self-trapped excitons (STEs) within an ordered J-aggregated triclinic lattice framework. Additionally, upon PB, a linear increase in STE PL intensity with laser power, along with the prolonged exciton lifetime, is observed for single-stranded T2T SAMRs, which distinguishes STE generation from lasing or amplified spontaneous emission. Remarkably, the emission wavelength remains stable across different laser excitation wavelengths (375, 405, 532 nm), heteromolecular systems, and various crystal sizes, underscoring the robustness of the STE state. These findings position T2T SAMRs as promising candidates for high-resolution, high-color-purity red-light sources.