Liquid-phase encapsulation of $\pi$-conjugated dyes in boron nitride nanotubes: Ensemble and single-nanotube optical characterization

Abstract

Boron nitride nanotubes (BNNTs) provide wide-bandgap, optically transparent one-dimensional hosts for molecular dyes, limiting direct electronic participation of the host. Whether dye@BNNT systems produce bright, well-defined J- or H-aggregates or instead heterogeneous emissive ensembles whose character depends on chain length and local packing remains only partly resolved. We address this question using ensemble extinction, photoluminescence, quantum-yield measurements, and TCSPC-derived radiative and non-radiative rates, together with polarization-resolved single-nanotube microscopy on encapsulated quaterthiophene, sexithiophene, octithiophene, and Nile Red, selected from a ten-dye screening. In the oligothiophene series, confinement modifies spectra and excited-state dynamics in a length-dependent manner, with all three oligothiophenes forming weakly emissive ensembles with suppressed effective radiative rates and 6T showing the strongest redistribution between effective radiative and non-radiative decay. The absence of radiative-rate enhancement or fluorescence-lifetime shortening across the series disfavors bright J-aggregate assignments. Polarization-resolved single-nanotube microscopy reveals strongly polarized emission, but with tube-to-tube and intratube variations, identifying oligothiophene@BNNTs as ordered yet structurally heterogeneous confined ensembles. Nile Red provides a complementary case in which the dominant response is dielectric tuning of a solvatochromic charge-transfer state rather than oligothiophene-like aggregate formation. These findings establish dye-filled BNNTs as optically quiet nanoconfined systems in which molecular ordering, dielectric confinement, and guest-guest coupling can be distinguished through combined ensemble and single-nanotube spectroscopy.