A star-shaped triazine-BODIPY trimer with fluorene spacers: synthesis, aggregation, and environment-dependent photophysical properties

Abstract

A novel star-shaped triazine-BODIPY trimer incorporating fluorene spacers (Tz-Fl-BDP) was successfully synthesized and well characterized. Experimental and computational investigations were carried out to elucidate its structural, electronic, and excited-state properties. The photophysical behavior of Tz-Fl-BDP was examined across solvents of varying polarity using absorption, steady-state, and time-resolved fluorescence, and dynamic light scattering (DLS) analyses. The molecule exhibited two dominant absorption bands at ∼417 and 530 nm, corresponding to higher-energy donor-to-acceptor charge-transfer transitions (ICT) and lower-energy localized BODIPY-centered π → π* transitions, respectively. Strong emission was observed in nonpolar solvents but progressively quenched in polar protic media owing to solvent-promoted non-radiative decay, while red-shifted, broadened emission in water is consistent with aggregation-induced quenching (AIQ). Time-resolved fluorescence and DLS studies correlated solvent-dependent aggregation with excited-state relaxation dynamics. A weak fluorescence quenching (1.47-fold) was observed upon addition of triethylamine (final concentration ≈ 1.25 mM) in THF, suggesting possible base-assisted non-radiative relaxation pathways that may involve possible PET interactions. In contrast, an increase in emission intensity was observed in viscous media (DMSO–glycerol mixture), consistent with viscosity-enhanced fluorescence arising from restriction of intramolecular motion (RIM). Collectively, these findings demonstrate that the introduction of a fluorene spacer into triazine-BODIPY frameworks enables environment-responsive fluorescence, providing new design insights for environment-responsive fluorophores with improved structural rigidity and stable emission behavior.