Polarization and symmetry of electronic transitions in long fluorescence lifetime triangulenium dyes

To fully exploit the capabilities of fluorescence probes in modern experiments, where advanced instrumentation is used to probe complex environments, other photophysical properties than emission color and emission intensity are monitored. Each dye property can be addressed individually as well as collectively to provide in-depth information unavailable from the standard intensity measurements. Dyes with long emission lifetimes and strongly polarized transitions enable the monitoring of lifetime changes as well as emission polarization (anisotropy). Thus experiments can be designed to follow slow dynamics. The UV and visible electronic transitions of a series of red-emitting dyes based on the triangulenium motif are investigated. We resolve overlapping features in the spectra and assign the orientation of the transition moments to the molecular axes. The result is the complete Jablonski diagram for the UV and visible spectral region. The symmetries of the studied dyes are shown to have a large influence on the optical response, and they are clearly separated into two groups of symmetry by their photophysical properties. The C_2v symmetric dyes, azadioxatriangulenium (ADOTA⁺) and diazaoxatriangulenium (DAOTA⁺), have high emission anisotropies, fluorescence lifetimes around 20 ns, and fluorescence quantum yields of ∼50%. The trioxatriangulenium (TOTA⁺) and triazatriangulenium (TATA⁺) dyes - nominally of D_3h symmetry - have fluorescence lifetimes around 10 ns lifetimes and fluorescence quantum yields of 10-15%. However, the D_3h symmetry is shown to be lowered to a point group, where the axes transform uniquely such that the degeneracy of the E′ states is lifted.