Investigating the Sulfur "Twist" on the Photophysics of DBD Dyes

The so-called DBD ([1,3]dioxolo[4,5-f][1,3]benzodioxole) dyes are a new class of fluorescent dyes, with tunable photophysical properties like absorption, fluorescence lifetime, and Stokes shift. With the development of sulfur based DBDs, this dye class is extended even further for possible applications in spectroscopy and microscopy. In this paper we are investigating the basic photophysical properties and their implications for future applications for S⁴-DBD as well as O⁴-DBD. On the basis of time-resolved laser fluorescence spectroscopy, transient absorption spectroscopy, and UV/vis-spectroscopy, we determined the rate constants of the radiative and nonradiative deactivation processes as well as the energy of respective electronic states involved in the electronic deactivation of S⁴-DBD and of O⁴-DBD. For S⁴-DBD we unraveled the triplet formation with intersystem crossing quantum yields of up to 80%. By TD-DFT calculations we estimated a triplet energy of around 13500-14700 cm^-1 depending on the DBD dye and solvent. Through solvent dependent measurements, we found quadrupole moments in the range of 2 B.