Theoretical Understanding of Photoluminescence and Singlet Oxygen Quantum Yields in a Few Halogenated Fluorescein Dyes

Visible-light absorbing metal-free organic dyes are of increasing demand for various optoelectronic applications because of their great structure-function tunability through chemical means. Several dyes also show huge potential in triplet photosensitization, generating reactive singlet oxygen. Understanding the structure-property relationships of many well-known fluorescein dyes is of paramount importance in designing next-generation energy efficient dyes, which is currently limited. For example, the role of heavy atoms in the excited-state deactivations is not fully understood for these dyes. Herein, 9 halogenated (Cl, Br, I) fluorescein dyes with varied halogen concentrations and positions are studied using time-dependent range-separated hybrid combined with polarizable continuum model with water dielectric for accounting of polarization and screening effects. Excited state energies of these dyes and their deactivations via radiative and non-radiative pathways are well described using 0-0 corrected excitation energies. Calculated results are in reasonable agreement with the available experimental data. However, no systematic correlation is found between the heavy-atom effect and calculated intersystem crossing/fluorescence rates. Not surprisingly, heavy-atom effect is found to be more pronounced in iodinated dyes compared to their brominated analogues. Halogen position also plays a critical role in determining the excited-state deactivation rates. All dyes show similar fluorescence rates of ~10⁸ $s^{-1}$ . Whereas, the intersystem crossing rates are much smaller and vary from ~10² to 10⁴ $s^{-1}$ . Nevertheless, lower fluorescence quantum yields for some dyes are attributed to the large internal conversion. Microscopic understanding on the excited-state properties of several halogenated fluorescein dyes reported here will aid in developing advanced fluorescein dye based energy efficient photosensitizers and also emitters.