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

Abstract 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 ~10 8 . Whereas, the intersystem crossing rates are much smaller and vary from ~10 2 to 10 4 . 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.