Heavy atom effect through chalcogen substitution in Red Nile dye: a theoretical investigation

A DFT and TDDFT study has been carried out on Red Nile dye (RN) and on two chalcogens-derivatives in which the carbonyl-oxygen was replaced by Sulfur-(SNR) and Selenium-(SeNR) in order to evaluate the effect of such substitution on their photophysical properties. Inspection of Type I and Type II photoreactions have been achieved exploring the absorption properties, computing spin–orbit coupling, proposing the most plausible deactivation channels leading to the population of excited triplet states and through the analysis of vertical electron affinities and ionization potentials. Our data show a useful bathochromic shift of the lowest energy 1ππ* absorption band as the size of the substituted atom increases combined with an enhancement of the SOC values for the main nπ* → ππ* S1 → T1 deactivation pathways, suggesting a more efficient ISC mechanism for the thio- and seleno-dye. Moreover, the introduction of selenium in the NR dye has the intriguing effect to promote also the electron transfer reaction producing the O2(−)• species through the TypeI mechanism, allowing us to propose that designed dye as a dual TypeI/II PS. Our study supports the strategy to afford heavy-atom effect by using chalcogens showing as it could be successfully exploited to achieve metal-free PSs with desirable properties.