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 1 & pi;& pi;* absorption band as the size of the substituted atom increases combined with an enhancement of the SOC values for the main n & pi;* & RARR; & pi;& pi;* S1 & RARR; 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(-)& BULL; 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.