Surface Loading\nDictates Triplet Production via Singlet\nFission in Anthradithiophene Sensitized TiO<sub>2</sub> Films

Singlet fission,\nthe process of transforming a singlet\nexcited\nstate into two lower energy triplet excited states, is a promising\nstrategy for improving the efficiency of dye-sensitized solar cells.\nThe difficulty in utilizing singlet fission molecules in this architecture\nis understanding and controlling the orientation of dyes on mesoporous\nmetal oxide surfaces to maximize triplet production and minimize detrimental\ndeactivation pathways, such as electron injection from the singlet\nor excimer formation. Here, we varied the concentration of loading\nsolutions of two anthradithiophene dyes derivatized with either one\nor two carboxylic acid groups for binding to a metal oxide surface\nand studied their photophysics using ultrafast transient absorption\nspectroscopy. For the single carboxylic acid case, an increase in\ndye surface coverage led to an increase in apparent triplet excited-state\ngrowth via singlet fission, while the same increase in coverage with\ntwo carboxylic acids did not. This study represents a step toward\ncontrolling the interactions between molecules at mesoporous interfaces.