Tuning singlet fission in amphipathic tetracene nanoparticles by controlling molecular packing with side-group engineering
Three tetracene derivatives with one (DPhTc-COOH), two (DPhTc-(COOH)2) or none (DPhTc) carboxylic groups are synthesized and prepared into colloidal nanoparticles in aqueous solution. Theoretical simulation shows that various molecular arrangements are adopted in these three nanoparticles. So different electronic coupling strengths between tetracene units are achieved in these three nanoparticles. Transient absorption spectra demonstrate that SF can be conducted in all nanoparticles but with distinguishing SF dynamics and efficiencies depending on the coupling strength among the adjacent tetracene units. In DPhTc-COOH NP with strong interaction, efficient SF can be proceeded fast, but the resulted triplet pair cannot dissociate into free triplets and recombine into ground state quickly. When the interaction decreases in DPhTc-(COOH)2 and DPhTc NPs, SF will become slowly and less efficient but the triplet pair can dissociate into free triplets or partially separated triplets. Our results demonstrate successfully that the number of carboxylic group influences the tetracene packing and coupling in nanoparticle, as well as SF kinetics and efficiency. Strong interactions between tetracene units is necessary for a quick and efficient SF, but the formed triplet pair cannot separate into long-lived triplet state. To get a high free triplet yield, elaborate tunning on the interaction among the tetracene molecules to achieve a quick dissociation for the triplet pair is necessary.