Tailoring triplet energy transfer and Förster resonance energy transfer in boron dipyrromethene modified lanthanide doped upconversion nanohybrids.

Tailoring the energy transfer (ET) processes, including the singlet energy transfer (SET) or triplet energy transfer (TET) process, in hybrid systems consisting of inorganic nanoparticles and organic molecules can offer a strategy for the design and synthesis of bright fluorescent probes for photodynamic therapy or photocatalytic applications. However, the weak absorption and low quantum yield of lanthanide doped upconversion nanoparticles (UCNPs) limited the performance of the molecule-UCNP hybrid system, and the complex mechanism of the ET process involving singlet and triplet excitons is yet to be fully understood. Here, we report on a strategy for the synthesis of boron dipyrromethene (BODIPY) modified UCNP nanohybrids to engineer the SET or TET processes, specifically 8-(4-carboxyphenyl)-3,5-(4-hydroxyl)styryl-1,7-tetramethyl-pyrromethene fluoroborate (BDP-1) and 8-(4-carboxyphenyl)-2,6-diiodo-3,5-(4-hydroxyl)styryl-1,7-tetramethyl-pyrromethene fluoroborate (IBDP-1). These nanohybrids exhibit enhanced upconversion performance with an 800-times increase in upconversion quantum yield (UCQY) and efficient singlet oxygen (1O2) generation under 980 nm excitation. Our quantum chemistry calculations suggest that the energy transfer in these systems takes place by the Förster resonance energy transfer (FRET) and back energy transfer (BET) in NaYbF4:2%Er3+@BDP-1 (UCNP@BDP-1), and that direct triplet energy transfer (TET) from Yb3+ to anchored IBDP-1 is the dominant energy transfer mechanism in NaYbF4:2%Er3+@IBDP-1 (UCNP@IBDP-1).