Ultrafast spectroscopy of size-dependent hole-transfer-mediated triplet energy transfer in a quantum dot-molecule complex

Triplet energy transfer (TET) from quantum dots to molecules has attracted tremendous attention in the last decade because the long-lived triplet state of molecules benefits various photophysical or photochemical processes and quantum dots (QDs), as the triplet sensitizer, provide prominent optical absorption and emission tunable through flexible size and composition control. Although several dynamical models for some certain QD-molecule complexes have been proposed, the TET mechanism in such complexes is still incomplete due to intricate energy level configuration. More specifics of TET being revealed is advantageous for its further applications. In this paper, a series of QD-molecule complexes composed of CdSe QDs with different diameters and 5-tetracene carboxylic acid (TCA) are assembled. The precise size control of CdSe QDs is realized by combining the transmission electron microscope and steady-state spectra. All CdSe-TCA complexes display ex-pected Type-II like band alignment, which favors the hole-transfer-mediated TET confirmed by a series of spectral measurements. On the surface, ultrafast spec-troscopy shows the charge dynamics, specifically the stepwise hole and electron transfer during TET, slow down with increasing size of the CdSe QDs. However, comprehensive analysis of the experimental results reveals that the hole transfer and the following electron transfer essentially depend on the energy level difference and wavefunction overlap between the QD donor and molecule acceptor rather than simply the QD size. The conclusions can facilitate our understanding of the TET mechanism in a QD-molecule complex and help optimize the performance of TET-related photoelectronic devices designed based on size control.