Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States of Anthracene Ligand Molecules

Abstract

The combination of CdSe semiconductor nanocrystals with 9-anthracene carboxylic acid ligands can sensitize triplet–triplet annihilation on the emitter molecule diphenylanthracene. This hybrid system has recently been shown to upconvert visible light (532 nm) to ultraviolet light (420 nm) (Huang, Z., et al. Nano Lett. 2015, 15, 5552–5557). In the current paper, time-resolved photoluminescence measurements are used to characterize the kinetics of the energy transfer from the CdSe exciton state to the triplet state of the anthracene ligand. We find that 9-anthracene carboxylic acid binds to CdSe according to Poisson statistics with a maximum number of 2–3 ligands per nanocrystal. The CdSe-to-ligand energy transfer rate is 1.5 × 107 s–1. The overall efficiency of the energy transfer appears to be limited by the presence of fast nonradiative decay channels in the nanocrystals and the low coverage of anthracene ligands, resulting from the specific ligand exchange conditions used in this paper. Possible strategies for improving this component of the hybrid upconversion system are discussed in light of these results.

Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules

“…28 In line with this, CdSe−ZnS core−shell NCs as sensitizers have been reported to enhance the upconversion QYs by a factor of 50 (to 1.4%) compared with core-only CdSe NCs, 29 as the trap states for nonradiative recombination on the CdSe core are passivated by the ZnS shell. Similarly, CdS−ZnS and PbS−CdS core−shell NCs were essential in achieving photon upconversion QYs of 5.2% and 8.4% for the production of UV and visible light, respectively.…”

Nanocrystal–Dye Interactions: Studying the Feasibility of Co‐Sensitization of Dyes with Semiconductor Nanocrystals

“…Usually,C dSe NCs show multi-exponential PL decay, [47,48] due to the presence of surface states and low-lying dark states. [49] These decay curves were best fitted with the sum of three exponentials,i n which the band edge recombination gives mediumr ange lifetime component (t 2 = 5-15 ns), whereas deep traps ands hallow traps provide fastest (t 1 < 5ns) andslowest (t 3 ! 30 ns) lifetime components, respectively.…”

Complementary Lock‐and‐Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer

“…It is well known that the excitedstate energy levels of anthracene are very sensitive to small changes in the molecular structure. [17] This lower rate of TET is perhaps due to the lower number of surface-bound transmitter ligands (ca. The overall T L1 !T Ln transition time decay constants at approximately 445 nm are 800 ps,7 85 ps,a nd 1050 ps for 2,2-PyAn, 2,3-PyAn, and 3,3-PyAn, respectively.W en otice that the decay time constant of the ESA at 437 nm of the nanocrystal photosensitizer in the CdSe/PyAn complexes (t q )isinversely correlated to the growth in the absorption of the triplet excited states on the transmitters at 445 nm (t TET ).…”

“…Communications troscopy,M ongin and co-workers determined the k TET value from CdSe NC to surface-bound 9-ACA to be about 2.0 10 9 s À1 , [5d] which is on the same order of magnitude as the value obtained in this study.Note that the average number of bound ligands, m,was about twelve in their case.Using timeresolved photoluminescence measurements,P iland and coworkers determined ar ate of energy transfer of about 1.5 10 7 s À1 for as imilar system, which is about two orders of magnitude smaller. [17] This lower rate of TET is perhaps due to the lower number of surface-bound transmitter ligands (ca. 2).Wehave considered electron transfer from ligands to CdSe NCs as another possible energy transfer mechanism.…”

“…However, f ET(PyAn!DPA) should be close to 1a st he concentration of DPAi s2 .1 mm,a nd TTAi s diffusion-limited here. [17,23] Thed iscrepancy may arise from the fact that the upconversion QY is measured in the presence of the annihilator whereas the f TET from TA is determined in the absence of the DPAe mitter. TheD PA annihilator is an energy sink for the triplets transferred to the PyAn transmitters and increases the efficiency of TET.…”