Determination of Upconversion Quantum Yields Using Charge-Transfer State Fluorescence of Heavy-Atom-Free Sensitizer as a Self-Reference

The efficiency of photon upconversion via triplet–triplet annihilation is characterized by an upconversion quantum yield (ΦUC); however, uncertainties remain for its determination. Here, we present a new approach for the relative measurement of ΦUC for green-to-blue upconversion using BODIPY–pyrene donor–acceptor dyad (BD1) as a heavy-atom-free triplet sensitizer. This new approach exploits broad fluorescence from a charge-transfer (CT) state of BD1, which possesses (i) a significant Stokes shift of 181 nm in dichloromethane and (ii) a comparably high CT-fluorescence quantum yield (Φref = 7.0 ± 0.2%), which is independent from oxygen presence and emitter (perylene) concentration while also exhibiting a linear intensity dependence. On the basis of this, we developed an upconversion reference using the BD1 sensitizer mixed with perylene (1 × 10–5 M/1 × 10–4 M) in dichloromethane. With this reference system, we investigated the performance of three BODIPY donor–acceptor dyads in the upconversion process and achieved one of the highest ΦUC of 6.9 ± 0.2% observed for heavy-atom-free sensitizers to date

Absolute quantum yield for understanding upconversion and downshift luminescence in PbF2_{2}:Er3+^{3+},Yb3+^{3+} crystals

The search for new materials capable of efficient upconversion continues to attract attention. In this work, a comprehensive study of the upconversion luminescence in PbF$_{2}$:Er$^{3+}$,Yb$^{3+}$ crystals with different concentrations of Yb$^{3+}$ ions in the range of 2 to 7.5 mol% (Er$^{3+}$ concentration was fixed at 2 mol%) was carried out. The highest value of upconversion quantum yield (ϕ$_{UC}$) 5.9% (at 350 W cm$^{-2}$) was found in the PbF$_{2}$ crystal doped with 2 mol% Er$^{3+}$ and 3 mol% Yb$^{3+}$. Since it is not always easy to directly measure ϕ$_{UC}$ and estimate the related key figure of merit parameter, saturated photoluminescence quantum yield (ϕ$_{UCsat}$), a method to reliably predict ϕ$_{UCsat}$ can be useful. Judd–Ofelt theory provides a convenient way to determine the radiative lifetimes of the excited states of rare-earth ions based on absorption measurements. When the luminescence decay times after direct excitation of a level are also measured, ϕ$_{UCsat}$ for that level can be calculated. This approach is tested on a series of PbF$_{2}$:Er$^{3+}$,Yb$^{3+}$ crystals. Good agreement between the estimates obtained as above and the directly experimentally measured ϕ$_{UCsat}$ values is demonstrated. In addition, three methods of Judd–Ofelt calculations on powder samples were tested and the results were compared with Judd–Ofelt calculations on single crystals, which served as the source of the powder samples. Taken together, the results presented in our work for PbF$_{2}$:Er$^{3+}$,Yb$^{3+}$ crystals contribute to a better understanding of the UC phenomena and provide a reference data set for the use of UC materials in practical applications

Improved photon absorption in dye-functionalized silicon nanocrystals synthesized via microwave-assisted hydrosilylation

Herein, we report a method to produce luminescent silicon nanocrystals (SiNc) that strongly absorb ultra-violet–visible light (300–550 nm) and emit in the near-infrared range (700–1000 nm) with a high photo-luminescence quantum yield (PLQY). Using microwave-assisted hydrosilylation and employing reactivechromophores–such as ethenyl perylene, ethynyl perylene and ethylene-m-phenyl BODIPY–we areable to achieve a 10- and 3-fold enhancement of the absorption in the blue and green spectral range,respectively. The investigated dyes function both as passivating agents and highly efficient antenna, whichabsorb visible light and transfer the energy to SiNc with an efficiency of >95%. This enhanced absorptionleads to a significant photoluminescence enhancement, up to∼270% and∼140% under excitation withblue and green light, respectively. Despite the gain in absolute brightness of the emission, we demon-strate that back energy transfer from the SiNc to the dyes leads to a decrease in the PLQY for dye-modified SiNc, as compared to unmodified SiNc. The synthesis of the SiNc-dye conjugates opens up newpossibilities for applications of this abundant and non-toxic material in thefield of solar energy harvesting,optical sensing and bioimagingviaachieving strong NIR PL excited with visible light

Rare-earth coordination polymers with multimodal luminescence on the nano-, micro-, and milli-second time scales

We present a coordination polymer based on rare-earth metal centers and carboxylated 4,4′-diphenyl-2,2′-bipyridine ligands. We investigate Y$^{3+}$, Lu$^{3+}$, Eu$^{3+}$, and a statistical mixture of Y$^{3+}$ with Eu$^{3+}$ as metal centers. When Y$^{3+}$ or Lu$^{3+}$ is exclusively present in the coordination polymer, biluminescence from the ligand is observed: violet emission from the singlet state (417 nm, 0.9 ns lifetime) and orange emission from the triplet state (585 nm, 76 ms (Y$^{3+}$) and 31 ms (Lu$^{3+}$)). When Eu$^{3+}$ is present in a statistical mixture with Y$^{3+}$, red emission from the Eu$^{3+}$ (611 nm, ∼500μs) is observed in addition to the ligand emissions. We demonstrate that this multi-mode emission is enabled by the immobility of singlet and triplet states on the ligand. Eu$^{3+}$ only receives energy from adjacent ligands. Meanwhile, in the broad inhomogeneous distribution of ligand energies, higher energy states favor singlet emission, whereas faster intersystem crossing in the more stabilized ligands enhances their contribution to triplet emission

Ratiometric Luminescent Thermometry with Excellent Sensitivity over a Broad Temperature Range Utilizing Thermally‐Assisted and Multiphoton Upconversion in Triply‐Doped La₂O₃:Yb³⁺/Er³⁺/Nd³⁺

A ratiometric optical thermometer based on triply‐doped La$_{2}$O$_{3}$:Yb$^{3+}$/Er$^{3+}$/Nd$^{3+}$ microcrystals is reported with a relative sensitivity above 1% K$^{-1}$ in the entire range from 300–700 K, and is between 1.8–0.7% K$^{-1}$ over the range 290–833 K. The 825 nm upconversion (UC) emission from the Nd$^{3+ 4}$F$_{5/2}$ level relies on thermally‐assisted energy transfer from Yb$^{3+}$; thus, unusually, the near‐infrared emission increases with increasing temperature in the relevant range. More typically, the two‐photon 660 nm UC from Er$^{3+ 4}$F$_{9/2}$ level decreases in intensity with increasing temperature due to increasing non‐radiative rates. The variation of fluorescent intensity ratio between these emissions is amplified by their opposite responses to temperature change leading to excellent sensitivity. Concurrently, the different pathways for the temperature response in the two emitting ions enable the high sensitivity to be maintained over an atypically broad temperature range. The wide separation in wavelength means that a standard silicon‐based monochrome camera with broad (inexpensive) band pass filters is sufficient to use this phosphor for thermography. The concept of combining thermally‐activated UC with classical Stokes‐shifted emission is demonstrated to provide combined features of excellent and broad‐range sensitivity plus excellent repeatability. Materials based on this concept are very promising for optical thermometry

BODIPY–pyrene donor–acceptor sensitizers for triplet–triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency

Triplet–triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin–orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY–pyrene heavy-atom-free donor–acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49–58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor–acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor–acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found

Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4_{4}:Yb3+^{3+},Er3+^{3+} nanocrystals and its role in enhancing upconversion luminescence

The sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with a broad and intense optical absorption is an interesting approach for efficient excitation-energy harvesting and enhancing the upconversion luminescence of such UCNCs. In this work, an ultrasmall (∼6.5 nm in diameter) β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$ core and related core@shell UCNCs were sensitized using six NIR-excitable cyanine dyes with a wide range of functional groups and optical properties. The greatest UC enhancement of 680-times was observed for the conjugate between the Cy 754 dye and β-NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs excited using a 754 nm laser. The enhancement was estimated relative to NaGdF$_{4}$:Yb$^{3+}$,Er$^{3+}$@NaGdF$_{4}$:10%Yb$^{3+},30$^{3+} core@shell UCNCs capped with oleic acid and excited using a similar intensity (75 W cm$^{-2}$) of a 980 nm laser. UC intensity measurements for identical dye-sensitized UCNCs carried out in methanol and in deuterated methanol under argon, as well as in air, allowed us to reveal the connection of the dye triplet states with UCNC sensitization as well as of the hydroxyl groups with quenching of the excited states of lanthanide ions. For UCNCs dispersed in methanol, the strong quenching UC luminescence was always observed, including core@shell UCNCs (with a shell of ∼2 nm). A strong influence of the triplet states of the dyes was observed for the two dyes Cy 754 and Cy 792 that bind firmly to UCNCs and allow the distances between the dye and the UCNC to be reduced, whereas the contribution of this sensitization pathway is very insignificant for Cy 740 and Cy 784 dyes that bind weakly to UCNCs