Limitation of room temperature phosphorescence efficiency in metal organic frameworks due to triplet-triplet annihilation

The effect of triplet-triplet annihilation (TTA) on the room-temperature phosphorescence (RTP) in metal-organic frameworks (MOFs) is studied in benchmark RTP MOFs based on Zn metal centers and isophthalic or terephthalic acid linkers (ZnIPA and ZnTPA). The ratio of RTP to singlet fluorescence is observed to decrease with increasing excitation power density. Explicitly, in ZnIPA the ratio of the RTP to fluorescence is 0.58 at 1.04 mW cm(−2), but only 0.42 at (the still modest) 52.6 mW cm(−2). The decrease in ratio is due to the reduction of RTP efficiency at higher excitation due to TTA. The density of triplet states increases at higher excitation power densities, allowing triplets to diffuse far enough during their long lifetime to meet another triplet and annihilate. On the other hand, the shorter-lived singlet species can never meet an annihilate. Therefore, the singlet fluorescence scales linearly with excitation power density whereas the RTP scales sub-linearly. Equivalently, the efficiency of fluorescence is unaffected by excitation power density but the efficiency of RTP is significantly reduced at higher excitation power density due to TTA. Interestingly, in time-resolved measurements, the fraction of fast decay increases but the lifetime of long tail of the RTP remains unaffected by excitation power density. This may be due to the confinement of triplets to individual grains, leading decay to be faster until there is only one triplet per grain left. Subsequently, the remaining “lone triplets” decay with the unchanging rate expressed by the long tail. These results increase the understanding of RTP in MOFs by explicitly showing the importance of TTA in determining the (excitation power density dependent) efficiency of RTP. Also, for applications in optical sensing, these results suggest that a method based on long tail lifetime of the RTP is preferable to a ratiometric approach as the former will not be affected by variation in excitation power density whereas the latter will be

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

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

Ultra-broadband near-infrared upconversion for solar energy harvesting

Upconversion – the absorption of two or more photons resulting in radiative emission at a higher energy than the excitation – has the potential to enhance the efficiency of solar energy harvesting technologies, most notably photovoltaics. However, the required ultra-high light intensities and the narrow absorption bands of lanthanide ions limit efficient solar utilisation. In this paper, we report results from exciting upconverters with concentrated sunlight at flux densities up to 2300 suns, where the radiation is restricted to photon energies below the bandgap of silicon (corresponding to a wavelength λ = 1200 nm). Upconversion to λ = 980 nm is achieved by using hexagonal erbium-doped sodium yttrium fluoride (β-NaYF4: Er3+) in a fluoropolymer matrix. Upconversion has a nonlinear relation with irradiance, therefore at a high irradiance a threshold occurs where the process becomes linear. For β-NaYF4:25%Er3+, we find a two-photon threshold under concentrated sunlight at 320 suns. Notably, this threshold is lower than under corresponding laser excitation and can be related to all resonantly excited Er3+ ion levels and excited stated absorption. These results highlight a pathway that utilises a far broader portion of the solar spectrum for photovoltaics