Critical Sensitizer Quality Attributes for Efficient Triplet–Triplet Annihilation Upconversion with Low Power Density Thresholds

Abstract

Triplet–triplet annihilation upconversion (TTAUC) is a power density-dependent process where photons of low energy are transformed into high energy ones. The most important attributes of efficient TTAUC are quantum yield ΦTTAUC, power density threshold I th (photon flux at which 50% of ΦTTAUC is achieved), and the upconversion shift of emitted photons (anti-Stokes shift). To date, approaches to balance these parameters have remained unclear. Herein, the cumulative effect of sensitizer triplet lifetime (τ0 S), sensitizer-annihilator triplet energy gap (ΔE T), and the total concentration of the sensitizer on the power density threshold at high TTAUC quantum yields is evaluated experimentally using Pt, Pd, and Zn tetraphenyl­porphyrin derivatives and a tetra-tert-butylperylene annihilator, and by kinetic rate modeling. The results suggest that a large energy gap (ΔE T ≥ 4 k B T) and long sensitizer triplet lifetime make the triplet–triplet energy transfer (TTET) extremely efficient and allow the utilization of high sensitizer concentrations for low I th. However, for large upconversion shifts, the triplet energy gap should be as small as possible. Smaller energy gap values result in slower forward TTET and faster reverse TTET, which together with high total sensitizer concentration can lead to a quenching of annihilator’s triplet state and therefore elevate the I th. In this regard, low concentration of a sensitizer is beneficial, making sensitizers with high molar extinction coefficients preferential. Sensitizers with a long living triplet state and a high molar extinction coefficient can work efficiently and have low I th at 0 k B T or even negative ΔE T. Kinetic rate modeling further helps to optimize the parameters for best possible TTAUC performance. Thus, the findings of the study pave the way for the design of TTAUC systems with superior performance, such as high ΦTTAUC at low excitation power densities with large anti-Stokes shift, for, for example, solar-driven photovoltaics, photocatalysis, bioimaging, and safe light-triggered drug-delivery systems.

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

“…We assumed that the values of ϕ UC > 0.5 × ϕ max UC can be relevant for practical applications. 68 Thus, the value of excitation intensity corresponding to ϕ UC ≈ 0.5 × ϕ max UC was used as I th and the values of 0.3 W cm −2 and 0.03 W cm −2 were estimated for BPD1 (Fig. S9b, ESI†) and BPD2 (Fig.…”

Triplet-triplet annihilation photon-upconversion in hydrophilic media with biorelevant cholesteryl triplet energy acceptors

“…A recent strategy in this regards proposed to achieve the TTA-UC processes in PEG200, which is a hydrophilic polymeric liquid that mimics cell membrane structures and intracellular conditions. [13][14][15][16] With this in mind, we are hereby reporting two new cholesteryl-based 9,10-diphenylanthracene Recently, a new family of triplet sensitizers: quinoidal naphthyl thioamides (QDN) (Scheme 1) were reported by our group. 17 QDN are structurally compatible with over-the-counter acceptors/fluorescent polyaromatic hydrocarbons such as perylene 18,19 and 9,10diphenylanthracene (DPA) which are routinely employed for TTA-UC processes.…”

Efficiency of Thermally Activated Delayed Fluorescence Sensitized Triplet Upconversion Doubled in Three‐Component System

“…Although the ternary system also showed a high threshold excitation intensity, this could be altered by changing the excitation wavelength, potentially altering the sensitizer concentration, or utilizing molecular design strategies to prearrange the sensitizer and acceptor components or enable their self‐assembly. [ 11,39,40 ] We also show that the use of 4CzIPN‐Br with a higher Φ ISC , but similar S 1 and T 1 energy levels to 4CzIPN, resulted in significantly lower delayed fluorescence quenching for the acceptors naphthalene, 1‐methylnaphthalene and p ‐terphenyl, suggesting the need for a greater understanding of the factors governing Φ TET between these molecules. While the excitation threshold intensity can still be improved by controlling the relative organization of the upconversion components, this research provides an example of an effective acceptor screening process, demonstrates the unique opportunities 3‐component upconversion blends, provides for tuning individual steps of upconversion, and highlights the need to control I th in the ternary TTA‐UC systems.…”

“…Some strategies proposed to decrease I th include altering the sensitizer concentration to reduce the amount of reverse TET back to the sensitizer , covalent attachment of the sensitizer and acceptor molecules to a backbone structure, and utilization of molecular self‐assembly. [ 11,39,40 ] Managing the I th will be important in order to take advantage of the ternary TTA‐UC system design for efficiently supplying triplet excitons to bright (high QY) emitters without sacrificing the low I th values which make TTA attractive.…”