Photonic Upconversion Materials for Organic Lanthanide Complexes: History
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Hong-Ju Yin
,
Zhong-Gui Xiao
,
Yansong Feng
,
Chang-Jiang Yao

Organic lanthanide complexes have garnered significant attention in various fields due to their intriguing energy transfer mechanism, enabling the upconversion (UC) of two or more low-energy photons into high-energy photons. In comparison to lanthanide-doped inorganic nanoparticles, organic UC complexes hold great promise for biological delivery applications due to their advantageous properties of controllable size and composition.

  • upconversion luminescence
  • organic lanthanide complexes
  • sensitizer
  • activator
  • mechanism

1. The Basic Concept of Organic Lanthanide-Based Upconversion

Organic upconversion (UC) materials mainly include two parts: an activator (accepter) and a sensitizer (donor). The UC phenomena are evoked by sensitizer-activator interactions.

1.1. Sensitizer

The role of the sensitizer is to absorb light from the pumping source and requires a relatively large absorption cross section (σabs), usually at least one order of magnitude higher than the associated activator. Until now, the three most commonly used sensitizers were Yb3+, Nd3+, and Er3+. The sensitizer Yb3+ directly interacts with the activator (e.g., Er3+, Tm3+, and Ho3+). While the sensitizers Nd3+ and Er3+ need the cooperation of Yb3+ to complete the whole process.

1.2. Activator

The activator is used to achieve upconversion luminescence (UCL) by using its stepped energy levels. Because each activator has its own unique energy level structure, its UCL shows sharp emission peaks with a differentiable spectroscopic fingerprint. Most of the Ln3+ ions can produce UCL, such as Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, and Tm3+. Until now, Er3+ ions have been the most efficient activators for green and red UCL. Er3+ ions can also be sensitizers because of their nearly perfect ladder-like energy levels. More interestingly, Er3+ ions occur simultaneously as sensitizers and activators and can produce strong UCL through self-sensitization.

1.3. The Mechanisms of Upconversion Luminescence

UCL mechanisms include excited state absorption (ESA), energy transfer upconversion (ETU), cooperative luminescence (CL), and cooperative upconversion (CU). ESA describes the simplest UC process, where the emission is caused by sequential absorption within the energy levels of a given ion. In principle, ESA could occur in many of the Ln3+ ions, such as Er3+, Dy3+, Tm3+, Ho3+, and Pr3+. ESA only plays an important role when the doping concentration of Ln3+ is relatively low, and materials based on the ESA mechanism are limited by low absorption and low UC emission intensity. ETU is recognized as the dominant mechanism in the most efficient UC system. UC can be realized from the activator (A) in a more excited state, while the sensitizer (S) returns to its ground state. Note that excited states sometimes transition to lower energy levels beyond the ground state, and this particular ETU process is also called cross relaxation (CR). Compared with ESA, ETU allows a much higher doping concentration of Ln3+, which offers a much stronger absorption ability to the material. The UC efficiency of ETU is usually more than two orders of magnitude higher than that of ESA. Cooperative upconversion (CU) is also known as cooperative sensitization upconversion (CSU) in some literature. One activator (A) ion receives energy from two nearby excited sensitizer (B) ions at the same time, and the excited activator relaxes back to its ground state by producing a UC photon. The rare cooperative luminescence (CL) lacks an activator-centered excited relay and is replaced by a virtual emissive state [1].

2. Applications

In recent decades, organic Ln(III) complexes based on UCL have been rapidly developed, and various Ln(III) complexes with different mechanisms have been prepared. These materials not only have excellent luminescence properties derived from Ln(III), but also exhibit favorable solubility, customizable particle size, and excellent film-forming characteristics, which make them have certain application potential in the fields of bioimaging and solar cells.

2.1. Bioimaging Application

NIR imaging has gained significant interest in the biomedical industry due to its advantages over traditional visible-light imaging [2]. Longer wavelength stimulation allows for improved tissue penetration by minimizing self-absorption and light scattering caused by the heterogeneity of human tissue [3]. In the last decades, numerous Ln complexes have been investigated as luminescence probes or imaging agents for bio-applications [4][5][6]. While several reviews on this subject have been published elsewhere [7][8][9][10][11][12], optical imaging has emerged as a precise method for observing cellular metabolism and physiological behavior [13]. For effective biological imaging, Ln(III) complexes require a long luminescence lifetime, bright luminescence, a high quantum yield, significant photostability and thermal stability, as well as a large Stokes shift. However, most Ln(III) complexes faced limitations in living cell bioimaging due to UV-radiation damage, limited exposure time, photobleaching, local bioluminescence scattering, and poor tissue penetration. UC materials offer notable advantages in the field of bioimaging, such as increased excitation penetration depth in vivo, eliminating autofluorescence, and preventing irradiated tissue damage. These materials efficiently convert low-energy, deep-penetrating light (longer wavelength) to higher-energy light (shorter wavelength). In this section, the overall research on bioimaging applications in vivo of organic Ln(III) complexes based on UC is summarized.
NIR radiation Yb(III) complexes have garnered significant attention in the field of multiphoton cell imaging. Despite their low luminescent quantum yield, Yb(III) complexes allowed excitation and emission in both the NIR-to-visible and NIR-to-NIR configurations [14][15]. NIR excitation by UC minimizes the scattering and autofluorescence from biological media, enhances the S/N ratio, and enables deep tissue penetration for imaging and bioanalysis [14][15][16]. In 2011, Wong and co-workers reported an Yb complex with exceptional luminescence properties in water and a remarkable two-photon cross-section. Interestingly, this complex was successfully utilized as a biological probe for imaging HeLa cells by two-photon microscopy, operating in the classic NIR-to-visible configuration for detection of residual ligand central emission [17]. Complexes [YbL16] were also identified as a 2P NIR biological probe with an emission wavelength of 980 nm, demonstrating a promising thick-tissue imaging probe [18]. In 2015, Maury et al. synthesized a samarium complex [SmL15] structurally similar to [YbL16]. The stained fixed cells could be imaged in the visible light and NIR spectrums. Under the same conditions, the 2P images obtained by [SmL15] in the NIR spectrum showed a similar distribution to that obtained with [YbL16]  [19].  In 2017, Maury et al. prepared Yb(III) complexes ([YbL17]Otf) based on the dimethyl cycline macrocyclic ligand and obtained high-quality images in both the classic NIR-to-vis configuration and the more challenging NIR-to-NIR configuration [20]. Yb(III) complexes offer potential for biological imaging and serve as effective light therapy agents for deep tissues due to their NIR excitation and emission and unique cooperative upconversion luminescence properties. In 2019, Patra and co-workers designed double-sensitized (L14/L13 and L5) Yb(III) complexes to modulate the desirable optical properties in the NIR region for bioimaging [21]. The cytosolic and nuclear localizations were monitored by utilizing their unique and intrinsic cooperative upconversion luminescence of Yb(III) ions in the NIR-to-visible region (λex = 950 nm) and multiphoton excitation (λex = 750 nm) using confocal fluorescence microscopy in HeLa and H460 cancer cells. The cellular uptake studies clearly demonstrated the cytoplasmic and nuclear localization of the complex. The NIR cytotoxicity of [Yb(L13)(L5)3] upon continuous 980 nm irradiation by a laser demonstrated its potential for PDT. These results provided an intelligent strategy for the development of light-responsive, highlight-stable Yb(III) probes for NIR therapy applications in biologically transparent phototherapeutic windows [21].
Bioavailability and cytotoxicity are important criteria for evaluating potential applications of bioimaging probes [22][23][24]. The advantages of organic lanthanide complexes over inorganic nanomaterials and organic molecules include tunable metal toxicity and easy modification [25]. For instance, when utilized in biological imaging, numerous visible/NIR emissive lanthanide complexes showed low toxicity [26][27][28][29][30][31][32]. Significant differences in the sensitivities of the applied cells as well as in the cytotoxic effects of various lanthanides on the same cell line were observed. By studying the interaction of Eu(III) with FaDu cells, Sachs and co-workers [30] found that the cytotoxicity of Eu(III) on FaDu cells was correlated to their cellular uptake, both being mainly concentration-dependent and only slightly time-dependent. They demonstrated that the cytotoxicity of Eu(III) on FaDu cells was mainly controlled by insoluble species. Additionally, organic lanthanide complexes can be structurally modified to simply increase the biocompatibility of the complexes and control the bioavailability of probe molecules [25]. Although bioimaging applications of organic lanthanide-based UC materials have been reported in a small number of cases, in-depth research concerning the impact of the lanthanide speciation on their cytotoxic behavior and bioavailability is still lacking.

2.2. Solar Cell Application

Recently, upconversion has gained recognition as a promising field in “third–generation photovoltaics”, which aims to overcome the efficiency limits of traditional single-threshold photovoltaic devices (Shockley–Queisser limit [33]) [34][35]. Silicon solar cells are crucial for renewable energy sources [36], and the use of Ln UC materials has shown potential due to their ability to absorb sunlight for silicon solar cells [37]. Planar luminescent layers embedded with LnCs in polymer substrates such as PVA, PMMA, and EVA were often used as spectral converters for silicon solar cells to improve PCE [38]. In this context, Wang and co-workers [39] developed a highly stable luminescent copolymer film consisting of the EuIII complex [Eu(CTAC)(ND)4, CTAC = hexadecyl trimethyl ammonium chloride, ND = 4-hydroxy-2-methyl-1,5-naphthyridine-3-carbonitrile] and EVA. Coating the surface of a large-area polycrystalline silicon solar cell (110 cm2) with a stable luminescent film resulted in an increased PCE of 15.06 to 15.57. The UC strategy has been proven to prolong the NIR response of the PVSCs by converting NIR light into visible light [40][41], which is subsequently reabsorbed by the perovskite photoactive layer to generate additional photocurrent in the PVSCs. Unlike the high light intensities required to integrate Ln(III)-based UCNPs into PVSCs, the TTA UC process in organic semiconductors can be efficient even at subsolar photon fluxes due to the energy stored in the long-lived triplet states. However, to our knowledge, the UCL of organic Ln(III) complexes has been rarely reported in PVSCs.
In addition, recent studies have highlighted the potential of organic–inorganic fluorescent materials containing LnIII for use in LED devices as light-converting materials [42][43]. However, there is limited research on the utilization of Ln organic complexes for UCL in LEDs.

This entry is adapted from the peer-reviewed paper 10.3390/ma16165642

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