Lanthanide Photoluminescence in Heterometallic Polycyanidometallate-Based Coordination Networks

Solid-state functional luminescent materials arouse an enormous scientific interest due to their diverse applications in lighting, display devices, photonics, optical communication, low energy scintillation, optical storage, light conversion, or photovoltaics. Among all types of solid luminophors, the emissive coordination polymers, especially those based on luminescent trivalent lanthanide ions, exhibit a particularly large scope of light-emitting functionalities, fruitfully investigated in the aspects of chemical sensing, display devices, and bioimaging. Here, we present the complete overview of one of the promising families of photoluminescent coordination compounds, that are heterometallic d–f cyanido-bridged networks composed of lanthanide(3+) ions connected through cyanide bridges with polycyanidometallates of d-block metal ions. We are showing that the combination of cationic lanthanide complexes of selected inorganic and organic ligands with anionic homoligand [M(CN)x]n− (x = 2, 4, 6 and 8) or heteroligand [M(L)(CN)4]2− (L = bidentate organic ligand, M = transition metal ions) anions is the efficient route towards the emissive coordination networks revealing important optical properties, including 4f-metal-centred visible and near-infrared emission sensitized through metal-to-metal and/or ligand-to-metal energy transfer processes, and multi-coloured photoluminescence switchable by external stimuli such as excitation wavelength, temperature, or pressure.


Introduction
Luminescent materials, that are able to emit the light due to absorption of photons, electric current, chemical reactions, or a mechanical action, are applied in the numerous aspects of science, technology, and everyday life. They are utilized in cathode ray or fluorescent tubes, X-ray detectors, lighting and display devices, optical communication, low-energy scintillation, optical storage, light conversion, photovoltaics, chemical sensing, bioimaging, and molecular thermometry [1][2][3][4][5][6]. Considering the construction of light-emitting devices, there are several particularly desired optical functionalities, including white-light emission (WLE), multi-coloured tunable emission, long-lived near-infrared (NIR) phosphorescence, and the non-linear optical property of up-conversion luminescence (UCL) [7][8][9][10]. These functionalities are efficiently realized by the photo-and electroluminescent materials, which are the most attractive in the application aspects [1][2][3][4][5][6].
Among the functional photoluminescent materials, the prime role has been played by traditional inorganic solids, such as oxides, fluorites, and silicates, that can incorporate emissive metal centres, mainly trivalent lanthanide ions revealing a wide range of photoluminescence ranging from UV for Gd 3+ , through visible for Eu 3+ or Tb 3+ , to NIR light for Nd 3+ or Yb 3+ [2]. Photoluminescence was also broadly investigated for organic molecules, especially those built of the expanded system of  Au) cyanido-bridged coordination polymers: (a) the views of the representative fragment of the structure together with the views of the whole network along a and c crystallographic directions, (b) the low-temperature (T = 20 K, λexc = 364 nm) pressure-dependent emission spectra of EuAu network, and the temperature-dependent emission spectra of TbAu compound (λexc = 350 nm). Colours for the structural diagrams: Ln, red; Ag/Au, dark blue; CN − , blue; H2O, grey [57,58,60]. Reprinted with permission from Inorg. Chem The further reduction of coordination dimensionality for lanthanide(III)-dicyanidometallates(I) compounds was achieved by the implementation of N,N,N-tridentate 2,2':6'2"-terpyridine (terpy) ligand. The resulting discrete dinuclear [Ln III (terpy)(H2O)(NO3)2][Au I (CN)2] (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) molecules exhibit the various lanthanide-dependent emission properties [71,72]. In the case of EuAu and TbAu, the dominant visible 4f-centred luminescence was detected due to the efficient intramolecular terpy-to-Ln 3+ and partial Au I -to-Ln 3+ energy transfer effects. It is accompanied by the residual broad green emission related to the transitions of the [Au I (CN)2] − 2 dimeric excimers that are formed between the cyanido-bridged molecules. The Au I -based emission is dominant for the other lanthanides where the ET processes to 4f metal centre are much less favoured [72]. In GdAu analogue involving Gd 3+ ion, not luminescent in the visible range, the [Au I (CN)2] − 2-based green emission is also observed but the emission spectrum is dominated by the other intense red photoluminescence, presumably assigned to the formation of an excited state exciplex of the closely stacked intermolecular terpy ligands [71].   The further reduction of coordination dimensionality for lanthanide(III)-dicyanidometallates(I) compounds was achieved by the implementation of N,N,N-tridentate 2,2':6'2"-terpyridine (terpy) ligand. The resulting discrete dinuclear [Ln III (terpy)(H 2 O)(NO 3 ) 2 ][Au I (CN) 2 ] (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) molecules exhibit the various lanthanide-dependent emission properties [71,72]. In the case of EuAu and TbAu, the dominant visible 4f-centred luminescence was detected due to the efficient intramolecular terpy-to-Ln 3+ and partial Au I -to-Ln 3+ energy transfer effects. It is accompanied by the residual broad green emission related to the transitions of the [Au I (CN) 2 ] − 2 dimeric excimers that are formed between the cyanido-bridged molecules. The Au I -based emission is dominant for the other lanthanides where the ET processes to 4f metal centre are much less favoured [72]. In GdAu analogue Molecules 2017, 22, 1902 5 of 30 involving Gd 3+ ion, not luminescent in the visible range, the [Au I (CN) 2 ] − 2 -based green emission is also observed but the emission spectrum is dominated by the other intense red photoluminescence, presumably assigned to the formation of an excited state exciplex of the closely stacked intermolecular terpy ligands [71].  [73] 1 cyanido-bridged skeleton, excluding the Ag-Ag or Au-Au short contacts; 2 ET = energy transfer; 3 MLCT = metal-to-ligand charge transfer; 4 terpy = 2,2 :6 2 -terpyridine; 5 bbp = bis(benzimidazole)pyridine. The interesting metal-metal-ligand interactions were observed in the series of coordination systems built of dicyanidoaurate(I) ions, and lanthanide(III) complexes with N,N,N-tridentate bis(benzimidazole)pyridine (bpp) ligands [73]. Depending on the synthetic conditions, the cyanido-bridged tetranuclear [ 2 ]·MeCN (Ln = Eu, Gd, Tb) chains were obtained. For Eu-and Tb-containing species the dominant 4f-centred photoluminescence was achieved, and the efficient bbp-to-Ln 3+ energy transfer was postulated, while the [Au(CN) 2 ] − ions are rather optically silent in this particular case. However, due to the present Au-Au interactions observed between the tetranuclear {LnAu 3 } molecules, the violet [Au(CN) 2 ] − -based emission could be detected but only for the GdAu analogue. Such 5d-centred photoluminescence was not observed for the cyanido-bridged LnAu chains lacking of the aurophilic interactions. All the related Gd-containing materials show additionally green emission of the bbp ligands. Photoluminescent dicyanidometallate-based coordination systems involving lanthanide ions are summarized in Table 1.

Tetracyanidometallates, [M II (CN) 4 ] 2− (M = Ni, Pd, Pt)
Among the square planar tetracyanidometallates(II) of group 10 metals, the tetracyanidoplatinate(II) ion arouses the greatest interest in the construction of photoluminescent materials as it reveals the strong visible emission. It originates from the metal-to-metal-to-ligand charge transfer (MMLCT) transitions of the one-dimensional [Pt II (CN) 4 ] 2− stacks controlled by the short Pt-Pt contacts [29]. As this emission is closely related to the alignment of square planar metal complexes, it was found to be strongly anisotropic, and the change of the light polarization induces the drastic 3change in the intensity of a few emission components assignable to the various electronic transitions. As a result, the colour of [Pt II (CN) 4 ] 2− photoluminescence is polarization-dependent, and varies in the broad range from approximately blue to red.
This phenomenon was nicely investigated for the family of classical cyanido-bridged [Ln III (H 2 O) n ] 2 [Pt II (CN) 4 ]·2{Pt II (CN) 4 }·9H 2 O (n = 6, Ln = La-Lu; n = 5.5, Ln = Eu, Tb) networks ( Figure 3, Table 2) [74][75][76][77][78][79]. They are constructed of the coordination layers of a 6-membered metal rings topology based on Pt II and Ln III centres bridged by cyanide ligands within the ab plane ( Figure 3a). The cyanido-bridged layers are further connected by the close Pt-Pt interactions giving the 1D [Pt II (CN) 4 ] 2− stacks arranged along c crystallographic axis. For most of the lanthanide(3+) ions, such LnPt networks reveal mainly the strong and polarization-dependent broad emission of the [Pt II (CN) 4 ] 2− stacks [74][75][76][77]. The emission colour and intensity were found to be dependent on the type of accompanied lanthanide(3+) ions due to the influence of the characteristic excited states of 4f metal ions amending the non-radiative processes within the bimetallic networks [75,77]. Moreover, the LuPt network exhibits the strong impact of the applied magnetic field on the Pt II -based photoluminescence due to the role of 4f orbitals of Lu III , while the other lanthanides show only very weak emission changes upon the variable magnetic field [76].  In contrast to the other 4f metal ions, SmPt and EuPt networks reveal the strong lanthanide photoluminescence in the visible range indicating the efficient [Pt II (CN)4] 2− -to-Ln 3+ energy transfer mechanism ( Figure 3b) [75,[77][78][79]. The polarization-dependent Pt II -based green to orange emission is still observed but significantly decreased through the radiationless energy transfer. The sensitization process towards Sm 3+ and Eu 3+ was reported to be sensitive to the hydrostatic pressure. Its increase induces the red shift of the Pt II -centred emission reducing the spectral overlap with Ln 3+ absorption states that leads to the gradual disappearance of the 4f-centred emission [78,79]. The similar effect is caused by the increase of temperature, clearly hampering the lanthanide photoluminescence [79].  [74]. They differ in the amount of [Pt II (CN)4] 2− ions occupying the space between the cyanido-bridged layers of an identical 6-membered metal rings topology. The sulfate-containing network shows the smaller number of tetracyanidoplatinates(II) ions resulting in the longer Pt-Pt distances within the one-dimensional metal-metal stacks which induces the significant blue shift of the polarization-dependent Pt II -centred emission [74]. Following the promising optical properties of 3D Ln III -[Pt II (CN)4] 2− networks, a considerable number of the related d-f cyanido-bridged coordination polymers revealing photoluminescent functionalities were reported ( Table 2) [80][81][82][83][84][85][86][87][88]. The insertion of 2,2':6'2"-terpyridine (terpy) into the self-assembled Ln III -Pt II system resulted in the one-dimensional [Ln III (terpy)(H2O)2(NO3)][Pt II (CN)4] ·n(solvent) (Ln = Eu, Tb) zig-zag chains ( Figure 4a) [81,87]. Under UV light excitation of various wavelengths, the EuPt chains exhibits exclusively the sharp emission peaks assigned to Eu III which clearly indicates the efficient dual acceptor-donor intramolecular energy transfer from both terpy ligand and [Pt II (CN)4] 2− ions to 4f metal centre (Figure 4b) [81]. On the contrary, the TbPt chains In contrast to the other 4f metal ions, SmPt and EuPt networks reveal the strong lanthanide photoluminescence in the visible range indicating the efficient [Pt II (CN) 4 ] 2− -to-Ln 3+ energy transfer mechanism ( Figure 3b) [75,[77][78][79]. The polarization-dependent Pt II -based green to orange emission is still observed but significantly decreased through the radiationless energy transfer. The sensitization process towards Sm 3+ and Eu 3+ was reported to be sensitive to the hydrostatic pressure. Its increase induces the red shift of the Pt II -centred emission reducing the spectral overlap with Ln 3+ absorption states that leads to the gradual disappearance of the 4f-centred emission [78,79]. The similar effect is caused by the increase of temperature, clearly hampering the lanthanide photoluminescence [79]. The strong influence of the Pt-Pt distances on the optical properties was proved by comparison of the closely related [Er III [74]. They differ in the amount of [Pt II (CN) 4 ] 2− ions occupying the space between the cyanido-bridged layers of an identical 6-membered metal rings topology. The sulfate-containing network shows the smaller number of tetracyanidoplatinates(II) ions resulting in the longer Pt-Pt distances within the one-dimensional metal-metal stacks which induces the significant blue shift of the polarization-dependent Pt II -centred emission [74].
The first reports on photoluminescent hexacyanido-bridged d-f materials were expanded by the implementation of various pyrimidine and pyridine derivatives that control the coordination Hexacyanidometallates of Co 3+ and Cr 3+ exhibit their own emission close to the edge between visible and NIR ranges, so they were expected to be good sensitizers for NIR-emitting lanthanide(3+) ions. It was accurately proved for the cyanido-bridged [Ln III (dmf) 4 [94,95]. The NdCr, NdCo, YbCr, and YbCo materials show the NIR emission of Nd 3+ or Yb 3+ , and the lack of the expected d-metal-centred luminescence which indicates the efficient Cr 3+ -to-Ln 3+ and Co 3+ -to-Ln 3+ intramolecular energy transfer processes.
The first reports on photoluminescent hexacyanido-bridged d-f materials were expanded by the implementation of various pyrimidine and pyridine derivatives that control the coordination topology [96][97][98][99][100]. The combination of 3-hydroxypyridine (3-OHpy) with Dy III and [Co III (CN) 6 ] 3− produced the cyanido-bridged [Dy III (3-OHpy) 2 6 ]·H 2 O zig-zag chains (Figure 5a), exhibiting the room temperature white-light Dy III emission realized by the complex excitation involving not only direct f-f transitions, but also Co 3+ -to-Dy 3+ and 3-OHpy-to-Dy 3+ energy transfer pathways (Figure 5b) [96]. This topology was also reported for trimetallic [Eu III x Tb III 6 ]·H 2 O materials showing the multi-coloured 4f-centred emission. The colour of luminescence is tuned between red, orange and yellow to green, by the compound's composition, that is the Eu/Tb ratio governing the intensity of red Eu 3+ and green Tb 3+ emission components ( Figure 5b) [97]. Multi-coloured emission is also realized for the trimetallic EuTbCo chains by the selection of the appropriate wavelengths of UV light controlling the intensities of Eu 3+ and Tb 3+ emission characteristics. Moreover, the 4f-centred photoluminescence is, here, enhanced by the energy transfer occurring from both [Co III (CN) 6 ] 3− and 3-OHpy to lanthanide(3+) ions, and the efficiencies of these radiationless effects play a vital role in the observed switchable emission.  The replacement of 3-hydroxypyridine by 4-hydroxypyridine (4-OHpy) in the Dy III -[Co III (CN)6] 3− system resulted in the dramatic change of the structure towards the cyanido-bridged [Dy III (4-OHpy)2(H2O)3][Co III (CN)6]·0.5H2O layered framework of a 6-membered metal rings topology. It exhibits the yellow Dy III emission sensitized by the [Co III (CN)6] 3− complex through intramolecular energy transfer. However, in contrast to 3-OHpy, the 4-OHpy ligand is not suitable sensitizer for Dy III , and its intrinsic greenish-blue phosphorescence was detected. As a result, the multi-coloured yellow to greenish-blue emission switchable by excitation light, governing the   6 ] 3− complex through intramolecular energy transfer. However, in contrast to 3-OHpy, the 4-OHpy ligand is not suitable sensitizer for Dy III , and its intrinsic greenish-blue phosphorescence was detected. As a result, the multi-coloured yellow to greenish-blue emission switchable by excitation light, governing the ligand-and Dy 3+ -based luminescent components, was achieved [98]. The layered network but of a square grid topology was prepared by the application of pyrimidine N-oxide (pmmo) with the Nd 3+ and [Cr III (CN) 6 Figure 6b) [100]. Under the UV excitation, they show typical NIR Yb 3+ emission, enhanced by the radiationless energy transfer from 3-pyone and [Co III (CN) 6 ] 3− to 4f metal ion, which proves that hexacyanidocobaltate(III) is particularly attractive photoluminescent metalloligand being a good sensitizer for both visible light-and NIR-emitting lanthanide(3+) ions. [100] 1 ET = energy transfer; 2 dmf = N,N-dimethylformamide; 3 3-OHpy = 3-hydroxypyridine; 4 4-OHpy = 4-hydroxypyridine; 5 pmmo = pyrimidine N-oxide; 6 3-pyone = 3-pyridone.

Octacyanidometallates, [M IV/V (CN) 8 ] 4−/3− (M = Mo, W)
Octacyanidometallate complexes of Mo(IV,V) and W(IV,V) are not emissive as their numerous d-d and charge transfer (ligand-to-metal or metal-ligand) electronic transitions were found to be rather photoreactive which was utilized in the construction of photomagnetic materials [23]. These cyanide complexes offer, however, very light yellow colour as their absorption bands are shifted to the UV range. It enables the observation of visible and NIR photoluminescence of accompanying emissive chromophores [32]. In effect, a considerable number of photoluminescent d-f coordination networks based on octacyanidometallates were reported (Figures 7 and 8 [103]. The emission of Nd 3+ was also possible to detect within the family of octacyanidometallate-based materials, as reported for the cyanido-bridged [Nd III (phen) 2 (dmf) 2   The interesting luminescent functionalites were presented for the three-dimensional hybrid inorganic-organic [Ln III 2(mpca)2(MeOH)2(H2O)6][Mo IV (CN)8]·xMeOH (Ln = Nd, Eu, Tb; mpca = 5-methyl-2-pyrazine carboxylic acid) networks where cyanido-bridged layers are connected in the third direction by organic mpca linkers [105]. They reveal the strong 4f-metal centred photoluminescence sensitized by mpca ligands while [Mo IV (CN)8] 4− serves as an inorganic linker stabilizing the 3D coordination network. Moreover, the Eu 3+ emission was found to be strongly sensitive to the amount of water molecules occupying the interstitial space within the crystal structure. Therefore, the emission intensity of the EuMo network increases significantly with decreasing humidity of the air around the solid sample, which makes this material an efficient humidity sensor working in the whole relative humidity range from 0 to 100% [106].   8 ]·xMeOH (Ln = Nd, Eu, Tb; mpca = 5-methyl-2-pyrazine carboxylic acid) networks where cyanido-bridged layers are connected in the third direction by organic mpca linkers [105]. They reveal the strong 4f-metal centred photoluminescence sensitized by mpca ligands while [Mo IV (CN) 8 ] 4− serves as an inorganic linker stabilizing the 3D coordination network. Moreover, the Eu 3+ emission was found to be strongly sensitive to the amount of water molecules occupying the interstitial space within the crystal structure. Therefore, the emission intensity of the EuMo network increases significantly with decreasing humidity of the air around the solid sample, which makes this material an efficient humidity sensor working in the whole relative humidity range from 0 to 100% [106].      [107,108]. Taking advantage of both luminescent Ln 3+ and pybox ligand, the Eu(iPr-pybox)W chains reveal the thermal switching between red Eu 3+ emission predominant at low temperature due to the effective ligand-to-metal energy transfer, and blue iPr-pybox-centred photoluminescence prevailing at room temperature, as the energy back transfer to ligand is operating at higher temperatures (Figure 7b) [107]. The GdW helices with iPr-pybox, and the more expanded ind-pybox ligands, exhibit exclusively the ligand-based red phosphorescence. The NdW chains show the 4f-metal-centred NIR emission enhanced by the pybox-to-Nd 3+ energy transfer process [108].
The rich scope of diverse lanthanides(3+) photoluminescence, and their interaction with organic 2,2'-bis(2-oxazoline) (box) organic chromophore was beautifully presented in the cyanido-bridged  (Figure 8a), reveals the excitation-dependent visible luminescence switchable between Tb 3+ green emission induced under the deep UV excitation of the interconfigurational d-f transition of the 4f-metal centre, and red box-based phosphorescence detected for the UV excitation around 340 nm that directs the energy mainly towards the ligand excited states (Figure 8b) [109]. Similar effect was found for the analogous TbMo system while the other members of the LnMo family showed lanthanide-dependent visible and/or NIR emission. For PrMo, SmMo, EuMo and HoMo, the box-to-Ln 3+ energy transfer induced the 4f-centred visible emission ranging from green for Ho 3+ , orange for Sm 3+ , to red for Pr 3+ and Eu 3+ . The ligand-to-metal energy transfer is also responsible for the observation of characteristic emission peaks in the NIR range for several compounds, PrMo, NdMo, SmMo, HoMo and YbMo layers [110].
The bimetallic 4d-4f coordination systems involving [Ru II (L)(CN) 4 ] 2− ions offer a rich structural diversity as the blocking L ligand and four potentially bridging cyanides can result in the various low dimensional molecular systems. Moreover, the large lanthanide(3+) ions reveal high coordination numbers that the supporting organic ligands could be also introduced. It was fruitfully presented for a series of zero-, one-and two-dimensional cyanido-bridged networks based on [Ru II (phen)(CN) 4 ] 2− (phen = 1,10-phenanthroline) anion [114]. Using the phen molecule as the ligand coordinated both to the 4d and 4f metal centres, the cyanido-bridged [Ln III (phen)(H 2 O) 3 ] 2 [Ru II (phen)(CN) 4 ]·14H 2 O (Ln = Nd, Gd, Er, Yb) layers of a corrugated honeycomb topology were obtained (Figure 9a). The GdRu layers reveal the strong MLCT red emission of [Ru II (phen)(CN) 4 ] 2− which is significantly quenched for the analogous NdRu, ErRu, and YbRu coordination frameworks due to the Ru 2+ -to-Ln 3+ energy transfer process (Figure 9b). The MLCT band completely disappears for NdRu as the high density of low-lying f-f excited states of Nd 3+ accepting the energy from Ru II . For ErRu and YbRu derivatives, the rate of d-f energy transfer, and the resulting the degree of quenching of the MLCT luminescence, decreased due to the smaller number of accessible excited f-f states (Figure 9b) [114]. The similar sensitized NIR emission was found for the other phen-containing cyanido-bridged hexanuclear K 2 [Ln III (phen) 2 (H 2 O)] 2 [Ru II (phen)(CN) 4 ] 4 ·n(solvent) (Ln = Pr, Nd, Er, Yb) molecules, prepared under the modified synthetic conditions. The application of other ancillary oligopyridine ligands, including 2,2':6'2"-terpyridine (terpy) and 2,2'-bipyrimidine (bpym), resulted in the formation of one-dimensional cyanido-bridged [ 4 ] 3 ·nsolvent (Ln = Nd, Er, Yb) networks of a ladder chain, and a hybrid chain of squares topologies, respectively. They all exhibit the characteristic 4f-metal-centetered NIR emission enhanced by the Ru 2+ -to-Ln 3+ energy transfer. It was proved that blocking polypyridine ligands hamper the coordination of cyanides and water molecules to the 4f metal ions, not only amending the structural topology but also visibly increasing lanthanide(III)-based emission lifetimes when compared to the networks without these supporting ligands [114].
The particularly efficient sensitization of both Nd 3+ and Yb 3+ was achieved for the heterometallic d-f coordination networks based on tetracyanidoruthenates(II) bearing the expanded aromatic system of hexaazatriphenylene (HAT) (Figure 10) [115,116]. The simplest metal-cyanide complex of this family, [Ru II (CN) 4 (Figure 10a) [115,116]. When Gd 3+ ion was used as the lanthanide centre, all the Ru II (HAT)-containing networks reveal the strong MLCT photoluminescence in the red range, which was substantially quenched by the insertion of NIR-emitting Nd 3+ and Yb 3+ ions. As a result, the strong near-infrared emission was observed, while the residual Ru II -based emission was around 100 times weaker than for the GdRu compounds (Figure 10b) [116].  [117]. This makes the tetracyanidoosmate(II) an even more efficient sensitizer for NIR-emitting lanthanide(3+) ions than the related tetracyanidoruthenate(II) due to the expected better spectroscopic overlap of Os 2+ -based emission with the absorption peaks of the 4f metal ions. This prediction was checked for the family of cyanido-bridged coordination systems, including hexanuclear Na 2 [Ln III (phen) 2  For all these species, the NIR lanthanide-based emission was sensitized by the Os 2+ -to-Ln 3+ energy transfer of distinguishable rates depending on 4f metal ion. The rates of Os 2+ -to-Ln 3+ energy transfer were found to be an order of magnitude faster than the rates of Ru 2+ -to-Ln 3+ energy transfer previously observed in similar heterometallic d-f cyanido-bridged networks [117].
The innovative luminescent functionality of heteroligand tetracyanidometallates was shown for the [Ru II ( t Bubpy)(CN) 4 ] 2− ion bearing the tert-butyl derivative of 2,2'-bipyridine [118,119]. This anion embedded in the bimetallic K[Ln III (H 2 O) 4 ][Ru II ( t Bubpy)(CN) 4 ] 2 ·8H 2 O (Ln = Pr, Nd, Sm, Eu) chain of {Ln 2 Ru 2 } molecular squares exhibits the orange emission of a typical metal-to-ligand charge transfer (MLCT) origin. The intensity of this photoluminescence detected for the bulk solid sample, and the polymeric film, was reported to be strongly dependent on the presence of gaseous amine molecules, which additionally sensitize the Ru II -based emission. Therefore, this material found the analytical application as an efficient and sensitive chemodosimetric detector of biogenic amine odorants including histamine, putrescine, spermidine, and ammonia, indicating a great potential of the polycyanidometallate-based d-f coordination networks in chemical and biochemical sensing based on the spectrofluorometric detection [119].
excitation, slightly weakened when compared with the reference cyanide-free {Er III 2(H2L)4} molecule due to the partial Er 3+ -to-Cu 2+ f-d energy transfer (Figure 11b). This work showed that a cuprous cyanide network serves as a stabilizing agent for the formation of unique NIR-emitting {Er III Cu II 2L12Cl2} clusters, non-isolable under cyanide-free synthetic conditions [120]. In all these materials, the broad green [Ir III (ppy)2(CN)2] − -based emission was detected, the strongest for the UV-emissive Gd 3+ , and much weaker for visible-light-emissive Eu 3+ , and NIR-emitting Nd 3+ . It suggested the presence of Ir 3+ -to-Eu 3+ and Ir 3+ -to-Nd 3+ energy transfer processes, but the sensitized Eu 3+ or Nd 3+ could not be detected indicating the additional quenching effect, presumably involving the water molecules existing in the crystal structure [121].
Non-heterometallic, however, cyanido-bridged photoluminescent coordination networks were obtained by combining trivalent lanthanide ions with inorganic tetracyanidoborate, [  The rare organometallic heteroligand dicyanidoiridate(III) anion, [Ir III (ppy) 2 (CN) 2 ] − , with four coordination sites blocked by two anions of 2-phenylpyridine (ppy), was also tested as a novel cyanide-containing metal complex for d-f photoluminescent materials [121]. The self-assembly of this specific dicyanidometallate with the excess of lanthanide(3+) ions resulted in a series of cyanido-bridged tetranuclear [ 2 ] 2 molecules which structural details depend on the attached lanthanide(3+) ions. In all these materials, the broad green [Ir III (ppy) 2 (CN) 2 ] − -based emission was detected, the strongest for the UV-emissive Gd 3+ , and much weaker for visible-light-emissive Eu 3+ , and NIR-emitting Nd 3+ . It suggested the presence of Ir 3+ -to-Eu 3+ and Ir 3+ -to-Nd 3+ energy transfer processes, but the sensitized Eu 3+ or Nd 3+ could not be detected indicating the additional quenching effect, presumably involving the water molecules existing in the crystal structure [121].
is played by its structurally modified derivative, that is pentafluoroethyltricyanidoborate(III) ion, [C2F5B III (CN)3] − [123]. This anionic building block combined with lanthanide(3+) ions resulted in the formation of a hydrated cyanido-bridged [Ln III {C2F5B(CN)3}3(H2O)3](Ln = La, Eu, Ho) layered framework showing the topology of cross-linked {Ln2B2}-square-based chains (Figure 12a). Upon thermal dehydration, this material could be transformed into the anhydrous three-dimensional cyanido-bridged [Ln III {C2F5B(CN)3}](Ln = La, Eu, Ho) hexagonal network built of nine-coordinated    (Figure 12a). Upon thermal dehydration, this material could be transformed into the anhydrous three-dimensional cyanido-bridged [Ln III {C 2 F 5 B(CN) 3 }](Ln = La, Eu, Ho) hexagonal network built of nine-coordinated Ln 3+ ions which coordination sphere is fully occupied by cyanide ligands of bridging [C 2 F 5 B III (CN) 3 ] − anions (Figure 12b). The LaB anhydrous network exhibits the broad greenish-blue UV-light-induced photoluminescence of tricyanidoborate(III) anions. This emission is also observed for HoB analogues but the intense reabsorption effect was detected as the several characteristic f-f lines situated within the broad emission band of the anion. It suggested the lack of efficient [C 2 F 5 B III (CN) 3 ] −to-Ho 3+ energy transfer. On the contrary, the sensitization effect was found for the hydrated and anhydrous EuB networks that show the strong 4f-metal-centred emission, and the complete quenching of the borate-based luminescence (Figure 12c). Due to the removal of water molecules, which presence typically decreases the lanthanide(3+) emission, the anhydrous phase reveals almost three times stronger photoluminescence than the related hydrated species. All these results proved that pentafluoroethyltricyanidoborate(III) anion is a promising luminescent cyanide building block for sensitization of the visible photoluminescence of lanthanide(3+) ions.

Conclusions
We have presented a detailed overview of lanthanide photoluminescence in coordination networks based on polycyanidometallates. To best of our knowledge, this is the first systematic catalogue of the state of the art in the investigation of photoluminescent d-f cyanido-bridged systems.
Selected groups of polycyanidometallates are suitable for construction of photoluminescent coordination frameworks that can explore intrinsic optical properties of lanthanide(3+) ions. Among them, the dicyanidometallates of Au I and Ag I are luminescent in the visible range due to the metal-to-ligand charge transfer (MLCT) transitions. Such emission, found in 4d/5d-4f coordination polymers, and facilitated by the presence of Au-Au interactions, was shown to be dependent on pressure and temperature. Dicyanidometallates are good sensitizers for visible emission of 4f-metal ions, and the resulting emission can be tuned by external stimuli such as pressure, temperature, or light excitation. Thus, the multi-coloured tunable photoluminescence is the main application of [M I (CN) 2 ]-based d-f systems, taking also into account the possible insertion of organic ligands, introducing additional energy transfer pathways, or separate emission components. Similar luminescence was achievable for tetracyanidoplatinate(II) ions revealing the visible emission due to the metal-to-metal-to-ligand charge transfer (MMLCT) transitions related to the Pt-Pt interactions. This emission is dependent on temperature, pressure, and light polarization due to the anisotropic character of these square planar polycyanido-metallates. [Pt II (CN) 4 ] 2− is a good sensitizer for some lanthanide(3+) ions, including Eu 3+ , Sm 3+ , or Tb 3+ . The tetracyanidoplatinate(II)-based systems offer a structural diversity due to the ancillary organic ligands, modifying the 4f-metal-centred emission.
The octahedral hexacyanidometallates of Co III and Cr III are luminescent in the red and NIR ranges, respectively. They can enhance NIR lanthanide(3+) emission by the energy transfer. [Co III (CN) 6 ] 3− can also sensitize visible 4f-metal-centred luminescence. They offer six cyanides to create extended networks, and leave the coordination sites on lanthanides that can be occupied by organic ligands. Thus, various optical functionalities were achieved, as exemplified by white light emission of Dy III -Co III compound, excitation-and composition-switchable multi-coloured emission in Eu III -Tb III -Co III chains, and dual-donor NIR emission in Nd III -Cr III and Yb III -Co III networks.
Despite their non-luminescent character, octacyanidometallates of Mo IV/V and W IV/V were fruitfully applied in the synthesis of photoluminescent d-f coordination systems. Due to the optical transparency in the Vis-NIR range, their cyanido-bridged networks with lanthanides could reveal the rich spectrum of intrinsic properties of 4f metal ions, and the sensitization effects on supporting organic ligands. [M(CN) 8 ] n− ions offer a great structural flexibility, so a number of d-f coordination networks could be rationally designed. As a result, such attractive functionalities as thermal switching of blue-red emission in Eu III -W V helices, excitation-tunable green-red luminescence in Tb III -W V layers, or humidity sensor based on red emissive Eu III -Mo IV network, were reported.
The heteroligand tetracyanidometallates of Ru II and Os II , [M II (CN) 4 (L)] 2− are photoluminescent in the visible range due to the metal-to-ligand charge transfer (MLCT) transition. These anions are good sensitizers for NIR-emitting lanthanide(3+) ions. Their emission energy can be tuned by the blocking ligand L, thus, the selection of appropriate organic part governs the efficiency of the Ru 2+ /Os 2+ -to-Ln 3+ energy transfer process. Metal center also plays a vital role, and the Os II -based cyanide complexes were proved to be better sensitizers for NIR emission than the analogous Ru II species. The ancillary organic ligands can be, here, inserted to amend the structural features, and facilitate the characteristics of energy transfer. Lately, the heteroligand tetracyanidoruthenate(II) embedded in d-f cyanido-bridged networks has successfully been applied in the chemodosimetric detection of biogenic amines utilizing a selective sensitization of its visible MLCT emission.
Among other cyanide-containing building blocks, the cuprous cyanide networks was found to stabilize NIR-emitting salen-based Er III -Cu III clusters, while the green emissive heteroligand dicyanidoiridate(III) bearing two blocking bidentate ligands were tested as possible sensitizers for lanthanide(3+) luminescence. In addition, the inorganic tetracyanidoborate anion was used for the construction of photoluminescent Tb III -and Dy III -based molecules, however, without a strong impact on the emission properties. On the contrary, its modified analogue, pentafluoroethyltricyanidoborate(III) ion, [C 2 F 5 B III (CN) 3 ] − is greenish-blue emissive, and efficiently sensitizes the Eu 3+ luminescence within the cyanido-bridged two-or three-dimensional coordination networks.
In summary, we have gathered all reported photoluminescent polycyanidometallate-based coordination systems incorporating lanthanide ions. We have shown that cyanide metal complexes are useful for the preparation of emissive solids revealing such functionalities as lanthanide-centred Vis-NIR photoluminescence, sensitized through metal-to-metal and/or ligand-to-metal energy transfer effects, and multi-coloured emission tuned by external stimuli of excitation wavelength, temperature, and pressure. There is still several challenges in this area, including the detailed investigation of structural and electronic features governing the sensitization process involving cyanide complexes, and the preparation of novel luminescent polycyanidometallates of improved optical properties, and affinity to lanthanide(3+) ions. The particularly promising future work comprises of searching for multifunctional photoluminescent materials that can exhibit the undiscovered physical cross-effects, as polycyanidometallate-based heterometallic coordination networks showed a diversity of physical properties, such as magnetic anisotropy leading to slow magnetic relaxation, magnetic ordering, spin transitions, catalytic activity, ferroelectricity, and ionic conductivity, that in some cases lead to the unprecedented physical phenomena [38][39][40][41][42]50].