Modulating the Inclusive and Coordinating Ability of Thiacalix[4]arene and Its Antenna Effect on Yb3+-Luminescence via Upper-Rim Substitution

The present work introduces the series of thiacalix[4]arenes (H4L) bearing different upper-rim substituents (R = H, Br, NO2) for rational design of ligands providing an antenna-effect on the NIR Yb3+-centered luminescence of their Yb3+ complexes. The unusual inclusive self-assembly of H3L− (Br) through Br…π interactions is revealed through single-crystal XRD analysis. Thermodynamically favorable formation of dimeric complexes [2Yb3+:2HL3−] leads to efficient sensitizing of the Yb3+ luminescence for H4L (Br, NO2), while poor sensitizing is observed for ligand H4L (H). X-ray analysis of the single crystal separated from the basified DMF solutions of YbCl3 and H4L(NO2) has revealed the transformation of the dimeric complexes into [4Yb3+:2L4−] ones with a cubane-like cluster structure. The luminescence characteristics of the complexes in the solutions reveal the peculiar antenna effect of H4L(R = NO2), where the triplet level at 567 nm (17,637 cm−1) arisen from ILCT provides efficient sensitizing of the Yb3+ luminescence.


Introduction
Calix[n]arenes and their thia-analogues continue to excite interest as a promising basis for design and synthesis of lanthanide complexes, which were successfully applied in developing nanosensors and contrasting agents [1,2]. The main advantage of a cyclophanic backbone is the feasibility of the structural diversifications, which allows embedding of different groups, in turn allowing either complex ability of the calixarene derivatives or tuning their acid/base and complexing properties through electronic effects of the substituents [3,4]. Moreover, the presence of the cyclophanic cavity results in unique inclusive complex formation mainly driven by the electron-donating ability of the cavity [5,6]. The present work is focused on upper-rim substituted thiacalix [4]arenes since preorganization of the four phenolic moieties provides excellent chelating properties towards lanthanide ions, followed by ligand-to-lanthanide energy transfer [1,[7][8][9]. It is worth noting that upperrim substitution of thiacalix [4]arenes was already documented as the tool to both increase their water solubility [9,10] and modify their sensitizing effect on lanthanide-centered luminescence [1,[7][8][9]. In particular, embedding of the bromine-substituents onto the upper rim of thiacalix [4]arenes allows to modify the ligand-centered triplet level responsible for [5,6]. The present work is focused on upper-rim substituted thiacalix [4]arenes since preorganization of the four phenolic moieties provides excellent chelating properties towards lanthanide ions, followed by ligand-to-lanthanide energy transfer [1,[7][8][9]. It is worth noting that upper-rim substitution of thiacalix [4]arenes was already documented as the tool to both increase their water solubility [9,10] and modify their sensitizing effect on lanthanide-centered luminescence [1,[7][8][9]. In particular, embedding of the bromine-substituents onto the upper rim of thiacalix [4]arenes allows to modify the ligand-centered triplet level responsible for feeding of the excited lanthanide-centered levels [8]. Incorporation of nitro-groups onto the upper rim of thiacalix [4]arene has also been reported [11][12][13][14].
Ytterbium compounds exhibiting near-infrared (NIR) luminescence are widely applied building blocks of nanomaterials for bioimaging and photothermal therapy [15][16][17][18][19][20][21][22]. This is due to the fact that Yb 3+ -centered luminescence exhibits the greatest intensity among other NIR-emitting lanthanide ions [23], which, in turn, derives from the large energy gap, 10250 cm −1 , between its emitting level and the ground state [24]. The poor feeding of the excited 2 F5/2-level of Yb 3+ due to forbiddance of f-f transitions raises a question of their feeding through ligand-to-metal [25,26] or metal-to-metal energy transfer and minimizing of radiationless transitions from the excited Yb 3+ -level to lower laying vibrational levels of ligands [27][28][29]. The reports of Iki et al. [30][31][32] highlight the advantage of the thiacalix [4]arene backbone in developing NIR-luminescent Yb 3+ complexes due to the specific rigid inner-sphere environment resulting from the sandwich-like coordination of the Yb 3+ ions between two phenolate rims of the thiacalix [4]arene derivatives.
However, an impact of the upper-rim substitution of thiacalix [4]arenes by the bromine and nitro-groups on the developing of bright NIR Yb 3+ -centered luminescence has not been highlighted. A combination of electron-withdrawing (NO2) and electron-donating moieties (OH, O − ) in thiacalix [4]arene molecules should produce an intraligand charge transfer (ILCT) absorption band in the visible range similar to that in the electronic spectra of nitrophenolates [33]. Literature data demonstrate fine examples of convenient excitation of Yb 3+ NIR luminescence by means of an ILCT absorption band in the visible range [25,26,34]. Thus, it is worth assuming that combination of the rigid inner-sphere ligand environment of Yb 3+ ions with excitation of an Yb 3+ -centered luminescence by means of an ILCT absorption band can be a tool to develop bright Yb 3+ -centered luminescence.
The present work represents thiacalix [4]arenes H4L(1-3) (Figure 1) with different upper-rim substituents (R = H, Br and NO2). The structural variation in the upper-rim substituents is aimed to highlight their impact on producing unique supramolecular structures, in turn derived from inclusive or coordinating abilities of the bromo-and nitrosubstituted thiacalix [4]arenes. Such structure variation is also aimed at distinguishing different structure effects on Yb 3+ -centered luminescence of the corresponding complexes, including: (1) structure rigidity effect derived from bulky substituents (Br, NO2); (2) interaction of the lone pairs of NO2 group with π * orbitals of the aromatic ring, which is known to quench Eu 3+ and Tb 3+ luminescence through shortening of the triplet excited state lifetime [35]; (3) participation of the triplet level arisen from the ILCT in the feeding of the low-energy excited states of Yb 3+ .  The unique supramolecular structures of bromo-substituted thiacalix [4]arene and the ytterbium complex of nitro-substituted thiacalix [4]arene determined by single-crystal X-ray diffraction (XRD) data will be discussed in correlation with the literature data and experimental results on complexation of Yb 3+ with thiacalix [4]arenes in DMF solutions. The coordination modes of Yb 3+ in the complexes will be revealed by computational modelling. Steady state and time-resolved Yb 3+ -centered luminescence will be correlated with both spectral properties and structural features of the ligands in order to recognize the impact of different factors, including the ILCT, on the luminescence of the complexes.

UV-Vis Absorption Behavior of H 4 L(3) and Crystal Structure of H 3 L(2) −
Discussion of the complex ability and antenna-effect of H 4 L(1-3) ligands in basified DMF solutions should be preceded by analysis of their acid-base behavior revealed through their spectral behavior at different concentrations of TEA. Both spectral and acid-base behaviors of H 4 L(1,2) in DMF solutions have already been published [7,8], and a correlation between deprotonation and spectral changes for H 4 L(3) was reported in aqueous solutions [12,13].
The electronic absorption spectra of 3 recorded in the neutral and basified DMF solutions are represented in Figure 2a. The enhanced electronic absorbance at 340-400 nm revealed from the spectrum of 3 in the neutral DMF solution (Figure 2a) is explained by the enhanced first step deprotonation of H 4 L(3) since similar spectral behavior of H 4 L(3) in the aqueous solutions was correlated with pK 1 = 2.75 [13]. The enhanced acidity of H 4 L(3) derives from both its cyclophanic structure and the electron-withdrawing effect of p-nitro-substituents, which differentiates H 4 L(3) from H 4 L(1,2). X-ray diffraction (XRD) data will be discussed in correlation with the literature data and experimental results on complexation of Yb 3+ with thiacalix [4]arenes in DMF solutions. The coordination modes of Yb 3+ in the complexes will be revealed by computational modelling. Steady state and time-resolved Yb 3+ -centered luminescence will be correlated with both spectral properties and structural features of the ligands in order to recognize the impact of different factors, including the ILCT, on the luminescence of the complexes.

UV-Vis Absorption Behavior of H4L(3) and Crystal Structure of H3L(2) −
Discussion of the complex ability and antenna-effect of H4L(1-3) ligands in basified DMF solutions should be preceded by analysis of their acid-base behavior revealed through their spectral behavior at different concentrations of TEA. Both spectral and acidbase behaviors of H4L (1,2) in DMF solutions have already been published [7,8], and a correlation between deprotonation and spectral changes for H4L(3) was reported in aqueous solutions [12,13].
The electronic absorption spectra of 3 recorded in the neutral and basified DMF solutions are represented in Figure 2a. The enhanced electronic absorbance at 340-400 nm revealed from the spectrum of 3 in the neutral DMF solution (Figure 2a) is explained by the enhanced first step deprotonation of H4L(3) since similar spectral behavior of H4L(3) in the aqueous solutions was correlated with pK1 = 2.75 [13]. The enhanced acidity of H4L(3) derives from both its cyclophanic structure and the electron-withdrawing effect of p-nitro-substituents, which differentiates H4L(3) from H4L(1,2). The low-energy band of H4L(3) is both red-shifted and more intensive (Figure 2a) than the shoulder at 340 nm for H4L(2) [8], which, in turn, is more pronounced than that of H4L(1) [8]. The spectral behavior of H4L(3) in the basified DMF solutions is characterized by the appearance of electronic absorbance at ∼450-500 nm ( Figure 2a). The red-shifting of the low-energy band of H4L(3) versus those of H4L(1,2) derives from the wellknown high electron-withdrawing effect of nitro-substituents, resulting in the appearance of the ILCT absorption band. It is worth noting that the electronic absorption of H4L(3) in the basified DMF solutions is in the longer wavelengths range, ∼450-500 nm compared to the absorption of p-nitrophenol (∼400-450 nm) in the alkaline solutions [33]. Thus, the cyclophanic structure of H4L(3) favors lower energy transitions, along with the effect of the p-nitro-substituents, which is the reason for the peculiar spectral behavior of H4L versus that of p-nitrophenol. As is evident from the titration plot in Figure 2b, two-step  [8], which, in turn, is more pronounced than that of H 4 L(1) [8]. The spectral behavior of H 4 L(3) in the basified DMF solutions is characterized by the appearance of electronic absorbance at~450-500 nm ( Figure 2a). The red-shifting of the low-energy band of H 4 L(3) versus those of H 4 L(1,2) derives from the well-known high electron-withdrawing effect of nitro-substituents, resulting in the appearance of the ILCT absorption band. It is worth noting that the electronic absorption of H 4 L(3) in the basified DMF solutions is in the longer wavelengths range,~450-500 nm compared to the absorption of p-nitrophenol (~400-450 nm) in the alkaline solutions [33]. Thus, the cyclophanic structure of H 4 L(3) favors lower energy transitions, along with the effect of the p-nitro-substituents, which is the reason for the peculiar spectral behavior of H 4 L versus that of p-nitrophenol. As is evident from the titration plot in Figure 2b, two-step deprotonation of H 4 L(3) is realized in the basified DMF solutions, while only one-step deprotonation was reported for H 4 L(1,2) [7,8].
The above-mentioned deprotonation of the phenolic rims of H 4 L(1-3) facilitates the electron-donating ability of their cavities, which promotes unique intermolecular inclusive interactions and coordination of metal ions. These interactions are clearly demonstrated by XRD analysis of the single crystals (H 3 L(2) − ·(CH 3 ) 2 NH 2 + ·DMF) grown from the DMF solutions of 2 basified by dimethylamine ( Figure 3). The thiacalixarene molecule in an almost perfect cone conformation with close values of dihedral angles of opposite aromatic rings is located in the general position of the triclinic unit cell ( Figure 3). However, the molecule loses its own C 4 symmetry due to its transformation into salt form, interaction with the solvent molecule ( Figure 3a) and specific inclusive interactions (Figure 3b). The crystal structure data (Table S1) and the parameters of the intra-and intermolecular interactions (Tables S2 and S3 and Figures S1-S4) are represented in the Supplementary Materials. electron-donating ability of their cavities, which promotes unique intermolecular inclusive interactions and coordination of metal ions. These interactions are clearly demonstrated by XRD analysis of the single crystals (H3L(2) − ·(CH3)2NH2 + ·DMF) grown from the DMF solutions of 2 basified by dimethylamine ( Figure 3). The thiacalixarene molecule in an almost perfect cone conformation with close values of dihedral angles of opposite aromatic rings is located in the general position of the triclinic unit cell ( Figure 3). However, the molecule loses its own C4 symmetry due to its transformation into salt form, interaction with the solvent molecule ( Figure 3a) and specific inclusive interactions (Figure 3b). The crystal structure data (Table S1) and the parameters of the intra-and intermolecular interactions (Tables S2 and S3 and Figures  An interesting feature of the intermolecular interaction revealed in single crystals is formation of peculiar centrosymmetric dimers of thiacalix [4]arenes ( Figure 3b). Substituent Br3 participates in the strong C-H…Br interaction with the hydrogen H3 of the phenyl ring of the neighboring molecule; the H3…Br3 distance is 3.03 Å. Pairwise incorporation of the bromine substituents designated as Br4 of one molecule into the cavity of another can be stabilized by four Br…π contacts with the four phenyl rings of the thiacalix [4]arene. However, in accordance with the IUPAC criteria [36], only one (Cg2(C9 ÷ C14)…Br4) of the four contacts can be called a halogen bond since the rest of them do not follow the rule of directionality of such bonds. Moreover, two of them have distances between the centers of the bromine and the nearest carbon atoms slightly more than the sum of their van der Waals radii (see Table S3 in Supplementary Materials). However, the formal criteria for such interactions used in the PLATON program [37] allows to consider all the contacts to the Br…π type (for the contact parameters, see Table S3).
It is worth noting the diversity of the Br…π contacts: in particular, the С-Br bond is directed to carbon С14 (Cg2(С9 ÷ С14)…Br4), while, in the contact Cg1(С2 ÷ С7)…Br4, the bromine atom is in an intermediate position between the center of the aromatic cycle and atom C7. In the case of contacts Cg3(С16 ÷ С21)…Br4 and Cg4(С23 ÷ С28)…Br4, the bromine atom is closer to the centers of aromatic rings, which are electron-deficient regions. The aforesaid provides one more example of the unique ability of the bromine An interesting feature of the intermolecular interaction revealed in single crystals is formation of peculiar centrosymmetric dimers of thiacalix [4]arenes ( Figure 3b). Substituent Br3 participates in the strong C-H . . . Br interaction with the hydrogen H3 of the phenyl ring of the neighboring molecule; the H3 . . . Br3 distance is 3.03 Å. Pairwise incorporation of the bromine substituents designated as Br4 of one molecule into the cavity of another can be stabilized by four Br . . . π contacts with the four phenyl rings of the thiacalix [4]arene. However, in accordance with the IUPAC criteria [36], only one (Cg2(C9 ÷ C14) . . . Br4) of the four contacts can be called a halogen bond since the rest of them do not follow the rule of directionality of such bonds. Moreover, two of them have distances between the centers of the bromine and the nearest carbon atoms slightly more than the sum of their van der Waals radii (see Table S3 in Supplementary Materials). However, the formal criteria for such interactions used in the PLATON program [37] allows to consider all the contacts to the Br . . . π type (for the contact parameters, see Table S3).
It is worth noting the diversity of the Br . . . π contacts: in particular, the C-Br bond is directed to carbon C14 (Cg2(C9 ÷ C14) . . . Br4), while, in the contact Cg1(C2 ÷ C7) . . . Br4, the bromine atom is in an intermediate position between the center of the aromatic cycle and atom C7. In the case of contacts Cg3(C16 ÷ C21) . . . Br4 and Cg4(C23 ÷ C28) . . . Br4, the bromine atom is closer to the centers of aromatic rings, which are electron-deficient regions. The aforesaid provides one more example of the unique ability of the bromine substituents to interact with both nucleophilic and electrophilic centers, which has gained great attention in the last decade [38].
The revealed short Br . . . π contacts are predominantly driven by polarized electrostatic attractions between the electron-deficient bromo-substituents and the electron donating cavity of H 3 L − (2) as a Lewis base [39,40]. It is worth noting that halogen bonds have already been highlighted as the driving force of inclusion of halogen-substituted benzenes into the cavities of calix [4]arene derivatives [41]. A search of the Cambridge Structural Database (CSD version 5.43, March 2022 release) for all structures containing upper-rim halogen substituted thia-and calix [4]arenes (68 hits) reveals the only example of dimeric or cog-like self-inclusion of distal substituted dibromocalix [4]arene bearing the two propoxy groups on the lower-rim [42]. This indicates that the present inclusive self-assembly based on halogen bonds is rather rare. Moreover, dimeric self-assembly is also stabilized by the C-H . . . Br interaction driven by the electron donating capacity of the bromo-substituents (Figure 3b and S1). Thus, the Janus-like nature of the bromine substituents provides additional interactions, which, along with S . . . π and π . . . π interactions, form the one-dimensional supramolecular motif ( Figure S1). Such chains are bound in a perpendicular direction due to Br . . . π and C-H . . . Br contacts ( Figure S2), forming a two-dimensional supramolecular motif-a layer of thiacalixarene dimeric fragments, where the dimeric structure shown in Figure 3b serves as a supramolecular synthon. Translation of such synthon in three directions forms a crystal packing as a whole (Figures S3 and S4). It is worth noting that the supramolecular packing is characterized by a sufficiently high packing factor of 0.717, which is closer to the upper limit of the packing factor values for crystals of organic compounds (0.65-0.75).

Complex Formation of H 4 L(1-3) with Yb 3+ Ions
Compounds H 4 L(1,2), previously represented as efficient ligands for Tb 3+ ions, provide their tight coordination followed by efficient sensitizing of terbium-centered luminescence [7,8]. Discussion of the complex formation of ligand H 4 L(3) in solutions is worth preceding by a presentation of the structure determined by XRD analysis of the single crystals grown from basified DMF solution containing H 4 L(3) and Yb 3+ ions in a 1:1 molar ratio.
The single crystals suitable for XRD analysis were grown through several months staying of the basified DMF solutions of YbCl 3 and H 4 L(3) mixed in a 1:1 ratio, while no single crystals appeared in the same conditions for ligands H 4 L(1,2). Moreover, in the case of the Yb(NO 3 ) 3 , we could not succeed in obtaining any crystals.
X-ray analysis of the separated single crystal revealed the large and strongly disordered structure in the monoclinic P21/n space group. A detailed description of the crystal structure data is in Supplementary Materials (Figures S5 and S6 and Table S4). The cell unit consists of two individual complexes in its composition with 2:1 (Yb:L) stoichiometry, although, as has been aforesaid, YbCl 3 and H 4 L(3) were mixed in a 1:1 ratio (Figure 4). Both of them contain a rather specific dimeric cubane-like structure of complex. The cluster coordination of four Yb 3+ ions with eight phenolates of two completely deprotonated p-nitro-thiacalix [4]arene anions (L 4− ) is stabilized by the bridge-like coordination of chloride ions with the coordination number of Yb 3+ ions equal to 6 and 7, according to Figure 4. Such values are rather scarce in comparison with coordination numbers 8 and 9 predominantly reported in the literature [43]. However, the data quality is insufficient to reveal such structural details as a location and number of water molecules supporting the structure.
Stabilization of transition metal ions clusters through coordination by thiacalix [4]arenes is well-represented by the review [44]. However, the cubane-like lanthanide coordination (Ln = Gd 3+ , Eu 3+ and Tb 3+ ) has been found only for sulfonylcalix [4]arene, where the cluster motif is supported by coordination of the lanthanide ions via both phenolates and sulfonyl oxygen atoms along with four bridging acetate ligands [44,45].
Both stoichiometry and structure of the cubane-like dimeric complex of Yb 3+ with ligand H 4 L(3) significantly differ from those of the dimeric terbium complexes formed by ligands 1 and 2 in the DMF solutions [7,8]. Thus, the revealed dimeric structure may either derive from the complex formation mode in solutions or be mainly affected by the crystal packing forces. However, the lanthanide contraction may be one more reason for specificity in the coordinative behavior of Yb 3+ versus its counterparts from the middle of the lanthanide series. Therefore, the complex formation of H 4 L(3) with Yb 3+ ions will be represented along with that of H 4 L(1,2) in the DMF solutions.
The intraligand electronic absorbance of H 4 L(1-3) is a convenient tool to reveal and compare their complex formation abilities towards lanthanide ions. The UV-Vis spectral data calculated and represented in the form of the Job's plots in Figure 5a demonstrate no specificity of Yb 3+ complex formation with ligands H 4 L(1) and H 4 L(2) in comparison with earlier obtained data for their Tb 3+ complexes [7,8]. The complex formation of Yb 3+ is accompanied by the deprotonation of two and three protons under their complex formation with H 4 L(1) and H 4 L(2) (Figure 5b), which is also in good agreement with the terbium complex formation.  Stabilization of transition metal ions clusters through coordination by thiacalix [4]arenes is well-represented by the review [44]. However, the cubane-like lanthanide coordination (Ln = Gd 3+ , Eu 3+ and Tb 3+ ) has been found only for sulfonylcalix [4]arene, where the cluster motif is supported by coordination of the lanthanide ions via both phenolates and sulfonyl oxygen atoms along with four bridging acetate ligands [44,45].
Both stoichiometry and structure of the cubane-like dimeric complex of Yb 3+ with ligand H4L(3) significantly differ from those of the dimeric terbium complexes formed by ligands 1 and 2 in the DMF solutions [7,8]. Thus, the revealed dimeric structure may either derive from the complex formation mode in solutions or be mainly affected by the crystal packing forces. However, the lanthanide contraction may be one more reason for specificity in the coordinative behavior of Yb 3+ versus its counterparts from the middle of the lanthanide series. Therefore, the complex formation of H4L(3) with Yb 3+ ions will be represented along with that of H4L (1,2) in the DMF solutions.
The intraligand electronic absorbance of H4L(1-3) is a convenient tool to reveal and compare their complex formation abilities towards lanthanide ions. The UV-Vis spectral data calculated and represented in the form of the Job's plots in Figure 5a demonstrate no specificity of Yb 3+ complex formation with ligands H4L(1) and H4L(2) in comparison with earlier obtained data for their Tb 3+ complexes [7,8]. The complex formation of Yb 3+ is accompanied by the deprotonation of two and three protons under their complex formation with H4L(1) and H4L(2) (Figure 5b), which is also in good agreement with the terbium complex formation. It should be noted that similar data for p-nitro-thiacalix [4]arene H4L(3) have not been reported. The addition of Yb 3+ to the basified solution of H4L(3) results in increased absorbance at 400 nm with the disappearance of the lower energy absorption bands at 450-500 nm (Figure 6a). Such spectral behavior provides a clear indication of Yb 3+ coordination via the lower phenolic rim of H4L as the reason for restricted charge transfer from phenolate to nitro-groups. The quantitative analysis of the spectral changes resulting from the concentration variation in both H4L(3) and Yb(NO3)3 through the Job-plotting (Figure 6b) indicates that the complex formation of Yb 3+ ions with H4L(3) in the basified DMF solutions predominantly occurs in 1:1 stoichiometry. However, similar with H4L (1, 2), the nonsymmetrical shape of the Job plot (Figures 5a and 6b) indicates that the 1:1 stoichiometry is contributed by the complex forms with 2:1 (Yb:L) stoichiometry. It is worth noting that the Job plots are indistinguishable in the solutions of Yb(NO3)3 and YbCl3, which points to  (1, 2), the non-symmetrical shape of the Job plot (Figures 5a and 6b) indicates that the 1:1 stoichiometry is contributed by the complex forms with 2:1 (Yb:L) stoichiometry. It is worth noting that the Job plots are indistinguishable in the solutions of Yb(NO 3 ) 3 and YbCl 3 , which points to predominance of the 1:1 stoichiometry in the recently prepared basified DMF solutions ( Figure S7).
It should be noted that similar data for p-nitro-thiacalix [4]arene H4L(3) have not been reported. The addition of Yb 3+ to the basified solution of H4L(3) results in increased absorbance at 400 nm with the disappearance of the lower energy absorption bands at 450-500 nm (Figure 6a). Such spectral behavior provides a clear indication of Yb 3+ coordination via the lower phenolic rim of H4L as the reason for restricted charge transfer from phenolate to nitro-groups. The quantitative analysis of the spectral changes resulting from the concentration variation in both H4L(3) and Yb(NO3)3 through the Job-plotting (Figure 6b) indicates that the complex formation of Yb 3+ ions with H4L(3) in the basified DMF solutions predominantly occurs in 1:1 stoichiometry. However, similar with H4L (1, 2), the nonsymmetrical shape of the Job plot (Figures 5a and 6b) indicates that the 1:1 stoichiometry is contributed by the complex forms with 2:1 (Yb:L) stoichiometry. It is worth noting that the Job plots are indistinguishable in the solutions of Yb(NO3)3 and YbCl3, which points to predominance of the 1:1 stoichiometry in the recently prepared basified DMF solutions (Figures S7).  Thus, similar to the other thiacalix [4]arenes H 4 L(1,2), the 1:1 complex stoichiometry is predominant in the complex formation of H 4 L(3) with Yb 3+ , followed by deprotonation of three phenolic moieties (Figure 6c). It is interesting that longer storage of the solutions with YbCl 3 resulted in obtaining crystals having the 2:1 stoichiometry. In accordance with Le Chatelier's principle, the phase separation of the crystals is the main driving force for both further deprotonation of the ligand and transformation of the complex stoichiometry from 1:1 to 2:1. It is also worth noting that the DMF molecule caps the cyclophanic cavities, and it should be considered as one more factor for stabilizing the structure.

Diffusion NMR Spectroscopy
The NMR spectral changes of ligands H 4 L(1-3) resulted from their complex formation in alkalized DMSO-d 6 solutions were analyzed for diamagnetic Lu 3+ ions in order to exclude the broadening of signals due to the paramagnetic effect of Yb 3+ ions. The interference of the signals arising from the different complex forms restricts the correct evaluation of self-diffusion coefficients in the case of H 4 L(1). Thus, the self-diffusion coefficients were obtained for the complexes with H 4 L(2,3) (Table 1). Their quantitative analysis allows estimating that the self-diffusion coefficient of ligand H 4 L(2) decreases by 16% under the complex formation with Lu 3+ (Table 1), while in the case of La 3+ , a more significant decrease (21%) was reported [8]. The self-diffusion coefficient for the thiacalix [4]arene H4L(3) under the complex formation with Lu 3+ becomes lower by 14% (Table 1). According to the literature data, the decrease of D s by~25% testifies to the dimerization of the molecules in the solutions [46][47][48]. Therefore, the less pronounced decrease in self-diffusion coefficients D s of ligands H 4 L(2,3) under the complex formation with Lu 3+ versus La 3+ ions indicates the greater contribution of the monomeric forms. In particular, the accumulation of dimeric complex forms is~65% and 55% for ligands H 4 L(2) and H 4 L(3), correspondingly.

Computational Modeling of the Yb 3+ Complexes with p-Nitrothiacalix[4]arene (H4L(3))
The DFT calculations were successfully applied in recognition of the impact of the complex stoichiometry and structure on its stability for the lanthanide complexes with the calix [4]arene and thiacalix [4]arene derivatives, including, in particular, the complexes

Computational Modeling of the Yb 3+ Complexes with p-Nitrothiacalix[4]arene (H 4 L(3))
The DFT calculations were successfully applied in recognition of the impact of the complex stoichiometry and structure on its stability for the lanthanide complexes with the calix [4]arene and thiacalix [4]arene derivatives, including, in particular, the complexes with ligands H 4 L(1) and H 4 L(2) [7,8]. The DFT calculations of ytterbium complexes with ligand 3 are based on the previously reported thermodynamically favorable structures of Tb 3+ complexes with ligands H 4 L(1) and H 4 L(2) with the assumption of the terbium coordination number (CN) being equal to 8. The CN-value of Yb 3+ can be either 8 or 7 in accordance with the well-known "lanthanide contraction effect". However, the literature data [49] reveal relatively small differences in the ionic radii of Yb 3+ (1.125 Å) and Tb 3+ (1.180 Å) ions. This, in turn, argues for the realization of CN = 8 for the Yb 3+ complexes with H 4 L(3) in the solutions [50].
The diversity of the complex formation modes is represented by both monomeric (1:1) and dimeric (2:2) complex forms.
The thermochemical parameters of complexes I, II and III are collected in Table 2. The ∆G 0 298 -values of monomeric complexes (I,II) formation indicate that they are thermodynamically favorable and mainly provided by the enthalpy contribution (Table 2). However, the dimeric complex formation (equilibriums 3, 4) is an entropically driven process, which differentiates it from the formation of monomeric complexes (I,II). The 2:2 complex formation undergoes coordination of each Yb 3+ ion via two oxygen and one sulfur atom of H 4 L(3), with further saturation of the coordination sphere of Yb 3+ by two DMF molecules (structure [Yb 2 HL 2 (DMF) 4 ]-(III) in Figure 8).   (Table 2). Thus, complex forms III and V are more thermodynamically favorable than I, II and IV. Nevertheless, the NMR diffusion results (Table 1) reveal that the aforesaid complex forms are in equilibrium with the diverse monomeric complex forms.

Luminescence Spectroscopy
The complex formation in the solutions is followed by the sensitizing of Yb 3+ -centered luminescence derived from 2 F 5/2 → 2 F 7/2 transition, with the main emission band centered at 980 nm and the secondary lines at 971, 996, 1025 and 1040 nm arisen from the main crystal field splitting ( Figure 9) [16,17,51]. The spectra in Figure 9 demonstrate that the Yb 3+ -centered luminescence is the greatest for the complexes with p-nitroand p-bromosubstituted thiacalix [4]arenes H 4 L(2,3) versus the complex with H 4 L(1). The intensity ratios of the bands at 971 and 996 nm, as well as of those at 1025 and 1040 nm, are well-known for their sensitivity to any changes in the inner-sphere ligand environment of Yb 3+ ions [51]. The ratios at 971 and 996 nm deviate within 1.56-1.72 for the studied ligands, while the ratio of the lower energy luminescence bands (1025 and 1040 nm) is somewhat greater for the complexes with ligand 3 versus those with 1 and 2, which argues for some peculiarity in the inner-sphere environment of Yb 3+ in the case of 3. It is worth noting the equilibration of the above-mentioned 2:2, 1:1 and 2:1 complexes as the factor influencing the inner-sphere environment of Yb 3+ ions in the complexes with ligands 2 and 3. Thus, the aforesaid deviation between the spectral patterns of the complexes and ligands 2 and 3 ( Figure 9) can be explained by the different ratios of the dimeric to monomeric complex forms.  Table 3). The τ-value of the complex with 1 is significantly shorter; thus, its correct measuring lies out of the present work's scope since the lowest limit of correct lifetime estimation is around 10 µs for a flash lamp used as the excitation source. Altogether, these facts argue for the small number of solvent molecules in the inner sphere environment of Yb 3+ ion for H4L (2,3) complexes, pointing to the significant contribution of 2:2 form in contrast to 1:1 form for H4L (1). The steady state intensities of the complexes correlate with their excited state lifetimes (Table 3), thus indicating that the radiationless losses are significantly lower for the complexes with H4L(2,3) versus those with H4L(1). It is worth noting that the similarity in τav-values of Yb 3+ in the complexes with ligands 2 and 3 allows to exclude the significant dissipation of the excitation energy in H4L(R = NO2) caused by the presence of the nitro-substituents as it was found for the lanthanide complexes with nitrobenzoates [35].  [7,8].
Low temperature phosphorescence measurements of [Gd2L2] 2− for H4L(3) were performed to evaluate the energy of the triplet level at 567 nm (17637 cm −1 ) ( Figure S8) and average lifetimes of the ligand-centered phosphorescence of the Gd 3+ (<τ> = 753 μs). The  Table 3). The τ-value of the complex with 1 is significantly shorter; thus, its correct measuring lies out of the present work's scope since the lowest limit of correct lifetime estimation is around 10 µs for a flash lamp used as the excitation source. Altogether, these facts argue for the small number of solvent molecules in the inner sphere environment of Yb 3+ ion for H 4 L(2,3) complexes, pointing to the significant contribution of 2:2 form in contrast to 1:1 form for H 4 L (1). The steady state intensities of the complexes correlate with their excited state lifetimes (Table 3), thus indicating that the radiationless losses are significantly lower for the complexes with H 4 L(2,3) versus those with H 4 L(1). It is worth noting that the similarity in τ av -values of Yb 3+ in the complexes with ligands 2 and 3 allows to exclude the significant dissipation of the excitation energy in H 4 L(R = NO 2 ) caused by the presence of the nitro-substituents as it was found for the lanthanide complexes with nitrobenzoates [35]. The excitation spectra of the complexes also reveal the difference between the ligands H 4 L(1-3) (Figure 9). In particular, the maximums of the excitation bands exhibit red shifting, which increases in the following series: H 4 L(1) (367 nm) < H 4 L(2) (386 nm) < H 4 L(3) (476 nm).
Low temperature phosphorescence measurements of [Gd 2 L 2 ] 2− for H 4 L(3) were performed to evaluate the energy of the triplet level at 567 nm (17,637 cm −1 ) ( Figure S8) and average lifetimes of the ligand-centered phosphorescence of the Gd 3+ (<τ> = 753 µs). The value is much lower than the previously reported triplet level energies for the complexes with ligands H 4 L(1) and H 4 L(2) (417 nm (23,981 cm −1 ) and 458 nm (21,834 cm −1 ), accordingly [8]. The energies of the triplet levels of the ligands increase in the following series: H 4 L(1) (417 nm) < H 4 L(2) (458 nm) < H 4 L(3) (567 nm) (Table 3), which is in good correlation with the above-mentioned red-shifting of the excitation bands.
Visible light excitation is especially important for NIR emitting materials employed in biochemistry and cell biology since living tissues are sensitive to UV irritation [34]. This indicates that the advantage of H 4 L(3) versus H 4 L(2) is the excitation of the bright NIR-luminescence by the lower energy irradiation. Nevertheless, the energy of the triplet level provides a relatively small impact on the Yb 3+ -luminescence of the complexes with ligands H 4 L(2,3). This argues for the effect of the upper-rim substituents R = NO 2 , Br on the restricted flexibility of the outer-sphere environment of Yb 3+ ions versus ligand H 4 L(1,R = H) as the main reason for the longer excited state lifetimes (Table 3) and brighter NIR luminescence (Figure 9).

Synthesis of Complex 3 with YbCl 3
The 2.85 mL of 4.5 mM solution of H 4 L (3) in DMF was mixed with 0.135 mL (0.1 M) of solution of ytterbium chloride hexahydrate in DMF. To this mixture, 0.015 mL (7.2 mM) of solution of TEA was added. The resulting solution was stored at room temperature for a few months and resulted in formation of yellowish needle-like crystals, which have been used for X-ray analysis.

Physical Measurements and Methods
Detailed descriptions of physical measurements and methods (electronic absorption, NMR experiments, MALDI-TOF mass spectrometry, crystal structure data, luminescence spectroscopy and quantum-chemical modeling) are presented in Supplementary Materials.

Conclusions
The present work revealed the impact of the bromo-and nitro-substituents embedded at the upper rim of thiacalix [4]arenes on a supramolecular package in crystals and solution behavior of both tetra-bromo and tetra-nitrothiacalix [4]arenes as well as their ytterbium complexes. It was shown that the upper-rim substituents (R = Br, NO 2 ) enhance deprotonation of the phenolic rims and generate the unusual inclusive self-assembly of H 3 L − (R = Br), revealed through single-crystal XRD analysis. Similar to non-substituted thiacalix [4]arenes, both tetra-bromo and tetra-nitrothiacalix [4]arenes coordinate Yb 3+ ions into 1:1, 2:2 and 2:1 (Yb:L) complex forms, but the antenna effect of the thiacalix [4]arene-based ligands on the Yb 3+ -centered luminescence is greatly enhanced by the bromo-and nitro-substituents. Quantum chemical study revealed thermodynamic favorableness of the formation of dimeric 2:2 (Yb:[HL] − ) complexes with a rigid structure as one of the reasons for efficient sensitizing of the Yb 3+ luminescence. X-ray analysis of the single crystal separated from the basified DMF solutions of YbCl 3 and H 4 L (R = NO 2 ) revealed transformation of the dimeric complexes into [4Yb 3+ :2L 4− ] ones with a cubane-like cluster structure.
Analysis of the time-resolved luminescence in correlation with the triplet energy levels revealed peculiarity in the antenna effect of H 4 L(R = NO 2 ), which is efficient sensitizing of the Yb 3+ luminescence by the triplet level of the ligand at 588 nm arisen from the ILCT without a significant decrease in the lifetime of the excited state caused by dissipation of an excitation energy in H 4 L(R = NO 2 ). However, the tetra-brominated ligand provides similar sensitizing due to the smaller radiationless losses of the ligand environment. Nevertheless, the red-shifting of the excitation wavelengths from 360-380 and 370-420 nm for H 4 L(R = H, Br) to 460-500 nm for H 4 L(R = NO 2 ) provides an advantage of H 4 L(R = NO 2 ) versus H 4 L(R = H, Br) in further bio-applications.