Comparison of Photophysical Properties of Lanthanide(III) Complexes of DTTA- or DO3A-Appended Aryl-2,2′-Bipyridines

New Tb(III) and Eu(III) complexes based on aryl-2,2′-bipyridine ligands with a cyclic DO3A chelating unit appended in the alpha position of the bipyridine core were synthesized. The photophysical properties of these complexes were compared with those of complexes of ligands with identical aryl-2,2′-bipyridine chromophores, but with an acyclic DTTA residue as an additional chelating site in the alpha position of the bipyridine core. The nature of the polyaminocarboxylic acid fragments was found to have a significant influence on the luminescence. For some of the Eu(III) complexes, upon the transition from acyclic DTTA- to the cyclic DO3A-appended ligands, a noticeable increase in the intensity of Eu(III) luminescence was observed, with an increase in the quantum yield of up to 2.55 times. In contrast, for most of the Tb(III) complexes, a similar transition resulted in a noticeable decrease in the luminescence intensity of the Tb(III) cation.

The ligands associated with these complexes have chromophore parts in their structure that are necessary for absorbing light, in order to transfer energy to the metal cation [8]. The chelating part also needs to be of rigid character to enable strong binding of the lanthanide cation. It should be noted that the presence of a water molecule in the coordination sphere of the lanthanide cation leads to quenching of luminescence due to the nonradiative relaxation of its excited state through energy transfer to the high-frequency oscillating O-H bonds of water molecules. To prevent this, complete filling of the coordination sphere of the lanthanide cation is required.
In this study, groups such as cyclic DO3A or acyclic DTTA were used as the polyaminocarboxylic acid fragments. However, there are very few reports of the preparation of complexes with a similar chromophore and residues of various polyaminocarboxylic acids in the literature. In one example, the photophysical properties of pairs of luminescent lanthanide complexes with a chromophore based on 5-phenyl-2,2 -bipyridine [19] and 9-methyl-1,10phenanthroline [11,12] were previously published, and a slight decrease in the quantum yield of the luminescence of europium(III) and terbium(III) cations upon passing from the DTTA to the DO3A residue was observed. In this work, for the first time, we have carried out a broader analysis of the influence of the nature of the polyaminocarboxylic acid, such as being cyclic (DO3A) or acyclic (DTTA), on the photophysical properties of the lanthanide(III) chelates obtained, comparing a number of different aryl-containing 2,2 -bipyridines as the ligands/chromophores.

Synthesis of the New Water-Soluble Lanthanide(III) Complexes
The synthesis of the target ligands and their complexes is shown in Scheme 1. The corresponding alpha-bromomethylbipyridines (shown at 1 in Scheme 1) were used as key intermediates. Further alkylation of the commercially available tris-tert-butyl ester of DO3A (2), carried out according to the previously reported procedure [19], made it possible to obtain the precursors of the ligands shown at 3. The structure of the compounds at 3 was confirmed through 1 H NMR and mass (electrospray) spectra. Thus, in the 1 H NMR spectra, signals from the protons of the arylbipyridine system, as well as signals from the DO3A fragment in the resonance region of the aliphatic protons, were observed. Separately, the signals from the protons of the methylene moiety, which connects the corresponding 2,2bipyridine fragment and the DO3A moiety, were observed in the region of 3.83-3.89 ppm. In this case, the change in the chemical shifts of these protons compared to those of the bromomethyl-substituted 2,2 -bipyridines shown at 1 was quite informative. The subsequent removal of tert-butyl protection in hydrochloric acid resulted in the ligands shown at 4 manifesting as hydrochlorides; their composition was established on the basis of elemental analysis data. Finally, the interaction of the trisodium salts of the DTTA-or DO3A-based ligands (obtained in situ) with the lanthanide chlorides led to the target Ln•4 complexes.
The subsequent removal of tert-butyl protection in hydrochloric acid resulted in the ligands shown at 4 manifesting as hydrochlorides; their composition was established on the basis of elemental analysis data. Finally, the interaction of the trisodium salts of the DTTA-or DO3A-based ligands (obtained in situ) with the lanthanide chlorides led to the target Ln•4 complexes.
Most of the complexes of the DTTA-appended ligands studied in this work have been described previously. However, some of them were synthesized for the first time; in particular, the complexes based on 5-aryl-2,2′-bipyridines. Thus, ligands 5d and 5e have been described by us earlier [20], and the synthesis of ligand 5b was performed on the basis of the corresponding 6-bromomethylbipyridine (1b) as described earlier [19], as a result of alkylation of the DTTA ester shown at 6 [21,22]. In the case of the 4-tolyl-containing ligand (5c), the 5-tolyl-2,2′-bipyridine-6-carboxylic acid methyl ester (7) reported above was used as a starting compound [23] (Scheme 2). The compositions of the new complexes Ln•4 and Ln•5 were confirmed on the basis of mass spectrometry (electrospray) and elemental analysis results. It should be noted that the structure of the Eu(III) complex Eu•4b bearing 5-(4-methoxyphenyl)-2,2 -bipyridine as a chromophore was previously confirmed by us by means of X-ray diffraction analysis [19] ( Figure 1). Thus, the simultaneous participation of the 2,2 -bipyridine moiety and the DO3A fragment in the chelation of the europium(III) cation was confirmed.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 12 The compositions of the new complexes Ln•4 and Ln•5 were confirmed on the basis of mass spectrometry (electrospray) and elemental analysis results. It should be noted that the structure of the Eu(III) complex Eu•4b bearing 5-(4-methoxyphenyl)-2,2′-bipyridine as a chromophore was previously confirmed by us by means of X-ray diffraction analysis [19] (Figure 1). Thus, the simultaneous participation of the 2,2′-bipyridine moiety and the DO3A fragment in the chelation of the europium(III) cation was confirmed.  [19]. Reprinted with the permission of [19]. Copyright © 2011 Elsevier B.V.

Photophysical Properties of the New Water-Soluble Lanthanide(III) Complexes
The results of the photophysical studies of the new complexes, as well as some earlier data, are summarized in Table 1. The quantum yield of the previously described Tb•5f complex was measured using standards considered to be more suitable than the quinine sulfate which was used previously [14], such as ([Ru(bpy)3]Cl2).   [19]. Reprinted with the permission of [19]. Copyright © 2011 Elsevier B.V.

Photophysical Properties of the New Water-Soluble Lanthanide(III) Complexes
The results of the photophysical studies of the new complexes, as well as some earlier data, are summarized in Table 1. The quantum yield of the previously described Tb•5f complex was measured using standards considered to be more suitable than the quinine sulfate which was used previously [14], such as ([Ru(bpy) 3 ]Cl 2 ).

Photophysical Properties of the New Water-Soluble Lanthanide(III) Complexes
The results of the photophysical studies of the new complexes, as well as some earlier data, are summarized in Table 1. The quantum yield of the previously described Tb•5f complex was measured using standards considered to be more suitable than the quinine sulfate which was used previously [14], such as ([Ru(bpy)3]Cl2). In regard to the confirmation of the participation of the 2,2 -bipyridine fragment in the chelation of the lanthanide (III) cations in the new complexes, we draw attention to the previously obtained europium complex Eu•10 based on 5-phenyl-2,2 -bipyridine with a DTTA moiety at the C5 position (Table 1) [21]. In this case, the chelation of the europium(III) cation by the bipyridine fragment is impossible. Analysis of the absorption maxima of the corresponding complexes Eu•10 and Eu•5a with the same chromophore demonstrated a noticeable bathochromic shift for the complex Eu•5a compared to chelate Eu•10 (327 nm vs. 304 nm). The absorption maxima in water for the corresponding complexes Ln•4 and Ln•5 with the same chromophore either differed by several nm or were the same. Taking into account the data from the XRD analysis of the complex Ln•4b [19] and these data, we may assume the involvement of the 2,2 -bipyridine fragment in the chelation of lanthanide(III) cations for the complexes termed Ln•5. The absorption spectra of the pairs of complexes Eu•5c/Eu•5c and Tb•5g/Tb•4g are shown in Figures 2 and 3.  Only for the complexes Ln•4j and Tb•5j with 1,10-phenanthroline as the chromophore was there a noticable difference between the absorption maxima (16 nm).
Moreover, the calculations based on the values of the luminescence lifetime in water and D 2 O for the complex Eu•10 showed the presence of two water molecules in the first coordination sphere of the europium(III) cation. This fact can be explained by the absence of the two coordination bonds with the nitrogen atoms of the 2,2 -bipyridine moiety [21].   Only for the complexes Ln•4j and Tb•5j with 1,10-phenanthroline as the chromophore was there a noticable difference between the absorption maxima (16 nm).
Moreover, the calculations based on the values of the luminescence lifetime in water and D2O for the complex Eu•10 showed the presence of two water molecules in the first coordination sphere of the europium(III) cation. This fact can be explained by the absence of the two coordination bonds with the nitrogen atoms of the 2,2′-bipyridine moiety [21].    As mentioned above, in the case of previously published complexes, there was no significant difference in the luminescence intensity of the terbium(III) and europium(III) cations, regardless of the nature of the additional chelating site (DTTA or DO3A). There was only a slight decrease in the quantum yields between the acyclic DTTA and the cyclic DO3A fragment. Based on these results, a conclusion could be made that the nature of the additional chelating site has an insignificant influence on the photophysical characteristics of the complexes.
However, all of the data collected by us are in strong contradiction to this assumption, since, in a number of cases, there is a significant difference between the values of the lanthanide (III) cation luminescence quantum yields for the corresponding Ln•4 and Ln•5 complexes. The relationship between the quantum yields of the complexes with either a cyclic (DO3A) or acyclic (DTTA) chelating part is shown in Figure 6, and the complexes are listed in decreasing order of this indicator in the corresponding series. Where there was a significant increase in the intensity of the lanthanide cation luminescence upon the transition to the DO3A-appended ligand, it is highlighted in red. The cases with a significant decrease in the intensity of the lanthanide (III) cation luminescence are highlighted in blue. As mentioned above, in the case of previously published complexes, there was no significant difference in the luminescence intensity of the terbium(III) and europium(III) cations, regardless of the nature of the additional chelating site (DTTA or DO3A). There was only a slight decrease in the quantum yields between the acyclic DTTA and the cyclic DO3A fragment. Based on these results, a conclusion could be made that the nature of the additional chelating site has an insignificant influence on the photophysical characteristics of the complexes.
However, all of the data collected by us are in strong contradiction to this assumption, since, in a number of cases, there is a significant difference between the values of the lanthanide (III) cation luminescence quantum yields for the corresponding Ln•4 and Ln•5 complexes. The relationship between the quantum yields of the complexes with either a cyclic (DO3A) or acyclic (DTTA) chelating part is shown in Figure 6, and the complexes are listed in decreasing order of this indicator in the corresponding series. Where there was a significant increase in the intensity of the lanthanide cation luminescence upon the transition to the DO3A-appended ligand, it is highlighted in red. The cases with a significant decrease in the intensity of the lanthanide (III) cation luminescence are highlighted in blue.
Thus, in a number of the europium(III) complexes, there was a significant increase in the luminescence of the europium(III) cation. Moreover, within the series studied, a record growth in the luminescence quantum yield was observed for the Eu•4e complex based on 5-(3-chlorophenyl)-2,2 -bipyridine. A somewhat smaller growth took place for the closest analogs of Eu•4e bearing 4-tolyl and 4-chlorophenyl substituents, such as Eu•4c and Eu•4d. For phenyl substituents, as previously described [19], no significant difference between the luminescence intensities of the Eu•4a and Eu•5a complexes was observed. An unexpectedly sharp drop in all the photophysical properties, including the luminescence lifetime in water and D 2 O, was observed for the complex Eu•4b based on the 4-methoxyphenyl-containing ligand. In the case of 4-aryl-2,2 -bipyridine ligands, a significant increase in the luminescence intensity was observed for the Eu•4h complex based on a 2-fluorophenyl-containing ligand (the ratio was 1.62). For the other two complexes, Eu•4g and Eu•4f, a decreased luminescence of the europium (III) cation was observed. In this regard, this effect seen in the complex of the 4-methoxyphenyl-containing ligand Eu•4f correlated well with the data for the Eu•4b complex, although the decrease in the luminescence intensity was not as significant as it was for the latter (the ratio was 0.678 instead of 0.103). A significant increase in the value of the quantum yield was also shown by the Eu•4i complex of the ligand based on 5-phenyl-2,2 -bipyridine with an additional chelating fragment in the C6 position (in the series studied, the observed ratio of 1.97 was the second highest result after that for the Eu•4e complex). When 1,10-phenanthroline was used as a chromophore, no significant changes in the luminescence intensity were observed upon passing to the DO3A-based complex (chelates Eu•4j and Eu•5j).
Molecules 2023, 28, x FOR PEER REVIEW 9 of 12 Thus, in a number of the europium(III) complexes, there was a significant increase in the luminescence of the europium(III) cation. Moreover, within the series studied, a record growth in the luminescence quantum yield was observed for the Eu•4e complex based on 5-(3-chlorophenyl)-2,2′-bipyridine. A somewhat smaller growth took place for the closest analogs of Eu•4e bearing 4-tolyl and 4-chlorophenyl substituents, such as Eu•4c and Eu•4d. For phenyl substituents, as previously described [19], no significant difference between the luminescence intensities of the Eu•4a and Eu•5a complexes was observed. An unexpectedly sharp drop in all the photophysical properties, including the luminescence lifetime in water and D2O, was observed for the complex Eu•4b based on the 4-methoxyphenyl-containing ligand. In the case of 4-aryl-2,2′-bipyridine ligands, a significant increase in the luminescence intensity was observed for the Eu•4h complex based on a 2-fluorophenyl-containing ligand (the ratio was 1.62). For the other two complexes, Eu•4g and Eu•4f, a decreased luminescence of the europium (III) cation was observed. In this regard, this effect seen in the complex of the 4-methoxyphenylcontaining ligand Eu•4f correlated well with the data for the Eu•4b complex, although the decrease in the luminescence intensity was not as significant as it was for the latter (the ratio was 0.678 instead of 0.103). A significant increase in the value of the quantum yield was also shown by the Eu•4i complex of the ligand based on 5-phenyl-2,2′bipyridine with an additional chelating fragment in the C6 position (in the series studied, the observed ratio of 1.97 was the second highest result after that for the Eu•4e complex). When 1,10-phenanthroline was used as a chromophore, no significant changes in the luminescence intensity were observed upon passing to the DO3A-based complex (chelates Eu•4j and Eu•5j).
We also studied the photophysical properties for the corresponding terbium(III) complexes of 4-aryl-2,2′-bipyridine chromophores, but further series could not be studied due to a lack of satisfactory sensitization of the terbium(III) cation by the other ligands We also studied the photophysical properties for the corresponding terbium(III) complexes of 4-aryl-2,2 -bipyridine chromophores, but further series could not be studied due to a lack of satisfactory sensitization of the terbium(III) cation by the other ligands under consideration, namely, those based on 5-aryl-2,2 -bipyridine. According to the data obtained, there was an absence of a noticeable increase in the luminescence intensity of the terbium(III) cation in case of DO3A-appended ligands. Only for the 4-methoxyphenyl-substituted ligand (complex Tb•4f) was there a slight increase in the luminescence quantum yield of the terbium(III) cation, where it was higher by a factor of 1.185. In two other cases, the 2-fluorophenyl-substituted ligand (complex Tb•4h) and the 3-chlorophenyl-substituted ligand (complex Tb•4g), a noticeable decrease in the quantum yield was observed. It is worth mentioning that the ratios of the luminescence quantum yield of the corresponding DTTA-containing Tb(III) complexes were 0.42 and 0.32, respectively. The same trend was observed for the Tb(III) complexes with 2-methyl-1,10-phenanthroline chromophore, such as in complexes Tb•4j and Tb•5j, and the observed ratio here was 0.73. The Tb•5i complex exhibited low terbium (III) luminescence intensity, with a quantum yield as low as 0.43%, while for the similar Tb•4i complex based on a DO3A-appended ligand, a decreased terbium luminescence was observed, although we were unable to reliably measure its quantum yield.
An analysis of the luminescence lifetimes in water and D 2 O for the pairs of Eu•4 and Eu•5 complexes under consideration showed that, for a number of them, when moving to DO3A-appended ligands, there is an increase in the luminescence lifetime in water and a decrease in D 2 O. In particular, this behaviour was observed for the complexes based on ligands 4a/5a, 4d/5d, and 4e/5e.
Detailed experimental procedures and characterizations of the products are presented in the Supplementary Materials.

Conclusions
In conclusion, for the first time, a broad comparison of the properties of Eu 3+ and Tb 3+ complexes based on ligands with the same aryl-2,2 -bipyridine series chromophores but having core additional chelating sites in the alpha position of the 2,2 -bipyridines, such as non-cyclic DTTA or cyclic DO3A residues, was carried out. In contrast to the previously published reports on the DTTA-and DO3A-based Ln(III) chelates, we found that the nature of the polyaminocarboxylic acid fragment had a significant influence on the Ln(III) luminescence in the resulting Ln(III) complex. For example, in the case of some Eu(III) complexes, passing from the DTTA-to the DO3A-appended ligands resulted in a noticeable increase in the intensity of the lanthanide(III) luminescence, with an increase in the quantum yield of up to 2.55 times. In contrast, for most of the DO3A-based Tb(III) complexes, a decrease in the luminescence intensity of the terbium(III) cation was observed. Most probably, upon the formation of macrocyclic Eu(III) complexes with DO3A-appended 2,2 -bipyridine ligands, a more efficient sensitization of the chelated Eu(III) cation takes place compared to the pseudo-cyclic Eu(III) complexes of the same ligands bearing a DTTA moiety.