Ingo Frommholz
email: ifrommholz@acm.org

Philipp Mayr
email: philipp.mayr@gesis.org

Guillaume Cabanac
email: guillaume.cabanac@univ-tlse3.fr

Suzan Verberne
email: s.verberne@liacs.leidenuniv.nl

Bibliometric-enhanced Information Retrieval

Keywords: Academic Search, Information Retrieval, Digital Libraries, Bibliometrics, Scientometrics

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Motivation and Relevance to ECIR

The aim of the BIR workshop series is to bring together researchers and practitioners from Scientometrics/Bibliometrics as well as Information Retrieval (IR). Scientometrics is a sub-field of Bibliometrics which, like IR, is, in turn, a sub-field of Information Science. Bibliometrics and Scientometrics are concerned with all quantitative aspects of information and academic literature [START_REF] Pritchard | Statistical bibliography or bibliometrics? [Documentation notes[END_REF], which naturally make them interesting for IR research, in particular when it comes to academic search and recommendation. In the 1960s, Salton was already striving to enhance IR by including clues inferred from bibliographic citations [START_REF] Salton | Associative document retrieval techniques using bibliographic information[END_REF]. In the course of decades, both disciplines (Bibliometrics and IR) evolved apart from each other over time, leading to the two loosely connected fields we know of today [START_REF] White | Visualizing a discipline: An author co-citation analysis of Information Science, 1972-1995[END_REF].

However, the exploding number of scholarly publications and the need to satisfy scholars' specific information needs led Bibliometric-enhanced IR to receive growing recognition in the IR as well as the Scientometrics communities. Challenges in academic search and recommendation became particularly apparent during the COVID-19 crisis and the need for effective and efficient solutions for scholarly search for and discovery of high-quality publications on that topic. Bibliometric-enhanced IR tries to provide these solutions to the peculiar needs of scholars to keep on top of the research in their respective fields, utilising the wide range of suitable relevance signals that come with academic scientific publications, such as keywords provided by authors, topics extracted from the full-texts, co-authorship networks, citation networks, bibliometric figures, and various classification schemes of science. Bibliometric-enhanced IR systems must deal with the multifaceted nature of scientific information by searching for or recommending academic papers, patents, venues (i.e., conference proceedings, journals, books, manuals, grey literature), authors, experts (e.g., peer reviewers), references (to be cited to support an argument), and datasets.

To this end, the BIR workshop series was founded in 2014 [START_REF] Mayr | Bibliometric-Enhanced Information Retrieval[END_REF] to tackle these challenges by tightening up the link between IR and Bibliometrics. We strive to bring the 'retrievalists' and 'citationists' [START_REF] White | Visualizing a discipline: An author co-citation analysis of Information Science, 1972-1995[END_REF] active in both academia and industry together. The success of past BIR events, as shown in Table 1, evidences that BIR@ECIR is a much needed interdisciplinary scientific event that attracts researchers and practitioners from IR and Bibliometrics alike.

Workshop goals/objectives

Our vision is to bring together researchers and practitioners from Scientometrics/Bibliometrics on the one hand and IR on the other hand to create better methods and systems for instance for academic search and recommendation. Our view is to expose people from one community to the work of the respective other community and to foster fruitful interaction across communities. Therefore, in the call for papers for the 2022 BIR workshop at ECIR, we will address, but are not limited to, current research issues regarding 3 aspects of the academic search/recommendation process:

1. User needs and behaviour regarding scientific information, such as:

-Finding relevant papers/authors for a literature review.

-Filtering high-quality research papers, e.g. in preprint servers.

-Measuring the degree of plagiarism in a paper.

-Identifying expert reviewers for a given submission.

-Flagging predatory conferences and journals.

-Understanding information-seeking behaviour and HCI in academic search. 2. Mining the scientific literature, such as:

-Information extraction, text mining and parsing of scholarly literature.

-Natural language processing (e.g., citation contexts).

-Discourse modelling and argument mining. 3. Academic search/recommendation systems, such as:

-Modelling the multifaceted nature of scientific information.

-Building test collections for reproducible BIR.

-System support for literature search and recommendation.

Target Audience and Dissemination

The target audience of the BIR workshops is researchers and practitioners, junior and senior, from Scientometrics as well as IR and Natural Language Processing (NLP). These could be IR/NLP researchers interested in potential new application areas for their work as well as researchers and practitioners working with bibliometric data and interested in how IR/NLP methods can make use of such data. BIR 2022 will be open for anyone interested in the topic. The 10th-anniversary edition in 2020 ran online with an audience peaking at 97 online participants [START_REF] Cabanac | Report on the 10th Anniversary Workshop on Bibliometric-enhanced Information Retrieval (BIR 2020)[END_REF]. BIR 2021, the 11th edition [START_REF]Proceedings of the 11th International Workshop on Bibliometric-enhanced Information Retrieval (BIR)[END_REF][START_REF] Frommholz | Report on the 11th Bibliometric-enhanced Information Retrieval Workshop (BIR 2021)[END_REF], attracted around 57 participants at peak times but a larger number throughout due to participants dropping in and out.

In December 2020, we published our third special issue emerging from the past BIR workshops [START_REF] Cabanac | Scholarly literature mining with Information Retrieval and Natural Language Processing: Preface[END_REF]. More special issues based on BIR workshops are planned.

As a follow-up of the workshop and following the tradition of previous years, the co-chairs will write a report summing up the main themes and discussions to SIGIR Forum [4, for instance] and BCS Informer, as a way to advertise our research topics as widely as possible among the IR community. As in the past, we plan to publish our accepted papers open-access in a CEUR Workshop Proceedings volume.

Peer Review Process and Workshop Format

Our peer review process will be supported by Easychair. Each submission is assigned to 2 to 3 reviewers, preferably at least one expert in IR and one expert in Bibliometrics or NLP. The programme committee for 2022 will consist of peer reviewers from all participating communities. Accepted papers are either long papers (15-minute talks) or short papers (5-minute talks). Two interactive sessions close the morning and afternoon sessions with posters and demos, allowing attendees to discuss the latest developments in the field and opportunities (e.g., shared tasks such as CL-SciSumm). We also invite attendees to demonstrate prototypes during flash presentations (5 minutes).

These interactive sessions serve as ice-breakers, sparking interesting discussions that, in non-pandemic times, usually continue during lunch and the evening social event. The sessions are also an opportunity for our speakers to further discuss their work. BIR has a friendly and open atmosphere where there is an opportunity for participants (including students) to share their ideas and current work and to receive feedback from the community. The list of PC members will be available on the BIR 2022 page.

Organisers and Programme Commitee

-

  Ingo Frommholz is Reader in Data Science at the University of Wolverhampton, UK. -Philipp Mayr is a team leader at the GESIS -Leibniz-Institute for the Social Sciences department Knowledge Technologies for the Social Sciences, Germany. -Guillaume Cabanac is an Associate Professor at the University of Toulouse, France. -Suzan Verberne is an associate professor at the Leiden Institute of Advanced Computer Science (LIACS) at Leiden University and group leader of Text Mining and Retrieval Leiden.

Table 1 .

 1 Overview of the BIR workshop series and CEUR proceedings

	Year	Conference	Venue		Papers	Proceedings
	2014	ECIR	Amsterdam,	NL	6	Vol-1143
	2015	ECIR	Vienna,	AT	6	Vol-1344
	2016	ECIR	Padua,	IT	8	Vol-1567
	2016	JCDL	Newark,	US	10 + 10 a	Vol-1610
	2017	ECIR	Aberdeen,	UK	12	Vol-1823
	2017	SIGIR	Tokyo,	JP	11	Vol-1888
	2018	ECIR	Grenoble,	FR	9	Vol-2080
	2019	ECIR	Cologne,	DE	14	Vol-2345
	2019	SIGIR	Paris,	FR	16 + 10 b	Vol-2414
	2020	ECIR	Lisbon (Online), PT	9	Vol-2591
	2021	ECIR	Lucca (Online), IT	9	Vol-2847

a with CL-SciSumm 2016 Shared Task; b with CL-SciSumm 2019 Shared Task


John P Karns

Svetlana V Eliseeva

Cassandra L Ward

Matthew J Allen

Stéphane Petoud

Jacob C Lutter

Near-infrared lanthanide-based emission from fused bis[Ln(III)/Zn(II) 14-metallacrown-5] coordination compounds

Keywords: Lanthanides, Near-infrared, Luminescence, Metallacrowns, Heterobimetallic Complexes, Singlecrystal X-ray Diffraction

A set of (Ln[14-MC Zn (II) N(quinHA) -5]) 2 Ln 2 Zn 2 (quinHA) 2 (ph) 2 (Hph) 4 (OH) 8 (H 2 O) 4 metallacrowns (Ln-1, Ln = Tb, Gd, or Yb; H 2 quinHA = quinaldic hydroxamic acid, H 2 ph = phthalic acid) have been synthesized via solution-state self-assembly. The MCs possess an uncommon topology within the metallacrown family where two rarely seen 14-metallacrown-5 moieties are fused by a Yb 2 Zn 2 (quinHA) 2 bridge. Moreover, Yb-1 analyzed in the solid state exhibits a characteristic near-IR luminescence signal arising from Yb 3+ 2 F 5/2 → 2 F 7/2 transition despite the proximity of high energy O-H oscillators.

 In addition, several Ln 3+ ions emit photons in the near-IR spectroscopic range that are of special interest for deep tissue imaging and telecommunications. 10 However, Ln 3+ have low absorbance because most f-f

3 transitions are parity forbidden. To counteract this limitation organic chromophoric ligands with large molar absorption coefficients can be used to sensitize Ln 3+ via energy transfer, leading to enhanced emission (antenna effect). [START_REF] Weissman | Intramolecular Energy Transfer The Fluorescence of Complexes of Europium[END_REF][START_REF] Uh | Novel Antennae for the Sensitization of near Infrared Luminescent Lanthanide Cations[END_REF] Metallacrowns (MCs) were first introduced by Pecoraro and Lah in 1989 as inorganic analogues to crown ethers, where the [-C-C-O-] n repeating unit is replaced by a [-M-N-O-] n unit to form a rigid macrocycle. [START_REF] Lah | The Tetranuclear Cluster Fe III [Fe III (Salicylhydroximato)(MeOH)(Acetate)] 3 Is an Analogue of M 3+ (9-Crown-3)[END_REF][START_REF] Pecoraro | Structural Characterization of [VO(Salicylhydroximate)(CH 3 OH)] 3 : Applications to the Biological Chemistry of Vanadium(V)[END_REF] M is typically a third-row metal ion, and the N-O binding unit is usually provided by hydroximates. [START_REF] Mezei | Structural and Functional Evolutions of Metallacrowns[END_REF][START_REF] Chow | Heterometallic Mixed 3d-4f Metallacrowns: Structural Versatility, Luminescence, and Molecular Magnetism[END_REF][START_REF] Lutter | Supramolecular Constructs With Potential in Extended Solids, Solution-State Dynamics, Molecular Magnetism, and Imaging[END_REF] Since 2011, MCs have been studied as antenna for Ln 3+ , [START_REF] Jankolovits | Assembly of Near-Infrared Luminescent Lanthanide Host(Host-Guest) Complexes With a Metallacrown Sandwich Motif[END_REF] typically inclding optically silent metals such as Zn 2+ and Ga 3+ to form the metallamacrocycle structure. Such Ln 3+ -based MCs have shown promises for optical imaging, orthogonal functionalization, nanothermometry, and white-light emission. [START_REF] Trivedi | Highly Emitting Near-Infrared Lanthanide "Encapsulated Sandwich" Metallacrown Complexes with Excitation Shifted toward Lower Energy[END_REF][START_REF] Chow | Ga 3+ /Ln 3+ Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains[END_REF][START_REF] Nguyen | One-Step Assembly of Visible and Near-Infrared Emitting Metallacrown Dimers Using a Bifunctional Linker[END_REF][START_REF] Martinić | Near-Infrared Optical Imaging of Necrotic Cells by Photostable Lanthanide-Based Metallacrowns[END_REF][START_REF] Martinić | Near-Infrared Luminescent Metallacrowns for Combined in Vitro Cell Fixation and Counter Staining[END_REF][START_REF] Lutter | A Unique Ln III {[3.3.1Ga III Metallacryptate} Series That Possesses Properties of Slow Magnetic Relaxation and Visible/Near-Infrared Luminescence[END_REF][START_REF] Lutter | Iodinated Metallacrowns: Toward Combined Bimodal Near-Infrared and X-Ray Contrast Imaging Agents[END_REF][START_REF] Lutter | Functionalization of Luminescent Lanthanide-Gallium Metallacrowns Using Copper-Catalyzed Alkyne-Azide Cycloaddition and Thiol-Maleimide Michael Addition[END_REF][START_REF] Salerno | Ga 3+ 8 Sm 3+ 2 , Ga 3+ 8 Tb 3+ 2 ] Metallacrowns Are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures[END_REF][START_REF] Eliseeva | 3+ White Light Emission Can Be Finely Controlled by Tuning the First Coordination Sphere of Ga 3+ /Dy 3+ Metallacrown Complexes[END_REF] Here, we present a series of Ln 3+ /Zn 2+ MCs (Ln[14-MC Zn II N(quinHA) -5]) 2 Ln 2 Zn 2 (quinHA) 2 (ph) 2 (Hph) 4 (OH) 8 (H 2 O) 4 (Ln-1, Ln = Tb, Gd, or Yb; H 2 quinHA = quinaldic hydroxamic acid, H 2 ph = phthalic acid, Figure 1) possessing a different structure that hasn't been reported so far. They contain two fused [14-MC-5] motifs and are able to sensitize Yb 3+ luminescence in the near-IR range.

The self-assembly synthesis of Ln-1 compounds was achieved via a stoichiometric addition of the appropriate components in N,N-dimethylformamide. A diffusion of water into N,Ndimethylformamide solutions of Ln-1 formed single crystals. X-ray diffraction confirmed the same unit cell parameters for each of the series of Ln-1. Detailed analysis was performed on Yb-1 that is comprised of four Yb 3+ , twelve Zn 2+ , ten quinHA 2-, four Hph -, two ph 2-, eight OH -, and four H 2 O. This structure contains two [14-MC-5] motifs that are spanned by a central Yb 2 Zn 2 (quinHA) 2 moiety (Figures S1 andS2, Supporting Information). The solid-state packing of Yb-1 molecules is stabilized by through-solvent intermolecular interactions (including hydrogen bonding and van der Waals interactions) and - stacking of adjacent quinHA 2-located on neighboring molecules.

Each metal ion was analyzed for its coordination geometry. In particular, coordination number (CN), shape, crystal shape measure parameter (CShM)), average bond length (d av ), and bondvalence sums (BVS) were determined (Table 1). Representations generated from the crystallographic data of each ligand field may be observed in Table 1 and Figure S3 (Supporting Information).

Two unique Yb 3+ cation sites can be distinguished, yet analysis with SHAPE v2.1 software on both Yb1 and Yb2 confirm eight-coordinate trigonal dodecahedral environments comprised of eight oxygens. [START_REF] Llunell | SHAPE version 2.1[END_REF] Bond-valence sums confirmed the +3 oxidation state for each of the two Yb 3+ cations. Yb1 is a part of the spanning moiety while Yb2 is a part of the [14-MC-5] motif. There are six Zn 2+ in the asymmetric unit, and all but Zn2 adopt a coordination number of five. Zn2 is in a six-coordinate octahedral coordination environment comprised of oxygen atoms. Zn1 and Zn3-Zn6 are all in five-coordinate coordination environments comprised of three oxygen and two nitrogen donor atoms. Analysis with SHAPE v2.1 suggests that the best coordination geometry to describe Zn1 and Zn4-Zn6 is square pyramidal while Zn3 is best described as a trigonal bipyramidal geometry. [START_REF] Llunell | SHAPE version 2.1[END_REF] Bond-valence sums confirm the +2 oxidation state for each Zn 2+ cation.

Zn1 is the only Zn 2+ cation that is a part of the spanning moiety, whereas the other five Zn 2+ cations are a part of the [14-MC-5] motif. ] "encapsulated sandwich" compounds. [START_REF] Trivedi | Highly Emitting Near-Infrared Lanthanide "Encapsulated Sandwich" Metallacrown Complexes with Excitation Shifted toward Lower Energy[END_REF][START_REF] Alexiou | From Monomer Zinc-Oxamato Complexes to Tetranuclear Inverse 12-Membered and Octanuclear 12-Membered Metallacrowns[END_REF][START_REF] Alexiou | Di-2-pyridyl Ketone Oxime in Zinc Chemistry: Inverse 12-Metallacrown-4 Complexes and Cationic Pentanuclear Clusters[END_REF][START_REF] Ostrowska | Complex Formation of Copper(II), Nickel(II) and Zinc(II) with Ethylophosphonoacetohydroxamic Acid: Solution Speciation, Synthesis and Structural Characterization[END_REF][START_REF] Marchiò | The Peculiar Behavior of Picha in the Formation of Metallacrown Complexes with Cu(II), Ni(II) and Zn(II) in Aqueous Solution[END_REF][START_REF] Jankolovits | Insight into the Structural Versatility of the Ln(III)[15-Metallacrown-5] Platform by Comparing Analogs with Ni(II), Cu(II), and Zn(II) Ring Ions[END_REF][START_REF] Jankolovits | Solvent Dependent Assembly of Lanthanide Metallacrowns Using Building Blocks with Incompatible Symmetry Preferences[END_REF][START_REF] Jankolovits | Assembly of Zinc Metallacrowns with an α-Amino Hydroxamic Acid Ligand[END_REF][45] The structure reported here stands out among these examples since examples of [14-MC-5]s are rare and Ln-1 series is only the second example reported. The first example of a [14-MC-5] was comprised of Mn 3+ and salicylhydroximate ligands that encapsulate a single Ln 3+ in their cavity, plus another Ln 3+ as a part of the MC ring. [START_REF] Boron | A Mixed 3d-4f 14-Metallacrown-5 Complex That Displays Slow Magnetic Relaxation through Geometric Control of Magnetoanisotropy[END_REF] Even though both compounds are [14-MC-5]s, there are significant differences in their structural compositions (Figure 2). Both [14-MC-5] compounds have a M-O-M "stutter" using a  3 -OH -or  3 -O 2-rather than a hydroximate group that induces the removal of a nitrogen atom from the standard M-N-O-M motif. Another difference that can be observed between the two structures is the presence of a mirror plane that bisects the molecule in Mn 3+ /Ln 3+ [14-MC-5] MC. Conversely, Yb-1 does not possess a mirror plane and maintains the M-N-O sequence for each of the four subunits. In addition, the Mn 3+ /Ln 3+ MC has a Ln 3+ in the [14-MC-5] MC ring, while that in the Ln-1 consists only of Zn 2+ ring metal ions. Lastly, the [14-MC-5] ring in Mn 3+ is planar since each Mn 3+ is located in an octahedral coordination environment with the 5,6-fused chelate motif of the salicylhydroximate ligand. In the case of Ln-1, both the square pyramidal Zn 2+ centers and the 5,5-fused chelate motif requires some bowling to complete the cycle with reasonable bond lengths. Compared to the H 2 quinHA and H 2 ph that possess absorption bands in the UV range up to 350 and 300 nm, respectively, absorption spectra of Ln-1 are significantly red-shifted and extend up to 475 nm. The broad lower energy absorption band with a maximum at 380 nm (ε 380 = 2.7 × 10 4 M -1 cm -1 ) could be assigned to intra-and/or inter-ligand charge transfer (ILCT) transitions considering that Zn 2+ usually does not participate in metal-to-ligand or ligand-to-metal charge transfer transitions and that the absorption spectra of Ln-1 are independent of Ln 3+ identity. Similar ILCT absorption bands were observed for Zn 2+ /Ln 3+ MCs with an 'encapsulated sandwich' structure assembled using quinHA 2-ligands. [START_REF] Trivedi | Highly Emitting Near-Infrared Lanthanide "Encapsulated Sandwich" Metallacrown Complexes with Excitation Shifted toward Lower Energy[END_REF] Diffuse reflectance spectra of Ln-1 formed with the different Ln 3+ are similar to each other and exhibit broad bands in the UV and visible range up to 550 nm (Figure S5, Supporting Information).

Luminescent properties of Ln-1 were analyzed in the solid state. Energy positions of the ligandcentered levels, in particular triplet states, employ Gd 3+ since the emissive state (32 200 cm -1 ) [START_REF] Carnall | Electronic Energy Levels of the Trivalent Lanthanide Aquo Ions. II. Gd 3+[END_REF] is too high in energy for many organic sensitizers. However, no phosphorescence could be detected upon excitation of Gd-1 in the range of 280-340 nm at 77 K in time resolved mode. On the other hand, Gd-1 exhibited broad-band emission in the range 450-750 nm arising from the ILCT in steady-state mode (Figure S6, Supporting Information). The energy position of the ILCT state was determined to be 21 410 cm -1 (467 nm) from the onset of the emission band of Gd-1, a value similar to the Zn 2+ /Ln 3+ 'encapsulated sandwich' MCs (21 560 cm -1 ). [START_REF] Trivedi | Highly Emitting Near-Infrared Lanthanide "Encapsulated Sandwich" Metallacrown Complexes with Excitation Shifted toward Lower Energy[END_REF] Upon excitation into ILCT band at 400 nm, Tb-1 exhibited only a broad-band emission in the range 450-750 nm (Figure S6, Supporting Information). The latter could be explained by a high probability of back energy transfer because of the energy match between the ILCT (21 410 cm -1 ) and the Tb 3+ 5 D 4 emitting level (20 500 cm -1 ) in Tb-1. On the other hand, Yb-1 upon excitation at 400 nm displays Yb 3+centered luminescence in the near-infrared range due to the 2 F 5/2 → 2 F 7/2 transition (Figure 3).

Excitation spectrum of Yb-1 upon monitoring emission at 980 nm resembles the diffuse reflectance one (Figure 3, solid vs. dashed black traces) and exhibits broad bands in the UV and visible ranges up to 550 nm pointing towards a sensitization of Yb 3+ through the MC scaffold. [START_REF] Uh | Novel Antennae for the Sensitization of near Infrared Luminescent Lanthanide Cations[END_REF] It should be noted that Yb 3+ near-infrared emission in Yb-1 could be observed despite the presence of water molecules in the first coordination sphere of Yb 3+ that often quench near-IR emitting Ln 3+ . [START_REF] Bünzli | Lanthanide Luminescence: Photophysical, Analytical and Biological Aspects[END_REF] The Yb 3+ -centered absolute quantum yield ( ) of Yb-1 upon excitation at 400 nm is 0.051( were recorded with  em = 980 nm. Emission spectra (red traces) were acquired with  ex = 400 nm.

The diffuse reflectance spectrum of Yb-1 is superimposed for comparison (dashed black trace).

The new Ln-1 MCs we report here represent a significant addition to the library of metallacrown structures not only as an example of a 14-MC-5 system, but also for the ability to sensitize near-IR emission from Yb 3+ . The compound is a fused bis[14-MC-5] that contains both quinHA 2-and ph 2-. Absorption spectra of Ln-1 display broad bands at lower energy due to intra-/inter-ligand

Figure 1 .

 1 Figure 1. Representation of the molecular structure of Yb-1 from single crystal diffraction data (a),

Figure 2 .

 2 Figure 2. Molecular representation of [14-MC-5] motifs established from crystallographic data for

Figure 3 . 1 .

 31 Figure 3. Normalized steady-state luminescence spectra of Yb-1. Excitation spectra (black traces)

Table 1 .

 1 Geometric information for metal cations in Yb

-1. Metal d av CN Crystallographic Representation Shape CShM a BVS 35,36 Addison Tau 37

	Zn4	2.052 5 [2 N quinHA ,	Square	0.59023 2.022	0.110
		2 O quinHA , 1	Pyramid
		O ph ]
	Zn5	2.035 5 [2 N quinHA ,	Square	0.88679 2.113	0.169
		2 O quinHA , 1	Pyramid
		O ph ]
	Zn6	2.048 5 [[2 N quinHA ,	Square	1.28931 2.038	0.103
		2 O quinHA , 1	Pyramid
		O OH ]]
			Trigonal	0.60167 2.950	-----
			Dodeca-
			hedron
	Zn1	2.066 5 [2 N quinHA ,	Square	2.06806 1.988	0.446
		2 O ph , 1 O OH ]	Pyramid
	Zn2	2.098 6 [1 O quinHA ,	Octa-	1.17399 2.050	-----
		3 O ph , 1 O OH ]	hedron
	Zn3	2.074 5 [2 N quinHA ,	Trigonal	2.20789 1.995	0.583
		1 O ph , 2 O OH ]	Bipyramid

Yb1 2.321 8 [5 O quinHA , 1 O ph , 1 O OH , 1 O water ] Trigonal Dodecahedron 1.01186 3.036 -----Yb2 2.337 8 [4 O quinHA , 2 O ph , 1 O OH , 1 O water ] a Smallest CShM out of all ideal geometries from SHAPE v2.1 analysis. 34 There are other examples of MCs formed with Zn 2+ , including [12-MC-4]s, [15-MC-5]s, and a bis[12-MC-4][12-MC-8

  ,[START_REF] Trivedi | Highly Emitting Near-Infrared Lanthanide "Encapsulated Sandwich" Metallacrown Complexes with Excitation Shifted toward Lower Energy[END_REF] due to the presence of water molecules directly bound to the lanthanide cation as well as self-quenching induced by proximal Yb 3+ .

	0.0 0.2 0.8 1.0 ms)300 0.4 0.6 Intensity (a.u.)	400	500	600 900	1000 1100
			Wavelength (nm)
					𝑄 𝐿 𝑌𝑏	1)%.
	Luminescence decays of Yb-1 were best fitted by a monoexponential function (Figure S7) where
	the luminescence lifetime is 714(8) nanoseconds. Both values of	𝑄 𝐿 𝑌𝑏	and τ obs of Yb-1 are smaller
	than the ones reported for the Zn 2+ /Yb 3+ 'encapsulated sandwich' MC (	𝑄 𝐿 𝑌𝑏	= 2.44(4)%;  = 47.8(4)

Funding Sources

Funding was provided by Wayne State University, the National Institutes of Health, La Ligue Contre le Cancer, Molécules Marines Métabolisme et Cancer from the Cancéropôle Grand Ouest, and the Institut National de la Santé et de la Recherche Médicale (INSERM). Notes The authors have no conflicts of interest to declare. ACKNOWLEDGMENT JCL acknowledges support from Wayne State University, MJA acknowledges support from the National Institutes of Health (EB027103). The work in France was supported by La Ligue Contre le Cancer (Comité du Loiret et d'Eure-et-Loir) and the network Molécules Marines, Métabolisme et Cancer from the Cancéropôle Grand Ouest. S.P. acknowledges support from the Institut National de la Santé et de la Recherche Médicale (INSERM).

charge transfer transitions. Yb-1 in the solid-state displays characteristic Yb 3+ emission in the near-IR range despite the presence of water molecules directly bound to the metal ion. Given the novelty of the structure and composition as well as the potential application of near-IR luminescence, Ln-1 is a notable MC compound.

ASSOCIATED CONTENT

The following files are available free of charge. Electronic Supplementary Information (ESI) available containing experimental details, additional spectroscopic information, and crystallographic parameters in word document format.

Single crystal diffraction data are available for Yb-1 in CIF format free of charge from www.ccdc.cam.ac.uk using deposition number 2125941.

AUTHOR INFORMATION

Corresponding Authors

* Svetlana V. Eliseeva (email: svetlana.eliseeva@cnrs.fr), Stéphane Petoud (email: stephane.petoud@inserm.fr) and Jacob C. Lutter (email: jclutter@umich.edu)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

SUPPORTING INFORMATION

The Supporting Information available:

Experimental details, additional spectroscopic information and crystallographic parameters (in PDF format).

Single crystal diffraction data are available for Yb-1 in CIF format free of charge from www.ccdc.cam.ac.uk using deposition number 2125941. ABBREVIATIONS MC, metallacrown; Ln 3+ , lanthanide; H 2 quinHA, quinaldic hydroxamic acid; H 2 ph, phthalic acid.

Table of Contents Synopsis

A fused set of rarely observed 14-metallacrown-5 moieties is described using single-crystal X-ray diffraction data, and near-infrared emission from Yb 3+ is observed in the solid state.


Svetlana V Elisseva
email: svetlana.eliseeva@cnrs-orleans.fr

Jordan R Travis

Sarah G Nagy

Alyssa M Smihosky

Collin M Foley

Abigail C Kauffman

Curtis M Zaleski
email: cmzaleski@ship.edu

Stéphane Petoud
email: stephane.petoud@inserm.fr

Visible and Near-infrared Emitting Heterotrimetallic Lanthanide-Aluminum-Sodium 12-Metallacrown-4 Compounds: Discrete Monomers and Dimers

The luminescent properties of two types of heterotrimetallic aluminum-lanthanide-sodium 12-metallacrown-4 compounds are presented here, LnNa(ben)4[12-MCAl(III)N(shi)-4] (LnAl4Na) and {LnNa[12-MCAl(III)N(shi)-4]}2(iph)4 (Ln2Al8Na2), where Ln = Gd III , Tb III , Er III , and Yb III , MC is metallacrown, ben -is benzoate, shi 3-is salicylhydroximate, and iph 2-is isophthalate. The aluminum-lanthanide-sodium metallacrowns formed with benzoate are discrete molecules while, upon replacement of the benzoate by the dicarboxylate isophthalate, two individual metallacrowns can be joined to form a dimer. In the solid state, the terbium complex of each structure type displays emission in the visible region, and the erbium and ytterbium complexes emit in the near-infrared. The luminescence lifetimes (τobs) and quantum yields have been collected under ligand excitation (𝑄 Ln L ) for both LnAl4Na monomers and Ln2Al8Na2 dimers. Several of these values tend to be shorter (luminescence lifetimes) and smaller (quantum yields) than the corresponding values recorded for the structurally similar gallium-lanthanide monomer and dimer 12-MC-4 molecules. However, the quantum yield value recorded for the visible emitting Tb2Al8Na2 dimer, 43.9%, is the highest value observed in the solid state to date for a Tb III based metallacrown.

Introduction

Metallacrowns (MCs), the functional and structural inorganic analogues to crown ethers, provide a pathway to self-assemble heterometallic macrocyclic molecules in a controllable and predictable manner. 1 Archetypal metallacrowns contain a metalnitrogen-oxygen repeat unit in a cyclic fashion akin to the carboncarbon-oxygen repeat unit of a crown ether. In addition, like crown ethers, metallacrowns contain a central cavity that can bind a metal ion via several oxygen atoms. While the same type of metal ion can be used for both the ring and the central cavity positions, it is commonplace for the ring metal ions and central cavity metal ion to be different. The choice of metal ions in these positions can dictate the properties of the metallacrown; thus, the molecules can be tailored to address the needs of a specific application. For example, the use of lanthanide ions in combination with transition metal ions can lead to the formation of 3d-4f heterometallic MCs with singlemolecule magnet properties. 2,3 In addition, the self-assembly process driving the formation of MCs allows the molecules to be functionalized while retaining structural similarities between related molecules. In a series of manganese-lanthanide 12-MC-4 complexes LnNaX4 [12-MCMn(III)N(shi)-4], where X is a carboxylate anion and shi 3-is salicylhydroximate, the structure of the MC can easily be conserved as the identity of the carboxylate is varied. 3,4 A large variety of carboxylate anions have been used including acetate, benzoate, trimethylacetate, hydroxybenzoates (2, 3, and 4 positions), and halogenated benzoates (chloro, bromo, and iodo in the 2, 3, or 4 positions). 3,4 Independent of the identity of the carboxylate anion, the MC has an overall square shape with a Mn-N-O repeat unit about the MC ring and lanthanide and sodium ions are captured on opposite sides of the central cavity. Thus, the properties of these molecules can be finely tuned by the judicious choice of the nature of the carboxylate anions. Indeed, when the magnetic properties of the DyNaX4[12-MCMn(III)N(shi)-4] complexes synthesized with acetate, benzoate, trimethylacetate, and salicylate were investigated, only the salicylate version behaved as a singlemolecule magnet. 3 Not only can the ligands of the metallacrowns be varied with structural integrity being maintained, but the ring metal ions can also be varied while retaining a structural similarity. If the ring manganese(III) ions of the LnNaX4[12-MCMn(III)N(shi)-4] structures are replaced with gallium(III), the analogous MCs can be produced. 5 One advantage of the gallium metal ion in MCs is that the luminescent properties of the central lanthanide ions can be exploited. It is well established that lanthanide ions can be highly luminescent with long-lived excited electronic states, have large energy differences between excitation and emission wavelengths (pseudo-Stoke shifts), and can emit in the UV, visible, and NIR regions. 6 For example, lanthanide clusters are known to be particularly luminescent materials with potential applications such as biological imaging and sensing. 7 Gallium(III) with its closed-shell electronic structure does not quench the excited states of lanthanide ions in MCs as would the manganese(III) ions of the [12-MCshi-4] molecules. Indeed, we have recently reported the luminescent properties of gallium-lanthanide [12-MCshi-4] molecules. 5,[8][9][10][11][12] A series of (Hpy)Ln(benzoate)4[12-MCGa(III)N(shi)-4] compounds, where Hpy + is pyridinium, are highly luminescent in both the visible and NIR. 5 In addition, the Sm III version can act as a sensor for G-quadruplex DNA 8 , and the Dy III analogue can emit white light. 9 These molecules are structurally similar to the DyNa(benzoate)4[12-MCMn(III)N(shi)-4] discussed above except with the use of the uncoordinated countercation pyridinium instead of a coordinated sodium ion. If the benzoate anions are replaced with the dicarboxylate isophthalate, two Ln [12-MCshi-4] units can be linked together to form a dimer, {Ln[12-MCGa(III)N(shi)-4]}2(iph)4. 10 These molecules are also highly luminescent and can meet the requirements of applications such as luminescent molecular nanothermometers. 10,11 In addition, if iodinated versions of isophthalate or salicylhydroximate are incorporated into the dimer, the molecules can be used as X-ray computed tomography (XCT) imaging agents. 12 Some of the dimers contain uncoordinated ammonium countercations while others have coordinated sodium ions. The sodium ions may bind to the central cavity of the 12-MC-4 moiety as in the monomer LnNaX4[12-MCMn(III)N(shi)-4] molecules but, in some instances, the sodium ions bind to the side of the dimer structures. The dimer motif can also accommodate Mn III ions in ring positions as we and others have also reported analogous structures with sodium as the counteraction. 2m,13 For the {LnNa[12-MCMn(III)N(shi)-4]}2(iph)4 dimers, the sodium cation has only been reported binding to the central cavity of the 12-MC-4 moiety.

Expanding on the gallium-lanthanide work, we decided to investigate aluminum-based metallacrowns. Aluminum has not generally been incorporated into the ring structure of metallacrowns as the research has mainly focused on d-block metals as opposed to p-block metals. Besides gallium, tin 14 and tellurium 15 are the only p-block metals that have been included in the archetype MC ring structure (N-O repeat unit). We questioned if the smaller ionic radius of Al III (calculated according to Shannon 16 as 0.535 Å for CN = 6) versus Mn III (0.645 Å; CN = 6 and high spin) and Ga III (0.620 Å; CN = 6) would induce the shrinking of the size of the MC cavity and prohibit the binding of the Ln III ion. However, the synthesis of aluminum-lanthanide-sodium metallacrowns proved to be facile chemistry, and we recently reported the structures of the analogous monomer LnNa(ben)4[12-MCAl(III)N(shi)-4] molecules and dimer {LnNa [12-MCAl(III)N(shi)-4]}2(iph)4 molecules, both with coordinated sodium ions. 17 A structural comparison of the aluminum and manganese Ln [12-MCshi-4] molecules reveals that there are minimal changes to the [12-MCshi-4] central cavity distances and that the nature of the ring metal ion does not significantly alter the MC framework. 17 Thus, these aluminumbased metallacrowns are structurally similar to the manganese and gallium versions discussed above. Herein we report the luminescent properties of the two types Ln[12-MCAl(III)N(shi)-4] compounds: LnNa(ben)4[12-MCAl(III)N(shi)-4] (LnAl4Na) and {LnNa[12-MCAl(III)N(shi)-4]}2(iph)4 (Ln2Al8Na2), where Ln = Gd III , Tb III , Er III , and Yb III .

Experimental

Synthesis of Compounds

The LnNa(ben)4[12-MCAl(III)N(shi)-4] and {LnNa[12-MCAl(III)N(shi)-4]}2(iph)4 compounds were prepared as previously reported. 17

Elemental Analyses

Analyses for TbAl4Na and YbAl4Na were performed by Atlantic Microlabs Inc (Norcross, Geogia, USA), and analyses for GdAl4Na, ErAl4Na, Gd2Al8Na2, Tb2Al8Na2, Er2Al8Na2, and Yb2Al8Na2 were performed at the Université de Rennes (Rennes, France).

Luminescence Measurements

Luminescence data were collected on solid state samples placed in 2.4 mm i.d. quartz capillaries (Fig. S1). Steady-state emission and excitation spectra were measured on a custom-designed Horiba Scientific Fluorolog 3 spectrofluorimeter equipped with either a visible photomultiplier tube (PMT) (220-850 nm, R928P; Hamamatsu) or a NIR PMT (950-1650 nm, H10330-75; Hamamatsu) upon excitation with a continuous Xenon lamp. All excitation and emission spectra were corrected for the instrumental functions. Luminescence lifetimes (τobs) were determined under excitation at 355 nm provided by a Nd:YAG laser (YG 980; Quantel). Signals were detected in the visible or NIR ranges using an iHR320 monochromator (Horiba Scientific) equipped with a Hamamatsu Please do not adjust margins Please do not adjust margins R928P and H10330-75 PMTs. The output signal from the detector was fed into a 500 MHz bandpass digital oscilloscope (TDS 754C; Tektronix), transferred to a PC for data processing with the program Origin 8 ® . Luminescence lifetimes are averages of at least three independent measurements. Ln III -centered quantum yields under ligands excitation (𝑄 Ln L ) at 330 nm were determined with the Fluorolog 3 spectrofluorimeter based on an absolute method with the use of an integration sphere (Model G8, GMP SA, Renens, Switzerland). Each sample was measured several times under comparable experimental conditions, varying the position of samples. Estimated experimental error for quantum yield determination is 10 %. Phosphorescence spectra of GdAl4Na and Gd2Al8Na2 in the solid state at 77 K were acquired on a Horiba-Jobin-Yvon Fluorolog 3 spectrofluorimeter in a time-resolved mode. Based on the Gaussian decomposition of the phosphorescence spectra the energies of the 0-0, 0-1, and 0-2 vibrational transitions were determined.

Results and discussion

Structural Description of the Metallacrowns

The single crystal X-ray structures of the LnNa(ben)4[12-MCAl(III)N(shi)-4] (LnAl4Na) and {LnNa[12-MCAl(III)N(shi)-4]}2(iph)4 (Ln2Al8Na2) compounds (Fig. 123), where Ln = Gd III , Tb III , Er III , and Yb III , have been previously reported. 17 Therefore, only an overview of the structures will be provided here. For both the LnAl4Na and Ln2Al8Na2 molecules, their respective structures are built upon a [12-MCAl(III)N(shi)-4] framework where four ring Al III ions and four N-O repeat units generate the metallacrown connectivity. The ligand salicylhydroximate (shi 3-) provides the N-O linkages and places the Al III ions at an angle of 90 o relative to each other via five-and sixmembered fused chelate rings. This organization generates an overall square metallacrown framework that is slightly domed away from the central Ln III ion. In both the LnAl4Na and Ln2Al8Na2 compounds, a Ln III ion is coordinated in the central MC cavity by the oxime oxygen atoms. In the LnAl4Na compounds, the Ln III ion is further tethered to the MC framework by four benzoate (ben -) anions that bridge via the carboxylate groups between the Ln III ion and the four ring Al III ions. If the benzoate anion is replaced by the dicarboxylate anion isophthalate (iph 2-), then two Ln[12-MCAl(III)N(shi)-4] units can be connected together to form a dimer of MCs: Ln2Al8Na2. The binding of the isophthalate is analogous to that of the benzoate as the dicarboxylate bridges between the central Ln III ion and the four Al III ions on each end of the dimer. For the LnAl4Na compounds, a sodium ion is also captured in the central cavity opposite of the Ln III ion. The coordination sphere of the sodium ion is completed by water molecules. For the Ln2Al8Na2 compounds, the sodium ions have two coordination modes. For Tb2Al8Na2 and Yb2Al8Na2, the sodium ions bind to the central cavity opposite of the Ln III ions as observed in the LnAl4Na compounds. However, for Er2Al8Na2, the sodium ions bind on the side of the dimer by coordinating to phenolate and carboxylate oxygen atoms belonging to two different shi 3-ligands and the carboxylate oxygen atom of an iph 2-ligand. In either of these binding positions, the coordination sphere of the sodium ions is completed by solvent molecules: water and/or DMF molecules. At this time, we have not rationalized the factors that govern the location where the sodium ions bind to the dimer MCs.

The Ln III ions in both LnAl4Na and Ln2Al8Na2 are eightcoordinate upon binding to four oxime oxygen atoms of the MC cavity and four carboxylate oxygen atoms belonging to either benzoate or iph 2-. A continuous shape measures (CShM) analysis (SHAPE 2.1 18 ) indicates that the coordination geometry around the lanthanide ion is best described as a square antiprism (D4d). 17 For both sets of compounds, the Al III ions are six-coordinate by binding two trans shi 3-ligands in the equatorial plane and the axial coordination is completed by a carboxylate oxygen atom of a benzoate or a iph 2-ligand and an oxygen atom of either a water or DMF molecule. The CShM analysis indicates an octahedral (Oh) geometry around the aluminum ions. 17 The sodium ions of LnAl4Na, Tb2Al8Na2, and Yb2Al8Na2 are nine-coordinate with a distorted spherical capped square antiprism geometry (C4v). 17 The coordination sphere is composed of four oxime oxygen atoms from the MC cavity, four water molecules, and one carbonyl oxygen atom from a disordered DMF molecule. For Er2Al8Na2, the sodium ions are six-coordinate, and the CShM analysis reveals that the geometry is best described as a severely distorted octahedron. 17 This journal is © The Royal Society of Chemistry 20xx

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Ln III -centered photophysical properties

Upon excitation into ligand-centered levels at 330 nm, TbAl4Na and Tb2Al8Na2 exhibit the characteristic green emission signals of terbium due to 5 D4→ 7 FJ (J = 6-0) transitions (Figs. 4 andS2), while the corresponding ytterbium (YbAl4Na and Yb2Al8Na2) and erbium (ErAl4Na and Er2Al8Na2) MCs show bands in the near-infrared (NIR) range arising from the 2 F5/2→ 2 F7/2 (940-1070 nm; Fig. 5) and from the 4 I13/2→ 4 I15/2 (1460-1620 nm; Fig. 6) transitions, respectively. Emission spectra of terbium in TbAl4Na and Tb2Al8Na2 are dominated by the 5 D4→ 7 F5 transition centered at 545 nm. Relative integral intensities of 5 D4→ 7 FJ/ 5 D4→ 7 F5 transitions (Table 1) are very similar for the monomer MC and the dimer MC. For the investigated monomer and dimer MCs, the Ln III crystal-field splitting of the f-f transitions remains the same which is consistent with the similarities of coordination environments and symmetries around Please do not adjust margins Please do not adjust margins the Ln III ion in both types of studied complexes. Excitation spectra of the MCs upon monitoring the emission at 545 nm for Tb III , 980 nm for Yb III , and 1525 nm for Er III are dominated by broad bands in the UV range up to 360 nm (Fig. 456) indicating an efficient sensitization of these Ln III ions through the energy levels of the chromophoric part of the MC scaffold. In addition, in the excitation spectrum of the erbium MCs, sharp bands assigned to f-f transitions throughout the visible range are observed, reflecting the possibility of direct excitation of the Er III ion(s). In the case of the terbium MCs, sharp features were detected at 360-380 nm which are, however, of very weak intensity compared to the broad ligand-centered bands. To quantitatively characterize the luminescence properties of the LnAl4Na and Ln2Al8Na2 MCs, absolute quantum yields were collected under ligands excitation (𝑄 Ln L ) on samples in the solid state. Observed luminescence lifetimes (τobs) were acquired upon ligand excitation, monitoring the signals of the lanthanide cations (Table 2). For the ytterbium and erbium MCs, the values of 𝑄 Ln L and τobs are essentially the same within experimental errors for the monomer molecules and for the dimers. For the terbium MCs, the luminescence lifetimes remain the same for both structure types. Remarkably, the absolute quantum yield 𝑄 Tb L increases by a factor 1.8 times from 25(1) % for TbAl4Na to 43.9(7) % for Tb2Al8Na2. It is important to note that, to date, the quantum yield for the terbriumaluminum dimer is thus far the highest value recorded for a Tb III MC in the solid state. The quantum yield values measured for the structurally similar monomer TbGa4 and dimer Tb2Ga8 metallacrowns are 34.7(1) and 31.2(2)%, respectively. 5,10 In addition, a [TbGa8(shi)8(OH)4]Na double-decker 12-MC-4 molecule, possessing four bridging --OH groups between the metallacrown moieties instead of a iph 2 -group has only a quantum yield of 35(2)% in the solid state. [START_REF] Salerno | [END_REF] The non-planar metallacrown Tb(H2shi)2(NO3)[12-MCGa(III)N(shi)-4](pyridine)4 has a quantum yield value of only 11.3(5)% in the solid state. 20 Lastly, the [3.3.1] metallacryptate TbGa6(H2shi)(Hshi)(shi)7(pyridine) has a quantum yield of merely 0.189(3)% in the solid state. 21Table 1. Integrated intensity ratios (I4-J) of the 5 D4→ 7 FJ transitions normalized with respect to the 5 D4→ 7 F5 transition for TbAl4Na and Tb2Al8 in the solid state for emission spectra collected at room temperature.

Determination of the Triplet State

In the spirit of rationalizing the energy transfer, the lowest triplet state (T1) of the organic ligands present in the MCs was analyzed. This electronic state is generally considered to play a major role in the sensitization of Ln III emission. 22,[START_REF] Bünzli | Comprehensive Inorganic Chemistry II[END_REF] It is therefore important to determine the energy of the T1 state. Phosphorescence spectra of the corresponding Gd III complexes are usually used for this purpose. The very high energy of the accepting energy level (32 000 cm -1 ) [START_REF] Carnall | [END_REF] of Gd III prevents its population through the donating energy levels of most commonly used organic chromophores. In addition, the heavyatom effect and the paramagnetic nature of Gd III facilitate intersystem crossing 25 , thus promoting the emission intensity from the triplet state. Phosphorescence spectra were acquired in time-resolved mode for GdAl4Na and Gd2Al8Na2 in the solid state upon excitation at 330 nm (Fig. 7) in order to remove the short-live contribution of the fluorescence emission arising from the singlet state. These spectra revealed broad bands in the range of 420-750 nm with an additional resolved vibronic structure for the 0-0, 0-1, and 0-2 transitions (Table 3). The lowest energy transition (0-0) is a good approximation of the T1 states of the MCs, and is located at 22 690 cm -1 for GdAl4Na and at 21 360 cm -1 for Gd2Al8Na2. These values are similar to the ones observed for the corresponding lanthanide-gallium MC analogues, i.e. 22 170 cm -1 for (Hpy)Gd(ben)4[12-MCGa(III)N(shi)-4] 5 , where Hpy + is pyridinium, 21 980 cm -1 for (NH4)2{Gd[12-MCGa(III)N(shi)-4]}2(iph)4 9 , and 22 385 cm -1 for {GdNa[12-MCGa(III)N(shi)-4]}2(iph)4. 12 Table 3. Energies of 0-0, 0-1 and 0-2 phonon transitions observed in the phosphorescence spectra of GdAl4Na and Gd2Al8Na2 collected in the solid state at 77 K under ligand excitation. While not exact structural analogues, a series of galliumlanthanide MCs has been reported by us. Gallium-lanthanide MC structures have been reported for both the discrete monomers and for the dimers. The structures are nearly isostructural to the aluminium-lanthanide MCs except for the nature of the countercation. In the LnAl4Na and Ln2Al8Na2 structures, sodium ions are coordinated to the MCs to provide the charge balance. For the analogous gallium-lanthanide MCs, the uncoordinated countercation is either pyridinium for the (Hpy)Ln(ben)4[12-MCGa(III)N(shi)-4] monomers 5 (LnGa4) or ammonium for the dimer (NH4)2{Ln[12-MCGa(III)N(shi)-4]}2(iph)4 molecules 10 (Ln2Ga8). One exception exists for the ytterbiumgallium dimer. In addition to the ammonium Yb2Ga8 dimer, the structure and solid-state luminescence data of the corresponding sodium version, {YbNa[12-MCGa(III)N(shi)-4]}2(iph)4

Gd III MC E (cm -1 ) 0-0 0-1 0-2 Δ0-1 Δ1-2
Please do not adjust margins Please do not adjust margins (Yb2Ga8Na2) has also been reported. 12 The comparison of the luminescence lifetimes and absolute quantum yields collected under ligand excitation (Table 2) reveals that for the erbium and ytterbium MCs, the gallium-based MCs tend to have longer lifetimes and higher quantum yield values than the analogous aluminum-based MCs. For the Er2Al8Na2 dimer, the quantum yield value is higher than the analogous Er2Ga8 dimer; however, the Er2Al8Na2 dimer has a shorter lifetime. This implies that the non-radiative deactivation pathways are likely more significant for the Er2Al8Na2 dimer, but the sensitization efficiency of the erbium ions is possibly better. For the terbium MCs, there is not an absolute trend.

TbAl4Na and TbGa4 have similar luminescence lifetimes, while TbGa4 has a higher quantum yield. Tb2Ga8 has a longer luminescence lifetime than Tb2Al8Na2, though Tb2Al8Na2 has a significantly higher value for the quantum yield by around 40% (Table 2). At this time, it is difficult to establish a direct comparison of the solid-state luminescence data between the different types of terbiumbased structures and to draw definitive conclusions. Part of the explanation lies in the amount of co-crystallized solvent molecules, in particular water, as the solvent is a strong source of non-radiative deactivation of the excited states of lanthanide cations. Moreover, the binding of the sodium ions to the Tb2Al8Na2 metallacrown may be a factor influencing the luminescent properties of this MC as the Tb2Ga8 metallacrown contains two uncoordinated ammonium countercations. More detailed investigations need to be performed to elucidate the exact mechanism underlying the behavior of the terbium-based metallacrowns. Lastly, the LnAl4Na and Ln2Al8Na2 compounds are unfortunately not readily soluble in common solvents, thus preventing any measurements in solution.

Conclusions

Two classes of 12-MC-4 compounds, LnNa(ben)4[12-MCAl(III)N(shi)-4] and {LnNa[12-MCAl(III)N(shi)-4]}2(iph)4, where Ln = Gd III , Tb III , Er III , and Yb III , were investigated for their luminescent properties in the visible and in the near-infrared. In all cases, we observed that the emissions of the studied lanthanide ions were sensitized through the electronic structures of this novel family of metallacrowns. For the near-infrared emitting Er III and Yb III ions, both the monomer and dimer versions of each metal type have fairly similar luminescence lifetimes and quantum yield values. The terbium-based MCs displayed emission in the visible region with the dimer version having a quantum yield 1.8 times larger than the one of the discrete monomer. The quantum yield of the terbium dimer MC has a remarkable value of 43.9%, which is as of date the highest value measured for a terbium metallacrown in the solid state. These results demonstrate that light-weight p-block metals can indeed be incorporated into the MC framework and are suitable for the generation of luminescent lanthanide compounds with promising properties.

  GdNa(ben)4[12-MCAl(III)N(shi)-4](H2O)4•3DMF•1.5H2O, GdAl4Na. Elemental analysis for C65H68Al4GdN7NaO28.5 (FW = 1691.43 g/mol): Found (%) H = 3.74, C = 45.79, N = 6.26; Calculated (%) H = 4.05, C = 46.16, N = 5.80. TbNa(ben)4[12-MCAl(III)N(shi)-4](H2O)4•6.5DMF, TbAl4Na. Elemental analysis for C75.5H89.5Al4N10.5NaO30.5Tb (FW = 1921.91 g/mol): Found (%) H = 4.76, C = 47.03, N = 7.55; Calculated (%) H = 4.69, C = 47.18, N = 7.65. ErNa(ben)4[12-MCAl(III)N(shi)-4](H2O)4•4DMF•1.5H2O, ErAl4Na. Elemental analysis for C68H75Al4ErN8NaO29.5 (FW = 1774.54 g/mol): Found (%) H = 4.00, C = 45.74, N = 6.70; Calculated (%) H = 4.26, C = 46.02, N = 6.31. YbNa(ben)4[12-MCAl(III)N(shi)-4](H2O)4•4.5DMF, YbAl4Na. Elemental analysis for C69.5H75.5Al4N8.5NaO28.5Yb (FW = 1789.84 g/mol): Found (%) H = 4.40, C = 46.54, N = 6.75; Calculated (%) H = 4.25, C = 46.64, N = 6.65. {GdNa[12-MCAl(III)N(shi)-4]}2(iph)4(DMF)2(H2O)8•6.5DMF•H2O, Gd2Al8Na2. Elemental analysis for C113.5H125.5Al8Gd2N16.5Na2O57.5 (FW = 3217.12 g/mol): Found (%) H = 3.57, C = 42.33, N = 7.49; Calculated (%) H = 3.93, C = 42.37, N = 7.18. {TbNa[12-MCAl(III)N(shi)-4]}2(iph)4(DMF)2(H2O)8•5DMF•2H2O, Tb2Al8Na2. Elemental analysis for C109H117Al8N15Na2O57Tb2 (FW = 3128.84 g/mol): Found (%) H = 3.40, C = 41.52, N = 7.11; Calculated (%) H = 3.77, C = 41.84, N = 6.71. {ErNa[12-MCAl(III)N(shi)-4]}2(iph)4(DMF)4(H2O)10•2.5DMF, Er2Al8Na2. Elemental analysis for C107.5H113.5Al8Er2N14.5Na2O56.5 (FW = 3108.97 g/mol): Found (%) H = 3.31, C = 41.14, N = 6.91; Calculated (%) H = 3.68, C = 41.53, N = 6.53. {YbNa[12-MCAl(III)N(shi)-4]}2(iph)4(DMF)2(H2O)8•6DMF•4.5H2O, Yb2Al8Na2. Elemental analysis for C112H129Al8N16Na2O60.5Yb2 (FW = 3275.21 g/mol): Found (%) H = 3.52, C = 40.67, N = 7.28; Calculated (%) H = 3.96, C = 41.07, N = 6.84.

Fig. 1 .

 1 Fig. 1. X-ray crystal structure of YbAl4Na with the metal ions displayed as spheres. (a) side view and (b) view along the pseudo-C4 axis through the Yb III and Na + ions with the metallacrown N-O connectivity between the Al III ions being highlighted. Hydrogen atoms, lattice DMF and water molecules, and disorder have been omitted for clarity. Color scheme: light blue -ytterbium; green -aluminum; yellow -sodium; red -oxygen; dark blue -nitrogen; and gray -carbon.

Fig. 2 .

 2 Fig. 2. Side view of the X-ray crystal structure of Yb2Al8Na2 with the metal ions displayed as spheres. The sodium ions (yellow) bind to the 12-MC-4 moieties opposite of the Yb III ions (light blue). See Fig. 1 for additional display details.

Fig. 3 .

 3 Fig. 3. Side view of the X-ray crystal structure of Er2Al8Na2 with the metal ions displayed as spheres. The sodium ions (yellow) bind to the side of the dimer instead of the MC cavity. See Fig. 1 for additional display details.

Figure 4 .

 4 Figure 4. Corrected and normalized excitation and emission spectra of TbAl4Na and Tb2Al8Na2 measured in the solid state at room temperature.

Figure 5 .

 5 Figure 5. Corrected and normalized excitation and emission spectra of YbAl4Na and Yb2Al8Na2 in the solid state measured at room temperature.

Figure 6 .a

 6 Figure 6. Corrected and normalized excitation and emission spectra of ErAl4Na and Er2Al8Na2 in the solid state measured at room temperature.

Figure 7 .

 7 Figure 7. Corrected and normalized phosphorescence spectra of GdAl4Na and Gd2Al8Na2 acquired in the solid state upon excitation at 330 nm and applying a 200 µs delay for the collection of the signal after the excitation flash at 77 K (black traces). 0-0 (red), 0-1 (blue) and 0-2 (magenta) phonon transitions were fitted with a Gaussian function.

Acknowledgements

CMZ would like to thank the following for financial support: the PASSHE Faculty Professional Development Council Grant Program, the Shippensburg University (SU) Faculty Professional Development Council Grant Program, the SU Student/Faculty Research Engagement Grant, the SU Miklausen-Likar Science Research Fund, the SU Summer Undergraduate Research Experience Program, and the SU and Shippensburg Foundation Undergraduate Research Program. The work in France was supported by La Ligue Contre le Cancer (comité du Loiret et d'Eure-et-Loir), the network ''Molécules marines, métabolisme et cancer'' from the Cancéropôle Grand Ouest, and la Région Centre Val de Loire. SP thanks the Institut National de la Santé et de la Recherche Médicale.

Author Contributions

Conflicts of interest

There are no conflicts to declare.