Tuning Photophysical Properties by p-Functional Groups in Zn(II) and Cd(II) Complexes with Piperonylic Acid

Aggregation between discrete molecules is an essential factor to prevent aggregation-caused quenching (ACQ). Indeed, functional groups capable of generating strong hydrogen bonds are likely to assemble and cause ACQ and photoinduced electron transfer processes. Thus, it is possible to compare absorption and emission properties by incorporating two ligands with a different bias toward intra- and intermolecular interactions that can induce a specific structural arrangement. In parallel, the π electron-donor or electron-withdrawing character of the functional groups could modify the Highest Ocuppied Molecular Orbital (HOMO)–Lowest Unocuppied Molecular Orbital (LUMO) energy gap. Reactions of M(OAc)2·2H2O (M = Zn(II) and Cd(II); OAc = acetate) with 1,3-benzodioxole-5-carboxylic acid (Piperonylic acid, HPip) and 4-acetylpyridine (4-Acpy) or isonicotinamide (Isn) resulted in the formation of four complexes. The elucidation of their crystal structure showed the formation of one paddle-wheel [Zn(μ-Pip)2(4-Acpy)]2 (1); a mixture of one dimer and two monomers [Zn(µ-Pip)(Pip)(Isn)2]2·2[Zn(Pip)2(HPip)(Isn)]·2MeOH (2); and two dimers [Cd(μ-Pip)(Pip)(4-Acpy)2]2 (3) and [Cd(μ-Pip)(Pip)(Isn)2]2·MeOH (4). They exhibit bridged (1, µ2-η1:η1), bridged, chelated and monodentated (2, µ2-η1:η1, µ1-η1:η1 and µ1-η1), or simultaneously bridged and chelated (3 and 4, µ2-η2:η1) coordination modes. Zn(II) centers accommodate coordination numbers 5 and 6, whereas Cd(II) presents coordination number 7. We have related their photophysical properties and fluorescence quantum yields with their geometric variations and interactions supported by TD-DFT calculations.


Introduction
The correlation between composition, structure and properties has always been the foundation of materials design. Understanding this interplay allows to comprehend the structure-properties relationship and make significant progress in fields inter alia luminescence [1], sensing [2] or photoelectrical conductivity [3,4], allowing to design prominent materials. In the field of optical materials, the conception of fluorescent light emitting materials based on discrete molecular complexes provides a crucial advantage compared to their polymeric analogues as they have better solvent processability [5].
Pyridine based fluorophores have been developed as a fitting family of ligands with which to design fluorescent complexes. They excel at being sensitive to electronic perturbation and are capable of coordinating both soft and hard metal ions [6]. However, they commonly present fluorescence quenching associated with intramolecular charge transfer (ICT) and intra-ligand charge transfer (ILCT) transitions, photoinduced electron [Zn(μ-Pip)(Pip)(Isn)2]2·2[Zn(Pip)2(HPip)(Isn)]·2MeOH (2). It crystallizes in the triclinic P-1 space group and comprises one dimeric and two monomeric structural units with two occluded MeOH molecules in the unit cell (Figure 2a). Such a structure is unusual and seems to be driven by strong interactions between the amide moieties, which also promoted the same behavior with Cu(II) [23]. Both the monomer and dimer have hexacoordinated Zn(II) atoms bearing [ZnO4N2] cores with a distorted octahedral geometry (S.I: Table S1) presenting an ata (average twist angle) of 58.41° for Zn(1A) and 58.81° for Zn(1B) [24,25] (Table 2, Figure 2b). The dimeric unit is composed of four Pip and four Isn ligands. In contrast, the monomer contains two Pip, one HPip and two Isn ligands. The Zn(II) centers in the dimer are joined by two bidentate bridged (μ2-η 1 :η 1 ) Pip ligands Supramolecular assembly forming 2D sheets promoted by π· · · π supported by C-H· · · π and C-H··O interactions. In detail, π· · · π (light orange) and C-H· · · π/C-H· · · O (light green) associations between Pip and 4-Acpy ligands. Table 1. Bond lengths (Å), bond angles ( • ) and intermolecular interactions in 1.
[Zn(µ-Pip)(Pip)(Isn) 2 ] 2 ·2[Zn(Pip) 2 (HPip)(Isn)]·2MeOH (2). It crystallizes in the triclinic P-1 space group and comprises one dimeric and two monomeric structural units with two occluded MeOH molecules in the unit cell (Figure 2a). Such a structure is unusual and seems to be driven by strong interactions between the amide moieties, which also promoted the same behavior with Cu(II) [23]. Both the monomer and dimer have hexacoordinated Zn(II) atoms bearing [ZnO 4 N 2 ] cores with a distorted octahedral geometry (S.I: Table S1) presenting an ata (average twist angle) of 58.41 • for Zn(1A) and 58.81 • for Zn(1B) [24,25] (Table 2, Figure 2b). The dimeric unit is composed of four Pip and four Isn ligands. In contrast, the monomer contains two Pip, one HPip and two Isn ligands. The Zn(II) centers in the dimer are joined by two bidentate bridged (µ 2 -η 1 :η 1 ) Pip ligands while the remaining two Pip units have a bidentate chelate coordination mode (µ 1 -η 2 ). Selected bond lengths and angles are displayed in Table 2. In addition, the Pip ligands in the monomer present µ 1 -η 2 and µ 1 -η 1 while HPip has a µ 1 -η 1 coordination mode. This µ 1 -η 1 coordination of the ligands is supported by an intramolecular O-H· · · O between the acidic proton of HPip and the uncoordinated carboxylate oxygen atom from the neighboring ligand.
Selected bond lengths and angles are displayed in Table 2. In addition, the Pip ligands in the monomer present μ1-η 2 and μ1-η 1 while HPip has a μ1-η 1 coordination mode. This μ1η 1 coordination of the ligands is supported by an intramolecular O-H···O between the acidic proton of HPip and the uncoordinated carboxylate oxygen atom from the neighboring ligand.    Simultaneously, the carboxylate O atom interacts with the MeOH molecule through an intermolecular O-H· · · O hydrogen bond. Besides, the aromatic rings from Isn stacks in an intramolecular π· · · π interaction ( Table 2).
The monomers and dimers form a 3D supramolecular net via amide· · · amide, N-H/C-H· · · O and π· · · π interactions (Table 3). Amide· · · amide piles monomers and dimers between themselves while Isn, through its N-H anti and m-H, form a dimer via association with an O atom from a chelate Pip ligand of a monomer and vice versa along [22] (Figure 3a). In addition, π· · · π interactions between Pip ligands stack monomers and dimers in an ordered sequence from Cg(3) to Cg(7) along the b axis ( Figure 3b).
The intermolecular assembly of 3 is guided by π···π interactions supported by three pairs of weak C-H···O associations between the Pip and 4-Acpy ligands. The A dimers are stacked via planar interactions in trios, complemented by a reciprocal C-H···O between m-H and Hmethyl from 4-Acpy and two carboxylate O atoms forming chains along the [100] direction. In contrast, B dimers only assemble via the equivalent C-H···O between Pip and 4-Acpy in the same axis. Finally, A···B dimers interact in pairs through π···π interactions between 4-Acpy and Pip ligands supported by C-H···O between the methylene H atoms from Pip and the O atoms from 4-Acpy along [21 2]. Both C-H···O and π···π interactions form 2D layers in (102) (Figure 6a,b). The dimeric units in 4 are held together by the amide· · · amide pattern engaged in a head-to-head disposition which orders the dimers in chains (A· · · A and B· · · B) parallel to the [10] direction. Besides, the N-H anti also partake in a double N-H· · · O and m-C-H· · · O associating the chains between A· · · B dimers (Figure 7a-g). Planar π· · · π interactions support the assembly of dimers along [10] direction (Figure 7h).

Structure and Geometric Evaluation
Detailed analysis of the geometric distortions present in Zn(II) and Cd(II) complexes have been performed using SHAPE [20,26] through the S parameter (S.I: Table S1). A recent structural search in the CCDC [27] has revealed that Zn(II) and Cd(II) mainly present coordination numbers ranging from 4 to 7, being scarce 3 and 8, the latter only affordable by Cd(II). The predominant structural motifs reported for Zn(II) are the dimeric µ-bridged [Zn 2 (CO 2 ) 2 ] units and the dimeric paddle-wheel [Zn 2 (CO 2 ) 4 ], bearing tetrahedral and octahedral geometries in [Zn 2 (CO 2 ) 2 ], or square pyramidal environments in [Zn 2 (CO 2 ) 4 ]. In turn, the most common motif in Cd(II) is the double bridged [Cd 2 (CO 2 ) 2 ], completed by two chelate ligands to form the [Cd 2 (CO 2 ) 2 ]−2+2 core and bearing coordination number 7 [12]. From these data it can be expected that complex 1 exhibits an almost ideal square pyramidal geometry (SPY-5, S = 0.240) [20] supported by the paddle-wheel structure that minimizes steric repulsion between the ligands. In the case of 2, both the monomer and dimer present uncommon structural motifs as the monomeric [Zn(CO 2 ) 3 ] and the dimeric [Zn 2 (CO 2 ) 4 ]. The two display the same ligand disposition and distorted octahedral geometry, which seems to be stabilized by strong intermolecular N-H· · · O (dimer) and intramolecular O-H· · · O (monomer) interactions. In the monomeric unit, the µ 1 -η 2 coordination mode combined with the intramolecular interaction between the HPip and Pip ligands force geometric constraints that accommodate the metal octahedral geometry (OC-6, S = 2.769 (Zn1A)). By the same token, this deviation (OC-6, S = 3.801 (Zn1B)) [20] is emphasized in the dimer by both µ 2 -η 1 :η 1 and µ 1 -η 1 -coordination modes of the Pip ligands. Cd(II) dimers  (4)) [20] since the equatorial plane arranged by µ 2 -η 2 :η 1 Pip linkers is almost equal. The strong double head-to-head amide· · · amide interaction between the dimers in 4 fix the Isn ligands and amend any distortion that cannot be minimized in complex 3, in which the interactions of 4-Acpy are weaker. Overall, the smaller radius of hexacoordinated Zn(II) (0.880 Å), with respect to heptacoordinated Cd(II) (1.17 Å) [28], accentuate the geometric constraints that occur during the formation of the dimeric arrays, which is reflected in the higher S value (3.801) of complex 2 compared to complexes 3 and 4 (S range between 1.995 and 2.448) [20].  Color codes: Cg(1) black; Cg(2) orange, Cg(5) dark blue; Cg(6) dark green; Cg (7) violet.
The dimeric units in 4 are held together by the amide···amide pattern engaged in a head-to-head disposition which orders the dimers in chains (A···A and B···B) parallel to the [010] direction. Besides, the N-Hanti also partake in a double N-H···O and m-C-H···O associating the chains between A···B dimers (Figure 7a-g). Planar π···π interactions support the assembly of dimers along [010] direction (Figure 7h).

Structure and Geometric Evaluation
Detailed analysis of the geometric distortions present in Zn(II) and Cd(II) complexes have been performed using SHAPE [20,26] through the S parameter (S.I: Table S1). A recent structural search in the CCDC [27] has revealed that Zn(II) and Cd(II) mainly present coordination numbers ranging from 4 to 7, being scarce 3 and 8, the latter only affordable by Cd(II). The predominant structural motifs reported for Zn(II) are the dimeric μ-bridged  [12]. From these data it can be expected that complex 1 exhibits an almost ideal square pyramidal geometry (SPY-5, S = 0.240) [20] supported by the paddle-wheel structure that minimizes steric repulsion between the ligands. In the case of 2, both the monomer and dimer present uncommon structural motifs as the monomeric [Zn(CO2)3] and the dimeric In detail, (b) amide· · · amide interactions between A dimers or (c) between B dimers; (d) MeOH· · · amide, (e) amide(A)· · · Pip(B) or (f) MeOH· · · Pip association; and (g) amide(B)· · · Pip(A) interactions. (h) π· · · π interactions between Pip rings.

Photophysical Properties
The absorption and emission properties of the complexes and ligands, as well as the references (L-tyrosine and phenanthrene), were recorded in a MeOH solution. The absorption was measured in the UV region of the spectra from 200 to 345 nm, while the emission was recorded between 270 and 450 nm at 298 K.
UV-Vis spectroscopy. To ensure the non-aggregation of the samples at the selected concentrations for the fluorescence experiments, we performed additive UV-Vis measurements within a concentration range from 1 × 10 −9 to 1 × 10 −5 M (Figure 8). references (L-tyrosine and phenanthrene), were recorded in a MeOH solution. The absorption was measured in the UV region of the spectra from 200 to 345 nm, while the emission was recorded between 270 and 450 nm at 298 K.
UV-Vis spectroscopy. To ensure the non-aggregation of the samples at the selected concentrations for the fluorescence experiments, we performed additive UV-Vis measurements within a concentration range from 1 × 10 −9 to 1 × 10 −5 M (Figure 8). The samples do not seem to present aggregation in this range of concentration. The absorption spectrum of 2 displays a significant change from 4.49 × 10 −8 M on, which has been ascribed to a change on the absorber rather than a change via aggregates formation. This is discussed below and supported by TD-DFT calculations. The absorption and emission maxima of complexes 1-4 (λ max-Abs and λ max-Em , respectively), have been identified, and their molar absorptivity (ε) and relative quantum yield (φ s ) calculated (Table 8). All of the wavelengths are given in nm. ∆λ are given in cm −1 . λ ex = excitation wavelength; λ max-em = maximum of emission; φ S = quantum yield. * Bands arising from a change in the absorber (the dimer in 2).
Photoluminescence. Heretofore, fluorescence measurements were performed using concentrations of 1.70 × 10 −9 M (1); 1.08 × 10 −8 M and 1.07 × 10 −7 M (2); 1.04 × 10 −7 M (3); and 1.01 × 10 −7 M (4), extracted from the UV-Vis results after ensuring their non-aggregation to minimize ACQ [7]; these samples were excited at the wavelength of their emission maxima. All of the relevant details have been summarized in Table 8. The spectra of 1 and 2 have single emission bands at 354 nm (1) and 344 or 318 nm (2) at being irradiated at 263 nm and 225 or 251 nm, respectively. The emission spectra of 3 and 4 present unfolded emission bands centered at 355 and 347 nm, at being irradiated at 315 and 226 nm, respectively. As displayed in the CIE 1931 chromaticity diagram [29], the resultant emission color of 1-4 (318-355 nm) is violet (Figure 9). Emission intensities increase in the order 1 < 2 < 3 < 4 considering the different concentrations used in the fluorescence experiments ( Figure 10). The relative quantum yields (ф s ) of the samples were calculated by way of comparison with two reference standards (L-tyrosine and phenanthrene) [30] using Equation (1): Emission intensities increase in the order 1 < 2 < 3 < 4 considering the different concentrations used in the fluorescence experiments ( Figure 10). The relative quantum yields (φ s ) of the samples were calculated by way of comparison with two reference standards (L-tyrosine and phenanthrene) [30] using Equation (1): where φ is the quantum yield, OD is the optical density (or absorbance) at the excited wavelength, I is the area under the curve of the emission spectra, and n is the refractive index of the solvent. In this study, L-tyrosine (φ ref = 0.14) [31] and phenanthrene (φ ref = 0.125) [32] has been used as  All of the wavelengths are given in nm. Δλ are given in cm −1 .λex = excitation wavelength; λmax-em = maximum of emission; фS = quantum yield. * Bands arising from a change in the absorber (the dimer in 2).

Electronic Calculations
DFT calculations. The geometric optimization of complexes 1-4 has been performed in a MeOH solution using the Polarizable Continuum Model (PCM). The resulting complexes containing 4-Acpy exhibited an improvement of the geometric evaluator S from 0.240 to 0.181 (1) and from 2.215 and 2.448 to 2.048 and 1.889 (3). In contrast, complexes 2 and 4 presented significant differences between the X-ray diffraction data and the geometry in MeOH solution. The monomeric array in compound 2 is already stabilized by strong O-H···O intramolecular interactions between HPip and Pip ligands and suffers minor geometric changes with S values varying from 2.769 to 2.311. However, the dimer in 2, which is stacked in chains by strong amide···amide intermolecular interactions, presents significant differences between when they are in a solid state compared to being in solu-

Electronic Calculations
DFT calculations. The geometric optimization of complexes 1-4 has been performed in a MeOH solution using the Polarizable Continuum Model (PCM). The resulting complexes containing 4-Acpy exhibited an improvement of the geometric evaluator S from 0.240 to 0.181 (1) and from 2.215 and 2.448 to 2.048 and 1.889 (3). In contrast, complexes 2 and 4 presented significant differences between the X-ray diffraction data and the geometry in MeOH solution. The monomeric array in compound 2 is already stabilized by strong O-H· · · O intramolecular interactions between HPip and Pip ligands and suffers minor geometric changes with S values varying from 2.769 to 2.311. However, the dimer in 2, which is stacked in chains by strong amide· · · amide intermolecular interactions, presents significant differences between when they are in a solid state compared to being in solution. The S values change from 3.801, which corresponds to an octahedral geometry, to 8.571, acquiring a trigonal prismatic geometry (S.I: Table S2). This geometrical change can be attributed to the strong intramolecular amide· · · amide interactions between the stacked Isn ligands that force the geometry of the Zn(II) metal nodes. A similar difference between the solid state and the MeOH solution is also present in complex 4; however, in this case, the bigger size of Cd(II) allows for a structural reorganization without such a marked difference in the S values, which shift from 2.015 and 1.995 to 3.367 and 2.288. Since MeOH has a solvent polarity parameter [34] ET(30) of 55.4 kcal·mol −1, this could be insufficient to predominantly establish the intermolecular interactions with Isn and prevent the intramolecular amide· · · amide association. Therefore, the dimers in complexes 2 and 4 are amenable to experience significant geometric changes and promote relaxation through non-radiative decays.
TD-DFT calculations. All of the calculated UV-Vis spectra of complexes 1-4 agree reasonably well with the experimental profiles ( Figure 12, S.I.: Figures S14-S16). The shift in the theoretical absorption spectra with respect to the experimental profiles is within the range of typical TD-DFT calculations (~0.3 eV) and are caused by computing the absorptions as vertical transitions [35]. Only transitions with a higher oscillator strength (f ) value have been selected for the molecular orbitals representation and natural transition orbitals (NTOs) analysis. The HOMO and LUMO outline as well as the energy gaps can be found in the S.I: Figure S17. Subsequently, the main contributors of the electronic transitions have been analyzed for each absorption band to identify the regions involved in it. The molecular orbitals of each set of transitions have been represented, as well as the corresponding NTOs.
Molecules 2022, 27, x FOR PEER REVIEW 19 of 28 energy gaps can be found in the S.I: Figure S17. Subsequently, the main contributors of the electronic transitions have been analyzed for each absorption band to identify the regions involved in it. The molecular orbitals of each set of transitions have been represented, as well as the corresponding NTOs. The TD-DFT results of the monomeric and dimeric forms present in complex 2 resulted in the ascription of the spectrum obtained at concentrations below 4.49 × 10 −8 M to the monomeric form, while the shape of the spectra of the dimer is closer to the one resulting from higher concentrations. Therefore, from the electronic calculations, it could be stated that the monomer-dimer ratio is displaced towards the monomeric form at lower concentrations while a mixture of both or even primacy of the dimer is observed. The HOMO and LUMO orbitals of 1-4 have π symmetry, being the HOMO along the Pip ligand while the LUMO is localized over the 4-Acpy and Isn linkers. This MO seclusion was previously observed [15] suggesting that by keeping the Pip linker constant, the incorporation of 4-Acpy ligand, bearing a more electron withdrawing functional group, Osc. strength The TD-DFT results of the monomeric and dimeric forms present in complex 2 resulted in the ascription of the spectrum obtained at concentrations below 4.49 × 10 −8 M to the monomeric form, while the shape of the spectra of the dimer is closer to the one resulting from higher concentrations. Therefore, from the electronic calculations, it could be stated that the monomer-dimer ratio is displaced towards the monomeric form at lower concentrations while a mixture of both or even primacy of the dimer is observed.
The HOMO and LUMO orbitals of 1-4 have π symmetry, being the HOMO along the Pip ligand while the LUMO is localized over the 4-Acpy and Isn linkers. This MO seclusion was previously observed [15] suggesting that by keeping the Pip linker constant, the incorporation of 4-Acpy ligand, bearing a more electron withdrawing functional group, would lower the energy of the LUMO and, thus, reduce the energy gap. The HOMO orbitals of these complexes are quite similar in energy, ranging from −8.312 eV to −8.018 eV since they are all located over the Pip ligands. In opposite, the LUMO orbitals present a significant difference in energy, which can be sorted into three groups. The monomer in 2 has the higher energy LUMO of −0. Since complex 1 does not present MLCT nor LMCT transitions, there appears to be a structural effect of the paddle-wheel that, in this case, minimizes electronic transitions on the Zn(II) metal center and hinders the charge transfers between Pip and 4-Acpy. The spatial arrangement of the ligands forced by the paddle-wheel leads to a greater separation between them, thus, avoiding intramolecular charge transfer transitions. The electronic transition states (TS) at the selected λ ex of complexes containing Isn (2 and 4) present a combination of LE over Isn with a strong contribution of ILCT transitions and a small contribution of MLCT to Isn in 4 (TS20 of the monomer, TS41 of the dimer in 2 or TS23 and 24 of 4); whereas, 4-Acpy complexes mainly present LE over 4-Acpy (TS8 and 19 of 1) or ILCT between them (TS5 and 6 of 3). Therefore, the combination of both intramolecular interactions and dimeric structure could directly affect the photophysical properties by promoting charge transfers instead of LE; emphasized in Isn complexes. The Supporting Information displays the complete data about geometry optimization (S.I.: Tables S3-S7 and Figures S28-S32).

Conclusions
A series of Zn(II) and Cd(II) complexes with HPip, 4-Acpy and Isn have been synthesized and fully characterized. Their crystal structure consists of one Zn(II) dimeric paddle-wheel (1); a mixture of one dimer and two monomers in the unit cell (2); and two dimers of Cd(II) (3 and 4). Their different nuclearity is based on different combinations of coordination modes of the Pip ligand: monodentate (µ 1 -η 1 , 2), bidentate chelate (µ 1 -η 2 , 2-4), bridged (µ 2 -η 1 :η 1 , 1 and 2) or both (µ 2 -η 2 :η 1 , 3 and 4) strongly influenced by intraand intermolecular interactions. In them, Zn(II) metal node displays coordination numbers of 5 (1) and 6 (2) while Cd(II) exhibits coordination number 7 (3 and 4). DFT geometric optimizations revealed dPy dependent geometrical changes, emphasized by the formation of intramolecular amide· · · amide interactions. TD-DFT results show HOMO-LUMO gap dependence on the dPy, being the shortest corresponding to the 4-Acpy complexes. NTOs analysis revealed LE character of absorptions in 1 and 3; whereas, in 2 and 4, ILCTs have a significant contribution. This has also been reflected in the φ S results in complex 1 being up to ten times higher than in 2. These results reflect how (i) hampering charge transfer transition by avoiding intramolecular π· · · π interactions; (ii) minimizing PET processes through coordination to Zn(II) and Cd(II); and (iii) changes in the p-substituents of dPy, can modulate the HOMO-LUMO gap and maximize the resulting S values.

Synthesis of Complexes 1-4
[Zn(µ-Pip) 2 (4-Acpy)] 2 (1). To a MeOH solution (15 mL) of Zn(OAc) 2 ·2H 2 O (250 mg, 1.14 mmol), the 4-Acpy (0.500 mL, 4.13 mmol) was added drop wise, and the mixture was stirred for 5 min. Then, a MeOH solution (35 mL) of HPip (379 mg, 2.28 mmol) was added drop wise. The resulting solution was stirred for 5 h 30 min until a yellowish powder precipitated. The suspension was cooled down for 15 min and filtered off. The solid was washed twice with cold MeOH (5 mL) and dried under vacuum. Suitable colorless crystals were obtained via slow evaporation of the mother liquors for 6 days. path length of 1 cm in the range of 190-345 nm. The molar absorptivity values were calculated and displayed as log(ε). The fluorescence measurements were carried out at 25 °C with a PerkinElmer LS 55 50 Hz fluorescence spectrometer (Perkin Elmer Inc., Shelton, CT, USA) using a 1 cm quartz cell, in MeOH solution. The samples were excited at their absorption maxima and the emission was recorded between 195 and 450 nm. The solid state photoluminescence measurement was recorded between 500 and 630 nm using a Varian Cary Eclipse Fluorescence spectrophotometer (Agilent, Santa Clara, CA, USA) and is given in nm. The UV-Vis and fluorescence spectra in solution, and the spectrum of 1 in solid state, were measured under air exposure. Both CIE 1931 chromaticity diagrams as well as the corrected dilution effects on the UV-Vis and fluorescence data were performed by means of Origin Pro 2019b software (OriginLab Corporation, Northampton, MA, USA).

Synthesis of Complexes 1-4
[Zn(μ-Pip)2(4-Acpy)]2 (1). To a MeOH solution (15 mL) of Zn(OAc)2·2H2O (250 mg, 1.14 mmol), the 4-Acpy (0.500 mL, 4.13 mmol) was added drop wise, and the mixture was stirred for 5 min. Then, a MeOH solution (     Methodology and computational details. All of the calculations were performed using Gaussian09 software version D.01 [39]. The geometry optimization of the ground state and vertical absorptions from the electronically excited state for 1-4 were completed using density functional theory (DFT) and time-dependent DFT (TD-DFT), respectively, using ωB97X-D [40,41] functional (S.I: Tables S3-S7 and Figures S28-S32). A correlation consistent basis set was used for the Zn, Cd, C, H, N and O atoms, the effective core potential CrenbL [42]. The MeOH solvation effects were incorporated using the polarizable continuum model-Linear Response (PCM-LR) [43,44]. Since either monomeric or dimeric arrays seem to be involved in absorption and emission depending on the concentration, the geometry of the monomer and the dimer in 2 were optimized separately. The frequencies were also computed for each optimized structure to ensure that the geometries corresponded to an energy minimum. The HOMO and LUMO energetic levels were firstly examined and then the energy gaps calculated. For 1-4, the first 80 vertical absorptions from the ground state to the excited states were calculated and only the most probable transitions, those with higher oscillator strength (f ) values, were selected for the electronic analysis. The shift in the theoretical absorption spectra, with respect to the experimental profile, is within the range of typical TD-DFT calculations (~0.3 eV) and are caused by computing the absorptions as vertical transitions [35].
The methodology for the generation of the MOs was the same as has been previously reported [15] (S.I: Figures S18-S22). The analysis of the electronic transitions was supported by the NTOs (S.I: Figures S23-S27) [45] to better identify and represent the main contributor molecular orbitals of each transition. The NTOs were generated using Multiwfn software [46] version 3.7 with an isovalue of 0.02.
Informed Consent Statement: Not applicable.