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Article

Luminescent Imidazo[1,5-a]pyridine Cores and Corresponding Zn(II) Complexes: Structural and Optical Tunability

by
G. Volpi
*,
A. Giordana
,
E. Priola
,
R. Rabezzana
and
E. Diana
Department of Chemistry, University of Torino, Via Pietro Giuria 7, 10125 Torino, Italy
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(9), 283; https://doi.org/10.3390/inorganics13090283
Submission received: 25 June 2025 / Revised: 31 July 2025 / Accepted: 22 August 2025 / Published: 25 August 2025
(This article belongs to the Section Organometallic Chemistry)
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Abstract

A new series of luminescent Zn(II) complexes based on mono- and bis-imidazo[1,5-a]pyridine ligands was synthesized to investigate the correlation between structural modifications and photophysical behaviour. Systematic variations in substituent groups, coordination geometry, and π-conjugation extent enabled precise tuning of absorption and emission properties. Spectroscopic analysis revealed that Zn(II) coordination enhances molecular rigidity and induces a conformational change in the ligands, resulting in improved quantum yields (up to 37%) and significant blue shifts in emission. Notably, in bis-ligand systems, each imidazo[1,5-a]pyridine unit retains its distinct emissive signature upon complexation, demonstrating their optical and electronic independence. This modular behaviour confirms that individual emissive centres can be predictably manipulated without mutual interference, offering a powerful design strategy for multichromophoric materials. Structural, vibrational, and mass spectrometric characterizations further corroborate the stability and coordination patterns of the synthesized complexes. These insights lay the groundwork for engineering efficient and tunable Zn(II)-based luminophores for applications in optoelectronics, sensing, and bioimaging.

1. Introduction

Luminescent zinc(II) complexes continue to attract significant interest for their potential implementation in optoelectronic devices [1,2,3,4], molecular sensors [5,6,7,8], and photofunctional materials [9,10,11,12]. The appeal of Zn(II) arises from its d10 electronic configuration, which precludes low-lying metal-centred excited states, thereby preserving the photophysical activity of the coordinated ligands. In this context, nitrogen-rich heteroaromatic scaffolds have proven highly versatile due to their rigid structure, high emission quantum yields, and modular coordination behaviour [8,12,13,14]. In this scenario, the imidazopyridine skeleton has previously shown promising results and versatility in the coordination and emission properties of the corresponding Zn(II) complexes [7,15,16,17].
In particular, within the imidazopyridine family, the peculiar imidazo[1,5-a]pyridine core offers multiple coordination sites through its fused aromatic rings, and its substitution pattern can be strategically modified to tune both electronic and steric properties [18,19,20,21]. These features render it a valuable platform for constructing mono-, bis-, and tris-chelate architectures with Zn(II), allowing for the systematic exploration of how ligand coordination and topology influence the final stoichiometry and geometry [22].
Within this framework, multidentate imidazopyridinic ligands incorporating additional donor sites (e.g., pyridine) enable the formation of coordination complexes with tunable geometric constraints and enhanced photophysical responses [18,23,24,25,26,27,28]. Variation in the spatial disposition of the donor atoms can lead to distinct binding modes and symmetry-breaking effects, which are known to significantly impact the electronic transitions and emission characteristics of the resulting metal complexes [19,29].
In this work, we report the synthesis and characterization of a new series of Zn(II) complexes with imidazo[1,5-a]pyridine ligands designed to promote different coordination geometries (Figure 1). To this end various imidazo[1,5-a]pyridine and multi-imidazo[1,5-a]pyridine ligands were directly combined with ZnCl2 to obtain the corresponding complexes, enhancing the blue-shift and optimizing the optical behaviour, in line with the recent interest in small luminescent organic molecules.
The study aims to elucidate the correlation between the ligand structure (including rigidity, optical behaviour, and structure) and the corresponding properties of the Zn(II) derivatives. A combination of spectroscopic and structural techniques is employed to assess the influence of complexation on the photophysical behaviour, with particular attention to emission maxima, quantum yields, and Stokes shifts. The obtained data provide insights into the structure–property relationships governing these systems and allow us to improve the design of new luminescent Zn(II) materials with tailored optical performance.
The development of such complexes is particularly relevant for applications requiring lightweight, non-toxic, and cost-effective emitters. Zinc-based luminophores incorporating N-heterocyclic ligands are being actively investigated for use in organic light-emitting diodes (OLEDs), light-emitting electrochemical cells (LECs), fluorescence-based sensors, and bioimaging probes. The ability to finely tune emission wavelength and quantum efficiency through rational ligand design positions these complexes as promising candidates for integration into next-generation photonic and diagnostic technologies.

2. Results and Discussion

2.1. Synthesis

The ligands A, AA, AB, B, and BB were prepared as previously reported, by a direct one-pot cyclization synthetic approach, condensing 2-benzoylpyridine or 2-pyridylketone with an opportune aldehyde (benzaldehyde for A and B or terephthalaldehyde for AA, AB and BB) in acetic acid and ammonium acetate. In particular, compounds have been prepared as previously reported (A [25], AA [23,24], AB [30], B [31], BB [23,24]). The significant substitution of the pendant pyridine (at position 1 on the imidazopyridine core) ensures the typical N–N bidentate coordination motif, which is well known for enabling effective complexation reactions with various metals.
The purity was assessed by TLC, 1H and 13C NMR spectroscopy, and high-resolution mass spectrometry (see Supporting Information).
The mono-chelate [Zn(B)Cl2] and [Zn(AB)Cl2] complexes were synthesized by reaction of ZnCl2 with a stoichiometric amount (1:1) of ligand in methanol, as previously reported [32,33]. Similarly, the [Zn2(BB)Cl4] complex was prepared by reacting ZnCl2 with a stoichiometric amount (2:1) of L in methanol, employing the same procedure previously reported for [Zn(B)Cl2]. ZnCl2 was selected as the starting reagent due to the presence of Cl− as a stable counter ion and coordinating ligand, as previously reported [15,16,32].
All the complexes are yellow crystalline powders and were characterized by mass spectrometry, IR (see Supporting Information), Raman, UV–Vis absorption and emission spectroscopies. Due to the extremely low solubility of these complexes in the common deuterated solvents, it was not possible to perform the NMR characterization as previously reported for similar products [15,29,32,33].

2.2. Optical Characterization

To improve the optical properties suitable for different technological applications, we have adopted three different strategies to modify the imidazo[1,5-a]pyridine emissive nucleus and the corresponding Zn(II) complexes. First, we compared different substituent groups (phenyl and pyridine) at position 3 on the imidazo[1,5-a]pyridine skeleton to achieve a useful hypsochromic shift in the absorption and emission spectra, as previously reported for different imidazo[1,5-a]pyridine ligands [18,34,35,36]. Secondly, we coordinated the pyridine pendant group and imidazo[1,5-a]pyridine core to achieve not only a convenient hypsochromic shift but also to improve the quantum yield, as previously reported in the case of Zn(II) complexes [32,33]. Finally, we modified the conjugated aromatic system by employing multiple imidazo[1,5-a]pyridine units, which is expected to influence the photophysical properties of the imidazo[1,5-a]pyridine nucleus. The modification of the π-system allowed us to evaluate the synergistic role of two imidazo[1,5-a]pyridine nuclei (and the corresponding Zn(II) complexes) in the optical performance.
The optical data obtained for the studied imidazo[1,5-a]pyridine products (A, AA, AB, B, BB) and for the corresponding Zn(II) complexes ([Zn(AB)Cl2], [Zn(B)Cl2], [Zn2(BB)Cl4]) in dichloromethane solution are collected in Table 1, while absorption and emission spectra are presented in Figure 2 and Figure 3.
Table 1. Absorption and emission data for the studied ligands and corresponding Zn(II) complexes in dichloromethane solutions (sh = shoulder).
Table 1. Absorption and emission data for the studied ligands and corresponding Zn(II) complexes in dichloromethane solutions (sh = shoulder).
CompoundAbsorption
(nm)		Excitation
(nm)		Emission
(nm)		Stokes Shift
[cm−1 (nm)]		Quantum Yield (%)
A306
342 sh
385 sh		3854845313
(178)		22
AA303
378		3774805692
(102)		10
AB331
379		379449 sh
468		5018
(89)		18
[Zn(AB)Cl2]326
379		379444
466 sh		3863
(65)		35
B325
380 sh		380435 sh
462		4671
(137)		19
[Zn(B)Cl2]325
362 sh
376
394 sh		376416 sh
440
462 sh		3868
(64)		32
BB330
387		380447 sh
466		4856
(79)		25
[Zn2(BB)Cl4]330
378		380440
466 sh		3588
(62)		37
The main absorption peaks were observed in the wavelength range between 320 nm and 400 nm, with almost no absorption beyond 450 nm. Usually, the compounds presented two main bands, one in the 280–310 nm range and a second one centred at 320–340 nm, except for A and B, in which a shoulder is observable at about 320–340 nm. The intensity of the absorption at 320–340 nm significantly decreased (from A and B to compounds AA, AB, and BB) as a clear consequence of the reduced conjugation. Moreover, the presence of pyridine pendant groups causes a remarkable blue shift (about 25 nm) as observable comparing A with B, as well as comparing AA with BB.
The corresponding Zn(II) complexes present two main absorptions. One in the 310–330 nm range, a second one centred at 360–380 nm, with a final shoulder at 390–400 nm. Compound [Zn(B)Cl2] shows an intense increase in the low-energy band at about 380 nm and a corresponding red shift if compared to the corresponding free ligand; contrarily, the complex [Zn2(BB)Cl4] shows a small blue shift (about 10 nm) of the main peak at 370–390 nm. Finally, comparison between AB and [Zn(AB)Cl2] shows no difference in the absorption profile except for an important decrease in the absorption intensity in the 310–330 nm range.
All the products display an intense fluorescence emission centred at about 450–500 nm in dichloromethane solution, characterized by quantum yields ranging from 10 to 37% (AA and [Zn2(BB)Cl4], respectively). These values are comparable to the best results previously reported in the literature for imidazo[1,5-a]pyridine products and derived complexes [7,29,32,33]. As previously reported, an intense increase in the quantum yield is appreciable after the complexation reaction of an imidazo[1,5-a]pyridine-based ligand with Zn(II), due to an important modification of the ligand conformation [32].
The emission maximum strongly depends on the chemical structure, ranging from 440 nm for compounds [Zn(B)Cl2] and [Zn2(BB)Cl4] to 484 nm for compound A. It is possible to obtain a 44 nm blue-shift in the emission band maximum by modifying the substituent groups on the imidazo[1,5-a]pyridine and employing these products as ligands towards Zn(II).
In general, all collected emissions exhibit a well-defined and intense structured band centred in the 440–480 nm range, each maximum flanked by two shoulders at about 40 nm before and after the main peak, which are more pronounced in the Zn(II) complexes compared to the free ligand (see Figure 3). This phenomenon provides clear evidence of the well-known intra-ligand π–π* character of the electronic transitions responsible for fluorescence, as anticipated for similar imidazo[1,5-a]pyridine ligands coordinated to a closed-shell Zn(II) ion [16,29]. This characteristic has been previously documented by us for similar mono-chelate Zn(II) complexes, resulting from the increased rigidity and conformational changes in the ligand (from transoid to cisoid) upon metal coordination [37]. The emission centred at 400–460 nm can be ascribed to an LC (π–π*) electronic transition, in agreement with the vibrational profile and with the assignment reported in the literature for Cu, Ir, and Re analogues [38,39].
[Zn(AB)Cl2] exhibits a less defined profile, attributed to the presence of one chelating imidazo[1,5-a]pyridine unit bearing a pyridine substituent, and one phenyl-substituted imidazo[1,5-a]pyridine unit in which the phenyl ring does not participate in coordination, thus reducing the overall rigidity upon complexation.
In general, as evidenced by comparing A and B (or AA and BB), the substitution of the pyridine substituent in place of phenyl causes a pronounced blue shift. Moreover, the reaction with Zn(II) ions to obtain the corresponding complexes increases the quantum yield and further causes a useful blue shift in the emission spectra at about 20 nm, as required for possible down-shifting applications.
Compound AA exhibits the lowest quantum yield in the series, highlighting the crucial role of the pyridine pendant group at position 1 of the imidazo[1,5-a]pyridine core. In general, each structural modification introduced has a clear impact on both the absorption and emission bands. The comparison between phenyl and pyridine substituents at position 1 reveals a noticeable hypsochromic shift, while the pyridine group also enables the chelating motif required for the synthesis of the corresponding Zn(II) complexes [24].
The comparison among compounds AA, AB, and BB demonstrates that each imidazopyridine unit behaves as an independent fluorophore, retaining its distinct photophysical identity even in the corresponding metal complexes ([Zn(AB)Cl2] and [Zn2(BB)Cl4]). In these products, the presence of the metal generally enhances the overall emission as a result of the increased molecular rigidity induced by coordination.

2.3. Structural Characterization

Ligand AB crystallizes in monoclinic space group type P21/n from a solution of DMF. The molecule presents a coplanar structure for the two imidazopyridine cores, with the central phenyl ring rotated with respect to this plane by 35.6 (5)° (Figure 4a). Although the two halves of the molecules are different due to the presence of one pyridinic nitrogen, a positional disorder of this site and the presence of a crystallographic inversion centre lying on the central aromatic ring make the nitrogen disordered on the four possible sites of the molecule. This can be the result of the absence of a strong directional interaction with this atom. This is clear in the crystal packing, which is dominated by π…π stacking and C-H…π weak hydrogen bonds (Figure 4b). However, the crystal packing does not follow the herringbone pattern of the ligand BB, although AB shows the same space group type as one of the two polymorphs studied, probably for different quadrupolar interactions (Figure 4b). The other crystal structures of molecules A, B and BB have been in-depth analyzed elsewhere, but for comparison, we can say that the dispersion forces are the main energetic components and that π…π stacking and C-H…π weak hydrogen bonds are the main motifs for all the structures [25]. The asymmetric unit of AB is reported in the Supporting Information (see Figure S1).

2.4. Vibrational Characterization

IR and Raman spectra of the synthetized complexes have been collected to confirm the formation of the complexes (see Supporting Information Figures S2–S5). The Raman spectra of L are used for comparison with the spectra of the metal complexes (in Figure 5). The most intense Raman signals of B, AB and BB ligands are skeletal modes, in the 1450–1600 cm−1 spectral region, that are slightly shifted to higher wavenumbers by the rigidity imposed by the metal coordination (see Table 2). The literature assignment of the ligands signals [32] indicates that those modes are mainly attributable to stretching mode of a specific ring: we can observe that signals related to phenyl ring (respectively, at 1603, 1612 and 1611 cm−1) maintain the same position in the spectra of the complexes, not being involved in coordination bonds, while modes of the pendant pyridine and imidazo[1,5-a]pyridine shift to higher energies after coordination. A similar behaviour is observed for breathing modes at around 1000 cm−1: In the spectrum [Zn2(BB)Cl4] a significative shift of ~20 cm−1 is observed, due to the formation of the two chelating rings [40], but in the spectrum of [Zn(AB)Cl2] the signal at 980 cm−1 maintains the same position, and so can be assigned to phenyl breathing mode, not involved in coordination. In the FIR spectra of the complexes, there are two strong peaks, which are absent in ligand ones, attributable to Zn–Cl stretching modes, and shifted at higher wavenumbers (~90 cm−1) with respect to mode of starting ZnCl2 (see Figure S5) [41]. A weak band is present in all three complexes at 230 cm−1 and can be assigned to the stretching Zn–N vibration, in accordance with literature [41].

2.5. Mass Spectrometry

Mass spectrometric characterization of the [Zn(B)Cl2] complex was already reported in a previous paper [33]. ESI-MS mass spectra of the [Zn(AB)Cl2] complex were recorded by using methanol as solvent. The [Zn(AB)Cl2] complex readily loses a Cl ligand, yielding the [Zn(AB)Cl]+ cation at m/z = 564. This ion was mass selected and subjected to MS/MS experiments; a representative spectrum is reported in Figure 6. The ion complex resulted quite stable and high voltage was required to provide fragmentation. As a result, ligand fragmentation occurred as well, and the ions produced are clearly detected in the spectra. The signal at m/z = 464 is attributed to the protonated ligand, [AB + H]+. Fragmentation of the complex with bond cleavage in the ligand occurs with formation of the C11N2H8 neutral moiety; its protonated form gives rise to a small peak at m/z = 169. The conceivable structures of both neutral and ion moieties formed by this bond cleavage are reported in Figure 6. Loss of the C11N2H8 fragment should produce a peak at m/z = 396; instead, the MS/MS mass spectra feature two peaks at m/z = 397 and 400, respectively. As reduction processes are frequently observed in ESI-MS, we assume that the ion fragment formed by the neutral loss acquires one or up to four hydrogen atoms. This gives rise to the peaks observed at m/z 397 and 400. The peak at m/z = 495 is assigned to the ionized AB ligand coordinated to a methanol molecule. Finally, the peak at m/z 232 is assigned to the doubly charged AB ligand.
The [Zn2(BB)Cl4] complex was much less soluble in solvents suitable for the ESI-MS device, and hence its mass spectrometric detection was rather difficult. However, a small peak was detected, which may be assigned to the [Zn2(BB)Cl3]+ ion coordinated to an ethanol molecule, since this species was used in the solvent mixture. Figure 7 reports the comparison between the experimental and the theoretical isotopic distributions. The agreement is not completely satisfying, probably due to the very low abundance of the experimental peak; therefore, this assignment should bear a note of caution.

3. Experimental Details

Materials and Techniques
All solvents and raw materials were used as received from commercial suppliers (Merk, Alfa Aesar (Haverhill, MA, USA), BASF (Ludwigshafen, Germany)) without further purification. TLC was performed on Fluka (Buchs, Switzerland) silica gel TLC-PET foils GF 254, particle size 25 nm, medium pore diameter 60 Å. Column chromatography was performed on Sigma-Aldrich (St. Louis, MO, USA) silica gel 60 (70–230 mesh).
1H and 13C NMR spectra were recorded on a Bruker (Billerica, MA, USA) Avance 200 spectrometer (1H NMR operating frequency 400 MHz), with chemical shifts referenced to residual protons of the solvent. The following abbreviations are used: s (singlet), d (doublet), t (triplet), dd (doublet of doublets), m (multiplet).
Mass spectra were recorded on a Thermo-Finnigan (San Jose, CA, USA) Advantage Max Ion Trap Spectrometer equipped with an electrospray ion source (ESI) in positive and negative ion acquisition mode.
UV–Vis absorption spectra were recorded on a Cary600 spectrometer (Agilent, Santa Clara, CA, USA). Emission absorption spectra were recorded on a Varian Cary Eclipse spectrometer (Agilent, Santa Clara, CA, USA). Relative quantum yield measurements were performed by comparing the integrated fluorescence intensities of the sample and a standard fluorophore of known quantum yield. The spectral response was corrected for the spectral sensitivity of the photomultiplier.
FT-Raman spectra were obtained with a Bruker Vertex 70 spectrometer, equipped with the RAMII accessory, by exciting with a 1064 nm laser. FT-ATR spectra in the Far Infrared (FIR) region were recorded with the same instrument, equipped with a Harrick MVP2 ATR cell and DTGS detector (Harrick Scientific Products, Inc., Pleasantville, NY, USA). The adopted resolution was equal to 4 cm−1 in all cases.
Single crystal of ligand AB was analyzed with a Gemini R Ultra diffractometer (Oxford Diffraction, Oxfordshire, UK) operating at 293(2) K, using a Cu-Kα source, λ = 1.54060 Å. Data collection and reduction were performed using the CrysAlisPro v42 software. The crystal structure was solved by direct methods and refined with the full matrix least-squares technique on F2 using the SHELXS-97 and SHELXL-97 programs. All non-hydrogen atoms were refined anisotropically; hydrogen atoms were placed in geometrical positions and refined using the riding model. Visualization of crystal structures has been performed using Mercury.
The crystallographic data for ligand AB have been deposited within the Cambridge Crystallographic Data Centre as supplementary publications, under the CCDC numbers 248201. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 25 June 2025), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

4. Conclusions

In this study, a novel series of luminescent imidazo[1,5-a]pyridine ligands and their corresponding Zn(II) complexes were synthesized and extensively characterized through spectroscopic, crystallographic, and mass spectrometric techniques. The design strategies, including variations in substituent patterns, coordination geometries, and π-conjugation extension, demonstrated a significant impact on the optical behaviour of the ligands and their metal complexes. Upon Zn(II) coordination, a marked increase in molecular rigidity and structural planarity was observed, correlating with an enhanced quantum yield and a distinct hypsochromic shift in emission spectra. These findings are consistent with the occurrence of intra-ligand transitions and the structural transformation of the ligand from transoid to cisoid conformations upon metal coordination. The study revealed that substitution with pyridine units not only modulates the electronic transitions but also facilitates metal chelation, leading to improved emission performance. Furthermore, bis-imidazo[1,5-a]pyridines, designed with multiple imidazo[1,5-a]pyridine units, retained their individual photophysical identities in the corresponding Zn(II) complexes, confirming the modularity and optical resilience of the system. The vibrational and mass spectrometric data further supported the coordination modes and stability of the synthesized complexes. Overall, this work elucidates key structure–property relationships that can guide the rational design of advanced Zn(II)-based luminescent materials for possible applications where low toxicity and emission tunability are critically desired. Future studies will address the solid-state properties and morphological features of the synthesized complexes, as these aspects are expected to play a crucial role in tuning the emission behaviour in thin films or solid-state devices. Such investigations will be essential for advancing the practical application of these materials in optoelectronic technologies, including OLEDs, sensors, and bioimaging.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13090283/s1, Figure S1. Asymmetric unit of AB; Table S1. Crystal data and structure refinement for AB. Table S2. Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for AB. Ueq is defined as 1/3 of of the trace of the orthogonalised UIJ tensor. Table S3. Anisotropic Displacement Parameters (Å2 × 103) for AB. The Anisotropic displacement factor exponent takes the form: -2π2[h2a*2U11+2hka*b*U12+…]. Table S4. Bond Lengths for AB. Table S5. Bond Angles for AB. Figure S2. ATR vibrational spectra of AB and the corresponding Zn(II) complexes. Figure S3. Raman vibrational spectra of AB and the corresponding Zn(II) complexes. Figure S4. ATR vibrational spectra of BB and the corresponding Zn(II) complexes. Figure S5. Vibrational spectra of ZnCl2 and the studied Zn(II) complexes: [Zn(AB)Cl2] and [Zn2(BB)Cl4]. Synthetic procedure for [Zn(AB)Cl2] and [Zn2(BB)Cl4]. Figure S6. Photographs of the obtained complexes in the solid state (powder): under ambient light (top) and under UV illumination at 254 nm (bottom). Figure S7. Photographs of the obtained complexes in the solid state (powder): under ambient light (top) and under UV illumination at 364 nm (bottom). Figure S8. Excitation and emission spectra of A in dichloromethane (about 10−5 M). Figure S9. Excitation and emission spectra of AA in dichloromethane (about 10−5 M). Figure S10. Excitation and emission spectra of AB in dichloromethane (about 10−5 M). Figure S11. Excitation and emission spectra of [Zn(AB)Cl2] in dichloromethane (about 10−5 M). Figure S12. Excitation and emission spectra of B in dichloromethane (about 10−5 M). Figure S13. Excitation and emission spectra of [Zn(B)Cl2] in dichloromethane (about 10−5 M). Figure S14. Excitation and emission spectra of BB in dichloromethane (about 10−5 M). Figure S15. Excitation and emission spectra of [Zn2(B)Cl4] in dichloromethane (about 10−5 M).

Author Contributions

Conceptualization, G.V.; Software, E.P.; Formal analysis, R.R.; Investigation, G.V.; Resources, E.D.; Data curation, E.P. and R.R.; Writing—original draft, G.V.; Writing—review & editing, A.G. and R.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors acknowledge support from Project CH4.0 under the MUR program “Dipartimenti di Eccellenza 2023–2027” (CUP: D13C22003520001).

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 13 00283 g001
Figure 1. General structures of studied imidazo[1,5-a]pyridine ligands and corresponding Zn(II) complexes (in blue unit B = 1-pyridinimidazo[1,5-a]pyridine and in green unit A = 1-phenylimidazo[1,5-a]pyridine units).
Figure 1. General structures of studied imidazo[1,5-a]pyridine ligands and corresponding Zn(II) complexes (in blue unit B = 1-pyridinimidazo[1,5-a]pyridine and in green unit A = 1-phenylimidazo[1,5-a]pyridine units).
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Inorganics 13 00283 g002
Figure 2. Normalized absorption spectra for the studied imidazo[1,5-a]pyridine ligands (A, AA, AB, B, BB) on the (left) and the corresponding Zn(II) complexes ([Zn(AB)Cl2], [Zn(B)Cl2], [Zn2(BB)Cl4]) on the (right) (dichloromethane solutions).
Figure 2. Normalized absorption spectra for the studied imidazo[1,5-a]pyridine ligands (A, AA, AB, B, BB) on the (left) and the corresponding Zn(II) complexes ([Zn(AB)Cl2], [Zn(B)Cl2], [Zn2(BB)Cl4]) on the (right) (dichloromethane solutions).
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Inorganics 13 00283 g003
Figure 3. Normalized emission spectra of the studied imidazo[1,5-a]pyridine ligands (A, AA, AB, B, BB) on the (left) and the corresponding Zn(II) complexes ([Zn(AB)Cl2], [Zn(B)Cl2], [Zn2(BB)Cl4]) on the (right) (dichloromethane solutions).
Figure 3. Normalized emission spectra of the studied imidazo[1,5-a]pyridine ligands (A, AA, AB, B, BB) on the (left) and the corresponding Zn(II) complexes ([Zn(AB)Cl2], [Zn(B)Cl2], [Zn2(BB)Cl4]) on the (right) (dichloromethane solutions).
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Figure 4. Molecular structure (a) and crystal packing (b) of ligand AB (grey: carbon; white: hydrogen; blue: nitrogen—ORTEP plot 80%).
Figure 4. Molecular structure (a) and crystal packing (b) of ligand AB (grey: carbon; white: hydrogen; blue: nitrogen—ORTEP plot 80%).
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Figure 5. Vibrational spectra of Zn(II) complexes: [Zn(AB)Cl2] (a), [Zn2(BB)Cl4] (b).
Figure 5. Vibrational spectra of Zn(II) complexes: [Zn(AB)Cl2] (a), [Zn2(BB)Cl4] (b).
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Figure 6. MS/MS spectrum of the [Zn(AB)Cl]+ cation (m/z = 564) with conceivable structures of fragments originated by one of the dissociation pathways of the [Zn(AB)Cl]+ cation.
Figure 6. MS/MS spectrum of the [Zn(AB)Cl]+ cation (m/z = 564) with conceivable structures of fragments originated by one of the dissociation pathways of the [Zn(AB)Cl]+ cation.
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Figure 7. Experimental (above) and theoretical (below) isotopic distributions of the [Zn2(BB)Cl3(EtOH)]+ ion.
Figure 7. Experimental (above) and theoretical (below) isotopic distributions of the [Zn2(BB)Cl3(EtOH)]+ ion.
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Table 2. Raman and Far IR (in italic) signals of ligands and related ZnCl2 complexes.
Table 2. Raman and Far IR (in italic) signals of ligands and related ZnCl2 complexes.
B[Zn(B)Cl2]BB[Zn2(BB)Cl4]AB[Zn(AB)Cl2]Assignment
1631 m1638 w1630 w1635 vw1632 w1638 w,shνC―N, νC―C
1603 s1604 vs1612 s1614 vs1611 vs 1611 s
1588 s1585 m1586 m
1533 vs1547 s 1548 m 1549 m
1523 s1533 s1528 m 1532 m1533 m
1507 s1513 vs1510 vs 1515 m1511 s1516 m
998 w
980 m		1023 m991 w
980 m		1028 w1000 w
980 m		1028 wνC―C
1018 w1016 w
1012 m1011 w996 m
994 s995 m980 w
331 s 315 m 324 mνZn―Cl
308 s 275 w 306 m
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Volpi, G.; Giordana, A.; Priola, E.; Rabezzana, R.; Diana, E. Luminescent Imidazo[1,5-a]pyridine Cores and Corresponding Zn(II) Complexes: Structural and Optical Tunability. Inorganics 2025, 13, 283. https://doi.org/10.3390/inorganics13090283

AMA Style

Volpi G, Giordana A, Priola E, Rabezzana R, Diana E. Luminescent Imidazo[1,5-a]pyridine Cores and Corresponding Zn(II) Complexes: Structural and Optical Tunability. Inorganics. 2025; 13(9):283. https://doi.org/10.3390/inorganics13090283

Chicago/Turabian Style

Volpi, G., A. Giordana, E. Priola, R. Rabezzana, and E. Diana. 2025. "Luminescent Imidazo[1,5-a]pyridine Cores and Corresponding Zn(II) Complexes: Structural and Optical Tunability" Inorganics 13, no. 9: 283. https://doi.org/10.3390/inorganics13090283

APA Style

Volpi, G., Giordana, A., Priola, E., Rabezzana, R., & Diana, E. (2025). Luminescent Imidazo[1,5-a]pyridine Cores and Corresponding Zn(II) Complexes: Structural and Optical Tunability. Inorganics, 13(9), 283. https://doi.org/10.3390/inorganics13090283

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