Design, Synthesis, and Investigation of the Photoelectric Properties of Glaucine Derivatives in Sensitized Solar Cells

Abstract

Two Zn(II) coordination compounds based on glaucine-derived Schiff bases were synthesized and investigated as potential materials for dye-sensitized solar cells (DSSCs). The structures of all compounds were established by X-ray diffraction analysis and quantum chemical modeling (DFT/TD-DFT). Their photophysical properties (absorption and luminescence spectra in solution and the solid state), electrochemical characteristics, and photovoltaic parameters in DSSC devices were studied. The highest power conversion efficiency (PCE ~5.18%) was demonstrated by the free ligands, which is attributed to their favorable absorption spectrum and optimal alignment of energy levels relative to the conduction band of TiO₂ and the redox couple of the electrolyte. The Zn(II) coordination compounds exhibited significantly lower efficiency (~2.1%). Impedance spectroscopy results indicated more efficient charge transfer at the TiO₂/dye/electrolyte interface for the organic derivatives.

1. Introduction

The need for environmentally friendly and renewable energy sources capable of meeting humanity’s growing energy demands is one of the most pressing challenges of our time. Solar energy, based on the direct conversion of solar radiation into electricity, plays a central role in addressing this challenge [1,2]. Despite the dominance of silicon-based photovoltaic technologies (first generation), their widespread adoption is limited by the high cost of materials and the energy-intensive production processes for high-purity silicon [3]. This has driven the development of alternative approaches, including thin-film solar cells (second generation) and emerging third-generation photovoltaic technologies [4].

The latter includes dye-sensitized solar cells (DSSCs), first reported by O’Regan and Grätzel in 1991 [5]. Their operating principle differs fundamentally from classical semiconductor p–n junctions. In DSSCs, the functions of light absorption and charge transport are separated: a photon is absorbed by a sensitizer molecule adsorbed onto the surface of a mesoporous oxide electrode (typically TiO₂), after which the excited electron is injected into the conduction band of the semiconductor. The oxidized dye is regenerated by a redox couple in the electrolyte (traditionally I⁻/I₃⁻), which is in turn reduced at the counter electrode [6,7]. Key advantages of DSSCs include the relatively low cost of materials and fabrication, the ability to operate under diffuse light conditions, and the potential for flexible, semi-transparent, and aesthetically appealing device architectures—offering opportunities for integration into building materials (BIPV) and wearable electronics [8].

The power conversion efficiency of a DSSC depends directly on the sensitizer’s ability to efficiently absorb sunlight over a broad spectral range and to inject electrons into the semiconductor. The energy levels of the sensitizer—the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)—must be optimally aligned with the conduction band of the semiconductor and the redox potential of the electrolyte [9]. Historically, the most efficient sensitizers have been ruthenium complexes and porphyrins, achieving PCEs exceeding 11–14% [10,11]. In recent years, however, research focus has shifted toward fully organic (metal-free) sensitizers. These materials contain no scarce or expensive metals, exhibit high molar extinction coefficients, and allow fine-tuning of their optical and electronic properties through molecular design—typically employing donor–π–bridge–acceptor (D–π–A) architectures [12,13,14]. Parallel efforts are intensively pursuing the improvement of other DSSC components: the replacement of conventional iodide-based electrolytes with more efficient cobalt or copper complexes to increase open-circuit voltage (Voc); the development of solid-state and gel-based hole-transport materials to address evaporation and leakage issues; and the design of novel photoanode and counter-electrode materials with enhanced morphological and charge-transport characteristics.

Of particular interest for the development of new DSSCs are molecules with natural or biomimetic structures, which may combine synthetic accessibility, unique optical properties, and potential for further functionalization. The alkaloid glaucine, possessing an extended aromatic system and sites amenable to functionalization, represents a promising chromophoric core for the design of a new class of sensitizers.

A comprehensive assessment of the practical potential of new materials requires moving beyond their initial laboratory characteristics and studying their behavior under real-world operating conditions. Environmental factors have a critical impact on the long-term stability of organic sensitizers. As highlighted in recent studies, the urban microclimate and local anthropogenic heat emissions create complex thermal landscapes that can significantly accelerate material degradation [15]. In this context, understanding the nonlinear mechanisms of heat accumulation becomes a prerequisite for predicting the spatiotemporal durability of organic photovoltaic devices in diverse geographical settings [16]. Furthermore, the development of environmentally benign dye derivatives is of key importance for implementing carbon footprint reduction strategies, particularly in their integration into facade systems designed to improve the energy efficiency of residential buildings [17].

The aim of the present work is the synthesis of a series of novel glaucine derivatives, investigation of their photophysical and electrochemical characteristics, and evaluation of their performance as sensitizers in DSSCs. The obtained data will enable the establishment of structure–property relationships for this class of compounds and identification of the most promising directions for their further optimization in the context of developing efficient and cost-effective photovoltaic devices.

2. Results and Discussion

2.1. The IR and 1H NMR Spectra

¹H NMR spectra of azomethines 1, 2 and their zinc(II) complexes 3, 4 allowed for the complete assignment of all proton signals (Figures S5–S8). For 1, the NH proton signal is observed at 11.66 ppm, and the CH=N proton signals appear at 8.93 and 9.12 ppm. For 2, the OH proton signal appears at 11.60 ppm, and the CH=N proton signals are observed at 9.18 and 9.22 ppm. Upon formation of zinc complexes 3 and 4, the signals corresponding to the NH and OH protons of the ligands, respectively, disappear. The CH=N proton signals in the zinc complexes are observed at 9.05, 9.15 ppm (for 3) and 9.11, 9.20 ppm (for 4) and remain essentially unchanged compared to those of the free ligands.

The IR spectra of compounds 1 and 2 (Figures S10 and S11) exhibit characteristic absorption bands, cm⁻¹: 2990 w (NH), 1597 s (CH=N), 1345 s (SO₂), 1168 s (SO₂) for 1; and 2928 w (OH), 1618 s, 1304 m (Ph–O) for 2. The zinc complexes 3 and 4 obtained from these ligands have a ZnL₂ composition according to elemental analysis. Their IR spectra (Figures S12 and S13) show characteristic changes indicative of chelate formation: the ν(C=N) absorption bands are slightly shifted to lower frequencies—1594 cm⁻¹ for 3 and 1603 cm⁻¹ for 4. The ν_as(SO₂) band in zinc complex 3 shifts to 1298 cm⁻¹, and the ν(SO₂) band to 1129 cm⁻¹. The ν(C=N) absorption band of zinc complex 4 is shifted by 15 cm⁻¹ to lower frequency relative to 2, appearing at 1603 cm⁻¹.

2.2. Single-Crystal X-Ray Characterization

Both compounds 1 and 2 are molecular organic crystals (Figure 1). Compound 1 crystallizes in the triclinic system, space group $P\overline{1}$ (2), while compound 2 crystallizes in the monoclinic system, space group $P{2}_1/n$ (14). The core of both molecules 1 and 2 consists of four fused rings forming a rigid planar framework. The six-membered ring C3C13C7C6C5N3 adopts an «armchair» conformation, with atom C5 displaced from the mean plane of the ring by 0.767 Å in 1 and 0.683 Å in 2. Four methoxy groups (–OCH₃) are attached to the aromatic rings at positions C9, C11, C17, and C18 and are not coplanar with the aromatic system. The phenyl rings (C24–C29) of the aldehyde moiety are twisted relative to the glaucine skeleton by 27.30° and 32.29° in 1 and 2, respectively. Selected bond lengths and angles for 1 and 2 are listed in

Table 1. Experimental and theoretical bond lengths (Å) and angles (º) in complexes 1 and 2.

Compound	1		2
Bond Lengths, Å
Bond	Exp.	Theory	Exp.	Theory
C1–C2	1.464(2)	1.451	1.467(2)	1.458
C1–N2	1.288(2)	1.296	1.276(2)	1.291
N1–N2	1.414(2)	1.387	1.405(1)	1.388
N1–C23	1.289(2)	1.288	1.281(2)	1.291
C23–C24	1.464(2)	1.458	1.451(2)	1.448
N4–C29/O5–C29	1.426(2)	1.404	1.350(2)	1.341
Angles, degr.
N1–C23–C24	125.0(1)	123.5	122.0(1)	122.2
N2–N1–C23	109.9(1)	115.2	111.6(1)	114.2
C1–N2–N1	112.9(1)	110.9	112.9(1)	112.2
C2–C1–N2	124.6(1)	127.9	123.3(1)	118.9
H6⋯N1–N2/H5⋯N1–N2	153.7	143.3	150.6	146.1
H6⋯N1–C23/H5⋯N1–C23	96.3	101.0	97.6	99.7

. The data in Table 1 clearly show that these geometric parameters are similar for the central fragment of the molecules, which contains the azomethine bond (CH=N).

Compound 4 is a monomeric zinc(II) complex bearing two molecules of the glaucine derivative 2. The zinc atom in 4 is coordinated by two nitrogen and two oxygen atoms from two different bidentate ligands, forming six-membered chelate rings (Figure 2).

The resulting coordination polyhedron ZnN₂O₂ is close to a slightly distorted tetrahedron, as can be seen from the geometric index τ₄ = 0.93 [18], which was determined by Formula (1):

$${\mathrm{\tau }}_4=\frac{{360}^{°}-(\mathrm{\alpha }+\mathrm{\beta })}{{360}^{°}-\mathrm{2}\mathrm{\theta }}$$

(1)

where α and β are the largest bond angles at the coordination center, and ϴ ≈ 109.5° is the tetrahedral angle. The bond lengths and bond angles at the coordination center are given in

Table 2. Experimental and theoretical bond lengths (Å) and angles (°) in complex 4.

Bond Lengths, Å			Angles, Degr.
Bond	Exp.	Theory		Exp.	Theory
Zn1–N1	2.010(1)	2.030	N1–Zn1–N2	116.88(5)	132.6
Zn1–N2	2.002(1)	2.030	N1–Zn1–O1	96.43(5)	93.8
Zn1–O1	1.906(1)	1.938	N1–Zn1–O2	114.64(5)	109.3
Zn1–O2	1.917(1)	1.943	N2–Zn1–O1	121.38(5)	111.0
N1–N3	1.402(1)	1.394	N2–Zn1–O2	96.41(5)	93.8
N2–N5	1.411(2)	1.395	O1–Zn1–O2	112.29(5)	118.5

. The average Zn–N and Zn–O distances are 2.006 Å and 1.912 Å, respectively, which are in good agreement with the data reported for analogous zinc complexes based on salicylaldehyde derivatives [19,20,21,22].

Density functional theory (DFT) calculations were performed to obtain optimized structures of compounds 1, 2, and 4, which were compared with X-ray diffraction data (Table 1 and Table 2). Correlation plots between the experimental and calculated structures for compounds 1, 2, and 4 are shown in Figures S1–S3 in the Supporting Information.

The comparison demonstrates that the computational method reproduces the crystallographic structures with high accuracy. A strong correlation between experimental and calculated bond lengths is observed (Figures S1–S3), with correlation coefficients R² = 0.9873 for 1, 0.9947 for 2, and 0.9633 for 4. Good agreement was also achieved for bond angles, with R² = 0.9408 (1), 0.9639 (2), and 0.8037 (4). The mean absolute deviations between calculated and experimental values for bond lengths and bond angles in compounds 1 and 2 are approximately 0.02 Å and 0.6°, respectively. For complex 4, these values are slightly larger, amounting to 0.05 Å and 1.6°.

The crystal structures of all compounds feature an extensive network of intermolecular hydrogen bonds (Figure 3). The characteristics of intra- and intermolecular hydrogen bonds are summarized in

Table 3. Parameters of selected hydrogen bonds in 1, 2 and 4.

H-Bonds	A⋯H, Å	D–A, Å	D–H⋯A, Degr.	Symmetry Code
1
N4–H⋯N1	2.093	2.797	137.2	x,y,z
C14–H⋯O3	2.081	2.784	129.6	x,y,z
C19–H⋯N2	2.164	2.863	129.5	x,y,z
C28–H⋯O5	2.447	2.947	112.7	x,y,z
C14–H⋯O4	2.499	3.010	112.2	x,y,z
2
O5–H⋯N1	1.939	2.658	146.1	x,y,z
C19–H⋯N2	2.263	2.905	125.7	x,y,z
C16–H⋯O3	2.096	2.777	129.0	x,y,z
C14–H⋯O4	2.295	2.861	117.0	x,y,z
4
C8–H⋯N3	2.114	2.816	129.5	x,y,z
C5–H⋯O5	2.064	2.769	129.7	x,y,z
C38–H⋯O9	2.102	2.785	127.5	x,y,z
C40–H⋯O10	2.345	2.905	115.5	x,y,z
C35–H⋯N5	2.213	2.878	126.5	x,y,z
C19–H⋯O6	2.446	2.972	113.2	x,y,z

. In compound 1, intramolecular hydrogen bonds N4–H4⋯N1, C19–H19⋯N2, and C16–H16⋯O3 play a key role in stabilizing the molecular conformation. In compound 2, strong intramolecular hydrogen bonds O5–H5⋯N1 and C16–H16⋯O3 serve an analogous function. Complex 4 also retains these hydrogen bonds characteristic of compound 2. These interactions contribute significantly to the flattened shape of the molecules. Such a planar conformation of molecules 1, 2 and 4 facilitates the formation of a layered crystal packing via intermolecular hydrogen bonds, as shown in Figure 3.

Intermolecular interactions in compounds 1, 2, and 4 were further analyzed by calculating the corresponding Hirshfeld surfaces [23,24,25,26]. Key topological parameters of the molecules—molecular volume (V), surface area (S), sphericity (G), and asphericity (Ω)—are listed in

Table 4. Hirshfeld surface shape characteristics for 1, 2 and 4.

Compounds	V, Å3	S, Å2	G	Ω
1	772.83	625.45	0.651	0.208
2	634.15	529.84	0.674	0.268
4	1283.57	945.30	0.604	0.020

.

According to the data in Table 4, the formation of complex 4 leads to a substantial increase in both molecular volume (by a factor of 2.02) and surface area (by a factor of 1.78) compared to azomethine 2. This increase is due to the incorporation of the Zn(II) ion and the ligands of its coordination sphere into the structure. Although azomethine 1 has larger absolute dimensions (V and S) than azomethine 2, it exhibits a different steric organization, which is reflected in its shape parameters. The sphericity parameter G reflects the evolution of geometric compactness. The higher G value for 2 (0.674) compared to azomethine 1 (0.651) indicates greater isotropy and more efficient space filling. In complex 4, the G value decreases to 0.604, suggesting the formation of a structure with an increased surface area-to-volume ratio. This is characteristic of a less dense molecular packing, which may offer greater potential for the formation of intermolecular contacts. The most significant transformation is observed for the asphericity parameter Ω. The high asphericity values of azomethines 2 (Ω = 0.268) and 1 (Ω = 0.208) are indicative of pronounced shape anisotropy and may lead to the formation of a specific type of packing arrangement, such as bilayer structures. Coordination results in a dramatic decrease in Ω for complex 4, down to 0.020, pointing to a transition toward a quasi-spherical, highly isotropic geometry. This is a direct consequence of the symmetric coordination sphere of the metal, which effectively compensates for the inherent elongation of azomethine 2.

Figure 4 shows the Hirshfeld surfaces (d_norm) mapped for molecules 1, 2, and 4. Different types of intermolecular contacts are indicated by color coding: contacts corresponding to the sum of van der Waals radii are shown in white; short hydrogen bonds are shown in red; and long, weak interactions are shown in blue.

Fingerprint plot analysis revealed that H⋯H contacts account for 48.1% (1), 50.8% (2), and 52.7% (4) of the total surface area, while C⋯H/H⋯C and O⋯H/H⋯O contacts contribute approximately 23.9% and 17.6%, respectively, for all complexes. These interactions make a significant contribution to the stability of the molecular structure and crystal packing (Figure 4).

In the aromatic systems of compounds 1 and 4, intermolecular π–π stacking interactions were identified and quantitatively characterized (

Table 5. Parameters of π⋯π- interactions in crystal packaging 1 and 4 (Cgⁱ—the centroid of the phenyl cycle; Cg-Perp—shortest distance from Cgi to j- planes of the neighboring cycle, α—the angle between the vector Cgⁱ–Cg^j and normal to j- plane, the shift is the distance between Cgⁱ and perpendicular projection Cg^j on the i- plane).

Interactions Cgi⋯Cgj	Cgi⋯Cgj, Å	Cg-Perp, Å	α, degr.	Shift, Å	Symmetryi
1
Cg(C7–C9, C11–C13)⋯Cg(C7–C9, C11–C13)i	3.559	3.505	0.0	0.618	1 − x, 2 − y, −z
Cg(C7–C9, C11–C13)⋯Cg(C2–C3, C12–C13, C15, C20)i	3.977	3.517	5.9	1.856	1 − x, 2 − y, −z
4
Cg(C33–C38)⋯Cg(C25, C26, C29–C32)i	3.669	3.544	6.3	1.332	−x, −y, 1 − z
Cg(C43–C48)⋯Cg(C2–C4, C9, C10, C17)i	3.783	3.618	12.6	1.779	−1 + x, y, z
Cg(C23–C25, C32–C34)⋯Cg(C23–C25, C32–C34)i	3.787	3.473	0.0	1.511	−x, −y, 1 − z
Cg(C23–25, C32–C34)⋯Cg(C33–C38)i	3.911	3.539	4.6	1.666	−x, −y, 1 − z

, Figure 5c,f). In structure 1, dimerization of the aromatic systems is observed, with three centroid–centroid (Cg⋯Cg) distances between the rings of the glaucine skeleton of 3.559 and 3.977 Å (Table 5, Figure 5c). In complex 4, four stacking interactions are present, also involving the rings of the glaucine skeleton, forming complex layered structures that contribute to the stabilization of the crystal lattice (Table 5, Figure 5f).

For complexes 1 and 4, the Hirshfeld surface curvature (Figure 5a,d) is characterized by extensive green flat regions, indicating a planar molecular arrangement. On the shape-index maps for complexes 1 and 4 (Figure 5b,e), pairs of red and blue triangles corresponding to these flat regions can be visualized, confirming the presence of strong π–π interactions (Table 5).

2.3. Electronic Absorption Spectra of Compounds

The UV–Vis absorption spectra of compounds 1–4 were examined both experimentally and theoretically using time-dependent density functional theory (TD-DFT). All compounds exhibit three distinct absorption bands in the ultraviolet and visible regions (Figure 6,

Table 6. Parameters of optical properties of compounds 1–4 in solution and the solid state.

Comp.	Absorption, CH2Cl2	Luminescence
		Solution in CH2Cl2			Solid Samples
	λ, nm	λ, nm	Lifetime, ns	Quantum Yield, %	λ, nm	Lifetime, ns	Quantum Yield, %
1	334, 406, 505	600	1.7	4.8	623	1.8	8.8
2	335, 410, 495	625	1.7	1.9	604	1.9	2.7
3	335, 406, 506	572	1.8	5.9	609	1.9	10.6
4	336, 410, 497	581	1.7	1.6	436, 601	1.6	1.3

). TD-DFT calculations, performed with an implicit solvation model for dichloromethane (CH₂Cl₂), enabled reliable assignment of the electronic transitions underlying each absorption feature.

The low-energy band (A, ~436–443 nm) in the spectra of the free ligands 1 and 2 corresponds to the HOMO→LUMO electronic transition. Analysis of the frontier orbital shapes (Figure 7a,b) indicates that this transition is an intra-ligand charge transfer (ILCT) of π→π type. The electron density is redistributed from a π-orbital delocalized over the aromatic ring system of the glaucine moiety to a π*-orbital localized predominantly on the aldehyde part of the ligand. The bands in the mid-UV region (B, ~336–340 nm) also arise from π→π* transitions. The higher-energy bands (C) have a mixed character, involving n→π* and π→π* transitions within the aromatic or heterocyclic fragments of the molecular framework.

Despite the similarity of the electronic absorption spectra of all compounds, the long-wavelength band A in the spectra of zinc complexes 3 and 4 undergoes a slight hypsochromic shift of about 10 nm relative to the corresponding band in the spectra of the free ligands, which is attributed to complexation.

TD-DFT calculations predict several electronic transitions in the visible and UV regions of the spectra for zinc complexes 3 and 4 (

Table 7. The experimental wavelengths (λ_exp), calculated wavelengths (λ_cal), energies (E), and oscillator strengths (f), involved molecular orbitals and their contributions for different electronic transitions in compounds 1–4 according to TD-DFT calculations.

Compound	λexp, nm	λ, nm	E, eV	Electronic Transitions, (Contribution, %)	f	Character
1	505 (A)	454.0	2.73	H→L (97%)	0.58	πGl→π*Ald,N
	406 (B)	386.7	3.21	H-1→L(92%)	0.33	πGl→π*Ald,N
	334 (C)	337.3	3.68	H→L + 3(43%) H→L + 1(20%)	0.40	n→π*Gl πGl,N→π*Ald,N
2	506 (A)	438.5	2.83	H→L (96%)	0.59	πGl→π*Ald,N
	406 (B)	379.0	3.27	H-1→L(91%)	0.37	πGl→π*Ald,N
	335 (C)	337.7	3.67	H→L(34%) H→L + 2(33%) H-3→L(33%)	0.39	πGl→π*Ald,N πGl→π*Ald πAld→π*Gl,N
3	495 (A)	483.9	2.56	H-1→L(92%)	0.41	πGl1→π*Ald1
		478.4	2.59	H→L + 1(95%)	0.34	πGl2→π*Ald2
	410 (B)	414.7	2.99	H-3→L(54%) H-2→L(37%)	0.18	πGl1,Ald1→π*Ald2, Gl2 πGl1,Ald1→π*Ald2, Gl2
		413.6	3.00	H-2→L + 1(89%)	0.31	πGl2,Ald2→π*Ald1, Gl1
		408.7	3.03	H-2→L(72%) H-3→L (34%)	0.15	πGl1,Ald1→π*Ald2, Gl2 πGl1,Ald1→π*Ald2, Gl2
	335 (C)	329.1	3.77	H-8→L (58%)	0.11	πGl1,2→π*Ald1,2
4	497 (A)	451.0	2.75	H→L (45%) H-1→L(30%)	0.41	πGl1→π*Ald2,Gl2 πGl1,2→π*Ald1,2
		444.8	2.79	H→L + 1 (64%) H-1→L(28%)	0.81	πGl1→π*Ald1 πGl1,2→π*Ald1,2
	410 (B)	395.8	3.13	H-2→L (58%) H-3→L (58%)	0.24	πGl1,Ald1→π*Ald2, Gl2 πGl1,Ald1→π*Ald2, Gl2
		393.1	3.15	H-2→L + 1 (49%) H-3→L + 1 (27%)	0.30	πGl2,Ald2→π*Ald1, Gl1 πGl2,Ald2→π*Ald1, Gl1
	336 (C)	338.3	3.67	H→L + 2 (32%) H-1→L + 2 (23%)	0.17	πGl1→π*Ald2,Gl2 πGl1,2→π*Ald1,2
		331.6	3.74	H-1→L + 4 (46%)	0.15	πGl1,Ald2→π*Gl2

). The most intense bands in the visible region (~495–497 nm) correspond to electronic transitions from the HOMO (HOMO, HOMO–1) to the LUMO (LUMO, LUMO+1). Analysis of the molecular orbital isosurfaces (Figure 8) indicates that these transitions are predominantly of interligand charge transfer (LLCT, π→π*) character. The absence of significant metal-to-ligand charge transfer (MLCT) transitions is expected, as the Zn(II) ions have a d¹⁰ configuration.

Analysis of the energy levels and shapes of the frontier orbitals (HOMO, LUMO) for 1–4 (Figure 7 and Figure 8) shows that, in general, the HOMO is localized on the glaucine fragment, while the LUMO is centered on the azomethine and aldehyde moieties of the ligand. This confirms that the low-energy electronic transitions occur with significant involvement of intra-ligand charge transfer between the donor (glaucine) and acceptor (azomethine fragment) modules of the molecule.

2.4. Photoluminescent Properties of the Compounds

The fluorescence maxima for compounds 1 and 3 in CH₂Cl₂ solution lie in the orange region (600, 625 nm), while those for compounds 2 and 4 lie in the yellow region (572, 581 nm) of the spectrum, respectively (Table 6, Figure 9).

For solid samples, a red shift (~20–30 nm) of the luminescence spectra is observed compared to solution. An exception is complex 3, for which a hypsochromic shift of 21 nm was recorded. The red shift of the fluorescence maxima in solid samples relative to solution is typically attributed to aggregation effects. For all compounds, both in solution and in the solid state, the fluorescence lifetimes are approximately 1.6–1.9 ns, which is characteristic of allowed singlet–singlet fluorescent transitions. The similarity of fluorescence lifetime values for compounds in different aggregation states is an important finding, indicating that intermolecular interactions in the solid state do not introduce new efficient excited-state quenching channels. Fluorescence quantum yields are noticeably higher (almost twofold) for compounds in the solid state compared to solutions, with the exception of 4, for which these values are close (1.3 and 1.6). It can also be noted that the quantum yields for the free ligands are higher than those for the zinc complexes, both in solution and in the solid state. The increase in quantum yield in the solid state for most samples is most likely associated with the restriction of molecular motions and the reduction of non-radiative relaxation.

2.5. Investigation of Photosensitization in Solar Cells

Based on the obtained photophysical and electronic properties, compounds 1–4 exhibit key characteristics that make them promising candidates for application as sensitizers in dye-sensitized solar cells (DSSCs). To evaluate the photovoltaic performance of the studied compounds 1–4, conventional DSSCs were fabricated (Figure 10).

For the fundamental assessment of the applicability of new organic compounds as photosensitizers, a comprehensive analysis of their redox and optical properties was performed (Figure 11). The key objective was the experimental determination of the energy positions of HOMO and LUMO relative to the energy levels of the semiconductor photoanode (TiO₂) and the redox electrolyte couple. The frontier orbital energy values and electrochemical properties of the synthesized dyes are summarized in

Table 8. HOMO and LUMO energy levels for compounds 1–4 determined by cyclic voltammetry.

Compounds	Eox, V	Energy HOMO, eV	Δ, eV	Energy LUMO, eV
1	0.79	−5.19	2.17	−3.02
2	1.04	−5.54	2.27	−3.27
3	0.81	−5.21	2.21	−3.00
4	1.05	−5.55	2.31	−3.24

.

The first oxidation potential (E_ox) recorded by cyclic voltammetry (Figure 11) corresponds to the removal of an electron from the HOMO of the dye. For conversion to the absolute energy scale (eV), the relationship E_HOMO = −(E_ox + 4.5)eV was used, correlating the potential relative to the standard hydrogen electrode with the energy level relative to vacuum.

The optical band gap (Δ), corresponding to the HOMO→LUMO transition, was determined from the long-wavelength edge of the absorption band in the UV-Vis region (Figure 6) using the formula ΔE = 1240/λ_edge (nm). The obtained Δ values ranged from 2.17 to 2.31 eV (Table 8), corresponding to an absorption edge in the visible region (~535–570 nm), characteristic of efficient sensitizers. The LUMO energy was calculated within the Franck–Condon model: E_LUMO = E_HOMO + Δ.

A critical condition for the operation of a dye-sensitized solar cell is the exothermicity of all charge transfer steps. Specifically, for efficient regeneration of the oxidized dye (D⁺) by the I⁻/I₃⁻ redox couple, the E_HOMO level of the dye must be more positive (lower in energy) than the redox potential of this couple. For all studied dyes 1–4, the E_HOMO levels lie in the range from −5.19 to −5.55 eV, which is below the accepted value of E_redox (I⁻/I₃⁻) = –4.80 –4.80 eV relative to vacuum. The more negative (lower) E_HOMO(D/D⁺) values for 1–4 relative to the redox potential of I⁻/I₃⁻ provide a positive thermodynamic driving force (Δ G_reg > 0.3 eV) for the reduction of the oxidized dye (D⁺ + I⁻ → D + I₃⁻). This condition promotes rapid dye regeneration, minimizing the accumulation of D⁺ and reducing recombination losses.

For successful electron injection from the excited state of the dye (D*) into the conduction band (CB) of TiO₂, the E_LUMO level of the dye must lie above the conduction band edge of TiO₂ (−4.00 eV). As can be seen from Table 8, the condition E_LUMO(D/D*) > E_CB(TiO₂) is satisfied for all dyes 1–4. The energy margin (ΔG_inj) ranges from ~0.7 to 1.0 eV, which is sufficient for efficient injection.

Thus, based on the electrochemical and spectral data, all studied dyes 1–4 meet the fundamental requirements for application in DSSCs. To finally evaluate their performance, photovoltaic characteristics (J–V curves, IPCE) were measured for the fabricated DSSCs. Figure 12a shows typical J–V curves, and

Table 9. The photovoltaic parameters of the DSSCs. JSC—short-circuit current density, VOC—open-circuit voltage, η—overall power conversion efficiency, FF- Fill Factor, Ohmic series resistance (R_S), charge transfer resistance (R_CT1) and recombination resistance (R_rec).

Compounds	Short-Circuit Current Density, JSC (mA cm−2)	Open-Circuit Voltage Voc (mV)	Overall Power Conversion Efficiency, η %	FF %	Rrec (Ω cm2)	RCT1 (Ω cm2)	Rs (Ω cm2)
1	11.24	633	4.83	72.2	11.74	4.88	21.44
2	12.18	675	5.18	73.8	11.90	4.75	21.31
3	5.74	628	2.12	70.7	22.51	4.26	21.80
4	7.61	634	2.08	70.4	24.95	4.20	21.74

summarizes their photovoltaic parameters.

The obtained photovoltaic data allow for a quantitative assessment of DSSC functionality and reveal performance differences among the various sensitizers (organic molecules 1, 2 and zinc coordination compounds 3, 4). As seen in Figure 12a and Table 9, organic sensitizers 1 (11.24 mA/cm²) and 2 (12.18 mA/cm²) exhibit significantly higher Jsc values compared to zinc coordination complexes 3 (5.74 mA/cm²) and 4 (7.61 mA/cm²), indicating their superior light-harvesting efficiency (LHE) within the working spectral range of solar radiation. The higher V_oc value of 2 (675 mV) compared to the other sensitizers (~630 mV) may suggest a more optimal alignment of its HOMO level relative to the redox potential of the electrolyte, minimizing thermodynamic losses during oxidized dye regeneration, as well as possible suppression of recombination processes between injected electrons and the oxidized form of the electrolyte or oxidized dye (e.g., due to molecular structure creating a spatial barrier). One of the main findings is the substantially higher (more than twofold) overall power conversion efficiency (PCE, η %) of the organic sensitizers compared to the zinc coordination complexes (4.83% and 5.18% vs. ~2.1%), owing to their superiority in both J_sc and V_oc. The improved photovoltaic performance of DSSCs with organic dyes (1, 2) relative to coordination compounds (3, 4) can be attributed to stronger absorption in the long-wavelength region of the spectrum (>550 nm), enabling organic sensitizers to utilize a larger portion of solar photons, and to a more favorable HOMO level alignment with the redox potential of the I⁻/I₃⁻ couple, facilitating rapid and efficient regeneration of the oxidized dye while minimizing recombination losses.

The incident photon-to-current efficiency (IPCE) spectra shown in Figure 12b characterize the wavelength-dependent efficiency of DSSCs in converting incident photons into electrons. The high maximum IPCE values (up to ~60%) observed for the fabricated DSSCs in the 400–500 nm region indicate high overall efficiency of the elementary processes—photon absorption, electron injection into the TiO₂ conduction band, and subsequent charge collection—suggesting favorable injection kinetics and minimal recombination losses in this spectral range. The IPCE spectra of all DSSCs show good agreement with the absorption spectra of the corresponding sensitizers in solution, confirming their key role in photocurrent generation and their effective adsorption onto the TiO₂ surface. The decline in IPCE in the long-wavelength region (>550 nm) correlates with the absorption band edge of the dyes and is attributed to their decreased molar extinction coefficient, and consequently lower LHE(λ), at these wavelengths. For the organic sensitizers 1 and 2, the shift of the absorption edge—and correspondingly the IPCE spectrum edge—to longer wavelengths compared to 3 and 4 directly accounts for their higher short-circuit current density (Jsc) under full solar illumination.

While the PCE of 5.18% for dye 2 demonstrates the potential of the glaucine core, it is notably lower than that of benchmark organic sensitizers like Y123 (>13%) or ruthenium complexes like N719 (11–12%) under similar conditions [27,28]. This lower efficiency is primarily attributed to the limited spectral response of our dyes, with an absorption onset below 600 nm, as evidenced by the IPCE spectra in Figure 12b. This restricts light harvesting in the red/near-IR region, where solar photon flux is significant. Furthermore, the absence of strong anchoring groups such as carboxylic or phosphonic acids in the molecular structure of 1 and 2 may lead to suboptimal electronic coupling with the TiO₂ surface and less stable dye adsorption, potentially affecting both electron injection efficiency and long-term device stability.

The dynamics of charge carriers under conditions of quasi-stationary electron injection into the mesoporous TiO₂ layer were investigated by electrochemical impedance spectroscopy. The obtained Nyquist plots (−Im(Z) vs. Re(Z)) (Figure 12c) exhibit two distinct semicircles, which is characteristic of DSSCs. These arcs correspond to different relaxation processes with distinct time constants and can be described by a Randles equivalent circuit with two-time constants. The first (high-frequency) semicircle (10³–10⁵ Hz) reflects the electrochemical process at the Pt/electrolyte interface. This process is associated with: R_CT1—the charge transfer resistance for the reduction of I₃⁻ to I⁻ at the catalytic surface of the platinum counter electrode. Its value is estimated from the diameter (chord) of the first semicircle. C_Pt—the capacitance related to the redistribution of electrolyte ions at the Pt electrode surface.

The second (low-frequency) semicircle (0.1–10³ Hz) corresponds to the charge transfer process at the TiO₂/dye/electrolyte interface. This process is associated with: R_rec—the recombination resistance of injected electrons in the TiO₂ conduction band with oxidized species in the electrolyte (primarily I₃⁻). The diameter of this semicircle is directly proportional to the magnitude of R_rec. A smaller diameter indicates lower resistance, meaning a faster charge transfer (recombination) process; C_μ—the chemical capacitance associated with the accumulation of electrons in the mesoporous TiO₂ film (change in the electron chemical potential). The characteristic frequency at the maximum of this semicircle allows estimation of the electron lifetime; Rs—the series resistance, determined by the intercept of the plot with the Re(Z) axis in the high-frequency region. It includes the ohmic resistance of the FTO glass, contacts, wires, and the ionic resistance of the electrolyte.

As can be seen from Figure 12c, the radius of the mid-frequency semicircle in the impedance spectra (R_rec), which characterizes the recombination resistance at the TiO₂/electrolyte interface, progressively decreases when going from the coordination complexes (3, 4) to the organic sensitizers (1, 2). This reduction in R_rec indicates a decrease in the energy barrier and/or an increase in the recombination rate of injected electrons with the oxidized species in the electrolyte (I₃⁻). The observed effect can be explained by the following factors:

• Influence on the conduction band: The organic dyes likely induce a more positive shift of the TiO₂ conduction band edge (due to surface dipoles or a different adsorption mechanism), which lowers the energy barrier for electron transfer to I₃⁻.
• Surface passivation: Although organic molecules might more effectively block recombination-active sites on the TiO₂ surface, in this case, this effect does not compensate for the dominant factor of the band shift.

The concurrent decrease in R_Pt indicates improved catalytic activity of the counter electrode in these cells. This factor is crucial because an enhanced reduction rate of I₃⁻ at the cathode facilitates more efficient regeneration of the oxidized dye (D⁺) by I⁻ ions, accelerating the entire charge transfer cycle.

It can be concluded that the increase in overall efficiency (PCE) of DSSCs with organic sensitizers is achieved despite faster recombination (smaller R_rec). The key factor is the significant increase in photocurrent density (J_sc), resulting from improved optical properties (broader and more intense absorption band) and possibly faster dye regeneration. This creates a balance where the gain from enhanced electron injection outweighs the losses due to somewhat faster recombination. Thus, impedance spectroscopy confirms the significant influence of the sensitizer nature on the kinetics of interfacial processes in the fabricated DSSCs. As for the stability of the devices, upon exploring resulting cells, the efficiency parameters remained stable within the margin of error for one to two weeks while the measurements were being carried out. High stability also corresponds to the high thermal stability of the studied compounds (up to a temperature of 250 °C) (Figure S17).

To contextualize the performance of our glaucine-based sensitizers within the broader landscape of modern photovoltaics, it is instructive to benchmark them against state-of-the-art inorganic and hybrid thin-film technologies. While direct comparisons are complicated by differing device architectures and operational principles, such an analysis highlights the unique advantages and current limitations of our materials.

The power conversion efficiency (PCE) of our best-performing organic dye, compound 2 (5.18%), is significantly lower than that of leading flexible perovskite solar cells (FPSCs), which have recently achieved certified efficiencies exceeding 23–25% [29,30]. These FPSCs benefit from the direct bandgap, exceptionally high absorption coefficients, and long carrier diffusion lengths inherent to metal halide perovskites. However, as extensively reviewed, the commercialization of FPSCs is hindered by concerns over operational stability, sensitivity to ambient conditions, and the toxicity of lead, necessitating stringent and costly encapsulation. In contrast, our DSSCs, while less efficient, operate robustly under diffuse light and ambient conditions without such demanding protection, offering a potential advantage in specific low-power, indoor, or building-integrated applications where longevity and environmental safety are paramount.

A more pertinent benchmark for eco-friendly innovation is provided by copper-based thin-film technologies, which are considered environmentally friendlier than CdTe and silicon-based solar cells due to their chemical stability and absence of toxic heavy metals [31]. Among these, the most widely used are ternary copper chalcogenides (CIS), quaternary copper chalcogenides (CIGS), and kesterites (CZTS), with bandgaps ranging from 1.0 to 1.2 eV [32], making them ideal for photoactive layers due to their high hole mobility, high absorption coefficient, and tunable bandgaps.

CIGS-based solar cells have achieved impressive efficiencies of around 23% at the cell level and approximately 19% at the module level [33], with the highest confirmed PCE of 23.35% demonstrated using cadmium-free buffer layers based on zinc sulfide (ZnS), which replace traditional CdS layers and provide better optical and electrical properties [34]. This achievement is particularly relevant in the context of our work, which emphasizes environmentally sustainable materials. However, CIS-based cells face efficiency limitations not exceeding 19% due to impurity issues caused by copper vacancies or antisite defects, which lower charge transport properties. Improving crystal quality, morphology control, larger grain size, and denser surfaces can enhance CIS PCE by up to 9% by reducing charge recombination and improving Voc and FF.

Kesterite CZTS cells, formed by the reaction of ternary copper chalcogenides with ZnS or ZnSe, exhibit moderate PCE around 12% [32]. The reason for their lower efficiency is Voc losses due to non-radiative recombination caused by defects in the bulk and film surface. As with CIS, larger grain sizes, uniform morphology, and higher crystallinity result in lower charge recombination and help restrict secondary phase formation in the photoactive layer.

This challenge of defect management in copper-based technologies is fundamentally analogous to the need for interfacial engineering in our DSSCs to suppress charge recombination at the TiO₂/dye/electrolyte interface. Although our organic dyes yield a higher short-circuit current density (Jsc ~11–12 mA/cm²) than typical CIS or CZTS devices, the superior charge-carrier dynamics of the inorganic absorber, once defects are passivated, result in a higher overall PCE. This comparison underscores that for our molecular dyes to compete, future design must not only broaden their absorption spectrum but also focus on minimizing recombination losses, potentially by incorporating stronger anchoring groups for better surface passivation and improved charge transport.

Finally, even when compared to mature thin-film technologies like flexible CIGS, which demonstrate commercial module efficiencies of 16–20% and excellent stability, our glaucine derivatives occupy a distinct niche. CIGS faces cost and scarcity issues with indium and gallium, while the use of cadmium in traditional buffer layers raises environmental concerns that are successfully addressed by transitioning to cadmium-free alternatives such as ZnS. Our purely organic sensitizers offer a pathway toward a more sustainable and potentially lower-cost alternative, albeit one that is currently at an earlier stage of development.

The performance gap relative to these advanced technologies can also be partially attributed to the charge transport layers used. Our DSSCs employ a standard I⁻/I₃⁻ redox electrolyte. The EIS data (Figure 12c) show that the charge transfer resistance at the Pt counter electrode (R_Pt) is lower for the organic dyes, indicating reasonably efficient dye regeneration kinetics. However, state-of-the-art FPSC and CIGS technologies leverage highly engineered transport layers and buffer layers—such as ZnS replacing CdS in CIGS, providing better optical and electrical properties, as well as doped PEDOT:PSS, NiOx, or SnO₂ in perovskite cells—that are often processed at low temperatures to enhance charge extraction and passivate interfacial defects. The record efficiencies in these systems are frequently enabled by interfacial modifiers that reduce trap-state density and improve energy level alignment. Furthermore, it is well-established that interfacial engineering, such as the use of co-adsorbents to form a blocking layer, or the application of core–shell structures on the TiO₂ photoanode to create an energy barrier, can further suppress charge recombination and enhance the net charge separation efficiency. These strategies represent promising avenues for future optimization of the devices based on the organic dyes 1 and 2. This suggests a clear direction for future work on our glaucine-based DSSCs: moving beyond simple dye optimization to a holistic device engineering approach. Exploring alternative redox couples (e.g., cobalt complexes) and employing interfacial passivation strategies on the TiO₂ photoanode, inspired by advances in cadmium-free buffer layers and advanced transport layers, could significantly enhance charge separation and boost overall device performance.

3. Materials and Methods

3.1. Starting Materials and General Characterization

The following commercially available solvents were used in this work: dimethylformamide, MeOH, EtOH, CH₂Cl₂, n-PrOH. The reagents used were: 7-formyldehydroglaucine (CAS 80941-67-9) prepared according to the procedure [35], zinc acetate dihydrate (CAS 5970-4-6), hydrazine hydrate (CAS 7803-57-6), 2-hydroxybenzaldehyde (CAS 90-02-8); 2-(N-tosylamino)benzaldehyde was synthesized according to the procedure [36].

Elemental analysis (C, H, N, and other elements) was performed on a Carlo Erba TCM 480 instrument (Cornaredo, Italy) using sulfanilamide as a reference standard. Metal content was determined gravimetrically at the analytical laboratory of the Institute of Physical and Organic Chemistry (Southern Federal University, Rostov-on-Don, Russia). Melting points were measured using the Kofler method.

Infrared spectra were recorded on a Varian Excalibur-3100 FT-IR spectrophotometer (Palo Alto, CA, USA) for powdered compounds. ¹H NMR spectra were measured on a Bruker Avance-300 spectrometer (Billerica, MA, USA, 300 MHz) at ambient temperature in DMSO-d₆, with the residual ²H solvent signal used as an internal standard.

Cyclic voltammetry (CV) measurements were performed in an anhydrous aprotic solvent (acetonitrile) containing a supporting electrolyte (tetrabutylammonium hexafluorophosphate, TBAPF₆). A platinum electrode was used as the working electrode, and the potentials were calibrated relative to a saturated calomel (silver chloride) electrode (n.c.e./h.c.e.), followed by conversion to the absolute vacuum energy scale

3.2. Synthesis

3.2.1. Synthetic Procedure for Hydrazone of 7-Formyldehydroglaucine

Hydrazone of 7-formyldehydroglaucine. A mixture of 3.81 g (0.01 mol) of 7-formyldehydroglaucine, 5 mL of hydrazine hydrate and 6 mL of n-propyl alcohol was boiled for 1 h. The precipitate that formed upon cooling was collected by filtration, washed with n-PrOH (3 × 2 mL), and dried in a vacuum oven at 100 °C. White powder, yield 2.65 g (67%), m.p. 153–155 °C. Found, %: C 66.90, H 6.35, N 10.54. For C₂₂H₂₅N₃O₄ calculated, %: C 66.82, H 6.37, N 10.63. ¹H NMR, DMSO-d₆, 300 MHz, δ (ppm): 2.75 (d, 3H, 3J = 8.7 Hz, N-CH₃), 3.10–3.13 (m, 2H, CH₂), 3.25–3.29 (m, 2H, CH₂), 3.80 (s, 3H, OCH₃), 3.86 (s, 3H, OCH₃), 3.89 (s, 3H, OCH₃), 3.95 (s, 3H, OCH₃), 6.18 (s, 1H, C_Ar-H), 6.70 (s, 1H, C_Ar-H), 7.13 (s, 0.4H, NH₂, E- or Z-isomer), 7.26 (d, 3H, 4J = 2.4 Hz, C_Ar-H), 7.60 (0.4H, NH₂, E- or Z-isomer), 8.37 (s, 0.6H, NH₂, Z- or E-isomer), 8.89 (s, 0.6H, NH₂, Z- or E-isomer), 9.09 (s, 0.4H, CH=N, E- or Z-isomer), 9.14 (s, 0.6H, CH=N, Z- or E-isomer). IR spectrum, selected bands, cm⁻¹: 3392 (NH₂), 2930, 2824 (CH₂), 1595 (CH=N), 1521, 1496, 1460, 1446, 1425, 1395, 1380, 1353, 1311, 1293, 1248, 1224, 1204, 1188, 1168, 1122, 1095, 1054, 1041, 1007, 984, 965, 925, 878, 863, 849, 837, 819, 790, 762, 744, 690, 655, 640, 619, 596, 574, 529, 511, 485, 466, 428. It was then used without additional purification to obtain azomethines 1 and 2. The ¹H NMR- and IR- spectra are presented in Supplementary Material (Figures S4 and S9).

3.2.2. Azomethines and Zn(II) Complexes Synthesis

A solution of 1.38 g (0.005 mol) 2-(N-tosylamino)benzaldehyde or 0.61 g (0.005 mol) of 2-hydroxybenzaldehyde in 20 mL of MeOH was added to a solution of 1.98 g (0.005 mol) of the hydrazone 7-formyldehydroglaucine in 10 mL of MeOH and the mixture was boiled for 1 h. The precipitates were filtered and recrystallized from a mixture of CH₂Cl₂: MeOH (1:2) (Scheme 1).

4-methyl-N-[2-[(E)-[(E)-(4,5,15,16-tetramethoxy-10-methyl-10-azatetracyclo [7.7.1.0^2,7.0^13,17]heptadeca-1(16),2,4,6,8,13(17),14-heptaen-8-yl)methylenehydrazono]methyl]phenyl]benzenesulfonamide (1). Orange crystals, yield: 2.88 g (85%). T_pl 208–209 °C. Anal. Calc. for C₃₆H₃₆N₄O₆S: C, 66.24; H, 5.56; N, 8.58. Found: C, 66.30; H, 5.62; N, 8.64%. ¹H NMR, DMSO-d₆, 300 MHz, δ (ppm): 2.26 (s, 3H, C-CH₃), 3.08 (s, 3H, N-CH₃), 3.15–3.19 (m, 2H, CH₂), 3.34–3.46 (m, 2H, CH₂), 3.81 (s, 3H, OCH₃), 3.82 (s, 3H, OCH₃), 3.95 (s, 3H, OCH₃), 3.95 (s, 3H, OCH3), 7.19–7.25 (m, 1H), 7.30–7.33 (m, 1H), 7.38–7.45 (m, 2H), 7.65 (d, 2H, J = 8.4 Hz, C_Ar-H), 7.81 (d, 1H, J = 8.4 Hz, C_Ar-H), 8.93 (s, 1H, CH=N), 9.12 (s, 1H, CH=N), 9.20 (s, 1H, C_Ar-H), 9.24 (s, 1H, C_Ar-H), 11.66 (s, 1H, NH). IR spectrum, selected bands, cm⁻¹: 3065, 2990, 2936, 2824, 1597, 1575, 1523, 1495, 1464, 1449, 1426, 1413, 1396, 1345, 1309, 1253, 1226, 1219, 1195, 1168, 1160, 1132, 1118, 1088, 1069, 1041, 1012, 983, 954, 885, 863, 836, 812, 799, 758, 722, 703, 660, 644, 621, 564, 547, 534, 509, 476, 455.

2-[(E)-[(E)-(4,5,15,16-tetramethoxy-10-methyl-10-azatetracyclo[7.7.1.0^2,7.0^13,17]heptadeca-1(16),2,4,6,8,13(17),14-heptaen-8-yl)methylenehydrazono]methyl]phenol (2). Orange crystals, yield: 2.00 g (80%). T_pl 181–182 °C. Anal. Calc. for C₂₉H₂₉N₃O₅: C, 69.72; H, 5.85; N, 8.41. Found: C, 69.68; H, 5.79; N, 8.48. ¹H NMR, DMSO-d₆, 300 MHz, δ (ppm): 3.00 (s, 3H, NCH₃), 3.14–3.17 (m, 2H, CH₂), 3.39–3.42 (m, 2H, CH₂), 3.80 (s, 3H, OCH₃), 3.91 (s, 3H, OCH₃), 3.94 (s, 3H, OCH3), 3.98 (s, 3H, OCH₃), 6.95–7.00 (m, 2H, C_Ar-H), 7.30 (s, 1H, C_Ar-H), 7.36 (t, 1H, 3J = 4.8 Hz, C_Ar-H), 7.71 (dd, 1H, 3J = 8.1 Hz, C_Ar-H), 9.05 (s, 1H, C_Ar-H), 9.11 (s, 1H, C_Ar-H), 9.18 (s, 1H, CH=N), 9.22 (s, 1H, CH=N), 11.60 (s, 1H, OH). IR spectrum, selected bands, cm⁻¹: 3853, 3751, 3734, 3689, 3628, 3175, 3048, 2983, 2928, 2901, 2829, 1618, 1595, 1576, 1520, 1492, 1462, 1426, 1413, 1395, 1378, 1351, 1305, 1250, 1200, 1174, 1155, 1129, 1085, 1065, 1038, 1009, 975, 860, 834, 762, 724, 648, 618, 574, 561, 408.

To a solution of 0.65 g (0.001 mol) of azomethine 1 or 0.5 g (0.001 mol) 2 in 30 mL of MeOH, respectively, 0.11 g (0.0005 mol) of zinc acetate dihydrate in 10 mL of MeOH was added and boiled for 2 h. The precipitates of the complexes were filtered and recrystallized from a mixture of CH₂Cl₂:MeOH (1:2). Dried in a vacuum drying cabinet at 150 °C (Scheme 1).

bis[N-(p-tolylsulfonyl)-2-[(E)-[(E)-(4,5,15,16-tetramethoxy-10-methyl-10-azatetracyclo[7.7.1.0^2,7.0^13,17]heptadeca-1(16),2,4,6,8,13(17),14-heptaen-8-yl)methylenehydrazono]methyl]anilino]zinc (3). Orange crystals, yield: 0.60 g (87%). T_pl > 250 °C. Anal. Calc. for C₇₂H₇₀N₈O₁₂S₂Zn: C, 63.17; H, 5.15; N, 8.19. Found: C, 63.21; H, 5.12; N, 8.21%. ¹H NMR, DMSO-d₆, 300 MHz, δ (ppm): 2.01 (s, 3H, CH₃), 2.44–2.48 (m, 1H, CH₂), 2.67 (s, 3H, NCH₃), 2.71–2.76 (m, 2H, CH₂), 2.97–3.01 (m, 1H, CH₂), 3.78 (s, 3H, OCH₃), 3.87 (s, 3H, OCH₃), 3.89 (s, 3H, OCH₃), 3.93 (s, 3H, OCH₃), 6.65 (br.s, 2H, C_Ar-H), 6.95–7.00 (m, 1H, C_Ar-H), 7.20 (m, 1H, C_Ar-H), 7.27–7.35 (m, 2H, C_Ar-H), 7.65 (d, 2H, 3J = 7.5 Hz, C_Ar-H), 7.92 (d, 1H, 3J = 7.5 Hz, C_Ar-H), 9.05 (s, 1H, CH=N), 9.15 (s, 1H, CH=N), 9.30 (s, 1H, CAr-H), 9.62 (s, 1H, C_Ar-H). IR spectrum, selected bands, cm⁻¹: 3592, 3517, 3127, 2934, 2822, 1594, 1563, 1521, 1462, 1445, 1394, 1298, 1289, 1253, 1227, 1199, 1129, 1084, 1066, 1040, 1008, 938, 900, 873, 858, 833, 811, 751, 661, 640, 578, 567, 545.

bis[2-[[(E)-(4,5,15,16-tetramethoxy-10-methyl-10-azatetracyclo[7.7.1.0^2,7.0^13,17]heptadeca-1(16),2,4,6,8,13(17),14-heptaen-8-yl)methylenehydrazono]methyl]phenoxy]zinc (4). Orange crystals, yield: 0.45 g (85%). T_pl > 250 °C. Anal. Calc. for C₅₈H₅₆N₆O₁₀Zn: C, 65.57; H, 5.31; N, 7.91. Found: C, 65.60; H, 5.30; N, 8.02%. 1H NMR, DMSO-d₆, 300 MHz, δ (ppm): 2.69 (s, 3H, NCH₃), 2.71–2.74 (m, 2H, CH₂), 2.88–2.90 (m, 2H, CH₂), 3.73 (s, 3H, OCH₃), 3.77 (s, 3H, OCH₃), 3.84 (s, 3H, OCH₃), 3.93 (s, 3H, OCH₃), 6.70 (t, 1H, 3J = 7.4 Hz, C_Ar-H), 6.79 (d, 1H, 3J = 8.4 Hz, C_Ar-H), 7.20 (s, 1H, C_Ar-H), 7.35–7.38 (m, 1H, C_Ar-H), 7.62 (dd, 1H, 3J = 8.1 Hz, 4J = 1.5 Hz, C_Ar-H), 8.94 (s, 1H, C_Ar-H), 9.00 (s, 1H, C_Ar-H), 9.11 (s, 1H, CH=N), 9.20 (s, 1H, CH=N). IR spectrum, selected bands, cm⁻¹: 3628, 3536, 3171, 3143, 3061, 2987, 2931, 2831, 2654, 1603, 1588, 1561, 1522, 1495, 1461, 1439, 1426, 1414, 1394, 1380, 1347, 1329, 1308, 1251, 1226, 1206, 1187, 1147, 1131, 1084, 1064, 1034, 1006, 983, 963, 882, 862, 738, 626, 579, 448.

The ¹H NMR (Figures S4–S8) and IR (Figure S9–S13) spectra of 1–4 are presented in Supplementary Material.

The phase purity of the synthesized compounds was confirmed by Powder X-ray Diffraction (PXRD). The experimental PXRD patterns for compounds 1, 2, and 4 showed excellent correspondence with the patterns simulated from their respective single-crystal structures (see Supplementary Materials, Figures S14–S16), confirming the high purity and crystallinity of the bulk samples.

3.3. Single-Crystal X-Ray Diffraction

Crystallographic data for compounds 1, 2, and 4 were obtained on an Agilent SuperNova diffractometer using a microfocus X-ray source with a copper anode and a two-dimensional Atlas S2 CCD detector. The reflections were collected and the unit cell parameters were determined and refined using a specialized software package called Chrysalispro (ver. 1.171.33.66) [37]. The structures were deciphered using the ShelXT program [38] and refined using the ShelXL program [39]. Hydrogen atoms are included in the refinement in the “rider” model in the isotropic approximation with dependent thermal parameters. Molecular graphics for structures 1, 2, and 4 were performed using the Olex2 software package (version 1.5) [40]. Crystal data and parameters for clarifying the structure of compounds 1, 2, and 4 are given in

Table 10. Crystallographic parameters and structure refinement statistics for compounds 1, 2 and 4.

	1	2	4
CCDC number	2524036	2524292	2524297
Empirical formula	C36H36N4O6S	C29H29N3O5	C58H56N6O10Zn
Formula weight	652.75	499.55	1062.45
Temperature [K]	99.99(10)	293(2)	100.15
Crystal system	triclinic	monoclinic	triclinic
Space group (number)	$P\overline{1}$ (2)	$P{2}_1/n$ (14)	$P\overline{1}$ (2)
a [Å]	8.22130(10)	7.08920(10)	9.57540(10)
b [Å]	12.6384(2)	24.9263(2)	12.33220(10)
c [Å]	16.6049(3)	14.64120(10)	22.8711(2)
α [°]	74.420(2)	90	97.2080(10)
β [°]	78.6330(10)	96.6410(10)	100.5220(10)
γ [°]	71.7100(10)	90	98.2200(10)
Volume [Å3]	1565.92(5)	2569.85(5)	2595.97(4)
Z	2	4	2
ρcalc [gcm−3]	1.384	1.291	1.359
μ [mm−1]	1.372	0.726	1.203
F(000)	688	1056	1112
Crystal size [mm3]	0.146 × 0.251 × 0.349	0.062 × 0.212 × 0.385	0.061 × 0.074 × 0.245
Crystal colour	yellow	orange	yellow
Crystal shape	prism	plate	prism
Radiation	Cu Kα (λ = 1.54184 Å)	Cu Kα (λ = 1.54184 Å)	CuKα (λ = 1.54184 Å)
2θ range [°]	5.57 to 151.92	7.04 to 151.96	7.33 to 152.54
Index ranges	−9 ≤ h ≤ 10 −14 ≤ k ≤ 15 −20 ≤ l ≤ 20	−8 ≤ h ≤ 8 −31 ≤ k ≤ 31 −18 ≤ l ≤ 18	−11 ≤ h ≤ 12 −15 ≤ k ≤ 15 −28 ≤ l ≤ 28
Reflections collected	45,632	37,704	73,562
Independent reflections	6499 Rint = 0.0406 Rsigma = 0.0224	5343 Rint = 0.0242 Rsigma = 0.0148	10,811 Rint = 0.0453 Rsigma = 0.0215
Completeness to θ = 67.684°	100.0%	100.0%	100.0%
Data/Restraints/Parameters	6499/0/434	5343/0/342	10,811/0/686
Absorption correction Tmin/Tmax (method)	0.558/1.000 (gaussian)	0.521/1.000 (gaussian)	0.751/1.000 (gaussian)
Goodness-of-fit on F2	1.046	1.059	1.065
Final R indexes [I ≥ 2σ(I)]	R1 = 0.0403 wR2 = 0.1084	R1 = 0.0390 wR2 = 0.1099	R1 = 0.0336 wR2 = 0.0854
Final R indexes [all data]	R1 = 0.0430 wR2 = 0.1118	R1 = 0.0424 wR2 = 0.1135	R1 = 0.0346 wR2 = 0.0861
Largest peak/hole [eÅ−3]	0.63/−0.53	0.21/−0.20	0.37/−0.54

.

3.4. Quantum-Chemical Calculations

All calculations were carried out in Gaussian 09W package [41] at the DFT level of theory. The molecular geometries of isolated molecules of the complexes were optimized by the DFT using Becke’s three-parameter exchange functional [42], Lee-Yang—Parr correlation functional (B3LYP) [43] and standard split-valence polarized 6–311G(d,p) basis set [44]. Stationary points were confirmed to correspond to minima of the potential energy surfaces, with no presence of imaginary frequencies or negative eigenvalues. The electronic UV spectra were simulated in CH₂Cl₂ by means of TD-DFT calculations, employing the B3LYP functional and the 6–31G(d,p) basis set, with solvent effects accounted for by the PCM model.

3.5. Fabrication of DSSCs

The fabrication procedure for DSSC devices comprised the following key steps:

• Preparation of the Photoanode. Fluorine-doped tin oxide (FTO)-coated glass was used as the conductive substrate. A TiO₂ nanoparticle paste (Solaronix T/SP, Aubonne, Switzerland, 15–20 nm particle size) was deposited onto the substrate by screen printing. The small particle size yields a highly porous mesostructure with a large specific surface area, essential for subsequent adsorption of a sensitizer monolayer. A scattering layer consisting of TiO₂ particles >100 nm in size was deposited atop the active layer; these larger particles efficiently scatter photons in the long-wavelength region, increasing the optical path length and the probability of light absorption within the active layer, thereby enhancing the overall external quantum efficiency of the device. The deposited layers were subjected to stepwise sintering in a muffle furnace under ambient atmosphere. An initial annealing step (250 °C, 45 min) served to remove organic dispersants and plasticizers from the paste, preventing film cracking. Subsequent high-temperature treatment (600 °C, 30 min) ensured sintering of the TiO₂ nanocrystals into a continuous mesoporous network with improved interparticle electron transport. Concurrently, crystallization of the amorphous TiO₂ phase into the predominantly anatase phase occurred, which possesses a favorable conduction band position for DSSCs and high electron mobility. After sintering and cooling to room temperature, the photoanodes were immersed in a 0.01 M solution of the dye under study in anhydrous dichloromethane (CH₂Cl₂) for 24 h. This prolonged dark incubation ensured equilibrium chemisorption of the dye molecules onto the TiO₂ surface. The resulting films were rinsed with acetone to remove unbound dye and then dried at 50 °C for 30 min. The active area and thickness of the photoanodes were 0.320 cm² and 10–12 µm, respectively.
• Counter Electrodes. Counter electrodes were fabricated on FTO substrates by thermal decomposition of chloroplatinic acid (H₂PtCl₆) at 450 °C. The resulting thin film of Pt nanoparticles serves as a highly active and stable catalyst for the regeneration of the redox couple in the electrolyte (converting of triiodide (I₃⁻) to iodide (I⁻)).
• Assembly of the Photoelectrochemical Cell. The photoanode and Pt counter electrode were hermetically sealed along the perimeter using a thermoplastic polymer film (Surlyn). This film provides both electrical insulation and mechanical adhesion, preventing electrolyte leakage.
• Electrolyte Injection. The redox electrolyte was introduced into the interelectrode space through a pre-drilled hole in the counter electrode. The hole was subsequently sealed with a thin glass cover slip (0.5 mm thickness) to prevent solvent evaporation and moisture ingress.
• Electrolyte Composition. The electrolyte composition was standard for the iodide/triiodide redox couple and included: 1-methyl-3-propylimidazolium iodide (PMII, 0.5 M) as the primary source of I⁻ ions and organic cations, forming an ionic liquid; lithium iodide (LiI, 0.1 M) to enhance ionic conductivity; in addition, Li⁺ cations contribute to charge screening on the TiO₂ surface, positively influencing the photovoltage; iodine (I₂, 0.05 M) as the oxidized form of the redox couple (electron acceptor at the cathode); 4-(tert-butyl)pyridine (TBP, 0.5 M), which passivates surface states on TiO₂ and adsorbs onto its surface to suppress recombination of injected electrons with the oxidized electrolyte species (I₃⁻), thereby increasing the open-circuit voltage; and guanidinium nitrate, which enhances the photovoltage via a positive shift of the TiO₂ conduction band and suppression of recombination. Acetonitrile was used as the solvent.

The photovoltaic current density–voltage (J–V) characteristics of the fabricated DSSCs were measured using a potentiostat/galvanostat (K3000 LAB, McScience, Suwon, Republic of Korea). The cells were illuminated with a class AAA xenon solar simulator (Model 96000, Newport, Irvine, CA, USA) calibrated to the standard AM 1.5G spectrum (air mass 1.5, global) at an intensity of 100 mW/cm² (1 sun). The use of a solar simulator ensures reproducible test conditions comparable to standard measurement protocols. To eliminate errors arising from scattered light and parasitic photocurrent originating from active areas outside the designated region, and to precisely define the incident power, a black opaque mask with an aperture of 0.320 cm² was placed over the cell surface. To ensure reliability and statistical significance of the results, three identical cells were fabricated and characterized. The averaged values of the key parameters (J_sc, V_oc, FF, PCE) are reported, enabling assessment of the reproducibility of the fabrication procedure and minimizing the influence of random errors.

Electrochemical impedance spectroscopy of the fabricated DSSCs was performed in a two-electrode configuration (working electrode: photoanode; counter electrode: Pt) under standard illumination from a solar simulator (AM 1.5G, 100 mW/cm²). Measurements were carried out at a constant forward bias of −0.75 V, corresponding to the operating potential range of the DSSC near the maximum power point (V_mp), where a steady-state photocurrent is established within the cell.

4. Conclusions

In this work, new Schiff base derivatives of the alkaloid glaucine (1, 2) and their corresponding zinc(II) coordination compounds (3, 4) were synthesized, structurally characterized, and investigated as sensitizers for TiO₂-based solar cells. The structures of the compounds were established by X-ray diffraction analysis and quantum chemical modeling (DFT/TD-DFT). The organic derivatives (1, 2) are molecules with a planar conjugated skeleton, facilitating intermolecular π–π interactions in the crystal. In complex 4, the zinc atom adopts a tetrahedral coordination environment, coordinated by two nitrogen atoms and two oxygen atoms from two different bidentate ligands. All compounds exhibit intense absorption in the visible region (maxima ~436–506 nm), arising from intraligand (ILCT, for 1, 2) or interligand (LLCT, for 3, 4) charge transfer transitions. The HOMO and LUMO energy levels, determined by cyclic voltammetry, satisfy the key thermodynamic criteria for DSSC operation for both organic and coordination compounds: the LUMO level lies above the TiO₂ conduction band (−4.00 eV), providing thermodynamic driving force for electron injection, while the HOMO level lies below the redox potential of the I⁻/I₃⁻ electrolyte couple (−4.80 eV), ensuring exothermic dye regeneration.

The highest power conversion efficiencies (PCE) were demonstrated by the organic dyes 1 (4.83%) and 2 (5.18%). The zinc coordination complexes 3 and 4 exhibited significantly lower efficiency (~2.1%). The advantage of the organic derivatives stems from their higher short-circuit current density (J_sc ~11–12 mA/cm² vs. ~5.7–7.6 mA/cm² for the complexes), which correlates with their superior light-harvesting ability in the 400–550 nm region, as confirmed by IPCE spectra, and possibly more effective adsorption onto the TiO₂ surface.

Impedance spectroscopy results revealed that devices based on organic sensitizers exhibit lower recombination resistance (R_rec) at the TiO₂/electrolyte interface compared to those with complexes, indicating somewhat accelerated recombination of injected electrons. Nevertheless, the efficiency gain from the significantly increased Jsc in organic dyes outweighs these losses. Additionally, cells with organic sensitizers showed lower charge transfer resistance for I₃⁻ reduction at the platinum counter electrode (RPt), indicating improved dye regeneration kinetics in this system.

This comprehensive study allows the conclusion that glaucine derivatives possess significant potential as sensitizers for DSSCs. The highest efficiency was achieved for the organic derivatives, highlighting the promise of this approach in molecular design. The obtained results contribute substantially to the fundamental basis for developing new efficient and cost-effective organic dyes for photovoltaic applications.