Influence of 2-Amino-4-methylpyridine and 2-Aminopyrimidine Ligands on the Malonic Acid-Cu(II) System: Insights through Supramolecular Interactions and Photoresponse Properties

Abstract

Two Cu(II)-malonate complexes with 2-amino-4-methylpyridine (complex 1) and 2-aminopyrimidine (complex 2) auxiliary ligands were synthesized, and their single-crystal X-ray diffraction structures were established. Change in the auxiliary ligand exhibits substantial structural variation in the present complexes. Complex 1 shows a one-dimensional anionic copper-malonate moiety connected by the malonate bridge, whereas complex 2 is a mononuclear one. For both the complexes, auxiliary ligands are attached with the Cu-malonate moiety through various noncovalent interactions. Optical band gap, electrical conductivity, and photosensitivity of complexes 1 and 2 were measured, but the values of electrical parameters of the complexes significantly differ from each other. However, the magnitudes of electrical parameters increase several times for both the complexes when they are exposed under visible light, though the values of light sensing parameters of complex 1 were found to be higher than those of complex 2. Density functional theory calculations for complex 1 were carried out to support the experimental result.

1. Introduction

The realm of transition metal complexes is considered to take up one of the forefront positions in the traditional field of chemistry and considered to cover almost all the relevant topics from structural to functional point of view. This extensive field of chemistry using transition metals and organic ligands as building blocks is enriched with high design ability and diversity of architectural motifs.¹⁻⁴ These complexes are capable of forming polymeric chains through coordination bonds;⁵⁻⁷ besides, they can also produce supramolecular polymers via weaker noncovalent interactions such as hydrogen bonding, lp···π, π···π stacking, π···cation, and π···anion interactions.⁸⁻¹² A preferred combination of these weak interactions along with covalent bonds form a supramolecular building block that takes an active participation in the field of drug delivery, gas storage, separation, barrier diode, sensing, and catalysis.¹³⁻¹⁶ Some specific noncovalent interactions involving aromatic systems such as π···π and salt bridge···π have immense influence in various chemical and biological processes as well.^17,18 Again, inclusion of additional aromatic N-donor auxiliary ligands in transition metal complexes creates diversity to the existing supramolecular structures as these N-donor ligands can participate in hydrogen bonding and π-ring containing weak interactions as well as coordinate to the metal.¹⁹⁻²¹

The complexes formed by the judicious choice of transition metal and organic ligands have shown promising applications as functional materials because of their fascinating structural features and supramolecular behavior.²²⁻²⁶ Various dicarboxylate or polycarboxylate ligands are the preferable choice for the formation of this type of complexes because of their several binding motifs and binding capability to the metal center in more than one way.²⁷⁻³⁰ Though the electrical properties of these complexes are not very striking because of the large number of insulating carbon atoms of the dicarboxylic acid present in molecular assembly, there are several pieces of evidence that the proper choice of secondary ligands can stimulate the charge transport phenomenon of the complexes.³¹⁻³³ The noncovalent interactions associated with the above said complexes often influence the structures of the complexes because they are able to further connect the coordinated structural motifs into high-dimensional supramolecular networks,^34,35 which also leads to the changes in electrical properties of the transition metal complexes.^36,37

In this present study, we have synthesized two copper(II)-malonate complexes with auxiliary ligands 2-amino-4-methylpyridine (for complex 1) and 2-aminopyrimidine (for complex 2) (Figure 1) in aqueous media. The role of weak noncovalent interactions is also explored from their single-crystal X-ray diffraction structural studies. Depending on the nature of the aromatic amine cation (auxiliary ligand) and binding mode of the malonate moiety, the Cu(II) units exhibit diverse topologies. For complex 1, malonate oxygen atoms bind to Cu(II) and two different metal centers are connected through carboxylate bridges, whereas complex 2 is a monomeric Cu(II)-malonate unit. For both complexes, auxiliary ligands are attached with the Cu-malonate moiety through various noncovalent interactions such as H-bonding, π···π, lp···π, and salt bridge···π and provide additional stability to the existing molecular assembly. Electrical and photosensing properties of both the complexes were also studied. Generally, the majority of porous metal–organic complexes are unable to afford any free charge carriers or low-energy pathways for charge transport and perform mostly like electrical insulators where conductivity is found to be very low.²² Only since the past couple of years, researchers tried to design novel structures, both porous and nonporous materials, which have been utilized to engender conductivity.^25,37−40 Electrical conductivities for our present complexes having partly delocalized systems are 4.83 × 10^–3 S m^–1 (for 1) and 2.02 × 10^–3 S m^–1 (for 2), which are comparable or even better than many reported values in the literature.^{25,37−39,41} Electrical characterization discloses the Schottky diode (SD) nature of both the present complexes, though the magnitude of electrical conductivity of complex 1 is found to be higher than that of complex 2. Recently, some of us reported two Cu(II) complexes made up of an aromatic dicarboxylic acid (phthalic acid) ligand for SD applications.²⁴ The complexes described herein show better performance in terms of electrical conductivity, photosensitivity, and on/off ratio.²⁴ Thus, the present complexes can be utilized as potential candidates for implementation in light-sensitive electronic devices.

Figure 1.

Scheme of the reactions for the preparation of the complexes.

2. Results and Discussion

2.1. Synthesis and Spectroscopic Characterizations

Complexes 1 and 2 were synthesized in aqueous media using copper(II) chloride dihydrate salt and malonic acid as primary ligands in the presence of ammonium hexafluorophosphate. 2-Amino-4-methylpyridine and 2-aminopyrimidine were used as auxiliary ligands for complex 1 and complex 2, respectively (Figure 1). Details of the synthesis procedure are given in the Experimental Section. Preliminary characterizations of two complexes were carried out using elemental analyses (C, H, N) and infrared (IR) spectra (Figures S1 and S2). Anal. Calcd for C₂₄H₃₂CuF₆N₆O₉P (complex 1): C, 38.08; H, 4.26; N, 11.10%. Found: C, 38.27; H, 4.14; N, 10.96%. Main IR absorption bands observed for complex 1 (cm^–1) are 3450 (s), 3310 (w, ν_N–H), 3126 (w, ν_Ar–C–H), 1674 (s, ν_C=O), 1576 (s), 1541 (s), 1428 (s, ν_Ar–C=C), 1357 (s, ν_Ar–C–N), 836 (s), 787 (s), 748 (s), 561 (s), 432 (s).

Anal. Calcd for C₂₂H₂₈CuF₁₂N₁₂O₈P₂ (complex 2): C, 28.05; H, 3.00; N, 17.84%. Found: C, 28.14; H, 3.12; N, 17.76%. Main IR absorption bands observed for complex 2 (cm^–1) are 3403 (w), 3333 (w, ν_N–H), 3172 (w, ν_Ar–C–H), 1675 (s, ν_C=O), 1555 (s), 1390 (s), 1343 (s, ν_Ar–C–N), 1192 (s), 962 (s), 830 (s), 744 (vs), 561 (vs).

2.2. Structural Description of Complex 1

The crystal structure of complex 1 (crystal data and structure refinement parameters are presented in Table S1) shows two different copper-malonate units where the copper atoms lie on inversion centers. In the diaquabis(malonato)copper(II) unit, the Cu(II) ion generates a distorted octahedron where two malonate oxygen atoms (O6 and O7) and their symmetric counterparts (O6* and O7*, * = 2 – x, −y, 1 – z) generate an equatorial plane and two water oxygen atoms O9 and O9* occupy the trans axial position. The other Cu(II) ion is in an octahedral coordination environment, where two malonate oxygen atoms (O2 and O3) and their symmetric counterparts (O2^# and O3^#, # = 1 – x, 1 – y, 1 – z) form the equatorial plane of bis(malonato)copper(II) unit and two trans axial carbonyl oxygen atoms O5 and O5^# from the diaquabis(malonato)copper(II) unit bind the metal center (Figure 2). In the asymmetric unit, there are two protonated 2-amino-4-methylpyridine, one unprotonated 2-amio-4-methylpyridine and one hexafluoridophosphate anion. In the equatorial plane, the Cu–O bond lengths vary from 1.923(3) to 1.966(3) Å, whereas the apical Cu(2)–O(9) and Cu(1)–O(5) bond lengths are much longer than the equatorial bond lengths (Table S2) but are in the normal range of values with malonate complexes reported earlier.⁴²⁻⁴⁵ The bond lengths and bond angles are included in Tables S2 and S3, respectively.

Figure 2.

ORTEP view and atom numbering scheme of complex 1 with displacement ellipsoid at the 30% probability. Unlabeled half-structural domain is generated through the symmetry operation.

In the solid state, the title structure is stabilized through N–H···O, O–H···O, N–H···F, C–H···O, and C–H···F hydrogen bonds and π···π stacking and carbonyl (lp)···π interactions (Tables S4–S6). Two copper-malonate units are interconnected through the carboxylate bridge for the formation of a one-dimensional (1-D) polymeric network (Figure S3) that is further strengthened by the O–H···O hydrogen bond between the water oxygen atom and carbonyl oxygen atom of the partner molecule. The parallel chains are then interconnected through weak C–H···O bonds by generating a R₂²(8) ring motif. Again, the water oxygen atom O9 plays as the donor to the carbonyl oxygen atom O4 in the molecule at (1 + x, y, z). These two kinds of hydrogen bonding contacts interconnect the parallel chains to form a two-dimensional (2-D) framework in the (110) plane (Figure S4). Again, in both sides of the polymeric chain, four protonated aminopyridine moieties are juxtaposed with the carboxylate oxygen atoms of the diaquabis(malonato)copper(II) unit to generate four salt-bridge (SB)^42,43,46 units. However, the amine nitrogen atom of the aminopyridine moiety acts as the double donor to bind the metal-coordinated carboxylate oxygen atoms of the bis(malonato)copper(II) unit. One SB unit of the diaquabis(malonato)copper(II) unit is juxtaposed to the other SB with a separation distance of 3.838 Å. Because of the self-complementarity, the π-cloud of the aminopyridine moiety is oriented toward the another SB unit with a separation distance of 3.619 Å. Therefore, an extended SB···SB/SB···π supramolecular network is evidenced (Figure 3). Moreover, the carbonyl oxygen atom O4 from the bis(malonato)copper(II) unit is oriented toward the π-face of the protonated aminopyridine ring with a separation distance of 3.664(3) Å, suggesting significant lone-pair···π interaction.^45,47−49 Finally, the aminopyridine rings are optimized through π⁺–π⁺ stacking⁵⁰⁻⁵² interactions with an interplanar spacing of 3.358 Å and a ring-centroid separation of 3.702(3) Å (Table S5). Therefore, lone-pair (lp)···π⁺/π⁺–π⁺ supramolecular network is responsible to perform a vital role in building the extended network structure (Figure 3).

Figure 3.

Perspective view of the extended supramolecular SB···SB/SB···π and lone-pair (lp)···π⁺/π⁺–π⁺ networks in complex 1.

2.3. Structural Description of Complex 2

The crystal structure of complex 2 shows one monomeric [Cu(C₃H₂O₄)₂(PF₆)] unit and one noncoordinated hexafluoridophosphate anion and four protonated 2-aminopyrimidine molecules (Figure 4). The coordination mode around the metal center can be best described as a distorted square-pyramid (τ = 0.024, eq S1) with the CuO₄F chromophore where the four-malonate oxygen atoms generate the basal plane and the axial position is occupied by the fluorine atom F7 of the hexafluoridophosphate anion having a bond length of 2.4933(14) Å (Table S2). The sixth empty coordination position is occupied by the F(6) atom of another hexafluoridophosphate anion having a bond length of 2.854(2) Å which is longer than Cu(1)–F(7) and is expected for a d⁹ system because of the pronounced Jahn–Teller effect.

Figure 4.

ORTEP view and atom numbering scheme of complex 2 with displacement ellipsoid at the 50% probability.

The crystal structure of complex 2 is stabilized through N–H···O, N–H···N, N–H···F, C–H···F, C–H···O hydrogen bonds and π–π stacking, anion···π, and carbonyl (lp)···π interactions (Tables S4–S6). In the first substructure, the malonate carbon atoms C(2) and C(5) act as donors to the carbonyl atoms O(5) and O(4) to generate a R₂²(8) dimeric ring. Interconnection of the dimeric rings leads the molecules to form an infinite chain along the [010] direction (Figure S5). The carboxylate oxygen atoms act as acceptors to the pyridine and amine nitrogen atoms of the protonated 2-aminopyrimidine ligand. The carbonyl atoms from the 1-D chains are oriented toward the π-cloud of the pyrimidine ring with a separation distance of 3.093(2) Å. This lone-pair (lp)···π interaction⁴⁷⁻⁴⁹ interconnects the parallel chains to form a 2-D supramolecular assembly (Figure S6). In another substructure, the protonated pyrimidine nitrogen atom of four 2-aminopyrimidine moieties is in contact with the three carbonyl and one carboxylate oxygen atoms, and three-amine nitrogen atoms from the 2-aminopyrimidine moieties are interconnected with the carboxylate oxygen atoms of the malonate moieties. Thus, two SB units are interconnected by a separation distance of 3.532 Å (Figure 5). Moreover, the molecular packing is such that the π⁺–π⁺ stacking interactions⁵⁰⁻⁵² between the aminopyrimidine rings are optimized with an interplanar spacing of 3.407 Å and a ring-centroid separation of 3.971(2) Å. In the opposite side, the pyrimidine rings are interconnected through the π⁺–π⁺ stacking interactions with a ring-centroid separation of 3.766(2) Å and an interplanar spacing of 3.361 Å. The combination of C–H···O hydrogen bonds and π⁺–π⁺ interactions results in a 2-D supramolecular framework in the (110) plane (Figure 5).

Figure 5.

Supramolecular framework generated through SB···SB and π⁺–π⁺ interactions in complex 2.

In the final substructure, the carbonyl oxygen atom O(1) is oriented toward the π-cloud of the (N2/C7/N3/C10/C9/C8) ring in the molecule at (1 + x, y, z) with a separation distance of 3.093(2) Å. Because of self-complementarity, the pyrimidine ring is in contact with another pyrimidine ring through face-to-face π⁺–π⁺ interaction with a separation distance of 3.971(2) Å. Finally, the F(9) atom of the hexafluoridophosphate anion binds the centroid of the 2-aminopyrimidine moiety with a separation distance of 3.089(2) Å. Therefore, the cooperativity of weak noncovalent interactions leads the molecules to generate lone-pair (lp)···π⁺/π⁺–π⁺/π⁺···anion assembly (Figure 6). These supramolecular networks generate a chain along the [100] direction. The self-complementary C–H···O hydrogen bonds interconnect the parallel chains to build a supramolecular layered framework in the (110) plane (Figure 6).

Figure 6.

Perspective view of the layered structure in complex 2 generated through the extended lone-pair (lp)···π⁺/π⁺–π⁺/π⁺···anion network.

2.4. Optical Characterization

In the present study, the solid-state absorbance spectra of the complexes (inset, Figure 7) have been recorded by preparing the thin films of well dispersion of complexes in dimethylformamide (see Pages S10 and S11) in the range 200–700 nm. The direct optical band gap (BG) has been estimated using Tauc’s equation (eq 1).⁵³

1

Here, “α”, “E_g”, “h”, and “ν” stand, respectively, for the absorption coefficient, BG, Planck’s constant, and the frequency of light. “A” is a constant which is considered as 1 for the ideal case.

Figure 7.

UV–vis absorption spectra (inset) and Tauc’s plots for (A) complex 1 and (B) complex 2.

By extrapolating the linear region of the plot (αhν)² versus hν to α = 0 absorption, the values of direct BG (E_g) have been evaluated as 3.38 and 4.16 eV for complexes 1 and 2, respectively (Figure 7).

The obtained BG for both the synthesized complexes and the presence of strong absorbance in the visible wavelength region signify some conducting nature of the synthesized complexes. Motivated from these analyses, the electrical characteristics of the synthesized complexes have been further studied.

2.5. Electrical Characterization

For analyzing the electrical properties, we measured the I–V characteristics of both the complex (1 and 2)-based multiple devices under dark conditions (Figure S7) and have calculated various major electrical parameters of these devices (Table S7). The obtained values of various parameters have been compared in Table S7, which indicate that the devices named as “Test device 1” for both the complexes (1 and 2) are the best devices among the others. Hence, we have selected “Test device 1” for both the complexes (1 and 2) as the final device and carried out all the electrical measurements to investigate the semiconducting properties of our synthesized complexes (1 and 2).

Figure 8 represents the I–V characteristics under both the dark and AM 1.5G radiation (illumination) conditions of the complex (1 and 2)-based final device.

Figure 8.

I–V characteristics curve and the log I vs voltage plot (inset) of complex (1 and 2)-based devices under both dark and illumination conditions.

The conductivities under the dark condition were found to be 1.53 × 10^–3 and 0.88 × 10^–3 (in S m^–1), respectively, for complex 1- and 2-based thin-film devices (Figure 8). However, with photoirradiation, the conductivity of the complex 1- and 2-based devices significantly improved to 4.83 × 10^–3 and 2.02 × 10^–3 S m^–1, respectively. These results depict that the conductivity of the complex 1-based device improves nearly three times more than in the dark conditions, and for the complex 2-based device, it improves approximately 2 times more than in the dark conditions.

The representative I–V characteristics (Figure 8) of the complex-based devices clearly exhibited a nonlinear curvature with highly influencing rectifying nature under illumination, which is the signature of a light sensing SD. The photosensitivities for the complex (1 and 2)-based devices have been calculated as 4.41 and 3.14, respectively. At ±2 V, the rectification ratios (I_on/I_off) have been found to be 39.45 and 30.23 under the dark condition and 97.48, 65.46 under the illumination condition for the complex 1- and 2-based devices, respectively.

The I–V characteristics of the complex (1 and 2)-based SDs have been further analyzed by thermionic emission theory and Cheung’s method⁵³ by considering following standard equations^53,54

2

3

Here, I₀, q, k, T, V, A, η, and A* stand for the saturation current, the electronic charge, Boltzmann constant, temperature in Kelvin, forward bias voltage, effective diode area, ideality factor, and effective Richardson constant, respectively. The effective diode area has been estimated as 7.065 × 10^–6 m², and the effective Richardson constant has been considered as 32 AK^–2 cm^–2 for all the devices.

At low bias, linearity in current is observed, which is consistent with the electron lifetime, whereas at higher bias, voltage deviation from linearity occurs for the change in diode series resistance (Figure 8). From Cheung, in terms of series resistance, the forward bias I–V characteristics can be expressed as⁵³

4

where the IR_S term is the voltage drop across series resistance of the devices. In this context, the series resistance can be determined from the following functions using eq 5(55,56)

5

Equation 5 can also be expressed as a function of I as

6

The series resistance (R_S) and ideality factor (η) for both devices under the dark condition have been determined (Table 1) from the slope and intercept of dV/dln (I) versus I plot (Figure S8A), respectively. The deviation of η (Table 1) from the ideal value (1) may be for the existence of inhomogeneities of Schottky barrier height and series resistance at the junction.^57,58

Table 1. Schottky Device Parameters for the Complex-Based Thin-Film Devices^a.

							series resistance RS
sample	condition	on/off	photosensitivity	conductivity (× 10–3 S m–1)	ideality factor	barrier height (eV)	from dV/dln I (Ω)	from H (Ω)
complex 1	dark	39.45	4.41	1.53	2.23 (0.01)	0.41 (0.01)	676 (11)	662 (10)
	light	97.48		4.83	1.44 (0.03)	0.36 (0.03)	354 (6)	338 (7)
complex 2	dark	30.23	3.14	0.88	2.89 (0.01)	0.46 (0.01)	715 (10)	693 (15)
	light	65.46		2.02	1.87 (0.02)	0.39 (0.02)	438 (7)	414 (8)

^aFigures in the brackets are the associated errors.

The value of barrier height (Ø_B) can be calculated from eq 6 using the just obtained η values. A plot of H versus I will lead to a straight line (Figure S8B) with the y-axis intercept being equal to ηØ_B, which also gives the value of series resistance. The measured Ø_B, η, and R_S under dark and light conditions for the metal (Al)–semiconductor (synthesized complexes) (MS) junctions are listed in Table 1. From the table, it is clear that in both methods (eqs 5 and 6) using Cheung’s function the R_S value remains almost the same. Each parameter for the complex (1 and 2)-based thin-film MS devices (Table 1) improves the performance after light soaking. The ideality factor for both complexes approached very close to 1 under illumination, generally indicating the less interfacial charge recombination and better homogeneity of Schottky junctions⁵³ under photoirradiation. The higher rectification ratio under the photoirradiation condition attributes lower barrier height and reduction in series resistance than in the dark condition indicates the large increase of photocurrent. These observations together portrayed the applicability of the synthesized complexes in the field of optoelectronic devices. From this viewpoint, electrical conductivity data of some recently reported Cu-complexes along with our synthesized Cu-malonate complexes are provided in Table S8. Comparing these values, it can be concluded that our reported complexes have adequate potentiality for implementation in different optoelectronic devices.

The above listed Schottky device parameters (Table 1) clearly reveal better performance of the complex 1-based device over the complex 2-based device. A justification for this behavior may be proposed based on the structural features and supramolecular behavior of the complexes. In complex 1 having a 1-D coordination polymeric structure (Figure S3), charge can move through continuous chains of covalent and coordination bonds (through bond conduction). On the other hand, in monomeric complex 2, charge transfer might involve much weaker C–H···O hydrogen bonding interaction in 1-D chains (Figure S5 and the discussion therein), which is less effective in the coordination polymeric system.²⁴ Noncovalent interactions present in their solid-state structure furnish higher dimensionality to both the complexes. In complex 1, the parallel chains are interconnected through O–H···O (1.99 Å) and C–H···O (2.38 Å) hydrogen bonding and generate a 2-D framework (Figure S4), whereas in complex 2, lone-pair (lp)···π [3.093(2) Å] interaction interconnects the parallel supramolecular chains to generate a 2-D supramolecular assembly (Figure S6). This implies that the shorter interchain distance in complex 1 may facilitate the electrical properties in complex 1 than in complex 2.²⁵ These facts together may enhance the charge transport properties in complex 1.

The charge transport phenomenon was further analyzed from the I–V plot in the logarithmic scale under both conditions (Figure 9A). log I versus log V curves (Figure 9A) exhibit two dissimilar slopes (region-I and region-II) with different values. In region-I (slope ≈ 1), current follows the relation I ∝ V (Ohmic regime), and in region-II (slope ≈ 2), current is proportional to V², typical of a trap free space-charge-limited current (SCLC) regime^53,59 which has been implemented here for the estimation of some vital parameters such as effective mobility (μ_eff), transit time (τ), and diffusion length of charge carrier (L_D).^55,56,59

Figure 9.

(A) log I vs log V and (B) I vs V² curves under both dark and illumination conditions for the complex (1 and 2)-based thin-film device.

Following the SCLC model, the effective carrier mobility was calculated from the higher voltage region of I vs V² plot (Figure 9B) by the Mott–Gurney equation,^53,56,59

7

where V, ε₀, ε_r, and d stand for the applied voltage, permittivity of free space, dielectric constant, and thickness (considered as ∼0.9 μm), respectively, of the film.

To calculate the ε_r value, capacitance versus frequency (Figure S9) has been plotted, and using the value of capacitance at the saturation level, ε_r has been calculated by the following equation⁵³

8

Here, C is the value of capacitance at saturation, L is the thickness of the film, and A is the effective area.

The values of capacitance at saturation point for complexes 1 and 2 are 1.23 × 10^–10 and 7.13 × 10^–11 F, respectively. Using these values, dielectric constants of complexes 1 and 2 have been evaluated as 1.97 and 1.14, respectively.

Transit time (τ) has been evaluated using eq 9 from the slope of the SCLC region (region-II) (Figure 9A).⁵³ Diffusion length of the charge carriers is further extracted from eq 10(53)

9

10

where D is the diffusion coefficient. Carrier lifetime has been extracted from the slope of region-II in log I versus log V graph, which is shown in Figure 9A. Using the Einstein–Smoluchowski equation,⁵³ the diffusion coefficient has been calculated as

11

All the estimated parameters from the SCLC region are listed in Table 2. Under irradiation, the increased mobility represents higher transport rate, and the number of charge carriers also increases under light. Again, the increased diffusion length under the same condition revealed that the charge carriers travel extra length before recombination, which direct the eventual increase in current displayed by the devices under light. From these estimated values, it can be clearly stated that the charge transport properties of the complexes quite improved after light soaking (Table 2).

Table 2. Charge Conducting Parameters of the Complex-Based Thin-Film Devices.

sample	condition	effective mobility μeff (× 10–9 m2 V–1 s–1)	transit time τ (× 10–5 s)	μeff τ (× 10–13 m2 V–1)	diffusion coefficient D (× 10–11 m2 s–1)	diffusion length LD (× 10–7 m)
complex 1	dark	2.65	11.11	2.94	6.62	1.21
	light	12.24	4.45	5.42	30.57	1.64
complex 2	dark	3.17	8.36	2.65	7.92	1.15
	light	11.43	3.04	3.46	28.85	1.31

2.6. Theoretical Rationalization

The density functional theory (DFT) calculation of the semiconductor properties has been carried out only for complex 1 because complex 2 exhibits an experimental BG higher than 4 eV, which is considered beyond the upper limit (4 eV) to belong to the wide BG semiconductor family. The structure of complex 1 was modeled using DFT and the experimental crystal lattice as the starting point for the optimization of the atomic positions. Standard band theory along with partial density of state (PDOS) calculations has been employed for the analysis of the properties of the optimized structure. For plotting the Kohn–Sham electronic energy levels as a function of the reciprocal space vector k, we have chosen a path along the first Brillouin zone (BZ) passing through a set of high symmetry points, as depicted in Figure 10.

Figure 10.

Electronic band structures for the ground state of complex 1. Points of high symmetry in the first BZ are labeled as follows: G = (0, 0, 0); F = (0, 0.5, 0); Q = (0, 0.5, 0.5); Z = (0, 0, 0.5).

A direct BG of 3.38 eV is obtained from UV–vis spectra, while the theoretical BG gives an underestimated value of 2.78 eV. The theoretical methodology gives the properties of the pure isolated material, whereas other effects, such as metal–semiconductor interface, semiconductor thickness, excitons, defects, and impurities, influence the experimental BG, which are obviously not considered in the theoretical calculations. Moreover, previous reports have shown that BGs are underestimated by DFT methods.⁶⁰

Our results show that the direct BG occurs in the Γ (0, 0, 0) point and belongs to the semiconductor family in agreement with experimental findings. The semiconductor nature of the material also agrees with the results obtained from DOS calculations that are shown in Figures 11 and 12.

Figure 11.

Total DOS calculation of complex 1. The lines represent the s-orbital character (blue), p-orbital character (red), and d-orbital character (green) of the atoms into the crystal.

Figure 12.

Calculated PDOSs in the solid state of complex 1. (A) α-Cu atoms, (B) malonic acid ligands, (C) 2-amino-4-methylpyridine ligands, and (D) hexafluorophosphate anions.

Figure 11 shows the total DOS of alpha and beta electrons of complex 1. The PDOS analysis shows the presence of isolated intermediate bands because of β electrons in the Cu 3d orbitals likely because of metal–ligand interactions,⁶¹ which have not been considered for the calculation of the BG and the subsequent analyses because they are very weakly populated.

The PDOS calculation (Figure 12) shows that the p-character of the malonic acid ligands and the d-character of Cu atoms (a lesser extent) dominate the valence bands. However, the conduction band is dominated by the p-character of the 2-amino-4-methylpyridine ligands. Hexafluorophosphate anions do not directly influence the upper and lower states of the BG in agreement with other hybrid materials.⁶²⁻⁶⁴

By means of DFT calculations, it is possible to relate the frequency dependence of an incident photon in a material with the calculation of the dielectric function ε(ω) (Figure 13).

Figure 13.

The left panel shows the real and imaginary (solid and dotted lines, respectively) parts of ε(ω) vs the photon energy in complex 1. The right panel shows the real and imaginary parts (solid and dotted lines, respectively) of optical conductivity vs the photon energy of complex 1. The incident “x”, “y”, and “z” directions of polarized radiation are represented in blue, green, and red lines, respectively.

How much material is polarized upon application of an electric field is given by the real part of ε(ω) and how much material is absorbed is given by the imaginary part. A photon-energy range of 0–10 eV has been used to measure the optical response in x, y, and z directions where the crystal is illuminated (ε_xx, ε_yy, ε_zz). In this material, the ε_xx and ε_yy components coincide, having the prominent peaks at 3.5, 4.4, 5.1, and 6.0 eV. However, the ε_zz component presents the most intense peaks at 4.4 and 5.0 eV, which can be assigned to transitions from the highest valence band to the lowest conduction band. The optical conductivity σ(ω) is convenient to study how the conductivity increases when the material is illuminated. The increment in photoconductivity and electrical conductivity is a consequence of the photon absorption.⁶¹ The calculation of σ(ω) (see Figure 13, right) shows that only particular wavelengths have noticeable optical response and that it is more important (higher intensity) in one direction into the crystal (green line). In particular, complex 1 starts to absorb at 2.0 eV, and the maximum peaks are located at 3.5, 4.3, 5.0, and 6.0 eV.

3. Conclusions

In conclusion, we have synthesized two Cu(II)-malonate complexes (complexes 1 and 2) with different auxiliary ligands, established their solid-state crystal structures, and explored the noncovalent interactions associated with their solid-state structures. We have also investigated the optical, dielectric, and electrical properties of both the complexes. The binding motifs and solid-state structure of the Cu(II)-malonate moiety are different in the two complexes. Again, the change in auxiliary ligands also alters the supramolecular behavior for complexes 1 and 2. These structural variations and different supramolecular behaviors may collectively affect the electrical properties of the complexes. All the parameters obtained from electrical characterization exhibit better result for complex 1 than for complex 2. In the case of complex 1, the polymeric structure and extended supramolecular network may be the reason behind its superior performance. In the Al/(complex 1, 2)/ITO configuration, Schottky device parameters of both complexes show better performance on illumination to visible light. Thus, it emerges that these kinds of complexes will have potentiality in fabricating optoelectronic devices.

4. Experimental Section

4.1. Physical Measurements

Elemental analyses (C, H, N) were performed on a PerkinElmer 240C elemental analyzer. IR spectra were recorded during the Fourier transform IR spectral measurement with the samples following the attenuated total reflectance technique on a PerkinElmer spectrometer (Spectrum Two). Solid-state UV–vis spectroscopy was carried out using a PerkinElmer UV–vis Lambda 365 instrument. The electrical characterization was performed with a Keithley 2635B source meter, interfaced with a personal computer.

4.2. Materials

All reactions were carried out under aerobic conditions and in aqueous medium. Malonic acid, copper(II) chloride dihydrate, 2-amino-4-methylpyridine, 2-aminopyrimidine, and ammonium hexafluorophosphate were purchased from Sigma-Aldrich Chemical Co. All chemicals used were of reagent grade and used without further purification. Freshly boiled, doubly distilled water was used throughout.

4.3. Synthesis of Complex 1

Copper(II) chloride dihydrate (0.170 g, 1.0 mmol) was dissolved in 20 mL of water and allowed to react with malonic acid (0.208 g, 2.0 mmol) in water (25 mL) at 60 °C. A warm aqueous solution (20 mL) of 2-amino-4-methylpyridine (0.432 g, 4.0 mmol) was added dropwise to the above solution with continuous stirring. Finally, a warm aqueous solution (20 mL) of ammonium hexafluorophosphate (0.652 g, 4.0 mmol) was added to it under stirring conditions. The reaction mixture thus obtained was heated at 60 °C for an hour with continuous stirring. The resulting blue solution was then cooled to room temperature and filtered to remove any undissolved materials. Mother solution was then left unperturbed for crystallization at room temperature. Block-shaped blue single crystals suitable for single-crystal X-ray structure analysis were obtained after a few weeks. The crystals were separated by filtration, washed with ice-cold water, and then air-dried, yield 0.497 g (66%).

4.4. Synthesis of Complex 2

Complex 2 was synthesized using identical methods to those for complex 1 in aqueous medium using copper(II) chloride dihydrate (0.170 g, 1.0 mmol), malonic acid (0.208 g, 2.0 mmol), 2-aminopyrimidine (0.380 g, 4.0 mmol), and ammonium hexafluorophosphate (0.652 g, 4.0 mmol). The reaction mixture was heated at 60 °C for an hour with constant stirring. The resultant mother liquor was then cooled, filtered, and left unperturbed for crystallization at room temperature. The yielded block shaped, blue single crystals suitable for X-ray structure analysis were separated after a few weeks. The crystals were washed with cold water and dried in the air, yield 0.573 g (61%).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02971.

• X-ray crystallography study; crystal data table, bond length, and bond angle table; hydrogen bonding parameters; geometrical parameters for π···π interactions; geometrical parameters for lp···π and anion···π interactions; solid-state UV–vis study; device fabrication; computational details; IR plot of complexes 1 and 2; polymeric chain in complex 1; 2-D network in complex 1; polymeric chain through C–H···O bonding in complex 2; supramolecular network in complex 2; I–V graphs; dV/dln I versus I and H versus I plot; capacitance versus frequency graph; comparison of electrical parameters of complex 1 and complex 2; and comparison of electrical conductivity data (PDF)

• Crystallographic data of complex 1 (CIF)

• Crystallographic data of complex 2 (CIF)

The authors declare no competing financial interest.