Effect of Organic Cation on Optical Properties of [A]Mn(H₂POO)₃ Hybrid Perovskites

Abstract

Hybrid organic–inorganic compounds crystallizing in a three-dimensional (3D) perovskite-type architecture have attracted considerable attention due to their multifunctional properties. One of the most intriguing groups is perovskites with hypophosphite linkers. Herein, the optical properties of six hybrid hypophosphite perovskites containing manganese ions are presented. The band gaps of these compounds, as well as the luminescence properties of the octahedrally coordinated Mn²⁺ ions associated with the ⁴T_1g(G) → ⁶A_1g(S) transition are shown to be dependent on the organic cation type and Goldschmidt tolerance factor. Thus, a correlation between essential structural features of Mn-based hybrid hypophosphites and their optical properties was observed. Additionally, the broad infrared luminescence of the studied compounds was examined for potential application in an indoor lighting system for plant growth.

1. Introduction

Hybrid organic–inorganic compounds with a three-dimensional (3D) perovskite structure constitute a very large family of materials. Their general formula is ABX₃, where A is an alkali metal or an organic cation, B is a divalent metal cation, and X denotes an organic or inorganic anion (halide, HCOO⁻, N₃⁻, CN⁻, N(CN)₂⁻, or H₂PO₂⁻). The 3D perovskite structure consists of corner-sharing BX₆ (X = Cl, Br, I, O, or N) octahedral units. Metal ions and linkers form a three-dimensional framework, and organic cations are accommodated inside the created voids. Due to their unique physical phenomena, such as ferroelectricity, multiferroicity, and dielectricity, as well as magnetic and luminescent properties, these compounds have been extensively studied in recent years [1,2,3,4,5,6,7,8,9,10,11]. Hybrid organic–inorganic perovskites (HOIPs) are frequently called multifunctional materials because they exhibit several physical phenomena at the same time [12,13,14,15,16,17,18,19]. As a consequence, HOIPs offer a wide range of potential applications for catalysis, gas storage, drug delivery, nonlinear optics, solid-state lighting, luminescent sensors, temperature sensors, biomedical photonics, and numerous other utilizations [12,13,15,19,20,21,22].

Lead halides are one of the most commonly investigated families of HOIPs [4,12,23,24,25]. For instance, lead halides containing methylammonium (MA⁺⁾ or formamidinium (FA⁺) cations attract great interest due to their promising application in solar cells [26,27,28,29], whereas methylhydrazinium (MHy⁺) lead halides have been reported to exhibit multiple nonlinear optical phenomena [4,24,30]. It is worth noting that the band gap value (E_g) and the position of the free exciton (FE) emission depend both on the halide used and the type of organic cation. Generally, the A-site organic cation is often ordered at low temperatures and becomes disordered as the temperature rises. Furthermore, organic cations have different sizes, shapes, and the ability to form hydrogen bonds (HBs) with the metal-ligand framework [31,32,33].

Three-dimensional hypophosphites are a relatively novel family of HOIPs. They were obtained for the first time in 2017 [34]. Unfortunately, the number of hypophosphite perovskites discovered is still low [34,35,36,37,38]. To date, only a few manganese, cadmium, and magnesium hypophosphite analogs have been reported [34,35,36,37,38]. Structural data revealed that metal–hypophosphite frameworks tend to have unconventional tilts, columnar shifts, and off-center positions of the organic cations [33,34], which may induce ferroelectricity or other functional properties [33,34,35]. Furthermore, some exhibit magnetic order, although this feature strictly depends on the type of organic cation, e.g., [MHy]Mn(H₂POO)₃ has an antiferromagnetic order near 6.5 K, [FA]Mn(H₂POO)₃ shows an antiferromagnetic order below 2.4 K, and [EA] Mn(H₂POO)₃ (EA⁺ = ethylammonium) exhibits a magnetic order at 7.0 K [36,38].

Manganese and cadmium hypophosphites can also show bright and broadband photoluminescence (PL) [38]. The optical properties of divalent manganese strongly depend on the crystal field. Due to the 3d⁵ configuration, Mn²⁺ electrons are in the outer orbital and are sensitive to changes in their immediate surroundings. Mn²⁺ in the tetrahedral coordination exhibits green emission characteristics for the ion in a weak crystal field, whereas in the octahedral site (strong crystal field), it possesses red infrared emission. It is well-known that the type of A-site cation strongly influences the structural and optical properties of hypophosphite perovskites. For this reason, the photoluminescence (PL) properties of six manganese hypophosphites, namely [DMA]Mn(H₂POO)₃, [EA]Mn(H₂POO)₃, [MHy]Mn(H₂POO)₃, [FA]Mn(H₂POO)₃, [Pyr]Mn(H₂POO)₃, and [IM]Mn(H₂POO)₃ (DMA⁺ = dimethylammonium, Pyr⁺ = pyrrolidinium, and IM⁺ = imidazolium) were investigated. Four ammonium cations are aliphatic, one is cyclic, and one is aromatic. Figure 1 presents the structural formulae and ionic radii of chosen ammonium cations. This paper presents some general remarks on the influence of the organic cations on the E_g value, PL band position, and thermal stability of the observed emission.

2. Results and Discussion

2.1. Crystal Structure

The difference in the ionic radius, the shape of the amine used, and the number of hydrogen bonds formed with the hypophosphite skeleton affect the crystal structure of the obtained compounds. Five of the six obtained compounds crystallize in the monoclinic system in the P2₁/n space group. Among these compounds, [FA]Mn(H₂POO)₃ is unique because it transforms at 178 K into another monoclinic system with the C2/c space group (polymorph I). Manganese ions occupy three non-equivalent positions in the [DMA] Mn(H₂POO)₃ structure; two positions in the EA, IM, and Pyr analogs; and one position in the FA compound. [MHy] Mn(H₂POO)₃ is the only hypophosphite that adopts the orthorhombic Pnma symmetry, with the manganese ions occupying only one independent site. Figure 2 presents a visualization of the crystal structures of the investigated hypophosphites. Further details of structural properties can be found in recently published papers [35,37,38], and in the next paragraph, the influence of the distortion of the nearest Mn²⁺ environment and the number of occupied sites on the PL properties of manganese hypophosphites is discussed. The description of sample synthesis and measurement technique is shown in Supplementary Materials.

2.2. Spectroscopic Properties

The absorption spectra of the investigated manganese hypophosphites were measured at room temperature. As can be seen in Figure 3a, they consist of strong host absorption at around 220 nm and less intense bands associated with the absorption of Mn²⁺ ions located in octahedral coordination. Thus, in Mn²⁺ ions, the absorption of the excitation energy occurs between the ⁶A_1g ground state and the ⁴T_1g(P) (275 nm), ⁴E(D) (339 nm), ⁴T_2g(D) (358 nm), ⁴A_1g and ⁴E_g(G) (404 nm), ⁴T_2g(G) (435 nm), and ⁴T_1g(G) (524 nm) excited levels [15,17,39]. The position of these bands is clearly visible for all obtained manganese hypophosphite perovskites. The optical band gap (E_g) was estimated using the Kubelka–Munk function. The determined E_g values are presented in the scheme shown in Figure 3b and listed in Table 1. The smallest E_g value of 4.98 eV is observed for [DMA]Mn(H₂POO)₃ and increases up to 5.32 eV for the [FA]Mn(H₂POO)₃ compound. At first glance, there is no relationship between the ionic radii of the ammonium cation and the E_g value. However, a more careful analysis shows that some general conclusions can be drawn. First of all, the samples containing linear amines and the samples comprising pyrrolidinium and imidazolium should be analyzed separately. Furthermore, it appears that the ionic radii of the chosen organic cation are insignificant; however, the value of the Goldschmidt tolerance factor (t_F) is of major importance. This geometrical parameter estimates the ion size mismatches that the perovskite structure is able to tolerate. To calculate the t_F parameters for the investigated manganese hypophosphites, the modified Goldschmidt equation was applied as appropriate for HOIPs [31,32]:

$$t_F=\frac{\left(r_{Aeff}+r_{xeff}\right)}{\sqrt{2}\left(r_B+0.5h_{xeff}\right)}$$

(1)

where r_B denotes the ionic radii of cation B in the perovskite structure, and r_ieff and h_ieff are the effective ionic radii and the effective height of the used organic amine (A) and ligand (X), respectively. The determined E_g values, together with the ionic radii of ammonium cations and t_F values, are given in Table 1. As can be seen, the E_g of [A]Mn(H₂POO)₃ increases as t_F decreases. In other words, the optical band gap is smaller for a more perfect perovskite structure. However, this relation is not valid for the cyclic or aromatic amines, as [Pyr]Mn(H₂POO)₃ has a slightly higher value of E_g than [IM]Mn(H₂POO)₃, in spite of its larger t_F.

The normalized PL spectra of the prepared hybrid hypophosphites registered at 80 K are presented in Figure 4a. The PL bands are very broad and range from 600 nm to 900 nm. The full width at the half maximum (FWHM) value of these bands is approximately 2200 cm⁻¹. After excitation under 266 nm, the electron moves to the ⁴T_1g(P) excited level (Figure 4b). Then, as a result of non-radiative depopulation, the electron is transferred to the ⁴T_1g(G) state. The radiative recombination of the electron from the ⁴T_1g(G) level to the ⁶A_1g(S) ground state generates intense red PL. The shape of the observed PL bands does not change with the applied organic cation, but the type of organic cation affects the band position (Table 1). The most redshifted PL is observed for the EA and MHy samples, with maxima at 688 nm and 686 nm, respectively. On the other hand, the emissions of the IM and FA compounds are positioned at 646 nm and 656 nm, respectively. PL of the DMA and Pyr analogs is located in the intermediate range, i.e., at 670 nm and 675 nm, respectively. It is worth mentioning that the emission of Mn-based dicyanamide perovskites was observed at lower wavelengths of about 626–629 nm, which is a result of using a shorter H₂POO⁻ ligand instead of the long dicyanamide linkers [17,40]. The presented redshift of the emission is a natural consequence of the increasing crystal field strength acting on Mn²⁺ ions. The analysis of the manganese octahedron deviation (δ) from the ideal shape and position of the PL band shows a certain relationship between these parameters for the FA, EA, and MHy compounds (Table 1). In particular, if the Mn²⁺ ions are located in an almost perfect octahedron, and the strong crystal field causes a large splitting of energy levels and a significant redshift of the emission. [DMA]Mn(H₂POO)₃ deviates from this trend because the generated PL is a superposition of three emission bands belonging to Mn²⁺ ions located at three non-equivalent sites in the crystal structure. This trend is also observed for the IM and Pyr analogs. The considerable deviation (δ, 1.35%) of manganese octahedra in [IM]Mn(H₂POO)₃ shifts the emission band to a lower wavelength (λ_em = 646 nm), whereas almost perfect octahedra in [Pyr]Mn(H₂POO)₃ (δ is only 0.18%) lead to a redshifted emission at λ_em = 675 nm. Regardless of the observed changes in the position of the band maxima, all samples exhibit red emission at 80 K. The chromatic coordinates of the investigated compounds are demonstrated in Figure 4c.

Table 1. List of the most important parameters of [A]Mn(H₂POO)₃ perovskites, such as ionic radius (IR) of the ammonium cations [41], tolerance factor (t_F), energy band gap (E_g) [36,37,38], the position of the PL band at 80 K (λ_max), octahedral deviation (δ) from the perfect shape [36,37,38], and quenching temperature (T_0.5)_.

	DMA+	EA+	MHy+	FA+	IM+	Pyr+
IR (pm)	272	344	264	253	320	258
tF	0.90	0.905	0.891	0.858	0.869	0.90
Eg (eV)	4.98	5.17	5.20	5.32	5.14	5.18
λmax (nm)	670	688	686	656	646	675
δ (%)	0.17	0.39	0.44	10.6	1.35	0.18
T0.5 (K)	100	130	122	135	135	115

Table 1. List of the most important parameters of [A]Mn(H₂POO)₃ perovskites, such as ionic radius (IR) of the ammonium cations [41], tolerance factor (t_F), energy band gap (E_g) [36,37,38], the position of the PL band at 80 K (λ_max), octahedral deviation (δ) from the perfect shape [36,37,38], and quenching temperature (T_0.5)_.

	DMA+	EA+	MHy+	FA+	IM+	Pyr+
IR (pm)	272	344	264	253	320	258
tF	0.90	0.905	0.891	0.858	0.869	0.90
Eg (eV)	4.98	5.17	5.20	5.32	5.14	5.18
λmax (nm)	670	688	686	656	646	675
δ (%)	0.17	0.39	0.44	10.6	1.35	0.18
T0.5 (K)	100	130	122	135	135	115

The broad emission localized in the near-infrared region can be attractive for plant growth. The absorbed photoenergy can be used by plants in the photosynthesis process. The main photopigments found in plants are chlorophyll A, chlorophyll B, phytochrome P_R, and phytochrome P_FR [42]. The first two types of photopigments mainly absorb blue light (400–500 nm), whereas P_R and P_FR are sensitive to red and far-infrared light, respectively [42]. As can be clearly seen in Figure 5, the emission generated from Mn-based hybrid hypophosphite perovskite covers almost the entire absorption range of the P_R and P_FR phytochromes. Nevertheless, it appears that [DMA]Mn(H₂POO)₃ emission coincides to the greatest extent with the absorption of both types of phytochrome. The biggest disadvantage is the low thermal stability of [A]Mn(H₂POO)₃ sample emissions (Figure 6 and Figure 7). At the moment, it seems impossible to construct a device that would keep the hypophosphite crystal at the temperature of 80 K.

The temperature-dependent PL spectra of the investigated samples recorded from 80 K to 320 K every 10 K are presented in Figure 6. Almost all hypophosphites exhibit the highest emission intensity at 80 K. However, the thermal stability of [IM]Mn(H₂POO)₃ and [EA]Mn(H₂POO)₃ emissions is quite different. At the beginning, the intensity of Mn²⁺ emission decreases and then increases. This abnormal thermal behavior can be explained by energy transfer between the Mn²⁺ ions located at two independent optical sites [36]. A common feature of all investigated perovskites is the asymmetric shape of the PL band, moving the band position to a higher wavelength with sample heating (Tab 7a). This behavior can be associated with the Mn²⁺ ions occupying more than one optical site and small changes in the crystal field strength with increasing temperature [23,36,43,44]. A careful analysis of the obtained results shows that the emission of the [DMA]Mn(H₂POO)₃ analog quenches much faster than the remaining samples. The quenching temperature, defined as the temperature at which intensity drops to half the initial value (T_0.5), is 100 K for the DMA compound (Figure 7b). [Pyr]Mn(H₂POO)₃ and [MHy]Mn(H₂POO)₃ exhibit significantly better thermal stability, with T_0.5 values of 115 K and 122 K, respectively. However, the emission of Mn²⁺ ions in the samples comprising EA⁺, FA⁺, and IM⁺ has the highest thermal stability. The emission of manganese ions is quenched by the crossing of the ⁴T_1g(G) excited state with the parabola of the ⁶A₁(S) ground level (Figure 7c). Nevertheless, temperature-dependent measurements proved that Mn²⁺ ion emission in the hypophosphite perovskites quenched faster than in the case of the Mn-based dicyanamides.

The presented results explain the influence of some key structural factors on the optical properties of the family of Mn-based hybrid hypophosphites. Figure 8 summarizes the correlation between structural parameters, such as t_F, octahedral deviation (δ), the number of Mn²⁺ sites, and optical features. As can be seen, the compounds comprising linear cations should be analyzed separately from those with cyclic organic cations. Furthermore, there is no doubt that there is a correlation between the Goldschmidt tolerance factor and the energy band gap value. The higher the t_F value, the lower the E_g, so the optical band gap decreases for the samples with an almost ideal perovskite structure. For the IM and Pyr analogs, the t_F and E_g values are comparable; therefore, it is impossible to define any rules. Analysis of the results allowed for the correlation of the octahedral deviation with the position of the PL band. The shift of the PL band is greater for manganese ions occupying the almost perfect octahedron. The results reported for [DMA]Mn(H₂POO)₃ do not comply with this rule due to the three independent optical sites of Mn²⁺ ions.

3. Conclusions

This paper presents the results of an investigation on the effect of organic cations on the PL properties of Mn²⁺ ions. Mn-based hybrid hypophosphites were synthesized by crystallization from solution. Five of the six obtained compounds crystallize in the monoclinic system in the P2₁/n space group. The only exception is [MHy]Mn(H₂POO)₃, which adopts an orthorhombic structure (Pnma space group). The obtained results indicate that the type of ammonium cation has a significant impact on the Goldschmidt tolerance factor value and the octahedral deformation parameter. As a consequence, the size of the energy band gap and the position of the emission band are tuned. It was found that the samples with a higher tolerance factor have a smaller energy band gap. Moreover, the reported redshift of the PL band position of the investigated compounds is associated with a smaller octahedral deformation. However, the temperature-dependent measurements prove that Mn²⁺ emission in the hypophosphite perovskites quenches faster than the PL of the Mn-based dicyanamides. The broad red PL of Mn²⁺ ions assigned to the ⁴T_1g(G) → ⁶A_1g(S) transition overlaps with the absorption of the phytochrome P_R and the phytochrome P_FR. Therefore, the investigated hypophosphites may be applied in an indoor lighting system for plant growth. However, a considerably disadvantage of these compounds is the low thermal stability of their PL.