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Journal of Molecular Modeling

, 24:230 | Cite as

Ligand-induced symmetry breaking and concomitant blueshift in the emission wavelength of an octahedral chromium complex

  • Manoj Majumder
  • Satadal Paul
  • Anirban MisraEmail author
Original Paper
  • 85 Downloads
Part of the following topical collections:
  1. International Conference on Systems and Processes in Physics, Chemistry and Biology (ICSPPCB-2018) in honor of Professor Pratim K. Chattaraj on his sixtieth birthday

Abstract

The resulting distortion of the octahedral symmetry of the complex [CrIII(NH3)6]3+ upon replacing the axial ligands with halides (i.e., weaker ligands) affects the stability of the doublet state with respect to that of the quartet ground state. This substitution affects the doublet-to-quartet transition responsible for phosphorescence. The position of the halide with respect to ammonia in the spectrochemical series is a major influence on the emission wavelength of the complex. The close proximity of fluorine and ammonia in the spectrochemical series leads to a blueshift in the emission wavelength when fluoride ions are introduced into the complex, thus providing a rational approach to the design of blue-phosphorescent materials, which are desirable for OLEDs used in full-color displays.

Graphical abstract

Shifts in the phosphorescence emission wavelength of an octahedral Cr(III) complex caused by axial ligand substitution. Replacing the axial ligands leads to a change in the relative positions of the axial and equatorial ligands in the spectrochemical series, which in turn induces a redshift or a blueshift in the emission wavelength

Keywords

Blueshift Quartet Luminescence OLED TDDFT 

Introduction

Luminescent materials have attracted considerable interest [1, 2, 3, 4, 5, 6] due to their multifarious applications in, for example, organic light-emitting diodes (OLEDS) [7, 8, 9, 10, 11, 12, 13, 14] and light-emitting electrochemical cells (LEECS) [15, 16, 17, 18], as light absorbers in dye-sensitized solar cells [19, 20, 21, 22], and in chemosensors, biosensors [23, 24, 25, 26, 27], lumophores for cell imaging [27, 28, 29, 30], and chemical photocatalysts [31, 32, 33, 34]. The suitability of luminescent materials for a specific application depends upon the stability of the system, the color purity of the emitted light, the quantum yield of the emission, the specific emission decay time, and several other factors. In the last few decades, considerable effort has been directed into identifying highly efficient emitters of the three primary colors (red, green, and blue), which are required in order to develop full-color displays. Although transition metal complexes that emit red and green light are abundant due to triplet harvesting [35, 36, 37], phosphorescent materials that emit blue light are relatively rare because a wide energy gap between the excited and the ground states is needed [38]. The strong spin–orbit coupling (SOC) that occurs in organometallic complexes containing heavy transition metals weakens the spin-forbiddenness of transitions between the ground and the excited states, so these systems partially fulfill the criteria of an ideal luminescent material [35, 39, 40]. However, the dd* transitions that take place in heavy metals strongly quench the luminescence of blue-phosphorescent materials [39, 41, 42, 43, 44, 45]. Another difficulty with heavy metals is triplet–triplet annihilation, which hinders the process of phosphorescence, making the material unfit for OLED applications [46, 47, 48, 49]. The best option for circumventing these difficulties with heavy metals is to use 3d transition metal complexes. For example, a Cu(I) complex was recently found by Yersin et al. [50] to be a blue emitter (λmax = 436 nm), although its emission decay time was too long to allow its use in high-brightness OLED applications. Hamada et al. reported that some Zn(II) complexes are greenish-white emitters [51]. However, among 3d transition metal complexes, Cr(III) systems have emerged as the most efficient luminescent materials due to their substantial quantum yields and their ability to both fluoresce and phosphoresce in liquid media at room temperature [52, 53, 54]. While there are a reasonable number of experimental reports on the photoresponsive behavior of Cr(III) complexes, a comprehensive theoretical account of the phosphorescence of an octahedral (Oh) Cr(III) complex is still awaited. All of these facts inspired us to theoretically investigate the luminescence properties of an octahedral Cr(III) complex.

In the first part of this paper, we introduce the hexa-amine Cr(III) system that we chose for this theoretical investigation. Subsequently, we provide the computational results for the absorption and emission wavelengths of this system, which are compared with corresponding experimental data in order to standardize the computational methodology. Octahedral Cr(III) complexes with weak ligands such as F, Cl, and Br have been found to exhibit fluorescence, and those with strong ligands such as carbonyl, ammonia, and cyano have been observed to yield phosphorescence [52]. However, a literature survey reveals a dearth of blue-emitting Cr(III) complexes compared to those emitting red or green. The second part of the paper summarizes our attempts to utilize the data obtained in the first part in order to identify an appropriate set of ligands for inducing a blueshift in the emission wavelength. Phosphorescence of a Oh Cr(III) complex involves excitation from the quartet ground state (Q) to the quartet excited state (Q′) followed by intersystem crossing (ISC) to the doublet state (D). The phosphorescence energy thus depends on the separation between the lowest doublet state (D) and the quartet ground state (Q), which is nearly equal to the absorption energy due to the Q → D transition [52]. Hence, in principle, excitation to the doublet state from the quartet ground state can lead to a blueshift in the emission wavelength. The doublet state can be attained for the Oh Cr(III) complex by lifting the degeneracy of the t2g orbitals. To do this, the symmetry of the Oh [CrIII(NH3)6]3+ complex is disturbed by replacing the axial ammonia ligands with weak halides (I, Br, Cl, and F). Thus, the doublet state can be induced by introducing asymmetry into the complex by substituting the axial ligands of [CrIII(NH3)6]3+. A trend in the change in the emission wavelength depending on the halogens included in the complex was observed. This study may therefore provide a useful guideline for the rational design of metal complexes with tailored luminescence behavior such as blue phosphorescence.

Computational methods

The B3PW91 functional was employed to optimize the geometries of the studied complexes in their ground states. This functional, based on generalized gradient approximations (GGAs), mixes some exact exchange with the trial DFT exchange–correlation functional, and works well for transition metals [55]. It has been successfully employed to study photophysical properties [56, 57]. However, to check how well other functionals perform, we also carried out the same set of calculations with the most popular GGA functional PBE and the meta-GGA functional TPSS. Since the hybrid GGA functional is higher than the pure GGA functional on the Jacob’s ladder of Perdew due to its ability to partially minimize the self-interaction energy, we present the results obtained using B3PW91 in the main text of this paper, while the PBE and TPSS results are given in the “Electronic supplementary material” (ESM). Hay and Wadt’s double-zeta effective core potential (LANL2DZ) was chosen for use as the basis set in all of the computations [58, 59]. Since the phosphorescence of the systems under investigation involves quartet and doublet spin states, an unrestricted formalism was adopted in the density functional theory (DFT) calculations. The absorption and emission energies of the reference systems were obtained using a time-dependent DFT (TDDFT) method coupled with the same exchange-correlation functional and basis set applied for geometry optimization of the ground quartet (Q) and doublet (D) states. The TDDFT approach offers a reliable route for the calculation of vertical electronic excitation spectra [60, 61], as demonstrated by the good agreement of experimental spectral properties with TDDFT results obtained for transition-metal complexes [62, 63, 64]. However, the solvent also plays an important role in determining excitation values. We calculated the energy values for different energy states (quartet, doublet, excited quartet) of the [CrIII(NH3)6]3+ complex and found that these states are only slightly more stable than expected from gas-phase calculations. Hence, in order to reduce computational effort, we performed gas-phase calculations in this work. That said, to get an accurate description of the quartet excited state (Q´) of each reference system, we resorted to a more sophisticated complete active space self-consistent field (CASSCF) method with an active space of (3, 5) and the LANL2DZ basis set. Using the initial guess obtained from the state-average calculation, the final geometry of the quartet excited state was then determined. Next, a single-point calculation was performed at the same DFT level for this optimized geometry of the excited-state molecule for comparison with the ground-state condition obtained at the DFT level. In each case, vibrational analysis was performed at the same level of DFT to check the structural stability of the system. All computations were done with the Gaussian 09W quantum chemical software package [65].

Results and discussion

The structural parameters and energies of the optimized geometries of [CrIII(NH3)6]3+ in its quartet ground state (Q), quartet excited state (Q´), and lowest-energy doublet state (D) are given in Table 1. In the quartet ground state, all of the Cr–NH3 bond lengths were found to be equal, indicating that the system has Oh symmetry. Molecular orbital (MO) analysis in the quartet ground state revealed that the main contributors to the highest occupied molecular orbital (HOMO) and the next two orbitals (HOMO – 1 and HOMO – 2) are the Cr d orbitals (dxz, dyz, and dxy). On the other hand, the main contributors to the lowest unoccupied molecular orbital (LUMO) and the LUMO + 1 orbitals are the Cr \( {d}_{z^2} \) and \( {d}_{x^2-{y}^2} \) orbitals (Fig. 1). Thus, in the octahedral field, we can confirm that the Cr MOs in this [CrIII(NH3)6]3+ complex split into t2g and eg orbitals (Fig. 1).
Table 1

Bond distances between Cr and the axial ligands (Cr–Lax) and between Cr and the equatorial ligands (Cr–Leq), and the energies of the [CrIII(NH3)6]3+ complex in the quartet ground state (Q), quartet excited state (Q´), and doublet ground state (D)

 

States

Cr–Lax (Å)

Cr–Leq (Å)

Relative energy (kcal/mol)

Open image in new window

Q

2.16

2.16

0

2.11

2.29

9.41

D

2.13

2.13

28.2

Fig. 1

t2g and eg orbitals in [CrIII(NH3)6]3+

According to an analysis of the TDDFT computations carried out for the quartet ground state of [CrIII(NH3)6]3+, the main excitations occur at 420 nm (Table 2), which is quite close to the experimental value of the absorption wavelength for this complex (Table 2) [52]. The main contribution to this absorption energy comes from electronic excitation from the occupied 37th spin-up orbital (α-HOMO) to the initially unoccupied 38th α-LUMO (Table S1a in the ESM). Thus, the quartet excited state (Q′) acquires the electronic configuration t2g2 eg1 due to spin-reserved electron excitation from the t2g to the eg level. The electronic configuration t2g2 eg1in the excited quartet state is supported by an elongation in the distance between Cr and each equatorial ligand compared to the corresponding distance in the quartet ground state (Q) (Table 1). In the quartet excited state, the 38th α-MO, which is primarily composed of the Cr \( {d}_{x^2-{y}^2} \) orbital, becomes populated through electronic excitation, and this occupation of the Cr \( {d}_{x^2-{y}^2} \) orbital leads to elongation in the equatorial (xy) plane. On the contrary, the axial Cr–NH3 distance remains almost unaltered in the quartet ground and excited states of the molecule (Table 1). Unlike the quartet excited state, the geometry in the doublet state is barely distorted compared to the quartet ground state (Table 1). In the doublet state, the electron distribution within the dxy, dyz, and dzx orbitals alters, but these orbitals are not directly oriented towards the ligands, so structural changes in the doublet state are nominal with respect to the quartet ground state (Table 1).
Table 2

Comparison of the computed absorption and emission wavelengths in nm (energies in cm−1) with experimental values for [CrIII(NH3)6]3+

 

Computed value

Experimental value [52]

Absorption (Q → Q′)

420 nm (23,804 cm−1)

21,550 cm−1

Emission (D → Q)

877 nm (11,403 cm−1)

15,120 cm−1

Then, to explore the mechanism for the intersystem crossing between the quartet excited state (Q′) and the doublet state (D), the vibrational modes of both states were closely examined. After a thorough analysis of the vibrational modes, the 20th mode of Q′ (vibrational energy = 379 cm−1) and the 16th mode of D (vibrational energy = 318 cm−1) were found to have oscillation vectors in the same direction (Fig. 2). The similar energies and oscillation patterns of these two vibrational modes suggest that the doublet and the quartet excited states may interconvert through these vibrations, as shown in Fig. 2. From the TDDFT computations for the lowest-energy doublet state, the emission wavelength for the D → Q transition was obtained as 877 nm, which is comparable to the experimental value for phosphorescence (Table 2) [52]. After standardizing the computational framework for estimating absorption and emission wavelengths, we shifted to the second part of this work, which was carried out using the same computational methodology as employed in the first part.
Fig. 2a–b

Oscillation vectors for a the 16th vibrational mode in the D state and b the 20th vibrational mode in the Q′ state of [CrIII(NH3)6]3+

As outlined in the “Introduction,” the axial ammonia ligands in [CrIII(NH3)6]3+ were substituted by (weaker) halogen ligands to disrupt the Oh symmetry of the complex. This was expected to lift the degeneracy of the singly occupied t2g orbitals, as shown in Fig. 3, and to induce the doublet state when the axial and equatorial ligands differed sufficiently in strength based on their positions in the spectrochemical series. Table 3 displays the structural parameters of the optimized geometries and the energies of all the halogen-substituted octahedral Cr(III) complexes.
Fig. 3

Orbital splitting patterns in the quartet ground state (Q), the quartet excited state (Q′), and the doublet state (D) of [CrIII(NH3)6]3+

Table 3

Distances between Cr and each axial ligand (Cr–Lax), and between Cr and each equatorial ligand (Cr–Leq), and energies of the complexes [CrIII(NH3)4(X)2]+ (X = I, Br, Cl, or F, shown in violet, brown, green, and light blue, respectively)

In the quartet ground state (Q), the diiodo-, dibromo-, and dichloro-substituted complexes were found to be less stable than [CrIII(NH3)6]3+, whereas the difluoro complex was more stable (Tables 1 and 3). The distance between the metal and each apical ligand (Cr–Lax) provides the explanation for this observation. For the first three substituted complexes, the Cr–Lax distance was larger than that in [CrIII(NH3)6]3+, whereas for [CrIII(NH3)4F2]+, the Cr–Lax distance was much shorter than that in [CrIII(NH3)6]3+ (Tables 1 and 3). Hence, in the present case, the bonding between the Cr and the axial ligands contributes significantly to the stability of the halogen-substituted complexes. In the quartet excited states (Q′) of all the halogen-substituted complexes, the bond distances in the equatorial (xy) plane were found to be stretched compared to those in the quartet ground state (Q), with little variation observed in the Cr–La distance. The bond stretching that occurs in the xy plane of the Q′ states compared to the Q states of these complexes can be attributed to electronic excitation in the \( {d}_{x^2-{y}^2} \) orbital, similar to the case for [CrIII(NH3)6]3+. To explore the stabilities of the complexes, we calculated the hardness of the quartet ground state for each complex, and the results are given in Table 4.
Table 4

Calculated hardness values (in eV) for the studied complexes (quartet ground state)

System

Hardness (eV)

[CrIII(NH3)6]3+

3.34

[CrIII(NH3)4(F)2]+

2.37

[CrIII(NH3)4(Cl)2]+

2.2

[CrIII(NH3)4(Br)2]+

2.18

[CrIII(NH3)4(I)2]+

1.15

From Table 4, we can conclude that [CrIII(NH3)4(F)2]+ is the most stable of the halide-substituted systems, as the largest hardness value implies the greatest stability [66]. Based on the TDDFT computations of the quartet ground states of the halogen-substituted complexes, the lowest transition was found to occur from the α-occupied HOMO (the 35th α-MO for the diiodo-, dibromo-, and dichloro-substituted complexes and the 37th α-MO for the difluoro-substituted complex) to the initially unoccupied LUMO (the 36th α-MO for the diiodo-, dibromo-, and dichloro-substituted complexes and the 38th α-MO for the difluoro-substituted complex), as indicated by Table S1 and Fig. S1 of the ESM. The absorption wavelengths of all the halogen-substituted complexes are given in Table 5. Also, for these complexes, HOMO to HOMO – 2 and LUMO to LUMO +1 belong to the t2g and eg orbitals, as also seen for [CrIII(NH3)6]3+ (Fig. S1a–h in the ESM). In all of the halogen-substituted complexes, there is a high possibility of intersystem crossing between the quartet excited state and the doublet state, as verified by the similar vibrational modes and energies of these two spin states (Table S2 in the ESM).
Table 5

Comparison of the absorption and emission wavelengths in nm (energies in cm−1) of the complexes [CrIII(NH3)4(X)2]+ (X = I, Br, Cl, or F)

Complex

Absorption (Q → Q′)

Emission (D → Q)

[CrIII(NH3)4I2]+

618.4 (16171)

1280.7 (7808)

[CrIII(NH3)4Br2]+

554.9 (18021)

1416.4 (7060)

[CrIII(NH3)4Cl2]+

502.2 (19912)

968 (10331)

[CrIII(NH3)4F2]+

555.4 (18004)

453.8 (22036)

According to TDDFT computations for the doublet state, the ligands control the emission frequency of each halogen-substituted complex. From an application perspective, the most interesting observation is the trend in the variation of the emission wavelength with the halogen ligands included in the complex, which can be explained by the different positions of the different halogens in the spectrochemical series. I and Br are weak ligands, in contrast to ammonia, which is a strong ligand. When weak ligands are placed along the z-axis, the dxz and dyz orbitals become more stable than the dxy orbital, so the threefold degeneracy of the t2g level is lifted (Fig. 3). This splitting of the t2g level stabilizes the doublet state compared to the quartet ground state, resulting in a decrease in the emission energy, as confirmed by Table 4. Hence, replacing the axial ligands in [CrIII(NH3)6]3+ with weak I or Br ligands causes a redshift in the emission wavelength. It is also interesting to note that the almost equal emission wavelengths of the diiodo and dibromo complexes correlate with the similar positions of I and Br in the spectrochemical series. On the other hand, a blueshift in the emission wavelength is observed for the difluoro compound with respect to the Oh [CrIII(NH3)6]3+ complex. This can again be explained by the relative strengths of the axial and equatorial ligands. Among the halogens, F is closest to ammonia in the spectrochemical series, meaning that F substitution of the axial ligands leads to a less asymmetric t2g than if I or Br ligands are inserted into the complex. This in turn implies that the doublet state becomes less favored, increasing the energy gap between the doublet state and the quartet ground state, which is reflected in an increased emission frequency. In line with this explanation, in the dichloro-substituted complex, the emission wavelength is between those of [CrIII(NH3)4F2]+ and [CrIII(NH3)4I2]+ or [CrIII(NH3)4Br2]+. This trend again correlates well with the position of chloride (i.e., between F and I, Br) in the spectrochemical series. In order to explore the variation in the emission wavelength caused by replacing the axial ligands with ligands other than halides, we also inserted OH and CN ligands. The former ligand is close to F and the latter is quite far from F in the spectrochemical series. The complex containing the stronger ligand, CN, exhibited emission in virtually the same region (676 nm) as [CrIII(NH3)6]3+, whereas the complex containing OH showed redshifted emission (1823 nm).

Conclusions

In the present work, the photophysical properties of an Oh Cr(III) complex was studied in the framework of density functional theory. In the first part of this study, the absorption and emission wavelengths for this complex were obtained using TDDFT, and were found to correspond well with the known phosphorescent behavior of the complex. In the second part of the study, the axial ligands in the Oh [CrIII(NH3)6]3+ complex were replaced with weaker ligands in order to disrupt the Oh symmetry of the complex. The presence of weaker ligands along the z-axis was expected to stabilize the dxz and dyz orbitals compared to the dxy orbital and thus lift the degeneracy of the t2g orbitals. The resulting distorted tetragonal structure undergoes electron redistribution, leading to the doublet state, as illustrated in Fig. 3. Since ensuring that the axial and equatorial ligands differ significantly in strength is the key to splitting the degenerate t2g orbitals, we investigated the effects of replacing the axial ligands of Oh [CrIII(NH3)6]3+ with I, Br, Cl, or F, which occur at quite different positions in the spectrochemical series with respect to NH3. The propensity for the doublet state is high for the diiodo and dibromo complexes due to the large difference in strength between the I or Br ligand and NH3. On the contrary, F and NH3 are close together in the spectrochemical series, so the doublet state cannot easily be achieved in the difluoro complex due to the almost equivalent strengths of the axial and equatorial ligands. This dependence of the likelihood of the doublet state on the difference in the strengths of the axial and equatorial ligands is supported by the relatively low emission energies of the diiodo and dibromo complexes and the high phosphorescence energy of the difluoro-substituted complex. The position of Cl (between F and I, Br) in the spectrochemical series correlates well with the emission frequency of the dichloro-substituted complex, which is also between the emission frequencies of the difluoro-substituted complex and those of the diiodo- and dibromo-substituted complexes. Thus, we can see that the emission frequency of the phosphorescence of the complex can be tuned by appropriately selecting the axial ligands of the complex, based on the difference in strength between those ligands and the equatorial NH3 ligands. The most important finding of the present work is the blueshift in the emission wavelength of [CrIII(NH3)4F2]+ as compared to the original Oh [CrIII(NH3)6]3+ complex. This blueshift in the emission wavelength to 454 nm may offer useful insights for the rational design of hitherto rare blue-emitting first-row transition metal complexes, which are desirable for OLED applications.

Notes

Acknowledgements

The Department of Science and Technology, India is gratefully acknowledged for its financial assistance. This work is dedicated to Professor P. K. Chattaraj on the occasion of his 60th birthday.

Supplementary material

894_2018_3768_MOESM1_ESM.docx (1.2 mb)
ESM 1 (DOCX 1271 kb)

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© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of North BengalDarjeelingIndia

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