Heteroleptic iridium(III) complexes bearing a coumarin moiety: an experimental and theoretical investigation

Abstract

Orange coloured complexes with the general formula [Ir(2-pypy)₂(L)] have been synthesized in excellent yields by reacting [Ir(2-pypy)₂(Cl)]₂ with HL in a ratio of 1 : 1 in a 1 : 1 mixture of ethanol and dichloromethane in an argon atmosphere. Herein, L⁻ is the deprotonated aldimine form of 7-hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde and 1-napthylamine (HL¹), 2-aminoanthracene (HL²) and 2-aminofluorene (HL³), respectively. The elemental analysis and ESI mass spectroscopic measurements ensured the formation of the desired complexes. The molecular structure of [Ir(2-pypy)₂(L¹)] was confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of the three Ir(III) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited state is associated with the ³IL and ³ML excited state. The emission-like transition is consistent with the strong ³ILCT and ³MLCT character.

---

Introduction

Organic light-emitting diodes (OLEDs)^1–6 having phosphorescent properties are of abiding interest due to their advantage both in display as well as their applications in lighting. These types of materials generate electro-phosphorescence easily from both singlet and excited states, which can help to attain the internal quantum efficiency up to a theoretical level of 100%.^1,2 In connection to this, a great effort has been given to the second- and third-row transition metal complexes with a d⁶ configuration, such as rhenium(I),⁷ ruthenium(II),⁸ osmium(II),⁹ rhodium(II)¹⁰ and iridium(III),¹¹ due to widespread application in the field of solar energy conversion,^12a,b sensing,^13–15 organic light emitting diodes and molecular electronics.^16–18 These metal ions show strong spin–orbit coupling, as a result the triplet metal to ligand charge transfer excited state (³MLCT) can emit molecular phosphorescence by borrowing the intensity of the singlet MLCT excited state.

Among these heavy metal ions, heteroleptic iridium(III) complexes bearing a coordinated phenylpyridine are still of high interest.¹⁹ The photophysical properties of such compounds are governed by the close environment around the metal ion, and a small change in the ligand architecture can strongly influence its chemistry. These demand further investigation of coordination chemistry of iridium to understand how a ligand environment modulates the details of structure, their electronic structures and their spectroscopic properties.

In the present study, we synthesized a series of 2-phenylpyridine (general abbreviation, 2-pypy) based heteroleptic iridium(III) mixed ligand complexes bearing a Schiff base derived from 7-hydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde and different amines. The complexes were characterized by IR, UV-vis, and ¹H NMR spectroscopic techniques. X-ray structures of a selected complex have been determined. The electrochemical behavior is also examined. The photophysical properties of the complexes were also investigated.

We also present a full density functional theory (DFT) and time-dependent density functional theory (TDDFT) investigation to provide a better insight into the geometry, electronic structure, and optical properties of these systems. Optimizations of the geometry of the singlet ground-state and triplet state were carried out using DFT calculations. TDDFT calculations of several singlet states have been performed for better understanding of the electronic origin of the absorption and emission spectra. The computational modeling of the NMR parameter is also of abiding interest, and such calculations by DFT has emerged as a promising approach for the prediction of nuclear shielding and coupling constants of NMR active nuclei.^20,21 Thus, we have also computed the proton and carbon NMR chemical shifts using the gauge-independent atomic orbital (GIAO)-DFT method, which is aimed at providing the definitive characterization of the complexes.

Experimental section

Materials

4-Methyl-7-hydroxy coumarin,^22a 4-methyl-7-hydroxy-8-formyl coumarin^22b and [Ir₂(2-pypy)₄Cl₂]^22c were prepared as reported in the literature. The three ligands that were used to synthesize the Ir(III) complexes were also prepared according to a literature procedure.^22d All the chemicals and solvents were analytically pure and used without further purification.

Physical measurements

UV-vis spectra were recorded on a Perkin-Elmer LAMBDA 25 spectrophotometer. IR spectra were obtained with a Perkin-Elmer L-0100 spectrophotometer. ¹H NMR spectra were obtained on a Bruker FT 300 MHz spectrometer. The atom numbering scheme used for ¹H NMR is same as that used in crystallography. Electrospray ionization mass spectrometry (ESI-MS) was carried out on a Micromass Qtof YA 263 mass spectrometer. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 series II analyzer and electrochemical measurements were carried out on a CHI 620A electrochemical analyzer using a platinum electrode under nitrogen atmosphere. Tetraethyl ammonium perchlorate (TEAP) was used as a supporting electrolyte and potentials were referenced to a standard calomel electrode (SCE) without junction correction. The emission data were collected on a Horiba FluroMax-4 fluorescence spectrometer. For all luminescence measurements, a slit width of 2 nm was used for both excitation and emission. Quantum yields of the complexes were determined in freeze–pump–thaw-degassed solutions of the complexes by a relative method using quinine sulfate in the same solvent as the standard [Φ_std = 0.54 (at 298 K) in 0.1 M H₂SO₄ at λ_ex = 350 nm]^23a by a standard method. The fluorescence lifetime was determined by a standard method.^23b The quantum yields were calculated using eqn (1):²⁴where Φ_r and Φ_std are the quantum yields of unknown and standard samples, A_r and A_std (<0.1) are the solution absorbances at the excitation wavelength (λ_ex), I_r and I_std are the integrated emission intensities, and η_r and η_std are the refractive indices of the solvent. Experimental errors in the reported luminescence quantum yields were about 10%. Time-correlated single-photon-counting (TCSPC) measurements were carried out for the luminescence decay of complexes in dichloromethane. For TCSPC measurement, the photoexcitation was made at 300 nm using a picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The fluorescence decay data were collected on a Hamamatsu MCP photomultiplier (R3809) and were analyzed using IBH DAS6 software. TGA analysis was carried out by a TA instruments SDT Q600 under nitrogen atmosphere at a flow rate of 100 ml min⁻¹ in a platinum crucible at a rate of 10° min⁻¹.

Computational details

The geometrical structures of the singlet ground state (S₀) and the lowest lying triplet excited state (T₁) were optimized by the DFT²⁵ method with a B3LYP exchange correlation functional²⁶ approach. The geometry of the complexes was fully optimized in solution phase without any symmetry constraints. There is a good agreement between the theoretical and experimental structures. On the basis of the optimized ground and excited state geometry structures, the absorption and emission spectra properties in dichloromethane (CH₂Cl₂) media were calculated by the time-dependent density functional theory (TDDFT)²⁷ approach associated with the conductor-like polarizable continuum model (CPCM).²⁸ We computed the lowest 50 singlet–singlet transitions, and results of the TD calculations were qualitatively very similar. The TDDFT approach had been demonstrated to be reliable for calculating spectral properties of many transition metal complexes.²⁹ Due to the presence of electronic correlation in the TDDFT (B3LYP) method, it can yield more accurate electronic excitation energies. Thus, TDDFT had been shown to provide a reasonable spectral feature for investigating our complex.

A relativistic effective core potential (ECP)³⁰ on Ir replaced the inner core electrons, leaving the outer core [(5s)²(5p)⁶] electrons and the (5d⁶) valence electrons of Ir(III). The calculation basis set aug-cc-pVDZ-pp was adopted as the basis set for Ir atoms. For H, C, N and O atoms, the 6-31+G(d) basis set was used for the optimization of both the ground state and the lowest lying triplet excited state geometries of all the complexes.

In addition, the ¹H NMR properties of the complexes were calculated with the magnetic field perturbation method as well as GIAO algorithm³¹ with the NMR = spin–spin keyword incorporated in the Gaussian 09W program. In the calculation, a basis set aug-cc-pVDZ-pp was adopted as the basis set for Ir atoms. For H, C, N and O atoms, the 6-31+G(d) basis set was employed. The relative chemical shift of a given nucleus X in the molecule was defined as δ^calc_X [ppm] = σ^ref_X − σ^calc_X, where TMS was used as a reference molecule optimized at the same level of theory.^32,33a To account for the solvent effect, we used the integral equation-formalism polarizable continuum model (IEFPCM) method.^33b,c

Finally, to understand the nature of excited states involved in absorption and emission processes, a natural transition orbital (NTO) analysis was performed for all the complexes. This approach provides the most compact representation of the electronic transitions in terms of an expansion into single particle orbitals by diagonalizing the transition density matrix associated with each excitation. Figures showing MOs, NTOs and the difference density plots were prepared using the GaussView 5.1 software. All the calculations were performed with the Gaussian 09 software package.³⁴ The Gausssum 2.1 program³⁵ was used to calculate the molecular orbital contributions from groups or atoms.

Crystallographic studies

A single crystal suitable for X-ray crystallographic analysis of the complex [Ir(2-pypy)₂(L¹)], 1, was obtained by diffusion from the dichloromethane–hexane solution of the complex. The X-ray intensity data were collected on a Bruker AXS SMART APEX CCD diffractometer (Mo K_α, λ = 0.71073 Å) at 293 K. The detector was placed at a distance of 6.03 cm from the crystal. A total of 606 frames were collected with a scan width of 0.3° in different settings of φ. The data were reduced in SAINTPLUS,³⁶ and an empirical absorption correction was applied using the SADABS package.³⁶ Metal atoms were located by the Patterson method and the rest of the non-hydrogen atoms emerged from successive Fourier synthesis. The structures were refined by a full matrix least-square procedure on F². The crystal was weakly diffracting in nature. The available observed data did not permit anisotropic refinement of all nonhydrogen atoms. Six carbon atoms of the naphthyl ring of the amine group were calculated in their fixed position and the remaining non-hydrogen atoms were refined anisotropically. Attempts to obtain a better data set were impeded by the poor crystal quality. Some of the carbon atoms were found to be refined with higher ADP probably due to poor and highly overlapping X-ray diffraction data. A large electron density value near the O2 and C20 atoms (refine_diff_density_max 4.47 for O2) and (refine_diff_density_max 4.10 for C20) was located at the end of the refinement cycle. However, the connectivity and other broad structural features of the complex were refined to a reasonable degree with respect to data quality and are undoubtedly correct.

The hydrogen atoms were included in calculated positions. All the calculations were performed using the SHELXTL V 6.14 program package.³⁷ Molecular structure plots were drawn using the Oak Ridge thermal ellipsoid plot ORTEP.³⁸ Relevant crystal data are given in Table 1.

Table 1 Crystal data and structure refinement parameters for complex 1·2H₂O

	[Ir(2-pypy)2(L^1)]·2H2O, 1·2H2O
a R1 = Σ∣∣Fo∣ − ∣Fc∣∣/Σ∣Fo∣.b wR2 = [Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]]^1/2.
Formula	C43H34N3O5Ir
Mr	864.93
Crystal system	Orthorhombic
Space group	Pna2(1)
a/Å	39.0382(14)
b/Å	9.2229(3)
c/Å	20.9573(8)
α/°	90.00
β/°	90.00
γ/°	90.00
V/Å^3	7545.6(5)
Z	8
Dcalcd/mg m^−3	1.523
μ/mm^−1	3.587
θ/°	2.4–27.5
T/K	293(2)
Reflns collected	17 270
R1,^a wR2^b [I > 2σ(I)]	0.0659, 0.1753
GOF on F^2	1.034

---

Synthesis

Three bidentate ligands, HL¹, HL² and HL³ [Chart 1] (general abbreviation HL), were used in this study as a monoanionic N, O donor ligand for the preparation of Ir(III) complexes. L¹, L² and L³ are the deprotonated form of the ligands.

Chart 1

The complexes were prepared by the same general methods (Scheme 1). The elemental analysis and ESI mass spectroscopic measurements confirm the formation of the synthesized complexes. Details are given here for a representative case.

Scheme 1 Schematic representation for the synthesis of the complexes 1, 2 and 3 by the same general methods. In the abovementioned diagram, Schiff base ligand (HL) is represented as N∩O.

[Ir(2-pypy)₂(L¹)], 1. A mixture of HL¹ (66 mg, 0.20 mmol) and [(2-pypy)₂Ir(μ-Cl)]₂ (214 mg, 0.20 mmol) in a 1 : 1 mixture of dichloromethane and ethanol (40 mL) was refluxed for 24 h under an argon atmosphere. After cooling to room temperature, the solvent was removed under reduced pressure. The crude mass was dissolved in a minimum volume of benzene and subjected to column chromatography on a silica gel column (60–120 mesh). An orange band was eluted using 20% acetonitrile in benzene solution. A yellow orange coloured solid was obtained after the removal of solvent under reduced pressure. The product on diffusion from dichloromethane–hexane afforded orange coloured crystals. Yield: 124 mg (75%). Elemental anal. calcd for C₄₃H₃₀N₃O₃Ir: C, 62.30; H, 3.65; N, 5.07. Found: C, 62.42; H, 3.72; N, 5.17. ESI-MS (CH₂Cl₂): m/z 852.34 [M + Na]⁺. IR_exptl (KBr, cm⁻¹): 1711, 1574, 1476, 1408, 1270, 1162; ν(C=N) 1604. ¹H NMR_exptl {300 MHz, CDCl₃, δ (ppm), J (Hz)}: 5.6 (H40, s), 7.8 (H33, s), 6.0 (H36, d, J = 10), 7.6 (H22, d, J = 9.1), 9.0 (H11, d, J = 10.6), 5.6–9.8 (25H, ArH), 3.6 (–CH₃, s). ¹H NMR_theor {δ (ppm), J (Hz)}: 5.0 (H40, s), 8.8 (H33, s), 5.9 (H36, d, J = 9.0), 7.6 (H22, d, J = 9.0), 9.1 (H11, d, J = 9.6) 5.9–9.0 (25H, ArH), 2.5 (–CH₃, s).

[Ir(2-pypy)₂(L²)], 2. Yield: 124 mg (70%). Elemental anal. calcd for C₄₆H₃₂N₃O₃Ir: C, 63.73; H, 3.72; N, 4.85. Found: C, 63.85; H, 3.79; N, 4.95. ESI-MS (CH₂Cl₂): m/z 868.38 [M + H]⁺. IR_exptl (KBr, cm⁻¹): 1711, 1467, 1397, 1221, 1270, 1152; ν(C=N) 1574. ¹H NMR_exptl {300 MHz, CDCl₃, δ (ppm), J (Hz)}: 5.5 (H43, s), 5.8 (H36, s), 6.2 (H39, d, J = 9.7), 7.7 (H22, d, J = 8.6), 8.8 (H11, d, J = 6.1), 5.6–9.8 (25H, ArH), 2.5 (–CH₂, s), 2.2 (–CH₃, s). ¹H NMR_theor {δ (ppm), J (Hz)}: 5.0 (H43, s), 5.6 (H36, s), 6.0 (H39, d, J = 9.0), 7.5 (H22, d, J = 7.8), 8.9 (H11, d, J = 5.8), 5.6–9.8 (25H, ArH), 2.0 (–CH₂, s), 1.5 (–CH₃, s).

[Ir(2-pypy)₂(L³)], 3. Yield: 123 mg (70%). Elemental anal. calcd for C₄₇H₃₂N₃O₃Ir: C, 64.22; H, 3.67; N, 4.78. Found: C, 64.34; H, 3.74; N, 4.88. ESI-MS (CH₂Cl₂): m/z 902.36 [M + Na]⁺. IR_exptl (KBr, cm⁻¹): 1717, 1515, 1476, 1397, 1270, 1152; ν(C=N) 1571. ¹H NMR_exptl {300 MHz, CDCl₃, δ (ppm), J (Hz)}: 5.6 (H44, s), 5.8 (H37, s), 6.8 (H40, d, J = 10.1), 7.8 (H22, d, J = 11), 9.0 (H11, d, J = 6.0), 5.6–9.8 (27H, ArH), 3.1 (–CH₃, s). ¹H NMR_theor {δ (ppm), J (Hz)}: 5.2 (H44, s), 5.0 (H33, s), 6.2 (H36, d, J = 9.9), 7.5 (H22, d, J = 10.6), 9.2 (H11, d, J = 5.5), 5.6–9.8 (25H, ArH), 2.6 (–CH₃, s).

Results and discussion

Crystal structures

The structure of [Ir(2-pypy)₂(L¹)] (1), crystallized as dehydrates, has been determined by single-crystal X-ray diffractometry. The complex crystallized in the Pna2(1) space group. The asymmetric unit of complex 1 consists of two geometrically similar but crystallographically distinct molecules and four water molecules.

A molecular view excluding the water molecule and hydrogen atoms is shown in Fig. 1. The numbering of the corresponding atoms in molecules 1 and 2 are n and n + 50, e.g., Ir1–O1 and Ir51–O51 (Table 2), respectively.

Fig. 1 ORTEP plot and atom labeling scheme of molecule 1 of complex 1. Hydrogen atoms and water molecules are omitted for clarity.

Table 2 Selected bond lengths (Å) and angles (°) for complex 1

Molecule 1		Molecule 2
Bond lengths (Å)
Ir1–C1	1.999(17)	Ir51–C51	2.001(17)
Ir1–C12	2.010(19)	Ir51–C62	1.978(18)
Ir1–N1	2.037(13)	Ir51–N51	2.071(15)
Ir1–N2	2.052(14)	Ir51–N52	2.053(15)
Ir1–N3	2.151(16)	Ir51–N53	2.188(16)
Ir1–O1	2.139(13)	Ir51–O51	2.141(13)
Bond angles (°)
C1–Ir1–C12	90.8(7)	C51–Ir51–C62	91.0(7)
C1–Ir1–O1	89.3(6)	C51–Ir51–O51	90.4(6)
C1–Ir1–N1	81.2(7)	C51–Ir51–N51	79.9(7)
C12–Ir1–N1	97.1(7)	C62–Ir51–N51	97.0(7)
C1–Ir1–N2	93.7(6)	C51–Ir51–N52	96.2(7)
C12–Ir1–N2	78.5(7)	C62–Ir51–N52	80.1(6)
N1–Ir1–N2	173.3(6)	N51–Ir51–N52	175.1(6)
C12–Ir1–O1	173.9(6)	C62–Ir51–O51	174.6(6)
N1–Ir1–O1	89.0(5)	N51–Ir51–O51	88.3(5)
N2–Ir1–O1	95.4(6)	N52–Ir51–O51	94.6(5)
C1–Ir1–N3	174.5(6)	C51–Ir51–N53	174.0(7)
C12–Ir1–N3	94.4(7)	C62–Ir51–N53	94.0(7)
N1–Ir1–N3	96.6(6)	N51–Ir51–N53	96.2(6)
N2–Ir1–N3	88.8(6)	N52–Ir51–N53	88.0(6)
N3–Ir1–O1	85.7(6)	N53–Ir51–O51	84.9(6)

---

The selected bond distances and angles for complex 1 are listed in Table 2, and the molecular views are shown in Fig. 1.

In complex 1, the ligand (HL¹) behaves as an O, N coordinating monoanionic ligand. The 2-phenylpyridine (2-pypy) moiety adopts a mutually eclipsed configuration. The geometry around the Ir(III) metal center is a distorted octahedral and it is characterized by N1–Ir1–N2 bond angle of 173.3(6)°. Ir–N1 and Ir–N2 bond distances appear at 2.037(13)° and 2.052(14)°, respectively. The ligand (HL¹) shows slightly elongated Ir–N distances of 2.151(16) than those of the trans oriented Ir–N1 and Ir–N2 distances of 2-phenylpyridine (2-pypy) ligand.

In summary, we can conclude that although the estimated standard deviations for both bond distances and angles are relatively high, the crystal structures confirm the present geometry of the species.

Thermogravimetric analysis of the crystalline sample of 1 was performed in the temperature range of 25–600 °C. The thermogravimetric analysis reveals that there is a weight loss at about 85 °C, and the observed percentage of weight loss is ∼4.1%, which is believed to be due to the loss of two lattice water molecules, as shown in ESI (Fig. S9†).

The complexes are diamagnetic and display well resolved ¹H spectra in CDCl₃ solution. The assignment of NMR spectra is based on intensity and spin–spin splitting structure of Ir(III) complexes.

The azo methine proton H33 (for complex 1) is observed as a singlet at 7.8 ppm, whereas the azo methine protons H36 (for complex 2) and H37 (for complex 3) are observed as a singlet at 5.8 ppm. H36 (for complex 1), H39 (for complex 2) and H36 (for complex 3) protons are observed at ∼6.0 ppm, 6.2 ppm and ∼6.0 ppm, respectively, as a doublet. In all the complexes, the –CH₃ protons are observed in the range of ∼2.2–3.6 ppm as a singlet. All the aromatic protons span in the range of ∼5.6–9.8 ppm for all the complexes. The experimentally observed values are well-correlated with the calculated values. The correlation diagram between the calculated and experimental ¹H NMR chemical shift of 1 is shown in Fig. 2 and for the remaining two complexes is given in ESI (Fig. S1(a)†). The corresponding NMR spectrum for complex 2 and 3 is given in ESI (Fig. S1(b) and (c)†), respectively.

Fig. 2 Linear correlation between the experimental and calculated ¹H NMR chemical shifts of complex 1.

Geometry optimization, electronic structure, and charge distribution

The complexes are diamagnetic in nature at room temperature indicating their singlet ground states associated with the t_2g⁶ electronic configurations. The geometry optimization for all the complexes was performed in solution phases in both ground singlet (S₀) and lowest lying excited triplet (T₁) spin states. Both the ground state and the lowest lying triplet excited state geometries for all the complexes were optimized using the basis set 6-31+G(d) for C, H, N and O atoms and aug-cc-pVDZ-pp for Ir atoms. The main optimized geometrical parameters for complex 1 are listed in Table 3. For other complexes, the main optimized geometrical parameters are given in ESI (Table S1†). The optimized structure of the complex 1 at singlet ground state is shown in Fig. 3, whereas the same for 2 and 3 are given in ESI (Fig. S2 and S3†).

Table 3 Selected optimized geometrical parameters for 1 in the ground (S₀) and lowest lying triplet (T₁) excited states at B3LYP levels using the basis set aug-cc-pVDZ-pp for Ir atoms

	1
	S0	T1
Bond lengths (Å)
Ir1–O1	2.171	2.186
Ir1–N3	2.210	2.117
Ir1–N2	2.060	2.054
Ir1–N1	2.078	2.082
Ir1–C12	2.012	2.017
Ir1–C1	2.022	2.048
C33–N3	2.613	2.637
Bond angles (°)
O1–Ir1–N3	85.1	81.7
N3–Ir1–N1	95.6	96.9
N1–Ir1–C1	80.2	80.4
N2–Ir1–C12	80.4	80.7
O1–Ir1–N1	87.3	90.6
N1–Ir1–N2	174.7	173.1
C1–Ir1–C12	88.7	88.8
C1–Ir1–O1	89.2	84.3
C12–Ir1–N1	97.5	95.5
C1–Ir1–N2	94.8	93.7
C12–Ir1–O1	174.3	169.8
N2–Ir1–O1	94.5	92.2
C1–Ir1–N3	173.2	165.7
N2–Ir1–N3	89.5	86.1
C12–Ir1–N3	97.2	105.3

---

Fig. 3 Optimized molecular structure of 1 at S₀ state. (Ir: cyan, N: blue, O: red, C: grey, H: white).

The modeled geometries possess a distorted octahedral arrangement around the Ir(III) center. The calculated structures are in excellent agreement with the experimental data for complex 1, for which X-ray data are available.

In complex 1, the 2-phenylpyridine (2-pypy) moiety mutually adopts an eclipsed configuration with the nitrogen atom N1 and N2 that reside at the trans location; the bond lengths of Ir–N1 and Ir–N2 are 2.078 and 2.060 Å, respectively, and the corresponding experimentally observed values are 2.037(13) and 2.052(14) Å. Atoms C12 and C1 are mutually in cis position with the Ir–C1 on the Ir atom, and the Ir–C12 distances are 2.022 and 2.012 Å, which are in very good agreement with the experimental values [Ir–C1 = 1.999(17) and Ir–C12 = 2.010(19)]. Thus, we can say that the slight discrepancy comes from the crystal lattice distortion existing in real molecules (Tables 2 and 3).

Upon excitation, the complexes experience a vertical transition from ground state to the singlet excited state and then undergo intersystem crossing to reach the triplet excited state, in which emission might occur. According to structure–property relationships, the structural changes would be expected between the S₀ and T₁ states.

The comparison of the S₀ and T₁ states geometries shows that, in the T₁ excited state, the metal–ligand bond lengths have perceptible changes. For example, in case of all the complexes, the calculated Ir–C1 bond has an elongation of about 0.02 Å. The Ir–O bonds have elongations of about 0.01 Å for complex 1 and 3, while for complex 2, it is elongated by 0.005 Å. The calculated bond angle O1–Ir–N3 shows only a slight change of about 3.4° in the case of 1.

The partial frontier molecular orbital compositions and energy levels of 1 in the singlet ground state (S₀) are listed in Table 4, while that for other complexes are given in ESI (Tables S2 and S3†). The partial molecular orbital diagram for all the complexes and the partial molecular orbital diagram with some isodensity frontier molecular orbitals that are mainly involved in the electronic transitions for complex 1 are shown in Fig. 4. For complexes 2 and 3, the partial molecular orbital diagrams with some isodensity frontier molecular orbital, which are mainly involved in the electronic transitions, are depicted in ESI (Fig. S4 and S5†).

Table 4 Frontier molecular orbital composition (%) in the ground state for 1

Orbital	Energy (eV)	Contribution (%)				Main bond type
		Ir	2-pypy	HL^1
				Imine	Aromatic system
LUMO+5	−1.317	1	15	0	84	π*(2-pypy) + π*(HL^1)
LUMO+4	−1.483	0	76	3	21	π*(2-pypy) + π*(HL^1)
LUMO+3	−1.715	5	1	0	95	π*(HL^1)
LUMO+2	−1.780	3	2	1	94	π*(HL^1)
LUMO+1	−1.904	1	72	24	3	π*(2-pypy) + π*(HL^1)
LUMO	−2.117	2	73	25	0	π*(2-pypy) + π*(HL^1)
HOMO	−5.471	43	23	3	31	d(Ir) + π(2-pypy) + π(HL^1)
HOMO−1	−5.701	41	36	1	21	d(Ir) + π(2-pypy) + π(HL^1)
HOMO−2	−5.797	38	14	1	48	d(Ir) + π(2-pypy) + π(HL^1)
HOMO−3	−6.134	6	84	4	6	π(2-pypy)
HOMO−4	−6.479	10	8	0	81	d(Ir) + π(HL^1)
HOMO−5	−6.581	16	25	0	59	d(Ir) + π(2-pypy) + π(HL^1)

---

Fig. 4 Left: partial molecular orbital diagram for complexes 1–3. The arrows indicate the HOMO–LUMO energy gaps. Right: partial molecular orbital diagram with some isodensity frontier molecular orbital mainly involved in the electronic transitions for complex 1.

In the ground state (S₀), the HOMO (H) and HOMO−1 are lying within the range of 0.15–0.23 eV for all the complexes. In all the complexes, the electron density in HOMO mainly reside on the d orbitals of metal (in the range of 5–43%), 2-phenylpyridine (2-pypy) (in the range of 23–38%) and the aromatic system (in the range of 24–31%) attached to C=N bonds along with some imine (C=N) character (3–4%). In the case of HOMO−1, the electron density mainly resides on the d orbitals of metal (in the range of 19–41%) and the cyclometalating 2-phenylpyridine (2-pypy) moiety (in the range of 31–62%); in the case of complex 1 and 2, the electron density in the case of HOMO−2 mainly resides on metal d orbital and Schiff base moiety, while for complex 3, the HOMO−2 is mainly composed by metal d orbital and cyclometalating 2-phenylpyridine (2-pypy). The energy difference between the HOMO and LUMO occurs in the range of 3.31–3.39 eV, and this energy gap is minimum for 3 (3.31 eV) and maximum for 1 and 2 (3.35 and 3.39 eV, respectively). It is believed that extended π-conjugation in the ligand frame is responsible for lowering of energy gap from 1–3 and it is highest for 2. In complexes 1 and 2, the HOMO−2 is lying at ∼0.32 eV below the HOMO, while for complex 3, this energy difference is 0.214 eV. HOMO−3 is mainly composed of 82% or more 2-phenylpyridine (2-pypy) moiety in case of complex 1 and complex 2, while that of complex 3 is composed of 37% metal d orbital, 15% 2-phenylpyridine (2-pypy) and 47% π (HL³) orbital.

For all the complexes, the HOMO−4 is mainly composed of a Schiff base moiety (in the range of 77–89%). The LUMO and LUMO+1 of all the complexes originate from mainly a π* orbital localized on the 2-phenylpyridine (2-pypy) moiety. The LUMO+2 orbital shows a strong contribution of the π* (HL¹) and π* (HL²) in the case of complex 1 and complex 2, respectively, while for complex 3, there is a strong contribution from the π* orbital of 2-phenylpyridine (2-pypy).

The most meaningful geometrical parameters in the T₁ state for complexes 1, 2 and 3 are listed in Table 3 and isodensity surfaces of the highest and lowest singly occupied molecular orbitals, namely, HSOMO and LSOMO, for 1, 2 and 3 at the relaxed T₁ geometry are shown in Fig. 5. Moreover, the corresponding electron spin density, which is defined as the difference between α and β spin contributions to the total electron density, is depicted in Fig. 5. The analysis of the singly occupied molecular orbitals at the T₁ geometry shows that the LSOMO is mainly located on the metal center resembling the HOMO of the corresponding S₀ geometry. On the other hand, π orbital of Schiff base moiety contributes primarily to HSOMO, and the HSOMO strongly resembles the corresponding LUMO+2 of the S₀ geometry. Nonetheless, as depicted in Fig. 4, the HSOMO appears to be different for 1 and 2 with respect to 3. Indeed, for complex 1 and 2, the orbital turns out to be confined mainly on the naphthalene and fluorene ring of Schiff base moiety, respectively, while for the compound 3, it is mostly located on the 2-phenylpyridine (2-pypy) moiety.

Fig. 5 Isodensity surface (iso-cutoff = 0.03) plots of the highest and lowest singly occupied molecular orbitals, denoted as HSOMO and LSOMO, respectively, along with the corresponding electron spin density for the complexes 1, 2 and 3 at their T₁-state geometry. Blue and green colors show regions of positive and negative difference between alpha and beta electron densities, respectively.

Absorption spectral properties

The absorption spectra of all the complexes were recorded in dichloromethane solution at room temperature. The complexes display several bands in solution. For all the cases, the lowest energy singlet vertical excitation (S₀ → S₁) falls in the range of 448–454 nm (f = 0.03–0.06). Moreover, such calculated values correspond well to the experimental absorption spectra (451–454 nm). Such excitation essentially for all the complexes consists of d(Ir)π(2-pypy)π(L) → π*(2-pypy)π*(L).

For all the complexes, the lowest computed absorption process corresponds to mainly HOMO → LUMO (69–93%) and can reasonably assigned to an admixture of metal-to-ligand charge-transfer (¹MLCT) transitions, spin-allowed π → π* (ligand centered, ¹ILCT) transitions and ligand-to-ligand charge transfer (¹LLCT) transitions.

Fig. 6 shows the experimental absorption spectra for all the complexes. The photophysical parameters of the complex 1, 2 and 3 are given in Table 5.

Fig. 6 Absorption spectra of complexes 1, 2 and 3 in dichloromethane at room temperature.

Table 5 Photophysical parameters of the complexes in dichloromethane solution at room temperature

Complex	λmax, nm (ε, M^−1 cm^−1)	λemi, nm	Φ (×10^−3)	kr, s^−1 (×10^6)	knr, s^−1 (×10^8)	τ1, (ns)	τ2, (ns)
1	263 (11 310), 361 (3747), 451 (893)	543	23.4	27.5	17.2	0.56	0.85
2	263 (11 293), 363 (3786), 451 (920)	398	30.2	5.43	1.74	1.37	5.56
3	259 (12 589), 360 (3377), 454 (900)	471	11.0	1.53	1.38	2.81	7.15
		440		1.57	1.41	1.88	6.98

---

For all the complexes, the absorption maxima in the region of 360–363 nm are mainly associated with mixed ¹MLCT and ¹ILCT transitions. The highest energy bands with maxima in the region of 259–263 nm for all the complexes are also mixed ¹MLCT and ¹ILCT transitions. These assignments were supported by theoretical calculations (from a spin density plot and also from NTO analysis).

Fig. 7 shows the accompanying electron density redistributions in complex 1, 2 and 3. From the spin density plots, we can conclude that the absorption bands in the region of 360–454 nm for all the complexes have a mixed ¹MLCT, ¹ILCT and ¹LLCT character.

Fig. 7 Difference electron density (iso-cutoff = 0.04) upon excitation from the ground state (S₀) to allowed singlet states (40 singlet to singlet excitations) for complexes 1, 2 and 3 determined with TD-DFT (B3LYP/CPCM-dichloromethane) calculations. Turquoise and purple colors show regions of decreasing and increasing electron density, respectively.

To analyze the nature of absorption, we performed an NTO analysis based on the calculated transition density matrices.⁴⁰ This method offers the most compact representation of the transition density between the ground and excited states in terms of an expansion into single-particle transitions (hole and electron states for each given excitation).

The calculated absorption energies associated with their oscillator strengths, the main configurations and their assignments as well as the experimental result of 1 are given in Table 6, whereas those for other complexes are given in ESI (Tables S4 and S5†). Table 6 confirms the usual assignments of all the absorption bands for complex 1.

Table 6 Main calculated optical transition for the complex 1 with composition in terms of molecular orbital contribution of the transition, vertical excitation energies, and oscillator strength in dichloromethane

Transition	Composition	E (eV)	Oscillator strength (f)	CI	λtheo (nm)	Assign	λexp (nm)
S0 → S1	HOMO → LUMO (88%)	2.7050	0.0392	0.663	458	^1MLCT/^1ILCT/^1LLCT	451
	HOMO → LUMO+1 (9%)			−0.216		^1ILCT/^1LLCT
S0 → S12	HOMO−3 → LUMO (78%)	3.3816	0.0751	0.625	366	^1ILCT/^1LLCT	361
	HOMO−3 → LUMO+1 (7%)			−0.190		^1ILCT/^1LLCT
	HOMO−2 → LUMO+2 (2%)			−0.101		^1MLCT/^1ILCT/^1LLCT
	HOMO → LUMO+3 (5%)			0.154		^1MLCT/^1ILCT/^1LLCT
S0 → S50	HOMO−10 → LUMO (3%)	4.4949	0.0070	−0.119	275	^1MLCT/^1ILCT/^1LLCT	263
	HOMO−9→ LUMO (3%)			0.117		^1ILCT/^1LLCT
	HOMO−9 → LUMO+2 (3%)			−0.124		^1ILCT/^1LLCT
	HOMO−7 → LUMO+2 (4%)			−0.141		^1ILCT/^1LLCT
	HOMO−5 → LUMO+4 (62%)			0.557		^1MLCT/^1ILCT/^1LLCT
	HOMO−4 → LUMO+4 (6%)			0.174		^1MLCT/^1ILCT/^1LLCT

---

Herein, we refer to the unoccupied and occupied NTOs as ‘‘electron’’ and ‘‘hole’’ transition orbitals, respectively. Note that NTOs are not the same as virtual and occupied MO pairs from the ground state calculations. Fig. 8 illustrates the natural transition orbitals (NTOs) for complex 1 and 2, whereas those for complex 3 are given in ESI (Fig. S6†). Based on the TDDFT NTOs analysis, the bands in the region of 259–454 nm for all the complexes can be characterized as possessing a mixture of MLCT, LLCT and ILCT states.

Fig. 8 Natural transition orbitals (NTOs) (iso-cutoff = 0.03) for the complexes 1 and 2, illustrating the nature of optically active singlet excited states in the absorption bands in the range of 263–451 nm. For each state, the respective number of the state, transition energy (eV), and the oscillator strength (in parentheses) are listed. Shown are only occupied (holes) and unoccupied (electrons) NTO pairs that contribute more than 25% to each excited state. All the transitions have a mixed MLCT/ILCT character: charge is transferred mainly from t_2g–π hole orbital to the π* orbital of the ligands.

As illustrated in Fig. 8, optical excitations occur from the occupied (hole) transition orbitals to the unoccupied (electron) transition orbitals. Hole NTOs contributing to the bands are localized on the Ir center along with π orbitals of ligands (t_2g–π), while the electron NTOs are mainly delocalized over the π* orbital of the ligand moiety.

Emission spectral properties

The emission spectral behavior of all the complexes was studied at room temperature in dichloromethane. The photophysical parameters are listed in Table 5. Fig. 9 represents the emission spectra of the complexes in a dichloromethane solution.

Fig. 9 Emission spectra of 1, 2 and 3 in a dichloromethane solution at room temperature.

At room temperature, in solution state, the quantum yield value of the complexes is in the range of (11.0–30.2) × 10⁻³.

Time-resolved luminescence spectra prove to be an important tool to understand the decay process and the emissive nature of the complexes. Thus, time-resolved luminescence spectra were recorded for all the complexes. All the complexes display a bi-exponential decay nature, and the decay plot for the complex 2 is shown in Fig. 10, while the same plot for other complexes are given in ESI (Fig. S7 and S8†). The fluorescence lifetime (τ) and radiative (k_r) and nonradiative (k_nr) decay rate constants are collected in Table 5.

Fig. 10 Changes in the time-resolved photoluminescence decay of complex 2 in dichloromethane at room temperature obtained with an excitation at 450 nm. The emission was monitored at 400 nm.

The photoluminescence property mainly originates from triplet state charge transfer transitions. Table 7 describes the calculated emission energies associated with their oscillator strengths, the main configurations and their assignments as well as the experimental results for 1 and 2, while the same for complex 3 is depicted in ESI (Table S6†). It is believed that the extended conjugation in the Schiff base moiety, as well as the rigidity of the molecule, is responsible for the fluorescence spectral change decay with the variation of the ligand. To elucidate the T₁ states of the complexes from a theoretical perspective, the electron spin density isosurfaces were studied by DFT calculations (Fig. 5). The isosurface of the spin density of complex 1 is localized mainly on the imine part of the HL¹ and on the metal. In the case of complex 2, the isosurface of the spin density is localized on 2-phenylpyridine (2-pypy) and HL², as well as on the metal. This is in agreement with the known ³MLCT/³LLCT excited state of cyclometalated Ir(III) complexes.^41,42 For complex 3, however, the spin density isosurfaces are exclusively localized on the anthracene/imine/aromatic moiety of HL³. In complex 3, the Ir(III) atom contributes considerably less to the spin density than it does in complex 1 and complex 2. Thus, the lowest-lying triplet excited state is mainly the ³IL-excited state (ligand-localized) for complex 3.

Table 7 Calculated triplet excited state of 1 and 2 in dichloromethane based on the lowest lying triplet state geometry. Main calculated vertical transitions with compositions in terms of molecular orbital contribution of the transition, vertical excitation energies and oscillator strength

Complex	Excitation	Composition	E (eV) (λ nm)	Oscillator strength (f)	CI	Assign	λexp (nm)
1	1	HOMO → LUMO+6	2.2490 eV (550 nm)	0.0009	−0.25610	^3MLCT/^3ILCT/^3LLCT	540
		HOMO−7→ LUMO			0.91445	^3MLCT/^3ILCT/^3LLCT
		HOMO−6 → LUMO			−0.24217	^3MLCT/^3ILCT/^3LLCT
		HOMO−5→ LUMO			0.11047	^3MLCT/^3ILCT/^3LLCT
2	1	HOMO−3→ LUMO+2	3.0435 eV (407 nm)	0.0347	0.14315	^3MLCT/^3ILCT/^3LLCT	398
		HOMO−1 → LUMO+1			−0.16170	^3MLCT/^3ILCT/^3LLCT
		HOMO−1 → LUMO + 2			0.21219	^3MLCT/^3ILCT/^3LLCT
		HOMO → LUMO+7			−0.19237	^3MLCT/^3ILCT/^3LLCT
		HOMO−11 → LUMO			−0.19791	^3MLCT/^3ILCT/^3ILCT
		HOMO−1 → LUMO+1			0.58634	^3MLCT/^3ILCT/^3LLCT
		HOMO−1 → LUMO+2			0.38638	^3MLCT/^3ILCT/^3LLCT
		HOMO−1 → LUMO+4			0.26232	^3MLCT/^3ILCT/^3LLCT
		HOMO−1 → LUMO+2			0.15074	^3MLCT/^3ILCT/^3LLCT
		HOMO → LUMO+4			−0.10726	^3MLCT/^3ILCT/^3LLCT

---

To analyze the nature of emission, we performed an NTO analysis based on the calculated transition density matrices. Fig. 11 illustrates the natural transition orbitals (NTOs) for all the three complexes. Based on our TDDFT NTOs analysis at the T₁ state, all the bands for complex 1 and 2 can be characterized as ³MLCT/³ILCT/³LLCT transitions, while for complex 3, it is mainly ³LC transitions because the electron density over the metal is very small.

Fig. 11 Natural transition orbitals (NTOs) (iso-cutoff = 0.03) for the complexes 1, 2, and 3, illustrating the nature of optically active triplet excited state. For each state, the transition energy (eV) and the oscillator strength (in parentheses) are listed. Shown are only occupied (holes) and unoccupied (electrons) NTO pairs that contribute more than 25% to each excited state.

As illustrated in Fig. 11, optical excitations occur from the occupied (hole) transition orbitals to the unoccupied (electron) transition orbitals. The hole and electron NTOs contributing to the bands are mainly metal–ligand and ligand localized, respectively.

From the NTO analysis, we can see that the electron density of the hole is predominantly located at the [d(Ir) + π(2-pypy) + π(HL¹)] for complex 1 and [d(Ir) + π(2-pypy) + π(HL²)] for complex 2, and the electron density of unoccupied (electron) transition orbitals is mainly located at the π* orbital of HL¹ and HL² for complex 1 and complex 2, respectively. Thus, we conclude that emission transition characters are from d(Ir) + π(2-pypy) + π(HL¹) to π*(2-pypy) + π*(HL¹) for complex 1 and from d(Ir) + π(2-pypy) + π(HL²) to π*(2-pypy) + π*(HL²) for complex 2. In the case of complex 3, both hole and electron NTOs contributing to the bands are mainly ligand localized.

Electrochemical studies

Cyclic voltammetry was performed for all the complexes in a dichloromethane solution at room temperature under a nitrogen atmosphere with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte using a Pt electrode as the working electrode and the potentials are referenced to the saturated calomel electrode (SCE) without junction correction. All the complexes exhibit a single ligand centered reduction wave and a metal centered oxidation wave, both of which are electrochemically irreversible in nature. For complex 2, the oxidation potential appears at ∼1.46 V, whereas for complex 1 and 3, it appears at 1.32 V and 1.15 V, respectively. Thus, it is evident that the oxidation potential decreases gradually. This result may be due to the presence of naphthalene ring and anthracene ring in case of complexes 1 and 3. Due to the presence of conjugation in the naphthalene ring and anthracene ring, the electron density at metal increases, rendering the complexes 1 and 3 easier to oxidize. The reduction potentials are −0.65 V, −0.52 V, and −1.75 V for complex 1, 2 and 3, respectively. Similar to the oxidation processes, the effect played on the reduction potentials of complexes 1 and 3 can be noticed where naphthalene ring and anthracene ring are present, respectively. The introduction of a naphthalene ring and anthracene ring in HL¹ and HL³, respectively, dramatically shifts the reduction potential toward more negative values, supporting the idea that the LUMO is most likely located on such moieties. It is also concluded from here that the cathodic shift in the case of complex 3 compared to complex 1 and complex 2 can be attributed to the destabilization of the HOMO of complex 3.

Conclusion

In summary, we have synthesized a family of monomeric 2-phenylpyridine based heteroleptic iridium(III) mixed ligand complexes bearing a Schiff base derived from 4-methyl-7-hydroxy-8-formyl coumarin. The complexes are characterized by different spectroscopic techniques, elemental analysis and X-ray structure determination. The present study investigated the ground- and excited-state geometries, NMR, absorption, and phosphorescence properties of three Ir(III) complexes by DFT and TDDFT methods. We specifically focused on the changes in electronic structure and optical properties of these molecules caused by changing the amine of the Schiff bases. TD-DFT investigation gave insights into the optical transitions involved in the excitation process. From our calculation results, we have characterized all the low-lying electronic states as a mixture of ILCT and MLCT in character. The nature of the transitions was also supported by a spin density difference map and natural transition orbital (NTO) analysis. The emission-like transition is associated with the admixture of strong ³MLCT and ³ILCT characters for complexes 1 and 2, whereas mainly ³ILCT character in complex 3, as demonstrated by electron spin density analysis. The effect of ancillary ligands on tuning the color emission is interpreted in terms of the raising/lowering of HOMOs and LUMOs as well as the gaps between them.

Acknowledgements

Financial assistance received from the Science & Engineering Research Board, New Delhi, India, is gratefully acknowledged. We are also thankful to the Department of Science and Technology, New Delhi, India, for the data collection on the CCD facility setup (Jadavpur University) under the DST-FIST program. We also acknowledge CAS, the Department of Chemistry, Jadavpur University and DST-PURSE program for other facilities. Amit Maity and Rupa Sarkar are also thankful to UGC, New Delhi for the research fellowship.

References

1. (a) M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403, 750–753; (b) S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem and K. Leo, Nature, 2009, 459, 234–238; (c) C. Wu, H. F. Chen, K. T. Wong and M. E. Thompson, J. Am. Chem. Soc., 2010, 132, 3133–3139; (d) K. Chen, C. H. Yang, Y. Chi, C. S. Liu, C. H. Chang, C. C. Chen, C. C. Wu, M. W. Chung, Y. M. Cheng, G. H. Lee and P. T. Chou, Chem.–Eur. J., 2010, 16, 4315–4327; (e) D. Chen, L. Han, D. Liu, K. Ye, Y. Liu, J. Zhang and Y. Wang, RSC Adv., 2015, 5, 18328; (f) Q.-L. Xu, X. Liang, S. Zhang, Y.-M. Jing, X. Liu, G.-Z. Lu, Y.-X. Zheng and J.-L. Zuo, J. Mater. Chem. C, 2015, 3, 3694.
2. (a) W. Y. Wong, C. L. Ho, Z. Q. Gao, B. X. Mi, C. H. Chen, K. W. Cheah and Z. Lin, Angew. Chem., Int. Ed., 2007, 46, 1558–1558; (b) Z. Q. Bian and C. H. Huang, Highly Efficient OLEDs with Phosphorescent Materials, Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim, Germany, 2008, pp. 391−420.
3. M. K. Nazeeruddin and M. Grätzel, in Photofunctional Transition Metal Complexes, ed. V. W. Yam, Springer, Berlin Heidelberg, 2007; vol. 123, p. 113.
4. F. Kessler, Y. Watanabe, H. Sasabe, H. Katagiri, M. K. Nazeeruddin, M. Gratzel and J. Kido, J. Mater. Chem. C., 2013, 1, 1070.
5. R. C. Evans, P. Douglas and C. Winscom, Coord. Chem. Rev., 2006, 250, 2093.
6. M. E. Thompson, P. E. Djurovich, S. Barlow and S. Marder, in Comprehensive Organometallic Chemistry III., ed. D. Crabtree, P. Michael and R. H. Mingos, Elsevier, Oxford, U.K., 2007, p. 101.
7. (a) R. N. Dominey, B. Hauser, J. Hubbard and J. Dunham, Inorg. Chem., 1991, 30, 4754–4758; (b) L. Sacksteder, M. Lee, J. N. Demas and B. A. DeGraff, J. Am. Chem. Soc., 1993, 115, 8230–8238.
8. (a) W. K. Smothers and M. S. Wrighton, J. Am. Chem. Soc., 1983, 105, 1067–1069; (b) Y. S. Wang, S. X. Liu, M. R. Pinto, D. M. Dattelbaum, J. R. Schoonover and K. S. Schanze, J. Phys. Chem. A, 2001, 105, 11118–11127.
9. Y.-L. Tung, P.-C. Wu, C.-S. Liu, Y. Chi, J.-K. Yu, Y.-H. Hu, P.-T. Chou, S.-M. Peng, G.-H. Lee, Y. Tao, A. J. Carty, C.-F. Shu and F.-I. Wu, Organometallics, 2004, 23, 3745–3748.
10. K. Shinozaki and N. Takahashi, Inorg. Chem., 1996, 35, 3917–3924.
11. (a) E. C. Constable, M. Neuburger, P. Rösel, G. E. Schneider, J. A. Zampese, C. E. Housecroft, F. Monti, N. Armaroli, R. D. Costa and E. Ortí, Inorg. Chem., 2013, 52, 885–897; (b) F. Spaenig, J.-H. Olivier, V. Prusakova, P. Retailleau, R. Ziessel and F. N. Castellano, Inorg. Chem., 2011, 50, 10859–10871; (c) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304–4312.
12. (a) W. J. Song, Z. F. Chen, M. K. Brennaman, J. J. Concepcion, A. O. T. Patrocinio, N. Y. Murakami Iha and T. J. Meyer, Pure Appl. Chem., 2011, 83, 749–768; (b) M. Grätzel, Acc. Chem. Res., 2009, 42, 1788–1798.
13. A. Kumar, S. S. Sun and A. J. Lees, Photophysics and photochemistry of organometallic rhenium diimine complexes, in Topics in Organometallic Chemistry, ed. A. J. Lees, Springer, New York, 2010, vol. 29, pp. 1–35.
14. M. K. Itokazu, A. S. Polo and N. Y. Murakami Iha, J. Photochem. Photobiol., A, 2003, 160, 27–32.
15. P. S. Wagenknecht and P. C. Ford, Coord. Chem. Rev., 2011, 255, 591–616.
16. T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz and H. E. Gaub, Science, 2002, 296, 1103–1106.
17. V. Balzani, A. Credi and M. Venturi, ChemPhysChem, 2008, 9, 202–220.
18. M. P. Santoni, G. S. Hanan, B. Hasenknopf, A. Proust, F. Nastasi, S. Serroni and S. Campagna, Chem. Commun., 2011, 47, 3586–3588.
19. (a) Y. Zhou, H. Gao, X. Wang and H. Qi, Inorg. Chem., 2015, 54, 1446–1453; (b) F. Monti, M. G. I. La Placa, N. Armaroli, R. Scopelliti, M. Grätzel, M. K. Nazeeruddin and F. Kessler, Inorg. Chem., 2015, 54, 3031–3042; (c) X. Yi, C. Zhang, S. Guo, J. Ma and J. Zhao, Dalton Trans., 2014, 43, 1672; (d) C. Li, S. Wang, Y. Huang, Q. Wen, L. Wanga and Y. Kanb, Dalton Trans., 2014, 43, 5595–5602; (e) V. H. Nguyen, R. S. H. Khoo and J. H. K. Yip, Inorg. Chem., 2015, 54, 2264–2277 and references therein.
20. (a) K. Wolinski, J. F. Hinton and P. Pulay, J. Am. Chem. Soc., 1990, 112, 8251–8260; (b) P. Taehtinen, A. Bagno, K. Klika and K. Pihlaja, J. Am. Chem. Soc., 2003, 125, 4609–4618; (c) F. Cloran, I. Carmichael and A. S. Serianni, J. Am. Chem. Soc., 2001, 123, 4781–4791.
21. (a) S. Basak, D. Chopra and K. K. Rajak, J. Organomet. Chem., 2008, 693, 2649; (b) S. Basak and K. K. Rajak, Inorg. Chem., 2008, 47, 8813; (c) P. Mondal, A. Hens, S. Basak and K. K. Rajak, Dalton Trans., 2013, 42, 1536–1549; (d) R. Sarkar, P. Mondal and K. K. Rajak, Dalton Trans., 2014, 43, 2859; (e) P. Mondal, R. Sarkar, A. Hens and K. K. Rajak, RSC Adv., 2014, 4, 38769; (f) R. Sarkar, A. Hens and K. K. Rajak, RSC Adv., 2015, 5, 15084.
22. (a) D. M. Manidhar, K. Uma Maheswara Rao, N. Bakthavatchala Reddy, Ch, S. Sundar and C. Suresh Reddy, J. Korean Chem. Soc., 2012, 56, 459; (b) D. R. Bender, D. Kanne, J. D. Frazier and H. Rapoport, J. Org. Chem., 1983, 48, 2709; (c) K. Nonoyama, Bull. Chem. Soc. Jpn., 1974, 47, 467–468; (d) D. Sarkar, A. Pramanik, S. Biswas, P. Karmakar and T. K. Mondal, RSC Adv., 2014, 4, 30666.
23. (a) B. Valuer, Molecular Fluorescence: Principle and Applications, Wiley-BCH, Weinbeim, 2001; (b) P. Mondal, A. Hens, S. Basak and K. K. Rajak, Dalton Trans., 2013, 1536–1549.
24. J. van Houten and R. J. Watts, J. Am. Chem. Soc., 1976, 98, 4853–4858.
25. R. Czerwieniec, A. Kapturkiewicz, R. Anulewicz-Ostrowska and J. Nowacki, J. Chem. Soc., Dalton Trans., 2001, 2756–2761.
26. H. C. Zhao, B. Mello, B.-L. Fu, H. Chowdhury, D. J. Szalda, M.-K. Tsai, D. C. Grills and J. Rochford, Organometallics, 2013, 32, 1832–1841.
27. A. S. Polo, M. K. Itokazu, K. M. Frin, A. O. T. Patrocinio and N. Y. Murakami Iha, Coord. Chem. Rev., 2006, 250, 1669–1680.
28. M. A. L. Marques and E. K. U. Gross, Annu. Rev. Phys. Chem., 2004, 55, 427–455.
29. Y. Wang, Y. Wang, J. Wang, Y. Liu and Y. Yang, J. Am. Chem. Soc., 2009, 131, 8839–8847.
30. (a) W. Koch and M. C. Holthausen, A Chemist’s Guide to Density Functional Theory, Wiley-VCH, Weinheim, Germany, 2000; (b) P. J. Hay and W. R. Walt, J. Chem. Phys., 1985, 82, 299.
31. C. Sundararajan, T. R. Besanger, R. Labiris, K. J. Guenther, T. Strack, R. Garafalo, T. T. Kawabata, D. Finco-Kent, J. Zubieta, J. W. Babich and J. F. Valliant, J. Med. Chem., 2010, 53, 2612–2621.
32. (a) K. Wolniski, J. F. Hilton and P. Pulay, J. Am. Chem. Soc., 1990, 112, 8251–8260; (b) R. Ditchfield, Mol. Phys., 1974, 27, 789–807; (c) R. Mcweeny, Phys. Rev., 1962, 126, 1028–1034; (d) F. London, J. Phys. Chem., 1937, 397–409.
33. (a) C. M. Rohlfing, L. C. Allen and R. Ditchfield, Chem. Phys., 1984, 87, 9–15; (b) M. Cossi, V. Barone, B. Mennucci and J. Tomasi, Chem. Phys. Lett., 1998, 286, 253–260; (c) E. Cancès, B. Mennucci and J. Tomasi, J. Chem. Phys., 1997, 107, 3032–3041.
34. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, (Revision A.1), Gaussian, Inc., Wallingford, CT, 2009.
35. E. D. Glendening, A. E. Reed, J. E. Carpenter and F. Weinhold, NBO, version 3.1, University of, Madison, 1995.
36. SMART, SAINT, SADABS, XPREP, SHELXTL, Bruker AXS Inc., Madison, WI, 1998.
37. G. M. Sheldrick, SHELXTL, v. 6.14, Bruker AXS Inc., Madison, WI, 2003.
38. C. K. Johnson, ORTEP Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976.
39. (a) S. J. Lee, K.-M. Park, K. Yang and Y. Kang, Inorg. Chem., 2009, 48, 1030; (b) E. Orselli, G. S. Kottas, A. E. Konradsson, P. Coppo, R. Fröhlich, L. de Cola, A. van Dijken, M. Büchel and H. Börner, Inorg. Chem., 2007, 46, 11082; (c) E. Orselli, R. Q. Albuquerque, P. Michel Fransen, R. Fröhlich, H. M. Janssenc and L. de Cola, J. Mater. Chem., 2008, 18, 4579.
40. R. L. Martin, J. Chem. Phys., 2003, 118, 4775–4777.
41. N. G. Park, G. C. Choi, Y. H. Lee and Y. S. Kim, Curr. Appl. Phys., 2006, 6, 620.
42. S. Fantacci and F. de Angelis, Coord. Chem. Rev., 2011, 255, 2704.

---

Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic file in CIF format for [Ir(2-pypy)₂(L¹)], 1; Fig. S1–S7 and Tables S1–S5. CCDC 1061614. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra08349d

---