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Understanding the Difference in Photophysical Properties of Cyclometalated Iridium(III) and Rhodium(III) Complexes by Detailed TDDFT and FMO Supports Siddhartha Pal, Sucheta Joy, Hena Paul, Snehasis Banerjee, Abhishek Maji, Ennio Zangrando, and Pabitra Chattopadhyay J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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Understanding the Difference in Photophysical Properties of Cyclometalated Iridium(III) and Rhodium(III) Complexes by Detailed TDDFT and FMO Supports Siddhartha Pal#, Sucheta Joy#, Hena Paul#, Snehasis Banerjee†, Abhishek Maji#, Ennio Zangrando‡ and Pabitra Chattopadhyay#* #

†

Department of Chemistry, Burdwan University, Golapbag, Burdwan-713104, West Bengal, India

Government College of Engineering and Leather Technology, Salt Lake Sector-III, Kolkata, 700098, India ‡

Department of Chemical and Pharmaceutical Sciences, Via Licio Giorgieri 1, 34127 Trieste, Italy

ABSTRACT: Two sets of new mono-nuclear cyclometalated complexes of the types [MIII(ppy)2(L)] (Ir1 and Rh1) and [MIII(bzq)2(L)] (Ir2 and Rh2) [where M = Rh/Ir; L = N-(furan-2-ylmethyl)-2pyridinecarboxamide; ppy = 2-phenylpyridine (1); bzq = benzo[h]quinoline (2);] are presented to understand the significant differences of emission behaviors between Ir(III) and Rh(III) complexes. The complexes were fully characterized by using physico-chemical and spectroscopic tools along with the detailed structural studies of Ir1 and Rh2 by single crystal X-ray diffraction. Structural analyses showed that in both complexes the metal exhibits a distorted octahedral geometry. The geometries, electronic structures, lowest-lying singlet-singlet absorptions, vertical singlet-triplet excitation and triplet-singlet emissions behavior of all the complexes (Ir1-2 and Rh1-2) were examined by means of DFT and TDDFT approaches. In order to find out the considerable differences in experimentally observed emission properties of Ir(III) (5d6) and Rh(III) (4d6) complexes, the radiative and non-radiative processes were ACS Paragon Plus Environment

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calculated and compared quantitatively and somewhere qualitatively. The results indicate that the smallest S1-T1 splitting energy (∆ES1-T1), large transition dipole moment (S1), and reverse order of energy states between the triplet metal-to-ligand charge transfer (3MLCT) and triplet metal-centered (3MC) states were responsible for higher radiative process of Ir(III) complexes than those of Rh(III) complexes. Since, non-radiative decay process plays an important role in the photo deactivation process, the potential energy profile for the deactivation process from 3MLCT via 3MC were explored. In addition to the relative energies of 3MLCT and 3MC states, the position of minimum energy 3MC/1GS crossing point (MECP) were also computed.

INTRODUCTION Over the past years, cyclometalated complexes of d6 transition metal ions with nitrogen-donor ligands have been the subject of significant interest in experimental and theoretical studies because of their outstanding photophysical and photochemical properties. These properties make them promising candidates for photochemical energy conversion studies. The excited state properties can be readily monitored either by changing the metal center, such as Ru(II), Rh(III), Os(II), or Ir(III), or by tuning the ligand framework.1-15 Ru(II) and Os(II) complexes containing nitrogen donor ligands especially have attracted a considerable attention for their applications as photosensitizers, or in the production of solar fuels by artificial photosynthesis, photoinduced intermolecular energy and electron-transfer processes, light driven molecular machines and decomposition of water. 16-20 The Ir(III) luminescent complexes have wide applications as phosphors in organic light emitting cells and diodes (OLEC and OLEDs), 21-27 sensors28-32 and luminescent labels for biomolecules,33-37 in contrast to Rh(III) similar derivatives, which have been poorly studied due to the lack of significant photophysical properties. 38-41 However, photoinduced electron and energy transfer processes were detected in supramolecular Ir(III) and Rh(III) cyclometalated species.42,43 Compared to organic fluorophores, luminescent metal complexes especially with heavy transition metal complexes are better probes for cellular imaging due to their superior photophysical properties with high photostability and relatively long lifetimes. Metal complexes show ACS Paragon Plus Environment

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considerable Stokes shifts for easy separation of excitation and emission wavelengths. Interestingly, in contrast to a reported single Rh(III) complex, there are several organometallic Ir(III) complexes recently used as bioimaging probes.40 As discussed above, a large number of bis-cyclometalated Ir(III) complexes of the type [Ir(N^C)2(N^N)] have strong luminescent properties, while, emissions from analogous Rh(III) complexes are rare. Till now, few studies were made to find the reasons for the non-emissive behavior of Rh(III) complexes.44-46 Three possible reasons are established: (i) The spin-orbit coupling (SOC) constant of rhodium is lower than that of iridium. (ii) Although Ir(IV) complexes are well-established, but the commonly found highest oxidation state for rhodium is Rh(III) not Rh(IV); as a result, the energy for metal-to-ligand charge transfer (MLCT) is higher for Rh(III) complexes.47,48 An increase in the energy of MLCT will serve to reduce radiative rate kr. (iii) The weaker ligand field strength of 4d rhodium than that of 5d iridium shall facilitate the metal-centered d-d excited 3MC states which are strongly distorted relative to the ground state and hence non-emissive. As a consequence, radiationless decay via d-d excited state becomes more accessible, which leads to almost non-emissive character of Rh(III). Keeping the above fact in mind, herein we assess the drastic difference of emission behavior between Rh(III) and Ir(III) complexes considering the experimental observations and theoretical calculations with respect to our synthesized complexes (Ir1-2 and Rh1-2). At present, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are essential to estimate the key features of the designed compound prior to synthesis and to predict the desired properties. These theoretical calculations may provide important evidences for the proper interpretation of crystallographic structures, absorption49,50 and emission spectra,50-53 and more interesting is the recent use of relativistic approximations for a better description of spin-orbit coupling.54-58 Therefore, in our study, DFT and TDDFT were performed to verify the obtained experimental results. To the best of our knowledge, the calculation of the geometrical parameters and absorption spectra of such complexes is routine work, but the computations of the excited states (3MLCT), the deactivation of 3MLCT via 3MC and the relative

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position of 3MC/1GS crossing point to explain the fact of non-emissive behaviors of Rh(III) complexes is challenging task. We have organized the theoretical part of the paper as follows: first, we studied the ground states and the molecular orbitals of all Ir(III) and Rh(III) complexes starting from their crystallographic structures. Then, the TD-DFT absorption spectra of the four complexes are discussed by assigning the main electronic transitions in comparison to experimental absorption spectra. Next, we studied the lowestlying triplet states and the corresponding phosphorescence spectra, and compared the emission efficiency of these complexes quantitatively. Finally, the energy barrier between 3MLCT and 3MC states, and the position of 3MC/1GS crossing point were evaluated. EXPERIMENTAL SECTION Time-Resolved Emission Spectra Experiments. The lifetime of the Rh1-2 were obtained by a HORIBA Jobin Yvon picosecond pulsed diode laser-based TCSPC spectrometer from IBH (UK) at λex = 395 nm with detector MCP-PMT. Emission from the sample was collected at a right angle to the direction of the excitation beam maintaining magic angle polarization (54.71). IBH DAS 6.2 data analysis software was used to fit the data to multiexponential functions after deconvolution of the instrument response function by an iterative reconvolution technique and here, reduced w2 and weighted residuals served as parameters for goodness of fit. Lifetime measurements of the Ir1-2 were performed using an Edinburgh FLS 980 Lifetime Spectrometer. The lifetime data was performed from the reconvolution fit analysis (Edinburgh F980 analysis software) on an Edinburgh instrument spectrometer. Computational Details. All calculations were performed with Gaussian 09 program package employing density functional theory (DFT) using the M06-2X59 and B3LYP60-62 hybrid functionals. The Stuttgart-Dresden (SDD) effective core potential (ECP) were used for the Ir(III) and Rh(III) atoms, and the split-valence 6-31G(d,p) basis set was applied for the other atoms (Scheme S1). The singlet and triplet structures were fully optimized by restricted and unrestricted formalisms, respectively, in vacuo and in acetonitrile solvent using the implicit SMD solvation model.63 The expected values calculated for

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were always smaller than 2.032. Geometry optimization was performed without any constraint, and the nature of all stationary points was confirmed by normal-mode analysis. The EDDM plots have been calculated using Gauss Sum 3.0.64 The minimum energy crossing point (MECP) between the S 0 and the 3

MC potential surfaces was optimized using Harvey´s algorithm65 as implemented in the ORCA software;

(F. Neese. Orca, an ab initio, DFT and semiempirical SCF-MO package 2.8.0 R2327, University of Bonn: Bonn, Germany, 2011). In this case, the B3LYP functional was employed in combination with the def2-svp basis set and the ECP-60-mwb Stuttgart/Dresden pseudo potential for Pt. To get relative energies for the MECP, single-point calculations with the 6-31G* basis set were performed. As stated, all calculations apart from the MECP optimization were carried out with the Gaussian09 program package. RESULTS AND DISCUSSION 1. Synthesis and Characterization of Ir(III) and Rh(III) Cyclometalates. From the reactions of the carboxamide derivative (L) with equimolar amount of dichloro-bridged dimer complexes [M(ppy)Cl]2 or [M(bzq)Cl]2 (where M= Ir(III) or Rh(III)) in boiling DMF (10 mL) for 5 h, the corresponding cyclometalated Ir(III)/Rh(III) complexes of formulation [M(ppy) 2(L)] (Ir1 and Rh1) and [M(bzq)2(L)] (Ir2 and Rh2) were isolated in solid state with good yield (Scheme 1). All the complexes were characterized by elemental analyses, ESI-MS spectra 1H-NMR spectra. Suitable X-ray quality single crystals of Ir1 and Rh2 were obtained on slow evaporation from a CH2Cl2/DMF solution.

Scheme 1. Synthesis of complexes Ir1-2 and Rh1-2. ACS Paragon Plus Environment

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All the complexes are soluble in CH3CN, DMF and DMSO, and stable both in solid and in solution. The spectroscopic and elemental analyses confirm the formulations of the complexes. The molar conductivity of freshly prepared solution (~1 x 10-3 M concentration) of Ir1 (ΛM = 41 M-1 cm-1), Ir2 (ΛM = 35 M-1 cm-1), Rh1 (ΛM = 45 M-1 cm-1) and Rh2 (ΛM = 42 M-1 cm-1) in DMF are fairly consistent with non-electrolyte species. The magnetic susceptibility measurements are in agreement with the diamagnetic nature of the complexes.

Figure 1. Ortep drawing (ellipsoids probability at 30%) of complexes (a) Ir1 and (b) Rh2 with the atom numbering scheme. 2. Structural description of complexes Ir1 and Rh2. As described in the experimental section, the Xray structural analyses of the solid state structure of these compounds revealed a formulation of [Ir1.DMF.2(H2O)] and [Rh2.0.75(H2O)]. The molecular structures of the complexes are depicted in Figure 1 and a selection of bond length and angles are listed in Table S2. In both complexes Ir1 and Rh2, the carboxamide ancillary ligand acts as a bidentate monobasic chelator through the nitrogen donors, differently from the bonding behavior of similar ligands with 3d metal ions. In fact, in reported literature, chelation towards the metal ion was exhibited by the carboxamide oxygen and the pyridine nitrogen with formation of square planar, square pyramidal, and octahedral complexes.66 The structural analysis evidenced that complex Ir1 crystallizes in triclinic space group P-1. The metal exhibits a distorted octahedral coordination sphere and, of the possible isomers, the configuration of the complex is such that the carbon donors of the 2-phenylpyridine ligands are in trans position to the ACS Paragon Plus Environment

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nitrogen donors of the pyridine carboxamide L ligand. This is confirmed by the considerably longer bond lengths of 2.151(4) and 2.154(4) Å of Ir-N bonds in the pyridine carboxamide ligand due to the trans effect of the carbon atoms. On the other hand, two ppy N atoms trans to each other have shorter Ir-N bond lengths of 2.051(3) and 2.057(3) Å. The remaining the Ir-C bond distances are close comparable within their esd’s, being 2.008(5) and 2.013(5) Å. The crystal packing shows the formation in the crystal of complex dimers connected by H-bonding scheme involving the lattice water molecules (Figure S7). The H-bond parameters of the packing are reported as supporting information (Table S3). Compound Rh2 crystallizes in orthorhombic space group Pna21 and the diffraction analysis reveals an octahedral complex having a configuration similar to that detected in Ir1. Here also, the Rh-N bond distances of the chelating pyridine carboxamide ligand (2.167(7) and 2.129(7) Å) are considerably longer than the Rh-N(bzq) ones (2.045(7) and 2.053(8) Å), again supporting the trans effect of benzoquinoline coordinating carbon atoms. Here the furanyl group reveals a intramolecular π-π interaction, stacking at 3.746(8) Å (centroid to centroid distance) with the central bzq ring C(4)-C(7)/C(35)/C(34). 3. UV-vis Study. Complexes Ir1-2 and Rh1-2 in CH3CN gave almost similar of absorption pattern (Figure 2). The maximum absorption peaks of the complexes in the ultraviolet part (from 210 to 300 nm) can be assigned to the spin-allowed ligand-centered π-π* transitions of the C∧N ligand and the weak bands observed between 300 and 470 nm can be attributed to the MLCT transitions. All these excitation spectra are further examined by computational analysis in the theoretical section.

Figure 2. Absorption spectra of the complexes Ir1-2 and Rh1-2. The measurements for all the complexes were carried out in CH3CN solution with a concentration of 0.5 x 10-4 mol/L at 298 K. ACS Paragon Plus Environment

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4. Photoluminescence. Changing the metal from Ir(III) (5d6) to Rh(III) (4d6) the complexes show a significant change in the emission intensity, and the respective photoluminescence spectra of complexes Ir1-2 and Rh1-2 are presented in Figure 3. Here the Ir(III) complexes showed high luminescence intensity, whereas the Rh(III) complexes did not display any significant luminescent character. Between the Ir(III) complexes, Ir2 showed a higher luminescence intensity with slight red shifted spectrum compared to Ir1, and the quantum yield is also slightly greater for Ir2 than that of Ir1 (Table 1). All the excited state phenomena are well described by the DFT approach and are in accordance with these experimental data.

Figure 3. Emission spectra of complexes Ir1-2 and Rh1-2. The measurements for the complexes were carried out in CH3CN solution with a concentration of 10-4 mol/L at 298 K; λex = 395 nm (Slit 5/5). 5. Time Resolve Spectra. In order to better clarify the dramatic difference in luminescence character, the excited state lifetimes of all the complexes are determined (Figure 4). The values and relative weightage are given in Table 1. Here it is worth noting that the lifetime of the Ir(III) complexes (Ir1 = 1.70 µs, Ir2 = 1.59 µs) is much greater than that of the Rh(III) derivatives (Rh1 = 2.69 ns, Rh2 = 2.10 ns), which again is in agreement with the high photo-luminescence character of Ir(III) complexes and the lifetimes are in accordance with the literature value.52

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Figure 4. Life time decay profiles for complexes Ir1-2 (left) and Rh1-2 (right). The measurements were carried out in CH3CN solutions with a concentration of 10-4 mol/L (Nano-LED used λex = 395 nm). 6. Theoretical Calculations. Ground States Properties. The ground state optimized geometries (S0) are shown in Table 2. To verify the reliabilities of the DFT functional, the results from B3LYP and M06-2X approaches are shown and compared with experimental X-ray data of Ir1 and Rh2 (Table S4), showing that B3LYP and M06-2X functionals provide similar accuracy. Therefore, from a geometrical point of view, the B3LYP level is sufficient and reliable for the Ir(III) and Rh(III) complexes under study. Also reports of literature 67,68 show that B3LYP accurately reproduces the energetics of these complexes. These results confirm for all the complexes a distorted octahedral geometry with two cyclometalated ppy or bzq ligands and one N^N ligand (L) chelating the Ir(III) and Rh(III) metal center of 5d 6 and 4d6 configuration, respectively. In the ground state geometry, Ir1-2 and Rh1-2 display two Ir/Rh-N(ppy) bond distances in the range 2.06-2.08 Å, while the other two Ir/Rh-N(L) distances (trans to C atoms) are considerably longer in between 2.172.22 Å. Upon the substitution of ppy by bzq, all the Ir/Rh-N and Ir/Rh-C bond distances remain practically unaffected. Bite angles of the chelating ligands play an important role in determining the photophysical properties of Ir/Rh metal complexes, 69 and a decrease in the bite value usually indicates faster non-radiative decay. Here, no noticeable change of bite angles was found for Ir and Rh complexes:

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Rh1 > Rh2, implying a higher kr value for Ir1 and Ir2. From eqn. (2), it can be concluded that complexes Ir1 and Ir2 with stronger SOC, better ISC rate and faster radiative decay provide to a higher photoluminescent quantum efficiency compared to Rh(III) complexes of this paper. On the other hand, eqn. (3) states that non-radiative decay should be smaller for Rh1-2 than Ir1-2 if the

values were considered alone. But according to our computational results, as stated before,

the 3MC states lie below the 3MLCT states for Rh1-2. Therefore, upon excitation of Rh1 and Rh2, 3

MLCT 3MC  S0 radiationless pathway was expected to be highly efficient. This would results in

high knr values for the Rh1 and Rh2 complexes. Thus the combining effect of SOC matrix, E em, µS1, 3

and reverse energy order of 3MLCT and

MC states are finally responsible for the high value of experimental PL of Ir1-2 (Table S15).

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Table 1. Life time details of complexes Ir1-2 and Rh1-2. Complex

B1

B2

B3

τ1

τ2

Χ2

τ3

τav

Ir1

12.58

87.42

-

5.83 µs

1.11 µs

-

1.09

1.70 µs

Ir2

26.15

73.85

-

4.57 µs

0.53 µs

-

1.08

1.59 µs

Rh1

35.38

35.03

29.59

1.54 ns

5.53 ns

0.69 ns

1.02

2.69 ns

Rh2

40.30

36.91

22.79

0.93 ns

4.67 ns

0.02 ns

1.02

2.10 ns

Table 2. Selected optimized parameters for complexes Ir1-2 and Rh1-2 in the ground state (S0), lowestlying triplet state (3MLCT), and photo-dissociated excited state (3MC). Of S0 data, the two values relate to gas and solvent phase, respectively. Ir1 Coord. bonds M−N1 M−N2 M−N3 M−N4 M−C1 M−C2

S0 2.048/2.083 2.076/2.089 2.213/2.212 2.185/2.196 2.026/2.030 2.042/2.042

Coord. bonds M−N1 M−N2 M−N3 M−N4 M−C1

S0 2.071/2.079 2.080/2.085 2.217/2.214 2.176/2.192 2.010/2.014

M−C2

2.030/2.029

S0(expt)b 2.051(3) 2.057(3) 2.151(4) 2.154(4) 2.008(5) 2.013(5)

3

Ir2 3

MLCT 2.091 2.076 2.186 2.068 2.06 2.041

Rh1 MLCT 2.073 2.081 2.185 2.091 2.025

MC 2.543 2.355 2.212 2.085 2.073

2.038

2.036

3

3

MC

S0

2.606 2.418 2.200 2.078 2.071 2.047

2.089 2.084 2.204 2.177 2.034 2.052

S0 2.088 2.087 2.200 2.184 2.035 2.051

Rh2 S0(expt)b 2.045(7) 2.053(8) 2.167(7) 2.129(7) 1.992(8) 1.995(10 )

3

MLCT 2.097 2.085 2.185 2.064 2.049 2.068

3

MLCT 2.107 2.084 2.266 2.167 1.989 2.055

3

MC

2.6 2.463 2.197 2.074 2.048 2.075

3

MC 2.547 2.389 2.208 2.167 1.988 2.055

b

X-ray data.

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Table 3. Calculated phosphorescent emission of the studied complexes in CH3CN media at the TDDFT method together with the experimental values Complexes

Ir1

a

excited state T1

 (nm)/(eV)

f

535/2.31

0

SCF (nm)

Eadia (nm)

550

490

E0,0 (nm)

521

configuration

LH(82%)

assignment ppy1+ppy2) d(Ir) +(L +ppy2) 3 MLCT/3LLCT/ 3ILCT

LH (57%), L H-1 (14%)

bzq1+bzq2)d(Ir) +(L+bzq) 3 MLCT/3ILCT/3LLCT

543

LH (93%)

(ppy) d(Ir) +(L) 3 MLCT/3ILCT

542

LH (92%)

bzq)d(Ir) +(L) 3 MLCT/3LLCT

Ir2

T1

627/1.97

0

604

503

575

Rh1

T1

763/1.62

0

727

524

Rh2

T1

750/1.65

0

719

524

expt.a (nm) 538

544 Nonemissive Nonemissive

This work

CONCLUSION In this study, we have used the detailed time dependent density functional theory (TDDFT) and frontier molecular orbital (FMO) analyses to validate the difference in photophysical behaviors of Ir(III) and Rh(III) complexes by presenting a joint computational and experimental study. To ascertain the fact, two sets of new mono-nuclear cyclometalated complexes of types [MIII(ppy)2(L)] (Ir1 and Rh1) and [MIII(bzq)2(L)] (Ir2 and Rh2) [where M = Rh/Ir] have been synthesized and structurally characterized. The DFT/TDDFT approach has helped us to assess the different experimentally observed phosphorescence efficiency of iridium(III) and rhodium(III) complexes. Although the geometry of iridium(III) and rhodium(III) complexes are similar, significant differences exist in the electronic structures between the 4d6 Rh(III) and 5d6 Ir(III) complexes. The main origins of phosphorescence were assigned to * for Rh1-2, while, *d for the Ir1-2. The higher spin-orbit coupling matrix, higher transition dipole moments µS1, smaller ES1-T1, reverse energy order of 3MLCT and 3MC states, higher activation barrier between 3MLCT and 3MC states, longer relative distance between 3MC and 1GS/3MC crossing point are responsible for the high values of experimental PL of Ir1-2 with respect to those of Rh1-2. We expect that the analysis discussed here will be helpful in guiding the development of new phosphorescent Rh(III) complexes. ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxx and contains all experimental studies, safety procedures, NMR and mass spectra together with detailed DFT calculations. CCDC no’s for [Ir1.DMF.2(H2O)] and [Rh2.0.75(H2O)] are 1512388 and 1512389, respectively. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-342-2558554 extn. 424. Fax: +91-342-2530452. Notes: The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by Department of Science and Technology, Govt. of West Bengal (DST, GoWB, vide project no. 698 (Sanc.)/ST/P/S & T/15-G/2015). S.J. wishes to thank the CSIR (New Delhi) for providing the fellowship. We are grateful to the honorable reviewers for their valuable comments for the improvement of this article.

REFERENCES (1) Flamigni, L.; Collin, J.; Sauvage, J. Iridium Terpyridine Complexes as Functional Assembling Units in Arrays for the Conversion of Light Energy. Acc. Chem. Res. 2008, 41, 857-871. (2) Imahori, H.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809-1818. (3) Chi, Y.; Chou, P.T. Transition-Metal Phosphors with Cyclometalating Ligands: Fundamentals and Applications. Chem. Soc. Rev. 2010, 39, 638-655.

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TOC Graphic The detailed time dependent density functional theory (TDDFT) and frontier molecular orbital (FMO) analyses have been used to understand the significant difference of photophysical behaviors of Ir(III) and Rh(III) complexes by presenting a joint computational and experimental study considering the emission behaviors of newly synthesized and structurally characterized cyclometalated complexes of types [MIII(ppy)2(L)] (Ir1 and Rh1) and [MIII(bzq)2(L)] (Ir2 and Rh2) [where M = Rh/Ir].

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