Metal/ligand-to-ligand Charge-transfer Research Articles

The Role of Intraligand Charge Transfer Processes in Iridium(III) Complexes with Morpholine-Decorated 4'-Phenyl-2,2':6',2″-terpyridine.

To investigate the impact of the electron-donating morpholinyl (morph) group on the ground- and excited-state properties of two different types of Ir(III) complexes, [IrCl3(R-C6H4-terpy-κ3N)] and [Ir(R-C6H4-terpy-κ3N)2](PF6)3, the compounds [IrCl3(morph-C6H4-terpy-κ3N)] (1A), 4[Ir(morph-C6H4-terpy-κ3N)2](PF6)3 (2A), [IrCl3(Ph-terpy-κ3N)] (1B) and [Ir(Ph-terpy-κ3N)2](PF6)3 (2B) were obtained. Their photophysical properties were comprehensively investigated with the aid of static and time-resolved spectroscopic methods accompanied by theoretical DFT/TD-DFT calculations. In the case of bis-terpyridyl iridium(III) complexes, the attachment of the morpholinyl group induced dramatic changes in the absorption and emission characteristics, manifested by the appearance of a new, very strong visible absorption tailing up to 600 nm, and a significant bathochromic shift in the emission of 2A relative to the model chromophore. The emission features of 2A and 2B were found to originate from the triplet excited states of different natures: intraligand charge transfer (3ILCT) for 2A and intraligand with a small admixture of metal-to-ligand charge transfer (3IL-3MLCT) for 2B. The optical properties of the mono-terpyridyl iridium(III) complexes were less significantly impacted by the morpholinyl substituent. Based on UV-Vis absorption spectra, emission wavelengths and lifetimes in different environments, transient absorption studies, and theoretical calculations, it was demonstrated that the visible absorption and emission features of 1A are governed by singlet and triplet excited states of a mixed MLLCT-ILCT nature, with a dominant contribution of the first component, that is, metal-ligand-to-ligand charge transfer (MLLCT). The involvement of ILCT transitions was reflected by an enhancement of the molar extinction coefficients of the absorption bands of 1A in the range of 350-550 nm, and a small red shift in its emission relative to the model chromophore.

Biological Evaluation of Platinum(II) Sulfonamido Complexes: Synthesis, Characterization, Cytotoxicity, and Biological Imaging.

Platinum-based compounds are actively used in clinical trials as anticancer agents. In this study, two novel platinum complexes, (C1 = [PtCl2(N(SO2quin)dpa)], C2 = [PtCl2(N(SO2azobenz)dpa)]) containing quinoline and azobenzene appended dipicolylamine sulfonamide ligands were synthesized in good yield. The singlet attributable to methylene CH2 protons of the ligands of C1 and C2 appears as two doublets in 1H NMR spectra, which confirms the presence of magnetically nonequivalent protons upon coordination to platinum. Structural data of N(SO2quin)dpa (L1), N(SO2azobenz)dpa (L2) and PtCl2(N(SO2quin)dpa) confirmed the formation of the desired compounds. Time-dependent density functional theory calculations suggested that the excitation of L1 show quin-unit-based π⟶π∗ excitations (i.e., ligand-centered charge transfer, LC), while C1 shows the metal-ligand-to-ligand charge-transfer (MLLCT) character. L1 displays intense fluorescence from the 1LC excited state, while C1 gives phosphorescence from the 3LC state. Mammalian cell toxicity of ligands and complexes was assessed with NCI–H292 nonsmall-cell lung cancer cells. Further, C1 and C2 showed significantly low IC50 values compared with N(SO2azobenz)dpa and PtCl2(N(SO2quin)dpa). Fluorescence imaging data of both ligands and complexes revealed the potential fluorescence activity of these compounds for biological imaging. All four compounds are promising novel candidates that can be further investigated on their usage as potential anticancer agents and cancer cell imaging agents.

Tuning excited state of bipyridyl platinum(II) complexes with bio-active flavonolate ligand: Structures, photoreactivity, and DFT calculations

Platinum(II) complexes with coordinated bio-active molecules that can undergo photo-induced oxygenation are finding applications in photoactivated chemotherapy (PCT). A series of bipyridyl(bpy) Pt(II) complexes with ligand flavonolate (fla-) [PtIIBpyFlaR][BF4] [BF4] (R = p- OMe (1), p-Me (2), p-H (3)) were prepared and characterized. Their photochemical and photophysical properties were investigated. Complexes 1–3 exhibit emission at ~530 nm, as compared to the analogous [PdIIBpyFlaR][BF4] with no emission upon irradiation. In addition, Pt(II) complexes undergo oxygenation reaction with oxygen with near visible light, indicating photocleavage of ligand flavonolate. Carbon monoxide(CO) release was observed by deoxymyoglobin from oxygenation of Pt(II) flavonolate complexes. Density functional theory calculations predict a mixed, Pt/fla- to bpy, metal/ligand-to-ligand charge transfer (ML-LCT) state in complexes 1–3.

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties.

The cis- and trans-dirhodium(II,II) complexes cis-[Rh2(μ-DTolF)2(μ-np)(MeCN)4][BF4]2 (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh2(μ-DTolF)2(μ-qxnp)(MeCN)3][BF4]2 [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh2(μ-DTolF)2(μ-qxnp)2][BF4]2 (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh2(μ-DTolF)2(μ-qxnp)2][BF4]2 (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm-1 and exhibits shorter lifetimes of the 1ML-LCT and 3ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The 3ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation.

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh2(μ-DTolF)2(μ-L)2][BF4]2, where DTolF = N,N'-di( p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh2(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

New Ru(ii) photocages operative with near-IR light: new platform for drug delivery in the PDT window.

A series of Ru(ii) complexes bearing the tridentate 2,6-di(quinolin-2-yl)pyridine (dqpy) ligand were designed to undergo photoinduced ligand dissociation with red/near-IR light. The complexes [Ru(dqpy)(L)(CH3CN)]2+, where L = 2,2'-bipyridine (bpy, 1), 4,4'dimethyl-2,2'-bipyridine (Me2bpy, 2), and 1,10-phenanthroline (phen, 3). Complexes 1-3 exhibit red-shifted lowest energy metal-to-ligand charge transfer (MLCT) absorption maxima at ∼600 nm, as compared to the corresponding tpy (2,2';6',2''-terpyridine) complexes with MLCT bands at ∼565 nm which appear as shoulders to the MLCT bands at ∼455 nm. This shift is attributed to the lower energy LUMO afforded by the dqpy ligand when compared to tpy, as evidenced by the shift of the first reduction wave to ∼0.3 V more positive potentials in the former. In addition, the lowest MLCT maximum of [Ru(dqpy)(acac)(CH3CN)]+ (4; acac- = acetylacetonate) is observed at 770 nm, attributed to the additional increase in energy of the HOMO afforded by the presence of the π-donating acac- ligand and supported by calculations. Complexes 1-3 undergo ligand substitution upon irradiation with red light, λ irr ≥ 610 nm, and the ligand substitution photochemistry of 4 is accessible with near-IR light, λ irr ≥ 715 nm and λ irr = 735 nm. Complexes 1-4 exhibit similar quantum yields of ligand exchange, Φ L, with 450 and 600 nm irradiation, however, that of 4 is 2-3 times greater than those measured for 1-3. This enhancement is explained by the difference in ligand contributions to the HOMO. Density functional theory calculations predict partial dqpy ππ* character in the MLCT states of 1-3 and a mixed Ru/acac- → dqpy metal/ligand-to-ligand charge transfer (ML-LCT) state in 4. The photoreactivity of 1-4 with tissue-penetrating red and near-IR light, together with their exceptional dark stability (>48 h), makes the new Ru(ii)-dqpy platform ideal for the development of new complexes for photoinduced drug release and for other applications that require broad absorption from the ultraviolet and visible ranges into the near-IR, such as solar energy conversion.

New Rh2(II,II) Complexes for Solar Energy Applications: Panchromatic Absorption and Excited-State Reactivity.

The new heteroleptic paddlewheel complexes cis-[Rh2(μ-form)2(μ-np)2][BF4]2, where form = p-ditolylformamidinate (DTolF) or p-difluorobenzylformamidinate (F-form) and np = 1,8-napthyridyine, and cis-Rh2(μ-form)2(μ-npCOO)2 (npCOO- = 1,8-naphthyridine-2-carboxylate), were synthesized and characterized. The complexes absorb strongly throughout the ultraviolet (λmax = 300 nm, ε = 20 300 M-1 cm-1) and visible regions (λmax = 640 nm ε = 3500 M-1 cm-1), making them potentially useful new dyes with panchromatic light absorption for solar energy conversion applications. Ultrafast and nanosecond transient absorption and time-resolved infrared spectroscopies were used to characterize the identity and dynamics of the excited states, where singlet and triplet Rh2/form-to-naphthyridine, metal/ligand-to-ligand charge-transfer (ML-LCT) excited states were observed in all four complexes. The npCOO- complexes exhibit red-shifted absorption profiles extending into the near-IR and undergo photoinitiated electron transfer to generate reduced methyl viologen, a species that persists in the presence of a sacrificial donor. The energy of the triplet excited state of each complex was estimated from energy-transfer quenching experiments using a series of organic triplet donors (E(3ππ*) from 1.83 to 0.78 eV). The singlet reduction (+0.6 V vs Ag/AgCl) potentials, and singlet and triplet oxidation potentials (-1.1 and -0.5 V vs Ag/AgCl, respectively) were determined. Based on the excited-state lifetimes and redox properties, these complexes represent a new class of light absorbers with potential application as dyes for charge injection into semiconductor solar cells and in sensitizer-catalyst assemblies for photocatalysis that operate with irradiation from the ultraviolet to ∼800 nm.

Exposing the Excited‐State Equilibrium in an IrIII Bichromophore: A Combined Time Resolved Spectroscopy and Computational Study

AbstractThe electronic structure and photophysical properties of a luminescent IrIII bis‐cyclometalated complex covalently attached to one 4‐piperidinyl‐1,8‐naphthalimide (PNI) chromophore through a coordinated 1,10‐phenanthroline, [Ir(ppy)2(phen‐PNI)](PF6), is presented. This bichromophore represents a new class of visible light‐harvesting IrIII complexes that exhibit markedly enhanced room‐temperature excited‐state lifetimes (τ = 8.8 ms) as a result of intervening ligand‐centered triplet states present on the pendant naphthalimide chromophore. In this IrIII complex, the intense singlet fluorescence of the pendant PNI chromophore is nearly quantitatively quenched and was found to sensitize the IrIII metal/ligand‐to‐ligand charge‐transfer (MLLCT) excited state. The excited state ultimately returns to the PNI chromophore as a long‐lived excited triplet that disposes of its energy by equilibrating with the photoluminescent IrIII MLLCT excited state. Evidence of the excited‐state equilibrium is provided through static and dynamic photoluminescence spectroscopy, transient absorption spectroscopy, and time‐dependent density functional theory calculations.

Evaluating the Extent of Intramolecular Charge Transfer in the Excited States of Rhenium(I) Donor–Acceptor Complexes with Time-Resolved Vibrational Spectroscopy

Excited states in transition-metal complexes, even in those featuring ligands with strong electron donating and accepting properties, often involve only partial charge transfer between the donor and acceptor ligands. The excited-state properties of [Re(bpy)(CO)3L](+) compounds were studied, where L is 4-dimethylaminobenzonitrile (Re4DMABN), 3-dimethylaminobenzonitrile (Re3DMABN), and benzonitrile (ReBN) using time-resolved infrared (TRIR) and electronic spectroscopy methods as well as electronic structure computations. The DMABN complexes exhibit strongly solvent-dependent luminescence; the excited state lifetime decreases from microseconds in dichloromethane to several nanoseconds in mixed MeOH:DCM (1:1) solvent. Despite the similarities in the solvent dependence of the excited state dynamics and redox properties for Re3DMABN and Re4DMABN, the nature of the lowest energy excited states formed in these two compounds is drastically different. For example, the lowest energy excited state for Re4DMABN in the mixed solvent is assigned to the (4DMABN → bpy) ligand-to-ligand charge transfer (LLCT) state featuring partial charge transfer character. An equilibrium between a 3DMABN intraligand triplet ((3)IL) and a metal-ligand-to-ligand charge transfer (MLLCT) state is found for Re3DMABN in the mixed solvent with the latter at ca. 400 cm(-1) lower energy. The origin of such a drastic difference between the states involved in Re4DMABN and Re3DMABN is attributed to a difference in the energies of polarized quinoidal resonance structures in 4DMABN and 3DMABN ligands.

Synthesis, characterization, electronic structure and catalytic activity of new ruthenium carbonyl complexes of N-[(2-pyridyl)methylidene]-2-aminothiazole

Reaction of ruthenium carbonyls, [Ru(CO)2Cl2]n/[Ru(CO)4I2] with bidentate Schiffs base ligands derived by the condensation of pyridine-2-carboxaldehyde with 2-aminothiazole in a 1:1 mole ratio in acetonitrile led to the formation of complexes having general formula [Ru(CO)2(L)X2] (X=Cl (1) and I (2)) (L=N-[(2-pyridyl)methylidene]-2-aminothiazole). The compounds have been characterized by various analytical and spectroscopic (IR, electronic and 1H NMR) studies. In acetonitrile solution the complexes exhibit a weak broad metal–ligand to ligand charge transfer (MLLCT) band along with ILCT transitions. The compounds are emissive in room temperature upon excitation in the ILCT band. The complexes exhibit a quasi-reversible one electron Ru(II)/Ru(III) oxidation couple at 1.44V for 1 and 0.94V for 2. Catalytic activity of these compounds is investigated to the oxidation of PhCH2OH to PhCHO, 2-butanol (C4H9OH) to 2-butanone, 1-phenylethanol (PhC2H4OH) to acetophenone, cyclopentanol (C5H9OH) to cyclopentanone, cyclohexanol to cyclohexanone, cycloheptanol to cycloheptanone and cycloctanol to cycloctanone using N-methylmorpholine-N-oxide (NMO) as oxidant. The catalytic efficiency of 2 is greater than complex 1 and well correlate with the metal oxidation potential of the complexes. DFT, NBO and TDDFT calculations are employed to explain the structural and electronic features and to support the spectroscopic assignments.

Photophysical properties of [Cu(binap)2]+ and [Pd(binap)2] complexes: A theoretical study

A theoretical study of the electronic and molecular structure of [Cu(binap)2]+ (1) and [Pd(binap)2] (2) complexes (binap=2,20-bis(diphenylphosphino)-1,10-binaphthyl) was performed. We examined the ground states and the lowest excited states (S1 and T1) in order to get insights about the luminescence processes of these systems. Geometry optimizations were performed at density functional theory DFT (PW91 functional) employing Slater type orbitals (STO) including two polarization functions (TZ2P) and scalar relativistic corrections via the ZORA Hamiltonian. The UV–Vis absorption spectra were simulated employing time-dependent density functional theory (TD-DFT) and solvent effects were estimated using the COSMO model.The calculated excitation energies are in good agreement with experimental data, which is reflected by the calculated UV–Vis spectra. The results suggest that the lowest-energy excited state in each compound corresponds to a ML-LCT (metal–ligand to ligand charge transfer) state. We constructed Jablonsky diagrams for the luminescence phenom. In (1), this process consists of flourescence originated from the S1 state and the phosphorescence from the T1 state. In (2) the emission energy only results in phosphorescence.We found mainly differences in the geometrical parameters that envolve the Pd metal. The structural changes observed in the calculated geometries of the first excited states respect to those of the ground state have significative relevance in the values of the calculated Stokes shifts showed by the complexes in their UV–Vis/emission spectra.

Synthesis, Structural, and Photophysical Investigation of Diimine Triscarbonyl Re(I) Tetrazolato Complexes.

The synthesis, structural, and photophysical properties of a novel family of neutral fac-[Re(N(∧)N)(CO)(3)(L)] complexes, where N(∧)N is either 2,2'-bipyridine or 1,10-phenanthroline and L is a para functionalized 5-aryltetrazolate [namely, 5-phenyltetrazolate (Tph(-)), 4-(tetrazolate-5-yl)benzaldehyde (Tbdz(-)), 5-(4-acetylphenyl)tetrazolate (Tacy(-)), and methyl 4-(tetrazolate-5-yl)benzoate (Tmeb(-))] are reported. The complexes were prepared by direct addition of the corresponding tetrazolate anion to the acetonitrile solvated fac-[Re(N(∧)N)(CO)(3)](+) precursor. NMR data demonstrate that the coordination of the metal fragment is regiospecific at the N2 atom of the tetrazolate ring. These conclusions are also supported by X-ray structural determinations. Photophysical data were obtained in diluted and deaerated dichloromethane solutions displaying broad and structureless profiles with emission maxima ranging from 566 to 578 nm. The absorption profiles indicate the presence of higher energy intraligand (IL) π-π* transitions and lower energies ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT). As the last two transitions are mixed, they are better described as a metal-ligand-to-ligand charge transfer (MLLCT), a result that is also supported by density functional theory (DFT) calculations. The complexes show excited state lifetime values ranging from 102 to 955 ns, with associated quantum yield between 0.012 and 0.099. Compared to the parent neutral chloro or bromo [Re(N(∧)N)(CO)(3)X], the complexes show a slightly improved performance because of the π accepting nature of the tetrazolato ligand. The metal-to-ligand backbonding is in fact depleting the Re center of electron density, thus widening the HOMO-LUMO gap and reducing the non-radiative decay mechanism in accordance with the energy gap law. Finally, the electron-withdrawing or donating nature of the substituent on the phenyltetrazolato ligand allows the fine-tuning of the photophysical properties.

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

AbstractHere, the photophysics and performance of single‐layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm bpy}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ and $\left[ {{\rm Ir}\left( {{\rm ppy}} \right)_{\rm 2} \left( {{\rm phen}} \right)} \right]^ + {\rm PF}_{\rm 6}^ -$ where ppy, bpy, and phen are 2‐phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the NˆN ligand has little effect on the emitting metal‐ligand to ligand charge‐transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m−2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.

Ligand-Selective Photodissociation from [Ru(bpy)(4AP)4]2+: a Spectroscopic and Computational Study

The new complex [Ru(bpy)(4AP)(4)](2+) (1), where bpy = 2,2'-bipyridine and 4AP = 4-aminopyridine, undergoes selective photodissociation of two 4APs upon light excitation of the metal-ligand-to-ligand charge-transfer (MLLCT) band at 510 nm. The photoproducts of the reaction are mer-[Ru(bpy)(4AP)(3)(H(2)O)](2+) (2a) and trans-(4AP)[Ru(bpy)(4AP)(2)(H(2)O)(2)](2+) (3a). Photodissociation occurs in two consecutive steps with quantum yields of phi(1) = (6.1 +/- 1.0) x 10(-3) and phi(2) = (1.7 +/- 0.1) x 10(-4), respectively. Complex 1 was characterized by combined spectroscopic and theoretical techniques. EXAFS experiments at the Ru K-edge (22 117 eV) of 1 in an aqueous solution gave a Ru-N distance of 2.09 +/- 0.01 A. Photoproducts were characterized by electronic spectroscopy, 1D and 2D NMR, and mass spectrometry. Singlet and triplet excited states of 1 were studied by density functional theory (DFT) and time-dependent DFT for characterizing the optical properties of the complex. In the singlet state, (1)MC (metal-centered) dissociative states lie 0.65 eV above the main (1)MLLCT transition in the visible region of the UV-vis absorption spectrum. In the triplet state, the energy difference between these states is not reduced. However, potential energy curves of singlet and triplet excited states of 1 along the Ru-N(axial 4AP) and Ru-N(equatorial 4AP) stretching coordinates show that the release of the first 4AP may occur from the triplet state by mixing of (3)MLLCT and (3)MC dissociative states. This mixing is favored when the Ru-N(equatorial 4AP) bond is elongated, explaining the formation of the photoproduct 2a.

Computational and Spectroscopic Studies of Re(I) Bipyridyl Complexes Containing 2,6-Dimethylphenylisocyanide (CNx) Ligand.

Density Functional Theory (DFT) calculations produce optimized geometries of the complexes [Re(CO)3(bpy)Cl] (1), [Re(CO)3(bpy)(py)](CF3SO3) (2), [Re(CO)3(bpy)(CNx)](CF3SO3) (3), and [Re(CO)(bpy)(CNx)3](CF3SO3) (4), where bpy = 2,2'-bipyridine, py = pyridine, and CNx = 2,6-dimethylphenylisocyanide in their ground and lowest-lying triplet states. The ground-state optimized geometry for the cation of [Re(CO)3(bpy)(CNx)](CF3SO3) (3) results in a Re-C (CNx) bond length of 2.10 Å, a Re-C (CO) bond length trans to CNx of 2.01 Å, and a Re-C (CO) bond length cis to CNx of 1.96 Å which compares favorably to the single-crystal analysis of a Re-C (CNx) bond length of 2.074(4) Å, a Re-C (CO) bond length trans to CNx of 1.971(4) Å, and Re-C (CO) bond length cis to CNx of 1.932(4) Å. The majority of the singlet excited-state energies calculated using Time-dependent Density Functional Theory (TDDFT) and Conductor-like Polarizable Continuum Model (CPCM) are metal-ligand-to-ligand charge transfer (MLLCT) states and are in good agreement with the UV-vis spectral energies for the complexes in ethanol. The complexes exhibit emission both at room temperature and at 77 K except 4 which is only emissive at 77 K. The 77 K emission lifetimes range from 3.9 μs for 1 to 8.8 μs for 3. The emissive lowest-lying triplet state is a (3)MLLCT state for complexes 1-3 but a triplet ligand-to-metal charge transfer ((3)LMCT) state for complex 4. The electronic, electrochemical, thermodynamic, HOMO-LUMO, and emitting-state energy gaps as well as the emission lifetimes increase in the order 1 < 2 < 3. A (3)d-d excited- state, which is located above the (3)LMCT state, accounts for the loss of room-temperature emission for complex 4.