Structural, Electronic, and Computational Studies of Heteroleptic Cu(I) Complexes of 6,6'-Dimesityl-2,2'-bipyridine with Ferrocene-Appended Ethynyl-2,2'-bipyridine Ligands.

Optical characterization and computational modeling of three ferrocene-appended ethynyl-2,2'-bipyridine ligands and the associated heteroleptic copper(I) complexes of 6,6'-dimesityl-2,2'-bipyridine are reported. These dyes have been studied using electrochemical analysis, electronic absorption, and Raman and resonance Raman spectroscopies, coupled with density functional theoretical approaches. For the complexes, optical spectra are dominated by a low energy copper(I) centered metal to ligand charge transfer (MLCT) transition; this is modulated by the presence of pendant ferrocene units and the extent of conjugation of the ferrocenyl bipyridine backbone. Electronic tuning due to ferrocene is shown to result in a redshift of the MCLT transition of up to ∼0.2 eV, while an elongation of conjugation appears to result in an increased MLCT intensity of around 50%.

Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids.

The synthesis of two bipyridine-hexa-peri-hexabenzocoronene (bpy-HBC) ligands functionalized with either (t)Bu or C12H25 and their Re(I) tricarbonyl chloride complexes are reported and their electronic properties investigated using spectroscopic and computational methods. The metal complexes show unusual properties, and we observed the formation of a long-lived excited state using time-resolved infrared spectroscopy. Depending on the solvent, this appears to be of the form Rebpy(•-)HBC(•+) or a bpy-centered π,π* state. TD-DFT calculations support the donor-acceptor charge transfer character of these systems, in which HBC is the donor and bpy is the acceptor. The ground state optical properties are dominated by the HBC chromophore with additional distinct transitions of the complexes, one associated with MLCT 450 nm (ε > 17 000 L mol(-1) cm(-1)) and another with a HBC/metal to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000 L mol(-1) cm(-1)). These assignments are also supported by resonance Raman spectroscopy.

Luminescent Cages: Pendant Emissive Units on [Pd2L4](4+) "Click" Cages.

The photophysics of a family of exo-functionalized [Pd2L4](4+) metallo-supramolecular cage architectures constructed from a tripyridyl 1,2,3-triazole backbone are reported. Several spectroscopic techniques are employed including both electronic (steady-state and transient absorption and emission) and vibrational (resonant and nonresonant Raman) methods. These experimental results are interpreted alongside simulated results from density functional theory calculations of the system's vibrational and electronic properties. The ligands and cages are shown to be essentially insulated from the exo-functionalization. They exhibit electronic transitions in the UV region and excited-state properties that are little affected by formation of the cage. Upon functionalization, characteristic Raman bands, electronic transitions, and emission bands associated with, and confined to, the substituent are observed.

"Tail" tuning of iron(II) spin crossover temperature by 100 K.

Two new Rdpt ligands featuring long "tails", padpt (N-4H-1,2,4-triazole-3,5-di(2-pyridyl)palmitamide) and hpdpt (4-(4-heptadecafluoroctylphenyl)-3,5-bis(2-pyridyl)-4H-1,2,4-triazole), were made and reacted with [Fe(II)(py)4(NCS)2] to give pinkish-red [Fe(II)(padpt)2(SCN)2] (1) and purple-red [Fe(II)(hpdpt)2(SCN)2] (2) as solvent-free crystals. Magnetic measurements reveal that both 1 and 2 exhibit complete and reproducible spin crossovers, with a far lower T1/2 for the amide-alkyl tailed 1 (182 K) than for the fluorocarbon tailed 2 (248 K), which in turn is far lower than the T1/2 of 290 K previously reported for the nonamide-alkyl tailed analogue [Fe(II)(C16dpt)2(SCN)2]·(2)/3H2O (3). Structure determinations for 1 and 2 in both the high spin (HS) and low spin (LS) states confirm the expected trans-NCS conformation and reveal that (a) the "tails" interdigitate and (b) the LS forms are less distorted than the HS forms (Σ = 58-70° vs 47-54°). DSC and Raman spectroscopy confirmed the high tail-dependence of the SCO events in 1 and 2, as well as in 3, with the Raman data giving T1/2 values of 190, 243, and 285 K, respectively. Bright orange single crystals of the solvatomorph [Fe(II)(hpdpt)2(SCN)2]·MeOH·H2O (2solv) were also structurally and magnetically characterized and, in contrast to 2, found to remain HS down to 4 K, providing further evidence of the huge impact of crystal packing on SCO. Both 1 and 2 form stable Langmuir films at an air-water interface, a single layer of which can be transferred to a solid support.

Intraligand charge-transfer excited states in Re(I) complexes with donor-substituted dipyridophenazine ligands.

The donor-acceptor ligands 11-(4-diphenylaminophenyl)dipyrido[3,2-a:2',3'-c]phenazine (dppz-PhNPh2) and 11-(4-dimethylaminophenyl)dipyrido[3,2-a:2',3'-c]phenazine (dppz-PhNMe2), and their rhenium complexes, [Re(CO)3X] (X = Cl(-), py, 4-dimethylaminopyridine (dmap)), are reported. Crystal structures of the two ligands were obtained. The optical properties of the ligands and complexes are dominated by intraligand charge transfer (ILCT) transitions from the amine to the dppz moieties with λabs = 463 nm (ε = 13 100 M(-1) cm(-1)) for dppz-PhNMe2 and with λabs = 457 nm (ε = 16 900 M(-1) cm(-1)) for dppz-PhNPh2. This assignment is supported by CAM-B3LYP TD-DFT calculations. These ligands are strongly emissive in organic solvents and, consistent with the ILCT character, show strong solvatochromic behavior. Lippert-Mataga plots of the data are linear and yield Δµ values of 22 D for dppz-PhNPh2 and 20 D for dppz-PhNMe2. The rhenium(I) complexes are less emissive, and it is possible to measure resonance Raman spectra. These data show relative band intensities that are virtually unchanged from λexc = 351 to 532 nm, consistent with a single dominant transition in the visible region. Resonance Raman excitation profiles are solvent sensitive; these data are modeled using wavepacket theory yielding reorganization energies ranging from 1800 cm(-1) in toluene to 6900 cm(-1) in CH3CN. The excited state electronic absorption and infrared spectroscopy reveal the presence of dark excited states with nanosecond to microsecond lifetimes that are sensitive to the ancillary ligand on the rhenium. These dark states were assigned as phenazine-based (3)ILCT states by time-resolved infrared spectroscopy. Time-resolved infrared spectroscopy shows transient features in which Δν(CO) is approximately -7 cm(-1), consistent with a ligand-centered excited state. Evidence for two such states is seen in mid-infrared transient spectra.

Vibrational spectroscopy as a probe of molecule-based devices.

This critical review discusses the applicability of vibrational spectroscopic techniques, specifically Raman and mid-infrared, to the study of molecule-based electronics through a series of examples. We focus on a number of devices currently of interest, such as solar cells, organic light emitting diodes, molecular junctions, switches and transistors. Infrared and Raman spectroscopic techniques and their variations, the main focus of this article, can be used to investigate properties such as crystallinity, multiphasic distributions in three dimensions, as well as lifetimes, structures and energetics of excited-states on ultrashort to very long timescales (210 references).

Application of terpyridyl ligands to tune the optical and electrochemical properties of a conducting polymer.

We present a simple and effective way of using metal and metal-ligand modifications to tune the electrochemical and optical properties of conducting polymers. To that end, a polyterthiophene functionalized with terpyridine moieties was synthesized and then the resulting film's surface or bulk was modified with different metal ions, namely Fe2+, Zn2+ and Cu2+ and terpyridine. The modification of the terpyridine functionalized polyterthiophene film by Fe2+ increased the absorptivity and electrochemical capacitance of the conducting polymer, and improved its conjugation. Further modification by Zn2+ and Cu2+ resulted in dramatically different spectroelectrochemical properties of the film. Moreover, the influence of the solvents (ACN and 1 : 1 ACN : H2O) in conjunction with the metal ion applied for the modification was found crucial for the electrochemical and optical properties of the films.

Chemically and electrochemically induced expansion and contraction of a ferrocene rotor.

A 2,2'-bipyridine-appended ferrocene rotor, 1,1'-di(5-yl-ethynyl-2,2'-bipyridine)ferrocene, can be switched from a folded/stacked (syn) conformation to an extended/unstacked (anti) conformation by the addition of [Cu(CH3CN)4](PF6) and 6,6'-dimesityl-2,2'-bipyridine. This extension and contraction process was completely reversible and could be triggered either chemically or electrochemically.

fac-Re(CO)3 complexes of 2,6-bis(4-substituted-1,2,3-triazol-1-ylmethyl)pyridine "click" ligands: synthesis, characterisation and photophysical properties.

The syntheses of the 4-n-propyl and 4-phenyl substituted fac-Re(CO)(3) complexes of the tridentate "click" ligand (2,6-bis(4-substituted-1,2,3-triazol-1-ylmethyl)pyridine) are described. The complexes were obtained by refluxing methanol solutions of [Re(CO)(5)Cl], AgPF(6) and either the 4-propyl or 4-phenyl substituted ligand for 16 h. The ligands and the two rhenium(I) complexes were characterised by elemental analysis, HR-ESMS, ATR-IR, (1)H and (13)C NMR spectroscopy and the molecular structures of both complexes were confirmed by X-ray crystallography. The electronic structure of the fac-Re(CO)(3) "click" complexes was probed using UV-Vis, Raman and emission spectroscopy, cyclic voltammetry and DFT calculations. Altering the electronic nature of the ligand's substituent, from aromatic to alkyl, had little effect on the absorption/emission maxima and electrochemical properties of the complexes indicating that the 1,2,3-triazole unit may insulate the metal centre from the electronic modification at the ligands' periphery. Both Re(I) complexes were found to be weakly emitting with short excited state lifetimes. The electrochemistry of the complexes is defined by quasi-reversible Re oxidation and irreversible triazole-based ligand reduction processes.