Pulsed Laser Photolysis and Thermodynamics Studies of Intramolecular Electron Transfer in Valence Tautomeric Cobalt <i>o</i>-Quinone Complexes

Four valence tautomeric complexes with the general composition [Co(3,5-DTBSQ)2(N⌒N)] have been investigated, where 3,5-DTBSQ- is the semiquinonate form of 3,5-di-tert-butyl-o-quinone. The diiminium ligand N⌒N is one of the following ligands: phen (1,10-phenanthroline, complex 1); bpy (2,2‘-bipyridine, complex 2); dmbpy (4,4‘-dimethyl-2,2‘-bipyridine, complex 3); dpbpy (4,4‘-diphenyl-2,2‘-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (ls) [CoIII(3,5-DTBSQ)(3,5-DTBCAT)(N⌒N)] at low temperatures to high-spin (hs) [CoII(3,5-DTBSQ)2(N⌒N)] at high temperatures, where the 3,5-DTBCAT2- ligand is the catecholate form of the o-quinone ligand. Variable temperature (150−340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give ΔH and ΔS values that characterize the valence tautomerism equilibrium. The ΔH values range from 21.33 to 38.36 kJ mol-1, whereas the ΔS values range from 60.6 to 133.8 J mol-1 K-1. Analogous to FeII spin crossover complexes, the cobalt valence tautomerism is an entropy driven process. Electronic and vibrational (changes in metal−ligand and ligand-based vibrations) entropy contributions are present that drive the ls-[CoIII(SQ)(Cat)] to hs-[CoII(SQ)2] conversion when the temperature is increased. Variable temperature electronic absorption spectra (320−820 nm) are presented for the complexes in toluene solution and for polystyrene-doped samples. The presence of isosbestic points indicates only two species are present. Optical spectra show that, compared to the situation in toluene solution, doping the cobalt tautomeric complexes into polystyrene leads to stabilization of the smaller ls-CoIII tautomer. Pulsed laser photolysis, both on the picosecond (90 ps pulse) and the nanosecond (24 ns pulse) time scales, were carried out for solutions of the complexes. The laser pulse into the 600 nm band of the ls-CoIII tautomer excites the complex to a ligand to metal charge transfer (LMCT) excited state. Some molecules rapidly intersystem cross to the hs-CoII state. The rate of back valence tautomerization, kbvt, for conversion from the hs-CoII to the ls-CoIII state can be measured. At room temperature in toluene solution, kbvt was determined with picosecond spectroscopy to be 6.08 × 107 s-1 for complex 3 and 6.71 × 108 s-1 for complex 4. Since no indication of a rise time was seen, the hs-CoII state is formed within the experimental resolution of 90 ps. The observation of a bleach and an absorption with identical kinetics when the probe wavelength is moved to either side of an isosbestic point confirms that the hs-CoII to ls-CoIII conversion is being monitored. Nanosecond laser spectroscopy was used to determine the temperature dependence of kbvt. For complex 4 in 2-methyltetrahydrofuran, data were collected in the 110.5−198.1 K range. There is evidence of a change from an Arrhenius-type behavior to a temperature-independent rate below ∼132 K. The quantum mechanical theory of radiationless transitions of Buhks and Jortner was used to fit the temperature dependence of kbvt. It appears the hs-CoII to ls-CoIII valence tautomerism may occur by quantum mechanical tunneling

Density Functional Study of the Valence-Tautomeric Interconversion Low-Spin [Co^III(SQ)(Cat)(phen)] ⇌ High-Spin [Co^II(SQ)₂(phen)]

The results of self-consistent field (SCF) nonlocal density functional molecular orbital calculations are presented for the various spin states and tautomeric forms of a cobalt complex with two o-quinone-derived ligands. In addition, new variable-temperature solution magnetic susceptibility, EPR, and electronic absorption data are presented to characterize the low-spin [CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [CoII(3,5-DTBSQ)2(phen)] valence-tautomeric interconversion, where 3,5-DTBSQ- and 3,5-DTBCat2- are the semiquinonate and catecholate forms of 3,5-di-tert-butyl-o-benzoquinone, respectively, and phen is 1,10-phenanthroline. The solution magnetic susceptibility data were fitted to give ΔH = 2238 cm-1 and ΔS = 118.1 J mol-1 K-1 for the ls-CoIII ⇌ hs-CoII equilibrium. Appreciable changes are seen in the electronic absorption spectrum as the complex changes between the two tautomeric forms. Unrestricted SCF calculations gave J = −594 cm-1 for the parameter characterizing the antiferromagnetic exchange interaction between hs-CoII ion (S = 3/2) and each of the two coordinated semiquinonate (S = 1/2) ligands in the hs-CoII tautomer. The calculations indicated that the ls-CoIII tautomer state is the most stable with an energy separation of ΔE = 4428 cm-1 (0.55 eV) between this ls-CoIII state and the S = 1/2 component of the hs-CoII spin ladder. This ΔE value compares favorably with the ΔH value evaluated from variable-temperature susceptibility data.The calculations indicate that, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the ls-CoIII and hs-CoII tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. Finally, the results of the calculations were used to assign the electronic transitions seen for the ls-CoIII and hs-CoII tautomers

Density Functional Study of the Valence-Tautomeric Interconversion Low-Spin [Co^III(SQ)(Cat)(phen)] ⇌ High-Spin [Co^II(SQ)₂(phen)]

The results of self-consistent field (SCF) nonlocal density functional molecular orbital calculations are presented for the various spin states and tautomeric forms of a cobalt complex with two o-quinone-derived ligands. In addition, new variable-temperature solution magnetic susceptibility, EPR, and electronic absorption data are presented to characterize the low-spin [CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [CoII(3,5-DTBSQ)2(phen)] valence-tautomeric interconversion, where 3,5-DTBSQ- and 3,5-DTBCat2- are the semiquinonate and catecholate forms of 3,5-di-tert-butyl-o-benzoquinone, respectively, and phen is 1,10-phenanthroline. The solution magnetic susceptibility data were fitted to give ΔH = 2238 cm-1 and ΔS = 118.1 J mol-1 K-1 for the ls-CoIII ⇌ hs-CoII equilibrium. Appreciable changes are seen in the electronic absorption spectrum as the complex changes between the two tautomeric forms. Unrestricted SCF calculations gave J = −594 cm-1 for the parameter characterizing the antiferromagnetic exchange interaction between hs-CoII ion (S = 3/2) and each of the two coordinated semiquinonate (S = 1/2) ligands in the hs-CoII tautomer. The calculations indicated that the ls-CoIII tautomer state is the most stable with an energy separation of ΔE = 4428 cm-1 (0.55 eV) between this ls-CoIII state and the S = 1/2 component of the hs-CoII spin ladder. This ΔE value compares favorably with the ΔH value evaluated from variable-temperature susceptibility data.The calculations indicate that, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the ls-CoIII and hs-CoII tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. Finally, the results of the calculations were used to assign the electronic transitions seen for the ls-CoIII and hs-CoII tautomers

Single-Molecule Spectroscopy of Interfacial Electron Transfer

It is widely appreciated that single-molecule spectroscopy (SMS) can be used to measure properties of individual molecules which would normally be obscured in an ensemble-averaged measurement. In this report we show how SMS can be used to measure photoinduced interfacial electron transfer (IET) and back electron transfer rates in a prototypical chromophore−bridge−electrode nonadiabatic electron transfer system. N-(1-hexylheptyl)-N‘-(12-carboxylicdodecyl)perylene-3,4,9,10-tetracarboxylbisimide was synthesized and incorporated into mixed self-assembled monolayers (SAMs) on an ITO (tin-doped indium oxide, a p-type semiconductor) electrode. Single-molecule fluorescence time trajectories from this system reveals “blinks”, momentary losses in fluorescence (>20 ms to seconds in duration), which are attributed to discrete electron transfer events:  electron injection from the perylene chromophore into the conduction band of the ITO leads to the loss of fluorescence, and charge recombination (back electron transfer) leads to the return of fluorescence. Such blinks are not observed when an electrode is not present. The fluorescence trajectories were analyzed to obtain the forward and back electron rates; the measured rates are found to lie in the millisecond to second regime. Different rates are observed for different molecules, but the lifetime distributions for the forward or back electron transfer for any given molecule are well fit by single exponential kinetics. The methodology used is applicable to a wide variety of systems and can be used to study the effects of distance, orientation, linker, environment, etc. on electron transfer rates. The results and methodology have implications for molecular electronics, where understanding and controlling the range of possible behaviors inherent to molecular systems will likely be as important as understanding the individual behavior of any given molecule

Single-Molecule Spectroscopy of Intramolecular Electron Transfer in Donor-Bridge-Acceptor Systems

It is widely appreciated that single-molecule spectroscopy (SMS) can be used to measure properties of individual molecules which would normally be obscured in an ensemble-averaged measurement. In this article, we show how SMS can be used to investigate intramolecular electron transfer (IET) processes in model dimer systems composed of two perylene chromophores connected via an adjustable bridge. The fluorescence behaviors of a large number of molecules are cataloged and the results statistically analyzed to gauge information about the range of behaviors of the ensemble. Single-molecule fluorescence time trajectories reveal “blinks”, momentary losses in fluorescence (>20 ms to seconds in duration), which are attributed to discrete IET excursions to the charge-separated (CS) state. We find that fluorescence blinking behavior is dependent on bridge length and chromophore geometry, which affect the electronic coupling and therefore the IET. The statistical trends observed in this analysis are used to corroborate the assignment of the blinking behavior to IET. These results and methodology have implications for molecular electronics, where understanding and controlling the range of possible behaviors inherent to molecular systems will likely be as important as understanding the individual behavior of any given molecule

NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO4)2] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) Å, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH were shown to adhere to the surface of the ZrP sheets and fill in the cracks between the sheets to a lesser extent. The measured heights of these polymer-coated multilayer films are 26(9) and 48(15) Å, respectively. These heights are consistent with theoretical estimates of ideally packed ionic films (28 and 48 Å, respectively). Dual-wavelength fluorescence NSOM imaging at 580 nm and >610 nm and near-field photobleach experiments were used to spatially resolve nanoscopic regions that display energy transfer between the layers

Self-Organized Perylene Diimide Nanofibers

A propeller-shaped perylene diimide trimer was synthesized and a simple evaporation method was used for the self-organization of trimer molecules into fluorescent nanofibers. The sizes of these fibersfrom 4 to 150 nm in diameterwere measured by atomic force microscopy and can be controlled by adjusting the concentration of the initial solution. The aspect ratios (length/height) are around 500. The plane of the trimer was determined by polarized scanning confocal microscopy to be perpendicular to the axis of the fibers, in agreement with molecular mechanics calculations. UV/vis and NMR spectroscopies were used to monitor concentration-dependent π−π stacking in solution. Single-fiber fluorescence imaging and spectroscopy were performed using a total internal reflection fluorescence microscope equipped with a digital color camera and imaging CCD spectrometer. Strongly red-shifted fluorescence from these fibers indicates a high degree of electronic delocalization, and breaking up this delocalization by photobleaching blue-shifts the emission toward that of an isolated noninteracting molecule. The delocalization along these nanofibers and the ability to study the electronic structure using fluorescence make them potentially useful in nanoscale devices, such as field effect transistors and photoconductors

NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO4)2] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) Å, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH were shown to adhere to the surface of the ZrP sheets and fill in the cracks between the sheets to a lesser extent. The measured heights of these polymer-coated multilayer films are 26(9) and 48(15) Å, respectively. These heights are consistent with theoretical estimates of ideally packed ionic films (28 and 48 Å, respectively). Dual-wavelength fluorescence NSOM imaging at 580 nm and >610 nm and near-field photobleach experiments were used to spatially resolve nanoscopic regions that display energy transfer between the layers

NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO4)2] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) Å, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH were shown to adhere to the surface of the ZrP sheets and fill in the cracks between the sheets to a lesser extent. The measured heights of these polymer-coated multilayer films are 26(9) and 48(15) Å, respectively. These heights are consistent with theoretical estimates of ideally packed ionic films (28 and 48 Å, respectively). Dual-wavelength fluorescence NSOM imaging at 580 nm and >610 nm and near-field photobleach experiments were used to spatially resolve nanoscopic regions that display energy transfer between the layers

NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO4)2] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) Å, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH were shown to adhere to the surface of the ZrP sheets and fill in the cracks between the sheets to a lesser extent. The measured heights of these polymer-coated multilayer films are 26(9) and 48(15) Å, respectively. These heights are consistent with theoretical estimates of ideally packed ionic films (28 and 48 Å, respectively). Dual-wavelength fluorescence NSOM imaging at 580 nm and >610 nm and near-field photobleach experiments were used to spatially resolve nanoscopic regions that display energy transfer between the layers