Surface-enhanced Raman scattering studies of rhodanines: Evidence for substrate surface-induced dimerization

The surface-enhanced Raman scattering (SERS) spectra of rhodanine adsorbed on silver nanoparticles have been examined using 514.5 and 632.8 nm excitation. There is evidence that, under the experimental conditions used, rhodanine undergoes a nanoparticle surface-induced reaction resulting in the formation of a dimeric species via the active methylene group in a process which is analogous to the Knoevenagel reaction. The experimental observations are supported by DFT calculations at the B3-LYP/cc-pVDZ level. Calculated energies for the interaction of the E and Z isomers of the dimers of rhodanine with silver nanoparticles support a model in which the (intra-molecular hydrogen bonded) E isomer dimer is of lower energy than the Z isomer. A strong band, at 1566 cm(-1), in the SERS spectrum of rhodanine is assigned to the nu(C=C) mode of the dimer species.

Developing Single-Laser Sources for Multimodal Coherent Anti-Stokes Raman Scattering Microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy has developed rapidly and is opening the door to new types of experiments. This work describes the development of new laser sources for CARS microscopy and their use for different applications. It is specifically focused on multimodal nonlinear optical microscopy—the simultaneous combination of different imaging techniques. This allows us to address a diverse range of applications, such as the study of biomaterials, fluid inclusions, atherosclerosis, hepatitis C infection in cells, and ice formation in cells. For these applications new laser sources are developed that allow for practical multimodal imaging. For example, it is shown that using a single Ti:sapphire oscillator with a photonic crystal fiber, it is possible to develop a versatile multimodal imaging system using optimally chirped laser pulses. This system can perform simultaneous two photon excited fluorescence, second harmonic generation, and CARS microscopy. The versatility of the system is further demonstrated by showing that it is possible to probe different Raman modes using CARS microscopy simply by changing a time delay between the excitation beams. Using optimally chirped pulses also enables further simplification of the laser system required by using a single fiber laser combined with nonlinear optical fibers to perform effective multimodal imaging. While these sources are useful for practical multimodal imaging, it is believed that for further improvements in CARS microscopy sensitivity, new excitation schemes are necessary. This has led to the design of a new, high power, extended cavity oscillator that should be capable of implementing new excitation schemes for CARS microscopy as well as other techniques. Our interest in multimodal imaging has led us to other areas of research as well. For example, a fiber-coupling scheme for signal collection in the forward direction is demonstrated that allows for fluorescence lifetime imaging without significant temporal distortion. Also highlighted is an imaging artifact that is unique to CARS microscopy that can alter image interpretation, especially when using multimodal imaging. By combining expertise in nonlinear optics, laser development, fiber optics, and microscopy, we have developed systems and techniques that will be of benefit for multimodal CARS microscopy.

Resonance Raman and lifetime studies on regioselectively deuteriated ruthenium (II) polypyridyl complexes

Two series of ruthenium(II) polypyridyl complexes [Ru(bipy)2(phpytr)]+ and [Ru(bipy)2(phpztr)]+ (where Hphpytr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyridine and Hphpztr = 2-(5-phenyl-1H-[1,2,4]triazol-3-yl)-pyrazine) are examined by electrochemistry, UV/Vis, emission, resonance Raman, transient resonance Raman and transient absorption spectroscopy, in order to obtain a more comprehensive understanding of their excited state electronic properties. The interpretation of the results obtained is facilitated by the availability of several isotopologues of each of the complexes examined. For the pyridine-1,2,4-triazolato based complex the lowest emissive excited state is exclusively bipy based, however, for the pyrazine based complexes excited state localisation on particular ligands shows considerable solvent and pH dependency.

Rapid, quantitative analysis of ppm/ppb nicotine using surface-enhanced Raman scattering from polymer-encapsulated Ag nanoparticles (gel-colls)

Rapid, quantitative SERS analysis of nicotine at ppm/ppb levels has been carried out using stable and inexpensive polymer-encapsulated Ag nanoparticles (gel-colls). The strongest nicotine band (1030 cm(-1)) was measured against d(5)-pyridine internal standard (974 cm(-1)) which was introduced during preparation of the stock gel-colls. Calibration plots of I-nic/I-pyr against the concentration of nicotine were non-linear but plotting I-nic/I-pyr against [nicotine](x) (x = 0.6-0.75, depending on the exact experimental conditions) gave linear calibrations over the range (0.1-10 ppm) with R-2 typically ca. 0.998. The RMS prediction error was found to be 0.10 ppm when the gel-colls were used for quantitative determination of unknown nicotine samples in 1-5 ppm level. The main advantages of the method are that the gel-colls constitute a highly stable and reproducible SERS medium that allows high throughput (50 sample h(-1)) measurements.

Resonance Raman and DFT studies of tetra-tert-butyl porphine: Assignment of strongly enhanced distortion modes in a ruffled porphyrin

The free-base form of tetra-tert-butyl porphine (TtBP), which has extremely bulky meso substituents, is severely distorted from planarity, with a ruffling angle of 65.5degrees. The resonance Raman spectrum of TtBP (lambda(ex) = 457.9 nm) and its d(2), d(8), and d(10) isotopomers have been recorded, and while the spectra show high-frequency bands similar to those observed for planar meso-substituted porphyrins, there are several additional intense bands in the low-frequency region. Density functional calculations at the B3-LYP/6-31G(d) level were carried out for all four isotopomers, and calculated frequencies were scaled using a single factor of 0.98. The single factor scaling approach was validated on free base porphine where the RMS error was found to be 14.9 cm(-1). All the assigned bands in the high-frequency (> 1000 cm(-1)) region of TtBP were found to be due to vibrations similar in character to the in-plane skeletal modes of conventional planar porphyrins. In the low-frequency region, two of the bands, assigned as nu(8) (ca. 330 cm(-1)) and nu(16) (ca. 540 cm(-1)), are also found in planar porphyrins such as tetra-phenyl porphine (TPP) and tetra-iso-propyl porphine (IPP). Of the remaining three very strong bands, the lowest frequency band was assigned as gamma(12) (pyr swivel, obsd 415 cm(-1), calcd 407 cm(-1) in do). The next band, observed at 589 cm-1 in the do compound (calcd 583 cm(-1)), was assigned as a mode whose composition is a mixture of modes that were previously labeled gamma(13) (gamma(CmCaHmCa)) andy gamma(11) (pyr fold(asym)) in NiOEP. The final strong band, observed at 744 cm(-1) (calcd 746 cm(-1)), was assigned to a mode whose composition is again a mixture of gamma(11) and gamma(13), although here it is gamma(11) rather than gamma(13) which predominates. These bands have characters and positions similar to those of three of the four porphyrin ring-based, weak bands that have previously been observed for NiTPP. In addition there are several weaker bands in the TtBP spectra that are also

Resonance Raman and surface-enhanced resonance Raman studies of polymer-modified electrodes which mimic heme enzymes

Iron-5,10,15,20-tetraphenylporphyrin (FeTPP) has been incorporated into films of a coordinating hydrogel polymer support medium, poly(gamma-ethyl-L-glutamate) (PEG) functionalised with imidazole pendant arms (PEG-Im), and studied in situ on silver electrodes using a combination of both resonance Raman (RR) and surface-enhanced resonance Raman (SERR) spectroscopy. The SERR spectra give information on the portion of the film close to the electrode surface while RR spectra probe the

Resonance Raman probing of the interaction between dipyridophenazine complexes of Ru(II) and DNA

Resonance Raman (RR) spectroscopy has been used to probe the interaction between dipyridophenazine (dppz) complexes of ruthenium(II), [Ru(L)(2)(dppz)](2+) (L = 1,10-phenanthroline (1) and 2,2-bipyridyl (2)), and calf-thymus DNA. Ground electronic state RR spectra at selected probe wavelengths reveal enhancement patterns which reflect perturbation of the dppz-centered electronic transitions in the UV-vis spectra in the presence of DNA. Comparison of the RR spectra recorded of the short-lived MLCT excited states of both complexes in aqueous solution with those of the longer-lived states of the complexes in the DNA environment reveals changes to excited state modes, suggesting perturbation of electronic transitions of the dppz ligand in the excited state as a result of intercalation. The most prominent feature, at 1526 cm(-1), appears in the spectra of both 1 and 2 and is a convenient marker band for intercalation. For 1, the excited state studies have been extended to the A and A enantiomers. The marker band appears at the same frequency for both but with different relative intensities. This is interpreted as reflecting the distinctive response of the enantiomers to the chiral environment of the DNA binding sites. The results, together with some analogous data for other potentially intercalating complexes, are considered in relation to the more general application of time-resolved RR spectroscopy for investigation of intercalative interactions of photoexcited metal complexes with DNA.

Time-resolved resonance Raman spectroscopy

Vibrational Raman spectroscopy is now widely recognized as a useful technique for chemical analysis. It has become increasingly popular for the characterization of stable species since the technology which underpins Raman measurements has matured. Time-resolved Raman spectroscopy has also become established as an excellent method for the characterization of transient chemical species but it is not so widely applied. However, the technical advances which have reduced the cost and increased the reliability of conventional: Raman systems can also be exploited in studies of transient species. In some cases it is just as straightforward to record the Raman-spectra of a short-lived transient species as it is to monitor a more stable sample. This raises the possibility of routinely adding time-domain Raman measurements to more conventional Raman techniques, increasing the selectivity of the analysis while retaining its ability to provide spectral information which is characteristic of the species under investigation.

Resonance-Raman probing of the interaction between dipyridophenazine complexes of ruthenium(II) and DNA

The resonance-Raman spectroscopic technique is an effective probe of the interaction between dipyridophenazine (dppz) complexes of ruthenium(II) and calf-thymus DNA, providing evidence that DNA addition results in changes to electronic transitions of the intercalating dppz ligand in both ground and excited states.

TIME-RESOLVED RESONANCE RAMAN-SPECTROSCOPY AND RAMAN SPECTROELECTROCHEMISTRY OF (CO)(5)W[4,4'-BPY]W(CO)(5), PROBED IN THE VISIBLE AND NEAR-INFRARED

Time-resolved resonance Raman spectroscopy of the lowest energy excited state of the 4,4'-bipyridyl ligand-bridged complex, [(CO)(5)W(L)W(CO5] (1), and Raman spectroscopy of electrochemically reduced 1, both give bands characteristic of the the L(.-) species. This confirms that the ligand L is negatively charged in the lowest energy exicited state which is therefore metal-ligand charge transfer (MLCT) in character. Raman spectra of the radical anion of 1 excited in the far red (800 nm) exhibited a band near 2050 cm(-1) due to a vco symmetric CO stretching mode, compared to the corresponding band at 2070 cm(-1) in the spectrum of the parent, uncharged complex. The lower vco in the reduced complex supports the recent finding by time-resolved IR spectroscopy of a similar frequency decrease for nu(CO) in the longest lived (MLCT) excited state of 1 which was attributed to electron/hole localisation in this state on the IR time scale.

TIME-RESOLVED ABSORPTION, INFRARED, AND RESONANCE RAMAN-SPECTRA OF THE COMPLEXES [RU(X)(R)(CO)(2)(ALPHA-DIIMINE)] (X=HALIDE R=ALKYL) - INFLUENCE OF X ON THE CHARGE-TRANSFER CHARACTER OF THE LOWEST EXCITED-STATE

Nanosecond time-resolved absorption (TA), resonance Raman (TR(3)), and infrared (TRIR) spectra are reported for several complexes [Ru(X)(R)(CO)(2)(alpha-diimine)] (X = Cl, Br, I; R = Me, Et; alpha-diimine = N,N'-diisopropyl-1,4-diaza-1,3-butadiene (iPr-DAB), pyridine-2-carbaldehyde-N-isopropylimine (iPr-PyCa), 2,2'-bipyridine (bpy)). This is the first instance in which the TA, TR(3), and TRIR techniques have been used to probe excited states in the same series of complexes. The TA spectra of the iodide complexes show a transient absorption between 550 and 700 nm, which does not depend on the solvent but shifts to lower energy in the order iPr-DAB > bpy > iPr-PyCa. This band is assigned to an intraligand transition. For the corresponding chloride and bromide complexes this band occurs at higher energy, most probably because of a change of character of the lowest excited state from XLCT to MLCT. The TRIR spectra show an increase in v(CO) (and k(CO)) on promotion to the excited state; however, the shifts Delta v(CO) show a decrease in the order Cl- > Br- > I-. The TR(3) spectra of the excited complexes [Ru(X)(R)(Co)(2)(iPr-DAB)] show v(s)(CN) of the iPr-DAB ligand 50-80 cm(-1) lower in frequency than for the complexes in their ground state. This frequency shift decreases in the order Cl- > Br- > I-, indicating a decrease of CT character of the lowest excited state in this order. However, going from X = Br to I, the effect on Delta v(CO) is much larger than the decrease of Delta v(s)(CN). This different effect on the CO- and CN-stretching frequencies is assigned to a gradual change in character of the lowest excited state from MLCT to XLCT when Cl- is replaced by Br- and I-. This result confirms a similar conclusion derived from previous resonance Raman and emission experiments on these complexes.

RESONANCE RAMAN-SPECTRA OF THE TRIPLET-STATE OF FREE-BASE TETRAPHENYLPORPHYRIN AND 6 OF ITS ISOTOPOMERS

The resonance Raman spectra of the ground state and the lowest excited tripler state of free-base tetraphenylporphyrin and six of its isotopomers have been obtained using two-color time-resolved techniques. Ground-state spectra were recorded using low-energy 447 nm probe laser pulses, and triplet-state spectra were probed, with similar pulses, 30 ns after high-energy excitation with 532 nm pump pulses. Polarization data on both the ground and triplet states are also reported. The resonance Raman spectrum of the triplet is very different from that of the ground state but the combination of extensive isotope substitution with polarization data allows bands in the ground state to be assigned and corresponding bands in the tripler state to be located. Isotope shifts of the same bands in the S-0 and T-1 states are similar, implying that the compositions of the vibrational modes do not change significantly on excitation. Two of the strongest bands in the T-1 spectra are associated with phenyl ring substituents; these are shifted less than 5 cm(-1) between the S-0 and T-1 states so that bonding in the phenyl substituents is barely affected by excitation to the T-1 state. The changes in position of the porphyrin ring bands are larger, but still only tens of cm(-1) or less, the main changes in the spectra being due to differences in relative band intensities in the two states. The relatively small shifts in the porphyrin ring band positions which are observed show that the excitation energy is not localized on a single small region of the molecule but is delocalized over the entire porphyrin skeleton. This picture of an excited species with high chemical reactivity, but with individual bonds only slightly perturbed from the ground state, is contrasted with molecules, such as benzophenone, where excitation causes a large perturbation in the bonding within a single functional group.