Michael W. George

Ultra: A Unique Instrument for Time-Resolved Spectroscopy

We report the development of a high-sensitivity time-resolved infrared and Raman spectrometer with exceptional experimental flexibility based on a 10-kHz synchronized dual-arm femtosecond and picosecond laser system. Ultrafast high-average-power titanium sapphire lasers and optical parametric amplifiers provide wavelength tuning from the ultraviolet (UV) to the mid-infrared region. Customized silicon, indium gallium arsenide, and mercury cadmium telluride linear array detectors are provided to monitor the probe laser intensity in the UV to mid-infrared wavelength range capable of measuring changes in sample absorbance of ΔOD ∼ 10−5 in 1 second. The system performance is demonstrated for the time-resolved infrared, two-dimensional (2D) infrared, and femtosecond stimulated Raman spectroscopy techniques with organometallic intermediates, organic excited states, and the dynamics of the tertiary structure of DNA.

Photoreactivity examined through incorporation in metal−organic frameworks

Metal-organic frameworks, typically built by bridging metal centres with organic linkers, have recently shown great promise for a wide variety of applications, including gas separation and drug delivery. Here, we have used them as a scaffold to probe the photophysical and photochemical properties of metal-diimine complexes. We have immobilized a M(diimine)(CO)(3)X moiety (where M is Re or Mn, and X can be Cl or Br) by using it as the linker of a metal-organic framework, with Mn(II) cations acting as nodes. Time-resolved infrared measurements showed that the initial excited state formed on ultraviolet irradiation of the rhenium-based metal-organic framework was characteristic of an intra-ligand state, rather than the metal-ligand charge transfer state typically observed in solution, and revealed that the metal-diimine complexes rearranged from the fac- to mer-isomer in the crystalline solid state. This approach also enabled characterization of the photoactivity of Mn(diimine)(CO)(3)Br by single-crystal X-ray diffraction.

EDF2: A density functional for predicting molecular vibrational frequencies

The majority of calculations of molecular vibrational spectra are based on the harmonic approximation but are compared (usually after empirical scaling) with experimental anharmonic frequencies. Any agreement that is observed in such cases must be attributable to fortuitous cancellation of errors and it would certainly be preferable to develop a more rigorous computational approach. In this paper, we introduce a new density functional model (EDF2) that is explicitly designed to yield accurate harmonic frequencies, and we present numerical results for a wide variety of molecules whose experimental harmonic frequencies are known. The EDF2 model is found to be significantly more accurate than other DFT models and competitive with the computationally expensive CCSD(T) method.

A combined theoretical and experimental study on the role of spin states in the chemistry of Fe(CO)5 photoproducts.

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragments (3)Fe(CO)(4) and (3)Fe(CO)(3) formed upon photolysis of Fe(CO)(5). Experimental data are provided for reactions in liquid n-heptane and in supercritical Xe (scXe) and Ar (scAr). Measurement of the temperature dependence of the rate of decay of (3)Fe(CO)(4) to produce (1)Fe(CO)(4)L (L = heptane or Xe) shows that these reactions have significant activation energies of 5.2 (+/-0.2) and 7.1 (+/-0.5) kcal mol(-1) respectively. Nonadiabatic transition state theory is used to predict rate constants for ligand addition, based on density functional theory calculations of singlet and triplet potential energy surfaces. On the basis of these results a new mechanism (spin-crossover followed by ligand addition) is proposed for these spin forbidden reactions that gives good agreement with the new experimental results as well as with earlier gas-phase measurements of some addition rate constants. The theoretical work accounts for the different reaction order observed in the gas phase and in some condensed phase experiments. The reaction of (3)Fe(CO)(4) with H(2) cannot be easily probed in n-heptane since conversion to (1)Fe(CO)(4)(heptane) dominates. scAr doped with H(2) provides a unique environment to monitor this reaction--Ar cannot be added to form (1)Fe(CO)(4)Ar, and H(2) addition is observed instead. Again theory accounts for the reactivity and also explains the difference between the very small activation energy measured for H(2) addition in the gas phase (Wang, W. et al. J. Am. Chem. Soc. 1996, 118, 8654) and the larger values obtained here for heptane and Xe addition in solution.

Infrared micro-spectral imaging: distinction of tissue types in axillary lymph node histology

BackgroundHistopathologic evaluation of surgical specimens is a well established technique for disease identification, and has remained relatively unchanged since its clinical introduction. Although it is essential for clinical investigation, histopathologic identification of tissues remains a time consuming and subjective technique, with unsatisfactory levels of inter- and intra-observer discrepancy. A novel approach for histological recognition is to use Fourier Transform Infrared (FT-IR) micro-spectroscopy. This non-destructive optical technique can provide a rapid measurement of sample biochemistry and identify variations that occur between healthy and diseased tissues. The advantage of this method is that it is objective and provides reproducible diagnosis, independent of fatigue, experience and inter-observer variability.MethodsWe report a method for analysing excised lymph nodes that is based on spectral pathology. In spectral pathology, an unstained (fixed or snap frozen) tissue section is interrogated by a beam of infrared light that samples pixels of 25 μm × 25 μm in size. This beam is rastered over the sample, and up to 100,000 complete infrared spectra are acquired for a given tissue sample. These spectra are subsequently analysed by a diagnostic computer algorithm that is trained by correlating spectral and histopathological features.ResultsWe illustrate the ability of infrared micro-spectral imaging, coupled with completely unsupervised methods of multivariate statistical analysis, to accurately reproduce the histological architecture of axillary lymph nodes. By correlating spectral and histopathological features, a diagnostic algorithm was trained that allowed both accurate and rapid classification of benign and malignant tissues composed within different lymph nodes. This approach was successfully applied to both deparaffinised and frozen tissues and indicates that both intra-operative and more conventional surgical specimens can be diagnosed by this technique.ConclusionThis paper provides strong evidence that automated diagnosis by means of infrared micro-spectral imaging is possible. Recent investigations within the authors laboratory upon lymph nodes have also revealed that cancers from different primary tumours provide distinctly different spectral signatures. Thus poorly differentiated and hard-to-determine cases of metastatic invasion, such as micrometastases, may additionally be identified by this technique. Finally, we differentiate benign and malignant tissues composed within axillary lymph nodes by completely automated methods of spectral analysis.

Development of a Broadband Picosecond Infrared Spectrometer and its Incorporation into an Existing Ultrafast Time-Resolved Resonance Raman, UV/Visible, and Fluorescence Spectroscopic Apparatus

We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, ∼150 cm−1 FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of ΔOD ∼ 10−4–10−5 with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.

Nanosecond Time-Resolved Infrared Spectroscopy with a Dispersive Scanning Spectrometer

A nanosecond time-resolved infrared spectroscopic system based on a dispersive scanning spectrometer has been constructed. This is an advanced version of a similar system reported in a previous paper; the time resolution has been improved from 1 μs to 50 ns and the sensitivity from 10−4 in intensity changes to 10−6. These have been achieved by the use of a high-temperature ceramic infrared light source, a photovoltaic MCT detector, and a low-noise, wide-band preamplifier developed specifically for the present purpose. Time-resolved infrared spectra of a few samples of photochemical and photobiological interests are presented to show the capability of the system. The origin of the thermal artifacts, which have been found to hamper the time-resolved infrared measurements seriously, is shown to be due to the transient reflectance change induced by a small temperature jump. The future prospect of time-resolved infrared spectroscopy is discussed with reference to other methods including infrared laser spectroscopy and Fourier transform infrared spectroscopy.

Cleaner Continuous Photo‐Oxidation Using Singlet Oxygen in Supercritical Carbon Dioxide

High pressure under the spotlight: A new milliliter-scale reactor is developed for using supercritical CO(2) to perform continuous photo-oxidation reactions. Changing from a traditional microliter-scale batch reaction to 8 hours of reaction using the new reactor gives a 3000-fold scale-up of the oxidation of alpha-terpinene (see picture).

Nanosecond time-resolved infrared spectroscopy: a comparative view of spectrometers and their applications in organometallic chemistry

Three time-resolved infrared (TRIR) spectrometers are described. These instruments are based on (i) a line-tunable CO laser (2000–1550 cm–1), (ii) a continuously tunable semiconductor diode laser (approximately 2250–1900 cm–1) and (iii) a modified and adapted dispersive IR spectrometer with Nernst glower (or globar) which, in principle, could cover all of the mid-IR region. The spectrometers differ in their inherent signal-to-noise ratio and their response. Although the ultimate performance can be achieved with the CO laser, the instruments are complementary, each with its own advantages. The performances of the three TRIR spectrometers were compared in a number of applications in organometallic photochemistry, including the IR detection of molecules in electronically excited states, the kinetic behaviour of intermediates in the reaction of (η5-C5H5)M(CO)4 compounds (M = V, Nb and Ta), an IR study of the photochemical reaction of CpMn(CO)3 within polyethylene film and solvent interactions in supercritical xenon solution and the detection of W(CO)5Xe.

Electrodeposition of metals from supercritical fluids

Electrodeposition is a widely used materials-deposition technology with a number of unique features, in particular, the efficient use of starting materials, conformal, and directed coating. The properties of the solvent medium for electrodeposition are critical to the techniques applicability. Supercritical fluids are unique solvents which give a wide range of advantages for chemistry in general, and materials processing in particular. However, a widely applicable approach to electrodeposition from supercritical fluids has not yet been developed. We present here a method that allows electrodeposition of a range of metals from supercritical carbon dioxide, using acetonitrile as a co-solvent and supercritical difluoromethane. This method is based on a careful selection of reagent and supporting electrolyte. There are no obvious barriers preventing this method being applied to deposit a range of materials from many different supercritical fluids. We present the deposition of 3-nm diameter nanowires in mesoporous silica templates using this methodology.