Structure and Reactivity of Organic Intermediates as Revealed by Time-Resolved Infrared Spectroscopy. In: Neckers DC, Von Bünau G, Jenks WS, editors. Advances in Photochemistry. Hoboken: John Wiley & Sons, Inc.; 2001. pp. 41-91. [DOI: 10.1002/9780470133576.ch2]

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR(3)), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S1 over 800 fs are followed by bond heterolysis in ∼1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ∼50 ps followed by intersystem crossing in ∼300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR(3) experiments, permitting a more clear assignment and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ∼4 K and observing a triplet species by cryogenic EPR spectroscopy.

Probing organometallic reactions by time-resolved infrared spectroscopy in solution and in the solid state using quantum cascade lasers

The photochemistry and photophysics of metal carbonyl compounds (W(CO)6, Cp*Rh(CO)2 (Cp* = η(5)-C5Me5), and fac-[Re(CO)3(4,4'-bpy)2Br] [bpy = bipyridine]) have been examined on the nanosecond timescale using a time-resolved infrared spectrometer with an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning it can monitor small transients with absorbance changes less than 10(-6) ΔOD. The C-H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we demonstrate that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements.