Shannon L. Studer

Practical and Convenient 355-nm and 337-nm Sharp-Cut Filters for Multichannel Raman Spectroscopy

The interference from Rayleigh and other elastic scattering is a serious problem for multichannel Raman spectroscopy. This problem becomes more serious if the surface subjected to the excitation reflects stray light, particularly in front-face excitation and in systems using rotating samples or applying the flow technique to avoid local heating and accumulation of the photoproduct. In order to eliminate this problem the conventional multichannel Raman spectrometer uses a specially designed triple polychromator equipped with a subtractive double-grating filter stage. However, due to the complicated optical arrangements, including three gratings and at least six mirrors, the throughput of the instrument is usually low. This is a severe disadvantage if one needs to detect extremely weak Raman signals. This is particularly true when time-resolved resonance Raman (TRR) spectroscopy is performed. In order to have an optimum TRR signal, the ideal method is to apply a single spectrograph equipped with a sharp line elimination filter tuned to the wavelength of the exciting line. Several filters such as chevron filters and colloid filters have been designed for this purpose. Unfortunately, most of these filters are only applicable in the visible region. In addition, the requirement of a high degree of collimation also reduces the throughput of the signal. Several commercially available sharp-cut long-wavelength bandpass filters and a recently developed J-aggregate filter applying organic dye molecules are also useful for the elimination of exciting lines in the visible region.

Studies of the triplet state of 2-(2'-hydroxyphenyl) benzothiazole

Abstract We present the first observation of the phosphorescence of 2-(2′-hydroxyphenyl) benzothiazole in nonpolar solvents. The phosphorescence has a maximum at 648 nm, and the lifetime was measured to be 1.95 ± 0.2 ms at 77 K in methylcyclohexane glass. The observed decay dynamics of the phosphorescence in combination with studies of the delayed fluorescence and two-step laser-induced fluorescence unambiguously prove that the observed triplet state is ascribed to the cis-keto form of HBT.

The role of the cis-keto triplet state in the proton transfer cycle of 2-(2′-hydroxyphenyl)benzothiazole

A two-step laser-induced fluorescence (TSLIF) study of 2-(2′-hydroxyphenyl)benzothiazole (HBT) in various solvents at <100 K revealed an unusually high yield of tautomer emission resulting from the T′2→S′1 intersystem crossing (ISC, prime denotes the tautomer state) of the cis-keto form. The T—S ISC yield was measured to be 5.2×10−4 in a 77 K MCH glass. A mechanism incorporating Tnπ*→Sππ* ISC is discussed. The identical spectra between non-time-resolved prompt tautomer emission and TSLIF gives further support for the assignment to the cis-keto triplet state.

Temperature-dependent study of the ground-state reverse proton transfer of 3-hydroxyflavone

Abstract Temperature-dependent studies of the rate of the ground-state reverse proton transfer of 3-hydroxyflavone have been performed. At room temperature, the rate of the reverse proton transfer is 6.9×10 4 s −1 in n -heptane. However, the reverse proton-transfer rate is 6.3×10 4 s −1 in a 77 K n -heptane Shpolskii matrix, which does not conform to predictions based on extrapolation of the liquid phase kinetic data. In the course of this study we have found that, unlike the steady-state tautomer emission, the tautomer emission measured by the two-step laser excitation experiment is not obscured by contributions due to hydrogen-bonding impurities in n -heptane Shpolskii matrices. An infrared spectrum of 3-HF in a 12 K argon matrix is presented for the first time. Photolysis of matrix-isolated 3-HF monitored by infrared spectroscopy fails to afford the ground-state tautomer.

Excited-state proton transfer spectroscopy of 3-hydroxypicolinamide

Abstract A rapid intramolecular proton transfer is observed in the lowest excited singlet state of 3-hydroxypicolinamide. A temperature-independent tautomerization rate of > 2×10 12 s −1 is deduced from steady-state and picosecond fluorescence measurements at both room temperature and 77 K. The lack of any detectable temperature dependence is indicative of a negligible barrier to tautomerization.

Studies of T2→S2 intersystem crossing for coumarins

Abstract Transient absorption and two-step laser-induced fluorescence studies of coumarin (CM), 7-hydroxy-4-methylcoumarin (7HMC) and 7-diethylamino-4-methylcoumarin (7DAMC) were performed to determine Φ ts , the yield of T 2 →S 1 ISC. Φ ts was measured to be 8.4×10 −4 and 1.4×10 −3 for 7HMC and 7DAMC, respectively. The results can be rationalized by the proposed mechanism incorporating T nπ * →S ππ * ISC.

PHOTOOXYGENATION OF 3-HYDROXYFLAVONE IN A 12 K O2 MATRIX

Abstract— We present an infrared study of 3‐hydroxyflavone in 12 K Ar and O2 matrices. Although it is extremely photostable in an Ar matrix, a remarkable oxygenation reaction for 3‐hydroxyflavone takes place upon photoexcitation in an O2 matrix. The primary photoproduct is concluded to be a keto‐hydroperoxide. On further photolysis the keto‐hydroperoxide affords the photostable secondary product 2‐(benzoyloxyl)benzoic acid, as well as CO and CO2 through two independent pathways.

Direct spectroscopic measurements of 1Δg O2 production by thermodecomposition and UV (266 nm) photolysis of benzaldehyde hydrotrioxide

Abstract The 1 Δ g O 2 chemiluminescence expelled from thermodecomposition as well as 266 nm photolysis of an oxygen rich molecule, benzaldehyde hydrotrioxide, has been observed. The 1 Δ g O 2 production through the excited-state proton (or hydrogen atom) transfer reaction is intriguing in terms of the mechanism. The probable explanation is the increase of the basicity for the carbonyl oxygen upon the π→π* transition, resulting in a great driving force for the proton (hydrogen atom) migration in the excited state.