Phosphorescence spectroscopy spectra

Luminescence spectroscopy involves three related optical methods fluorescence, phosphorescence, and chemiluminescene. These methods utilize excited molecules of an analyte to give a species whose emission spectrum can provide information about the molecule. In fluorescence, atoms can be excited to a higher energy level by the absorption of photons of radiation. Some features of luminescence methods are increased sensitivity (in the order of three magnitudes smaller than absorption spectroscopy), larger linear range of concentration, and method selectivity (Parsons 1982). 

During the past five years two research disciplines of optical spectroscopy and magnetic resonance have merged when it became evident that at low temperatures, microwave radiation of resonance frequencies with the zero-field (zf) transitions of the lowest triplet state could have observable effects on the phosphorescence intensity as well as the spectrum. Quantitative information can then be obtained from these phosphorescence-microwave multiple-resonance experiments from which the magnetic, the radiative, and the nonradiative as well as the structural properties of the triplet state can be determined. 

The method of phosphorescence microwave double resonance (PMDR) spectroscopy is based, like the two other methods discussed above, on c.w. excitation of the Pd(2-thpy)2 compound at low temperature. Additionally, micro-wave irradiation is applied, whereby the frequency is chosen to be in resonance with the energy separation between the two substates I and III of 2886 MHz. With this set-up, one monitors the phosphorescence intensity changes in the course of scanning the emission spectrum. Technically, the phosphorescence spectrum is recorded by keeping the amplitude-modulated microwave frequency at the constant value of 2886 MHz and by detecting the emission spectrum by use of a phase-sensitive lock-in and signal averaging procedure (e.g. see [61, 75,90]). 

The information obtained from the phosphorescence microwave double resonance (PMDR) spectroscopy nicely complements the results deduced from time-resolved emission spectroscopy. (See Sect. 3.1.4 and compare Ref. [58] to [61 ].) Both methods reveal a triplet substate selectivity with respect to the vibrational satellites observed in the emission spectrum. Interestingly, this property of an individual vibronic coupling behavior of the different triplet substates survives, even when the zero-field splitting increases due to a greater spin-orbit coupling by more than a factor of fifty, as found for Pt(2-thpy)2. 

Double-resonance Spectroscopy.—A review has been given of double-resonance methods in spectroscopy.378 Attention will be focused here on optically (usually phosphorescence) detected magnetic resonance experiments (ODMR). Microwave-optical double-resonance experiments have been carried out on the spectrum of gaseous N02,379 permitting assignment of the rotational = 0—4 side-bands of the 493 nm band. 

The lowest excited states of sulfoxides 1 and 2, the corresponding sulfides, and sulfones were studied by photoelectron spectroscopy (PES), voltammetry, absorption, and emission spectroscopy [18]. (Simpler sulfoxides, both saturated and aromatic, have also been studied by PES [19].) The phosphorescence spectrum of 1 and its small AEs r of about 6 kcal/mol are both typical of an aromatic nii ketone. The same data for 2 are more consistent with a Tt triplet. 

Even with simple UV-VIS spectroscopy, the exchange interaction In of a naphthalene dimer in naphthalene, i.e. in a mini-exciton, could be detected as a spUtting of the 0,0 transition in the phosphorescence spectrum, yielding 1.2 0.2cm [18]. 

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