The Phosphorescence Excitation Method

The very weak Tm - So transitions are hard to observe directly by absorption spectroscopy. Even with long cells, the high concentrations required present solubility — and what is more important — purity problems. An impurity of 1 10 may give rise to absorption bands which have the same intensity as the expected Ti So absorption. The experimental conditions, therefore, have to be chosen to allow an increase of the Ti- - So oscillator strength to be achieved through perturbation by paramagnetic species (O2 or NO) or heavy atoms. Alternatively, an indirect method, phosphorescence excitation spectroscopy, which has high sensitivity and selectivity, may be applied. 

A mirror-image relation (Fig. 16) between the Ti - So absorption spectrum and the Ti- Sq emission spectrum can be expected whenever there is a relatively small geometry change between ground and first triplet states, and S and T1 have similar vibrational spectra. 

Ti So spectra can be determined by scanning the sample with monochromatic light and by monitoring the phosphorescence intensity at a fixed wavelength 

The toted number of phosphorescence photons iVphos emitted per unit time is given by  

The method was first applied by Rothman, Case, and Kearns to the determination of the Ti- -So absorption spectrum of 1-bromonaphthalene. Sixteen photochemically active aromatic ketones cind aldehydes have been investigated by Kearns and Case Transitions from So to two triplet states were located and assigned as n,n ) and 

---

Let us consider in detail a recent paper by Jones, Kearns, and Wing which represents one of the most thorough investigations of singlet-triplet transitions and reveals much of the power of the phosphorescence excitation method. 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

---