Effects of substitution by auxochromic groups on the electronic spectra of aromatic compounds are discussed. Depending on the location and direction of the singlet-singlet (S-S) relative to the triplet-triplet (T-T) oscillators within the molecular frame, substitution by an auxochromic group may cause T-T absorption bands to move into or out of the fluorescence region, or they may be left unchanged. Predictions can thus be made of potential dye laser materials based on substitutional effects on the spectral properties of aromatic compounds. The predictions are incorporated in a graph that presents possible arrangements of low-energy SS oscillators relative to a T-T oscillator located in the fluorescence region of the aromatic compound. Depending on short- or long-axis substitution, the red shifts of the S-S and T-T absorption bands result in two promising constellations (structures) for low T-T absorption over the fluorescence region. The laser action properties of coumarin and some of its derivatives are discussed.

Prediction of laser action properties of organic dyes from their structure and the polarization characteristics of their electronic transitions

: The 'exciplex' mechanism of radiation emission from a dye affords one means of producing a broad fluorescent spectrum without adding to the absorption spectrum. It relies on a change of equilibrium properties in the excited state compared with those of the ground state, and the fluorescence is not from the same chemical species absorbing the radiation. Such a system may show a considerable displacement of the fluorescence and absorption spectra so that the threshold input power for lasing may be reduced and the range of tuning can be extended. Also, by a careful selection of conditions, emissions from both the dye and the exciplex may appear, thereby permitting an even greater tuning range. Two mechanisms apply: the 'proton exciplex', which relates to changes in conventional acidity and basicity in the excited state; and the 'molecular exciplex', which relates to changes in electron-acceptor and electron-donor properties(i.e., changes in Lewis acidity and basicity). Wide tuning range (blue-violet to yellow-green, 122 nanometers) is reported for 4,8-dimethylumbelliferone. It is proposed that this dye operates by a double proton exciplex mechanism, wherein the molecule is both a stronger acid and a stronger base in the excited state. Peculiarities in the lasing action of 9-aminoacridine are discussed.

R. S. Hughes

Papers

A Search for Some Wide-Range Tunable Dye Laser Systems Working on the 'Exciplex' Principle.