Benzocoumarins, a New Family of Laser Dyes

Coumarin compounds are efficient fluorescers and several of them have been tested for their laser properties1–5. Of particular interest is the wide tunability resulting from the tautomerism of 7-hydroxycoumarins. The structure of 4-methylumbelliferone (4MU) (I) or 4-methyl-7-hydroxy-coumarin is shown in Fig. 1. We have designed, synthesized and tested the hydroxybenzocoumarins for their spectral and laser properties. We have designed these compounds with the intention of obtaining a compound with the wide laser tunability of 7-hydroxycoumarins but with a tunability range beyond the green region of the spectrum which coumarin laser dyes cover. The increase in the π conjugation length in organic molecules is in general accompanied by the bathochromic shift in the absorption and emission spectra. The tautomerism mechanism which we have proposed suggested that the conjugation length between the carbonyl and the hydroxyl groups is the major factor determining the fluorescence λmax. Therefore, the extension of the conjugation from hydroxycoumarins to hydroxybenzocoumarins was made between those groups. (The only benzocoumarin compound reported in the literature as a laser dye is 3-carboxy-5,6-benzocoumarin, which does not contain any hydroxyl group5.) Also, the tautomerism mechanism is satisfied only if the hydroxyl group is placed at specific carbon atoms of benzocoumarin. We have synthesized these compounds, which can be prepared in one step from the commercially available chemicals. Compounds II-VI shown in Fig. 1 have been prepared. Compounds III, V and VI are reported for the first time in the literature. (By the Chemical Abstracts nomenclature system, III, V and VI are 9-hydroxy-1 -trifluoromethyl-3H-naphtho[2,1-b]pyran-3-one, 8-hydroxy-4-trifluoromethyl-2H-naphtho[1,2-b]pyran-2-one and 8-hydroxy-4-phenyl-2H-naphtho[1,2-b]pyran-2-one, respectively.)