Origin of the dual-band laser emission from 7-diethylamino-4-methylcoumarin (C1) in solution: effect of hydrogen-bonding interaction by hydroxylic molecules

Ultraviolet absorption measurements on 7-diethylamino-4-methylcoumarin (Cl) and its rigid analogue C102 in which the 7-amino group is structurally prevented from twisting in n-butyl acetate (nBuOAc) solutions containing submolar concentrations of n-butyl alcohol showed weak 2:1 alcohol:dye complexation. These results contrast with those from fluorescence measurements which revealed a 1:1 alcohol:dye excited state complex for the Cl dye. These results together with N2 laser excited amplified spontaneous emission (ASE), and laser gain measurements on Cl and C102 in nBuOAc with alcohol or acetonitrile co-solvent show that dual ASE from Cl originates from a quasi-equilibrium concentration of hydrogen-bonded or acetonitrile-solvated excited state complex. The results are inconsistent with a TICT-nBuOAc exciplex for Cl in nBuOAc. The laser gain of the twin ASE peaks at 420 nm and 455 nm from Cl can be tuned by adding a small quantity of hydroxylic co-solvent to dried nBuOAc. ASE was found to be a selective probe for a H-bonded Cl complex whose fluorescence is normally obscured by that from the "free" dye. Key words: aminocoumarins, dual amplified spontaneous emission, excited state hydrogen-bonding complex, laser gain.