Triplet-singlet emission in fluid solutions. Phosphorescence of eosin

Abstract

This paper describes a high-sensitivity spectrophosphorimeter which will record very weak long-lived emission spectra and will measure the ratio of long-lived luminescence to fluorescence down to values of 10-5. It describes tests with solutions of eosin in glycerol and ethanol within the temperature range +70°C to -196°C and discusses the advantages of using the technique to investigate singlet-triplet conversion and triplet quenching.Phosphorescence and delayed fluorescence emission bands were observed from eosin even in ethanol at temperatures as high as +70"C. Quantum efficiencies for triplet formation and activation energies and frequency factors for the triplet to upper singlet transition were determined. Approximate rates for the three intersystem crossing processes were derived. Crossing from triplet to ground state is independent of viscosity so long as the solution does not set to a rigid glass. There is some evidence of intersystem crossing from an upper vibrational level of the excited singlet state.Since the pioneering work of Lewis and co-workers with fluorescein in boric acid glass,l a long-lived luminescence has been observed from many other solutes in rigid media, and its interpretation in terms of triplet-singlet transitions is now well established. In contrast, there have been few reported cases of long-lived luminescence in fluid solution at room temperature. This is mainly due to the long radiative lifetime of the triplet states and their consequent susceptibility to internal quenching and to quenching by minute traces of oxygen or other impurities.2 However, as long ago as 1930, Mlle. Boudin observed a weak long-lived luminescence from solutions of eosin in glycerol at room temperature.3 Using a visual phosphoroscope she found the phosphorescence from a 0-005 % w/v solution to have a lifetime of about 1 msec and an intensity of about 1/400 of the fluorescence. We have repeated and extended her measurements, partly as a test of a new photoelectric spectrophosphorimeter, and partly as a preliminary to a search for triplet-singlet emission from other substances in fluid solution at room temperature. Our first experiments ( fig. 1) showed that there was indeed a visible long-lived luminescence from solutions of eosin in glycerol, but that in addition t o the visible band there was a second band in the far red. At room temperature the intensity of the visible band was of the same order of magnitude as that reported by Mlle. Boudin, but its intensity relative to the second band was strongly temperature dependent. This behaviour seemed to be analogous to that observed by Lewis and co-workers for fluorescein in boric-acid glass. The results to be described confirmed this view and it was found that the long-lived luminescence bands could still be observed in a solvent as fluid as ethanol at room temperature (see fig. 2).The possibility of determining triplet-singlet emission intensities and lifetimes over a wide range of temperature, both in fluid and rigid media, provides a means of investigating th...

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Rational Design of TADF Polymers Using a Donor–Acceptor Monomer with Enhanced TADF Efficiency Induced by the Energy Alignment of Charge Transfer and Local Triplet Excited States

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Intersystem-crossing and excited-state absorption in eosin Y solutions determined by picosecond double pulse transient absorption measurements

“…An enhanced intersystem crossing occurs due to the heavy atom effect of Br [19,20]. Phosphorescence [21] and delayed fluorescence [22,23] have been observed even at room temperature. Reported values of the quantum yield of triplet formation <f>T and of the fluorescence quantum yield <ft F are listed in table 1.…”

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