Triplet–triplet (T–T) absorption spectra of organic compounds (e.g., dyes) are conveniently obtained by the steady-state method employing an ultraviolet/blue cw laser as an excitation source. The large depletion of molecules in the ground singlet state allows accurate determination of molar triplet extinction coefficients ∊T without the knowledge of additional kinetic parameters. An approximation method is presented, which allows reasonably accurate determination of the concentration of triplet molecules [and consequently ∊T(λ)] at the onset of the singlet-singlet (S–S) absorption. This method is of interest in cases in which strong S–S and strong T–T absorption overlap. The ∊T of brilliant sulphaflavine in the region from 4700 to 6520 Å was measured. As the excitation source, the 4416-Å line of a 50-mW He–Cd cw laser was used. A 45% depletion of the singlet-ground-state population was observed.
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