Fluorescence of acetylenic hydrocarbons

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Abstract
A structureless fluorescence with a half‐width of ∼6.5 kK has been observed from optically excited alkylated acetylenes (from propyne to 1‐, 2‐, 3‐, and 4‐octynes) in cyclohexane solution. The emission maxima of the 1‐alkyne series (propyne, 1‐butyne, 1‐pentyne, etc.) are all at 370 nm, and those of the 2‐, 3‐, and 4‐alkyne series (2‐butyne, 3‐hexyne, 4‐octyne, etc.) are all at 388 nm, regardless of the number of carbon atoms, NC, in the molecule. The fluorescence is subject to concentration quenching the efficiency of which is much higher for 1‐alkynes than for 2‐, 3‐, and 4‐alkynes. The fluoresence quantum yields, extrapolated to zero concentration, of 1‐, 2‐, 3‐, and 4‐alkynes are found to be independent of NC, and are 0.013, 0.014, 0.009, and 0.009, respectively. The fluorescence spectra in the vapor phase are very similar to those in solution except for a slight blue shift (by ∼5 nm). The observed emissions are interpreted as originating from the lowest singlet excited states (?) of the alkynes, the abnormally large Stokes shift (≳20 kK) being a consequence of the large disparity in equilibrium nuclear configurations of ground and excited states.